The Application of a Solar–Air-Source Heat Pump Dual-Supply Heating System in a High-Cold Area in China

Abstract

Solar energy is the most available renewable resource and has great potential for various applications. Solar heat pumps are limited when operating alone due to weather and the unstable and intermittent nature of solar energy. The idea of combining a solar collector with an air-source heat pump (ASHP) is proposed to solve the problems. Taking solar energy and air energy as the heat source of the system can improve the heat collection efficiency and heating performance coefficient of the dual-supply heating system in realizing the efficient and stable operation of the whole unit. This study investigated the application of a solar–air-source heat pump (S/AS-HP) dual-supply heating system for a residential building in Lhasa and compared the winter work performance and economic benefits of three different heating forms: solar–air-source heat pump, air-source heat pump, and solar–electric auxiliary heating systems. A simulation model was established and analyzed using the measurement data. The results show that during the heating period, the temperature of the primary side water supply is stable at about 60 °C, and the total operating hours of the solar collector period accounted for 26.9% of the total hours, of which 23.16% were independent operating hours. The average COP_sys of the solar–air-source heat pump is 4.21, which greatly improves work performance compared with other systems, and the annual cost is the lowest among the three forms for the same service life at 41,824.6 CNY, saving 9.6% and 35.1% per year compared with the air-source heat pump system and the solar–electric auxiliary heating system. Considering conditions in alpine regions, S/AS-HP systems are the optimal heating solution for single buildings.

1. Introduction

1.1. Background

With the increasing contradiction between energy and global economic development, countries around the world rely on the continuous adjustment of energy strategic layouts and attach more value to the development and utilization of clean energy, among which, solar–air-source heat pump (S/AS-HP) systems have attracted much attention. Solar collectors can be coupled with an air-source heat pump (ASHP) in different forms, which can be divided into direct expansion solar–air-source heat pumps (DX-S/AS-HPs) and indirect expansion solar–air-source heat pumps (series, parallel, and hybrid). Solar energy and energy from the environment (air energy) are used as the heat source of the system to make up for the instability and intermittency of the solar system when operating alone due to weather and natural law factors. S/AS-HPs also improve heat collection efficiency and the heating performance coefficient of the dual-supply heating system so as to achieve the efficient and stable operation of the overall unit.

