Assessing the Potential and Benefits of Domestic Solar Water Heating

Assessing the Potential and Benefits of Domestic Solar Water Heating System Based on Field Survey Shuwen Niu ,a,b Zhenguo Hong,b Wenli Qiang,b Yingdong Shi,b Man Liang,b and Zhen Lib a Key Laboratory of Western China’s Environmental Systems (Ministry of Education), Lanzhou University, Lanzhou 730000, China b College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, China Published online 00 Month 2017 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.12827 Domestic solar water heating system (DSWHS) without auxiliary energy is widely used in loess hilly region of China. However, there is a lack of studies on DSWHS in this region. This article assessed the potential and benefits of DSWHS on the basis of field survey. Results indicate that a DSWHS with evacuated tube collectors (ETC DSWHS), with a collecting area of 2.09 m2 and an average thermal efficiency of 40%, can supply heat of 3661.7 MJ during its valid period of 263 d. A household with an average of 4.9 people consumes annually 11,043.5 kg of hot water supplied by DSWHS. A total of 1238.7 MJ of energy is needed to heat so much water. The actual thermal efficiency of DSWHS is only 13.53%. Clearly, there is a great potential to improve the energy efficiency of DSWHS. DSWHS has low economic benefit because of its low actual thermal efficiency. However, it has enormous environmental benefits and significant social impacts. The 1238.7-MJ energy provided by it is equal to that provided by 211.6-kg raw coal, which can emit 454.99-kg CO2, 3193.79-g SO2 during use. Rural residents are recommended to use more hot water supplied by DSWHS through increasing the frequency of bathing and cloth washing. The government should further promote DSWHS utilization by driving C 2017 American Institute of Chemical Engineers Envicosts down. V ron Prog, 00: 000–000, 2017

Keywords: domestic solar water heating system (DSWHS), resource potential assessment, economic feasibility, environmental benefit, social benefit

INTRODUCTION

The excessive usage of fossil fuels has caused chain environmental consequences worldwide, including air pollution and global warming [1]. It is compelling to use renewable energy from the perspective of sustainable development. Solar energy, as one type of renewable energy, has been more and more widely used [2]. Currently, there are three main ways to use solar energy. First, photovoltaic conversion is one of the fastest growing renewable energy technologies in the world. During photovoltaic conversion, solar radiation can be directly converted into electricity [3]. Second, photothermal conversion technology could be utilized in solar collectors, solar thermal plants, and so on. Third, photochemical conversion enables the conversion of solar energy into thermal and electrical energy. Artificial energy conversion is being developed [4]. Additional Supporting Information may be found in the online version of this article. C 2017 American Institute of Chemical Engineers V

Some hybrid system about solar energy are also utilized or analyzed to meet sustainable development [5]. Advantages and Disadvantages of Solar Energy Use Solar energy has some unique advantages. (a) Wide distribution [6], (b) free of charge [6], (c) no greenhouse gas (GHG) emission, the environmental impact is relatively low [7], (d) great potential, (e) low dependence on public facilities [8], (f) wide application, such as water heating, space heating, and photovoltaic generation, and (g) no noise or smell during usage. However, solar energy also has the following obvious disadvantages. (a) Low energy density [6]. (b) Intermittent supply. It cannot be collected at night or during cloudy and rainy days [8]. (c) The requirement of a large area of land for installation of solar energy device. This limits its use in regions with high population density [6,7]. (d) High cost of storage and conversion at large scale. These characteristics seriously affect the utilization of solar energy. In households, hotels, factories and other recipients, energy is mainly used to heat water [9]. The domestic solar water heating system (DSWHS) is most widely used to heat water using solar energy and is also one of the most successful renewable energy technologies [10]. During the last two decades, DSWHSs have been commercialized in many counties worldwide [11]. Therefore, there have been many reports about DSWHS. Literature Review on DSWHS Three types of stationary collectors are commonly used in DSWHS: flat plate collectors (FPCs), evacuated tube collectors (ETCs), and compound parabolic collectors (CPCs) [12]. Among them, ETCs are mostly utilized for small-scale water heating applications [13]. Previous reports mainly focused on performance evaluation and feasibility analysis of DSWHS [1,14–18]. The technical evaluation of DSWHS shows that solar energy accounts for considerable percentage of energy consumption of a family for heating water in the Mediterranean countries [14]. According to Tunisian weather data, a total solar radiation of 4653.13 MJ/m2 is collected annually by the 3.4-m2 ETC DSWHS, and the average solar fraction can reach 84.4% if auxiliary energy is supplied to the ETC DSWHS [19]. The annual energy output of DSWHS ranges from 1260 to 2880 MJ/m2 in Greece [20]. One of the most critical factors affecting the dissemination of DSWHSs is their financial viability [11], which directly influences residents’ willingness to pay for them. In a typical Tunisian household with 4–5 persons, the annual savings in

