Modeling and performance analysis of a house heating system with a ground coupled heat pump

Transcription

1 Energy Education Science and Technology Part A: Energy Science and Research 1 Volume (issues) 8(): Modeling and performance analysis of a house heating system ith a ground coupled heat pump Recep Yumrutas 1,*, Mazhar Unsal 1 University of Gaziantep, Department of Mechanical Engineering, Gaziantep, Turkey Halic University, Department of Industrial Engineering, Sisli, Istanbul, Turkey Abstract Received: 14 March 11; accepted: 7 April 11 This study deals ith modeling and performance analysis of a house heating system ith ground coupled heat pump. The heating system consists of an underground thermal energy storage (TES) tank, a heat pump and a house. An analytical model is presented for finding the thermal performance of the heating system. The model developed is based on formulations for the transient heat transfer problem outside the TES tank, for the energy needs of the heat pump and for the house. The solution of the problem formulation is obtained using a similarity transformation technique. Expressions for energy consumption of the heat pump and house are derived as a function of inside design air, ambient air and TES temperatures. An interactive computer program based on the analytical model is prepared for finding hourly variation of ater temperature in the TES tank, coefficient of performance (COP) of heat pump and timespan required to attain an annually periodic operating condition. Results indicate that 5 to 7 years ill be a sufficient timespan required for the system to attain an annually periodic operating condition. Keyords: Thermal energy storage; Energy storage; House heating; Ground coupled heat pump Sila Science. All Rights Reserved. 1. Introduction Large scale consumption of fossil fuels depletes petroleum and natural gas resources, affects the environment negatively [1-4], increases CO, SO x and NO x emissions to the atmosphere as ell as expenditure of capital for countries importing fossil fuels [5-7]. Additionally, political and economical uncertainties and depletion of energy sources create an unpreventable increase in the prices of petroleum and natural gas [8]. For these reasons, many governments have decided to strengthen their national efforts to increase the deployment of energy conservation technologies and increase utilization of reneable energy sources [5]. It is therefore desirable to use reneable energy sources to decrease the utilization of the fossil fuels and to eliminate their negative effects. Efficient usage of the energy and energy sources is also of great importance. Application of heat pump systems can be a solution contributing positively to the efficient utilization of fossil fuel energy sources. Widespread utilization of * Corresponding author. Tel.: ; fax: address: yumrutas@gantep.edu.tr (R. Yumrutas).

2 67 R. Yumrutas, M. Unsal / EEST Part A: Energy Science and Research 8 (1) such energy efficient systems ill decrease utilization of fossil fuels and their negative effects on the environment. Ground Coupled Heat Pump (GCHP) systems have been gaining popularity as they reduce energy consumption, initial investment, operating and maintenance costs as compared to conventional air source heat pump systems [8-1]. Also, they are environment-friendly, causing less CO emission than their conventional alternatives [11]. They can make significant contributions to reductions in electrical energy usage, and allo more effective demand side management schemes. GCHP systems are generally considered more attractive over other reneable energy sources because of the year round stable underground temperatures [1-14]. Since the temperature in deep earth is very stable, the soil is an ideal heat sink in the summer and an ideal heat source in the inter[15]. Hoever, compared to air source heat pump systems, GCHP systems have not been idely used. This may be attributed to comparatively higher installation costs and ground area requirements. Many experimental and theoretical investigations have been conducted in the past, related ith the design, analysis, and optimization of TES systems and GCHP systems for space heating and cooling applications. Dincer [16, 17] reported methods and applications describing and assessing TES systems, as ell as their economical, energy conservation and environmental aspects. Zhang [15] developed a model of a space heating and cooling system ith a surface ater pond having an insulating cover that serves as the heat source in the inter and heat sink in the summer. They considered three running modes to analyze the interaction of the seasonal heat charge and discharge hich can save about 16% compressor ork compared ith the modes hich operate for heating and refrigeration individually. Zogou and Stamatelos [18] investigated the effects of climatic conditions on the performance of heat pump systems for space heating and cooling and concluded that armer Mediterranean conditions are ell suited for these applications. Yumrutaş and Ünsal [1] developed a computational model for the analysis of ground coupled heat pump space heating system ith a hemispherical storage tank as the ground heat source. Yumrutas et al. [13] presented an analytical and computational model for a solar assisted heat pump heating system ith an underground cylindrical storage tank. Yumrutaş et al. [14] developed a computational model for determining annual periodic performance of a cooling system utilizing a ground coupled chiller and a spherical underground TES tank. Esen et al. [19] developed a numerical model of a GCHP system using the ground as a source of heat, and installed the physical system. They observed that numerical results agree ith their experimental results. İnallı et al. and İnallı [, 1] studied solar assisted heat pump space heating systems ith underground spherical and cylindrical TES tanks. They obtained annually periodic solution of the heat conduction problem outside the storage tank, and presented temperature variation of ater in the tank and performance of the heating system for the annually periodic operating conditions. There are several analytical and computational models for space heating and cooling system ith a heat pump and a TES tank [1-14,, 1]. They used different solution techniques for solving the heat transfer problem outside the TES tank. In the analytical models, temperature field problem outside the TES tank ere solved using annual periodic values of outside air temperatures and solar radiation on horizontal surface. They obtained performance of the heating system for only the annually periodic operating conditions. There are no analytical and/or computational model in the literature based on hourly data investigating such systems during the transitional timespan beginning ith the time of installation and ending ith the annually periodic operating regime and this problem is the subject of the present investigation. Analytical and computational models based on hourly

