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Efficiency Maine Team IV Subcooling a Fujitsu 15RLS2 Mini-Split Heat Pump
Capstone Design Report May 8, 2014
Submitted to:
Michael L. Peterson, Ph.D. “In partial fulfillment of requirements for MEE 488, Spring 2014”
CC:
Murray Callaway, M.A. “In partial fulfillment of requirements for ECP 488, Spring 2014”
Submitted by:
Samuel M. Prentiss, Matt D. H. Strong, Bruce L. Tuttle
Mechanical Engineering Department College of Engineering
University of Maine 5711 Boardman Hall
Orono, ME 04469-5711
ii
Abstract Five mechanical engineering capstone teams at the University of Maine have taken part in a yearlong
project to evaluate the cost effectiveness of Efficiency Maine’s electricity energy usage programs and to
help educate the public on the more obscure technical aspects of available energy products. Efficiency
Maine Team IV has designed, constructed, and tested a device, designated as the Subcooler Unit, that
can increase the efficiency of a residential air-to-air mini-split heat pump running in heating mode by up
to 66 percent corresponding to a COP of 4.4. This device extracts thermal energy from the liquid
refrigerant after it condenses in the heat pump’s indoor unit, resulting in both an increased system
coefficient of performance and compressor energy efficiency ratio. Intended as a retrofit for an existing
air-to-air mini-split heat pump or as part of a new installation, the Subcooler Unit is easily incorporated
in the heat pump cycle by placing it in line with the copper liquid refrigerant line connecting the mini-
split heat pump’s indoor and outdoor unit. The device, consisting of a refrigerant-to-air heat exchanger,
an inline centrifugal fan, various ductwork components, and a smart control system, was produced for
under $1000 resulting in a payback period of nine months. The Subcooler Unit was installed with a
Fujitsu 15RLS2 mini-split heat pump at the Service Building A, which is located at the University of Maine
in Orono, Maine, where both the Subcooler Unit and the Fujitsu 15RLS2 were monitored and
performance characteristics recorded and analyzed. Through completion of this project
iii
Acknowledgment Michael Peterson, Ph.D. Efficiency Maine Team IV would like to thank Dr. Peterson for his dedication to the capstone class and
for his insightful real world engineering knowledge.
James C. Labrecque Efficiency Maine Team IV would like to thank James C. LaBrecque for the initial conception of the
Subcooler Unit and his unwavering support throughout the entire capstone process.
Karen Fogarty As the Mechanical Engineering Departments Administrative Assistant, Karen was responsible for
processing all purchase requests. Without her dedication, this project could not have happened.
Alcoil Efficiency Maine Team IV would like to thank Alcoil for donating the aluminum micro-channel coil heat
exchanger coil to our capstone project. The coil’s unique small size paired with its high heat transfer
capacity was pivotal in making our design successful. We would especially like to thank Jim Bogart at
Alcoil for his helpful advice during the design process.
Stewart Harvey Stewart Harvey is the Executive Direct of Facilities at the University of Maine and was instrumental in
making our installation a reality.
Professor Murray Callaway Professor Murray Callaway provided helpful advice while writing this report and during the development
of our team website and open house poster.
Justin Poland, Ph.D. Dr. Poland provided helpful advice regarding the measurement of an airstream velocity profile, heat
pump theory, and economic analysis methods.
Senthil Vel, Ph.D Professor Vel donated an Arduino Uno micro controller board and a SPDT relay module board and
provided advice regarding our smart control system.
Authur Pete Author Pete helped with welding thermocouple beads and provided helpful Crosby Laboratory advice.
R J Morin Inc R J Morin Inc. was responsible for installing the Fujitsu 15RLS2 and Subcooler Unit at Service Building A.
University of Maine Electrical Shop The University of Maine Electrical Shop and Nate Emerson in particular, was responsible for all electrical
wiring for the installation at Service Building A.
iv
Table of Contents Abstract ......................................................................................................................................................... ii
Acknowledgment ......................................................................................................................................... iii
List of Figures .............................................................................................................................................. vii
List of Tables ................................................................................................................................................ ix
Individual Contributions to Report Sections ................................................................................................. x
Nomenclature .............................................................................................................................................. xi
1 Introduction .......................................................................................................................................... 1
1.1 Energy Capstone Projects and the Efficiency Maine Trust ........................................................... 1
1.2 Air-to-Air Mini-Split Heat Pumps .................................................................................................. 1
1.2.1 The Real Alternative Energy .................................................................................................. 1
1.2.2 How a Heat Pump Works ...................................................................................................... 2
1.2.3 Heat Pump COP, EER, and SEER, and HSPF ........................................................................... 3
1.2.4 Heat Pump Advantages ......................................................................................................... 4
1.2.5 Heat Pump Disadvantages .................................................................................................... 4
1.3 Heat Pump Subcooling and Increasing System Efficiency ............................................................ 4
1.4 Desired Outcome .......................................................................................................................... 5
2 Design Description ................................................................................................................................ 6
2.1 Increase Heat Pump Efficiency ...................................................................................................... 6
2.2 Subcooler Unit Mechanical Componentry .................................................................................... 8
2.3 Subcooler Unit Power and Control Electronics ........................................................................... 12
2.4 Arduino IDE Code for Smart Controls ......................................................................................... 16
2.5 Installation at Service Building A ................................................................................................. 19
2.6 Testing and Instrumentation ...................................................................................................... 23
2.7 Budget for Subcooler Unit, Fujitsu 15RLS2, and Installation ...................................................... 24
3 Design Process .................................................................................................................................... 25
3.1 Initial Conception ........................................................................................................................ 25
3.2 Refrigeration Theory ................................................................................................................... 25
3.2.1 Sensible and Latent Heat .................................................................................................... 25
3.3 System Performance Prediction ................................................................................................. 26
3.3.1 Fujitsu 15RLS2 without Subcooler Unit ............................................................................... 26
3.3.2 Fujitsu 15RLS2 with Subcooler Unit .................................................................................... 29
v
3.4 Subcooler Unit Design Mechanical Componentry Specifications ............................................... 32
3.4.1 SolidWorks .......................................................................................................................... 32
3.4.2 Alcoil Micro-Channel Coil .................................................................................................... 32
3.4.3 Square Duct Assembly ........................................................................................................ 33
3.4.4 Fantech FG 6XL Centrifugal Inline Duct Fan ........................................................................ 34
3.4.5 Actuated Dampers .............................................................................................................. 34
3.4.6 Transition Ducts .................................................................................................................. 35
3.4.7 Various Ductwork Components .......................................................................................... 35
3.5 Subcooler Unit Electrical Power and Controls ............................................................................ 35
3.5.1 Electrical Power Componentry ........................................................................................... 35
3.5.2 Electrical Control Componentry .......................................................................................... 36
3.5.3 Arduino IDE Development .................................................................................................. 37
3.6 Finding an Installation Location .................................................................................................. 38
4 Final Design Testing and Evaluation.................................................................................................... 40
4.1 Testing and Evaluation Objectives .............................................................................................. 40
4.2 Regarding Subcooler Unit and Fujitsu 15RLS2 Evaluation .......................................................... 41
4.3 Test Trials .................................................................................................................................... 41
4.4 Subcooler Unit Testing and Evaluation ....................................................................................... 42
4.4.1 Measuring Airstream Velocity Profile ................................................................................. 42
4.4.2 Measuring Subcooler Unit Airstream Inlet and Outlet Temperatures ............................... 45
4.4.3 Measuring Subcooler Unit Refrigerant Inlet and Outlet Temperatures ............................. 46
4.4.4 Calculating Subcooler Unit Input Power ............................................................................. 48
4.4.5 Calculating Heat of Rejection from Subcooler Unit ............................................................ 48
4.4.6 Calculating Subcooler Unit Operating Efficiency ................................................................ 53
4.5 Fujitsu 15RLS2 Testing and Evaluation ........................................................................................ 55
4.5.1 Measuring Electrical Power Input to Fujitsu 15RLS2 Heat Pump ....................................... 55
4.5.2 Measuring Fujitsu 15RLS2 Indoor Unit Airstream Temperatures ....................................... 57
4.5.3 Calculating Heat of Rejection from Fujitsu 15RLS2 Indoor Unit ......................................... 58
4.6 Overall Installed System Evaluation ............................................................................................ 60
4.7 Economic Analysis ....................................................................................................................... 62
4.7.1 Payback Period on Installation ............................................................................................ 62
4.7.2 Payback Period on Manufactured Device ........................................................................... 64
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4.7.3 Comparison of Other Heating Fuel Sources ........................................................................ 64
4.7.4 Economic Analysis Summary ............................................................................................... 65
5 Conclusions ......................................................................................................................................... 66
5.1 Effect of Subcooling on Fujitsu 15RLS2 ....................................................................................... 66
5.2 Subcooler Unit Future Design ..................................................................................................... 66
5.3 Possible Solution to Heating Energy Demand in Maine ............................................................. 67
6 References .......................................................................................................................................... 68
Appendix A: Subcooler Unit Drawing Package ........................................................................................... 69
Appendix B: Power and Control Electrical Schematic ................................................................................. 69
Appendix C: Arduino IDE Code for Controls................................................................................................ 69
Appendix D: Installation Drawing Package ................................................................................................. 69
Appendix E: Subcooler Unit and Installation Parts Specification Budget ................................................... 69
Appendix F: System Prediction Calculation Spreadsheet ........................................................................... 69
Appendix G: MEE 443 Mechanical Laboratory III Experiment Report ........................................................ 69
Appendix H: Data Reduction Spreadsheet .................................................................................................. 69
Appendix I: Economic Analysis Spreadsheet .............................................................................................. 69
vii
List of Figures Figure 1. Vapor-compression cycle describing refrigerant states. .............................................................. 3
Figure 2. Fujitsu 15RLS2 with Subcooler Unit system conceptual schematic. ............................................ 7
Figure 3. Pressure-enthalpy diagram describing effects of subcooling a Fujitsu 15RLS2 MSHP. ............... 8
Figure 4. Assembled Subcooler Unit from left to right: two 6 in. actuated dampers, one WYE duct, one 6
in. round to 15 in. square transition duct, (Square duct assembly including filter rack, pleated air filter,
and micro-channel coil), 15 in. square to 6 in. round transition duct, and one Fantech FG 6XL centrifugal
inline duct fan. .............................................................................................................................................. 9
Figure 5. Alcoil all-aluminum micro-channel heat exchanger coil. ........................................................... 10
Figure 6. Square duct assembly to house micro-channel coil, filter rack with door, and pleated air filter.
.................................................................................................................................................................... 10
Figure 7. Transition ducts positioned on either side of the square duct assembly (6 in. round to 15 in.
square). ....................................................................................................................................................... 11
Figure 8. Fantech FG 6XL Centrifugal Inline Duct Fan. .............................................................................. 11
Figure 9. Honeywell 6 in. 24 VAC actuated dampers. ............................................................................... 12
Figure 10. Electrical Enclosure for Subcooler Unit Containing all Power and Controls Componentry. .... 12
Figure 11. Din rail mounted 5 amp breaker, hot terminal strip, neutral terminal strip, terminal
connections, and one 120 vac to 24 vac transformer located within Subcooler Unit’s electrical enclosure.
.................................................................................................................................................................... 13
Figure 12. Toggle switches to select smart or manual Subcooler Unit control and fan speed adjuster. . 14
Figure 13. 102 VAC receptacle, 9 VDC adapter, Arduino Uno micro controller board, SPDT relay module
board, standard solder less bread board, and two DS18B20 digital temperature sensors leads connected
by means of a 4.7K axial lead resistor and 3-pin screw type terminal block. ............................................. 15
Figure 14. Sample DS18B20 digital temperature sensor and three sample 4.7K ohm resistors. ............. 15
Figure 15. Sample 0.5 in. Carflex conduit and two straight fittings. ......................................................... 16
Figure 16. Two DS18B20 digital temperature sensors located before and after the micro-channel coil to
measure Subcooler Unit airstream temperatures. ..................................................................................... 16
Figure 17. Light dependent resistor (LDR) used to detect light and dark in the photocopier room. ....... 17
Figure 18. Diagram representing smart control system logic. ................................................................... 18
Figure 19. Service Building A at the University of Maine. ........................................................................... 19
Figure 20. Subcooler Unit installed in photocopier room (Dryer duct is run to outside for testing
purposes). ................................................................................................................................................... 19
Figure 21. The two unistrut channels bolted to wall in photocopier room to support Subcooler Unit. .. 20
Figure 22. Fujitsu 15RLS2 outdoor unit mounted on the outside brick wall of Service Building A. ......... 21
Figure 23. Fujitsu 15RLS2 indoor unit mounted to wall in Work Control Center located in Service
Building A. ................................................................................................................................................... 21
Figure 24. Refrigerant lines to connect Fujitsu 15RLS2 outdoor unit and indoor unit with the Subcooler
Unit.............................................................................................................................................................. 22
Figure 25. Mount for Fujitsu 15RLS2 and hole in wall to run insulated copper refrigerant lines and
electrical lines. ............................................................................................................................................ 22
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Figure 26. Insulated copper liquid refrigerant lines diverted from Fujitsu 15RLS2 through ceiling in Work
Control Center to connect to Subcooler Unit. ............................................................................................ 23
Figure 27. Fujitsu 15RLS2 MSHP with Subcooler Unit conceptual system schematic with key refrigerant
states labled. ............................................................................................................................................... 27
Figure 28. Predicted Subcooler Unit heat output. .................................................................................... 30
Figure 29. Predicted Fujitsu 15RLS2 indoor unit heat of rejection due to subcooling liquid R-410A. ...... 31
Figure 30. Predicted System (Subcooler Unit and Fujitsu 15RLS2) COP. .................................................. 32
Figure 31. FG series performance characteristics supplied by Fantech. ................................................... 34
Figure 32. Temporary 6 in. flexible duct to draw Subcooler Unit airstream from outside. ...................... 42
Figure 33. Hot Wire Anemometer and Traversing Guide Apparatus. ....................................................... 43
Figure 34. Subcooler Unit and Additional Ducting for Airstream Profile Experiment .............................. 44
Figure 35. Velocity measurement locations specicified by the Log-Linear Rule for Circular ducts .......... 44
Figure 36. Plot showing fully developed airstream velocity profile in Subcooelr Unit exit duct for two test
trials. ........................................................................................................................................................... 45
Figure 37. Type T thermocouple inserted in Subcooler Unit to measure inlet airstream temperature. .. 46
Figure 38. Two Type T thermocouple attached to inlet and outlet 0.625 in. micro-channel connections
respectively ................................................................................................................................................. 47
Figure 39. USB thermocouple data logger to measure Subcooler Unit's inlet and outlet airstream
temperatures and inlet and outlet R-410A temperatures. ......................................................................... 47
Figure 40. Subcooler Unit heat output as a function of outdoor temperature. ....................................... 52
Figure 41. Subcooler Unit Efficiency plotted as a function of outdoor temperature ............................... 54
Figure 42. Curve fit 6th degree polynomial for Fujitsu 15RLS2 input work as a function of outdoor
temperature. ............................................................................................................................................... 56
Figure 43. Fujitsu 15RLS2 inddor unit outfitted with type T thermocouples. ........................................... 58
Figure 44. Second USB thermocouple data logger to measure and record temperatures associated witht
he Fujitsu 15RLS2 indoor unit. .................................................................................................................... 58
Figure 45. Fujitsu 15RLS2 heat output as a function of outdoor temperature ......................................... 60
Figure 46. Installed system (Subcooler Unit and Fujitsu 15RLS2) COP plotted as a function of outdoor
temperature ................................................................................................................................................ 61
Figure 47. Annual average heating cost for various fuel sources in Maine. ............................................. 65
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List of Tables Table 1. Summarized Budget for Efficiency Maine Team IV Capstone Design Project. ............................ 24
Table 2. Description of six unique R-410A refrigerant states with respect to vapor compression cycle. 27
Table 3. Variables used to determine heat of rejection from the Subcooler Unit. .................................. 49
Table 4. Engineering constants used in data reduction. ........................................................................... 49
Table 5. Coefficient values and respective units for 6th degree polynomial curve fit of Fujitsu 15RLS2
input power ................................................................................................................................................. 57
Table 6. Variables used to calculate heat of rejection from Fujitsu 15RLS2 indoor unit. ......................... 59
Table 7. Donations to Efficiency Maine Team IV project. ......................................................................... 62
x
Individual Contributions to Report Sections This final design report is a collaboration of Efficiency Maine Team IV’s three team members. The details
of who wrote each report section can be seen below. If a team member wrote all subsections of a
certain section, only the section will be shown.
Samuel M. Prentiss Sam was responsible for compiling the design report and for a majority of the editing.
The following sections were written by Sam:
1.2, 1.3, 2.1, 2.2, 2.3, 2.6, 3.2, 3.3, 3.4, 4.1, 4.2, 4.3, 4.4.4, 4.4.5, 4.4.6, 4.5, 4.6
Sam helped to write the following sections:
1.1, 1.4, 3.1, 3.5.1, 3.5.2, 4.4.2, 4.4.3, 5.1, 5.2, 5.3
Sam was responsible for Appedixes A, F, G, and H
Matt D. H. Strong The following sections were written by Matt:
2.7, 4.4.1, 4.7
Matt helped to write the following sections:
1.1, 1.4, 3.1, 4.4.2, 4.4.3, 5.2, 5.3
Matt was responsible for Appedixes A, G, and I.
Bruce L. Tuttle The following sections were written by Bruce:
2.4, 2.5, 3.5.3, 3.6
Bruce helped to write the following sections:
1.1, 3.1, 3.5.1, 3.5.2, 5.1, 5.2
Bruce was responsible for Appedixes B, C, D, and G.
xi
Nomenclature Engineering MSHP mini-split heat pump.
HVAC heating, ventilating, and air conditioning.
COP Coefficient of Performance.
EER Energy Efficiency Ratio.
SEER Seasonal Energy Efficiency Ratio.
BTU/hr British thermal unit per hour.
W Watt.
ton 12,000 BTUh of Cooling Capacity.
CFM Cubic Feet Per Minute of fluid flow.
lbm Pound Mass.
