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Annual dynamic thermal performance of solar water heaters: A case study in China's Jiangsu Province
Accepted Manuscript
Annual Dynamic Thermal Performance of Solar Water Heaters:A Case Study in China’s Jiangsu Province Juan Shi , KaiWei Lin , ZhenQian Chen , Hao Shi PII: DOI: Reference:
S0378-7788(17)32980-8 10.1016/j.enbuild.2018.04.048 ENB 8521
To appear in:
Energy & Buildings
Received date: Revised date: Accepted date:
27 November 2017 16 March 2018 23 April 2018
Please cite this article as: Juan Shi , KaiWei Lin , ZhenQian Chen , Hao Shi , Annual Dynamic Thermal Performance of Solar Water Heaters:A Case Study in China’s Jiangsu Province, Energy & Buildings (2018), doi: 10.1016/j.enbuild.2018.04.048
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Annual Dynamic Thermal Performance of Solar Water Heaters: A Case Study in China’s Jiangsu Province Juan Shi 1, KaiWei Lin 1, ZhenQian Chen*1,2, Hao Shi 3 1. School of Energy and Environment, Southeast University, Nanjing 210096, China 2. Jiangsu Provincial Key Laboratory of Solar Energy Technology, Nanjing 210096, China
*
Corresponding email: [email protected]
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3. MCC Huatian Engineering & Technology Corporation, Nanjing 210019, China Tel/Fax number:86-25-83790626
Abstract：In our search for a better socioeconomic path for water heater usage in China, we
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studied and compared the annual dynamic thermal performance of solar water heaters (SWHs) in the Jiangsu province. Experiments are conducted in two cities within Jiangsu—Nanjing and Lianyungang—spanning two climate zones. The results show that SWHs have good thermal performance in Jiangsu. The SWHs in Lianyungang show better thermal performance than in Nanjing, because they receive greater solar radiant energy. Also, from a thermal
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perspective, vacuum tube solar water heaters (VTSWHs) perform better throughout the year
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than flat-plate solar water heaters (FPSWHs). Meanwhile, taking socioeconomics into account, gas-assisted flat-plate solar water heaters (GA-VTSWHs) have better performance
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compared to other types of water heaters. Based on these results, the implications for developing and applying SWHs are derived. This study can be used to optimize operation of
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the household hot water supply system and use it more efficiently.
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Keywords：Solar water heater, thermal performance, China, utilization efficiency
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1. Introduction As socioeconomic development and living standards improve in China (especially the eastern part), hot water consumption steadily increases [1]. Projections show this will be an upward trend for some time, so to meet future demands, we must evaluate the availability of energy sources [2]. Currently, fossil fuels are the primary energy sources, but to avoid or
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diminish their adverse impact on the environment—and to find a more efficient way to heat water—it is important to identify alternative energy sources [3–4]. For example, Chow et al. argue that heating water with electricity or natural gas is inefficient, because the desired hot water temperature is usually under 60C [5]. For such applications, renewable energy is a viable alternative. It is estimated that renewable energy contributes to one-fourth of total
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global energy consumption. Such energy sources are a much ―cleaner‖ way to contribute toward the global energy demand, offering less pollution [6–7].
China has shown a longstanding interest in solar power with SWH development beginning in the 1950s, as SWHs have many advantages over conventional water heating.
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For example, SWHs emit negligible greenhouse gases, have a low payback period, and are easy to install [9–10]. The energy crisis in the 1970s accelerated the country’s investment in
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renewable energy. By the 1980s, the solar energy industry in China was on par with developed nations [11]. In the past three decades, China has promoted solar energy’s use. In
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1992, after the United Nations Conference on Environment and Development, the Chinese government took the lead in formulating the 21st Century Agenda of China and encouraged
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the solar energy industry’s expansion. With active support from the government, China's renewable energy consumption to total energy consumption increased from 7.2% in 2000 to
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11.4% in 2014. China had invested nearly US$90 billion in renewable energy in 2014. As of 2012, there are more than 3,000 SWH enterprises in China [12], making it one of the largest solar heater markets in the world. In China, both vacuum tube solar water heaters(VTSWHs) and flat plate solar water heaters (FPSWHs) are widely used. According to Tang et al. [13], VTSWHs account for 88% of the market share in China. The data indicates that China's VTSWHs occupy a large amount of the market of total SWHs in the world. The total installed SWHs in China are in excess of 2
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180,400 megawatts thermal (MWth) in 2012. In 2011, the installed capacity in China accounted for 84.0% of the world’s total capacity [14]—a significant increase from 57.2% in 2000. China is now a dominant SWH producer. Compared to the rest of the world—especially western countries—the share of the FPSWHs market was far greater than the VTSWHs in Japan, Germany, and Australia. What’s more, the share even reached up to 100%, because FPSWHTs offer good thermal efficiency, appearance, and stability. That said,
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in China domestic SWH research institutions and enterprises have promoted VTSWHs’ use, and VTSWHs offer various advantages such as a better heat-loss coefficient, antifreeze performance, and so on. Thus, VTSWHs are now mainstream on the SWH market.
In recent years, researchers conducted many experiments to study SWHs’ thermal
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performance. Li and Yang [15] introduced a type of assisted solar heating system, and made a comparison among SWHs, air-source heat pump water heaters (ASHP), and traditional water heaters. Tang et al. [16] used two VTSWHs systems with different tilt angles to study the influence of the tilt angle on the SWHs. They found that even though two systems absorbed
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different amounts of heat, the daily solar thermal conversion efficiency was almost equal. Chow et al. [17] surveyed the thermal performance of a single-phase open thermosyphon
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system and the two-phase closed thermosyphon system. The experimental and simulation study are both done with two solar water heaters. It turns out that the two-phase closed
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thermosyphon solar-collector costs less to run. Grant and Pradeep [18] introduced a new method for measuring the annual consumption of gas-assisted SWHs with an in-series gas
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booster. Different from the existing standard, the variation of inlet water temperature, flow rate, and environment’s temperature were considered for evaluating the energy consumption
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of gas-assisted SWH. To discover the factors affecting the SWHs’ performance, Zhang et al. [19] studied VTSWHs’ performance according to Chinese standards. In Europe, solar performance tests are based on EN 1295-2 [20] (the European standard
for solar thermal collector testing), to ensure the reliability of test experiments. For example, Dawit et al. [21] considered both open- and closed-loop solar water systems, using CFD simulation to study the influence of inter-temperature, the environment’s temperature, solar radiation density, and mass flow rate on SWHs’ efficiency. Then they proved the simulation results using the experimental results. Likewise, in China today the widespread use of SWHs 3
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is also largely dependent on government support and guidance. According to the Chinese Academy of Building Sciences, when conducting a comprehensive evaluation of green buildings, solar thermal energy has been given enough attention as an important part of renewable energy [22]. Furthermore, according to the DGJ32/J 71-2014 Standard of Engineering Construction in the Jiangsu province, buildings with fewer than six floors are required to install a solar hot water supply system [23].
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At present, experiments on SWHs’ thermal performance are mainly conducted using a standard condition instead of cloudy/rainy days. Hence, no ―real-life‖ systematic measurement and analysis exists for SWHs’ performance. That is why we are studying the annual dynamic thermal performance of SWHs in the Jiangsu province, and why we installed
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online experimental platforms for different kinds of SWHs in different climate zones. In this paper, we compared the dynamic thermal performance (such as the coefficient of thermal performance, solar fraction, or the solar energy protection ratio) of SWH systems. We also compare economic and environmental performance. This information can be used to optimize
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the household’s hot water supply system and increase efficiency in its use.
2.1 Experimental System
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2. Methodology
The Jiangsu province is located in the eastern part of China, with a relatively higher
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socioeconomic level. Thus, SWHs are prevalent in this area. As Figure 1(a) shows, Jiangsu is typically in hot summers & cold winters climate area [24]. According to Zhou’s survey [25],
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the overall solar energy resources in Jiangsu province are lower compared to western China, with average radiation at 4,200–5,400 megajoules (MJ)/(m2•a). As Figure 1(b) shows, solar
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energy resources in the cities of Lianyungang, Yancheng, and Suqian are relatively rich. The total radiation can reach above 5,000 MJ/(m2•a). However, the solar energy distribution in the cities of Nanjing, Changzhou, Wuxi, and Suzhou are less than the northern Jiangsu province. The average radiation is only 4,700 MJ/(m2•a).
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(a)The climate zone map for architectural design in China
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(b)Solar radiation density in Jiangsu Province
Figure 1. Experimental platform locations.
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To study the characteristic of SWHs in different climate areas with different solar radiation density, we chose Nanjing (the hot summer and cold winter climate area; 118°22′E– 119°14′E, 31°14′N–32°37′N) and Lianyungang (cold climate area; 118°24′E–119°48′E, 34°00′N–35°70′N) to set up a real-time experimental platform for household SWHs. In each
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city, VTSWHs and FPSWHs are installed to compare different SWHs’ thermal performance. The water temperature of the storage tank, the amount of solar radiation, and the dynamic
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characteristics of SWHs in different areas are analyzed. Figure 2 describes the experimental system. The setup contains the SWH, measurement,
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and data collection systems. The SWH system is comprised of a solar collector and water storage tank. The measurement system includes an irradiation instrument and thermocouple.