3 R. Yumrutas, M. Unsal / EEST Part A: Energy Science and Research 8 (1) operation are developed in the present study to determine performance parameters of a house heating system utilizing a heat pump and a spherical underground TES tank. These parameters are long term hourly variation of ater temperature in the tank over all years beginning ith the time of installation of such a system and the annual COP of heat pump. The computational model is used to find the timespan required for the system to attain an annually periodic operating condition as ell as the long-term performance parameters.. Description of the heating system The heating system shon in Fig. 1 consists of three main components, hich are a seasonal thermal energy storage (TES) tank, a heat pump and a house to be heated in inter season. In this section, each component of the heating system is explained briefly. The most important component of the heating system is the underground TES tank hich provides energy savings and contributes to reductions in environmental pollution[5]. Energy is transferred from ground to ater in the TES tank, and then this energy is extracted by the heat pump for heating of the house. The energy exchange beteen the heat pump and the ground ill improve performance of the heat pump heating system. The ground is considered as large energy storage medium ith large mass and stable temperature [1-14]. Therefore, the TES tank ill be used as long-term storage or heat source of the heat pump in the heating season. The TES is considered to be spherical in shape and buried under ground. It is assumed that ater is to be used as a storage medium in the tank due to high specific heat of ater and high capacity rates for thermal charge and discharge. The high heat capacity and lo cost of ater often makes the ater as an appropriate choice for TES systems. Fig. 1. House heating system ith a heat pump and an underground TES tank. 1. Compressor,. Condenser, 3. Expansion valve, 4. Evaporator, 5. Circulation pumps. The heat pump is another important part of the system, hich is coupled to the TES tank. The ground coupled heat pump technology can achieve higher energy efficiency for air-conditioning than conventional airconditioning systems because the underground environment provides higher temperatures for heating and experiences less temperature fluctuation than ambient air temperature variations[]. Energy from the TES tank is extracted by the heat pump for space heating during only the heating season. The heat pump operates to keep the house at the required inside design air temperature. It supplies the total heating requirements of the house by extracting energy from the tank. The house is the last component of the heating system. It is an insulated house that needs 1 kw of energy at the design condition. Heating requirements of the house ill be supplied by panel type radiators and ater is circulated through the radiators. Thermal energy to the radiators is supplied by the heat pump condenser. 3. Modeling of the heating system The heating system consisting of underground TES tank, a house, heat pump and its components is schematically shon in Fig. 1. In order to develop an analytical model, it is necessary to make thermal analysis for all components of the heating system. Therefore, a model is developed by analyzing each component of the

