Research and development of the hybrid ground-coupled heat pump technology in China

Renewable Energy xxx (2015) 1e12

Contents lists available at ScienceDirect

Renewable Energy journal homepage: www.elsevier.com/locate/renene

Research and development of the hybrid ground-coupled heat pump technology in China Min Guo a, Nairen Diao a, *, Yi Man a, Zhaohong Fang a, b a b

Ministry of Education Key Laboratory of Renewable Energy Utilization Technologies in Buildings, Shandong Jianzhu University, Jinan 250101, China Shandong Zhongrui New Energy Technology Co. Ltd., Jinan 266071, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 March 2015 Received in revised form 22 July 2015 Accepted 10 August 2015 Available online xxx

The hybrid ground-coupled heat pump (HGCHP) systems with supplemental heat rejecter/supplier can effectively solve heat imbalance problems in the subsurface, and consequently improve the operation performance of the geothermal systems. For example, solar energy and/or industrial waste heat may be used as stable heat sources for underground heat storage in northern China with higher heating load, and cooling towers are installed to release heat into the air in southern China, where more cooling demand is needed. This paper reviews and discusses different HGCHP systems, which have been applied in China. And based on the heat transfer model of vertical borehole heat exchangers (BHE) for HGCHP systems, physical and mathematical models of multistage series circuits are developed to illustrate the heat transfer process of the underground thermal storage. A set of parameters, such as borehole spacing, heat recharging rate fractions and thermal properties of soils, which affect the thermal performance of the ground heat exchangers are analyzed, and the optimal solutions are discussed for engineering application. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Ground-coupled heat pump Hybrid system Seasonal heat storage Cooling tower Heat transfer analysis

1. Introduction In recent decades, due to the support from the government, contributions of researchers and engineers and the growing concern about the air pollution from coal-fired boilers, groundcoupled heat pump (GCHP) technologies have improved dramatically and aroused more and more interest in China. According to national statistics, by the end of 2014, the completed GSHP systems which are used for space heating are applied for a total area of more than 300 million square meters [1], while the figure in 2009 was only 100 million. And by 2020, the total amount of used geothermal energy will equal to 50 million tons of standard coal [2]. There are five climate zones in China, therefore, the climate characteristics from different region vary widely. The buildings in the northern region have much more annual heating demand. If GCHP systems are applied to these buildings, the heat extracted

* Corresponding author. Present address: Fengming Road, Shandong Jianzhu University, Jinan 250101, China. E-mail address: [email protected] (N. Diao).

from underground in winter is far greater than the heat injected in summer. The situation in the southern region is on the contrary. The imbalance between the cumulative amount of heat injection and extraction in the northern and southern regions restricts the wide application of GSHP systems. In order to solve heat imbalance problems in the subsurface, the HGCHP systems with supplemental heat rejecter/supplier were invented. The operation performance of the hybrid systems improves a lot, and therefore, they can be widely used across the country. For example, solar energy and industrial waste heat are used as supplemental heat sources for underground heat storage in northern region, and cooling towers are installed to release heat into the air in southern region. According to the scale of the projects, the financial situation, and the heating/cooling loads of the buildings, HGCHP systems with cooling tower are generally connected in two configurations, namely in-parallel connection and in-series connection. In China, the research on HGCHP mainly focuses on the designing theories and methods, different system forms, performance analysis and control strategies etc. Man et al. discussed the feasibility of using HGCHP systems in regions with hot summers and warm winters [3], and they investigated the use of nocturnal radiation cooling as supplemental heat rejecter [4]. Guo et al. studied the optimization

http://dx.doi.org/10.1016/j.renene.2015.08.021 0960-1481/© 2015 Elsevier Ltd. All rights reserved.

Please cite this article in press as: M. Guo, et al., Research and development of the hybrid ground-coupled heat pump technology in China, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.08.021

2

M. Guo et al. / Renewable Energy xxx (2015) 1e12

Nomenclature a cp H k M ql rb Tb Tf

ground thermal diffusivity (m2/s) fluid specific heat (J/kg K) borehole depth (m) ground thermal conductivity (W/m K) mass flow rate of circulating fluid (kg/s) heat flow per unit length of pipe (W/m) borehole radius (m) borehole wall temperature (
C) fluid temperature (
C)

Greek symbols recharging heat rate fraction ε borehole heat transfer efficiency t time (s) Q dimensionless fluid temperature

b

of underground loop systems with an objective of minimum investment [5]. Cui et al. simulated the performance of HGCHP system for domestic hot water supply [6]. Li et al. proposed a GCHP system using the absorption chiller, which can reduce the imbalance between heating and cooling load in cold climate region [7]. Hu et al. discussed the optimal intermittent operation strategies of HGCHP [8]. By using TRNSYS software, Fan et al. analyzed the impact of several important design parameters on a small HGCHP system performance [9]. Based on the study of artificial neural network, Wang et al. established a model for HGCHP system, and studied the control strategy [10]. In addition, there are a number of studies about experiments and simulations of solar energy assisted HGCHP system [11e14]. Different from the single GCHP system, the performance of hybrid system depends not only on long-term ground temperature evolution, but also the short-time high-frequency temperature response of BHEs. Li et al. proposed a composite-medium linesource model to simulate the high frequency temperature response of HGCHP systems [15e17], and they also established full-scale temperature response function for heat transfer from different time scales and proposed an entropy generation minimization method [18,19]. There are various types of industrial waste heat and solar energy collectors. In most industrial production processes, the industrial waste heat are generally featured by delivering thermal energy of over 50
C and requiring a relatively lower return temperature, such as 20
C. Obviously, the large temperature drop required by the production process cannot be implemented through the conventional parallel connection of boreholes which can mostly provide a maximum temperature difference of 5
C [20e22]. Based on this, a novel borehole configuration of multistage-series circuits is proposed is this study, it could not only satisfy the requirement of the large temperature drop by the industrial process but also make full use of industrial waste heat. Previous studies mainly focus on the analysis of underground temperature distribution of boreholes with conventional parallel connection [23e30], while in this study, the heat transfer of a new borehole configuration of multistageseries circuits is analyzes. Fig. 1 illustrates the two different borehole connection configurations, i.e. the parallel connection for GCHP systems and the multistage-series connection for the seasonal thermal storage of industrial waste heat.

Fig. 1. Left: conventional GHE in parallel connection; right: multistage-series GHE for the thermal storage based on the industrial waste heat.

