A Review on the Research and Development of Solar-Assisted Heat Pump for Buildings in China

Abstract

The building sector accounts for over 40% of global energy consumption. The utilization of renewable energy systems such as the solar-assisted heat pump (SAHP) in buildings has been shown to improve building energy efficiency and achieve carbon neutrality. This paper presents a review of the research and development of solar-assisted heat pumps for buildings in China. It firstly introduces the different stages of solar-assisted heat pump research. Secondly, the research on different types of heat pumps, the core components of heat pumps, the computer software used, and the economic feasibility evaluation of solar-assisted heat pumps are presented. Thirdly, the application of SAHPs in practical projects is examined and relevant regulations, standards, and policies for solar-assisted heat pump development in China are highlighted. Finally, recommendations for the future development of solar-assisted heat pumps in China are suggested.

1. Introduction

Since the energy shortage and oil crisis in the 1970s, it has been evident that any development at the expense of environmental deterioration is not sustainable [1]. The increase in building energy consumption is one of the most important factors that contribute to energy shortages, as buildings consume up to 45% of the primary energy globally [2]. With rapid urbanization, the total energy consumption in China continued to grow since 2005 and surpassed the United States in 2009 [3]. According to the statistics from the China Building Energy Conservation Association, the total building energy consumption reached 2.233 billion tons of standard coal in China in 2019, accounting for 46% of the total national primary energy consumption [4]. In addition, carbon emissions in the building sector were 50% of the national carbon emissions [4]. To reduce carbon emissions from fossil fuels, China has increased its investment in renewable energy, such as solar energy. It has been at the forefront of the utilization of renewable energy for sustainable development. Since 2013, China surpassed Europe as the investment leader in the renewable energy industry. In 2017, China’s total installed solar power generation capacity reached 53 GW, accounting for half of the global total solar power generation [5]. In addition, China’s total investment in renewable energy reached $8.34 × 10¹⁰, far exceeding the $5.55 × 10¹⁰ investment in the USA, ranking China first in the world [5]. According to the World Energy Statistics yearbook, the world’s solar power generation capacity has increased by 127 GW in the past 20 years [6]. China is the largest contributor to the growth of renewable energy (1.0 EJ/year), followed by Europe (0.7 EJ/year) and the United States (0.4 EJ/year). Figure 1 presents the comparison of solar power generation in China, the United States, Europe, and Japan. It can be seen that, since 2017, China has become the largest solar power-generating country in the world, with an installation capacity reaching 253.8 GW in 2020.

Figure 1. Solar photovoltaic power generation from 2010 to 2020 in various countries [6].

Since the issue of the “Green Building Evaluation Standard” in 2006, building energy efficiency has received wide attention in China [7]. In particular, renewable energy application in buildings is regarded as one of the focuses in the future [8]. Professor Jiang proposed the transition to a low-carbon energy system based on renewable energy in 2017 [9]. China has set a target to reach the peak of carbon dioxide emissions by 2030 and carbon neutrality by 2060 [10]. Therefore, it is critical to adopt clean energy sources in the building sector to reduce carbon emissions. Of many sources of carbon emission, the heating, ventilation, and air-conditioning (HVAC) system accounts for 50%–70% of building energy consumption [11]. Therefore, it is an important area to be considered in building energy reduction. Since many emerging energy resources come from solar energy, the rational development and utilization of solar energy resources are essential to solving future energy shortage problems [12]. The application of the solar-assisted heat pump in buildings can reduce building energy consumption by utilizing solar energy to power the HVAC system/improve the HVAC system’s efficiency.

The solar heat pump technology was developed with the breakthrough in monocrystalline silicon cells and selective solar absorptive coatings in the 1950s using solar energy as the heat source [13]. Since then, the concept of the direct expansion solar-assisted heat pump (DX-SAHP) and other types of solar-assisted heat pump systems (SAHP) with single or multiple heat sources including solar energy have been proposed [14,15]. Typically, a SAHP system is an integration of a traditional heat pump and solar thermal panels, which function as a low-temperature heat source. The heat produced is used to feed the evaporator. Due to a higher evaporator temperature, their coefficients of performance (COPs) are much higher than those of traditional heat pumps. In addition, with multiple heat sources, SAHPs can work stably under different climatic conditions [16]. A large number of experimental and theoretical studies and system optimizations have subsequently been conducted in developed countries [17,18,19]. By comparison, China’s research on SAHPs started later in the 1980s [20]. At the beginning of the 21st century, Ji and Pei et al. [15] conducted comprehensive research on photovoltaic electric/thermal heat pumps. Since then, many universities and research institutes have carried out studies on SAHPs leading to great progress theoretically and practically [21].

China has a vast territory with rich solar energy resources [22]. There are five climatic regions in China: the severe cold region, cold region, hot summer and cold winter region, mild region, and hot summer and warm winter region [23]. The total annual solar irradiations vary in the range of 3340 MJ/m²~8400 MJ/m² across the countries, with a median value of 5852 MJ/m². High solar irradiations are distributed across different climatic regions including Qinghai (severe cold region), Xinjiang (severe cold region and cold region), southern Ningxia (cold region), Gansu (cold region), southern Inner Mongolia (severe cold and cold region), northern Shanxi (cold region), Liaoning (severe cold region), southeastern Hebei (cold region), southeastern Shandong (cold region), southeastern Henan (cold region), western Jilin (severe cold region), central and southwestern Yunnan (mild region), southeastern Guangdong (hot summer and warm winter region), southeastern Fujian Guangdong (hot summer and warm winter region), eastern and western Hainan Island (hot summer and warm winter region), and Qinghai-Tibet Plateau(severe cold region and cold region). Although they have abundant solar energy resources, in most areas their economy is less developed. In the more developed regions of China, the advantages of solar energy resources are not obvious, and SAHP development is driven by the local heating energy demand and financial support for clean energy systems from governments [24].

The research and development of SAHP and its application in buildings in China are quite different from that of other countries due to China’s complex climatic conditions and economic situation. Therefore, it is important to carry out a comprehensive and systematic review of the research and development of SAHP in China. Firstly, this paper introduces the different stages in the research and development of SAHP for buildings in China. Secondly, the research on different types of heat pumps, the core components of heat pumps, the computer software used, and the economic feasibility evaluation of solar-assisted heat pumps is presented. Thirdly, the application of SAHP in practical projects is examined and the relevant regulations, standards, and policies for solar-assisted heat pump development in China are highlighted. Finally, recommendations for the future developments of solar-assisted heat pumps in China are suggested.

2. Stages of SAHP Research in China

Developed countries such as the United States, Japan, and Denmark have conducted a large number of studies on SAHP since it was first proposed in 1955. The research on SAHP started late in China, and the renewable energy applications in China are still in a rapid development stage [25]. The following section summarizes the publication trend for SAHP research in China in recent decades and divides it into four stages according to the research topics and timelines.

2.1. Research Publication Trend from 1981 to 2021

This review searched the literature published in databases including CNKI, Web of Science, Baidu, and Google Scholar and focused on SAHP research in China from 1986 to 2021. A total number of 1224 publications were found (Figure 2), and after careful selection within the areas of the building environment, energy consumption, numerical simulation, experimental and theoretical analysis, policy and system optimization, based on the quality and authority, e.g., from core collections of the Chinese literature database, government official websites, and the Science Citation Index (SCI) database, 153 representative papers and electronic documents were selected. It should be noted that the primary purpose of this review is to focus on the research and development of SAHPs in the building sector, and therefore theoretical analyses of the application of heat pumps in other areas such as drying and dehumidification were excluded.

Figure 2. Publication trends from 2010 to 2021.

From Figure 2, it can be observed that fewer than ten papers were published annually before 2000, meaning renewable energy resources did not receive much attention during that time period. In 2005, the enactment of the Law of Renewable Energy of the People’s Republic of China greatly supported the large-scale application of solar, thermal, and photovoltaic electric power in China [26]. This appears to have stimulated research on SAHP with relatively steady growth in publications. With the implementation of the National Tenth Five-Year Plan, the development of clean energy represented by solar, thermal, and photovoltaic power generation was formulated as an important national energy development strategy. With governmental support, more and more studies were devoted to SAHPs [27]. The rapid rise of the solar industry in China was attributed to the favorable world economic environment and government policies. However, during the period from 2008 to 2014, the emergence of the global financial crisis had a significant impact on the new energy industry. Meanwhile, due to a lack of innovations and core technologies, the overall quality and technological level were low, compared with developed countries, leading to overproduction in solar-related industries in China [28]. In 2013, photovoltaic product demand was less than 60% of its output, and the polysilicon industry supply exceeded 67% of market demand by the end of the year [29]. The bankruptcy of the largest photovoltaic company in China was indicative of the overproduction of PV industries reaching their peak level. This fluctuation appears to be reflected in the number of publications during this same period. With the promulgation of relevant subsidy policies for the solar energy industry by the central government after 2014, research on SAHPs again became more prominent and the number of publications has been on the rise since 2015.

2.2. Research Topics in Different Periods

From the literature survey, it was found that Chinese scholars began to carry out experimental studies on the SAHP manufactured by Hitachi in 1985 and then gradually conducted some theoretical research on SAHP [30]. According to the research topics and timelines, the SAHP research can be divided into four stages.

