Original Paper

Early design strategy for hybrid ground source heatpump systems for a commercial buildingGyuyoung Yoon

Graduate School of Design and Architecture, Nagoya City University, Nagoya, Aichi, Japan

Correspondence

Gyuyoung Yoon, Graduate School of Design and Archi-tecture, Nagoya City University, Nagoya, Aichi, Japan.Email: yoon@yoonlab.net

Received April 30, 2020; Accepted August 19, 2020

doi: 10.1002/2475-8876.12192

Abstract

In this study, the design strategy for hybrid ground source heat pump (GSHP)

systems consisting of GSHP and air source heat pump (ASHP) is investigated with

a focus on the early design stages of an air conditioning system for a commercial

building. A design procedure for determining the capacity ratio of the GSHP and

ASHP is presented. Then, conservative values of the design parameters for the

design process are described. A case study conducted according to the introduced

procedure showed that the capacity ratio of GSHP and ASHP is influenced by the

magnitude of the peak load of the building and building site conditions, as well

as the configuration of ground source heat exchangers. Furthermore, an effort to

reduce the peak load would increase the capacity ratio of the GSHP. It was

shown that the capacity ratio would be approximately set by the site conditions,

which can be a dominant constraint on GSHP capacity design. Finally, in the ini-

tial design phase, a design practice with conservative design values was insti-

tuted that would help in determining the capacity ratio and the configuration of

the ground source heat exchangers.

Keywords

capacity ratio, hybrid ground source heat pump, renewable thermal energy

1. Introduction

Following the increase in demand for energy savings and CO2

emission reductions in recent years, application of not onlyrenewable power energy but also renewable thermal energy,such as geothermal, solar thermal, and snow-melting heat, hasbecome attractive as a solution. The Japanese government haspromoted geothermal heat as a promising energy source forthe future. As a result, its utilization is expected to becomecommonplace.With a focus on geothermal heat utilization, the Japanese

government launched the Basic Energy Plan.1 This plan aimsat improving the energy supply and demand structure, increas-ing the use of distributed energy systems, and engenderingsupport for these systems. In addition, the promotion of heatutilization as an application of renewable energy sources hasencouraged the use of geothermal heat sources.Figure 1 shows the number and percentage of installations

of geothermal utilization units.2 Among the 6,877 cases intotal, air-circulation systems constitute 1919 cases, accountingfor 27.9% of installed units. Water-circulation systems accountfor 25.9%, or 1781 cases, whereas ground source heat pump(GSHP) systems account for 2230 cases, or 32.4% of the total.

Vertical type ground heat exchangers (GHEXs) are mainlyused in Japan because of their small installation space require-ments. For similar reasons, adoption of hybrid GSHP systemscombined with an air source heat pump (ASHP) is a realisticsolution because of the restrictions imposed on building sites,especially in urban areas and high-density districts.Generally, hybrid GSHP systems consist of GHEXs, solar

thermal collectors, cooling towers, and other parts. They serveas a useful solution to compensate for significant imbalancesin temperatures arising from heat rejecters and extractors inhot or cold climates.3,4 Kim et al.5 suggested that the hybridapplication of a water-to-air heat pump could be considered asboth a supplemental heat sink and a source for GSHPs. Namet al.6 and Pardo et al.7 further confirmed that a combined sys-tem with ground and air sources shows higher energy effi-ciency than conventional GSHPs. Regarding spacerequirements for installation of GHEXs, the hybrid applicationof an ASHP could be an affordable solution in urban areaswhere installation space is limited.Previous research on the design procedure of GSHP systems

shows a series of design processes from planning during theinitial design phases to the implementation phase.8-11 TheGeo-Heat Promotion Association of Japan13 also describes the

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the originalwork is properly cited.© 2020 The Authors. Japan Architectural Review published by John Wiley & Sons Australia, Ltd on behalf of Architectural Institute of Japan.

