The Effect of Coupling Solar Thermal System and Geothermal Heat Pump Systems in Areas with Unbalanced Heating and Cooling Demand

Home , Ground source heat pump, Heat pump, Solar thermal collector

Article The Effect of Coupling Solar Thermal System and Geothermal Heat Pump Systems in Areas with Unbalanced Heating and Cooling Demand

Jihyun Hwang 1,2, Doosam Song 3,* and Taewon Lee 2,*

1 Department of Construction Environmental System Engineering, Sungkyunkwan University, Suwon 16419, Korea; [email protected] 2 Department of Fire Safety Research, Korea Institute of Civil Engineering and Building Technology, Goyang 10223, Korea 3 School of Civil, Architectural Engineering, and Landscape Architecture, Sungkyunkwan University, Suwon 16419, Korea * Correspondence: [email protected] (D.S.); [email protected] (T.L.)

Abstract: Geothermal source heat pump (GSHP) systems as renewable energy systems are being more frequently installed as part of the zero-energy building drive. However, in South Korea, where a large amount of heating load can be required, maintaining high system performance by using only a GSHP is difﬁcult owing to the gradual degradation of its thermal performance. The performance of a solar-assisted GSHP system was therefore experimentally analyzed and compared with a GSHP-only system. The results showed that the heating coefﬁcient of performance of the GSHP-only operation was 5.4, while that of the solar-assisted GSHP operation was 7.0. In the case of the GSHP-only system, the maximum temperature of the heat pump water supply on the heat source side was initially 13.1 ◦C, but this rapidly decreased to 11.4 ◦C during operation. For the solar-assisted GSHP system, the temperature of the water supply to the heat source side of the heat pump was controlled at 15–20.9 ◦C, and the power consumption for system operation was reduced by about 20% compared with that for the GSHP-only system. Much higher temperatures could be supplied when solar heat is used instead of ground heat, as solar heat contributes to the performance improvement of the heat Citation: Hwang, J.; Song, D.; Lee, T. pump system. The Effect of Coupling Solar Thermal System and Geothermal Heat Pump Keywords: geothermal source heat pump; solar thermal system; heating; coupling; performance Systems in Areas with Unbalanced analysis Heating and Cooling Demand. Energies 2021, 14, 31. https://dx.doi.org/ 10.3390/en14010031

Received: 17 November 2020 1. Introduction Accepted: 21 December 2020 There is a global trend toward zero-energy buildings in order to minimize buildings’ Published: 23 December 2020 energy consumption and reduce carbon dioxide emissions. As of 2020, the South Korean government has mandated the construction of new buildings with a total floor area of Publisher’s Note: MDPI stays neu- 1000 m2 or more as zero-energy buildings. Zero-energy buildings are buildings that waste less tral with regard to jurisdictional claims energy by having improved insulation and air-tightness, and minimize energy consump- in published maps and institutional tion through energy production using renewable energies, such as solar and geothermal afﬁliations. energies. It is envisaged that by 2030, all newly constructed buildings larger than 500 m2 will be zero-energy buildings, thus emphasizing the importance of new and renewable energy sources.

Copyright: © 2020 by the authors. Li- Solar and geothermal energies are renewable and can signiﬁcantly contribute to censee MDPI, Basel, Switzerland. This achieving zero-energy buildings by generating energy on-site or off-site. Since 2004, article is an open access article distributed the South Korean government has mandated the installation of renewable energy systems, under the terms and conditions of the whereby a part of a building’s energy demand in terms of new construction, extension, or 2 Creative Commons Attribution (CC BY) renovation of public buildings of equal to or larger than 1000 m is realized by renewable license (https://creativecommons.org/ energy; this percentage increased from 10% in 2011 to 30% in 2020. In South Korea, licenses/by/4.0/). the installation of a geothermal heat pump system as a renewable energy system [1].

Energies 2021, 14, 31. https://dx.doi.org/10.3390/en14010031 https://www.mdpi.com/journal/energies Energies 2021, 14, 31 2 of 15

Geothermal systems, ground heat source systems, or ground-coupled heat pump (GCHP) systems are widely used in the USA, central and northern Europe, and northwestern China for space heating [2,3]. In addition, the application of GCHPs has increased in multi-residential building projects in South Korea [4]. Geothermal source heat pumps (GSHPs) provide heating and cooling by exchanging heat with the soil. The advantage of such a system is that soil temperature remains constant across time. The performance of GSHPs is high in the case of Toronto, Canada, where the heating and cooling loads are balanced. However, in South Korea and Edmonton, Canada, the annual heating demand is larger than the cooling demand, and thus the heating performance of GSHPs decreases with time owing to soil thermal imbalances [5]. Furthermore, room temperatures go below the set point and the GSHP does not function normally for years of operation [6]. Conventional studies show that the average ground temperature could decline by 3–12 ◦C, and then the coefficient of performance could decline by approximately 0.50–2.20 [7–13]. Therefore, the thermal imbalance of GSHPs in cold regions must be addressed. Qian and Wang [8] proposed ground heat exchanger (GHE)-modified solutions, such as by increasing borehole space. A larger borehole space indicates a larger soil heat- retaining ability [14]. System-modified solutions combine assistance energy with a conventional GSHP to enhance the thermal input to the ground or reduce the thermal output from the ground. The integration of gas boilers can achieve economic and energy savings over conventional GSHPs [15]. Furthermore, the coupling of solar thermal energy with GSHPs can enhance the heating performance by approximately 3.75 [16]. To help look for solutions to problems of inefficiency with GSHPs, in this study, the introduction of Section1 described the background, necessity, domestic and international research trends, and structure of this manuscript. The experimental method was described in Section2. Section3 was analyzed a solar-assisted GSHP operating in the winter season in South Korea through field measurements. Section4 summarizes the results of the analysis through field measurements.

2. Method 2.1. Overview of the Target Building The analyzed building, located in Hwaseong City, South Korea, was completed in 2006. Table1 lists key data of the analyzed building, which comprises steel frames and glass-wool composite panel (100 mm) wall structures. The building has two offices on the second floor, and the third and fourth floors were used for conducting the experiments. The total area of the building is 1725 m2, with total heating and cooling loads being 236.2 and 283.7 kWh, respectively. The heating and cooling heat source was a GSHP, the performance of which deteriorated over time.

Table 1. Key data of the analyzed building.