The first stage of the study lasted from 1985 to 2000. The main research topics and timelines are shown in Figure 3. At this stage, the research mainly focuses on direct expansion heat pumps. Although few research outcomes were presented during this period, they did provide important references for future theoretical and experimental research. The representative research results of the first stage are the performance outcomes of the DX-SAHP system. The technical difficulty of this research stage is that, in conventional SAHPs, the solar collector and heat pump are used to operate as two separate units with high energy losses.

Figure 3. Timeline of SAHP research between 1985 and 2000.

The second stage was from 2001 to 2005 when the research on the SAHP system expanded considerably. Figure 4 categorizes the areas of research during this period. It can be seen from Figure 4 that the research primarily focuses on the collector and other heat pump components of the direct expansion system [31]. At the same time, preliminary research was carried out on different types of SAHPs, such as multiple heat sources and non-direct expansion heat pumps, which lays good foundations for subsequent studies [32]. The representative research results of the second stage are the optimization of the heat exchanger and solar heat collector. The technical difficulties of this research stage are the lack of more environmentally friendly refrigerants, standard system application specifications, and fixed heat sources, which make it impossible to greatly improve the thermal efficiency of the system.

Figure 4. Categories and contribution proportions of SAHP research between 2001 and 2005.

The third stage is from 2006 to 2015, which is a period of development with some fluctuation. During this period, fruitful theoretical achievements were made in the types, theories, project applications, and simulations of SAHPs. The main research categories and corresponding contributions are presented in Figure 5. It can be observed that a large proportion of the studies still focus on direct expansion systems and a small proportion of them concentrates on experimental research on non-direct expansion systems. In addition, multi-functional SAHPs such as the photovoltaic thermal (PV/T) system, the photovoltaic solar energy system, ground source auxiliary, and other new types of SAHPs have become the focus of research in this period due to their high efficiency and ability to meet different operating conditions [33,34]. The representative research results of the third stage are the design of multiple heat source systems, e.g., solar-assisted ground source heat pumps (SGHP), and the emergence of various high-efficiency heat and electricity cogeneration systems. The technical difficulties of this research stage are the low system thermal efficiency and low system application rate due to cost factors such as material prices, inadequate information to gather collectors’ lifespans and performances under high temperature operating conditions, and the experimental verification of numerical models.

Figure 5. Categories and contribution proportions of SAHP research between 2006 and 2015.

The fourth stage is from 2016 to now, which is again a period of rapid development. Figure 6 presents the research categories and contribution proportion during this time. It can be observed that the research focuses switched to different types of heat pumps, multi-functional heat pumps, and the intelligent control of heat pumps. Based on the direct-expansion system, new systems with different connections on the main components of the heat pumps and combinations with various heat sources have been explored, such as the PV/T system, the sewage source heat pump system, the air and solar energy heat pump compound machine, etc. [35,36,37]. In addition, during this period, significant achievements have been made in the application of refrigerants, and many practical projects on SAHPs were carried out. Furthermore, the optimal control of SAHPs has gradually become the research focus. The representative research results of the fourth stage are on the enhancement of the efficiency of the photovoltaic/thermal (PV/T) collector, e.g., micro-channel heat pipes and concentrating heat collectors, as well as automated and intelligent control of the SAHP system. The technical difficulties of this research stage are the integration of wind and geothermal heat sources into SAHP systems and optimization of the system configuration, the integration of terminal units and the SAHP system, and a lack of specific guidelines for SAHP application in the residential sector.

Figure 6. Categories and contribution proportions of SAHP research between 2016 and 2021.

The research progress on SAHP systems in China can be summarized in Figure 7. It can be concluded that SAHP research in China has undergone significant advancements in heat source selection, theoretical optimization, numerical simulation, etc. New types of systems such as PV/T-SAHP and geothermal auxiliary SAHP systems were developed based on direct expansion and non-direct expansion SAHP systems. Excellent achievements were made after 2015 when the Chinese government proposed a continuous transformation of energy structures and high-quality development with the “Paris Agreement” being signed in December 2015 [38].

Figure 7. Research progress on SAHP in China.

More research publications and diverse topics on SAHP can be expected in the next decade as the Chinese government has proposed its goals of achieving carbon peak and carbon neutrality at the United Nations Climate Conference in 2020. Meeting the carbon neutrality targets will bring significant investment in new renewable energy research and projects, which will stimulate Chinese researchers to carry out more in-depth research on SAHP.

3. Research on SAHP

3.1. Research on the Types of Heat Pumps

SAHP technology is an effective combination of solar collectors and traditional heat pumps. The SAHP is based on the transformation of the heat exchanger of the traditional heat pump by utilizing solar energy as the heat source, or combined with other energy sources, to improve the coefficient of performance of the heat pump. From the ways of solar energy utilization, SAHP systems can be divided into photovoltaic-solar-assisted heat pumps (PV-SAHP), photothermal-solar-assisted heat pumps (PT-SAHP), and photovoltaic/thermal-solar-assisted heat pumps (PV/T-SAHP).

The PV-SAHP systems can be divided into direct solar-assisted heat pumps (DX-SAHP), indirect solar-assisted heat pumps (IX-SAHP), and PV/T-SAHP systems, according to the different connection methods and heat collection media. In the DX-SAHP system, the refrigerant flows into the solar collector directly and is heated. The collector is the heat source for evaporation. In the IX-SAHP system, the solar collector and the heat pump evaporator operate independently, and heat is absorbed through a heat exchanger. According to the differences in the connection between the solar heat collection cycle and the heat pump cycle, they can be classified as series, parallel, and dual heat source heat pumps [39,40]. The PV/T-SAHP system simultaneously utilizes solar energy, electric energy, and other forms of ambient energy. PV/T modules are used as a heat collection evaporator combined with an SAHP cycle to realize the comprehensive utilization of solar energy, photoelectricity, and heat, and improve the overall efficiency of the heat pump system [41]. The PV/T system will be one of the focuses of future research.

Table 1 lists the main components for the different types of SAHP systems.

Table 1. Classification and system components of SAHP systems.

Classification	System Components	Ref.
PV-SAHP	PV evaporator (photovoltaic and heat pump evaporation), photovoltaic conversion device (inverter), auxiliary evaporator, heat pump system (evaporator, condenser, compressor, expansion valve), public grid, building heating, and domestic hot water system.	[39]
DX-SAHP	Solar collector, heat exchanger, pump, heat pump system (evaporator, condenser, compressor, expansion valve), domestic hot water system, and hot water storage tank (optional).	[40]
IX-SAHP (Series)	Flat plate collector, compressor, condenser, evaporator, expansion valve, pump, and domestic hot water system.	[42]
IX-SAHP (Parallel)	Flat plate collector, compressor, condenser, expansion valve, water pump, hot water inlet, hot water outlet, evaporator, and hot water tank.	[42]
IX-SAHP (Hybrid)	Flat plate collector, compressor, condenser, evaporator, expansion valve, water pump, hot water inlet, and hot water outlet.	[42]
PV/T-SAHP	Solar air collector (photovoltaic module, heat absorption plate, airflow channel, edge and back insulation layer, metal frame), compressor, hot water storage tank with a built-in condensing coil, and expansion valve.	[41]

The SAHP system combines solar energy utilization and building energy supply through the heat pump cycle. In the DX-SAHP system, the refrigerant flows directly through the collector/evaporator and then passes through various components such as the compressor to complete a cycle [39]. For IX-SAHP, the solar collector absorbs heat and transfers it to the system evaporator through the heat exchanger. The refrigerant absorbs heat in the evaporator, evaporates, and then passes through the compressor, condenser, and throttle valve to complete a cycle [42]. Compared with IX-SAHP, the application of the collector in the DX-SAHP system makes the system structure more simplified and compact for the following reasons: (1) the refrigerant in the solar collector directly absorbs heat and vaporizes, leading to higher thermal performance; (2) at the same time, the working fluid in the collector is refrigerant instead of water, which can prevent the freezing problem of solar collectors on cold nights [39]; (3) for the collector, the refrigerant absorbs heat and evaporates in the collector, which can maintain a low collector temperature and effectively improve the efficiency of the collector. The disadvantage of DX-SAHP is that the thermal performance of the system is closely related to the change in solar radiation intensity. As the daily solar radiation intensity can vary from 0 W/m² to 800 W/m², the thermal performance of the system fluctuates greatly [43]. Figure 8a–d display the system diagrams for DX-SAHP and IX-SAHP in series, parallel, and hybrid connections. In a series system, the solar collector and the heat pump evaporator are connected in series and exchange heat through an intermediate medium. In the parallel system, the solar heat collection system and the heat pump system are connected in parallel, and both can produce hot water [43]. Figure 8e presents the system diagram of a PV/T-SAHP system. Compared with normal photovoltaic-assisted solar heat pumps, the PV/T systems address both the cooling need of photovoltaic modules and the heat absorption need of the evaporator. The solar energy utilization efficiency is significantly improved when the photoelectric and photothermal conversions are carried out at the same time. In addition, the working temperature of the photovoltaic cell is decreased, and the photoelectric efficiency is increased. As the heat source of the heat pump, the PV/T heat collection module increases the evaporation temperature and evaporation pressure of the working fluid of the evaporator so that the coefficient of performance of the heat pump can be improved [44].

