Research on the Performance and Energy Saving of Solar-Coupled Air Source Heat Pump Heating System: A Case Study of College Dormitory in Hot Summer and Cold Winter Zone

Abstract

As a densely populated area, college student dormitories consume a large amount of electricity every year to heat the domestic hot water used by students. Applying solar energy to hot water systems can effectively alleviate this situation. This paper first conducts a simulation of the hot water load and the calculation of the available area of the solar roof in a dormitory building of a certain university. Then, different solar-coupled air source heat pump systems were designed, and simulation models of the two systems were established. The thermal performance parameters and solar energy utilization of the two systems were discussed, and the energy efficiency, economy, and environmental protection of the two systems were analyzed. The results show that after coupling with the solar collector, the system operation time is shortened by 26.2%, the annual performance coefficient is 3.4, which is 0.8 higher than that of the original system, and the annual heating energy consumption is reduced by 24.4%. In contrast, the annual energy self-sufficiency rate of the photovoltaic coupled with air source heat pump system is 94.6%, achieving nearly zero energy consumption for heating.

1. Introduction

Buildings consume approximately 36% of society’s total energy [1], and heating, ventilation, and air conditioning (HVAC) systems account for the largest share of buildings at 37% [2]. University student dormitories, as densely populated places, have a large requirement for domestic hot water. A large amount of electricity is consumed each year to heat domestic hot water, and the reduced performance of electric heating equipment in winter results in lower hot water discharge temperatures, reducing thermal comfort for personnel [3]. Solar collectors effectively collect solar energy and convert it into usable energy for a variety of purposes, such as heating air for HVAC systems and heating domestic water [4,5,6,7,8]. However, the working conditions of solar collectors are unstable, so electric heating equipment must be used in conjunction with heating. Heat pump units are a very good option for energy conservation and environmental protection, and air source heat pumps are often preferred due to their ease of installation and lower cost [9].

In recent years, research on solar-coupled air source heat pump systems has made progress. In the research of direct expansion solar-coupled air source heat pumps, Sahar et al. [10] comparatively analyzed the annual COP and energy consumption of two direct expansion heat pump water heating systems, namely solar-coupled heat pumps and air source heat pumps. Thiago et al. [11] conducted an experimental study on the capillary direct expansion-type solar-coupled heat pump. Cai et al. [12] proposed an air-source hybrid solar-coupled heat pump system with finned tube evaporators and collectors in series. Sun et al. [13] compared the advantages of direct expansion solar-coupled heat pumps over traditional air source heat pumps.

In the research of hot water systems, Ma et al. [14] proposed and designed a solar-coupled air source heat pump domestic hot water supply system. Its performance and economy were verified through numerical simulation and experiments, and the optimal design parameters were determined through economic optimization analysis. Li et al. [15] conducted experiments and simulation studies on the technical and economic performance of the system under three modes: solar water heater heating, air source heat pump heating, and solar water heat pumps (heat pumps that use hot water from solar collectors as heat). Zheng et al. [16] used TRNSYS 18 software to build a SAASHP system that meets the hot water demand for the bathing of 480 students, providing a method for the economic design of domestic SAASHP systems. Li et al. [17] reported the investigation results on the application of solar-coupled air source heat pump systems in hot water production in Hong Kong. Zheng et al. [18] proposed a new dynamic operation method (DOM) for SAASHP systems and took the hot water supply in university dormitories as an example to study its performance and feasibility.

In terms of combining phase-change materials, Ni et al. [19] proposed a phase-change material-solar-coupled air source heat pump system composed of an air source heat pump, phase change materials, and a solar collector. Jin et al. [20] proposed a dual-source solar heat pump latent heat storage hot water system utilizing renewable energy. Zheng et al. [21] studied the performance of the combined system of the solar-coupled air source heat pump and the PCM floor. Li et al. [22] developed a novel composite phase-change material (CPCM) for efficient heat storage in SAASHP systems.

