The Potential and Challengesof Solar Boosted Heat Pumps for
Domestic Hot Water Heating
Stephen Harrison Ph.D., P. Eng., Solar Calorimetry Laboratory,
Dept. of Mechanical and Materials Engineering, Queen’s University, Kingston, ON, Canada
Solar CalorimetryLaboratory
Solar CalorimetryLaboratory
Background• As many groups try to improve energy efficiency in residences,
hot water heating loads remain a significant energy demand.
• Even in heating-dominated climates, energy use for hot water
production represents ~ 20% of a building’s annual energy
consumption.
• Many jurisdictions are imposing, or considering regulations,
specifying higher hot water heating efficiencies.
– New EU requirements will effectively require the use of either heat
pumps or solar heating systems for domestic hot water production
– In the USA, for storage systems above (i.e., 208 L) capacity, similar
regulations currently apply Canadian residential sector energy
consumption (Source: CBEEDAC)
Solar CalorimetryLaboratory
Solar and HP water heaters
• Both solar-thermal and air-source heat pumps can achieve efficiencies above 100% based on their primary energy consumption.
• Both technologies are well developed, but have limitations in many climatic regions.
• In particular, colder ambient temperatures lower the performance of these units making them less attractive than alternative, more conventional, water heating approaches.
• Another drawback relates to the requirement to have an auxiliary heat source to supplement the solar or heat pump unit, particularly, during cold or overcast periods.
Solar
Collector
Glycol/water
Anti-freeze
Circulation Loop
Roof Line
Heat
Exchanger
Cold Mains
Water Inlet
Electric Pump
Hot Water
to Load
Electric Pump
(optional)
Auxiliary
Heater
Water
Storage
Tank
Outdoor Fan-coil
Evaporator
Building
Wall
Cold Mains
Water Inlet
Hot Water
to Load
Electric Pump
(optional)
Auxiliary
Heater
Condenser
Split Heat Pump
ExpansionValve
Compressor
Outdoor
Ambient Air
Water
Storage
Tank
Solar CalorimetryLaboratory
Compact Air-source HPWH Indoor Fan-coil
Evaporator
Cold Mains
Water Inlet
Hot Water
to Load
Auxiliary
Heater
Compact Heat Pump
Water Heater
Ambient Air
Wrap-around
Condenser
• The use of compact air-source heat pump water heaters (HPWHs) is well
established.
• Most draw heat from the surrounding environment and are best suited for
mild climates where they may be placed outdoors or in an unheated garage.
• In cold regions, they must be located in a heated space to avoid
high standby-losses or freezing, thereby shifting the water heating load to
the space heating.
• To alleviate this, “split systems” with outdoor fan-coil evaporators can be
used or outdoor-air can be ducted into the indoor unit, however, cold
outdoor temperatures can lower overall capacity and COP.
Solar CalorimetryLaboratory
WHY SB-HPWH
Ambient Air
Warm Climate
Cold Climate
Ambient Air
ExpansionValve
Compressor
Outdoor
Ambient Air
HPWH Outdoor HPWH Indoor “Split” HPWH SB-HPWH
Solar CalorimetryLaboratory
Solar Boosted HPs
• Solar boosting HP output has been extensively studied and the benefits in improved performance are well established but depend on climate and the type of solar collector used.
• Many configs: series vs parallel; direct and indirect; dual source etc.
• draw-backs to some SB-HPWH in terms of installation and operation, e.g., direct systems may need refrigeration connections on roof tops.
• Indirect systems use conventional anti-freeze loops but require and an additional pump
• the most common configuration uses unglazed solar collectors that can collect ambient energy during low sun periods.
– These are simple and efficient but performance will drop during very
cold or overcast periods.
Solar
Collector
Glycol/water
Anti-freeze
Circulation Loop
Roof Line
Cold Mains
Water Inlet
Electric Pump
Hot Water
to Load
Electric Pump
(optional)
Auxiliary
Heater
Evaporator Condenser
Heat Pump
ExpansionValve
Compressor
Water
Storage
Tank
Evaporator
Solar Collector
Roof Line
Cold Mains
Water Inlet
Hot Water
to Load
Electric Pump
(optional)
Auxiliary
Heater
Condenser
Heat Pump
ExpansionValve
Compressor
Water
Storage
Tank
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14
Effi
cie
ncy
(η)
(Tf -Ta)/GT
Typical Efficiency Curves of Solar Collectors
Unglazed Collector Single Glazed, Flat Plate Evacuated Tube
• Improvements to collector performance and HP COP
ΔT = 30, G=600W/m2ΔT = 6, G=600W/m2
• Cooling the solar collector significantly improves its efficiency
• Increasing the HP’s evaporator temperature increases HP COP
• Collector area can be halved relative to Solar DHW.
• high-performance solar panels (with glazed and insulated absorbers), may be used but limit “non-solar”, air-source capacity; reducing their benefit.
Solar CalorimetryLaboratory
Motivation to Solar-boost a HP
Solar CalorimetryLaboratory
Example Performance comparison
• Freeman and Bridgeman studied an Indirect Solar-Assisted HPWH
• Compared performance for various climates, e.g., Toronto, Montreal and Vancouver.
• Collector area and overall cost was reduced
• System performance was more uniform over year.