Environmental Progress & Sustainable Energy (Vol.00, No.00) DOI 10.1002/ep

Month 2017 1

Figure 1. Distribution of sampling sites in the study region.

electrical energy consumption resulting from the use of FPC and ETC are about 1316 and 1459 kWh/yr with a payback period of around 8 and 10 yr (based on electric water heater), respectively [15]. In Greece, solar energy cannot compete financially with heat produced by oil and natural gas. The DSWHS could reduce winter peak electrical demand by 13% and final energy demand by 27% in Zimbabwe [16]. In addition, it is the most inexpensive heater for domestic use in Jordan [17]. The most important benefit of renewable energy systems is that they can reduce environmental pollution from the burning of fossil fuels [14]. In Canada with colder climates, if all eligible domestic hot water systems were to be replaced by DSWHS, 22.7 PJ of end-use energy and 1 Mt of GHG emissions would be reduced from the residential sector [18]. These studies indicate that DSWHS has not only good economic feasibility but also good social and environmental benefits [5,12,21]. DSWHS Application in China In China, the coal-dominated energy structure has caused many environmental problems. Coal accounted for 67.5% of the total energy consumption in 2013 [22]. Fortunately, there are abundant solar resources in China [23]. To increase the proportion of renewable energy consumption, the Chinese government strongly supports the development and utilization of solar energy, including photovoltaic generation, spatial heating and cooling, DSWHS, etc. [2,24]. China’s DSWHS ownership increased from 3.5 million m2 in 1998 to 414 million m2 in 2014 [25]. At present, China has become the largest country to manufacture and utilize DSWHS in the world. Thus, it is important to assess the feasibility of DSWHS. The Knowledge Gap in the Existing Literatures In previous studies, economic and environmental life cycle assessment was used to evaluate the performance of solar heating system. Some software tools such as Gemis, Ecoinvent 2.0, and TRNSYS were applied to model analysis [9,12,26–28]. With these tools, the energy output can be 2 Month 2017

calculated conveniently according to the system details. Further, the efficiency of DSWHS was calculated by modeling approach and scenario analysis [17,29]. In existing empirical analysis, DSWHS is equipped with auxiliary energy component so that it can continuously supply hot water. However, the supply of hot water may be interruptedly in reality. The deficiencies in the existing literatures are as follows. (a) There are few reports about the amount of energy from DSWHS actually used by households. (b) Most DSWHSs are not configured with auxiliary energy component, so hot water is supplied intermittently. This case has not been investigated to date. (3) There is a lack of studies on DSWHS in China, especially empirical research. In this article, field survey was conducted to investigate the actual hot water use in households with DSWHSs on the basis of daily family life. Then, we calculated the amount of hot water actually consumed by households, on which basis we analyzed the economic feasibility, environmental and social benefits of DSWHS. Main contributions of this study are as follows: (a) calculating the amount of energy from DSWHS actually used by households; (b) assessing the potential as well as the economic, environmental, and social benefits of DSWHS without auxiliary energy component; (c) explaining why DSWHS ownership increases quickly in the study region; and (d) proposing some measures and policy suggestions to promote DSWHS use. REGIONAL CHARACTERISTICS

Our sampling sites are distributed in 12 towns and 15 villages in the Western Loess Plateau, China. The whole study region includes three counties and two districts of Gansu Province (Figure 1). The region is a typical loess hilly region with crisscross gullies, sparse vegetation and most severe loss of water and soil [30]. In addition, it is a typically semiarid warm temperate zone with mean annual temperature of 8.9–11.08C. Furthermore, this region has an abundant and reliable supply of solar resource with mean annual sunshine


Figure 2. Schematic diagram of an ETC DSWHS.

time of 1843.5–2469.4 h and a total solar radiation of about 5500 MJ/m2 [31]. It is an ancient agricultural region in China. However, it is underdeveloped due to low land productivity. In 2014, rural per capita net income and urban per capita disposable income in this region were 747.5 USD and 2757.1 USD, respectively, which were far lower than those in other regions. Local residents depended heavily on biomass fuels for a long time. In recent years, the local government has been encouraging the use of solar cookers and biogas digesters since 2003 [32]. Thus, DSWHS application becomes increasingly popular in this region. At present, DSWHS ownership for households is 25% in rural area and 35% in urban area. Households in this region are switching from traditional fuels to multiple fuels, which enables higher energy efficiency. METHODS AND DATA