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The irradiation instrument is used to measure the solar radiation intensity. The K-type sheathed thermocouple is used to measure the water temperature in the SWH system, as well
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as the outdoor environment temperature. The experimental results are scanned by a data acquisition instrument and sent to a computer via I/O interface.
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Figure 2. The experimental system’s schematic.
Each experiment is conducted from 8:00 a.m. to 8:00 a.m. the next day. Normally, solar radiation is abundant to heat the water during the periods of 8:00 a.m. to 4:00 p.m. The
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solar-collector gains heat and delivers it to the storage tank. In the course from 8:00 p.m. to 8:00 a.m. next day, heat loss occurs in the water storage tank, which results in the water
2014 to Sept. 2015.
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temperature’s decrease. The experiment is conducted over the course of a year, from Sept.
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2.2 Experimental Equipment
In the experiment, both FPSWHs and VTSWHs with similar storage volume are
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installed in Nanjing and Lianyungang as the main experimental equipment. SWHs used in the experiment are all water-storage-type water heaters. Table 1 shows the parameters of SWHs
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in the experiment.
Table 1. Solar water heaters’ (SWHs’) parameters in the experiment.
Location Latitude Type
Manufacturer
Nanjing
Lianyungang
31°14’-
34°00'35°70'
32°37' Vacuum Flat plate tube Light Sunrain 6
Flat plate Sunrain
Vacuum tube Sunrain
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Volume（L）
150
140
120
150
Absorbing area(m2)
2
2.25
2.06
2.5
Tilt angels（°）
45
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2.3 Experimental Measurement The experimental setup records the environment’s temperature, along with the water temperature in different parts of the storage tank and solar radiation density. Through such data, the following four indexes are calculated: the coefficient of thermal performance (CTP), heat-collecting efficiency, the solar fraction, and the solar energy protection ratio.
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(1) Coefficient of thermal performance (CTP)
According to the Chinese National Standard (CNS) GB/T19141-2011 [26], the coefficient of thermal performance is used to evaluate the performance of SWHs, which can be calculated by the following equation:
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CTP Qg / Qm a U c / U m ,
(1)
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where Qm is the minimum useful heat gained from the unit area of the household’s SWH collector restricted by GB/T19141, given as MJ/m2. Um is the maximum heat loss of the
equal to 0.9.
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SWH system restricted by GB/T19141, given as W/m3•K. a is the proportional coefficient,
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Qg is the useful heat gain from the unit area of the household’s SWH collector under experimental conditions, give as MJ/m2. It can be calculated as
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Qg mwc p (Tw2 Tw1 ),
(2)
where Qg are the effective solar heat gains of the system, given as MJ/m2. And mw is the mass of water in the storage tank, given in kilograms (kg). Then cp is the specific heat capacity of water, given as kJ/kg•C. Tw1 is the initial water temperature in the storage tank, in terms of C. Tw2 is the final water temperature in the storage tank, given in C. Uc is the average SWH system heat loss under the experimental conditions, given as W/m3•K. It can be calculated as 7
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(3)
where w is the density of water, given as kg/m3. Ta is the ambient temperature, given in C. (2) Heat-collecting efficiency(HCE) The heat-collector efficiency (η) is the ratio of the system’s effective solar heat gain to the collector plate’s total solar radiation. It can reflect the solar energy-absorbing ability of
Qg / Qt 100%,
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the collector and can be calculated as follows: (4)
where Qt is the total solar radiation on SWHs collector, given as MJ/m2. η’s heat-collecting efficiency is given as a percentage. (3) Solar fraction
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To study the SWH system’s energy efficiency, a solar fraction (f) is applied here to account for the ratio of effective heat supported by SWH over the total required heating load from the water heater system.
f （Qg Q l）/ L 100%,
(5)
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where Ql is the water system’s heat loss, measured in MJ. L is the total required heating load
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from the water heater system, also measured in MJ. (4) Solar energy protection ratio
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Considering the remaining heat utilization, the solar energy protection ratio (ε) is introduced here to measure the independence of the SWHs on the auxiliary equipment. It can
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be calculated with the following formula:
D / DZ 100%,
(6)
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where D is the number of days when no auxiliary equipment is required. DZ is the total number of days.
3. Results and Discussion 3.1 Water Temperature Variation Figure 3 shows the dynamic system water temperature variation throughout the year. The temperatures of the environment, initial water, and final water for the SWHs are calculated based on a large number of experimental data for each month. 8
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(d) VTSWHs in Nanjing
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Figure 3. Annual dynamic water temperature variation.
As Figure 3 shows, the water temperature changes as the environment’s temperature
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varies. Meanwhile, results show that the VTSWHs’ performance is superior to FPSWHs’, with an added 6C to the final water temperature in both Lianyungang and Nanjing.
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Moreover, the final water temperature of VTSWHs and FPSWHs in Lianyungang is much higher than in Nanjing because of the higher solar radiation density in Lianyungang.
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Figure 4 shows the weather’s effect on the SWHs’ water temperature, based on the experimental results from March 2015. It is apparent that the weather greatly affects the tank
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water temperature. The water temperature Tw2 on a sunny day is about 12C higher than it is on a cloudy day. Meanwhile, Figure 4 also shows that the water temperature of VTSWHs is higher than FPSWHs because of the different apparatus structure.
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T w2 (oC)
50 40 30 FPSWHs, Sunny, Ta =8.55℃
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VTSWHs, Sunny, Ta =8.55℃
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VTSWHs, Cloudy, Ta =8.12℃
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Figure 4. The weather’s influence on the SWH’s water temperature.
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3.2 Coefficient of Thermal Performance (CTP)
Considering the comprehensive performance of SWHs, the coefficient of thermal performance (CTP) is used to evaluate the thermal performance of different SWHs. According to Eq(1), we show the result in Table 2; the CTP of both SWHs in Lianyungang
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and the VTSWHs in Nanjing reached more than 0.5. According to CNS GB 26969-2011, when CTP is more than 0.5, SWHs system can reach Energy Efficiency Grade 1 (CTP ≥ 0.5).
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However, the CTP of FPSWHs in Nanjing is 0.48, which is Energy Efficiency Grade 2 (0.32 ≤ CTP ≤ 0.5) [27]. Generally, the CTP of the SWH system in Nanjing is lower than
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Lianyungang’s because the solar radiation density in Nanjing is lower than Lianyungang’s.
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Table 2. Coefficient of thermal performance (CTP) of SWHs.
CTP
Type
Lianyungang
FPSWHs
0.48
0.55
VTSWHs
0.55
0.64
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Nanjing
3.3 Heat-Collecting Efficiency The solar-collector efficiency is the ratio of the system’s effective solar heat gain relative to the collector plate’s total solar radiation. Essentially, this ratio indicates the collector’s ability to absorb solar energy. As Figure 5 shows, the heat-collecting efficiency of VTSWHs 10
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exceeds FPSWHs by about 9%, indicating that the VTSWHs is better than FPSWHs on the collecting solar energy. On the other hand, the heat-collecting efficiency in winter is significantly lower than that in other seasons, which is an obstacle for SWHs to meet the needs of hot water throughout the year. One reason is that the heat loss in the collector and storage tank increases in winter. Additionally, the heat-collecting efficiency in Nanjing is lower than Lianyungang’s, which is influenced by local weather and the SWH manufacturing
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3.4 Solar Fraction
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Figure 5. SWHs’ annual heat-collecting efficiency.
Considering the total heat load and weather conditions for a household hot water, we
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introduce the solar fraction to assess the SWH system’s energy efficiency. We can calculate this using the ratio of effective heat supported by SWH over the total required heating load
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from the water heater system, as detailed in Eq (5). Figure 6 shows the results. The solar fraction in two climate zones of VTSWHs are both more than 50%, while those of FPSWHs
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can barely reach about 42%. Moreover, the solar fraction of Lianyungang is higher than Nanjing with an additional 5%, as a result of each city’s climatic differences. Lianyungang is in a cold, dry climate area with a fair amount of sunshine. Nanjing has hot summers and cold winters, with rainy, cloudy weather in the lower solar fraction in June and July.
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f (%)
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t (m)
t (m)
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(b) Nanjing
Figure 6. Solar fraction of different SWHs.
3.5 Solar Energy Protection Ratio
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Although the solar fraction takes the total heat load and weather conditions into account, it does not quantitatively describe users’ satisfaction. The solar fraction only considers one day’s thermal utilization. However, in a real living situation, the remaining heat in the water tank can be used next day. We therefore introduce a solar energy protection ratio. Eq(6)
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calculates and describes the independence of the SWHs on the auxiliary equipment. As Figure 7 shows, the solar energy protection ratio can reach up to 75% or more in the summer,
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which means that it can generally satisfy a household’s hot water requirements without needing additional assistance. At the same time, because of Lianyungang’s climate
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characteristics, its performance is better than Nanjing’s. 100
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Figure 7. Solar energy protection ratio of different SWHs.
Table 3 compares the solar energy protection ratio and solar fraction. The solar energy protection ratio is higher than the solar fraction, because the heat remaining in the tank at the 12
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end of the day can be used the next day in the summer. Moreover, the VTSWHs hold a higher solar energy protection ratio compared to FPSWHs. The solar fraction and solar energy protection ratio of FPSWHs is close, but the gap between the solar energy protection ratio and solar fraction of VTSWHs is larger. The reason why is that VTSWHs perform better in cloudy weather and retain more heat. Table 3. SWHs’ annual solar fraction and solar energy protection ratio.
