- Email: [email protected]
Transient performance characteristics of a hybrid ground-source heat pump in the cooling mode
Applied Energy 123 (2014) 121–128
Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Transient performance characteristics of a hybrid ground-source heat pump in the cooling mode Joo Seong Lee, Honghee Park, Yongchan Kim ⇑ Department of Mechanical Engineering, Korea University, Anam-Dong, Sungbuk-Ku, Seoul 136-713, Republic of Korea
h i g h l i g h t s The transient characteristics of a hybrid ground-source heat pump were measured. A heat storage bath was adopted to simulate the ground thermal condition. The hybrid operation improved the performance under degraded ground conditions. The optimum set-point temperature was suggested at various cooling conditions.
a r t i c l e
i n f o
Article history: Received 28 October 2013 Received in revised form 17 January 2014 Accepted 15 February 2014 Available online 15 March 2014 Keywords: Hybrid ground-source heat pump Transient state COP Optimum Heat storage bath Ground thermal degradation
a b s t r a c t The objective of this study is to compare the transient performance characteristics between a groundsource heat pump (GSHP) and a hybrid ground-source heat pump (HGSHP), accounting for the degradation of the ground thermal condition during long-term operation. A heat storage bath for the ground heat exchanger (GHE) was adopted to simulate the transient characteristics of the ground thermal condition. In transient state, the performances of the HGSHP and GSHP were measured by changing the fluid flow rate (FFR) through the supplementary plate heat exchanger (SPHE) and the set-point temperature of the hybrid operation. The optimum FFR and the optimum set-point temperature of the HGSHP were determined as 8 kg min1 and 30 °C, respectively. At the optimized conditions, the average COP of the HGSHP increased by 7.2% compared with that of the GSHP. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The energy consumed during space cooling and heating has become an important issue in the design of commercial and residential buildings [1]. An air-source heat pump (ASHP) has been applied frequently for space cooling and heating. Its performance, however, ultimately degrades during operation at either very low or high ambient air conditions. A ground-source heat pump (GSHP) rejects heat into the ground or extracts heat from the ground via a ground heat exchanger (GHE), such as the vertical type shown in Fig. 1(a). The performance of an ASHP is highly dependent on ambient temperature, while that of the GSHP on the ground temperature. A GSHP provides higher performance than an ASHP because the ground heat source offers more favorable operating conditions [2–5]. Therefore, the GSHP has been widely applied in residential and commercial buildings [6,7]. ⇑ Corresponding author. Tel.: +82 2 3290 3366; fax: +82 2 921 5439. E-mail address: [email protected] (Y. Kim). http://dx.doi.org/10.1016/j.apenergy.2014.02.056 0306-2619/Ó 2014 Elsevier Ltd. All rights reserved.
However, during long-term operation, the thermal condition of the ground around the GHE can be degraded by the thermal imbalance between heat rejection and extraction in the GHE [8,9]. In cold-weather regions such as Beijing, the average ground temperature around the GHE decreased from 14 °C to 10 °C during a 10year operation of a GSHP [10]. In hot-weather areas such as Hong Kong, the borehole wall temperature of the GHE increased on average from 23 °C to 41 °C during a 10-year operation of a conventional GSHP [11]. Increased ground temperature degrades the heat transfer performance of the GHE and thus, decreases the performance of a GSHP in the cooling mode. Therefore, the performances of the GHE and GSHP during long-term operation must be evaluated in transient state. The transient heat transfer characteristics of a GHE and their effects on the transient system performance of the GSHP have been studied by many researchers [12–19]. Eskilson [12] presented a method to predict the temperature distribution of the ground around a GHE during long-term operation. He introduced a transient temperature response of the borehole, referred to as long time step (LTS) g-functions.
122
J.S. Lee et al. / Applied Energy 123 (2014) 121–128
Nomenclature ASHP C COP EEV EFT FFR FFRS GHE GPHE GSHP HE HGSHP HRG HRS IPHE LFT _ m OPHE P_ PHE Q_ SPHE ST
air-source heat pump specific heat at constant pressure (kJ kg1 K1) coefficient of performance electronic expansion valve entering fluid temperature (°C) fluid flow rate (kg min1) fluid flow rate through SPHE (kg min1) ground heat exchanger ground plate heat exchanger ground-source heat pump heat exchanger hybrid ground-source heat pump heat rejection rate of GPHE (W) heat rejection rate of SPHE (W) indoor plate heat exchanger leaving fluid temperature (°C) mass flow rate (kg min1) outdoor plate heat exchanger power (W) plate heat exchanger cooling capacity or heat rejection rate (W) supplementary plate heat exchanger set-point temperature (°C)
In this study, a hybrid ground-source heat pump (HGSHP), as shown in Fig. 1(b), is proposed to overcome thermal imbalance under degraded ground thermal conditions by incorporating the GSHP with supplementary heat rejecters or extractors. The supplementary equipment in the HGSHP controls some portion of the heat rejection or extraction rate of the GHE to reduce the cooling or heating load imposed on the GHE. Therefore, the HGSHP lessens the deterioration of the ground thermal condition, increasing the system performance during long-term operation. However, the HGSHP can be applied when the energy saving potential by the hybrid operation exceeds the additional energy consumption of the supplementary equipment.
T THR
temperature (°C) total heat rejection rate (W)
Greek letter D difference Subscripts amb ambient c cooling comp compressor f fluid G ground heat exchanger heater heater hs heat sink heat exchanger in in out out p pressure P parallel pump pump S serial tot total wb water bath
Most previous studies on the HGSHP were conducted to determine the optimum capacity and control strategy of the supplementary equipment during long-term operation using simulation methods [8,10,11,19–25]. However, there are hardly any experimental studies on the transient performance improvement of the HGSHP under optimized hybrid operation with consideration of degraded ground thermal conditions. The objective of this study is to compare the transient performance characteristics of the GSHP with those of the HGSHP with consideration of degraded ground thermal conditions during long-term operation. A heat storage bath was adopted to simulate the transient characteristics of ground thermal conditions. The transient performances of the
Fig. 1. Schematic diagrams of the GSHP and HGSHP.
123
J.S. Lee et al. / Applied Energy 123 (2014) 121–128
M
25.0 St op
Pu mp
ON
OF HT 2 F
HT 1
Constant temperature bath for heat sink Constant temperature bath for supplementary equipment Pump
Cooling mode Heating mode
25.0 St op
Pu mp
ON
OF HT F 2
T4
HT 1
Thermocouple
P
Pressure transducer
T5 : EFT of SPHE T6 : LFT of SPHE T7 : EFT of IPHE T8 : LFT of IPHE
T1 : LFT of GHE T2 : EFT of GPHE T3 : LFT of GPHE T4 : EFT of GHE
T1
T
T3
Ground loop
M
Heat storage bath
Turbine flow meter
M
Pump
T2
Supplementary loop
T6
EEV
Check valve
T5
T
Pump
EEV driver
T
T
T
GPHE T P
SPHE
Check valve
Primary loop
4-way valve Compressor driver
EEV
Filter dryer
T P
Mass flow meter
T T8
T P
T
T
Turbine flow meter M
T P
IPHE
T7
Indoor loop
25.0 Sto p
Pu mp
HT1
ON
OF F
HT2
Accumulator T
Compressor
Pump
Constant temperature bath for the indoor load
Fig. 2. Schematic diagram of the experimental setup.
GSHP and HGSHP were measured and analyzed under degraded ground thermal conditions.
2. Experimental setup and test procedure 2.1. Experimental setup Fig. 2 shows the schematic diagram of the test setup to measure the transient performances of the HGSHP and GSHP. The HGSHP was composed of a rotary compressor, an indoor plate heat exchanger (IPHE), two outdoor plate heat exchangers (OPHEs), and two electronic expansion valves (EEVs) for cooling and heating modes, respectively. The HGSHP using R-410A had the rated cooling capacity of 7.0 kW at the compressor frequency of 70. The OPHEs consisted of a ground plate heat exchanger (GPHE) of a ground loop and a supplementary plate heat exchanger (SPHE) of a supplementary loop for the hybrid operation mode. Table 1 shows the specifications of the tested HGSHP. The HGSHP had two operation modes, namely GSHP and HGSHP. In the GSHP mode, the supplementary flow loop was not operated even though the refrigerant passed the SPHE. In the HGSHP mode with the operation of the supplementary flow loop, the refrigerant flowed through the SPHE and experienced heat transfer [26]. The operation mode was changed from the GSHP to HGSHP when the ground temperature increased beyond a setpoint temperature. For an actual HGSHP, a boiler can be used as the supplemental equipment in the heating mode, while a cooling tower can be used in the cooling mode. A constant temperature bath for the heat sink in the ground loop and a constant temperature bath in the supplementary loop were adopted to simulate the operating conditions in the GHE and in the supplementary equipment, respectively. The constant temperature bath in the indoor loop was used to impose the cooling or heating loads. Water was selected as working fluid for the secondary fluid loops, in which an inverter-driven pump and a manual-type needle valve to control the fluid flow rate (FFR) supplied to the plate heat exchangers (PHEs) were included.
A data acquisition system monitored temperatures, pressures, mass flow rate, volumetric flow rates, and powers in transient state. A mass flow meter was installed between the OPHEs and EEVs to measure the refrigerant mass flow rate. Three volumetric flow meters were installed for measuring FFRs in the secondary fluid flow loops, respectively. The specifications and uncertainties of the sensors are presented in Table 2.
