Research on Energy Savings of an Air-Source Heat Pump Hot Water System in a College Student’s Dormitory Building

Abstract

Centralized hot water systems are commonly installed in college student dormitories, representing a typical application for such systems. To achieve sustainable and environmentally friendly heating solutions, air-source heat pump hot water systems have gained attention for their high efficiency and energy-saving characteristics. By implementing heat pump technology, China could make significant progress towards achieving its carbon neutrality goals by reducing energy consumption and associated emissions. In this study, the heating performance of an air-source heat pump hot water system was tested in the field over the course of a year at a university in Chongqing, China. A simulation model was constructed using TRNSYS software, and a time-sharing control strategy was proposed to analyze the system’s operating characteristics and energy-saving performance. Results showed a 6% increase in unit annual average Coefficient of Performance (COP) and annual electricity savings of 10,027 kW·h, with an energy-saving rate of 8.77% after time-sharing control. The study highlights the significant economic and environmental benefits of adopting sustainable energy solutions, particularly in the context of increasing global greenhouse gas emissions.

1. Introduction

Against the backdrop of resource and environmental constraints and the global shift towards low-carbon transformation, the active promotion of green and low-carbon development and the reduction in carbon emission intensity has become imperative for China to build an ecological civilization [1]. This is in line with the global efforts towards sustainable development and the urgent need to reduce carbon emissions, particularly in the face of environmental challenges such as climate change. According to statistics, the total energy consumption of buildings in China has exceeded 33% of the total social energy consumption, of which the energy consumption for domestic hot water accounts for about 20% of the total building energy consumption, and the energy consumption for domestic hot water in commercial buildings can even reach up to about 40% [2]. Air-source heat pump hot water systems have the characteristics of high efficiency, energy-saving, environmental protection, and strong adaptability [3]. The heating efficiency of producing the same amount of hot water is 2–3 times that of traditional boilers [4]. It has already achieved widespread application in China, and the further application of heat pump technology will have a significant energy-saving and emission reduction effect on China’s future goal of peaking carbon emissions and achieving carbon neutrality.

Scholars, both domestic and foreign, have conducted research on air-source heat pump systems in two main aspects. Firstly, studies have been conducted on the performance of air-source heat pump hot water systems. Silva et al. [5] conducted a sensitivity analysis of air-source heat pump systems in nine cities in Australia, analyzing their COP, heat output, thermal loss, thermal storage capacity, and set temperature of the water tank. The researchers found that the energy consumption and water supply temperature of the air-source heat pump hot water system are related to the unit’s COP and electricity cost. Yokoyama et al. [6] used experiments and numerical simulations to analyze the impact of environmental factors such as ambient dry-bulb temperature and tap water temperature on the performance of CO₂ heat pump hot water systems in urban areas. Kara et al. [7] conducted theoretical analysis and research on the transient characteristics of water tanks in hot water systems using a mathematical model. They obtained the influence of system performance parameters on the duration of heating and the water tank set temperature, and they determined the optimal flow range of the circulation pump. Ding et al. [8] proposed a fine-tuned system operating strategy based on the hourly variation of user-side heat load. By considering various influencing factors and optimizing the campus’s actual situation, the researchers found that the system could save 38.9% of energy consumption and increase the performance coefficient by 2.24 times when the operating strategy was at the optimal state point, without affecting user thermal comfort, providing a reference for the optimized operation parameter setting of heat pump hot water units. Tangwe et al. [9] predicted the linear relationship between the COP of air-source heat pump systems and the predictive factors in different hot water usage scenarios using mathematical models and simulation applications. They found that environmental conditions are the main factor contributing to COP, with a weight of five times that of other factors. Amirirad et al. [10] analyzed the application of air-source heat pumps in severe cold regions and obtained the impact of the performance of air-source heat pump units and other critical factors on the overall energy consumption of buildings, providing an accurate prediction of the performance of air-source heat pump water heaters (ASHPWH) in severe cold regions. Kim et al. [11] studied the hourly heat exchange of the entire system, including the unit and the hot water pipe network, through a power model of an air-source heat pump system. They found that the system’s heat loss is proportional to the size of the water tank, while the instantaneous performance value is inversely proportional to the size of the water tank. Amirirad et al. [12] analyzed the application of air-source heat pump hot water systems in cold regions using the TRNSYS software. They found that the ambient temperature and humidity have a significant impact on ASHPWH, and the system achieves the best COP at 4 °C and 35% RH. The system increases cooling energy consumption by 3% annually but still saves 55% compared to traditional electric water heaters. Pospisil et al. [13] controlled the system’s start and stop by predicting the ambient temperature in the next 48 h, ensuring that the system runs at the highest temperature period of the day to improve system efficiency. Wang Y et al. [14] from Tianjin University analyzed the COP of an air-source heat pump system under different ambient dry bulb temperature conditions using an enthalpy difference test bench. They found that the ambient temperature and water temperature in the tank are key factors affecting ASHPWH performance.

