In order to utilize solar energy to meet the heating demands of a rural residential building during the winter in the northwestern region of China, a hybrid heating system combining solar energy and coal was built. Multiple experiments to monitor its performance were conducted during the winter in 2014 and 2015. In this paper, we analyze the efficiency of the energy utilization of the system and describe a prototype model to determine the thermal efficiency of the coal stove in use. Multiple linear regression was adopted to present the dual function of multiple factors on the daily heat-collecting capacity of the solar water heater; the heat-loss coefficient of the storage tank was detected as well. The prototype model shows that the average thermal efficiency of the stove is 38%, which means that the energy input for the building is divided between the coal and solar energy, 39.5% and 60.5% energy, respectively. Additionally, the allocation of the radiation of solar energy projecting into the collecting area of the solar water heater was obtained which showed 49% loss with optics and 23% with the dissipation of heat, with only 28% being utilized effectively.
In rural areas of northern China, the energy consumption for heating buildings has reached 1 × 109 tce, which occupies a large portion of the 56% of the total energy consumption, of which coal comprises 74% [
It is widely known that there are two major types of solar heating technology, namely, passive and active. Passive solar energy technology is the earliest pattern for heating buildings by the utilization of solar energy, which uses building orientation, glass windows, and other building materials for collecting, storing, and utilizing solar energy to achieve indoor thermal comfort [
The solar-assisted heat pump is another important active solar heating technology that has attracted much attention. Han et al. [
There have also been many studies related to innovative solar heating systems. Chen et al. [
Economic analysis has been performed for solar heating systems as well. Ziemele et al. [
Recently, numerous rural buildings are being built in northwest China; however, the majority of them lack solar heating technology since they are located in areas where coal is still the primary heating resource. Although many renewable systems have appeared, few studies have focused on rural residential buildings in cold regions of northwest China. Those buildings have unique characteristics: scattered distribution and weak energy-conservation performance dominate; however, sufficient space exists for installation of solar energy utilization devices, and it is easy to utilize local biomass energy resources.
A complementary heating system combining solar energy and coal for a rural household building has been established in a new socialist countryside called Zhangma, Minqin County, Gansu Province (latitude 38°34′N, longitude 103°3′E). Neither the solar energy utilization device nor the coal stove in the system is highly sophisticated, but they are the easiest to operate and most economical model in practice for the heating of rural household buildings. The experiment on the performance of the system was carried out from December 10, 2014, to March 30, 2015. The aim of the present work was to analyze the efficiency of the energy utilization of the system and to reveal the combined effects of multiple factors on solar energy utilization.
Figure
Floor plan of the building (mm).
The building’s previous heating system was a natural circulation heating system that consisted of a coal stove installed in the kitchen and radiators fixed in other rooms, but not in the hall. Six solar water heaters were added in the new complementary heating system; the existing coal stove and solar water heaters are shown in Figure
The complementary heating system.
Coal-fired stove
Solar water heaters
To analyze the efficiency of the energy utilization of the system, both the input and output energy must be measured or calculated (Using the prototype model, the average thermal efficiency of the furnace is obtained. Based on the measured coal quality, the amount of heat absorbed by the solar energy and released into the room. The energy input to the building is calculated based on the inputs coal and the solar energy.). The intensity of the surface facing south, with an inclined angle of 45° and which receives the maximum solar energy in winter, was measured by a pyranometer. The weight of coal consumed by the stove was also weighed by a platform of the solar radiation on balance every day, and the caloric value of the coal was calculated in the laboratory at Lanzhou University of Technology. The inlet/outlet temperature and the circulation flow rate of the hot water were measured to calculate the energy outputs supplied by the solar water heaters. Moreover, the ambient temperature, the temperature in the five indoor rooms (except the hall), and the temperature of the hot water in one of the storage tanks in the middle line were measured. A solar water heater, used to heat the digester, was employed as a reference that faced south and was not shaded in order to study the influence of the orientation and shade effects on thermal performance. The inlet/outlet temperature, circulation flow rate, and water temperature in the storage tank of the reference solar water heater were also measured. All the measurements were recorded automatically by a data acquisition system (Agilent 349702, Agilent Technologies, USA) every 10 s. The detailed parameters of the measuring instruments used in this study are shown in Table
The detailed parameters of instruments.
