Applied Energy 116 (2014) 76–85

Contents lists available at ScienceDirect

Applied Energy

journal homepage: www.elsevier .com/ locate/apenergy

Optimization of operating parameters of ground source heat pumpsystem for space heating and cooling by Taguchi method and utilityconcept

0306-2619/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.apenergy.2013.10.065

⇑ Corresponding author. Tel.: +91 1332 285635.E-mail address: krimufme@iitr.ernet.in (K. Murugesan).

T. Sivasakthivel a, K. Murugesan a,⇑, H.R. Thomas b

a Department of Mechanical and Industrial Engineering, Indian Institute of Technology Roorkee, Roorkee, Indiab Geoenvironmental Research Centre, Cardiff School of Engineering, Cardiff University, Cardiff, UK

h i g h l i g h t s

� Ground Source Heat Pump (GSHP) technology is suitable for both heating and cooling.� Important parameters that affect the GSHP performance has been listed.� Parameters of GSHP system has been optimized for heating and cooling mode.� Taguchi technique and utility concept are developed for GSHP optimization.

a r t i c l e i n f o

Article history:Received 3 May 2013Received in revised form 14 October 2013Accepted 27 October 2013

Keywords:Ground source heat pumpCoefficient of performanceHeatingCoolingTaguchi methodUtility concept

a b s t r a c t

Use of ground source energy for space heating applications through Ground Source Heat pump (GSHP)has been established as an efficient thermodynamic process. The electricity input to the GSHP can bereduced by increasing the COP of the system. However, the COP of a GSHP system will be different forheating and cooling mode operations. Hence in order to reduce the electricity input to the GSHP, an opti-mum value of COP has to be determined when GSHP is operated in both heating and cooling modes. Inthe present research, a methodology is proposed to optimize the operating parameters of a GSHP systemwhich will operate on both heating and cooling modes. Condenser inlet temperature, condenser outlettemperature, dryness fraction at evaporator inlet and evaporator outlet temperature are considered asthe influencing parameters of the heat pump. Optimization of these parameters for only heating or onlycooling mode operation is achieved by employing Taguchi method for three level variations of the aboveparameters using an L9 (34) orthogonal array. Higher the better concept has been used to get a higherCOP. A computer program in FORTAN has been developed to carry out the computations and the resultshave been analyzed for the optimum conditions using Signal-to-Noise (SN) ratio and Analysis Of Variance(ANOVA) method. Based on this analysis, the maximum COP for only heating and only cooling operationare obtained as 4.25 and 3.32 respectively. By making use of the utility concept both the higher values ofCOP obtained for heating and cooling modes are optimized to get a single optimum COP for heating andcooling modes. A single optimum COP value of 3.92 has been obtained for both space heating and coolingoperations.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

In the recent decades, energy consumption for building sectorhas increased in multifold around the world. Efforts are beingmade to develop alternate energy sources for meeting the demandof building heating and cooling loads. One of the best alternateways is the use of ground source energy, which is green and sus-tainable. This energy can be utilized using ground source heat

pump (GSHP) system, which is well established in Western andEuropean countries for space heating applications. However, inAsian countries like India, South Korea, Japan and China therequirement is more on cooling than heating. For making this tech-nology suitable for use in such countries, the GSHP has to meetboth heating and cooling loads. This can be achieved by harnessingthe temperature variation in the ground during summer and win-ter with respect to the ambient. The seasonal temperature varia-tion within the ground can be attributed to the solar radiation.Half of the solar radiation received by earth is stored in the ground.This results in ground temperature variation with climatic

Nomenclature

_m mass flow rate (kg/s)T temperature (�C or K)h specific enthalpy (kJ/kg)_E rate of energy (kW)_Q rate of heat transfer (kW)_W rate of work, power (kW)

I current (A)V voltage (V)

Subscriptscond condensercomp compressorw waterevap evaporatorsys systemout outletGSHP ground source heat pumpANOVA analysis of variance

NV number of variablesP preference numberGHX ground heat exchangerr refrigerantCOP coefficient of performanceHP heat pumpin inlet2, 3 State pointsact actualS/N signal to noise ratioL levelsU utility valueTRT thermal response test

Greek symbolsg efficiencycos/ power factor

T. Sivasakthivel et al. / Applied Energy 116 (2014) 76–85 77

conditions and depth of ground. At deeper layers, the ground tem-perature remains almost constant throughout the year and is usu-ally higher than that of the ambient air during the cold months ofthe year and lower during the warm months [1,2]. GSHP systemsuse some electricity to extract heat from the ground and deliverto the space to be heated, thus they indirectly contribute lessgreenhouse gas generation than the conventional heating systems.In many developing countries, in the absence of GSHP, conven-tional electric resistance heaters are used for space heating in win-ter and air conditioners for space cooling in summer. With increasein average temperature of earth due to global warming, the needfor space cooling systems in summer season is augmented, result-ing in significant consumption of centrally generated electricity.Hence there is an urgent need to find alternative systems for thepresent air conditioners and heaters. Ground Source Heat Pump(GSHP) will be a viable solution as it can be used for both heatingand cooling purpose with lesser electricity consumption and withan indirect benefit of reduction in green house gas emissions.