Figure 8. Different types of SAHPs. (a) Schematic diagram of DX-SAHP; (b) Series connection of IX-SAHP; (c) Parallel connection of IX-SAHP; (d) Hybrid connection of IX-SAHP; (e) PV/T-SAHP [43].

3.2. Research on the Core Components of Heat Pumps

The SAHP system follows the basic reversed Carnot cycle, assuming that the refrigerant gas compression is adiabatic and reversible, so that there is no pressure loss outside the compressor and throttling device, and there is no heat exchange with the ambient environment except with the evaporator and condenser. The theoretical cycle of DX-SAHP can be described as two isobaric heat transfer processes of isentropic compression and adiabatic throttling [42]. In practice, due to the complexity of the system and environmental factors, overheating, subcooling, and pressure drops exist during the refrigerant cycle; therefore, it is much more complicated than the reversed Carnot cycle [40]. The solar collector/evaporator, compressor, and piping system are the core parts of the SAHP. Their performance directly affects the operating efficiency of the entire system. Therefore, they will be separately discussed in the following sections. Meanwhile, the research on heat exchangers and refrigerant flow characteristics will be presented.

3.2.1. Solar-Collector/Evaporator

Current PV/T collector/evaporator research focuses on the heat exchanger where the bare-plate structure is common for most evaporators [45,46,47,48,49,50,51]. As the finned tube structure has the advantages of material saving, lightweight, and high heat exchange efficiency, the tube-fin structure heat collecting evaporator receives wide attention [43]. Figure 9 provides a schematic diagram of a solar collector module. Figure 10 is the cross-sectional view of a heat pipe of a PV collector/evaporator, with Table 2 summarizing the investigations on evaporator-related components by Chinese scholars. Based on the comparison of the systems’ COP, a significant improvement in the system performance can be found after the optimization of the materials and structural configurations of the evaporator.

Figure 9. Structure layered diagram of PV/T collector/evaporator [40]. (a) Structure diagram; (b) Sectional view.

Figure 10. Cross-sectional view of the heat pipe of the PV collector/evaporator [41].

Table 2. Investigations on evaporator components of the SAHP system.

Author	Year	System Type	Note	Average COP	Ref.
Ji et al.	2009	PV/T DX SAHP	The system can produce electricity and heat at the same time. The performance of the heat pump was tested, and the numerical model was validated. The photovoltaic efficiency (instantaneous rate of conversion from solar energy to electricity) of the system was >12% during the period of measurement (from 8:00 a.m. to 16:00 a.m.), which was higher than other types of photovoltaic/thermoelectric systems.	3.5	[45]
Song	2020	DX SAHP with Novel Fresnel Photovoltaic + Triethylene Glycol (TEG) Hybrid Evaporator	Experimental investigation was conducted, and it was shown that under the irradiation of 800 W/m2, the maximum photovoltaic power generation can be increased by 43.77 W, and the electrical efficiency can be increased by 1.40%. Meanwhile, the COP of the hybrid system can reach 7.89.	7.89	[44]
Xu et al.	2006	Dual-source DX-SAHP	Simulation was performed for a system with a tube-fin structure evaporator, and it was shown that the overall energy efficiency for hot water heating was improved, and the system worked properly even during rainy days and produced hot water at 55 °C under all weather conditions.	4.6~5	[46]
Qin et al.	2018	New Direct Expansion Variable Frequency Fin-tube Solar/Air Assisted Heat Pump Water Heater	System performance was tested with a solar simulator in the enthalpy difference laboratory, and the results show that the energy consumption of the compressor was almost not affected by ambient temperature, solar irradiation, and operating mode. Energy consumption of the evaporator and COP increases with the increase in ambient temperature and solar irradiation. It was recommended to operate the compressor at 60 Hz under ambient temperatures of 20 °C to 30 °C and average solar irradiation of 900 W/m2.	<4.15	[47]
Long et al.	2019	New Water Refrigerant SAHP System	The system is configured with a tube-fin evaporator. It can obtain heat from low-temperature water easier than the traditional water refrigerant evaporator and, therefore, has better performance in utilizing air sources and solar energy through heat pumps in cold seasons.	3.87~4.45	[48]
Cai et al.	2019	Series DX-SAHP system	The system is configured with a tube-fin structure evaporator and a bare plate structure collector in series. The average COP increases from 2.78 to 3.31 with the ambient temperature rising from 5 °C to 30 °C and from 2.71 to 3.22 with solar radiation increasing from 100 W/m2 to 300 W/m2.	2.71 *	[49]
Ji et al.	2020	DX-SAHP system with tube-fin structure heat-collecting evaporator	The influence of environmental parameters on the operation of the system is compared with that of the bare plate structure system. It was found that the COP of the bare-plate type system and the finned-tube type DX-SAHP increased by 6.6% and 16.2%, respectively, with the ambient temperature rising from 5.0 °C to 15.0 °C. Under frost conditions, the COP of the finned-tube type system decreases from 1.72 to 1.54 with relative humidity rising from 50% to 90%, while the COP of the bare-plate type system increases by 16.3%, with the relative humidity rising from 70% to 90%.	2.56~2.58	[50]
Wang and Quan	2015	Dual-source heat pump (DSHP) system	The whole system is equipped with two evaporators, and the heat of the evaporators can be provided by an air source or solar energy. It was found that COPs of the dual-heat-source operating mode and single-heat-source operating mode varied in the range of 1.70 to 3.27 and 1.45 to 3.18, with mean values of 2.49 and 2.24, respectively.	4.08	[51]

* At an ambient temperature of 10 °C and solar irradiation of 100 W/m².

The collector/evaporator is responsible for heat collection. At the same time, the working fluid evaporates in the collector/evaporator and absorbs heat from solar thermal conversion and ambient air which increases the evaporator temperature and improves the system COP. Therefore, they are the key components to exploit solar radiation to improve the system’s efficiency. The technical difficulties in the research on collector/evaporators lie in how to improve the absorptivity of the collector/evaporator and their heat transfer performance. The corresponding solutions are to use finned tube structures with high heat exchange efficiency and selective absorption coating for the collector/evaporator.

The above literature survey shows that the improvement of the collector/evaporator and the load matching between the evaporator/condenser and the compressor have always been the focus of the research. Many studies have been carried out on the structural arrangement of vertical copper–aluminum finned tube collector evaporators and fins. Moreover, investigations have been conducted on the arrangement between the heat-absorbing plate and the evaporator, refrigerant selection, and structural optimization of the collector/evaporator. The evaporator structure and material selection have a great influence on the operation of the whole DX-SAHP system.

3.2.2. Operation and Thermal Characteristics of the Compressor

The operating frequency, solar irradiation intensity, and ambient air temperature are the three most important factors that affect the performance of the SAHP. In particular, the performance of the compressor plays a key role in the system’s performance [52]. The matching between different components is very important, especially the matching between the area of the collector/evaporator and the compressor capacity [53]. Table 3 lists the theoretical investigations on the characteristics of the compressor by Chinese scholars. It can be found that the system COP can be improved by adjusting the compressor speed under different working conditions.

Table 3. Theoretical investigations on the characteristics of the compressor.

Scholar	Year	System Type	Note	Average COP	Ref.
Cai and Li	2019	New air source hybrid SAHP	Experiment was conducted on a new type of air-source hybrid SAHP and results from the experiment show that the compressor has the largest exergy loss in the heat pump system.	5.02	[49,54]
Kong et al.	2020	DX-SAHP hot water heater system	Under steady state and actual working conditions, the influence of different compressor speed adjustment methods on system performance is analyzed, and it is shown that the compressor speed has little effect on the heating power demand of the system, and a reasonable compressor speed adjustment method will help improve the energy efficiency and stability of DX-SAHP systems.	4.6	[55]
Pei and Ji	2007	Photovoltaic SAHP	High-frequency mode helps improve the condensing capacity and photovoltaic and thermal efficiency. Low-frequency mode results in a reduction in compressor power consumption and the compression ratio. Therefore, the compressor frequency should increase with the increase of solar radiation intensity to improve the thermal efficiency of the system.	3.8–4.2	[56]
Li and Huang	2021	SAHP	The effect of compressor speed on the performance of the SAHP system under different operating conditions was studied experimentally, and it was shown that when the solar radiation intensity was low, increasing the compressor speed could significantly improve the heating capacity of the system. When the ambient temperature is low, increasing the compressor speed will slightly reduce the system’s COP, but it can help improve the system’s heating capacity.	7.7	[57]
He	2019	PV-SAHP test bench	The effect of compressor frequency on system heating performance and system power generation under different weather conditions was verified. The results show that the average value of solar irradiation intensity and ambient air temperature does not have an obvious impact on the compressor operation frequency and average COP.	4.4–4.7	[58]
Dong	2015	DX-SAHP water heater test bench	System performance was obtained via a data acquisition and monitoring system in Qingdao. It is concluded that the COP of the SAHP system can be above 6.0 in summer, and the COP in cold winter can be above 4.0.	4.0–4.38 in winter mode	[59]
Dong et al.	2013	DX-SAHP	Variation of the compressor capacity with solar radiation intensity was simulated, and the authors concluded that the adjustment of the compressor frequency can greatly improve the system performance of the SAHP.	>3.0	[60]
Qin et al.	2017	DX-SAHP test bench	The effect of compressor frequency on system performance and frost formation under different modes was tested. It was shown that when the compressor frequency is 60 HZ, the system COP under the air energy mode are greater than that in the dual-source mode and vice versa when the compressor frequency is 45 Hz or 30 Hz.	Up to 3.54, 45 Hz, dual-source mode.	[53]
Li et al.	2007	New DX-SAHP hot water system	Simulation and validation of the system performance under various operating conditions were carried out in Shanghai. The results show that, due to the use of the capacity adjustment method of the inverter compressor, the COP of the system can reach 5.29 ~6.93 on sunny days in autumn, and the COP of the system can reach 3.11 on a rainy night with an ambient temperature of 17.1 °C.	Up to 5.29~6.93 in autumn	[61]

