Experimental Study on the Combined Heat Storage and Supply of Air/Water-Source Heat Pumps

Abstract

As the application of renewable energy becomes increasingly extensive, heat pump technology with renewable energy as the heat source is achieving good results. Air-source heat pumps and water-source heat pumps can be widely used in cold areas. In this work, an integrated combined storage and supply system of an air-source heat pump and a water-source heat pump was studied, and the heating characteristics of the system at the beginning, middle, and end of the heating period were examined. It was found that, when the outdoor temperature of the system was very low, the efficiency of the combined storage and supply system reached the highest value of 2.57 when the source-side water tank was kept at 30 °C, and the performance of the combined storage and supply system was better than that of the air-source heat pump and the water-source heat pump in cold regions. Meanwhile, the independent storage of the air-source heat pump and the combined storage and supply system can be used for heating at the beginning and end of the heating period.

1. Introduction

With the continuous acceleration of urbanization, building heating accounts for a large proportion of the energy consumption of the building industry [1], but it still has a lot of room for development [2]. The use of renewable energy technologies in buildings causes less environmental pollution [3]. According to statistics, coal power accounted for 58.4% of China’s total power generation and 43.8% of the total installed capacity in 2022 [4]. By the end of 2022, the country’s installed capacity stood at 12 billion kilowatts, with wind and solar contributing 3.65 and 3.93 billion kilowatts, respectively, for a total renewable energy generation of 2700 TWh [5]. Heat pump sales have shown a stable growth trend over the past decade in China. The installed capacity of building heat pumps will reach 575 million kilowatts by 2030, and it will exceed 1.1 billion kilowatts by 2050 [6]. The total installed capacity of renewable energy in China has reached 1.213 billion kW, and it accounted for 47.3% of the country’s total installed power generation capacity at the end of 2022, overtaking that of coal for the first time [7]. Therefore, renewable energy is the most economical and practical way to break away from fossil fuels [8].

Air-source heat pumps are an important type of energy-saving technology researched in recent years [9], but evaporator frost has been the main factor plaguing their development [10]; hence, in order to study the defrosting problem of air-source heat pumps and their application in cold regions, Huihui Zhong et al. [11] conducted tests and numerical analyses on the anti-frosting performance of dual-source evaporators. The outcomes demonstrated that, when the operating temperature of the evaporator was higher than the critical frosting temperature, the evaporator could prevent frost in winter. Pavel Míšek et al. [12] studied gas absorption heat pumps (GAHPs), which use air as a low-temperature heat source, and they found that this form of heating can reduce fossil fuel consumption. Additionally, compared with the heat produced by gas condensing boilers, GAHPs could reduce CO₂ emissions by 52 kg by generating 1 MWh of heat. Hernández et al. [13] analyzed the thermal zoning system of an air/water heat pump system with fan coil units based on TRNSYS. Additionally, they examined the non-zoned duct fan coil unit and individual fan coil units and they analyzed the influence of thermal zoning on the heat pump’s performance from the perspective of energy consumption. R. O’Hegarty et al. [14] reviewed and analyzed the performance of air/water heat pumps, and they found that the heating seasonal performance of the monitored case study heat pump was 2.49, mainly attributed to pipeline losses and inaccurate weather data. Lim et al. [15] exploited underground air as a heat source for winter greenhouse agriculture and compared the economics of air/water heat pumps (AWHPs) with those of traditional air heaters, and it was revealed that AWHPs with air preheating can save more than 70% of the total heating costs. Kowalski et al. [16] applied TRNSYS to the waste heat recovery of air/water electric compressor heat pumps in multi-residential buildings at five locations, and they found that the most energy-efficient system was the one with the lowest heating system temperature.