2.2. Heat storage bath for the GHE The heat storage bath, as shown in Fig. 3(a), was composed of a water bath, a tube-type heat exchanger for the GHE, and another tube-type heat exchanger for the heat sink connected with a constant temperature bath. Table 3 shows the specifications of the heat storage bath for the GHE. The U-tube in the in situ GHE was simulated by the heat exchanger for the GHE. The grout and soil in the in situ GHE were simulated by the water bath and the heat sink of the heat storage bath, respectively. The water bath
Table 1 Specifications of the HGSHP and fluid pump. Components
Specifications
Compressor
Type Refrigerant Frequency Rated input power (at 60 Hz) Rated capacity (at 60 Hz)
BLDC rotary R-410A 30–110 1818
Hz W
5881
W
Heat exchanger (outdoor and indoor)
Type Size Volume (path 1/path 2)
Plate 95 85 325 640/670
mm cm3/cm3
Expansion valve
Type Rated capacity Range
EEV 2.2 0–500
RT Pulse
Type Rated flow rate Rated head
Centrifugal 3 27
m3 h1 m
Fluid pump
Unit
124
J.S. Lee et al. / Applied Energy 123 (2014) 121–128
Table 2 Specifications and uncertainties of the sensors.
Table 3 Specifications of the heat storage bath.
Item
Specifications
Power meter
Range Accuracy
0–300 V/0–10 A ±0.01% 1
Mass flow meter
Range Accuracy
0–360 kg h ±0.1% of full scale
Pressure transducer
Range Accuracy
0–5.0 MPa ±0.1% of full scale
Thermocouple
Range Accuracy
200 to 200 °C ±0.2 °C
Turbine flow meter
Range Accuracy
0.36–3.96 m3 h1 ±0.5% of full scale
dT wb _ f ;G C p;f ;G ðT f ;G;in T f ;G;out Þ ¼m dt _ f ;hs C p;f ;hs ðT f ;hs;out T f ;hs;in Þ m
Specifications
Water bath
Shape Size
Rectangular section 60 60 60
mm3
Type
9 Rows, 4 Steps, 4 Paths Serpentine shape 12.7 19.20
mm m
5 Rows, 5 Steps, 5 Paths Serpentine shape 15.9 12.75
mm m
HE for GHE
Outer diameter Length HE for heat sink
temperature, which indicates the ground temperature around the U-tube, was calculated by averaging 5-point temperatures of the T-type thermocouples serially located near the wall of the water bath, because the water bath temperature was stratified from the bottom to the top position. The heat accumulation rate in the water bath was determined by the difference between the heat rejection rate of the GHE and the heat removal rate of the heat sink. Since the capacity of the heat storage bath is significantly smaller than that of the ground source in a true GSHP, the operation-period in the simulated system stands for accelerated time. Based on the energy balance within the heat storage bath assuming the perfectly insulated and well-mixed water bath, the governing equation was derived as,
ð1Þ
where q and Cp denote the density and specific heat at constant pressure, respectively; V and Twb are the volume and temperature _ is the water mass flow rate through the heat of the water bath; m exchanger; TG and Ths mean the fluid temperatures within the heat exchangers for the GHE and the heat sink, respectively. As shown in Fig. 3(a), a thermal response test of the heat storage bath was conducted to determine the capacity of the heat sink. The electric heater was inserted in the ground loop to impose the
Type Outer diameter Length
Unit
cooling load of the GHE. The governing equation of the heat storage bath for the thermal response test can be simplified as,
qwb V wb C p;wb
dT wb _ f ;hs C p;f ;hs ðT f ;hs;out T f ;hs;in Þ ¼ P_ heater m dt
2.3. Test procedure The transient tests of the GSHP and HGSHP in the cooling mode were started respectively at the initial ground temperature, and the performance degradations with operation time were measured. As the ground temperature increased beyond the limit, the operation mode was changed from the GSHP to HGSHP to prevent
Constant temperature bath for heat sink
M
25.0 St op
Pu mp
HT 1
ON
OF F
HT 2
EFT of heat sink
Qhs Ground loop
HE for heat sink
o
T
Qamb
T T
T Tin
Pump T
T
HE for GHE QG
Water
Heat storage bath
Electric heater
T
T Tout
T
Pump
40 38 Tin of thermal response test 36 34 Tout of in-situ GHE 32 30 Tout of thermal response test 28 26 24 22 20 18 16 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Time (h) Power supplier
ð2Þ
The inlet (Tin) and outlet temperature (Tout) of the fluid of the GHE were measured for 6 h from the start to the end at 1-s intervals. The heater power was determined as 4.5 kW. The entering fluid temperature (EFT) and the FFR of the heat sink heat exchanger were controlled to satisfy the temperature profile of the thermal response test of the in situ GHE [27], as shown in Fig. 3(b). Table 4 shows the ground thermal condition and specifications of the GHE in the thermal response test of the in situ GHE. When the EFT and FFR of the heat sink were 3 °C and 28 kg min1, respectively, Tout of the thermal response test of the heat storage bath was very similar to that of the in situ site, which showed a maximum temperature difference of less than 1 °C in the whole operating range. Therefore, the EFT and the FFR of the heat sink were determined as 3 °C and 28 kg min1, respectively.
Fluid temperature ( C)
qwb V wb C p;wb
Components
(b) Thermal response
(a) Schematic diagram Fig. 3. Schematic diagram and thermal response of the heat storage bath.
125
J.S. Lee et al. / Applied Energy 123 (2014) 121–128 Table 4 Ground thermal condition and specifications of the GHE in the thermal response test. Parameters
Specifications
Unit
Number of boreholes Borehole depth Borehole radius Soil specific heat Soil density Soil thermal conductivity Soil heat capacity Soil diffusivity Grout specific heat Grout density Grout thermal conductivity Pipe radius Pipe thermal resistance Fluid specific heat Fluid density Average air temperature
1 150 0.076 0.909 2640 11.088 2399.76 0.146 3.5454 1100 2.916 0.016 0.41 4.19 1000 12.9
m m kJ kg1 K1 kg m3 kJ h1 m1 K1 kJ m3 K1 m2 day1 kJ kg1 K1 kg m3 kJ h1 m1 K1 m K W1 kJ kg1 K1 kg m3 °C
severe performance degradation. The set-point temperature (ST) indicates the leaving fluid temperature (LFT) of the GHE when the operation mode changes from the GSHP to HGSHP. Table 5 shows the test conditions for the HGSHP as well as the GSHP. The initial temperature of the water bath and the LFT of the GHE were 13.5 °C and 15.5 °C, respectively. The EEV was controlled to obtain the superheat of 5 °C at the compressor inlet. The FFR of the supplementary loop was changed from 4 to 16 kg min1 in 4 kg min1 increments. The EFT of the SPHE was fixed at 30 °C based on the typical operating condition presented in the ASHRAE Handbook [28]. The set-point temperature of the LFT of the GHE in the HGSHP was changed from 27.5 °C to 40 °C in 2.5 °C increments. The FFRs of the GPHE and IPHE were fixed at the optimum value of 20 kg min1, which yielded the maximum COP with the variation of FFR under the standard test condition for the GSHP [29]. The measured data in transient state were recorded continuously for 60 min in 1-s intervals. As shown in Eq. (3), cooling capacity was determined with the FFR and the temperature difference across the IPHE. The heat rejection rates in the GPHE (HRG) and SPHE (HRS) were also calculated using Eq. (3). The total heat rejection rate (THR) of the HGSHP was calculated by summing the HRG and HRS. The total power and cooling COP were determined using Eqs. (4) and (5), respectively.
