Simulation of hybrid ground-coupled heat pump with domestic hot water heating systems using HVACSIM+

Energy and Buildings 40 (2008) 1731–1736

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Simulation of hybrid ground-coupled heat pump with domestic hot water heating systems using HVACSIM+ Ping Cui a,*, Hongxing Yang a, Jeffrey D. Spitler b, Zhaohong Fang c a

Department of Building Services Engineering, The Hong Kong Polytechnic University, Hong Kong, China School of Mechanical Engineering, Oklahoma State University, USA c Ground Source Heat Pump Research Center, Shandong University of Architecture and Engineering, Jinan, China b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 22 December 2007 Accepted 1 March 2008

A hybrid ground-coupled heat pump (HGCHP) with domestic hot water (DHW) supply system has been proposed in this paper for space cooling/heating and DHW supply for residential buildings in hot-climate areas. A simulation model for this hybrid system is established within the HVACSIM+ environment. A sample system, applied for a small residential apartment located in Hong Kong, is hourly simulated in a typical meteorological year. The conventional GCHP system and an electric heater for DHW supply are also modeled and simulated on an hourly basis within the HVACSIM+ for comparison purpose. The results obtained from this case study show that the HGCHP system can effectively alleviate the imbalanced loads of the ground heat exchanger (GHE) and can offer almost 95% DHW demand. The energy saving for DHW heating is about 70% compared with an electric heater. This proposed scheme, i.e. the HGCHP with DHW supply, is suitable to residential buildings in hot-climate areas, such as in Hong Kong. ß 2008 Elsevier B.V. All rights reserved.

Keywords: Hybrid ground-coupled heat pump Domestic hot water Ground heat exchanger Thermal storage water tank

1. Introduction Air conditioning (A/C) and hot water supply contribute to a significant percentage of the total energy consumption in Hong Kong. According to a report of the ‘‘Hong Kong Energy End-use Data’’ conducted by the Electrical and Mechanical Services Department [1] of the Hong Kong SAR Government, air conditioning plays a dominant role in energy consumption in Hong Kong, accounting for 28% and 22% of the total energy end-uses respectively in commercial and residential buildings in 2004. In addition, about 23% of the total energy end-use in residential buildings was consumed to heat domestic hot water (DHW) in 2004. The growing energy shortage has now become a worldwide crisis. Consequently, more efforts should be made to reduce the considerable energy consumption for air conditioning and hot water heating. Ground-coupled heat pump (GCHP) systems, which use renewable energy stored in the ground to offer space cooling and heating, are known for energy conservation and environmental friendliness. However, higher capital cost of excessively larger ground heat exchanger (GHE) or limited land area has restricted to a large extent the wider applications of this technology in cooling-dominated buildings, which reject more * Corresponding author. Tel.: +852 2766 7801; fax: +852 2774 6146. E-mail addresses: PingCuia*[email protected]">PingCuia*[email protected] (), [email protected] (H. Yang), [email protected] (J.D. Spitler), [email protected] (Z. Fang). 0378-7788/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2008.03.001

heat to the ground than that extracted from the ground on an annual basis in hot-climate areas. In recent years, some concerns have been raised about application of hybrid ground-coupled heat pump (HGCHP) systems with a supplemental heat rejecter in cooling-dominated buildings [2–4]. Incorporating a supplemental heat rejecter can reduce a fair amount of heat rejected into the ground and then effectively balance the ground thermal loads, which can consequently reduce the capital cost of the system and improve the operation performance. Another economical and practical way to reduce the high capital cost of the GCHP system is to preheat a portion of DHW using the excess condensation heat through addition of a desuperheater to the heat pump unit [5]. The desuperheater is a small, auxiliary heat exchanger that uses superheated gas from the compressor to heat water. In summer, when the HGCHP system is in the cooling mode, the desuperheater uses excess heat that would otherwise be expelled to the ground to heat domestic water virtually for free. In winter, more heat can be extracted from the ground to simultaneously provide space heating and DHW heating. Kavanaugh [6] reported a similar project of the GCHP system with a desuperheater and concluded that the cost savings were very considerable based on the utility bill. Actually, there are a great number of factors that affect the HGCHP performance, such as the heat pump capacity, various profiles of DHW usage, the continuously changing environmental conditions, the building loads and the long time-scale heat transfer of the GHE. Therefore, it is necessary to use an hourly simulation

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As is shown in Fig. 1, this system consists of a water-to-water heat pump equipped with a desuperheater, a GHE, a thermal storage water tank and water pumps. The cooling and heating capacities of the heat pump unit are 4.5 and 4.9 kW, respectively. The thermal tank, which is installed with a supplemental resistance heating element, is used to store DHW. The volume of the thermal tank is 120 l. According to ASHRAE handbooks and the local DHW usage profile, the average daily DHW demand per person for a typical residential apartment is about 60 l. Hence, generally speaking, the sample system with DHW heating can basically meet the requirements of space cooling, heating and DHW demand for a family of two persons. The borehole field of the GHE in the system consists of two vertical and two inclined boreholes with a tilted angle of 208 arranged in a rectangular configuration. The distance between two adjacent boreholes on the ground surface is 4 m. Each borehole has the diameter of 110 mm and the depth of 30 m. The underground temperature is relatively constant at 22 8C in Hong Kong. 3. Simulation model The major components including the heat pump (with desuperheater), GHE, thermal storage tank and pumps are modeled individually in the simulation system. Fig. 1. Schematic diagram of the HGCHP with DHW heating system.