°F Degree Fahrenheit.
sec Second.
min Minute.
hr Hour.
h Specific Enthalpy in BTU per lbm.
CP Specific Heat at Constant Pressure of Air.
IDE Code Integrated Development Environment Code.
LDR Light dependent resistor.
Economics SBC System Benefit Charge.
RGGI Regional Greenhouse Gas Initiative.
T&D Transmission and Distribution of Electrical Energy.
1
1 Introduction
1.1 Energy Capstone Projects and the Efficiency Maine Trust At the beginning of the 2013-2014 academic school year, five capstone groups comprised of seniors majoring in
Mechanical Engineering at the University of Maine were tasked to evaluate the cost effectiveness of Efficiency
Maine’s electricity energy usage programs.
Efficiency Maine is a quasi-public independent trust committed to reducing the energy usage in the State of
Maine. A significant portion of the funding for Efficiency Maine comes from the System Benefit Charge (SBC) and
the Regional Greenhouse Gas Initiative (RGGI). SBC money comes from an additional charge applied to
customers on their monthly electrical bill and RGGI money comes from carbon allowances purchased by
electricity production plants. Due to these two chief sources of funding sources, the primary focus of the trust
has been on reducing electricity usage in Maine. To the informed individual this is an ironical goal since
electricity usage in Maine is unusually low. Furthermore, legislation has been underway to expand Maine’s
electric grid.
Several of Efficiency Maine programs, and proposed savings to residents and reduction in greenhouse gas
emissions, have fallen short of their mark. This combined with the increased costs of electricity in Maine led to
the formation of the five energy capstone projects. It was desired that each group perform an unbiased analysis
free of bureaucratic policy constraints. This analysis could take the form of a transportable demo unit or a
system installation at a public building, which would both help educate the public on the more obscure technical
aspects of available energy products.
Our capstone group, Efficiency Maine Team IV, sought to design and fabricate a miniature air handling device to
increase the efficiency of a residential air-to-air mini-split heat pump (MSHP). This device, to be incorporated
into the MSHP, has been deemed the Subcooler Unit. In addition to testing and validating the subcooling
concept, a thorough economic analysis has also been completed evaluating the cost effectiveness of Efficiency
Maine Team IV’s MSHP project.
1.2 Air-to-Air Mini-Split Heat Pumps
1.2.1 The Real Alternative Energy
Electrical energy production from both photovoltaic panel systems and wind turbine farms has received high
praise as alternative energy solutions. Both energy production methods are considered green, as they receive all
of their energy from the environment and have zero carbon emissions (Disregarding device manufacturing).
Photovoltaic panel systems convert incident solar energy to electrical energy and wind turbines convert kinetic
energy in the wind to electrical energy. The key word is availability. Regardless of how technologically advanced
either of these two energy production methods become, they are still inertly dependent upon the environment.
How often is it cloudy? How often does the wind not blow?
Thermal energy in the environment is the solution to an energy fickle source. Even though air in the atmosphere
may feel “cold” by human standards, it is far warmer than air existing at negative 460° F (absolute zero). This
means that even on a cold winter’s night, heat can be extracted from the atmospheric air. Heat pumps are able
2
to accomplish this feat. Since many of the refrigerants used in heat pumps boil (evaporate) at significantly
negative temperatures, they are able to extract heat from the environment. Day or night, sunny or cloudy, calm
or windy, heat pumps are unyielding in their ability to procure thermal energy from the environment. So instead
of being useful only for a fraction of the year, heat pumps can run as needed, when needed, with little regard to
Mother Nature’s whims.
1.2.2 How a Heat Pump Works
A heat pump is a device that operates on the thermodynamic process known as the vapor-compression cycle
and is able to transfer thermal energy (heat) in the opposite direction than that thermal energy would normally
flow. This means that a heat pump can absorb heat from a cold space and deliver it to a warmer space. A heat
pump is able to accomplish this by means of external electrical energy delivered to a compressor and that the
refrigerant flowing within the heat pump has unique properties. R-410A, a refrigerant used in many residential
heat pumps, evaporates at -55.3 degrees Fahrenheit thus allowing it to absorb heat from a cold space [1].
Some common examples of a heat pump include a household refrigerator, an automobile air conditioner, or a
window air conditioner. Less well known devices include geothermal heat pumps and MSHPs, which are both
capable of cooling or heating a residential space or commercial building. Mini-split means that the heat pump is
capable of reversing the refrigerant flow within the device and is thus capable of either moving thermal energy
from outside to inside (heating) or from inside to outside (cooling), depending on comfort level desired by the
building’s occupants. By essentially combining two appliances into one compact attractive package, MSHPs have
become ever more popular as residential heating and cooling solutions. Due to the two-face capability of a
modern MSHP, the terms condenser and evaporator for the outdoor and indoor unit respectively typically used
in the heating, ventilation and air conditioning (HVAC) industry have become obsolete. The two units are now
referred to as outdoor and indoor units and can act as a condenser or evaporator depending on what mode the
MSHP is set to. In heating mode, the indoor unit acts as the condenser and the outdoor unit acts as the
evaporator, and vice versa for cooling mode.
The vapor-compression cycle can be described in four main simplified steps (Figure 1). While a MSHP can either
heat or cool a building, the vapor compression cycle will be described for heating mode. Since this process is
cyclic, it is necessary to choose an arbitrary point in the cycle to start with. For this illustration, the vapor-
compression cycle will begin with circulating refrigerant entering the heat pump’s compressor as a vapor. The
vapor is then compressed and exits at a higher pressure and higher temperature. This hot, pressurized vapor
then enters the condenser (indoor unit), where it changes state to a liquid, thus giving off heat. The liquid
refrigerant then passes through an expansion valve, where its pressure decreases dramatically and then travels
outside to the evaporator, where it changes state to a vapor again and thus extracts heat from the environment.
This vapor then begins the cycle again and enters the compressor.
3
Figure 1. Vapor-compression cycle describing refrigerant states.
Source: “Heat Pumps: Harnessing the Energy of Nature”, 2014. [Online]. Available: http://www.carmichaelbrowns.co.uk/heat-pumps/.
[Accessed 22 4 2014]
Figure 1 and the previous description of the vapor-compression cycle are very simplistic and do not describe any
superheating and subcooling that may occur.
1.2.3 Heat Pump COP, EER, and SEER, and HSPF
Heat pumps are the only man made device that has an operating efficiency greater than 100%. This is due to the
fact that for every unit of electrical energy inputted to the heat pump two to three more units of thermal energy
are absorbed from the environment. All of these energy units combine and are delivered to the desired heat
sink location. In short, a heat pump is able to take “free” energy from the atmosphere.
The coefficient of performance (COP) of a heat pump is the ratio of the thermal energy transfer rate delivered
by the heat pump to the heat sink divided by the electrical energy rate delivered to the heat pump (Compressor,
fans, and controls). Since both numerator and denominator have units of energy per time, a heat pump’s COP is
a unit less parameter. In the case of a MSHP running in heating mode, the MSHP’s COP can be calculated as
follows:
Eq.1
Typical COP values for modern MSHPs are in the 2.5 to 3.5 range and are heavily dependent on outdoor
temperature. The energy efficiency ratio (EER) rating of a heat pump is used in the HVAC industry to characterize
a heat pumps cooling ability and is defined as the ratio of the thermal energy input to the heat pump’s
COPHeat_PumpHeat_Output
Electrical_Work_Input
4
evaporator in BTU/hr to the electrical energy input to the heat pump in Watts. Thus, an EER rating has units of
BTU per Watt-hr.
The seasonal energy efficiency ratio (SEER) rating for a heat pump is defined as the ratio of the cooling output
for a season in BTUs divided to the total electrical energy input in Watt-hr during the same period. The heating
seasonal performance factor (HSPF) is the ratio of heating output for a season in BTUs divided to the total
electrical energy input in Watt-hr during the same period. While a heat pump’s COP and EER rating are used to
characterize instantaneous performance, SEER and HSPF are used to characterize seasonal temperature
variations and on/off cycling.
1.2.4 Heat Pump Advantages
Modern residential MSHPs are attractive in design and incorporate variable speed compressors and fans
allowing for the exact amount of heating or cooling to be delivered to the building. These variable speed
compressors paired with advanced control systems allow for very constant indoor temperatures to be attained.
Past mini-split heat pumps did not have variable speed compressors and fans, and thus could only operate at full
speed which often resulted in inefficient operating conditions and wasted electrical energy. Fans located on
ductless MSHPs indoor and outdoor units are quite efficient. The ductless design results in lower static pressure
for the fans to overcome.
1.2.5 Heat Pump Disadvantages
Although there are significant advantages to heat pumps, there are some problems surrounding them. A MSHP’s
operating performance drops with outdoor temperature. Once the outdoor temperature drops low enough, the
heat pump will shut off automatically. Although some MSHPs are designed to be able to operate at subzero
temperatures, a secondary heat source is often required. Another problem with heat pumps is the social stigma
surrounding them. Unreliable performance paired with exorbitant initial purchase and maintenance costs has
seriously deterred potential customers in the past. Modern residential MSHP design has alleviated these
concerns.
1.3 Heat Pump Subcooling and Increasing System Efficiency Efficiency Maine Team IV’s senior capstone design project for the Mechanical Engineering Department at the
University of Maine consists of subcooling a one and a quarter ton Fujitsu 15RLS2 mini-split heat pump while it is
running in heating mode. This means extracting thermal energy from the flowing liquid R-410A refrigerant after
it condenses in the indoor unit and before it passes through the electronic expansion valve and enters the
outdoor unit’s coil (evaporator). Subcooling (sensible heat) refers to lowering the temperature of the liquid
refrigerant from its condensing temperature and typically does not occur within a stock Fujitsu mini-split heat
pump. Subcooling liquid refrigerant, with respect to heat pump cycles, is not a new concept and has been
employed by the refrigeration industry for many years to increase the capacity of evaporator units located in
food coolers and freezers.
Theoretically the R-410A refrigerant should completely condense in the stock Fujitsu 15RLS2’s indoor unit and
thus enter the flowing through the heat exchanger coil in the Subcooler Unit as a liquid. By subcooling the liquid
refrigerant, thermal energy that is typically not extracted from the liquid refrigerant, is obtained. As a result of
the liquid refrigerant giving off more thermal energy from subcooling, it is in turn able to extract more heat from
5
the environment when it passes through the electronic expansion valve and enters the evaporator. Have your
cake and eat it too! By delivering more heat to the heat sink and increasing the capacity of the heat pump’s
outdoor unit (evaporator), subcooling increases the COP of the heat pump, as well as the Energy Efficiency Ratio
(EER) of the compressor respectively.
Efficiency Maine Team IV’s advisor James C. LaBrecque was the source of inspiration for the capstone project. To
subcool the liquid R-410A refrigerant, a Subcooler Unit has been designed, consisting of a heat exchanger micro-
channel coil heat exchanger produced by Alcoil, a centrifugal inline duct fan produced by Fantech, various
galvanized duct components and a smart control system. When installing the Fujitsu 15RLS2, the liquid
refrigerant line was diverted and connected to the heat exchanger micro-channel coil located in the Subcooler
Unit. When in operation, an airstream drawn by the centrifugal fan is guided through the ductwork and passes
over the heat exchanger coil. Since the entering airstream is cooler than the entering refrigerant, the airstream
is heated and the refrigerant is cooled. This heated airstream is then delivered to the desired heat sink.
1.4 Desired Outcome The primary purpose of this capstone project was to determine if incorporating the Subcooler Unit with a Fujitsu
15RLS2 MSHP was practical from both a performance and economic sense. To perform successfully from a
design standpoint, the Subcooler Unit had to deliver more thermal energy than consume electrical energy. Since
the Subcooler Unit only consumes approximately 150 W of electrical power while running, fulfilling this
objective is trivial. To preform successfully economically, the Subcooler Unit had to deliver many more times
thermal energy than consume electrical energy and thus compensate for its initial capital cost. After installing
the Subcooler Unit with the Fujitsu 15RLS2, and performance data measured and recorded, a detailed economic
analysis could be prepared.
It was also desired that the complete installed system (Subcooler Unit and Fujitsu 15RLS2) be a permanent
install and help offset current heating expenses at the location. For accessibility and transparency reasons an
office building at the University of Maine was selected so that the system could easily be toured by the public.
Following this defining characteristic of the project, it was decided that a smart control system be developed to
automate the operation of the Subcooler Unit. This smart control system signals the inline centrifugal fan to shut
off if the Fujitsu 15RLS2 is running in defrost or cooling mode, cognizant of building make up air requirements,
and is capable of determining if the airstream should be drawn from inside or outside the building.
From initial conception, it was believed that the Subcooler Unit could eventually be manufactured as a retrofit
product for any existing residential mini-split heat pump or for a new installation. With further development and
higher production, the cost could be reduced so as to provide the consumer with a relatively short payback
period. For our purposes, we do not intend to go into future production, but solely wished to establish, test, and
evaluate the concept and provide the public with a clear understanding of the energy savings associated with
MSHPs and subcooled MSHPs. Although a prototype, it was desired that the Subcooler Unit would still be
economically sensible and have a realistic payback period.
6
2 Design Description
2.1 Increase Heat Pump Efficiency The Subcooler Unit is a retro fit device for an existing MSHP or to be incorporated with a new MSHP installation.
The Subcooler Unit will increase the efficiency of the entire heat pump cycle when running in heating mode by
roughly 27% on average. In HVAC terms, this means increasing the average COP of the heat pump cycle from
roughly 3 to 3.8. The Subcooler Unit has found to increase the system COP to as much as 4.4. A Fujitsu 15RLS2
MSHP, with R-410A as the active refrigerant, has been selected to be installed with the Subcooler Unit so that
the effects on the MSHP’s performance can be determined.
The concept of subcooling a Fujitsu 15RLS2 MSHP to increase the COP derives from the theory that when a stock
Fujitsu 15RLS2 MSHP is running in heating mode there is typically unused thermal energy in the liquid
refrigerant after it condenses and exits the indoor unit’s heat exchanger coil. The Subcooler Unit has been
designed to extract this unused heat from the liquid refrigerant and transfer it to a flowing airstream by means
of a refrigerant-to-air coil heat exchanger and a centrifugal fan. Theoretically, the refrigerant should enter and
exit the Subcooler Unit as a liquid. To effectively transfer thermal energy from the liquid refrigerant to the
airstream, an aluminum micro-channel coil produced by Alcoil has been selected because of its small physical
size and high rated capacity for heat transfer.
Since the R-410A refrigerant flowing through the heat exchanger in the Subcooler Unit remains a liquid, all heat
transfer from the R-410A refrigerant to the airstream is considered sensible heat and the process of the liquid
refrigerant decreasing in temperature is subcooling. All phase change from a vapor to a liquid (condensing) has
already occurred within the Fujitsu 15RLS2’s indoor unit.
To incorporate the Subcooler Unit into the heat pump cycle of the Fujitsu 15RLS2, the liquid copper refrigerant
line exiting the Fujitsu 15RLS2’s indoor unit is diverted and connected to the upper (inlet) connection on the
micro-channel coil located in the Subcooler Unit. The lower (outlet) connection on the micro-channel coil in the
Subcooler Unit connects to a second liquid copper refrigerant line, which is connected to the MSHP's outdoor
unit. A schematic plan of the Subcooler Unit and Fujitsu 15RLS2 MSHP system cycle, with major components and
piping shown has, can be seen on the following page (Figure 2). When the Fujitsu 15RLS2 is in heating mode, the
liquid refrigerant coming from the indoor unit will “drain” down through the micro-channel coil. When the
Fujitsu 15RLS2 is in cooling mode, the liquid refrigerant coming from the outdoor unit will “ascend” up through
the micro-channel coil. The Subcooler Unit will be shut off when the Fujitsu 15RLS2 is running in cooling mode,
so the addition of the micro-channel coil in the MSHP system essentially acts as additional refrigerant piping.
7
Figure 2. Fujitsu 15RLS2 with Subcooler Unit system conceptual schematic.
The effect of subcooling on a MSHP running in heating mode can be represented by a pressure-enthalpy
diagram describing the state of the refrigerant at every point in the vapor compression cycle. Specific enthalpy,
designated by the letter h, is the amount of thermal energy (heat) in one unit of mass of a certain substance. In
the English Engineering Unit system, specific enthalpies are typically expressed in BTU/lbm. The specific enthalpy
of a substance at a certain state consists of the substances internal energy per unit mass, plus the product of the
pressure and specific volume of the system, thereby making it a property defined by the substance’s state. An R-
410A pressure enthalpy diagram representing the installed system (Subcooler Unit and Fujitsu 15RLS2)
operating in heating mode can be seen below (Figure 3).
8
Figure 3. Pressure-enthalpy diagram describing effects of subcooling a Fujitsu 15RLS2 MSHP.
Partial Source: DuPont Suva, "suva.dupont.com," April 2005. [Online]. Available:
http://www2.dupont.com/Refrigerants/en_US/assets/downloads/k05718_Suva410A_pressure_enthalpy_eng.pdf. [Accessed 22 4 2014].
From Figure 3, it can be seen that after the R-410 completely condenses to a liquid in the Fujitsu 15RLS2’s indoor
unit and gives of heat (States B to C), additional heat is transferred to the air as the liquid R-410A subcools in the
Subcooler Unit (States C to Z). Furthermore it can be seen that by subcooling the liquid R-410A, the capacity of
the Fujitsu 15RLS2’s evaporator increases (States D to A). The additional thermal energy that is removed by
subcooling can then by extracted from the environment.
2.2 Subcooler Unit Mechanical Componentry Efficiency Maine Team IV has designed a device based on the subcooling process described above, that is
relatively compact in size, attractive in appearance, simple to integrate into the Fujitsu 15RLS2 MSHP cycle, and
capable of transferring heat from the flowing R-140A liquid refrigerant to an airstream. The Subcooler Unit
consists of an Alcoil brazed aluminum refrigerant-to-air micro-channel coil heat exchanger, a 6 in. diameter
Fantech FG 6XL inline centrifugal duct fan, a square duct assembly housing for the micro-channel coil and a
pleated air filter, two 6 in. diameter Honeywell actuated dampers, various 6 in. ductwork components including
two 90 degree bends, one WYE connector, and one 60 in. straight duct section.