The evaporating temperature of the refrigerant is also an important factor affecting the operation efficiency of the system, and Tan et al. [17] tested and simulated the refrigerant evaporation temperature and the heating performance of heat pumps, and they found that the heating capacity and thermal stability of the air/water heat pump system were higher in Xiangtan City, which was taken as an example. Artuso et al. [18] studied the performance of air/water reversible R744 heat pumps and simulated the typical operation of heat pump configurations using a numerical model. Due to the low inlet water temperature, the COP was found to be the highest during domestic hot water operation. Afshari et al. [19] conducted a study on the impact of the compressor cooling fan of heat pumps at various fan speeds. They utilized refrigerant R134a as the circulating fluid and examined the COP, P-h diagrams, efficiency, and thermodynamic properties for performance optimization. Diaz et al. [20] integrated thermoelectric air-to-air heat pumps with heat recovery devices based on passive houses, and they discovered that enhancing the heat exchanger between the thermoelectric module and the cooling airflow was more convenient than increasing the number of modules to elevate the cooling capacity. Leonzio et al. [21] investigated an air-source heat pump for high-temperature water heating and analyzed an industrial ASHP employed for high-temperature water heating, and its economic advantage and the outcomes indicated that the former ASHP had superior efficiency and saved operating costs. Min Kyu Kim et al. [22] verified the effect of physical parameters of air/water-source heat pump R32 on mass flow rate, and the correlation equation was found to be consistent with a comparative study.

Multi-source heat pump systems are able to improve the operational efficiency of the system and have achieved good results. Air and water sources as renewable energy sources can greatly reduce carbon emissions. Rastegarpour et al. [23] established a COP prediction model, and they verified the accuracy and application potential of the model by conducting experiments on different operating modes of air/water-source heat pumps. Panigrahi et al. [24] and Jafargholi et al. [25] designed an air/water dual-source evaporator and verified the energy efficiency of switching between different operating modes for space heating and equipment defrosting using different methods. Clauß et al. [26] investigated the energy flexibility potential of air-source heat pumps and direct electric heating in Norwegian single-family detached houses, and it was found that, by modifying the temperature set point, energy usage could be notably increased. Correa et al. [27] established an air/water heat pump model to study heating and domestic hot water in Chilean residences. They took into consideration the annual energy requirements of Concepcion and Santiago, which were 3721 kWh and 3541 kWh, respectively. Xu et al. [28] studied the effect of the frequent startup of an air/water heat pump on system efficiency based on an ANN model, and they found that it could improve system performance and reduce system energy consumption. Li et al. [29] studied a new dual-source system that can reduce carbon emissions compared with conventional heat pump systems and has good environmental benefits for heating in different areas. Bruno et al. [30] tested and adjusted the data correlation of an air/water heat pump experimental device. The function between the nominal and actual EER in the CR function was identified by modifying the correction coefficient. Xu et al. [31] proposed a new air/water dual-source composite evaporator and verified the performance coefficients of different operating modes through a composite heat pump system. Additionally, a comparison revealed that the performance coefficient of the composite heat pump system could be increased by 6.3–9.8%. Olympios et al. [32] studied the cost of air/water heat pumps designed, using the UK as an example, and verified the correlation between the cost and performance of the heat pumps in all aspects, and determined the optimal design of heat pump systems. Heinz et al. [33] compared the performance of air/water-source heat pump systems with and without desuperheaters, and they found that electrical energy savings of 3.3–5.2% could be achieved with desuperheaters.

The above review demonstrates the characteristics of the operation of air-source heat pump, water-source heat pump, and air/water-source heat pump systems. These studies could provide a good reference for the development of heating in cold areas. Although there are some research results on coupled heat pumps, there are fewer studies on the use of hot water storage tanks to store hot water produced by air-source heat pumps in cold weather during the day, or in better conditions, and provide it to water-source heat pumps as a low-level heat source. In this paper, the experimental electricity is mainly used to supply heat to the building at peak and valley tariffs, which can meet the heating demand of the building throughout the day in cold regions. In the middle of the heating period, the air-source heat pump is used as an auxiliary heat source to improve the comprehensive COP value of the system operation, and the experiments are conducted to mainly study the following two aspects:

• Verify the feasibility of the independent storage and supply of air-source heat pumps and joint storage and supply operation system of air/water-source heat pumps.
• Calculate the COP of the joint operation system using an air-source heat pump for heating in the early stage (ending) of heating and an air/water-source heat pump for storage in the middle stage of heating, and determining the optimal source-side water temperature for operation.

