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Article

An Environmental and Economic Assessment for Selecting the Optimal Ground Heat Exchanger by Considering the Entering Water Temperature

Department of Architectural Engineering, Yonsei University, Seoul 120-749, Korea
*
Author to whom correspondence should be addressed.
These authors contributed equally to this work.
Academic Editor: Hossam A. Gabbar
Energies 2015, 8(8), 7752-7776; https://doi.org/10.3390/en8087752
Received: 29 June 2015 / Revised: 23 July 2015 / Accepted: 23 July 2015 / Published: 29 July 2015
(This article belongs to the Special Issue Energy Conservation in Infrastructures)

Abstract

In order to solve environmental problems such as global warming and resource depletion in the construction industry, interest in new renewable energy (NRE) systems has increased. The ground source heat pump (GSHP) system is the most efficient system among NRE systems. However, since the initial investment cost of the GSHP is quite expensive, a feasibility study needs to be conducted from the life-cycle perspective. Meanwhile, the efficiency of GSHP depends most significantly on the entering water temperature (EWT) of the ground heat exchanger (GHE). Therefore, this study aims to assess the environmental and economic effects of the use of GHE for selecting the optimal GHE. This study was conducted in three steps: (i) establishing the basic information and selecting key factors affecting GHE performances; (ii) making possible alternatives of the GHE installation by considering EWT; and (iii) using life-cycle assessment and life-cycle cost, as well as comprehensive evaluation of the environmental and economic effects on the GHE. These techniques allow for easy and accurate determination of the optimal design of the GHE from the environmental and economic effects in the early design phase. In future research, a multi-objective decision support model for the GSHP will be developed.
Keywords: ground source heat pump; ground heat exchanger; entering water temperature; life-cycle assessment; life-cycle cost ground source heat pump; ground heat exchanger; entering water temperature; life-cycle assessment; life-cycle cost

1. Introduction

Excessive carbon dioxide (CO2) emissions are currently becoming the major cause of climate change problems such as global warming and abnormal climates. Efforts to reduce CO2 are being established across the world through the strengthening of environmental regulations such as the Kyoto Protocol. Meanwhile, South Korea imports around 97% of the total amount of energy presently used domestically. As more than 25% of these imports are used for buildings, CO2 emissions from buildings can no longer be neglected. According to the 2013 Annual End-Use Energy Statistics, the total energy consumption of high energy consumption buildings that use 2,000 tons of equivalent (toe) each year reached 2,307,000 toe and 10,083,000 ton-CO2, respectively. School facilities showed energy consumption of 336,000 toe and CO2 emission of 1,397,000 ton-CO2, which accounted for around 15% of the total amount used by buildings [1].
As the seriousness of the energy problem grows, the importance of alternative energy or new renewable energy (NRE) systems become more urgent [2,3,4,5,6,7,8,9,10,11,12,13,14]. In particular, a ground source heat pump (GSHP) system is a highly efficient NRE system for heating and cooling the building [15,16,17,18]. This system relies on a relatively constant ground temperature and can transfer the earth’s heat into a building during the winter, and transfer heat out of the building during the summer [19,20,21,22,23,24,25]. A typical GSHP system consists of a ground heat exchanger (GHE) to collect/reject heat to the ground heat pump to heat/cool the building, and pump(s) to circulate a thermal fluid between the heat pumps and the GHE [26,27,28,29,30].
There have been various studies on the environmental and economic effects of GSHP systems in a building from the following three perspectives: (i) the environmental effects of the GSHP systems were analyzed under various conditions; (ii) the economic effects of the GSHP systems in a building were analyzed; and (iii) the environmental and economic effects of GSHP systems were analyzed at the same time.
First, various studies have focused on the environmental effects of the GSHP system in a building [31,32,33,34,35]. Genchi et al. [36] assessed CO2 payback of a GSHP system in Shinjuku, Tokyo, Japan that consumed a large amount of electric energy. The total CO2 emissions from the installation and operation of GSHP system were estimated to be 67,701 and 33,935 tons respectively. That means the GSHP system would result in a reduction of 54% of the CO2 emissions (39,519 ton-CO2) compared with air source heat pumps (ASHP). If an ASHP system installed within an area of 1 km2 in this region is replaced with a GSHP system, CO2 payback-time is estimated at 1.7 years. Blum et al. [37] conducted an analysis of unit-area CO2 savings from a GSHP system. Its scope was limited to 1105 installed GSHP systems. Compared with conventional heating systems, the minimum average total annual CO2 savings of all installed GSHP systems is up to 2000 tons/year due to the subsidy program which promoted GSHP systems for private one-family, two-family, and terraced houses for which the heating demand is less than 17 kW in Baden-Wurttemberg, Germany. The additional CO2 emissions are primarily associated with the affluent suburbs of the most densely populated area in the region. Saner et al. [38] conducted not only an analysis of CO2 and energy, but also life-cycle assessment (LCA) of overall environmental damages in terms of resource depletion, human health and ecosystem quality. The analysis result showed that, although a GSHP system normally contributes to the reduction of CO2 emissions, the GSHP system can cause water pollution.
Second, the economic effects of the GSHP system were analyzed under various conditions [39,40,41,42,43,44]. Zhu et al. [45] analyzed the economic effect of a GSHP system including life-cycle cost analysis. Life-cycle cost analysis is based on the Monte Carlo simulation. A GSHP system installed in Pensacola, U.S. was selected as a case. According to the analysis results, the effectiveness was highest when the subsidy for initial investment cost and operation cost was provided, compared with a conventional single-zone split system using heat pumps. However, where an incentive system did not exist, the feedback period was analyzed as more than 15 years. Sekine et al. [46] introduced a cast-in-place concrete pile (energy pile) method to reduce the high initial investment cost, and to improve the efficiency of the GSHP system. According to the analysis result, this system gained a competitive advantage with US$0.79/W, compared with US$3/W of the conventional system.
Third, the environmental and economic effects of GSHP systems were analyzed at same time [47,48,49]. Nagano et al. [47] developed a tool to analyze GSHP system performances, life-cycle cost (LCC) and life-cycle CO2 (LCCO2).They conducted a comparative evaluation of residential buildings in Sapporo, Japan using thermal response tests. For the case that the building energy loads during the winter are satisfied, they also compared the facility systems of other heat sources and their consumptions and conducted a cost analysis accordingly. Results of the calculations showed that the LCCO2 of the GSHP system is 2038 kg-CO2/year which is less than half compared to conventional oil boiler systems. Also, the GSHP system can reduce LCC by 50,000–90,000 Japanese yen/year. Self et al. [49] compared GSHP systems with facility systems of other heat sources in terms of LCC and LCCO2. According to the analysis result, the GSHP system was by far lower in CO2 emissions than other heat source systems.
In summary, there are several limitations in the previous studies: (i) most previous studies focused only on CO2 emissions of a GSHP system; (ii) there are no previous studies focusing on the comparison analysis of the environmental and economic assessments of different entering water temperatures (EWT) of a ground heat exchanger (GHE); and (iii) there are no previous studies on the optimal GHE design alternative in terms of integrated environmental and economic assessment. This study aimed to solve these limitations.
To address these challenges, this study defined the research scope as follows: (i) the scope of environmental impact assessment was defined by life-cycle stages (including material manufacturing, use and maintenance); (ii) the scope of environmental impact categories were placed into six categories (including resource depletion potential (RDP), global warming potential (GWP), ozone layer depletion potential (ODP), acidification potential (AP), eutrophication potential (EP), and photochemical oxidation potential (POCP)); and (iii) the scope of analysis focused on GHE. Based on the defined research scope, this study aims to assess the environmental and economic effects of the use of GHE for selecting the optimal GHE. This study was conducted in three steps: (i) Establishing the basic information and selecting key factors affecting GHE performances; (ii) Creating possible alternatives for GHE installation while considering EWT; (iii) Using life-cycle assessment and life-cycle cost, as well as comprehensive evaluation of the environmental and economic effects on the GHE (refer to Figure 1).
Figure 1. Framework of the research.
Figure 1. Framework of the research.
Energies 08 07752 g001