Measured parameters | Measuring instruments | Range | Precision |
---|---|---|---|
Temperature | Pt-100 temperature sensor | −50 |
±0.1°C |
Flow rate for building | LWGY-20 turbine flowmeter | 0.7 |
±0.45% |
Flow rate for digester | LWGY-15 turbine flowmeter | 0.4 |
±0.45% |
Solar radiation | TBQ-2 pyranometer | 0 |
±2% |
Coals weight | Platform balance | - | ±0.2 kg |
The energy output supplied by the coal stove was difficult to measure directly, so a prototype model for calculating the thermal performance of the coal stove was set up; it will be described in detail in Section
There are several studies that refer to the efficiency of traditional natural circulation coal stoves. Jiang [
Chinese National Standard JGJ 132-2001, entitled “Standard for Energy Efficiency Inspection of Heating Residential Buildings” [
From (
When the coal-fired stove is used for heating only,
In practice, there are three heating models, namely,
Durations and results for
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Duration |
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|
Duration |
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17:00 pm on 12 to 2:30 am on 13 Dec, 2014 | 32.3 | 9 to 11 Dec, 2014 | 5.2 | 6.4 | |
17:30 to 24:00 pm on 2nd Jan, 2015 | 42.1 | 4 Jan, 2015 | 5.9 | ||
17:00 to 23:00 pm on 5 Jan, 2015 | 36.1 | 38.1 | 6 Jan, 2015 | 7.2 | |
17:00 to 23:00 pm on 5 Jan, 2015 | 44.0 | 17 Jan, 2015 | 7.5 | ||
19:50 pm on 15 to 1:50 am on 16 Feb, 2015 | 36.1 | 27 to 28 Jan, 2015 | 7.1 | ||
31 Jan to 3 Feb, 2015 | 6.5 | ||||
6 Feb, 2015 | 5.5 |
In fact, the heat-collection capacity and the efficiency of the solar water heater are influenced by a series of factors, such as solar radiation, ambient temperature, collecting temperature, and orientation. A relational expression for the determination of the daily heat-collection capacity is provided in Chinese National Standard GB/T 18708-2002, which is
The duration for calculating the heat-collection capacity during the daytime is valid when the solar radiation intensity is higher than 120 W/m2; moreover, the circulation pumps may sometimes work in the duration, given that
As mentioned above, a solar water heater at its best working conditions is employed as a reference, and its thermal behavior will be realized using the same method. For convenience, a single solar water heater used for heating the building was designated SI, and the output energy of each solar water heater is considered to be equal; the reference one was designated SII.
SI and SII are calculated by the following:
All the related data were dealt with using multiple linear regression models in Excel. The analytical results are calculated using (
The relationship of heat-collecting capacity between solar radiation and temperature difference.
When solar energy is transformed into thermal energy, a part of it is transported into the building for space heating, but another part is lost to the ambient because the water temperature is always higher than the ambient temperature, so it is important to know the heat-loss coefficient of the storage tank. The Chinese National Standard GB/T 18708-2002 also provides an experimental method for determining the heat-loss coefficient of a storage tank, as follows:
The following experimental conditions are required by Chinese National Stand GB/T 18708-2002: Static Conditions; that is, the uniform temperature field being kept above 50°C; natural cooling for 8 h and nine uniform temperature points to calculate; and an average wind speed of less than 4 m/s. Six durations in all were in accordance with these conditions, and the results obtained are shown in Table
The heat loss coefficient of the storage tank.
