Optimization of solar assisted ground source heat pump system for space heating application by Taguchi method and utility concept

Energy and Buildings 82 (2014) 296–309

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Optimization of solar assisted ground source heat pump system for space heating application by Taguchi method and utility concept Vikas Verma, K. Murugesan ∗ Department of Mechanical and Industrial Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, Uttarakhand, India

a r t i c l e

i n f o

Article history: Received 26 March 2014 Received in revised form 14 July 2014 Accepted 14 July 2014 Available online 22 July 2014 Keywords: Coefficient of performance Taguchi method Utility concept Solar collector area Ground heat exchanger length

a b s t r a c t In the present research, a methodology is proposed to optimize the solar collector area and ground heat exchanger length for achieving higher COP of Solar Assisted Ground Source Heat Pump (SAGSHP) system using Taguchi method and utility concept. Four operating parameters for solar collector and four parameters for ground heat exchanger have been selected with mixed level variation using an L18 (21 , 37 ) orthogonal array. The key parameters such as solar collector area, ground heat exchanger length and COP of the SAGSHP system are optimized to predict the best levels of operating parameters for maximum COP of SAGSHP system. Lower the better concept has been used for the solar collector area and ground heat exchanger length whereas higher the better concept has been employed for the COP of SAGSHP system and the results have been analyzed for the optimum conditions using signal-to-noise (SN) ratio and ANOVA method. Computations were carried out for 18 experimental trial runs by considering 2 ton heating load in winter season. The optimum COP for SAGSHP was estimated to be 4.23 from the utility concept, which is 8.74% higher than the optimum COP predicted by Taguchi optimization. Optimization of solar collector area and ground heat exchanger length by the utility concept has shown only about 2.3% reduction in area and 1.6% reduction in length respectively compared to those values optimized by the Taguchi method. © 2014 Elsevier B.V. All rights reserved.

1. Introduction It is estimated that around one fourth of total energy consumption in the world is spent for space heating and cooling applications. Currently most of the conventional space heating devices consume electricity, which is a high grade energy commonly generated by power plants using fossil fuels that emit a huge amount of greenhouse gases. Global awareness on the alarming rise in greenhouse gas emissions has forced the researchers to focus on the development of space heating and cooling systems which are environment friendly with less emission of greenhouse gases, at the same time, consume reduced electrical energy input. Renewable energy source is a great opportunity to fulfill our demand for space heating. Solar energy stored in the ground as ground source energy has been extracted for space heating through ground source heat pump systems [1–3] and they are the most popular technology for space heating in Western and European countries. GSHP system is a

∗ Corresponding author. Tel.: +91 1332 285635. E-mail addresses: [email protected], [email protected] (K. Murugesan). http://dx.doi.org/10.1016/j.enbuild.2014.07.029 0378-7788/© 2014 Elsevier B.V. All rights reserved.

good alternate technology for providing space heating and also consumes less amount of electrical energy. Direct solar radiation also can be used for space heating with the help of a heat pump, known as solar coupled heat pump (SCHP) system. Liang et al. [4] employed air source heat pump coupled with 40 m2 area of solar collector for their experimental work. They observed that the use of solar collector has resulted in 24% energy saving without significant increase in the COP of the heat pump system. Kayagusuz et al. [5] analyzed a solar coupled heat pump system using latent heat thermal energy storage tank filled with phase change material (PCM) for storing heat absorbed by a flat plate solar collector. They considered different areas for the solar collector and found that the SCHP with a solar collector area of 30 m2 achieved a COP of 5 to 7 for the heat pump and 3.5 to 7 for the case of SCHP. They also reported an increase in collector and storage efficiency of 50–90% and 50–60%, respectively. Bakirci and Yuksel [6] carried out experimental thermal performance analysis of a solar source heat pump system for residential heating in cold climate region with the main objective of reducing solar collector area. They found that a solar collector area of 20 m2 gave an average COP of 3.8 and 2.9 for the heat pump and the whole system respectively and observed that the collector efficiency varied from 33% to 47% with flat plate solar

V. Verma, K. Murugesan / Energy and Buildings 82 (2014) 296–309

Nomenclature A C COP cos ϕ d D FR FCU GCHP GHX h IT I J K L m NV Q R r S/N SCHP SAGSHP T UL V W Z T  

area (m2 ) specific heat capacity (J/kg K) coefficient of performance fraction factor installation depth of GHX (m) diameter of GHX pipe (mm) heat removal factor fan coil unit ground coupled heat pump ground heat exchanger heat transfer coefficient (W/m2 K) solar radiation (W/m2 ) current (A) level thermal conductivity (W/mK) length of ground heat exchanger (m) mass flow rate(kg/s) parameters energy gain (kW) thermal resistance (m2 K/W) radius (mm) signal-to-noise ratio solar coupled heat pump solar assisted ground source heat pump temperature (◦ C) overall loss coefficient (W/m2 K) voltage (V) power input (kW) enthalpy (J/kg) temperature difference in solar collector (◦ C) efficiency (%) temperature difference in GHX (◦ C)

Greek letters  reflectivity absorptivity ˛ Subscripts a ambient c solar collector comp compressor condenser cond g glass cover i inner inlet in o outer out outlet

collector when the outdoor temperature varied from −10.8 ◦ C to 14.6 ◦ C. Yang et al. [7] carried out a numerical simulation study to determine the optimum collector area for maximum COP of SAHP system. They considered different areas varying from 10 m2 to 30 m2 and observed that when the collector area was maximum, the heat pump produced heat with maximum COP. Zhao et al. [8] demonstrated mathematical optimization of solar coupled heat pump system and ground coupled heat pump system using metric method and obtained 16.7% improvement in COP for heat pump and found optimized COP of 3.9 and 3.2 for the solar coupled heat pump system and ground coupled heat pump respectively. They also showed 17.2% saving in energy and 11.8% saving in the total cost of the system.