2. Establishing the Basic Information and Selecting Key Factors Affecting Ground Heat Exchanger (GHE) Performances

Prior to designing the GSHP system, the basic information for the GSHP system installation needs to be established. The study defines the region, facility type, and power supply system type where the system is to be implemented. Furthermore, the study selects key factors affecting GSHP system performances. The study establishes the information of regional factors, design factors (GHE), and system factors (heat pump) linked with the facility to design the optimal alternative.

2.1. Regional Factors of a Ground Source Heat Pump (GSHP) System

Ground thermal conductivity, ground heat capacity and ground temperature are considered regional factors. These factors affect GSHP system performances, and the value varies depending on regions. In addition, as borehole drilling costs incurred in construction are different according to the types of soil, these factors should be investigated prior to GSHP system installation.

2.2. Design Factors of GHE for a GSHP System

GHE is largely assessed by the characteristics of the borehole, such as the number and arrangement of boreholes, borehole spacing, grout conductivity, U-tube type and size, EWT, etc. and accounts for a large proportion of cost in the initial investment of GSHP system. Each component affects the GSHP system performances individually and decides GHE performances through interaction. The details of each factor are as follows (refer to Figure 2 and Table 1):
Borehole length (BL): Borehole length usually affects largely the performances and cost of GHE, and, accordingly, multi-studies are being conducted on the design of optimal length. The optimal length is decided by all the factors affecting GHE such as underground environment, GHE components and G-function.
Number of boreholes and arrangement: The number and arrangement of boreholes is a factor that can affect the total length of the borehole and accounts for a significant portion of the cost. Besides, as the distribution of temperature transferred to underground varies according to a type of arrangement such as L-, U- and rectangle types, this factor affects the performances of boreholes.
Borehole spacing (BS): As heat capacity differs according to the type of ground, optimal spacing should be designed to prevent a reduction in GHE performances caused by intersection of the scopes of ground-source heats emitted and absorbed by each borehole.
Borehole diameter (BD): Borehole diameter is designed in consideration of the U-pipe through which fluid flows and the volume of grout that fills a borehole. As borehole thermal resistance is higher with an increase in the volume of grout, the performance decreases. If its volume is too small, the inner components protected by grout can be impaired. Thus, it is necessary to make proper thickness.
U-pipe spacing (PS), pipe size and pipe type: It is necessary to combine these factors to meet temperature load in consideration of the speed of fluid that flows inside a pipe and its related heat transfer capacity. These factors, which are related to the flowing fluid the pipe directly touches, require a strength and durability above a certain level.
Grout conductivity: Grout conductivity is an element that constitutes a borehole. The higher its thermal conductivity, the lower the overall borehole resistance.
Fluid type, flow rate: Fluid type and flow rate are variables of heat transfer that occur while fluid flows in a pipe. The usual mix with other material keeps fluid from freezing.
Table 1. Overview of key factor.
Table 1. Overview of key factor.
CategoryKey Factor (Unit)References
Regional FactorGround temperature (°C), Soil type, Ground thermal conductivity (W/mK), Ground heat capacity (kJ/K·m3)[50,51,52,53]
Ground Heat ExchangerBorehole length (BL) (m), Borehole spacing (BS) (m), Borehole diameter (BD) (mm), U-pipe spacing (PS) (mm), Number of boreholes: arrangement, Grout conductivity (W/mK), Borehole thermal resistance (K/(W/m)), Pipe type, Pipe size, Fluid type, Flow rate (L/s), Entering water temperature (°C)[54,55,56,57,58,59,60,61,62]
Heat PumpCapacity (kW), Power input (kW), Heat of rejection (kW), Heat of extraction (kW), Coefficient of performance, Energy efficient rating, Entering water temperature (°C)[27,63,64,65]
Figure 2. Components of Ground Source Heat Pump (GSHP) system.
Figure 2. Components of Ground Source Heat Pump (GSHP) system.
Energies 08 07752 g002
Borehole thermal resistance: Borehole thermal resistance, which is the thermal resistance of the overall borehole determined by a mix of the above factors, affects the design of a borehole.
Entering water temperature (EWT): EWT, which is an indicator that can evaluate the final performance by a mix of each key factor, is the GHE outlet temperature that meets the energy demand of the building [66]. To calculate the EWT, Equation (1) was used [67]. EWT is designed to provide water at a high temperature in the case of heating, and at a low temperature in the case of cooling, which can minimize the load of a heat pump. Hence, the design process of the GHE will be a core factor in the design of the overall GSHP system.
E W T = T g + q i 2 m c p + Q i R b H + 1 2 π L i = 1 N ( Q i Q i 1 ) g { ( t i t i 1 ) t s , r b H }
where, EWT is GHE outlet temperature, Tg is ground temperature, q is the net heat rejection rate, i is the index to denote the end of a time step, m is mass flow rate, Cp is specific heat of the water, Qi is step heat rejection pulse, Rb is borehole thermal resistance, L is borehole length, H is borehole depth, t is time, ts is time scale, rb is borehole radius, k is ground thermal conductivity.
In this case, the total length of a borehole can be calculated by taking into account the energy and EWT necessary for the cooling and heating of the building (refer to (Equations (2) and (3))) [67].
L C = ( C a p a c i t y C o o l i n g ) ( C O P C 1 C O P C ) ( R p + R s R u n F r a c t i o n h ) E W T M a x T g . M a x . A n n u a l
L H = ( C a p a c i t y   H e a t i n g ) ( C O P H 1 C O P H ) ( R p + R s R u n F r a c t i o n h ) T g . min . A n n u a l E W T min
where, Lc is required bore length for cooling, Lh is required bore length for heating, Capacity is capacity of ground source heat pump, COP is coefficients of performance of ground source heat pump, Rp is thermal resistance of pipe, Rs is thermal resistance of soil, RunFractionh is runtime of heat pump, EWT is the heat pump entering liquid temperature

2.3. System Factors (Heat Pump) of GSHP System

A heat pump is directly linked to GHE; therefore, the heating and cooling loads of a target facility should be first identified. In this case, a design of heat pump can be conducted by taking into account hourly load, average daily load or average monthly load. Meanwhile, the EWT, which is the temperature of water entering from the GHE, has a considerable effect on the efficiency of a heat pump. Thus, a proper heat pump should be designed in conjunction with EWT and the energy demand of the building.

3. Creating Possible Alternatives for the GHE Installation by Considering Entering Water Temperature (EWT)

3.1. Selection of a Facility for Case Study

To conduct an environmental- and economic-effect assessment for the GHE in this study, a facility that has actually applied the GSHP system was selected as a case study. The following criteria were applied to selection:
According to the 2013 Annual End-Use Energy Statistics, the total energy and CO2 emissions from high energy consumption buildings that use 2000 toe per year reached 2,307,000 toe and 10,083,000 ton-CO2, respectively. The energy used by the schools among them reached 336,000 toe with CO2 emissions of 1,397,000 ton-CO2, which accounted for around 15% of the total amount used by buildings [1].
The GSHP system among an NRE system can be installed underground and designed around the systematic characteristics. Accordingly, the region with the lowest high building density should be selected.
A building that fully uses an air cooling and heating GSHP system was selected to calculate the environmental and economic effects of a GSHP system according to GHE scenarios.
Based on the above criteria, the “Y” university gym facility located in Seoul, South Korea was selected. “Y” university gym facility introduced a 422 kW GSHP system (installation capacity: 204 kW for cooling; and 218 kW for heating) in December 2012 and has replaced 100% of the annual heating and cooling energy consumption with the GSHP system. Table 2 provides data about the gym of “Y” university with regard to its characteristics and energy consumption.
Table 2. Overview of Target facility.
Table 2. Overview of Target facility.
CategoryUniversity Facilities
Year established2012
LocationSeoul
Building typeEducational facility
Electricity systemOn-grid
Heating systemIndividual heating
Progressive taxNo
Floor space of gym1197.54 m2
Major energy serviceGround source heat pump (GSHP)
Installation of capacityCooling: 204 kW/Heating: 218 kW
BoreholeLength: 143 m/hole: 22 EA