General Schematic of ETC DSWHS ETC DSWHS consists of four major components, including ETCs, storage tank, water-pipe, and steel frame (Figure 2). Steel frame supports and allows the installation of DSWHS on roofs (Figure 3). DSWHS is passive type solar water heater, requiring no pumping for circulation of water. Water is circulated by natural convection in DSWHS. Collector absorbs solar radiation. Cold water flows into evacuated tubes from the storage tank and is thus heated. Because of lower density of hot water, the heated water rises and enters storage tank again, and colder water runs down into evacuated tubes. Such circulation lasts for a long time, and all water in the storage tank can be heated. Figure 2 shows how ETC DSWHS works. Methods Our working ideas is shown in Figure 4. For the potential analysis, total annual solar radiation per unit area (PI), the available energy obtained annually by DSWHS (PII) and the energy from DSWHS actually utilized by a household per

year (PIII) were considered and calculated. Substitution analysis was employed to calculate the amount of coal, crop straw, LPG, and electricity that can produce the same amount of energy as PIII: Finally, the economic, environmental, and social benefits of DSWHS were assessed on above basis.

The Energy from DSWHS Actually Used by the Households If h1 is the thermal conversion efficiency (%) of solar collector specified by the manufacture, the available energy (He1 , MJ/m2) obtained annually by an ETC DSWHS (PII) could be calculated as follows: He1 5Hts 3h1

(1)

He1 calculated by Eq. (1) is only a theoretical value. Hot water in the storage tank would get cold if it was not used in time. Thus, He2 was proposed here to indicate the hot water in storage tank actually used by the households, which can be assessed through field survey. From February to June 2015, a questionnaire survey was conducted concerning DSWHS use in the study region. We interviewed residents face to face, learned about the technical performance of the DSWHS installed in their homes and recorded relevant information included the type, size, brand, and prices of the DSWHS, family members, living habits, situations of DSWHS use, and so on. We interviewed 469 households and obtained 371 effective questionnaires. Specifications, technical parameters and prices of DSWHSs vary a lot. For example, DSWHS with a lower price may have a larger storage tank and collecting area, and collecting area is not strictly proportional to the number of glass tubes. Therefore, we statistically analyzed the technological and economic attributes of DSWHSs and calculated the weighted average according to frequency distribution of these samples.


Month 2017 3

Figure 3. ETC DSWHS installed on roofs. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 4. Assessing potential and benefits of DSWHS.

On the basis of field survey, the energy from DSWHS actually used by household (He2 ) and the actual thermal efficiency (h2 ) of DSWHS can be calculated as follows: He2 ðTh1 -Tl Þ3S1 3c1ðTh2 -Tl Þ3S2 3c h2 5 5 Hts Hts

(2)

where h2 is the actual thermal efficiency of DSWHS (%), He2 is the amount of energy from DSWHS actually used by household (MJ), Th1 is water temperature in the outlet for bath (8C), Th2 is water temperature in the outlet for daily washing and laundry (8C), Tl is the inlet water temperature (8C), S1 is the amount of hot water consumed for bath (L), S2 is the amount of hot water consumed for daily washing and 4 Month 2017

laundry (L), and c is the specific heat capacity of water (MJ/(kg8C)). The Economic and Environmental Benefits of DSWHS The respective amount of coal, biomass energy, LPG and electricity, which can produce the same amount of energy as He2, were calculated. According to the prices and emission factors of these energy sources, the economic and environmental benefits of DSWHS can be quantitatively assessed: Fi 5

He2 1 3 hi Ui

(3)

where He2 is the amount of energy from DSWHS (MJ), Fi is the amount of the ith type of energy source (kg, kWh), Ui is


the unit caloric value of the ith type of energy source (MJ), and hi is the actual thermal efficiency of the ith type of energy source (%). The service life of the contemporary DSWHS is estimated to be about 15 yr [1,14]. Economic and environmental benefits of a DSWHS can be expressed as follows, respectively: Ri 5153Fi 3pi

(4)