297

Sometimes the heat obtained from solar collector is not sufficient for heating during the winter season, in such situations the solar collector can be coupled with a ground coupled heat pump system, in which the heat stored in the ground also can be exploited along with the heat received from the solar collector. Such a system is called solar assisted ground source heat pump (SAGSHP). The idea of SAGSHP was first proposed by Metz [9]. After the experimental and theoretical works carried out on SAGSHP at Brookhaven National Laboratory, USA [10], many researchers showed interest on SAGSHP research. Han et al. [11] carried out numerical simulation of SAGSHP system with latent heat energy storage in severely cold area for which they obtained an average COP of 3.28 for heating with the highest value of 5.95. Yuehong et al. [12] carried out theoretical and experimental studies using vertical double-spiral coil (VDSC) for the ground heat exchanger in order to effectively decrease the temperature interference between the interior and exterior coil pipe. They reported an average COP of 2.78 for the SAGSHP system. Rad et al. [13] proposed a hybrid ground source heat pump system combined with a solar thermal collector for space heating applications and observed that the use of 6.81 m2 area of solar collector had resulted in 15% reduction in length of GHX and this is estimated a reduction of 7.64 m length of GHX per square meter area of solar collector. The use of solar collector has also reduced the cost around 3.7––7.6% lower than the conventional GSHP system. Chen et al. [14] optimized the long term operation of a solar assisted ground coupled heat pump system used for space heating and domestic hot water (DHW) production with the help of optimization and numerical simulation using TRNSYS. Their simulation results showed that the long term yearly average space heating efficiency was improved by 26.3% over the traditional ground coupled heat pump (GCHP) system. Chen and Hongxing [15] carried out a simulation study on solar assisted ground coupled heat pump system under the specified load condition using a collector area of 40 m2 and a borehole length of 264 m. The COP of the system was found to be 3.55. Wang and Qi [16] carried out an experimental study to analyze the performance of underground thermal storage system in a solar-ground coupled heat pump system for residential buildings. They achieved an efficiency of 40% for the underground thermal storage system based on the total solar radiation and 70% efficiency for the solar collectors. They also reported the ratio between the tank volume and the area of solar collectors should be in the range of 20 to 40 L/m2 . Wang et al. [17] showed that the overall COPsys of solar assisted ground source heat pump (SAGSHP) system and hybrid solar ground source heat pump (HSGSHP) system can be improved by injecting heat into the borehole and by this method the system’s electrical energy demand could be reduced by 32%. Coskun et al. [18] investigated the effect of control parameters on the system performance on waste heat recovery application using mechanical heat pump by Taguchi optimization technique using L27 orthogonal array. They also analyzed the effect of control factors and their two way interactions on response were modeled via regression and correlation analysis with coefficient of determination (R2 ) values of COPsys and COPhp as 77% and 79.7%, respectively. Li and Yang [19] developed a mathematical model to study the performance of solar assisted air source heat pump systems for hot water production in Hong Kong and verified their results with experimental data and observed that simulation results were reasonable. They found that the tilt angle of solar collector had little impact on the optimum flow rate. Park et al. [20] analyzed the comparison of performance between GSHP and HGSHPS arranged in parallel and series configurations with varying leaving fluid temperature (LFT) of the GHX. They noted that with a leaving fluid temperature of 40 ◦ C, the COP of HGSHP was observed to be respectively 18% and 6% higher performance compared to GSHP and HGSHP systems. Sivasakthivel et al. [21] optimized the operating parameters of a ground source heat pump system for

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space heating and cooling applications using Taguchi method and utility concept. Based on this analysis the maximum COP for heating and cooling were obtained as 4.25 and 3.32, respectively. Further using utility concept a single optimum COP value for both heating and cooling was obtained as 3.92, which amounts to 8.4% less efficient for heating and 15.4% more efficient for cooling. The application of Taguchi and utility concept has been well established for problems related to manufacturing engineering [22–24]. A detailed literature survey indicates that only when a ground coupled heat pump is coupled with solar collectors/solar energy thermal storage system, the real benefit of reduction in electricity consumption can be achieved by suitably selecting the solar collector area and ground heat exchanger length. Only a very few research works have investigated the optimization of either solar collector area or ground heat exchanger length. To the best knowledge of the authors no work has been published to find optimum collector area and ground heat exchanger length, at the same time achieving the maximum COP. In order to obtain a significant reduction in work input or increase in COP of the SAGSHP, both solar collector area and ground heat exchanger length have to be optimized. In the present work it is proposed to employ Taguchi optimization technique and utility concept to optimize solar collector area and ground heat exchanger length to achieve maximum COP of SAGSHP. For this purpose four influencing factors for solar collector area and four for ground heat exchanger length are considered for the analysis. The details of the methodology followed and the results obtained are discussed in the following sections.

2. System description A solar assisted ground source heat pump system (SAGSHPS) with a combination of flat plate solar collector and horizontal ground heat exchanger with a heating load capacity of 2 ton shown in Fig. 1 is considered for the present analysis. The performance of solar collector mainly depends on the area of solar collector (m2 ) and GHX performance depends on the length of ground heat exchanger (m). The heat pump consists of a compressor, condenser, expansion valve, evaporator and fan coil. In the SAGSHP system, the solar collector is combined with the ground heat exchanger where water is considered as working fluid to extract heat from solar collector as well as from the ground through GHX. Refrigerant R-22 is considered as a working fluid in the heat pump system. While using the SAGSHP for heating mode, the evaporation of the refrigerant takes place using the heat absorbed from the ground through the ground heat exchanger and solar collector. The heat rejected in the condenser is used for space heating using the fan coil unit and then the refrigerant is sent back to evaporator through an expansion valve. In this work Taguchi method and utility concept have been employed to optimize the area of the solar collector, GHX length and COP of the SAGSHP system by performing the calculations using thermodynamic analysis. The following temperatures were assumed at various state points in the SAGSHP: temperature of water at inlet to GHX = 8 ◦ C, ground temperature = 14 ◦ C, thermal conductivity of soil = 2.07 W/m K.

˛ 1 − (1 − ˛)g

˛ = UL =

1 r + i ln hi k

(2)

r  o

ri

+

ri 1 ro ho

(3)

3.2. Area of solar collector (AC ) Ac =

Qu IT ˛ − UL (Ti − Ta )

(4)

Using Eqs. (1) and (2) in Eq. (4)



Ac = IT

F (˛)IT − FR UL T

˛ 1−(1−˛)g

 R −

1 hi

+

ri K

ln

 ro  ri

+

ri 1 ro ho



(5) (Ti − Ta )

where “Generic” values for glazed solar collectors are provided with FR ˛ = 0.68 and FR UL = 4.9 W/m2 ◦ C. These values correspond to the test results for thermodynamic collectors [26].The solar collector efficiency can be calculated using the following relation [27]: c =

Qu Ac IT

(6)

After the calculation of solar collector area and efficiency, the length of horizontal ground heat exchanger can be calculated by performing an energy balance on the differential ground heat exchanger pipe element under steady state conditions. The inlet and outlet temperatures of ground heat exchanger fluid (water) can be easily measured and hence the following expression for the length of ground heat exchanger has been obtained in terms of these temperatures and the resistance of soil, fluid convection and pipe material [28]: 3.3. Length of ground heat exchanger (L)