Area Cooling Load Heating Load Operation Zone (m2) (kWh) (kWh) Time (h) 1 Ofﬁce (1) 470 111.0 131.5 8 2F 2 Ofﬁce (2) 366 86.4 102.4 8 3 Laboratory 510.8 38.8 49.8 Intermittent 3F, 4F 4 Hallway 378.2 — — — Total 1725 236.2 283.7

2.2. System Conﬁgurations Figure1 illustrates a schematic of the solar-assisted GSHP system analyzed in this study. It consists of a geothermal source heat pump system, GHE, and solar thermal collecting system comprising solar collectors, high- and low-temperature heat storage tanks, and circulation pumps. The heating medium (a mixture of water and antifreeze), Energies 2021, 14, x FOR PEER REVIEW 3 of 15

2.2. System Configurations Figure 1 illustrates a schematic of the solar-assisted GSHP system analyzed in this Energies 2021, 14, 31 3 of 15 study. It consists of a geothermal source heat pump system, GHE, and solar thermal collecting system comprising solar collectors, high- and low-temperature heat storage tanks, and circulation pumps. The heating medium (a mixture of water and antifreeze), heat storageheat storage medium medium (water), (water), and GHE and GHEheating heating medium medium (a mixture (a mixture of water of water and and antifreeze) antifreeze) areare separated separated through through four four sets sets of of plate plate heat heat exchangers. exchangers.

FigureFigure 1. 1.SchematicSchematic of ofsolar-assisted solar-assisted geothermal geothermal heat heat pump pump system. system.

TableTable 22 lists lists the the specifications speciﬁcations of of the the GSHP GSHP system system installed installed in in the the analyzed analyzed building. building. FourFour water-to-water water-to-water heat heat pumps areare installed,installed, with with heating heating capacities capacities of 47.5of 47.5 and and 110.1 110.1 kWh. kWh.The The total total cooling cooling and and heating heating capacities capacities of the of fourthe four heat heat pumps pumps were were 262.4 262.4 and and 315.1 315.1 kWh, kWh,respectively. respectively. Soil wasSoil used was asused the as geothermal the geothermal source. source. A closed-loop A closed-loop vertical GHEvertical was GHE used, wasand used, 29 boreholes and 29 boreholes with diameters with diameters of 150 mm of were 150 installed mm were at installed a vertical at depth a vertical of 150 depth m. The oftotal 150 m. length The oftotal the length GHE was of the 4350 GHE m, andwas the4350 temperature m, and the oftemperature the hot water of the supply hot water during supplyheating during was setheating to 45–48 was◦ C.set to 45–48 °C.

Table 2. SpeciﬁcationsTable 2. Specifications of the geothermal of the source geothermal heat pump source system. heat pump system.

Geothermal Source PowerPower Consumption Consumption QuantityGeothermal Cooling Source CapacityQuantity HeatingCooling CapacityHeating Heat Pump System (kW)(kW) (EA)Heat Pump System(kWh) (EA) Capacity(kWh) Capacity (Water-to-Water) (Water-to-Water) (kWh) (kWh) CoolingCooling Heating Heating Heat Pump-1, 2 2 39.1 47.5 10.1 14.0 Heat Pump-1, 2 2 39.1 47.5 10.1 14.0 Heat Pump-3, 4 2 Heat Pump-3, 92.14 2 92.1 110.1 110.1 25.2 25.2 34.9 34.9

Table 3 lists the configuration of the flat-plate solar thermal collector and the double- evacuatedTable tube3 lists solar the thermal configuration collector of that the flat-platewere installed solar thermalto compensate collector for and the thelimitation double- ofevacuated the existing tube GSHP solar system. thermal The collector thermal that ener weregy installed output toof compensatethe flat-plate for collector the limitation and evacuatedof the existing tube collector GSHP system. and efficiency The thermal are base energyd on the output manufacturer’s of the flat-plate Korean collector Industrial and Standardsevacuated (KS) tube certificate collector [17]. and efficiency are based on the manufacturer’s Korean Industrial StandardsSolar collectors (KS) certificate receive [ 17solar]. radiant energy and transfer it to the flowing fluid. The useful energy collected from a solar collector is given by Equation (1), and the collector efficiency is given by Equation (2) [18]:

Energies 2021, 14, 31 4 of 15

Table 3. Classiﬁcation and speciﬁcation of the two solar thermal collector types.

Classification Specification Collector area 2.0 m2 Dimensions 2000 (L) × 1000 (W) × 80 (H) mm Flat-plate type Collecting efficiency 71.51% Given solar gain 2.0 kWh/m2 per day (clear-sky condition) Collector area 3.02 m2 Double- Dimensions 1990 (L) × 1516 (W) × 134 (H) mm evacuated Collecting efficiency 75.09% tube type Given solar gain 2.6 kWh/m2 per day (clear-sky condition)

Solar collectors receive solar radiant energy and transfer it to the flowing fluid. The useful energy collected from a solar collector is given by Equation (1), and the collector efficiency is given by Equation (2) [18]:

Qu = Ac[IT(στ) − UL(Ti − Ta)] (1)

E = F0m(στ) − F0mUL[(Ti − Ta)/IT] (2) 2 where, Qu is the useful energy gain of solar collector [W], Ac is the collector area [m ], 2 IT is the global solar insolation at the collector plane [W/m ], στ is the transmittance 2 ◦ absorbance product [-], UL is the overall heat loss factor [W/m · C], Ti is the fluid inlet ◦ ◦ 0 temperature [ C], Ta is the outdoor air temperature [ C], F m is the mean temperature collector efficiency factor [-], and E is the collector efficiency [-]. The total collector area of the flat-plate collector is 35 sheets × 2 m2 = 70 m2, and its averaged collecting efficiency and useful solar heat gain are 71.51% and 2.0 kWh/m2 per day, respectively. Moreover, the total area of the double-evacuated tube collector is 15 sheets × 3.02 m2 = 45.3 m2, and its averaged collecting efficiency and useful solar heat gain are 75.09% and 2.6 kWh/m2 per day, respectively. Tanks were used for the conservation of heat energy or hot water for use when needed. One was a high-temperature heat storage tank, and the other was a low-temperature heat storage tank. The high-temperature tank (capacity 4 tons) first stored the heat obtained from the solar collectors, while the low-temperature tank (capacity 6 tons) performed secondary heat storage. The hot water in the high-temperature tank can be used for heating or hot water supply, while the hot water in the low-temperature tank can be used for the heat source of the heat pump or the heat sent to the ground through the ground heat exchanger for seasonal thermal energy storage. In the solar thermal system, the circulation pump is controlled and operated based on the difference between the outlet water temperature of the solar collector and the internal temperature (water in the tank) of the high-temperature tank. This system can be operated in three ways, as shown in Figures2–4. Figure2 shows the typical existing GSHP operation mode. Figure3 depicts the mode in which the collected solar heat is supplied to the heat pump through the low-temperature heat storage tank heat exchanger to supply the heating medium with a higher temperature to the heat source side of the GSHP. In this case, the temperature of the low-temperature (secondary) heat storage tank should be higher than that of the GHE; otherwise, the system will operate in the existing geothermal heat pump mode. Figure4 depicts the operation mode in which solar heat is supplied to and stored in the ground as a seasonal borehole thermal energy storage system. In the absence of any demand for the collected solar heat and operation of the heat pump for cooling, such as in the pre-winter months, the heat collected by the solar thermal system is supplied to the underground space through the GHE. This improves the heat exchange efficiency of the geothermal source system during soil thermal imbalances. Energies 2021, 14, x FOR PEER REVIEW 5 of 15 Energies 2021, 14, x FOR PEER REVIEW 5 of 15 Energies Energies2021 2021, 14, 14, 31, x FOR PEER REVIEW 5 ofof 1515

Figure 2. Geothermal-only heat pump heating operation mode. Figure 2. Geothermal-only heat pump heating operation mode. FigureFigure 2.2.Geothermal-only Geothermal-only heatheat pumppumpheating heatingoperation operationmode. mode.