Type
Lianyungang
Nanjing
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Location
FPSWHs VTSWHs FPSWHs VTSWHs
f（%）
45
60
39
ε（%）
46
66
42
61
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4. Socioeconomic Analysis
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Based on the aforementioned SWHs’ thermal performance, here we study the SWHs’ socioeconomic analysis. Several water heaters are compared with SWHs to find better solutions for household hot water systems. The water heaters for comparison are electric
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water heaters, gas water heaters, ASHP, electrical assistant solar water heaters (EA-SWHs),
heaters (AHSP-SWHs). 4.1 Economic Analysis
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gas assistant solar water heaters (GA-SWHs) and air-source heat pump assistant solar water
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The household hot water supply is taken as an example in Nanjing and Lianyungang.
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According to the survey, the average demand of daily household hot water is 60 L per human, so for a family of three people it is 180 L, and the required hot water temperature is 55C
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[28–29]. Therefore, the total required auxiliary heating load can be calculated as follows:
Lk c p mr Tm ,
(7)
where Lk is the auxiliary heating heat load of a family in one day. For SWHs, the temperature difference ΔTm = (55C - Tw2). For other water heaters, the temperature difference ΔTm = (55C - Tw1). According to the survey, the electricity price is 0.528 Yuan/kilowatt hour (kWh) and the gas price is 2.4 Yuan/m3 in the Jiangsu province. Taking 10 years into account, we can calculate the annual operating cost, total investment, and payback period [30]. Figure 8 shows 13
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the results. As Figures 8a and 8b show, the electric water heater (EWH) has the highest operating costs and requires the highest total investment. Compared to the EWH, the EA-SWH’s payback period is far less than any other water heater. By economic comparison, we see that the EA-VTSWH and GA-VTSWH perform better among all types of SWHs. 1500
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Water heater
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(c) Pay-back period Figure 8. Economic analysis of different water heaters.
4.2 Environmental Benefits As environmental problems become more severe, the standard for evaluating water heaters must consider minimizing not only the costs but also the environmental impact. For example, using electricity or gas to heat or drive a compressor also requires a coal-fired power plant to generate electricity. We can calculate the consumption of standard coal as follows: 14
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C E 0.404 G 1.2,
(8)
where C is the consumption of standard coal, given in kg. E is electric power consumption, given in kWh. G is gas consumption, given in m3. Figure 9(a) shows the total consumption of standard coal in 10 years, with consumption varying among different water heaters. It is obvious that, compared to others, the EWH far
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exceeds the total consumption of standard coal among water heaters. Moreover, it can be found that SWHs using gas or ASHP as auxiliary heating are more beneficial compared to traditional EASWHs. To further understand the social effect of SWHs, the carbon dioxide
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Total carbon dioxide emission in 10 years (t)
emissions of different water heaters are calculated as an important indicator.
Water heater
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(a) Total consumption of standard coal in 10 years (b) Total carbon dioxide emission in 10 years Figure 9. Different water heaters’ social effect analysis.
Assuming that all the electricity is generated from coal-fired power stations, then 1 kg of
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standard coal equals to about 1.41 kg of raw coal [31]. Meanwhile, 1 kg of raw coal will release 2.66 kg of carbon dioxide, which means that the consumption of 1 kg of standard coal
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will bring 3.525 kg of carbon dioxide. Besides, the complete combustion of per-cubic-meter natural gas of standard coal will release 1.92 kg of carbon dioxide [32]. Figure 9(b) shows the amount of carbon dioxide emissions of different water heaters. Compared to traditional EWHs, the use of SWHs can reduce emissions of carbon dioxide effectively by more than 65%. Moreover, as one type of clean energy, the emissions can be further reduced by using gas as auxiliary heating. Among all the water heaters, although the consumption of standard coal of GA-VTSWHs is slightly higher than that of ASHP-VTSWHs, the carbon dioxide 15
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emissions of GA-VTSWHs are much lower than any other water heaters. From this analysis, it is obvious that the GA-VTSWHs perform better compared with other water heaters, considering socioeconomic effects. Therefore, for long-term energy savings and emission reductions, it is important to popularize SWHs’ usage.
5. Implications for SWHs’ Development and Use
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5.1 Research on Users’ Satisfaction with SWHs A survey among SWH users was conducted in Yingong Villa, which is one of the demonstration areas for SWHs in Nanjing. More than 200 participants responded to the survey. According to the survey (Figure 10(a)), respondents showed greater preference for purchasing water heaters that are cost-effective, have long lifecycles, are energy-efficient, and
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are safe for daily usage. About 95% of respondents thought that SWHs were energy-efficient, while more than 60% thought that SWHs were cost-effective and safe for domestic use. By comparing the advantage of SWHs in Figure 10(b) to the distributions in Figure 10(a), it was evident that SWHs are ideally suited for residents’ hot water supply needs. In general, all of
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the respondents were either satisfied or ambivalent about using SWHs (Figure 10(c)). In light of such positive feedback, we recommend that policymakers promote SWHs’
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household use. However, successfully using SWHs for households hinges on addressing some of the heating systems’ inefficiencies. For example, issues with frozen water pipes in
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Nanjing have not been resolved yet. According to the survey (Figure 10(d)), almost half of the respondents in Nanjing complained about freezing water pipes in winter. Moreover,
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one-fifth of residents complained about the heating time required to bring the cold water to a usable temperature of 50C. These issues highlight the need for improving the heating
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systems’ design, especially the thermal insulation of plumbing pipes. Moreover, auxiliary heating is necessary to accelerate SWHs’ heating speed.
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100
100
80
Frequency
Frequency (%)
60
40
20
Frequency
60
40
20
0
0 Energy-saving
Cost-effective
Safe
Satisfied service
Cost-effective
(a)Factors that residents consider while purchasing a water heater
Satisfied 49%
Water pipe frozen 49%
Affected by the weather 12%
Safe
(b)Advantages of SWH recognized by residents
Slowly water temperature rising 20%
(c)User satisfaction about SWH
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Low water temperature 10%
Energy-saving
Extremely satisfied 8%
Unsatisfied aftermarket service 4%
Often failing 5%
Long lifecycle
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Frequency (%)
80
General 43%
(d)Common complaints from consumers about SWH
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Figure 10. Summary of SWH survey results.
5.2 Key Problems in Developing SWHs
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Compared to traditional water heaters, SWHs have greater potential for saving energy and reducing emissions. However, there are several problems associated in the promotional
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process that make SWHs unappealing to consumers. First, the SWH market in China is at an immature stage. Requirements for SWHs’ manufacturing can be achieved easily, which
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results in numerous similar SWH products on the Chinese market. Yet with the absence of standardization, the quality of SWHs fluctuates significantly among different manufacturers,
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leading to low consumer-confidence levels. Although a large number of low-cost SWH manufacturers have helped promote the development of the SWH industry, policymakers should strengthen regulations by formulating national standards for SWH products. Also, with the increased level of urbanization in China, residential buildings are being replaced by multistory building. Unlike individual residential homes, residents of such buildings only have access to a limited open area: usually a balcony that might not be facing direct sunlight. This limited space for setting solar collectors presents a challenge for 17
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installing a VTSWH. Aesthetically, too, VTSWHs are not preferred. The limitations of VTSWHs can be addressed by using FPSWHs, but these have a higher initial investment cost. The FPSWH industry technology in developed countries is more efficient than in China. FPSWHs cover a wide area of a total building area in developed countries, and the collector effect is more efficient compared to the VTSWHs that are usually employed in China. Therefore, high-efficiency FPSWHs for indigenous use should be explored.
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In the rural areas, policymakers should vigorously develop the use of GA-VTSWHs. Considering the long-term economic and environmental benefits, GA-VTSWHs are preferable to any other water heaters. However, there are still several unresolved issues with GA-SWHs. First, the complex design of GA-SWHs make them more difficult to install than
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EA-SWHs. Furthermore, GA-SWHs rely on the availability of a gas network, thereby limiting their application to residential units that receive service from the gas network. Research on simpler designs, along with reducing the production costs, should be encouraged for GA-VTSWHs. For areas that are not connected to gas networks, EA-SWHs can be
5.3 Suggestions for SWHs’ Use
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promoted as a viable option.
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Government policy plays a key role in the development and wide-scale deployment of SWHs. Hence, we make the following suggestions for selecting SWHs. SWHs should be
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used for water heating when there is enough space and abundant sunshine, and electricity or gas should be used as auxiliary energy if SWHs are unable to meet the hot water
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requirements. Regarding FPSWHs, we note that these are suitable for water heating when indoor space is limited or a higher indoor aesthetic is required. In general, we recommend
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taking the entire solar energy reuse plan into account, from the building design to the building’s whole lifecycle [33]. And when using FPSWHs, antifreeze must be taken if the minimum operating temperature is below 0℃. In addition, FPSWHs are more suitable for high-rise buildings, as they can be mounted on the wall outside. In contrast, VTSWHs would be a good choice if the minimum operating temperature is below 0℃ or there are wide enough spaces.
