Chapter 2: Literature review
Chapter 2Literature ReviewThis chapter will provides a bird-eyes
view of the GSHP system literature with emphasis on the design and
operational studies of GSHP systems. An analysis of representative
literature can identify which aspects have been studied
extensively, and which aspects have received the least attention.
Literature reviewed so far can be classified into three groups:
experimental work, which includes laboratory and in situ
experiments; modelling of GHEs, which is divided into analytical
and numerical methods; and the rest of the literature focusing on
GSHP system-related studies, which include analysis, comparison,
operation, installation, design, simulation, and optimization of
GSHP or hybrid GSHP (HGSHP) systems, etc.2.1 Properties of soilThe
efficiency of the heat transfer between the ground heat exchanger
and the ground is strongly dependent on the thermodynamic
characteristics of the soil. The thermal conductivity, the density,
the specific heat, the porosity and the hydraulic conductivity
should be investigated to study the thermal performance of GSHP
systems.The measurement of these thermal properties are not easy,
especially for vertical GHEs design, which usually passes through
several soil layers and all these types need to be identified
correctly. In order to measure these properties, recently, many
researchers have focused their attention on development of in situ
measurement methods (Low et al 2014). Several experimental
apparatus were first developed and reported in USA and Europe
(Gehlin, 2002). After that, many researchers have developed mobile
test facilities for this purpose, in different regions of the
world, including Latin America (Roth et al 2004), Canada (Marcotte
et al 2008), China (Wang et al 2010) and elsewhere. Table 2.1 shows
the thermal conductivity and thermal diffusivity of typical soils
identified (ASHRAE, 2011). As can be seen in Table 2.1, thermal
conductivity varies with the content of water in the soil. The soil
moisture is an important parameter influencing the soil thermal
properties. When air between soil particles is replaced by water,
the contact resistance will be reduced to some extent. The research
work of Leong et al (1998) suggested the GSHP performance is
strongly dependent on the soil moisture content, the higher the
soil moisture value, the better performance of the heat pump. Table
2.1 Typical thermal properties of soil. Thermal conductivity
(W/mK)Thermal diffusivity (m2/day)
Soils
Heavy clay, 15% water1.4 to 1.90.042 to 0.061
5% water1.0 to 1.40.047 to 0.061
Light clay, 15% water0.7 to 1.00.055 to 0.047
5% water 0.5 to 0.90.056 to 0.056
Heavy sand, 15% water2.8 to 3.80.084 to 0.11
5% water2.1 to 20.093 to 0.14
Light sand, 15% water1.0 to 2.10.047 to 0.093
5% water 0.9 to 10.055 to 0.12
Rocks
Granite 2.3 to 3.70.084 to 0.13
Limestone 2.4 to 3.80.084 to 0.13
Sandstone 2.1 to 3.50.65 to 0.11
Shale, wet 1.44 to 2.40.065 to 0.084
dry 1.0 to 2.10.055 to 0.074
2.2 Performance evaluation of GSHP system Similar with the
performance evaluation of traditional heat pump systems, several
performance indexes are used for evaluating the energy performance
of ground source heat pump systems. COP (Coefficient of
Performance)The COP is the ration of the rate of energy delivered
to the rate of energy supplied to do that work for a complete
operating heat pump plant. SEER (Seasonal Energy Efficiency
Ratio)SEER is the ratio of the total cooling output during a normal
usage period for cooling (in Btu) to the total energy input (in
Watt-hour) during the same operation period which can be used to
determine the seasonal energy efficiency of heat pumps during
heating and cooling seasons.
(2.1)Up to now, many performance evaluations of various types of
ground source heat pump systems have been conducted simulatively
and experimentally. E.M.R (1989) concluded based on a massive
studies of systems in Canada that the coefficient of performance
(COP) of a typical ground source heat pump is 2.9 to 3.2 at 2C in
Canada. . Sanner etal.(2003)reviewed the early development of GSHP
systems for commercial buildings, and pointed that the utilization
of GSHP systems in commercial applications offered some economic
and environmental advantages. Hamada etal.(2007) described a GSHP
system using friction piles as heat exchangers for air conditioning
of a building for both office and residential use. Long-term space
heating operation measurements found that the average coefficient
of performance (COP) for space heating was high at 3.9, and the
seasonal primary energy reduction rate reached 23.2% compared with
a typical air-conditioning system. Michopoulos etal.(2007)
presented a performance evaluation of a GSHP system with vertical
ground heat exchanger in parallel connection for air-conditioning a
public building in northern Greece. It was proved that the energy
demand of the system was significantly lower, compared to that of
conventional heating and cooling systems. The primary energy
required by the system for heating was estimated to be lower by 45%
and 97% (period average) as compared to that of air-to-water heat
pump based and conventional oil boiler respectively. In cooling
mode the relevant differences were estimated at 28% and 55% for
air-to-water and air-to-air heat pump based systems. Hwang
etal.(2009)presented the cooling performance of a
water-to-refrigerant type GSHP system installed in a school
building in Korea. The average cooling coefficient of performance
and overall COP of the GSHP system were found to be around 8.3 and
5.9 at 65% partial load condition, respectively. While the air
source heat pump system, which had the same capacity with the GSHP
system, was found to have the average COP of 3.9 and overall COP of
3.4, implying that the GSHP system was more efficient than the air
source heat pump system Karabacak etal.(2011)have reported the
cooling performance of a GSHP system consisting of a 225m vertical
single U-tube ground heat exchanger. After a cooling period the COP
(Coefficient of Performance) of the heat pump and the system was
found in the range of 3.14.8 and 2.13.1, respectively. They had
also reported a heat injection rate between 27 and 93W/m.A
comparison study conducted by Self et al (2012) showed great
advantages of GSHP system. Results of the comparison of COP of
different heating systems are shown in Table 2.2. Table 2.2 Typical
equivalent COP comparisons results for heating systems.Types of
systemCOP
Ground source heat pumps3-5
Air source heat pump2.3-3.5
Electric baseboard heaters1
Mild-efficiency natural gas furnace0.78-0.82
High-efficiency natural gas furnace0.88-0.97
2.3 Modelling of Ground Source Heat Pump SystemsIn the past
decades, many researches have been focused on the development of
mathematical models of ground heat exchangers to evaluate and
predict the heat transfer phenomena in the ground to facilitate the
design and operation investigations of GSHP systems. The main
objective of the GHE modelling is to determine the temperature of
the heat carrier fluid, which is circulated between the U-tubes and
the heat pump, under certain operating conditions. A design goal is
then to control the temperature rises of the ground and the
circulating fluid within acceptable limits over the system
lifespan. Several literature reviews on GHE models have been
reported (Florides et al 2007; Yang et al 2010).2.3.1 Modelling of
vertical ground heat exchangersModelling GHEs is important, which
allows system dynamic simulations to be performed. For GSHP system,
simulation is an important tool for system optimization design
purpose as well as for investigating long-term system performance.
