Scholarly article on topic 'Volume design of the heat storage tank of solar assisted water-source heat pump space heating system'

Volume design of the heat storage tank of solar assisted water-source heat pump space heating system Academic research paper on "Civil engineering"

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{"Solar assisted water-source heat pump" / "space heating" / "heat storage tank" / "system simulation"}

Abstract of research paper on Civil engineering, author of scientific article — Xiaodong Guo, Haiwen Shu, Jin Gao, Fei Xu, Chen Cheng, et al.

Abstract The paper presents a method to design the volume of the heat storage tank of the solar assisted water-source heat pump space heating (SAWHPSH) system. With the area of the solar thermal collector calculated according to the China national design code for solar heating system, the space heating load and the solar radiation are considered simultaneously in this method. The design method of the volume of the solar heat storage tank is elaborated afterwards. In order to show the advantage of this design method, TRNSYS software was used to simulate the same SAWHPSH case project with the volume of the heat storage tank designed by both the method brought out in the paper and the one described in the China national design code. Performances of the system designed by the two different methods are investigated and the results show that the SAWHPSH system with the heat storage tank volume designed by the method presented in the paper has a higher average system COP.

Academic research paper on topic "Volume design of the heat storage tank of solar assisted water-source heat pump space heating system"

I Available online at www.sciencedirect.com I #

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scienceDirecr Engineering

Procedia Engineering 205 (2017) 2691-2697

www.elsevier.com/locate/procedia

10th International Symposium on Heating, Ventilation and Air Conditioning, ISHVAC2017, 1922 October 2017, Jinan, China

Volume design of the heat storage tank of solar assisted water-source heat pump space heating system

Xiaodong Guoa, Haiwen Shua*, JinGaob, Fei Xua, Chen Chenga, Zeyuan Suna, Dongbin Xua

aSchool of Civil Engineering, Dalian University of Technology, Dalian, Liaoning Province 116024, China bCivil and Architecture Design Institute of Dalian University of Technology, Dalian, Liaoning Province 116024, China

Abstract

The paper presents a method to design the volume of the heat storage tank of the solar assisted water-source heat pump space heating (SAWHPSH) system. With the area of the solar thermal collector calculated according to the China national design code for solar heating system, the space heating load and the solar radiation are considered simultaneously in this method. The design method of the volume of the solar heat storage tank is elaborated afterwards. In order to show the advantage of this design method, TRNSYS software was used to simulate the same SAWHPSH case project with the volume of the heat storage tank designed by both the method brought out in the paper and the one described in the China national design code. Performances of the system designed by the two different methods are investigated and the results show that the SAWHPSH system with the heat storage tank volume designed by the method presented in the paper has a higher average system COP.

© 2017 The Authors. Published by Elsevier Ltd.

Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Air Conditioning.

Keywords: Solar assisted water-source heat pump; space heating; heat storage tank; system simulation

1. Introduction

As the energy consumed by the buildings increases year by year, the utilization of clean and renewable energy is increasingly imperative. Because of the characteristics of high efficiency and environmental sustainability, Water-Source Heat Pump is widely used. However, in most northern cities in China, relative low water temperature in heating season leads to heating capacity reduction and performance deterioration. In most part of China, Solar Energy is

* Corresponding author.

E-mail address:shwshw313@sma.com

1877-7058 © 2017 The Authors. Published by Elsevier Ltd.

Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and

Air Conditioning.

10.1016/j.proeng.2017.10.196

abundant. So one solution to solve the problem above is adding a solar energy system to the heat pump system. So that the hot water heated by solar energy can be used to raise the temperature of water entering the evaporator of the heat pump or directly supply heating to the end users.

Many studies have focused on how the heat storage tank of solar assisted water-source heat pump system influences the performance of the combined system. J.W. Macarthur [1] analyzed the payback period of the tandem solar energy heat pump system and found that the volume of the tank and the area of the collector are the key factors. Y. Kuang [2] had an experimental study on solar assisted water-source heat pump system and found that the heat storage tank is an important component of the combined system. Y. Li [3] used TRNSYS to simulate a solar energy and air/water source heat pump combined heating system with double evaporators heat pump and double water tanks. The results showed that the use of double water tank made the system have a high solar fraction, and even in bad weather the solar fraction could reach 21.26%. L. Cheng [4] studied solar combined heat pump radiant floor heating system and found that for different regions and different buildings the best volume of the tank per collector area is different. With the volume of the heat storage tank increases, the efficiency of the system first rises and then falls. J.G. Cheng [5] studied solar assisted low temperature water source heat pump and concluded that when the collector area is 49% of the heating area and the volume of the tank is 100L per collector area, the system had the best performance and the solar fraction is up to 99.8%.