The Fantech FG 6XL inline centrifugal duct fan is responsible for drawing an airstream through the Subcooler
Unit. Two Honeywell actuated dampers are incorporated at the Subcooler Unit’s inlet to allow for the airstream
to be drawn from either inside or outside and are connected to both branches of the WYE connector
9
respectively. The WYE duct’s third opening is connected to the 6 in. round connection on one of the two
transition ducts, which is then connected to the inlet side of the 15 in. square duct assembly. Within the square
duct assembly, the airstream first passes through the pleated air filter and then across the micro-channel coil.
The subcooler design has incorporated the pleated air filter before both the micro-channel coil and centrifugal
fan to avoid any fouling issues to either the coil or the fan. The second transition duct is attached to the exit side
of the 15 in. square duct assembly.
A picture of the assembled Subcooler Unit, orientated so that the airstream enters on page right and exits on
page left, including the 15 in. square duct assembly, centrifugal fan, transition ducts, actuated dampers, and
WYE duct with components labeled has been included (Figure 4).
Figure 4. Assembled Subcooler Unit from left to right: two 6 in. actuated dampers, one WYE duct, one 6 in. round to 15 in. square transition duct, (Square duct assembly including filter rack, pleated air filter, and micro-channel coil), 15 in. square to 6 in. round
transition duct, and one Fantech FG 6XL centrifugal inline duct fan.
The major mechanical componentry and subassemblies included in the Subcooler Unit will now be presented in
more detail, beginning with the micro-channel coil and progressing outward. This micro-channel coil is the
Subcooler Unit’s fundamental component. The micro-channel coil donated by Alcoil consists of two
manifolds/headers, connected by 33 flat micro-channel tubes with louvered fins (24 fins per in.) spanning these
tubes. The overall dimensions of the micro-channel coil are 12.2 in. wide by 14.2 in. tall by 1.9 in. thick. Two
0.625 in. diameter copper tubes are connected to the two headers respectively. The micro-channel coil has
been designed so that the two headers are orientated horizontally and that the liquid R-410A enters the upper
header and exits the lower header (Figure 5).
Square Duct Assembly
Transition Duct
Actuated Damper 2
Pleated Air Filter
WYE Duct
Airstream Outlet
Centrifugal Fan Actuated
Damper 1
Transition Duct
10
Figure 5. Alcoil all-aluminum micro-channel heat exchanger coil.
The 15 in. square duct assembly measures 12 in. long and consists of the micro-channel coil, a filter rack with a
hinged access door, the 14 in. nominal square pleated air filter, and a drip pan with a 0.5 in. threaded male
fitting located underneath the micro-channel coil. The 15 in. square duct housing was constructed from 24
gauge galvanized steel sheet metal and was tungsten inert gas (TIG) welded together. The Subcooler Unit design
has the airstream entering the 15. in. square duct assembly so that it passes through the pleated air filter first
and then passes through the micro channel coil. The micro-channel coil is orientated such that the R-410A will
enter the upper header and leave the lower header when the Fujitsu 15RLS2 is running in heating mode. Twin
City Sheet Metal was contracted to build this 15 in. square duct assembly (Figure 6).
Figure 6. Square duct assembly to house micro-channel coil, filter rack with door, and pleated air filter.
Top Manifold
5/8 in. Copper Connection
Bottom Manifold
Micro-channel Tubes Connected With Fins
Filter Door
Square Duct Housing
Pleated Air Filter
Filter Rack
Micro-Channel Coil
Support Strut
Refrigerant Inlet
Refrigerant Outlet
5/8 in. Copper Connection
11
Two 15 in. square to 6 in. round transition ducts measuring 12 in. long and constructed from 24 gauge
galvanized sheet metal are attached on either side of the 15 in. square duct assembly using sheet metal screws.
Twin City Sheet Metal was contracted to build these two transition ducts (Figure 7). The two transition ducts
measure 12 in. long so that minimal airstream flow obstructions occur.
Figure 7. Transition ducts positioned on either side of the square duct assembly (6 in. round to 15 in. square).
One Fantech FG 6XL inline centrifugal duct fan is used to pull an airstream through the pleated air filter and then
across the micro-channel coil (Figure 8).This centrifugal fan is supplied with a 110 VAC source.
Figure 8. Fantech FG 6XL Centrifugal Inline Duct Fan.
Two 6 in. diameter Honeywell actuated dampers are included in the Subcooler Unit so that the airstream can
either be drawn from outside or inside (Figure 9). This design feature will allow for a better understanding on
the effects of inlet airstream temperature on the heat of rejection from the micro-channel coil. Each damper is
controlled by a 24 VAC source.
12
Figure 9. Honeywell 6 in. 24 VAC actuated dampers.
Various 6 in. diameter galvanized steel duct components are also included in the Subcooler Unit to confine and
direct the airstream as needed and include straight sections, two 90 degree bends, and one WYE connector.
Sheet metal screws were used to connect all Subcooler Unit components together. Using sheet metal screws
instead of rivets allows for easy disassembly of the Subcooler Unit if needed. A detailed description of the
development of the Subcooler Unit design will be presented in the Design Process Section (Section 3) of this
report. Section 2.5 will describe how the Subcooler Unit was installed at Service Building A at the University of
Maine. For a detailed drawing set of the Subcooler Unit, including all key components, refer to Appendix A.
2.3 Subcooler Unit Power and Control Electronics The Subcooler Unit includes a Type II Nema enclosure containing all power and control electrical componentry
to supply and control the Fantech FG 6XL centrifugal fan and two Honeywell actuated dampers. The enclosure is
supplied by a 120 VAC source by means of heavy duty industrial insulated cord and plugged into a wall
receptacle. From the Type II Nema enclosure, the Fantech FG 6XL centrifugal fan is supplied by a 110 VAC source
and is rated at 1.48A and the two Honeywell actuated dampers are each supplied by a 24VAC source and are
rated at 0.25 A. The testing portion of this report will present the actual voltage and current values for these
components. This Type II Nema enclosure is mounted on a wall in close proximity to the Subcooler Unit (Figure
10). The enclosure’s lid, back plate and main box are grounded.
Figure 10. Electrical Enclosure for Subcooler Unit Containing all Power and Controls Componentry.
Electrical Enclosure
0.5 in. Carflex Conduit and Fittings
120 VAC Recipticle
13
Electrical power components inside of the Type II Nema enclosure are mounted on one 24 in. long section of
standard 35mm din rail attached to the enclosure’s back plate. While many components came with din rail
specific mounts, several custom din rail mounts were fabricated for various components. This back plate can be
removed if serious electrical componentry changes need to be made. Otherwise, all componentry inside of the
enclosure are quite accessible and easy to work on. Electrical power components include the following:
One 5 Amp breaker
Fourteen single pass through 1492 J style terminal blocks
Several terminal block jumper strips cut to different lengths (two lengths of 4, and 4 lengths of 2)
Eight terminal block end anchors
Nine terminal block barriers
One 120 VAC to 24 VAC transformer
One Fantech fan speed controller
Three on-off-on heavy duty toggle switches
One triple-receptacle120 VAC convenience outlet.
All electrical power componentry is connected by 14 AWG, 16 AWG, or 18 AWG stranded copper wire. The din-
mounted 5A breaker is included in the enclosure for safety reasons to limit current drawn by the Subcooler Unit
(Figure 11). Two sets of four single pass through 1492 J style terminal blocks, each connect by a jumper strip,
separated by barriers and secured by anchors are used as the enclosures hot and neutral bus bars respectively
and can also be seen below installed in the enclosure. Four sets of two single pass through 1492 J style terminal
blocks, each connect by a jumper strip, separated by barriers and secured by anchors are used when three
independent wires need to be connected together and can also be seen below installed in the enclosure. One
120 VAC to 24 VAC transformer is included in the enclosure, so that the two Honeywell dampers can be
actuated, and can be seen below installed in the enclosure.
Figure 11. Din rail mounted 5 amp breaker, hot terminal strip, neutral terminal strip, terminal connections, and one 120 vac to 24 vac transformer located within Subcooler Unit’s electrical enclosure.
5 Amp Breaker
120 VAC to 24 VAC Transformer
Neutral Terminal Strip
Hot Terminal Strip
Wire Connections
Ground Block
14
The Subcooler Unit control system incorporates the ability to switch between manual and smart controls. This
manual control system is critical for performance evaluations of the Subcooler Unit. Three heavy duty toggle
switches are used to switch between manual and smart controls and can be seen attached to their mounting
plate below (Figure 12). The top toggle switch allows for the user to select manual or smart damper controls, the
middle toggle switch allows the user to control the dampers when the upper toggle switch is set to manual
damper control, and the bottom toggle switch is used to allow the user to switch between manual or smart
centrifugal fan control. Regardless of which control scheme is being used, the centrifugal fan speed can be
manually adjusted by means of a knob style speed controller and the centrifugal fan can also shut off at any
time. This Fantech speed controller can also be seen below installed in the enclosure (Figure 12).
Figure 12. Toggle switches to select smart or manual Subcooler Unit control and fan speed adjuster.
Electrical control componentry inside of the Type II Nema enclosure is also mounted on the same section of
standard 35mm din rail and includes one Arduino Uno micro controller board and one SainSmart 2-Channel
SPDT relay module board (Figure 13). The Arduino Uno controls the SPDT relay module board, which consists of
two SPDT (Single Pole Double Throw) relays. One relay controls the two dampers at the intake of the Subcooler
Unit and the other controls the fan, whether it was on or off. The Subcooler Unit control assembly also includes
two waterproof DS18B20 digital temperature sensors used to measure the airstream temperature before and
after the micro-channel coil and one LDR to detect light in the room. A solderless bread board is also included in
the enclosure to allow for proper connection of the two DS18B20 digital temperature sensors and the LRD to the
Arduino Uno. This non-solder bread board, showing connections to the DS18B20 digital temperature sensors by
means of a 4.7K ohm axial lead resistor and 3-pin screw type terminal block, can also be seen below installed in
the enclosure. The breadboard was attached using two pieces of Velcro for easy removal if necessary. The
breadboard was also used for a 5 VDC power rail, as well as a ground rail.
Smart or Manual Damper Selection
Fan Speed Adjustment
Smart or Manual Fan Selection
Manual Damper Selection
15
Figure 13. 102 VAC receptacle, 9 VDC adapter, Arduino Uno micro controller board, SPDT relay module board, standard solder less bread board, and two DS18B20 digital temperature sensors leads connected by means of a 4.7K axial lead resistor and 3-pin screw
type terminal block.
Efficiency Maine Team IV chose the DS18B20 Digital Temperature Sensors because they require no external
components besides a resistor, have sufficient accuracy for the project demands, measure temperatures within
the range they would be exposed to in this application, and are relatively robust and inexpensive (Figure 14).
Figure 14. Sample DS18B20 digital temperature sensor and three sample 4.7K ohm resistors.
All connections to the type II Nema enclosure, except for the110 VAC supply, use 0.5 in. carflex conduit straight
or 90deg bend fittings and conduit (Figure 15). The two waterproof DS18B20 digital temperature sensors leads
were also run into the enclosure using the Carflex conduit with only one fitting.
SPDT Relay Board
Arduino Uno Micro Controller
9 VDC Adapter
Solderless Breadboard
DS18B20 Wire Leads
4.7K Resistor and Terminal Block
Sample 4.7K Resistors
DS18B20 Digital Temperature Sensor
and Lead
16
Figure 15. Sample 0.5 in. Carflex conduit and two straight fittings.
Figure 15 shows a sample section of 0.5 in. conduit and two straight 0.5 in. Carflex fittings. For a detailed power
and control electrical schematic for the Subcooler Unit refer to Appendix B.
2.4 Arduino IDE Code for Smart Controls To program the Arduino Uno, integrated development environment (IDE) coding was used. The objective of the
control system is to control the fan and dampers based on real-time conditions. To control the fan two DS18B20
Digital Temperature Sensors are used. These temperature sensors are located before and after the micro-
channel coil and provide the input data for fan control (Figure 16). To interface with these sensors both the IDE
One Wire and Dallas Temperature libraries are used. These libraries make coding with these sensors much more
user friendly.
Figure 16. Two DS18B20 digital temperature sensors located before and after the micro-channel coil to measure Subcooler Unit airstream temperatures.
To control the dampers a light dependent resistor (LDR) is used. This is located in the NEMA enclosure box and
provides the input data for damper control (Figure 17).
DS18B20 Sensor to Measure Airstream Outlet Temperature
DS18B20 Sensor to Measure Airstream Inlet Temperature
17
Figure 17. Light dependent resistor (LDR) used to detect light and dark in the photocopier room.
Source: Rapid Electronics, "rapidonline.com," 2014. [Online]. Available: http://www.rapidonline.com/electronic-components/12mm-light-
dependent-resistor-non-rohs-82713/. [Accessed 5 May 2014].
To interface with the LDR, multiple variables were created. Variable LDR is defined as the pin the LDR is
connected to on the Uno, variable “LDRValue” is created to store the LDR values and has an initial value of zero,
variable light was created to represent the LDR value when the light in the photocopier room is on. These
variables are necessary to control the damper in the main loop.
The portion of the main loop of the program that controls the fan is based on the temperature sensor readings.
At the beginning of the loop the fan is turned on using a “digitalWrite” function to set the fan pin to low, which
turns it on. Values from the temperature sensor readings are requested and assigned to variables. These
variables are “Input” and “Output” and correspond to temperature before and temperature after the airstream
flows over the micro channel coil, respectively. There is also a third variable, “DeltaT”, which is defined as the
value of “Output” minus the value of “Input”. The value of this variable dictates the state of the fan pin. The fan
pin is either “high” or “low”, high meaning power is not supplied (fan is off) and low meaning power is supplied
(fan is on).
The other portion of the main loop of the program provides damper control. This portion begins by executing an
“analogRead” function on the LDR pin and assigns the LDR pin value to “LDRValue”. “LDRValue” dictates the
state of the damper pin. The damper pin can be either “high” or “low”. A value of “high” does not supply power
to the pin and a value of “low” does. When the pin is set to “high” the indoor damper closes and the outside
damper is left open. When the pin is set to “low” the outdoor damper closes and the indoor damper opens.
Both pins being controlled have two possible states, “high” or “low”. However, when the outside damper is
open there is a check on the variable “Output” to ensure the air entering the building isn’t too cold. This leads to
five conditional ‘if’ statements to ensure all relevant factors are accounted for (Figure 18).
18
Figure 18. Diagram representing smart control system logic.
Figure 18 shows the three ‘if’ statements that control the damper. ‘If’ one and two use Boolean operators. This
allows two operands in one statement. This statement is only true if both operands are true. For statement one
to be true the light in the room must be on. When the “LDRValue” is greater than or equal to the value of the
variable light the light in the room is on. The other operand is that the outlet airstream temperature is greater
than or equal to a minimum temperature variable. When these operands are both true a “digitalWrite” function
that sets the damper pin to high, resulting in the closing of the indoor damper, is executed.
For statement two to be true like statement one, the light in the room must be on. The same operand as in the
previous statement is used to detect whether the light is on or not. The second operand is that the outlet
airstream temperature is less than the minimum temperature variable. When both these operands are true a
digitalWrite function is executed. This function sets the damper pin to low, resulting in the inside damper being
opened and the outside being closed. The final statement controlling the dampers is true when the light is off.
When the “LDRValue” is less than the variable light the light in the room is off. When this is true a digitalWrite
function that sets the damper pin to low, resulting in the closing of the outdoor damper, is executed.
Figure 18 shows the two ‘if’ statements that control the fan. For statement four to be true the “DeltaT” must be
greater than two. When this occurs the fan is left on. For statement five to be true the “DeltaT” must be less
than or equal to two. When this is true a “digitalWrite” function is executed. This function sets the fan pin to
high, resulting in the fan being turned off. All statements have delay function in them. This results in the loop
being executed every ten minutes. After the loop is a subroutine. This subroutine converts the temperature
sensor values to a Fahrenheit temperature. For a listing of the IDE code for the Subcooler Unit smart control
system refer to Appendix C.
19
2.5 Installation at Service Building A The final design plan was installing the Fujitsu 15RLS2 with Subcooler Unit at the University of Maine’s Service
Building A (Figure 19).
Figure 19. Service Building A at the University of Maine.
The Subcooler Unit was installed in the Photocopier room (Figure 20). It was installed there because the room
contains an exhaust fan that removes air from the room and creates the need for makeup air that the Subcooler
Unit provides. The flexible hose seen in Figure 20 is to allow for an airstream to be drawn from outside for
testing purposes. This is only an element in the installation. Eventually a hole will be cut in the roof above the
Subcooler Unit and a water tight intake installed, which will connect to the upper damper on the Subcooler Unit.
Figure 20. Subcooler Unit installed in photocopier room (Dryer duct is run to outside for testing purposes).
Electrical Enclosure
Subcooler Unit
Condensate Pump
Unistrut Support
Hose to Draw Airstream from
Outside (Temporary)
20
The electrical enclosure was anchored to the wall below the Subcooler through the use of four drywall butterfly
anchors. The Subcooler Unit was bolted to two Unistrut channels that were bolted to the door frame wall
approximately eight feet high in the photocopier room (Figure 21).
Figure 21. The two unistrut channels bolted to wall in photocopier room to support Subcooler Unit.
The Fujitsu 15RLS2 MSHP consists of two units, one located inside and one located outside. Both were placed on
the wall mounts provided by Fujitsu. The Fujitsu 15RLS2 MSHP was installed in the Work Control Center room. It
was installed there because the room was in need of an improved heating and cooling system. The outdoor unit
of the Fujitsu 15RLS2 was placed on its mount that is located outside on the brick wall between the two
windows in the Work Control Center approximately 2 feet off the ground (Figure 22).
Unistrut Bracket Bolted to Wall Stud
21
Figure 22. Fujitsu 15RLS2 outdoor unit mounted on the outside brick wall of Service Building A.
The indoor unit of the Fujitsu 15RLS2 was placed on its mount on the wall that is located between the two
windows in the Work Control Center (Figure 23).
Figure 23. Fujitsu 15RLS2 indoor unit mounted to wall in Work Control Center located in Service Building A.
