1. Introduction
Reducing energy consumption and minimizing environmental pollution are crucial concerns globally, with the construction industry being a major contributor to energy consumption and carbon emissions [
1]. According to statistics, heating accounts for over 75% of the final energy consumption in the building and industrial sectors [
2]. Therefore, reducing the energy consumption of buildings and their associated greenhouse gas (GHG) emissions is essential for combating global warming. Currently, residents in cold regions generally use traditional heating methods in winter, such as burning coal and wood. However, these methods consume a lot of energy, cause significant pollution, and negatively impact residents’ health [
3]. Some buildings utilize ground-source heat pumps, solar thermal systems, air-source heat pumps, and passive design strategies for heating [
4]. However, ground-source heat pumps have high costs and site limitations [
5], solar thermal systems are prone to pipe freezing and rupture in low-temperature environments [
6], and air-source heat pumps can frost over in cold conditions, increasing operational costs [
7]. Passive design strategies are typically used to reduce heating demands but rarely meet the full heating load of buildings [
8]. However, as one of the most widely used low-carbon heating technologies, air-source heat pumps (ASHPs) consume 55–70% less energy than electric heating systems and emit 12% less CO
2 than gas boilers [
9]. Considering the equipment costs, system complexity, and operational stability, combining passive design strategies with air-source heat pump systems holds great potential for winter heating in cold regions [
10].
Passive design strategies typically involve improving building conditions based on the local climate to optimize indoor environments while reducing energy demand [
11]. Many researchers have proposed various passive strategies for cold conditions, such as optimizing building shape and orientation [
12], enhancing building airtightness [
13], passive solar heating [
14], optimizing door and window materials [
15], using phase-change materials (PCMs) [
16,
17], and Trombe walls [
18,
19]. Solar energy is widely distributed in nature; sunspaces, as a passive solar heating technology, have significant advantages in terms of energy saving, environmental protection, and sustainable development [
20]. During building operation, sunspaces can collect solar energy to increase indoor temperatures and heat adjacent rooms [
21]. Yang et al. [
22] studied the improvement of rural cave dwellings with sunspaces, showing that adding sunspaces can increase the air temperature of the main room by 1.0 °C on weekdays and by 4.3 °C on holidays. Li et al. [
23] evaluated the heating performance of sunspaces in rural houses in Lhasa (a cold region with abundant solar resources) through measurements. The results showed that the average temperature of the bedrooms in the south-facing rooms reached 16.91 °C, and the sunspaces in these rooms alone could meet the heating needs. Ma L et al. [
24] used Energy Plus to simulate the heating performance of sunspaces in rural houses in Anda City (a cold region with scarce solar resources). They found that compared to reference buildings, the heating energy consumption of sunspaces was reduced by 4.6%. However, sunspaces as a passive heating strategy struggle to heat all rooms and have fluctuating solar energy availability throughout the day. Therefore, in the face of high heating demands in cold regions, active technologies such as air-source heat pumps are also needed in addition to passive strategies.
Air-source heat pumps (ASHPs) are typically compression-type heat pumps that rely on the Carnot cycle theorem, using outdoor air as the heat source. By consuming a small amount of high-grade energy such as electricity, they produce a large amount of low-grade heat energy, achieving energy savings [
25]. In cold regions, the heating performance of ASHPs during winter is mainly limited by ambient temperature. At low temperatures, the coefficient of performance (COP) and heating capacity of the heat pump are affected [
26]. Currently, there are many methods to improve the low-temperature adaptability of ASHPs, such as using scroll compressors with flash tanks [
27], adopting two-stage compression systems [
28], using cascade ASHP compression [
29], and optimizing outdoor coil fins [
30]. However, these methods still cannot effectively solve the problems faced by ASHPs under extremely low temperatures. Many researchers have proposed the use of solar-assisted air-source heat pump (SAASHP) systems for combined heating, which collect and utilize solar energy to compensate for and reduce the impact of ambient temperature on ASHP heating performance [
31]. Abbasi B et al. [
32] conducted a study that mainly explored a thermal performance comparison between glass-covered and non-glass-covered direct expansion solar-assisted heat pump (DX-SAHP) water heaters, and examined the system performance under different ambient temperatures and solar radiation intensities. The results showed that the non-glass-covered system had a higher COP throughout the year, especially during winter when sunlight is scarce. The glass-covered system could only protect the collector plate from heat loss in winter, and the payback period of the system was less than five years. Li et al. [
33] conducted a comparative analysis using TRNSYS 16 software to simulate the performance of a system combining an air-source heat pump (ASHP) and a solar evacuated-tube water heater (SETWH) with that of a system combining a micro heat pipe photovoltaic/thermal system (MHP-PV/T). The MHP-PV/T-ASHP system increased the solar fraction and system efficiency ratio by 19% and 2.2 times, respectively, and improved the primary energy saving rate by 12.3%. Han et al. [
34] studied the performance of solar and heat pump combined heating systems. The results showed significant energy-saving effects compared to traditional distributed coal-fired heating and hot water supply, with a maximum of 43.55%. Although the heating performance of solar-coupled heat pump systems has been widely discussed, current combined heating systems mainly use water for heat storage. However, due to the instability and discontinuity of solar energy supply throughout the day, there is still room for improvement in 24 h heating.