In general when a GSHP system is designed to operate for heat-ing mode, it will produce higher COP during heating operation.Thermodynamic [3–6] and experimental [7–10] investigations ofGSHP have been reported for space heating applications by manyresearchers in the past. Balbay and Esen [11] proposed GSHP basedbridge and pavements heating to clear snow during winter in Tur-key. They used 30 m, 60 m and 90 m vertical borehole heatexchangers for heating purpose. The effect of depth of boreholeson the performance of GSHP for heating was also investigated.The results show that the GSHP system was able to successfully re-move the snow from the bridges and pavements. Esen et al. [12]carried out experimental study on a GSHP system coupled withhorizontal ground heat exchanger. They evaluated the perfor-mance by calculating COP of the system and also created a numer-ical model to predict the temperature distribution in the vicinity ofthe heat exchanger and their numerical results compared close totheir experimental data. Esen and Yuksel [13] studied the possibil-ity of using various renewable energy sources for green househeating and they concluded that GSHP also can be used for greenhouse heating. Considerable amount of research has been carriedout to study the performance of GSHP system in cooling modes[14–17] as well. In the literature some of the experimental andthermodynamic analysis of GSHP for both heating and cooling alsohas been presented [18–20]. Montagud et al. [21] studied the

performance of GSHP system for space heating and coolinginstalled in a university building. Along with experimental studythey created a model in TRNSYS and GLHEPRO software and mon-itored the operating performance for seven years to see the consis-tency between experimental data and numerical data. For thispurpose they recorded the outlet temperature of the borehole heatexchanger and their experimental data were in close agreementwith their numerical data. Some of the researchers [22–27] appliedneural network and fuzzy logic techniques to predict the perfor-mance of GSHP systems. Gang and Wang [28] applied static anddynamic models of ANN to predict the exit temperature of GHX.They concluded that compared to static model, dynamic modelwas able to predict the exit temperature accurately and also theyproposed a control strategy for GSHP to operate between GHXand cooling tower. Michopoulos et al. [29] studied the operationalperformance of a GSHP system installed in Northern Greece forheating and cooling modes. They observed that during one cyclicoperation the maximum seasonal COP was observed to be 5.2and 4.5 for heating and cooling modes respectively. It is very clearfrom the literature and also from the basic working principle of aGSHP system that when the same GSHP is operated in heatingand cooling mode operations, the COP in both the cases will notbe the same. If COP is not optimized for a GSHP for combinedmodes of operations, then whatever energy input gained duringheating mode operation obtained by the use of GSHP will be lostwhile operating in the cooling mode. However, a single maximumvalue of COP can be obtained for both heating and cooling modeoperations of the GSHP by optimizing the design parameters ofthe heat pump.

In order to save electricity input to the GSHP system operatingboth in heating and cooling modes, the system parameters have tobe optimized to achieve a higher COP. The influencing parametersof GSHP on the COP can be classified into four groups (Fig. 1):ground heat exchanger parameters, heat pump parameters, groundparameters and climate and distribution parameters. Optimizingthese parameters to get high performance is an important aspectin the performance analysis of GSHP systems. With regard to opti-mization of GSHP operating parameters, few research works havebeen published. Ramniwas et al. [30] applied Taguchi techniqueto optimize the operating parameters of a GSHP system for spaceheating application. They observed that the condenser outlet tem-perature plays an important role in controlling the value of COP of

Borehole Heat Exchanger Inlet temperature Outlet temperature Pipe diameter Pipe thermal conductivity Heat exchanger fluid Borehole diameter Borehole depth Borehole resistance Type of heat exchanger Heat exchange material

Heat Pump Condenser inlet temperature Compressor efficiency Condenser outlet temperature Evaporator outlet temperature Mass flow rate of refrigerant Type of refrigerant Evaporator inlet temperature Condenser pressure Evaporator pressure

Climate and Distribution System

Ambient temperature Wind speed FCU mass flow rate of air Humidity Solar radiation Room temperature

Ground Properties Soil thermal conductivity Soil moisture ground water movement Soil diffusivity Soil temperature Ground resistance Grouting material Grouting properties

GSHP Performance

Fig. 1. Cause and effect diagram for GSHP system.

78 T. Sivasakthivel et al. / Applied Energy 116 (2014) 76–85

a GSHP system when operated in heating mode. Kavanaugh andRafferty [31] developed a model to optimize the ground heat ex-changer loop length and additional heat source and sink for a hy-brid system in cooling mode operation. The optimization of looplength for cooling dominated building was carried out by consider-ing the heating load of the building. Park et al. [32] took a study onthe optimization of Hybrid GSHP with parallel configuration. Theparameters considered for optimization were: flow rate of refriger-ant, flow rate of the ground heat exchanger fluid and the flow rateof the fluid in the cooling tower. After optimization of the param-eters, the COP was observed to be 21% higher than the normalGSHP. Kjellsson et al. [33] studied the optimal combination ofGSHP and solar collectors for heating and hot water production.The analysis was carried out by using TRNSYS software. The bestcombination observed was: use the solar collector for hot waterproduction in summer and for recharging the ground in winter.Some research has been carried out on thermo economic optimiza-tion of GSHP [34,35] for reducing the cost associated with GSHP.

A detailed literature survey on GSHP research indicates thatonly very few research works have been reported on the study ofGSHP for combined modes of operation. As regard to the optimiza-tion of GSHP parameters are concerned, works have been reportedeither only for heating or only for cooling mode operations. To thebest knowledge of the authors, GSHP parameters optimization forheating and cooling mode operations is not available in the litera-ture. The challenge in optimizing the GSHP parameters for bothheating and cooling involves combined optimization of the opti-mized parameters obtained for heating and cooling modes sepa-rately. This process cannot be achieved by the Taguchi methodwhich is limited to optimization for single objective function.Hence there is a necessity to make use of utility concept, whichis well established in manufacturing [36–39] as well as in concreteresearch [40,41]. In the present work an attempt is made to deter-mine the optimum level of heat pump parameters that will give anoptimum value of COP for both heating and cooling mode opera-tions of a GSHP system. For this purpose four heat pump parame-ters have been considered at three levels based on experimentally

observed parameters on the experimental set-up available at theauthors’ institute. Detailed discussions on the Taguchi methodimplementation for heating mode alone and cooling mode alonefollowed by the use of utility concept to determine the final opti-mum COP of the GSHP system for heating and cooling mode oper-ations are discussed in the following sections.