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process from design to construction and operation of an open-loop GSHP system. In particular, a research group at HokkaidoUniversity9 described in detail the planning and field surveystages, while also confirming design concepts, setting of designtarget values, and arrangement of design conditions. However,hybrid systems consisting of both GSHPs and ASHPs are notconsidered in those design procedures.Furthermore, Kavanaugh,14 Chiasson and Yavuzturk,15 and

Hackel et al.16 have described a design method for hybridGSHP systems that have a supplemental heat sink and heatsource. According to Kavanaugh, the size of the supplementalheat source or sink should be based on the excessive heatextraction or rejection caused by the GSHP while the requiredGHEX length is determined considering the cooling load orheating load.The hybrid system targeted in this study can be implemented

using these design or optimization methods. However, thereare site constraints, such as complicated boundary conditionsin urban areas where the installation space for GHEXs is lim-ited. To ensure practical design, it is helpful to determine thecapacity of the GSHP based on the constraints on the site inthe initial stages of the design phase.There are various methodologies for the design of GHEXs.

Direct and iterative solutions include Earth Energy Designer,17

GLHEPro,18 Ground Loop Design,19 and Ground Club.20 More-over, the ground heat transfer methodology is widely employeddue to its simplicity in calculating the partial load factor, annualaverage heat transfer rate, and effective thermal resistance ofGHEXs, as shown in the ASHRAE handbook,12 and the methodbased on simple steady heat transfer by Ingersoll et al.21

The present study developed a procedure for the early designof a hybrid GSHP. In this paper, the design procedure is pre-sented, and conservative values of various design parametersapplied in the design procedure are investigated. These valuescan be useful to a system designer for deciding the capacityratio of a GSHP. Finally, design case studies were carried outin the early design phase to show how various building siteconditions influence the capacity ratio and the combination ofGHEX configurations.

2. Design Procedure for Hybrid Ground source heat pump

Figure 2 depicts an air conditioning system that incorporates ahybrid GSHP system. The system contains 2 different types ofheat sources, the ground source and air source. In addition, a

solar thermal collector and cooling tower as supplemental heatsources or heat sink equipment would be installed to balancethe heat extraction and heat injection in the ground. This studyfocuses on determining the capacity ratio of the GSHP andASHP.A simple method is suitable in the initial design phase

because many of the parameters are yet to be undetermined.Thus, simple steady heat transfer21 has been adapted in thisstudy.

Qcond ¼Qc� COPcþ1ð Þ=COPc,Qevap ¼Qh� COPh�1ð Þ=COPh,

whereQcond: heat pump condenser heat rate to ground (W),Qevap: heat pump evaporator heat rate from ground (W),Qc: building design cooling block load (W),Qh: building design heating block load (W),COPc: cooling mode coefficient of performance (dimension-

less), andCOPh: heating mode coefficient of performance (dimension-

less).According to the equations shown above,8 the heat pump con-

denser (or evaporator) heat rate in the equation can be deter-mined by the building design cooling (or heating) block loadand the coefficient of performance (COP) of the heat pump.Assuming that the heat pump condenser (or evaporator) heat rateis equal to the heat extraction/injection rate of GHEXs and thebuilding design cooling (or heating) block load is replaced withheat pump cooling (or heating) capacity, a relational expressionfor heat pump cooling (or heating) capacity and total GHEXlength can be derived as shown in equations (1) and (2). Due toits simplicity, this method is most relevant for the early designphase targeted in this study. It should be noted, however, thatprevious studies9 have indicated that this method is useful whenthe heat load pattern is relatively simple, such as in the case of adetached house, so it is only valid in the initial design phasewhen the annual heat load is undetermined.Based on the simple steady heat transfer, the design proce-

dure in Table 1 can be used for estimating the capacity ratioof the GSHP to the total capacity of heat source equipment inthe initial design phase. Thus, a system designer can define thecapacity ratio of the hybrid GSHP by following the proceduredescribed below.

