Study on Heating Energy Efficiency and Optimization of Solar-air Source Heat Pump

This paper studies the heating energy efficiency and optimization of a solar-air source heat pump (SASHP). Through system simulation, the energy efficiency of the SASHP is compared with other heating methods. The results show that SASHP has higher energy efficiency. We also found that in the SASHP, the air source heat pump (ASHP) is involved in heating 46% of the time. Therefore, we suggest optimizing the energy consumption of the ASHP to improve the system’s energy efficiency. In addition, lowering the supply water temperature can also reduce the system’s heating energy consumption. When the supply water temperature is lowered from 60°C to 48°C, the COP is increased from 1.98 to 2.83, which is a 31% improvement. Our research provides theoretical support for the application of SASHP and provides new ideas for achieving green and low-carbon heating.


Introduction
Energy is a crucial factor in the advancement of human civilization and is the material foundation of economic development.With the acceleration of China's economic expansion and the increasing demand from various industries, the problem of energy shortage has become increasingly prominent and has formed a contradiction with economic development, which mutually restrict each other [1].China's energy consumption system has long been dominated by coal, with the use of multiple energy sources in a complementary and comprehensive utilization structure.Although China has abundant fossil energy reserves, the large population and low per capita ownership result in total energy consumption exceeding production, and demand continues to increase, which has strengthened the country's dependence on energy imports.In recent years, the intensification of international trade conflicts has increased pressure on energy imports.[2] Upon analyzing the breakdown of energy usage, it becomes evident that a considerable portion is attributed to building energy consumption.According to relevant statistical data, building energy consumption accounted for 21.11% of the total national energy consumption in 2017, with operational energy consumption accounting for more than 95%, of which heating and domestic hot water supply are important components [3].Clean heating technology can achieve energy-saving and emission-reduction goals by selecting advanced energy-consuming equipment and using clean energy as a heat source to improve the overall heating efficiency of the system.Promoting clean heating technology can reduce fossil energy consumption, reduce environmental pollution, improve living conditions, and enhance residents' happiness.
Clean and renewable energy sources, such as solar energy and ASHPs, hold immense potential.Solar energy is a widely used clean energy source because it does not produce any pollutants and can be directly utilized when the sun is shining.ASHPs, on the other hand, are devices that use the heat in the air for heating or cooling without the need for fossil fuels, making them highly environmentally friendly.In practical applications, solar energy and ASHPs are often combined to meet the demand for hot water and heating, which can improve energy efficiency and reduce energy consumption and emissions to a certain extent.Therefore, researching the combined application of solar energy and ASHPs has high theoretical and practical significance.By studying the combined application of the two, more ideas and methods can be provided for the development and utilization of clean energy, and greater contributions can be made to improving the environment and reducing energy consumption.Currently, there are many research results on the SASHP.
Hussein et al. [4] examined how the design parameters of flat reflectors impact the efficiency of flatplate collectors.The findings indicated that aligning the reflector installation angle with the optimal angle for noon working conditions resulted in the highest daily efficiency.Zhang et al. [5] proposed a dual-function flat-plate collector that can be used in three different operating modes, including air, water, and air-water hybrid medium.By establishing a mathematical model, they investigated the effect of mass flow rate on the heat transfer performance in the three operating modes.The experimental findings demonstrated that the air-water heating mode yielded the highest performance for the collector, achieving an impressive average overall thermal efficiency of 73.4%.Employing this collector can lead to a substantial enhancement in the utilization of solar thermal energy.Verma et al. [6] designed a single spiral channel by changing the flow channel shape of the flat plate solar collector, and the experimental results showed that compared with the traditional parallel flow channel design, the single spiral channel has higher collection efficiency and smaller flow channel pressure loss, and can also reduce the material cost by about 30%.Saffarian et al. [7] studied the enhanced heat transfer effects of a flat plate collector with various flow channel shapes and nanofluids.The results showed that a collector using corrugated pipes and a nanofluid containing 4% CuO/H 2 O could achieve a 78.25% increase in heat transfer coefficient.Woo et al. [8] introduced a frost calculation method for determining the initiation time of defrosting ASHP units, and the effectiveness of this approach was confirmed through experimental data validation.Zhang et al. [9] analyzed the technical feasibility and economic practicality of a lowtemperature ASHP in extremely cold areas using mathematical modeling methods.The study showed that the system can operate stably and reliably with outdoor temperatures as low as -15℃ and can meet residential heating needs while consuming less energy compared to other heating methods.Wang et al. [10] designed an ASHP unit using a new type of two-stage variable speed scroll compressor and conducted experimental tests in various ambient temperatures and compressor frequency conditions.The experimental results showed that the system can achieve efficient and stable operation at low temperatures.Its heating performance reached 68.1% of the rated value at an ambient temperature of -20℃, and the heating performance coefficient reached 2.0.Hengel et al. [11] proposed an ASHP system with steam injection and variable frequency control.Experimental data confirmed that the system can effectively reduce start-stop losses, and its heating efficiency is 14.8% higher than that of a conventional system, which can save 13.5% of electricity and has great potential for promotion.Sterling et al. [12] utilized TRNSYS to simulate the behavior of a solar-based system incorporating a heat pump as an indirect auxiliary heat source.The system featured dual water tanks for heat storage, enabling a comparison of electricity consumption and operational costs among the heat pump system, traditional solar water heaters, and household electric water heaters.The study maintained a constant system load and water supply temperature.The results demonstrated that the heat pump system, as an auxiliary heat source, exhibited the lowest electricity consumption and operational cost compared to traditional solar water heaters and household electric water heaters.
This article takes a building in Lhasa City as the research object and uses TRNSYS software for system modeling to study the heating energy efficiency and optimization of the SASHP.