For this a reliable and feasible heat transfer analysis model of
GHE is required. Practically, the heat transfer process of a
vertical GHE is analysed in two separated zones. One is the
soil/rocks outside the borehole. The other is the zone inside the
borehole, including the grout, U-tube pipes and the circulating
fluid inside the pipes. Outside the vertical GHEsCurrently, there
have been a number of models that can predict transient heat
transfer in outside zone of vertical U-tube GHE (Yang et al 2010).
The models are mostly based on either some analytical solutions
like line source heat source theory proposed by Ingersoll and
Plass(1948) and cylindrical heat source theory first presented by
Carslaw and Jaeger(1959)and Ingersoll et al.(1954)and later refined
by Deerman and Kavanaugh(1991) or numerical solutions like the one
proposed by Eskilsons(1987)and Hellstrom(1991)that were used for
designing vertical boreholes used in GCHP systems. The existing
simplest analytical solutions are the line source model from
Ingersoll and Plass(1954)and the cylindrical source model from
Carslaw and Jaeger(1959). Both models assume infinite length for
borehole, and no steady-state occurs. Some important expressions
regarding the simplest analytical solutions can be found in Table
2.3. Inside the vertical GHEsExcept simulation of the transient
heat conduction of solid/rock outside the borehole, another
important part isolated for analysis is the region inside the
borehole, including grouting materials, the arrangement of flow
channels and the circulating fluid inside the pipes. The heat
transfer within a borehole depends not only on the arrangement of
flow channels but also thermal properties of grouted materials and
adjacent surrounding soils. Thermal processes between the
heat-carrying fluid and the ground are composed of three parts: 1)
Convective heat transfer between the circulating fluid and the
surface of pipes; 2) Conductive heat transfer through the pipes; 3)
Conductive heat transfer through the grouting material.With the
steady state assumption, they can be characterized by steady
thermal resistances, and sum of them yields an effective
fluid-to-ground thermal resistance Rb.
(2.1)where Rb is thermal resistance of the borehole, Rf is the
convective resistance of the fluid, Rp is the conductive resistance
of the pipes, Rg is the conductive resistance of the grout.Thus the
steady-state borehole thermal resistance Rb can be calculated as
the ratio between the heat flux and the temperature difference
between the circulating fluid and the borehole wall:
(2.2)where: Rb is thermal resistance of the borehole, q is the
heat flux per length of borehole, Tf is the average circulating
fluid temperature.There also have been a number of models existed
to determine the borehole thermal resistance (Lamarche et al 2010).
Important expressions of the models are also summarised in Table
2.3. Table 2.3 Summary of the important expressions of vertical
GHEs models. Region Expressions of modelsReferences
Outside the vertical GHEInfinite line source model
Borehole is modelled as a line of heat sources or sinks of
infinite length (Carslaw and Jaeger, 1959; Ingersoll et al.
1954).
Infinite cylindrical model
; It is based on the solution for a constant heat flux
considering two pipes as one coaxial infinite long pipe within a
borehole with infinite length (Carslaw and Jaeger, 1946, 1959;
Ingersoll et al. 1954).
Inside the vertical GHEEmpirical model
This expression is based on the shape factor of heat conduction
and fitting experimental data (Paul, 1996).
Line source approximation
Line-source formula used in the popular DST program (Pahud et
al., 1996).
Quasi-three dimensional models
This model considers the variation of fluid temperature along
the borehole depth (Diao, et al 2004).
2.3.2 Modelling of horizontal ground heat exchangersCompared
with vertical ground heat exchangers, quite few theoretic
simulation analyses of horizontal heat exchangers have been derived
so far, which mostly due to the complexity of nature problems.