The installation involved connecting the three units with refrigerant copper tubing. The 0.25 in. vapor line was
fed from the outdoor unit into the ceiling and back down through the wall to the indoor unit (Figure24).
Fujitsu 15RLS2 Outdoor Unit
Unit Disconnect
Fujitsu 15RLS2 Indoor Unit
Slim Duct
Mounting Brackets
22
Figure 24. Refrigerant lines to connect Fujitsu 15RLS2 outdoor unit and indoor unit with the Subcooler Unit.
The standard installation process would involve running the 0.25 in. copper liquid refrigerant line from the
indoor unit to the outdoor unit. For this installation it was necessary to divert the liquid refrigerant line. After
exiting the indoor unit the 0.25 in. liquid line was fed back into the wall, up through the ceiling where it was then
run down the hallway to the photocopier room, and fed to the inlet of the micro-channel coil through the wall.
Two pictures showing how the 0.25 in. liquid line were run through the ceiling and down the hallway can be
seen below (Figure25).
Figure 25. Mount for Fujitsu 15RLS2 and hole in wall to run insulated copper refrigerant lines and electrical lines.
The 0.25 in. copper refrigerant line leaving the outlet of the micro-channel coil was then ran back down the
hallway and connected to the outdoor unit. The line was fed through the wall in the Photocopier room ran up to
the ceiling down the hallway back to the Work Control Center room and fed to the outside unit (Figure26).
Hole to run Refrigerant and Electrical Lines
Fujitsu 15RLS2 Indoor Unit
Mounting Bracket
0.25 in. Copper Liquid and 0.5 in.
Copper Vapor Refrigerant lines
0.25 in. Copper Liquid Line to Subcooler Unit
23
Figure 26. Insulated copper liquid refrigerant lines diverted from Fujitsu 15RLS2 through ceiling in Work Control Center to connect to Subcooler Unit.
This process of diverting the refrigerant lines enabled the integration of the Subcooler Unit with the Fujitsu
15RLS2 unit. To supply power to the Fujitsu 15RLS2, a 220VAC line was necessary. To do this two 110VAC
opposite phase breakers were ran from the control panel to a unit disconnect located near the outdoor unit,
from the unit disconnect the unit was powered with the necessary 220 VAC. The electrical components within
the Nema enclosure managed the power supply to the Subcooler Unit. These electrical components were
supplied form an industrial grade power cord plugged into an 110VAC wall receptacle. All electrical lines
connecting components on the Subcooler Unit to the Subcooler Unit’s electrical enclosure were protected with
0.5 in. Carfllex conduit with and 0.5 in. fittings. The mechanical components were installed by RJ Morin. The
electrical components were installed by Nate Emerson and other workers from the University of Maine’s
Electrical Shop. For a detailed drawing set of the Subcooler Unit and Fujitsu 15RLS2 installation at the Facilities
Management building at the University of Maine refer to Appendix D.
2.6 Testing and Instrumentation Besides the clear benefit of providing efficient heating and cooling for Service building A, the primary goal of our
capstone project was to analyze the performance of a Fujitsu 15RLS2 integrated with our Subcooler Unit. To
accomplish this goal, significant instrumentation was included with the installation to monitor performance. This
instrumentation includes the following:
Two type T thermocouples measuring airstream inlet and outlet temperatures in the Subcooler Unit
respectively.
Two type T thermocouples measuring R-410A inlet and outlet temperatures in the Subcooler Unit
respectively.
One type T thermocouple measuring airstream inlet temperature for the Fujitsu 15 RLS2 indoor unit
respectively.
Two type T thermocouples measuring airstream outlet temperatures for the Fujitsu 15 RLS2 indoor unit
respectively (This was done to get an average).
Two 0.25 in. Copper Refrigerant lines
Connect Subcooler Unit
24
One type T thermocouple measuring outdoor temperature near Service Building A.
Two USB thermocouple data loggers to record thermocouple measurements for the Subcooler Unit and
Fujitsu 15RLS2 respectively.
One TED device measuring power input to the Fujitsu 15RLS2 (This includes compressor, indoor fan, and
outdoor unit fan power). (Failed).
An induction clamp-on style ammeter and multimeter to measure current drawn and voltage drop for
various components.
These instrumentation components will be described in detail in the Testing and Evaluation (Section 4) portion
of this report.
2.7 Budget for Subcooler Unit, Fujitsu 15RLS2, and Installation Efficiency Maine Team IV design project budget is provided by the University of Maine Mechanical Engineering
Department. The Budget for The Subcooler Unit is separated into four categories:
Duct Assembly
Power, Controls, and Electronics
Instrumentation and Installation.
Fujitsu 15RLS2 MSHP
The duct assembly consists of all the ductwork and air-handling equipment necessary to inexpensively construct
the Subcooler Unit, as well as two copper-reducing-bushings required to connect the liquid refrigerant line of
the Fujitsu 15RLS2 MSHP to the inlet and outlet of the micro-channel coil [FIGURE NUMBER HERE]. The budget
for power controls electronics consists of all electronic components necessary to provide power to and control
the Fantech Fan and two Honeywell dampers. The budget for the installation is an estimate for the labor to hire
a contractor to install the Fujitsu 15RLS2 MSHP, and the instrumentation is all purchased components required
to test the MSHP with Subcooler Unit.
Table 1. Summarized Budget for Efficiency Maine Team IV Capstone Design Project.
Summarized Budget for Efficiency Maine Team IV Capstone Design Project
Subcooler Duct Components $772.11
Subcooler Power Electronics and Controls $225.93
Subcooler Instrumentation and Installation $1,703.67
Fujitsu 15RLS2 MSHP $1,600.00
Total $4,301.71
Not included in Table 1 is an Omega Engineering HHF2005HW Hot Wire Anemometer and 64 feet of Omega
Engineering Type-T thermocouple wire. For a detailed parts list and budget for the Subcooler Unit design project
refer to Appendix E.
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3 Design Process
3.1 Initial Conception During the beginning of the fall 2013 semester our capstone team was planning to interface an evacuated tube
solar collector system with a GE Geospring hybrid hot water heater (Hybrid signifies air-to-water heat pump
with supplementary electrical resistance heaters) by means of a large water storage tank and several heat
exchangers. It was concluded that the initial cost of the solar collector system eclipsed any benefits gained from
the design and therefore decided to pursue a much cheaper and simpler option with a faster payback period.
This is when the concept of subcooling the liquid refrigerant in a MSHP was presented by project advisor James
LaBrecque. After some consideration, it was decided that building and testing a Subcooler Unit would be both a
beneficial learning experience and potentially positive for MSHP owners in Maine looking to increase efficiency
of current or new installations.
It was also considered that the Subcooler Unit would be ideal for providing makeup air for a building. In the
HVAC industry, makeup air is air that must be brought in from outside of a building to replace air being removed
from within the building, via a bathroom exhaust fan, a clothes-dryer exhaust fan, or a kitchen hood fan. The
Subcooler Unit would have provided the free-heat generated from subcooling to the GE Hybrid electric water
heater, which would absorb the free-heat during the latent heat of evaporation that occurs to heat the water in
the tank. Due to budget concerns and the suggestion to solely focus on creating and testing the Fujitsu 15RLS2
MSHP with the Subcooler Unit, it was decided to scrap the GE Hybrid electric water heater.
From the initial conception of the Subcooler Unit, several design cycles ensued and a final prototype model for
the project was settled upon. The main controlling factor was the physical size of the Alcoil micro-channel coil
that would be used in the Subcooler Unit. This heat exchanger coil dictated the size of the square duct housing
that contains the coil and pleated air filter and therefore the size of the two transition ducts used to connect to
the round duct componentry. Following Alcoil‘s order confirmation for a micro-channel coil, they sent us
detailed drawings of the coil allowing us to adjust our design as needed.
3.2 Refrigeration Theory
3.2.1 Sensible and Latent Heat
Before the prediction theory for the Subcooler Unit can be presented, it is necessary to explain some
fundamental terminology that is critical to understanding how a MSHP operates. A majority of the heat transfer
released or absorbed by a MSHP occurs when the refrigerant flowing inside changes state from either vapor
condensing to a liquid or liquid evaporating to a vapor, known as the latent heat of condensation and latent heat
of vaporization respectively and both occur at a unique constant temperature. While latent heat transfer is a
constant temperature process and cannot be measured with a thermometer, sensible heat transfer results in a
change in temperature of the substance and can be measured by a thermometer. When people refer to heat
transfer they are typically considering sensible heat.
Now that the basics of latent and sensible heat have been established, the different states that the liquid or
vapor refrigerant are in need to be considered. These states are directly connected to the concept of latent heat
and sensible heat. Throughout the Fujitsu 15RLS2 MSHP cycle, the R-410A refrigerant flowing within can exist as
26
a superheated vapor, a saturated vapor, a saturated liquid, or a subcooler liquid. R-410A existing in a saturated
liquid or saturated vapor state means that that liquid or vapor is at temperature that is condensed or
evaporated at respectively. Latent heat transfers associated with these phase changes occur at these saturation
temperatures. Superheated vapor implies that the vapor exists at a temperature higher than its boiling
saturation temperature. Subcooled liquid implies that the liquid exists at a temperature lower than its
condensing saturation condensing temperature.
The process of water boiling at 212° F at sea level is an effective model to illustrate sensible heat, latent heat,
superheated vapor, and subcooled liquid. A sample of water existing at 100° F is termed subcooled since it is at a
temperature lower than the temperature at which it will boil. If heat is slowly added to this subcooled liquid
water, it will increase in temperature until reaching the boiling (saturation) temperature of 212° F. Until this
point no phase change has occurred and all heat transfer associated is in the form of sensible heat. Once the
water is heated to 212° F it will remain at that temperature until it has completely evaporated to a vapor. This
evaporation has occurred at a constant temperature and has resulted in a change of state of the water and is
therefore a latent heat process. If the vapor water remains at exactly 212° F it is termed a saturated vapor and
will condense if the temperature is decreased. If heat continues to be applied to the water sample, then the
water vapor will increase in temperature and become a superheated vapor.
3.3 System Performance Prediction
3.3.1 Fujitsu 15RLS2 without Subcooler Unit
Fujitsu General Limited provides heating capacity and cooling capacity tables for each of the 9RLS2, 12RLS2, and
15RLS2 models in their Design and Technical Manual [2]. These tables are a compilation of very precise
calorimetry experiment procedures conducted by Fujitsu. For a certain indoor fan setting, these tables provide
either heating or cooling capacity in Btu/h and total input power in kW for four different indoor temperatures
ranging from 60 to 75 degrees Fahrenheit and nine different outdoor temperatures ranging from -5 to 59
degrees Fahrenheit. It then follows that these tables can be used to understand the performance of these
Fujitsu models, and even predict an R-410A mass flow rate if certain assumptions are made. These capacity
tables provide heating cycle performance data for indoor temperatures.
While the following two performance prediction sections (3.3.1 and 3.3.2) for a Fujitsu 15RLS2 without and with
Subcooler Unit respectively will only present the equations used, Appendix F contains an Excel spreadsheet
containing all calculations for all possible outside temperature conditions.
The following prediction analysis of a Fujitsu 15RLS2 without Subcooler Unit running in heating mode will follow
basic thermodynamic procedures so as to predict an R-410A mass flow rate for a wide range of operating
conditions. This R-410A mass flow rate will be used in the following sub section to predict the effect of
subcooling on the Fujitsu 15RLS2 heating cycle. Since this prediction analysis only considers the Fujitsu 15RLS2
running in heating mode, the indoor unit will be designated as the condenser and the outdoor unit will be
designated as the evaporator. These terms will be used throughout the two following prediction subsections.
A representation of the system diagram is used to better communicate the cycle (Figure 27). Figure 27 shows
the Fujitsu 15RLS2 including the micro-channel coil located inside of the Subcooler Unit with five unique
refrigerant states labeled. For this prediction analysis of the Fujitsu 15RLS2 without Subcooler Unit, it will be
27
assumed that the micro-channel coil has no effect on the flowing R-410A. The flow of the R-410A when the
Fujitsu 15RLS2 is running in heat mode is designated by the dashed arrows in Figure 27.
Figure 27. Fujitsu 15RLS2 MSHP with Subcooler Unit conceptual system schematic with key refrigerant states labeled.
The letters A, B, C, D, E and Z seen in Figure 27 denote a specific state that the R-410A is in with respect to the
vapor compression cycle (Table 2). Table 2 gives a description of each of these states. For this analysis, state B,
the vapor discharged from the compressor is unimportant. As previously mentioned, this prediction analysis of
the Fujitsu 15RLS2 without Subcooler Unit will assume no change in R-410A properties between states C and Z.
Table 2. Description of six unique R-410A refrigerant states with respect to vapor compression cycle.
State Description
A Superheated Vapor at TA = TE + (10°F Superheat)
B Compressor Discharge (TB > 120°F)
C Saturated Liquid at 120°F (TC = SDT)
D Low Pressure Side of EEV (hD = hC) or (hD = hZ with Subcooler)
E Saturated Vapor at TE = (Toutside – TD) & (TE = SST)
Z Subcooled Liquid below 120°F (Only when including subcooler)
SDT stands for saturated discharge temperature and SST stands for saturated suction temperature, both
common industry terms that are relative to the heat pump’s compressor.
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Throughout this analysis and the following one for incorporating the Subcooler Unit, the subscripts noSC or SC will
be used to clarify the absence or presence of a Subcooler Unit respectively. In some cases, either subscript is
unnecessary as the value is the same for a Fujitsu 15RLS2 cycle with or without a Subcooler Unit.
This analysis makes several assumptions for the heat pump cycle. A condensing temperature of 120 degrees
Fahrenheit is assumed. This is the constant temperature that the R-410A in the heat pump’s condenser (indoor
unit’s coil) is at. This value was advised by team advisor James C. LaBrecque from his experience with 15RLS2
Fujitsu MSHPs. Thus, the prescribed subcooling temperature used in the following analysis of the Fujitsu 15RLS2
with Subcooler Unit is relative to this temperature. While some R-410A subcooling may already occur in the
stock Fujitsu 15RLS2 condenser, this analysis assumes that state C is all saturated liquid at 120° F and not
subcooled liquid.
Although the capacity tables provided by Fujitsu General Limited contains performance data for indoor
temperature ranging from 60° F to 75° F, only the performance information for the 70° F indoor temperature
was used in this prediction analysis. This is because the indoor temperature in the Work Control Center room in
Service Building A is approximately 70° F.
A temperature difference (TD) on the Fujitsu 15RLS2 evaporator (outdoor unit) of 10° F will be defined. This
means that the refrigerant entering the evaporator will be 10° F less than the prescribed outdoor temperature.
If the R-410A was not at a lower temperature than the outside air, it would not be able to pull any thermal
energy from the air.
A superheat value of 10° F on the saturated vapor exiting the evaporator is also assumed. This means that after
the R-410A completely evaporates in the evaporator, it is then heated an additional 10° F. Akin to the subcooling
value, this superheat value is deemed sensible heat.
As with any heat pump or refrigeration cycle analysis, this analysis assumes that the specific enthalpy of the R-
410A on the low pressure side of the electronic expansion valve is equal to the specific enthalpy of the R-410A
on the high pressure side. Using these defined states described in Figure 27 and Table 2 and the aforementioned
assumptions, R-410A specific enthalpies with units of BTU/lbm are looked up in a respective table and defined
with correct subscripts as:
Eq.2
Eq.3
Eq.4
Eq.5
Eq.6
Since this analysis only considers the Fujitsu 15RLS2 running in heating mode, the following two heat transfer
rates are defined as:
Eq.7
hA hsuperheatedvapor_10F_superheat
hC hliq_120F
hD hC
hE hsatvapor_at_Tevaporator
hZ hsubcooled_liquid_at_Tz
Qcondenser Qindoor_unit
Qevaporator Qoutdoor_uint
29
Eq.8
From the provided heating capacity (condenser heat output) and power input to the Fujitsu 15RLS2, the
corresponding COP of the stock Fujitsu 15RLS2 MSHP is calculated as follows:
Eq.9
A first law of thermodynamics equation is then written for the entire heat pump system and solved for the heat
transfer rate into the evaporator as follows:
Eq.10
From this heat transfer rate into the evaporator and the defined R-410A specific enthalpies of states A and D,
the mass flow rate of the R-410A refrigerant in lbm/hr is predicted as:
Eq.11
This mass flow rate will be used in the following analysis, where the Subcooler Unit is incorporated into the
Fujitsu 15RLS2 MSHP cycle.
3.3.2 Fujitsu 15RLS2 with Subcooler Unit
In a stock Fujitsu 15RLS2 MSHP, the R-410A goes straight outside to the outdoor unit by means of a 0.25 in.
copper line after condensing to a saturated liquid in the heat pump’s indoor unit (condenser). Upon reaching the
outdoor unit, the R-410A passes through an electronic expansion valve, expands and enters the outdoor unit’s
coil (evaporator). The Subcooler Unit is incorporated into the Fujitsu 15RLS2 MSHP cycle so that after the R-410A
condenses it is routed to the Alcoil micro-condenser coil located in the Subcooler Unit, where an airstream is
simultaneously drawn across the micro-channel coil. After passing through the coil and transferring thermal
energy to the flowing airstream, the subcooled R-410A continues on to the outdoor unit.
This prediction analysis of a Fujitsu 15RLS2 with Subcooler Unit assumes that the R-410A mass flow rate and
condenser heat of rejection are the values predicted from the analysis of the Fujitsu 15RLS2 without Subcooler
Unit. This is because the addition of the Subcooler Unit theoretically does not affect either of these values.
However, the addition of the Subcooler Unit does increase the capacity of the heat pump’s evaporator.
Subcooling is commonly used in refrigeration systems for this exact reason, but typically this “free heat” is
wasted.
For this Subcooler Unit analysis, the enthalpy of the subcooled liquid refrigerant at a specified temperature TZ is
used (Equation 12). TZ is the temperature that the liquid R-410A is subcooled to from 120° F after it completely
condenses to a saturated liquid within the Fujitsu 15RLS2’s indoor unit. The Excel prediction calculation
spreadsheet seen in Appendix G analyzes the heat pump cycle with Subcooler Unit for subcooling values of 10° F
to 100° F subcooling (from 120° F). It should be noted that 100° F subcooling is not very realistic. A far more
realistic value subcooling value is 60° F (Tz = 40°F).