The compressor is a fundamental component of the SAHP system, as it is responsible for circulating the refrigerant throughout the system. The choice of the compressor may deeply affect the system’s performance and reliability. The technical difficulties of the research on compressors are how to match the compressor with solar heat gain under different climatic conditions while improving the system performance and also avoiding short-cycling under low load conditions. The solutions are to use variable frequency compressors and using dynamic frequency control.

The above literature survey demonstrates that lots of work has been carried out on the variation of working conditions of the SAHP under ambient environments. It can be summarized that (1) the dynamic matching of the compressor, collector/evaporator, and other components affects the overall performance of the SAHP system, and it is crucial for the improvement of the system COP; (2) the dynamic frequency adjustment strategy on the performance of the compressor is highly sensitive to regional environmental and weather factors. Simply adjusting the operating frequency of the compressor does not lead to a significant improvement in the thermal performance of the system. Further research can be carried out to obtain the optimal variable capacity control strategy by considering multiple influencing factors. In addition, more in-depth experimental tests are needed to optimize the operation modes of multi-functional composite heat pumps such as PV and PV/T heat pumps.

3.2.3. Refrigerant and Its Flow Characteristics

Chlorofluorocarbons (CFCs) are widely used in refrigeration cycles due to their excellent thermodynamic and chemical properties. Considering the ozone depletion potential and the impact of chlorofluorocarbon-containing refrigerants, including CFCs, Hydrochlorofluorocarbons (HCFCs), and Hydrofluorocarbons (HFCs), on the atmospheric environment, the search for suitable environment-friendly and high energy performance refrigerants has been one area of the research focus [62]. Many studies have been conducted on the characteristics of refrigerants in DX-SAHP systems. In China, the production of R11 and R12 has been prohibited, and the production of R22 is based on a quota and will be prohibited in 2030 [63]. Potential alternative refrigerants for R12 include R134A, R152A, R142B, etc., among which R134A is widely used in heat pump air conditioners [64]. Studies have shown that SAHP systems using R-12 and R-22 have the highest COP, and R-134A has the best performance among all the alternative refrigerants. Compared with R-134A, the COPs of all mixed azeotropic refrigerants can be lower by up to 20% [65]. An important research direction in China is to study the use of environmentally friendly refrigerants and their flow characteristics in the heat pump system. Among them, the refrigerant mass flow rate adjustment is of great importance for the efficient operation of the heat pump system [65]. Table 4 shows the recent investigations on the refrigerant of SAHP by Chinese scholars. It can be found that most of the studies focus on R134a, R290a, and R410a. The systems with R134a have the highest COPs under different working conditions. Therefore, it can be considered the best refrigerant and will be widely used in SAHPs.

Table 4. Recent investigations on the refrigerant of SAHP by Chinese scholars.

Author	Year	System Type	Note	Average COP	Ref.
Zhao et al.	2000	DX-SAHP	Simulation was performed for the system with R134a and R12 as refrigerants, and the results show that the COP of the R134a system could reach 4.0~6.5 with a discharge temperature of the compressor lower than that of the R12 system.	4.0~6.5	[66]
Kong et al.	2018	DX-SAHP	The control strategy of the DX-SAHP system with R134a as the refrigerant was developed by adjusting the degree of superheating and the control method of the DX-SAHP system with variable frequency compressor was also developed.	3.9–5.22	[67]
Kong et al.	2020	DX-SAHP	Experimental investigation was carried out on two DX-SAHP systems with R134a and R290 as the refrigerants, respectively. The results show that the average COP of the system using the R290 refrigerant is higher than that of the R134a system under winter conditions.	2.72	[68]
Wang et al.	2018	SAHP	Theoretical analysis was performed on the system performance of the SAHP using R1234yf and R134a as the refrigerants. The results show that the COP of the dual-source heat pump system using R134a is about 2% to 3.4% higher than that of the R1234yf system. R1234yf as an alternative refrigerant of R134a has broad application prospects.	5.61 (R1234yf) 6.68 (R134a)	[69]
Yang et al.	2020	DX-SAHP	An experimental study was carried out for a DX-SAHP using R290 as refrigerant under actual weather conditions for a whole year. The annual average heating power was found to be 1358.6 W. The maximum, minimum, and annual average system COP were found to be 5.99, 2.04, and 3.88, respectively.	3.88	[70]
Ouyang	2012	Dual-source CO2 heat pump	A theoretical analysis and optimization study of a dual heat source solar-assisted transcritical CO2 heat pump system suitable for winter conditions in north China was performed.	4.97	[71]
Kong et al.	2010	DX-SAHP	A DX-SAHP experimental system using propane as the refrigerant was developed.	4.2	[72]
Li et al.	2020	new solar-assisted heat pump multifunctional composite machine (SAHPMCM)	The working principle of the new solar-assisted heat pump multifunctional composite machine (SAHPMCM) is introduced. The PID control method on the expansion valve, system setting, and experimental results on the corresponding COP were given.	4.5	[36]
Zhang	2014	DX-SAHP	A study was carried out on the effects of refrigerant charge and system design parameters on the performance of the heat pump. It was found that when the area of the solar collector is 6.0 m2, the length of the condenser tube is 70 m, and the inner diameter of the condenser tube is 9 mm, 70–80% of the refrigerant is distributed in the evaporator and condenser, and the most suitable refrigerant charge is 1.65–1.75 kg.	3.5–5.5	[73]
Kong et al.	2017	DX-SAHP	A simulation program of the DX-SAHP water heater system using R410a was developed, and the influence of five parameters, including the refrigerant charge on the thermal performance of the system, was analyzed under fixed superheat at the collector/evaporator outlet.	4.1–6	[74]
Zhang et al.	2013	DX-SAHP	A mathematical model of the refrigerant charge was developed, and the simulation results show that most of the refrigerant exists in the heat exchanger, of which over 50% remains in the condenser. In order to avoid refrigerant leakage, a refrigerant charge of 1.65–1.75 kg was recommended.	3.8	[73]
Qi et al.	2014	Solar-air energy dual-source integrated heat pump system	A Tandon cavitation coefficient calculation model was developed under a Maple environment and the R134a refrigerant charge was obtained which was close to the actual optimum refrigerant charge.	3.7–4.5	[75]
Kong et al.	2021	DX-SAHP	A neural network model was developed to predict the mass flow rate of the R290 refrigerant. It was found that the increase of solar radiation intensity leads to an overall increase in the mass flow rate of R290; however, with the increase in ambient temperature, the influence of solar radiation intensity on the mass flow rate of refrigerant decreased.	4.38–5.69	[76]
Wang	2021	DX-SAHP	A new subcooling control system was proposed, and the influence of the subcooling degree on the new system was studied. The results show that, with the increase in subcooling degree, the heat supply per unit of refrigerant mass flow will increase, and the heating coefficient will increase first and then decrease. When the subcooling degree is 4 K, the heating COP is the highest. When the subcooling degree is controlled within the range of 2 K~ 5 K, the heating COP is maintained at the high-level range.	4.0~6.5	[77]

The refrigerant is the working fluid in the refrigeration cycle and, as such, is highly important. The technical problem of the research is to find refrigerants with a low environmental impact that leads to high system COP. Environmentally friendly refrigerants are the potential solutions to solve these problems.

Based on the above literature survey, it can be seen that due to the environmental problems caused by HCFCs refrigerants such as R22 and R12, searching for new green and environmentally friendly refrigerants to replace traditional refrigerants has become the focus of the research. Alternative refrigerants including R134a, R152a, R142b, R410a, etc., have been proposed as replacement refrigerants. Among these, it is worthwhile to mention that many studies focused on the application of the R134a refrigerant in the DX-SHAP, especially its influence on energy consumption and system COP. In addition, theoretical and experimental studies on the application of new refrigeration cycles such as transcritical CO₂ to SAHP systems, as well as research on the flow characteristics of new environmentally friendly refrigerants in PV/T-SAHP systems, are relatively few in China. As the physical properties, charging capacity, and type of refrigerant are important factors affecting the COP and energy consumption of the system, more investigation on its operating performance and system optimization are needed.