_ f C f DT f Q_ c ¼ m
ð3Þ
P_ tot ¼ P_ comp þ P_ pump
ð4Þ
COP ¼
Q_ c _Ptot
ð5Þ
Uncertainty analysis was performed for the cooling capacity and cooling COP based on ANSI/ASME PTC 19.1 [30]. The uncertainties of the cooling capacity and COP were estimated as ±3.72% and ±3.73%, respectively. 3. Results and discussion 3.1. Comparison of transient performance between the HGSHP and GSHP Transient performance tests of the GSHP and HGSHP were conducted after controlling the thermal response of the heat storage bath corresponding to the transient thermal response of the borehole in the in situ GHE [27]. Fig. 4 shows the increase in the water bath temperature according to operation time due to the heat accumulation by the difference between the heat rejection rate of the GHE and the heat removal rate of the heat sink. The water bath temperature reached approximately 26 °C after 60 min of operation in the GSHP mode with the initial temperature of 13.5 °C. The higher water bath temperature meant more deteriorated ground thermal condition of the GSHP. At the initial stage, the water bath temperatures of the GSHP and HGSHP were nearly the same because the HGSHP operated in the GSHP mode before the hybrid operation. However, when the LFT of the GPHE increased beyond the set-point temperature of 35 °C (at approximately 19 min), the water bath temperature of the HGSHP decreased gradually compared with that of the GSHP because the HGSHP rejected heat into both the ground loop through the GPHE and the supplementary loop through the SPHE. Therefore, the heat accumulation rate in the water bath of the HGSHP was lower than that of the GSHP after the hybrid operation. After 60 min operation, the water bath temperature of the HGSHP decreased by 4.3%, compared with that of the GSHP, so the degradation of ground thermal environment can be delayed by the HGSHP operation. At all FFRs through the SPHE, the water bath temperature of the HGSHP decreased by 3.3% on average, compared with that of the GSHP during the hybrid operation. It should be noted that the long-term operation of the in situ GSHP was accelerated in this transient test to speed up degradation of ground thermal conditions. Fig. 5 shows the heat rejection rates of the GSHP and HGSHP according to operation time. The HRGs in the GSHP and HGSHP increased rapidly with operation time, and then reached to their maximum values at 15 min. After the peak point, the HRG of the GSHP operated nearly constantly at the maximum value with slight fluctuations, while the HRG of the HGSHP started to decrease gradually due to the partial heat rejection into the supplementary loop under hybrid operation. At FFR of 8 kg min1 through the SPHE, the HRG of the HGSHP decreased by 7.1%, compared with
Table 5 Test conditions for the GSHP and HGSHP. Parameters
Value
Compressor frequency (Hz) EEV opening Set-point temperature of LFT of the GHE (°C)
70 Controlled to obtain 5 °C superheat – 27.5, 30, 32.5, 35(baseline), 37.5, 40 30 12 20 (Optimized value) 4, 8, 12(baseline), 16 20 (Optimized value) 3 28 13.5 15.5 60
EFT of the SPHE (°C) Fluid temperature entering the indoor heat exchanger (°C) Mass flow rate in the ground loop (kg min1) Mass flow rate in the supplementary loop (kg min1) Mass flow rate in the indoor heat exchanger (kg min1) Fluid temperature entering the HE for the heat sink (°C) Mass flow rate entering the HE for the heat sink (kg min1) Initial temperature Operation time (min)
GSHP mode HGSHP mode-Parallel
Water bath temperature (°C) LFT of the GHE (°C)
J.S. Lee et al. / Applied Energy 123 (2014) 121–128
9000
HGSHP
240
24
200
22 160
20
120
18 16
80 Hybrid operation of HGSHP
14
40
o
ST = 35 C
12 10
FFR of SPHEX = 8 kg min
0
5
-1
0
Capacity of GSHP Capacity of HGSHP
8500
Cooling capacity (W)
26
Heat accumulation rate (kW)
Water bath temperature (oC)
GSHP
1900
7500
1800
7000
1700 1600
6500
1500
Hybrid operation of HGSHP
6000
ST = 35 C -1 FFR of SPHEX = 8 kg min
5500 5000
0
5
10 15 20 25 30 35 40 45 50 55 60
loops. Upon the start of the hybrid operation, the total power of the HGSHP became instantly higher than that of the GSHP due to the additional power of the pump in the supplementary loop. However, after several minutes into the hybrid operation, the total power of the HGSHP decreased gradually compared with that of the GSHP due to the decreased compressor power. During the hybrid operation period, the average total power of the HGSHP decreased by 1.4% compared with that of the GSHP at FFR of 8 kg min1 through the SPHE. Besides, the COP of the GSHP decreased gradually with operation time due to the deterioration of the ground thermal condition. The COP of the HGSHP started to increase rapidly upon start of the hybrid operation because of the increased cooling capacity. Therefore, during the hybrid operation period, the average COP of the HGSHP was 10.4% higher than that of the GSHP at FFR of 8 kg min1 through the SPHE, mainly due to the increased average cooling capacity by 8.9%. Fig. 8 shows the average performances of GSHP and HGSHP during the whole operation time (60 min) according to the FFR through the SPHE. The FFR of 0 kg min1 through the SPHE indicates the GSHP operation mode. As the FFR through the SPHE increased from 0 to 4 kg min1, the average cooling capacity of the HGSHP increased rapidly and the average compressor power decreased slightly, resulting in the rapid increase of the COP. As the FFR through the SPHE increased from 4 to 16 kg min1, the average cooling capacity of the HGSHP increased and peaked at FFR of 8 kg min1. The average total power of the HGSHP increased slightly with FFR due to the increase in the additional power of the pump in the supplementary loop. At FFR of 16 kg min1 through the SPHE, the average total power of the HGSHP was
COP of GSHP COP of HGSHP
ST = 35 C
Total power of GSHP Total power of HGSHP
4000
-1
3000
HRG of GSHP
2000
HRS of GSHP HRS of HGSHP Total heat rejection rate of HGSHP
HRG of HGSHP
Hybrid operation of HGSHP 4.5
o
ST = 35 C -1 FFR of SPHE = 8 kg min
4.0
0
1900 1800 1700 1600
3.5
1500
1000
1400
3.0
0
2200
2000
o
FFR of SPHE = 8 kg min
2300
2100
5.0
COP
Heat rejection rate (W)
5.5
7000
5000
1200
Fig. 6. Variations of the cooling capacity and compressor power with time.
6.0
Hybrid operation of HGSHP
1300
Time (min)
8000
6000
1400
o
Time (min)
that of the GSHP. In addition, as the hybrid operation started, the HRS in the HGSHP increased rapidly. Therefore, during the hybrid operation period, the THR of the HGSHP was on average 8.5% higher than that of the GSHP. In the cooling mode, the HGSHP was more effective than the GSHP in rejecting heat from the heat pump unit. Fig. 6 shows the cooling capacities and compressor powers of the GSHP and HGSHP according to operation time. The cooling capacity of the GSHP rapidly increased at the initial stage, and then decreased gradually due to the deterioration of the ground thermal condition according to operation time. The cooling capacity fluctuated because the EEV was controlled to satisfy 5 °C superheat condition. The compressor power of the GSHP increased gradually due to the increase in the discharge pressure of the compressor under degraded thermal conditions of the water bath. In the HGSHP, as the hybrid operation mode started at the set-point temperature of 35 °C (approximately 19 min from the start), the cooling capacity of the HGSHP increased rapidly, exceeding that of the GSHP, due to the partial heat rejection through the supplementary loop (SPHE). Besides, the compressor power of the HGSHP decreased substantially compared with that of the GSHP during the hybrid operation period because of the decreased condensing pressure. During the hybrid operation period, the average cooling capacity of the HGSHP increased by 8.9%, and the average compressor power decreased by 4.4%, compared with that of the GSHP at FFR of 8 kg min1 through the SPHE. Fig. 7 shows the COPs and total powers of the GSHP and HGSHP according to operation time. Total power includes the compressor power and the pump powers of the ground and supplementary
2000
8000
10 15 20 25 30 35 40 45 50 55 60
Fig. 4. Variations of the water bath temperature and heat accumulation rate with time.
2100
Comp. power of GSHP Comp. power of HGSHP
1300 5
10 15 20 25 30 35 40 45 50 55 60
Time (min) Fig. 5. Variation of the heat rejection rate with time.
Compressor power (W)
280
28
0
5
10 15 20 25 30 35 40 45 50 55 60
Time (min) Fig. 7. Variations of the total power and COP with time.
Total power (W)
126
127
J.S. Lee et al. / Applied Energy 123 (2014) 121–128 7300
o
ST = 35 C 7200 7100
Average capacity in transient Average COP in transient Average total power in transient Average comp. power in transient COP in steady [9]
3.5 3.4
7000 6900 6800 2000 1900
3.3
1800
3.2
1700
0
4
8
12
16 -1
Fluid flow rate of SPHE (kg min ) Fig. 8. Variations of the average performances of the HGSHP with the fluid flow rate of SPHE.
higher than that of the GSHP. Therefore, the average COP of the HGSHP peaked at FFR of 8 kg min1, which was determined as the optimum FFR through the SPHE. At the optimum FFR and set-point temperature of 35 °C, the average COP of the HGSHP increased by 6.9%, compared with that of the GSHP. The optimum FFR through the SPHE in the transient state was the same as the result of the steady state test carried out for the fixed ground condition [9]. 3.2. Effects of the set-point temperature on the performance of the HGSHP Fig. 9 shows the compressor power and total power of the HGSHP according to the set-point temperature (ST) under hybrid operation at FFR of 12 kg min1 through the SPHE. As the set-point temperature increased from 30 °C to 40 °C, the initiation time of the hybrid operation in the HGSHP increased from 8 min to 55 min, which indicated that the hybrid operation of the HGSHP started at a more degraded ground thermal condition. At all setpoint temperatures during the hybrid operation, the compressor power of the HGSHP decreased rapidly and then became lower than that of the GSHP. This decrease was due to the decreased discharge pressure from the reduced temperature of the water bath by the hybrid operation, which meant the delay in the deterioration of ground thermal condition. For the entire duration of the operation at FFR of 12 kg min1 through the SPHE, the average compressor powers of the HGSHP at set-point temperatures of 30 °C and 40 °C decreased by 3.6% and 1.2%, respectively, compared
2400
8500
o
Hybrid operation at ST=30 C o Hybrid operation at ST=35 C
2250
Power (W)
2100 1950 1800 Hybrid operation o at ST=40 C
1650 GSHP
1500
o
HGSHP, ST=30 C
Total power
1350
o
Comp. power
1200
o
HGSHP, ST=35 C
5
o
Time (min) Fig. 9. Variations of the compressor power and total power of the HGSHP with the set-point temperature in the hybrid operation.