Secondly, studies have been conducted on control strategies for air-source heat pump water heaters. Marincowitz et al. [15] proposed a fine-grained system operation strategy based on the hourly changes of user-side heat load. When the system operates at the optimal state point, the energy consumption can be reduced by 38.9% without affecting the user’s thermal comfort, and the system performance coefficient is also improved by 2.24 times. Lu et al. [16] proposed a temperature-based control strategy and optimized it for a university campus in a cold region, achieving 38.18% energy savings while meeting the heating demand. Fischer et al. [17] studied the impact of different control strategies and boundary conditions on the performance of air-source heat pump hot water systems. By using a predictive control method, the annual operating cost of the system can be reduced by 6–11%, resulting in a 2–4% cost reduction. Zhou et al. [18] proposed an ASHP temperature and hydraulic balance control strategy that not only improved the heating effect of the system but also saved 38.6% of heat and 31.5% of electricity consumption, as well as reducing environmental emissions by 37.2%. Jing [19] established a TRNSYS simulation model based on actual heating operation data from a campus, and by adopting a time-sharing optimization control strategy, the theoretical energy-saving rate of the entire heating system can reach as high as 47.76%. The Improvement of controls for centralized hot water plants in buildings plays a crucial role in different energy scenarios and diverse climate conditions. It holds significant importance not only in terms of its relationship with heating systems but also in meeting health requirements, ensuring occupant comfort, and promoting energy efficiency and conservation [20].

This study investigates the effects of time-sharing control on the operation of an air-source heat pump hot water system in a university student dormitory building in Chongqing, China. The study employs on-site testing and simulation validation using TRNSYS software to objectively evaluate the energy efficiency of the building’s hot water system through energy consumption analysis and provide reference data for the design of hot water systems in other buildings.

By improving the control methods of the hot water system, time-sharing control is added on the basis of water level control and water temperature control, so as to set the time period for students to use water intensively, which not only reduces the heat loss of the system and improves the energy efficiency of the system but also protects the environment. By identifying opportunities for energy savings and increased efficiency, this study underscores the importance of sustainable practices in building energy management, particularly in the higher education sector. The findings of this study may have significant implications for the development of sustainable and eco-friendly heating systems in other regions with similar climatic conditions.

2. Experimental Activity

2.1. The Investigated System

This study focuses on the testing and analysis of an air-source heat pump hot water system in a college student dormitory building located in Chongqing, China. The dormitory building, which is designed to accommodate 960 people, has 8 floors, with 30 four-person rooms on each floor. In a bid to replace the original natural gas boiler, the dormitory building was one of the first schools in Chongqing to renovate its hot water system using an air-source heat pump unit. Chongqing is characterized by a hot summer and cold winter climate. The system supplies 45 °C hot water to the student apartment building’s bathrooms continuously for 24 h. All the equipment is installed at the top of the dormitory building, as illustrated in Figure 1.