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6. Conclusion In this work, we established experimental platforms to measure the real-time annual thermal performance of SWHs in the cities of Nanjing and Lianyungang, within China’s Jiangsu province. We carried out this year-long experimental study using different kinds of water heaters, and compared the dynamic thermal performance (such as the coefficient of
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thermal performance, solar fraction, and the solar energy protection ratio) of SWH systems. Results show that overall, SWHs perform well in the Jiangsu province; however, the SWHs in Lianyungang have a better thermal performance than Nanjing’s because they receive more solar radiant energy. We also observed that VTSWHs have a better thermal performance throughout the year compared to FPSWHs. Meanwhile, from a socioeconomic perspective,
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most SWHs are far preferable to traditional water heaters. Among all the SWHs, GA-VTSWHs performed the best over a long period compared to other types of water heaters. These results are helpful for developing and determining usage for future SWHs in China. Our experimental conclusions are supported by a large amount of data collected over a
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long duration. The experiment also considers the impact of climate on SWHs’ performance, with different climate zones selected in the experiment to ensure accuracy. A particularly
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innovative aspect of this paper is that it presents the concept of a solar energy protection ratio. Employing this ratio to compare the actual satisfaction of using VTSWHs and FPSWHs from
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a practical point of view strengthens our conclusions, and provides helpful data for other SWH simulation experiments. These results also act as a reference point for selecting water
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heaters in Jiangsu, helping policymakers make more informed decisions. However, there is still much work to be done. For example, we did not measure the hot water circulation flow
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rate of SWHs in this experiment, but we plan to in future work.
Acknowledgement This study is financially supported by National Natural Science Foundation of China (No. 51606037), Natural Science Foundation of Jiangsu Province (No. BK20160687), China’s Manned Space Program (TZ-1), and Building energy saving special guidance fund of Jiangsu and the Research Funds of Key Laboratory of Heating and Air Conditioning, The Education Department of Henan Province. 19
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References [1] E. Koepf, I. Alxneit, C. Wieckert, A. Meier, A review of high temperature solar driven reactor technology: 25 years of experience in research and development at the Paul Scherrer Institute, Applied Energy 188 (2017) 620-651. [2] H.M. Abd-ur-Rehman, F.A. Al-Sulaiman, Optimum selection of solar water heating
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(SWH) systems based on their comparative techno-economic feasibility study for the domestic sector of Saudi Arabia, Renewable and Sustainable Energy Reviews 61 (2016) 336-349.
[3] F. Urban, S. Geall, Y. Wang, Solar PV and solar water heaters in China: Different
AN US
pathways to low carbon energy, Renewable and Sustainable Energy Reviews 64 (2016) 531-542.
[4] H. Zou, H. Du, J. Ren, B.K. Sovacool, Y. Zhang, G. Mao, Market dynamics, innovation, and transition in China's solar photovoltaic (PV) industry: A critical review, Renewable and
M
Sustainable Energy Reviews 69 (2017) 197-206.
[5] T. Chow, Y. Bai, Z. Dong, K. Fong, Selection between single-phase and two-phase
ED
evacuated-tube solar water heaters in different climate zones of China Solar Energy 98 (2013) 265-274.
PT
[6] A.H. Al-Badi, M.H. Albadi, Domestic solar water heating system in Oman: Current status and future prospects, Renewable and Sustainable Energy Reviews 16 (2012) 5727-5731.
CE
[7] D. Nugent, B.K. Sovacool, Assessing the lifecycle greenhouse gas emissions from solar PV and wind energy: a critical meta-survey, Energy Policy 65 (2014) 229–244.
AC
[8] S. Rashidi, J.A. Esfahani, A. Rashidi, A review on the applications of porous materials in solar energy systems, Renewable and Sustainable Energy Reviews 73 (2017) 1198-1210. [9] Z.M. Omara, A.E. Kabeel, A.S. Abdullah, A review of solar still performance with reflectors, Renewable and Sustainable Energy Reviews 68 (2017) 638-649. [10] R. Singh, I.J. Lazarus, M. Souliotis, Recent developments in integrated collector storage (ICS) solar water heaters: A review, Renewable and Sustainable Energy Reviews 54 (2016) 270-298. [11] X. Fang, D. Li, Solar photovoltaic and thermal technology and applications in China, 20
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Renewable and Sustainable Energy Reviews 23 (2013) 330-340. [12] R. Hu, P. Sun, Z. Wang, An overview of the development of solar water heater industry in China, Energy Policy 51 (2012) 46-51. [13] R. Zheng, J. Nie, Analysis for Marketization Development Prospect of Large-scale Solar Heating Combisystems in China, Energy Procedia 70 (2015) 574-579. [14] S. Qiu, M. Ruth, S. Ghosh, Evacuated tube collectors: A notable driver behind the solar
CR IP T
water heater industry in China, Renewable and Sustainable Energy Reviews 47 (2015) 580-588.
[15] R. Tang, Y. Yang, W. Gao, Comparative studies on thermal performance of water-in-glass evacuated tube solar water heaters with different collector tilt-angles, Solar
AN US
Energy 85 (2011) 1381-1389.
[16] F. Mauthner, W. Weiss, Solar Heat Worldwide, Markets and Contribution to the Energy Supply 2011. IEA Solar Heating & Cooling Programme (2011) http://www.iea-shc.org/ Data/Sites/1/publications/Solar-Heat-Worldwide-2013.pdf. (accessed 17.06.21)
M
[17] H. Li, H. Yang, Potential application of solar thermal systems for hot water production in Hong Kong, Applied Energy 86 (2009) 175-180.
ED
[18] R. Tang, Y. Yang, W. Gao, Comparative studies on thermal performance of water-in-glass evacuated tube solar water heaters with different collector tilt-angles, Solar
PT
Energy 85 (2011) 1381-1389.
[19] T. Chow, Z. Dong, L. Chan, K.F. Fong, Y. Bai, Performance evaluation of evacuated tube
CE
solar domestic hot water systems in Hong Kong, Energy and Buildings 43(12) 2011 3467-3474.
AC
[19] B. Grant, B. Pradeep, New test method for gas boosters with domestic solar water heaters, Solar Energy 86(1) (2012) 78-86. [21] X. Zhang, S. You, W. Xu, M. Wang, T. He, X. Zheng, Experimental investigation of the higher coefficient of thermal performance for water-in-glass evacuated tube solar water heaters in China, Energy Conversion and Management 78 (2014) 386-392. [22] CEN, European Standard prEN 12975-2:2000, European Committee for Standardisation (2000). [23] G.G. Dawit, M. Pinakeswar, P.S. Robi, CFD and experimental investigation of flat plate 21
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solar water heating system under steady state condition, Renewable Energy 106 (2017) 24-36. [24] Chinese Academy of Building Sciences, Green building evaluation technical rules, Beijing, China: China architecture & building press (2015). (in Chinese) [25] Jiangsu Provincial Department of Housing and Urban-Rural Development, Design standard of thermal-environment & energy conservation for residential buildings in Jiangsu
CR IP T
province. Standard of Engineering Construction in Jiangsu Province DGJ32/J 71-2014, Jiangsu, China (2014). (in Chinese)
[26]CNS, Thermal design code for civil building, Chinese National Standard GB/50196, China (1993). (in Chinese)
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[27] Y. Zhou, W. Wu, Y. Hu, Q. Fang, G. Liu, Evaluation of available solar energy resources in Jiangsu Province, Renewable Energy Resources 28 (2010) 10-13. (in Chinese) [28] CNS, Technical conditions for domestic solar water heating systems, Chinese National Standard GB/T19141,China (2011).(in Chinese)
[29]CNS, Energy efficiency limit and energy efficiency rating of household solar water
M
heating system. Chinese National Standard GB 26969-2011, China (2011). (in Chinese) [30] K. Kerrigan, H. Jouhara, G.E. O’Donnell, A.J. Robinson, A naturally aspirated convector
67 (2013) 187-194.
ED
for domestic heating application with low water temperature sources, Energy and Buildings
PT
[31] C.E. John, G.S. Lukas, Domestic hot water consumption estimates for solar thermal system sizing, Energy and Buildings 58 (2013) 58-65.
CE
[32] W. Li, G. Song, M. Beresford, B. Ma, China's transition to green energy systems: The economics of home solar water heaters and their popularization in Dezhou city, Energy
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Policy 39 (2011) 5909-5919. [33] CNS, Technical conditions for domestic solar water heating systems, Chinese National Standard GB/T 2589-2008,China (2008).(in Chinese) [34] State Environmental Protection Administration, Practical Handbook for Discharge Declaration and Registration, Beijing, China: China Environmental Science Press (2004). (in Chinese) [35] J. Shi, W. Su, M. Zhu, H. Chen, Y. Pan, S. Wan, Y. Wang, Solar water heating system 22
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integrated design in high-rise apartment in China, Energy and Buildings 58 (2013) 19-26.