Duration |
|
|
|
---|---|---|---|
2014-12-21 | 0:00–8:00 | 4.17 | 4.10 |
2014-12-26 | 0:00–8:00 | 4.70 | |
2015-01-30 | 0:00–8:00 | 4.38 | |
2015-02-11 | 0:00–8:00 | 3.77 | |
2015-02-14 | 0:00–8:00 | 3.59 | |
2015-02-15 | 0:00–8:00 | 4.01 |
The calculated values ranged from 3.59 to 4.70 W/K, and the error might have resulted from some unmeasured parameters, such as the wind speed. The average value of 4.10 W/K is therefore taken as the heat-loss coefficient of the storage tank. Otherwise, another Chinese National Standard, GB/T 19141-2011 [
The energy-conservation equation for a single solar water heater can be expressed as follows:
Using the above equations, the allocation of the solar energy projected onto the collector area of a solar water heater for heating the building during the experiment period was obtained. Figure
The variations of water temperature, ambient temperature, and solar radiation intensity of a typical duration.
The total radiation of solar energy projected onto a solar water heater was 7046 MJ, of which 49% is lost to optics and 23% to heat dissipation, with only 28% being utilized efficiently. When compared to the reference solar water heater, the portion of optical loss was only 33% in the same period, which shows that orientation and shade have the largest influence on the heat-collection capacity of a solar water heater.
In our view, there are two main factors that cause the high heat loss. The first is the location of the storage tank. Although the heat-loss coefficient is lower than that required by the Chinese National Standard, the storage tank always loses heat to the environment because the water temperature is higher than the ambient temperature. The second factor causing the high heat loss is the mismatch between the working temperature of the radiator and the collecting temperature of the solar water heater. Therefore, when the water temperature is too low to heat the building, the circulation pump will be shut down, and the heat energy cannot then be utilized, but only released to the environment.
The energy utilization efficiency of a complementary heating system using solar energy and coal for a rural household building in northwest China was analyzed. The prototype model shows that the average thermal efficiency of the stove is 38%, which means that the portion of energy that enters into the building from solar energy and coal is 60.5% and 39.5%, respectively. A relational expression for the daily heat-collection capacity of the solar water heaters and the heat-loss coefficient of the storage tank was obtained. When compared to the reference solar water heater, it was found that the orientation of the solar water heaters and the shade between them have the largest influence on heat-collecting capacity. Regarding the solar energy projected onto the collecting area of the solar water heater, 49% was lost to optics and 23% to heat dissipation, with only 28% being used effectively. The mismatch between the working temperature of the radiator and the collecting temperature of the solar water heater and the location of storage tanks are considered to be the main factors that led to the high heat loss.
Several items must be considered for a solar heating system applied to rural household buildings in the future. It can be seen from the above analysis that the portion of SI is 16% higher than that of SII, which is caused by the orientation of the solar water heaters and the shade between them. Despite the influence from each of not being distinguished clearly in this research, it is highly recommended to consider a reasonable design of the building originally, especially regarding the layout of solar collectors, as well as for employing the passive technique. On the other hand, the radiant floor heating technology seems to be common feature of new era. Such advanced techniques can decrease the collecting temperature of the solar collectors and improve the collecting efficiency. The work of this paper is of great significance for the next study of the influence of multifactor coupling on the heat-collecting performance of solar collectors. Continuous and stable use of solar energy to meet the user’s multilevel energy demand, not only for the improvement of people’s livelihood in northwest China and the protection of the ecological environment of great value but also of international renewable energy, has important academic value and significance.
The authors declare that they have no conflicts of interest.
This work was funded by the National High Technology Research and Development Program of China (863 Program) (2014AA052801), the National Natural Science Foundation of China (51676094), Major S&T Special Projects of Gansu Province (1502NKDA007), International S&T Cooperation Projects of Gansu Province (1604WKCA009), Natural Science Foundation of China (no. 51706128), Scientific Research Program Funded by Shaanxi Provincial Education Department (Program no. 17JS018), the Natural Science Foundation of Gansu Province (1508RJYA097), the National Natural Science Foundation of China (51166008), and Gansu Construction Science and Technology Project (JK2014-48).