A Study on the Efficiency Improvement of Multi-Geothermal Heat Pump

energies

Article A Study on the Efﬁciency Improvement of Multi-Geothermal Heat Pump Systems in Korea Using Coefﬁcient of Performance

Young-Ju Jung 1, Hyo-Jun Kim 2, Bo-Eun Choi 2, Jae-Hun Jo 3 and Young-Hum Cho 4,*

1 Green Building Center, Korea Productivity Center Quality Assurance, 32 Saemoonan 5th street, Jongno-gu, Seoul 03170, Korea; [email protected] 2 Department of Architectural Engineering, Graduate School of Yeungnam University, 280 Daehak-Ro, Gyeongsan, Gyeongbuk 38541, Korea; [email protected] (H.-J.K.); [email protected] (B.-E.C.) 3 Department of Architectural Engineering, Inha University, 100 Inha-ro, Nam-gu, Incheon 22212, Korea; [email protected] 4 School of Architecture, Yeungnam University, 280 Daehak-Ro, Gyeongsan, Gyeongbuk 38541, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-53-810-3081; Fax: +82-53-814-3675

Academic Editors: Nyuk Hien Wong and Chi-Ming Lai Received: 8 January 2016; Accepted: 13 April 2016; Published: 12 May 2016

Abstract: The Korean government is fostering a renewable energy industry as a means of handling the energy crisis. Among the renewable energy systems available, geothermal energy has been highlighted as highly efficient, safely operable and relatively unaffected by outdoors air conditions. Despite the increasing use of renewable energy, the devices using renewables may not be operating appropriately. This study examined current problems in the operation of a geothermal heat pump (GHP) system. The efficiency of a geothermal heat pump system to studied to maximize the operation plan. Our study modelled the target building and analyzed the energy using TRNSYS, which is a dynamic energy simulation tool, to apply the coefficient of performance (COP) and evaluate the operation method. As a result, the GHP total energy consumption from the COP control method was reduced by 46% compared to the current operation. The proposed control method was evaluated after applying the system to a building. The results showed that efficient operation of a geothermal heat pump system is possible.

Keywords: geothermal heat pump (GHP); coefﬁcient of performance (COP); TRNSYS; operation method; renewable energy; building energy; hybrid operation

1. Introduction Indispensable for national economic growth, energy needs to be supplied on a stable basis. The demand for energy is expected to increase in the future with the advances in industrialization, quality of life, etc. On the other hand, the reserves of fossil fuels will reach their limits, which will eventually lead to not only an imbalance between demand and supply and the strategic imperialization of energy, but also to increased environmental contamination and Earth temperature due to the emission of greenhouse gases (GHG) from fuel combustion. The Intergovernmental Panel on Climate Change (IPCC) reported that the total emissions of greenhouse gases should be reduced to 40%–70% by 2050 across the world compared to 2010. According to the report, greenhouse gases increased drastically from 2000 to 2010, caused primarily by increased industrialization, population, life quality improvement, etc. In terms of energy, the emissions are due mainly to the increasing energy consumption in the industrial and building sectors [1].

Energies 2016, 9, 356; doi:10.3390/en9050356 www.mdpi.com/journal/energies Energies 2016, 9, 356 2 of 19

Any problem with the energy supply will give rise to social problems and serious losses to national economies. For example, in South Korea, which has a very high energy consumption, imports cover 95% of the energy demand [2]. Therefore, specific alternatives and countermeasures must be established to cope systematically with any energy supply problem. In this regard, the South Korean government is focusing its investments on renewable energy and energy efficiency as it sees the expansion of renewable energy supplies as a positive alternative, because renewable energy can both solve the environmental problems and ensure the stability of the energy supply. In South Korea, there has been a mandate in place since 2004 for public institutions to use renewable energy [3]. Currently, renewable energy must cover 10% or more of the expected energy consumption in buildings with a total area of 1000 m2 or higher. A geothermal system is highly efficient, environmentally friendly, has a lengthy service life, and has low maintenance costs. Accordingly, geothermal heat pump systems have been addressed by many studies, which have focused on the efficiency enhancement and performance evaluation of underground heat exchangers. On the other hand, geothermal systems have not been used efficiently after installation at a number of buildings in Korea. Mostly, they are in hybrid operation with existing heating sources, and take the form of a partial load that is responsible for the air conditioning of a building part. In addition, many systems have overcapacity due to the compulsory installation ratios [4]. This results in energy wastage and economic losses. Nevertheless, there are few studies of hybrid operation with existing heating sources. To resolve this problem, this paper proposes an operation method to use geothermal heat pump systems efficiently in combination with existing heat sources. Several studies have evaluated the performance of geothermal heat pump systems. m published a comprehensive overview of the current international legal status for the use of shallow geothermal energy. In their study, an international survey was performed using a questionnaire sent to more than 60 countries worldwide. Luo et al. [6] and Rees and Curtis [7] examined the importance of the research trends and the design of a GHP system. Nam [8] evaluated system performance through the dynamic energy simulation of heat pump systems using solar heat and geothermal heat, and made a comparative study of the changes in system performance under different climate conditions in two regions. Hwang et al. [9] confirmed the hybrid operation possibility and performance through an experimental performance evaluation. Son et al. [10] predicted the performance of air conditioning and heating through a simulation after assuming the installation of a geothermal heat pump system instead of the existing facilities in a building. Jang [11] performed an experiment to analyze the performance of the system using EES and TRNSYS. In addition, many studies have examined the efficiency of ground heat exchangers, ground circulating water, and temperature. Park [12] measured the change in ground temperature according to depth and analyzed the performance of the heat exchanger and the geothermal heat pump to check the optimal design of a geothermal heat pump system, identify the operating conditions, and determine whether a ground well is suitable for the system. Jung et al. [13] analyzed the coefficient of performance (COP) and consumption energy of a geothermal heat pump unit by the change in flow of ground circulating water. Lee [14] devised an optimal design method by analyzing the design load and consumption data in a geothermal system as the main source with a focus on electricity, air conditioning, and heating energy consumption in the target building. These studies were performed on geothermal heat pump systems from different perspectives, but few studies have examined a hybrid operation method for a geothermal heat pump system with an existing heat source. Bakirci et al. [15] examined the performance of a solar-GSHP system with vertical ground handling equipment (GHE). The results suggested that this solar-GSHP system can be used for heating applications in the cold climate regions of Turkey. Yang [16] assessed the performance characteristics of a vertical U-bend direct-expansion ground source (geothermal) heat pump system (DX GSHPS) for both heating and cooling. Kim et al. [17] reported that the design of a proper indoor temperature for variable outdoor conditions is important for maintaining high system performance and reliability in a hybrid solar-geothermal CO2 heat pump system. Ozgener and Hepbaslib [18] examined the energetic and exergetic modeling of GSHP systems for system analysis and performance assessments. Energies 2016, 9, 356 3 of 19

