Exploratory research on the field application of solar assisted air source heat pump system for rural buildings in Xinjiang

ABSTRACT

A time-controlled solar assisted air source heat pump (SAHP) system was proposed in accordance with the climate characteristics of Xinjiang. Combined with an actual project of rural office building in Xinjiang, the time-controlled SAHP system test study was carried out . Test results showed that, on a typical day in the slightly cold period (SCP), the heat collection efficiency of the solar collectors was 68.12%, the solar energy utilization rate was 84.22%, and the indoor temperature increased by 5.2°C. On a typical day in the bitter cold period (BCP), the time-controlled SAHP system could provide more than 70% of the heating consumption. Furthermore, the feasibility of the time-controlled SAHP system was evaluated based on the data of the SAHP system and the traditional coal-fired boiler system in the same period. In comfort, the indoor temperature maintained at 14–20.2°C, and the average temperature was 18°C. In economy, the coal consumption of the time-controlled SAHP system was 18.34 kg/m² during the heating period, which was 25.0% of that of the traditional heating method. In environmental protection, the time-controlled SAHP system could reduce CO₂ emissions by 149.10 kg/m², NO_x emissions by 4.47 kg/m², and SO₂ emissions by 2.24 kg/m² during the heating period.

Highlights

• A feasible SAHP system for rural areas in Xinjiang has been clarified

• An experimental study was carried out to verify comfort and economy.

• Operating characteristics of the various parts have been analyzed.

• The influence of climate on the system has been discussed.

• Curves fitted by statistical data show a better prediction performance.

KEYWORDS:

• Flat-plate collectors
• air source heat pump
• energy consumption
• thermal comfort
• financial analysis

1. Introduction

Indoor air pollution caused by civil combustion of solid fuels is a major health hazard in China’s rural environment (Li et al. 2016a). The inefficient operation of a large number of indoor civil boilers has caused air pollution and respiratory diseases for users (Chen et al. 2016). Household combustion of solid fuels, as the second most essential environmental risk factor for premature deaths in China, has caused about 605 thousand premature deaths in 2016 (Chen et al. 2018). The traditional heating methods need to be changed in China’s Xinjiang province. There are about 80,000 people relying on coal-fired boilers for heating in the suburbs and rural areas. Questionnaires showed that each household in Shihezi city of Xinjiang burned up to 10 tons of coal in winter. That caused a large amount of coal consumption and air pollution.

Recently, a number of renewable technologies have been proposed to address the current environmental challenge (Johansson et al. 1993), including solar thermal electric technology (Duffie, Beckman, and Worek 1994), biomass conversion technology (Heidari et al. 2018), purified coal technology (Melikoglu 2018) and heat pump technology (Shi et al. 2019). The solar assisted air source heat pump (SAHP) system has been extensively investigated and applied to develop novel heating systems (Ni et al. 2016). Since the air source heat pump (ASHP) is a renewable and energy-efficient device in the sector of sanitary hot water production, the SAHP system can provide stable, continuous heating for buildings even the efficiency of solar collectors is reduced due to the unstable sunshine hours and seasonal fluctuations (Tangwe, Simon, and Meyer 2016; Morrison, Anderson, and Behnia 2004; Cansino et al. 2011). Typical SAHP systems can be divided into three categories based on their structure: series system, parallel system, and complex system (Day and Karayiannis 1994). Precious study, evaluated the performance and correlation of different integrated systems using solar thermal and heat pumps (Hadorn 2012; Stark, Loose, and Drück 2014). The results showed that 70% of SAHP systems were parallel systems, 7% were series systems, and 21% were complex systems. In general, the solar thermal system and the ASHP system operate as a unit in the series system (Hatheway and Converse 1981). Nuntaphan, Chansena, and Kiatsiriroat (2009) proved that the coefficient of performance (COP) of a typical series system in Thailand was between 2.5 and 5.0. Moreover, Li and Yang (2010) investigated the application of series SAHP systems in Hong Kong and found that the COP of the system was decreased over the heating process. In contrast to series system, the solar collector system and ASHP system operated independently in the parallel system (Kuang, Wang, and Yu 2003). Panaras, Mathioulakis, and Belessiotis (2013) found that the parallel SAHP system for space heating had a monthly energy-savings ratio of approximately 52% and the ASHP unit has a dead-band temperature of −7°C. Huan et al. (2019) evaluated the performance of different SAHP systems in the bathroom of colleges in Xi’An and test result showed that the parallel system outperformed the serial system. Similarly, Du et al. (2019) speculated that the average COP of a parallel system was 2.77 in China’s Shandong, by building a model in TRNSYS, which was higher than that of the series system. Ahmad et al. (2013) pointed out that designing a reasonable control center management parallel system could maintain better thermal comfort and less temperature fluctuations, and would save more than 9% of energy.