1.2. Literature Review

Researchers at home and abroad have conducted a large number of studies to explore the applications of their component heating systems in buildings and learn how to further enhance the heating performance of the systems and reduce operating costs. Deng [1] established a heating system combining a solar collector and an ASHP with electric auxiliary heating equipment according to the heating policy of “coal to electricity” successively issued in northern China. Under three different electricity price policies, the TRNSYS software was used to simulate typical application scenarios for the system and analyzed the influence of different operating factors on the annual cost. Based on an original hot water system in a student apartment, Zhou et al. [2] proposed a renovation scheme to solve problems such as a low solar energy utilization rate, an inability to realize real-time start–stop controls based on outdoor meteorological parameters, and high-energy consumption from power components. After the renovation, the energy-saving and operational efficiency of the system was significantly improved. Yang [3] took the office area of 130 m² in Jinan as the research model and built a test bed to study the operational performance, energy-saving, and emission reduction benefits of an S/AS-HP system. The results showed that the average indoor temperature of the system remained above 22 °C during the heating season, and the average energy efficiency ratio was 2.08, which had good environmental benefits. He et al. [4] used a combined operation of parabolic trough collectors and an ASHP system to meet the daily heating needs of residents in a plateau area. Comparing the operational efficiency and economic benefits of the system under different heat storage strategies and equipment parameters in the actual building, the influence of the two on the system operation was simply analyzed, and the design parameters of the system’s equipment structure were optimized by using transient simulation software combined with the orthogonal experiment method. Guo et al. [5] conducted experimental tests on an S/AS-HP dual-supply system under five different working conditions and fitted the experimental data to obtain the relationship between the COP of the air-source heat pump and ambient temperature, heat collection efficiency, and sunshine intensity. On this basis, four representative cities in different climate regions were selected to calculate the annual comprehensive energy efficiency coefficient of the system. A comparative analysis of the applicability of the system in different climate zones showed that the system is more energy-efficient than the traditional electric heating method, and the operational efficiency of the system is highest in hot-summer and warm-winter areas, so it can be popularized. Wu et al. [6] constructed a mathematical model for an S/AS-HP hot water unit. According to the operation characteristics of the unit, the evaporator was revised. Based on the existing mathematical model of the evaporator, the influence of radiation intensity was added to modify the calculation model of the evaporator in the unit, which can be used to solve the height of the frost layer and the effective thermal conductivity of the evaporation side of the system. Dust and radiation correction coefficients were also introduced to further improve the practical value of the model. Sterling et al. [7] used TRNSYS to simulate the performance of an indirect-heat-pump-assisted solar domestic hot water system. The system was set with two water tanks for heat storage and compared with traditional solar water heaters and domestic electric water heaters under the condition that the system load and supply water temperature remained unchanged. The heat pump, as an auxiliary heat source, had the lowest power consumption and operating costs. Kegel et al. [8] studied the heating schemes of three different types of independent houses in the same area in cold-winter and warm-summer areas of Canada and determined the influence of different building heat loads on the performance and life-cycle costs of solar-assisted heat pumps. By comparing solar-assisted heat pumps, traditional ASHPs and ground-source heat pumps, they analyzed heating system types suitable for different types of houses in these regions. Panaras et al. [9] developed a complete modular solar heat pump hot water system calculation model based on the MATLAB program, which can be used for the dynamic simulation of system application scenarios that are difficult to test in experiments to evaluate the long-term performance of the system. The analysis showed that the setting value of the starting temperature of the heat pump is an important parameter affecting performance. Poppi et al. [10], based on the relevant data of commercial S/AS-HP systems on the market, studied the influence of heat pump circulation, heat storage modules, and system integration modes on system electricity consumption by taking two houses with different building standards located in the climate zone of Zurich and Carcassonne as objects. The system can be reformed under given economic and system boundary conditions to improve cost-effectiveness. Dong et al. [11] found that traditional solar heating systems have high requirements for the installation space, which cannot meet the heating needs of users in high-rise buildings, and exposed water pipes were prone to burst. In order to make up for the above shortcomings, they proposed an S/AS-HP-integrated heating system using R407c as the refrigerant. The average COP of the system is 14.9% higher than that of the traditional ASHP, which has more market promotion value.

1.3. Novelty and Aims

There is research value in integrating commonly used solar collectors and dual-heat-source heat pumps. However, most of the previous studies are limited to the performance optimization of coupled systems, and there are still few research cases of continuous and stable heating in residential buildings in the high-cold areas of China using flat-plate collectors coupled with ASHPs. The economic and environmental benefits should also be considered in studies, which are important evaluation indicators for heating equipment in alpine regions.

Lhasa was selected as a representative city in the cold plateau region of China. This paper analyzes the factors affecting the performance of S/AS-HP systems combined with the climate characteristics and heating demands of buildings in Lhasa and compares the technical performance and economic benefits of three different heating forms: solar–air-source heat pump, air-source heat pump, and solar–electric auxiliary heating systems, aiming to solve the heating demand problems of residents in this high-cold region by using renewable energy. The results are not only valuable for the national strategies of clean heating to improve the living quality of residents and promote ‘‘double carbon” but also important for reducing the consumption of traditional fossil energy and environmental pollution. The feasibility of the application of S/AS-HP in plateau cold climate regions is investigated using simulated data, and it also serves as a significant reference for future research.

2. Materials and Methods

2.1. Target Heating Building Description

The model of this paper is a residential building located in Lhasa, Tibet Autonomous Region. The building has a total of 4 floors without underground construction. Each floor is 2.8 m high, and the total area of the building is 1611.2 m². To comply with the Code for Thermal Design of Civil Buildings (GB50176-2016), the thermal performance parameters of various envelope structures of the target building were determined by using the relevant design provisions of standard JGJ26-2018 [12,13] according to the climate zoning of Lhasa, and the specific settings are listed in

Table 1. Enclosure structures of the target building.

Name	Enclosure Structure	Heat Transfer Coefficient (W/m2·K)
Outwalls	Cement mortar, high-density expanded polystyrene board insulation, concrete block	0.38
Roofs	Fine stone concrete, cement mortar, asphalt waterproofing membrane, expanded perlite, reinforced concrete	0.22
Windows	Low-E insulating glass (air) (6 + 12A + 6)	1.9
Doors	Stainless steel with wood finish	-

. The heating time was 120 days from 15 November of the first year to 15 March of the second year. During this period, the buildings are heated 24 h a day. The outdoor climate parameters are based on the typical annual climate parameters. The building’s interior design parameters were set according to standard JGJ26-2018; the indoor design temperature in winter was 18 °C, and the relative humidity range was 40~60%, which was set to 50%. The indoor wind speed was less than 0.3 m/s, the air change rate was 0.5 times per hour, and the fresh air index was 30 m³/h per capita.