3.1. Heat pump model model to analyze the operation performance of this hybrid system for a given building under given weather conditions. The main objective of this study is to develop an hour-by-hour simulation model for the complex system including all major components in the GCHP and DHW heating systems within a component-based modeling environment (HVACSIM+). The annual power consumption of the whole system will be calculated and meaningful comparisons with a conventional GCHP and electric hot water heater will be made.

A simple equation-fit model of the heat pump unit with a desuperheater can be developed using the experimental data. The following assumptions are made prior to fitting the model equations. The water flow rates through the heat pump unit are assumed constant since the thermophysical changes of pure water with temperature are relatively insignificant and neglected in the simulation model. The chilled water supply temperature is assumed to be constant at 7 8C. Based on the aforementioned assumptions and experimental data, the equation-fit model can determine the heat pump power consumption, load side heat transfer rate (Qload) and COP, which are a function of heat pump entering water temperature (EWT) from the GHE and entering DHW temperature (EDHWT) from the thermal tank. All the variables fitted in the heat pump model have the identical expression of equations. Taking the example of the heat pump power consumption, the equation fitted is given as follows:

2. System description The studied system may operate in four different modes, i.e., cooling with DHW heating, cooling only, heating with DHW and heating only. However, to date, few manufacturers can provide the comprehensive performance data of a heat pump unit with a desuperheater for all the operation modes. A small water-to-water heat pump equipped with a desuperheater was developed by a manufacturer according to our research requirements. Experiments for all the operation modes were undertaken. This small prototype system including the GCHP and DHW heating, which is basically complied with the capacity of the heat pump unit, was developed for the purpose of space heating, space cooling and hot water heating, as shown in Fig. 1. Based on the capacity of the heat pump unit, a sample apartment of an area of 30 m2 in Hong Kong is used for simulation purpose.

Power ¼ a1 þ a2 EWT þ a3 EWT2 þ a4 EDHWT þ a5 EDWT2 þ a6 EWT  EDHWT

(1)

where a1–a6 are coefficients fitted using the experimental data, which are listed in Table 1.

Table 1 Summary of the identified coefficients in the fitted equations Modes

Variables

Cooling with DHW

COP Power Qload

Heating with DHW

COP Power Qload

Cooling Heating

COP COP

a1 7.4378 0.47911 4.9478

a2

a3

a4

a5

a6

Relative error (%)

0.14049 0.03131 0.02163

1.842e-3 1.39e-4 6.14e-5

0.03521 4.06e-4 5.026e-3

5.83e-4 5.22e-6 8.8e-5

1.02e-3 9.46e-7 2.7e-5

7.6 0.46 1.42

4.48e-4 2.3e-4 3.63e-4

0.01188 0.01 0.14183

1.08e-4 6.26e-5 1.21e-3

.00044 1.51e-4 3.55e-4

1.16 1.51 3.2

0 0

1.82 0.82

2.4967 1.5151 0.671

0.051978 0.0111 0.07927

6.8442 1.9465

0.1742 0.034646

0.001402 0.000045

0 0

0 0

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A correction factor, which is defined as the ratio of the actual cooling/heating loads of the building to the heat transfer rate of the fitted load, can be used to offset the deviation of the fitted values from the actual ones. f ¼

Q act;load Q fit;load

(2)

The heat pump power consumption fitted from experimental data should be corrected by multiplying the correction factor. Considering the free DHW output from the system, the conventional expression of the COP can be revised accordingly: COP ¼

Q act;load þ Q DHW Power

(3)