Principle of SASHP
The main equipment of SASHP includes solar collectors, ASHPs, thermal storage tanks, heating water tanks, water-water heat exchangers, circulating water pumps, domestic hot water supply equipment, and terminal heating equipment.The system operates in parallel with solar collectors and ASHP through thermal storage tanks as an intermediate medium.This allows for the joint operation of the system while prioritizing and fully utilizing solar energy resources.It also addresses the problem of solar energy not being able to serve as a continuous and stable heat source, thus improving the system's reliability and applicability.The schematic diagram of SASHP is shown in Figure 1.It should be noted that in selecting the components of the system, it is necessary to ensure that the ASHP, as a joint heating source of the system, can operate stably even in extreme situations where there is no solar energy available.
The supply heating mode of SASHP can be divided into four types: 1.When there is sufficient solar energy during the day, the ASHP is turned off and the solar collector operates independently.It heats the water in the storage tank to a pre-set temperature and provides the required heat for residential heating and hot water.The surplus solar energy generated is accumulated in the water tank in the form of sensible heat.
2. In cases where the solar radiation intensity is insufficient to independently heat the water in the tank, the air-source heat pump is activated as a supplementary heat source to ensure the consistent operation of the system.
3. During nighttime or periods of cloudy and rainy days, when the solar radiation intensity falls below the threshold for the startup conditions of the solar collector loop, the operation of the solar collector is halted.However, if the temperature of the hot water in the storage tank satisfies the heating demands, the heat stored within the tank is utilized to sustain the system's operation.
4. When the solar collector is turned off and the temperature of the hot water in the storage tank is below the required supply temperature, the air-source heat pump is used as an independent heat source for the system.

Analysis of outdoor environmental parameters
Lhasa is the capital of Tibet, located at 91°13' east longitude and 29°67' north latitude, in the southwest of the Qinghai-Tibet Plateau.With an average altitude of 3650 meters, the city has abundant solar energy resources and mostly clear weather throughout the year.According to the Chinese Building Climate Zone Division, Lhasa belongs to the plateau cold climate zone, characterized by mostly clear weather, low precipitation, cold and dry winters, and large temperature differences between day and night.To better understand the climate conditions of the heating area, this paper uses the Meteonorm software to obtain meteorological data for a typical year in Lhasa from the meteorological database, focusing on analyzing the meteorological parameters that affect building heat load calculation.As shown in Figure 2, the outdoor dry-bulb temperature in Lhasa ranges from -13.8℃ to 28.3℃, with an annual average temperature of 9.4℃ and a maximum temperature difference of 42.1℃.The heating season in this region lasts for 120 days, from 11/15 to 3/15 of the following year.Lhasa has advantageous geographical conditions, with high altitude, low air pressure, clean and thin air, and high atmospheric transparency, providing abundant material conditions for the utilization of solar thermal technology.The export results of the meteorological data show that the hourly solar radiation intensity throughout the year in the region is presented in Figure 3, with the maximum total solar radiation intensity of 4586.86 kJ/(h•m 2 ), i.e., 1274.16W/m 2 .The average daily sunshine hours are more than 3000 NESP-2023 Journal of Physics: Conference Series 2592 (2023) 012022 hours per year, and 78% of the sunshine time during the heating season has a solar radiation intensity above 250 W/m 2 , which can be effectively utilized by solar collectors.Therefore, the solar energy resources in Lhasa can satisfy the building heating and domestic hot water demands during the day for the residents.However, due to the intermittent and unstable characteristics of solar energy, air-source heat pumps need to be used for joint heating.

System simulation model establishment
The TRNSYS software was employed to model a SASHP.The system diagram, depicted in Figure 4, illustrates the interconnections and relationships between various modules.These modules, with specific functionalities and roles, were connected sequentially based on the system schematic diagram.Relevant parameters were set to simulate the actual operation of the system.The completed system simulation model was represented in a specific configuration, enabling comprehensive analysis and evaluation of the system performance.