The model was simulated for the whole year (0–8760 h), and the time step was set as 1 h so that the hourly heating load of the building during the heating season could be obtained. According to the results (Figure 1), the heat load of the building was large for January–February during the whole heating period, and the outdoor ambient temperature was low here too. The maximum hourly heating load of the target building was 66.8 kW, and the cumulative heating load was 75,720 kwh.

2.2. System Description

Considering the shortage of traditional fossil energy, geographical advantages, and environmental protection policies in the plateau region of China, this paper proposes a scheme for using an S/AS-HP dual-supply heating system to meet the heating demands of users. The main components of the dual-supply heating system include the solar collector, the ASHP, the heat storage water tank, the water heating tank, the heat exchanger, the circulating water pump, and the terminal heating equipment. The dual-supply heating system connects solar collectors and air-source heat pumps in parallel through a heat storage water tank as an intermediate medium to improve the reliability and applicability of system operation, which can make full use of solar energy. The schematic diagram of the dual-supply heating system is shown in Figure 2. The ASHP, as the combined heating source of the system, needs to ensure that the system can still run stably without solar energy. The heat of the ASHP needs to meet the maximum heating load of the building during the heating season.

There are four types of heating modes in the dual-supply heating system: (1) The ASHP is closed when the solar energy is sufficient during the day, and the solar collector runs as an independent system. The water in the tank is heated to the preset temperature to provide heat for the residential building and domestic hot water. The excess heat generated by solar energy will still be stored in the tank through the sensible heating of the water. (2) If the irradiation intensity of the solar energy is low, that is, not enough to provide heat to warm the water in the tank alone, it is necessary to open the ASHP as a supplementary heat source for heating to ensure the stable operation of the system. (3) At night or on rainy days, when the solar radiation intensity is lower than before, it cannot meet the starting conditions of the solar heat collection cycle, and the solar collector will stop running. In the meantime, if the temperature of the heat storage water tank meets the heating conditions, the heat stored in the hot water tank is preferred to maintain the system’s operations. (4) Under the closed condition of the solar collector, when the temperature of the heat storage water tank is lower than the temperature of domestic hot water, the ASHP is used as the independent heat source of the system.

2.3. Design of S/AS-HP Dual-Supply Heating System

2.3.1. Solar Collector

The solar collector is the most important part of the dual-supply heating system. Its area directly affects the amount of solar radiation absorbed by the system and the initial investment of the heating system, which is the key factor in determining the energy savings and cost of the system. The accuracy of the collector area calculation is crucial to the system. A flat plate solar collector was used in the S/AS-HP system.

Solar heat pump systems can be divided into direct systems and indirect systems. Direct systems generally mean that the heated water in the solar collector channel is sent directly to the customer side for use or transported to the heat storage water tank for storage, with no heat transfer process in between. The hot water stored in the tank is circulated through pipes for heating or to provide domestic hot water for the user. Indirect systems generally refer to the heat transfer between the working medium heated in the collector and the water used in the tank through the heat exchange equipment. Then, the hot water in the tank is recycled and put to use. According to GB50495-2019, the calculation method for determining a collector area in two different systems can be obtained [14,15,16], and the specific calculation process is as follows:

$$A_c=\frac{86400Q_{j}f}{J_T{\eta }_{ct}\left(1-{\eta }_L\right)}$$

(1)

$$A_{IN}=A_c^{\prime }\left(1+\frac{U_L{A}_c^{\prime }}{U_{hx}{A}_{hx}}\right)$$

(2)

$$A_{hx}=\frac{\left(1-{\eta }_{hl}\right)Q_{hx}}{\epsilon U_{hx}\Delta t_j}$$

(3)

$$Q_{hx}=\frac{kfQ}{3600S_y}$$

(4)