 L = mw cw RTotal ln

win wout

(7)

where RTotal = Rcon + Rpipe + Rsoil

(8)

Considering a horizontal pipe of inside diameter Di and outside diameter Do , buried at a depth, ‘d’ from the ground surface, expressions for different thermal resistances can be expressed as follows: 3.4. Thermal resistance due to convective heat transfer for flow of water in the pipe 1 Di hw

Rcon =

(9)

3.5. Thermal resistance due to conduction through pipe thickness

 ln Rpipe =

Do Di

 (10)

2 Kpipe

3.6. Thermal resistance due to conduction in soil 3. Thermodynamic and heat transfer equations Rsoil =

3.1. Solar collector plate The following equation relates the rate of useful energy, Qu collected by a flat plate solar collector to the design parameters of the solar collector and meteorological conditions [25]: Qu = FR {IT (˛) − UL (Ti − Ta )}

(11)

where, S is the conduction shape factor of the pipe [29] expressed as 2

S= ln

(1)

1 SKsoil

 2d   2d 2 Do

+

Do

−1

(12)

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299

Fig. 1. Schematic diagram of SAGSHP system.

3.7. COP of heating systems In the present analysis, the COP of heat pump and SAGSHP systems are computed and are defined as follows: COP of heat pump =

Qcondenser Wcomp

COPSAGSHP of system = Qcondenser = (Z2 − Z3 )

Qcondenser Wcomp + Wpump

(13) (14)

consideration. Based on the number of control parameters and levels, the appropriate orthogonal array (OA) has to be selected and the OA will specify the optimum number of trial runs to be performed. Finally the results obtained for each trial run have to be analyzed using signal to noise ratio (S/N), ANOVA and response table. There are three types of performance characteristics used for analyzing S/N: lower the better, higher the better and nominal the best. ANOVA is used to find out the percentage contribution of individual parameters in the experiment. The results obtained in the

(15)

Wcomp and Wpump are computed based on the electrical energy input to the respective components. In SAGSHP system heat collected from GHX and solar collector act as heat source. 4. Methodology 4.1. Taguchi technique Taguchi optimization is an experimental optimization technique that uses the standard orthogonal arrays in matrix form of experiments which gives an optimum number of experimental trial runs with “optimum setting” of control parameters and using this technique the best level of each parameter can be found. There are different phases for applying Taguchi optimization which are shown in Fig. 2. In this method initially the number of control parameters and levels have to be identified for the system under

Fig. 2. Flow chart for Tguchi optimization procedure.

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above analysis will provide information on the influential parameters and their levels. Generally a confirmation run is carried out using the optimum parameters and levels to verify the results using the response and ANOVA tables. In the Taguchi optimization technique, for the given number of control parameters, the minimum number of experiments to be conducted can be determined by using the following relation: NTaguchi = 1 + NV (J − 1)

(16)

where, NTaguchi is the number of experimental trial runs to be conducted. In the present analysis, NV = 8 and mixed level was used, hence L18 (21 , 37 ) has been used in the optimization technique. As the main objective of the present research is to find out the optimum solar collector area and ground heat exchanger length for the SAGSHP, the lower the better criterion has been selected so that the initial cost of SAGSHP is reduced.

Lower the better S/NLB = −10 log

1 2 yi r r



(17)

i=1

where, Yi = performance value at a given observation, i = number of repetitions in a trial. When the solar collector area and ground heat exchanger length are optimized with the help of Taguchi optimization technique, the optimum COP of SAGSHP can be computed using the optimized parameters and their levels by the implementation of utility concept. 4.2. Utility concept Utility is one of the best optimization techniques to evaluate the optimum parameters of SAGSHP system. In utility the performance parameters are characterized by individual performance parameters. The final optimum parameters of SAGSHP system are obtained by combining all the expected performance parameters. This combination is known as utility of a system. In this paper the overall utility of a SAGSHP system is assumed to be the sum of all utilities of heating obtained by solar collector and GHX and performance of SAGSHP system defined by the COP. This is expressed by the following equation [19–26]: U(X1 , X2 , . . ., Xn ) = f (U1 (X1 ), U2 (X2 ). . .. . . Un (Xn ))

(18)

where U(X1 ,X2 ,. . .,Xn ) is overall utility of ‘n’ performance parameters. All the performance parameters are not related to each other and also independent of other parameters. Here the overall utility is the summation of all the individual utilities, hence the utility function can be expressed as U(X1 , X2 , ....., Xn ) =

n 

Ui (Xi )

(19)

i=1

Performance parameters are considered on priority basis. These priorities can be calculated by fixing of weighting factors for each parameter. The common form of weighted utility equation is U(X1 , X2 , ....., Xn ) =

n 

Wi Ui (Xi )

(20)

i=1

The sum of weight of all the performance parameters is equal to 1 [30]. Higher the better concept is associated for finding the signal to noise ratio. In utility concept preference scale has to be constructed for heating parameters of SAGSHP system and weighting factors has to be assigned with the preferences scale for calculating the overall utility value. As per the literature [22,31,32] the preference numbers are assumed to range between 0 and 9. In SAGSHP system the minimum preference number is 0 and the maximum

preference number is set as 9. In a logarithmic scale the preference number is represented as Pi = A log

Xi 

Xi

(21)

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

9 log

X∗ Xi

(22)

where, Xi is the optimum performance value of SAGSHP, X i is the minimum acceptable performance value of SAGSHP and A is a constant. The weighting factor has to satisfy the following condition so that the SAGSHP system performance can be calculated: n 

Wi = 1

(23)

i=1

Wi = WSolar collector area + WGHXlenth + WCOP of SAGSHP system

(24)

where, WSolar collector area , WGHX length and WCOP of SAGSHP system are the weights assigned to the response of the SAGSHP system performance parameters and now the overall utility can be calculated as U=

n i

Wi Pi

(25)