The annual solar total radiation in Xinjiang is about 5500 MJ·m⁻², and the annual sunshine duration is over 3200 hours (Ke and Fanneng 2010). So the SAHP systems have great potential to be used in the heating of rural buildings in Xinjiang. The heating season generally lasted from October to April, in Shihezi city of Xinjiang (Khodakarami and Ghobadi 2016). However, the air quality index (AQI) in the heating season was higher than that of other seasons (See Figure 1), and the AQI increased significantly with a decrease in the ambient temperature. Previous studies showed that solar energy could be used as a clean power source to reduce the impacts of air pollutants, but the efficiency of solar energy was affected by the levels of air pollution.

Figure 1. Ambient temperature and air quality fluctuations with time in Shihezi in recent four years. All data are summarized from China national meteorological information center.

So far, numerous studies on SAHP have been carried out in cold regions. However, few researches focus on SAHP in severe cold region due to the low heat collection efficiency caused by low outdoor temperature, large heating load, and long heating period. Therefore, the time-controlled operation mode for SAHP was proposed in severe cold region. And the time-controlled operation mode consisted of work time mode and leave time mode. The system composition, operation characteristics, system benefits and other factors of the SAHP were analyzed. The relation between environment parameters and operation parameters was investigated and discussed. Furthermore, the performance of the time-controlled SAHP system was evaluated by comparing with the conventional coal-fired heating system.

2. Experiments

2.1. System design

The proposed SAHP heating system in this study was driven by solar and air source and designed for small public buildings in Xinjiang. As shown in Figure 2, the fully integrated system consisted of solar collectors, an ASHP system, a water storage tank, a control center and three water pumps. In addition, radiant floor heating (RFH) system was installed in the test room.

Figure 2. Schematic layout of the experimental set-up for the SAHP system. Supply fluids was painted red and flow back fluids was painted blue.

Three water pumps without relying on solenoid valves were designed for the system in rural areas, and each device was controlled by the temperature change of each measuring point. The heating period was divided into two periods depending on the ambient temperature and the levels of air pollution. The first period was a slightly cold period (SCP) (Xue et al. 2017), and the ambient temperature was above −5°C in October, November, March, and April. The second period was a bitter cold period (BCP), and the ambient temperature was below −5°C in December, January, and February.

2.2. Experimental equipment

As shown in Figure 3), six flat-plate collectors with a total area of 12.72 m² were installed on the roof of the test house. The weight of the ASHP (Figure 3)) was 145 kg and the rated electric power was 4.2 kW. A water storage tank with the volume of 500 L was used (Figure 3)). Considering that the outdoor temperature was often lower than 0°C during the experiment, 70% mass ethylene glycol anti-icing agent was used in the outdoor pipeline of the system, and the indoor pipeline was filled with demineralized water. The heat transfer between the anti-icing fluid and the demineralized water was obtained by a small copper tube of more than 40 m in the water storage tank. The experimental RFH system, with double spiral tubing, was made of three groups of ground heating pipes with the outer diameter of 20 mm, using polypropylene fixed to the office floor, as shown in Figure 3). The interval distance between the ground heating pipes was 150 mm and the total length was approximately 285 m.

Figure 3. The main composition of SAHP system. (a) Solar collectors. (b) ASHP system. (c) The water storage tank (d) RFH system.