Furthermore, in other research aspects, Zhang et al. [23] proposed and experimented with an indirect expansion solar-coupled air source heat pump system. Yang et al. [24] simulated the operational performance of a differential solar-coupled air source heat pump in a typical year in the London area using TRNSYS 17. Martin et al. [25] deduced that solar-coupled air source heat pumps demonstrated better energy efficiency and lower capital costs among all considered housing types. Wang et al. [26] evaluated the performance of various air source heat pump systems regarding solar energy resources. Li et al. [27] proposed a solar-coupled air-source heat pump time-sharing heating system, which can adapt more effectively to the local climate characteristics and the living habits of residents. Zhang et al. [28] conducted a comprehensive study on the development prospects of solar-coupled air source heat pump systems in China. Hou et al. [29] proposed a solar-coupled multi-source heat pump drying system. Yang et al. [30] completed the numerical simulation of the solar-coupled air source heat pump heating system. Yang et al. [31] selected a single-family residence as the reference building and demonstrated that low-temperature heating could significantly reduce the electricity consumption of this heating system. Zhu et al. [32] proposed a double-nozzle ejector vapor compression cycle for a solar-coupled air source heat pump system. Tolga et al. [33] examined three heating systems, namely the textile solar-coupled air source heat pump, the flat plate solar-coupled air source heat pump, and the air source heat pump, which were adopted to heat the storage tank, and the energy, fire, and economic indicators of the three different heating systems were compared. Yang et al. [34] reviewed the latest progress of solar-coupled air source heat pumps in China from aspects such as system configuration, solar collectors, thermal energy storage, and defrosting methods, and made prospects for future research. Liang et al. [35] proposed a new type of flexible operation solar-coupled air source heat pump system. Ma et al. [36] utilized the advantages of multi-tank layout to conduct an optimization study on the solar-coupled air-source heat pump hot water system.

In addition, photovoltaic (PV) coupled heat pump systems have been studied during recent years. Ming-En et al. [37] established a multisectoral community energy system that included photovoltaic and heat pump systems. Richard et al. [38] combined photovoltaic and different heat pump systems. In addition, C. Roselli et al. [39], Altti et al. [40], Francisco et al. [41], and Sangmu et al. [42], have studied PV coupled HVAC systems. In particular, studies have shown that hybrid renewable energy systems can not only meet but can sustainably exceed future energy demand [43].

Through a literature review, it can be found that the research on solar-coupled air source heat pump systems is basically concentrated at the residential building level, and the research in cold and severely cold regions accounts for a relatively large proportion. The heating habits in university dormitories are different from those in residential buildings, and there are few studies on energy-saving heating in university dormitories. Therefore, it is necessary to study the potential of renewable energy systems for hot water heating in dormitories. Therefore, the research on solar-coupled air source heat pump hot water systems in university dormitories has certain value. This study comprises the following work: Firstly, a simulation of the hot water load and the calculation of the available area of the solar roof were conducted for the dormitory building of a certain university. Then, two sets of solar-coupled air source heat pump systems were designed to provide hot water for the dormitories, as well as the TRNSYS simulation system. The thermal performance parameters and solar energy utilization of the two systems were discussed, and the energy efficiency, economy, and environmental protection of the two systems were analyzed. This research can provide valuable design references for the design of solar-coupled air source heat pump hot water systems in colleges and universities.

2. Methods

2.1. Overview of the Target Building

The object of the building is located on the campus of a university in a hot summer and cold winter zone, and the type is a graduate student dormitory. The existence of a large number of small and varied air-conditioned thermal zones does not lend itself to centralized heating or cooling by central air-conditioning. However, hot water demand is relatively more concentrated, both in terms of the timing of hot water use and heating temperatures. The total hot water load for the dormitory building was calculated by the DeST 2.0 software. Firstly, a DeST model of a typical building was created based on the actual building as shown in Figure 1.

Next, the hot water load demand for the dormitory building was calculated. This is shown in Figure 2, where the hot water load demand of the dormitory varies significantly with outdoor meteorological parameters, with the maximum hourly hot water load demand reaching 3708 kW. The total annual hot water load demand for this dormitory building is 6906 MWh.