The main objective of implementing the utility concept is to determine the maximum COP of SAGSHP system using the optimum length of GHX and solar collector area. The important key steps to be followed in the utility concept are (i) use the Taguchi orthogonal array to calculate the optimal value of each selected parameter, (ii) select a preference scale optimal and minimum acceptance values for each optimum value, (iii) assign weight (WSolar collector area , WGHX length , WCOP of SAGSHP system ), (iv) calculate the overall utility values for all the L18 experimental trial runs, (v) find S/N ratios by using higher-the-better concept, (vi) find out the optimum utility value and (vii) compare utility value and the value optimized by the Taguchi method. 5. Results and discussions The main aim of this study is to optimize the solar collector area and the ground heat exchanger length in order to obtain the optimum value for the COP of SAGSHP system, which is influenced by several parameters like solar intensity, transmissivity of the glass cover, absorptivity of the material, reflectivity of the plate, thermal conductivity of the solar collector pipe, inner radius of solar collector pipe, water inlet temperature of collector, overall heat loss coefficient, thermal conductivity of GHX pipe, ground heat exchanger length, material of GHX, installation depth of GHX, inner radius of GHX, specific heat capacity of liquid, mass flow rate of water flowing in solar collector and ground heat exchanger. Among these parameters some of the parameters are controllable in the optimization process and others are fixed. As a first attempt, in this paper only eight parameters have been considered for the analysis in order to achieve optimum COP of SAGSHP. Initially the parameters are optimized to obtain optimized solar collector area and ground heat exchanger length using Taguchi method and these parameters are optimized to achieve optimum COP for space heating using the utility concept. 5.1. Taguchi method – control factors for space heating The main objective of the Taguchi experiments is to optimize the operating parameters of SAGSHP to achieve maximum coefficient of performance (COP) with minimum solar collector area

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301

Table 1 Parameters and their levels. Label

Parameters

Level

A B C D E F G H

Thermal conductivity of GHX pipe (W/m K) Installation depth of GHX (m) Inner radius of GHX pipe (mm) Specific heat capacity of liquid (J/kg K) Reflectivity of glass cover (−) Mass flow rate of water (kg/s) Thermal conductivity of solar collector pipe (W/m K) Inner radius of solar collector pipe (mm)

1

2

3

0.10 1 19.05 3991 0.12 0.3 16 19.05

0.42 1.5 25.4 4187 0.16 0.4 54 25.4

– 2 31.75 4288 0.24 0.5 110 31.75

Table 2 Taguchi L18 (21 ,37 ) standard orthogonal array. Experimental number

Factor (A)

Factor (B)

Factor (C)

Factor (D)

Factor (E)

Factor (F)

Factor (G)

Factor (H)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2

1 1 1 2 2 2 3 3 3 1 1 1 2 2 2 3 3 3

1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3

1 2 3 1 2 3 2 3 1 3 1 2 2 3 1 3 1 2

1 2 3 2 3 1 1 2 3 3 1 2 3 1 2 2 3 1

1 2 3 2 3 1 3 1 2 2 3 1 1 2 3 3 1 2

1 2 3 3 1 2 2 3 1 2 3 1 3 1 2 1 2 3

1 2 3 3 1 2 3 1 2 1 2 3 2 3 1 2 3 1

and minimum ground heat exchanger (GHX) length for space heating application. Hence, in order to calculate the ground heat exchanger length and solar collector area, the following eight operating parameters are considered as control factors: (A) thermal conductivity of GHX pipe, (B) installation depth of GHX, (C) inner radius of GHX pipe, (D) specific heat capacity of GHX fluid (water), (E) reflectivity of glass cover, (F) mass flow rate of GHX fluid (water), (G) thermal conductivity of solar collector pipe, and (H) inner radius of solar collector pipe. Among these, seven parameters are considered at three levels and one parameter at two levels as summarized in Table 1. Hence L18 orthogonal array has been chosen for deciding the experimental lay out. The general layout of L18 Orthogonal Array (OA) is shown in Table 2 and the parameter matrix of random

experimental trial runs are shown in Table 3. Each trial run shown in the above table provides the set of required data for calculating the solar collector area, ground heat exchanger length and COP for space heating application and the calculated values of solar collector area, ground heat exchanger length and COP of the SAGSHP are shown in Table 4. 5.2. Taguchi method – signal to noise ratio In Taguchi method once the required trial runs are carried out, then the next step is to convert the trial run results into signal to noise ratio (S/N). The term signal illustrates the preferable effect for the output, that is, solar collector area, heat exchanger length and

Table 3 Taguchi L18 experimental plan. Experimental number

Factor (A)

Factor (B)

Factor (C)

Factor (D)

Factor (E)

Factor (F)

Factor (G)

Factor (H)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.42 0.42 0.42 0.42 0.42 0.42 0.42 0.42

1 1 1 1.5 1.5 1.5 2 2 2 1 1 1 1.5 1.5 1.5 2 2 2

19.05 25.40 31.75 19.05 25.40 31.75 19.05 25.40 31.75 19.06 25.40 31.75 19.05 25.40 31.75 19.05 25.40 31.75

3991 4187 4288 3991 4187 4288 4187 4288 3991 4288 3991 4187 4187 4288 3991 4288 391 4187

0.12 0.16 0.25 0.16 0.25 0.12 0.12 0.16 0.24 0.24 0.12 0.16 0.24 0.12 0.16 0.16 0.24 0.12

0.3 0.4 0.5 0.4 0.5 0.3 0.5 0.3 0.4 0.4 0.5 0.3 0.3 0.4 0.5 0.5 0.3 0.4

16 54 110 110 16 54 54 110 16 54 110 16 110 16 54 16 54 110

19.05 25.4 31.75 31.75 19.05 25.40 31.75 19.05 25.40 19.05 25.4 31.75 25.40 31.75 19.05 25.40 31.75 19.05

302

V. Verma, K. Murugesan / Energy and Buildings 82 (2014) 296–309 Table 5 Signal to noise ratio for solar collector area, GHX length and COP of SAGSHP system.

Table 4 Area of solar collector, length of GHX and COP of SAGSHP system. Experimental number

Area of solar collector (m2 )

Length of GHX (m)

COP of SAGSHP system

Experimental number

S/N ratio of solar collector area

S/N ratio of GHX length

S/N ratio COP of SAGSHP system

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

7.59 7.70 7.74 7.69 7.72 7.63 7.78 7.62 7.64 7.68 7.77 7.59 7.58 7.71 7.75 7.76 7.55 7.72

515.327 406.86 351.53 490.5 409.27 425.51 476.99 482.33 413.67 332.77 298.109 319.06 390.59 330.12 302.30 354.09 388.13 329.979

3.65 3.68 3.73 3.65 3.67 3.69 3.64 3.65 3.71 3.72 3.73 3.77 3.64 3.75 3.73 3.71 3.73 3.76

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

−17.6053 −17.7409 −17.7836 −17.7279 −17.7570 −17.6554 −17.8257 −17.6417 −17.6723 −17.7141 −17.8162 −17.6098 −17.5960 −17.7487 −17.7953 −17.7976 −17.5697 −17.7620