Figure captions
Figure 2 Schematic of experimental system
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Figure 1 The experimental system’s schematic
Figure 3 Annual dynamic water temperature variation
Figure 4 The weather’s Influence on the SWH’s water temperature Figure 5 SWHs’ annual heat-collecting efficiency
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Figure 6 Solar fraction of different SWHs
Figure 7 Solar energy protection ratio of different SWHs
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Figure 8 Economic analysis of different water heaters Figure9 Different water heaters’ social effect analysis
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Tables
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Figure10 Summary of SWH survey results
Table 1 Solar water heaters’ (SWHs’) parameters in the experiment
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Table 2 Coefficient of thermal performance (CTP) of SWHs Table 3 SWHs’ annual solar fraction and solar energy protection ratio
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Annual Dynamic Thermal Performance of Solar Water Heaters:A Case Study in China’s Jiangsu Province Juan Shi , KaiWei Lin , ZhenQian Chen , Hao Shi PII: DOI: Reference:
S0378-7788(17)32980-8 10.1016/j.enbuild.2018.04.048 ENB 8521
To appear in:
Energy & Buildings
Received date: Revised date: Accepted date:
27 November 2017 16 March 2018 23 April 2018
Please cite this article as: Juan Shi , KaiWei Lin , ZhenQian Chen , Hao Shi , Annual Dynamic Thermal Performance of Solar Water Heaters:A Case Study in China’s Jiangsu Province, Energy & Buildings (2018), doi: 10.1016/j.enbuild.2018.04.048
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Annual Dynamic Thermal Performance of Solar Water Heaters: A Case Study in China’s Jiangsu Province Juan Shi 1, KaiWei Lin 1, ZhenQian Chen*1,2, Hao Shi 3 1. School of Energy and Environment, Southeast University, Nanjing 210096, China 2. Jiangsu Provincial Key Laboratory of Solar Energy Technology, Nanjing 210096, China
*
Corresponding email: [email protected]
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3. MCC Huatian Engineering & Technology Corporation, Nanjing 210019, China Tel/Fax number:86-25-83790626
Abstract：In our search for a better socioeconomic path for water heater usage in China, we
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studied and compared the annual dynamic thermal performance of solar water heaters (SWHs) in the Jiangsu province. Experiments are conducted in two cities within Jiangsu—Nanjing and Lianyungang—spanning two climate zones. The results show that SWHs have good thermal performance in Jiangsu. The SWHs in Lianyungang show better thermal performance than in Nanjing, because they receive greater solar radiant energy. Also, from a thermal
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perspective, vacuum tube solar water heaters (VTSWHs) perform better throughout the year
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than flat-plate solar water heaters (FPSWHs). Meanwhile, taking socioeconomics into account, gas-assisted flat-plate solar water heaters (GA-VTSWHs) have better performance
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compared to other types of water heaters. Based on these results, the implications for developing and applying SWHs are derived. This study can be used to optimize operation of
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the household hot water supply system and use it more efficiently.
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Keywords：Solar water heater, thermal performance, China, utilization efficiency
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1. Introduction As socioeconomic development and living standards improve in China (especially the eastern part), hot water consumption steadily increases [1]. Projections show this will be an upward trend for some time, so to meet future demands, we must evaluate the availability of energy sources [2]. Currently, fossil fuels are the primary energy sources, but to avoid or
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diminish their adverse impact on the environment—and to find a more efficient way to heat water—it is important to identify alternative energy sources [3–4]. For example, Chow et al. argue that heating water with electricity or natural gas is inefficient, because the desired hot water temperature is usually under 60C [5]. For such applications, renewable energy is a viable alternative. It is estimated that renewable energy contributes to one-fourth of total
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global energy consumption. Such energy sources are a much ―cleaner‖ way to contribute toward the global energy demand, offering less pollution [6–7].
China has shown a longstanding interest in solar power with SWH development beginning in the 1950s, as SWHs have many advantages over conventional water heating.
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For example, SWHs emit negligible greenhouse gases, have a low payback period, and are easy to install [9–10]. The energy crisis in the 1970s accelerated the country’s investment in
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renewable energy. By the 1980s, the solar energy industry in China was on par with developed nations [11]. In the past three decades, China has promoted solar energy’s use. In
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1992, after the United Nations Conference on Environment and Development, the Chinese government took the lead in formulating the 21st Century Agenda of China and encouraged
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the solar energy industry’s expansion. With active support from the government, China's renewable energy consumption to total energy consumption increased from 7.2% in 2000 to
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11.4% in 2014. China had invested nearly US$90 billion in renewable energy in 2014. As of 2012, there are more than 3,000 SWH enterprises in China [12], making it one of the largest solar heater markets in the world. In China, both vacuum tube solar water heaters(VTSWHs) and flat plate solar water heaters (FPSWHs) are widely used. According to Tang et al. [13], VTSWHs account for 88% of the market share in China. The data indicates that China's VTSWHs occupy a large amount of the market of total SWHs in the world. The total installed SWHs in China are in excess of 2
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180,400 megawatts thermal (MWth) in 2012. In 2011, the installed capacity in China accounted for 84.0% of the world’s total capacity [14]—a significant increase from 57.2% in 2000. China is now a dominant SWH producer. Compared to the rest of the world—especially western countries—the share of the FPSWHs market was far greater than the VTSWHs in Japan, Germany, and Australia. What’s more, the share even reached up to 100%, because FPSWHTs offer good thermal efficiency, appearance, and stability. That said,
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in China domestic SWH research institutions and enterprises have promoted VTSWHs’ use, and VTSWHs offer various advantages such as a better heat-loss coefficient, antifreeze performance, and so on. Thus, VTSWHs are now mainstream on the SWH market.
In recent years, researchers conducted many experiments to study SWHs’ thermal
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performance. Li and Yang [15] introduced a type of assisted solar heating system, and made a comparison among SWHs, air-source heat pump water heaters (ASHP), and traditional water heaters. Tang et al. [16] used two VTSWHs systems with different tilt angles to study the influence of the tilt angle on the SWHs. They found that even though two systems absorbed
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different amounts of heat, the daily solar thermal conversion efficiency was almost equal. Chow et al. [17] surveyed the thermal performance of a single-phase open thermosyphon
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system and the two-phase closed thermosyphon system. The experimental and simulation study are both done with two solar water heaters. It turns out that the two-phase closed
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thermosyphon solar-collector costs less to run. Grant and Pradeep [18] introduced a new method for measuring the annual consumption of gas-assisted SWHs with an in-series gas
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booster. Different from the existing standard, the variation of inlet water temperature, flow rate, and environment’s temperature were considered for evaluating the energy consumption
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of gas-assisted SWH. To discover the factors affecting the SWHs’ performance, Zhang et al. [19] studied VTSWHs’ performance according to Chinese standards. In Europe, solar performance tests are based on EN 1295-2 [20] (the European standard
for solar thermal collector testing), to ensure the reliability of test experiments. For example, Dawit et al. [21] considered both open- and closed-loop solar water systems, using CFD simulation to study the influence of inter-temperature, the environment’s temperature, solar radiation density, and mass flow rate on SWHs’ efficiency. Then they proved the simulation results using the experimental results. Likewise, in China today the widespread use of SWHs 3
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is also largely dependent on government support and guidance. According to the Chinese Academy of Building Sciences, when conducting a comprehensive evaluation of green buildings, solar thermal energy has been given enough attention as an important part of renewable energy [22]. Furthermore, according to the DGJ32/J 71-2014 Standard of Engineering Construction in the Jiangsu province, buildings with fewer than six floors are required to install a solar hot water supply system [23].
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At present, experiments on SWHs’ thermal performance are mainly conducted using a standard condition instead of cloudy/rainy days. Hence, no ―real-life‖ systematic measurement and analysis exists for SWHs’ performance. That is why we are studying the annual dynamic thermal performance of SWHs in the Jiangsu province, and why we installed
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online experimental platforms for different kinds of SWHs in different climate zones. In this paper, we compared the dynamic thermal performance (such as the coefficient of thermal performance, solar fraction, or the solar energy protection ratio) of SWH systems. We also compare economic and environmental performance. This information can be used to optimize
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the household’s hot water supply system and increase efficiency in its use.
2.1 Experimental System
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2. Methodology
The Jiangsu province is located in the eastern part of China, with a relatively higher
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socioeconomic level. Thus, SWHs are prevalent in this area. As Figure 1(a) shows, Jiangsu is typically in hot summers & cold winters climate area [24]. According to Zhou’s survey [25],
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the overall solar energy resources in Jiangsu province are lower compared to western China, with average radiation at 4,200–5,400 megajoules (MJ)/(m2•a). As Figure 1(b) shows, solar
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energy resources in the cities of Lianyungang, Yancheng, and Suqian are relatively rich. The total radiation can reach above 5,000 MJ/(m2•a). However, the solar energy distribution in the cities of Nanjing, Changzhou, Wuxi, and Suzhou are less than the northern Jiangsu province. The average radiation is only 4,700 MJ/(m2•a).
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(a)The climate zone map for architectural design in China
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(b)Solar radiation density in Jiangsu Province
Figure 1. Experimental platform locations.
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To study the characteristic of SWHs in different climate areas with different solar radiation density, we chose Nanjing (the hot summer and cold winter climate area; 118°22′E– 119°14′E, 31°14′N–32°37′N) and Lianyungang (cold climate area; 118°24′E–119°48′E, 34°00′N–35°70′N) to set up a real-time experimental platform for household SWHs. In each
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city, VTSWHs and FPSWHs are installed to compare different SWHs’ thermal performance. The water temperature of the storage tank, the amount of solar radiation, and the dynamic
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characteristics of SWHs in different areas are analyzed. Figure 2 describes the experimental system. The setup contains the SWH, measurement,
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and data collection systems. The SWH system is comprised of a solar collector and water storage tank. The measurement system includes an irradiation instrument and thermocouple.
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The irradiation instrument is used to measure the solar radiation intensity. The K-type sheathed thermocouple is used to measure the water temperature in the SWH system, as well
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as the outdoor environment temperature. The experimental results are scanned by a data acquisition instrument and sent to a computer via I/O interface.
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Figure 2. The experimental system’s schematic.