Several major difficulties were proposed (Mei, 1986):1) the lack of
knowledge of soil thermal conductivity and diffusivity and moisture
migration of a given location;2) the effect of seasonal temperature
variation on the shallow depths of ground;3) uncertain thermal
resistance due to lack of close contact of the coil with the
soil;4) the effect of ground coil size, configuration and
material.Despite these, modelling of horizontal loop heat
exchangers have been done for many years, mathematical models are
available to assist in the design of horizontal loop heat
exchangers. IGSHPA (1998) provided the thermal response functions
for many different ground-loop configurations. Claesson et al
(1983) also proposed a line-source theory based model for ground
heat exchanger which requires the estimated specification of
line-source strength. Mihalakakou et al.(1994)presented a model in
which the ground surrounding the pipe and the pipe itself are
described in polar co-ordinates. The model was solved in the TRNSYS
(a modular energy system simulation program) environment and
validated with good results. Lin et al (2005) developed a plane
source heat transfer model to analyse the heat transfer phenomenon
of slinky horizontal loop heat exchangers to assist the design
process. A semi-analytical model for serpentine horizontal ground
heat exchangers was proposed and validated by Philippe et al
(2011). Computer-aided simulation tools are also available,
finite-element simulator is most commonly used simulation engine to
analyse the heat transfer of horizontal loop heat exchangers in
various configurations (Cingedo et al 2012, Fujili et al 2012,
Simms et al 2014). Based on the literature reviewed, simulation
models developed for horizontal loop heat exchangers are obviously
not as sophisticated as those for vertical ground heat exchangers,
it is also hard to conclude a universal simplified analytical model
for horizontal loop heat exchangers due to the major difficulties
mentioned above. 2.4 Design and operational optimization of GSHP
systemsAs reviewed above, GSHP (Ground source heat pump) can be
regarded one of the promising technologies for space heating and
cooling applications. In general, a GHE can be either a horizontal
or a vertical loop system. Horizontal GHEs are normally buried
under the ground at a depth of 1 2 m while vertical GHEs are
drilled at a depth of 20 200 m. Selection of GHE configurations for
heat pump applications depends on the availability of resources
i.e. water, land etc. In general, GSHP users tend to believe that
vertical GSHP systems are more efficient than the horizontal GSHP
systems because the variation in ambient temperature will have more
influence on the horizontal GSHP systems that are buried at shallow
depth compared to the deep vertical GHEs. Thus closed-loop GSHPs
vertical heat exchangers are particularly considered. In both
cases, a large amount of research have been done concerning the
design and operation of GSHP systems and are briefly reviewed in
below sections. 2.4.1 GSHP systems design and operational
approaches2.4.1.1 Design considerationsDesign of GSHP system is a
complicated and important issue since the design has a direct
influence on the performance of the GSHP system. Eskilson (1987)
and Hellstrm (1991) provided a detailed thermal analysis of heat
extraction boreholes and describe important parameters in their
performance. The five most important parameters identified in the
performance of a borehole heat exchanger are the soil thermal
conductivity, the borehole thermal resistance, the undisturbed soil
temperature, the heat extraction (and rejection) rates, and the
mass flow rate of the heat carrier fluid. The thermal performance
of a borehole heat exchanger is proportional to the thermal
conductivity of the ground. A considerable amount of research has
been conducted over the past decades regarding in-situ testing (or
thermal response testing) to determine earth thermal conductivity
for use in design and simulation tools. The borehole thermal
resistance is defined by a number of design variables including the
composition and flow rate of the fluid, borehole diameter, pipe
material, arrangement of the flow channels, and grout material. The
large thermal resistance of the individual borehole will reduce the
heat transfer rate between the heat carrier fluid and the
surrounding soil, thus increases the length of the GHE. As a
result, it is desirable to keep the borehole thermal resistance at
a minimum value. The undistributed soil temperature is normally
takes as an average in current design and simulation tools. The
required borehole depth is essentially proportional to the
temperature difference between this temperature and the minimum (or
maximum) design heat pump entering fluid temperature. Precise
identification of this temperature is essential to the design of
GHEs. The heat extraction or rejection rate directly influences the
design capacity of GHEs, which is normally determined based on the
peak building thermal load. The mass flow rate is actually included
in the borehole thermal resistance calculation, but it is important
to keep the flow rate large enough to ensure turbulent flow
(Eskilson, 1987). Based on the discussion above, the basic design
routine for the different types of GSHPs are summarized as follows
(Kvavnaugh et al 1997; Abdeen 2008):1) Determination of the
cooling/heating design loads of the building;2) Select a properly
sized heat pump system;3) Select a type of indoor air distribution
system;4) Select the proper air supply diffusers and registers and
return grillers for the air distribution system;5) Size the indoor
air distribution system;6) Estimate the energy requirements of the
building:a. The buildings heating and cooling loadsb. The type and
size of heat pump equipment selectedc. The climate and soil thermal
characteristics7) Estimate the ground heat exchanger loads:a.
Select a ground heat exchanger configuration:a) horizontal or
verticalb) parallel or series flow arrangementb. Select plastic
pipe, considering:a) materialb) sizec) diameterd) lengthe)
circulating fluid pressure lossc. Estimate ground heat exchanger
length;d. Select circulating pump(s).2.4.1.2 GHE sizing methods In
general, design the GSHP systems refers to sizing equipment to meet
a desired result by accomplishing the fundamental task of properly
sizing the ground heat exchangers (GHEs). One design method is
based on the solution of the equation for heat transfer from a
cylinder buried in the earth. This equation was developed and
evaluated by Carslaw and Jaeger (1959) and was suggested by
Ingersoll and Zobel (1954) as an appropriate method of sizing
ground heat exchangers. Kavanaugh (1985) adjusted the method to
account for the U-bend arrangement and hourly heat rate variations.