Therefore, the DHW side heat transfer rate (QDHW) can be obtained using Eq. (3). The source side heat transfer rate (Qsource) can be consequently obtained using energy balance. 3.2. GHE model The GHE simulation model is a complex mathematical problem, which should be treaded as a three-dimensional and transient heat transfer process. The heat transfer process around a borehole may usually be analyzed in two separate regions: one is the solid soil/ rock region outside the borehole, where heat conduction has to be treated as a transient process, and another is the region inside the borehole, including the grout, the U-tube pipes and the circulating fluid inside the pipes. In the first region, the long time-step and short time-step approaches are incorporated into the simulation model. An explicit analytical solution of the finite line source in semi-finite medium based on the Eskilson’s numerical approach [7] is employed for convenient calculation of the thermal resistance outside the borehole for long time steps [8]. Following the definition in Eskilson’s approach, the temperature rise on the wall of a single borehole caused by a heat pulse can be calculated as follows:   q at r b Tb  T0 ¼ l g (4) ; 2 2kp H H where T0 means the initial temperature of the soil, i.e. the annual mean temperature of the soil; k and a denote the thermal conductivity and thermal diffusivity of the soil respectively; H and rb are the borehole length and radius; ql is the heating rate per borehole length. The so called g-function for a single borehole represents the non-dimensional borehole wall temperature response:

g



at r ; H2 H



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It should be noticed that the finite line source model is not applicable to simulating the GHEs in short time intervals (usually less than a few hours) because of the assumption of line source. A short time-steps model based on the finite element numerical approach is developed as an extension of the long time-step model and is utilized in this simulation [9]. The main objective of the thermal analysis for the second region is to determine the temperature of the heat carrier fluid in the Utubes. A quasi-three-dimensional model has been developed with the consideration of the fluid temperature variation along the borehole depth and the thermal interference of the up and down Utubes. The resistance inside the borehole can also be found accounting for the thermal interference between the two legs of the U-tube [10]:   H 1 1 Rb ¼  (7) MC e 2 where M and C are the mass flow rate and specific heat of the circulating fluid; e is derived from the outlet and inlet temperatures, which is referred as efficiency of the borehole. The detailed expressions of the model have been discussed by Zeng et al. [10]. The entire GHE model also uses the techniques of spatial superimposition for sequential temporal superimposition for arbitrary heating/cooling loads of the systems as proposed by Eskilson [7]. 3.3. Thermal storage tank model The following assumptions are made in the thermal storage tank model.  The water temperature in the tank is assumed to well-mixed and equal to the outlet water temperature;  The radiation heat loss on the tank surface is neglected. The governing equation for the tank model is given: rVcp

dT ˙ p ðT in  TÞ ¼ UAðT ambient  TÞ þ mc dt

(8)

where r and cp denote the water density and specific heat capacity at temperature T; V and m˙ are the tank volume and water mass flow rate through the tank; UA is the overall heat transfer coefficient of the tank (5 W/K); Tambient means the ambient air temperature. To simplify the simulation process, the daily DHW usage time is set to be within 9:00–10:00 p.m. for shower and the thermal tank is

2pk½Tðr; 0:5H; tÞ  T 0  ql 8 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffi9 Z 1> = h 1 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼  d > 2 0 > 2 2 : ; H ðr=HÞ2 þ ½0:5  ðh=HÞ ðr=HÞ2 þ ½0:5 þ ðh=HÞ ¼

(5)

refilled with make-up water at ambient temperature before 10 p.m. This means the HGCHP system will stop heating the DHW from 9 to 10 p.m. no matter whether the DHW temperature achieves the set point or not. The resistance heating element can automatically operate when a thermostat detects the DHW supply temperature is still below the set value (50 8C) until 9 p.m.

Once the temperature distribution of a single borehole is determined, it can be superimposed in space to obtain the temperature distribution of the whole borehole field. For a GHE of m boreholes the representative temperature rise on the wall of a borehole concerned can then be calculated accordingly. "  #  m1 X  at r i  ql at r b g þ (6) Tb  T0 ¼ ; g ; 2pk H2 H H2 H i¼1

3.4. Circulation water pump model

where ri is the distance between the ith borehole and the borehole concerned.

A simplified simulation model for water pumps is employed because of their constant water flow rate and relatively small

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Fig. 3. Annual ambient air temperatures in the TMY in Hong Kong. Fig. 2. Annual building loads in the TMY in Hong Kong.

power consumption compared to that of the heat pump. ˙ p mD Power ¼ rh

(9)

where, Dp and h denote the pressure increase and the efficiency of the water pump. 3.5. Building loads and ambient air temperature The annual hourly loads of the sample building in the typical meteorological year (TMY) of 1989, shown in Fig. 2, are calculated using the building energy simulation software, the HTB2 [11]. Fig. 3 illustrates the average hourly ambient air temperature during the TMY. 3.6. Simulation of hybrid GCHP with DHW using HVACSIM+ The system is constructed in the HVACSIM+ modeling environment, which stands for ‘HVAC Simulation Plus other systems’. It was initially developed by the National Institute of

Standards and Technology (NIST) [12] and updated to a visual interface version using an event driven approach [13]. It is capable of modeling HVAC systems, HVAC controls, buildings, energy management systems and other thermal systems. The HVACSIM+ represents HVAC elements as individual component, such as fans, pumps, pipes, etc., connected to form a complete system, which allows users to develop new models and introduce them in the package to simulate them in various configurations. A number of modular components based ground source heat pump systems have been developed by a research group in U.S.A. [14,4,15]. The entire system is configured in the visual modeling environment (HVACSIM+), as shown in Fig. 4. All the components in the system are represented as icons and pictures. The boundary parameters include the cooling/heating loads, ambient air temperature, and water flow rates in the GHE and DHW loops. The input variables of each component are connected either to the output variables of other components or to specified boundary conditions according to the system operation principle.