3.2. Establishment of a Basic GHE Energy Model

To analyze the effect of the GSHP system on the target building, a basic energy model that reflects the current energy consumption pattern should be established. Toward this end, this study used the energy simulation software program: (i) GLHEpro which was developed by the International Ground Source Heat Pump Association (IGSHPA) in the U.S. [68]; (ii) EnergyPlus which was developed by the U.S. Department of Energy [69]. GLHEpro uses an incremental search method to change the borehole depth and then runs a simulation. The max and min EWT results are compared with the desired values, iterating until it finds a solution that matches EWT requirements. EnergyPlus was developed to be available at any stage of the design process. In this study, “Y” university gym facility has replaced 100% of the annual heating and cooling energy consumption with the GSHP system. As a result, electric energy is consumed only by a heat pump that connects GHE to the “Y” university gym facility. Therefore, in this study, a basic energy model should be established for electricity energy consumption of the heat pump.
To verify the feasibility of the energy simulation results, the coefficient of variation of the root mean square error (CV(RMSE)) wasused (Equation (4)). As shown in Figure 3, the CV(RMSE) values were 19.26% within the tolerance limits (25%) [7]. Thus, it was determined that the basic model was feasible.
CV(RMSE) = i = 1 n (y estimate_usage,i y actual_usage,i ) 2 n 1 y ¯ actual_usage × 100
where, yestimate_usage,i is electricity consumption of each month (simulation-based data), yactual_usage,i is electricity consumption of each month (actual data), yactual_usage is average of actual electricity consumption for 1 year, n is the number of data (months).
Figure 3. (CV)RMSE: electricity consumption of heat pump (“Y”university gym facility).
Figure 3. (CV)RMSE: electricity consumption of heat pump (“Y”university gym facility).
Energies 08 07752 g003

3.3. Establishment of GHE Design Scenarios Considering EWT

In Section 2, this study established the basic information and selecting key factors affecting GHE performances. In this case, GHE is a system that provides heat to the heat pump in the building or recovers heat through direct heat exchange from heat sources. GHE should be designed to satisfy the energy demand of the building and comprehensively consider the effects of key factors in order to create an optimal scenario in terms of environmental and cost effectiveness.
In order to design a GSHP system that can satisfy the energy demand of the target facility, scenarios for GHE were created in a condition of the existing facility (i.e., heat pumps and land). As mentioned above, GHE is designed based on EWT and the characteristics of boreholes. The number of GHE scenarios available in a situation when EWT fluctuates, increases considerably with by taking into account the energy demand of the target facility, the EWT and the characteristics of boreholes (refer to Table 3, Table A1, Table A2, Table A3 and Table A4). For a direct comparison with a case with an existing GHE (Scenario #4), GHE scenarios were selected on the basis of borehole length according to a change in EWT for a GHE that satisfies the energy demand of target facility. Furthermore, the characteristics of a borehole except for the length were fixed to analyze the change (refer to Table 4). Based on five scenarios as follows, an environmental and economic assessment was conducted to select the optimal GHE in terms of environmental and economic effects.

4. Environmental and Economic Assessment for Selecting the Optimal GHE by Considering EWT

4.1. System Boundary Conditions for Life-Cycle Assessment (LCA) and Life-Cycle Cost (LCC) Analyses

Since the initial investment cost of the GSHP system is quite expensive, the feasibility study needs to be conducted from the life-cycle perspective. Moreover, in recent years, environmental implication has become one of the primary concerns of many governments. Therefore, this study conducted the life-cycle environmental and economic assessment and the following two methods were used: (i) Life-cycle assessment (LCA), a method of calculating the environmental impacts throughout the whole life cycle of a product; (ii) Life-cycle cost (LCC) analysis, a method of calculating the economic value throughout the whole life cycle of a product.
Table 3. Scenarios (length of borehole) of ground heat exchanger (GHE) taking into consideration (entering water temperature) EWT (23 °C) and characteristics of borehole.
Table 3. Scenarios (length of borehole) of ground heat exchanger (GHE) taking into consideration (entering water temperature) EWT (23 °C) and characteristics of borehole.
Maximum Entering Water Temperature: 23 °C, Minimum Entering Water Temperature: 5 °C
Number of Boreholes: Arrangement (U)Borehole SpacingGrout Conductivity (W/mK)U-Tube Type and SizeU-Tube Type and SizeU-Tube Type and Size
PN10, DN25PN10, DN32PN10, DN40
Borehole DiameterBorehole DiameterBorehole Diameter
125 mm150 mm125 mm150 mm125 mm150 mm
U-Pipe SpacingU-Pipe SpacingU-Pipe Spacing
3 mm20 mm3 mm20 mm3 mm20 mm3 mm20 mm3 mm20 mm3 mm20 mm
22 (EA): 8 × 84 m1 W/mK249.1227.3251.9232.6212.1194.1215.1198.7182.5168.3185.2173.3
1.4 W/mK216.7196.9217.7199.7189.0174.6189.8177.1166.2154.2167.0156.5
1.8 W/mK195.8181.1195.8182.4174.7162.1166.6163.0155.8145.3155.2145.9
5 m1 W/mK247.6225.4250.5231.1210.4192.5213.4197.2180.3166.0183.1170.8
1.4 W/mK214.9195.2216.1198.1187.3172.2188.2174.8164.0152.2164.8154.4
1.8 W/mK194.4179.0194.3180.2172.3159.8172.0160.7153.8143.6153.3144.2
6 m1 W/mK246.6224.0249.5229.9209.1191.4207.8195.8179.2165.1181.9169.7
1.4 W/mK212.8194.3214.7196.9186.0171.1182.3173.9163.0151.1163.8153.4
1.8 W/mK193.2177.9193.1179.0171.5158.7166.6159.6152.7142.5152.2143.1
Table 4. Target scenarios (length of borehole) of GHE considering EWT and characteristics of borehole.
Table 4. Target scenarios (length of borehole) of GHE considering EWT and characteristics of borehole.
Number of ScenarioMax./Min. Entering Water TemperatureNumber of Boreholes: Arrangement (U)Borehole SpacingGrout Conductivity (W/mK)U-Tube Type and SizeBorehole DiameterU-Pipe SpacingBorehole Length
Scenario #130/5 °C22 (EA): 8 × 86 m1.8 W/mKPN10, DN25150 mm20 mm90 m
Scenario #227/5 °C107 m
Scenario #325/5 °C123 m
Scenario #4 (exist GHE)23/5 °C143 m
Scenario #520/5 °C195 m