Figure 1. Number and percentage of installations of geothermal units

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At first, the peak cooling and heating load of a building aredefined according to conservative values, while the thermalload calculation is not carried out in the early design phase.Next, the available cooling and heating capacity of the GSHPcan be estimated considering the building-coverage ratio, heatextraction/injection rate of GHEXs, and COP of the heatpump. Here, the COP value should be obtained for the maxi-mum heating or cooling capacity considering the performancecharacteristics of hot- or chilled-water temperatures.The building-coverage ratio is an important constraint for

assessing the capacity of a GSHP with respect to the maximumlength of GHEXs possible for a specific site. The capacityratio of the GSHP to the total capacity of heat source equip-ment is estimated using the following equation:

i For a GHEX around the building:

λ1 ¼HP=PL�100, (1)

HP ¼ 1�X=100ð Þ�S�d1� l1�q1�COP= COP�1ð Þ,PL ¼ PLi�Y=100�S�α=100:

ii For a GHEX under the building:

λ2 ¼HP=PL�100, (2)

HP ¼X=100�S�d2� l2�q2�COP= COP�1ð Þ,PL ¼ PLi�Y=100�S�α=100:

iii For a GHEX both around and under the building:

λ¼ λ1þλ2: (3)

The underground space required for the installation of theGHEXs would be around or under the building site. In thisprocedure, a borehole-type heat exchanger would be appliedaround the building, while an energy pile would be installedunder the building (See Figure 3).

3. Parameters in the Design Procedure

The design parameters were investigated using the referencesand related literature, which provide conservative values forimplementation of the design procedure.Figure 4 shows the peak-load intensity of 599 buildings built

between about 2008 and 2014 in Japan, which was obtainedfrom the technical sheet of facilities provided by the Societyof Heating, Air conditioning, and Sanitary Engineers ofJapan.22 The 599 samples include offices, schools, and othercommercial buildings located across a wide range from theTohoku area to the Kyushu area, where cooling degree daysrange from 59 to 315 and heating degree days range from 961to 2026. The median values of the cooling and heating loadare 173.2 and 143.2 W/m2, respectively, and most buildingshave a value <400 W/m2.Figure 5 shows the peak-load intensity by the main purpose

of the 599 buildings. These values can be used to determinethe peak load for cooling and heating in the early design stage,before more specific parameters are determined.Figure 6 shows the rated COPs for GSHPs obtained from 7

manufacturers in Japan and 57 heat pumps. The ratedCOP ranges between 3 and 5. Therefore, a system designercan consider this range for the COP of a heat pump. This ten-tative value can then be updated after the details are finalized.Figure 7 compares the cooling capacity of heat pumps and

the total length of their GHEXs, as determined from 36 heatpump systems with boreholes and 11 heat pump systems withenergy piles.23 This figure shows that the total length of theheat exchanger increases as the heat pump cooling capacityincreases. The heat extraction/injection rate ranges from 40 to240 W/m in both the borehole and energy pile GHEX types.It should be assumed by evaluating the effective thermal conduc-

tivity of the ground, which is evaluated by the soil properties andwater content from the borehole investigation results, or by con-ducting thermal response test. However, in the early design stages,these are often not yet implemented. The system designer has nochoice but to rely on the conservative value or empirical values.Regarding to borehole, the UK standard24 states that it ranges

from 25 to 43 W/m depending on the different effective thermalconductivity of ground. Also, according to literature,25 68 to82 W/m for the United States (in case of single U tubes),

Figure 2. Air conditioning system with hybrid ground source heat pump system

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75 W/m for Switzerland, and 30 to 70 W/m for Austria,depending on water content and temperature range of use. And,Germany has a range of 20 to 60 W/m depending on the effec-tive soil conductivity. In Japan, the reference9 states that it isaround 40 W/m according to the actual results so far. It alsopoints out that the value for the energy pile is around 100 W/m.Therefore, a system designer can set the heat extraction/injec-tion rate referring to these empirical values and then refine itlater with an actual value as the design phase progresses.Figure 8 shows the distribution of the building-coverage and

floor-area ratios in the buildings in 3 large cities in Japan.26

The data suggest that a large number of buildings have a 60%building-coverage ratio and a 200% floor-area ratio. This iscommon in these cities, namely Tokyo, Nagoya, and Osaka.