2. Design of the Experimental System

The experimental platform was situated in two interconnected laboratories in an experimental building of a university in Changchun, China. The total area of the laboratories amounted to 124.4 m², with a floor height of 4.2 m. The heating terminals adopted the form of radiators, and the maximum heat load reached 10.6 kW, according to 2022 measurement data.

2.1. Configuration of the Experimental System

The experimental platform of the air/water-source heat pump (AWSHP) combined heat storage and supply system consisted of an air/source-heat pump (ASHP), a water-source heat pump (WSHP), a low-temperature heat storage water tank, a high-temperature water tank, and several water pumps and valves. The evaporator side of the WSHP was considered the source side, and the condenser side of the WSHP was considered the user side. A schematic diagram and the connection sequence of the equipment and pipelines are shown in Figure 1.

2.2. Determination of the Experimental Equipment

The ASHP, WSHP, and circulating pump models were selected according to the heat load requirements of the laboratory, and they are shown in

Table 1. Data parameters for ASHP and WSHP selection.

Heat Pump Type	Model Number	Rated Heating Capacity (kW)	Maximum Output Power (kW)	Refrigerant
ASHP	MACS050ER5	14.8	4.0	R410A
WSHP	AQSW0041-DHW	15.1	4.0	R410A

and

Table 2. Circulating water pump selection.

Model Number	Water Pump Model	Pump Flow Rate (m3/h)	Pump Head (m)	Operating Frequency (Hz)	Output Power (W)
Circulating water pump	LRW-16	1.5	5–10	50	95

.

Table 3. Range, resolution, and precision of the measurement equipment in the experiments.

Instrument	Range	Resolution	Precision
Electromagnetic flow meter ZH-LDE250W	0.3–10 m/s	0.01 L/min	±0.5 ± 1.0%
Digital thermometer GJD-200LCD	−50–200 °C	0.1 °C	±0.1%
Temperature Sensors Rs485	−40–80 °C	0.1 °C	≤0.1%

shows the range, resolution, and precision of the experimental test equipment. The volume of the two hot water storage tanks with thermometers was 1 m³. As a high-precision digital display thermometer, a GJD-200LCD was selected. Figure 2 presents a diagram of the experimental platform, from left to right, the names of the equipment are mainly water-source heat pumps, air-source heat pumps, user-side tanks, source-side tanks.

The adopted battery digital thermometer, GJD-200LCD, uses a copper-plated nickel probe, and it has a high precision and common temperature range, which can meet different temperature measurement needs. The selected temperature sensor, Rs485, is waterproof and has an anti-interference circuit; it can be connected to a computer to check the data and alarm status at any time, and it has a variety of output methods.

3. Experimental Method

The heating period was divided into the initial heating period, the middle heating period, and the terminal heating period to accommodate various climatic circumstances and heating requirements. The initial heating period spanned from November to 15 December, and the terminal heating period spanned from the beginning of February to early April. While the outdoor temperature typically ranges from −5 °C to −15 °C in Changchun, the mid-heating period usually commences in the latter half of December and persists until around the beginning of February, and the temperature is generally below −15 °C.

3.1. Independent Storage and Supply Experiment of Air-Source Heat Pump

In this part of the study, an experiment was conducted to mainly investigate the heat storage performance of the ASHP under different outdoor temperatures. The experiment was conducted in different heating periods, and the COPs were compared. An experimental schematic diagram is shown in Figure 3.

The water of the ASHP was heated in the storage tank to the designated temperature. The outdoor temperature, operation duration, and temperature fluctuations within the storage tank were recorded, and the COP₁ of the air-source heat pump could be obtained. The calculation formula of COP₁ is presented in Equation (1):

$${COP}_1={\displaystyle \frac{Q_1}{W_1}}={\displaystyle \frac{c_{p}m∆T_1∆{\tau }_1}{W_1}}$$

(1)

where $Q_1$ is the ASHP heat exchange capacity, J; $W_1$ is the ASHP power consumption, J; $c_p$ is the specific heat capacity of the water at a constant pressure, $\mathrm{J}/(\mathrm{k}\mathrm{g}·\mathrm{K})$; $m$ is the mass flow rate, kg/s; ΔT₁ is the difference in working temperature before and after the heat storage of the water tank, K; and Δτ₁ is the air-source heat pump working time, s.