where $A_c$ is the calculated area of the direct expansion solar collector (m²); $Q_j$ is the system design heat load (W); $f$ is the solar guarantee rate (%), which takes a value according to the climate zone; $J_T$ is the average solar radiation amount in December on the local solar collector daylighting surface (J/(m²·d)); ${\eta }_{ct}$ is the average heat collection efficiency (%); ${\eta }_L$ is the heat loss rate of the pipeline and heat storage device (%), and the empirical value ranges from 10% to 20%, taking 15%; $A_{IN}$ is the calculated area of the indirect expansion solar collector (m²); $A_c^{\prime }$ is the calculated area of the collector according to the direct system formula (m²); $U_L$ is the total heat loss coefficient of a collector (W/(m²·°C)); $U_{hx}$ is the heat transfer coefficient of the heat exchange equipment (W/(m²·°C)); $A_{hx}$ is the heat transfer area of the indirect system heat-exchange equipment (m²); η_hl is the heat loss rate of the pipeline, which ranges from 0.02 to 0.05 according to experience, and 0.03 is adopted in this paper; $Q_{hx}$ is the heat of the heat-exchange equipment (kW); $\epsilon$ is the influence coefficient of the structure, which ranges from 0.6 to 0.8, and is 0.7 in this case; $U_{hx}$ is heat transfer coefficient of the heat-exchange equipment (W/(m² °C)); $\Delta t_j$ is the heat transfer temperature difference (°C); $k$ is the variation coefficient of solar irradiance, which ranges from 1.5 to 1.8, and is 1.6 in this case; $Q$ is the average daily heat supply of the system (kJ); and $S_y$ is the average annual sunshine hours in the region where the system is located (h).

The dual-supply heating system is direct and indirect, so it needs to be divided into two parts when calculating the collector area. The solar energy guarantee rate is 50%. According to the meteorological parameters provided in the appendix of the standard, the average annual sunshine hours in this area are 8.6 h, and the average daily solar radiation amount per unit area of the daylighting surface is 25.025 × 10⁶ (J/(m²·d)). Finally, the total area of the solar collector is 171.18 m².

2.3.2. Heat Storage Water Tank

The heat storage water tank in the dual-supply heating system is used to store the excess heat collected by the solar collector during the daytime operation of the system; it can provide heating for users at night so that solar energy can be used across several periods. The volume of the heat storage water tank is calculated according to the following:

$$V_s=\frac{Q_u-Q_1}{{\rho }_{r}C\Delta t_{xr}}$$

(5)

where $V_s$ is the volume of the heat storage water tank (m³); $Q_u$ is the average effective daily heat collection of the collector (kJ/d); $Q_1$ is the total average daily heat consumption of the heating system (kJ/d); ${\rho }_r$ is the density of water (kg/L); $C$ is the specific heat of the water, [kJ/(kg °C)]; and $\Delta t_{xr}$ is the average heat storage temperature difference (°C)

The volume of the heat storage water tank isfound to be 19.36 m³ through preliminary calculation, and we finally settled on 20 m³ in the model. This system is a short-term heat storage system; the value range of the heat storage water tank per unit lighting area of the collector is 40 L/m²~300 L/m² according to the provisions of GB50495-2019, and the value of the solar heat storage water tank corresponding to this system is 116 L/m², in line with the standard provisions [17].

2.4. Simulation Models

2.4.1. TRNSYS Model

TRNSYS is a transient system simulation program developed by the Solar Energy Laboratory (SEL) of the University of Wisconsin–Madison in the United States. With the joint efforts of several European research institutes, the program has been gradually improved into a complete and scalable simulation environment. The TRNSYS software itself comes with a library of standard modules, while the TESS application library developed by the U.S. Thermal Energy Research Center can be used by extending the program package. Different modules can realize the simulation of the corresponding equipment in the system. A simulation of the system can be realized by selecting the required modules in Simulation Studio, setting the parameters, connecting the modules according to the system principle, and controlling the system operation mode using the temperature control module. TRNSYS allows for the dynamic simulation of the operating conditions of a wide range of systems, such as HVAC systems, solar systems (solar–thermal and photovoltaic), non-conventional air conditioning systems (e.g., soil-source heat pumps, water-source heat pumps), boiler heating systems, building-by-building energy consumption, etc., and it enables the further multi-parameter optimization of systems, as well as the prediction of system operating costs.

The required modules must be selected from the standard module library and the TESS library of TRNSYS 17 (

Table 2. Main simulation modules of the system.