Figure 3. Collected solar heat supplied to the heat pump for space heating. FigureFigure 3.3. CollectedCollected solarsolar heatheat suppliedsupplied toto thethe heatheat pumppump forfor spacespace heating.heating. Figure 3. Collected solar heat supplied to the heat pump for space heating.

Figure 4. Collected solar heat stored in the ground storage system (seasonal(seasonal boreholeborehole thermalthermal energyenergy storage).storage). Figure 4. Collected solar heat stored in the ground storage system (seasonal borehole thermal energy storage). Figure 4. Collected solar heat stored in the ground storage system (seasonal borehole thermal energy storage).

Energies 2021, 14, 31 6 of 15

In this study, the three aforementioned operational modes were accomplished for the hybrid GSHP assisted by a solar thermal collector for the analyzed building. The performance of the solar-assisted GSHP system was analyzed for a period of 143 days, from 15 November 2019 to 6 April 2020. The supply setting temperature on the demand side was 45 ◦C on the representative day when the GSHP-only was operated; the supply setting temperature on the demand side was 46.5 ◦C on the representative day when the solar-assisted GSHP was operated. The outdoor air temperature and the heating load of the two systems were similar. The flow rate of the heat source and demand side uses the constant flow rate supply approach according to the operation of the circulation pumps. Solar heat collection is operated by the differential temperature control system when the temperature difference between the internal temperature of the high-temperature tank and the outlet temperature of the solar collector is 5 ◦C. A three-way valve was used to control the operation of the existing geothermal system and the operation of the combined system. To analyze the performance of the system, the coefficient of performance (COP) and seasonal performance factor (SPF) were used, and these are calculated by Equations (3)–(5) [10,19]:

COP = Q1/W1,HP (3) COPsystem = Q1/(W1,HP +W1,Pumps (4)

where Q1 is the heating load [kWh/day], W1, HP is the power consumption (heat pump) [kWh/day], and W1, Pumps is the power consumption (pumps) [kWh/day]

SPF = Q2/(W2,HP+W2,Pumps) (5)

where Q2 is the heating load [kWh/seasonal], W2 is the power consumption (heat pump) [kWh/seasonal], W2, Pumps is the power consumption (pumps) [kWh/seasonal]. Table4 displays the monitoring points of the solar thermal and geothermal systems. A total of 44 data points each in the areas comprising the solar thermal and geothermal system were monitored. To measure the temperature, the heat storage tank was equipped with a thermocouple (T type) sensor. A digital thermometer with a digital liquid crystal display was used for displaying the temperature of the geothermal heat pump supply and return to allow the overseer to directly check the temperature at the site. The manufacturer guarantees a thermometer accuracy of ±0.3 ◦C(−60 to 60 ◦C) with the thermocouple (T type) sensor and ±0.15 ◦C (180 to 80 ◦C) with the digital thermometer. An ultrasonic flowmeter was used to measure the flow rate. The ultrasonic flowmeter was certified as accurate to ±1% (full scale). The thermometer and ultrasonic flowmeter were calibrated by the Korea Laboratory Accreditation Scheme (KOLAS), an organization that calibrates and inspects according to standards set by the International Organization for Standardization (ISO). An ELITEpro XC power metering data logger was used to measure the electric power consumption. The measurement interval was set to 1 min, and the result files were compiled in daily units. The accuracy of the power meter was ±0.06% (full scale), and it was certified by the Korea Communication Commission (KCC), which provides certification for input, output, and storage devices of data and communication messages. In the selection of the measuring devices used in this study, the appropriateness of the temperature range and flow rate range measured in this study were analyzed. All result values are values excluding the measurement equipment uncertainty so consider this part. Energies 2021, 14, 31 7 of 15

Table 4. Measuring points of the solar thermal and geothermal systems.

Items Geothermal Source Heat Pump (GSHP) Solar Thermal Collector Ground heat exchangers 1, 2, 3 Ground hot water supply Solar-collector circulation Flow rate Load1 hot water supply Solar hot water supply Load2 hot water return High-temp. heat source tank bottom1/bottom2, High-temp. heat source tank top1/top2, Low-temp. heat source tank bottom1/bottom2, Low-temp. heat source tank top1/top2, Ground heat 1 input, Collector1 input/Collector2 input/Collector3 Ground heat 2 input, input/Collector4 input, Collector1 Ground heat 3 input, output/Collector2 output/Collector3 Ground heat output, output/Collector4 output, Collector integrated HP-1A Geothermal input/output, supply/return, HP-1A hot water supply/return, Temperature High and low temp. tank middle, HP-1B Geothermal input/output, High and low temp. tank output, HP-1B hot water supply/return, High-temp. tank 1nd input/output, HP-2A Geothermal input/output, Low-temp. tank 1nd input/output, HP-2A hot water supply/return, High-temp. tank 2nd input/output, HP-2B geothermal input/output, High-temp. tank 2nd supply/return, HP-2B hot water supply/return Low-temp. tank 2nd input/output, Low-temp. tank 2nd supply/return, Hot water tank input/output, Hot water tank return/supply Main, HP-1A (compressor), HP-1B (compressor), HP-2A (compressor), HP-2B (compressor), P4-A Pump, Electric power Solar 1st Pump, Solar 2nd Pump, P4-B Pump, P4-C Pump, P4-D Pump, P5-A consumption Hot water convection pump Pump, P5-B Pump, P6 A,B Pump, P7-A Pump, P7-B (S.B) Pump, P8-A Pump, P8-B Pump, P3-A Pump, P3-B Pump Outdoor temperature, Outdoor humidity, Solar Insolation