The system comprises five high-temperature direct heating units and one medium-temperature circulating unit, along with insulation water tanks, water pumps, and pipeline valves. As illustrated in Figure 2, it operates in two modes: direct heat water production and circulation heat preservation. In the direct heat water production mode, the water level value of the water tank is automatically controlled. When the water level of the water tank is lower than 75%, the direct heat water production mode initiates. Cold water enters the high-temperature direct heat cycle unit from the tap water pipe, and the generated high-temperature hot water is directly supplemented into the water tank until it reaches 100% water level before closing.

In the circulation heat preservation mode, the circulation heat preservation mode adopts automatic temperature control. When the water level of the water tank reaches the set value, if the hot water temperature in the insulation water tank is lower than 40 °C, the circulating insulation mode starts. During this mode, the direct heat mode of the same unit will not be activated. The mode circulates the substandard hot water in the insulation water tank through the circulation pipeline to the high-temperature direct heating unit and the medium-temperature circulating unit, with the circulation temperature difference set to around 5 °C. After heating, the hot water re-enters the insulation water tank until the water temperature in the tank reaches the set temperature value and then stops. When the temperature sensor controlling the return water in the pipeline detects that the stored water temperature in the pipeline is lower than 40 °C, the hot water booster pump starts. The hot water in the water tank will then enter the pipeline to flush back the stored water in the pipeline to the insulation water tank, ensuring that the stored water temperature in the pipeline reaches the set value.

2.2. Description of Testing Rig

The primary objective of this study is to determine the COP of an air-source heat pump hot water unit in both the direct heat water production mode and circulation heat preservation mode, under typical daily environmental conditions in Chongqing over the course of a year. The hot water system being tested is located in a college student dormitory building in Chongqing and includes two types of units: five high-temperature direct heating hot water units (model RSJ-380/S-820) with a rated heating capacity of 38.5 kW, and a brand-new medium-temperature circulating hot water unit (model RSJ-380/MSN1-H (E2)) with a rated heating capacity of 37.5 kW.

To carry out this test, a range of high-precision instruments were utilized, such as PT100 adhesive temperature sensors, paperless recorders, ultrasonic flow meters, three-phase clamp power meters, and temperature and humidity data loggers. The main model parameters of these instruments are presented in

Table 1. Main test instrument parameters.

Instrument	Model	Parameter	Quantity	Test Subject
Adhesive temperature sensor	PT100	−70–200 °C	4	In/Out Water Temperature
Paperless recorder	RX4116A	−80–200 °C	2	In/Out Water Temperature
Ultrasonic flowmeter	DTFX1020	DN20 mm–4000 mm	2	Flow rate
Three-phase clamp power meter	3169-20/21	75.00 W–900.0 kW	2	Unit power
Temperature and humidity recorder	UX100-011	−20–70 °C	12	Ambient dry bulb temperature

. This study aims to provide valuable insights into the performance of the air-source heat pump hot water unit and contribute to the development of sustainable energy systems.

Photos of the main testing instruments are illustrated in Figure 3.

2.3. Testing Procedure

According to the heating principle of air-source heat pumps, the heating performance coefficient $CO{P}_H$ of the unit is calculated according to the following formula [21,22]:

$$CO{P}_H=Q_H/W_H$$

(1)

where $CO{P}_H$ represents the heating performance coefficient of the unit; $Q_H$ represents the average heating capacity of the unit during the test period, in kW; $W_H$ represents the average input power of the unit during the test period, in kW.

The average heating capacity $Q_H$ of the unit during the test period is calculated as follows:

$$Q_H=V\rho C\Delta t_w/3600$$

(2)

where $V$ represents the average flow rate of the unit in m³/h; $\rho$ represents the density of water in kg/m³; $C$ represents the specific heat capacity of water at constant pressure in kJ/(kg·°C); and $\Delta t_w$ represents the temperature difference between the inlet and outlet water of the unit in °C.

2.4. Data Analysis

2.4.1. Data Processing

To ensure representative test conditions, typical days were selected for each month, and the average dry bulb temperatures for Chongqing were obtained from the National Meteorological Data Center, as illustrated in Figure 4. Specifically, the average temperature during December was found to be 10.0 °C.