• Identified need for variable capacity compressor to accommodate seasonal operation
0
10
20
30
40
50
60
70
0 1 2 3 4 5 6 7 8 9 10 11 12
Month of the Year
Sola
r F
racti
on
(%
)
SAHP-Glazed
SAHP-Unglazed
SDHW
0
20
40
60
80
100
0 2 4 6 8
Collector Area (m2)
Coll
ecto
r E
ffic
iency (
%)
SAHP-Glazed
SAHP-Unglazed
SDHW
10
20
30
40
50
60
0 2 4 6 8
Collector Area (m2)
Sola
r F
ract
ion
(%
)
SAHP-Glazed
SAHP-Unglazed
HP Base
SDHW
Collector Area (m2)
Collector Area (m2)
45
50
55
60
65
1 2 3 4 5 6 7 8
Collector Area (m2)
Sola
r F
racti
on (
%)
Toronto
Vancouver
Montreal
Solar fraction versus collector
area for unglazed SAHP systems.
Collector Area (m2)
Solar CalorimetryLaboratory
• new systems are being studied that include dual- or tri-mode
solar collectors that act as efficient solar- or air-source
evaporators and may even include Photovoltaic/thermal
absorbers.
• Combined with new system configurations and components (e.g.,
new variable speed, high-efficiency compressors), fully integrated,
high performance, solar/HP hybrid water heaters are possible.
• PV/Thermal Panels offer many opportunities to “piggyback” on
existing PV infrastructure (installation and mounting, etc.)
• PV/Thermal panels offer high solar efficiency by delivering heat
and electricity.
New Approaches to SB-HPWH
Venting channel allows energy collection during low-
solar periods
Solar Boosted Heat Pump (Collector Options)
Unglazed Collector• By using an unglazed solar absorber it is
possible to collect more heat from the ambient air (particularly during periods with low solar input) however the solar contribution will be less.
• If collector temperature can be kept very near or sub-ambient solar collector efficiency can be greater than 100%.
• The low temperature requirement may reduce heat pump COP and trade-off’sneed to be assessed.
UnglazedSolar Thermal Collector
Dual Mode Vented Collector• maximizes both ambient and solar
with a vented glazed solar collector that allow ambient air to circulate next to the absorber plate
• The collector can reach higher temperatures during sunny periods (this was done for Team Ontario’s Solar Decathlon Entry, 2013) and still draw heat from the ambient air.
Solar CalorimetryLaboratory
The Challenge: PV/Thermal HP Water heater• Solar conversion efficiency of PV/Thermal devices
can be very high as solar energy, not directly converted to electricity, is converted to heat and this can be extracted for heating purposes.
• The addition of a heat pump to this combination allows solar panel operation at low temperatures (even sub-ambient) increasing both electric and thermal conversion efficiency.
• The COP of the Heat pump “leverages” the electrical input of the system while increasing the thermal output of the system
• The successful integration of these systems, their controls and the utilization of the PV generated electricity are areas requiring research and development.
Compressor
Expansion Valve
Hot water
storage
Auxiliary
Heating
Element
To Load
Water Mains
Supply
Refrigerant
Loop
Natural
Convection
Loop
Electric Pump
Col
lect
ors
Anti-freeze
Collector Loop
Eva
po
rato
r
Co
nd
en
se
r
Direct power to Compressor or Grid
HP SDHW
PV-Thermal Modules
Direct DC power to TEM HP
PVT Solar Boosted HP vs Air Source
PVT V-C HP DHW
𝐅𝐄𝐑 =19502000 = 97.5%
𝐄𝐟𝐟𝐜𝐨𝐥 =450+1500
2400 = 81%
𝐂𝐎𝐏𝐇𝐏 =1900450 = 4.2
𝐀𝐫𝐞𝐚𝐂𝐨𝐥𝐥𝐞𝐜𝐭𝐨𝐫 = 3 m2
𝐅𝐄𝐑 =14502000 = 72.5%
𝐂𝐎𝐏𝐇𝐏 =1900650 = 2.9
Sola
r =
80
0 W
/m2
x 3
m2
= 2
40
0 W
SinkSource
Hea
t =
20
00
W
1900
Air-SOURCE HP DHW (Split system)
Hea
t =
20
00
W
AC Grid 650 W
Air-source1450 W
1900
Ou
tdo
or
Am
bie
nt
Air
Example Energy Flows for a heating load of 2 kW
Solar CalorimetryLaboratory
Competing Water Heating Options
Domestic Hot Water Heating Approaches
Heat Pump
Solar Thermal
Direct/No Freeze Protection
ICS (Integral Collector Storage)
Indirect/Freeze
Protected
Electric Resistance
Oil/Gas Fueled
Storage Heaters
On Demand(no storage)
Central Home
Point of use
Air Source HP
Split System
Integrated System
Solar Source HP
Solar PV
Direct/Series
(Solar Evaporator)
Indirect/Series Heat Loop
PV/Thermal Solar Panels (2 & 3 Mode)
PV Power Electric Resistance Htr.
PV HP power fans,
compressor etc.
Traditional
Parallel System
Oil/gas and electric resistance
Solar CalorimetryLaboratory
Conclusion
• As energy efficiency in buildings increases, space heating requirements may be reduced through improved building practices
• In the limit domestic hot water heating will persist as a significant load and major contributor to peak load demands.
• to be competitive Solar Boosted Heat Pumps must be able to deliver:– high temps, energy efficiency, ease of installation, good cost
performance, load shifting.
• There is tremendous potential but still work to do!
Thank you!
Questions:a) Is there a role for PV/Thermal technology for HPWH?b) Can solar boosted PVT HPWHs “piggyback” on PV infrastructure
to accelerate deployment?b) Should PVT-HPWHs send power to the “grid” or self-power?
Solar CalorimetryLaboratory