o
HGSHP, ST=40 C Hybrid operation o at ST=40 C
5.5 5.0
7000 4.5
6500
Hybrid o operation at ST=35 C
6000 5500
4.0 3.5
Hybrid o operation at ST=30 C
5000
HGSHP, ST=40 C
10 15 20 25 30 35 40 45 50 55 60
6.0
o
HGSHP, ST=30 C
HGSHP, ST=35 C ,
7500
4500 0
GSHP ,
8000
Cooling capacity (W)
COP
3.6
Capacity, Power (W)
3.7
with those of the GSHP. During the stable hybrid operation period, the total power of the HGSHP decreased slightly compared with that of the GSHP at all set-point temperatures due to the additional power of the pump in the supplementary loop. Therefore, the decrease in the average total power of the HGSHP below that of the GSHP was very small, less than 0.4% at FFR of 12 kg min1 through the SPHE. Fig. 10 shows the cooling capacity and COP of the HGSHP according to the set-point temperature of the hybrid operation at FFR of 12 kg min1 through the SPHE. At all set-point temperatures, the cooling capacity of the HGSHP increased rapidly upon start of the hybrid operation due to the additional heat rejection through the supplementary loop. During the hybrid operation period, the cooling capacity of the HGSHP remained nearly constant with slight fluctuations due to the decrease in the heat rejection to the ground. The average cooling capacities of the HGSHP at set-point temperatures of 30 °C, 35 °C, and 40 °C increased by 6.4%, 5.2%, and 0.2%, respectively, compared with that of the GSHP at FFR of 12 kg min1 through the SPHE. At the set-point temperature of 30 °C, the COP degradation of the HGSHP decreased gradually with operation time due to the higher cooling capacity of the HGSHP than the GSHP. As the set-point temperature increased beyond 35 °C, the COP of the HGSHP increased instantly over that of the GSHP just after the start of the hybrid operation due to the rapid reduction of the heat rejection rate in the ground. The average COPs of the HGSHP at set-point temperatures of 30 °C, 35 °C, and 40 °C increased by 6.0%, 5.0%, and 0.8%, respectively, compared with those of the GSHP at FFR of 12 kg min1 through the SPHE. Fig. 11 shows the average performances of the HGSHP for the entire duration of the operation (60 min) according to the set-point temperature at FFRs of 8 kg min1 and 12 kg min1 through the SPHE, respectively. The higher set-point temperature indicated a more delayed hybrid operation. As the set-point temperature increased from 27.5 °C to 40.0 °C at FFR of 12 kg min1, the average cooling capacity of the HGSHP peaked at the set-point temperature of 30 °C. As the set-point temperature increased from 32.5 °C to 40 °C at FFR of 12 kg min1, the average cooling capacity of the HGSHP decreased by 6.1% due to the decreased cooling effect from the delayed hybrid operation, while the average total power decreased by 0.6%. Therefore, the average COP of the HGSHP showed a trend similar to that of the average cooling capacity. At FFR of 8 kg min1 for set-point temperatures from 27.5 °C to 35 °C, the average COP was slightly higher than that at FFR of 12 kg min1. However, the average COP decreased more rapidly when the setpoint temperature increased beyond 35 °C due to the delayed hybrid operation of the HGSHP. At FFR of 8 kg min1, the HGSHP gave the maximum average COP at the set-point temperature of 30 °C.
COP
3.8
0
5
3.0
10 15 20 25 30 35 40 45 50 55 60
Time (min) Fig. 10. Variations of the cooling capacity and COP of the HGSHP with the set-point temperature in the hybrid operation.
128
J.S. Lee et al. / Applied Energy 123 (2014) 121–128
3.8
7200
COP
3.6
7100 7000
3.5
6900 Average capacity, FFRS = 12 kg min -1
3.4
6800
Average total power,FFRS = 12 kg min-1
3.3
Average COP, FFRS = 12 kg min -1
2000
Average COP, FFRS = 8 kg min -1
1975
3.2
Capacity, Power (W)
7300
3.7
3.1
References
7400
1950 27.5
30.0
32.5
35.0
37.5
40.0
1925
o
Set-point temperature of hybrid operation ( C) Fig. 11. Variations of the average performances of the HGSHP with the set-point temperature in the hybrid operation.
Therefore, the optimum set-point temperature of the HGSHP was determined as 30 °C. At the optimum set-point temperature of 30 °C and the optimum FFR of 8 kg min1, the maximum average COP of the HGSHP was 3.76, which was 7.2% higher than that of the GSHP.
4. Conclusions In this study, the transient performance characteristics of the HGSHP were tested in the cooling mode by changing the FFR through the SPHE and the set-point temperature of the hybrid operation. The optimum set-point temperature and optimum FFR of the SPHE were determined under the standard design condition for the cooling tower [28]. A heat storage bath for the GHE was used to simulate degradation of the ground thermal condition with operation time. During the hybrid operation period up to 60 min, the ground temperature condition in the HGSHP decreased by 3.3%, compared with that in the GSHP. The average capacity and COP of the HGSHP peaked at FFR of 8 kg min1 through the SPHE, which was determined as the optimum FFR. The performance enhancement of the HGSHP over the GSHP was highly dependent on the set-point temperature. The average cooling capacity and COP of the HGSHP peaked at the set-point temperature of 30 °C regardless of the FFR through SPHE. Therefore, the optimum setpoint temperature of the HGSHP was determined as 30 °C. At the optimum FFR of 8 kg min1 and the optimum set-point temperature of 30 °C, the average COP of the HGSHP during 60 min operation was 3.76, which was 7.2% higher than that of the GSHP. As the set-point temperature increased beyond 35 °C, the average COP for 60 min operation decreased rapidly due to the delayed hybrid operation of the HGSHP.
Acknowledgments This work was supported by the Human Resources Development Program (No. 20124010203250) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea Government Ministry of Trade, Industry and Energy.
[1] Nam Y, Ooka R, Shiba Y. Development of dual-source hybrid heat pump system using ground water and air. Energy Build 2010;42:909–16. [2] Sanner B. Shallow geothermal energy, Geo-heat center bulletin. Geo-Heat Center: Klamath Falls, OR; 2001. [3] Florides G, Kalogirou S. Ground heat exchangers – a review of systems, models and applications. Renew Energy 2007;32(15):2461–78. [4] Michopoulos A, Bozis D, Kikidis P, Papakostas K, Kyriakis NA. Three-years operation experience of a ground source heat pump system in Northern Greece. Energy Build 2007;39(3):328–34. [5] Wang H, Qi C. Performance study of underground thermal storage in a solar ground coupled heat pump system for residential buildings. Energy Build 2008;40(7):1278–86. [6] Yang H, Cui P, Fang Z. Vertical-borehole ground-coupled heat pumps: a review of models and systems. Appl Energy 2010;87:16–27. [7] Self SJ, Reddy BV, Rosen MA. Geothermal heat pump systems: Status review and comparison with other heating options. Appl Energy 2013;101:341–8. [8] Yavuzturk C, Spitler JD. Comparative study to investigate operating and control strategies for hybrid ground source heat pump systems using a short time-step simulation model. ASHRAE Trans 2000;106(2):192–209. [9] Park H, Lee JS, Kim W, Kim Y. Performance optimization of a hybrid ground source heat pump with the parallel configuration of ground heat exchanger and a supplemental heat rejecter. Int J Refrig 2012;35:1537–46. [10] Chen X, Lu L, Yang H. Long term operation of a solar assisted ground coupled heat pump system for space heating and domestic hot water. Energy Build 2011;43:1835–44. [11] Man Y, Yang H, Spitler JD, Fang Z. Feasibility study on novel hybrid ground coupled heat pump system with nocturnal cooling radiator for cooling load dominated buildings. Appl Energy 2011;88:4160–71. [12] Eskilson P. Thermal analysis of heat extraction boreholes. Ph.D. Thesis. Sweden:University of Lund; 1987. [13] Yavuzturk C, Spitler JD, Rees SJ. A Transient two-dimensional finite volume model for the simulation of vertical U-tube ground heat exchangers. ASHRAE Trans 1999;105(A):465–74. [14] Hellstrom G. Duct ground heat storage model manual for computer code. Sweden: Department of Mathematical Physics, University of Lund; 1989. [15] Lamarche L, Beauchamp B. New solutions for the short-time analysis of geothermal vertical boreholes. Int J Heat Mass Transfer 2007;50:1408–19. [16] Bandyopadhyay G, Gosnold W, Mann M. Analytical and semi-analytical solutions for short-time transient response of ground heat exchangers. Energy Build 2008;40:1816–24. [17] Bauera D, Heidemanna W, Dierschb HJG. Transient 3D analysis of borehole heat exchanger modeling. Geothermics 2011;40:250–60. [18] Choi JC, Lee SR, Lee DS. Numerical simulation of vertical ground heat exchangers: Intermittent operation in unsaturated soil conditions. Comput Geotechn 2011;38:949–58. [19] Yavuzturk C. Modeling of vertical ground loop heat exchangers for ground source heat pump systems. Doctoral thesis, Oklahoma State University: Stillwater, Oklahoma; 1988. [20] Sagia Z, Rakopoulos C, Kakaras E. Cooling dominated hybrid ground source heat pump system application. Appl Energy 2012;94:41–7. [21] Chen X, Yang H. Performance analysis of a proposed solar assisted ground coupled heat pump system. Appl Energy 2012;97:888–96. [22] Xu X. Simulation and optimal control of hybrid ground source heat pump systems. Doctoral Thesis, Oklahoma State University: Stillwater, Oklahoma; 2007. [23] Hackel S, Nellis GN, Klein SK. Optimization of cooling-dominated hybrid ground-coupled heat pump systems. ASHRAE Trans 2009;115(1):565–80. [24] Man Y, Yang H, Wang J. Study on hybrid ground-coupled heat pump system for air-conditioning in hot-weather areas like Hong Kong. Appl Energy 2010;87:2826–33. [25] Chiasson AD. Simulation and design of hybrid geothermal heat pump systems. Doctoral thesis, University of Wyoming: WY; 2007. [26] Park H, Lee JS, Kim W, Kim Y. The cooling seasonal performance factor of a hybrid ground-source heat pump with parallel and serial configurations. Appl Energy 2013;102:877–84. [27] Kim W, Park H, Kim Y. Development of an electric circuit transient analogy model in a vertical closed loop ground heat exchanger. Kor J Air-Cond Refrig 2012;24(4):306–14. [28] ASHRAE. ASHRAE Handbook-HVAC systems and equipment. Atlanta: American Society of Heating Refrigerating and Air-conditioning Engineers; 2008 [Chapter 39]. [29] ISO 13256-2. Water-source heat pump testing and rating for performance: Part 2: Water-to-water and brine-to-water heat pumps. International Standards Organization; 1998. [30] ANSI/ASME PTC 19.1. Measurement uncertainty. NY: American Society of Mechanical Engineers; 1985.
Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Transient performance characteristics of a hybrid ground-source heat pump in the cooling mode Joo Seong Lee, Honghee Park, Yongchan Kim ⇑ Department of Mechanical Engineering, Korea University, Anam-Dong, Sungbuk-Ku, Seoul 136-713, Republic of Korea
h i g h l i g h t s The transient characteristics of a hybrid ground-source heat pump were measured. A heat storage bath was adopted to simulate the ground thermal condition. The hybrid operation improved the performance under degraded ground conditions. The optimum set-point temperature was suggested at various cooling conditions.
a r t i c l e
i n f o
Article history: Received 28 October 2013 Received in revised form 17 January 2014 Accepted 15 February 2014 Available online 15 March 2014 Keywords: Hybrid ground-source heat pump Transient state COP Optimum Heat storage bath Ground thermal degradation
a b s t r a c t The objective of this study is to compare the transient performance characteristics between a groundsource heat pump (GSHP) and a hybrid ground-source heat pump (HGSHP), accounting for the degradation of the ground thermal condition during long-term operation. A heat storage bath for the ground heat exchanger (GHE) was adopted to simulate the transient characteristics of the ground thermal condition. In transient state, the performances of the HGSHP and GSHP were measured by changing the fluid flow rate (FFR) through the supplementary plate heat exchanger (SPHE) and the set-point temperature of the hybrid operation. The optimum FFR and the optimum set-point temperature of the HGSHP were determined as 8 kg min1 and 30 °C, respectively. At the optimized conditions, the average COP of the HGSHP increased by 7.2% compared with that of the GSHP. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The energy consumed during space cooling and heating has become an important issue in the design of commercial and residential buildings [1]. An air-source heat pump (ASHP) has been applied frequently for space cooling and heating. Its performance, however, ultimately degrades during operation at either very low or high ambient air conditions. A ground-source heat pump (GSHP) rejects heat into the ground or extracts heat from the ground via a ground heat exchanger (GHE), such as the vertical type shown in Fig. 1(a). The performance of an ASHP is highly dependent on ambient temperature, while that of the GSHP on the ground temperature. A GSHP provides higher performance than an ASHP because the ground heat source offers more favorable operating conditions [2–5]. Therefore, the GSHP has been widely applied in residential and commercial buildings [6,7]. ⇑ Corresponding author. Tel.: +82 2 3290 3366; fax: +82 2 921 5439. E-mail address: [email protected] (Y. Kim). http://dx.doi.org/10.1016/j.apenergy.2014.02.056 0306-2619/Ó 2014 Elsevier Ltd. All rights reserved.
However, during long-term operation, the thermal condition of the ground around the GHE can be degraded by the thermal imbalance between heat rejection and extraction in the GHE [8,9]. In cold-weather regions such as Beijing, the average ground temperature around the GHE decreased from 14 °C to 10 °C during a 10year operation of a GSHP [10]. In hot-weather areas such as Hong Kong, the borehole wall temperature of the GHE increased on average from 23 °C to 41 °C during a 10-year operation of a conventional GSHP [11]. Increased ground temperature degrades the heat transfer performance of the GHE and thus, decreases the performance of a GSHP in the cooling mode. Therefore, the performances of the GHE and GSHP during long-term operation must be evaluated in transient state. The transient heat transfer characteristics of a GHE and their effects on the transient system performance of the GSHP have been studied by many researchers [12–19]. Eskilson [12] presented a method to predict the temperature distribution of the ground around a GHE during long-term operation. He introduced a transient temperature response of the borehole, referred to as long time step (LTS) g-functions.
122
J.S. Lee et al. / Applied Energy 123 (2014) 121–128
Nomenclature ASHP C COP EEV EFT FFR FFRS GHE GPHE GSHP HE HGSHP HRG HRS IPHE LFT _ m OPHE P_ PHE Q_ SPHE ST
air-source heat pump specific heat at constant pressure (kJ kg1 K1) coefficient of performance electronic expansion valve entering fluid temperature (°C) fluid flow rate (kg min1) fluid flow rate through SPHE (kg min1) ground heat exchanger ground plate heat exchanger ground-source heat pump heat exchanger hybrid ground-source heat pump heat rejection rate of GPHE (W) heat rejection rate of SPHE (W) indoor plate heat exchanger leaving fluid temperature (°C) mass flow rate (kg min1) outdoor plate heat exchanger power (W) plate heat exchanger cooling capacity or heat rejection rate (W) supplementary plate heat exchanger set-point temperature (°C)
In this study, a hybrid ground-source heat pump (HGSHP), as shown in Fig. 1(b), is proposed to overcome thermal imbalance under degraded ground thermal conditions by incorporating the GSHP with supplementary heat rejecters or extractors. The supplementary equipment in the HGSHP controls some portion of the heat rejection or extraction rate of the GHE to reduce the cooling or heating load imposed on the GHE. Therefore, the HGSHP lessens the deterioration of the ground thermal condition, increasing the system performance during long-term operation. However, the HGSHP can be applied when the energy saving potential by the hybrid operation exceeds the additional energy consumption of the supplementary equipment.
T THR
temperature (°C) total heat rejection rate (W)
Greek letter D difference Subscripts amb ambient c cooling comp compressor f fluid G ground heat exchanger heater heater hs heat sink heat exchanger in in out out p pressure P parallel pump pump S serial tot total wb water bath
Most previous studies on the HGSHP were conducted to determine the optimum capacity and control strategy of the supplementary equipment during long-term operation using simulation methods [8,10,11,19–25]. However, there are hardly any experimental studies on the transient performance improvement of the HGSHP under optimized hybrid operation with consideration of degraded ground thermal conditions. The objective of this study is to compare the transient performance characteristics of the GSHP with those of the HGSHP with consideration of degraded ground thermal conditions during long-term operation. A heat storage bath was adopted to simulate the transient characteristics of ground thermal conditions. The transient performances of the
Fig. 1. Schematic diagrams of the GSHP and HGSHP.
123
J.S. Lee et al. / Applied Energy 123 (2014) 121–128
M
25.0 St op
Pu mp
ON
OF HT 2 F
HT 1
Constant temperature bath for heat sink Constant temperature bath for supplementary equipment Pump
Cooling mode Heating mode
25.0 St op
Pu mp
ON
OF HT F 2
T4
HT 1
Thermocouple
P
Pressure transducer
T5 : EFT of SPHE T6 : LFT of SPHE T7 : EFT of IPHE T8 : LFT of IPHE
T1 : LFT of GHE T2 : EFT of GPHE T3 : LFT of GPHE T4 : EFT of GHE
T1
T
T3
Ground loop
M
Heat storage bath
Turbine flow meter
M
Pump
T2
Supplementary loop
T6
EEV
Check valve
T5
T
Pump
EEV driver
T
T
T
GPHE T P
SPHE
Check valve
Primary loop
4-way valve Compressor driver
EEV
Filter dryer
T P
Mass flow meter
T T8
T P
T
T
Turbine flow meter M
T P
IPHE
T7
Indoor loop
25.0 Sto p
Pu mp
HT1
ON
OF F
HT2
Accumulator T
Compressor
Pump
Constant temperature bath for the indoor load
Fig. 2. Schematic diagram of the experimental setup.
GSHP and HGSHP were measured and analyzed under degraded ground thermal conditions.
2. Experimental setup and test procedure 2.1. Experimental setup Fig. 2 shows the schematic diagram of the test setup to measure the transient performances of the HGSHP and GSHP. The HGSHP was composed of a rotary compressor, an indoor plate heat exchanger (IPHE), two outdoor plate heat exchangers (OPHEs), and two electronic expansion valves (EEVs) for cooling and heating modes, respectively. The HGSHP using R-410A had the rated cooling capacity of 7.0 kW at the compressor frequency of 70. The OPHEs consisted of a ground plate heat exchanger (GPHE) of a ground loop and a supplementary plate heat exchanger (SPHE) of a supplementary loop for the hybrid operation mode. Table 1 shows the specifications of the tested HGSHP. The HGSHP had two operation modes, namely GSHP and HGSHP. In the GSHP mode, the supplementary flow loop was not operated even though the refrigerant passed the SPHE. In the HGSHP mode with the operation of the supplementary flow loop, the refrigerant flowed through the SPHE and experienced heat transfer [26]. The operation mode was changed from the GSHP to HGSHP when the ground temperature increased beyond a setpoint temperature. For an actual HGSHP, a boiler can be used as the supplemental equipment in the heating mode, while a cooling tower can be used in the cooling mode. A constant temperature bath for the heat sink in the ground loop and a constant temperature bath in the supplementary loop were adopted to simulate the operating conditions in the GHE and in the supplementary equipment, respectively. The constant temperature bath in the indoor loop was used to impose the cooling or heating loads. Water was selected as working fluid for the secondary fluid loops, in which an inverter-driven pump and a manual-type needle valve to control the fluid flow rate (FFR) supplied to the plate heat exchangers (PHEs) were included.
A data acquisition system monitored temperatures, pressures, mass flow rate, volumetric flow rates, and powers in transient state. A mass flow meter was installed between the OPHEs and EEVs to measure the refrigerant mass flow rate. Three volumetric flow meters were installed for measuring FFRs in the secondary fluid flow loops, respectively. The specifications and uncertainties of the sensors are presented in Table 2.