Eij 5153Fi 3fij

(5)

where Ri is the economic benefit (USD) of DSWHS if it replaces the ith type of energy source for water heating, pi is the unit price of the ith type of energy source (USD), Eij is the emission of jth pollutant during the use of the ith type of energy source (kg), and fij is the emission factor of jth pollutant during the use of the ith type of energy source (GHG and pollutants emissions were included in the calculation of environmental benefit) [33,34]. Further, we can assess the economic feasibility of DSWHS through the cost–benefit analysis. When the net present value (NPV) is equal to zero, the initial capital investment is just recovered. NPV is defined as: n X NPV5 t51

Ct 2I ð11rÞt

(6)

where I is the initial capital investment, Ct is the aggregate costs incurred in year (i), r is the social discount rate, and t is the service life of DSWHS. THE ECONOMIC, ENVIRONMENTAL, AND SOCIAL BENEFITS OF DSWHS

Performance of the ETC DSWHS The total annual solar radiation is 5474.52 MJ/m2 in the study region [31]. According to previous reports [23,35], the area with a solar radiation of 5040–6300 MJ/(m2a) is relatively suitable for solar energy use, meaning the primary potential (PI) of the study region is considerable. In this study, no auxiliary energy is used for DSWHS, and residents stop using the ETC DSWHS in winter when mean daily temperature is below 08C to prevent its glass tubes from being frost-cracked. Therefore, we calculated the number of days when DSWHS was used (Figure 5). Results show that DSWHS can be used for about 263 d in each year, which is then defined as the valid period of DSWHS. According to Supporting Information, we calculated the total annual solar radiation (Hts ) received by the inclined plane and the available energy (He1) obtained by an ETC DSWHS (see Supporting Information). Results show that Hts is 5750.6 MJ/m2, which is more than that obtained by horizontal surface. Hts is 4380 MJ/m2 during the valid period (263 d), accounting for 76.2% of the total radiation. Thus, an ETC DSWHS with a collecting area of 2.09 m2 is able to obtain a total radiation of 9154.2 MJ during the valid period. According to field investigation, the thermal efficiency (h1 ) of ETC DSWHS specified by manufacturers is 35%–45%, and the average value of 40% was adopted in this article. Then, the available solar radiation converted by inclined plane is 1752 MJ/m2, and that converted by the ETC DSWHS with a collecting area of 2.09 m2 is 3661.7 MJ. This amount of heat energy is called the second potential (PII), which is the available energy source for household. The Energy Actually Used by Residents According to Their Daily Life From the field survey, we obtained some information related to DSWHS use, including family size (persons), using

Figure 5. The usage period of DSWHS in study region.

years (yr), number of class-vacuum tubes, collecting area (m2), tank capacity (L) and the prices of DSWHS (including purchase and installation costs, USD). By statistical analysis of 371 effective questionnaires, the distribution of family size and the attributes of ETC DSWHS are shown in Figure 6. These parameters vary a lot and do not strictly match between sample households. There is an obvious difference between adults and children in the frequency of bathing. Adults bathe one to two times each month, while children bathe three to four times. However, there is no significant difference in the frequency of bathing, daily washing, and cloth washing among sample households. These data reflect the actual use of hot water by local residents. The average values of family sizes and of above attributes of DSWHS were calculated. On this basis, S1 and S2 in Eq. (2) can be calculated, and the amount of energy from DSWHS actually used by household can also be calculated (Table 2). According to field survey, the water temperature in the outlet is 428C for bath (Th1 ) and 378C for daily washing and laundry (Th2 ). In our study region, the average temperature is 11.948C from March 4th to November 21st, which was taken as the water temperature in the inlet (Tl ). Residents usually bathe and wash clothes on sunny days, so there is a time interval (several days) between two times of bathing or cloth washing. As shown in Table 2, the frequency of bathing and cloth washing is low. Note that hot water is basically always used for daily washing. When the weather is cloudy or rainy for two or more days, however, DSWHS cannot supply hot water for daily washing. According to the local meteorological data [31], three are no more than three days with such weather in each month. Therefore, 27 days per month was taken as hot water usage period for daily washing. The results show that a household with an average of 4.9 people consumes annually hot water of 11,043.5 kg, consisting of 3636.9 kg of hot water (428C) used for bath (S1) and 7406.6 kg of hot water (378C) used for daily washing and laundry (S2). Thus, the average amount of hot water used per household per day is only 42 kg. Total energy used for heating 11,043.5 kg of water is 1238.7 MJ, which is defined as the amount of energy from DSWHS actually used by a household during the valid period. However, DSWHS can supply a total energy of 3661.7 MJ during the valid period in each year. According to Eq. (2), the actual thermal efficiency of DSWHS (h2 ) is only 13.53%, which is about one-third of the thermal efficiency of DSWHS specified by manufacture (h1 , 40%). This indicates that more hot water can be used