2. Thermodynamic modeling of GSHP

2.1. Governing equations

For a general steady flow process, the four balance equations,namely mass, energy, entropy and exergy balance equations areapplied to study the performance of the system in terms of heatinteractions, the rate of exergy decrease and the energy and exergyefficiencies.

The governing equations of mass and energy conservation for asteady flow are [42,43]:X

_min ¼X

_mout ð1Þ

where _m is mass flow rate and the subscript ‘in’ stands for inlet and‘out’ stands for outlet.

The general equation of energy balance based on First Law ofThermodynamics can be written asX

_Ein ¼X

_Eout ð2Þ

In more detail it can be written as

_Q þX

_minhin ¼ _W þX

_mouthout ð3Þ

Power input to compressor can be calculated by

_Wcomp;act ¼_Wcomp

gor _Wcomp;act ¼

1:732IVcos/1000

ð4Þ

Heating and cooling capacity of the heat pump can be calcu-lated by (Fig. 2)

Fig. 2. Schematic diagram of GSHP for (a) space cooling and (b) space heating.

T. Sivasakthivel et al. / Applied Energy 116 (2014) 76–85 79

_Q cond: ¼ _mrðh2 � h3Þ and _Q evap: ¼ _mrðh2 � h3Þ ð5Þ

The COP of the GSHP unit and the overall heating system(COPsys) is calculated by the following equations:

COPHP ¼_Q cond:

_Wcomp

; COPsys ¼_Q cond:

_Wcomp þ _Wpump þ _W fan

ð6Þ

COPsys;act ¼_Q cond:

_Wcomp;act þ _Wpump;act þ _W fan;act

ð7Þ

The COP of the GSHP unit and the overall cooling system(COPsys) is calculated by the following equations

COPHP ¼_Q evap:

_Wcomp

; COPsys ¼_Q evap:

_Wcomp þ _Wpump þ _W fan

ð8Þ

COPsys;act ¼_Q evap:

_Wcomp;act þ _Wpump;act þ _W fan;act

ð9Þ

2.2. System description

A vertical ground source heat pump with a cooling load capacityof 1.5 ton is considered for our analysis as shown in Fig. 2. The fig-ure shows the schematic diagram of the GSHP system for spaceheating and cooling. The system consists of a compressor, con-denser, expansion valve, evaporator, fan coil unit, ground heat ex-changer and a reversing valve. Fig. 3 shows the photographicpictures of the ground heat exchanger unit, heat pump unit and in-door unit (fan coil unit) of the experimental setup. The system usesR22 as a refrigerant; the operating pressures of the systems are:0.35 MPa and 1.689 MPa. In this analysis the room temperatureis assumed be 22 �C and the ambient temperature during summerand winter are assumed to be 40 �C and 5 �C respectively. Theground temperature during winter and summer is assumed to beconstant at 21 �C. The different operating parameters of the GSHPhave to be optimized in order to achieve maximum COP for bothheating and cooling mode operations. The values of control param-eters and their levels were taken from the experiments carried outfor heating and cooling mode operations.

2.3. Uncertainty analysis

The uncertainty analysis is important to estimate the accuracyof the results from the experiments. In the present study uncer-tainties are involved in the measurement of temperature, flow,pressure and power input to compressor, circulating pump andfan coil unit. Following uncertainties were observed by calibrationof the instruments: error in temperature: ±0.2%, pressure: ±1.4%,flow rate: ±2%, power input to compressor: ±1.00%, power inputto pump: ±1.6%, power input to fan coil unit: ±3%.

3. Methodology

3.1. Taguchi technique

Taguchi optimization is an experimental optimization tech-nique that uses the standard orthogonal arrays for forming the ma-trix of experiments. By using this matrix it will help us to getmaximum information from minimum number of experimentsand also the best level of each parameter can be found. The majorsteps of implementing the Taguchi method are given in Ref. [44]. Indata analysis, signal-to-noise (S/N) ratios are used to calculate theresponse. There are three types of performance characteristicsused for analyzing S/N: lower the better, higher the better andnominal the better. The use of ANOVA (analysis of variance) is tofind out the percentage contributions of individual parameters.

3.2. Taguchi – design of experiments

For the computational purpose, four parameters at three levelsare considered for the Taguchi method. The factors to be studiedare mentioned in Table 1. Before selecting an orthogonal array,the minimum number of experiments to be conducted can be fixedby using the following relation,

NTaguchi ¼ 1þ NVðL� 1Þ ð10Þ

where NTaguchi is the number of experiments to be conducted, NV isthe number of variables and L is the number of levels. In this anal-ysis, NV = 4 and L = 3. Hence a minimum of nine computational tri-als have to be conducted. The standard orthogonal arrays availableare L4, L8, L9, L12, L16, L18, etc. According to the Taguchi design con-cept L9 orthogonal array is chosen for our computations and thestructure of the array is shown in Table 2. Each computational trialis performed as per standard L9 array table. The optimization of the

Fig. 3. Photos of (a) GHX unit, (b) heat pump unit and (c) indoor unit.

Table 1Control factors and their levels.

Parameters Condenserinlet temp(�C)

Condenseroutlet temp(�C)

Dryness fractionat evaporatorinlet

Evaporatoroutlet temp(�C)

Labels A B C DLevel 1 70.00 40.00 0.24 6.000Level 2 75.00 42.00 0.27 10.000Level 3 80.00 44.00 0.30 14.000

Table 2Taguchi L9 (34) standard orthogonal array.

Experimental number Factor (A) Factor (B) Factor (C) Factor (D)

1 1 1 1 12 1 2 2 23 1 3 3 34 2 1 2 35 2 2 3 16 2 3 1 27 3 1 3 28 3 2 1 39 3 3 2 1

80 T. Sivasakthivel et al. / Applied Energy 116 (2014) 76–85

observed values is determined by using the signal-to-noise (S/N) ra-tios and analysis of variance (ANOVA). Table 3 shows the standardexperimental design for L9 orthogonal array. In the Taguchi method,all the S/N ratio values are calculated based on the concept of higherthe better.