In all the design methods, parameters that must be known include:
the thermal properties and undistributed temperature of the soil,
the thermal properties and flow rate of the heat transfer fluid,
the coefficient of performance (COP) of the heat pump equipment at
the design conditions, the minimum and maximum design entering
fluid temperatures to the heat pump equipment, and the building
loads distribution over time. Several methods for sizing the GHEs
of GSHP system are summarized below.1) Rules of thumbs methodThe
rule of thumb approximations had been in vogue for a long time,
which were discussed by Ball et al. (1983). Rules of thumbs is also
referred as the specific installed thermal outputs or specific heat
extraction in W/m, the values of the specific heat rate are
obtained based on extensive analysis of monitored systems. Some
international guidelines for dimensioning vertical and horizontal
GSHP systems are summarized below (Rosen, 2001): In United States,
68-82 W/m are reported for vertical GHEs with single U-tubes; In
German, for vertical GHEs, 20-25 W/m are recommended for soil
thermal conductivity less than 1.5 W/mK, 50-60W/m for medium
thermal conductivity and 70-84 W/m for soil thermal conductivity
greater than 3.0 W/mK. Across Europe in general, the average heat
rate is estimated at 62 W/m for vertical GHEs with single U-tube.
For the entire borefield, Robert et al (2014) estimated the range
of acceptable load per unit of total length is between 30 W/m and
130 W/m. For horizontal GHEs, 50-100 W/m are recommended for slinky
trench, while 15-30 W/m are reported for single-pipe trench. Based
on the rules of thumbs method, ASHRAE (2011) summarized the
recommended trench lengths for the various types of commonly used
excavation methods, and is shown in table 2.4.Such rules of thumb
may be a good start point and provides great convenience for
design, however, excessive reliance on these rules is dangerous. As
mentioned in section 2.1.1.1, the performance of a closed loop GSHP
system depends on many parameters, a simple rule of thumb (a
certain number of watts per drilled meter) will be too
simplistic.TypePitchGround temperature (C)
m of Pipe per m Trench/Bore 7 to 88 to 1111 to 1313 to 1515 to
1717 to 19 19 to 21
Horizontal GHE6-pipe /6-pitch spiral616141314161720
4-pipe /4-pitch spiral419171717192226
2-pipe226242224263035
Vertical GHE19 mm pipe216151415161720
25 mm pipe215141314151619
32 mm pipe2141312.513141517
Table 2.4 Recommended lengths of trench or bore per kW for
residential GSHPs.
2) IGSHPA methodThe IGSHPA modelling procedure is also built
around Kelvins line source theory, and is mainly used for the
design of vertical GHEs. Bose (1984) sizes the ground heat
exchanger length for the coldest and the hottest month of the year
and then calculates the seasonal performance and system energy
consumption using the monthly bin method of energy analysis. The
IGSHPA approach defines the ground formation resistance of a single
vertical heat exchanger as follows:
(2.3) where rb is the borehole radius, ks is the soil thermal
conductivity, is the simulation time, E1(x) is the exponential
integral function, N is the borehole number, s is the soil density,
and cs is the specific heat of the soil.The methodology also allows
for the calculation of ground formation resistance for multiple
vertical heat exchangers by superimposing the thermal resistive
effects of adjacent heat exchangers and adding the total effect to
the ground formation resistance of a single pipe of an equivalent
radius. The IGSHPA approach calculates the annual heating and
cooling run fractions based on heat pump maximum and minimum
entering fluid temperatures. Bose (1984), and Cane and Forgas
(1991) recommend that a design minimum entering fluid temperature
Tf,min of 1.1C to 4.4C above the coldest outdoor air temperature at
a given geographical location and essentially assume 37.8 C as the
first approximation for the maximum entering fluid temperature
Tf,max. Equations determined the total length of the GHEs are
listed below:For heating,
(2.4)For cooling,
(2.5)where Qc,h and Qc,c are the heating and cooling capacity,
COPc and COPh are the coefficient of performance of heat pump in
heating and cooling performance. 3) ASHRAE methodIngersoll and
Zobel (1954) derived the design method that can be used to
handlethese shorter-term variations. It uses the following
steady-state heattransfer equation:
(2.6)where q is the heat transfer rate, L is the required
vertical GHE length, ts is the undistributed soil temperature, tf
is the fluid temperature, R is the effective thermal resistance of
soil. Kavanaugh and Rafferty (1997) modified the equation to
represent the variable heat rate of a ground heat exchanger by
using a series of constant heat-rate pulses. Calculations of the
required borefield lengths for cooling and heating is based on Eqs
(2.7) and (2.8).For heating,
(2.7)For cooling,
(2.8)where qa can be calculated from Eq (2.9).