−54.2417 −52.1889 −50.9193 −53.8128 −52.2404 −52.5783 −53.5703 −53.6670 −52.3332 −50.4430 −49.4875 −50.0776 −51.8345 −50.3737 −49.6089 −50.9825 −51.7797 −50.3697

11.2601 11.3211 11.4458 11.2667 11.2992 11.3570 11.2316 16.4581 11.3875 11.4050 11.4539 11.5383 11.2411 11.4899 11.4510 11.3974 11.4429 11.5038

COP of SAGSHP system and the term noise represents the undesirable effects on the above outputs. In this research work, signal to noise ratio values are calculated by using lower the better concept for solar collector area and ground heat exchanger length, whereas the COP of the system is calculated by using higher the better concept for space heating application using the SAGSHP system and the computed S/N values are shown in Table 5. Then the response tables (Tables 6–8) for all the three outputs are computed by taking the average of the S/N values of these three outputs given in Table 5. The ranking shown in the last rows of Tables 6–8 for solar collector area, ground heat exchanger and COP of SAGSHP system respectively demonstrate the order of influence of the parameters on the output, rank 1 being the most influencing parameter and 8 being the least influencing parameter. According to the results of S/N ratio shown in Tables 6–8 the best combination set of parameters for space heating has been determined by selecting the level with the lowest value for each factor of solar collector and ground heat exchanger and the highest value for each factor for the COP

of SAGSHP system. Thus in Table 6 the optimum levels of the considered eight parameters are A1 (thermal conductivity of GHX pipe is 0.10), B1 (installation depth of GHX pipe is 1), C1 (inner radius of GHX pipe is 19.05), D1 (specific heat capacity of liquid is 3991), E3 (reflectivity of glass cover is 0.24), F1 (mass flow rate of water is 0.3), G1 (thermal conductivity of solar collector pipe is 16) and H1 (inner radius of solar collector pipe is 19.05). Similarly using the values shown in Table 7, the optimum parameter-levels for ground heat exchanger length can be selected as A2 (thermal conductivity of GHX pipe is 0.42), B1 (installation depth of GHX pipe is 1), C3 (inner radius of GHX pipe is 31.75), D3 (specific heat capacity of liquid is 4288), E1 (reflectivity of glass cover is 0.12), F3 (mass flow rate of water is 0.5), G1 (thermal conductivity of solar collector pipe is 16) and H1 (inner radius of solar collector pipe is 19.05). Following the procedure adopted for solar collector area and ground heat exchanger length, the optimum parameter levels for the COP of SAGSHP system are chosen as A1 (thermal conductivity of GHX pipe is 0.10), B3 (installation depth of GHX pipe is 2), C2 (inner

Table 6 Response table for solar collector area. Level

A

B

C

D

E

F

G

H

1 2 3 Delta Rank

−17.71 −17.71

−17.71 −17.71 −17.71 0.00

−17.71 −17.71 −17.71 0.00

−17.70 −17.72 −17.72 0.03

−17.74 −17.72 −17.68 0.05

−17.61 −17.73 −17.80 0.18

−17.70 −17.72 −17.72 0.02

−17.71 −17.71 −17.71 0.00

0.00

Table 7 Response table for ground heat exchanger length. Level

A

B

C

D

E

F

G

H

1 2 3 Delta Rank

−52.84 −51.55

−51.23 −51.74 −52.12 0.89

−52.48 −51.62 −50.98 1.50

−51.88 −51.71 −51.49 0.38

−51.72 −51.72 −51.72 0.00

−52.36 −51.59 −51.13 1.23

−51.71 −51.71 −51.71 0.00

−51.76 −51.76 −51.76 0.00

2.29

Table 8 Response table for COP of SAGSHP. Level

A

B

C

D

E

F

G

H

1 2 3 Delta Rank

11.89 11.44

11.40 11.35 12.24 0.89

11.30 12.24 11.45 0.94

11.38 11.36 12.26 0.90

11.38 12.24 11.37 0.87

12.22 11.40 11.38 0.84

11.40 11.37 12.23 0.86

12.23 11.36 11.40 0.87

0.46

V. Verma, K. Murugesan / Energy and Buildings 82 (2014) 296–309

303

Table 9 ANOVA: Taguchi method – solar collector area. Source

DF

Sum of squares (SS)

Mean of squares (MS)

F

P

% Contribution

A B C D E F G H Error Total

1 2 2 2 2 2 2 2 2

0.000000 0.000013 0.000012 0.002068 0.008972 0.012533 0.001758 0.000015 0.000036

0.000000 0.000006 0.000006 0.001034 0.004486 0.051266 0.000879 0.000008 0.000018

0.00 0.36 0.32 57.24 248.36 2838.24 48.67 0.43

0.996 0.737 0.756 0.017 0.004 0.000 0.020 0.702

0 0.049 0.031 8.235 35.287 49.427 6.918 0.053

radius of GHX pipe is 25.4), D3 (specific heat capacity of liquid is 4288), E2 (reflectivity of glass cover is 0.16), F1 (mass flow rate of water is 0.3), G3 (Thermal conductivity of solar collector pipe is 110) and H1 (inner radius of solar collector pipe is 19.05). The optimum-level combination of all the parameters for solar collector area, ground heat exchanger length and the COP of SAGSHP can be summarized as A1B1C1D1E3F1G1H1, A2B1C3D3E1F3G1H1 and A1B3C2D3E2F1G3H1, respectively. The S/N values given in Tables 6–8 are also depicted in the form of illustrations in Figs. 3–5 for better understanding the influencing parameters. Fig. 3 shows the S/N ratio variation for different levels for solar collector area. As parameters A, B, and C are related to the ground heat exchanger length, they do not exhibit any effective change in S/N ratios in the above figure. Fig. 4 illustrates the S/N variations at different levels for the parameters A–H related to the performance of ground heat exchanger length. It is observed that only the parameters A, B, C, D and F only influence the length of ground heat exchanger. Fig. 5 shows the optimum parameter level combinations for COP of SAGSHP system. 5.3. Taguchi method – ANOVA analysis ANOVA is used to calculate the relative magnitude of each parameter in terms of percentage contribution of overall response. In ANOVA analysis results for the degree of freedom (DF), sum of squares (SS), mean of squares (SS), F ratio, P ratio and percentage

contribution by different control factors for SAGSHP system are shown in Tables 9–11 respectively for solar collector area, ground heat exchanger length and COP of SAGSHP system. Among this F ratio is used to identify the parameters that have significant effect on the solar collector area, COP of the system and ground heat exchanger length. Sum of squares (SS) and Degree of freedom (DF) have been calculated by using the following equation: SS =

1 2



2 2 (sum of S/N ratio level I) +(sum of S/N ratio level I)

+(sum of S/N ratio level I)2 − C.F

 Correction factor (C.F) =

sum of





S 2 N

N

where, N = total number of experiments (N = 18), degree of freedom = level − 1. 5.3.1. Solar collector area In order to achieve the optimum solar collector area, the proper levels of controllable and uncontrollable parameters have to be selected. The most important controllable parameters are specific heat capacity and mass flow rate of fluid (water) in collector tubes which flows inside the solar collector. The other controllable parameters are reflectivity of glass cover, thermal conductivity of solar collector pipe, inner and outer radius of solar collector pipe.