3. Results and Discussion 3.1. Operation Behaviors of GSHP with and without the Solar Thermal System Figure5a,b shows the operation behaviors of the GSHP without and with the solar thermal collector, respectively. To compare the performances of the two systems, the operation results obtained for a day with similar outdoor temperature conditions were selected. The changes in the heat pump source, demand-side water supply, returning-water temperature and ﬂow rate, power consumption, and outdoor temperature were collected over time. The power consumption was calculated as the sum of the heat and circulation pumps at the heat source and demand sides, respectively. The operation pattern of a typical heat pump system, including the intermittent operation, according to the outdoor temperature, is shown in Figure5. In the GSHP-only system (Figure5a), the maximum temperature of the heat pump supply water at the heat source side was initially 13.1 ◦C but rapidly decreased to 11.4 ◦C during operation. The supply water temperature for heating at the demand side was maintained at approximately 45 ◦C, which is the set value. Energies 2021, 14, x FOR PEER REVIEW 7 of 15

High-temp. heat source tank bottom1/bottom2, High-temp. heat source tank top1/top2, Low-temp. heat source tank bottom1/bottom2, Ground heat 1 input, Low-temp. heat source tank top1/top2, Ground heat 2 input, Collector1 input/Collector2 input/Collector3 input/Collector4 Ground heat 3 input, input, Collector1 output/Collector2 output/Collector3 out- Ground heat output, put/Collector4 output, Collector integrated supply/return, HP-1A Geothermal input/output, High and low temp. tank middle, Temper- HP-1A hot water supply/return, High and low temp. tank output, ature HP-1B Geothermal input/output, High-temp. tank 1nd input/output, HP-1B hot water supply/return, Low-temp. tank 1nd input/output, HP-2A Geothermal input/output, High-temp. tank 2nd input/output, HP-2A hot water supply/return, High-temp. tank 2nd supply/return, HP-2B geothermal input/output, Low-temp. tank 2nd input/output, HP-2B hot water supply/return Low-temp. tank 2nd supply/return, Hot water tank input/output, Hot water tank return/supply Main, HP-1A (compressor), HP-1B (compressor), HP-2A (compressor), HP-2B Electric (compressor), P4-A Pump, power P4-B Pump, P4-C Pump, P4-D Pump, P5- Solar 1st Pump, Solar 2nd Pump, con- A Pump, P5-B Pump, P6 A,B Pump, P7- Hot water convection pump sump- A Pump, P7-B (S.B) Pump, tion P8-A Pump, P8-B Pump, P3-A Pump, P3-B Pump Outdoor temperature, Outdoor humidity, Solar Insolation

Energies 2021, 14, x FOR PEER REVIEW 8 of 15

(a)

(b)

Figure 5. Operation results of the GSHP systems: (a) without and (b) with the solar thermal collector. Figure 5. Operation results of the GSHP systems: (a) without and (b) with the solar thermal collector. In the solar-assisted GSHP system, the heat source supply from the solar thermal system occurred in the afternoon because the water temperature in the low-temperature The power consumption was calculated as the sum of the heat and circulation pumps (T ) heat storage tank was higher than that of the GHE outlet water in the afternoon. If the at the heat sourceLT and demand sides, respectively. The operation pattern of a typical heat water temperature in the low-temperature heat storage tank was higher than 25 ◦C (the set- pump system, including the intermittent operation, according to the outdoor temperature, point temperature), the accumulated solar heat was supplied to the heat pump for heating is shown in Figure 5. between 13:09 and 16:54, as shown in Figure5b. The temperature of the supply water to In the GSHP-only system (Figure 5a), the maximum temperature of the heat pump the heat pump source side was controlled within 15–20.9 ◦C. Consequently, much higher supply water at the heat source side was initially 13.1 °C but rapidly decreased to 11.4 °C temperatures could be supplied when solar heat was used instead of ground heat; the solar during operation.heat contributesThe supply towater the performance temperature improvementfor heating at of the the demand heat pump side system. was The supply maintained attemperature approximately at 45 the °C, demand which is side the was set value. 46.5 ◦ C, which was slightly higher than that for In the solar-assistedthe GSHP-only GSHP case. system, The solar-assisted the heat source operation supply required from the a solar lower thermal power consumption system occurredbecause in the the afternoon GHE circulation because the pump water was temperature not required in for the operation. low-temperature (TLT) heat storage tankTable was5 shows higher the than system that of performance the GHE outlet of the water GSHP in the with afternoon. and without If the solar the water temperaturethermal system. in the low-temperature The amount of heat heat supplied storage fortank heating, was higher power than consumed 25 °C by the heat (the set-point temperature),and circulation the pumps, accumulated and COPs solar are heat listed was supplied based on to the the operation heat pump results for in Figure5. heating betweenIn the 13:09 case and of the16:54, solar-assisted as shown in GSHP, Figure a slightly5b. The largertemperature amount of of the heat supply was supplied even water to the heatthough pump the source daily side average was cont outdoorrolled temperature within 15–20.9 was °C. slightly Consequently, higher. much This is because the higher temperaturesdifference could of thebe supplied outdoor air when temperature solar heat on was that used day instead was large. of ground In addition, heat; the COP of the the solar heat heatcontributes pump forto the the performanc GHSP-onlye case improvement was heating of load/powerthe heat pump consumption system. The (heat pump) = supply temperature1431.8 kWh/265.6at the demand kWh side = 5.4, was while 46.5 that°C, which for the was system slightly including higher the than circulation that pump was for the GSHP-onlyheating case. load/power The solar-assisted consumption operation (heatpump required + pumps) a lower = 1431.8power kWh/(265.6consump- + 93.4) kWh tion because the GHE circulation pump was not required for operation. Table 5 shows the system performance of the GSHP with and without the solar thermal system. The amount of heat supplied for heating, power consumed by the heat and circulation pumps, and COPs are listed based on the operation results in Figure 5. In the case of the solar-assisted GSHP, a slightly larger amount of heat was supplied even though the daily average outdoor temperature was slightly higher. This is because the difference of the outdoor air temperature on that day was large. In addition, the COP of the heat pump for the GHSP-only case was heating load/power consumption (heat pump) = 1431.8 kWh/265.6 kWh = 5.4, while that for the system including the circulation pump was heating load/power consumption (heat pump + pumps) = 1431.8 kWh/(265.6 + 93.4) kWh = 4.0. In contrast, the COP of the solar-assisted GSHP system was heating load/power consumption (heat pump) = 1542.5 kWh/221.1 kWh = 7.0, and was 1542.5 kWh/(221.1 + 66.9) kWh = 5.4 when the circulation pump was used. These results indicate the improved performance of the solar-assisted GSHP system.

Energies 2021, 14, 31 9 of 15

= 4.0. In contrast, the COP of the solar-assisted GSHP system was heating load/power consumption (heat pump) = 1542.5 kWh/221.1 kWh = 7.0, and was 1542.5 kWh/(221.1 + 66.9) kWh = 5.4 when the circulation pump was used. These results indicate the improved performance of the solar-assisted GSHP system.