Each experiment is conducted from 8:00 a.m. to 8:00 a.m. the next day. Normally, solar radiation is abundant to heat the water during the periods of 8:00 a.m. to 4:00 p.m. The
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solar-collector gains heat and delivers it to the storage tank. In the course from 8:00 p.m. to 8:00 a.m. next day, heat loss occurs in the water storage tank, which results in the water
2014 to Sept. 2015.
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temperature’s decrease. The experiment is conducted over the course of a year, from Sept.
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2.2 Experimental Equipment
In the experiment, both FPSWHs and VTSWHs with similar storage volume are
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installed in Nanjing and Lianyungang as the main experimental equipment. SWHs used in the experiment are all water-storage-type water heaters. Table 1 shows the parameters of SWHs
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in the experiment.
Table 1. Solar water heaters’ (SWHs’) parameters in the experiment.
Location Latitude Type
Manufacturer
Nanjing
Lianyungang
31°14’-
34°00'35°70'
32°37' Vacuum Flat plate tube Light Sunrain 6
Flat plate Sunrain
Vacuum tube Sunrain
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Volume（L）
150
140
120
150
Absorbing area(m2)
2
2.25
2.06
2.5
Tilt angels（°）
45
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2.3 Experimental Measurement The experimental setup records the environment’s temperature, along with the water temperature in different parts of the storage tank and solar radiation density. Through such data, the following four indexes are calculated: the coefficient of thermal performance (CTP), heat-collecting efficiency, the solar fraction, and the solar energy protection ratio.
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(1) Coefficient of thermal performance (CTP)
According to the Chinese National Standard (CNS) GB/T19141-2011 [26], the coefficient of thermal performance is used to evaluate the performance of SWHs, which can be calculated by the following equation:
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CTP Qg / Qm a U c / U m ,
(1)
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where Qm is the minimum useful heat gained from the unit area of the household’s SWH collector restricted by GB/T19141, given as MJ/m2. Um is the maximum heat loss of the
equal to 0.9.
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SWH system restricted by GB/T19141, given as W/m3•K. a is the proportional coefficient,
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Qg is the useful heat gain from the unit area of the household’s SWH collector under experimental conditions, give as MJ/m2. It can be calculated as
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Qg mwc p (Tw2 Tw1 ),
(2)
where Qg are the effective solar heat gains of the system, given as MJ/m2. And mw is the mass of water in the storage tank, given in kilograms (kg). Then cp is the specific heat capacity of water, given as kJ/kg•C. Tw1 is the initial water temperature in the storage tank, in terms of C. Tw2 is the final water temperature in the storage tank, given in C. Uc is the average SWH system heat loss under the experimental conditions, given as W/m3•K. It can be calculated as 7
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(3)
where w is the density of water, given as kg/m3. Ta is the ambient temperature, given in C. (2) Heat-collecting efficiency(HCE) The heat-collector efficiency (η) is the ratio of the system’s effective solar heat gain to the collector plate’s total solar radiation. It can reflect the solar energy-absorbing ability of
Qg / Qt 100%,
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the collector and can be calculated as follows: (4)
where Qt is the total solar radiation on SWHs collector, given as MJ/m2. η’s heat-collecting efficiency is given as a percentage. (3) Solar fraction
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To study the SWH system’s energy efficiency, a solar fraction (f) is applied here to account for the ratio of effective heat supported by SWH over the total required heating load from the water heater system.
f （Qg Q l）/ L 100%,
(5)
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where Ql is the water system’s heat loss, measured in MJ. L is the total required heating load
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from the water heater system, also measured in MJ. (4) Solar energy protection ratio
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Considering the remaining heat utilization, the solar energy protection ratio (ε) is introduced here to measure the independence of the SWHs on the auxiliary equipment. It can
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be calculated with the following formula:
D / DZ 100%,
(6)
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where D is the number of days when no auxiliary equipment is required. DZ is the total number of days.
3. Results and Discussion 3.1 Water Temperature Variation Figure 3 shows the dynamic system water temperature variation throughout the year. The temperatures of the environment, initial water, and final water for the SWHs are calculated based on a large number of experimental data for each month. 8
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(d) VTSWHs in Nanjing
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Figure 3. Annual dynamic water temperature variation.
As Figure 3 shows, the water temperature changes as the environment’s temperature
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varies. Meanwhile, results show that the VTSWHs’ performance is superior to FPSWHs’, with an added 6C to the final water temperature in both Lianyungang and Nanjing.
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Moreover, the final water temperature of VTSWHs and FPSWHs in Lianyungang is much higher than in Nanjing because of the higher solar radiation density in Lianyungang.
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Figure 4 shows the weather’s effect on the SWHs’ water temperature, based on the experimental results from March 2015. It is apparent that the weather greatly affects the tank
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water temperature. The water temperature Tw2 on a sunny day is about 12C higher than it is on a cloudy day. Meanwhile, Figure 4 also shows that the water temperature of VTSWHs is higher than FPSWHs because of the different apparatus structure.
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60
T w2 (oC)
50 40 30 FPSWHs, Sunny, Ta =8.55℃
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VTSWHs, Sunny, Ta =8.55℃
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VTSWHs, Cloudy, Ta =8.12℃
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20 t (h)
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28
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Figure 4. The weather’s influence on the SWH’s water temperature.
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3.2 Coefficient of Thermal Performance (CTP)
Considering the comprehensive performance of SWHs, the coefficient of thermal performance (CTP) is used to evaluate the thermal performance of different SWHs. According to Eq(1), we show the result in Table 2; the CTP of both SWHs in Lianyungang
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and the VTSWHs in Nanjing reached more than 0.5. According to CNS GB 26969-2011, when CTP is more than 0.5, SWHs system can reach Energy Efficiency Grade 1 (CTP ≥ 0.5).
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However, the CTP of FPSWHs in Nanjing is 0.48, which is Energy Efficiency Grade 2 (0.32 ≤ CTP ≤ 0.5) [27]. Generally, the CTP of the SWH system in Nanjing is lower than
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Lianyungang’s because the solar radiation density in Nanjing is lower than Lianyungang’s.
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Table 2. Coefficient of thermal performance (CTP) of SWHs.
CTP
Type
Lianyungang
FPSWHs
0.48
0.55
VTSWHs
0.55
0.64
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Nanjing
3.3 Heat-Collecting Efficiency The solar-collector efficiency is the ratio of the system’s effective solar heat gain relative to the collector plate’s total solar radiation. Essentially, this ratio indicates the collector’s ability to absorb solar energy. As Figure 5 shows, the heat-collecting efficiency of VTSWHs 10
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exceeds FPSWHs by about 9%, indicating that the VTSWHs is better than FPSWHs on the collecting solar energy. On the other hand, the heat-collecting efficiency in winter is significantly lower than that in other seasons, which is an obstacle for SWHs to meet the needs of hot water throughout the year. One reason is that the heat loss in the collector and storage tank increases in winter. Additionally, the heat-collecting efficiency in Nanjing is lower than Lianyungang’s, which is influenced by local weather and the SWH manufacturing
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process.
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3.4 Solar Fraction
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Figure 5. SWHs’ annual heat-collecting efficiency.
Considering the total heat load and weather conditions for a household hot water, we
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introduce the solar fraction to assess the SWH system’s energy efficiency. We can calculate this using the ratio of effective heat supported by SWH over the total required heating load
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from the water heater system, as detailed in Eq (5). Figure 6 shows the results. The solar fraction in two climate zones of VTSWHs are both more than 50%, while those of FPSWHs
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can barely reach about 42%. Moreover, the solar fraction of Lianyungang is higher than Nanjing with an additional 5%, as a result of each city’s climatic differences. Lianyungang is in a cold, dry climate area with a fair amount of sunshine. Nanjing has hot summers and cold winters, with rainy, cloudy weather in the lower solar fraction in June and July.
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t (m)
t (m)
(a) Lianyungang
(b) Nanjing
Figure 6. Solar fraction of different SWHs.
3.5 Solar Energy Protection Ratio
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Although the solar fraction takes the total heat load and weather conditions into account, it does not quantitatively describe users’ satisfaction. The solar fraction only considers one day’s thermal utilization. However, in a real living situation, the remaining heat in the water tank can be used next day. We therefore introduce a solar energy protection ratio. Eq(6)
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calculates and describes the independence of the SWHs on the auxiliary equipment. As Figure 7 shows, the solar energy protection ratio can reach up to 75% or more in the summer,
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which means that it can generally satisfy a household’s hot water requirements without needing additional assistance. At the same time, because of Lianyungang’s climate
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characteristics, its performance is better than Nanjing’s. 100
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Figure 7. Solar energy protection ratio of different SWHs.
Table 3 compares the solar energy protection ratio and solar fraction. The solar energy protection ratio is higher than the solar fraction, because the heat remaining in the tank at the 12
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end of the day can be used the next day in the summer. Moreover, the VTSWHs hold a higher solar energy protection ratio compared to FPSWHs. The solar fraction and solar energy protection ratio of FPSWHs is close, but the gap between the solar energy protection ratio and solar fraction of VTSWHs is larger. The reason why is that VTSWHs perform better in cloudy weather and retain more heat. Table 3. SWHs’ annual solar fraction and solar energy protection ratio.