Some scholars have found that integrating phase change heat storage (PCHS) with solar-assisted air-source heat pump (SAASHP) systems can improve the heating stability and efficiency of SAASHP systems by leveraging PCHS technology [
35]. These methods include incorporating phase change materials (PCMs) with the evaporation side, condensation side, or heat exchanger of the SAASHP system [
36]. Han et al. [
37] investigated the performance of a solar and heat pump combined heating system based on PCHS and found that it could save up to 43.55% of energy compared to traditional distributed coal-fired heating and hot water supply systems. J. Gao et al. [
38] proposed a solar-coupled air-source heat pump system with phase change heat storage. Their experiments demonstrated that the average power consumption of the compressor was 1.87 kW, a reduction of 21.1%, and the system’s average coefficient of performance was 5.42, an increase of 143.0% over the original system. Qu et al. [
39] discovered that incorporating an annular energy storage heat exchanger and solar energy into the heat pump system significantly improved the thermal performance under low-temperature conditions. Additionally, Ni et al. [
40] conducted experimental research on the annual operation of a solar and ASHP heating system using a three-pipe energy storage heat exchanger. They found that higher temperatures and flow rates of hot water on the solar side could significantly enhance the heating energy efficiency of the system.
The existing literature shows that extensive research has been conducted on passive heating and the optimization of ASHP-based systems. However, these systems are complex and have issues with operational stability. To address these problems, we propose a coupling heating scheme that integrates an ASHP with passive heating. This system mainly consists of an air-source heat pump unit, a heat storage tank, heating terminals, and a passive solar room. The air-source heat pump provides heating for the building, while the passive solar room creates suitable working conditions for the heat pump. During the daytime in winter, the passive solar room collects solar energy, raising the internal temperature and providing a favorable working environment for the air-source heat pump [
41]. The high-temperature air inside the passive solar room serves as the heat source for the heat pump, requiring less air to obtain the same amount of heat compared to outdoor operation, thereby significantly improving the heating efficiency of the air-source heat pump. Additionally, the favorable ambient temperatures reduce the likelihood of frost formation on the heat pump’s evaporator. To address the issue of insufficient solar energy at night, we propose using phase change materials and heat storage media in the building to store excess heat during the day and release it at night, thereby meeting the heating demand throughout the day. This study focuses on three key aspects: analyzing the heating performance of coupled systems on the coldest day of winter in severe cold regions and evaluating their energy supply capacity and power consumption; comparing the energy consumption and economic benefits of coupled systems with traditional ASHP systems to verify their energy-saving effects; and exploring the application of phase change materials in heating systems to enhance system stability and sustainability through heat storage and release mechanisms. The working principle of the system is illustrated in
Figure 1. This paper uses software such as DesignBuilder and TRNSYS for simulation and mathematical analysis, comparing the coupling scheme with traditional schemes to explore the heating performance and energy-saving effects of the ASHP and passive heating coupling system in rural houses in extremely cold regions.
4. Conclusions
This paper studies the feasibility of air-source heat pump and passive heating coupled systems for heating in severely cold regions, and analyzes the performance of two modes (Mode 1: traditional air-source heat pump system; Mode 2: passive heating combined with air-source heat pump system) through simulation.
(1) Under the conditions of the coldest day in severely cold regions, the air-source heat pump and passive heating coupled system’s total heat output for the entire day is 99.41 kWh, which can meet the energy requirement of 86.67 kWh to maintain an indoor temperature of 20 °C throughout the day.
(2) Due to the excessively low ambient temperature, the traditional ASHP system cannot meet the normal operating temperature for an extended period, resulting in the need for inefficient electric heating water tanks, consuming large amounts of electricity. In Mode 2, the air-source heat pump’s outdoor unit is placed in a passive sunroom, utilizing the higher ambient temperature inside the sunroom, significantly improving the ASHP’s COP value and thereby reducing the heat pump’s power consumption. Analysis shows that the average power consumption of the air-source heat pump and passive heating coupled system is reduced by 66.88% compared to the traditional system, demonstrating excellent energy-saving effects.
(3) The study shows that to address the issue of not being able to capture solar energy at night, excess heat from passive heating during the day is proposed to be stored for night-time heating, achieving stable heating and energy savings. Compared to traditional heating methods, this system greatly reduces energy consumption while ensuring comfort, further quantifying the application potential of air-source heat pumps in cold regions.
Due to the inability to simulate the operating state of the air-source heat pump in an additional sunroom in this study, continuous dynamic simulation could not be achieved. The reliance on traditional mathematical calculations blurred the fluctuations and losses in heat transfer. Future research should focus on the specific design and simulation of heat storage and release, optimizing the design through real system operation on test benches.