Month 2017 5

Figure 6. Distributions of family size and parameters of ETC DSWHS.

and there is a potential to improve the efficiency of DSWHS use. As mentioned earlier, residents tend to reduce the frequency of bathing and cloth washing when indoor temperature is low, so DSWHS is not used in winter. There is no district heating system in rural area, and local residents often use coal stove and Kang [36] to keep warm. Therefore, there is a low demand of hot water in winter. Hot water for daily washing and laundry in winter is heated by a stove or electricity. According to the field survey, a typical household uses 1605 kg of coal every year, and 70% of the coal is used for heating water, cooking, and keeping warm in winter. Benefit Analysis The Economic Benefit of DSWHS

The thermal efficiencies of raw coal, crop straw, LPG, and electricity are different (Table 3). According to previous report [32], we calculated the amount of each energy source that can produce the same amount of energy as that from DSWHS actually used by household according to Eq. (3). Further, we calculated the economic benefit of DSWHS according to Eq. (4). The energy from DSWHS actually used 6 Month 2017

by household is 1238.7 MJ per year. To produce this amount of energy, 211.6 kg of raw coal, 411.5 kg of crop straws, 41.1 kg of LPG, and 430.1 kWh of electricity is needed, respectively. According to current market prices of these energy sources, they should cost 397.1, 227.2, 794.6, and 522.3 USD during a period of 15 yr (service life of DSWHS), respectively (Table 3). The prices of ETC DSWHSs ranges from 264.5 to 956.8 USD (including purchase and installation costs), and the average price is 506.9 USD. The annual maintenance cost is about 2.94 USD, which is close to that reported by Han et al. [37]. According to the above analysis, the average cost of DSWHSs is higher than that of raw coal and crop straw, lower than that of LPG, and close to that of electricity. In addition, NPVs of DSWHS replacing coal, crop straws and electricity remains negative (0) in the 12th year (Figure 7-1). This indicates that DSWHS has poor economic feasibility. This is because residents only use a part of hot water that is supplied by DSWHS and its actual thermal efficiency (h2 ) is quite low. If the actual thermal efficiency of DSWHS (h2 ) is equal to the thermal efficiency specified by manufacture (h1 ), meaning all hot water supplied by DSWHS in a household can be


Table 1. Family size and main parameters of ETC DSWHS.

Term

Family size (persons)

Using years (yr)

Glass tubes (number)

Collection areas (m2)

Tank capacity (L)

Prices (USD)

Max Mean Min

10 4.9 1

15 4.1 0.1

10 17.4 25

30.8 2.09 1.2

200.0 139.5 80.0

956.8 506.9 265.0

Table 2. Application situation of ETC DSWHS. Term Number of person Frequency (time/month) Hot water use (L/time) Total water use (L/yr, 263 days†) Total water use (L/yr, 12 months) Water temperature (8C)

Adult bath

Child bath

Daily washing

Washing clothes

3.4 1.43 41.29* 1735.8 2409.0 42

1.5 3.55 41.29* 1901.1 2638.4 42

4.9 27 23.7** 5528.3 7672.3 37

4.9 3.9 55.7** 1878.3 2606.8 37

*It is water consumption for bathe per person per time. **It is water consumption of the whole family for daily washing and laundry. † 263 days are converted into equivalent 8.647 months.

Table 3. Parameters in Eq. 3 and Eq. 4 and results.

Fuel Coal Crop straws LPG Electricity

Ui (MJ/kg, MJ/kWh)

pi (USD/kg, USD/kWh)

gi

Fi † (kg, kWh)

Fi †† (kg, kWh)

Ri † (USD)

Ri †† (USD)

20.908 15.05* 50.2 3.6

0.85/6.7934 0.25/6.7934 8.75/6.7934** 0.55/6.7934

28% 20% 60% 80%

211.6 411.5 41.1 430.1

625.5 1216.5 121.6 1271.4

397.1 227.2 794.6 522.3

1173.9 671.5 2348.7 1544.0

Notes: 1 USD 5 6.7934 RMB. *The average of wheat and maize stalk. **105 RMB of a bottle 12 kg LPG. † Calculating with He 5 1238.7 MJ. †† Calculating with He 5 3661.7 MJ.