Type
Lianyungang
Nanjing
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Location
FPSWHs VTSWHs FPSWHs VTSWHs
f（%）
45
60
39
ε（%）
46
66
42
61
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4. Socioeconomic Analysis
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Based on the aforementioned SWHs’ thermal performance, here we study the SWHs’ socioeconomic analysis. Several water heaters are compared with SWHs to find better solutions for household hot water systems. The water heaters for comparison are electric
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water heaters, gas water heaters, ASHP, electrical assistant solar water heaters (EA-SWHs),
heaters (AHSP-SWHs). 4.1 Economic Analysis
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gas assistant solar water heaters (GA-SWHs) and air-source heat pump assistant solar water
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The household hot water supply is taken as an example in Nanjing and Lianyungang.
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According to the survey, the average demand of daily household hot water is 60 L per human, so for a family of three people it is 180 L, and the required hot water temperature is 55C
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[28–29]. Therefore, the total required auxiliary heating load can be calculated as follows:
Lk c p mr Tm ,
(7)
where Lk is the auxiliary heating heat load of a family in one day. For SWHs, the temperature difference ΔTm = (55C - Tw2). For other water heaters, the temperature difference ΔTm = (55C - Tw1). According to the survey, the electricity price is 0.528 Yuan/kilowatt hour (kWh) and the gas price is 2.4 Yuan/m3 in the Jiangsu province. Taking 10 years into account, we can calculate the annual operating cost, total investment, and payback period [30]. Figure 8 shows 13
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the results. As Figures 8a and 8b show, the electric water heater (EWH) has the highest operating costs and requires the highest total investment. Compared to the EWH, the EA-SWH’s payback period is far less than any other water heater. By economic comparison, we see that the EA-VTSWH and GA-VTSWH perform better among all types of SWHs. 1500
900 600 300
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0
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Water heater
Water heater
(a) Annual operation costs
(b) Total investment in ten years
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G
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A
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Total costs (Yuan)
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H H EW W TS -V H EA SW T -V A G P SH A H W H G W PS H W TS
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H W
V P-
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PS
PS
F P-
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G
SH
A
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0
Water heater
(c) Pay-back period Figure 8. Economic analysis of different water heaters.
4.2 Environmental Benefits As environmental problems become more severe, the standard for evaluating water heaters must consider minimizing not only the costs but also the environmental impact. For example, using electricity or gas to heat or drive a compressor also requires a coal-fired power plant to generate electricity. We can calculate the consumption of standard coal as follows: 14
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C E 0.404 G 1.2,
(8)
where C is the consumption of standard coal, given in kg. E is electric power consumption, given in kWh. G is gas consumption, given in m3. Figure 9(a) shows the total consumption of standard coal in 10 years, with consumption varying among different water heaters. It is obvious that, compared to others, the EWH far
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exceeds the total consumption of standard coal among water heaters. Moreover, it can be found that SWHs using gas or ASHP as auxiliary heating are more beneficial compared to traditional EASWHs. To further understand the social effect of SWHs, the carbon dioxide
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9
6
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W H
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ED
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V P-
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Total consumption of standard coal (t)
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Total carbon dioxide emission in 10 years (t)
emissions of different water heaters are calculated as an important indicator.
Water heater
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(a) Total consumption of standard coal in 10 years (b) Total carbon dioxide emission in 10 years Figure 9. Different water heaters’ social effect analysis.
Assuming that all the electricity is generated from coal-fired power stations, then 1 kg of
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standard coal equals to about 1.41 kg of raw coal [31]. Meanwhile, 1 kg of raw coal will release 2.66 kg of carbon dioxide, which means that the consumption of 1 kg of standard coal
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will bring 3.525 kg of carbon dioxide. Besides, the complete combustion of per-cubic-meter natural gas of standard coal will release 1.92 kg of carbon dioxide [32]. Figure 9(b) shows the amount of carbon dioxide emissions of different water heaters. Compared to traditional EWHs, the use of SWHs can reduce emissions of carbon dioxide effectively by more than 65%. Moreover, as one type of clean energy, the emissions can be further reduced by using gas as auxiliary heating. Among all the water heaters, although the consumption of standard coal of GA-VTSWHs is slightly higher than that of ASHP-VTSWHs, the carbon dioxide 15
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emissions of GA-VTSWHs are much lower than any other water heaters. From this analysis, it is obvious that the GA-VTSWHs perform better compared with other water heaters, considering socioeconomic effects. Therefore, for long-term energy savings and emission reductions, it is important to popularize SWHs’ usage.
5. Implications for SWHs’ Development and Use
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5.1 Research on Users’ Satisfaction with SWHs A survey among SWH users was conducted in Yingong Villa, which is one of the demonstration areas for SWHs in Nanjing. More than 200 participants responded to the survey. According to the survey (Figure 10(a)), respondents showed greater preference for purchasing water heaters that are cost-effective, have long lifecycles, are energy-efficient, and
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are safe for daily usage. About 95% of respondents thought that SWHs were energy-efficient, while more than 60% thought that SWHs were cost-effective and safe for domestic use. By comparing the advantage of SWHs in Figure 10(b) to the distributions in Figure 10(a), it was evident that SWHs are ideally suited for residents’ hot water supply needs. In general, all of
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the respondents were either satisfied or ambivalent about using SWHs (Figure 10(c)). In light of such positive feedback, we recommend that policymakers promote SWHs’
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household use. However, successfully using SWHs for households hinges on addressing some of the heating systems’ inefficiencies. For example, issues with frozen water pipes in
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Nanjing have not been resolved yet. According to the survey (Figure 10(d)), almost half of the respondents in Nanjing complained about freezing water pipes in winter. Moreover,
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one-fifth of residents complained about the heating time required to bring the cold water to a usable temperature of 50C. These issues highlight the need for improving the heating
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systems’ design, especially the thermal insulation of plumbing pipes. Moreover, auxiliary heating is necessary to accelerate SWHs’ heating speed.
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100
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Frequency
Frequency (%)
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Frequency
60
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0 Energy-saving
Cost-effective
Safe
Satisfied service
Cost-effective
(a)Factors that residents consider while purchasing a water heater
Satisfied 49%
Water pipe frozen 49%
Affected by the weather 12%
Safe
(b)Advantages of SWH recognized by residents
Slowly water temperature rising 20%
(c)User satisfaction about SWH
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Low water temperature 10%
Energy-saving
Extremely satisfied 8%
Unsatisfied aftermarket service 4%
Often failing 5%
Long lifecycle
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Frequency (%)
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General 43%
(d)Common complaints from consumers about SWH
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Figure 10. Summary of SWH survey results.
5.2 Key Problems in Developing SWHs
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Compared to traditional water heaters, SWHs have greater potential for saving energy and reducing emissions. However, there are several problems associated in the promotional
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process that make SWHs unappealing to consumers. First, the SWH market in China is at an immature stage. Requirements for SWHs’ manufacturing can be achieved easily, which
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results in numerous similar SWH products on the Chinese market. Yet with the absence of standardization, the quality of SWHs fluctuates significantly among different manufacturers,
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leading to low consumer-confidence levels. Although a large number of low-cost SWH manufacturers have helped promote the development of the SWH industry, policymakers should strengthen regulations by formulating national standards for SWH products. Also, with the increased level of urbanization in China, residential buildings are being replaced by multistory building. Unlike individual residential homes, residents of such buildings only have access to a limited open area: usually a balcony that might not be facing direct sunlight. This limited space for setting solar collectors presents a challenge for 17
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installing a VTSWH. Aesthetically, too, VTSWHs are not preferred. The limitations of VTSWHs can be addressed by using FPSWHs, but these have a higher initial investment cost. The FPSWH industry technology in developed countries is more efficient than in China. FPSWHs cover a wide area of a total building area in developed countries, and the collector effect is more efficient compared to the VTSWHs that are usually employed in China. Therefore, high-efficiency FPSWHs for indigenous use should be explored.
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In the rural areas, policymakers should vigorously develop the use of GA-VTSWHs. Considering the long-term economic and environmental benefits, GA-VTSWHs are preferable to any other water heaters. However, there are still several unresolved issues with GA-SWHs. First, the complex design of GA-SWHs make them more difficult to install than
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EA-SWHs. Furthermore, GA-SWHs rely on the availability of a gas network, thereby limiting their application to residential units that receive service from the gas network. Research on simpler designs, along with reducing the production costs, should be encouraged for GA-VTSWHs. For areas that are not connected to gas networks, EA-SWHs can be
5.3 Suggestions for SWHs’ Use
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promoted as a viable option.
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Government policy plays a key role in the development and wide-scale deployment of SWHs. Hence, we make the following suggestions for selecting SWHs. SWHs should be
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used for water heating when there is enough space and abundant sunshine, and electricity or gas should be used as auxiliary energy if SWHs are unable to meet the hot water
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requirements. Regarding FPSWHs, we note that these are suitable for water heating when indoor space is limited or a higher indoor aesthetic is required. In general, we recommend
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taking the entire solar energy reuse plan into account, from the building design to the building’s whole lifecycle [33]. And when using FPSWHs, antifreeze must be taken if the minimum operating temperature is below 0℃. In addition, FPSWHs are more suitable for high-rise buildings, as they can be mounted on the wall outside. In contrast, VTSWHs would be a good choice if the minimum operating temperature is below 0℃ or there are wide enough spaces.