2. Solar energy assisted HGCHP systems 2.1. Heat transfer analysis of vertical BHE with multistage-series circuits 2.1.1. Assumptions for the heat transfer model As for a single borehole, the heat transfer process can be directly analyzed by means of the same mathematical methods as the conventional vertical borehole in GCHP systems. The finite line source heat transfer model commonly used is employed in this study. To keep the problem analytically manageable, the theoretical model of the multistage-series GHE is based on the following simplified assumptions: 1) The ground is regarded as a homogeneous medium with a uniform initial temperature t0, and its thermophysical properties do not change with temperature. 2) The heat transfer in the ground is assumed to be carried out solely by conduction with the neglect of groundwater advection, using an effective ground thermal conductivity. The moisture migration in the ground is also negligible. 3) Each circuit is composed of a number of individual boreholes connected in parallel with the same geometric parameters and thermal properties. 4) During the entire heat storage and extraction period, the heat flow rate and continuous time vary, but the total heat content remains the same. And for large thermal storage system, with the help of heat pumps, the amount of extracted heat equals to the storage amount.

2.1.2. Heat transfer model for heat storage and its solution For a given GHE configuration with multistage series, the main objective of the thermal analysis is to determine the optimal heat injection/extraction rate (Qi) of each independent circuit and to 00 consequently obtain the inlet/outlet temperatures (ti0 ; ti ) of the circulating fluid of each circuit. Fig. 2 describes the physical model of the multistage series, where, Rbi and ti are the borehole thermal resistance and the fluid temperature of the ith circuit, respectively. Based on the energy conservation law, the injection heat rate to the surrounding ground by ith circuit is equivalent to the heat rate

Fig. 2. The physical model of multistage series.

Please cite this article in press as: M. Guo, et al., Research and development of the hybrid ground-coupled heat pump technology in China, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.08.021

M. Guo et al. / Renewable Energy xxx (2015) 1e12

transferred from the circulating fluid in the pipe to the borehole wall. The equation for the energy balance is given:

  8 00 0 ¼ Mc t  t Q > i i i > < > > : Qi ¼

00

ti0 þ ti  tbi 2

!,

(1)

ti0 ¼ tbi þ Rbi bi Q þ bi Q =ð2McÞ 00 ti ¼ tbi þ Rbi bi Q  bi Q =ð2McÞ

(2)

    8 > t t  t  t > f 1 b f 1 f 2 dt > f1 > > ¼ þ Mc > < dz RD RD 1 12     > > > tf 2  tb tf 2  tf 1 dtf 2 > > > ¼ þ : Mc dz RD RD 2 12

ð0  z  HÞ

(3)

Two conditions are necessary to complete the solution:

z ¼ 0; tf 1 ¼ tf0 z ¼ H; tf 1 ¼ tf 2 R R R2

(4) R R R2

R R R2

11 22 11 22 11 22 D D 12 12 12 where RD . R11 and R22 are R12 1 ¼ R22 R12 , R2 ¼ R11 R12 , R12 ¼ the thermal resistance between the circulating fluid and the borehole wall, and R12 is the resistance between the two pipes. The general solution of this problem is derived by Laplace transformation, which is slightly complicated in form. At the instance of the symmetric placement of the U-tube inside the borehole, the temperature profiles in the two pipes were illustrated by Diao et al. (2004). For the purpose of practical applications an alternative param00 eter ε ¼ ðtf0  tf Þ=ðtf0  tb Þ is derived from the temperature profiles, which is named as the heat transfer efficiency of the borehole. It 00 should be noticed that tf0 and tf are the entering/exiting fluid temperatures to/from the U-tube. From the derived temperature profile the more accurate heat conduction resistance between the fluid inside the U-tube and the borehole wall can be calculated by,

Rb ¼

  H 1 1  Mc ε 2

(6) It is noticed that the borehole wall temperature varies with time and borehole depth. The temperature at the middle of the borehole depth (z ¼ 0.5H) is usually chosen as its representative temperature. For the n-stage series, following the scheme shown in Fig. 2, the fluid outlet temperature of circuit 1 corresponds to the inlet temperature of circuit 2 and so on. Therefore, the coupling conditions are given as follows: 00

where, Q is the total injection heat rate by the GHE and bi ¼ Qi/Q is the recharging heat rate fraction which is defined as a ratio of the recharging heat rate occupied by the ith circuit to the total heat rate. The following two subsections presents the heat transfer models inside and outside the boreholes to achieve the thermal resistances and the borehole wall temperature as well. The quasi 3-D model of the heat transfer inside the borehole is employed to calculate the total borehole thermal resistance, which takes into account the 2-D heat conduction in the transverse crosssection as well as the convective heat transfer in the axial direction by the fluid inside the U-tubes. The energy equilibrium equations can be written for up-flow and down-flow of the circulating fluid:



1 1 0 0 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2 2 þðzhÞ 2 þðzþhÞ 8 r r A erfc@ A9 pffiffiffiffi erfc@ 2pffiffiffiffi > at 2 at > > ZH> = < ql qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi tb ¼ dh þ t0 > > 4pkg > ; r 2 þ ðz  hÞ2 r 2 þ ðz þ hÞ2 > 0 :

Rbi

where, tbi is the representative borehole wall temperature of the ith circuit. The inlet and outlet temperatures of the ith circuit can be easily derived according to Eq. (1):



3

(5)

Based on the finite line source model, the temperature on the borehole wall, where r ¼ rb, was given by Zeng et al. (2002):

00

00

0 t1 ¼ t20 ; /ti ¼ tiþ1 ; /tn1 ¼ tn0

The objective function is defined as:

D ¼ jD1 j þ jD2 j þ / þ jDn1 j 00

00

0 where, D1 ¼ t1  t20 , Di ¼ ti  tiþ1 ði ¼ 1; /; n  1Þ, It is noticeable that the objective function is a function of the variables bi (i ¼ 1, …,n1), i.e. b¼(b1,b2,/bn) according to Eq. (2). The final aim of the thermal analysis for the GHE with n-stage series circuits is to seek the unique value of recharging heat rate fraction (i.e.bi) and the inlet/outlet temperatures of each circuit for every time step with the constrain conditions. Theoretically, once the value of the objective function approaches to zero, the simulation model can determine the inlet/outlet temperatures of each circuit in the multistage series as well as the recharging heat fractionbi. The calculation program begins with an initial value of bi, for example an assumption of equal distribution, bi ¼ 1/n for the first time step. Then, the inlet/outlet temperatures of each circuit (ti0 and 00 ti ) can be estimated using the thermal resistances both inside and outside of the boreholes. The initial value of the objective function D ¼ (b1,b2,/bn1) will be derived for the first time step. Based on the downhill simplex method, an optimal minimum value of the objective function will be found, which satisfies the constraint conditions. Finally, the recharging heat rate and the inlet/outlet temperatures of each circuit can be obtained for the first time step. For the next time steps, the thermal resistances of the ground will be calculated based on the previous calculated heat rates and the current heat rate for each circuit. The same procedure can be employed to complete the whole time steps.