Table 5. Performance of the GSHP with and without the solar thermal collector.

Power Coefﬁcient of Average Outdoor Consumption Performance Air Temp. during Heating Load (kWh/day) Analyzed Day Operation (kWh/day) ◦ Heat Heat Heat and ( C) Pumps Pump Pump Circulation Pumps GSHP-only 23 December 2019 5.0 1431.8 265.6 93.4 5.4 4.0 Solar-assisted GSHP 31 January 2020 5.3 1542.5 221.1 66.9 7.0 5.4

The temperature of the water supplied to the heat pump increased when it was coupled with the solar thermal system, and the power consumption of the circulation pump reduced as the hot water in the solar heat storage tank was directly supplied to the heat pump without traversing the relatively long GHE. The integration of the GSHP with the solar system reduced the power consumption of the heat pump by approximately 17% (GSHP-only power consumption (heat pump)-Solar-assisted GSHP power consumption (heat pump)/GSHP-only power consumption (heat pump) = {(265.6 − 221.1) kWh/265.6 kWh}·100 = 17%) and the pump power by approximately 28% (GSHP-only power consumption (pumps)-Solar-assisted GSHP power consumption (pumps)/GSHP-only power consumption (pumps) = {(93.4 − 66.9) kWh/93.4 kWh}·100 = 28%). Moreover, the total power consumption decreased by approximately 20%. (GSHP-only power consumption (heat pump+pumps)-Solar-assisted GSHP power consumption (heat pump+pumps)/GSHP-only power consumption (heat pump + pumps) = {(265.6 + 93.4 − 221.1 + 66.9) kWh/(265.6 + 93.4) kWh}·100 = 20%). Figure6 shows the operation results of the solar thermal system on the analyzed day when the solar-assisted GSHP system operated. The changes in the solar collector inlet and outlet water temperatures, circulation ﬂow rate, and temperatures of the high- and low-temperature solar heat storage tanks over time, were collected along with the solar radiation and outdoor temperatures. The combined system operation started at approximately 10:00 and ended at 16:00. The total and the maximum solar radiation of the analyzed day were 604.5 and 110 W/m2, respectively. The temperatures varied depending on the solar radiation, and the total amount of solar heat collected on that day was 1315.2 kWh. The amount of heat supplied to the heat pump from 13:06 when the water temperature of the solar heat storage tank exceeded 25 ◦C was 823.4 kWh. Approximately 63% of the collected solar heat was used as the heat source for the heat pump; this value corresponds to 38% of the total daily heating load. Energies 2021, 14, x FOR PEER REVIEW 10 of 15

Energies 2021, 14, x 31 FOR PEER REVIEW 1010 of of 15 15

Figure 6. Operation results of the solar thermal system on the analyzed day.

Figure 7 shows the temperature variation of the supply water at the heat source side of the heat pump over time on the analyzed day when the geothermal and solar-assisted operations were performed. In the case of the GSHP-only operation, the supply water temperature was approximately 11.5 °C, while in the solar-assisted operation, the temper-

ature to the heat pump ranged from 20.9 °C to 15 °C with an average of 18.1 °C during FigureFigureoperation. 6. 6. OperationOperation The results resultsdifference of of the the in solar solar the ther thermaltemperaturmal system systeme between on on the the analyzed analyzed the geothermal-only day. day. and solar-assisted systems was 6.6 °C. Figure7 shows the temperature variation of the supply water at the heat source Figure 87 showsshows the the heat temperature supplied fromvariation the heat of thesource supply to the water heat atpump the heatover sourcetime on side side of the heat pump over time on the analyzed day when the geothermal and solar- ofthe the analyzed heat pump day whenover time both on operations the analyzed were dayperformed. when the The geothermal amount of and heat solar-assisted supplied operationsassisted operations were performed. were performed. In the case In theof the case GSHP-only of the GSHP-only operation, operation, the supply thesupply water was generally larger when the solar-assisted ◦operation was performed compared with temperaturewater temperature was approximately was approximately 11.5 °C, 11.5 whileC, in while the solar-assisted in the solar-assisted operation, operation, the temper- the that of the GSHP-only system. Moreover, the total amount◦ of heat◦ supplied was 151.3 and ◦ aturetemperature194.9 tokWh the duringheat to the pump the heat GSHP-onlyranged pump rangedfrom and 20.9 fromsolar-assisted °C 20.9to 15 C°C to GSHPwith 15 anC operations, withaverage an average of respectively, 18.1 °C of 18.1during C operation.duringshowing operation. approximately The difference The 23% difference in increasethe temperatur in in the heat temperature foe rbetween the solar-assisted betweenthe geothermal-only theoperation. geothermal-only This and shows solar-as- and ◦ sistedsolar-assistedan improvement systems systemswas in the6.6 performance°C. was 6.6 C. of the heat pump system. Figure 8 shows the heat supplied from the heat source to the heat pump over time on the analyzed day when both operations were performed. The amount of heat supplied was generally larger when the solar-assisted operation was performed compared with that of the GSHP-only system. Moreover, the total amount of heat supplied was 151.3 and 194.9 kWh during the GSHP-only and solar-assisted GSHP operations, respectively, showing approximately 23% increase in heat for the solar-assisted operation. This shows an improvement in the performance of the heat pump system.

Figure 7. 7. SupplySupply water water temperature temperature at the at heat the heatsource source side of side the heat of the pump heat over pump time over on the time ana- on the analyzedlyzed day. day.

Figure8 shows the heat supplied from the heat source to the heat pump over time on the analyzed day when both operations were performed. The amount of heat supplied was generally larger when the solar-assisted operation was performed compared with that of the GSHP-only system. Moreover, the total amount of heat supplied was 151.3 and 194.9 kWh during the GSHP-only and solar-assisted GSHP operations, respectively, Figureshowing 7. Supply approximately water temperature 23% increase at the inheat heat source for theside solar-assistedof the heat pump operation. over time Thison the shows ana- lyzedan improvement day. in the performance of the heat pump system.

Energies 2021, 14, x FOR PEER REVIEW 11 of 15

EnergiesEnergies 20212021, ,1414, ,x 31 FOR PEER REVIEW 11 11of of 15 15

Figure 8. Amount of heat supplied to the heat source side of the heat pump.