4.1.1. Establishment of System Boundary and Assumptions for LCA

To assess the life-cycle environmental effect, the LCA process was conducted by following four steps defined by ISO 14040: (i) Goal and scope definition; (ii) Life-cycle inventory (LCI) analysis; (iii) Life-cycle impact assessment (LCIA); (iv) Results and interpretations [70,71,72]. The system boundary of the LCA showed the depth and breadth as follows (refer to Figure 4): (i) Life-cycle stages (including material manufacturing, use and maintenance stage); (ii) Building industry (including building level, product level, and material level); (iii) Life-cycle inventory (including emissions from material manufacturing, resources use for material manufacturing, and emissions from energy combustion used in whole life cycle); (iv) Phase of design process (including design development stage); (v) Life-cycle impact categories (including RDP, GWP, ODP, AP, EP, and POCP).
Figure 4. System boundary of the LCA.
Figure 4. System boundary of the LCA.
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Step 1. Goal and scope definition: Based on the information available in whole life cycle, environmental impact generated from the material manufacturing phase and use and maintenance stage are analyzed. The functional unit is defined as “the entire building supplied from design and use and maintenance for a whole service life”.
Step 2. LCI analysis: Using the LCI analysis results by life-cycle phase, the environmental-impact substances can be calculated. First, using input-output (I-O) LCA, energy source quantity utilized to produce the material for each life-cycle phase was calculated. Second, employing a process-based LCA, the national LCI database of South Korea established that the environmental-impact substances produced in the material and energy production process can be calculated (refer to Equation (5)) [71,72].
E i = k ( Q E k × E P i k + Q E k × E C i k )
where Ei is the emission of substance (i), QEk is the quantity of energy source (k), EPik is the emission factor of substance (i) emitted in producing one unit of energy source (k), and ECik is the emission factor of substance (i) emitted in consuming one unit of energy source (k).
Step 3. Life-cycle impact assessment: LCIA converts the environmental-impact substances from the LCI analysis into the environmental impacts. LCIA is made up four categories: (i) Classification; (ii) Characterization; (iii) Normalization; (iv) Weighting [70,71]. In this study, classification and characterization were used to calculate the characterized environmental impact. The process of classification and characterization was calculated by Equation (6) [71,72]. To calculate characterized impacts (CCIl), the characterization factor (CFl,i) of each substance is required. Based on the “environmental labeling type III” standard, the characterized environmental impacts on six environmental-impact categories (i.e., RDP, GWP, ODP, AP, EP, and POCP) were presented.
C C I l = i E i × C F l , i
where CCIl is the characterized impact of impact category (l), Ei is the emission of substance (i), and CFl,i is the characterization factor of substance (i) to impact category (l).
Step 4. Results and interpretations: Using the estimated environmental impact, environmental and economic values are calculated. Environmental cost signifies the cost generated using end-point LCA methodology [70,73,74]. In this study, the environmental-cost conversion factor proposed in EPS 2000 was used to convert the environmental impact to environmental cost [75] (refer to Table A5). By analyzing the relative degree of the impact on the global environment of the environmental-impact categories, all the environmental impacts can be converted into environmental cost.

4.1.2. Establishment of System Boundary and Assumptions for LCC

For the calculation of the life-cycle economic value using LCC analysis, the following system boundaries and assumptions were established: (i) analysis approach; (ii) analysis period; (iii) interest rate; and (iv) significant cost of ownership (refer to Table A6) [71].
Analysis approach: For the analysis approach, net present value (NPV) was selected for the LCC analysis. NPV is the method used to convert the future value of a design alternative into the present value by considering the discount rate and the time value (refer to Equation (7)). If NPV > 0, the project is deemed feasible; if NPV = 0, the break-even point is deemed to have been reached [76].
N P V = t = 0 n B E S t + B E T t ( 1 + r ) t t = 0 n C I t + C R r t + C R t t ( 1 + r ) t
where NPV is the net present value, BESt is the benefit from the energy savings in year t, BETt is the benefit from the emissions trading in year t, CIt is the cost of the initial investment in year t, CRrt is the cost of the repair in maintenance phase in year t, CRtt is the cost of the replacement in maintenance phase in year t, r is the real discount rate, and n is the period of the life-cycle analysis.
Analysis period: Generally, the analysis period for the LCC analysis can be established based on the service life of a product, which is based on the building’s structural type [71]. In this study, a 40-year time frame was used for the analysis period of the LCC.
Interest rate (refer to Equation (8)): In this study, the real discount rate was calculated using the nominal interest rate and various inflation rates (refer to Table A6) [71]. It can be used for converting various benefits and costs into present values.
i = ( 1 + i n ) ( 1 + f ) 1
where i is the real discount rate, in is the nominal interest rate, and f is the inflation rate (i.e., electricity price growth rate, gas price growth rate, carbon dioxide emission trading price growth rate).
Significant cost of ownership: From the life-cycle perspective, the initial investment cost and the use and maintenance cost need to be considered. The material consumption information was collected from the bill of quantities of the GHE and heat pump. The energy consumption information, on the other hand, was established through energy simulation, which was allocated to the energy cost among the use costs. Meanwhile, the repair rate, repair cycle, and replacement cycle of each material should be considered to calculate the cost in the maintenance phase. In this study, resources such as “Public Procurement Service”, “Ministry of National Defense” and “Implementing Regulations of the Housing Act in Korea (Appendix 5)”, which are provided by respectable institutions, were used [71].