2.2. Results and discussion 2.2.1. Effect of borehole spacing on the thermal performance The heat losses to the surrounding ground are roughly proportional to the area exposed to the surroundings. For an industrial thermal storage system, the larger size the buried boreholes are, the smaller relative heat losses to the surrounding ground will be. A smaller spacing between boreholes leads to a smaller storage volume and a significant thermal interference between boreholes. On the contrary, a larger borehole spacing can reduce the thermal shortcut among the boreholes and enhance the thermal performance of the heat injection/extraction process. The fluid inlet/ outlet temperatures for two values of borehole spacing (4 m and 5 m) are monthly simulated for 10 years operation, as shown in Fig. 3. As expected, the fluid temperatures in the case of the 5 m spacing are obviously lower than those in the case of 4 m spacing. Consequently, the temperature of the storage volume with a larger spacing is lower compared to the closer spacing, which leads to a

Please cite this article in press as: M. Guo, et al., Research and development of the hybrid ground-coupled heat pump technology in China, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.08.021

4

M. Guo et al. / Renewable Energy xxx (2015) 1e12

Fig. 3. Effect of borehole spacing on thermal performance (Tin1 and Tout1 are the temperatures of the fluid entering and exiting the GHE for 5 m spacing; Tin2 and Tout2 are the temperatures for 4 m spacing).

higher thermal performance. However, a large borehole spacing will increase the capital cost for a vast amount of land. Therefore, it is necessary to determine a reasonable borehole spacing considering both the capital cost and the thermal performance of the system. 2.2.2. Effect of heat recharging rate fractions of different circuits Fig. 4 illustrates the variations of the heat rate fractions of threestage series circuits against the simulation time. As Fig. 4 shows, the fraction of the first circuit is significantly higher than the second one and consequently the second one is higher than the third one because the temperature gradient between the first circuit and the surrounding ground is highest compared to the other two circuits. The highest value of the fraction of the first circuit reaches 0.58 at the beginning of the operation. It is noticed that the sum of the fractions of the three circuits equals one. 2.2.3. Effect of thermal properties of different soils The ground thermal properties are the critical factors, which can thermally influence to a large extent the storage performance of the GHE. In this study, three types of soils (clay, sandy clay and

siltstone) are selected as storage mediums for comparisons of their individual heat transfer capability. The thermal properties of these soils are listed in Table 1. Fig. 5 shows the sensitivity of the injection heat rate per unit length of borehole to the soil thermal properties for the three cases of soils during the two years of continuous recharging. Evidently, all the heat transfer rates of the three kinds of soils are decreased with time due to the gradually reduced temperature gradient between the fluid and the soil. The siltstone behaves the highest heat flux because of its largest density and conductivity. The heat transfer rate of the sandy clay is larger at the beginning of the heat recharge because of its larger thermal diffusivity and becomes lower compared to the clay at the end of the simulation. This is due to the lower heat capacity of the sandy clay, which results in a higher temperature rise among the surrounding soil and consequently a performance degradation. Therefore, the recharging heat performance of the GHE in the soil is comprehensively influenced by the thermal conductivity and heat capacity of the soil. 3. HGCHP system with cooling tower as supplemental heat rejecter As mentioned, when the GCHP system is utilized for cooling load dominated buildings, the heat rejected into ground will accumulate around the ground heat exchangers, which results in degradation of system energy performance. One of the available options to resolve this problem is to utilize hybrid GCHP systems to reject the accumulated heat by supplemental heat rejecters. The scope of Section 3 is limited to the HGCHP system with supplemental heat rejecter for cooling load dominated buildings. The conventional supplemental heat rejecter of the HGCHP system is a cooling tower. As shown in Fig. 6, the conventional HGCHP system mainly consists of a heat pump unit, a GHE, a cooling tower, a plate heat exchanger and a domestic hot water (DHW) production device. Although the HGCHP system shows considerable promise for cooling load dominated buildings, it has not been widely applied due to the design of the HGCHP system is a complex optimization problem with tradeoff between sizing the GHE and the cooling tower as well as selection of system operation control strategy. It should be noted that the cooling tower, requires additional energy consumption and periodic maintenance. The maintenance and operation costs due to the added cooling tower will increase the whole HGCHP system's life cycle cost if it is not properly designed. Besides, the actual amount of heat transferred to and from the ground through the GHE varies with the building's air conditioning load which may result in short time step temperature fluctuations of the water circulating inside the GHE and then affect the COP of the heat pump. If the time-of-day electricity rates are applicable, the impact of the fluctuating performance on the system operating costs may be even more significant. Furthermore, selection of the cooling tower's cooling capacity as well as its control strategy, i.e., under what conditions to operate the cooling tower, and the short-time impact of control strategies on the operation performance of the conventional HGCHP system are complex. Therefore, in order to properly design and apply the conventional HGCHP system, a practical short time step simulation model and control strategy of the conventional HGCHP system by analyzing the heat transfer process of its main components should be established and verified by experimental data. 3.1. Simulation model establishment of the HGCHP system

Fig. 4. Recharging heat rate fractions with simulation time.

3.1.1. Simulation model of heat pump unit In order to simulate the performance of the heat pump on an

Please cite this article in press as: M. Guo, et al., Research and development of the hybrid ground-coupled heat pump technology in China, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.08.021

M. Guo et al. / Renewable Energy xxx (2015) 1e12

5

Table 1 Thermal properties of different soils. Soil type

Density kg/m3

Heat capacity J/kg
C

Thermal conductivity W/m
C

Thermal diffusivity 106 m2/s

clay Sandy clay siltstone

1285 1925 2570

1200 1000 1556

0.8 2.2 3.2

0.519 1.143 0.8

polynomial curves are employed in this study to fit these functions. 3.1.2. Simulation model of cooling tower A concise model based on Merkel's enthalpy theory and the Effectiveness-NTU method which predigests the operation of the cooling tower into a stable process is used in this study. The precision of this model has been validated by comparing with manufacturers' data.