COPnoSC
Qcondenser
Wfujitsu15RLS2_input
Qevaporator_noSC Qcondenser Wfujitsu15RLS2_input
mdot_R410A
Qevaporator_noSC
hA hD
30
Using this specific enthalpy, hZ, the heat transfer rate from the micro-channel coil located in the Subcooler Unit
is calculated as:
Eq.12
The negative sign in Equation 12 is present so as to make Qsubcooler a positive value. It is known that that the
subcooler is rejecting heat, therefore proper thermodynamic sign convention will be ignored (Qsubcooler should be
a negative value). The overall predicted heat of rejection from the Subcooler Unit is found by adding the
electrical power input to the Subcooler Unit’s fan to the heat of rejection from the micro-channel coil as seen
below:
Eq.13
For a wide range of subcooling values, the predicted heat of rejection from the Subcooler Unit has been plotted
as a function of outdoor temperature (Figure 28).
Figure 28. Predicted Subcooler Unit heat output.
The new heat transfer rate into the outdoor unit’s coil (evaporator) is calculated as:
Eq.14
Alternatively the new heat transfer rate into the evaporator is calculated by adding the heat of rejection from
the Subcooler Unit to the heat input to the stock Fujitsu 15RLS2’s evaporator as:
Qmicrochannel_coil mdot_R410A hZ hC
Qsubcooler Qmicrochannel_coil Wfan
Qevaporator_sc mdot_R410A hA hD_sc
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Eq.15
For a wide range of subcooling values, the predicted heat input to the Fujitsu 15RLS2 evaporator has been
plotted as a function of outdoor temperature (Figure 29).
Figure 29. Predicted Fujitsu 15RLS2 indoor unit heat of rejection due to subcooling liquid R-410A.
The new COP of the heat pump due to the effects of the subcooler can be calculated as:
Eq.16
Attention should be paid to the additional term WSubcoolerFan in the denominator of Equation 16. Until enough
subcooling actually occurs, the additional heat benefit gained will not outweigh the additional electric work
input to the Subcooler Unit’s centrifugal fan and the COP could actually decrease compared to a heat pump
without a subcooler. It has been found that it only takes about 20 degree Fahrenheit subcooling for this to
happen. Therefore this is not a problem, since these subcooling values are easily attainable. For a wide range of
subcooling values, the predicted COP of Fujitsu 15RLS2 with Subcooler Unit has been plotted as a function of
outdoor temperature (Figure 30).
Qevaporator_sc Qevaporator_nosc Qsubcooler
COPsc
Qsondenser Qsubcooler
Wfujitsu15RLS2_input Wsubcooler_fan
32
Figure 30. Predicted System (Subcooler Unit and Fujitsu 15RLS2) COP.
For the complete Excel prediction spreadsheet refer to Appendix F. This spreadsheet also predicts Subcooler
Unit airstream outlet temperatures with inlet airstream temperature as outdoor temperature or user specified.
3.4 Subcooler Unit Design Mechanical Componentry Specifications
3.4.1 SolidWorks
From initial conception of the Subcooler Unit, SolidWorks CAD modeling software was used for successive
design iterations. Besides providing a useful visual aid for conception purposes, detailed drawing sets are
created of various components included in the Subcooler Unit. These were helpful for conveying our design to
Stewart Harvey and when Twin City Sheet Metal was contracted to fabricated the transition ducts and the
square duct assembly.
3.4.2 Alcoil Micro-Channel Coil
From the complete analysis of the Fujitsu mini-split heat pump with subcooler, it had been deemed that
approximately 2000 watts (6824.3 BTUh) was a realistic heat transfer rate that could be extracted from the
liquid R-410A refrigerant using the Subcooler Unit. Following this theoretical prediction, a refrigerant to air heat
exchanger needed to be selected. Following advice from James C. LaBrecque, it was decided that an all-
33
aluminum heat exchanger coil manufactured by Alcoil, utilizing micro-channel tube technology, would be the
appropriate choice for the application.
Compared to traditional fin and tube heat exchangers, micro-channel tube heat exchangers incorporate flat
micro-channel tubes in parallel position, fins located between the parallel tubes, and two manifolds that
connect to the micro-channel tubes on either end. These all-aluminum components are then joined together
using a nitrogen charged brazing furnace.
In November of 2013 Alcoil was contacted. The predicted theoretical heat of rejection from the micro-channel
coil, as well as the manufacture specified CFM for the chosen centrifugal duct, the airstream inlet temperature,
the inlet temperature of the liquid R-410A were all provided to Alcoil’s vice president Jim Bogart. Following this
communication, Alcoil decided that they could fabricate a coil from stock off the shelf components and that they
would donate it to Efficiency Maine Team IV. In late February of 2014, the micro-channel arrived at the
University of Maine.
3.4.3 Square Duct Assembly
From initial conception, the Subcooler Unit was based around a square or rectangular duct assembly that would
house both the Alcoil micro-channel coil and filter rack for pleated air filter. This assembly was designed so that
the airstream would pass through the air filter and then across the micro-channel coil. The option of a hinged or
removable door was also included so that the pleated are filter could easily be changed. Following the prediction
calculations, which resulted in an approximate size of the micro-channel coil needed, a better estimate of the
size of the square duct assembly began to develop. Initially the square duct assembly was designed to house a 9
in. nominal by 4 in. nominal serpentine micro-channel coil advertised in Alcoil’s off-the-shelf product line.
Following further communication with Alcoil and confirmation of a final micro-channel coil size, it was decided
that a square duct assembly measuring 15 in.by 15 in. would be appropriate. Although the micro-channel coil
provided by Alcoil is rectangular in overall dimension, it was decided that a square duct assembly, instead of a
rectangular duct assembly, was preferred for airstream efficiency and construction reasons. An extra sheet
metal flange was included to hold the micro-channel in the correct position. Foreseeing installation factors, the
square duct assembly was designed so that the two 0.625 in. copper connections to the micro-channel coil were
on the opposite side of the filter door.
Following this choice for the size of the square duct assembly a 14 in. nominal square pleated air filter was
selected. A 14 in. nominal square pleated filter was selected over a 15 in. nominal square filter for availability
reasons. Efficiency Maine team IV purchased a six pack of air filters, assuming an effective running period of
around four months. The rated face velocity for the 14 in. nominal filter is much higher than the airstream
velocity present in the square duct assembly, consequently under sizing the filter is a negligible choice. A drip
pan beneath the micro-channel coil was incorporated in the assembly for potential condensation that might
occur when the Fujitsu 15RLS2 was running in cooling mode. The micro-channel coil and the air filter rack were
positioned 4 in. apart and each 4 in. from the end of either square duct. Thus the total length of the square duct
assembly is 12 in.
Following these final design decisions, the SolidWorks 3D model was modified and technical drawing updated.
Twin City Sheet Metal in Brewer, ME was contacted to build the square duct assembly. Twin City specializes in
custom ductwork and frequently incorporates large and small filter racks in square and rectangular ductwork
components. Upon reviewing drawing and consulting with Efficiency Maine team IV, Twin City agreed to build
34
the square duct assembly and the micro-channel coil and a sample air filter was delivered to Twin City shop in
Brewer, ME. The micro-channel coil was provided to Twin City so that they could fabricate the proper flange to
secure the coil in place and so they could install the coil in the square duct while assembling. Attempting to
mount the micro-channel coil into the square duct assembly would have been difficult and result in an inferior
fit. Along with the filter rack, Twin City incorporated a hinged door with two latches so that the air filter could be
changed. The drip pan beneath the micro-channel coil was made from the same galvanized sheet metal used to
form the rest of the square duct assembly.
3.4.4 Fantech FG 6XL Centrifugal Inline Duct Fan
Assuming that there would be static pressure in the Subcooler Unit duct system due to the pleated air filter and
micro-channel coil, it was determined that it was necessary to use an inline centrifugal duct fan instead of a
typical inline duct booster fan. Although inline duct booster fans can produce large CFM volumetric flow rates,
they are incapable of overcoming large static pressure existing in a duct system, and performance drops
significantly. From specifications for the micro-channel coil and pleated air filter, an estimation of the static
pressure in the Subcooler Unit of about 0.6 in. W.G. was predicted.
A Fantech FG 6XL inline centrifugal duct fan was selected for the Subcooler Unit. The manufacture Fantech
specifies that this fan can produce about 369 aCFM at 0.6 in. W.G. static pressure (Figure 31). Figure 31 is a
graph showing the CFM performance values as a function of present static pressure for Fantech’s FG series
centrifugal inline duct fans.
Figure 31. FG series performance characteristics supplied by Fantech.
Source: Fantech. Lenexa, KS 66215 “FG Series Brochure Item #: 450459” Rev Date 111011. September 2010.
From Figure 29, it can be seen that the rated CFM’s of a FG 6XL and a FG 8XL are very similar. For this reason the
FG 6XL was selected, as it would provide comparable performance at a reduced price. Selecting a 6 in. diameter
fan instead of an 8 in. diameter fan also would decrease the cost of other duct work needed for the Subcooler
Unit, specifically the actuated dampers. From this performance data provided by Fantech, the static pressure in
the Subcooler Unit ductwork can be estimated or the calculated mass flow rate compared with the advertised.
3.4.5 Actuated Dampers
From initial conception, it was decided that the ability to draw the airstream from inside or outside was a critical
characteristic that the Subcooler Unit needed to have. To achieve this goal, two 6 in. diameter Honeywell
35
actuated dampers were selected. The 6 in. diameter for the actuated dampers was selected after the size of the
Fantech centrifugal fan was finalized. By incorporating one 120 VAC to 24 VAC voltage transformer in the
Subcooler Unit control panel, these two dampers were powered and controlled. Both dampers are designed so
that round damper inside each spring returns to an open state when not receiving power and actuate to a
closed state when they are receiving power. Foam inserts within both dampers ensure a relatively airtight seal.
3.4.6 Transition Ducts
The sizing for the two round-to-square transition ducts resulted from the final design selection for both the 15
in. square duct assembly and the Fantech FG 6XL centrifugal fan with 6 in. diameter connections. A smooth
transition from 6 in. round duct to 15 in. square duct made from 24 gauge galvanized sheet metal was
conceptualized and modeled in SolidWorks and Twin City Sheet Metal was contracted to fabricate the two
pieces. Twin City commonly fabricates transition ducts and made two very quality pieces. When Twin City was
again contracted to build the 15 in. square duct assembly, one of the transition ducts was brought back to their
shop to ensure proper fit.
3.4.7 Various Ductwork Components
Three 6 in. diameter straight duct sections 5 ft. in length and four 6 in. diameter 90° bend ducts were purchased
before installation details were finalized. The final installation at Service Building A only used a portion of one of
the 5 ft. duct sections and two of the 90° bend ducts. The 90° bend ducts are comprised of several articulating
pieces, allowing for other angle bends to be achieved.
3.5 Subcooler Unit Electrical Power and Controls
3.5.1 Electrical Power Componentry
A Hoffman electrical enclosure with a hinged cover including a glass front panel was donated to Efficiency Maine
Team IV by James LaBrecque to house all electrical power and control componentry for the Subcooler Unit. This
enclosure includes a removable back plate, allowing for access when modifications to the componentry need to
be made.
For a clean, organized design and for construction purposes, all electrical and power componentry was bought
or made to be mountable to the standard 35 mm din rail. One strip of standard 35 mm din rail cut to 24 in. was
bolted to the enclosures back plate. One 5 amp breaker was selected to limit current drawn by the Subcooler
Unit. Although, the Subcooler Unit only draws about 1.5 Amps when it is running, the current needed to be
limited in case someone made contact with a live wire. One 110 VAC to 24 VAC voltage transformer was
selected to provide the two actuated dampers with the correct voltage. The transformer was directly bolted to
the enclosure’s back plate for heat dissipation reasons.
Fourteen single pass through 1492 J-style terminal blocks, eight terminal block end anchors, nine terminal block
barriers and several screw type terminal jumper strips were purchased from Horizon Solutions LLC. in Bangor,
ME. Two sets of four terminal blocks, each connected by a jumper strip cut to a length of four poles were used
for the hot and neutral buses for the enclosure. Terminal block barriers and terminal block end anchors were
included ensuring that the terminal blocks remained securely in place.
36
Four sets of two terminal blocks each connected by a jumper strip cut to a length of two poles were
incorporated for various wire connections that needed to be made in the enclosure. Instead of using a screw cap
to connect three wires, one set of two terminal blocks were used resulting in clean, traceable wiring. The first of
such pairs connected one line from the 24 VAC transformer to one line from each of the two actuated dampers.
The second and third pairs connected the other 24 VAC line from each actuated damper to the respective two
lines to control each damper. For each actuated damper one of these two lines came from the manual controls
and the other from the smart controls. The fourth pair connected the hot line to the centrifugal fan to the
manual control and smart control inputs.
A Fantech fan speed controller was included in the enclosure so that the CFM created by the Subcooler Units fan
could be adjusted. Although included for customized performance, the Subcooler Unit was only tested with the
fan set at its highest speed.
Three on-off-on heavy duty toggle switches were included to allow for switching between manual and smart
controls for the Subcooler Unit’s fan and two actuated dampers. This manual control system is critical for
performance evaluations of the Subcooler Unit. The first of the three toggle switches allow for the user to select
manual or smart damper controls, the second toggle switch allows the user to control the dampers when the
upper toggle switch is set to manual damper control, and the third toggle switch is used to allow the user to
switch between manual or smart centrifugal fan control. Regardless of which control scheme is being used, the
centrifugal fan speed can be manually adjusted by means of a knob style speed controller and the centrifugal fan
can also shut off at any time. In sequential order, these three toggle switches were mounted to a plate that then
clipped to the standard 35 mm din rail.
One din rail mountable triple-receptacle120 VAC convenience outlet was included in the enclosure so that the 9
VDC adapter for the Arduino Uno micro controller board could be plugged in. The two spare receptacles could
be used for other control components in future Subcooler Unit modifications.
All componentry is connected by 14 AWG, 16 AWG, or 18 AWG stranded copper wire, color specific to 120 VAC
hot, 120 VAC neutral, 24 VAC, and ground. The enclosure itself, as well as the lid and removable back plate are
all connected to a ground terminal. This ground terminal is also connected to the buildings ground.
3.5.2 Electrical Control Componentry
Electrical control componentry inside of the Hoffman electrical enclosure is also mounted on the same section
of standard 35mm din rail and includes one Arduino Uno micro controller board and one SainSmart 2-Channel
SPDT relay module board. The Arduino Uno controls the SPDT relay module board, which consists of two SPDT
(Single Pole Double Throw) relays. One relay controls the two dampers at the intake of the Subcooler Unit and
the other controls the fan, whether it was on or off. The Subcooler Unit control assembly also includes two
waterproof DS18B20 digital temperature sensors used to measure the airstream temperature before and after
the micro-channel coil. A light dependent resistor (LDR) is also included in the control assembly to detect
whether a light was on. A non-solder bread board is also included in the enclosure to allow for proper
connection of the two DS18B20 digital temperature sensors to the Arduino Uno, the LDR, as well as for several
LED’s to be connected. The breadboard was attached using two pieces of Velcro for easy removal if necessary.
The breadboard was also used for a 5 VDC power rail, as well as a ground rail.
For a detailed power and control electrical schematic for the Subcooler Unit refer to Appendix B.
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3.5.3 Arduino IDE Development
To have a successful permanent install, the Subcooler Unit needs a control system that is able to smart control
the fan and dampers based on current conditions. The fan can be either on or off. The fan is desired to be on
when the Fujitsu 15RLS2 MSHP is running in heat mode. The fan is desired to be off when the Fujitsu 15RLS2
MSHP is in defrost mode or cooling mode. This is desired because when the Fujitsu 15RLS2 MSHP is in defrost or
cooling modes, it is not cost or energy effective to have the fan on.
The dampers control which inlet the airstream comes from. There is a damper that controls air coming from
inside and a damper that controls air coming from outside. The outside damper is open with the inside damper
closed when the room’s exhaust fan is on. This is done because when the exhaust fan is on, there is a need for
makeup air. When there is no need for makeup air the outdoor damper is closed and the inside damper is
opened.
An Arduino Uno programmable microcontroller is used because it meets the demands of the controls system
and Efficiency Maine Team IV is familiar with this microcontroller as well as the Arduino integrated development
environment (IDE). A Sainsmart 5V 2-channel solid state relay is used because it is compatible with Arduino and
the relay contains two single pole double throw (SPDT) relay switches, one relay switch controlling the fan the
other controlling the dampers.
The IDE has a main loop that runs once every ten minutes. Ten minutes is subjective and can be changed if
future results indicate doing so. At the beginning of the loop the fan is turned on for a time defined by variable
“fanon”. This is currently equal to one minute, but can be changed if a more accurate time interval is found. The
fan must be on because there is currently no other way to determine what state the Fujitsu 15RLS2 MSHP is
running in.
The use of two One Wire DS18B20 Digital Temperature Sensors provides the input data for fan control. To make
coding with these sensors easier Dallas Temperature and OneWire Arduino libraries are used [3]. These libraries
are used because they make the coding with these sensors much more user friendly. A program to find the serial
numbers was also used [4]. The serial numbers were needed because these sensors are connected to one pin on
the Arduino and the serial numbers identify one from the other. These particular sensors were used because
these sensors have sufficient accuracy for the project demands, measure temperatures within the range they
would be exposed to in this application, and are relatively inexpensive. Most importantly, they can be used to
determine what state the Fujitsu 15RLS2 MSHP is in. This is done by putting one sensor before the micro-
channel coil and one after it. To store these temperature values, variables are created in the IDE code. These
were “Inlet” and “Outlet” and correspond to the sensor before and the sensor after the micro-channel coil,
respectively. A third variable, “DeltaT” is defined as the “Outlet” minus the “Inlet” and is used to determine the
state of the Fujitsu 15RLS2 MSHP. When the value of “DeltaT” is less than or equal to two, the fan will be turned
off because theFujitsu 15RLS2 MSHP is deemed to be in defrost or cooling mode. This may not be the exact
value because that value was not known at the time of the creation of the controls system. However, if found
this value can be changed to the correct value. When the value of “DeltaT” is greater than two the fan is left on,
because this deems that the Fujitsu 15RLS2 MSHP is in heat mode. This results in a total of two statements that
control the fan each of which contains a delay defined by “fanoff”, which is currently equal to eight minutes.