4 67 R. Yumrutas, M. Unsal / EEST Part A: Energy Science and Research 8 (1) system in this section. This model consists of a solution of the transient heat transfer problem outside the tank, expressions for coefficient of performance (COP) of the heat pump, and heat requirements of the heat pump and the house. Expressions for hourly temperature variation of ater in the TES are obtained by solving the problem outside the tank. The solution procedure for the TES tank problem is presented in this study. Expressions for energy requirement of the house are presented. Expressions for the energy requirement of the heat pump and its COP are given in the third section Solution of the transient heat transfer problem for the TES tank The TES tank is an important part of the heating system. Water in the tank serves as the heat source in inter. Tank ater temperature, T (t), changes continually throughout the year due to heat exchange among the tank ater and the soil [15]. We make the folloing 4 assumptions: (1) TES tank is spherical in shape, () the ater is initially equal to the deep ground temperature T, ater in the tank is fully mixed at a spatially lumped time varying temperature T(t), (3) the soil is homogenous ith constant thermal properties, (4) the TES tank is located deep enough so that the far field ground temperature aay from the storage is taken to be constant and equal to the deep ground temperature, T. In this section, formulations of the transient heat transfer problem and its solution procedure are presented. The heat transfer problem outside the spherical TES tank is given in spherical coordinates as follos: T T 1 T r r r t (1) T(R, t) = T (t) () T(, t) = T (3) T(r, ) = T (4) The energy balance for the TES tank is equal to the difference beteen energy storage in the tank and heat loss from the tank to ground, hich is: Q dt cv dt T ka (R,t) r (5) here,, c, and V are density, specific heat, and volume of the ater in the tank, respectively, and k, R, and A are thermal conductivity of the surrounding soil, tank radius, and tank surface area, respectively. The transient heat transfer problem is transferred to dimensionless form by using folloing dimensionless variables. r x R T T T t R Q q 4 R kt, T T T a T a T T c p 3 c (6) here x,, and q are dimensionless parameters of radial distance, time, temperature, and net energy input to the store, respectively, and and c are the density and the specific heat of the ground, respectively. Subscripts and a stand for ater and ambient air, respectively. When the dimensionless variables are applied to the problem given by Eq.(1-5), the folloing dimensionless formulation of the problem is obtained as: x x x (7) (x, ) = () (8) (, ) = (9) T(x, ) = (1) d q p (1, ) d x (11)

5 R. Yumrutas, M. Unsal / EEST Part A: Energy Science and Research 8 (1) In order to simplify the problem given by (7-11), the folloing transformation is introduced: ( x, ) x (x, ) (1) Application of (1) to (7-11), yields: x (1, ) = () (14) (, ) = (15) (x, ) = (16) d q p (1, ) (1, ) d x (17) This dimensionless transient heat transfer problem is solved by introducing the folloing similarity transformation. (13) x 1 (18) The solution for constant is: x 1 ( x, ) 1 Erf (19) The general solution is obtained as: ( x 1 x, ) () 1 Erf d ( ) x 1 1 Erf d () If the general solution is differentiated ith respect to dimensionless variables x, the result evaluated at x=1 and substituted into (17), the folloing integro-differential equation is obtained. d q p d ( ) d( ) d d ( ) (1) Eq. 1 can be expressed in the folloing finite difference form. p q( n ) ( ) n 1 ( n p 1 1 ) 1 n i1 ( i1 ) ( ) (n i) i () Eq. () can be used to calculate the dimensionless ater temperature in the TES tank. The q() term in Eq. () represents the dimensionless net heat extraction rate from the tank. The dimensionless heat extraction rate, q() is equal to the difference beteen heat pump ork and the heat requirement of the house, hich may be expressed as: ( ) q( ) qh( ) (3)

6 674 R. Yumrutas, M. Unsal / EEST Part A: Energy Science and Research 8 (1) here, () and q h () are the dimensionless heat pump ork and heat requirement of the house during heating season, respectively. The dimensionless heat pump ork is given in the next section. 3.. Energy requirement of the house Another component of the heating system is the house to be heated in inter season. It is knon that heat loss of a space is equal to its energy requirement. In order to find the energy requirement of the house, it is necessary to define expressions for the heat loss. Energy need of the house in heating season may be expressed as: Q (t) (UA) h h T T (t) i a (4) here, (UA) h is the UA-value of the house, T i and T a (t) are the inside design air and hourly ambient air temperatures, respectively. The energy requirement of the house, Q h (t) is supplied through the load side heat exchangers or radiators hich may also be expressed by: Q (t) (UA) h he h i T T (5) here (UA) he and T h are UA-value for the load side heat exchanger and temperature of fluid in the load side heat exchanger used in the house, respectively. The energy need of the house can be supplied by the heat pump, Eqs. (4) and (5) are combined ith heat pump expressions to be given in the folloing section. 3.. Derivation of heat pump ork The heat pump is another essential part of the heating system hich operates on a cycle consisting mainly of an evaporator, a compressor, a condenser, and an expansion device. Heat is supplied to the house by the condenser. The heat loss may be expressed as: Q h (t) W(t)(COP) (6) here, COP is the coefficient of performance of the heat pump for space heating. It is defined as Qh(t) Qh(t) COP (7) W(t) Q (t) Q (t) h L The COP can also be given as product of Carnot efficiency ( c ) and Carnot COP c. COP c COP c (8) here, the c is knon as ratio of actual COP of a heat pump to Carnot COP c. The Carnot COP c at any time is given as a function of source (T ) and sink (T h ) temperatures. T (t) COP h c Th (t) T (t) (9) When the Eq. (9) is inserted in the Eq. (8), then, a unique expression can be obtained as a function of c, T and T h. T (t) COP h c Th (t) T (t) (3) The c is equal to unity for a Carnot heat pump cycle. When the Eqs. (8) and (9) are combined and solved for T h (t), and T h (t) then substituted into Eq. (3) and dimensionless parameters in Eq. (6) are used, COP can be expressed as:

7 R. Yumrutas, M. Unsal / EEST Part A: Energy Science and Research 8 (1) u COP c i a ( ) i ( ) u ( ) 1 i a i (31) In this study, energy requirement of the heat pump is important for the energy balance of the TES tank given by the Eq.(3). Eqs. (4) and (31) are inserted into Eq. (6) and heat pump ork is obtained in dimensionless form []. a ( ) i u i a ( ) u ( ) ( ) i (3) c i a i 1 here, i is dimensionless inside design air temperature, and a () and () are dimensionless hourly outside air and ater temperature in the TES tank, respectively. The parameter given in Eqs. (31-3) is ratio of (UA) h - value for the house to (UA) he -value for load side heat exchanger, hich is; (UA) h Th T u i (UA) he Ti Tad (33) 4. System parameters and computation procedure It is required to compute performance parameters of the heating system. The parameters are hourly and yearly variation of ater temperature in the TES tank, energy requirement of the house and heat pump, and COP of the heat pump. The computation procedure is explained in this section. A computer program in Fortran 9 based on the present analytical model as prepared. The program is used to determine the performance parameters. Several data are required as inputs to run the program. The data are for the house, heat pump, TES tank, climate and geological structure surrounding the TES tank. The computation procedure are described and explained in the folloing subsections House Data for the house is used in the numerical computations to find energy requirement of the house and temperature of the ater in the TES tank. The energy need of the house during the heating season is estimated using Eq. (8) for each hour. Design heat load of a typical house located in Gaziantep is assumed to be equal to 1 kw in the heating season. Energy requirement of circulation pumps are neglected since they are very small compared to ork consumption of the heat pump compressor. The inter inside (T i ), outside (T ad ) design air and hot ater (T h ) temperatures are taken o C, -9 o C and 55 o C, respectively. For the design condition, product of heat transfer coefficient and area for the house, (UA) h is calculated to be 345 W/ o C using Eq. (4). Ratio of (UA) values for the house and radiator, u is calculated as 1. using Eq. (33) hen the values of T i, T ad and T h for the city of Gaziantep are taken as o C, -9 o C and 55 o C, respectively. 4.. Heat pump Performance of the heat pump depends on source and sink temperatures, and size of the heat pump. Zogou and Stamatelos [1] recommend that Carnot efficiency ( c or CE) varies beteen.3 and.5 for small electric heat pumps. Since the heat pump used in this study is a small type heat pump, mean value of the CE is taken as.4 unless otherise specified. Also, in order to examine the effect of the CE values on the store temperature and COP values, three CE values of,3,.4 and.5 are used in present calculations Thermal energy storage (TES) tank The TES tank is assumed to be filled ith ater and is fully mixed. Initial ater temperature is assumed to be equal to the deep ground temperature hich as taken as 15 C. It is assumed that there is no resistance beteen the tank and the geological structure. Tank volumes of 4, 5, 6, and 7 m 3 ere considered in the present study Climate Climatic data effects performance of the heating system, since energy requirement of the house and heat