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6. Conclusion In this work, we established experimental platforms to measure the real-time annual thermal performance of SWHs in the cities of Nanjing and Lianyungang, within China’s Jiangsu province. We carried out this year-long experimental study using different kinds of water heaters, and compared the dynamic thermal performance (such as the coefficient of
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thermal performance, solar fraction, and the solar energy protection ratio) of SWH systems. Results show that overall, SWHs perform well in the Jiangsu province; however, the SWHs in Lianyungang have a better thermal performance than Nanjing’s because they receive more solar radiant energy. We also observed that VTSWHs have a better thermal performance throughout the year compared to FPSWHs. Meanwhile, from a socioeconomic perspective,
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most SWHs are far preferable to traditional water heaters. Among all the SWHs, GA-VTSWHs performed the best over a long period compared to other types of water heaters. These results are helpful for developing and determining usage for future SWHs in China. Our experimental conclusions are supported by a large amount of data collected over a
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long duration. The experiment also considers the impact of climate on SWHs’ performance, with different climate zones selected in the experiment to ensure accuracy. A particularly
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innovative aspect of this paper is that it presents the concept of a solar energy protection ratio. Employing this ratio to compare the actual satisfaction of using VTSWHs and FPSWHs from
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a practical point of view strengthens our conclusions, and provides helpful data for other SWH simulation experiments. These results also act as a reference point for selecting water
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heaters in Jiangsu, helping policymakers make more informed decisions. However, there is still much work to be done. For example, we did not measure the hot water circulation flow
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rate of SWHs in this experiment, but we plan to in future work.
Acknowledgement This study is financially supported by National Natural Science Foundation of China (No. 51606037), Natural Science Foundation of Jiangsu Province (No. BK20160687), China’s Manned Space Program (TZ-1), and Building energy saving special guidance fund of Jiangsu and the Research Funds of Key Laboratory of Heating and Air Conditioning, The Education Department of Henan Province. 19
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References [1] E. Koepf, I. Alxneit, C. Wieckert, A. Meier, A review of high temperature solar driven reactor technology: 25 years of experience in research and development at the Paul Scherrer Institute, Applied Energy 188 (2017) 620-651. [2] H.M. Abd-ur-Rehman, F.A. Al-Sulaiman, Optimum selection of solar water heating
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(SWH) systems based on their comparative techno-economic feasibility study for the domestic sector of Saudi Arabia, Renewable and Sustainable Energy Reviews 61 (2016) 336-349.
[3] F. Urban, S. Geall, Y. Wang, Solar PV and solar water heaters in China: Different
AN US
pathways to low carbon energy, Renewable and Sustainable Energy Reviews 64 (2016) 531-542.
[4] H. Zou, H. Du, J. Ren, B.K. Sovacool, Y. Zhang, G. Mao, Market dynamics, innovation, and transition in China's solar photovoltaic (PV) industry: A critical review, Renewable and
M
Sustainable Energy Reviews 69 (2017) 197-206.
[5] T. Chow, Y. Bai, Z. Dong, K. Fong, Selection between single-phase and two-phase
ED
evacuated-tube solar water heaters in different climate zones of China Solar Energy 98 (2013) 265-274.
PT
[6] A.H. Al-Badi, M.H. Albadi, Domestic solar water heating system in Oman: Current status and future prospects, Renewable and Sustainable Energy Reviews 16 (2012) 5727-5731.
CE
[7] D. Nugent, B.K. Sovacool, Assessing the lifecycle greenhouse gas emissions from solar PV and wind energy: a critical meta-survey, Energy Policy 65 (2014) 229–244.
AC
[8] S. Rashidi, J.A. Esfahani, A. Rashidi, A review on the applications of porous materials in solar energy systems, Renewable and Sustainable Energy Reviews 73 (2017) 1198-1210. [9] Z.M. Omara, A.E. Kabeel, A.S. Abdullah, A review of solar still performance with reflectors, Renewable and Sustainable Energy Reviews 68 (2017) 638-649. [10] R. Singh, I.J. Lazarus, M. Souliotis, Recent developments in integrated collector storage (ICS) solar water heaters: A review, Renewable and Sustainable Energy Reviews 54 (2016) 270-298. [11] X. Fang, D. Li, Solar photovoltaic and thermal technology and applications in China, 20
ACCEPTED MANUSCRIPT
Renewable and Sustainable Energy Reviews 23 (2013) 330-340. [12] R. Hu, P. Sun, Z. Wang, An overview of the development of solar water heater industry in China, Energy Policy 51 (2012) 46-51. [13] R. Zheng, J. Nie, Analysis for Marketization Development Prospect of Large-scale Solar Heating Combisystems in China, Energy Procedia 70 (2015) 574-579. [14] S. Qiu, M. Ruth, S. Ghosh, Evacuated tube collectors: A notable driver behind the solar
CR IP T
water heater industry in China, Renewable and Sustainable Energy Reviews 47 (2015) 580-588.
[15] R. Tang, Y. Yang, W. Gao, Comparative studies on thermal performance of water-in-glass evacuated tube solar water heaters with different collector tilt-angles, Solar
AN US
Energy 85 (2011) 1381-1389.
[16] F. Mauthner, W. Weiss, Solar Heat Worldwide, Markets and Contribution to the Energy Supply 2011. IEA Solar Heating & Cooling Programme (2011) http://www.iea-shc.org/ Data/Sites/1/publications/Solar-Heat-Worldwide-2013.pdf. (accessed 17.06.21)
M
[17] H. Li, H. Yang, Potential application of solar thermal systems for hot water production in Hong Kong, Applied Energy 86 (2009) 175-180.
ED
[18] R. Tang, Y. Yang, W. Gao, Comparative studies on thermal performance of water-in-glass evacuated tube solar water heaters with different collector tilt-angles, Solar
PT
Energy 85 (2011) 1381-1389.
[19] T. Chow, Z. Dong, L. Chan, K.F. Fong, Y. Bai, Performance evaluation of evacuated tube
CE
solar domestic hot water systems in Hong Kong, Energy and Buildings 43(12) 2011 3467-3474.
AC
[19] B. Grant, B. Pradeep, New test method for gas boosters with domestic solar water heaters, Solar Energy 86(1) (2012) 78-86. [21] X. Zhang, S. You, W. Xu, M. Wang, T. He, X. Zheng, Experimental investigation of the higher coefficient of thermal performance for water-in-glass evacuated tube solar water heaters in China, Energy Conversion and Management 78 (2014) 386-392. [22] CEN, European Standard prEN 12975-2:2000, European Committee for Standardisation (2000). [23] G.G. Dawit, M. Pinakeswar, P.S. Robi, CFD and experimental investigation of flat plate 21
ACCEPTED MANUSCRIPT
solar water heating system under steady state condition, Renewable Energy 106 (2017) 24-36. [24] Chinese Academy of Building Sciences, Green building evaluation technical rules, Beijing, China: China architecture & building press (2015). (in Chinese) [25] Jiangsu Provincial Department of Housing and Urban-Rural Development, Design standard of thermal-environment & energy conservation for residential buildings in Jiangsu
CR IP T
province. Standard of Engineering Construction in Jiangsu Province DGJ32/J 71-2014, Jiangsu, China (2014). (in Chinese)
[26]CNS, Thermal design code for civil building, Chinese National Standard GB/50196, China (1993). (in Chinese)
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[27] Y. Zhou, W. Wu, Y. Hu, Q. Fang, G. Liu, Evaluation of available solar energy resources in Jiangsu Province, Renewable Energy Resources 28 (2010) 10-13. (in Chinese) [28] CNS, Technical conditions for domestic solar water heating systems, Chinese National Standard GB/T19141,China (2011).(in Chinese)
[29]CNS, Energy efficiency limit and energy efficiency rating of household solar water
M
heating system. Chinese National Standard GB 26969-2011, China (2011). (in Chinese) [30] K. Kerrigan, H. Jouhara, G.E. O’Donnell, A.J. Robinson, A naturally aspirated convector
67 (2013) 187-194.
ED
for domestic heating application with low water temperature sources, Energy and Buildings
PT
[31] C.E. John, G.S. Lukas, Domestic hot water consumption estimates for solar thermal system sizing, Energy and Buildings 58 (2013) 58-65.
CE
[32] W. Li, G. Song, M. Beresford, B. Ma, China's transition to green energy systems: The economics of home solar water heaters and their popularization in Dezhou city, Energy
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Policy 39 (2011) 5909-5919. [33] CNS, Technical conditions for domestic solar water heating systems, Chinese National Standard GB/T 2589-2008,China (2008).(in Chinese) [34] State Environmental Protection Administration, Practical Handbook for Discharge Declaration and Registration, Beijing, China: China Environmental Science Press (2004). (in Chinese) [35] J. Shi, W. Su, M. Zhu, H. Chen, Y. Pan, S. Wan, Y. Wang, Solar water heating system 22
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integrated design in high-rise apartment in China, Energy and Buildings 58 (2013) 19-26.
Figure captions
Figure 2 Schematic of experimental system
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Figure 1 The experimental system’s schematic
Figure 3 Annual dynamic water temperature variation
Figure 4 The weather’s Influence on the SWH’s water temperature Figure 5 SWHs’ annual heat-collecting efficiency
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Figure 6 Solar fraction of different SWHs
Figure 7 Solar energy protection ratio of different SWHs
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Figure 8 Economic analysis of different water heaters Figure9 Different water heaters’ social effect analysis
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Tables
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Figure10 Summary of SWH survey results
Table 1 Solar water heaters’ (SWHs’) parameters in the experiment
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Table 2 Coefficient of thermal performance (CTP) of SWHs Table 3 SWHs’ annual solar fraction and solar energy protection ratio
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