Table 10 ANOVA: Taguchi method – ground heat exchanger length. Source

DF

Sum of squares (SS)

Mean of squares (MS)

F

P

% Contribution

A B C D E F G H Error Total

1 2 2 2 2 2 2 2 2

23.5636 2.3996 6.7933 0.4439 0.1027 4.6312 0.0021 0.1464 0.0686

23.5636 1.1996 3.3966 0.2220 0.0513 2.3156 0.0010 0.0732 0.0343

686.85 34.97 99.01 6.47 1.50 64.50 0.03 2.13

0.001 0.028 0.010 0.134 0.401 0.015 0.971 0.319

61.5334 6.189677 17.40616 1.14353 0.269191 13.118 0.005306 0.334735

Table 11 ANOVA: Taguchi method – COP of SAGSHP system. Source

DF

Sum of square (SS)

Mean of square (MS)

F

P

% contribution

A B C D E F G H Error Total

1 2 2 2 2 2 2 2 2

0.9355 2.9630 3.0952 3.1871 2.9746 2.7465 2.8682 2.8849 2.8321

0.9355 1.4815 1.5476 1.5935 1.4873 1.3732 1.4341 1.4425 1.4161

0.66 1.05 1.09 1.13 1.05 0.97 1.0 1.02

0.502 0.489 0.478 0.471 0.488 0.508 0.497 0.495

4.666 13.987 13.749 14.875 13.974 12.871 12.987 12.891

V. Verma, K. Murugesan / Energy and Buildings 82 (2014) 296–309

-17 .9

-17 .9

-17.8

-17 .8

SN ratio

SN ratio

304

-17 .7

-17 .7 -17.6

-17.6 -17.5

-17.5 0

0.1

0.2

0.3

0.4

0.5

0.5

1

Thermal Conductivity of GHX Pipe (W/mK)

2

2.5

-17.9

-17.9

-17.8

SN ratio

-17.8

SN ratio

1.5

Installation Depth of GHX Pipe (m)

-17.7

-17.7 -17.6

-17.6

-17.5

-17.5 17

21

25

29

3900

33

4000

4100

4200

4300

4400

Specific Heat Capacity of Liquid (J/KgK)

Inner Radius of GHX Pipe (mm)

-17.75 -17.8

-17.7145

SN ratio

SN ratio

-17.85

-17.679

-17.75 -17.7 -17.65 -17.6

-17.6435 0

0.1

0.2

0.1

0.3

0.2

0.3

0.4

0.5

0.6

Mass Flow Rtae of Water (Kg/s)

Reflectivity of Glass Cover -17.9

-17.7554

SN ratio

SN ratio

-17.8

-17.7199

-17.7 -17.6 -17.5

-17.6844 0

50

100

150

Thermal Conductivity of Solar Collector Pipe (W/mK)

17

21

25

29

33

Inner radious of Solar Collector Pipe (m)

Fig. 3. Mean of S/N ratios for solar collector area.

The uncontrollable parameters are taken as FR (˛) = 0.68 and FR UL = 4.90 (W/m2 ◦ C) where FR is the collector heat removal factor,  is transmissivity of the cover, ˛ is absorptivity, UL is the overall heat loss coefficient of the collector. The average solar intensity and average ambient temperature of the given location for heating period are assumed to be 4.01 kW/m2 and 14.85 ◦ C respectively. The heat energy contribution from the solar collector is assumed to be 3.5 kW that is 50% of the total heating load. The calculated solar collector area varies from 7.55 m2 to 7.77 m2 . From the ANOVA results shown in Table 9 for solar collector area, the higher to lower order

percentage contribution of parameters can be arranged as FEDGHBCA. The highest contribution comes from the mass flow rate of water (F) with 49.4% followed by the reflectivity of glass cover (E) with 35.3%. 5.3.2. Ground heat exchanger length For the ground heat exchanger the controllable parameters are thermal conductivity of ground heat exchanger pipe, installation depth of GHX, inner/outer radius of GHX pipe, specific heat capacity of GHX fluid (water) which flows in the ground heat exchanger

V. Verma, K. Murugesan / Energy and Buildings 82 (2014) 296–309

-52.94

-52.2242 -51.9242

SN ratio

-52.44

SN ratio

305

-51.94

-51.6242 -51.3242

-51.44 0

0.1

0.2

0.3

0.4

-51.0242

0.5

0.5

1

Thermal Conductivity of GHX pipe (W/mK)

1.5

2

2.5

Installation Depth of GHX (m) -51.9837

-52.6

-51.8797

SN ratio

SN ratio

-52.1 -51.6

-51.7758 -51.6718 -51.5678

-51.1

-51.4639 15

20

25

30

3950

35

4250

4350

-52.6

-51 .87 -51 .81

-52.2

SN ratio

SN ratio

4150

Specific Heat Capacity of Liquid (J/Kg K)

Inner Radius of GHX Pipe (mm)

-51 .75 -51 .69

-51.8 -51.4

-51 .63

-51

-51 .57 0.08

0.13

0.18

0.23

0.25

0.28

0.3

0.35

0.4

0.45

0.5

0.55

Mass Flow Rate of Water (Kg/s)

Reflectivity of Glass Cover -51 .76

-51 .81

-51 .71

-51 .76

SN ratio

SN ratio

4050

-51 .66

-51 .71

-51 .66

-51 .61 0

25

50

75

100

125

Thermal Conductivity of Solar Collector Pipe (W/m K)

14

19

24

29

34

Inner Radius of Solar Collector Pipe (mm)

Fig. 4. Mean of S/N ratios for ground heat exchanger length.