4.2. Optimal GHE in Terms of Environmental and Economic Effects

Among the 540 GHE design alternatives (U arrangement and 22 EA borehole) (refer to Table 3 and Table A1, Table A2, Table A3 and Table A4), the analysis results of five scenarios including existing GHE system are presented as an example (refer to Table 4). As shown in Table 4, the existing GHE (scenario #4) has a maximum EWT of 23 °C and the characteristics of the borehole are 22 EA boreholes, 6 m borehole spacing, 1.8 W/mK grout conductivity and 143 m borehole length. For a direct comparison with the existing GHE (Scenario #4), GHE scenarios were selected on the basis of borehole length according to a change in EWT; furthermore, the characteristics of the borehole except for the length were fixed to analyze the change.
Table 5 shows the results of the design alternative analysis on the GHE in the target facility. The analysis results can be presented in the following three aspects: (i) life-cycle environmental cost; (ii) life-cycle economic cost; and (iii) life-cycle environmental and economic cost. Table A6 shows the boundary conditions of LCC and LCA.
First, life-cycle environmental cost: Saving effect of life-cycle environmental cost of Scenario #3 was determined at 2.2% compared with the existing GHE (Scenario #4). Although the initial investment environmental cost is higher than that of the existing GHE, the operation and maintenance environmental cost is lower than that of the existing GHE.
Table 5. Life-cycle environmental and economic cost of scenarios.
Table 5. Life-cycle environmental and economic cost of scenarios.
ScenarioClassificationEnvironmental Impact CategoryEnvironmental CostEconomic CostTotal Cost
RDPGWPODPAPEPPOCP
(kg-Sb-eq)(kg-CO2-eq)(kg-CFC11-eq)(kg-SO2-eq)(kg-PO43-eq)(kg-C2H4-eq)
Scenario #1Initial cost260151,144-8046222723,13694,638117,774
O and M cost1275724,427-12442312108,246104,664212,909
Total cost1535875,572-2047293229131,381199,302330,683
Scenario #2Initial cost288168,762-8986925025,819103,296129,115
O and M cost1150653,432-1122209297,63896,079193,716
Total cost1437822,194-2020277252123,457199,375322,831
Scenario #3Initial cost315186,416-9937627428,508111,969140,477
O and M cost1096622,723-1069199293,04992,421185,469
Total cost1411809,139-2062275276121,557204,390325,947
Scenario #4Initial cost352210,223-11218530532,133123,610155,743
O and M cost1085616,970-1059197292,18991,824184,014
Total cost1437827,193-2180282307124,322215,434339,756
Scenario #5Initial cost439265,811-141910737940,600150,977191,578
O and M cost966549,094-943175282,04783,713165,760
Total cost1405814,905-2362282381122,647234,690357,337
Notes: Unit (US$); operation and maintenance phase (O and M); resource depletion potential (RDP); global warming potential (GWP); ozone layer depletion potential (ODP); acidification potential (AP); eutrophication potential (EP); and photochemical oxidation potential (POCP).
Second, life-cycle economic cost: Saving effect of life-cycle economic cost of Scenario #1 was determined to be 7.5% as compared to the existing GHE (Scenario #4). Although the initial investment cost is higher than that of the existing GHE, the operation and maintenance cost is lower than that of the existing GHE.
Third, life-cycle environmental and economic cost: Saving effect of total cost of Scenario #2 was determined to be 5.0% compared with the existing GHE (Scenario #4). Although the initial investment cost is higher than that of the existing GHE, the operation and maintenance cost is lower than that of the existing GHE.
In conclusion, compared to the existing design, the design alternatives of GHE in the target facility were shown to be advantageous in the life-cycle environmental and economic aspects. All of the initial costs tend to decrease when max EWT decreases; however, all of the O and M costs tend to increase when max EWT increases. In other words, as the amount of EWT that enters a heat pump is reduced during the summer, the efficiency of a heat pump becomes higher and borehole length becomes greater.