Module	TRNSYS Type
Meteorological data reading module	Type15-6
Solar collector	Type1b
Air-source heat pump	Type941
Heat storage water tank	Type158
Heat exchanger	Type5b
Water pump	Type114
Diverter	Type11f
Tee valve	Type11h
Calculation	/
Control module	Type2b
Terminal simulation module	Type682
Data reader	Type9e
Integrating module	Type24
On-line plotter	Type65a

), and the simulation modules are connected in turn according to the system’s schematic diagram and the interrelationship between modules. The relevant parameters are set based on the actual system’s operations, and, thus, the simulation model of the completed system can be connected. In this article, a flat-plate solar collector was used in winter, and the solar energy guarantee rate was 50%. The inclination angle of the collector was 30°, the collector area was 172 m², and the volume of the heat storage tank was 20 m³. The simulation model of the solar–air-source heat pump dual-supply heating system was built to analyze and study the performance of the system.

2.4.2. Model Assumptions

According to the selection of the above system models, the following assumptions can be made for the established calculation model for the convenience of calculation:

(1) Water is an incompressible single-phase flow with constant physical properties and is analyzed using a steady-state one-dimensional model in the system.

(2) The system uses the pump as the circulating power, so the tank is full of water during operations. There are temperature layers in the heat storage water tank, and the temperature of each layer is evenly distributed. There are temperature differences between the layers.

(3) In the heat exchange process, the mass flow rate of the heat transfer medium flowing through the heat exchanger channel is constant, and the heat transfer coefficient of the heat exchanger is a constant value.

(4) Pipe heat loss is taken into account in the simulation, and it is assumed that the tank does not age with time.

2.5. Thermal Performance Criteria

COP_sys is used in this paper to comprehensively evaluate the overall performance of the dual-supply heating system. COP_sys represents the ratio of accumulated heat production to the total power consumption of the unit during the operating period of the system. Its calculation process can be expressed as follows:

$${\mathrm{COP}}_{\mathrm{sys}}=\frac{Q_U+Q_{\mathrm{hp}}}{W_{\mathrm{hp}}+\sum W_p}$$

(6)

where ${\mathrm{COP}}_{\mathrm{sys}}$ is the heating performance coefficient of the system; $Q_U$ is the effective heat collection of the collector during the operation period (kWh); $Q_{\mathrm{hp}}$ is the heating capacity by the ASHP during the operation period (kWh); $W_{\mathrm{hp}}$ is the power consumption of the ASHP during the operation period (kWh); and $\sum W_p$ is the total power consumption of the water pump during the operation period (kWh).

2.6. Economic Performance of the System

The system benefits need to consider both the initial investment and operating costs of the equipment, so the annual cost was chosen as the objective function to bring the system optimization results more in line with the actual needs of engineering. The annual cost is to convert the initial investment and annual operating cost of the system into the equivalent cost at the end of each year. It means that the system design variables are the best combination when the annual cost is the lowest. The annual cost is calculated according to Equation (7), and the system equipment’s initial investment and operating costs are taken according to

Table 3. Estimation of initial investment and operation costs of the system equipment.

Name	Price
Solar collector (flat-plate solar)	430 CNY/m2
Heat storage water tank	900 CNY/m3
Air-source heat pump	CNY 50,000
Water pump	CNY 650
Heat exchanger	CNY 2000
electricity price	0.675 CNY/kWh

.

$$AC={\displaystyle \sum }_{j=1}^n\frac{\alpha {\left(1+\alpha \right)}^{\beta }}{{\left(1+\alpha \right)}^{\beta }-1}{Z}_j+Y_r+Y_y$$

(7)

where $AC$ is the annual cost; $n$ is the number of equipment types included in the heating system; $\alpha$ is the internal returns ratio, taken as 8%; $\beta$ is the service life of the heating equipment, taken as 15 years; Z_j is the amount of investment in the heating equipment, CNY; $Y_r$ is the annual cost of fuel/electricity for heating; and $Y_y$ is the operation and maintenance cost, taken as 2% of the initial investment in the equipment for calculation.

3. Results and Discussion

3.1. Validation of Simulation Results

The computational results were used to validate the simulation results. As shown in Figure 3, the hourly heating load per unit area can be calculated. The results show that the maximum value is 47.3 W, which is consistent with the design thermal index and reflects the good thermal performance of the building envelope. The change law is consistent with the trend of outdoor ambient temperature changes over time. Therefore, the simulation model has good accuracy and can be used to evaluate the performance of the heating system.