Table 3Taguchi L9 (34) experimental plan.

Experimental trial runnumber

Condenser inlettemp (A)

Condenser outlettemp (B)

Dryness frainlet (C)

1 70.00 40.00 0.242 70.00 42.00 0.273 70.00 44.00 0.304 75.00 40.00 0.275 75.00 42.00 0.306 75.00 44.00 0.247 80.00 40.00 0.308 80.00 42.00 0.249 80.00 44.00 0.27

Higher the better S=N ðdBÞ ¼ �10 � log101n

Xn

i¼1

1y2

i

!ð11Þ

where Yi is the performance value at observation and i is the num-ber of repetitions in a trial.

3.3. Utility concept

The optimum COP of a GSHP system for both heating and cool-ing can be evaluated using the utility concept which characterizesthe utility of a system based on the utility values of individual per-formance parameters. The final performance index of the GSHP isobtained by combining all the expected performance parameterstogether. This combined performance index is called the utility ofa system. In this paper the overall utility of a GSHP system is thesum of utilities of heating and cooling. In general, if the perfor-mance of a system is measured by a number of performanceparameters represented by, Hi, where i varies from 1 to n, thenthe overall utility function, U can be written as [36]

UðH1;H2 . . . HnÞ ¼ f ðU1ðH1Þ;U2ðH2Þ; . . . ;UnðHnÞÞ ð12Þ

where U(H1, H2, . . ., Hn) is the overall utility of ‘n’ performanceparameters. By considering that all the performance parametersare independent and there is no interaction between the parame-ters and the overall utility is a linear function of sum of individualutilities, then the utility function can be written as

UðH1;H2; . . . ;HnÞ ¼Xn

i¼1

UiðHiÞ ð13Þ

Depending upon the performance requirement, the perfor-mance parameters can be given priorities. The priorities can bemeasured by assigning the weighting factors for each performanceparameters. The general form of weighted utility equation is

ction at evaporator Evaporator outlettemp (D)

Space heatingCOP

Space coolingCOP

6.00 4.056 3.6610.00 3.978 3.58914.00 3.942 3.51314.00 4.149 3.6486.00 4.093 3.39610.00 4.036 3.72510.00 4.242 3.45414.00 4.185 3.7836.00 4.128 3.531

T. Sivasakthivel et al. / Applied Energy 116 (2014) 76–85 81

UðH1;H2; . . . ;HnÞ ¼Xn

i¼1

WiUiðHiÞ ð14Þ

where Wi is the weight assigned to the performance parameters andthe sum of the weights for all the performance parameters is equalto 1 [40]. The signal-to-noise ratio associated with the utility valueis higher the better type. To calculate the utility value for differentGSHP performance parameters, a preference scale has to be con-structed for heating and cooling parameters and to calculate theoverall utility value the weighting factors has to be assigned withthe preference scale. As per the literature [36–39] the preferencescale has to be logarithmic. The preference numbers are constructedbetween 0 and 9. The minimum acceptable factor for GSHP perfor-mance is set at a preference number of 0 and for the best factor thepreference number is set as 9. In a logarithmic scale the preferencenumber is represented as

Pi ¼ A logHi

H0ið15Þ

Arbitrarily, one can choose A value such that Pi = 9 at Hi = H*,where H* is the optimum value of Hi. So,

A ¼ 9log H�

H0i

ð16Þ

where Hi is the optimum performance value of GSHP, H0i is the min-imum acceptable performance value of GSHP and A is a constant. Toassign a weighting factor for calculating GSHP performance, the fol-lowing conditions have to be satisfied.

Xn

i

Wi ¼ 1 ð17Þ

Wi ¼Wheating þWcooling ð18Þ

where Wheating, Wcooling, are the weights assigned to the response ofthe GSHP performance parameters. In this study equal importancehas been given to the weighting factors. Therefore Wheating =Wcooling = 0.5 and the overall utility can be calculated as

U ¼Xn

i¼1

WiPi ð19Þ

The major steps involved in implementing the utility concept are (i)Use the Taguchi matrix experimental design and analysis to find outthe optimal value of each of the selected performance parameters,(ii) Make a preference scale for each response based on their opti-mal value and minimum acceptable value, (iii) Assign weights(Wheating and Wcooling) based on the preference, (iv) Find the overallutility values for different experimental trial conditions, (v) Find S/Nratios by using higher-the-better concept, (vi) Find out theoptimum parameters for optimum utility value, (vii) calculate indi-vidual performance based on step (vi) and (viii) compare confirma-tion trial value with predicted value.

Table 4Signal to noise ratio for space heating and cooling.

Experiment No. Space heating COP S/N value

1 4.056 12.1612 3.978 11.9933 3.942 11.9144 4.149 12.3585 4.093 12.2406 4.036 12.1197 4.242 12.5518 4.185 12.4339 4.128 12.314

4. Results and discussions

4.1. Taguchi method – analysis of the S/N ratio for space heating andcooling

The main objective of the Taguchi experiment is to optimize theoperating parameters of GSHP to achieve maximum coefficient ofperformance (COP) for space heating or cooling. The four importantoperating parameters considered here are: condenser inlet temper-ature (A), condenser outlet temperature (B), dryness fraction atevaporator inlet (C), evaporator outlet temperature (D), each factorhaving three levels and these values are given in Table 1. The lay-out of L9 Orthogonal Array (OA) and the parameter-level matrix ofrandomly run experimental trial runs are presented in Table 2.Table 3 shows the actual values of levels of the above fourparameters for calculating the COP for all the 9 trial runs and theCOP values calculated using these data for heating and coolingmodes are also presented in the above table. The S/N ratio valuesof space heating and cooling are calculated using the higher thebetter concept. The calculated signal-to-noise ratio values for spaceheating and cooling are provided in Table 4. The average responsesfor S/N ratios for each level of parameters for heating and coolingare summarized in Tables 5 and 6 and they are also shown in plotsin Figs. 4 and 5 respectively. From the S/N ratio Tables 5 and 6 andFigs. 4 and 5, the best set of combination parameters for spaceheating or cooling can be determined by selecting the level withthe highest value for each factor. Thus, the best combination ofthe operating parameters are found to be A3 (A at level 3), B1(B at level 1), C3 (C at level 3), D1 (D at level 1), for space heatingand for space cooling the best combinations are A1, B3, C1, and D3.Similar trends for the respective best combination of parameterscan also be observed from Figs. 4 and 5.