2.2. Area of Solar Energy Available

The total usable roof area of the dormitory building is 47,639 m^2, but the usable solar area is only 18,872 m². This is mainly due to the presence of various obstacles and the defects of the roof structure. The detailed calculation of the solar energy available area on the roof of the dormitory is shown in Figure 3: Firstly, the building roof modular segmentation in the satellite map is done using QGIS 3.26.1 software. Secondly, the roof obstacles in the module are identified using Labelme 4.5.13 software. Finally, the available area of solar energy is simulated by Python software.

2.3. System Design

Due to the simplicity and relatively low cost of installation of air source heat pumps. Therefore, the ASHP system was chosen as the main equipment for hot water heating in the dormitory. Solar thermal collectors and photovoltaic installations are used as two different forms to assist ASHP system. Solar collectors convert solar radiation into heat energy, thereby reducing the operating pressure of the ASHP and opening up new sources of energy and reducing costs. The specific process is that when the outlet temperature of the solar collector is higher than the outlet temperature of the heated water tank (HWT), then the circulating water pump pumps the collected solar heat into the HWT for heat exchange. The ASHP system collects heat from the air, uses a small amount of electricity to raise the temperature, and then delivers the heat to the HWT for heating. The PV coupled ASHP system is the same as the ASHP system in terms of heating form, but from the energy point of view, the PV power generation can offset part of the heating energy consumption and, thus, realize low-energy heating. The two forms of solar-coupled ASHP systems are shown in Figure 4, with different system module divisions and energy flows indicated by different line colors:

2.4. System Control Strategies

The control strategies of this system are mainly composed of solar collector system and air source heat pump heating system. As illustrated in Figure 5, when the collector exit temperature is 8 °C higher than the HWT exit temperature, the circulating water pump of the collector module will turn on to channel the heat into the HWT. When the collector outlet temperature is 2 °C lower than the HWT outlet temperature, the pump will stop running [16]. At the heating end, the air source heat pump system will turn on when the HWT exit temperature is below 50 °C and will stop when the HWT exit temperature is above 60 °C.

2.5. Mathematical Model

Thermal energy Q_u from collector installations [44]:

$$Q_{\mathrm{u}}=\stackrel{·}{m}\times c_{\mathrm{p}}\times (T_{\mathrm{out}}-T_{\mathrm{in}})$$

(1)

where ṁ is the mass flow, c_p is the specific heat, and T_out and T_in are inlet and outlet water temperatures.

Collector thermal efficacy ηt is denoted by [45]:

$${\eta }_{\mathrm{t}}={\displaystyle \frac{Q_{\mathrm{u}}}{G\times A}}$$

(2)

where G shows the amount of solar radiation; A is the size of the solar collector.

The amount of electricity generated by PV is expressed as [46]:

$$E_{\mathrm{pv}}={\eta }_{\mathrm{e}}\times ({\alpha }_{\mathrm{c}}\times {\tau }_{\mathrm{G}})\times G\times A$$

(3)

where η_e is the PV efficiency, α_c is the photovoltaic absorptivity, and τ_G is the transmissivity.

The reference PV efficiency is represented as [47]:

$${\eta }_{\mathrm{e}}={\eta }_0\times [1-{\beta }_1\times (T_{\mathrm{p}}-T_0)]$$

(4)

where η₀ and T₀ reference photovoltaic efficiency and temperature; β1 is the photovoltaic efficiency coefficient; and T_p is the surface temperature.

The heat pump’s COP is determined by the following equation [48]:

$$CO{P}_{\mathrm{g}}={\displaystyle \frac{Q_{\mathrm{load}}}{N_{\mathrm{g}}}}$$

(5)

where COP_g is the heat pump’s COP, Q_load is the building heating load, and N_g is the energy consumption of the heat pump.

The heating system’s COP is derived from the following equation [49]:

$$CO{P}_{\mathrm{S}}={\displaystyle \frac{Q_{\mathrm{load}}}{N_{\mathrm{g}}+\sum N_{\mathrm{j}}}}$$

(6)

where COP_S denotes the coefficient of performance of the system and ∑N_j indicates the power consumption of the other pumps.