Heat storage tank is one of the important components in the SAWHPSH system. The volume of the tank directly relates to the heat collecting efficiency of solar collectors and the performance of the system. This paper presents a method to design the volume of the heat storage tank. Four typical days in heating period are chosen to analyze the space heating load and the solar radiation. Then the meteorological data of these days are used to calculate the volume of the heat storage tank with the area of the solar thermal collector calculated according to the China national design code for solar heating system. The same SAWHPSH case project designed by the meteorological data of four typical days are simulated to compare the performance of the SAWHPSH system. Then the same SAWHPSH case project with the heat storage tank volume designed by China national design code [6] is also compared. In this study COP is the decision parameter to analyze the performance of each system.

2. Method

The SAWHPSH system studied in the paper is showed in Figure 1. The time of the charging and discharging cycle of this combined system is one day. The system has four operation modes. In mode1 (M1), the solar hot water in the tank is directly used to heat the users. In mode2 (M2), the water in the tank is used to heat the source side liquid of the heat pump. In mode3 (M3), the water in the tank is used to heat the evaporator of the heat pump by directly flowing into it. In mode4 (M4), only the heat pump operates to heat the users. The operation mode of SAWHPSH system changes according to the average temperature of the tank which is showed in table 1. In the table, Ts is the heat supply temperature, Th is the highest temperature of the fluid that is allowed to enter the evaporator of the heat pump, and Tw is the temperature of the water source.

Fig. 1. Schematic diagram of SAWHPSH system

Table 1. Different operation modes corresponding with the average temperature of the tank

Average temperature of the tank Operation mode

T>Ts M1

Th<T<Ts M2

Tw<T<Th M3

T<Tw M4

The time of the charging and discharging cycle of this combined system is set as one day. In designing the volume of the heat storage tank, some typical days in the heating period will be selected and the meteorological data of these typical days are used to calculate the volume of the heat storage tank. Four typical design days were selected in the study.

• Day 1 (D1) is the day when its total solar radiation equals to the monthly average daily total solar radiation of the minimum radiation month;

• Day 2 (D2) is the day when its total solar radiation equals to the average daily total solar radiation during the heating period;

• Day 3 (D3) is the day when its average temperature equals to the average temperature during the coldest month;

• Day 4 (D4) is the day) when its average temperature equals to the average temperature during the heating period. If D3 or D4 is selected, the daily total solar radiation of the chosen typical day should not be lower than average

daily total solar radiation of the minimum irradiation month significantly. In case that the weather of the typical day is extremely bad, the meteorological data are inappropriate to be used to design the tank. If this happens, choose another day as the typical day or just use the monthly average daily total solar radiation of the minimum radiation month as the typical day's total radiation. Then Eq. (1) was used to calculate the volume of the storage tank.

V=Q (1)

where Qc is the total heat collected by the collector, kJ, which can be calculated by Eq. (2), and c is the specific heat of the water in the tank, kJ/(kg K); p is the density of the water in the tank, kg/m3; At is the utilization temperature difference, °C, which is the difference between the heat supply temperature and the average temperature of the heat pump's heat source during the heating period.

Qc = ARtVcd (1-n) (2)

where A is the area of the collector, m2; Rt is the total radiation on the plane of the solar collector per unit area, kJ/m2; ncd is the thermal efficiency of the collector, %; is the thermal lost rate of the tank and the pipeline, %.

3. Results

Take an office building in Dalian as an example. The heating area of this building is about 12,900m2 and the hourly heat load during the heating period is showed in Fig. 2. The design heating load of the building is 470kW and a seawater heat pump is selected as the heat source with the heating capacity of 498kW. The water supply/return temperatures are 45/40 C. The area of the collector is calculated to be 1,235m2 according to the national design code.

According to the method described above, the first step of calculating the volume of the heat storage tank is to choose the typical days. In the case project, Jan. 9, Jan. 25, Dec. 1 and Jan. 19 are selected as D1, D2, D3 and D4 respectively. The total solar radiation on the collector surface, effective solar heat collected and the total heat load of the building in typical days are shown in Fig. 3. The calculated volumes of the volumes of the heat storage tank under the four different typical days are in Table 2. The utilization temperature difference is calculated as Ts+3-Tw=45+3-5=43(C), where T'w is the average seawater temperature in the project and the temperature dead band is supposed to

be 3 °C. The highest temperature of the water in the tank is set as 60 °C so that the water in the heat storage tank can be used directly to heat the users.