3.2.4. Performance of the Dual-Source Heat Pump

The heat exchange performance of the heat pump system is very important to improve the operation performance of the heat pump and antifog in cold winter. Chinese scholars have carried out experimental and theoretical studies on the heat exchange performance for different types of SAHPs, including solar-assisted air source heat pumps, solar-assisted soil-source heat pumps, solar-assisted water source heat pumps, etc. [78,79]. Table 5 lists the relevant studies on the performance of the dual-source SAHPs by Chinese scholars.

Table 5. Relevant studies on the heat exchange performance of SAHPs by Chinese scholars.

Author	Year	System Type	Note	Ref.
Huang et al.	2017	Solar-assisted air source heat pump	The mathematical model of the solar collector was developed. The study shows that the energy efficiency ratio of the system can be improved by 1~2 with solar radiation.	[78]
Cai	2019	Air source solar-assisted heat pump (AS-SAHP)	Through theoretical analysis and experimental validation, it was concluded that with the increase in solar radiation intensity on the collector/evaporator side, the system COP can be significantly increased with a relatively small increase in power consumption.	[49]
Wan et al.	2020	Solar and air dual heat source two-stage compression CO2 heat pump water heater system	The component model and the thermal physical property parameter model of the refrigerant were developed. Simulation results show that when the flow rate of solar hot water is 0.6 m3/h and the outdoor environment is 15 °C, the system COP increase by 11.6%, compared with the air source heat pump.	[80]
Hou	2008	SAHP	Economic analysis was performed for SAHPs with air source and three conventional energy sources as auxiliary heat sources. The results show that in Lanzhou, the air source heat pump water heating system has economic advantages over other systems.	[81]
Yang et al.	2008	Solar-assisted soil-source heat pump	The heat storage characteristics of soil for the U-shaped buried tube heat exchanger were carried out through simulation and experimental validation. It was shown that the intermittent heat storage method is beneficial to the recovery of soil temperature and further improves the heat storage efficiency of soil.	[82]
Wanget al.	2008	Solar-ground coupled heat pump system (SGCHPS)	Through experiment and computer simulations, it was concluded that the underground heat storage performance of SGCHPS strongly depends on the intensity of solar radiation and the ratio between the volume of the water tank and the area of the solar collector. It is suggested that the reasonable ratio of the volume of the water tank to the area of the solar collector should be within the range of 20~40 L/m2.	[83]
Han et al.	2008	SGCHPS	A computer program on the SGCHPS was developed, and the simulation results show that the annual average heating performance coefficient of the system under combined solar and soil source energy operation mode was 2.674.	[84]
Xu et al.	2015	SGCHPS	The computer model was developed under the TRNSYS environment, and the heating mode operation was validated with an experiment. It was concluded that under the combined heating mode, the soil temperature at different buried depths mainly depends on the solar radiation intensity, and a high soil temperature can ensure high system energy performance.	[85]
Gao et al.	2020	SGCHPS	The imbalance efficiencies of ground source heat pumps (SGHP) were analyzed, and Beijing, Harbin, and Zhengzhou were selected as representative cities from cold regions, severe cold regions, and hot summer and cold winter region analysis. The results show that the operating time for SGCHPS was shorter than SGHP, with more energy reduction, and it can keep the unbalance rate at 1% after system optimization.	[86]
Yi	2009	Solar-assisted water source heat pump	The experimental study shows that under the testing conditions, the COP of the system in the heating and cold storage mode was in the range of 5.8~6.2.	[87]
Qu	2015	Solar-assisted water source heat pump	A computer model was developed to simulate the application of the system in a villa in Beijing under the TRNSYS environment. The results show that when the temperature difference between the supply and return water of the collector was at 7 °C, the COP of the system can reach 2.47.	[88]
Li	2011	Solar-assisted underground water source heat pump	An economic analysis for the application of a solar-assisted underground water source heat pump system in a residential building in Shenyang was performed. Compared with oil-fired boilers and electric boilers, the proposed system is more energy efficient. However, it is less economical than coal-fired boilers but with minimal environmental pollution.	[89]
Ma	2020	Solar-air-wastewater multi-energy complementary heat pump system	Through computer simulation and experimental validation, it was shown that the system is applicable in hot summer and cold winter regions. When there is not enough space for a solar collector or the heating capacity of the system is not enough to meet the demand of the building, it can be considered to add a sewage water source as auxiliary energy.	[90]

Based on the above survey of the literature, it can be concluded that the coupling of solar energy and other heat sources needs to take advantage of the abundance of energy sources in the specific region. This is because multi-energy-source heat pumps usually have higher COPs than that of single-source heat pumps. Few papers have focused on the intelligent control of the SAHPs; although, it is very important to match the dynamic user demand and achieve energy saving in practice. More studies are needed to optimize the capacity matching among all the components of the heat pump to achieve high system energy efficiency.

3.3. Research on the Performance of SAHP

Different commercially available software and programming language have been used by scholars to conduct simulations of the performance of the SAHP system, among which TRNSYS was most favored by the researchers due to its ability to dynamically simulate the annual building thermal load, heating/air conditioning system operation, solar energy system, ground source, hybrid connection SAHP performance, etc. Table 6 lists representative numerical studies on SAHP systems and the programs used by the researchers.

Table 6. Numerical studies on the SAHP system by Chinese scholars.

Author	Year	System	Program and Results	Ref.
Cai	2015	PV/T-assisted heat pump water heating system (PV/TA + HPWH)	A computer model was developed under the MATLAB environment using the Simulink platform. It was validated with experimental data in June 2013 and showed sufficient confidence under fluctuating solar radiation conditions.	[91]
Dai et al.	2017	Hybrid photovoltaic solar-assisted loop heat pipe/heat pump system (PV-SALHP/HP)	A computer program was written to simulate the performance of the PV-SALHP/HP system. The results show that, on typical sunny days in spring and autumn, the operation under the hybrid LHP/HP mode leads to 40.6% energy saving compared with the operation under the HP mode. Note: Software platform not specified.	[92]
Zhou et al.	2020	Micro-channel PV/T modules based direct-expansion SAHP system	A unified calculation computer program was written in C language to simulate the annual performance of a novel DX-SHAP system with microchannel PV modules for space heating. The results of the simulation were validated with daily experimental data with a maximum error of 7.2%.	[93]
Luo et al.	2020	Non-direct expansion solar-assisted air source heat pump	A computer program was written under the TRNSYS environment to simulate the performance of the system installed in residential houses in Shangri-La. The relationship between the heat pump capacity and the heating area was obtained.	[94]
Wang	2020	SAHP hot water system	A computer program was written under the TRNSYS environment to evaluate the energy and economic performance of the system with an energy-efficient conservation operation strategy.	[95]
Shen	2015	Parallel SAHP system	A computer model was developed under the TRNSYS environment for the parallel SAHP system, and the annual operation performance of the system was simulated.	[96]
Yin	2013	SGCHPS	Mathematical models for each component of the system were developed, and a computer program was written under the TRNSYS environment. The performance of different system connection methods was analyzed, and it was recommended to set the location of the solar energy supplement hot spot between the outlet of the heat pump unit and the inlet of the buried pipe.	[97]
Li	2017	Solar-assisted air source heat pump integrated domestic hot water system (SASIHPDHW)	A computer program was written under the TRNSYS environment to simulate the performance of the SASIHPDHW system, and it was validated with experimental results with a difference of less than 10%. The annual average COP of the system was found to be 4.12, which is 11.7% higher than that of an air-source heat pump.	[98]
Wei et al.	2019	SAHP	A computer program was written under the MATLAB environment using the Simulink platform, and the impact of solar radiation intensity, area of the collector/evaporator, etc. were analyzed.	[99]
Guan et al.	2009	Solar-storage and ground source heat pump hot water supply system	A computer model was developed under the TRNSYS environment to optimize the solar water heating system of a demonstration project in Tianjin. The results show that solar energy and renewable energy (solar energy + ground source energy) accounted for 62.8% and 86.5% of the total heat supply, respectively.	[100]

Figure 11 lists the simulation platforms/programming languages adopted by the researchers. It can be found that a large proportion of the computer models were developed under the TRNSYS/MATLAB environment. TRNSYS is most favored by researchers for whole system performance analysis, while MATLAB is welcome for analyzing the operating characteristics of a certain component in the SAHP system and the dynamic energy performance under different environmental conditions. The CFD software, such as ANSYS and Fluent, is used to perform analysis of indoor thermal and humidity environments.

Figure 11. Simulation platforms/programming languages adopted by the researchers.

From the literature survey, it can be concluded that current studies focus on whole system operation performance or component level simulation, or indoor environmental condition analysis. Few studies have been conducted to investigate the optimal control strategies and their impact on indoor thermal and humidity, which could be the future direction of research.