2.3. The test house

As shown in Figure 4, the SAHP system was tested in a small isolated, and single office with an area of 54.66 m² in Shihezi city of Xinjiang (44°24ʹN,86°00ʹE). The building refurbishment project completed before the installation of the SAHP system, as shown in Figure 5. The south and west walls of the experimental room were the external facades consisting of the original 370 mm brickwork and an additional 120 mm expanded Polystyrene (EPS) insulation. The heat transfer coefficient (U-value) of the external walls was 0.302 W·m⁻²·K⁻¹. The north and east walls connected to other adjacent rooms were the internal walls. The U-value of the internal walls, including the original 240 mm brickwork and an additional 120 mm EPS insulation, were 0.319 W·m⁻²·K⁻¹. In addition, on the basis of the original 130 mm concrete roof, 100 mm extruded polystyrene (XPS) insulation and 200 mm concrete were added. The final U-value of the roof was 0.235 W·m⁻²·K⁻¹. The windows were converted into double glazed windows with a U-value of 2.82 W·m⁻²·K⁻¹.

Figure 4. Details of the experimental room.

Figure 5. Photo of the experimental office room. (a) Pre-renovation test building in August 2018. (b) Testing building after renovation in April 2019.

2.4. The control center and measuring system

Figure 6 presents the control logic diagram of the control center. The control center allowed users to select manual or automatic mode through the touch screen according to their heating requirements. In this study, the time-controlled operating mode was divided into two parts, work time and leave time. During the work time (from 7:00 to 19:30), the primary goal of the SAHP system was to ensure that indoor temperature was around 18°C by using solar energy preferentially. Additionally, indoor temperature was controlled between 16°C and 20°C by the RFH system. When the solar collectors’ temperature was higher than 35°C and the water storage tank temperature, the solar system started to heating. Then, the collectors stopped working until the temperature met the conditions. When the water storage tank temperature was lower than 30°C, the ASHP was turned on until the temperature of the water storage tank reached 40°C. During the leave time (from 19:30 to 7:00), the system needed to prevent the ground heating tube from freezing, and keep the equipment operating normally when the indoor temperature was below 14°C.

Figure 6. Control logic diagram of control center. Control program was written by GX-Works2 software. USB/485 Converter was used to connect computers and read secure digital memory cards.

Figure 7

Figure 7. Indoor instrument arrangement.

shows the layout of measurement points in the test room. The experiment was from October 15th, 2018 to April 15th, 2019. A series of parameters were measured during the experiment, including solar radiation, ambient temperature, relative humidity, indoor temperature, indoor surface temperature, indoor heat flow, and other system parameters. And system parameters were automatically recorded in secure digital memory card every two minutes, such as flow rate, inlet temperature, outlet temperature, useful energy gain of the system. The system was considered to be in steady-state conditions due to the change of ambient temperature was slow. Moreover, indoor temperature and relative humidity were measured every 10 min by a multichannel temperature and humidity monitor. Black globe temperature was measured by a black globe temperature monitoring device. The horizontal distance of the indoor monitor was more than 0.5 m from the wall, while the vertical distance was 1.5 m from the ground. Ambient temperature and solar radiation were recorded every 30 min by an automatic weather station installed on the roof. Table 1 shows the technical information of the sensors used in the experiments.

Table 1. Test instruments and parameter accuracy.

Parameters	The versions of instruments	Measuring range	Measuring accuracy
Indoor temperature	JTR08B	−40~80°C	±0.3°C
Relative humidity	JTR08B	0~100%	±1.5%
Black globe temperature	JTR04	−20~125°C	±0.5°C
Ambient temperature	Vantage Pro2	−40~65°C	±0.5°C
Solar radiation	Vantage Pro2	0~1800 W/m^2	±5.0%

3. Results

3.1. Operating characteristics of the solar collectors

The fluctuations of the ambient temperature and the indoor temperature are shown in Figure 8, as well as the inlet and outlet temperature of the solar collectors during SCP and BCP periods.

Figure 8. Operating variations of the solar collectors from SCP and BCP. (a) Temperature on October 25th, 2018 of SCP. (b) Temperature on December 7th, 2018 of BCP. (c) Solar power on October 25th, 2018 of SCP. (d) Solar power on December 7th, 2018 of BCP.