Salvalai [19] reported the results of a parameter estimation study of a water-to-water heat pump model, which implemented in the IDA Indoor Climate and Energy (IDA-ICE) simulation environment, and was presented and validated using experimental data. Ally et al. [20,21] provided excellent data on the energy savings and the fraction of energy that could be extracted from the ground for both space heating and water heating. They reported the advanced building envelope characteristics, instrumentation, data acquisition, tabulated data collected, analysis, measures of performance, and conclusions for ground-coupled heat pumps for reducing the energy footprint in buildings. Chen et al. [22] discussed the use of an underground water-source water-loop heat-pump (UWSWLHP) air-conditioning system for tall apartment buildings in Beijing. Sivasakthivel et al. [23] provided extensive data from India highlighting the energy and CO2 savings potential of the GSHP system. In the case of GSHP, the CO2 emissions were between 4022 million kg and 12,071 million kg, resulting in approximately a 24.54% per year reduction. Yoo et al. [24] examined the hybrid operation of an existing heat source and geothermal heat pump system, adding the opening/closing extent of the AHU to the control point and proposing a sequential control by time deferment. On the other hand, studies have focused on the design of geothermal heat pump systems, the performance evaluation of individual systems, and the improvement in efficiency of ground heat exchangers. In South Korea, most buildings used geothermal pumps and existing heat source systems simultaneously, but there is a shortage of studies on how to operate the hybrid heat source. Some relevant studies did not present any specific control method considering the performance of heat pumps. Therefore, the present study developed a sequential operation method of multi-geothermal heat pump systems using the COP to improve their efficiency. Moreover, this study compared the existing operation method with the proposed method through simulation. The existing heat sources in buildings are chiefly in hybrid use with a geothermal heat pump system, but the problem is that the high-efficiency geothermal heat pump system mainly takes a partial load form, being responsible for the air conditioning of a building part, and is designed separately. Therefore, this study selected a target building with a geothermal heat pump system, analyzed its operation status, checked the problems, and proposed an operation method to improve the efficiency of the multi geothermal heat pump system. This study modelled the target building and analyzed the energy using TRNSYS, a dynamic energy simulation tool, to apply the COP and evaluate the operation method.

2. Obligatory Renewable Energy Installation System In Korea, the Obligatory Renewable Energy Installation System of public buildings is the main applicable legal regime. The Ministry of Trade, Industry and Energy enacted and implemented the “Act on the Promotion of the Development, Use and Diffusion of New and Renewable Energy”. Pursuant to its Article 12, Section 2 and Enforcement Decree Article 15, Section 1, a part of the expected annual total energy consumption at any public organization building whose total area for new construction, or extension for reconstruction is 1000 m2 or higher must be supplied with energy produced by renewable energy facilities. Table1 lists the rates.

Table 1. Renewable energy supply duty rates in Korea.

Year 2011–2012 2013 2014 2015 2016 2017 2018 2019 2020 or Later Supply duty rate (%) 10 11 12 15 18 21 24 27 30

The obligatory supply rates of renewable energy can be calculated using Equation (1). The expected energy consumption is calculated by Equation (2), Tables2 and3 using the total area of the building and unit energy consumption, purpose-specific correction coefficient, and area coefficient [3]:

Renewable energy production amount Renewable energy supply duty rate p%q “ ˆ 100 (1) Expected energy consumption amount Energies 2016, 9, 356 4 of 19

Expected energy consumption amount “ (2) Total ﬂoor area ˆ Energy consumption rate ˆ Correction factor ˆ Area coefﬁcient

Table 2. Building purpose-speciﬁc unit energy consumption and correction coefﬁcients.

Unit Energy Consumption Purpose-Specific Category (kWh/m2¨ year) Correction Coefficient Correctional and military facilities 392.07 1.64 Public purpose Broadcasting and communication facilities 490.18 1.31 Business facilities 371.66 1.73 Culture and assembly facilities 412.03 1.56 Religious facilities 257.49 2.50 Medical facilities 643.52 1.00 Education and research facilities 231.33 2.78 Educational and Facilities for the elderly and infirm 175.58 3.67 social facilities Training facilities 231.33 2.78 Sports facilities 235.42 2.73 Cemetery-related facilities 234.99 2.74 Sightseeing and rest facilities 437.08 1.47 Funeral hall 234.99 2.74 Sales and operation facilities 408.45 1.58 Transport facilities 374.47 1.72 Commercial purpose Business facilities 374.47 1.72 Lodging facilities 526.55 1.22 Recreation facilities 400.33 1.61

Table 3. Regional coefﬁcients according to climate conditions in Korea.

Category Seoul Incheon Daegu Busan Gwangju Daejeon Jeju Regional coefﬁcient 1.00 0.97 1.04 0.93 1.01 1.00 0.97

Similarly, renewable energy was distributed and expanded, but the systems are often operated without knowing even the efﬁciency or load distribution ratio. In addition, renewable facilities are often responsible for partial loads and designed for overcapacity to meet legal standards. Therefore, it is necessary to improve the energy efﬁciency using renewable energy resources preferentially.