3.4. Economic and Feasibility Evaluation

The economic and feasibility evaluation of the SAHP system is also an important area of research. Although the SAHP systems utilize renewable energy, the installation of such systems requires extra costs [100,101]. In addition, the SAHP system might not be able to provide enough cooling and heating when solar radiation is low. In this case, an auxiliary heating/cooling system would be needed, which would result in additional investment costs. Therefore, the economic and feasibility evaluation of the SAHP system is critical. Table 7 lists the relevant studies of SAHP from Chinese scholars.

Table 7. Economic and feasibility evaluation of SAHP.

Author	Year	Focus	Conclusion	Ref.
Yu	2007	Environmental benefits of SAHP	It was found that after the introduction of the environmental protection tax, the price of coal power will rise significantly, the economy of SAHP will become worse due to the need of electricity for auxiliary heating, and the environmental benefits of SAHPs are worse than natural gas boilers.	[102]
Feng	2012	Economic and Environmental Benefit Evaluation of PV-SAHP Water Heaters (PV-SAHPWH)	PV-SAHPWH has the highest initial cost and lowest operating cost in the Nanjing area compared with other hot water systems, leading to the lowest lifecycle cost and highest environmental benefits.	[103]
Luo et al.	2009	Thermal performance evaluation and economic analysis of air source heat pump-assisted heating solar hot water system	Through reasonable component size matching, the COP of the air source heat pump unit can still reach 2.5 to 3.3 in the coldest month. The use of the system can lead to great energy saving potential compared with other cold/heat sources and is economically feasible.	[104]
Jing	2015	Energy saving potential, economical, and environmental analysis of the air source heat pump-assisted solar water heating system	Compared with oil-fired and gas-fired hot water boiler systems, the energy consumption and dynamic annual cost of this system are both the lowest.	[105]
Liu	2011	Economic and environmental benefits of air source heat pump-assisted solar water heating system	Compared with other conventional hot water systems, the air source heat pump-assisted solar water heating system led to the lowest annual operating energy consumption and the best economic and environmental benefits in Chongqing.	[106]
Peng	2010	Economic and environmental benefits of air source heat pump-assisted solar water heating system	Compared with other conventional water heating systems applied in the Yangze River and the Huai River region, the solar energy guarantee rate is 46%, and the energy consumption is the lowest. The environmental benefits of the system are comparable to the gas-fired boiler system.	[107]
Hou et al.	2008	Economic impact factors of heat pump-assisted solar centralized hot water system	Compared with the other three alternative conventional solar energy auxiliary heat source systems, while considering the impact of the increase in energy price, the use of air source heat pumps as auxiliary heat sources has strong economic competitive advantages, as well as social and environmental benefits in large-scale application in the cold regions in northwest China.	[81]

From Table 7, it can be found that most of the investigations were on the energy saving potential and economical and environmental analysis of the system, especially the air source heat pump-assisted solar water heating system as it is common and easy to be implemented. Analysis of other types of systems is relatively rare and very little research could be found on exergy analyses of the system, which could be the future research direction.

4. Application of SAHP

The application of SAHP in a certain region is related to the solar power generation capacity in that region. Figure 12 presents the solar power generation by region in China in 2019. It can be observed that solar power generation concentrates in the northwest, east coast, and north China regions. The solar power generation in the northern region of China (north of Qinling and Huaihe River) is much higher than that in the southern region, which could be due to the better local solar energy resources and more supportive government policies in northern regions. As a result, more SAHP projects were developed in northern regions. Table 8 lists the representative SAHP projects in China since 2001 that have been implemented and put into operation. The projects were selected from the national and local government official websites. Those local renewable demonstration projects have been completed and put into operation with proven energy-saving data and benefits. These projects demonstrate innovation and economic and environmental protection benefits compared with traditional projects.

Figure 12. Solar power generation by region in China in 2019 (TWh) [3].

Table 8. List of representative SAHP projects in China.

Year	Project Name	Location	Project Information	Operation Benefits	Comparison with Traditional System	Ref.
2004	Olympic Village SAHP Hot Water System	Chaoyang District, Beijing	The system was designed to supply hot water to the Olympic Village prepared for the 2008 Beijing Olympic Games. Hot water heating was mainly through solar energy and assisted by a heat pump.	An automatic intelligent controller was adopted with real-time performance detection. Automation control was realized.	Compared with the oil-fired boiler heating system, the energy-saving ratio is more than 60%, and, compared with the electric heating system, the energy-saving ratio is 90%.	[101]
2004	Solar air conditioner heat pump system in Beijing Tianpu Group Industrial Park	Chuangye Road, Lucheng Industrial Zone, Daxing District, Beijing	An absorption lithium bromide solar air conditioning system was adopted. It was the largest solar air-conditioning system among the new energy demonstration buildings in China at that time, and it is also one of the buildings with the largest percentage of renewable energy usage in the total energy consumption of China.	The air conditioning and heating requirements of the new energy demonstration building are satisfied, and the heat collection efficiency in winter and summer reaches 0.2 and 0.4, respectively.	Compared with the traditional gas boiler, 1600 m3 of natural gas can be saved every year.	[108]
2006	Solar system of a holiday hotel in Pudong New Area, Shanghai	Pudong New Area, Shanghai	The system adopts a solar collector and supplementary heating equipment, an intelligent control system, an SAHP water heater, hot a water tank, etc., with a design power for hot water of 1745 kW.	As dual heat sources (solar and air energies) are used, even when solar irradiation intensities are low, the energy consumed by the system is only 50% of that of the solar hot water system.	A total of 2,076,390 m3 of natural gas is saved during the 15-year life span with a CO2 emissions reduction of 6,175,183 kg.	[109]
2011	Asian Games City Solar Water Source Heat Pump Project	Panyu, Guangzhou	The system is designed to supply domestic hot water for residential buildings (1.14 million m2) by solar heating in the Asian Games City and provide cooling by river water source (from the Lijiang River) heat pump for some single buildings (250,000 m2). It is listed as a national renewable energy building application demonstration project.	The water source heat pump is used as an auxiliary heat source for solar energy. The highest daily water consumption during the competition could still be met when there was no solar heat collection.	Compared with gas heating, 5723 m3 of natural gas with 260 kg of CO2 emission was reduced during the operation period.	[110]
2015	Shijiazhuang City Ground Source Heat Pump + Solar + Energy Storage System Project	No. 88, South Second Ring East Road, Shijiazhuang City	Solar energy and ground source heat pump systems combined with energy storage technology are used to provide heating, cooling, and domestic hot water for industrial and government buildings.	Cooling demand was satisfied by the ground source heat pump, and domestic hot water demand was satisfied by the solar energy system with an auxiliary electric heater.	Energy savings were equal to 79 t of standard coal every year, CO2 and other carbon oxide emissions were reduced by more than 188 t, NOx emissions were reduced by 0.7 t, and SOx and other sulfur oxide emissions were reduced by 1.32 t.	[111]
2015	Combined cooling, heating, and power demonstration project using ground source heat pump coupled solar energy in Ruzhou City, Henan Province	Ruzhou City, Henan Province	The system was designed to provide cooling, heating, and power to the Ruzhou No. 2 High School, and the project was awarded first place in the Henan Province Construction Science and Technology Progress Award.	The cooling and heating energy consumption of the building was significantly reduced by using geothermal and solar energy. The overall system COP was in the range of 3.8~4.4 all year-round. Meanwhile, the soil thermal imbalance problem was solved.	Compared with the coal-fired boiler + electric heating system before retrofit, the yearly operation cost was reduced by CNY 664,700 (135 t of standard coal). Compared with the coal-fired boiler system, the investment cost was reduced by CNY 410,000.	[112]
2016	Beijing Daxing Village Household Solar Energy + Air Source Heat Pump Heating Retrofit Project	Daxing, Songzhuang Town, Tongzhou District, Beijing	The system was designed to provide heating and domestic hot water for the working farmer with a heating area of 60 m2 and hot water demand of 250 L/d. The system was able to operate under ultra-low temperatures.	The system COP was able to reach 2.2 under low-temperature conditions in winter.	The energy savings on the daily electricity consumption was about 25 kWh (CNY 1500 of electricity in heating season).	[113]
2019	Solar + Water Source Heat Pump Heating Project of Caina Township Government, Qushui County, Lhasa, Tibet	Caina Township, Lhasa, Tibet	The system was designed to provide heating needs of government office buildings and dormitory buildings in a heating area of 5830 m2 with a design heating temperature of 18°C in winter, and the number of heating days was 150.	The heating COP of the water source heat pump was 2.0. The overall system heating COP was 3.85. The leaving water temperature from the collector reached 73.6 °C, which can satisfy the heating and hot water needs in winter.	During the test in a heating season, energy savings of equal to 128 t of standard coal and CO2 emission reduction of 448.9 t were achieved. CNY 7.16 × 106 cost savings can be achieved compared with electric heating in a 20-year life span.	[114]
2020	Multi-energy source cooling and heating system retrofit project of Liaoning Building	Liaoning Building, Shenyang City, Liaoning Province	A retrofit project was designed for the cooling and heating system of the Liaoning Building. A water source heat pump was used for cooling and heating the air-conditioning system. An electric heat storage boiler was used to supply hot water to the radiator for heating. A solar heating system was used to provide domestic hot water. At the same time, natural gas was used as the steam heat source for the kitchen and washing machine.	The water source heat pump system was preferred for heating using the electric heat storage boiler as an auxiliary system during the valley’s electricity demand period. The solar energy system was preferred for producing domestic hot water. The maximum system heating COP reached 4.1 and the average heating COP of the water source heat pump was 3.6.	Compared with the coal boiler system, annual energy savings equal to 1977 t of standard coal, and cost savings of CNY 3.4 × 106 were achieved.	[115]

Figure 13 and Figure 14 provide an overview of the Beijing Daxing Village Household Solar Energy + Air Source Heat Pump Heating Retrofit Project and the Solar + Water Source Heat Pump Heating Project of Caina Township Government, respectively.