consumed, the energy from DSWHS actually used by the household will reach 3661.7 MJ. Then, the economic benefit of DSWHS would be considerable (Table 3). This indicates that there is a great potential to increase the economic benefit of DSWHS. When h2 5h1 , the NPVs of DSWHS replacing LPG, electricity, raw coal, and crop straw will become positive in 3.5, 4.5, 7.6, and 15.3 yr, respectively (Figure 7-2). If household utilizes auxiliary energy, the valid period of DSWHS will be 365 d rather than 263 d. Then, the household can obtain useful heat of 2300.2 MJ per year from a DSWHS. However, we did not find any case where auxiliary energy was used for DSWHS during field survey in the study region. According to Hazami et al. [19], the annual auxiliary solar radiation of the ETC DSWHS is 962.65 MJ/m2. Thus, the DSWHS with a collecting area of 2.09 m2 needs auxiliary energy of 2011.9 MJ/yr. However, this is almost equal to the total energy used for bath, daily washing, and cloth washing (2300.2 MJ). Auxiliary energy, which is provided by electricity, is mainly needed in winter. Obviously, using auxiliary energy is uneconomic, so residents just stop using DSWHS in winter. In short, the economic benefit of DSWHS is low, but it has a potential to increase.

The Environmental Benefits of DSWHS

The most important benefit of DSWHS is that it can mitigate environmental pollution [14]. ETC DSWHS without using auxiliary energy does not produce any GHG or pollutants during its use. Coal, crops straw, LPG, and electricity are main conventional energy resources in this area. They can emit many GHG and pollutants during usage. As mentioned above, a household with an average of 4.9 people consumes annually 11,043.5 kg of hot water that is supplied by ETC DSWHS. Heating this amount of water needs 1238.7 MJ of energy. If this amount of energy is provided by conventional energy sources, 211.6 kg of coal, 411.5 kg of crops straw, 41.1 kg of LPG, and 430.1 of kWh electricity are needed per year, respectively. Then, the emissions of various GHG and pollutants when these non-clean energy sources are used could be calculated according to emission factors. The results are shown in Table 4. Among them, crop straw is a type of renewable energy, but it discharges the largest amount of CO2 (557.71 kg), N2O (28.8 g), CH4 (2031.6 g), and TSP (3748.3g) when it is directly used. Coal discharges the largest amount of NOx (454.99g) and SO2 (3193.79 g). LPG has the least emissions during usage. Coal and crop straw are burned directly in conventional cooking


Month 2017 7

Figure 7. NPVs of DSWHS replacing raw coal, biomass, LPG and electricity in two cases: (a) h2 51/3h1 and (b) h2 5h1 .

Table 4. The reduction of GHG and pollutants emissions if using DSWHS in a year. GHG and pollutant Coal (211.6 kg) Crops straw (411.5 kg) LPG (41.1 kg) Electricity (430.1 kWh)

CO2 (kg)

N2O (g)

NOx (g)

SO2 (g)

CH4 (g)

TSP (g)

454.99 557.71 129.73 206.16

5.85 28.80 1.27 3.01

375.29 345.55 96.66 111.85

3193.79 106.50 0.03 1363.44

582.89 2031.6 2.26 302.52

259.51 3748.31 * 127.07

Note: TSP is total suspended particle. *It is ignored too small.

ranges in the study area. This results in serious indoor environment pollution. Large amounts of GHG and pollutants emissions can be avoided by using DSWHS instead of conventional energy sources. Thus, large-scale application of DSWHS can bring enormous environmental benefits, including reducing emission, mitigating climate change, improving indoor environmental quality and thus reducing health hazards to residents. The Social Benefits of DSWHS

In this study, the most important social influence of DSWHS is that it changes residents’ lifestyles, especially increasing their frequency of bathing and cloth washing (Table 5). Rural residents rarely bathed or washed their clothes before DSWHS was installed in their homes. However, they have begun to bathe and wash clothes regularly and change their living habits since the installation of DSWHS. The amount of hot water used for bathing and cloth washing in a household with a DSWHS is 5515.2 kg/yr, more than twice the amount when a household does not have a DSWHS (2158.2 kg). In addition, using DSWHS to supply hot water enables improved air condition and sanitary condition in kitchen than using crop straw and coal to heat water. The larger amount of hot water consumption and the improved conditions in kitchen indicate improved healthcare for rural residents and higher quality of life. More importantly, rural residents learn the idea of low-carbon life through DSWHS use. DISCUSSION AND POLICY IMPLICATION