Fig. 5. Effect of the soil thermal properties on recharging heat rate per unit borehole length.

short time step basis, it is more feasible to build the functions between its COP and effusing fluid temperature versus its corresponding entering fluid temperature according to the heat pump manufacturer’ data. The least square method and the 2nd power

3.1.3. Simulation model of GHE Due to its complications and long term effect, the heat transfer process of the GHE is separated into two parts: the process outside boreholes, where heat transfer is treated as a transient process; and the process inside borehole, where heat transfer is considered as a steady process. Transient heat transfer around boreholes of the GHE is analyzed here by a 2-D model established by Zeng et al. [31], which assumes the borehole is a line-source with finite length, and releases heat at a constant rate per length. Generally, the GHE contains more than one borehole, and each borehole has its own temperature response based on its location in the GHE. In order to ensure the effective heat transfer capacity of the GHE, the typical borehole wall temperature response is employed to represent the borehole wall temperature response of the whole GHE in this study. Based on our previous research [32], the temperature response to a constant step heat current ql is shown in Eq. (7)

Fig. 6. The conventional HGCHP system with cooling tower.

Please cite this article in press as: M. Guo, et al., Research and development of the hybrid ground-coupled heat pump technology in China, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.08.021

6

M. Guo et al. / Renewable Energy xxx (2015) 1e12

8 0qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1,rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1,rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Z1 < r 2 r 2 ðrb =HÞ2 þ ð0:5  H0 Þ2 ðrb =HÞ2 þ ð0:5 þ H 0 Þ2 ql b b 0 Þ2  erfc@ A A qffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffi qe ¼ þ ð0:5  H þ ð0:5 þ H 0 Þ2 erfc@   4kp : H H 2 2 2 at H 2 at H 0 1,rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi3 1, 2 0q ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 0qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  2  2 N N r 2 X X rj H þ ð0:5  H0 Þ2 rj H þ ð0:5 þ H0 Þ2 j 2 0 A A 4erfc@ 4erfc@ qffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffi þ ð0:5  H Þ 5  þ   H 2 at H2 2 at H2 j¼1 jsi j¼1 jsi rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi39 = r 2 j þ ð0:5 þ H 0 Þ2 5 dH 0 ; H (7)

In order to calculate the borehole wall temperature response to practical heat transfer current which varies from time to time and to integrate this analytical solution into our computer program, the g-function which represents the non-dimensional temperature response of borehole wall to the step heat current, and further the f-function which represent the non-dimensional

The practical mutative heat flow currents imposed on boreholes from 0 to t can be approximated by the sum of a serious of rectangular pulse heat currents. Further, a single pulse heat current can be approximated by superposition of two step heat currents with the initial one ql0 set to be 0. According to the superimposing theory, the temperature response of the borehole wall at moment t can be deduced:

2 3 ∞ ∞ ∞ X



1 X 1 4X q¼ q $gðt  ti1 Þ  ql0 $gðt  t0 Þ  qlj $g t  tj 5 q  qli1 $gðt  ti1 Þ ¼ 2pk i¼1 li 2pk i¼1 li j¼1 ∞ ∞ 1 X 1 X q $½gðt  ti1 Þ  gðt  ti Þ ¼ q $f ðt  ti1 Þ ¼ 2pk i¼1 li 2pk i¼1 li

temperature response of borehole wall to the pulse heat current are employed:

gðtÞ ¼ 2pkqe =ql ;

f ðt  ti1 Þ ¼ gðt  ti1 Þ  gðt  ti Þ;

(8)

(9)

For the experimental test rig in Section 3.2, there are two boreholes. The borehole wall temperature response Tb at moment t after the HGCHP system is activated can be calculated by Eq. (10) according to the pulse heat current qli.

9 8 0qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1 0qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1 > > 2 2 > > 2 2 0 0 > > ðrb =HÞ þ ðZ=H  H Þ ðrb =HÞ þ ðZ=H þ H Þ > > > A A > @ @ > > q q ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi erfc erfc > >  2  2 > > > > > > Þ H Þ H 2 aðt  t 2 aðt  t > > i1 i1 > > > > ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi q q  > > > > > > 2 2 2 2 0 0 > > > > ðrb =HÞ þ ðZ=H  H Þ ðrb =HÞ þ ðZ=H þ H Þ > > > > > > > > q ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi q ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 1 1 0 0 > > > > > > 2 2 2 2 0 0 > > þ ðZ=H  H Þ þ ðZ=H þ H Þ ðr=HÞ ðr=HÞ > > > A A > @ > > erfc@ q q ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi erfc > >  2  2 > > > > > > 2 aðt  ti1 Þ H 2 aðt  ti1 Þ H > > > > > > ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi q q  þ > > > > > > 2 2 1 2 2 0 0 > > Z < ∞ = ðr=HÞ ðr=HÞ þ ðZ=H  H Þ þ ðZ=H þ H Þ X 1 Tb ðtÞ ¼ T0 þ qli dH 0 q ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi q ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 1 0 0 > > 4kp i¼1 > > 2 2 0 2 0 2 > > ðrb =HÞ þ ðZ=H  H Þ ðrb =HÞ þ ðZ=H þ H Þ > 0 > > A A> > > erfc@ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi erfc@ > > > >  2ffi  2ffi > > > > Þ H Þ H 2 aðt  t 2 aðt  t > > i i > > > > qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi þ  > > > > > > 2 2 2 2 > > 0 0 > > ðr ðr þ ðZ=H  H Þ þ ðZ=H þ H Þ =HÞ =HÞ > > b b > > > > > > q ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi q ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi > > 1 1 0 0 > > > > 2 2 0 2 0 2 > > > > ðr=HÞ þ ðZ=H  H Þ ðr=HÞ þ ðZ=H þ H Þ > > > > A A @ @ q q ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi erfc > > erfc > >   > > 2 2 > > Þ H Þ H 2 aðt  t 2 aðt  t > > i i > > > > > > ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi q q þ  > > > > > > 2 2 0 2 0 2 ; : ðr=HÞ þ ðZ=H  H Þ ðr=HÞ þ ðZ=H þ H Þ

(10)

Please cite this article in press as: M. Guo, et al., Research and development of the hybrid ground-coupled heat pump technology in China, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.08.021

M. Guo et al. / Renewable Energy xxx (2015) 1e12

7

Fig. 7. Schematic diagram of the HGCHP experimental rig.

Taking the fluid axial convective heat transfer and thermal “short-circuiting” among U-tube legs into account, the quasi-3-D model of the boreholes established by Zeng et al. [33] is utilized in this study to calculate the temperature of fluid effusing and entering the GHE.

experimental data in Section 3.3.

. i  00 Tf ¼ Tb þ ql $H$Q m$cp $ 1  Q i h .  00 Tf0 ¼ Tb þ ql $H m$cp $ 1  Q

The HGCHP experimental rig is installed inside the laboratory building of the Hebei University of Engineering, and it mainly consists of five components, i.e., the heat pump unit, the GHE, the cooling tower, the fan coil units, the circulating water pumps and the data acquisition instruments, as shown in Fig. 7.