Figure 8) shows the operation of solar collectors in the SCP on October 25th, 2018. Specifically, the solar radiation recorded by the Vantage Pro2 weather station was 11.32 MJ m⁻² on that day. The ambient average temperature was about 3.11°C. During the operation of the solar collectors (from 11:00 to 18:30), the temperature increased rapidly from 11:30 to 15:00. Meanwhile, the heat collection efficiency reached the peak at 13:30 with an instantaneous solar heating rate of 5.22 kW (Figure 8)). The RFH system, which worked from 9:30 to 19:00, increased the indoor temperature by 5.2°C using the solar energy as the only heat source. During the test, the total heat gains of the solar collectors reached 98.08 MJ, the heat collection efficiency of the solar collectors maintained at a high level of 68.12%, and the solar fraction of the day reached 84.22%. Furthermore, in sunny weather with sufficient solar radiation, the requirement of warm water for indoor RFH system was ensured by the solar collectors from the beginning to the end of heating. The excess heat was stored in the water storage tank to meet the heating needs of the next day.

Figure 8) reveals that the operation of solar collectors in the BCP is an example on December 7th, 2018. The total solar radiation was 7.58 MJ·m⁻² on that day and the ambient average temperature was around −18.08°C. The low ambient temperature caused the solar collector to collect heat only between 12:30 and 15:30. As shown in Figure 8), the solar collector had relatively low heat collection capacity, and the average heating rate hovered around 2.56 kW. When the indoor floor heating worked from 8:00 to 19:00, the running heat by solar collectors was not enough for warming the test office room. On that day, the total heat gain of solar collectors was 38.41 MJ, the collection efficiency of solar collectors maintained at 39.85%, and the solar fraction was only 27.31%. In the BCP period, the efficiency of solar collectors dramatically reduced due to the low solar radiation and the extremely low outdoor ambient temperature. In this case, the heat needed for indoor heating was supplemented by the ASHP system.

The different performance of solar collectors in SCP and BCP was mainly caused by the change of environmental parameters. In steady-state, the performance of a solar collector was described by an energy balance and the useful energy gain was an important parameter reflecting energy balance (Duffie, Beckman, and Worek 1994). Typically, the average energy gain per unit per day of solar collectors is given as:

(1) $q_{\mathrm{s}\mathrm{o}\mathrm{l}}=\eta I_{\mathrm{T}}$(1)

where $q_{\mathrm{s}\mathrm{o}\mathrm{l}}$ is the average energy gain (MJ·m⁻²·d⁻¹), $\eta$ is the collection efficiency of solar collectors (%) and $I_{\mathrm{T}}$ is the total incident solar radiation (MJ·m⁻²·d⁻¹).

According to the Equation (1)(1) $q_{\mathrm{s}\mathrm{o}\mathrm{l}}=\eta I_{\mathrm{T}}$(1) , 145 valid sample data were analyzed to study the relationship between $q_{\mathrm{s}\mathrm{o}\mathrm{l}}$ and $I_{\mathrm{T}}$. As shown in Figure 9, there was a highly linear relationship between the incident solar radiation and the energy gain of solar collectors when the area of solar collectors was a constant (Hussain, Dincer, and Zubair 2004). So the average energy gain is shown in Equation (2)(2) $q_{\mathrm{s}\mathrm{o}\mathrm{l}}=0.75212\times I_{\mathrm{T}}-2.81867(q_{\mathrm{s}\mathrm{o}\mathrm{l}}>0)$(2) :

(2) $q_{\mathrm{s}\mathrm{o}\mathrm{l}}=0.75212\times I_{\mathrm{T}}-2.81867(q_{\mathrm{s}\mathrm{o}\mathrm{l}}>0)$(2)

Figure 9. Energy gain of solar collectors with the change of horizontal radiation

3.2. Operating characteristics of the ASHP system

The typical rainy weather during SCP and BCP was selected for analysis of the ASHP system. Taking November 12th, 2018 during SCP as an example, the ambient average temperature of the day was 0.69°C, as shown Figure 10). In addition, the heating capacity was 131.91 MJ and the power consumption was 17.0 kW·h (Figure 10)). In fact, the actual power consumption of the ASHP system was lower than that of the rated electric power. Furthermore, the calculated average daily COP was 2.15. The average temperature difference was 8.1°C for between supply water and return water during the operation, with 7.5 h running time. The heating power of the ASHP system decreased with the increasing running time, while the power consumption was relatively stable. It was also observed that the COP of the ASHP system decreased over the running time.