4.2. Taguchi method – ANOVA

ANOVA is used to estimate the relative significance of eachparameter in terms of percentage contribution on the overall re-sponse. The ANOVA table contains the degrees of freedom, sumof squares (SS), variance and percentage contribution. The param-eters with higher percentage contributions are ranked higher interms of importance in the experiment and also have significant ef-fects in controlling the overall response. The variance, percentagecontribution and sum of squares of the control factors, A, B, Cand D were computed for space heating and cooling with conditionof maximum COP in both the cases and they are shown in Tables 7and 8. It can be observed from Table 7 that for space heating, themaximum variance and percentage contributions are observedonly in factor A (condenser inlet temperature) and factor B (con-denser outlet temperature), whereas the contributions from factorsC and D are insignificant. Among the two factors, factor A contrib-utes 74% and factor B contributes 25.7% and the same observationcan be made from Fig. 4 as well. The parameter which affects the

(dB) Space cooling COP S/N value (dB)

3.66 11.2693.589 11.0993.513 10.9133.648 11.2413.396 10.6193.725 11.4223.454 10.7663.783 11.5563.531 10.957

Table 5Mean of S/N ratios for space heating.

Level Condenser inlettemp (A)

Mean of S/Nratios

Condenser outlettemp (B)

Mean of S/Nratios

Dryness fraction atevaporator inlet (C)

Mean of S/Nratios

Evaporator outlettemp (D)

Mean of S/Nratios

1 70.00 12.02 40.00 12.357 0.24 12.221 6.00 12.2312 75.00 12.24 42.00 12.22 0.27 12.222 10.00 12.2213 80.00 12.43 44.00 12.116 0.30 12.223 14.00 12.231

Table 6Mean of S/N ratios for space cooling.

Level Condenser inlettemp (A)

Mean of S/Nratios

Condenser outlettemp (B)

Mean of S/Nratios

Dryness fraction atevaporator inlet (C)

Mean of S/Nratios

Evaporator outlettemp (D)

Mean of S/Nratios

1 70.00 11.094 24.00 11.092 0.24 11.416 6.000 10.9492 75.00 11.094 28.00 11.092 0.27 11.100 10.000 11.0963 80.00 11.094 32.00 11.093 0.30 10.766 14.000 11.237

11.9

12.1

12.3

12.5

12.7

69 74 79 84

Mea

n of

S/N

rat

ios

Condensor Inlet Temp (ºC)

11.9

12.1

12.3

12.5

12.7

39 41 43 45

Mea

n of

S/N

rat

ios

Condensor Outlet Temp (ºC)

11

11.5

12

12.5

13

0.32

Mea

n of

S/N

rat

ios

Dryness fraction at Evaporator Inlet

11

11.5

12

12.5

13

4 8 12 16

Mea

n of

S/N

rat

ios

Evaporator outlet Temp(ºC)

0.23 0.26 0.29

Fig. 4. Mean of S/N ratios for space heating – Taguchi method.

10

10.5

11

11.5

12

68 72 76 80

Mea

n of

S/N

rat

ios

Condenser inlet Temp (ºC)

10

10.5

11

11.5

12

23 26 29 32

Mea

n of

S/N

rat

ios

Condenser Outlet Temp (ºC)

10.4

10.7

11

11.3

11.6

0.22 0.25 0.28 0.31

Mea

n of

S/N

rat

ios

Dryness fraction at Evaporator inlet

10.5

10.75

11

11.25

11.5

4 8 12 16

Mea

n of

S/N

rat

ios

Evaporator outlet Temp (ºC)

Fig. 5. Mean of S/N ratios for space cooling – Taguchi method.

82 T. Sivasakthivel et al. / Applied Energy 116 (2014) 76–85

Table 7ANOVA for space heating.

Factors Degree offreedom

Sum ofsquares (SS)

Variance %Contribution

Condenser inlet temp (A) 2 0.253 0.126 74.000Condenser outlet temp (B) 2 0.088 0.044 25.713Dryness fraction at

evaporator inlet (C)2 0.000 0.000 0.128

Evaporator outlet temp (D) 2 0.001 0.000 0.159Error 0Total 8 0.341

Table 8ANOVA for space cooling.

Factors Degree offreedom

Sum ofsquares

Variance %Contribution

Condenser inlet temp (A) 2 0.000 0.000 0.000Condenser outlet temp (B) 2 0.000 0.000 0.009Dryness fraction at

Evaporator inlet (C)2 0.634 0.317 83.561

Evaporator outlet temp (D) 2 0.125 0.062 16.429Error 0Total 8 0.758 100.000

T. Sivasakthivel et al. / Applied Energy 116 (2014) 76–85 83

space cooling performances are factor C (dryness fraction at evap-orator inlet) and factor D (evaporator outlet temperature) and thecontributions of other two factors, A and B are negligible. The per-centage contribution of factors C and D for space cooling are83.56% and 16.42% respectively. The following formulae have beenused to calculate the sum of squares (SS), variance and degree offreedom:

SS ¼ 12fðsum of S=N ratio level IÞ2

þ ðsum of S=N ratio level IIÞ2

þ ðsum of S=N ratio level IIIÞ2 � C:Fg ð20Þ

Correction factor ðC:FÞ ¼ðsum of S

NÞ2

Nð21Þ

where N is the total number of experiments (N = 9)

Degree of Freedom ¼ Level� 1 ð22Þ

Variance ¼ SSDOF

ð23Þ

Table 10Utility concept – signal to noise ratio.