Pathways Increasing the Multiple Benefits of DSWHS In our study region, solar energy has been widely used for many purposes, including lighting, indoor warming, water heating and cleaning [38,39]. DSWHS has become a 8 Month 2017

part of local people’s life. According to the analysis of economic benefits, DSWHS is a common consumer goods rather than a good investment product. However, the DSWHS with auxiliary energy has promising application according to NPV analysis. The study of Kalogirou shows a payback time of 2.7 yr [14], the study of Hazami et al. shows a payback time of 10 yr [19], and the study of Kaldellis shows a payback period of 5.6–6.5 yr [1]. These studies are based on 120 L hot water per household with four people per day and more hot water use can result in a reasonable economic feasibility. In this article, we assessed the economic benefit of DSWHS according to the amount of hot water actually consumed by a household. Results indicate that there is a great potential to increase its economic benefit. This can be achieved by using more hot water, which is related to the actual thermal efficiency of DSWHS. Pathways that can increase the amount of hot water used by households are as follows: (a) using hot water for cooking after water purification; (b) increasing the frequency of bathing and cloth washing; (c) improving the insulation efficiency of storage tank of DSWHS so that the use period of hot water can be prolonged. If the actual thermal efficiency of DSWHS reaches the thermal efficiency specified by manufacture (40%), the economic benefit of DSWHS will be great. The Reasons that DSWHS Ownership Increases Quickly The economic feasibility of DSWHS is poor, but why does its ownership increase quickly in the study region? The possible reasons are as follows. (a) Residents have the willingness to improve their quality of life with income increase, especially improving indoor environmental quality. (b) Most residents consider DSWHS as a common consumer goods rather than an investment product, so it is purchased to meet some life demands. (c) DSWHS is easier to use than coal and crop straw and is cheaper than LPG and electricity. (d) The


Table 5. The influence of hot water consumption by DSWHS. Term Number of person Frequency (time/month)* Total water use (L/yr, 263days)* Frequency (time/month)** Total water use (L/yr, 263days)**

Adult bath

Child bath

Washing clothes

Total

3.4 1.43 1735.8 0.6 734.4

1.5 3.55 1901.1 1.31 701.5

4.9 3.9 1878.3 1.5 722.3

5515.2 2158.2

*They are the conditions after installed DSWHS;. **They are the conditions when there is no a DSWHS.

local government offers some subsidies for low-income families. (e) Using DSWHS can reduce the use of electricity and fossil fuels. Policy Discussion In 2014, per capita net income in rural region was 747.5 USD. At present, DSWHS, a one-off payment of 506.9 USD, is still expensive for low-income family. Therefore, related policy should be formulated to increase the use of DSWHS. The specific suggestions are as follows: (a) the government should provide subsidies to DSWHS buyers, especially low-income families, similar to the case of encouraging biogas construction. These subsidies could be regarded as the payment for environmental benefits since using DSWHS can reduce GHG and pollutants emissions [40]. (b) To reduce the purchase and installation costs of DSWHS, government agencies can procure a large number of DSWHSs, or the government can offer discount and users pay in installments. (c) According to previous reports [41,42], emissions trading is conducted between enterprises with emission of GHS and DSWHS users through the carbon emissions trading system. Enterprises provide funds for DSWHS users, thus GHG emission reduction is owed to enterprises. (d) The application of solar energy should be included in the plan concerning the reconstruction of village and town. Then, the spatial layout of buildings, DSWHSs, water supplying pipelines and other facilities can be optimized. CONCLUSIONS

(1) More hot water should be used to increase the efficiency of DSWHS use. Loess hilly region of Gansu Province of China can receive abundant solar radiation, with a total annual solar radiation of 5474.52 MJ/m2. According to local meteorological data, the available annual solar radiation absorbed by the inclined plane with a tilt angle of 458 to the horizontal is 5750.6 MJ/m2, which is more than that received by the horizontal surface. Field survey shows that local residents stop using DSWHS in winter. Besides, no case where auxiliary energy is used to support DSWHS is found. Thus, the valid period of DSWHS utilization is 263 d per year. The available solar radiation obtained by the inclined plane is 4380.0 MJ/m2 during the valid period. According to the thermal efficiency of DSWHS specified by the manufacture (h1 , 40%), an ETC DSWHS with a colleting area of 2.09 m2 can supply heat of 3661.7 MJ. According to the statistical analysis of hot water use in this region, a household with an average of 4.9 people annually consumes hot water of 11043.5 kg supplied by DSWHS for bath, daily washing and laundry (i.e., only 42 L per day). Heating this amount of water needs 1238.7 MJ of energy. Among the 3661.7 MJ of energy supplied by DSWHS, only 1238.7 MJ is used. Thus, the actual thermal efficiency of DSWHS (h2 ) is only 13.53%, which is one-third of the thermal efficiency specified by the manufacturer (h1 , 40%). This