A light dependent resistor (LDR) is used to control the dampers. This is used because when the light is on, the
exhaust fan is also on. During this period the outdoor damper will be open and the indoor damper will be closed.
38
This state allows for the needed makeup air to be provided. However, there is a check on the outlet air
temperature to the room. If the air entering the room is too cold, then the outdoor damper will close and the
indoor damper will be opened. This minimum temperature is given a variable “TempMin” and is currently equal
to 50o F. When the light in the room is off, the exhaust fan is also off. When the LDR determines the light is off,
the indoor damper is opened and the outdoor damper is closed. This results in three statements that control the
dampers. Each statement has a delay defined by “damperdelay” and is currently equal to a minute. This
combined with the other two delay variables result the aforementioned ten minute loop.
For a listing of the IDE code for the Subcooler Unit smart control system refer to Appendix C.
3.6 Finding an Installation Location The installation process began by finding a site that fit the parameters outlined by Professor Peterson. These
parameters were that the installation site was public. Other criteria for the installation location was that the
public place was local, preferably on the University of Maine campus, that the system would be installed at the
site permanently and that the site was in need of an improved heating and cooling system. The first site that
met these criteria was the University Garage.
After finding a possible installation site, approval from Professor Peterson was needed. After a brief meeting,
the approval was given. The next approval needed was from the Associate Director of Facilities Management,
Mr. Geremy Chubbuck P.E., because he was the person who could give approval to install in the building. After
the first meeting, Mr. Chubbuck seemed optimistic about the project. He asked for a drawing package for the
possible installation. Following the completion of the drawing set was another meeting. This meeting was more
technical and dealt with the many factors involved with the installation. This meeting was somewhat of a
setback; because the 220V line necessary to power the Fujitsu heat pump would be provided through the use of
an existing welding outlet. This meant the installation would not be as permanent as hoped for. However,
because at the time there was no alternative installation site, this was accepted. When it was time to start the
actual installation process, Mr. Jim Labrecque, the project advisor came to the installation site. He immediately
determined this site was not acceptable for the installation. The University Garage was an industrial site that
was very dirty. The Fujitsu heat pump being used in this project was not designed for industrial use and because
of that would not work in this application.
A new installation site was needed. The University of Maine’s Service Building A, located adjacent to the
University Garage also met the specifications for the installation site. After receiving approval from Professor
Peterson to install at Service Building A, approval from Executive Director of Facilities & Capital Management
Services, Mr. Stewart Harvey, P.E. was needed, because he was the person who could give the approval
necessary to install in the building. At the conclusion of the first meeting, Efficiency Maine Team IV had the
approval. The next step in the installation process was a drawing set in AutoCAD. An electrical schematic was
also required; this was done in TinyCAD.
The indoor unit of the Fujitsu MSHP was installed in the Work Control Center. This was the installation site
because there was an inefficient air conditioner in the room and the room was in need of an improved heating
and cooling system. The outdoor unit was located outside this room.
39
The Subcooler Unit was installed in the photocopier room located down the hall from the Work Control Center.
This room was in need of makeup air because in this room there was an exhaust fan that ran whenever the light
was on. Exhaust fans take air out of the room and this air is generally not replaced. The Subcooler Unit would
replace this air.
Due to University and State laws much of the installation was done by University employees or contracted out.
The Fujitsu 15RLS2 MSHP installation was done by RJ Morin. They were chosen because they have done work for
the University of Maine in the past and Mr. Harvey has hired them before. The electrical work needed for this
installation was done by the University of Maine’s Electric Shop, with much of the work being done by Mr. Nate
Emerson.
40
4 Final Design Testing and Evaluation
4.1 Testing and Evaluation Objectives To accurately measure and analyze the performance of the Subcooler Unit, Fujitsu 15RLS2, and overall installed
system, many different independent variables have been monitored and recorded.
These 18 objectives have been divided into three categories including, Subcooler Unit testing and evaluation,
Fujitsu 15RLS2 testing and evaluation, and overall installed system evaluation. Each of these three major
subdivisions and their respective objectives can be seen in list form below. Details of each testing and evaluation
subdivision will be presented in the following sub sections.
Subcooler Unit Testing and Evaluation
1. Measure airstream velocity profile at the Subcooler Unit outlet using a hot wire anemometer inserted
into the ductwork using a vertical traversing apparatus.
2. Measure and record Subcooler Unit inlet and outlet airstream temperatures using two type T
thermocouples and a USB-5104, battery-powered 4-channel thermocouple data logger.
3. Measure and record R-410A (saturated or subcooled liquid state) temperature at the inlet and outlet of
the Alcoil micro-channel coil using two type T thermocouples affixed to the copper refrigerant lines, as
well as the previously mentioned USB thermocouple data logger.
4. Measure voltage drop across the entire Subcooler Unit, as well as across the Fantech FG 6XL centrifugal
inline duct fan, the two Honeywell actuated dampers, and the Arduino Uno using a clamp style induction
ammeter.
5. Measure current drawn by the entire Subcooler Unit, as well as by the Fantech FG 6XL centrifugal inline
duct fan, the two Honeywell actuated dampers, and the Arduino Uno using a clamp style induction
ammeter.
6. Determine mass flow rate of the airstream drawn through the Subcooler Unit.
7. Determine amount of liquid R-410A refrigerant occurring in micro-channel coil in Subcooler Unit.
8. Determine electrical power consumed by the entire Subcooler Unit, as well as by the Fantech FG 6XL
centrifugal inline duct fan, the two Honeywell actuated dampers, and the Arduino Uno.
9. Determine heat rejection from micro-channel coil located in the Subcooler Unit using a first law of
thermodynamics analysis and specified control volume on the Subcooler Unit.
10. Determine overall heat of rejection from the Subcooler Unit to the photocopier room.
11. Determine operating efficiency of the Subcooler Unit.
Fujitsu 15RLS2 Testing and Evaluation
1. Measure and record inlet and outlet airstream temperatures for the indoor unit using three type T
thermocouples and a second USB-5104, battery-powered 4-channel thermocouple data logger.
2. Measure and record power input to Fujitsu 15RLS2 unit using The Energy Detective device.
3. Determine airstream volumetric flow rate through 15RLS2 from Fujitsu manufacture data.
4. Determine average outlet airstream temperature from the two airstream outlet temperatures
measured.
5. Determine the heat of rejection from the Fujitsu 15RLS2 indoor unit by applying a first law of
thermodynamic equation to the airstream passing through the indoor unit.
41
6. Determine COP of Fujitsu 15RLS2 MSHP.
Overall Installed System Evaluation
1. Determine COP of overall installed system including Subcooler Unit and Fujitsu 15RLS2.
4.2 Regarding Subcooler Unit and Fujitsu 15RLS2 Evaluation The variable speed compressor, which powers the Fujitsu 15RLS2, creates certain complexities when trying to
evaluate the performance of the installed system at Service Building A. It is this variable speed compressor that
makes modern MSHP’s far more efficient than their outdated counterparts that could either run at a fixed speed
or be turned off. To monitor and record the variable power input to the Fujitsu 15RLS2, a certain device with a
reputable performance background was selected. The device, which will be described in section 4.4.2, ended up
having a faulty module. Time and cold weather constraints prompted Efficiency Maine Team IV to not seek a
replacement device.
Following this obstacle to begin testing, it was decided that the performance of the whole system (Fujitsu
15RLS2 and Subcooler Unit) would only be analyzed when the Fujitsu 15RLS2 was running at or near its highest
capacity. Due to this assumption, a trend line correlating input power to the Fujitsu 15RLS2 to outdoor
temperature could be generated from manufacture performance data supplied by Fujitsu General Limited for
the 15RLS2 model. Thus, only raw test data collected from each test trial exhibiting peak performance
characteristics was selected for use in the data reduction and analysis. The method of generating this trend line,
as well as the various constraints associated with it is presented in section 4.4.1.1.
Excluding the airstream velocity profile in the subcooler outlet duct, all graphical representations of measured
and reduced data in this report section are plotted as functions of outdoor temperature. Although several other
choices could be used, outdoor temperature is regularly used in the industry when representing performance
characteristics of MSHPs.
4.3 Test Trials After the Subcooler Unit and Fujitsu 15RLS2 MSHP were installed at Service Building A, five test trials were
completed. Each trial lasted anywhere from 12 to 48 hours long and covered a wide range of outdoor
temperatures. Although the system was not running until April 14th, Maine’s erratic weather patterns allowed
for test data to be collected for outdoor temperatures ranging from a minimum of 26° F to a maximum of 64° F.
Before the fourth and fifth test trials, Stewart Harvey had a temporary length of6 in. flexible duct connected to
one of the Subcooler Unit’s inlets and ran outside (Figure 32). This temporary flexible duct would allow for a
Subcooler Unit airstream to be drawn from outside, permitting for a more thorough understanding of
subcooling.
42
Figure 32. Temporary 6 in. flexible duct to draw Subcooler Unit airstream from outside.
Following completion of testing, the length of flexible duct was removed. As previously mentioned, a straight
section of 6 in. steel duct will eventually be run through a hole cut in the roof of service building A.
4.4 Subcooler Unit Testing and Evaluation
4.4.1 Measuring Airstream Velocity Profile
The airstream velocity profile at the Subcooler Unit outlet in the round duct is measured to determine an
average velocity the mass flow rate of the air. The velocity profile is measured according to ASHRAE standards,
as indicated in the Objective Section of this proposal report, using an Omega Engineering hot wire anemometer.
To accurately measure the airstream velocity with the hot wire anemometer, an apparatus constructed out of a
twelve-inch ruler, a six-inch hose clamp, and two aluminum “guides” that are clamped onto the ruler is used to
traverse the six inch round duct with said hot wire anemometer .The probe shaft of the hot wire anemometer is
inserted via a hole, and the two aluminum guides apparatus will keep the probe shaft perpendicular to the
airflow, which is necessary to achieve accurate results the hot-wire anemometer. The apparatus for inserting the
hot wire anemometer was inspired by a similar device that was used to traverse a much smaller PVC pipe with a
pitot-static tube to determine the airstream velocity profile in the Undergraduate Mechanical Engineering
course “Mechanical Laboratory I” (Figure 33). Figure 33 shows the anemometer apparatus and one of the
members of Efficiency Maine Team IV using the apparatus to conduct the experiment. For this experiment, a
datum is taken at the bottom surface of the interior of the duct wall.
43
Figure 33. Hot Wire Anemometer and Traversing Guide Apparatus.
The velocity profile is measured at a location in the ducting of the Subcooler Unit where the airstream has
reached fully developed flow. According to ASHRAE standards, flow in a duct is fully developed if it is 7.5
hydraulic diameters downstream and 3 hydraulic diameters upstream from any flow disturbances. The average
airstream velocity is determined by applying the Log-Linear Rule for flow in circular ducts using the measured
data collected with the hot wire anemometer.
For this experiment, the Subcooler Unit is equipped temporarily with a 10-foot length of galvanized steel round
ducting, as to allow the airstream flow to become fully developed for accurate testing. A hole is drilled in the
ducting 7.5-feet from the centrifugal fan which is the last downstream disturbance in the airstream, and the
collar-and-ruler assembly is placed around the ducting to allow traversing of the duct with the hot-wire
anemometer. The Subcooler Unit, equipped with the additional ducting can be set up on two experiment
benches in Crosby Lab (Figure 34).
Traversing Guide Apparatus
Hot Wire Anemometer Probe
Hot Wire Anemometer Display
44
Figure 34. Subcooler Unit and Additional Ducting for Airstream Profile Experiment
With these conditions met, the airstream velocity profile in the round duct is measured using the Log-Linear
Rule for Circular ducts (Figure 35).
Figure 35. Velocity measurement locations specicified by the Log-Linear Rule for Circular ducts
(Source: ASHRAE Fundamentals Handbook, Chapter 36.13)
Figure 35 illustrates where to make airflow measurements in a round duct with any given interior diameter to
determine an average airstream-flow-rate in the duct as recommended by ASHRAE. Summing the airstream flow
rates at each location and dividing by the total number of measurements taken determines the average-air-
stream flow rate in the ducting.
Subcooler Unit
10 ft. Section of 6 in. Round Duct
Anemometer Guide Apparatus and Hot Wire Anemometer
45
The airstream velocity test is conducted two times to assure accurate results. With the tests completed, it is
possible to view the airstream velocity in the duct of the Subcooler Unit (Figure 36).
Figure 36. Plot showing fully developed airstream velocity profile in Subcooelr Unit exit duct for two test trials.
A 4 ft. long section of 6 inch round duct work is incorporated in the Subcooler Unit after the centrifugal fan.
Thus, it is believed that the airstream will be essentially fully developed when it is measured at the subcooler
duct outlet. Using values illustrated in Figure 36, the average airstream velocity in the ducting is determined to
be 1716 feet per minute.
4.4.2 Measuring Subcooler Unit Airstream Inlet and Outlet Temperatures
The Subcooler Unit airstream temperature is measured with two type T thermocouples positioned at the inlet
and outlet of the Subcooler Unit. The first thermocouple, measuring the inlet airstream temperature, is inserted
inside the round duct portion of the inlet transition duct so as to measure the inlet air temperature regardless of
the damper position (Drawing airstream from inside or outside) (Figure 37).
46
Figure 37. Type T thermocouple inserted in Subcooler Unit to measure inlet airstream temperature.
The second type T thermocouple, measuring the outlet airstream temperature, is inserted in the round duct
after the square duct. This second thermocouple is inserted in the exit round duct in a similar manner to the
thermocouple measuring the inlet temperature. Both type T thermocouples are connected to a 4 channel USB
thermocouple data logger, which can record up to 1.6 million data points.
4.4.3 Measuring Subcooler Unit Refrigerant Inlet and Outlet Temperatures
The temperature of the working refrigerant, R-410A, is measured at the inlet and the outlet of the micro-
channel coil using two type T thermocouples (Figure 38). One thermocouple is placed on the inlet and one is
placed on the outlet of the liquid refrigerant line. Since the refrigerant line is copper, a material with high
thermal conductivity, the temperature of the line is assumed to be the same temperature of the refrigerant.
Each thermocouple is covered with thermal grease and foam pipe insulation and electrical tape, so as to prevent
the indoor air from influencing the temperature reading at the refrigerant inlet and outlet. The temperature of
the working refrigerant is measured with the same USB thermocouple data logger measuring the inlet and
outlet air temperatures recording temperature data for both thermocouples once per every three minutes.
Type T Thermocouple to Measure Airstream
Inlet Temperature
47
Figure 38. Two type T thermocouple attached to inlet and outlet 0.625 in. micro-channel connections respectively
The USB data logger measuring the Subcooler Unit inlet and outlet airstream temperatures and inlet and outlet
R410A temperatures can be seen below (Figure 39).
Figure 39. USB thermocouple data logger to measure Subcooler Unit's inlet and outlet airstream temperatures and inlet and outlet R-410A temperatures.
After running a performance test trial, the recorded temperature data from the 4 channel USB thermocouple
data logger for the four the type T thermocouples can be easily exported as CSV file to Excel.
Type T Thermocouple to Measure R-410A Inlet
Temperature
R-410A Inlet Temperature
Airstream Outlet Temperature
Airstream Inlet Temperature
R-410A Outlet Temperature
Type T Thermocouple to Measure R-410A Outlet
Temperature
48
4.4.4 Calculating Subcooler Unit Input Power
To determine the power drawn by the Subcooler Unit, the total current drawn by the Subcooler Unit was
measured using a clamp style induction ammeter and the voltage drop across the Subcooler Unit was measured
using a standard multimeter. After measuring the current in amperes and the voltage drop in VAC the power
drawn by the Subcooler Unit could be calculated by multiplying the two values as seen below in Equation 17.
Eq.17
This total power input to the Subcooler Unit will be used when calculating the Subcooler Unit efficiency, as well
as the installed system’s COP. Since the centrifugal fan is set to a certain speed, and that one of two actuated
dampers will always be powered, it will be assumed that the total power drawn by the Subcooler Unit is a
constant. This assumption was verified after the total power drawn by the Subcooler Unit was measured on
several different occasions.
The power input to the Subcooler Unit as a whole includes power input to the Fantech FG-6XL centrifugal duct
fan, power input to one of the two Honeywell actuated dampers (only one damper is selected at a time), and
power input to the Arduino micro-controller, which also powers the SPDT relay module board. Each of these
individual power inputs could be found by applying the same ammeter and multimeter to correct wires and
terminal blocks respectively. The power drawn by each of the three components can then be found the same
way as Equation 18.
Eq.18
Eq.19
Eq.20
It can be seen from Equation 20 that the power drawn by the Arduino micro controller board is parasitic when
compared to the power drawn by the fan and dampers.
4.4.5 Calculating Heat of Rejection from Subcooler Unit
To accurately analyze the airstream flowing through the Subcooler Unit, a first law of thermodynamics equation
can be written on an appropriate control volume. This control volume is defined as encompassing the entire
Subcooler Unit with boundaries set a few inches away from the duct inlets and outlets. Due to this assumption,
the kinetic energy term due to the airstream velocity at either inlet (outside or inside) is considered to be zero.
In contrast, the kinetic energy term of the flowing air at the outlet is included. There is electrical work input to
the centrifugal fan that is passing into this control volume boundary and is defined as Wfan, a negative value by
thermodynamic sign convention. Since the Subcooler Unit is relatively short and since the airstream flowing
within is moving quickly, the defined control volume will be considered to be adiabatic. Perhaps there are small
heat gains (to ambient) from the airstream before the micro-channel coil and small heat losses (to ambient)
after, but they can both be considered negligible. The two mass flow rates passing the boundary of the defined
control volume are the airstream mass flow rate mdot_air and the R-410A mass flow rate mdot_R410A. All of these
variables with corresponding units can be seen below in tabular form (Table 3). The following data reduction
theory section will be conducted using the English engineering unit system and therefore several conversions
are needed and will be described as needed.