4.3. Taguchi method – confirmation test

Confirmation trial runs were carried out based on the calculatedoptimum parameters for heating and cooling modes, which is tab-ulated in Table 9. The confidence interval (CI) can be calculated toestimate the range of performance parameters. In this study 95% ofconfidence interval is considered; the formula to evaluate CI is gi-ven in Eq. (24).

Table 9Confirmation trial run.

Sl.No

GSHPParameters

Optimumparameters

Predictedoptimumvalue

95% of predictedconfidence intervals

Confirmationresults

1 Heating A3B1 C3D1 4.253 3.702 < heating < 4.467 4.4282 Cooling A1B3 C1

D33.321 3.101 < cooling < 3.521 3.458

CI ¼ Fað1; feÞVe1

Neffþ 1

R

� �� �12

and Neff ¼N

1þ TDOFð24Þ

where F(a, 1, fe) is the F value from the F table from any statisticalbook at the required confidence interval and at a degree of freedomof 1, fe the error in degree of freedom; Ve the error variance; R thenumber of repetitions for confirmation experiment; N the totalnumber of experiments; TDOF is the total degree of freedom. In thisexperimental study interaction between the parameters are notconsidered. The predicted values of COP for optimum parametersof space heating and cooling are found to be 4.25 and 3.32 respec-tively. The confirmation test values are approximately 4% higherthan the predicted value.

4.4. Utility concept – analysis of S/N ratio for space heating and cooling

From the Taguchi analysis two different set of values of opti-mum parameters were obtained for space heating and cooling inorder to obtain maximum COP for heating and cooling. Such anal-ysis will be useful to find the optimum parameters when the GSHPis operating either in heating or cooling mode. However, when theGSHP is employed for both space heating and cooling modes in ayear round operation, then it is highly essential to estimate theoptimum parameters for both the modes of operation and also todetermine a single value of COP for the system. This can beachieved with the help of utility concept, which is widely used inmanufacturing techniques. In a given application, there may bethree possibilities of heating/cooling load requirements, the totalheating load may be lower than the cooling load, or higher thanthe cooling load or equal to the cooling load. In the present analysisit is assumed that the heating load is equal to the cooling load.Hence the weighting factor for heating and cooling is assumed tobe equal to 0.5. Using Eqs. (15)–(19), the utility values of the GSHPsystem have been computed for the four operating parameters A, B,C and D at three levels and their corresponding S/N values are alsocomputed for the nine trial runs and are given in Table 10. Higherthe better concept has been employed to calculate the S/N ratio.

Table 11 presents the mean of S/N ratios of the four operatingparameters, A, B, C, and D at three levels. From the table it is clearthat all the four parameters have some effects on the performanceparameters when the GSHP system is operated in heating and cool-ing mode operations. The maximum value of the mean S/N valueindicates the optimum value of the operating parameters. Hencefrom Table 11, the best combinations of the parameters to get opti-mum COP for space heating and cooling can be found as A3 B1 C1 D3,which is also confirmed by plots of S/N values for all the four param-eters shown in Fig. 6. Among the four parameters, condenser inlettemperature and dryness fraction at evaporator inlet have higherdegree of influence on GSHP performance when it is operated forboth heating and cooling. If we rank them in order, the effect ofdryness fraction at evaporator inlet will be the first parameter, thencondenser inlet temperature, condenser outlet temperature andevaporator outlet temperature. Hence while designing a GSHP for

Experiment no. Utility value S/N value (dB)

1 10.406 20.3452 8.060 18.1263 6.384 16.1014 11.911 21.5185 7.536 17.5436 10.876 20.7297 10.954 20.7918 14.259 23.0819 10.000 20.000

Table 11Utility concept – mean of S/N ratios.

Level Condenser inlettemp (A)

Mean of S/Nratios

Condenser outlettemp (B)

Mean of S/Nratios

Dryness fraction atevaporator inlet (C)

Mean of S/Nratios

Evaporator outlettemp (D)

Mean of S/Nratios

1 70.00 18.191 40.00 20.885 0.24 21.386 6.000 19.2972 75.00 19.930 42.00 19.584 0.27 19.882 10.000 19.8823 80.00 21.291 44.00 18.944 0.30 18.145 14.000 20.234

18.000

19.000

20.000

21.000

22.000

68.00 73.00 78.00 83.00

Mea

n of

S/N

rat

ios

Condenser Inlet Temp (ºC)

17.000

18.500

20.000

21.500

23.000

39.00 41.00 43.00 45.00

Mea

n of

S/N

rat

ios

Condenser Outlet Temp (ºC)

16.000

17.500

19.000

20.500

22.000

0.22 0.25 0.28 0.31

Mea

n of

S/N

rat

ios

Dryness fraction at Evporator inlet

18.000

19.000

20.000

21.000

22.000

3.000 7.000 11.000 15.000

Mea

n of

S/N

rat

ios

Evaporator outlet Temp (ºC)

Fig. 6. Mean of S/N ratio for utility concept.

Table 12ANOVA for utility concept.

Factors Degree of freedom Sum o

Condenser inlet temp (A) 2 14.483Condenser outlet temp (B) 2 5.874Dryness fraction at Evaporator inlet (C) 2 15.776Evaporator outlet temp (D) 2 1.346Error 0Total 8 37.479

84 T. Sivasakthivel et al. / Applied Energy 116 (2014) 76–85

both heating and cooling applications these parameters have to beconsidered in this order of preference as the most important designparameters.