(2.9)The terms used in the above equations are explained
below:Fsc = Short-circuit heat loss factor, which accounts for heat
loss due to heat transfer between the two different legs of the
U-tube in the borehole.Lc,tot = Required ground-loop length to meet
the shaved cooling load.Lh,tot = Required ground-loop length to
meet the shaved heating load.PLFm = Part-load factor during design
month, which represents the fraction of equivalent full load hours
during the design month to the total number of hours in that
month.qa = Net annual average heat transfer to the ground.Qlc =
Building design cooling block loadQlh = Building design heating
block load.Rsa = Effective thermal resistance of the ground; annual
pulse.Rsd = Effective thermal resistance of the ground; daily
pulse.Rsm = Effective thermal resistance of the ground; monthly
pulse.Rb = Thermal resistance of the borehole.ts = Undisturbed
ground temperature.tp = Temperature penalty (change in ground
temperature over a long run which is due to the thermal
interference between adjacent boreholes).tfi = Water temperature at
heat pump inlet;tfo = Water temperature at heat pump outlet.Cfc and
Cfh = Correction factors that account for the amount of heat
rejected or absorbed by the heat pumps. The values depend on the
respective EER and COP of the units and are provided in the design
manual.EFLHc and EFLHh = Annual equivalent full-load cooling and
heating hours.2.4.1.3 Status of current design tools for GSHP
systemDesign of GHEs for GHP systems in commercial buildings is
generally done using a software program. For single-zone,
residential systems, design tables can be used. Software programs
vary widely in calculation approach and simplifying assumptions
necessary for efficient calculation, and thus result in widely
varying accuracy. Table 2.5 is a non-exhaustive list of
commercially-available software design programs for GHE (Chiasson
2007).
Table 2.5 List of Software Programs for GHE
design.SoftwaresVendor
CLGS Intl. Ground-Source Heat Pump Assoc., Stillwater, OK,
USA
ECA Elite Software, Inc., Bryan, TX, USA
Earth Energy Designer (EED) University of Lund, Sweden
Lund Programs University of Lund, Sweden
GEOCALC Ferris State University, Big Rapids, MI, USA
GeoDesigner ClimateMaster, Oklahoma City, OK, USA
GchpCalc Energy Information Services, Tuscaloosa, AL, USA
GL-Source Kansas Electric Utility, Topeka, KS, USA
GLHEPRO Intl. Ground-Source Heat Pump Assoc., Stillwater, OK,
USA
Ground Loop Design (GLD)Gaia Geothermal; GBT, Inc., Maple Plain,
MN, USA
2.4.2 Operational strategies of GSHP systemHVAC systems
operational strategies design is to maintain the temperature,
humidity, etc. in buildings. Automatic control primarily modulates,
stages or sequences mechanical equipments to satisfy building load
requirements and safe equipment operation. In practical projects,
control loops can use digital, pneumatic, mechanical, electrical,
and electric control devices to meet different control purposes.
Development of an operational strategy of GSHP system is
complicated to some extent, as the control should be considered
into building level, heat pump level and ground loop level
(Verhelst, 2012). Building level control is to ensure the
temperature, humidity, etc. of the buildings are within the
acceptable range of setting values. Research focused on the
building level control are normally to develop simplified, accurate
building models to predicting both the thermal comfort and the
heating, cooling load. Thermal comfort is a function of the
operative temperature Top, which in turn is a weighted sum of the
room air temperature and the radiative temperature. An accurate
prediction of Top requires a detailed building model which
distinguishes between convective and radiation heat transfer
processes into and inside the building zones. In the review work of
Verhelst (2012), studies of Wimmer (2004) and Bianchi (2006) on
floor heating systems indicated that a third-order or even a
second-order lumped capacitance model is able to capture the
control relevant dynamics imposed by the floor heating time
constant in a well-insulated heavy-weight residential building. The
capacity of the zone air, inner walls and outer walls are all
lumped to one capacity at an average zone temperature. The impact
of the solar gains on the heating load are taken into account by
adding a positive temperature difference to the ambient air
temperature. The study of Zhai et al (2012) also indicated the
strong effects of the indoor temperature on the system performance
of GSHP system. Either the heat rejected to soil in the cooling
mode or the heat extracted from soil in the heating mode is
evidently affected by the set value of indoor temperature. With the
increase of indoor set temperature, the imbalance of earth energy
decreases.The decreased imbalance ratio would be beneficial to the
long-time operation of such GSHP system.In heat pump level control,
extension research effort in the field of developing and evaluating
optimal control strategies of cooling or heating dominated,
air-conditioned buildings. e.g. (Ahn et al 2001; Jin et al 2005; Ma
et al 2009; Li et al 2013; West et al 2014, Sichilalu et al 2014).