3.2. Storage and Supply Experiment of the Water-Source Heat Pump

The purpose of this experiment was to study the effects of different source and user-side water temperatures on the water-source heat pump and to investigate the thermal storage performance. An experimental schematic diagram is shown in Figure 4.

In this experiment, different source and user-side water temperatures were set. The temperatures of the two water tanks could be regulated and controlled through the ASHP in the experimental platform system, and the user-side tank was heated to the preset temperature by the source-side tank of the WSHP, which was heated by the ASHP until the heat pump stopped working. The heating times and temperature variations in the source and user-side water tanks were recorded during the experiment, and the performance coefficient of the WSHP (COP₂₎ could be calculated using Equation (2):

$${COP}_2={\displaystyle \frac{Q_2}{W_2}}={\displaystyle \frac{c_{p}m·T_2∆{\tau }_2}{W_2}}$$

(2)

where $Q_2$ is the ASHP heat exchange capacity, J; $W_2$ is the ASHP power consumption, J; $c_p$ is the specific heat capacity of the water at a constant pressure, $\mathrm{J}/(\mathrm{k}\mathrm{g}·\mathrm{K})$; $\mathsf{\Delta }{T}_2$ is the difference between the temperature of the water in and out of the WSHP, K; $m$ is the mass flow rate, kg/s; and $∆{\tau }_2$ is the WSHP working time, s.

3.3. Storage and Supply Experiment of AWSHP System

During the experiments in various heating periods, the source-side water was heated and its temperature maintained by the ASHP. Changes in the WSHP in other scenarios were recorded by altering the source-side water temperature. A schematic diagram of the experiment is presented in Figure 5.

The combined storage system continued to operate at low temperatures to ensure heating the building, and since the heat exchange of the combined storage and supply system is composed of $Q_1$ and the power consumed by the water-source heat pumps, $W_2$, and the COP of the combined storage system is calculated by Equation (4):

$$Q_2=Q_1+W_2$$

(3)

$$COP={\displaystyle \frac{Q_2}{W_1+W_2}}={\displaystyle \frac{{COP}_1{COP}_2}{\frac{{Q_2{COP}_1+Q}_1{COP}_2}{Q_2}}}={\displaystyle \frac{{COP}_1{COP}_2}{{COP}_1+{COP}_2-1}}$$

(4)

where COP is the performance coefficient of the combined storage and supply system, COP₁ is the performance coefficient of the ASHP, COP₂ is the performance of coefficient the WSHP, $Q_1$ is the amount of heat exchanged on the source side of the water-source heat pump, $W_2$ is the power consumed by the water-source heat pump in the combined storage and supply system, and $W_1$ is the power consumed by the air-source heat pump in the combined storage and supply system.

4. Experimental Process and Discussion

4.1. Independent Storage and Supply Experiment of ASHP

The initial temperature of the heat storage water was 15 °C. The experimental time frames were the middle heating period, with experiments conducted on 22 January 2022, and the end heating period, with experiments conducted on 1 March 2022, because the outdoor temperature difference between the early heating period and the end heating period was not significant. The start times for the experiments in the early (ending) heating period were 4:00 a.m., 11:00 a.m., and 8:00 p.m., and the ASHP was activated at these times. The heat storage water was heated to approximately 40 °C, and the working duration and COP₁ of the heat pump were recorded. The work efficiency of the ASHP in different heating periods is presented in

Table 4. ASHP efficiency at different times and temperatures.

Time		Outdoor Temperature (°C)	Ending Temperature (°C)	Temperature Rise (°C)	Thermal Storage Time (min)	COP1
22 January	4:00	−29.9	30.1	15.1	169	1.7
	11:00	−20.2	29.4	17.4	108	2.13
	20:00	−22.4	30.5	16.5	104	1.98
1 March	4:00	−14.2	31.1	15.1	169	2.4
	11:00	−10.4	30.5	17.4	108	2.73
	20:00	−4.2	30.4	16.5	104	3.3

, and the heat pump COP₁ change curve is shown in Figure 6.