2.2. Heat storage bath for the GHE The heat storage bath, as shown in Fig. 3(a), was composed of a water bath, a tube-type heat exchanger for the GHE, and another tube-type heat exchanger for the heat sink connected with a constant temperature bath. Table 3 shows the specifications of the heat storage bath for the GHE. The U-tube in the in situ GHE was simulated by the heat exchanger for the GHE. The grout and soil in the in situ GHE were simulated by the water bath and the heat sink of the heat storage bath, respectively. The water bath
Table 1 Specifications of the HGSHP and fluid pump. Components
Specifications
Compressor
Type Refrigerant Frequency Rated input power (at 60 Hz) Rated capacity (at 60 Hz)
BLDC rotary R-410A 30–110 1818
Hz W
5881
W
Heat exchanger (outdoor and indoor)
Type Size Volume (path 1/path 2)
Plate 95 85 325 640/670
mm cm3/cm3
Expansion valve
Type Rated capacity Range
EEV 2.2 0–500
RT Pulse
Type Rated flow rate Rated head
Centrifugal 3 27
m3 h1 m
Fluid pump
Unit
124
J.S. Lee et al. / Applied Energy 123 (2014) 121–128
Table 2 Specifications and uncertainties of the sensors.
Table 3 Specifications of the heat storage bath.
Item
Specifications
Power meter
Range Accuracy
0–300 V/0–10 A ±0.01% 1
Mass flow meter
Range Accuracy
0–360 kg h ±0.1% of full scale
Pressure transducer
Range Accuracy
0–5.0 MPa ±0.1% of full scale
Thermocouple
Range Accuracy
200 to 200 °C ±0.2 °C
Turbine flow meter
Range Accuracy
0.36–3.96 m3 h1 ±0.5% of full scale
dT wb _ f ;G C p;f ;G ðT f ;G;in T f ;G;out Þ ¼m dt _ f ;hs C p;f ;hs ðT f ;hs;out T f ;hs;in Þ m
Specifications
Water bath
Shape Size
Rectangular section 60 60 60
mm3
Type
9 Rows, 4 Steps, 4 Paths Serpentine shape 12.7 19.20
mm m
5 Rows, 5 Steps, 5 Paths Serpentine shape 15.9 12.75
mm m
HE for GHE
Outer diameter Length HE for heat sink
temperature, which indicates the ground temperature around the U-tube, was calculated by averaging 5-point temperatures of the T-type thermocouples serially located near the wall of the water bath, because the water bath temperature was stratified from the bottom to the top position. The heat accumulation rate in the water bath was determined by the difference between the heat rejection rate of the GHE and the heat removal rate of the heat sink. Since the capacity of the heat storage bath is significantly smaller than that of the ground source in a true GSHP, the operation-period in the simulated system stands for accelerated time. Based on the energy balance within the heat storage bath assuming the perfectly insulated and well-mixed water bath, the governing equation was derived as,
ð1Þ
where q and Cp denote the density and specific heat at constant pressure, respectively; V and Twb are the volume and temperature _ is the water mass flow rate through the heat of the water bath; m exchanger; TG and Ths mean the fluid temperatures within the heat exchangers for the GHE and the heat sink, respectively. As shown in Fig. 3(a), a thermal response test of the heat storage bath was conducted to determine the capacity of the heat sink. The electric heater was inserted in the ground loop to impose the
Type Outer diameter Length
Unit
cooling load of the GHE. The governing equation of the heat storage bath for the thermal response test can be simplified as,
qwb V wb C p;wb
dT wb _ f ;hs C p;f ;hs ðT f ;hs;out T f ;hs;in Þ ¼ P_ heater m dt
2.3. Test procedure The transient tests of the GSHP and HGSHP in the cooling mode were started respectively at the initial ground temperature, and the performance degradations with operation time were measured. As the ground temperature increased beyond the limit, the operation mode was changed from the GSHP to HGSHP to prevent
Constant temperature bath for heat sink
M
25.0 St op
Pu mp
HT 1
ON
OF F
HT 2
EFT of heat sink
Qhs Ground loop
HE for heat sink
o
T
Qamb
T T
T Tin
Pump T
T
HE for GHE QG
Water
Heat storage bath
Electric heater
T
T Tout
T
Pump
40 38 Tin of thermal response test 36 34 Tout of in-situ GHE 32 30 Tout of thermal response test 28 26 24 22 20 18 16 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Time (h) Power supplier
ð2Þ
The inlet (Tin) and outlet temperature (Tout) of the fluid of the GHE were measured for 6 h from the start to the end at 1-s intervals. The heater power was determined as 4.5 kW. The entering fluid temperature (EFT) and the FFR of the heat sink heat exchanger were controlled to satisfy the temperature profile of the thermal response test of the in situ GHE [27], as shown in Fig. 3(b). Table 4 shows the ground thermal condition and specifications of the GHE in the thermal response test of the in situ GHE. When the EFT and FFR of the heat sink were 3 °C and 28 kg min1, respectively, Tout of the thermal response test of the heat storage bath was very similar to that of the in situ site, which showed a maximum temperature difference of less than 1 °C in the whole operating range. Therefore, the EFT and the FFR of the heat sink were determined as 3 °C and 28 kg min1, respectively.
Fluid temperature ( C)
qwb V wb C p;wb
Components
(b) Thermal response
(a) Schematic diagram Fig. 3. Schematic diagram and thermal response of the heat storage bath.
125
J.S. Lee et al. / Applied Energy 123 (2014) 121–128 Table 4 Ground thermal condition and specifications of the GHE in the thermal response test. Parameters
Specifications
Unit
Number of boreholes Borehole depth Borehole radius Soil specific heat Soil density Soil thermal conductivity Soil heat capacity Soil diffusivity Grout specific heat Grout density Grout thermal conductivity Pipe radius Pipe thermal resistance Fluid specific heat Fluid density Average air temperature
1 150 0.076 0.909 2640 11.088 2399.76 0.146 3.5454 1100 2.916 0.016 0.41 4.19 1000 12.9
m m kJ kg1 K1 kg m3 kJ h1 m1 K1 kJ m3 K1 m2 day1 kJ kg1 K1 kg m3 kJ h1 m1 K1 m K W1 kJ kg1 K1 kg m3 °C
severe performance degradation. The set-point temperature (ST) indicates the leaving fluid temperature (LFT) of the GHE when the operation mode changes from the GSHP to HGSHP. Table 5 shows the test conditions for the HGSHP as well as the GSHP. The initial temperature of the water bath and the LFT of the GHE were 13.5 °C and 15.5 °C, respectively. The EEV was controlled to obtain the superheat of 5 °C at the compressor inlet. The FFR of the supplementary loop was changed from 4 to 16 kg min1 in 4 kg min1 increments. The EFT of the SPHE was fixed at 30 °C based on the typical operating condition presented in the ASHRAE Handbook [28]. The set-point temperature of the LFT of the GHE in the HGSHP was changed from 27.5 °C to 40 °C in 2.5 °C increments. The FFRs of the GPHE and IPHE were fixed at the optimum value of 20 kg min1, which yielded the maximum COP with the variation of FFR under the standard test condition for the GSHP [29]. The measured data in transient state were recorded continuously for 60 min in 1-s intervals. As shown in Eq. (3), cooling capacity was determined with the FFR and the temperature difference across the IPHE. The heat rejection rates in the GPHE (HRG) and SPHE (HRS) were also calculated using Eq. (3). The total heat rejection rate (THR) of the HGSHP was calculated by summing the HRG and HRS. The total power and cooling COP were determined using Eqs. (4) and (5), respectively.