3. Multi-Geothermal Heat Pump System Operation Method Using COP Geothermal heat pumps are installed according to the mandated renewable energy installation system, but the focus is often placed on the obligatory capacity alone rather than the efficient use of the system. Although efficient and environmentally friendly, heat pump systems are often responsible for the partial load of a building. The independent use of a geothermal heat pump system often focuses on the mere adjustment to meet the legal capacity obligations rather than the building load, which leads to overdesign compared to a shared load. This paper proposes an operation method to save energy by upgrading the device efficiency using COP, using the remaining energy of a geothermal heat pump system preferentially, and reducing the use of the existing heat source in a selected building.

3.1. Analysis of Existing Multi-Geothermal Heat Pump System The target is a university hospital building located in Gyeongsangbuk-do. The building has a total area of approximately 81,928 m2 and consists of three stories below ground and nine stories above ground. The system operates a 530 RT geothermal heat pump according to the obligatory renewable energy installation system. Table4 provides an overview of the target building. The building has twelve 530 RT geothermal heat pumps, and three 800 RT and one 500 RT absorption chiller-heaters. As shown in Figure1, the building has separate spaces for the geothermal heat pump and absorption chiller-heater, and the heat source water produced is supplied to the air handling unit (AHU) applicable zones. The absorption chiller-heater is responsible for the entire Energies 2016, 9, 356 5 of 19 geriatric ward and the higher part of the main building, while the geothermal heat system air-conditions the lower part of the main building and the cancer center. A total of 32 AHUs are present, of which Energies 2016, 9, 356 5 of 18 seven and 25 AHUs correspond to the geothermal heat pump and the absorption chiller-heater, respectively.air‐conditions The operating the lower amountpart of the control main isbuilding a variable and controlthe cancer method center. for A thetotal amount of 32 AHUs of equipment are accordingpresent, to of the which indoor seven load and and 25 is AHUs established correspond by both to the the geothermal geothermal heat heat pump pump and andthe absorption the absorption chiller-heaterchiller‐heater, based respectively. on the cold The and operating hot water amount supply control temperatures. is a variable control method for the amount of equipment according to the indoor load and is established by both the geothermal heat pump and the absorption chillerTable‐ 4.heaterSummary based of on the the building cold and information. hot water supply temperatures.

ListTable 4. Summary of the building information. Contents BuildingList nameContents K University Hospital BuildingBuilding purpose name K University Medical Hospital and funeral services NumberBuilding of purpose ﬂoors ThreeMedical stories and funeral below, services nine above the ground BuildingNumber of area floors Three stories below, nine above9737 mthe2 ground TotalBuilding area area 9737 m281,929 m2 Total area Main building—three81,929 stories m2 below, nine above the ground Hospital purpose Main building—threeGeriatric ward—One stories below, story nine below, above seven the aboveground the ground Hospital purpose GeriatricCancer ward—One ward—two story below, stories seven below, above ﬁve the above ground the ground Area using geothermal heat pump Cancer ward—two stories below,2263 five above m2 the ground 2 GeothermalArea using heatgeothermal pump heat capacity pump 2263 m 530 RT 1 Geothermal heat pump capacity 530 RT 1 1 RT: Refrigeration Ton. 1 RT: Refrigeration Ton.

FigureFigure 1. System 1. System diagram diagram of of the the hybrid hybrid operation operation method for for the the geothermal geothermal heat heat pump pump system system and and absorption chiller‐heater. absorption chiller-heater. The geothermal heat pump system is not used efficiently due to system separation from the Theabsorption geothermal chiller‐ heatheater. pump A problem system is isencountered not used efﬁcientlydue to the dueoperating to system amount separation control of from the the absorptiongeothermal chiller-heater. heat pump system A problem by the is cold encountered and hot water due supply to the temperatures; operating amount every geothermal control of the geothermalsystem is heat operated pump continually system by to meet the cold the set and temperature, hot water even supply in the temperatures; case of a low load. every geothermal system is operated continually to meet the set temperature, even in the case of a low load. 3.2. Existing Multi‐Geothermal Heat Pump System 3.2. ExistingThis Multi-Geothermal study used TRNSYS Heat 17, Pump a dynamic System energy simulation tool, to evaluate the performance of the multi‐geothermal heat pump system installed at the target building. As shown in Figure 2, the This study used TRNSYS 17, a dynamic energy simulation tool, to evaluate the performance of the target building was modelled using the TRNSYS 3D of Google Sketch up, the detailed data was multi-geothermal heat pump system installed at the target building. As shown in Figure2, the target

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Energies 2016, 9, 356 6 of 18 building was modelled using the TRNSYS 3D of Google Sketch up, the detailed data was entered and theentered HVAC and system the HVAC established system using established Trnbuild modelingusing Trnbuild and Simulation modeling Studio and modeling.Simulation Tables Studio5 andmodeling.6 list the Tables equipment 5 and 6 data list andthe equipment the indoor roomdata and conditions, the indoor respectively. room conditions, respectively.

Figure 2. Target building TRNSYS 3D modeling. Figure 2. Target building TRNSYS 3D modeling.

Table 5. Geothermal heat pump and absorption chiller and heater data information. Table 5. Geothermal heat pump and absorption chiller and heater data information. Lists Geothermal Heat Pump Absorption Chillers and Heater ListsCooling 46.24 Geothermal RT Heat Pump Absorption800 USRT Chillers and Heater Capacity Cooling 46.24 RT 800 USRT Capacity Heating 48.86 RT 2,419,500 Kcal/h Leaving Cooling Heating7 °C 48.86 RT 2,419,5007 °C Kcal/h temperature Heating Cooling45 °C 7 ˝C60 °C 7 ˝ C Leaving temperature ˝ ˝ Entering Cooling Heating12 °C 45 C12 °C 60 C Cooling 12 ˝C 12 ˝C Enteringtemperature temperature Heating 40 °C 55 °C Heating 40 ˝C 55 ˝C Table 6. Individual room set points. Table 6. Individual room set points. Temperature (°C) Humidity (%) Room TemperatureSummer (˝C)Winter Summer HumidityWinter (%) Room Ward Summer26 Winter23 Summer55 45 Winter DepartmentWard of pharmacy 26 26 2322 55 55 45 45 DepartmentOutpatient of pharmacy department 2626 2222 55 55 45 45 Outpatient departmentOperating room 2624 2224 50 55 50 45 OperatingRecovery room room 2424 2425 55 50 50 50 Recovery room 24 25 55 50 Office 26 20 55 45 Ofﬁce 26 20 55 45 AssemblyAssembly hall hall 2626 2020 55 55 45 45