At the end of the heating period, the average COP₁ of the heat storage of the ASHP could be sustained above 2.4 when the outdoor temperature remained above −15 °C. Nevertheless, during the middle of the heating period, with the decrease in the outdoor temperature, the COP₁ decreased. When the outdoor temperature was lower than −20 °C, the average COP₁ could merely be maintained at about 2. As the outdoor temperature decreased in the middle stage of heating, the heating water temperature of the ASHP could not meet the heating demand; thus, the ASHP heat storage and supply system is well-suited for operation under the condition of a relatively high outdoor temperature at the beginning (end) of the heating period but it is not conducive under the condition of a relatively low outdoor temperature in the middle of the heating period.

4.2. Water-Source Heat Pump Storage and Supply Experiments

Based on the experimental principle, this experiment was divided into five groups. For each group, the initial working temperature of the source-side water tank was set to 20 °C, 25 °C, 30 °C, 35 °C, and 40 °C, and the source-side water tank was preheated to these temperatures by the ASHP. In each group of experiments, the initial working temperature of the user-side water tank was set to 15 °C, 25 °C, and 35 °C for sequential heating. The heating times and temperature variations were recorded, and the COP₂ of the WSHP at each temperature is presented in

Table 5. WSHP efficiency at different source and user-side water temperatures.

Source-Side Starting Temperature (°C)	User-Side Starting Temperature (°C)	User-Side End Temperature (°C)	COP2
20	15	40.5	5.3
	25	42.6	4.8
	35	45.0	4.5
25	15	40.0	4.7
	25	43.2	4.6
	35	46.6	4.5
30	15	42.0	6.3
	25	48.7	6.2
	35	49.0	4.6
35	15	41.2	5.7
	25	46.5	5.4
	35	49.1	4.6
40	15	45.8	6.1
	25	50.0	6.0
	35	49.6	4.6

.

As shown in Table 5, the WSHP could heat the user-side water to a temperature of 50 °C, and the WSHP COP was the largest at a source-side water temperature of 30 °C. When the ASHP maintained the WSHP source-side water tank temperature at 30 °C, in the air/water-source heat pump (AWSHP) combined storage and supply system, the WSHP was able to maintain a good heating effect. The changes in the WSHP COP with the source and user-side water temperatures are shown in Figure 7. It was found that the maximum COP₂ of the WSHP was 6.3, the minimum was 4.5, and the average COP₂ was 5.1, which was caused by the ASHP. The COP₂ was higher when the user-side water temperature was 15 °C than when it was 25 °C or 30 °C, and the higher the user-side water temperature, the lower the COP₂. Therefore, in an air/water heat pump combined heat storage system, the ASHP could stabilize the water temperature on the source side of the WSHP at 30 °C to sustain the long-term and highly efficient operation of the WSHP. Additionally, the experiment revealed that, when the source-side water temperature was lower than 6 °C, the WSHP stopped working, and when the water supply was insufficient, the WSHP could not be used for separate storage and heat supply.

4.3. Experiments on Air/Water-Source Heat Pump Combined Storage and Supply System

4.3.1. Storage and Supply Experiment at Different Source-Side Water Temperatures

The purpose of this experiment was to verify the COP variation with the water temperature of the source-side water tank of the system in the combined heat storage and supply state. The experiment was conducted on 5th March and 6th March, 2022. Initially, the water temperature on the source side of the water-source heat pump was set to 15 °C, and the ASHP was employed for heating and maintaining at 25 °C. The experiment was conducted at 8:00, 12:00, and 16:00. When the system reached a temperature of 45 °C, it was halted, then the COPs of the ASHP and WSHP were recorded, and the comprehensive COP of the system was calculated. The experimental data are shown in

Table 6. Experimental results with source-side water temperature at 25 °C.