Figure 13. Overview of the Beijing Daxing Village Household Solar Energy + Air Source Heat Pump Heating Retrofit Project [113].

Figure 14. Overview of the Solar + Water Source Heat Pump Heating Project of Caina Township Government [114].

The above projects point out that in regions with sufficient radiation during the heating season, DX-SAHP is suggested for seasonal heating. In regions with sufficient annual irradiation throughout the year, it is advisable to use solar heating assisted with a dual-source heat pump for heating. In cold regions that required heating most of the time in the year and with insufficient radiation, a combination of a non-direct expansion heat pump system with thermal storage technology as an auxiliary heat source is recommended. Furthermore, most projects are implemented in coastal areas in the east and inland areas with moderate solar energy resources and developed economies. Although solar energy resources are abundant in the northwest regions, southwest regions, and Tibetan regions, very few SAHP projects could be found in these regions. Therefore, to help reach carbon peak and carbon neutrality, it is recommended to promote SAHP projects in these regions.

5. Regulations, Standards, and Policies Related to SAHP

The solar energy industry is part of both the renewable energy industry and the energy-saving industry. Currently, the regulations, standards, and policies for SAHP mainly focus on solar thermal utilization and water heaters. Compared with large-scale solar heat collection projects, the standards and technical regulations of different types of SAHPs and hot water systems still need improvement. It is also important to increase the formulation and implementation of economic policies for SAHPs [116].

5.1. Regulations and Standards for SAHP in China

Table 9 lists the relevant regulations for SAHP in China. It can be seen that early regulations mostly focused on solar energy hot water systems due to their easy installation. “Technical specification for solar photovoltaic and thermal heat pump system (T/CECS 830-2021)” [117] is the first relatively complete technical regulation on the utilization of SAHP in the past ten years. In addition, the mandatory policies of relevant laws and regulations on SAHPs mainly focus on solar thermal utilization.

Table 9. Relevant regulations and standards in China.

Implementation Date	Regulation	Status	Note	Ref.
2009.6.1	Specifications of air source heat pump-assisted domestic solar water heating system (GB/T 23889-2009)	Current	This specifies the terms and definitions, classification and nomenclature, technical requirements, measurement and test methods, inspection rules and documentation, marking and packaging of air source heat pump assisted domestic solar water heating systems.	[118]
2011.9.29	Test methods for the solar-plus-supplementary water heating system (tank capacity more than 0.6 m3) (GB/T 26973-2011)	Current	The performance test method for solar water heating systems with auxiliary energy (the volume of the water storage tank is greater than 0.6 m3) is specified. It does not apply to solar water heating systems with auxiliary heating by heat pumps.	[119]
2012	Technical code for solar air conditioning system of civil buildings (GB 50787-2012)	Current	Mandatory specification for design calculations of solar collector systems.	[120]
2018	Technical standard for solar water heating system of civil buildings (GB 50364-2018)	Current	Mandatory specifications for solar collector systems, hot water supply systems, and auxiliary heat source systems.	[121]
2019.6.4	Test method for domestic direct-expansion solar heat pump water heating system NB/T 10155-2019	Current	It specifies the test methods for DX-SAHP hot water systems for household and similar purposes and specifies the classification and coding, technical requirements, test methods, and inspection rules.	[122]
2019	Technical standard for solar heating system (GB50495-2019)	Current	Mandatory specification for solar heating design.	[123]
2021.8.1	Technical specification for solar photovoltaic and thermal heat pump system T/CECS 830-2021	Current	This standardized the design, installation, commissioning, acceptance, operation, and maintenance of the solar photovoltaic heat pump system and gave a detailed system description for each component of the solar photovoltaic heat pump system as well as the design, installation, and system selection.	[117]
2021	Notice of the Ministry of Housing and Urban-Rural Development on Printing and Distributing the Atlas of Passive Solar Heating in Rural Areas (Trial) and the Technical Guidelines for the Application of Household Air Source Heat Pump Heating (Trial)	Current	Guide the project development of solar energy and heat pump systems in northern rural areas through “Atlas of Passive Solar Heating in Rural Areas (Trial)” and “Technical Guidelines for Application of Household Air Source Heat Pump Heating (Trial)”	[124]

With the proposal of new national strategies for reaching carbon peak and carbon neutrality, a complete technical specification and design standard is needed for solar thermal utilization and SAHP application to support and promote the extensive application of SAHPs in China. In addition, there are very few mandatory policies for solar energy and heat pump engineering technology. Therefore, in the future development of SAHPs, the government still needs to formulate and improve relevant laws and regulations and develop practical guidelines for implementation accordingly.

5.2. Government Financial Subsidy Policy for Solar Industry

The solar photovoltaic industry in China has great development potential with policy-based financial support and market-based financial support. However, due to the high investment risk and uncertain yield in the renewable energy market, market-based financial support is not yet mature enough. Therefore, setting up economic policies that can provide guidance and incentives to the solar energy industry is very important at the early stage [125]. For example, the average photovoltaic power generation cost is about six times that of thermal power. However, among all primary energy sources, the proportion of solar power generation energy has steadily increased. By 2020, the annual solar energy utilization nationwide reached over 1.140 × 10⁸ tons of equivalent standard coal [126]. From 2012 to 2021, the proportion of solar power generation increased from 0.07% to 4% of the total national power generation [127].

The SAHP industry belongs to the new energy industry and renewable energy industry, and the government has been providing special subsidies to solar photovoltaic cells and modules [128]. Article 25 of the “Renewable Energy Law of the People’s Republic of China”, revised in 2009, states that the renewable energy development and utilization projects listed in the National Renewable Energy Industry Development Guidance Catalog are eligible for loan application and can receive financial assistance with discounted loans from financial institutions [26].

Table 10 lists the financial subsidy policies for the photovoltaic solar energy industry from the central government. Financial subsidies for the solar energy industry mainly include subsidies for initial investment, on-grid tariffs, and financial subsidies for technology research and development and personnel training in the photovoltaic industry. The subsidy policies come from many different ministries, such as the National Development and Reform Commission, Energy Administration, and China Development Bank, and the subsidies are not unified [129].

Table 10. Financial subsidy policies for the photovoltaic solar energy industry from the central government.

Time	Solar Subsidy Policy	Policy on Subsidy	Ref.
2009	“Golden Sun” Demonstration Project	This stipulates two subsidy methods for centralized photovoltaic power generation and distributed photovoltaic power generation. The subsidies for centralized photovoltaic power generation and distributed photovoltaic power generation are in the range of CNY 0.75 to 1 CNY/kWh, and 0.35 CNY/kWh, respectively.	[130]
2016	Notice on the adjustment of the benchmark on-grid tariff of photovoltaic power generation and onshore wind power generation	The benchmark on-grid electricity price (tax included) for new photovoltaic power plants in 2017 for Class I, II, III, and Tibet Autonomous Region areas would be lowered to 0.65 CNY/kWh, 0.75 CNY/kWh, 0.85/kWh, and 1.05 CNY/kWh, respectively.	[131]
2020	Notice of the National Development and Reform Commission on Matters Concerning the 2020 On-grid Electricity Price Policy for Photovoltaic Power Generation	For industrial and commercial distributed photovoltaic power, it included financial subsidies in 2020 based on the power generation scale, and for household distributed photovoltaic power, the subsidy is 0.08 CNY/kWh.	[128]

In addition to the central government’s subsidies, local governments have also introduced a large number of relevant subsidy policies to encourage the development of the solar industry, which are added to the national subsidies. The subsidies offered by the local governments can be mainly divided into power demand/consumption subsidies and one-time investment subsidies. With the support from the subsidies, the Return On Investment (ROI) of distributed photovoltaic power generation projects can be significantly improved [125]. Table 11 lists the representative subsidies from local governments.

Table 11. Subsidies from different local governments.