_ f C f DT f Q_ c ¼ m
ð3Þ
P_ tot ¼ P_ comp þ P_ pump
ð4Þ
COP ¼
Q_ c _Ptot
ð5Þ
Uncertainty analysis was performed for the cooling capacity and cooling COP based on ANSI/ASME PTC 19.1 [30]. The uncertainties of the cooling capacity and COP were estimated as ±3.72% and ±3.73%, respectively. 3. Results and discussion 3.1. Comparison of transient performance between the HGSHP and GSHP Transient performance tests of the GSHP and HGSHP were conducted after controlling the thermal response of the heat storage bath corresponding to the transient thermal response of the borehole in the in situ GHE [27]. Fig. 4 shows the increase in the water bath temperature according to operation time due to the heat accumulation by the difference between the heat rejection rate of the GHE and the heat removal rate of the heat sink. The water bath temperature reached approximately 26 °C after 60 min of operation in the GSHP mode with the initial temperature of 13.5 °C. The higher water bath temperature meant more deteriorated ground thermal condition of the GSHP. At the initial stage, the water bath temperatures of the GSHP and HGSHP were nearly the same because the HGSHP operated in the GSHP mode before the hybrid operation. However, when the LFT of the GPHE increased beyond the set-point temperature of 35 °C (at approximately 19 min), the water bath temperature of the HGSHP decreased gradually compared with that of the GSHP because the HGSHP rejected heat into both the ground loop through the GPHE and the supplementary loop through the SPHE. Therefore, the heat accumulation rate in the water bath of the HGSHP was lower than that of the GSHP after the hybrid operation. After 60 min operation, the water bath temperature of the HGSHP decreased by 4.3%, compared with that of the GSHP, so the degradation of ground thermal environment can be delayed by the HGSHP operation. At all FFRs through the SPHE, the water bath temperature of the HGSHP decreased by 3.3% on average, compared with that of the GSHP during the hybrid operation. It should be noted that the long-term operation of the in situ GSHP was accelerated in this transient test to speed up degradation of ground thermal conditions. Fig. 5 shows the heat rejection rates of the GSHP and HGSHP according to operation time. The HRGs in the GSHP and HGSHP increased rapidly with operation time, and then reached to their maximum values at 15 min. After the peak point, the HRG of the GSHP operated nearly constantly at the maximum value with slight fluctuations, while the HRG of the HGSHP started to decrease gradually due to the partial heat rejection into the supplementary loop under hybrid operation. At FFR of 8 kg min1 through the SPHE, the HRG of the HGSHP decreased by 7.1%, compared with
Table 5 Test conditions for the GSHP and HGSHP. Parameters
Value
Compressor frequency (Hz) EEV opening Set-point temperature of LFT of the GHE (°C)
70 Controlled to obtain 5 °C superheat – 27.5, 30, 32.5, 35(baseline), 37.5, 40 30 12 20 (Optimized value) 4, 8, 12(baseline), 16 20 (Optimized value) 3 28 13.5 15.5 60
EFT of the SPHE (°C) Fluid temperature entering the indoor heat exchanger (°C) Mass flow rate in the ground loop (kg min1) Mass flow rate in the supplementary loop (kg min1) Mass flow rate in the indoor heat exchanger (kg min1) Fluid temperature entering the HE for the heat sink (°C) Mass flow rate entering the HE for the heat sink (kg min1) Initial temperature Operation time (min)
GSHP mode HGSHP mode-Parallel
Water bath temperature (°C) LFT of the GHE (°C)
J.S. Lee et al. / Applied Energy 123 (2014) 121–128
9000
HGSHP
240
24
200
22 160
20
120
18 16
80 Hybrid operation of HGSHP
14
40
o
ST = 35 C
12 10
FFR of SPHEX = 8 kg min
0
5
-1
0
Capacity of GSHP Capacity of HGSHP
8500
Cooling capacity (W)
26
Heat accumulation rate (kW)
Water bath temperature (oC)
GSHP
1900
7500
1800
7000
1700 1600
6500
1500
Hybrid operation of HGSHP
6000
ST = 35 C -1 FFR of SPHEX = 8 kg min
5500 5000
0
5
10 15 20 25 30 35 40 45 50 55 60
loops. Upon the start of the hybrid operation, the total power of the HGSHP became instantly higher than that of the GSHP due to the additional power of the pump in the supplementary loop. However, after several minutes into the hybrid operation, the total power of the HGSHP decreased gradually compared with that of the GSHP due to the decreased compressor power. During the hybrid operation period, the average total power of the HGSHP decreased by 1.4% compared with that of the GSHP at FFR of 8 kg min1 through the SPHE. Besides, the COP of the GSHP decreased gradually with operation time due to the deterioration of the ground thermal condition. The COP of the HGSHP started to increase rapidly upon start of the hybrid operation because of the increased cooling capacity. Therefore, during the hybrid operation period, the average COP of the HGSHP was 10.4% higher than that of the GSHP at FFR of 8 kg min1 through the SPHE, mainly due to the increased average cooling capacity by 8.9%. Fig. 8 shows the average performances of GSHP and HGSHP during the whole operation time (60 min) according to the FFR through the SPHE. The FFR of 0 kg min1 through the SPHE indicates the GSHP operation mode. As the FFR through the SPHE increased from 0 to 4 kg min1, the average cooling capacity of the HGSHP increased rapidly and the average compressor power decreased slightly, resulting in the rapid increase of the COP. As the FFR through the SPHE increased from 4 to 16 kg min1, the average cooling capacity of the HGSHP increased and peaked at FFR of 8 kg min1. The average total power of the HGSHP increased slightly with FFR due to the increase in the additional power of the pump in the supplementary loop. At FFR of 16 kg min1 through the SPHE, the average total power of the HGSHP was
COP of GSHP COP of HGSHP
ST = 35 C
Total power of GSHP Total power of HGSHP
4000
-1
3000
HRG of GSHP
2000
HRS of GSHP HRS of HGSHP Total heat rejection rate of HGSHP
HRG of HGSHP
Hybrid operation of HGSHP 4.5
o
ST = 35 C -1 FFR of SPHE = 8 kg min
4.0
0
1900 1800 1700 1600
3.5
1500
1000
1400
3.0
0
2200
2000
o
FFR of SPHE = 8 kg min
2300
2100
5.0
COP
Heat rejection rate (W)
5.5
7000
5000
1200
Fig. 6. Variations of the cooling capacity and compressor power with time.
6.0
Hybrid operation of HGSHP
1300
Time (min)
8000
6000
1400
o
Time (min)
that of the GSHP. In addition, as the hybrid operation started, the HRS in the HGSHP increased rapidly. Therefore, during the hybrid operation period, the THR of the HGSHP was on average 8.5% higher than that of the GSHP. In the cooling mode, the HGSHP was more effective than the GSHP in rejecting heat from the heat pump unit. Fig. 6 shows the cooling capacities and compressor powers of the GSHP and HGSHP according to operation time. The cooling capacity of the GSHP rapidly increased at the initial stage, and then decreased gradually due to the deterioration of the ground thermal condition according to operation time. The cooling capacity fluctuated because the EEV was controlled to satisfy 5 °C superheat condition. The compressor power of the GSHP increased gradually due to the increase in the discharge pressure of the compressor under degraded thermal conditions of the water bath. In the HGSHP, as the hybrid operation mode started at the set-point temperature of 35 °C (approximately 19 min from the start), the cooling capacity of the HGSHP increased rapidly, exceeding that of the GSHP, due to the partial heat rejection through the supplementary loop (SPHE). Besides, the compressor power of the HGSHP decreased substantially compared with that of the GSHP during the hybrid operation period because of the decreased condensing pressure. During the hybrid operation period, the average cooling capacity of the HGSHP increased by 8.9%, and the average compressor power decreased by 4.4%, compared with that of the GSHP at FFR of 8 kg min1 through the SPHE. Fig. 7 shows the COPs and total powers of the GSHP and HGSHP according to operation time. Total power includes the compressor power and the pump powers of the ground and supplementary
2000
8000
10 15 20 25 30 35 40 45 50 55 60
Fig. 4. Variations of the water bath temperature and heat accumulation rate with time.
2100
Comp. power of GSHP Comp. power of HGSHP
1300 5
10 15 20 25 30 35 40 45 50 55 60
Time (min) Fig. 5. Variation of the heat rejection rate with time.
Compressor power (W)
280
28
0
5
10 15 20 25 30 35 40 45 50 55 60
Time (min) Fig. 7. Variations of the total power and COP with time.
Total power (W)
126
127
J.S. Lee et al. / Applied Energy 123 (2014) 121–128 7300
o
ST = 35 C 7200 7100
Average capacity in transient Average COP in transient Average total power in transient Average comp. power in transient COP in steady [9]
3.5 3.4
7000 6900 6800 2000 1900
3.3
1800
3.2
1700
0
4
8
12
16 -1
Fluid flow rate of SPHE (kg min ) Fig. 8. Variations of the average performances of the HGSHP with the fluid flow rate of SPHE.
higher than that of the GSHP. Therefore, the average COP of the HGSHP peaked at FFR of 8 kg min1, which was determined as the optimum FFR through the SPHE. At the optimum FFR and set-point temperature of 35 °C, the average COP of the HGSHP increased by 6.9%, compared with that of the GSHP. The optimum FFR through the SPHE in the transient state was the same as the result of the steady state test carried out for the fixed ground condition [9]. 3.2. Effects of the set-point temperature on the performance of the HGSHP Fig. 9 shows the compressor power and total power of the HGSHP according to the set-point temperature (ST) under hybrid operation at FFR of 12 kg min1 through the SPHE. As the set-point temperature increased from 30 °C to 40 °C, the initiation time of the hybrid operation in the HGSHP increased from 8 min to 55 min, which indicated that the hybrid operation of the HGSHP started at a more degraded ground thermal condition. At all setpoint temperatures during the hybrid operation, the compressor power of the HGSHP decreased rapidly and then became lower than that of the GSHP. This decrease was due to the decreased discharge pressure from the reduced temperature of the water bath by the hybrid operation, which meant the delay in the deterioration of ground thermal condition. For the entire duration of the operation at FFR of 12 kg min1 through the SPHE, the average compressor powers of the HGSHP at set-point temperatures of 30 °C and 40 °C decreased by 3.6% and 1.2%, respectively, compared
2400
8500
o
Hybrid operation at ST=30 C o Hybrid operation at ST=35 C
2250
Power (W)
2100 1950 1800 Hybrid operation o at ST=40 C
1650 GSHP
1500
o
HGSHP, ST=30 C
Total power
1350
o
Comp. power
1200
o
HGSHP, ST=35 C
5
o
Time (min) Fig. 9. Variations of the compressor power and total power of the HGSHP with the set-point temperature in the hybrid operation.