Figure 10. Operating variations of the ASHP system from SCP and BCP. (a) Temperature on November 12th, 2018 of SCP. (b) Temperature on December 13th, 2018 of BCP. (c) Power on November 12th, 2018 of SCP. (d) Power on December 13th, 2018 of BCP.

In contrast, Figure 10) summaries the operating characteristics of the ASHP system on December 13th, 2018 during the BCP. The average ambient temperature of the day was −15.07°C. Moreover, Figure 10) shows that the heat load was 189.32 MJ, the power consumption was 32.2 kW·h, and the average daily COP was 1.63. The average temperature difference was 8.2°C between the supply water and return water of the ASHP system during the operation, which was similar to the operating performance in the SCP. The running time of the system was 11.5 h, which was larger than that in the SCP. This was mainly due to the decrease of ambient temperature that led to the increase of heat consumption of building envelope and longer running time of the ASHP system.

A mathematical model was established to describe the operation of the ASHP, as shown in Figure 11. Briefly, heat was pumped from the air source at lower temperature to the water at higher temperature through the vapor compression cycle during the operation of the ASHP system (Ahmad et al. 2013). And, COP is generally associated with ambient temperature and the temperature of the water storage tank (Rahman, Smith, and Fumo 2016; Karsli 2007). The temperature of the water storage tank was very stable; however, the tank temperature changed sharply when the water was used by the RFH system at the beginning of the working hours (Banister et al., 2014). Therefore, the COP of the ASHP system was determined by ambient temperature (Lloyd and Kerr 2008). According to the data collected during the heating period, a linear relationship between the COP and the ambient temperature was obtained with the temperature ranging from −25°C to 15°C, the linear regression equation was given as

(3) $CO{P}_{\mathrm{A}\mathrm{S}\mathrm{H}\mathrm{P}}=0.03304\times T_{\mathrm{a}}+2.10522$(3)

Figure 11. COP of the ASHP system with the change of ambient temperature

where $CO{P}_{\mathrm{A}\mathrm{S}\mathrm{H}\mathrm{P}}$ is the coefficient of performance of ASHP and $T_{\mathrm{a}}$ is the ambient temperature (°C).

3.3. Impact of energy consumption of the experimental office room

In order to compare the daily envelope heat loss of the experimental office during the SCP and BCP, a heat flux meter was used to test the indoor heat dissipation. And a typical day from each period was selected as shown in Figure 12. For the roof, the thermal losses were not increased because of thermal insulation and the snow. Compared with SCP, the thermal losses of the south wall, the west wall, and the window in BCP increased by 67.4%, 97.4%, and 75%, respectively. Compared with the external walls, the thermal losses of internal walls were less affected by ambient temperature, such as the north wall and east wall. The heat consumptions of buildings from the SCP to the BCP was affected by the temperature change those parts of the buildings that were in direct contact with the surrounding environment. Therefore, the ambient temperature was the key factor in building heat consumption (Lei, Tan, and Wang 2017).

Figure 12. Heat consumption data in experimental room of November 12th, 2018 and January 11th, 2019.

The sensible energy demand of the zine was measured by the Ultrasonic heat meter. According to the collected data, a linear correlation between the sensible energy demand of the zine and the ambient temperature was obtained, as shown in Figure 13. The linear regression equation is given as:

(4) $q_{\mathrm{r}}=-0.10830\times T_{\mathrm{a}}+2.19162$(4)

Figure 13. Sensible energy demands of zone with the change of ambient temperature

where $q_{\mathrm{r}}$ is the sensible energy demand of zone.

3.4. Contrast with the local traditional coal-fired heating

In order to verify the feasibility of using the SAHP system to replace local traditional coal-fired boilers in local building heating, a series of data were collected for analysis, such as indoor temperature, relative humidity, and energy consumption. The experiment was conducted during the normal working hours of the office building. As shown in Figure 14), traditional coal-fired boilers needed manual addition of coal stored in a separate room hourly. And the large amount of coal dust caused serious pollution to the surrounding environment. Through thermal imaging, the heat distribution of the SAHP system in the vertical direction (Figure 14)) was more uniform than the coal-fired heating system (Figure 14)).