Location	Policy on Subsidies	Ref.
Wuxi, Jiangsu	A one-time subsidy of 200,000 CNY/MW for distributed photovoltaic power stations.	[132]
Beijing	The subsidy standard is 0.4 CNY/kWh (tax included) for building integrated photovoltaic (BIPV) projects. For photovoltaic power generation projects in general industrial, commercial, and agricultural residential buildings, the subsidy standard is 0.3 CNY/kWh (tax included).	[133]
Shanghai	A subsidy of 0.4 CNY/kWh for a home-owner, and a subsidy of 0.25 CNY/kWh for an enterprise, for a period of 5 years. In 2014, the total newly added distributed photovoltaic power generation in Shanghai receiving subsidies is 200 MW.	[134]
Jiangsu	During the period from 2012 to 2015, if the photovoltaic power generation projects newly put into operation in the province did not receive national financial subsidies, they would be given subsidies of 1.2 CNY/kWh in 2014 and 1.15 CNY/kWh in 2015 from the provincial government.	[135]
Luoyang, Henan	For distributed photovoltaic grid-connected power generation projects completed before the end of 2015, and preferentially using components produced by enterprises in Luoyang City, a reward of 0.1 CNY/W will be given according to their installed capacity, and the reward will continue for 3 years.	[136]
Shangluo, Shaanxi	A 5% tax refund to some photovoltaic and ancillary product manufacturers registered locally. A 5% refund of labor service tax to enterprises that install photovoltaic generators of no less than 50 MW.	[137]
Hangzhou, Zhejiang	A subsidy of 0.05 CNY/kWh will be given to the photovoltaic grid-connected ground power stations that are constructed on schedule from 2016 to 2018, and the subsidy will be continued for 5 years (60 months). The annual subsidy for a single project does not exceed 5 × 106 RMB, and the annual subsidy for the same enterprise does not exceed 1.0 × 107 RMB.	[138]

According to the analysis of fiscal policies in Table 10 and Table 11, the number of subsidies from the state and local governments provided to the solar energy industry, especially the photovoltaic industry, is steadily increasing. At the same time, with the national macro policy, the supporting photovoltaic policies in local regions are more active. However, due to poor project supervision and the low entry barrier of this industry, there have been problems such as fraudulent subsidies and fake projects that disrupt the market order, which discourage the enthusiasm of the enterprise [139]. Therefore, financial subsidies for the solar energy industry should move reasonably towards investment subsidies, electricity price subsidies, and consumption subsidies in order to stimulate and develop the clean energy consumption markets [132].

5.3. Fiscal, Tax, and Financial Policies for the Photovoltaic Industry

In terms of income tax, the National Development and Reform Commission has added solar photovoltaic products to the “National Environmental Protection Industry Equipment Product Catalog” which was released in 2010. With this, relevant enterprises can enjoy tax reductions and exemptions in terms of investment credits and accelerated depreciation for equipment [116]. For Value-Added Tax (VAT), when the scale of distributed photovoltaic power plants is small and the monthly online sales revenue is less than CNY 20,000, VAT is exempt. When the annual sales revenue exceeds CNY 500,000, VAT must be paid. According to the notice of the PV power VAT policy issued by the Ministry of Finance and the State Taxation Administration, taxpayers can enjoy a 50% refund of VAT immediately after collection, and enterprises can be exempt from a certain percentage of income tax within a certain period of time [129].

In recent years, the China Development Bank has provided various support for distributed photovoltaic project implementation [125]. The financing methods for solar photovoltaic enterprises mainly include equity financing, debt financing, policy financing, financial leasing, internet crowdfunding, internet wealth management and third-party financing [140], among which bank loans are the primary financing method. In terms of policy financing, the China Development Bank is the leading provider for the new energy industry [141]. As of August 2013, the China Development Bank has provided loans of CNY 4.105 × 10¹¹ to photovoltaic solar energy, and the Bank of China offered CNY 3.01 × 10¹² green finance in 2014, mainly to support wind power and solar photovoltaic industry [142]. As of March 2017, a total of 60 banks across the country have launched “photovoltaic loans” to support the development of the photovoltaic industry, mainly from Zhejiang, Jiangsu, Jiangxi, Shandong, Shanxi Province, etc. [143].

Multiple guidelines and policies have been issued for the use of clean energy in building design and renovation in a number of domestic provinces and cities. For example, the economic policies issued in Hubei, Shandong, and other provinces and cities provide guidance on projects related to solar energy + “multi-energy complementary heat utilization”, cooling, heating, electricity trigeneration, solar/air source energy, ground source, and other clean energy sources.

6. Recommendations for Future Improvement

The vigorous development of the SAHP industry requires the formulation of government policies, the R&D and innovation vitality of enterprises, the fairness and improvement of the market, and the improvement of relevant laws and regulations. Therefore, a good development environment will help the SAHP industry in China step into a more promising future. Some recommendations for future improvement are elaborated as follows.

6.1. Expansion of Research Directions on SAHP

In addition to the traditional research directions on SAHPs, the following areas can be explored in the future: (1) SAHPs + intelligent control and remote monitoring systems, (2) multi-source heat pumps, (3) advanced heat storage and exchange unit (HSEU) technology, (4) advanced machine learning and multi-objective evolutionary optimization models that can be used for performance prediction and optimization of SAHP technology, (5) energy-efficient and low-carbon operation of heat pumps [144]. In addition, in-depth R&D on solar panel arrays, heat pumps with heat recovery, thermal storage and thermal storage technologies, and specific technology case studies can help identify potential ways for promoting the development of SAHPs to meet the requirements of a carbon-neutrality target. For example, the combination of soil source and solar energy heat pump technology, which has dual heat sources, can make the system more flexible and reliable and reduce the power consumption of the compressor [145]. Therefore, the combination of digitalization and intelligence technology for sustainable development may become a future research direction for SAHP.

6.2. Strengthen Industrial Chain Development and Increase the Output of Fundamental R&D

The development of any industry is inseparable from innovation and scientific research outputs. The proportion of transformation of theoretical achievements into practical application projects is relatively low in China, showing a large gap compared with developed countries such as the United States and Europe. Therefore, the experiences from developed countries need to be used. For example, Japan has put great emphasis on the development and protection of core technologies during the development of the solar energy industry with staged support from the state and local governments. During the early development stage, the government subsidizes enterprises to promote technology research and development. After the technology is mature, solar energy products can be introduced into the market, which will effectively reduce the cost of industrial promotion [118]. The Chinese government should increase investment in fundamental research and development of solar energy utilization technology in order to lead breakthroughs in key technologies such as solar cell materials as soon as possible [146].

6.3. Establish Positive Interaction between Local Government and Enterprises and Optimize Industrial Structure

To promote the development of the solar energy industry, it is crucial to building a benign interaction between local governments, the market, and enterprises. The local governments should create a healthy market environment, formulate rules for fair marketing, and replace traditional planning methods with more market economic methods [118]. They also must actively fill the loopholes and fix shortcomings in the market industry chain. It is necessary to supervise the solar energy industrial chain and utilize the advantages of local resources to increase resource utilization efficiency [147]. More policies should be introduced to improve the effectiveness of the role of the local government in SAHP industry development.

To optimize and upgrade the industrial structure, the process of production factors such as capital, labor, land, and technology should flow from the production sectors with low value-added, poor efficiency, and high consumption to those with high value-added, high efficiency, and low consumption. Large solar energy companies with a low production cost and high efficiency can be encouraged to merge with small companies with a high production cost and low efficiency to create a more competitive and vigorous market [148]. More specifically, more support should be provided to the development of the equipment manufacturing industry, parts production enterprises, and technical research and development institutions [149].

6.4. Develop Specific Fiscal and Financial Subsidy Policy for SAHP Industry

So far, no specific fiscal and financial subsidy policies have been developed for the SAHP industry in China. Most of the existing subsidy policies were developed for the solar photovoltaic industry. However, there are some macro policies for the promotion of solar water heaters and solar heating [150,151]. Therefore, the government should develop more flexible and specific subsidy policies for the SAHP industry to promote the application of SAHP in buildings.

The subsidy policies can be made concerning the following aspects: (1) Product subsidy: the government compensates for some of the SAHP products in order to reduce their expenses while increasing their output. As a result, production and consumption grow, but the price remains the same; (2) consumer subsidy: the government subsidizes the consumers to incentivize them to use more SAHP products; (3) employment subsidy: the government gives this incentive to SAHP companies and organizations in order to enable them to provide more job opportunities. The above subsidy policies should be provided based on market needs to avoid overproduction.

7. Conclusions

This paper provides a systematic review of the research, application, regulations, standards, and financial policies related to solar-assisted air heat pumps for buildings in China. Recommendations for the future development of the solar energy industry in China are also provided. The following conclusions can be made based on the literature review:

(1)

Current research focuses on the theoretical and experimental investigation of the performance of the main components of the solar air heat pump system, in particular the collector/evaporator, compressor, and heat exchanger, and the characteristics of the refrigerant. More attention should be paid to the intelligent control and optimal operation of the system and integration with buildings to achieve maximum energy savings. In addition, more comprehensive economic and feasibility evaluation studies should be carried out for different types of SAHP systems;

(2)

Due to the uneven distribution of solar energy resources and economic growth, the development of SAHP should take advantage of regional resources and complement solar energy with other clean energy resources. The selection of the appropriate type of SAHP in a particular region should consider the climate conditions of that region. It is also important to strengthen the cooperation between enterprises and local governments to increase resource utilization efficiency for the development of SAHP projects.

(3)

There is a lack of specific fiscal and financial subsidy policies for SAHP. To promote the application of SAHP in buildings, the government should develop relevant subsidy policies according to the market needs. The subsidy should cover product subsidies, consumer subsidies, and employment subsidies.