and the uncontrollable parameters are thermal conductivity of soil (2.07 W/m K), ground temperature (14 ◦ C), water inlet temperature to the GHX (8 ◦ C), density and dynamic viscosity of GHX fluid and the heat energy to be supplied by the ground heat exchanger (3.5 kW) that is 50% of the total heating load. From the values shown in Table 10 for the ground heat exchanger length, the important contributing parameters are found to be thermal conductivity and inner radius of GHX pipe. From this table the GHX contributing parameters ranking can be obtained as ACFBDHEG. According to

these series one can easily notice that the thermal conductivity of ground heat exchanger pipe (A) contributes a highest percentage of 61.5%. 5.3.3. COP of the SAGSHP system In order to achieve the optimum COP of the SAGSHP system, the appropriate levels of controllable and uncontrollable parameters have to be selected. The uncontrollable parameters are solar intensity, thermal conductivity of soil, ambient

306

V. Verma, K. Murugesan / Energy and Buildings 82 (2014) 296–309

12.5

SN ratio

SN ratio

12

11.5

12 11.5 11

11 0

0.1

0.2

0.3

0.4

0.8

0.5

12.2

12.2 12

SN ratio

SN ratio

2.3

12.4

12.4

11.8 11.6

12 11.8 11.6 11.4

11.4 11.2

11.2 15

20

25

30

35

3950

Inner Radius of GHX Pipe (mm)

4000

4050

4100

4150

4200

4250

4300

Specific Heat Capacity of Liquid (J/Kg K)

12.4

12.4

12.2

12.2

12

SN ratio

SN ratio

1.8

Installation Depth of GHX pipe (m)

Thermal Conductivity of GHX pipe (W/mK)

11.8 11.6 11.4

12 11.8 11.6 11.4 11.2

11.2

11 0.05

0.1

0.15

0.2

0.25

0.25

Reflectivity of Glass Cover

0.3

0.35

0.4

0.45

0.5

0.55

Mass Flow Rate of Water (Kg/s)

12.4

12.6 12.4 12.2 12 11.8 11.6

12.2

SN ratio

SN ratio

1.3

12 11.8 11.6 11.4

11.4 11.2

11.2 0

20

40

60

80

100

120

Thermal Conductivity of Solar Collector pipe (W/mK)

14

19

24

29

34

Inner Radius of Solar Collector Pipe (mm)

Fig. 5. Mean of S/N ratios for COP of SAGSHP system(Taguchi method).

temperature and soil properties. Table 11 shows the ANOVA results for the COP of SAGSHP system. It can be observed from these tabular values that almost all the parameters contribute equally except thermal conductivity of GHX pipe. The specific heat capacity of liquid (D) contributes a highest percentage of 14.9%. 5.4. Taguchi method – confirmation test The solar collector area was analyzed based on the average solar intensity for Indian climatic conditions and calculations have been performed for all the 18 experimental trial runs using the data given in Table 3. The calculated values of solar collector

area, length of ground heat exchanger and COP of solar assisted ground source heat pump (SAGSHP) are shown in Table 4. It can be observed from these results that the solar collector area, length of GHX and COP of the SAGSHP system range varies from 7.55 m2 to 7.77 m2 , 298.109 m to 515.327 m and 3.64 to 3.77, respectively. The COP of SAGSHP system depends on both the solar collector area and ground heat exchanger length. Based on the best set of operating parameters among the L18 Taguchi array, the optimum area of solar collector, length of ground heat exchanger and COP of SAGSHP system are computed as 7.34 m2 , 280.52 m and 3.86, respectively. Among these the optimum value of solar collector area and ground heat exchanger are found to be the minimum of the 18 values given in Table 4. However, the predicted

V. Verma, K. Murugesan / Energy and Buildings 82 (2014) 296–309 Table 12 Utility concept – signal to noise ratio. Experimental number

Utility value

S/N value (dB)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

29.63032 24.2491 21.32576 28.97218 24.35576 28.53967 28.45898 29.45913 24.7638 18.85714 16.81795 17.94968 21.29014 19.59168 17.04217 21.24159 22.58862 19.79395

29.4347 27.6939 26.5781 29.9850 27.7320 29.1090 29.0844 29.0585 27.8763 25.5095 24.5155 25.0811 26.5636 25.8414 24.6305 26.5437 27.0778 25.9306

307

D, E, F, G and H at mixed levels and their corresponding S/N values are also computed for 18 trial runs using the higher the better concept and the results are given in Table 12. The response table for the COP of SAGSHP system using these values are shown in Table 13. It can be seen that all the eight parameters have some effect on the coefficient of performance parameter. The optimum parameter levels contributions for achieving maximum COP for the SAGSHP system are found to be as A1 (thermal conductivity of GHX pipe is 0.10 W/m K), B3 (installation depth of GHX pipe is 2 m), C1 (inner radius of GHX pipe is 19.05 mm), D3 (specific heat capacity of liquid is 4288 J/kg K), E2 (reflectivity of glass cover is 0.16), F1 (mass flow rate of water is 0.3 kg/s), G3 (thermal conductivity of solar collector pipe is 110 W/m K) and H1 (inner radius of solar collector pipe is 19.05 mm) and series of parameters based on ranking is AFBEGCDH. The maximum value of the mean S/N value indicates the optimum value of the operating parameters. Fig. 6 shows the S/N variations for all the parameters to achieve maximum COP of SAGSHP system. It can be noticed that all the parameters contribute for better performance of the SAGSHP system as already predicted by the values shown in Table 13. The best combinations of the parameters obtained for maximum COP is summarized as A1B3C1D3E2F1G3H1.

COP of SAGSHP system is higher than the COP values shown in Table 4.

5.6. Utility concept – ANOVA analysis 5.5. Utility concept – signal to noise ratio ANOVA analysis also has been carried out for the calculations performed using the utility concept. Table 14 represents the percentage contribution of each parameter on the performance of SAGSHP system and the series of parameters based on percentage contribution is found to be in the order AEFBGCHD, in that thermal conductivity of ground heat exchanger is found to play a prime role and its percentage contribution is 42.56% followed by the reflectivity of glass cover for solar collector with 10.77%. It is clear from the above observations that if optimum COP for heating mode of SAGSHP system has to be achieved, then the selection of appropriate values for thermal conductivity of GHX pipe and reflectivity of glass cover of solar collector become important. For the data of mixed levels of eight operating parameters considered in the present research, the optimum value of COP is found to be 4.23 with 95% confidence interval.