00

h

00

(11)

3.2. HGCHP experimental rig

00

where the Tf and Tf0 denote the temperature of fluid effusing and entering the GHE respectively, the Tb denotes the temperature of borehole wall, which can be calculated by the model of heat transfer outside the boreholes. 3.1.4. Operation control strategies The operation control strategies investigated in this paper can be categorized into four categories: (1) set point control strategy, i.e. to operate the cooling tower when the entering fluid temperatures of the heat pump exceeds a set value; (2) dry bulb temperature difference control strategy, i.e. to operate the cooling tower when the difference between the entering fluid temperature of heat pump and the dry bulb temperature of ambient air exceed a set value; (3) wet bulb temperature difference control strategy, i.e. to operate the cooling tower when the difference between the entering fluid temperature of heat pump and the wet bulb temperature of ambient air exceed a set value; (4) operating time control strategy, i.e. to operate the cooling tower during the specified period of time. 3.1.5. Operation simulation of the whole HGCHP system The performance of the whole HGCHP system operated with various control strategies can be simulated using an independently developed computer calculation program based on the simulation models with FORTRAN language. The simulation results calculated by this computer program are shown and compared with

3.2.1. Heat pump unit The heat pump utilized in this experimental rig is a water-towater scroll hermetic compressor unit with R134a as refrigerant. The cooling capacity, power input and motor rotation speed of the heat pump of rated capacity established by America Refrigeration Institute are 21.8 kW, 6.54 kW and 2900r/min, respectively.

3.2.2. GHE configuration The GHE of this HGCHP experimental rig has two vertical boreholes whose depths and diameters are 120 m and 150 mm, respectively. The U-pipes’ outside and inside diameters are 32 mm and 25 mm, respectively, and instead of series-connection, they are connected in parallel with PPR pipes above the ground. Boreholes are completely backfilled by grout mixed with drilling mud, cement and sand in specific proportions. According to the on-site geological conditions report, the ground belongs to moisture saturated soil, and the detailed type is: claypan layer from surface to 30 m deep, pebble gravel layer from 30 m to 50 m deep and sand layer from 50 m to 120 m deep. The average temperature, thermal conductivity and thermal diffusivity of ground from surface to 120 m deep are tested to be 16.10
C, 2.40  103 kW/(m.K) and 0.91  106 m2/ s, respectively. The thermal conductivity and thermal diffusivity of the grout are 2.42  103 kW/(m.K) and 1.03  106 m2/s, respectively.

Please cite this article in press as: M. Guo, et al., Research and development of the hybrid ground-coupled heat pump technology in China, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.08.021

8

M. Guo et al. / Renewable Energy xxx (2015) 1e12

Table 2 Detailed control parameter and time schedule of HGCHP system experiments. Control strategy Strategy 1

No No No No No No No No No No No No No No No No

Strategy 2

Strategy 3

Strategy 4

1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4

Control Parameter

Experimental time

EFT >30
C turn on, EFT<29
C turn off the cooling tower EFT >32
C turn on, EFT<31
C turn off the cooling tower EFT >34
C turn on, EFT<33
C turn off the cooling tower EFT >35
C turn on, EFT<34
C turn off the cooling tower EFT- Tdb >2
C turn on, EFT- Tdb <1
C turn off the cooling tower EFT- Tdb >3
C turn on, EFT- Tdb <2
C turn off the cooling tower EFT- Tdb >4
C turn on, EFT- Tdb <3
C turn off the cooling tower EFT- Tdb >5
C turn on, EFT- Tdb <4
C turn off the cooling tower EFT- Twb >9
C turn on, EFT- Twb <8
C turn off the cooling tower EFT- Twb >10
C turn on, EFT- Twb <9
C turn off the cooling tower EFT- Twb >11
C turn on, EFT- Twb <10
C turn off the cooling tower EFT- Twb >12
C turn on, EFT- Twb <11
C turn off the cooling tower 12:00 < time<8:00 turn on the cooling tower 14:00 < time<8:00 turn on the cooling tower 16:00 < time<8:00 turn on the cooling tower 18:00 < time<8:00 turn on the cooling tower

16 20 24 28 02 06 10 14 18 22 26 30 03 07 11 15

3.2.3. Cooling tower As mentioned before, the open-circuit cooling tower cooperated with isolation plate heat exchanger is employed as supplemental heat rejecter of the HGCHP system test rig. The low noise glass fiber reinforced plastic reverse-flow cooling tower is selected, with rated cooling capacity of 9.5 kW. 3.2.4. Circulating water pumps The water circulating loops of the GCHP experimental rig consists of the cooling water loop, the chilled water loop, and the cooling tower circulating pump. Three centrifugal pumps with rated flow in 5.5 m3/h, 2.5 m3/h and 1.9 m3/h are selected for these three circulating loops respectively. These three variable-speed

Jun 8:00e18 Jun 8:00 Jun 8:00e22 Jun 8:00 Jun 8:00e26 Jun 8:00 Jun 8:00e30 Jun 8:00 Jul 8:00e04 Jul 8:00 Jul 8:00e08 Jul 8:00 Jul 8:00e12 Jul 8:00 Jul 8:00e16 Jul 8:00 Jul 8:00e20 Jul 8:00 Jul 8:00e24 Jul 8:00 Jul 8:00e28 Jul 8:00 Jul 8:00e01 Aug 8:00 Aug 8:00e05 Aug 8:00 Aug 8:00e09 Aug 8:00 Aug 8:00e13 Aug 8:00 Aug 8:00e17 Aug 8:00

pumps are controlled by three identical frequency converters, which can change the flow rate of circulating water by changing the spin frequency of the circulating pumps. 3.2.5. Fan coil units Five fan coil units are installed in parallel in the system on two floors of the building as terminal units of the HGCHP experimental rig. The total air conditioned floor area attached to these five fan coil units is 250 m2. 3.2.6. Data acquisition system The data acquisition system consists of the temperature acquisition system, the pressure acquisition system, the flow rate

Table 3 Experimental results of the HGCHP system operation performances. Control strategy

Cooling provision system (kWh)

Energy consumption of heat Energy consumption of pump unit (kWh) circulating pumps (kWh)

Energy consumption of cooling tower (kWh)

Average COP of heat Average COP of pump unit HGCHP system

Strategy No 1 1 No 2 No 3 No 4 Strategy No 2 1 No 2 No 3 No 4 Strategy No 3 1 No 2 No 3 No 4 Strategy No 4 1 No 2 No 3 No 4