Finally, the distance between individual boreholes rangedfrom 5 to 7 m based on the ASHRAE Handbook’s12 recom-mendation. In case of the distance is <5 m, it should be paidattention for thermal interaction between individual bores.Regarding the long-term operation, if necessary, the installa-tion of cooling towers and auxiliary heat sources could be con-sidered to mitigate temperature penalty caused by theextraction/injection heat imbalance.

4. Case Study on Capacity Ratio

In this section, the influence of the hybrid GSHP system on thecapacity ratio is shown. The different design parameters areintroduced through the proposed design procedure. It is assumed

Table 1. Design procedure for estimating the capacity ratio of a hybrid ground source heat pump system

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Figure 3. Types of ground heat exchangers considered in this study

Figure 4. Peak-load intensity of 599 buildings

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that the target buildings are in urban areas like Tokyo. The heatsource equipment capacity is determined by the cooling peakload. The case study considers the influence of the heat pumpCOP, peak-load intensity, and building site conditions.

4.1 Rated COP of the heat pump

Figure 9 shows that the capacity ratio for the GSHP changeswith changes in the heat pump COP and peak-load intensity.

The capacity ratio increases with decreasing peak-load inten-sity. It is almost 50% at 50 W/m2 of peak-load intensity for aGHEX around the building and over 80% for a GHEX underthe building. However, no significant influence on the capacityratio is observed for different COP ratings for the heat pump.Consequently, it can be said that to increase the available

Figure 5. Peak-load intensity by main purpose of buildings

Figure 6. Rated COPs for ground source heat pumps

Figure 7. Cooling capacity and total length of ground heat exchang-ers

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capacity of a GSHP, peak heat load reduction is more benefi-cial than increased heat pump COP.

4.2 Building site conditions

Figure 10 shows the capacity ratio of the GSHP for variousfloor-area ratios and building-coverage ratios. In both types ofGHEX (around and under the building), the capacity ratioincreases with smaller floor-area ratios and smaller building-

coverage ratios due to the decreased floor-area ratio of thebuilding. In contrast, the capacity ratio increases with smallerbuilding-coverage ratio when GHEXs are placed around thebuilding and with larger building-coverage ratio when GHEXsare placed under the building. With a floor-area ratio of 200%and building-coverage ratio of 60%, the capacity ratio is12.4% for GHEXs around the building and 23.3% for GHEXsunder the building. It can be concluded that the estimated

Figure 8. Frequency of building-coverage ratio and volume ratio in major Japanese cities

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capacity ratio above about 12.4% and 23.3% would be approx-imately established by these site conditions, and it can be adominant constraint during the determination of GSHP capac-ity in urban areas.

4.3 Configuration of ground heat exchanger

Figure 11 shows the capacity ratio of a GSHP for variousGHEX spacings, lengths, and configurations. The capacity ratiodecreases with greater GHEX spacing due to the increasedheat transfer area of GHEXs. The length and number ofGHEXs are vital parameters in terms of the initial cost of aGHEX system. Hence, it should be chosen carefully consider-ing the cost performance of the system.In addition, the capacity ratio increases as the length and

number of GHEXs increases and the peak-load intensitybecomes smaller. Changes in the configuration of GHEXs havea greater impact on enlarging the capacity ratio than changesin the COP of heat pumps.Consequently, it can be seen that an effort to reduce the

peak load would lead to a higher capacity ratio, and alsohigher energy performance of the system.