From the Taguchi analysis the optimum values for solar collector area, GHX length and COP of SAGSHP system were obtained for space heating application. However, when the SAGSHP is employed for space heating mode in a year round operation, it is highly essential to estimate the optimum values for solar collector area and GHX length which will give the maximum value for the COP of SAGSHP system in order to reduce the running cost of the system. This can be achieved with the help of utility concept, which is widely used in manufacturing techniques. In a given heating mode application, there may be two possibilities for heating, one by using the solar collector for which the collector area needs to be optimized and another is the ground heat exchanger for which the length has to be optimized. In the present analysis the utility values of the SAGSHP system have been computed for eight operating parameters A, B, C,

Table 13 Response table for COP of SAGSHP system by utility concept. Level

A

B

C

D

E

F

G

H

1 2 3 Delta Rank

29.95 25.74

26.47 28.14 28.93 2.46 3

28.69 28.32 26.53 2.15 6

28.09 27.01 28.44 1.43 7

27.32 29.33 26.89 2.44 4

29.05 27.97 26.51 2.54 2

27.08 27.18 29.27 2.19 5

28.38 27.05 28.11 1.3 8

4.21 1

Table 14 ANOVA:COP of SAGSHP system by utility concept. Source

DF

Sum of squares (SS)

Mean of squares (MS)

F

P

% Contribution

A B C D E F G H Error Total

1 2 2 2 2 2 2 2 2

79.624 18.943 15.913 6.615 20.405 19.499 18.302 5.938 4.046

79.624 9.472 7.956 3.308 10.202 9.749 9.151 2.969 2.023

39.36 4.68 3.93 1.63 5.04 4.82 4.52 1.47

0.024 0.176 0.203 0.380 0.165 0.172 0.181 0.405

42.56525 10.50741 8.495845 3.584712 10.77788 10.60124 9.68867 3.779021

308

V. Verma, K. Murugesan / Energy and Buildings 82 (2014) 296–309

29.5 29 28.5

SN ratio

SN ratio

30.5 30 29.5 29 28.5 28 27.5 27 26.5 26 25.5

28 27.5 27 26.5 26

0

0.1

0.2

0.3

0.4

0

0.5

29

SN ratio

SN ratio

1.5

2

2.5

28.1

28 27.5 27

27.8 27.5 27.2

26.5

26.9

26 10

14.5

19

23.5

28

3900

32.5

4000

4100

4200

4300

Specific Heat Capacity of Liquid (J/kgK)

Inner radius of GHX pipe (mm) 29.5

29.5

29

29

28.5

28.5

SN ratio

SN ratio

1

28.4

28.5

28 27.5

28 27.5 27

27

26.5

26.5

26 0.09

0.14

0.19

0.24

0.29

0.25

0.3

0.35

0.4

0.45

0.5

0.55

Mass Flow Rate of Water (Kg/s)

Reflectivity of glass Cover 29.5

28.5

29

28.2

28.5

27.9

SN ratio

SN ratio

0.5

Installation Depth of GHX (m)

Thermal Conductivity of pipe (W/mK)

28 27.5 27

27.6 27.3 27

26.5

26.7 13

33

53

73

93

113

Thermal Conductivity of solar collector pipe (W/mK)

10

14.5

19

23.5

28

32.5

Inner Radius of Solar Collector Pipe (mm)

Fig. 6. Mean of S/N ratios for COP of SAGSHP system (Utility concept).

6. Conclusions In the present research work, the performance of a solar assisted ground source heat pump (SAGSHP) has been analyzed using Taguchi technique and utility concept. The design parameters are optimized to obtain the optimum solar collector area and ground heat exchanger length for space heating application with optimum

COP. In this technique eight parameters that influence the performance of the solar collector and ground heat exchanger at mixed level operation were considered. Hence the orthogonal array L18 has been employed to obtain the random experimental trial runs. Simulation results were obtained for all the 18 trial runs for SAGSHP system with the main objective of obtaining the optimum COP for SAGSHP with optimum solar collector area and ground heat

V. Verma, K. Murugesan / Energy and Buildings 82 (2014) 296–309

exchanger length. Based on the results the following conclusions have been arrived at: • The optimized solar collector areas are found to be 7.34 m2 and 7.17 m2 using the Taguchi method and the utility concept respectively for space heating application. Also the optimum length of ground heat exchanger (GHX) is estimated as 280.52 m and 276 m from Taguchi method and utility concept respectively for space heating. • Thermal conductivity of the ground heat exchanger pipe is found to be a more influential parameter for the design of GHX length because its percentage contribution is found to be around 61.5%. For the solar collector, the contributing parameters are found to be the mass flow rate of water and reflectivity of the glass cover with contributions of 49.4% and 35.3%, respectively. • Based on the Taguchi optimization it is found that the specific heat capacity of ground heat exchanger liquid is an insignificant parameter in deciding the length of ground heat exchanger. Similarly for the solar collector the inner radius of solar collector pipe is found to be an insignificant parameter in deciding the area of solar collector. • With the help of Taguchi and utility concept the optimum values of control parameters have been determined to obtain optimum COP from solar assisted ground source heat pump system (SAGSHP). • The optimum COP for SAGSHP was estimated to be 4.23 from the utility concept, which is 8.74% higher than the optimum COP predicted by Taguchi optimization. • Optimization of solar collector area and ground heat exchanger length has shown only about 2.3% reduction in solar collector area and 1.6% reduction in GHX length compared to the values predicted by the Taguchi method. Acknowledgments The first author is thankful to the Ministry of Human Resources and Development, Government of India, for providing the fellowship for pursuing PhD at Indian Institute of Technology Roorkee, Roorkee, India. References [1] O. Ozyurt, D.A. Ekinci, Experimental study of vertical ground source heat pump performance evaluation for cold climate in Turkey, Applied Energy 83 (2011) 1257–1265. [2] H.J. Wang, Q. Zhao, J. Wu, B. Yang, Z. Chen, Experimental investigation on the operation performance of a direct expansion ground source heat pump system for space heating, Energy and Buildings 61 (2013) 349–355. [3] U. Cakir, K. Comakli, O. Comakli, S. Karsli, An experimental exergetic comparison of four different heat pump systems working at same conditions: as air to air, air to water, water to water and water to air, Energy 58 (2013) 210–219. [4] C.H. Liang, X.S. Zhang, X.W. Li, X. Zhu, Study on the performance of a solar assisted air source heat pump system for building heating, Energy and Buildings 43 (2011) 2188–2196. [5] K. Kaygusuz, O. Comakli, T. Ayhan, Solar assisted heat pump systems and energy storage, Solar Energy 47 (1991) 383–391. [6] K. Bakirci, B. Yuksel, Experimental thermal performance of a solar source heat pump system for residential heating in cold climate region, Applied Thermal Engineering 31 (2011) 1508–1518.

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