3.2. Hot Water Temperature

The temperature changes in the supply and return water and domestic hot water in the dual-supply heating system are shown in Figure 4. The domestic hot water supply temperature has certain fluctuations, but it is stable at about 60 °C, consistent with the design value. When heating the building, the temperature of the water supply is basically 50~55 °C, and the temperature difference in the water supply and the return fluctuates between 5 and 10 °C. This variation trend is consistent with the variation trend of the building heat load. Because the building envelope has a certain heat storage capacity, the temperature difference in the water supply and the return shows a certain lag in time. In the middle period of heating (January to February), due to the continuous decrease in outdoor ambient temperature, the heating load of the building and the heat loss of the water tank increase, the temperature of the water supply decreases slightly, and the temperature difference between the supply and return water also increases. The maximum value is 16.89 °C, which appears at 8 am on 13 January. Furthermore, the corresponding ambient temperature is −13.8 °C, which is the lowest temperature for the whole year. The design of the temperature is reasonable, and it can meet the demands of both building heating and domestic hot water supplies throughout the analysis.

3.3. Operation Mode and Time Ratio

Figure 5 shows the start/stop signals for the different heating modes in the operation of the system in January, where 1 means start and 0 means stop. The system heating mode is divided into four types: solar collector heating mode (mode 1), ASHP heating mode (mode 2), S/AS-HP heating mode (mode 3), and heat storage water tank heating mode (mode 4). Mode 1 ran for 7 h from 11:00–18:00 on 16 January, and all the heat in the system came from solar radiation energy collected by the collector. The operation times of mode 2 were 3:00~10:00 and 22:00~2:00, a total of 11 h. During this period, the solar radiation intensity was not enough to heat the water to the opening condition of the heat collection cycle, and the hot water temperature in the heat storage water tank was lower than the water supply temperature. The system load was provided by the ASHP. Mode 3 ran for 1 h from 10:00 to 11:00. At this moment, the solar radiation intensity kept increasing, and the temperature difference between the hot water at the outlet of the solar collector and the return water of the heat storage water tank was greater than 8 °C. The solar collector started to work, but it was not enough to provide all the heat for the system, so the air-source heat pump was needed as a combined heat source to maintain the operation of the system. Mode 4 ran for 5 h from 18:00 to 22:00 and 2:00 to 3:00. The heat storage water tank met the independent operation conditions and released the solar energy stored in the form of hot water during the day to provide heat for the system. It is not difficult to see from the analysis of the results that the total time that the ASHP participated in the operation was 12 h, accounting for 50%.

Table 4. Total operation time and proportion of different heating modes in the heating period.

Heating Mode	Mode 1	Mode 2	Mode 3	Mode 4
Total duration (h)	667	1229	109	875
Proportion (%)	23.16	42.68	3.78	30.38
Average daily operation time (h)	5.56	10.24	0.91	7.29

presents statistics on the total operation time of different heating modes during the heating period and calculates the percentage of each mode. The total operation time of the solar collector was 776 h, accounting for 26.9% of the total heating period, of which the operation time of mode 1 was 667 h, accounting for 23.16%. The duration of heating through mode 2 was 1229 h, accounting for 42.68%. As can be seen from Figure 6, the distribution ratio of the running time of each mode in the system in November, December, and March was reasonable.

3.4. Comprehensive Energy Efficiency Ratio of Dual-Supply Heating System

The daily average COP_sys of the system during the heating period is shown in Figure 7. The maximum and minimum COP_sys values of the system were 6.657 and 2.76, respectively, and the average COP_sys was 4.21 for the whole heating period. As can be seen from

Table 5. Monthly average COP_sys of the system.

Month	Total Heat Supply of System (kWh)	Total Energy Consumption of System (kWh)	Monthly Average COPsys of the System
November	18,314.012	3730.179	4.910
December	38,903.242	8519.245	4.567
January	41,016.888	9955.527	4.120
February	34,438.802	9259.452	3.719
March	17,060.497	4105.584	4.155

, COP_sys changed little in the early stage of heating; the monthly average COP_sys from November to January was greater than four, while the monthly average COP_sys fluctuated significantly from February to March due to the great change in solar radiation intensity. The monthly average COP_sys in February was the minimum value of 3.72 in the heating period.