APF (Annual performance factor) is represented as:

$$APF={\displaystyle \frac{Q_{\mathrm{Total}}}{E_{\mathrm{Total}}}}$$

(7)

where Q_Total is the amount of heat provided by the heating system throughout the year, and E_Total is the power consumption of the system throughout the year.

The payback period (P) is calculated as [50]:

$$P={\displaystyle \frac{\mathsf{\Delta }{C}_{\mathrm{i}}}{C_{\mathrm{saving}}}}$$

(8)

where C_saving is the cost savings of the proposed system compared to the original system. ΔC_i is difference in initial investment.

Carbon emissions from electricity consumption are expressed as [50]:

$$C=\sum A_{\mathrm{j}}\cdot E{F}_{\mathrm{j}}$$

(9)

where A_j is the energy consumption, kWh; EF_j is the carbon emission factor of electrical energy, kgCO₂/kWh; and C is the carbon emissions. The EF_j factor here is taken as 0.57 kgCO₂/kWh.

3. Heating System Simulation

3.1. Simulation Setup Parameters

TRNSYS systems are often used to simulate the operating parameters of HVAC systems or solar systems, and the simulation system generally consists of several components. The components required for collector-coupled ASHP system and PV PV-coupled ASHP system are shown in

Table 1. TRNSYS parameters.

Simulation Components	Type	Component Function
Weather parameters	Type 15	Typical meteorological parameters of Wuhan throughout the year.
Solar collector component	Type 73	Collector slope is 45°. Collector fin efficiency factor is 0.7 [27].
PV component	Type 562f	Photovoltaic power generator.
Circulating water pump	Type 114	Energy transmission.
Schedule	Type 14h	Schedule.
Calculator	Equation	Customized modules.
Air source heat pump	Type 941	Devices for improving air energy.
Combiner valve	Type 11h	Combiner.
Controller	Type 165	Temperature difference controller.
Load data	Type 9e	Building load.
Air conditioning ends	Type 682	Load files.
Water storage tank	Type 4c	Heating installations.
Integrator	Type 24	Data cumulative.
Plate heat exchangers	Type 91	Heat exchangers.
Printer	Type 65c	Data output.

:

3.2. System Model

The simulation model of the collector-coupled ASHP system is shown in Figure 6. It mainly consists of the solar heating loop, indicated in red, and the ASHP heating loop, indicated in green, as well as the heating-end loop, indicated in orange. The control module, the calculation module, and the output module are also essential parts. In addition, in Figure 7, the green loop represents the photovoltaic power generation system.

4. Results and Discussion

4.1. Outdoor Meteorological Parameters

The hour-by-hour meteorological parameters of Wuhan throughout the year are shown in Figure 8, and the climate of Wuhan is in the hot-summer and cold-winter zone. The year-round outdoor maximum temperature can reach 38.7 °C, the average air temperature is 17.5 °C, and the mean outdoor water temperature is 19.3 °C. Moreover, the intensity of solar radiation in Wuhan can reach 3590.6 kJ/h·m², and solar energy resources are relatively abundant, which is conducive to the exploitation of solar energy [51].

4.2. System Thermal Performance Parameters

The heating temperatures and temperature differences for the collector coupled ASHP system are shown in Figure 9a, and the heating temperatures for the PV coupled ASHP system are shown in Figure 9b. The trend in heating temperature differences is essentially the same for both systems, with a maximum difference of no more than 5 °C. As for the heating temperature, the auxiliary heating effect of the collector causes fluctuations in the exit temperature, while the magnitude and trend are related to the strength of solar radiation. The fluctuations are smaller in winter and larger in summer. The average heating temperature was 56.9 °C for the collector-coupled ASHP system and 55.5 °C for the PV-coupled ASHP system. The relatively high average heating temperature is due to the higher outlet temperature of the solar collectors during times of high solar radiation, which raises the system heating temperature.

The collector-coupled ASHP system has three heating modes: collector heating, ASHP heating, and synergistic heating. Figure 10 shows the ASHP start signal with and without collector assistance, as well as the collector start signal. The ASHP runtime with collector assistance was reduced by 507 h, or 26.2%, relative to that without collector assistance. This means that the collector provided the rest of the required heat, and the collector was run for a total of 760 h. The life of ASHP is inversely proportional to the running time. The assistance of a collector not only relieves the pressure of ASHP operation but also effectively extends the life of ASHP.