-o 400

"TO 300

2001000

Jan. time

Fig. 2. The hourly load during the heat period

| Total radiation on the collector I Effective heat collected I Total heat load

Jan.9 Jan.25 Dec.1 Jan.19

Typical day

Fig.3. Total solar radiation on the collector surface, effective heat collected and the total heat load of the four typical days

Typical Day At c P Effective heat collected V

fC] [kJ/(kg-K)] [kg/m3] [kJ] [m3]

D1 3,168,508.63 18

D2 6,273,692.34 34

43 4.2 1000

D3 7,489,680.00 41

D4 6,019,098.93 33

The volume of the heat storage tank that is calculated to be 120m3according to the national design code. Then the same case of SAWHPSH with different heat storage tank volumes are simulated using TRNSYS software to analyze the performance of the systems. The simulation model of the SAWHPSH system in TRNSYS is shown in Fig.4, and the control strategy of the system is shown in Fig. 5.

Fig. 4. TRNSYS model of SAWHPSH system

Fig. 5. The flow chart of the control strategy of the SAWHPSH system

The average COP values of the system with different heat storage tank volumes during the heating period were calculated and shown in Table 3.

2696 Xiaodong Guo et al. /Procedia Engineering 205 (2017) 2691—2697

Table 3. The average COP of the SAWHPSH during the heating period

Category Average COP of the System during the Heating Period Improving Rate of COP (Compared to Design Code)

D1 2.86 34.91%

D2 2.91 37.39%

D3 2.67 25.74%

D4 2.87 35.59%

Design Code 2.12 0

4. Discussion

According to Table 2, among the volumes calculated by the meteorological data of different typical days, the maximum volume is 41m3(D3), and the minimum is 18m3(D1). However, all the four calculated volumes of the heat storage tank are much smaller than the one designed by the method in the China national design code. According to Table 3, among the COP values calculated according to the four typical days, the maximum value is 2.91(D2), and the minimum value is 2.67(D3). But they are all higher than the system with a tank volume designed by the method in the China national design code. The study shows that not only the volume of the heat storage tank can be reduced significantly using the method presented in the paper but also the energy efficiency of the SAWHPSH system can be improved as well. However, as to which typical day can provide the best result and its reason still needs further research.

5. Conclusions

The authors present a method to design the volume of the tank of the SAWHPSH system. The method requires to choose a typical day in the heating period in order to use the meteorological data of the typical day to calculate the volume of the tank. Then the method is compared with the one brought out in the China national design code by simulating the same SAWHPSH case project designed by two different methods using TRNSYS software. The results show:

• The volume of the solar heat storage tank has a significant effect on the performance of the SAWHPSH system. Much research effort needs to be made to find the best method to design the volume of the tank in order to improve the performance of the SAWHPSH system.

• The design method of the volume of solar heat storage tank presented in the paper has much better performance than the one in the China national design code and the highest energy saving rate reaches 37.39% in the case project.

Acknowledgements

This work is funded by the China national key research and development program- Intergovernmental cooperation in science and technology innovation (2016YFE0114500) and the Science and Technology Project of Dalian urban and Rural Construction Committee.

References

[1] J.W. Macarthur. Performance analysis and cost optimization of a solar assisted heat pump system. Solar Energy. 21(1) (1978) 1-9.

[2] Y. Kuang, R. Wang, L. Yu. Experimental study on solar assisted heat pump system for heat supply. Energy Covers Manage, 44(7)(2003) 10891098.

[3] Y. Li, L Feng, Y. Yuan. The application of solar energy and air/water source heat pump combined heating system in the railway station plateau cold area. Refrigeration and Air Conditioning, 30(20) (2016) 225-228.

[4] L. Cheng. Research of characteristics and optimization of solar heat pump radiant floor heating system. Taiyuan University of Technology, 2011, Shanxi

[5] J. Cheng. A study for the application of solar energy and water source heat pump in Sitang. Southwest Jiaotong University, 2005, Chengdu

[6] Ministry of Housing and Urban-Rural Development of the Republic of China, GB 50495-2009 Technical code for solar heating system. China Architecture & Building Press, 2009, Beijing