o
HGSHP, ST=40 C Hybrid operation o at ST=40 C
5.5 5.0
7000 4.5
6500
Hybrid o operation at ST=35 C
6000 5500
4.0 3.5
Hybrid o operation at ST=30 C
5000
HGSHP, ST=40 C
10 15 20 25 30 35 40 45 50 55 60
6.0
o
HGSHP, ST=30 C
HGSHP, ST=35 C ,
7500
4500 0
GSHP ,
8000
Cooling capacity (W)
COP
3.6
Capacity, Power (W)
3.7
with those of the GSHP. During the stable hybrid operation period, the total power of the HGSHP decreased slightly compared with that of the GSHP at all set-point temperatures due to the additional power of the pump in the supplementary loop. Therefore, the decrease in the average total power of the HGSHP below that of the GSHP was very small, less than 0.4% at FFR of 12 kg min1 through the SPHE. Fig. 10 shows the cooling capacity and COP of the HGSHP according to the set-point temperature of the hybrid operation at FFR of 12 kg min1 through the SPHE. At all set-point temperatures, the cooling capacity of the HGSHP increased rapidly upon start of the hybrid operation due to the additional heat rejection through the supplementary loop. During the hybrid operation period, the cooling capacity of the HGSHP remained nearly constant with slight fluctuations due to the decrease in the heat rejection to the ground. The average cooling capacities of the HGSHP at set-point temperatures of 30 °C, 35 °C, and 40 °C increased by 6.4%, 5.2%, and 0.2%, respectively, compared with that of the GSHP at FFR of 12 kg min1 through the SPHE. At the set-point temperature of 30 °C, the COP degradation of the HGSHP decreased gradually with operation time due to the higher cooling capacity of the HGSHP than the GSHP. As the set-point temperature increased beyond 35 °C, the COP of the HGSHP increased instantly over that of the GSHP just after the start of the hybrid operation due to the rapid reduction of the heat rejection rate in the ground. The average COPs of the HGSHP at set-point temperatures of 30 °C, 35 °C, and 40 °C increased by 6.0%, 5.0%, and 0.8%, respectively, compared with those of the GSHP at FFR of 12 kg min1 through the SPHE. Fig. 11 shows the average performances of the HGSHP for the entire duration of the operation (60 min) according to the set-point temperature at FFRs of 8 kg min1 and 12 kg min1 through the SPHE, respectively. The higher set-point temperature indicated a more delayed hybrid operation. As the set-point temperature increased from 27.5 °C to 40.0 °C at FFR of 12 kg min1, the average cooling capacity of the HGSHP peaked at the set-point temperature of 30 °C. As the set-point temperature increased from 32.5 °C to 40 °C at FFR of 12 kg min1, the average cooling capacity of the HGSHP decreased by 6.1% due to the decreased cooling effect from the delayed hybrid operation, while the average total power decreased by 0.6%. Therefore, the average COP of the HGSHP showed a trend similar to that of the average cooling capacity. At FFR of 8 kg min1 for set-point temperatures from 27.5 °C to 35 °C, the average COP was slightly higher than that at FFR of 12 kg min1. However, the average COP decreased more rapidly when the setpoint temperature increased beyond 35 °C due to the delayed hybrid operation of the HGSHP. At FFR of 8 kg min1, the HGSHP gave the maximum average COP at the set-point temperature of 30 °C.
COP
3.8
0
5
3.0
10 15 20 25 30 35 40 45 50 55 60
Time (min) Fig. 10. Variations of the cooling capacity and COP of the HGSHP with the set-point temperature in the hybrid operation.
128
J.S. Lee et al. / Applied Energy 123 (2014) 121–128
3.8
7200
COP
3.6
7100 7000
3.5
6900 Average capacity, FFRS = 12 kg min -1
3.4
6800
Average total power,FFRS = 12 kg min-1
3.3
Average COP, FFRS = 12 kg min -1
2000
Average COP, FFRS = 8 kg min -1
1975
3.2
Capacity, Power (W)
7300
3.7
3.1
References
7400
1950 27.5
30.0
32.5
35.0
37.5
40.0
1925
o
Set-point temperature of hybrid operation ( C) Fig. 11. Variations of the average performances of the HGSHP with the set-point temperature in the hybrid operation.
Therefore, the optimum set-point temperature of the HGSHP was determined as 30 °C. At the optimum set-point temperature of 30 °C and the optimum FFR of 8 kg min1, the maximum average COP of the HGSHP was 3.76, which was 7.2% higher than that of the GSHP.
4. Conclusions In this study, the transient performance characteristics of the HGSHP were tested in the cooling mode by changing the FFR through the SPHE and the set-point temperature of the hybrid operation. The optimum set-point temperature and optimum FFR of the SPHE were determined under the standard design condition for the cooling tower [28]. A heat storage bath for the GHE was used to simulate degradation of the ground thermal condition with operation time. During the hybrid operation period up to 60 min, the ground temperature condition in the HGSHP decreased by 3.3%, compared with that in the GSHP. The average capacity and COP of the HGSHP peaked at FFR of 8 kg min1 through the SPHE, which was determined as the optimum FFR. The performance enhancement of the HGSHP over the GSHP was highly dependent on the set-point temperature. The average cooling capacity and COP of the HGSHP peaked at the set-point temperature of 30 °C regardless of the FFR through SPHE. Therefore, the optimum setpoint temperature of the HGSHP was determined as 30 °C. At the optimum FFR of 8 kg min1 and the optimum set-point temperature of 30 °C, the average COP of the HGSHP during 60 min operation was 3.76, which was 7.2% higher than that of the GSHP. As the set-point temperature increased beyond 35 °C, the average COP for 60 min operation decreased rapidly due to the delayed hybrid operation of the HGSHP.
Acknowledgments This work was supported by the Human Resources Development Program (No. 20124010203250) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea Government Ministry of Trade, Industry and Energy.
[1] Nam Y, Ooka R, Shiba Y. Development of dual-source hybrid heat pump system using ground water and air. Energy Build 2010;42:909–16. [2] Sanner B. Shallow geothermal energy, Geo-heat center bulletin. Geo-Heat Center: Klamath Falls, OR; 2001. [3] Florides G, Kalogirou S. Ground heat exchangers – a review of systems, models and applications. Renew Energy 2007;32(15):2461–78. [4] Michopoulos A, Bozis D, Kikidis P, Papakostas K, Kyriakis NA. Three-years operation experience of a ground source heat pump system in Northern Greece. Energy Build 2007;39(3):328–34. [5] Wang H, Qi C. Performance study of underground thermal storage in a solar ground coupled heat pump system for residential buildings. Energy Build 2008;40(7):1278–86. [6] Yang H, Cui P, Fang Z. Vertical-borehole ground-coupled heat pumps: a review of models and systems. Appl Energy 2010;87:16–27. [7] Self SJ, Reddy BV, Rosen MA. Geothermal heat pump systems: Status review and comparison with other heating options. Appl Energy 2013;101:341–8. [8] Yavuzturk C, Spitler JD. Comparative study to investigate operating and control strategies for hybrid ground source heat pump systems using a short time-step simulation model. ASHRAE Trans 2000;106(2):192–209. [9] Park H, Lee JS, Kim W, Kim Y. Performance optimization of a hybrid ground source heat pump with the parallel configuration of ground heat exchanger and a supplemental heat rejecter. Int J Refrig 2012;35:1537–46. [10] Chen X, Lu L, Yang H. Long term operation of a solar assisted ground coupled heat pump system for space heating and domestic hot water. Energy Build 2011;43:1835–44. [11] Man Y, Yang H, Spitler JD, Fang Z. Feasibility study on novel hybrid ground coupled heat pump system with nocturnal cooling radiator for cooling load dominated buildings. Appl Energy 2011;88:4160–71. [12] Eskilson P. Thermal analysis of heat extraction boreholes. Ph.D. Thesis. Sweden:University of Lund; 1987. [13] Yavuzturk C, Spitler JD, Rees SJ. A Transient two-dimensional finite volume model for the simulation of vertical U-tube ground heat exchangers. ASHRAE Trans 1999;105(A):465–74. [14] Hellstrom G. Duct ground heat storage model manual for computer code. Sweden: Department of Mathematical Physics, University of Lund; 1989. [15] Lamarche L, Beauchamp B. New solutions for the short-time analysis of geothermal vertical boreholes. Int J Heat Mass Transfer 2007;50:1408–19. [16] Bandyopadhyay G, Gosnold W, Mann M. Analytical and semi-analytical solutions for short-time transient response of ground heat exchangers. Energy Build 2008;40:1816–24. [17] Bauera D, Heidemanna W, Dierschb HJG. Transient 3D analysis of borehole heat exchanger modeling. Geothermics 2011;40:250–60. [18] Choi JC, Lee SR, Lee DS. Numerical simulation of vertical ground heat exchangers: Intermittent operation in unsaturated soil conditions. Comput Geotechn 2011;38:949–58. [19] Yavuzturk C. Modeling of vertical ground loop heat exchangers for ground source heat pump systems. Doctoral thesis, Oklahoma State University: Stillwater, Oklahoma; 1988. [20] Sagia Z, Rakopoulos C, Kakaras E. Cooling dominated hybrid ground source heat pump system application. Appl Energy 2012;94:41–7. [21] Chen X, Yang H. Performance analysis of a proposed solar assisted ground coupled heat pump system. Appl Energy 2012;97:888–96. [22] Xu X. Simulation and optimal control of hybrid ground source heat pump systems. Doctoral Thesis, Oklahoma State University: Stillwater, Oklahoma; 2007. [23] Hackel S, Nellis GN, Klein SK. Optimization of cooling-dominated hybrid ground-coupled heat pump systems. ASHRAE Trans 2009;115(1):565–80. [24] Man Y, Yang H, Wang J. Study on hybrid ground-coupled heat pump system for air-conditioning in hot-weather areas like Hong Kong. Appl Energy 2010;87:2826–33. [25] Chiasson AD. Simulation and design of hybrid geothermal heat pump systems. Doctoral thesis, University of Wyoming: WY; 2007. [26] Park H, Lee JS, Kim W, Kim Y. The cooling seasonal performance factor of a hybrid ground-source heat pump with parallel and serial configurations. Appl Energy 2013;102:877–84. [27] Kim W, Park H, Kim Y. Development of an electric circuit transient analogy model in a vertical closed loop ground heat exchanger. Kor J Air-Cond Refrig 2012;24(4):306–14. [28] ASHRAE. ASHRAE Handbook-HVAC systems and equipment. Atlanta: American Society of Heating Refrigerating and Air-conditioning Engineers; 2008 [Chapter 39]. [29] ISO 13256-2. Water-source heat pump testing and rating for performance: Part 2: Water-to-water and brine-to-water heat pumps. International Standards Organization; 1998. [30] ANSI/ASME PTC 19.1. Measurement uncertainty. NY: American Society of Mechanical Engineers; 1985.