3.2. System Performance Analysis 3.2.1. Influence of the Supply Temperature on Heat Pump Performance Figure 9 shows the performance (COP) of the heat pump according to the supply temperature on the heat source side for both the GSHP-only and the solar-assisted GSHP operations. It is generally known that the inlet temperature on the heat source side of the geothermal heat pump through GHE ranges from 10 to 20 °C during the heating operation [20]. FigureFigure 8. 8. AmountAmount of of heat heat supplied supplied to to the the heat heat source source side side of of the the heat heat pump. pump. The supply temperature on the heat source side ranged from 9.5 to 12.8 °C in the case 3.2.of3.2. the System System geothermal Performance Performance system Analysis Analysis operation in this study. In contrast, for the solar-assisted system operation,3.2.1. Inﬂuence the temperature of the Supply range Temperature was 14.6–26.1 on Heat °C. Pump Performance 3.2.1. Influence of the Supply Temperature on Heat Pump Performance AsFigure the supply9 shows temperature the performance on the (COP)heat source of the side heat increased, pump according the performance to the supply (COP) Figure 9 shows the performance (COP) of the heat pump according to the supply oftemperature the heat pump on the tended heat sourceto increase side foralmost both linearly. the GSHP-only The COP and of thethe solar-assisted heat pump ranged GSHP temperature on the heat source side for both the GSHP-only and the solar-assisted GSHP fromoperations. 4.8 to 6.0 It and is generally 5.4 to 7.4 known for thethat GSHP-only the inlet and temperature solar-assisted on the GSHP heat operations, source side re- of operations. It is generally known that the inlet temperature on the heat source side of the spectively.the geothermal In the heatcase pumpof the throughsolar-assisted GHE operation, ranges from the 10COP to 20of the◦C duringheat pump the heatingsignifi- geothermal heat pump through GHE ranges from 10 to 20 °C during the heating operation cantlyoperation improved. [20]. [20]. The supply temperature on the heat source side ranged from 9.5 to 12.8 °C in the case of the geothermal system operation in this study. In contrast, for the solar-assisted system operation, the temperature range was 14.6–26.1 °C. As the supply temperature on the heat source side increased, the performance (COP) of the heat pump tended to increase almost linearly. The COP of the heat pump ranged from 4.8 to 6.0 and 5.4 to 7.4 for the GSHP-only and solar-assisted GSHP operations, respectively. In the case of the solar-assisted operation, the COP of the heat pump significantly improved.

FigureFigure 9. 9. HeatHeat pump pump performance performance according according to to the the supply supply temperature temperature on on the the heat heat source source side side for for bothboth the the geothermal-only geothermal-only and and solar-assisted solar-assisted operations. operations.

AfterThe supply analyzing temperature the seasonal on theperformance heat source factor side ranged(SPF) for from the 9.5heating to 12.8 period◦C in from the case 15 Novemberof the geothermal 2019 to system6 April operation 2020, the in SPF this for study. the InGHSP-only contrast, forcase the was solar-assisted the heating system load duringoperation, heating the temperature period/power range consumption was 14.6–26.1 during◦C. the heating period (heat pump + As the supply temperature on the heat source side increased, the performance (COP) of the heat pump tended to increase almost linearly. The COP of the heat pump ranged from 4.8 to 6.0 and 5.4 to 7.4 for the GSHP-only and solar-assisted GSHP operations, respectively. FigureIn the 9. case Heat of pump the solar-assisted performance operation,according to the the COP supply of thetemperature heat pump on signiﬁcantlythe heat source improved. side for both theAfter geothermal-only analyzing the and seasonal solar-assisted performance operations. factor (SPF) for the heating period from 15 November 2019 to 6 April 2020, the SPF for the GHSP-only case was the heating loadAfter during analyzing heating the period/power seasonal performance consumption factor during (SPF) thefor the heating heating period period (heat from pump 15 November+ pumps) =2019 29,122.6 to 6 April kWh/(5145.3 2020, the + SPF 1893.4) for kWhthe GHSP-only = 4.1. In contrast, case was the the SPF heating of the load solar- during heating period/power consumption during the heating period (heat pump +

Energies 2021, 14, x FOR PEER REVIEW 12 of 15

EnergiesEnergies 20212021,, 1414,, 31x FOR PEER REVIEW 1212 of 1515 pumps) = 29,122.6 kWh/(5145.3 + 1893.4) kWh = 4.1. In contrast, the SPF of the solar-assisted GSHP system was heating load during heating period / power consumption during heating period (heat pump + pumps) = 19,742 kWh/(3077 + 982.4) kWh = 4.9. pumps) = 29,122.6 kWh/(5145.3 + 1893.4) kWh = 4.1. In contrast, the SPF of the solar-as- assistedFigure GSHP 10 shows system the was heat heating supplied load to the during heat heatingpump on period the heat / powersource consumptionside for both sisted GSHP system was heating load during heating period / power consumption during theduring GSHP-only heating and period solar-assisted (heat pump GSHP + pumps) operatio = 19,742ns. In kWh/(3077 the case of +the 982.4) geothermal kWh = 4.9.opera- heating period (heat pump + pumps) = 19,742 kWh/(3077 + 982.4) kWh = 4.9. tion, theFigure amount 10 shows of heat the heatsupplied supplied to the to theheat heat pump pump was on determined the heat source in response side for both to the the Figure 10 shows the heat supplied to the heat pump on the heat source side for both heatingGSHP-only load, and while solar-assisted the supply GSHPtemperature operations. on the In heat the casesource of theside geothermal did not significantly operation, the GSHP-only and solar-assisted GSHP operations. In the case of the geothermal opera- changethe amount owing of to heat the suppliedstructural to limitation the heat pumpof the geothermal was determined system. in responseIn the case to of the the heating solar- tion, the amount of heat supplied to the heat pump was determined in response to the assistedload, while operation, the supply the supply temperature temperature on the va heatried source more sidewidely, did notand signiﬁcantlythe amount of change heat heating load, while the supply temperature on the heat source side did not significantly suppliedowing to could the structural respond limitation to the heating of the load. geothermal system. In the case of the solar-assisted change owing to the structural limitation of the geothermal system. In the case of the solar- operation,The amount the supply of heat temperature supplied to varied the heat more pump widely, was and not therelated amount to the of heatsupply supplied water assisted operation, the supply temperature varied more widely, and the amount of heat temperature,could respond and to it the varied heating according load. to the heating load at that time. supplied could respond to the heating load. The amount of heat supplied to the heat pump was not related to the supply water temperature, and it varied according to the heating load at that time.

FigureFigure 10. 10. HeatHeat supplied supplied to to the the heat heat pump pump with with the the suppl supplyy temperature temperature on on the the heat heat source source side side for for bothboth the the geothermal-only geothermal-only and and solar-assisted solar-assisted operations. operations.