Comparison and analysis of different heating methods
Simulations were conducted using TRNSYS for different heating systems: SASHP, ASHP, and solarelectric auxiliary heating (SEAH).To ensure a fair comparison, only the heating source was changed during the simulations.The energy saving and economic comparison results for the three systems are shown in Figures 5 and 6.According to Figure 5, the monthly average COP of the SASHP is larger than the others, and it remains above 3.8 throughout the heating season.The average COP is 1.7 times that of the ASHP, indicating that the system has better operating performance.SEAH has the worst energy efficiency, with an average COP of 1.5.As shown in Figure 6, the electricity consumption of the ASHP and SEAH during operation are 1.7 and 2.4 times that of the SASHP, respectively, greatly increasing their operating costs and reducing their economic viability.Therefore, considering the performance and economics of the various heating systems, SASHP is the optimal heating solution.

Analysis of operating time under different heating modes
There are a total of four heating modes for the system: Mode 1: Solar collector heating mode.During this period, the system's heat is entirely derived from the solar radiation energy collected by the collector.
Mode 2: ASHP heating mode.Throughout this period, when the solar radiation intensity is insufficient and the temperature of the hot water in the thermal storage tank is lower than the required supply water temperature, the ASHP exclusively fulfills the heating load of the system.
Mode 3: Combination of solar and ASHP heating modes.During this period, the solar radiation intensity continues to increase, but the available solar radiation is not enough to meet the entire heating demand.The ASHP functions as the combined heat source to maintain system operation.
Mode 4: Storage tank heating mode.In this period, the storage tank satisfies the autonomous operational requirements and utilizes the stored solar energy in the form of hot water to provide heating.
Table 1 presents data on the overall duration of various heating modes throughout the heating season, along with the corresponding percentages for each mode.The total operating time of the solar collector is 776 hours, representing 26.9% of the total heating period, with a mode 1 operating time of 667 hours, constituting 23.16% of the total duration.The system's utilization of mode 2 for heating spans 1, 229 hours, which constitutes 42.68% of the total duration.The energy consumption of the ASHP will greatly determine the energy consumption of the system.Therefore, optimizing the energy consumption of the air-source heat pump can achieve good results in improving the system's energy efficiency.

Analysis of the impact of supply water temperature on heat pump system efficiency
To examine the effect of the supply water temperature on the system's energy usage, the ASHP was simulated for supply water temperatures of 48℃, 55℃, and 60℃ during the entire heating season in the ASHP heating mode.The COP changes are shown in Figure 8. Figure 8 reveals that under the same environmental conditions, the COP of the ASHP is negatively correlated with the supply water temperature, that is, the higher the supply water temperature is, the lower the COP of the system is.The average COP values of the system at 48℃, 55℃, and 60℃ water temperatures during the entire heating season are 2.83, 2.45, 1.98, which decrease by 13% and 31% compared to 48℃.This is because at the same environmental temperature, the evaporation temperature and pressure remain unchanged, and when the hot water temperature rises, the condensation temperature and pressure also increase, which in turn increases the compression ratio of the compressor, increases the input power of the compressor, and ultimately increases the system, COP.

Conclusion
This article conducted a simulation study on SASHP and obtained the following conclusions: 1.By simulating different heating systems, namely SASHP, ASHP, and SEAH, the results showed that the COP of the SASHP was higher than the other two systems, consistently maintaining above 3.8, and the SEAH had the worst energy-saving performance.
2. In Lhasa, where solar energy resources are abundant, the time when only solar collectors provide all the heat for the system accounts for only 23% of the total operating time of the system.When ASHP is used for heating, including ASHP separate heating and SASHP joint heating, the time accounts for 46% of the total operating time of the system.The energy consumption of the ASHP will largely determine the energy consumption of the entire SASHP.Therefore, it is recommended to start with the optimization of the ASHP energy consumption to improve the system's energy efficiency.
3. Regarding the operating conditions of the ASHP, the COP is negatively correlated with the supply water temperature, meaning that the higher the supply water temperature, the lower the system COP.
The average COP values of the heat pump system at supply water temperatures of 48℃, 55℃, and 60℃ during the entire heating season were 2.83, 2.45, and 1.98, which decreased by 13% and 31% compared to 48℃.By selecting an appropriate supply of water temperature, the energy-saving effect of the SASHP can be further improved.

Figure 2 .
Figure 2. Hourly variation of outdoor dry bulb temperature in Lhasa.

Figure 3 .
Figure 3. Hourly variation of solar radiation intensity in Lhasa.

Figure 5 .
Figure 5.Comparison of monthly average heating performance coefficient of each system.

Figure 6 .
Figure 6.Comparison of total power consumption of each system.

Figure 8 .
Figure 8. Hourly COP under different supply water temperatures.

Table 1 .
Total operation time and proportion of different heating modes in the heating period.
Figure 7. Distribution of operating hours for different heating modes by month.