4.5. Utility concept – ANOVA

ANOVA analysis also has been carried out for the calculationsperformed on the utility concept. Table 12 presents the percentagecontribution of each parameter on the performance of GSHP, inthat dryness fraction at evaporator inlet is observed to play a majorrole and its percentage contribution is 42.09%, then the condenserinlet temperature, which contributes 38.64%. In total the percent-age contribution of condenser outlet temperature and evaporatoroutlet temperature is only 19.55%. It is clear from the above obser-vations that if we want to get an optimum COP for both heatingand cooling modes of a GSHP system, the dryness fraction at evap-orator inlet becomes a critical parameter that needs to be con-trolled. For the data of three levels of the four operatingparameters, A, B, C and D, the predicted optimum value of COP isfound to be 3.92 with 95% confidence interval and the optimumcombination of the parameters are: A3 B1 C1 D3. It is observed thatcompared to the situation where the GSHP was operated only forheating, this system operates less efficient by 8.4% and 15.4% moreefficient compared to the system operating only for cooling.

5. Conclusions

In this study, a methodology is proposed to optimize the perfor-mance of a ground source heat pump system for space heating andcooling applications using Taguchi method and utility concept. Forthis purpose four operating parameters at three levels of operationof the GSHP system were considered. Hence, the orthogonal arrayof L9 with four parameters and three levels has been selected andcomputations were carried out for all the nine trial runs. The S/Nratio and the ANOVA were employed to optimize the operatingparameters. Totally nine computational trial runs were carriedout using a computer program in FORTRAN. By applying the Tagu-chi technique, initially the operating parameters were optimizedfor only heating and for only cooling. The procedure was extendedto optimize the operating parameters for combined heating andcooling mode operations using utility concept. Based on the re-sults, the following conclusions have been arrived at:

� Using the Taguchi optimization analysis it is found that con-denser inlet and outlet temperatures are the most influencing

f squares Variance Pure SS Percentage contribution

7.242 14.483 38.6442.937 5.874 15.6727.888 15.776 42.0930.673 1.346 3.591

100.000

T. Sivasakthivel et al. / Applied Energy 116 (2014) 76–85 85

parameters for achieving the maximum COP during only heat-ing whereas the dryness fraction at evaporator inlet and outlettemperature of evaporator are the important parameters thatcontribute for the maximum COP during only cooling.� The optimum COP of the heat pump predicted using the opti-

mized parameters by the Taguchi method for only heatingand for only cooling are found to be 4.25 and 3.32 respectively.Implementation of utility concept has made possible to deter-mine the optimum parameters common for combined heatingand cooling and the COP for this case is estimated as 3.92, whichlies between the values of COP obtained for only heating and foronly cooling (that is 8.4% less efficient than for only heating and15.4% more efficient than for only cooling).� For combined heating and cooling mode operations, using the

utility concept, it is found that the dryness fraction at evapora-tor inlet is the most influential parameter whereas the evapora-tor outlet temperature is the least influencing parameter.� The present optimization technique based on the combined

Taguchi and utility concept has predicted the COPs with 95%confidence interval and hence this method can be easily imple-mented for optimum design of GSHPs for combined heating andcooling mode operations to achieve optimum COP.

Acknowledgments

The first author is thankful to the Ministry of Human Resourcesand Development, Government of India, for providing the fellow-ship for pursuing PhD at Indian Institute of Technology Roorkee,Roorkee, India.

References

[1] Omer AM. Ground-source heat pumps systems and applications. Renew SustEnergy Rev 2008;12:344–71.

[2] Ochsner K. Geothermal heat pumps: a guide for planning and installing. 1sted. London: Earthscan Publications Ltd.; 2007.

[3] Ozgener L, Hepbasli A, Dincer I. Exergy analysis of two geothermal districtheating systems for building applications. Energy Convers Manage2007;48:1185–92.

[4] Hepbasli A, Akdemir O. Energy and exergy analysis of a ground source(geothermal) heat pump system. Energy Convers Manage 2004;45:737–53.

[5] Ozgener O, Hepbasli A. Experimental performance analysis of a solar assistedground-source heat pump greenhouse heating system. Energy Build2005;37:101–10.

[6] Bi Yuehong, Guo Tingwei, Zhang Liang, Chen Lingen. Solar and ground sourceheat-pump system. Appl Energy 2004;78:231–45.

[7] Bakirci Kadir. Evaluation of the performance of a ground-source heat-pumpsystem with series GHE (ground heat exchanger) in the cold climateregion. Energy 2010;35:3088–96.

[8] Wood CJ, Liu H, Riffat SB. An investigation of the heat pump performanceandground temperature of a pile foundation heat exchanger system for aresidential building. Energy 2010;35:4932–40.

[9] Fan R, Jiang Y, Yao Y, Ma Z. Theoretical study on the performance of anintegrated ground-source heat pump system in a whole year. Energy2008;33:1671–9.

[10] Trillat-Berdal V, Souyri B, Fraisse G. Experimental study of a ground-coupledheat pump combined with thermal solar collectors. Energy Build2006;38:1477–84.

[11] Balbay A, Esen M. Experimental investigation of using ground source heatpump system for snow melting on pavements and bridge decks. ScientificResearch and Essays 2010;5:3955–66.

[12] Esen H, Inalli M, Esen M. Numerical and experimental analysis of a horizontalground-coupled heat pump system. Build Environ 2007;42:1126–34.

[13] Esen M, Yuksel T. Experimental evaluation of using various renewable energysources for heating a greenhouse. Energy Build 2013;65:340–51.