3.2.2.Figure ThePower 10. amountHeat Consumption supplied of heat to suppliedthe According heat pump to theto with the heat theOperation pump supply was temperature Mode not related on the to theheat supplysource side water for temperature,both the geothermal-only and it varied and according solar-assisted to theoperations. heating load at that time. Figure 11 shows the daily power consumption for the GSHP-only and solar-assisted GSHP3.2.2. Poweroperation Consumption conditions. AccordingFor comparison to the, Operationthe daily power ModeMode consumption was reviewed according to the heating load after 13:00, that is, when the daily solar heat connection Figure 11 shows the daily power consumption forfor thethe GSHP-onlyGSHP-only andand solar-assistedsolar-assisted operation was possible. As the daily heating load increased, the power consumption in- GSHP operation conditions. For comparison,comparison, the daily power consumption was reviewed creased almost linearly, and the solar-assisted operation resulted in a lower power con- according to the heatingheating loadload afterafter 13:00,13:00, thatthat is,is, whenwhen thethe dailydaily solarsolar heatheat connectionconnection sumption than that in the case of the GSHP-only operation. operation was possible. possible. As As the the daily daily heating heating load load increased, increased, the the power power consumption consumption in- increasedcreased almost almost linearly, linearly, and and the the solar-assi solar-assistedsted operation operation resulted resulted in a inlower a lower power power con- consumptionsumption than than that that in the in thecase case of the of theGSHP-only GSHP-only operation. operation.

Figure 11. Power consumption according to the operation mode.

EnergiesEnergies 20212021,, 1414,, 31x FOR PEER REVIEW 1313 of of 1515

Figure 1212 showsshows the the comparison comparison of ofthe the average average daily daily power power consumptions consumptions in the in case the caseof the of geothermal-only the geothermal-only and andsolar-assisted solar-assisted operation operation conditions. conditions. The Thepower power consumption consump- tionincludes includes the consumption the consumption by the by heat the heatand andcirculation circulation pumps. pumps. The annual The annual power power con- consumptionsumption was was 58,276 58,276 kWh kWh for forthe thegeothermal-only geothermal-only operation operation and and 45,661 45,661 kWh kWh for forthe theso- solar-assistedlar-assisted operation. operation. As As observed, observed, the the po powerwer consumption consumption was was r reducededuced by by about 22% in the coupled operation.

Figure 12. Averaged daily power consumption according to the operation modes. Figure 12. Averaged daily power consumption according to the operation modes. 4. Conclusions 4. Conclusions In recent years, with the increase in zero-energy buildings in South Korea, geothermal sourceIn heatrecent pump years, (GSHP) with the systems increase are beingin zero-energy installedas buildings a new type in South of a renewable Korea, geother- energy system.mal source However, heat pump the annual(GSHP) heating systems demand are being of installed South Korean as a new buildings type of is a larger renewable than theenergy cooling system. demand, However, and thethe heatingannual heating performance demand of GSHPsof South decreases Korean buildings with time is owing larger tothan thermal the cooling imbalances demand, in the and surrounding the heating soil. performance To derive aof more GSHPs efficient decreases system, with a solar- time assistedowing to GSHP thermal was imbalances experimentally in the analyzed surrounding in this soil. study. To derive The performance a more efficient of this system, system a wassolar-assisted analyzed GSHP for a period was experimentally of 143 days from analyz 15 Novembered in this study. 2019 toThe 6 Aprilperformance 2020, and of this the findingssystem was of thisanalyzed study for are a as period follows. of 143 days from 15 November 2019 to 6 April 2020, and the findingsThe performances of this study of are two as GSHP follows. systems, one with and one without a solar thermal collector,The performances were compared of bytwo considering GSHP systems, the operation one with resultsand one with without similar a solar outdoor thermal tem- peraturecollector, conditions.were compared The heating by considering COP of thethe geothermal-onlyoperation results operation with similar was outdoor 5.4, whereas tem- thatperature of the conditions. solar-assisted The operationheating COP was of 7.0. the A geothermal-only significant improvement operation inwas the 5.4, heat whereas pump performancethat of the solar-assisted was therefore operation demonstrated was 7.0. when A significant the GSHP wasimprovement connected in to athe solar heat thermal pump system.performance In addition, was therefore the power demonstrated consumption when for the the GSHP system was operation connected was to a reduced solar ther- by aboutmal system. 20% compared In addition, with the the power GSHP-only consumpt system.ion for the system operation was reduced by aboutIn the 20% case compared of the GSHP-only with the GSHP-only system, the system. maximum temperature of the heat pump ◦ supplyIn waterthe case on of the the heat GSHP-only source side system, was 13.1 the maximumC initially; temperature however, it rapidlyof the heat decreased pump ◦ tosupply 11.4 waterC during on the operation. heat source However, side was in 13.1 the °C solar-assisted initially; however, geothermal it rapidly GSHP decreased system, theto 11.4 supply °C during water operation. temperature However, to the heat in the pump solar-assisted heat source geothermal side was controlled GSHP system, between the ◦ 15supply and 20.9waterC. temperature Much higher to temperaturesthe heat pump could heat besource supplied side was when controlled solar heat between was used 15 insteadand 20.9 of °C. ground Much heat, higher and te themperatures solar heat could contributes be supplied to the when performance solar heat improvement was used in- of thestead heat of pumpground system. heat, and the solar heat contributes to the performance improvement of the heatThe pump COP variationsystem. with the supply temperature at the heat source side of the heat pumpThe was COP analyzed; variation the with COP the of the supply heat pumptemperature almost at linearly the heat increased source withside anof increasethe heat inpump the was supply analyzed; temperature. the COP The of the COP heat of pump the heat almost pump linearly ranged increased from 4.8 with to an 6.0 increase for the geothermal-onlyin the supply temperature. operation butThe improvedCOP of the to he 5.4–7.4at pump for theranged solar-assisted from 4.8 to operation. 6.0 for the geothermal-onlyThe seasonal operation performance but improved factor (SPF)to 5.4–7.4 for the for heating the solar-assisted period (140 operation. days) showed that the SPFThe for seasonal the GHSP-only performance case factor was 4.1, (SPF) while for the the SPF heating for the period solar-assisted (140 days) GSHP showed system that wasthe SPF 4.9. for the GHSP-only case was 4.1, while the SPF for the solar-assisted GSHP system was 4.9.

Energies 2021, 14, 31 14 of 15

Comparisons were made for the average daily power consumption in the case of the geothermal-only and solar-assisted operation conditions. The results showed that the power consumption includes the consumption by the heat and circulation pumps showing a reduction by approximately 22% in the coupled operation of the solar thermal system. In areas where a high heating load is required and the heating performance of GSHPs decrease with time owing to soil thermal imbalances, it is difﬁcult to maintain high system performance. Consequently, the use of a solar thermal coupled heat pump system can be a good solution to guarantee better operation performance. In future work, an optimal design and operation simulation analysis will be undertaken. In addition, a comprehensive analysis of the investment profitability of the optimization system will be conducted.