Wsubcooler Isubcooler Vsubcooler
Wcentrifugal_fan Ifan Vfan 1.16A( ) 113 VAC( ) 131.08W
Wdamper Idamper Vdamper 0.33A( ) 26VAC( ) 8.58 W
Warduino IArduino VArduino 0.01A( ) 113( ) 1.13W
49
Table 3. Variables used to determine heat of rejection from the Subcooler Unit.
Independent Variable Units
Wfan Watts
Vair_outlet Feet per Minute
Diaround_duct Inches
Tair_outlet Degrees Fahrenheit
Tair_inlet Degrees Fahrenheit
TR410A_outlet Degrees Fahrenheit
TR410A_inlet Degrees Fahrenheit
Dependent Variable
mdot_air Pound Mass per Hour
Qmicrochannel_coil BTU per Hour
Since the R-410A refrigerant is in a liquid state at both the inlet and outlet of the Alcoil micro-channel coil, the
corresponding specific enthalpies in BTU/lbm can be found in an R-410A thermodynamic properties table
provided by DuPont. Although the R-410A liquid at the outlet of the micro-channel coil will exist in a subcooled
state, not saturated, the enthalpy values are essentially the same for both at the same temperature. The three
engineering constants Cp_air, ρair, and gc_eng used the analysis of the Subcooler Unit can be seen below (Table 4).
Table 4. Engineering constants used in data reduction.
CP_air ρair gc_eng
0.24 BTU/lbm-°F 0.075 lbm/ft3
32.174 lbm-ft/lbf-s2
It should also be noted that the airstream mass flow rate and the velocity are dependent upon each other. The
mass flow rate of the air can be calculated from the average airstream velocity of the airstream profile recorded
at the subcooler outlet duct, the cross sectional area of this outlet duct, and the defined density of the airstream
flowing through the subcooler as seen in Equation 21 below.
Eq.21
The formula for the cross sectional area of the 6 in. round exit duct, as well as two needed conversion factors to
convert form square inches square feet and from minutes to hours , can be substituted into Equation 21 so that
an airstream mass flow rate with units of lbm/hour results and can be seen below in Equation 22.
Eq.22
The two needed conversion factors used in Equation 22, so that a mass flow rate in lbm/hour results, are as
follows:
mdot_air vair_outlet Areaoutlet air
mdot_air vair_outlet
4
Diaround_duct2
1 ft
2
144 in2
60 min
1 hr
air
50
Eq.23
Eq.24
From the defined airstream mass flow rate, as well as previous definitions and assumptions, a first law of
thermodynamics equation written on the defined control volume encompassing the Subcooler Unit is as follows:
Due to the previously mentioned adiabatic assumptions, Qsubcooler is essentially zero. The electrical power rate
Wfan is a negative value in thermodynamic sign convention since it is into the defined control volume. Equation
25 employs three conversion equalities, so that units are consistent throughout. The first corresponds to the
centrifugal fan electrical work input and the second corresponds to the kinetic energy term due to the airstream
velocity at the subcooler ductwork exit. These three conversion equalities are as follows:
Eq.26
Eq.27
Eq.28
Equation 25 can then be solved for the heat of rejection from the Alcoil micro-channel coil (R-410A mass flow
rate multiplied by difference in inlet and outlet specific enthalpies) and can be seen below in Equation 29. It
should be noted that the R-410A enthalpies at the coil inlet and outlet have been rearranged, so that the left
side of Equation 29 (i.e. the heat of rejection from the micro-channel coil) is a positive value.
11 ft
2
144 in2
160 min
1 hr
Qsubcooler Wfan
3.412BT U
hr
1 W
mdot_air Cp_air Tair_outlet Tair_inlet
vair_outlet2 1 min
2
3600s2
778.169lbf ft
1 BT U
2 gc_eng
mdot_R410A hR410A_outlet hR410A_inlet
1
3.412BT U
hr
1 W
11 min
2
3600s2
1778.169lbf ft
1 BTU
Eq.25
51
The left side of Equation 29 is equal to the heat of rejection from the micro-channel coil and can be assigned the
value Qmicrochannel_coil as follows:
Eq.30
Equation 22 for the airstream mass flow rate and Equation 30 for the heat of rejection form the micro-channel
coil can then be substituted into Equation 29 and is as follows:
Analogous to Equation 29, Equation 31 is also arranged so that Qmicrochannel_coil is a positive value. If a new control
volume was defined as only encompassing the Alcoil micro-channel coil, this heat of rejection would be negative
by thermodynamic sign convention (heat is leaving the micro-channel coil). Since it is known that this heat of
rejection is from the micro-channel coil to the airstream in the subcooler duct system, it has been defined as
positive for simplicity. The R-410A refrigerant mass flow rate through the micro-channel coil can then be solved
from Equation 30 as follows:
Eq.32
Equation 31 can then be used to reduce data recorded from the Subcooler Unit and to predict the heat rejection
from the micro-channel coil. It should be noted that it is critical for the heat of rejection from the micro-channel
coil to be greater than the electrical work input to the centrifugal fan for any benefit to occur. If this was not the
case, the coefficient of performance of the overall heat pump cycle would actually decrease.
In Equation 31 only the heat of rejection from the micro-channel coil was found. To accurately analyze the entire
installed system in a later section, the centrifugal fan’s effect on the airstream needs to be incorporated. This
can be done by simply adding the power input to the centrifugal fan to the heat rejection from the micro-
channel coil and can be seen below in Equation 32. It is important to note that the input power to the
centrifugal fan is a negative value by thermodynamic sign convention and therefore a double negative exists in
Equation 32. This total heat rejection from the Subcooler Unit will be deemed QSubcooler.
mdot_R410A hR410A_inlet hR410A_outlet mdot_air Cp_air Tair_outlet Tair_inlet
vair_outlet2 1 min
2
3600s2
778.169lbf ft
1 BT U
2 gc_eng
Wfan
3.412BT U
hr
1 W
Qmicrochannel_coil mdot_R410A hR410A_inlet hR410A_oulet
Qmicrochannel_coil vair_outlet
4
Diaround_duct2
1 ft
2
144 in2
60 min
1 hr
air Cp_air Tair_outlet Tair_inlet
vair_outlet2 1 min
2
3600s2
778.169lbf ft
1 BT U
2 gc_eng
Wfan
3.412BT U
hr
1 W
mdot_R410A
Qmicrochannel_coil
hR410A_inlet hR410A_oulet
Eq.29
Eq.31
52
Eq.33
A conversion is included in Equation 33 to convert the fan input power from Watts to BTUhr. The Subcooler Unit
heat of rejection has been plotted as a function of outdoor temperature for the five test performance trials
(Figure 40). It should be noted that since the static pressure present in the system is relatively low (roughly 0.6
inches W.G.), the centrifugal fan’s effect on increasing the airstream temperature is relatively low. If a nozzle
was placed at the subcooler duct outlet, a higher static pressure would result and therefore a higher airstream
temperature rise.
Figure 40. Subcooler Unit heat output as a function of outdoor temperature.
Due to the time of year that the system was installed, no performance data was collected for outdoor
temperatures below 20° F. From Figure 40, it can be seen that the Subcooler Unit heat output ranged between
4,590 and 7,420 BTU/hr when the entering airstream is drawn from inside Service Building A at a temperature of
75° F, between 8,530 to 11,440 BTU/hr when the entering airstream is drawn from outside Service Building A at
a temperature of 60° F, and between 13,740 and 14,780 BTU/hr when the entering airstream is drawn from
outside Service Building A at a temperature of 30° F. Thus, indicating that drawing in a colder Subcooler Unit
airstream results in greater heat output from the Subcooler Unit.
Qsubcooler Qmicrochannel_coil Wfan
3.412BT U
hr
1 W
53
These heat output rates exceeding one ton of heat indicate that some condensing of the R-410A in the micro-
channel coil is occurring. This is due to incomplete condensation occurring in the Fujitsu 15RLS2’s indoor unit. To
alleviate this problem, additional R410A needs to be added to the installed system (Subcooler Unit and Fujitsu
15RLS2). This task was not achieved due to recognition of the problem occurring at the end of the spring
semester.
When the Subcooler Unit airstream was pulled from inside Service Building A at a temperature of 75° F, the
airstream’s temperature increase ranged from 13.5° F to 18.5° F. When the Subcooler Unit airstream was pulled
from outside Service Building A at a temperature of 60° F, the increase of the airstream’s temperature increase
ranged from 22.5° F to 30.5° F. When the Subcooler Unit airstream was pulled from outside Service Building A at
a temperature of 30° F, the increase of the airstream’s temperature increase ranged from 37.5° F to 39.0° F.
These increased Subcooler Unit airstream temperature differentials are synonymous with higher heat output.
Often the temperature increase of the airstream through the Subcooler Unit exceeded the temperature
decrease of the R-410A though the micro-channel coil. This is another indication that refrigerant condensation is
occurring in the micro-channel coil.
4.4.6 Calculating Subcooler Unit Operating Efficiency
The operating efficiency in percent form of the Subcooler Unit can be calculated by dividing the heat of rejection
from the Subcooler Unit by the total power consumed by the Subcooler Unit and can be seen below in Equation
33.
Eq.34
The Subcooler Unit operating efficiency has been plotted as a function of outdoor temperature for the five
completed test trials (Figure 41).
SubcoolerUnitefficiency
Qsubcooler
Wsubcooler
100
54
Figure 41. Subcooler Unit Efficiency plotted as a function of outdoor temperature
From Figure 41, it can be seen that the Subcooler Unit operates between 910 percent and 1475 percent
efficiency when the entering airstream is drawn from inside Service Building A at a temperature of 75° F,
between 1695 percent to 2275 percent efficiency when the entering airstream is drawn from outside Service
Building A at a temperature of 60° F, and between 2730 and 2940 percent efficiency when the entering
airstream is drawn from outside Service Building A at a temperature of 30° F. Thus, indicating that drawing in a
colder Subcooler Unit airstream results in greater heat output from the Subcooler Unit.
When the Subcooler Unit airstream was pulled from inside Service Building A at a temperature of 75° F, the
airstream’s temperature increase ranged from 13.5° F to 18.5° F. When the Subcooler Unit airstream was pulled
from outside Service Building A at a temperature of 60° F, the increase of the airstream’s temperature increase
ranged from 22.5° F to 30.5° F. When the Subcooler Unit airstream was pulled from outside Service Building A at
a temperature of 30° F, the increase of the airstream’s temperature increase ranged from 37.5° F to 39.0° F.
These increased Subcooler Unit airstream temperature differentials are synonymous with higher heat output.
The fact that the Subcooler Unit operating efficiencies are such large magnitudes is indicative of the “free heat”
claim made about the Subcooler Unit. Solely looking at the Subcooler Unit, the only electrical power input is to
the centrifugal fan. The mass flow rate of the R-410A can be ignored in the Subcooler Unit efficiency equation
since the heat that is removed from the R-410A is then regained from the environment. Theoretically the
Fujitsu’s compressor and indoor unit should not notice the presence of the Subcooler Unit.
55
For detailed experiment report for MEE 443 Mechanical Lab III, outlining the methodology used to measure the
heat of rejection from the Subcooler Unit refer to Appendix G.
4.5 Fujitsu 15RLS2 Testing and Evaluation
4.5.1 Measuring Electrical Power Input to Fujitsu 15RLS2 Heat Pump
4.5.1.1 The Energy Detective FAILURE
To accurately measure the power input to the variable speed compressor located in the Fujitsu 15RLS2’s
outdoor unit a device called The Energy Detective, or TED, is implemented into this design project.
To measure the electrical input energy necessary to run the compressor of the Fujitsu 15RLS2, The Energy
Detective (TED) was selected. TED is an electricity monitor that collects and records amperage and voltage data
to determine power consumed. TED is able to measure the electricity consumed by the heat pump by placing
the two current transformer clips around the incoming conductors inside the breaker box providing electricity to
the compressor, which is located on the outdoor unit of the heat pump. The current transformer clips are
connected to a Measuring Transmitting Unit, which sends the electricity usage data to an Energy Control Center
via Powerline Carrier Communication, which passes through the existing power lines.
The Energy Control Center is a data-storing hub that stores and analyzes the electricity data obtained by the
current transformers, and is embedded with “Footprints Data”, which is the electricity data logger software that
can be connected to and viewed on any personal computer or laptop. Footprints Data records and stores the
electricity data in either a monetary or kilowatt-per-hour basis in a CSV file, which can be opened at any time
without stopping the logging of electricity data with a program such as Microsoft Excel.
TED is an attractive choice for measuring the electricity power input to the heat pump because it monitors and
records the electricity usage of the heat pump in real time, and because it is inexpensive. The electricity usage
being recorded in real times allows one to perform a transient first law of thermodynamics analysis on the entire
heat pump unit, determining the coefficient of performance of the heat pump as a function of time and outdoor
air temperature. With TED installed, it is also easy for one with a less technical background to monitor the
monetary amount required to run the Fujitsu 15RLS2 Heat Pump. According to manufacturer specifications, TED
can monitor up and save ten years of energy usage data and is calibrated in factory to be accurate in measuring
electricity within plus or minus two percent.
After installing TED at Service Building A and consulting with TED technical support personnel several times, it
was determined that one of the TED components devices was faulty. Due to time constraints, Efficiency Maine
Team IV had to abandon TED as a method for monitoring power input to the Fujitsu 15RLS2. In its place a 6th
degree polynomial was developed to predict power input to the Fujitsu 15RLS2 as a function of outdoor
temperature.
4.5.1.2 Determining Electrical Power Input to Fujitsu 15RLS2 using 6th Degree Polynomial
Following the TED failure, it was determined that a 6th degree polynomial fit to manufacture data supplied by
Fujitsu General Limited that correlates the input power to the 15RLS2 to outdoor temperature, would be
sufficient for system evaluations. As described in Section 3.3.2, the power input to the Fujitsu 15RLS2 should not
increase due to the addition of the Subcooler Unit to the heat pump cycle and therefore using power input
56
values for a stock Fujitsu 15RLS2 should correlate reasonably well to the installed system. These nine data pairs
(Fujitsu 15RLS2 input power and corresponding outdoor temperature) were plotted in Microsoft Excel and a 6th
degree polynomial best fit trend line was selected for the data set (Figure 42).
Figure 42. Curve fit 6th degree polynomial for Fujitsu 15RLS2 input work as a function of outdoor temperature.
This 6th degree polynomial for input work as a function of outdoor temperature presented visually in Figure 40
can be seen below in Equation 35.
Eq.35
For each of the seven constants seen Equation 35, values to twenty decimal places and respective units have
been presented (Table 5). Each constant has been expressed to 20 decimal places for accuracy reasons. If each
constant is expressed to only five decimal, inaccurate input power values will result.
Wfujitsu A Toutdoor6
B Toutdoor
5
C Toutdoor
4
D Toutdoor3
E Toutdoor
2
F Toutdoor G
57
Table 5. Coefficient values and respective units for 6th degree polynomial curve fit of Fujitsu 15RLS2 input power
Constant Value Units
A -0.00000011459772208955 W/°F6
B 0.00009319727673573470 W/°F5
C 0.01144965288332710000 W/°F4
D 0.51679312641702300000 W/°F3
E -9.76919414534158000000 W/°F2
F 81.92982303912180000000 W/°F
G 2,007.24245041627000000000 W
This 6th degree polynomial trend line has an R2 value of 0.9738383838383838, signifying that about 97 % of the
data points used to create the trend line are a good fit. For the purposes of this evaluation this polynomial trend
line will be deemed sufficient.
To confirm the accuracy of the 6th degree polynomial to predict actual power input, current drawn and voltage
drop readings for the Fujitsu 15RLS2 were measured at various times during each test trial. It was found that the
Fujitsu 15RLS2 was actually drawing less power than predicted, indicating that it was never running at its peak
level. The existing heating system in Service Building A was probably responsible for this. Following this
discovery, a 10 percent reduction factor was used for the Fujitsu 15RLS2 input power for all data reduction
calculations.
4.5.2 Measuring Fujitsu 15RLS2 Indoor Unit Airstream Temperatures
To determine the heat of rejection from the indoor unit when the Fujitsu 15RLS2 is running in heating mode, the
airstream flowing through the unit was analyzed. Three Type T thermocouples were attached to the indoor unit;
one where the airstream enters the unit and two where the airstream exits the unit. Two Type T thermocouples
were placed at the exit so that an average temperature could be found. A fourth Type T thermocouple was ran
through a small gap in an adjacent window to read the outdoor temperature. These four temperatures were
recorded using a second USB thermocouple data logger. This USB data logger was placed in the Work Control
room and has a built in thermocouple to read the ambient temperature (Figure 43). Figure 43 shows the
thermocouples mounted to the Fujitsu 15RLS2 and connected to the 4 channel USB thermocouple data logger.
58
Figure 43. Fujitsu 15RLS2 inddor unit outfitted with type T thermocouples.
The 4 temperatures in degrees Fahrenheit are displayed on the four channel USB data logger (Figure 44).
Figure 44. Second USB thermocouple data logger to measure and record temperatures associated witht he Fujitsu 15RLS2 indoor unit.
4.5.3 Calculating Heat of Rejection from Fujitsu 15RLS2 Indoor Unit
Independent and dependent variables used in the data reduction analysis to calculate the heat of rejection from
the Fujitsu 15RLS2 indoor unit can be seen below in tabular form (Table 6).
USB Thermocouple Data Logger
Fujitsu 15RLS2 Indoor Unit
Airstream Inlet Temperature
Two Type T Thermocouple
s Measuring outlet
Temperatures
Airstream Outlet 2 Temperature
Airstream Outlet 1 Temperature
Outdoor Temperature
59
Table 6. Variables used to calculate heat of rejection from Fujitsu 15RLS2 indoor unit.