3.5. Comparison of Different Schemes

Three different heating forms of S/AS-HP, air-source heat pump, and solar–electric auxiliary (S/EA) heating systems were simulated using TRNSYS. To make the comparison results more reasonable, it was first necessary to clarify the system variables; set the simulation to change only the system heat source form; and keep parameters such as solar collector area, heat storage tank volume, air-source heat pump, and related auxiliary accessories consistent. The COP_sys and the annual cost value were selected as the system’s comprehensive performance and economic evaluation indexes. The optimal heating solution was found through comparison. The energy-saving and economic comparison results of each system are shown in Figure 8 and Figure 9.

As can be seen in Figure 8, the monthly average COP_sys of the solar–air-source heat pump was always above 3.7, which was larger than the other two systems, and the average COP_sys during the heating period was 1.7 times higher than that of the air-source heat pump system, which had a better system operation performance. The solar–electric auxiliary heating system was the least energy-efficient, with an average COP_sys of only 1.4 during the heating period. By comparing the costs of the three systems (Figure 9a), we concluded that the initial investment for the solar–air-source heat pump system was the highest, at CNY 151,790, and the lowest was for the air-source heat pump system, at CNY 78,900. The difference between the two is large, indicating that solar collectors occupy a larger proportion of the initial investment in the system. Therefore, the design and selection of equipment must be comprehensive in consideration of energy efficiency and the economy in the actual project rather than the pursuit of high solar energy guarantee rates; otherwise, it will lead to excessive initial investments, and the system economy will not be guaranteed.

Although the solar–air-source heat pump system has a larger initial investment, the annual cost is the lowest among the three for the same service life, at CNY 41,824.6, saving 9.6% and 35.1% per year compared with the air-source heat pump system and the solar–electric auxiliary heating system when analyzed over the whole life cycle of the system. As shown in Figure 9b, the air-source heat pump system and the solar–electric auxiliary heating system consume 1.7 and 2.4 times more electricity than the solar–air-source heat pump, respectively, during operation, which greatly increases their operating costs and is not economical. Therefore, comparing the performance and economy of each heating system, the solar–air-source heat pump system is the best heating solution.

4. Conclusions

In this article, the winter work performance and economic benefits of the solar–air-source heat pump combined heating system were studied in Lhasa, Tibet, a typical high-cold area in China. The main conclusions are as follows:

(1) First, combined with the climatic characteristics of Lhasa, a residential building located in Lhasa was taken as the research model, and TRNSYS was used to simulate the dynamic change rule of the hourly heat load of the building. The maximum hourly heat load of the building during the heating period was 66.8 kW, the maximum heat load per unit area was 47.3 W, and the accumulated heat load of the building during the whole heating period was 75,720 kWh.

(2) The working principle and control mode of the solar–air-source heat pump dual-supply heating system are both described. Based on the analysis results of the dynamic variation of the heat load of the target building, the solar collector and heat storage water tank were designed and calculated. The calculation results show that the total area of the solar collector was 171.18 m², and the volume of the heat storage water tank was 19.36 m³.

(3) The simulation model of the solar–air-source heat pump dual-supply system was built to analyze the system’s operational performance. The results show that the water temperature of the domestic hot water supply system was stable at about 60 °C, the water temperature of the building was basically 50~55 °C, and the temperature difference in the supply and return water fluctuated around 5~10 °C. The variation trend is consistent with the variation trend of the building heat load, which can meet the demands of building heating and the domestic hot water supply. The total running time of the solar collector during the heating period accounted for 26.9% of the total heating period, in which the independent running time accounted for 23.16%. The maximum average daily heat collection efficiency was 55.27%, and the average value of the whole heating period was 40.6%. The maximum COP_sys value was 6.657, the minimum COP_sys value was 2.76, and the average COP_sys value was 4.21 during the whole heating period.

(4) Through the simulation of three different forms of heating systems, we concluded that the COP_sys of a solar–air-source heat pump is larger than that of an air-source heat pump system or a solar–electric auxiliary heating system; this value always remained above 3.7, and the energy-savings of the solar–electric auxiliary heating system were the worst. In terms of the full life-cycle analysis of the system, the annual cost of the solar–air-source heat pump system is the lowest among the three at CNY 41,824.6 for the same service life, saving 9.6% and 35.1% per year compared with the air-source heat pump system and the solar–electric auxiliary heating system.

(5) The study of the solar–air-source heat pump dual-supply heating system in this paper is beneficial to the expansion of renewable energy applications in cold areas, the transformation of energy structures, and the construction of an environmentally friendly society.