Author Contributions: Conceptualization, T.L.; Data curation, D.S., J.H.; Formal analysis, J.H.; Funding acquisition, T.L.; Investigation, J.H.; Methodology, J.H., T.L.; Project administration, D.S., T.L.; Resources, T.L., J.H.; Software, T.L., J.H.; Supervision, D.S., T.L.; Validation, D.S., J.H.; Visualization, D.S., J.H.; Writing—original draft, J.H.; Writing—review & editing, D.S.,T.L., J.H. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the Urban Architecture Research Project (20AUDP-B099686- 06) funded by the Ministry of Land, Infrastructure and Transport in 2020. Informed Consent Statement: Informed consent was obtained from all subjects involved in the study. Data Availability Statement: Data available in a publicly accessible repository. Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Abbreviations COP Coefﬁcient Of Performance SPF Seasonal Performance Factor E Electricity consumption [kW] ◦ TOA Outdoor Air Temperature [ C] 2 RS Solar Radiation [W/m ] TS_H Supply water Temperature for Heating TR_H Return water Temperature for Heating 3 FH Water Flow rate for Heating [m /h] 3 FHP_G Water Flow rate for Heat Pump from Geothermal [m /h] 3 FHP_S Water Flow rate for Heat Pump from Solar collecting [m /h] ◦ TS_HP Supply water Temperature for Heat Pump [ C] ◦ TR_HP Return water Temperature for Heat Pump [ C] ◦ THT Water Temperature of the High-Level Heat storage Tank [ C] ◦ TLT Water Temperature of the Low-Level Heat storage Tank [ C] ◦ TS_S Supply water Temperature for Solar collector [ C] ◦ TR_S Return water Temperature for Solar collector [ C] 3 FS Water Flow rate for Solar collector [m /h]

References 1. Joo, B.K. Geothermal heating and cooling systems, initial investment and operating costs are excessively higher than gas heating and cooling systems. Gas News, 2020. Available online: http://www.gasnews.com/news/articleView.html?idxno=91688 (accessed on 15 November 2020). 2. Sanner, B.; Karytsas, C.; Mendrinos, D. Current status of ground source heat pumps and underground thermal energy storage in Europe. Geothermics 2003, 32, 579–588. [CrossRef] 3. Hepbasli, A.; Akdemir, O.; Hancioglu, E. Experimental study of a closed loop vertical ground source heat pump system. Energy Convers. Manag. 2003, 44, 527–548. [CrossRef] 4. Kang, E.C. Invigorating the Geothermal Market, “Communal Housing” will be responsible for vitalizing the geothermal marker. Korea Heating Air-Conditioning & Renewable News, 1 October 2015. Available online: http://www.kharn.kr/news/article.html?no= 392 (accessed on 15 November 2020). 5. Shahed, A.M.; Harrison, S.J. Preliminary review of geothermal solar assisted heat pumps. In Proceedings of the 4th Annual Canadian Solar Buildings Conference, Toronto, ON, Canada, 25–27 June 2009. Energies 2021, 14, 31 15 of 15

6. Lohani, S.P.; Schmidt, D. Comparison of energy and exergy analysis of fossil plant, ground and air source heat pump building heating system. Renew. Energy 2010, 35, 1275–1282. [CrossRef] 7. Yu, T.; Liu, Z.; Chu, G. Influence of intermittent operation on soil temperature and energy storage duration of ground-source heat pump system for residential building. In Proceedings of the 8th International Symposium on Heating, Ventilation and Air Conditioning—Volume 2: HVAC&RComponent and Energy System; Li, A., Zhu, Y., Li, Y., Eds.; Springer: Berlin/Heidelberg, Germany, 2014; pp. 203–213. 8. Qian, H.; Wang, Y. Modeling the interactions between the performance of ground source heat pumps and soil temperature variations. Energy Sustain. Dev. 2014, 23, 115–121. [CrossRef] 9. Retkowski, W.; Ziefle, G.; Thöming, J. Evaluation of different heat extraction strategies for shallow vertical ground-source heat pump systems. Appl. Energy 2015, 149, 259–271. [CrossRef] 10. Liu, Z.; Xu, W.; Qian, C. Investigation on the feasibility and performance of ground source heat pump (GSHP) in three cities in cold climate zone, China. Renew. Energy 2015, 84, 89–96. [CrossRef] 11. You, T.; Wu, W.; Wang, B.; Shi, W.; Li, X. Dynamic soil temperature of ground-coupled heat pump system in cold region. In Proceedings of the 8th International Symposium on Heating, Ventilation and Air Conditioning—Volume 2: HVAC&RComponent and Energy System; Li, A., Zhu, Y., Li, Y., Eds.; Springer: Berlin/Heidelberg, Germany, 2014; pp. 439–448. 12. Dai, L.; Li, S.; DuanMu, L. Experimental performance analysis of a solar assisted ground source heat pump system under different heating operation modes. Appl. Eng. 2015, 75, 325–333. [CrossRef] 13. Shang, Y.; Dong, M.; Li, S. Intermittent experimental study of a vertical ground source heat pump system. Appl. Energy 2014, 136, 628–635. [CrossRef] 14. Tian, Y.; Wei, W.; Wenxing, S.; Baolong, W.; Xianting, L. An overview of the problems and solutions of soil thermal imbalance of ground-coupled heat pumps in cold regions. Appl. Energy 2016, 177, 515–536. 15. Ni, L.; Song, W.; Zeng, F.; Yao, Y. Energy saving and economic analyses of design heating load ratio of ground source heat pump with gas boiler as auxiliary heat source. In Proceedings of the 2011 International Conference on Electric Technology and Civil Engineering (ICETCE), Lushan, China, 22–24 April 2011; pp. 1197–1200. 16. Chen, X.; Yang, H.X.; Lu, L. Experimental studies on a ground coupled heat pump with solar thermal collectors for space heating. Energy 2011, 36, 5292–5300. 17. Korean Agency for Technology and Standards. Solar Thermal Collectors, KS B 8295. 2015; pp. 30–36. Available online: https://standard.go.kr/KSCI/standardIntro/getStandardSearchView.do?menuId=919&topMenuId=502&upperMenuId=50 3&ksNo=KSB8295&tmprKsNo=KSB8295&re (accessed on 15 November 2020). 18. Khaled, Z.; Amenallah, G.; Ramzi, S.; Chakib, K. Solar thermal systems performances versus flat plate solar collectors connected in series. Engineering 2012, 4, 881–893. 19. Hong, P.J.; Seoung, L.W.; Kim, Y.K. The cooling seasonal performance factor of a hybrid ground-source heat pump with parallel and serial configurations. Appl. Energy 2013, 102, 877–884. 20. Lim, H.J. Performance evaluation and economic estimation of ground source heat pump cooling and heating system. J. Energy Eng. 2004, 13, 296–300.