385.18

90.23

29.51

6.85

4.27

3.04

393.78

93.69

29.51

4.43

4.20

3.09

390.59

97.25

29.51

2.94

4.02

3.01

376.31

98.09

29.51

2.82

3.84

2.89

391.92

92.04

29.51

6.80

4.26

3.05

401.38

94.76

29.51

5.23

4.24

3.10

393.55

96.06

29.51

4.97

4.10

3.02

394.92

100.32

29.51

2.20

3.94

2.99

402.01

93.87

29.51

6.68

4.28

3.09

408.69

96.81

29.51

4.04

4.22

3.14

405.39

98.21

29.51

3.74

4.13

3.08

404.07

99.91

29.51

3.30

4.04

3.04

409.33

97.06

29.51

6.04

4.22

3.09

409.01

98.29

29.51

4.40

4.16

3.09

400.26

100.97

29.51

3.93

3.96

2.98

394.74

101.19

29.51

3.22

3.90

2.95

Please cite this article in press as: M. Guo, et al., Research and development of the hybrid ground-coupled heat pump technology in China, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.08.021

M. Guo et al. / Renewable Energy xxx (2015) 1e12

acquisition system and the power consumption acquisition system. An industrial control computer serves as the data logger of the acquisition system. The ADAM-4015 6-channel RTD modules with modbus are selected for temperature acquisition. The ADAM-4017 16-bit 8-channel analog modules are selected for the pressure sensors, flow rate and power consumption acquisition. The ADAM4520 converter serves as communication module with the computer. The HMI/SCADA program IFIX, which is a superior proven real time information management and SCADA solution, is utilized as the software of the acquisition system. 3.3. Experimental results and comparison with simulative results 3.3.1. Control parameters and test time arrangement The HGCHP system cooling operation tests were carried out in June to August with four kinds of operation control strategies mentioned in Section 3.1.4. Each control strategy was investigated with four different control parameters. The HGCHP system with each operation conditions was operated and tested continuously for 48 h due to the total test time limitation. The detailed tests arrangement is shown in Table 2 (EFT denotes the entering fluid temperature of heat pump unit; Tdb denotes the dry bulb temperature of ambient air; Twb denotes the wet bulb temperature of ambient air). The temperature distribution of borehole, the temperature and flow rate of circulating water, the power consumption, the cooling capacity and the refrigerant pressure of the HGCHP

9

experimental system operated with these control strategies were recorded. 3.3.2. Experimental data of HGCHP system operation performance We measured the indoor temperature during the experiment. The measured data show that, through the control of the heat pump operation, the indoor temperature varies in a very small range (±0.5
C) for different strategies. All the four control strategies of cooling tower are capable to meet the demand of the heat pump operation, but possibly, with different system performances, which can be seen in Table 3. The cooling performance of the GCHP system is represented by the COP of the heat pump unit and the COP of the system. The results show that the HGCHP system controlled by the strategy 3 performs better compared with the system controlled by other three strategies, and the HGCHP system operated with the control strategy 3 for the control parameter No. 2 possesses the highest average system COP. That is to say, for pursuing the highest system COP, the control strategy 3 is the optimal one among the investigated control strategies. This is because for the wet bulb temperature difference control strategy, the cooling tower is controlled according to the difference between entering fluid temperature and wet bulb temperature of ambient air, which is the key parameter for the cooling tower's operation. The cooling tower is turned on certain weather condition. Therefore, more heat can be rejected with smaller energy consumption of cooling tower. From experimental results, the HGCHP system operated with

Fig. 8. Operation performance of HGCHP system operated with optimal control strategy.

Please cite this article in press as: M. Guo, et al., Research and development of the hybrid ground-coupled heat pump technology in China, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.08.021

10

M. Guo et al. / Renewable Energy xxx (2015) 1e12

Table 4 Comparison between experimental data and simulative results of HGCHP system. Average operation parameters

Experimental data

Simulative results

Comparison between experimental data and simulative results

Temperature of fluid entering the GHE (
C) Temperature of fluid effusing the GHE (
C) Temperature of fluid entering the GHE (
C) Temperature of fluid effusing the GHE (
C) Borehole wall temperature (
C) COP of heat pump unit COP of HGCHP system

33.76 30.22 34.34 33.28 24.82 4.22 3.14

33.35 30.04 34.48 33.22 24.59 4.26 3.14

1.20% 0.59% 0.41% 0.18% 0.94% 1.00% 0.29%

the temperature difference control strategies (strategy 3 and strategy 2) possess higher system COP compared the systems operated with the operating time control strategy (strategy 4) and the set point control strategy (strategy 1). However, the system COP difference between these four control strategies with their respective optimal control parameters is not very significant. For the temperature difference control strategies, complex measurement and control instruments are required, and the maintenance costs of the cooling tower may increase due to it's frequently startup-shutdown. For easy operation, a HGCHP system without auto-control device can use the operating time control strategy (strategy 4) and appropriate operation time of the cooling tower can be chosen according to the characteristics of the building loads and local climatic conditions. 3.3.3. Comparison between experimental data and simulative results In order to validate the accuracy of the developed simulative model and computer calculation program, the comparison between experimental data and simulative results are carried out for the operation performance of the HGCHP system operated with different control strategies. The comparisons for the optimal control strategy (Strategy 3, No 2) are plotted in Fig. 8. The comparisons shown in Fig. 8 denote that the analytical results agree very well with the experimental data in system operation performance. The relative deviation in operation parameters of the HGCHP system operated with optimal control strategy are compared in Table 4. It is indicated that, for the HGCHP system operated with optimal control strategy, the relative deviation of the average operation parameters is less than 1.2%. The accuracy of the developed simulation model and computer program is high enough for engineering applications. 3.4. Case study of HGCHP systems 3.4.1. Heating and cooling loads of the sample building The sample building selected is an office estate located in

eastern China where the weather is hot in summer and cold in winter with extreme temperatures of 37
C and 10
C respectively. The total floor area of this building is 4,850 m2. The TMY weather data is used and the building loads are calculated on an hourly basis by the DEST, which is a convenient and reliable tool to calculate the air-conditioning load of buildings located in various climate regions. According to the calculation results, this sample building is a cooling-dominated one and the ratio between its annual accumulated cooling load and heating load is 1.59.