For this type of buildings, the focus potential primarily lies at
development of operational models or sequences of different HVAC
equipments, such as, determining the logic of charging and
discharging of active thermal energy storage devices (e.g., ice
storage), or the logic of switching between active cooling, free
cooling and night ventilation and the capacity operations of
chillers, heat pumps etc. In general, the control methods can be
summarized into three types: constant setpoint based, temperature
differential based and schedule based.Yavuzturk and
Spitler(2000)discussed a comparative research method which
investigates the advantages and disadvantages of various hybrid
GSHP systems operating and control strategies. One of the operating
and control strategies is about recharging the soil. It is based on
cooling storage in the ground to avoid a long-term temperature
rise. The cooling storage effect is achieved by operating the
cooling tower for six hours during the night. In addition, one of
the advantages of this system operating and control strategy is
that it uses the peak and valley electric charges to improve
economical efficiency. And one of the disadvantages is heat on the
flow of water might be carried and transported from the cooling
tower to the soil if the outdoor wet bulb temperature is higher
than the soil around the GHE. Man et al. (2011)proposed a similar
method, which proposes a novel hybrid GSHP (HGSHP) system with
nocturnal cooling radiator (NCR) serves as supplemental heat
rejecters. NCR is activated under ideal meteorological conditions
to diminish the heat accumulation around GHE. The water circulation
pump is activated from 10:00 pm to 6:00 am to pump the circulating
water flow through the NCR installed on the roof for rejecting the
accumulated heat around GHE. At 6:00 am, the circulation pump is
turned off. In order to reduce the heat rejected into ground and
take full advantage of waste heat, this HGSHP system is designed to
produce domestic hot water by a desuperheater. Ouyang et
al.(2012)proposed a new operating and control strategy aimed at
decreasing soil temperature during night. Under this control
strategy, the cooling tower is connected with the condenser of heat
pump through the heat exchanger, and the GHE is connected with the
evaporator of heat pump, so that the soil could be cooled at night
to relieve the soil heat accumulation problem. In this way, the
coefficient of performance (COP) of HGSHP system can be improved
obviously in the day time. Because the extra energy is consumed by
the heat pump at night, the total electricity consumption of the
system will be increased. However, it uses the peak and valley
electric charges to improve economical efficiency as the
aforementioned control strategy. Yang et al (2014) analyzed the
intermittent operation strategies of a hybrid ground-source heat
pump system with double-cooling towers for hotel buildings. On the
basis of hotel load patterns, four operating conditions were
designed for this system including one continuous condition and
three intermittent conditions conducted 20 years simulation in
TRNSYS. Results showed the optimal intermittent operating condition
favored both energy consumption reduction and soil temperature
recovery.When the GSHP system is designed to cover the entire
heating and cooling demand, the control at ground loop level is
straightforward. The ground loop can be used permanently only when
the temperature limits are met at the end of the design life time,
normally chosen 20 to 25 years. In order to facilitate the optimal
operation, massive efforts have been invested in developing
different models of GHEs to accurately predict fluid temperature in
the ground loop. Mathematical models have been reviewed in detail
in section 2.3. However, a GHE model with high accuracy temperature
prediction is extremely difficult because of the dimensionality and
complexity of the heat exchange process underground. A two
dimensional infinite line-source model used by Michopoulos and
Kyriakis(2009)to predict water temperature exiting the GHE had a
bias at 2C on average. In order to minimize the bias to further
extent, an artificial neural network (ANN) model of GHE was
established by Gang et al (2013). Based on the ANN model built,
Gang et al (2014) proposed a new control strategy to compare the
cooling water temperature exiting the ground heat exchanger
predicted by ANN model and cooling tower directly. Four years
performance of the hybrid ground source heat pump system controlled
based on the new method is calculated and compared with another two
frequently used methods (Schedule based and temperature
differential based). Results show that the new control method is
more energy efficient and can make full use of the heat exchange
advantage of outdoor air and the soil. 2.4.3 Optimization research
concerning GSHP systems A majority research of optimization are
focused either on developing and modifying models of GSHP system,
or on evaluating and comparing different design/operational
strategies and then recommending the best strategies. Zogou and
Stamatelos(1998)studied the design optimization of heat pump
systems to examine the effect of climatic conditions. They
considered northern and southern parts of Europe for their
analysis. Their study reveals that milder climates of the
Mediterranean and subtropical climates are found to be favorable
for a heat pump system. Spitler etal. (2005) performed simulation
and optimization for different components of a GSHP system. They
considered the effect of heating and cooling loads of the buildings
on the optimization of heat exchanger length when the GSHP system
was operated for 20 years. Their optimization results enabled them
to maintain the entering water temperature to the heat pump at the
design value. Kjellsson etal. (2010)optimized a solar assisted GSHP
system with a vertical GHX installed in a dwelling. Their results
reveal that using solar collector for hot water production in
summer and recharging the ground in winter is the optimal
combination. Hackel etal. (2008) investigated the optimization of a
hybrid GSHP system using TRNSYS (transient system simulation)
simulation studio and concluded that for cooling dominated
buildings the hybrid system should be sized to meet the heating
demand. Park etal.(2011)and(2012) performed optimization of a
hybrid GSHP with parallel configuration of a GHX and compared with
a non-hybrid GSHP system. They found that hybrid GSHP system was
21% more efficient than the conventional GSHP system and also they
optimized the hybrid GSHP using RSM (response surface methodology).
Bazkiaei etal.(2013)proposed a method to optimize a horizontal GHX
system by using homogenous and non-homogenous soil profiles. Based
on their study, they concluded that the performance of GHX
installed in soil with non-homogenous profile has better extraction
and dissipation rates compared to the soil with homogenous profile.
These studies above lack explicit optimization objectives and
optimization strategies, but, provide fundamental theories for
further development of optimization methodology of the GSHP system.
In recent years, a number of systematic optimization research on
GSHP system have been carried out. 2.4.3.1 Optimization
parametersThe performance of a GSHP system depends on many
parameters such as geological condition, pipe material, carrier
fluid property, pipe diameter, mass flow rate in the GHE, distance
between the pipes, boreholes and borehole diameters, and operation
configurations etc. (Cho et al 2014). Fig 2.1 depicts the
classification of design and operating parameters of GSHP system.
Optimization of these parameters is important to improve energy
performance and reduce the upfront and running cost of the GSHP
system (Garber et al 2013). The following section describes
previous research work on the optimization of GSHP systems.