indicates that there is a great potential to increase the efficiency of DSWHS use. (2) The economic benefit of DSWHS is low, but it can be improved. The average price of DSWHS is 506.9 USD and the average annual maintenance cost of it is 2.9 USD. The 1238.7 MJ of energy from DSWHS actually used by household per year is equal to that produced by 211.6 of kg raw coal, 411.5 kg of crop straws, 41.1 kg of LPG, and 430.1 kWh of electricity, respectively. When DSWHS is installed to replace coal, crop straw, and electricity, respectively, for heating water, the cost of the investment cannot be recovered within the service life of the DSWHS (15 yr). However, the cost of the investment can be recovered in the 12th year if DSWHS is installed to replace LPG for heating water. This indicates that DSWHS has poor economic feasibility, which is due to the low consumption of hot water by residents. If residents consume all hot water supplied by DSWHS, the economic benefit of it would be greatly increased. The payback times of DSWHS replacing LPG, electricity, raw coal, and crop straw are shortened to 3.5, 5.6, 7.6, and 15 yr, respectively. (3) Using DSWHS can bring about great environmental benefit: energy conservation and emission reduction. The 1238.7 MJ of energy from DSWHS used by a household per year is equivalent to that produced by 211.6 kg of raw coal (or 411.5 kg crops straw, 41.1 kg LPG, and 430.1 kWh electricity), which can emit 454.99 kg of CO2, 5.85 g of N2O, 375.29 g of NOx, 3193.79 g of SO2 and 582.89 g of CH4 during use. If more residents install DSWHSs in their homes, the emissions of GHG and pollutants will be greatly reduced. (4) Using DSWHS can bring about significant social benefits. After DSWHS is used for supplying hot water, the quality of indoor environment is improved, which can help improves residents’ health conditions. In addition, rural residents have increased the frequency of bathing and cloth washing and have been changing other living habits since the installation of DSWHS. This indicates a historical progress toward modern civilization in the study area. (5) Policy suggestions are also given here. To improve the actual thermal efficiency of DSWHS and create more benefits, households should be encouraged to utilize more hot water. For example, hot water can be used for cooking after water purification. Also, the government should provide some subsidies for DSWHS buyers as payment for the environmental benefits brought by DSWHS. ACKNOWLEDGMENTS

The authors would like to thank all the members conducting the field survey. They are Ye Liqiong, Li Na, Zhang Yong, Lin Xiao, Dou Chengwu, Ji Yaqiang, and Wei Fangli. We are also grateful to four anonymous reviewers for their valuable suggestions, and National Natural Science Foundation of China for financial support during this study (Grant No. 41171437).


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NOMENCLATURE

c DSWHS Eij ETC FPC Fi fij GHG Hb H Hts Hbt Hdt Hrt He He1 He2 LPG NPV PI PII Pi Ri S1 S2 TSP Th1 Th2 Tl Ui d h1 h2 hi / u xst xss

Specific heat capacity of water (J/(kg*8C)) Domestic solar water heater system Emission (kg, g) Evacuated tube collectors Flat plate collectors Amount of energy resources (kg, kWh) Emission factor Greenhouse gas Beam component (MJ/m2) Total solar radiation (MJ/m2) Solar radiation received on the inclined plane (MJ/m2) Beam component on the inclined plane (MJ/m2) Diffuse component on the inclined plane (MJ/m2) Reflected component on the inclined plane (MJ/ m2) Useful heat gained by an ETC DSWH Available heat (MJ) Useful heat (MJ) Liquefied petroleum gas Net present value (USD) Primary potential Second potential Unit price of energy resources (USD) Economic benefit (USD) Hot water consumption for bath (l) Hot water consumption for washing (L) Total suspended particle (g) Water temperature in the outlet for bath (8C) Water temperature in the outlet for washing (8C) Water temperature in the inlet (8C) Unit caloric value of fuels (MJ) Solar declination angle (8) Thermal efficiency (%) Actual thermal efficiency (%) Thermal efficiency of fuels (%) Local altitude (8) Collector slope (8) Angle at sunset on horizontal plane (8) Angle at sunset on inclined plane (8)

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