5. Conclusions

This study aims to assess the environmental and economic effects of the use of GHE for selecting the optimal GHE. This study was undertaken in three steps: (i) establishing the basic information and selecting key factors affecting GHE performances; (ii) making possible alternatives of the GHE installation by considering EWT; and (iii) using life-cycle assessment and life-cycle cost for a comprehensive evaluation of the environmental and economic effects on the GHE.
By conducting a multilateral analysis on the results of the energy simulation, the energy generation effect considering EWT was evaluated by applying the GSHP system. Furthermore, LCA (e.g., RDP, GWP, ODP, AP, EP, and POCP) with the analysis of the LCC was conducted to assess the environmental and economic effects of the implementation of the GSHP system to “Y” university gym facility using NPV methods (refer to Table A6). The analysis results can be summarized as follows:
Life-cycle environmental cost: Saving effect of life-cycle environmental cost of Scenario #3 was determined to be 2.2% compared with existing GHE (Scenario #4).
Life-cycle economic cost: Saving effect of life-cycle economic cost of Scenario #1 was determined at 7.5% compared with existing GHE (Scenario #4).
Life-cycle environmental and economic cost: Saving effect of total cost of scenario #2 was determined at 5.0% compared with existing GHE (Scenario #4).
In conclusion, all of the initial costs tend to decrease when max EWT (for cooling system) decreases; however, all of the O and M costs tend to increase when max EWT increases. In other words, as there is less EWT entering a heat pump during the summer, the efficiency of a GSHP system increases and the borehole lengthens.
The results of this study could benefit potential GSHP system users and give new value in terms of system application in several ways: (i) decide which location is proper for the implementation of the GSHP system considering the characteristics of the regional factor; (ii) maximize the environmental and economic benefit and the efficiency by considering key design factors such as EWT and characteristics of borehole.
Meanwhile, the following research is recommended for future studies: (i) sensitivity analysis of the GSHP system considering the recent trends in the reduction of the initial investment cost and government’s subsidy; (ii) a multi-objective optimization system for the ultimate decision maker to analyze uncountable scenarios in terms of several key factors (EWT and characteristics of borehole).

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP; Ministry of Science, ICT & Future Planning) (No.NRF-2015R1A2A1A05001657).

Author Contributions

All authors read and approved the manuscript. All authors contributed to this work, discussed the results and implications and commented on the manuscript at all stages. Jimin Kim discussed the main idea behind the work and reviewed and revised the manuscript. Taehoon Hong gave precious advice on the establishment of framework as well as design process. Myeongsoo Chae made the model with the co-authors and explicated the GSHP system more thoroughly. Choongwan Koo made the LCC and LCA analysis models for assessing the alternative of GSHP system. Jaemin Jeong led the development of the paper and conducted the LCC and LCCO2 analyses.

Conflicts of Interest

The authors declare no conflict of interest.