Fig.2.1 Design and operating parameters summarized.2.4.3.2
Optimization objective functionsObjective function is a key
component formulating an optimization methodology. In mathematics,
optimization is the discipline concerned with finding inputs of a
function that minimize or maximize its value, which may be
subjected to constraints. Based on the literature reviewed, the
objective functions adopted in the optimization study of GSHP
system can be categorized into two categories: economic and
thermodynamic. Objective functions derived based on economic aspect
are total cost, total life cycle cost or the system COP/EER etc.
Thermodynamic objective functions are the system irreversibility,
the exergy loss and entropy/enthalpy generation etc. Fig.1 below
shows the classification of main optimization objectives in the
literature.
Fig 2.2 Classification of objective function used.
Economic optimization of GSHP systems is normally based on the
thermoeconomic analysis, which incorporates the associated costs of
the thermodynamic inefficiencies in the total product cost of an
energy system which can help designers to find out the cost
formation process in the energy system (Bejan et al 1996). A
powerful thermoeconomic analysis, evaluation, and optimization
techniques have been refined and applied by researchers around the
world to solve practical problems in the design and improvement of
energy system (Frangopoulos 1987). In the literature, there are a
number of economic analysis methods used to evaluate and optimize
GSHP systems. These include the life cycle cost method (Kreith et
al 2008), net benefits (net present worth) method (Peterson et al
2012), payback method, benefit-to-cost (or savings-to-investment)
ratio method, internal rate-of-return method, overall
rate-of-return method, exergy and cost energy mass method, and the
analytical hierarchy process (Nikolaidis et al 2009). The earliest
work using thermoeconomic analysis on heat pump system was proposed
by Wall (1985). He analysed a heat pump systems and pointed out
that a thermoeconomic optimization is an economic optimization in
conjunction with thorough thermodynamic description of the system.
Since then, great efforts have been put in this area, and in 1996,
Bejan et al (1996) established the principles and methodologies of
thermoeconomics and provided guidelines to perform thermoeconomic
analysis.Zhao et al (2003) put forward an integrated optimal
mathematical model by analysing the operating characteristics of
the groundwater heat pump and then optimized the system with an
objective function of the annual total costs according to technical
and economic optimal principle. In Khan et al work (2004), the
authors reported on a simulation procedure implemented in HVACSIM+
and a life cycle cost analysis and gives example result for a
typical Canadian residential building. The life cycle cost analysis
was based on the electricity costs for the heat pump and
circulating pump and first costs for the heat pump, circulating
pump, grout, borehole drilling, U-tube, and antifreeze. Esen et al.
(2006) has reported a detailed techno-economic analysis of a ground
source heat pump system and six conventional heating systems for
the climate conditions of Turkey in heating season of 20022003. In
hot climates such as in Turkey, GSHPs represent a viable
alternative to ASHPs and conventional space cooling and heating
systems because of their higher operating efficiency, especially
during the cooling season. Further, Pulat et al (2009) conducted an
experimental study of horizontal ground source heat pump
performance for mild climate in Turkey, the economic analysis also
indicated that GSHP system was more cost effective than the all
other conventional heating systems. Sanaye and Niroomand (2009;
2010) developed a thermal-economic optimal design method and
utilize the model to optimize a vertical ground-coupled heat pump
and a horizontal ground-coupled heat pump system respectively. The
objective function was the sum of annual operating and investment
costs of the system, and was minimized by using NelderMead and
genetic algorithm optimization methods separately to guarantee the
validity of the optimization results. Kalinci et al (2008)
conducted a study dealing with the determination of optimum pipe
diameters based on economic analysis and the performance analysis
of geothermal district heating systems along with pipelines using
energy and exergy analysis methods. They found that the nominal
diameter of DN300 pipeline has the minimum cost of US $561856.906
per year, with the energy efficiency and exergy efficiency values
of 40.21% and 50.12%, respectively. A probability based approach
was adopted in Garber et al.s study (2013), to evaluate the
economical feasibility and CO2 savings of a full-size GSHP system
as compared to four alternative HVAC system configurations. Results
showed that potential savings from a GSHP system largely depend on
projected HVAC system efficiencies, and gas and electricity prices,
and the GSHP sized to meet the full design load with an auxiliary
back up was potentially the most cost efficient configuration.
Robert et al (2014) established a new design method in order to
optimal size the vertical ground heat exchangers of ground source
heat pump system. The procedure relies on total cost minimization,
and includes a series of different initial costs (e.g., drilling,
excavation, heat pump, and piping network) as well as the operation
cost (energy). Retkowski et al (2014) developed a new mixed-integer
nonlinear programming (MINLP) approach to optimal design GSHP
system. The mathematical model applied includes the calculation of
the total annual costs (TAC) and the coefficient of performance to
obtain estimates of both economic and ecological relevance to
design an optimal equipment set-up. A case study was performed to
validate the effectiveness of the method, the results showed that
the TAC can be reduced by more than 10%. Morrone et al analyzed the
energy and economic savings using geothermal heat pumps in
different climates by performing a numerical study on 20 years GSHP
operation. The results implied that the economical profit of GSHPs
is more difficult to achieve in mild climates than in cold ones.
Conversely, greenhouse gas (GHG) emission reduction is found to be
larger in mild climates than in cold ones. In situ experiment study
was also conducted by Cerve ra-Vzquez et al (2015) to optimize the
water circulation pumps frequency of ground source heat pump
systems by improving the energy performance of the system. Results
show that energy savings up to 32% can be obtained by applying this
optimization methodology.Thermodynamic optimization of the GSHP
systems are usually studied using the second law of thermodynamics
or exergy analysis. Exergy analysis is a powerful tool in the
design, optimization and performance evaluation of energy systems.