Figure 11 shows the full-year, time-by-time COP changes for ASHP with and without PT assistance. There is little difference in the COP of the ASHP between the two forms. The mean COP for the collector-coupled ASHP was 3.1, while the mean COP for the uncoupled ASHP was 2.9. The overall trend of heating COP varies with outdoor meteorological parameters, being lower in winter and higher in summer. Moreover, the APF (Annual performance factor) of the collector-coupled ASHP system was 3.4, the APF of the ASHP alone was 2.6, and the addition of the collector improved the APF by 0.8. Finally, the average COP of the collector-coupled ASHP system was compared with that of residential heating studies in the same zone. The results showed that the system’s average COP was higher than the 2.67 reported by Li et al. and the 2.5 reported by Zhang et al [27]. This demonstrates that the collector-coupled ASHP system has a performance advantage when used for hot water heating in university dormitories.

4.3. Solar Energy Utilization Analysis

These two systems correspond to two different forms of solar energy exploitation, and the different utilization of solar energy is shown in Figure 12. Figure 12a illustrates the thermal efficiency and energy production of the collector at various time periods. The system received a total of 21,159 MWh of solar radiation throughout the year and produced a total of 2216 MWh of heat throughout the year, with a solar thermal efficiency of 10.5% throughout the year. The maximum hour-by-hour solar thermal utilization efficiency can be up to 42.2%, and the overall efficiency trend also varies with outdoor meteorological parameters. Figure 12b shows the variation of solar PV utilization efficiency. It also received 21,159 MWh of solar radiant energy while producing a cumulative total of 2534 MWh for the year, with an average photovoltaic efficiency of 12.0% for the year and a maximum hour-by-hour photovoltaic efficiency of 12.2%. Compared to the year-round trend in solar thermal efficiency, the year-round trend in PV efficiency is the opposite, showing a higher trend in winter and a lower trend in summer. This is mainly because of the high surface temperature of the PV panels in summer, which reduces the PV efficiency.

4.4. Energy Efficiency Analysis

Figure 13a illustrates the heat production share of different heat sources in the collector-coupled ASHP system. The ASHP cumulatively produces 5538 MWh of thermal energy, which occupies 71.4%, and the remaining heat contribution corresponds to the solar fraction, which is 28.6%. Figure 13b shows the cumulative energy consumption and cumulative electrical energy output of the PV-coupled ASHP system. The orange area is where energy consumption is greater than capacity, and the green area is where capacity is greater than consumption; the two scenarios are alternating. Ultimately, energy self-sufficiency reached 94.6% for the year.

Figure 14a illustrates the annual heating energy consumption of the two models. The energy consumption of the two systems does not differ much in winter, which is mainly due to the limited amount of heat produced by the collectors in winter when the solar radiation is weak. The energy-saving advantages of the collector-coupled ASHP system became progressively more apparent during the transition season and summer months. The ASHP system combined with the collector consumes 2026 MWh of electricity for the year, compared to 2679 MWh for the ASHP system alone. This means that with the addition of collector synergy heating, the annual heating energy consumption is reduced by 24.4%. Furthermore, Figure 14b compares the month-by-month net energy consumption of two different forms of the system. PV coupled ASHP systems in 6 months, in which the system’s energy production was greater than the system’s energy consumption, achieving zero energy heating. The net system energy consumption for the year was only 145.1 MWh, a 92.8% reduction compared to the collector-coupled ASHP system.

4.5. Economic and Environmental Benefits

As shown in Figure 15, the economic analysis compares two systems. Expanding the existing ASHP system with 18,872 m² of solar collectors requires an investment of CNY 7,548,800. After 1 year of combined operation, the heating cost for the solar collector-coupled ASHP system is CNY 1,175,196. This is CNY 378,624 less than the CNY 1,553,820 needed annually for the standalone ASHP system, a 24.4% reduction. The payback period for the solar collector investment is 19.9 years. Despite the long payback period, the ASHP system, coupled with collectors, is expected to have an extended lifespan and improved stability. Additionally, its performance under extreme heating conditions will be better ensured.