Experiment Start Time	Outdoor Temperature (°C)	Time-Consumption (min)	Source-Side Tank Temperature (°C)	User-Side Tank Temperature (°C)	WSHP COP	ASHP COP	Combined Storage and Supply System COP
8:00	−5.5	72	25.9	46.8	5.5	3.4	2.37
12:00	−6.8	70	26.6	45.1	5.3	3.23	2.27
16:00	−6.7	73	25.3	46.7	4.8	3.3	2.23

.

Next, the WSHP source-side water temperature was set to 35 °C to achieve a heat pump source-side water temperature greater than 30 °C. The experiment was conducted on March 6 at 8:00, 12:00, and 16:00. The above experiments were repeated, and the corresponding data were recorded. The experimental results are shown in

Table 7. Experimental results with source-side water temperature at 35 °C.

Experiment Start Time	Outdoor Temperature (°C)	Time-Consumption (min)	Source-Side Tank Temperature (°C)	User-Side Tank Temperature (°C)	WSHP COP	ASHP COP	Combined Storage and Supply System COP
8:00	−7.7	85	35.7	46.8	6.5	2.72	2.15
12:00	−9.8	81	36.6	47.1	6.2	2.47	1.99
16:00	−9.6	83	35.3	46.7	6.4	2.55	2.05

.

Figure 8 shows the comprehensive COP of the combined storage and supply system at three source-side water temperatures. When the source-side tank temperature was 25 °C, the ASHP had the best performance, and when the source-side tank temperature was 35 °C, the water-source heat pump had a better performance, but the ASHP performance was poor, when the source-side tank temperature was 30 °C, and although the performance of the ASHP and the WSHP was relatively poor, the economic performance of the air/water-source heat pump combined storage and supply system was better. Through calculations of the system at the source-side tank temperature of 30 °C, it was found that the system’s integrated efficiency reached the highest value of 2.34. Through the experiment revealed that in the heating period, the ASHP could be improved by enhancing the thermal quality of the source side of the tank of the WSHP to improve the combined storage and supply system of the integrated COP, and it is able to meet the needs of the building in the heating demand of the heating period.

4.3.2. Storage and Supply Experiments of the Combined System at the Beginning (End) of the Heating Period

The outdoor temperatures at the beginning (end) of the heating period were relatively proximate, and 4:00, 12:00, and 20:00 on 27 November and 2 March were selected to study the combined heat storage and supply system.

Table 8. COP of the ASHP and WSHP at the beginning (end) of the heating period.

Experimental Conditions				Source-Side Tank	ASHP	WSHP
Date of Experiment	Experimental Time	Time-Consumption (min)	Outdoor Temperature (°C)	Ending Temperature (°C)	COP1	COP2
2 March	4:00	87	−4.9	32.5	3.12	\
				46.2	\	6.3
	12:00	84	−3.8	31.2	3.3	\
				46.3	\	6.5
	20:00	86	−3.7	30.1	3.4	\
				46.1	\	6.4
27 November	4:00	87	−9.5	28.8	2.89	\
				46.8	\	6.4
	12:00	89	−2.4	30.5	3.51	\
				46.1	\	6.1
	20:00	85	−3.8	29.9	3.39	\
				46.7	\	6.5

displays the experimental data of the ASHP and the WSHP in the heating beginning (ending) period. The source- and user-side tank starting water temperatures were set to 15 °C in the experiment, and the ending temperatures were set to approximately 30 °C and 46 °C, respectively. The outdoor temperature, operation duration, and temperature variations were studied, and it was found that they greatly influenced the COP of the heat pump.

Figure 9 and Figure 10 display the COPs of the ASHP, WSHP, and AWSHP. Figure 9 displays the COP variations at the beginning of the heating period, and it was found that the COPs of air/water heat pump at 4, 12, and 20 o’clock were 2.23, 2.48, and 2.48, respectively. Figure 10 shows the COP transformations at the end of the heating period, and it was found that the combined operation system’s COPs at 4, 12, and 20 o’clock were 2.34, 2.43, and 2.47, respectively. Figure 9 and Figure 10 both show that, when the ASHP preheated the source-side tank of the WSHP, the performance of the WSHP improved rapidly. Furthermore, because the system was combined, the combined operation system was able to maintain better performance in the heating period. The figure shows that the COP of the AWSHP system could be maintained above 2.3, and, at the beginning (ending) of the heating period, the ASHP was selected as the heating mode due to the high outdoor ambient temperature.