Fig. 4. System configuration in the visual HVACSIM+ environment.

P. Cui et al. / Energy and Buildings 40 (2008) 1731–1736

Fig. 5. Hourly variations of EWT and ExWT for the HGCHP system.

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Fig. 7. Annual power consumption of the HGCHP.

3.7. Simulation of GCHP system and electric heater using HVACSIM+ To better investigate the operation performance of the HGCHP with DHW heating system and to evaluate the impact of adding a desuperheater on the GHE, a conventional GCHP system and an electric heater which is very commonly used for DHW heating in Hong Kong are also constructed and simulated in HVACSIM+. It is referred to a base case compared to the HGCHP system. 4. Simulation results and discussions The annual simulations of the HGCHP system and the base case are conducted under the HVACSIM+ environment. Some critical performance parameters are obtained and plotted in Figs. 5–11. Figs. 5 and 6 illustrate the variations of hourly heat pump entering and exiting water temperatures (EWT/ExWT) from/to the GHE, respectively, in one simulation year. The maximum peak EWT of the HGCHP system is about 37 8C, whereas the EWT of the base case reaches a peak of 52 8C. As can be seen from the curves, the EWT for the HGCHP system almost returns back to its initial value after the first year of heating/cooling circle. By contrast, a temperature increase of 3 8C for the base case is observed after one year operation. This demonstrates that, for the HGCHP system, the effect of the desuperheater on alleviating the thermal imbalance of the GHE is quite significant.

Figs. 7 and 8 present the energy consumptions of the HGCHP system and the conventional GCHP system, which only take into account the power consumed by the heat pump and GHE water pump. A comparison of Figs. 7 and 8 shows that the power consumption of the HGCHP system is significantly lower than that of the base case in cooling mode due to the lower EWT of the

Fig. 6. Hourly variations of EWT and ExWT for the GCHP system (base case).

Fig. 9. Supplemental electricity consumption for DHW heating in HGCHP system.

Fig. 8. Annual power consumption of the GCHP system (base case).

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strongly influenced by the ambient air temperature, an annual hourly simulation for the hybrid system has been performed for a small residential apartment located in Hong Kong. To compare the HGCHP system performance with conventional systems, a GCHP system for space cooling/heating and an electric heater for DHW supply are also modeled and simulated on an hourly basis within the HVACSIM+. Some specific conclusions of this case study are described as follows: 1. The HGCHP system can effectively alleviate the imbalanced loads of the GHE which exhibits lower EWT to the heat pump than that from the GCHP system. 2. The general energy saving of the HGCHP system for space cooling/heating is about 3.4% compared to the GCHP system for the first running year. 3. The HGCHP can offer almost 95% of total DHW demand in this case study along with about 70% energy saving compared to the electric heater. Fig. 10. Electrical heater power consumption for DHW heating (base case).

An analysis of the simulation data shows that the HGCHP with DHW supply system has significantly better performance relative to the conventional systems, which is thus a good choice to apply the HGCHP with DHW supply in residential buildings in hotclimate areas, such as in Hong Kong. Acknowledgement The research is funded by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (project no. PolyU 5302/06E) Reference

Fig. 11. DHW supply temperature to the thermal water tank.

HGCHP system. However, for the heating mode, the former is obviously higher than the latter because of higher heating requirements relative to the base case. As a whole, the annual total energy consumption of the HGCHP system for space cooling and heating shows a slight reduction of 3.4% compared to the base case. Fig. 9 shows the hourly supplemental electricity consumed in the thermal tank for the HGCHP system during the simulation year. The annual total power consumption of the supplemental electric heater is only 90 kWh. In addition, the circulation DHW pump power consumption is found to be 328 kWh for the whole year. For the electric heater, the accumulative power consumption for the DHW heating is about 1350 kWh, as shown in Fig. 10. Therefore, the total energy consumption of the HGCHP system for DHW heating is significantly reduced by 70% relative to the electric heater during the first year of operation. Fig. 11 presents the hourly DHW supply temperature to the thermal tank, which demonstrates the HGCHP system can offer almost 95% of the DHW demand for two persons all the year round. 5. Conclusions A simulation model of the HGCHP with DHW heating system has been developed within the HVACSIM+ environment. Since the DHW usage profile has a remarkably hourly fluctuation and is

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