Figure 14. Photos and thermal imaging of different systems. (a) Photo of the room with coal-fire heating boiler. (b) Thermal imaging of the room with coal-fire heating boiler. (c) Photo of the room with the SAHP system. (d) Thermal imaging of the room with the SAHP system.

Figure 15) depicts the change of indoor temperature and relative humidity in test room heated by a coal-fired boiler. The indoor average temperature was 16.8°C, while the highest temperature was 25.5°C and the lowest temperature was only 9.5°C. The extremely high heating and cooling rates of this coal-fired boiler directly affected the indoor air temperature and it had very limited effect on the thermal storage of building envelope, so the indoor temperature difference as high as 9.7°C. In contrast, the variation range of indoor relative humidity was small, with fluctuation between 20% and 44% and an average humidity of 32.2%. The frequency of indoor relative humidity fluctuation, was relatively high, and the indoor relative humidity was low because the heat source was dry when the coal was burned for heating.

Figure 15. Indoor temperature and relative humidity in test room. (a) Heating by a coal-fired boiler. (b) Heating by the SAHP system.

When the SAHP system was used for the heating, as shown in Figure 15), the average indoor temperature was 18.0°C with the temperature peaked at around 18:00 and the lowest temperature occurred around 8:00. The RFH system, controlled by the automatic control system, heated the building’s envelope firstly, and then increased the indoor air temperature. Therefore, the indoor temperature was relatively stable. The highest temperature was 20.8°C, the lowest temperature was 13.8°C, and the highest difference in temperature was 7°C. Considering the working hours of office buildings, the indoor temperature difference during working hours should be lower than that in the whole day. In addition, the indoor average humidity was 50.5%. The highest and lowest values of humidity were 56% and 43%, respectively, it indicated the humidity fluctuation was mild. In the second half of the test period, the relative humidity fluctuated with temperature, and the minimum humidity usually occurred at the highest indoor temperature.

Figure 16 presents the schematic diagram of human thermal sensation based on predicted mean vote (PMV) model (Wang et al. 2010, 2018b). The model described people’s voting on the thermal sensation scale and the prediction of the percentage of dissatisfaction with heat by incorporating a few parameters including metabolic rate, clothing insulation, air temperature, radiant temperature, airspeed, and humidity (Enescu 2017). A total of 4363 valid sample data were collected in working hours during the heating period in the test rooms, and analyzed under metabolic rate of 1.2 met and a clothing thermal resistance of 1.0 clo. The average PMV of the test room heated by a coal-fired boiler was −1.6 and the corresponding thermal sensation was from cold to cool (see Figure 15). In contrast, the average PMV of the test room heated by the SAHP system was −0.5 and the corresponding thermal sensation was between slightly cool and neutral, and it followed the Poisson distribution. The PMV of the SAHP-heated room was affected by the auto start and stop of the control center, and the thermal comfort of the room was better.

Figure 16. Schematic diagram of human thermal sensation

The traditional coal-fired boilers were selected as the benchmark when the energy-saving benefits of SAHP was evaluated. The energy consumption of the heat-engine plants in this region was at 292 g·(kW·h)⁻¹ (Li et al. 2016b), and it could be converted into coal consumption according to calculations. The coal consumption of the test room was calculated based on the building energy consumption. The lower heating value of standard coal was 29,307 kJ·kgce⁻¹, and the efficiency of traditional residential coal-fired boilers was 25% (Huang et al. 2017). The SAHP system mainly consumed electric energy. Figure 17 shows the consumption of coal-fired system and SAHP system. It can be seen that the coal consumption of the SAHP system was much lower than that of the coal-fired boiler during the SCP, and it was equivalent to 10.8% of the coal consumption of coal-fired system. During the BCP, ASHP undertook the entire heating demand for test room, and the power consumption increased with the decrease of the ambient temperature. The coal consumption of the SAHP system was equivalent to 29.2% of that using the coal-fired boiler. Throughout the heating period, the heating coal consumption index of the rest room actually reduced to 18.34 kg/m², saving 55.02 kg of coal per unit area. The SAHP system in one heating period reduced CO₂ emissions by 149.10 kg/m², NO_X emissions by 4.47 kg/m², SO₂ emissions by 2.24 kg/m², and reducing air pollution caused by burning in winter.

Figure 17. Consumption of standard coal with different months.