3.2.1. Control Methods for the Existing Geothermal Heat Pump 3.2.1. Control Methods for the Existing Geothermal Heat Pump A control was established based on the operating amount control by the set temperature of the A control was established based on the operating amount control by the set temperature of existing geothermal heat pump system through the system composition of a geothermal heat pump, the existing geothermal heat pump system through the system composition of a geothermal heat temperature sensor and AHU. The strategy followed was to operate the system if a set indoor pump, temperature sensor and AHU. The strategy followed was to operate the system if a set indoor temperature reaches its upper limit (Tset + d) with a temperature sensor attached to the cold and temperature reaches its upper limit (Tset + d) with a temperature sensor attached to the cold and water water pipe of the geothermal heat pump system. The operating amount control was established pipe of the geothermal heat pump system. The operating amount control was established according to according to whether the set temperature is met or not. Make on/off control is set up to stop operation whether the set temperature is met or not. Make on/off control is set up to stop operation if the set if the set temperature reaches its lower limit (Tset − d). Figure 3 presents the control algorithm. temperature reaches its lower limit (Tset ´ d). Figure3 presents the control algorithm.

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Figure 3. Control methods for an existing GHP.

(1) If no heat pump system is currently operated: Operate a heat pump, if its leaving water temperature is higher than its set temperature. Subsequently,Operate a determine heat pump, the ifoperation its leaving of a waternext heat temperature pump according is higher to whether than its to set control temperature. the set Subsequently,temperature. Maintain determine a current the operation status, ofunless a next the heat leaving pump water according temperature to whether is higher to control than the set temperature. Maintain a current status, unless the leaving water temperature is higher than the set(2) temperature.If a heat pump system is currently operated: (2) StopIf a heat the pumpoperation system of isthe currently heat pump, operated: if its leaving water temperature is higher than its set temperature. If the heat pump leaving cold water temperature is higher than its setpoint, operate a next Stopheat thepump operation additionally of the to heat adjust pump, the setpoint. if its leaving Stop waterthe operation temperature of the is heat higher pump than system its set temperature.sequentially, if If the the leaving heat pump water leaving temperature cold water is lower temperature than the setpoint. is higher than its setpoint, operate a next heat pump additionally to adjust the setpoint. Stop the operation of the heat pump system sequentially,3.2.2. Performance if the leavingEvaluation water of temperatureExisting Geothermal is lower Heat than thePump setpoint.

3.2.2.The Performance coefficient Evaluation of performance of Existing (COP) Geothermal is used as a Heat criterion Pump to judge the heat efficiency of a heat source system. As shown in Figure 4, the maximum COP was 4.2 and the energy consumption was The coefﬁcient of performance (COP) is used as a criterion to judge the heat efﬁciency of a heat lowest at the maximum COP according to the simulation result of the geothermal heat pump system source system. As shown in Figure4, the maximum COP was 4.2 and the energy consumption was in the target building. lowest at the maximum COP according to the simulation result of the geothermal heat pump system Figure 5a shows the summer cold water entering and leaving temperatures and COPs of the in the target building. existing geothermal heat pump system. The cold water leaving and entering temperatures were on Figure5a shows the summer cold water entering and leaving temperatures and COPs of the average 7.51 °C and 7.91 °C, respectively, and the COPs were a maximum of 4.1 with an average of 3.55. existing geothermal heat pump system. The cold water leaving and entering temperatures were on Figure 5b shows the winter hot water entering and leaving temperatures and the COPs of the average 7.51 ˝C and 7.91 ˝C, respectively, and the COPs were a maximum of 4.1 with an average of 3.55. existing geothermal heat pump system. The hot water leaving and entering temperatures were on Figure5b shows the winter hot water entering and leaving temperatures and the COPs of the average 43.46 °C and 41.71 °C, respectively, and the COPs were a maximum of 4.2 and an average of existing geothermal heat pump system. The hot water leaving and entering temperatures were on 3.59. The COP did not reach the maximum value of 4.2 and remained around 3.5 on average average 43.46 ˝C and 41.71 ˝C, respectively, and the COPs were a maximum of 4.2 and an average throughout the year on account of the operating amount control of the geothermal heat pump of 3.59. The COP did not reach the maximum value of 4.2 and remained around 3.5 on average system for the set temperature control of the cold and hot water supplied to the load side. throughout the year on account of the operating amount control of the geothermal heat pump system for the set temperature control of the cold and hot water supplied to the load side.

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Figure 4. COP and energy consumption relationship inin thethe GHPGHP system.system. Figure 4. COP and energy consumption relationship in the GHP system.

(a) (a)

(b) (b) Figure 5. Entering and leaving temperatures and COPs of the existing geothermal heat pump system: FigureFigure 5. 5.Entering Entering and and leaving leaving temperaturestemperatures and and COPs COPs of of the the existingexisting geothermalgeothermal heatheat pumppump system:system: (a) Summer; (b) Winter. (a(a)) Summer; Summer; ( b(b)) Winter. Winter.

The operating amount control by the set temperature is simple and reflects the room load The operating amount control by the set temperature is simple and reflects the room load indirectly. On the other hand, even when there is no load in the conditioning space, the system is indirectly. On the other hand, even when there is no load in the conditioning space, the system is

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The operating amount control by the set temperature is simple and reﬂects the room load indirectly. On the other hand, even when there is no load in the conditioning space, the system is operated to meet the set temperature of the leaving water temperature, leading to unnecessary energy consumption. Geothermal heat pumps are operated individually, leading to a waste of energy and the lowering of the COP. This problem makes it necessary to establish ways of reducing energy wastage without limiting the set point of the leaving water temperature. Therefore, an energy-efﬁcient system control can be established if the geothermal heat pump system is controlled based on the maximum COP.