This analysis can be used to identify the main sources of
irreversibility (exergy loss) and to minimize the generation of
entropy in a given process where the transfer of energy and
material take place (Bejan 2006). Piechowski (1996) first
introduced a relatively new approach to optimize a ground heat
exchanger (GHE), which was based on the second law of
thermodynamics. The proposed method of designing a GSHP system was
to accurately size a GHE not only to local soil conditions but also
to the building thermal characteristics. Ozgener et al (2004)
conducted a series of exergy analysis and performance assessment on
ordinary GSHP and hybrid GSHP systems. They established an
energetic and exergetic modeling and utilized the actual thermal
data taken from the system to evaluate the system performance
through energy and exergy efficiencies, exergetic improvement
potential, as well as some other thermodynamic parameters. Bi et al
(2009) presents a comprehensive exergy analysis of three circuits
and whole system of a GSHP for both building heating and cooling
modes to search out the key potential energy saving components.
Results showed that the GHEs normally have minimum exergy
efficiency and thermodynamic perfection, indicating great potential
of design optimization from the aspect of thermodynamic
performance. A new method for determining the optimized dimensions
of a ground source heat pump system (GSHPS) heat exchanger is
developed by Marzbanrad et al (2010), optimum length and diameter
for the heat exchanger is determined for different mass flow by
using entropy generation minimization method to reduce the exergy
destroy and power consumption of the system, thus the heat pump
efficiency increases. Ally et al (2012) presented an exergy and
energy analysis on the horizontal ground-source heat pump system
operated in a low-energy test house. The average monthly rate of
entropy production and percent entropy contribution for each
segment of the system were calculated through the exergy analysis,
meanwhile, the monthly average COPs were also summarized. Li et al
(2013)provided analytical expressions for optimizing flow velocity
and borehole length by applying the entropy generation minimization
method for GSHPs with a single U-tube. Their analyses indicated the
existence of optimum parameters based on pure heat transfer and
thermodynamics ground. Yekoladio (2013) studied on the design,
performance analysis and optimization of a downhole coaxial heat
exchanger for an enhanced geothermal system. The objective function
is minimization of heat transfer and fluid friction
irreversibilities, in terms of entropy generation number. An
optimum diameter ratio of the coaxial pipes for minimum pressure
drop in both limits of the fully turbulent and laminar fully
developed flow regime was determined and observed to be nearly the
same irrespective of the flow regime. Furthermore, an optimum
geothermal mass flow rate and an optimum geometry of the downhole
coaxial heat exchanger were determined for maximum net power
output.The above reviewed work on thermodynamic optimization or
economic optimization only considers one objective function. Some
scholars adopted multi-objective approaches, to optimize both
thermodynamic and thermoeconomic objectives simultaneously. An
earlier study proposed by Hepbasli (2004) conducted an energy and
exergy analysis of a GSHP system. The results provided specific
thermodynamic assessment analysis of a GSHP system for volume
heating and cooling modes, and gave useful energy transfers between
the components, and the exergy consumptions in the GSHP system and
its components for the average calculated values have been achieved
from the experimental studies. Sayyaadi et al. (2009) proposed a
multi-objective optimization of a vertical U-tube GSHP system to
minimize both the total levelized cost of the system product and
the exergy destruction of the system. Seven temperature differences
(e.g. between inlet brine and sub-cooled refrigerant in the
condenser, between the outlet air and superheated refrigerant in
the evaporator, etc.) and the pipe diameter of the GHE were chosen
as the decision variables. The sensitivities of the interest rate,
operating hours and the cost of electricity for the optimization
were also studied. Shi (2012) developed a thermoeconomic model for
analysis and optimization of a seawater source heat pump (SWHP)
system in a residential building. The modelling results indicated
that the exergy loss and EER increasing by 22.7% and 13.9%
respectively using thermoeconomic optimization compare to
thermodynamic optimization, but annual production costs reduce by
29.1%. As can be seen from the literature reviews above, the
efforts of research on the optimization of GSHP systems has
increased dramatically in recent years and the optimization focus
has gradually developed from single-objective to more comprehensive
multi-objective level for GSHP systems. However, due to the
complexities of an accurate building load evaluation, the variation
of ground and climate conditions and the complex correlations among
all these parameters in GSHP system, etc. More efforts are still
necessary in developing optimization strategies for GSHP
systems.2.5 ConclusionA detailed literature review around this
thesis topic covering the simulation model establishment, operation
simulation and design, control optimization of the GSHP system. A
simple summary can be deduced:1) Analytical and numerical models
have been developed for modelling and dimensioning ground heat
exchangers. The analytical GHE models are usually used for long
time period simulation and not suitable for the short time response
calculation. Numerical models are not suitable for direct
incorporation in a building simulation program with hourly or
sub-hourly time steps due to the computational time requirements.2)
Several design procedures and software are commercially available
for sizing the vertical ground loop heat exchanger. However, due to
the simplifications and assumptions made in these design methods,
the design results are necessary to be further optimized. 3)
Several operational strategies of GSHP system are available now,
including constant setpoint based, temperature differential based
and schedule based. As reviewed, the currently available
operational strategies might be far from optimal and not generally
applicable, operational optimization is still worth to be further
investigated.1