3.5. Financial analysis

Compared with traditional coal-fired heating, the time-controlled SAHP system proposed in this paper had a benefit of CNY 2 546. After the system was installed, no additional investment was required except for routine maintenance. In addition, the traditional heating and SAHP systems were evaluated by calculating the net present value (NPV). NPV was defined as the difference between the present value of cash inflows and the present value of cash outflows (Baptista, Melo, and Rolim 2014). A detailed description of NPV was proposed by Ma et al. (2016). The specific cost of a typical system for rural building was CNY 12000 and the specific cost of RFH was CNY 50 m⁻². In addition, the construction cost of traditional coal-fired system was CNY 4 000. The electricity cost and Coal cost of the region were CNY 0.39 (kW·h)⁻¹ and CNY 0.7 kg⁻¹, respectively. The financial analysis and the SAHP system were summed (see Table 2).

Table 2. Financial data the SAHP system.

Year	Discounted factor	Net present value	Present value of benefit	Present value of net benefit	Accumulated net benefit
		(CNY)	(CNY)	(CNY)	(CNY)
0	1	−18054.4	0	−18054.4	−18054.4
1	0.97	2622.62	2472.07	2472.07	−15582.33
2	0.94	2701.3	2400.07	2400.07	−13182.26
3	0.92	2782.34	2330.16	2330.16	−10852.1
4	0.89	2865.81	2262.29	2262.29	−8589.81
5	0.86	2951.78	2196.4	2196.4	−6393.41
6	0.84	3040.33	2132.43	2132.43	−4260.98
7	0.81	3131.54	2070.32	2070.32	−2190.66
8	0.79	3225.49	2010.02	2010.02	−180.64
9	0.77	3322.25	1951.47	1951.47	1770.83
10	0.74	3421.92	1894.63	1894.63	3665.47
11	0.72	3524.58	1839.45	1839.45	5504.92
12	0.7	3630.32	1785.88	1785.88	7290.79
13	0.68	3739.23	1733.86	1733.86	9024.65
14	0.66	3851.4	1683.36	1683.36	10708.01
15	0.64	3966.94	1634.33	1634.33	12342.34

Meanwhile, a comparison of SAHP heating system and traditional coal-fired boiler heating was gained (see Table 3). It can be seen that the use of SAHP system for heating improved the thermal comfort of the room, reduced coal consumption, and thereby reduced polluting gas emissions. In addition, the payback period of the system investment is 8.1 years. In summary, the SAHP system had great feasibility to replace traditional coal-fired boilers.

Table 3. Comparison of SAHP heating and traditional coal-fired boiler heating.

	Traditional coal-fired boiler heating	SAHP system heating
The highest indoor temperature during the test (°C)	25.5	20.2
The lowest indoor temperature during the test (°C)	9.5	14
Average indoor temperature during the test (°C)	16.8	18
Indoor temperature difference within a day (°C)	9.7	5.4
Average indoor relative humidity during the test period (%)	20–40	43–56
PMV	−1.6	−0.5
Coal consumption (kg/m^2)	73.37	18.34

4. Discussion

4.1. SAHP application in severe cold area

The COP of the SAHP system was lower than the photovoltaic air source heat pump (PV/T-ASHP) system (Wang et al. 2020, 2018a). Therefore, SAHP systems were more popular in rural areas due to its economic considerations (Li et al. 2019). Meanwhile, several simulation studies were carried out to investigate the system performance of the cryogenic environment, and the result provided reference for long-term operation of outdoor systems (Qiu, Xu, and Cai 2018; Panagiotidou, Aye, and Rismanchi 2019). And our result indicated that the heating efficiency of the time-controlled SAHP system reduced firstly and then stabilized during SCP or BCP. The heating efficiency fluctuation of the ASHP was often neglected in long-term operation under bitterly cold conditions.