3.3. Multi Geothermal Heat Pump System Control Method Using Coefﬁcient of Performance (COP)

3.3.1. COP Monitoring Method This study evaluated a method to effectively apply low-cost and high-level COP monitoring to a building control and diagnosis system to effectively control the geothermal heat pump system. A calculation process was extracted using Equations (3)–(5) to obtain the necessary factors in the case of the COP calculation.

(1) Step 1. Determine Power Consumption Model

. . 1{m Vsuc NVsuc Vsuc c Pdis η “ . “ . “ “ 1 ´ ´ 1 (3) υ V 1 ´ c P VD NVD D «ˆ suc ˙ ff . . . . V η V η NV m “ suc “ v D “ v D (4) vsuc Vsuc vsuc pm´1q{m . . . m P W “ mw “ m p v dis ´ 1 (5) suc suc m ´ 1 P # «ˆ suc ˙ ff+ . . where, η = volumetric efficiency; VD = compressor displacement rate; N = the number of suction v . 3 strokes per unit time (rpm); VD = the volume of the suction chamber(s) (m ); Vsuc = actual volumetric flow rate evaluated at the suction condition; m = empirical parameter; Psuc = pressure at the suction . . condition; W = power consumption; m = mass rate of refrigerant flow; vsuc = specific volume at suction condition; w = specific work; c = clearance factor.

(2) Step 2. The performance of a geothermal heat pump is generally represented by COP and can be calculated using Equation (6):

Q COP “ . (6) W . where, Q = capacity (kW); W = power consumption (kW).

3.3.2. Sequential Control Methods by Coefficient of Performance in a Multi-Geothermal Heat Pump The control of a geothermal heat pump system is made possible by the highest-efficiency and lowest energy. High efficiency can be produced with the lowest energy, if sequential control is applied to the geothermal heat pump system based on the optimal COP. In Figure4, the maximum COP was 4.2 in the geothermal heat pump system of the target building. This study evaluated a method to control the geothermal heat pump system sequentially to maintain the maximum COP. Figure6 shows a sequential control method algorithm of the geothermal heat pump by COP. Energies 2016, 9, 356 10 of 18

Stop the operation of the first heat pump if its leaving water temperature is higher than its set temperature (Tw < Tset + d). Operate the following heat pump because the load of a room is not Energies 2016, 9, 356 10 of 19 borne if the COP of the first heat pump is higher than its set value (COPHP > COPset + d).

Start

Input : Tw, Tset, COPHP, COP , S set Tw : Heat pump supply cold water temperature Tset : Heat pump supply cold water setpoint COPHP : Heat pump COP COP : Heat pump COP setpoint Call Tw set S : Heat pump operating signal (1 = on, 0 = off)

Tw≤Tset

S1 = 0 S1 = 1

Yes COPHP1≥COPset

S2 = 1

S2 = 0 COPHP1≥COPset Yes

FigureFigure 6. SequentialSequential control control method algorithm by the COP.

(1) FigureIf no heat 7 shows pump a system comparison is currently of energy operated: consumption between existing and sequential control methods at a geothermal heat pump. HP 4 satisfied most loads when the sequential operation methodOperate by COP a heat was pumpapplied if to its a leavingmulti‐geothermal water temperature heat pump is system. higher Even than itswhen set the temperature load of a (buildingTw > Tset is + met d). Operateby the mere the following high‐efficiency heat pump geothermal if the COP heat of pump, the first both heat heat pump‐source does systems not bear are the operatedload of a room,and thus even energy when itis reaches wasted. the It was COP SETpossible. Maintain to use the a currentgeothermal status, heat unless pump the system leaving better, water sincetemperature there was is higher less air than conditioning the set temperature. load than the design capacity of the geothermal heat pump system. Therefore, it will be possible to improve energy efficiency, if the remaining energy of the geothermal(2) If a heat heat pump pump system system is currently is used to operated: lower the load distribution ratio if the existing absorption chiller‐heater and maximize the use of the geothermal heat pump system. Stop the operation of the first heat pump if its leaving water temperature is higher than its set temperature (Tw < Tset + d). Operate the following heat pump because the load of a room is not borne if the COP of the first heat pump is higher than its set value (COPHP > COPset + d). Figure7 shows a comparison of energy consumption between existing and sequential control methods at a geothermal heat pump. HP 4 satisfied most loads when the sequential operation method by COP was applied to a multi-geothermal heat pump system. Even when the load of a building is met by the mere high-efficiency geothermal heat pump, both heat-source systems are operated and thus energy is wasted. It was possible to use a geothermal heat pump system better, since there was less air conditioning load than the design capacity of the geothermal heat pump system. Therefore, it will be possible to improve energy efficiency, if the remaining energy of the geothermal heat pump system is used to lower the load distribution ratio if the existing absorption chiller-heater and maximize the use of the geothermal heat pump system.

Figure 7. Comparison of the energy consumption at GHPs.

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Stop the operation of the first heat pump if its leaving water temperature is higher than its set temperature (Tw < Tset + d). Operate the following heat pump because the load of a room is not borne if the COP of the first heat pump is higher than its set value (COPHP > COPset + d).

Start

Tw≤Tset

S1 = 0 S1 = 1

Yes COPHP1≥COPset

S2 = 1

S2 = 0 COPHP1≥COPset Yes

Figure 6. Sequential control method algorithm by the COP.

Figure 7 shows a comparison of energy consumption between existing and sequential control methods at a geothermal heat pump. HP 4 satisfied most loads when the sequential operation method by COP was applied to a multi‐geothermal heat pump system. Even when the load of a building is met by the mere high‐efficiency geothermal heat pump, both heat‐source systems are operated and thus energy is wasted. It was possible to use a geothermal heat pump system better, since there was less air conditioning load than the design capacity of the geothermal heat pump system. Therefore, it will be possible to improve energy efficiency, if the remaining energy of the geothermal heat pump system is used to lower the load distribution ratio if the existing absorption Energies 2016, 9, 356 11 of 19 chiller‐heater and maximize the use of the geothermal heat pump system.