Independent Variable Units
Wfujitsu Watts
AirCFM_Fujitsu CubicFeet per Minute
Tair_outlet_Fujitsu Degrees Fahrenheit
Tair_inlet_Fujitsu Degrees Fahrenheit
Dependent Variable
mdot_air Pound Mass per Hour
Qindoor_unit_fujitsu BTU per Hour
From manufacturer data from Fujitsu, the CFM of the airstream through the indoor unit is specified for different
fan speed comfort levels (high, medium, low). All performance tests were performed with the Fujitsu 15RLS2
indoor unit set at the highest fan speed. At this fan speed, Fujitsu General Limited specifies a CFM flowing
through the indoor unit of 559 ft.3 /min. From this CFM prescribed by Fujitsu, the density of the airstream
flowing through the indoor unit of 0.075 lbm/ft3, and an equality to convert from minutes to hours, the mass
flow rate of the airstream flowing through the indoor unit in lbm/hr can be found as follows:
Eq.36
The heat of rejection from the Fujitsu 15RLS2 indoor unit in BTU/hr can then be calculated as follows:
The heat of rejection from the Fujitsu 15RLS2 indoor unit has been plotted as a function of outdoor temperature
for the five completed test trials can be seen below (Figure 45).
mdot_air_fujitsu AirCFM_fujitsu60 min
1 hr
air
Qindoor_unit_fujitsu mdot_air_fujitsu Cp_air Tair_outlet_fujitsu Tair_inlet_fujitsu Eq.37
60
Figure 45. Fujitsu 15RLS2 heat output as a function of outdoor temperature
From Figure 45 it can be seen that the Fujitsu 15RLS2 rarely operated at peak capacity, as indicated by the
indoor unit heat output. During the first test trial, when the outdoor temperature ranged from 35° F to 39° F,
the indoor unit reached peak performance specified by Fujitsu and actually exceeded it. This is further
confirmation of the 10 percent reduction factor used when predicting the power input to the Fujitsu 15RLS2.
4.6 Overall Installed System Evaluation To determine the COP of the installed system, which includes the Fujitsu 15RLS2 and Subcooler Unit, when
running in heating mode, all heat transfer rates into Service Building A and all electrical power inputs to both the
Fujitsu 15RLS2 and Subcooler Unit need to be considered. By using the heat of rejection from both the Fujitsu
15RLS2 and Subcooler Unit, electrical power input to both the Fujitsu 15RLS2 and Subcooler Unit. Equation 16
for the COP of the entire system when running in heating mode was presented in Section 3.3.2 and will be
repeated for clarity.
Eq.16
COPsystem_heatmode
Qindoor_unit_fujitsu Qsubcooler
Wfujitsu Wsubcooler
61
The system’s COP has been plotted as a function of outdoor temperature for the five completed test trials
(Figure 44). Various stock Fujitsu 15RLS2 COP values corresponding to outdoor temperatures ranging from 14° F
to 59° F have also been included for comparisons sake.
Figure 46. Installed system (Subcooler Unit and Fujitsu 15RLS2) COP plotted as a function of outdoor temperature
From Figure 44 it can be seen that for the range of outdoor temperatures that the installed system was tested
at, the installed system’s COP is at times substantially greater than the stock Fujitsu 15RLS2’s COP values at
corresponding outdoor temperatures.
When the Subcooler Unit’s airstream pulled in outdoor air at 30°F, the max COP of the installed system was 3.9,
a 66 percent increase from a stock Fujitsu 15RLS2 MSHP. When the Subcooler Unit’s airstream pulled in outdoor
air at 60°F, the max COP of the installed system was 4.4, a 22 percent increase from a stock Fujitsu 15RLS2
MSHP. Based on the performance data, an expected average COP of around 3.8 is highly realistic.
For the complete data reduction spreadsheet for the installed system (Subcooler Unit and Fujitsu 15RLS2)
including raw performance data refer to Appendix H.
62
4.7 Economic Analysis
4.7.1 Payback Period on Installation
The main objective of the economic analysis of Efficiency Maine Team IV’s capstone project is to characterize
the Fujitsu 15RLS2 with Subcooler Unit’s ability to be used as a heat source for Maine residencies. That is, to
determine the total monetary savings associated with using the Subcooler Unit to a Fujitsu 15RLS2 MSHP that is
to be used as a space heater in a residential setting. For this report, the related savings associated with the
Subcooled MSHP are determined as a function price-per- million BTUh. The micro-channel coil heat of rejection
is how much free heat is attained from subcooling, and therefore provides replaces” purchased heat” with “free
heat”.
The installation and testing of the Subcooler Unit at the Facilities Management Office building at the University
of Maine campus is a way to determine the heat of rejection from the Subcooler Unit and as a way to determine
its performance of the MSHP during a sample of a heating season in Maine. A simple pay- back period is
determined by considering capitol cost of the Subcooler Unit and dividing it by annual average return.
It should be mentioned that the economic analysis in this report is written and based off of heating data from a
small fraction of the heating season in Maine. Due to this, many assumptions are made and data is extrapolated
as to predict the annual monetary savings, as well as the annual reduction in carbon dioxide emissions. A simple
payback period for this design can be determined by looking at the capitol cost of the design project divided by
the average annual return.
Eq.38
In this section, the construction, installation, and instrumentation of the Subcooler Unit and MSHP are
considered the capitol costs. Not included in the capital cost are the donations Efficiency Maine Team IV
received (Table 7).
Table 7. Donations to Efficiency Maine Team IV project.
Description of Item Donated Donor Amount
Arduino Uno& SPDT Relay Professor Senthil Vel $250
Brazed Aluminum Micro-Channel subcooler Coil Alcoil $300
Electronic Hardware Samuel M. Prentiss $50
Inline Electricity Filter for TED Electricity Monitor and Discount The Energy Detective $66
NEMA Enclosure, Type II Jim LaBrecque $40
Labor for Installation UM Facilities Management $800
The annual average return is the difference between the annual cost of heating your home with a Fujitsu
15RLS2, and heating your home with a Subcooled Fujitsu 15RLS2. As seen in section 4.4 of this report, it has
been determined that the average heat of rejection of the Subcooler Unit is 7000 BTUh, which increases the
average COP of the MSHP to 3.8. This heat of rejection value represents how much less heat needs to be
Simple_PaybackCapitol_Cost
Annual_Average_Return
63
purchased per heating season, because it is now being provided to the living space at no additional electrical
energy cost. It is also assumed that an average of 97 million BTU/hr is consumed per-year per-residency in
Maine [5+. For this simple analysis, the MSHP without Subcooler Unit’s COP is considered to be 3.
Before the annual cost of heating a residency in Maine with a MSHP, or a Subcooler Unit with MSHP can be
determined, it is necessary to convert the performance of both systems into a common cost factor, the
monetary value per million BTU/hr.
Eq.39
Eq.40
With the price per million BTUh determined, it is possible to determine the annual heating cost by multiplying
the Subcooled MSHP cost factor, and MSHP cost factor by the amount of million BTU/hr consumed per year per
residency.
Eq. 41
MSHP_Annual_Cost MSHP_Cost_Factor 97MMBTU
yr Eq.42
Now, the average annual savings can be determined by subtracting the Subcooled MSHP annual heating cost
from the MSHP annual heating cost.
Eq.43
Eq.44
By inserting the capital cost of the installation equations 43 into equation 44, it is possible to see that the
payback period for the installation is a mere 3.2 years, or 176 weeks.
Annual_Average_Return MSHP_Annual_Cost Subcooled_MSHP_Annual_Cost
Subcooler_Rejection_Average 700BTU
hr
Subcooled_MSHP_Annual_Cost Subcooled_MSHP_Cost_Factor 97MMBTU
yr
Simple_PaybackCapitol_Cost
Annual_Average_Return
Subcooled_MSHP_Cost_Factor
0.15$
kW hr
3412kW
BTU
hr
Subcooled_MSHP_Average_COP
100000
BTU
hr
MMBTU
hr
MSHP_Cost_Factor
0.15$
kW hr
3412kW
BTU
hr
MSHP_Average_COP
100000
BTU
hr
MMBTU
hr
64
4.7.2 Payback Period on Manufactured Device
The Subcooler Unit created and evaluated by Efficiency Maine Team IV has the potential to be a manufactured
device that can be sold to the general public. The Subcooler Unit can conceivably be retrofitted to any existing
air-to-air residential heat pump, or installed with a new unit to increase savings associated with the heat pump
while it is running in heating mode. Efficiency Maine Team IV has been lucky enough to be working with supplier
such as Alcoil, who has provided the design team with the micro-channel coil responsible for subcooling the
refrigerant. Alcoil is a leading manufacturer of low-cost aluminum Micro-Channel heat exchangers, and has
ample ability for both low volume and high volume production, making them an attractive choice for a
manufacturing partner if the Subcooler Unit becomes mass-produced and is available to be purchased by the
general public.
The proposed cost of the manufactured Subcooler Unit, based on price estimate for Micro-Channel provided by
Alcoil and the various ducting components required for the Subcooler Unit is approximately $1000 USD. Another
way of determining the cost of the manufactured unit is to subtract the cost of the Fujitsu 15RLS2 MSHP and
Instrumentation from the total cost of the budget as indicated In section 2.7 of this report.
Cost_Manufactured_Subcooler Total_Installation_Budget Cost_Instrmentation Cost_15RLS2( ) Eq.45
From equation W, the cost of proposed manufactured Subcooler Unit is $997.50.
The payback period for the manufactured device can now be determined by dividing the proposed cost of the
manufactured Subcooler Unit by the same average annual return as defined in section 4.7.1
Eq.46
By inserting equation W into equation U, it can be seen that the proposed payback period for the manufactured
Subcooler Unit is 0.74 years or approximately 39 weeks.
4.7.3 Comparison of Other Heating Fuel Sources
It is possible to approximate the annual heating cost of other heating fuel sources in Maine using the same
method as illustrated in Eq. Y and applying each respective fuel source’s cost factor *6].
Payback_Period_Manufactured_SubcoolerCost_Manufactured_Subcooler
Annual_Average_Return
65
Figure 47. Annual average heating cost for various fuel sources in Maine.
It should be noted that Figure 47 does not take into consideration the cost to heat domestic water.
4.7.4 Economic Analysis Summary
In summary, it is clear that the Fujitsu 15RLS2 with Subcooler Unit has promising potential as a space heater in
Maine. The Fujitsu 15RLS2 with Subcooler Unit will increase performance of the MSHP while it is running, but a
residency in Maine would still require an auxiliary heating source to provide heat during extreme cold
temperatures. Both payback periods in this economic analysis base the annual average return off of the
difference between a MSHP and a Subcooled MSHP. If one was to determine annual average return based off of
the savings associate with switching from a fossil fuel for example, the annual average return would be much
higher because the annual heating costs associated with fossil fuels is much higher than that of the annual
heating costs associated with a MSHP. At the very least, the underlying purpose of this simple economic analysis
is to raise excitement in the topic of MSHPs with Subcooler Units reducing annual heating costs for Maine
residencies.
For the economic analysis calculation spreadsheet refer to Appendix I.
66
5 Conclusions
5.1 Effect of Subcooling on Fujitsu 15RLS2 From performance data collected from the installed system at Service Building A at the University of Maine, it
has been determined that subcooling an air-to air mini-split heat pump is a viable method for increasing the COP
of a MSHP running in heating mode. While other methods may exist to increase efficiency, integrating a
Subcooler Unit into a MSHP cycle is relative simple and economically feasible. Based on the performance data,
an expected average COP of around 3.8 is highly realistic.
When the Subcooler Unit’s airstream pulled in outdoor air at 30°F, the max COP of the installed system was 3.9,
a 66 percent increase from a stock Fujitsu 15RLS2 MSHP. When the Subcooler Unit’s airstream pulled in outdoor
air at 60°F, the max COP of the installed system was 4.4, a 22 percent increase from a stock Fujitsu 15RLS2
MSHP.
The remarkable heat output from the Subcooler Unit is not as clear cut as it seems. These heat transfer rates
indicate that the system is slightly undercharged with R-410A. An undercharged system would result in
incomplete condensing of the high pressure, high temperature R-410A in the Fujitsu 15RLS2’s indoor unit. As
such, condensing is probably occurring within the micro-channel coil in the Subcooler Unit. Heat transfer due to
phase change (condensing) is much higher than heat transfer solely due to subcooling (sensible heat). This
conclusion is further supported because the capacity of the Fujitsu 15RLS2 indoor unit was consistently below
Fujitsu General manufacture specified rates. While this can be partly attributed to the Fujitsu 15RLS2 not
running at full capacity due to the additional heat source in Service Building A, an undercharged system is highly
likely. During the first test trial performed by Efficiency Maine Team IV, the Fujitsu 15RLS2 indoor unit did reach
and even exceed peak capacity. This test trial occurred when the outdoor temper
The fact that the Subcooler Unit operating efficiencies are such large magnitudes is indicative of the “free heat”
claim made about the Subcooler Unit. Solely looking at the Subcooler Unit, the only electrical power input is to
the centrifugal fan. The mass flow rate of the R-410A can be ignored in the Subcooler Unit efficiency equation
since the heat that is removed from the R-410A is then regained from the environment. Theoretically the
Fujitsu’s compressor and indoor unit should not notice the presence of the Subcooler Unit. This is why
subcooling is such a remarkable process. In what other energy process can heat be removed and then
immediately replaced for free!
The Subcooler Unit provides additional heat to the residency, thus lowering the annual heating cost. The
individual payback period of the Subcooler Unit built for this project is 0.74 years and the payback period of the
entire installation is 3.2 years.
5.2 Subcooler Unit Future Design First and foremost, it is desired that following seniors in mechanical engineering will take up the Subcooler Unit
project and be able to perform a more in depth and lengthy analysis. As mentioned, charging additional R-410A
to the installed system is critical to understanding the MSHP performance associated with pure subcooling.
Secondly, real monitoring of power input to the Fujitsu 15RLS2 is a must. The TED failure was a major setback to
our project. Only selecting test data exhibiting peak performance characteristics is an inadequate test method.
67
Moreover, it is hoped that the Subcooler Unit that has been built, instrumented, tested, and evaluated in this
report is a prototype for improved Subcooler Units to come. Future designs of the Subcooler Unit will be much
less cumbersome with ducting, while still providing the airstream required to lower the temperature of the
active liquid refrigerant.
Future designs should focus on developing more sophisticated control algorithms that allow for wireless data
acquisition, and wireless manual override to simplify the testing procedure, and to allow for performance testing
to occur off-site. Also, future control systems should focus on being able to detect when the Fujitsu 15RLS2
MSHP is in heat mode and use that to turn the centrifugal fan on.
Finally, it is hoped that in the future there will be relationships established with whole sale suppliers of ducting,
micro-channel coils, fans, and all other componentry required to assemble the Subcooler Unit, as to lower the
capitol cost of the Subcooler Unit with hopes that it can forever be a product made and sold in Maine.
5.3 Possible Solution to Heating Energy Demand in Maine Mini-split heat pumps offer an enticing and realistic solution to the high energy demands faced by our state.
Currently the State of Maine spends an outrageous amount on heating, predominately in the form of home
heating oil or kerosene. As a result of this heating trend, electricity usage is remarkably low during heating
months in Maine. Although electricity production costs are extremely low due to the prevalence of natural gas
(used by a majority of Maine’s power plants) the residential customer’s electricity bill is still quite high due to
transmission and distribution (T&D) costs. These high costs arise from a veritable transmission and distribution
monopoly that exists in Maine and the need for standby power plants when peak demand periods exist. For the
electrical utility, such as Emera or Central Maine Power, these T&D costs could be reduced by increasing the
amount of electricity they sell and by eliminating the standby plants that may only run for a few hours a day for
a few days a year. During the heating season, these standby plants exist for the unique demand encountered
when the coldest temperatures occur. Cumulatively, these coldest temperatures (less than 10 degrees
Fahrenheit) only amount for about 24 hours yearly, indicating that it is unrealistic to have a grid based on such
extreme conditions.
A smart grid incorporating MSHPs and existing oil furnaces for peak demand periods has the potential to reduce
the transmission and distribution costs associated with reserve power generation plants and relatively low
electricity consumption. When peak electricity demands exist in heating months, this smart grid would signal
MSHPs to shut off and the oil, propane, or natural gas back up to automatically turn on and begin heating the
home. Subsequently, the availability of electricity in the winter in Maine would be taken advantage of and the
need for standby power plants would be diminished, thus reducing the cost of electricity, as well as reducing
carbon emissions associated with fossil fuel heating methods. Furthermore, including Subcooler Units with all
new MSHP installations in the state could save even more energy.
68
6 References [1] DuPont Suva, "suva.dupont.com," October 2004. [Online]. Available:
http://www2.dupont.com/Refrigerants/en_US/assets/downloads/h64422_Suva410A_thermo_
prop_eng.pdf. [Accessed 8 2 2014].
[2] Fujitsu General Limited, "master.ca," 2014. [Online]. Available:
http://www.master.ca/documents/7Fujitsu_Technical_Manual_9_12_15RLS2.pdf. [Accessed April
2014].
[3] Arduino, "arduino.cc," Arduino, 2014. [Online]. Available: http://arduino.cc/. [Accessed 27 4 2014].
[4] Hacktronics, "hacktronics.com," Hacktronics, 2014. [Online]. Available:
http://www.hacktronics.com/Tutorials/arduino-1-wire-address-finder.html. [Accessed 19 4 2014].
[5] "www.maine.gov," 14 5 2014. [Online]. Available: http://www.maine.gov/energy/fuel_prices/heating-
calculator.php. [Accessed 27 4 2014].
[6] "The Climate Registry," 8 5 2014. [Online]. Available:
http://www.theclimateregistry.org/downloads/2014/02/2014-Climate-Registry-Default-Emissions-
Factors.pdf. [Accessed 27 4 2014].
69
Appendix A: Subcooler Unit Drawing Package Link
Appendix B: Power and Control Electrical Schematic Link
Appendix C: Arduino IDE Code for Controls Link
Appendix D: Installation Drawing Package Link
Appendix E: Subcooler Unit and Installation Parts Specification Budget Link
Appendix F: System Prediction Calculation Spreadsheet Link
Appendix G: MEE 443 Mechanical Laboratory III Experiment Report Link
Appendix H: Data Reduction Spreadsheet Link
Appendix I: Economic Analysis Spreadsheet Link
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