Appendix

Table A1. Scenarios (length of borehole) of GHE considering EWT (30 °C) and characteristics of borehole.
Table A1. Scenarios (length of borehole) of GHE considering EWT (30 °C) and characteristics of borehole.
Maximum Entering Water Temperature: 30 °C, Minimum Entering Water Temperature: 5 °C
Number of Boreholes: Arrangement (U)Borehole SpacingGrout Conductivity (W/mK)U-Tube Type and SizeU-Tube Type and SizeU-Tube Type and Size
PN10, DN25PN10, DN32PN10, DN40
Borehole DiameterBorehole DiameterBorehole Diameter
125 mm150 mm125 mm150 mm125 mm150 mm
U-Pipe SpacingU-Pipe SpacingU-Pipe Spacing
3 mm20 mm3 mm20 mm3 mm20 mm3 mm20 mm3 mm20 mm3 mm20 mm
22 (EA): 8 × 84 m1 W/mK157.4142.4159.3145.8134.1123.0135.7126.3114.7105.7116.4108.8
1.4 W/mK136.5125.1137.1126.9119.1109.7119.7111.3104.296.9104.898.2
1.8 W/mK124.3113.9124.3114.6109.8101.5109.6102.197.991.897.592.2
5 m1 W/mK156.1141.4157.9144.7133.2121.9134.8125.1113.5104.3115.3107.5
1.4 W/mK135.6123.9136.2125.8117.9108.5118.5110.1102.995.7103.497.0
1.8 W/mK123.2112.7123.1113.4108.5100.2108.4100.896.690.696.391.0
6 m1 W/mK155.4140.7157.3144.0132.4120.8134.0124.2112.7103.8114.4106.9
1.4 W/mK134.9123.0136.5124.8117.1107.8117.7109.4102.395.3102.996.6
1.8 W/mK122.3111.9122.2112.7107.999.8107.7100.396.290.195.990.4
Table A2. Scenarios (length of borehole) of GHE considering EWT (27 °C) and characteristics of borehole.
Table A2. Scenarios (length of borehole) of GHE considering EWT (27 °C) and characteristics of borehole.
Maximum Entering Water Temperature: 23 °C, Minimum Entering Water Temperature: 5 °C
Number of Boreholes: Arrangement (U)Borehole SpacingGrout Conductivity (W/mK)U-Tube Type and SizeU-Tube Type and SizeU-Tube Type and Size
PN10, DN25PN10, DN32PN10, DN40
Borehole DiameterBorehole DiameterBorehole Diameter
125 mm150 mm125 mm150 mm125 mm150 mm
U-Pipe SpacingU-Pipe SpacingU-Pipe Spacing
3 mm20 mm3 mm20 mm3 mm20 mm3 mm20 mm3 mm20 mm3 mm20 mm
22 (EA): 8 × 84 m1 W/mK186.5169.1188.5173.1158.4144.6160.7148.5135.7122.4133.7125.8
1.4 W/mK161.8147.0162.7149.3140.3130.4141.0132.2124.1112.9121.5114.4
1.8 W/mK146.1134.9146.0135.7130.5120.8130.3121.5114.0107.0113.5107.4
5 m1 W/mK185.0167.5187.2171.5156.8143.3159.0147.1132.0122.1133.6125.6
1.4 W/mK160.0145.7161.0147.9139.1129.1139.8131.0120.5112.3121.1113.9
1.8 W/mK144.8133.8144.8134.5129.2119.4129.0120.0113.4106.3113.0106.7
6 m1 W/mK184.1166.9186.3170.8156.0142.5158.2146.3131.9121.9133.6125.3
1.4 W/mK159.3144.9160.0147.1138.3128.0139.0129.9120.1111.9120.8113.5
1.8 W/mK144.1132.9144.0133.7128.0118.3127.9118.9113.0105.9112.6107.0
Table A3. Scenarios (length of borehole) of GHE considering EWT (25 °C) and characteristics of borehole.
Table A3. Scenarios (length of borehole) of GHE considering EWT (25 °C) and characteristics of borehole.
Maximum Entering Water Temperature: 23 °C, Minimum Entering Water Temperature: 5 °C
Number of Boreholes: Arrangement (U)Borehole SpacingGrout Conductivity (W/mK)U-Tube Type and SizeU-Tube Type and SizeU-Tube Type and Size
PN10, DN25PN10, DN32PN10, DN40
Borehole DiameterBorehole DiameterBorehole Diameter
125 mm150 mm125 mm150 mm125 mm150 mm