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3.4. Hybrid Operation Method for a Geothermal Heat Pump System and Existing Heat Source This paper proposes a hybrid operation method by integrating the distribution with the absorption chiller‐heater and the multi‐geothermal heat pump system. The hybrid operation method involves supplying the heat source amount for each heat source system to a header and distributing it to each AHU. If the load is small due to the connection of the heat source system, the geothermal heat pump system can be considered to make air conditioning possible. Therefore, it will be necessary to set the set temperatures of the two heat source systems equally. This will be an operation method for improving the energy efficiency to use the remaining energy of the geothermal heat pump system preferentially, reduce the use of the existing heat source, as well as maximize the Figure 7. Comparison of the energy consumption at GHPs. operation of the geothermal heat pump system and minimize the use of the existing heat source, i.e.,

the absorption chiller‐heater. Figure 9 shows the hybrid operation method in an algorithm. 3.4. Hybrid Operation Method for a Geothermal Heat Pump System and Existing Heat Source (1) If no heat pump system is currently operated: This paper proposes a hybrid operation method by integrating the distribution with the absorption Operate a heat pump if its leaving water temperature is higher than its set temperature chiller-heater and the multi-geothermal heat pump system such as Figure8. The hybrid operation (Tw > Tset + d). Operate the following heat pump if the COP of the first heat pump does not bear the methodload involves of a room, supplying even when the it heatreaches source the COPSET. amount Maintain for each the heat current source status system unless to the a headerleaving and distributingwater temperature it to each AHU.is higher If thethan load the set is smalltemperature. due to the connection of the heat source system, the geothermal heat pump system can be considered to make air conditioning possible. Therefore, it (2) If a heat pump system is currently operated: will be necessary to set the set temperatures of the two heat source systems equally. This will be an operationStop method the operation for improving of the first the energyheat pump efﬁciency if its leaving to use water the remaining temperature energy is higher of thethan geothermal its set heat pumptemperature system (Tw preferentially, < Tset + d). Operate reduce the the following use of the heat existing pump heat because source, the load as well of a as room maximize is not the borne if the COP of the first heat pump is higher than its set value (COPHP > COPset + d). Begin the operation of the geothermal heat pump system and minimize the use of the existing heat source, i.e., operation of the absorption chiller‐heater if no load is borne, even when the geothermal heat pump the absorption chiller-heater. Figure9 shows the hybrid operation method in an algorithm. system is operated at the maximum.

Figure 8. System diagram on the hybrid operation method for GHP system and absorption Figurechiller 8. System‐heater. diagram on the hybrid operation method for GHP system and absorption chiller-heater.

Energies 2016, 9, 356 12 of 19 Energies 2016, 9, 356 12 of 18

Start

TW : Heat Pump supply cold water temperature Tw,set : Heat Pump supply cold water setpoint Input :TW, Tw,set , COPHP1 : Heat Pump COP COPHP1,COPset, SH,one, SA, COPset : Heat Pump COP setpoint SH,on, SH,onmax SH,one: One Heat Pump operating signal (1 = on, 0 = off) S : Absorption chiller operating signal Call T A Room (1 = on, 0 = off) SH,on : The number of heat pumps turned on. SH,onmax : The maximum number of the heat pump can be turned on.

Yes TW=Tr,set No

SH,one = 0 SH,one = 1

Yes COPHP1=COPset No

S = 1 SH,on=SH,onmax No A Yes

SH,one = 1

Figure 9. Hybrid operation method for GHP system and absorption chiller‐heater. Figure 9. Hybrid operation method for GHP system and absorption chiller-heater.

4. Evaluation of Geothermal Heat Pump System Operation Method Using COP (1) If no heat pump system is currently operated: According to the existing operation method, the multi‐geothermal heat pump system was subjectedOperate to operating a heat pump amount if itscontrol leaving based water on the temperature cold and hot is higherwater supply than its temperatures, set temperature and (wasTw >operated Tset + d ).in Operate system theseparation following from heat the pump existing if the heat COP source, of the i.e. first, the heat absorption pump does chiller not bear‐heater. the loadTherefore, of a room, this evenstudy when applied it reaches a sequential the COPSET. control Maintain using the the COP current to statusimprove unless the theefficiency leaving of water the temperaturemulti‐geothermal is higher heat than pump the system, set temperature. and proposed a hybrid operation method with an integral system to use the geothermal heat pump system with more efficiency than the existing heat source. (2)Table If 7 a provides heat pump a summary system is of currently the simulation operated: CASE. Stop the operation of the first heat pump if its leaving water temperature is higher than its set Table 7. Simulation CASE. temperature (Tw < Tset + d). Operate the following heat pump because the load of a room is not borne if the COP ofExisting the first CASE heat pump is higher than its set value (COPHPProposed > COPsetCASE + d). Begin the operationSequential of Control the absorption by Coefficient chiller-heater of Performance if no in load isSequential borne, even Control when by Coefficient the geothermal of Performance heat pump in systemMulti is operated Geothermal at the Heat maximum. Pump System Multi Geothermal Heat Pump System + + 4.Separation Evaluation Operation of Geothermal Method for Heat Geothermal Pump System Heat OperationHybrid Operation Method Using Method COP for Geothermal Heat Pump System and Absorption Chiller‐Heater Pump System and Absorption Chiller‐Heater According to the existing operation method, the multi-geothermal heat pump system was subjected4.1. Analysis to of operating Temperatures amount and COP control at GHP based System on the cold and hot water supply temperatures, and was operated in system separation from the existing heat source, i.e., the absorption chiller-heater. Therefore,Figure this 10a study shows applied the results a sequential obtained control by applying using thethe COP sequential to improve operation the efficiency method by of the multi-geothermalsummer COP to the heat geothermal pump system, heat and pump proposed system. a hybridThe leaving operation and entering method with temperatures an integral were system on toaverage use the 12.33 geothermal °C and 13.33 heat pump°C, respectively. system with The more COPs efficiency were on thanaverage the existing4.1 with heata maximum source. of Table 4.2.7 providesFigure a summary 10b presents of the the simulation results obtained CASE. by applying the sequential operation method by the winter COP to the geothermal heat pump system. The leaving and entering temperatures were on average 42.3 °C and 41.5 °C, respectively. The COPs were on average 4.12 with a maximum of 4.2. In both summer and winter, the COP was improved with sequential control in the multi‐geothermal heat pump system.