[14] Karabacak R, Acar SG, Kumsar H, Gökgöz A, Kaya M, Tülek Y. Experimentalinvestigation of the cooling performance of a ground source heat pump systemin Denizli, Turkey. Int J Refrig 2011;34:454–65.

[15] Hwang Y, Lee JK, Jeong YM. Cooling performance of a vertical ground-coupledheat pump system installed in a school building. Renew Energy2009;34:578–82.

[16] Jeon J, Lee S, Hong D, Kim Y. Performance evaluation and modeling of a hybridcooling system combining a screw water chiller with a ground source heatpump in a building. Energy 2010;35:2006–12.

[17] Kizilkan O, Dincer I. Exergy analysis of borehole thermal energy storagesystem for building cooling applications. Energy Build 2012;49:568–74.

[18] Yu X, Wang R, Zhai X. Year round experimental study on a constanttemperature and humidity air-conditioning system driven by ground sourceheat pump. Energy 2011;36:1309–18.

[19] Bi Y, Wang X, Liu Y, Zhang H, Chen L. Comprehensive exergy analysis of aground-source heat pump system for both building heating and coolingmodes. Appl Energy 2009;86:2560–5.

[20] Aikins KA, Choi JM. Current status of the performance of GSHP (ground sourceheat pump) units in the Republic of Korea. Energy 2012;47:77–82.

[21] Montagud C, Corberán JM, Ruiz-Calvo F. Experimental and modeling analysisof a ground source heat pump system. Appl Energy 2013;109:328–36.

[22] Esen H, Inalli M, Sengur A, Esen M. Modelling a ground-coupled heat pumpsystem using adaptive neuro-fuzzy inference systems. Int J Refrig2008;31:65–74.

[23] Esen H, Inalli M, Sengur A, Esen M. Artificial neural networks and adaptiveneuro-fuzzy assessments for ground-coupled heat pump system. Energy Build2008;40:1074–83.

[24] Esen H, Inalli M, Sengur A, Esen M. Forecasting of a ground-coupled heat pumpperformance using neural networks with statistical data weightingpreprocessing. Int J Therm Sci 2008;47:431–41.

[25] Esen H, Inalli M, Sengur A, Esen M. Modeling a ground-coupled heat pumpsystem by a support vector machine. Renew Energy 2008;33:1814–23.

[26] Esen H, Inalli M, Sengur A, Esen M. Performance prediction of a ground coupledheat pump system using artificial neural networks. Expert Syst Applic2008;35:1940–8.

[27] Esen H, Inalli M, Sengur A, Esen M. Predicting performance of a ground-sourceheat pump system using fuzzy weighted pre-processing-based ANFIS. BuildEnviron 2008;43:2178–87.

[28] Gang W, Wang J. Predictive ANN models of ground heat exchanger for thecontrol of hybrid ground source heat pump systems. Appl Energy2013;112:1146–53.

[29] Michopoulos A, Bozis D, Kikidis P, Papakostas K, Kyriakis AN. Three-yearsoperation experience of a ground source heat pump system in NorthernGreece. Energy Build 2007;39:328–34.

[30] Ramniwas K, Murugesan K, Sahoo PK. Optimization of operating parameters ofground source heat pump using Taguchi Method. In: 23rd IIR Conference,Prague, Czech Republic; August 21–26, 2011.

[31] Kavanaugh SP, Rafferty K. Ground source heat pumps—design of geothermalsystems for commercial and institutional buildings, ASHRAE, Atlanta; 1997.

[32] Park H, Lee JS, Kim W, Kim Y. Performance optimization of a hybrid groundsource heat pump with the parallel configuration of ground heat exchangerand a supplemental heat rejecter. Int J Refrig 2012;35:1537–46.

[33] Kjellsson E, Hellstrom G, Perers B. Optimization of systems with thecombination of ground-source heat pump and solar collectors in dwellings.Energy 2010;35:2667–73.

[34] Sanaye S, Niroomand B. Thermal-economic modeling and optimization ofvertical ground-coupled heat pump. Energy Convers Manage2009;50:1136–47.

[35] Sayyaadi H, Amlashi EH, Amidpour M. Multi-objective optimization of avertical ground source heat pump using evolutionary algorithm. EnergyConvers Manage 2009;50:2035–46.

[36] Singh H, Kumar P. Optimizing multi-machining characteristics throughTaguchi’s approach and utility concept. J Manuf Technol Manage2006;17:255–74.

[37] Dubey AK. Multi-response optimization of electro-chemical honing usingutility-based Taguchi approach. Int J Adv Manuf Technol 2009;41:749–59.

[38] Kumar J, Khamba JS. Multi-response optimization in ultrasonic machining oftitanium using Taguchi’s approach and utility concept. Int J Manuf Res2010;2:139–60.

[39] Kumar P, Barua B, Gaindhar JL. Quality optimization (multi-characteristic)through Taguchi’s technique and utility concept. Qual Reliab Eng Int2000;16:475–85.

[40] Rahim A, Sharma UK, Murugesan K, Sharma A, Arora P. Optimization of post-fire residual compressive Strength of concrete by Taguchi method. J Struct FireEng 2012;3:169–80.

[41] Rahim A, Sharma UK, Murugesan K, Sharma A, Arora P. Multi-responseoptimization of post-fire residual compressive strength of high performanceconcrete. Constr Build Mater 2013;38:265–73.

[42] Hepbasli A. Thermodynamic analysis of a ground source heat pump system fordistrict heating. Int J Energy Res 2005;29:671–87.

[43] Esen H, Inalli M, Esen M, Pihtili K. Energy and exergy analysis of a ground-coupled heat pump system with two horizontal ground heat exchangers. BuildEnviron 2007;42:3606–15.

[44] Chen YH, Tam SC, Chen WL, Zheng HY. Application of Taguchi method in theoptimization of laser micro-engraving of photo masks. Int J Mater ProdTechnol 1996;11:333–44.