8 676 R. Yumrutas, M. Unsal / EEST Part A: Energy Science and Research 8 (1) pump is expressed as function of inside design air temperature and hourly outside air temperatures. Therefore, the design temperature and variation of the outside air temperature are important. The outside design air temperature is equal to -9 o C for Gaziantep, Turkey, hich is used to find UA of the house. The inside design air temperature as taken as o C. The hourly outside air temperatures for Gaziantep ere used in the numerical calculations. They ere obtained from the Meteorological Station located in Gaziantep, and stored in a computer data file Geological structure Thermophysical properties of geological structures surrounding the TES tank are also significant for performance of the heating system since the amount of heat exchange beteen the TES tank and the structures depends on the thermophysical properties. The heat exchange directly affects the temperature of ater in the TES tank, hich is related ith performance of the heating system. Therefore, data for the thermophysical properties of the structures are necessary for the determination of the performance parameters of the system. Because granite gives good system performance, thermophysical properties of granite are used in the present study. Its properties ere obtained from Ozisik [3] and listed in Table 1. These data are given as a data statement in the computer program. Table 1. Properties of the geological structure Conductivity Diffusivity Specific heat Heat capacity Earth Type (W/mK) (m /sec) (J/kgK) (kj/m 3 K) Granite x Computation procedure Hourly heat requirement of the house and compressor ork are firstly calculated by using assumed storage temperature, inside design air and hourly outside air temperatures. The heat pump is operated only at times hen the dimensionless tank temperature, () is less than the critical dimensionless temperature given by: u( i a ) i. Net dimensionless energy extracted from the TES tank, q(), is computed next using Eq. (3), the dimensionless hourly energy requirement of the house, q h (), and dimensionless heat pump ork (). By using the net extracted energy, hourly store temperature is calculated from Eq. (), numerically. Hourly, monthly and yearly COP of the heat pump are calculated lastly. These calculations continue hour by hour until yearly temperature distribution in the storage tank reaches the annually periodic operating regime. 4. Results and discussions Computer program prepared using the model given in Section 3 is used to determine hourly ater temperature in the TES tank, number of years to attain the annually periodic operational regime and Coefficient of Performance (COP) of heat pump for the house heating system ith a load consisting of one house located in Gaziantep, Turkey. Effects of the system parameters such as Carnot Efficiency (CE), tank volume and radiator temperature on the storage temperature and heat pump COP ere examined using thermophysical properties of granite. Results obtained from the numerical computations are given in figures and are discussed in the folloing subsections The Carnot efficiency (CE) Carnot Efficiency (CE) is an important parameter that has significant effect on the storage temperature and performance of the heat pump. It is equal to the ratio of the actual COP of a heat pump and the ideal COP. Zogou and Stamatelos [18] recommended values for the CE that varies from.3 to.5 for small electric heat pumps. Since the heating system considered in this study is a small heating system and a small heat pump, the CE values ere taken

9 July Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Storage temperature, o C R. Yumrutas, M. Unsal / EEST Part A: Energy Science and Research 8 (1) accordingly. Figs. and 3 depict the effects of CE on store temperature and COP of the heat pump for CE values of.3,.4 and.5. The Fig. indicates monthly variation of ater temperature during the tenth year of operation. During this year, the system operates on an anually periodic regime. The figure shos that higher CE values yield smaller ater temperature variation [14]. A higher CE means that a higher amount of heat from the tank is extracted at the same heating load. As a result of this, ater temperature in the TES decreases. Effect of CE on the storage temperature is small as seen in Fig. 3. Hoever, it has a strong effect on the COP as seen in Fig. 4. Values of the COP are 3.15, 4.13 and 5.1 for respective CE values of.3,.4 and.5 during the first year of operation. The COP decreases ith years up until the attainment of anually periodic operating regime. Effect of the CE on the ater temperature agrees ith results of the Ref.[14]. 1 8 CE=.3 CE=.4 CE=.5 4 Fig.. Effect of CE on the storage temperature during the tenth year. 4.. Annually periodic operational regime The system operation is transient up until the attainment of the annually periodic operational regime. Energy exchange beteen the heat pump and TES tank, and beteen the TES tank and surrounding earth are not equal perior to attainment of the annually the periodic regime. Temperature of ater in the storage decreases belo the deep ground temperature as a result of the extracted energy from the storage and heat flos from earth into the storage. During the first fe years, the extracted and transferred energy are not equal and the store temperature gradually reaches the annually periodic values. When the energy exchanges are equal, then the storage temperature does not change and the system reaches the annually periodic condition. Figs. 4 and 5 are depicted in order to sho the evolution toard the annually periodic regime. Fig. 4 shos annual variation of ater temperature in the tank ith respect to years. Storage temperature varies rapidly during especially the first, second and third years of operation. After the third year of operation, annual temperatures continue to decrease up until the sixth or seventh year of operation, and they do not change thereafter. Seventh year can be considered as the year of attainment of the annually periodic regime. Evolution toard the annually periodic regime is also depicted in Fig. 5. Temperature variation is reasonably high especially during the first and second year, but it does not change after the sixth or seventh year of operation, since storage temperatures during the seventh year are closer to those of the sixth year. These figures give us important information regarding the magnitude of the transient period prior to the attainment of the annually periodic regime Thermal energy storage (TES) tank volume Months Storage tank volume affects performance of the system. Figs. 6 through 8 are depicted to examine effects of the tank volume on the storage temperature and heat pump COP. Fig. 6