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Table 7. Simulation CASE.

Existing CASE Proposed CASE Sequential Control by Coefﬁcient of Performance in Sequential Control by Coefﬁcient of Performance in Multi Geothermal Heat Pump System Multi Geothermal Heat Pump System + + Separation Operation Method for Geothermal Heat Hybrid Operation Method for Geothermal Heat Pump System and Absorption Chiller-Heater Pump System and Absorption Chiller-Heater

4.1. Analysis of Temperatures and COP at GHP System Figure 10a shows the results obtained by applying the sequential operation method by the summer COP to the geothermal heat pump system. The leaving and entering temperatures were on average 12.33 ˝C and 13.33 ˝C, respectively. The COPs were on average 4.1 with a maximum of 4.2. Energies 2016, 9, 356 13 of 18

(a)

(b)

Figure 10. Entering and leaving temperatures and COPs of GHP system by COP sequential control: Figure 10. Entering and leaving temperatures and COPs of GHP system by COP sequential control: (a) (a) Summer; (b) Winter. Summer; (b) Winter.

4.2. Analysis of the Indoor Environment Figure 10b presents the results obtained by applying the sequential operation method by the winterFigures COP to11 the and geothermal 12 present heat graphs pump showing system. the The representative leaving and indoor entering temperature temperatures in summer were on averageand winter 42.3 with˝C and the 41.5indoor˝C, temperature respectively. set The to COPs 24 °C. were In hospitals, on average it 4.12is more with important a maximum to keep of 4.2. the building occupants comfortable than in other types of buildings, and it is necessary to control the indoor temperature properly. According to the simulation, it was possible to control the indoor temperature within the set difference with both methods. The nearer the method drew to the sequential operation method, the more the difference in the average indoor temperature was reduced. This shows that it is possible to control the indoor temperature comfortably and stably in an indoor heated environment.

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In both summer and winter, the COP was improved with sequential control in the multi-geothermal heat pump system.

4.2. Analysis of the Indoor Environment Figures 11 and 12 present graphs showing the representative indoor temperature in summer and winter with the indoor temperature set to 24 ˝C. In hospitals, it is more important to keep the building occupants comfortable than in other types of buildings, and it is necessary to control the indoor temperature properly. According to the simulation, it was possible to control the indoor temperature within the set difference with both methods. The nearer the method drew to the sequential operation method, the more the difference in the average indoor temperature was reduced. This shows that it is possible to Energiescontrol 2016, 9, the356 indoor temperature comfortably and stably in an indoor heated environment. 14 of 18

(a)

(b)

FigureFigure 11. Indoor 11. Indoor temperature temperature in in the the existing CASE: CASE: (a )( Summer;a) Summer; (b) Winter. (b) Winter.

(a)

Figure 12. Cont.

Energies 2016, 9, 356 14 of 18

(a)

(b) Energies 2016, 9, 356 15 of 19 Figure 11. Indoor temperature in the existing CASE: (a) Summer; (b) Winter.

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Figure 12. Cont.

(b)

Figure 12. Indoor temperature in the proposed CASE: (a) Summer; (b) Winter. Figure 12. Indoor temperature in the proposed CASE: (a) Summer; (b) Winter.

4.3. Analysis of Energy Consumption 4.3. Analysis of Energy Consumption Figure 13 presents a graph comparing the energy consumption of the geothermal heat pump Figure 13 presents a graph comparing the energy consumption of the geothermal heat pump systems using the existing and proposed operation methods. Each heat pump showed an increase in systems using the existing and proposed operation methods. Each heat pump showed an increase energy consumption according to the sequential operation method by the COP and the proposed in energy consumption according to the sequential operation method by the COP and the proposed operation method, which preferentially uses a high‐efficiency geothermal heat pump system in the operation method, which preferentially uses a high-efﬁciency geothermal heat pump system in the system integration with an absorption chiller‐heater. system integration with an absorption chiller-heater. Figure 14 shows the total energy consumption of the geothermal heat pump system and the absorption chiller-heater. In terms of energy consumption, compared to the existing operation method, which uses the geothermal heat pump system and the absorption chiller-heater in system separation, the geothermal heat pump system showed an approximately 35% increase, while the absorption chiller-heater showed an approximately 45% decrease, according to the hybrid operation method of the geothermal heat pump system and the absorption chiller-heater. As a result, the total energy consumption decreased by approximately 40%. The sequential operation by the COP made it possible

Figure 13. Energy consumption of each GHP in hybrid GHP Operation Method.

Figure 14 shows the total energy consumption of the geothermal heat pump system and the absorption chiller‐heater. In terms of energy consumption, compared to the existing operation method, which uses the geothermal heat pump system and the absorption chiller‐heater in system separation, the geothermal heat pump system showed an approximately 35% increase, while the absorption chiller‐heater showed an approximately 45% decrease, according to the hybrid operation method of the geothermal heat pump system and the absorption chiller‐heater. As a result, the total energy consumption decreased by approximately 40%. The sequential operation by the COP made it possible to operate the geothermal heat pump system efficiently, and the hybrid operation with the absorption chiller‐heater made it possible to bear the load, to apply the remaining energy of the

Energies 2016, 9, 356 15 of 18

(b)

Figure 12. Indoor temperature in the proposed CASE: (a) Summer; (b) Winter. Energies 2016, 9, 356 16 of 19 4.3. Analysis of Energy Consumption to operateFigure the 13 geothermal presents a heatgraph pump comparing system the efﬁciently, energy and consumption the hybrid of operation the geothermal with the heat absorption pump chiller-heatersystems using made the existing it possible and to proposed bear the operation load, to apply methods. the remaining Each heat energy pump ofshowed the geothermal an increase heat in pumpenergy system consumption to the load according of the existingto the sequential absorption operation chiller-heater, method and by lower the COP the shareand the ratio proposed of load, asoperation well as reducemethod, the which energy preferentially consumption uses of a the high existing‐efficiency heat geothermal source by maximizing heat pump thesystem use ofin the geothermalsystem integration heat pump with system. an absorption chiller‐heater.