However, the obstacle to application of time-controlled SAHP system was the low cost of traditional coal-fired boilers. But the indoor air pollution was obvious. To reduce air pollution from traditional coal-fired boilers, it was reasonable for us to believe that the time-controlled SAHP could be an optional way to improve the living conditions of residents

4.2. Impacts of season change on the system

In this study, we determined the operating parameters of the time-controlled SAHP system and the parameters impacts on the overall system energy consumption. Particularly, test result showed that the environmental parameters were closely related to the operating parameters of the system, and it could be used to predict the energy consumption of similar systems (Li and Kao 2017). For the solar part, the days of high solar radiation in SCP significantly outnumbered the BCP, which was consistent with the trend of ambient temperature change. Therefore, the solar collector could provide most of the room heating during SCP, but the solar collector could not meet the heating demand during BCP. The COP of the ASHP system was highly correlated with ambient temperature (R = 0.786), and it was greatly affected by seasonal changes (Lu et al. 2010; Mehla and Yadav 2018). The linear model was also consistent with the experimental data this time-controlled SAHP system, demonstrating great potential for future applications.

4.3. Future developments

There were some limitations in this study. Firstly, only environmental temperature was considered when studying the influence of environmental factors on COP of ASHP system. Therefore, the regression equation obtained could not be used for accurate prediction and design, although there was a high correlation between climate parameters and COP of the ASHP system. Secondly, the enclosure of the experimental room had been renovated to satisfy the requirements of China’s Design Standard (GB 50189-2015), however, it still faced high heat consumption costs because of the bitter cold climates (China MoHaU-RCotPsRo 2015). Finally, the solar fraction was also far lower than the predicted value in BCP. Therefore, when ambient temperature decreased, higher heat consumption cost was required to obtain an indoor temperature of 18°C in BCP. And it led to a decrease of COP for the ASHP system.

It is expected that the pre-stored energy technologies can be adopted to reduce the burden of the ASHP system in BCP; therefore, future work should focus on reducing the cost of the system and developing new solar seasonal heat storage technologies suitable for cold regions.

5. Conclusions

The time-controlled SAHP system was investigated in an office building in the cold and dry climate of Xinjiang to assess its ability and stability. The main conclusions of this paper are summarized below:

1. Solar collectors have undertaken 80% of the consumption for the heating room during SCP (October, November, March, and April). And the total heat collection of solar collectors could reach 98.08 MJ in one day. The heat collection efficiency of the collectors could reach 68.12%, and the solar energy utilization rate could reach 84.22%. Meanwhile, the indoor temperature increased by 5.2°C relying on solar collectors.

2. The ASHP system provided more than 70% of heating consumption for the building heating during BCP (December, January, and February). And the total heat collection of solar collectors was 38.41 MJ in one day. The heat collection efficiency of the collectors was 39.85%, and the solar energy utilization rate was 27.31%. At this time, the heating needed to be supplemented by ASHP system, because the solar collector did not meet the heating demand.

3. The building energy consumption increased and the COP of the system decreases with the decrease of ambient temperature. Therefore, it took longer time for the room to warm up. The daily power consumption of ASHP system was 32.2 kWh during BCP, and it was about twice of that during SCP. The average daily COP of ASHP during BCP was 75.81% of that during SBP. In addition, through the analysis of ASHP system, the linear regression equation between ASHP system performance coefficient and ambient temperature was obtained, which provided calculation reference for predicting ASHP system electricity consumption.

4. The SAHP system provided steadily higher indoor thermal comfort index compared with the traditional coal-fired boiler. In comfort, the indoor temperature fluctuated between 14°C and 20.2°C, and the average temperature was 18°C. In environmental protection, the coal consumption reduced to 18.34 kg/m² using the SAHP system, and it was 75.0% lower than that of traditional coal-fired boiler heating. Meanwhile, CO₂, NO₂, and SO₂ emissions were reduced by 149.10 kg/m², 4.47 kg/m², and 2.42 kg/m², respectively. In economy, the payback period of SAHP was 8.1 years, and the traditional coal-fired boilers could be replaced by the SAHP.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Correction Statement

This article has been republished with minor changes. These changes do not impact the academic content of the article.

Additional information

Funding

This work was supported by the Xinjiang Production and Construction Corps Southern Xinjiang Key Industry Support Program Project [2019DB007]; and Shihezi University Innovation and Development Project [CXFZ202010].

Notes on contributors

Jie Li

Jie Li, Associate professor. Mainly engaged in research on green buildings and building energy efficiency.

Mingkang Wang

Mingkang Wang, Mainly engaged in research on renewable energy and building energy conservation.

Xin Xu

Xin Xu, Research fields: Architectural thermal engineering