10 Storage temperature, o C Storage temperature, o C COP 678 R. Yumrutas, M. Unsal / EEST Part A: Energy Science and Research 8 (1) CE=.3 CE=.4 CE= Years Fig. 3. Effect of Carnot Efficiency (CE) on the heat pump COP during the tenth year First year Second year Third year Fifth year Seventh year 5 Months Fig.4. Variation of TES tank temperature ith operation time 15 September December March Fig.5. Variation of storage temperature ith operation time indicates annual variation of ater temperature in the TES tank ith tank volume. It can be seen in this figure that storage temperature increases ith the tank volume, and also amplitude of the ater temperature decreases hen the tank size is increased [1]. The smallest tank size gives the loest store temperatures at the end of the inter season. These are in good agreement ith the results given in Yumrutas and Unsal [1]. When the storage volume is equal to 4 m 3, storage temperature drops belo the o C at the end of February and March. A larger tank is desired, since it ill allo beter heat flo from the earth keeping the storage temperature Years

11 Storage temperature, o C Storage temperature, o C R. Yumrutas, M. Unsal / EEST Part A: Energy Science and Research 8 (1) higher. Hoever, a larger tank means a greater initial cost and the economic aspects should be considered during the selection of tank size. During February and March, COP of the heat pump is loer than those of other months and TES tank ith a volume of 5 m 3 is acceptable since loer storage volume is desired for minimum initial cost. Therefore, this volume as used in the folloing numerical calculations. Effect of the tank volume on store temperature is shon in Fig. 7, hich indicates annual variation of storage temperature ith years during March for storage volumes of 4, 5, 6 and 7 m 3. The figure shos that the temperature is reasonably high during the first three years. But it reduces thereafter and stays constant after the sixth year. This increase (decrease) in temperature ith the volume ill increase (decrease) COP of the heat pump as seen in Fig. 8. COP of the heat pump changes in parallel to the storage temperature. Fig. 8 shos effects of tank size on annual COP of the heat pump for different storage sizes. Although increasing the tank size gradually improves the performance, the magnitude of the increase in COP decreases at the larger tank sizes. The COP does not change after the sixth year folloing the attainment of the annually periodic oprational regime. It is observed that the tank size has a small effect on annual the COP of the heat pump at the larger tank sizes. 1 8 V=4 m3 V=5 m3 V=6 m3 V=7 m3 4 July Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Months Fig.6. Variation of annual storage temperature ith tank volume during the tenth year V=4 m3 V=5 m3 V=6 m3 V=7 m Years Fig.7. Variation of storage temperature ith operation time during March.