3.4.2. Design and simulation of HGCHP system At the beginning of the design, proper length of its GHE and the capacity of its cooling tower must be determined. The criterion for designing the GHE and cooling tower is that the peak Entering Fluid Temperature (EFT) to heat pump unit from the GHE must be lower than the required peak value, which is chosen according to the heat pump unit utilized in the system and this value is 32
C in this study. Another criterion for the HGCHP system design is the system cost which includes both the initial and the operating costs. By simulating with proposed model, the design method to satisfy the annual heating load of the cooling-dominated building just by the GHE and to satisfy its annual cooling load by the GHE cooperated with the CT is selected. The total length of the GHE can be chosen based on the annual heating load and the capacity of the CT can be sized according to the difference between cooling and heating load. After the HGCHP system is properly designed, the complex effect of various control strategies on the system performance also needs to be analyzed in detail. In order to determine which control strategy is better, the hourly running data of the HGCHP system operated with four investigated control strategies are simulated and compared. According to the average price of local energy and equipments, the initial and operating costs of the traditional GCHP system and the HGCHP system operated under four control strategies are simulated by our computer program. The calculation results are compared in Table 5. As shown, the lowest operating cost can be obtained when the HGCHP system running under control strategy 3, e.g. the CT is

Table 5 Cost comparisons between GCHP system and HGCHP system operated under four control strategies. Item

Annual operating time of CT(h) Highest temperature of water entering the heat pump(
C) Annual energy consumption (kWh) Annual operating cost ($) Borehole number of GHE Initial cost of GHE($) Initial cost of CT ($) Initial cost of heat pump and circulation pump ($) System initial cost ($) Sum of Initial cost and ten years operating cost ($)

HGCHP system

GCHP system

Control strategy 1

Control strategy 2

290 31.77 106794 6675 30 30000 312.5 25625 55938 122684

555 31.60 101385 6337

119303

Control strategy 3 1194 27.23 93577 5849

114423

Control strategy 4 830 31.49 111465 6967

125603

none 31.30 113034 7065 50 50000 none 75625 160247

Please cite this article in press as: M. Guo, et al., Research and development of the hybrid ground-coupled heat pump technology in China, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.08.021

M. Guo et al. / Renewable Energy xxx (2015) 1e12

11

Fig. 9. Operation performance of HGCHP system and GCHP system in ten years operating.

controlled based on the difference between the temperature of water entering the heat pump and the wet-bulb temperature of ambient air. This strategy activating the CT under the most advantageous weather condition can make full use of ambient air to cool the cooling water and reject the imbalanced heat effectively so that higher COP of the heat pump can be obtained and the degradation of system performance can be avoided in the long term running. Comparisons between optimal HGCHP system and the traditional GCHP system denote that the optimal HGCHP system can reduce 35% initial cost and 20.79% operating cost in the first year. Due to the performance degradation of the GCHP system year by year, the economical benefit of the HGCHP system is more obvious in long term running. The optimal HGCHP system can save 44.69% operating cost and 40.05% total cost compared with the common GCHP system in ten years operating. The comparison between HGCHP system and GCHP system on the temperature of borehole wall, and COP of system in ten years operating are shown in Fig. 9. As expected, for this cooling-dominated building, the borehole wall temperature of the GCHP system will increase and the performance of the whole system will experience obvious degradation over time. By contrary, the ground temperature around GHE in the HGCHP system is close to initial value, and accordingly, the COP of the heat pump remains high after ten years operation. Simulation results indicate that, a properly designed HGCHP system can achieve good performance of air-conditioning system in coolingdominated buildings.

4. Conclusions The multistage series connected ground heat exchange is the effective heat storage method for HGCHP system based on the high temperature difference heat storage. The presented method not only can resolve the heat storage of industrial waste heat or solar energy collector in summer, but also can obtain the higher temperature drop of circulating fluid inside GHE. By extracting heat from ground for heating, the system can maintain ground temperature field balance as well as waste heat storage and utilization. For region hot in summer and warm in winter, the HGCHP system supplemented with cooling tower can avoid heat accumulation in the ground around GHE, maintain the ground temperature field balance, and improve the system energy efficiency.

4.1. 1. HGCHP System with solar collector as supplemental heater (1) The heat transfer model of multistage series connected ground heat exchanger is established based on the 3D geometry model of traditional U pipe and heat accumulator. The optimal solution to objective function of three stage series connected GHE suitable for engineering applications is obtained:

b1 ¼ 0:357800;

b2 ¼ 0:332749;

b3 ¼ 0:309451

(2) The heat storage with high temperature difference should take the endurance capacity of GHE into account. For the GHE made of traditional PE pipe, the operation temperature must be lower than 80
C, so the industrial waste heat stored should in lower temperature. The heat storage with higher temperature needs further research. (3) During period of heat storage (extraction), the temperature around boreholes of GHE will experience increase (decrease) continuously with the accumulation of operation time, especially for the first level of borehole group entering the circulating fluid, the temperature of ground around GHE is highest (lowest), and the heat exchange efficiency is lowest. For improving this situation, the flow direction of circulating fluid can be changed periodically to uniform the heat exchange efficiency of different level borehole group and to improve the total heat exchange intensity of GHE.

4.2. 2. HGCHP System with cooling tower as supplemental heat rejecter (1) For the HGCHP system test rig operated with the four control strategies, the wet bulb temperature difference control strategy, for which the cooling tower is on when the difference between the entering fluid temperature of the heat pump and the wet bulb temperature of ambient air exceeds 10
C, and the cooling tower is off when this temperature difference is lower than 9
C is the optimal control strategy for obtaining the highest system COP. The HGCHP system

Please cite this article in press as: M. Guo, et al., Research and development of the hybrid ground-coupled heat pump technology in China, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.08.021

12

M. Guo et al. / Renewable Energy xxx (2015) 1e12

operated with this optimal control strategy has the average system COP of 3.14. (2) The accuracy of the simulation model and computer program developed for the HGCHP system in this study has been verified by experimental results. For the HGCHP system operated with the optimal control strategy, the relative deviation of the average operation parameters between the experimental data and simulative results is less than 1.2%. It is accurate enough to utilize the simulation model and computer program for engineering applications. (3) The advantage for using the HGCHP system compared with the GCHP system is obvious for cooling-dominated buildings. Comparisons between optimal HGCHP system and the traditional GCHP system denote that the optimal HGCHP system can reduce 35% initial cost and 20.79% operating cost in the first year. Due to the performance degradation of the GCHP system year by year, the disparity between these two systems' performance increases with the operation time period.