4.3.3. Experiment of Combined Storage and Supply System in the Middle of the Heating Period

The experimental date for the mid-heating period was chosen as 22 January 2022, and the outdoor temperature was relatively low on the test day; hence, the experimental method was carried out in the same way as that at the end of the heating period. The experimental data of the ASHP and WSHP in the middle of the heating period were analyzed, and the system operation COPs were computed. The results are displayed in

Table 9. COP of the air-source heat pump and water-source heat pump in medium-term heating period.

Source-Side Tank				ASHP	WSHP
Experimental Time	Time-Consumption (min)	Ending Temperature (°C)	Outdoor Temperature (°C)	COP1	COP2
0:00	112	26.5	−27.0	1.9	\
		45.3		\	4.7
4:00	124	27.2	−29.9	1.78	\
		43.4		\	4.0
8:00	108	28.1	−26.7	1.8	\
		46.7		\	5.1
12:00	106	26.8	−19.7	2.24	\
		46.8		\	5.3
16:00	117	28.5	−16.5	2.37	\
		46.1		\	4.7
20:00	120	25.9	−22.4	2.12	\
		46.5		\	4.6

, in addition to the operation duration, temperature difference, and outdoor temperature.

Table 9 shows that the ending water temperature of the user-side tank was maintained at 46 °C, and the WSHP stopped working due to the low-temperature water; hence, the source-side temperature of the WSHP had to be increased by the ASHP in the combined system. Figure 11 shows that the COP₁ of the ASHP decreased substantially during the mid-heating period, with an average value of 2.04. Concurrently, its capacity to maintain the temperature of the source-side water tank decreased accordingly, and at the conclusion of the experiment, the water temperature of the source-side water tank was below 30 °C. The COP₂ of the WSHP also decreased in comparison with the experimental data at the end of the heating period, with an average value of 4.7. The comprehensive COP of the combined heat storage and supply system was 1.67.

5. Conclusions

This study established an air/water-source heat pump combined heat storage and heating system and carried out an experimental investigation into the heat storage and heating capacity of this system. This encompassed independent heat storage experiments of the ASHP at various outdoor temperatures during different heating periods, heat storage experiments of the WSHP with various source and user-side water temperatures, and combined heat storage capacity experiments. The following conclusions could be drawn from the experiments.

The experimental results of the ASHP showed that the COP of the system was 2.8 in the initial heating stage and only 1.94 in the middle heating stage. Due to the difference in the outdoor temperature, the COP of the ASHP in the initial (final) heating period was higher than that of the independent storage and supply in the middle heating period. Additionally, the COP of the ASHP working independently when the outdoor temperature was lower than −20 °C in the middle of heating did not meet the national requirements for heating.

The experimental results of the WSHP indicated that a too-high or too-low source-side water temperature caused the WSHP to stop working, and this would cause residents to have a bad heating effect; therefore, the WSHP is not suitable for independent heat storage heating under conditions of insufficient water. Furthermore, when the source-side water temperature was 30 °C, it was found that the COP of the combined storage and supply system reached the maximum, 2.34; therefore, in the combined storage and supply system, the ASHP needed to increase the water temperature at the source side of the WSHP and maintain it at 30 °C, and then the combined storage and supply system had the best effect. The whole experiment showed that the independent storage and supply of the ASHP could maintain a good heating effect in the beginning (ending) heating stage. In the middle heating stage, the combined storage system of the air/water-source heat pump could achieve a better heating effect.

This study investigated air-source heat pump independent storage and supply and an air/water combined storage and supply heat pump system, and it verified the system effect in each heating period through the performance coefficient, thereby providing a good reference value for the heating of clean energy buildings in northern China. Due to the lack of automatic control of the temperature, time, and water level of the water tank in the system, further research could be carried out on these aspects.