Energies 2016, 9, 356 16 of 18 geothermal heat pump system to the load of the existing absorption chiller‐heater, and lower the share ratio of load, as well as reduce the energy consumption of the existing heat source by maximizing theFigure use of 13. the Energy geothermal consumption heat ofpump each system. GHP in hybrid GHP Operation Method.

Figure 14. EnergyEnergy consumption of the absorption chiller chiller-heater‐heater and GHP System.

4.4. Relationship between COP and Load at Geothermal Heat Pump System Figure 1515 presentspresents the the results results of of the the analysis analysis of the of COPthe COP relationship relationship according according to the to load. the Under load. Underthe same the load same condition, load condition, a heat pump a heat showed pump ashowed higher performancea higher performance in the case in of the the case application of the applicationof sequential of sequential control by control COP. The by COP. existing The operationexisting operation method method shows shows that the that heat the pump heat pump has a has low a lowefficiency efficiency because because the on/offthe on/off control control is used is used according according to the to coldthe cold water water temperature temperature irrespective irrespective of the of theefficiency efficiency of the of heat the pump.heat pump. On the On other the hand,other sequentialhand, sequential control bycontrol the COP by the showed COP high showed efficiency high efficiencybecause the because control the is perfomedcontrol is perfomed in a good efficiencyin a good stateefficiency with thestate optimal with the COP. optimal COP.

Figure 15. COP and load relationship in the GHP system.

5. Conclusions A geothermal heat pump system was installed to meet the legal standards under the renewable energy installation duty system, but it is not used efficiently due to over‐design and a designed separation from the existing heat source. Therefore, the present study proposed a sequential operation method by COP to maximize the efficiency of the geothermal heat pump system. Subsequently, this study proposed an operation method to maximize and minimize the use of a high‐efficiency geothermal heat pump system and the existing heat source, respectively, and evaluated it through a simulation:

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geothermal heat pump system to the load of the existing absorption chiller‐heater, and lower the share ratio of load, as well as reduce the energy consumption of the existing heat source by maximizing the use of the geothermal heat pump system.

Figure 14. Energy consumption of the absorption chiller‐heater and GHP System.

4.4. Relationship between COP and Load at Geothermal Heat Pump System Figure 15 presents the results of the analysis of the COP relationship according to the load. Under the same load condition, a heat pump showed a higher performance in the case of the application of sequential control by COP. The existing operation method shows that the heat pump has a low efficiency because the on/off control is used according to the cold water temperature irrespective of the efficiency of the heat pump. On the other hand, sequential control by the COP showed high Energies 2016, 9, 356 17 of 19 efficiency because the control is perfomed in a good efficiency state with the optimal COP.

FigureFigure 15.15. COP and load relationship inin thethe GHPGHP system.system.

5.5. Conclusions Conclusions AA geothermalgeothermal heatheat pumppump systemsystem waswas installedinstalled toto meetmeet thethe legallegal standardsstandards underunder thethe renewablerenewable energyenergy installationinstallation dutyduty system,system, but it isis notnot usedused efficientlyefficiently duedue toto over-designover‐design andand aa designeddesigned separationseparation fromfrom the the existing existing heat heat source. source. Therefore, Therefore, the present the present study proposed study proposed a sequential a sequential operation methodoperation by method COP to by maximize COP to the maximize efficiency the of theefficiency geothermal of the heat geothermal pump system. heat pump Subsequently, system. thisSubsequently, study proposed this study an operation proposed method an operation to maximize method and to minimizemaximize theand use minimize of a high-efficiency the use of a geothermalhigh‐efficiency heat geothermal pump system heat and pump the existing system heat and source, the existing respectively, heat source, and evaluated respectively, it through and aevaluated simulation: it through a simulation: (1) The geothermal heat pump of the target building was operated to adjust the supply temperature on the side of load to the set point, and allow on/off control. Controlled by the supply temperature, the geothermal heat pump was operated, irrespective of the room space load, which led to a waste of energy. Therefore, this study implemented the sequential operation of the geothermal heat pump by COP monitoring based on the optimal COP per geothermal heat pump. When the COP was 4.2, the consumption of the least energy and the highest efficiency were achieved. (2) The existing heat source was operational with a resulting waste of energy, even when the entire load was met by the geothermal system due to the system separation between the geothermal system and existing heat source system. Therefore, this study proposed an operation method to maximize the operation of the geothermal heat pump system by integrating the system, to enhance the efficiency, and minimize the use of an existing heat source. (3) The system integration method preferentially uses a high-efficiency geothermal heat pump system because there is an integration of headers between the geothermal heat system and the absorption chiller-heater. The room load is removed using the absorption chiller-heater if the room load is not removed using the geothermal heat pump system alone. Compared to the existing operation method, the geothermal heat pump system increased its energy consumption by approximately 35%, and the absorption chiller-heater consumption decreased by 45%. Accordingly, the total energy consumption was reduced by approximately 40%, which showed that the method contributed to an improvement in energy efficiency. The present study proposed a system integration operation method with the existing heat source, the absorption chiller-heater, by applying the sequential operation by COP to improve the efficiency of the geothermal heat pump system. The proposed operation method met the room heat environment demands, and reduced the energy consumption, compared to the existing operation method. Follow-up studies will make it possible to efficiently operate the geothermal heat pump system by applying the proposed operation method to real buildings and evaluating the results. Energies 2016, 9, 356 18 of 19

Acknowledgments: This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology. (NRF-2014R1A1A2058912). Author Contributions: All authors contributed to this work. Young-Ju Jung performed the result analysis of simulation and wrote the major part of this article. Hyo-Jun Kim conducted the energy simulation. Bo-Eun Choi summarized simulation data. Jae-Hun Jo performed the result discussion and gave technical support. Young-Hum Cho was responsible for this article and gave conceptual advice. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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