Similarly, adding 18,872 m² of photovoltaic installations on the basis of the existing ASHP system would require an investment cost of approximately CNY 9,436,000. The grid-connected revenue generated by the photovoltaic device in a year is approximately CNY 1,392,973. Compared with the standalone ASHP system, the net operating cost of the PV-ASHP system for the whole year is CNY 160,847, a year-on-year decrease of 89.6%. Finally, the payback period for the installation of PV is only 6.8 years. However, PV will not enhance the heating performance of ASHP.

Figure 16 shows a comparison of the annual carbon emissions of the three systems. The traditional ASHP system has an annual net carbon dioxide emission of 1,527,030 kg, while the solar collector-coupled ASHP system has an annual net carbon dioxide emission of 1,154,934 kg. After adding the solar collector, the system’s carbon emissions have decreased by 372,096 kg. The annual net carbon dioxide emission of the PV-coupled ASHP system was only 82,878 kg, which was 1,444,152 kg less than that of the ASHP system alone and 1,072,056 kg less than that of the solar collector.

4.6. Discussion

Finally, the energy-saving and economic performance of the system under different roof laying ratios were discussed. As shown in Figure 17, during the process of increasing the proportion of solar collectors laid on the roof from 50% to 100%, the operating energy consumption and operating costs of the system gradually decreased. It can be found here that the reduction in energy consumption and operating costs from 50% to 75% of the land area is greater than that from 75% to 100%. However, the initial investment cost curve of the collector rises with a constant slope. Therefore, it is necessary to optimize the solar collector coupled with an air source heat pump system in the future to balance the heating performance of the system and the initial investment cost.

As shown in Figure 18, during the process of increasing the PV laying ratio from 50% to 100%, the system’s operating energy consumption, operating costs, and initial investment costs basically all increase or decrease at a constant slope. When the PV installation area reaches 100% of the available solar energy area on the roof, the system’s annual net energy consumption is 145.1 MWh, and the net operating cost is CNY 168,047. The photovoltaic power generation of the system can offset 94.6% of the system’s power consumption, theoretically achieving a nearly zero-energy consumption building.

5. Conclusions

• The ASHP runtime with collector assistance was reduced by 507 h, or 26.2%, relative to that without collector assistance. The assistance of a collector not only relieves the operating pressure of ASHP but also effectively extends the service life of ASHP. In addition, the APF of the collector-coupled ASHP system was 3.4, whereas the APF of the air source heat pump system without PT assistance was 2.6, and the addition of the collector increased the APF by 0.8. And the collector-coupled ASHP system has a solar fraction of 28.6%.
• The collector-coupled ASHP system has an average solar thermal efficiency of 10.5% throughout the year, with a maximum hour-by-hour solar thermal efficiency of up to 42.2%. The cumulative annual generation reached 2534 MWh from the PV coupled ASHP system, with an average photovoltaic efficiency for the year of 12.0% and a maximum hour-by-hour photovoltaic efficiency of 12.2%.
• The collector-coupled ASHP system reduces heating energy consumption by 24.4% throughout the year. The PV-coupled ASHP system achieved 94.6% annual energy self-sufficiency, reducing net consumption by 92.8% compared to the collector coupled configuration.
• Installing a solar collector device on the basis of the existing air source heat pump can reduce the annual operating cost by CNY 378,624, a year-on-year decrease of 24.4%. The installation of PV devices can reduce operating costs by 89.6% annually, and the payback period for installing PV is only 6.8 years. In addition, installing solar collectors can reduce carbon emissions by 372,096 kg annually, and the net carbon dioxide emissions after installing PV are only 82,878 kg.

Both systems have their own advantages, and the collector’s greater advantage lies in the synergistic heating aspect, which can both relieve the pressure of ASHP operation and extend the service life of ASHP, as well as slow down the frost phenomenon of ASHP in winter and, thus, improve the stability of heating. The advantage of PV lies in the energy aspect, which generates greater energy savings through photovoltaic power generation without interfering with the heating of the system, achieving near-zero energy heating.