12 COP 68 R. Yumrutas, M. Unsal / EEST Part A: Energy Science and Research 8 (1) V=4 m3 V=5 m3 V=6 m3 V=7 m Years Fig.8. Effect of storage tank volume on the heat pump COP. 5. Conclusions The computational analysis based on the analytical model presented in this paper appears to be useful in determining long term performance of a house heating system utilizing heat pump ith an underground spherical TES tank. The estimated storage tank temperatures and COP of the heat pump considering different values for the system parameters such as Carnot efficiency (CE) and storage volume sho that this system is thermodynamically more advantageous over conventional air source heat pump systems. The folloing additional important conclusions are obtained from this study. 1. A 1 percent increase in the CE value gives only a small change in TES temperature but a significant increase in the COP. While the ater temperature increases beteen.5 o C and 1 o C approximately, this increase causes beteen 18 to 5 percent increase in the COP.. There are small differences in tank temperature and COP of the heat pump beteen 5 th and 7 th years of operation. It is observed that the annually periodic operating regime is attained in approximately 7 years for the system parameters considered in this study. 3. Amplitude of storage temperature increases hen the storage size decreases. 4. Storage tank ith a volume of 5 m 3 is a reasonable size for the granite, a house ith a heating load of 1 kw, a COP of the heat pump beteen 3 and 5, and Gaziantep climatic conditions. Finally, it is recommended that a comprehensive thermoeconomic analysis should be carried for optimum design of the system considered taking into account the timespan until the annually periodic operating regime, the tank size, earth structure around the tank and other system characteristics. Nomenclature c specific heat of soil, J kg -1 K -1 c specific heat of ater in the tank, J kg -1 K -1 COP c coefficient of performance for Carnot heat pump COP coefficient of performance of heat pump k thermal conductivity of soil, Wm -1 K -1 r radial distance from the tank center, m R tank radius, m q dimensionless energy extracted rate from the tank Q h heat loss from the house, W heat transfer from the heat exchanger, W Q he p t T T a T d T h T i dimensionless c product time, s soil temperature, K ambient air temperature, K design ambient air temperature, K temperature of ater in radiator, K design inside air temperature, K

13 R. Yumrutas, M. Unsal / EEST Part A: Energy Science and Research 8 (1) T ater temperature in the storage tank, K T deep earth temperature, K (UA) h the product of heat transfer coefficient and area for the house, W K -1 (UA) he the product of heat transfer coefficient and area for the heat exchanger, WK -1 V volume of the tank, m 3 dimensionless compressor ork W compressor ork, W x dimensionless radial distance thermal diffusivity of soil, m s -1 c Carnot efficiency dimensionless temperature a dimensionless ambient air temperature dimensionless design inside air temperature i 4Rk/(UA) h density of soil, kg m -3 density of ater, kg m -3 dimensionless time dimensionless temperature References [1] Saidur R. Energy, economics and environmental analysis for chillers in office buildings. Energy Educ Sci Technol Part A 1;5:1 16. [] Demirbas A. Social, economic, environmental and policy aspects of biofuels. Energy Educ Sci Technol Part B, 1;: [3] Darici B, Ocal FM. The structure of European financial system and financial integration. Energy Educ Sci Technol Part B, 1;: [4] Demirbas A. Energy issues in energy education. Energy Educ Sci Technol Part A 11;7:9. [5] Novo AV, Bayon JR, Castro-Fresno D, Rodriguez-Hernandez J. Revie of seasonal heat storage in large basins: Water tanks and gravel ater pits, Appl Energy 1;87: [6] Balat H. Prospects of biofuels for a sustainable energy future: A critical assessment. Energy Educ Sci Technol Part A 1;4: [7] Guohui Gan SB, Riffat CSA. Chong, A novel rainater ground source heat pump-measurement and simulation. Appl Thermal Eng 7;7: [8] Basaran T, Sumbul N. The performance of a cool storage system in various operation conditions and its economic analysis Energy Educ Sci Technol Part A 11;6: [9] Hepbasli A. Performance evaluation of a vertical ground-source heat pump system in Izmir,, Turkey. Int J Energy Res ;6: [1] Hepbasli A, Akdemir O, Hancioglu E. Experimental study of a closed loop vertical ground source heat pump system. Energy Convers Manage 3;44: [11] Zeng H, Diao N, Fang Z. Heat transfer analysis of boreholes in vertical ground heat exchangers, Int J Heat Mass Trans 3;46: [1] Yumrutas R, Unsal M. Analysis of solar aided heat pump systems ith seasonal thermal energy storage in surface tanks. Energy, ;5: [13] Yumrutas R, Kunduz M, Ayhan T. Investigation of thermal performance of a ground coupled heat pump system ith a cylindrical energy storage tank. Int J Energy Res 3;7: [14] Yumrutas R, Kanoglu M, Bolatturk A, Bedir SB. Computational model for a ground coupled space cooling system ith an underground energy storage tank. Energy Build 5;37: [15] Zhang H-F, Ge X-S, Ye H. Modeling of a space heating and cooling system ith seasonal energy storage. Energy, 7;3:51 58.

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