U-Pipe SpacingU-Pipe SpacingU-Pipe Spacing
3 mm20 mm3 mm20 mm3 mm20 mm3 mm20 mm3 mm20 mm3 mm20 mm
22 (EA): 8 × 84 m1 W/mK212.6192.9215.4197.0181.5166.3184.1170.4155.1142.6157.5146.8
1.4 W/mK185.2168.8186.1171.3161.3148.1162.1150.3140.8132.2141.5133.8
1.8 W/mK168.0154.0168.0155.0148.1137.7147.9138.3133.4125.1132.9125.6
5 m1 W/mK211.2191.5213.8195.7179.8164.4182.2168.6153.4141.2155.7145.3
1.4 W/mK183.5166.9184.4169.5159.3146.4160.2148.6139.5130.8140.1132.6
1.8 W/mK166.1152.3166.1153.3146.5136.5146.3137.1132.1123.5131.6124.0
6 m1 W/mK210.1190.6212.8197.8178.8163.6181.1167.7182.9140.3154.8144.4
1.4 W/mK182.4166.3183.4168.5158.4145.5159.3147.7138.5129.6139.2131.5
1.8 W/mK165.3151.4165.2152.4145.6135.5145.4136.1130.9122.1130.4122.6
Table A4. Scenarios (length of borehole) of GHE considering EWT (20 °C) and characteristics of borehole.
Table A4. Scenarios (length of borehole) of GHE considering EWT (20 °C) and characteristics of borehole.
Maximum Entering Water Temperature: 23 °C, Minimum Entering Water Temperature: 5 °C
Number of Boreholes: Arrangement (U)Borehole SpacingGrout Conductivity (W/mK)U-Tube Type and SizeU-Tube Type and SizeU-Tube Type and Size
PN10, DN25PN10, DN32PN10, DN40
Borehole DiameterBorehole DiameterBorehole Diameter
125 mm150 mm125 mm150 mm125 mm150 mm
U-Pipe SpacingU-Pipe SpacingU-Pipe Spacing
3 mm20 mm3 mm20 mm3 mm20 mm3 mm20 mm3 mm20 mm3 mm20 mm
22 (EA): 8 × 84 m1 W/mK338.8306.9342.7314.1287.2263.6291.4270.1247.3230.1250.9236.0
1.4 W/mK293.8267.9295.4271.3255.8237.6257.2240.7226.7209.7228.5213.0
1.8 W/mK266.5245.9266.4247.2237.7220.9237.5222.1211.9198.8211.1199.1
5 m1 W/mK337.1305.4341.1312.4285.5261.8289.6268.7245.5227.2249.0233.9
1.4 W/mK291.6266.1293.3269.9254.0235.5255.4238.8224.2206.9225.4210.6
1.8 W/mK264.6243.8264.5245.3235.7218.3235.4219.5209.6195.9208.8196.6
6 m1 W/mK335.9304.2340.0311.2284.3260.5288.4267.7274.3225.5247.6232.3
1.4 W/mK290.3264.9292.0268.9252.8234.0254.1237.0222.2205.1223.7208.7
1.8 W/mK263.4242.3263.2243.8234.2216.4233.9217.6207.3194.1206.5194.9
Table A5. Environmental cost conversion factor.
Table A5. Environmental cost conversion factor.
Environmental ImpactEnvironmental Cost Conversion Factor
resource depletion potential (RDP)2.439 US$/kg-Sb-eq
global warming potential (GWP)0.167 US$/kg-CO2-eq
ozone layer depletion potential (ODP)145.172 US$/kg-CFC11-eq
acidification potential (AP)0.032 US$/kg-SO2-eq
eutrophication potential (EP)0.029 US$/kg-PO43-eq
photochemical oxidation potential (POCP)2.675 US$/kg-C2H4-eq
Table A6. Boundary conditions of LCC and LCA.
Table A6. Boundary conditions of LCC and LCA.
ClassificationDetailed ClassificationDetailed Description
Analysis ApproachPresent Worth Method (NPV40)
Analysis Period40 years
Realistic Discount RateInterest3.30%
Electricity0.66%
Gas0.11%
KCERs2.66%
Significant Cost of OwnershipInitial construction costInitial investment cost
Operation and maintenance costReplacement/repair cost
Energy consumption cost
Operation and maintenance benefitGas savings, electricity savings
Benefit from KCERs

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