According to Figure 11(a), when the peak-load intensityis 200 W/m2, 12.4% of the capacity ratio would be obtainedwith a 100-m long GHEX. At the same time, the samecapacity ratio could be obtained with a 50-m GHEX if thepeak-load intensity can be mitigated by 100 W/m2. In thesame manner, 17.8% of the capacity ratio would beobtained with 0.02 units/m2 (equal to a spacing distance5 m) of GHEX instead of 0.04 units/m2 (equal to 7 m) ifthe peak-load intensity can be reduced by 100 from 200 W/m2, as shown in Figure 11(b).

4.4 Design for capacity ratio of the ground source heat pump in

urban area

The above analysis showed that the capacity ratio of GSHPsystems is significantly affected by the GHEX specificationsand not by the COP of the heat pump. In addition, it wasfound that the building-coverage ratio and floor-area ratiowere frequently at 60% and 200%, respectively, in represen-tative Japanese cities. Therefore, given the conditions listedin Table 2, the capacity ratio is expressed by equations (4)and (5).

Figure 9. Capacity ratio of ground source for different heat pump COP values

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(i) For GHEXs installed around a building:

λ1 ¼ 22:2�d1� l1�q1=PLi (4)

(ii) For GHEXs installed under a building;

λ2 ¼ 33:3�d2� l2�q2=PLi (5)

For a GHEX installed around a building, if a systemdesigner intends to achieve 20% of the capacity ratio with thepeak-load intensity fixed at 150 W/m2, the term of d1 × l1 ×q1 should be 135 W/m2 according to equation (4). For aGHEX installed under a building, the term of d2 × l2 × q2should be 90 W/m2 according to equation (5).

5. Conclusions

A design procedure for an air conditioning system incorporat-ing a hybrid GSHP system has been presented. The

conservative values for each design parameter that should beconsidered in the early design stages were introduced. Thecase-study results showed that an effort to reduce the peakload would increase the capacity ratio of the GSHP. In addi-tion, it was found that the capacity ratio of the GSHP was sig-nificantly affected by the GHEX specification and not by theCOP of the heat pump. However, the heat pump COP signifi-cantly influences the energy performance of the system, andhence, should be considered not only in terms of capacity ratiobut also energy efficiency. In future studies, conservative val-ues for the design parameters should be obtained by morepractical approaches and sufficient data should be gathered to

Figure 10. Capacity ratio of ground source under different GHEX types and building coverage ratios.

Table 2. Case-study conditions

Building-coverage

ratio [%] X

Floor-area

ratio [%] Y

Air-conditioned floor-

area ratio [%] α COPc

80 200 75 5

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validate their reliability, because these values are extremelyhelpful to a system designer who employs the proposed proce-dure. In addition, the expected effect in terms of energy sav-ings and improvements in energy efficiency by introducinghybrid GSHP systems with different capacity ratios should beprovided in the design procedure to allow the system designerto make an appropriate decision.

Disclosure

The author have no conflict of interest.

Abbreviations

Q Heat transfer rate (W)Y Floor-area ratio (%)S Site area (m2)PL Peak-load (W)PLi Peak-load intensity (W/m2)α Air-conditioned floor-area ratio (%)

X Building-coverage ratio (%)d Number of ground heat exchangers per area (units/m2)q Heat extraction/injection rate per unit of the ground heat

exchanger (W/m)l Length of each ground heat exchanger (m)COP Coefficient of performance (dimensionless)λ Capacity ratio of the ground heat source heat pump (%)* Subscripts – 1: Around the building, 2: Under the

building, c: cooling, h: heating, GHEX: Ground heatexchanger

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How to cite this article: Yoon G. Early design strategyfor hybrid ground source heat pump systems for acommercial building. Jpn Archit Rev. 2020;00:1–11.https://doi.org/10.1002/2475-8876.12192

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