[12]

[13]

[14]

[15]

[16]

[17]

[18]

[19]

References [1] Ground source heat pump network, The development of the ground source heat pump industry year growth speed will not less than 20% in future, http:// www.dyrbw.com/Details.aspx?mid¼12&cid¼12&id¼49197. [2] PRC Central Government Website, Instrument of the National Energy Bureau, Ministry of Finance, Ministry of Land and Resources, Ministry of housing and urban rural development on promotion of the development of geothermal energy utilization, http://www.gov.cn/zwgk/2013-02/07/content_2329361. htm. [3] Yi Man, Hongxing Yang, Jinggang Wang, Study on hybrid ground-coupled heat pump system for air-conditioning in hot-weather areas like Hong Kong, Appl. Energy 87 (2010) 2826e2833. [4] Yi Man, Hongxing Yang, Jeffrey D. Spitler, Zhaohong Fang, Feasibility study on novel hybrid ground coupled heat pump system with nocturnal cooling radiators for cooling load dominated buildings, Appl. Energy 88 (2011) 4160e4171. [5] Hui Guo, Zuming Chen, Chaohe Chen, Lixing Ding, Optimal match of ground heat exchanger in hybrid ground coupled heat pump system, Acad. J. Railw. Sci. Eng. 4 (2009) 64e69. [6] Ping Cui, Hongxing Yang, Jeffrey D. Spitler, Zhaohong Fang, Simulation of hybrid ground-coupled heat pump with domestic hot water heating systems using HVACSIMþ, Energy Build. 40 (2008) 1731e1736. [7] Wei Wu, Baolong Wang, Tian You, Wenxing Shi, Xianting Li, A potential solution for thermal imbalance of ground source heat pump systems in cold regions: ground source absorption heat pump, Renew. Energy 59 (2013) 39e48. [8] Jing Yang, Linghong Xu, Pingfang Hu, Na Zhu, Xuepeng Chen, Study on intermittent operation strategies of a hybrid ground-source heat pump system with double-cooling towers for hotel buildings, Energy Build. 76 (2014) 506e512. [9] Rui Fan, Yan Gao, Li Hua, Xiaoxi Deng, Jie Shi, Thermal performance and operation strategy optimization for a practical hybrid ground-source heatpump system, Energy Build. 78 (2014) 238e247. [10] Wenjie Gang, Jinbo Wang, Shengwei Wang, Performance analysis of hybrid ground source heat pump systems based on ANN predictive control, Appl. Energy 136 (2014) 1138e1144. [11] Wei Bo Yang, Ming Heng Shi, Hua Dong, Numerical simulation of the

[20] [21] [22]

[23]

[24] [25]

[26] [27]

[28]

[29] [30]

[31]

[32] [33]

performance of a solar-earth source heat pump system, Appl. Therm. Eng. 26 (2006) 2367e2376. Zongwei Han, Maoyu Zheng, Fanhong Kong, Fang Wang, Zhongjian Li, Tian Bai, Numerical simulation of solar assisted ground-source heat pump heating system with latent heat energy storage in severely cold area, Appl. Therm. Eng. 28 (2008) 1427e1436. Enyu Wang, Alan S. Fung, Chengying Qi, Wey H. Leong, Performance prediction of a hybrid solar ground-source heat pump system, Energy Build. 47 (2012) 600e611. Lanhua Dai, Sufen Li, Lin DuanMu, Xiangli Li, Yan Shang, Ming Dong, Experimental performance analysis of a solar assisted ground source heat pump system under different heating operation modes, Appl. Therm. Eng. 75 (2015) 325e333. Min Li, A.C.K. Lai, New temperature response functions (G functions) for pile and borehole ground heat exchangers based on composite-medium linesource theory, Energy 38 (2012) 255e263. Min Li, A.C.K. Lai, Analytical Model for short-time responses of borehole ground heat exchangers: Model development and validation, Appl. Energy 104 (2013) 510e516. Yi Yang, Min Li, Short-time performance of composite-medium line-source model for predicting responses of ground heat exchangers with single Ushaped tube, Int. J. Therm. Sci. 82 (2014) 130e137. Min Li, P. Li, V. Chan, A.C.K. Lai, Full-scale temperature response function (Gfunction) for heat transfer by borehole ground heat exchangers (GHEs) from sub-hour to decades, Appl. Energy 136 (2014) 197e205. Min Li, A.C.K. Lai, Thermodynamic optimization of ground heat exchangers with single U-tube by entropy generation minimization method, Energy Convers. Manag. 65 (2013) 133e139. Yanshun Yu, Zuiliang Ma, Simulation research on the thermal storage and release process of ground, Sol. Energy Acad. J. 26 (4) (2005) 487e492. Qing Gao, Ming Li, Yan Jiang, Analysis on underground storage and heat transfer efficiency of energy, Therm. Sci. Technol. 5 (3) (2006) 216e221. Nairen Diao, Yaozhong Zhang, Chengmiao Gao, Weiqun Shi, Ping Cui, Analysis of extra cold utilization of ground coupled heat pump system, Acad. J. Shandong Jianzhu Univ. 02 (2012) 133e136. Min Li, A.C.K. Lai, Analytical solution to heat conduction in finite hollow composite cylinders with a general boundary condition, Int, J. Heat. Mass Transf. 60 (2013) 549e556. Nairen Diao, Zhaohong Fang, Ground Coupled Heat Pump Technology, Higher Education Press, 2006, p. 10. Zhaohong Fang, Rairen Diao, Hongxing Yang, Wenke Zhang, Combined conduction and advection heat transfer around a buried coil, in: Proceedings of the 8thInternational Symposium on Heat Transfer, ISHT8-07-09, Beijing, 2012. Zhaohong Fang, Research and Utilization of ground source heat pump technology, Acad. J. Shandong Jianzhu Univ. 27 (10) (2012) 8e11. Nairen Diao, Ping Cui, Zhaohong Fang, Heat transfer analysis method of vertical borehole ground heat exchanger, Ground Source Heat. Pump. 5 (2012) 5e10. Ping Cui, Nairen Diao, Fangfang Zhang, Zhaohong Fang, Design principle and simulation software of vertical borehole ground heat exchanger, Ground Source Heat. Pump. 6 (2012) 5e13. Nairen Diao, Ping Cui, Zhaohong Fang, Design and optimization of vertical borehole ground heat exchanger, Ground Source Heat. Pump. 8 (2012) 5e12. Zhaohong Fang, Nairen Diao, Ping Cui, Heat transfer analysis of spiral pipe buried in medium with uniform seepage flow, Ground Source Heat. Pump. 9 (2012) 6e10. Heyi Zeng, Nairen Diao, Zhaohong Fang, A finite line-source model for boreholes in geothermal heat exchangers, Heat Transfer Asian Res. 31 (7) (2002) 558e567. Yi Man, Hongxing Yang, Zhaohong Fang, Study on hybrid ground-coupled heat pump systems, Energy Build. 40 (2008) 2028e2036. Heyi Zeng, Nairen Diao, Zhaohong Fang, Heat transfer analysis of boreholes in vertical ground heat exchangers, Int. J. Heat Mass Transf. 46 (23) (2003) 4467e4481.

Please cite this article in press as: M. Guo, et al., Research and development of the hybrid ground-coupled heat pump technology in China, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.08.021