Through experimental testing, three points were set up around the air-source heat pump hot water unit to test the ambient dry bulb temperature on typical days from 9 December to 11 December, and the specific temperature change is illustrated in Figure 5. By calculation, the average temperature of these three days is 10.4 °C, which is close to 10.0 °C from the National Meteorological Data Center, so typical daily data from the National Meteorological Data Center are reliable for its monthly calculation.

To ensure comprehensive data collection, the direct heat water production mode of the system was tested during three time periods each day, namely 10:00–12:00 in the morning, 14:00–18:30 in the afternoon, and 23:00–23:40 at night, with an average duration of about seven hours and ten minutes per day. The circulation heat preservation mode was tested during similar time periods, mainly from 9:00–12:00 in the morning, 14:00–17:00 in the afternoon, and 20:30–22:00 at night, with an average duration of about seven hours and thirty minutes per day. The hourly data curves for water flow rate, inlet and outlet water temperature, inlet and outlet water temperature difference, and power from December 9 December to 11 December are illustrated.

Figure 6 illustrates the hourly data curves for water flow rate, inlet and outlet water temperature, inlet and outlet water temperature difference, and power during the test period. The air-source heat pump hot water system operated in direct heat water production mode and circulation heat preservation mode, with the former producing water at a flow rate of approximately 1.19 m³/h in the morning, 0.78 m³/h in the afternoon, and 0.75 m³/h in the evening, while the latter had a flow rate of about 6.05 m³/h in the morning, 6.25 m³/h in the afternoon, and 6.25 m³/h in the evening. The inlet and outlet water temperature differences were around 18.35 °C, 29.97 °C, and 29.03 °C in the morning, afternoon, and evening, respectively, for the direct heat water production mode, and about 4.89 °C, 4.78 °C, and 4.90 °C in the morning, afternoon, and evening, respectively, for the circulation heat preservation mode. During operation, the power remained relatively constant, at around 8.44 kW for the direct heat water production mode, and about 11.85 kW for the circulation heat preservation mode.

2.4.2. System COP of Different Operating Modes

Based on the test results from 9 to 11 December, the heating performance coefficients of the air-source heat pump hot water unit in the direct heat water production mode and the circulation heat preservation mode during three different time periods on a typical day in December can be obtained [23]. The calculation results are illustrated in Figure 7.

In the same way, the typical days of other months of the year were tested, and the outdoor environment dry bulb temperature range of the whole year is 5–30 °C. The heating performance coefficient of the air-source heat pump water heater unit in the direct heat water production mode and the circulation heat preservation mode varies with the outdoor environment dry bulb temperature, as illustrated in Figure 8.

According to the calculation results, it can be seen that during the operation of the unit in direct heat water production mode, as the outdoor dry bulb temperature rises, the system’s heating performance coefficient increases. The test results were fitted with a polynomial, and the relationship between the COP of the unit in direct heat water production mode and the outdoor dry bulb temperature $T_w$ is as follows:

$$COP=2.1\times {10}^{(-5)}\times x^4-1.4\times {10}^{(-3)}\times x^3+2.8\times {10}^{(-2)}\times x^2+0.12\times x+2.5$$

(3)

According to the calculation results, it can be seen that during the operation of the unit in circulation heat preservation mode, as the outdoor dry bulb temperature rises, the system’s heating performance coefficient increases. The test results were fitted with a polynomial, and the relationship between the COP of the unit in circulation heat preservation mode and the outdoor dry bulb temperature $T_w$ is as follows:

$$COP=-0.9\times {10}^{(-5)}\times x^4+7.8\times {10}^{(-4)}\times x^3-2.5\times {10}^{(-2)}\times x^2+0.36\times x+1.1$$

(4)

According to the test results, the annual average COP of the direct heat water production mode is higher than that of the circulation heat preservation mode. Therefore, reasonably increasing the proportion of running time of the direct heat water production mode can effectively improve the energy-saving effect of the system.

2.4.3. Annual Energy Consumption

Figure 9 illustrates the electricity consumption patterns of the direct heat water production and circulation heat preservation modes, highlighting their dependence on the outdoor temperature and academic calendar. Notably, the electricity consumption in December is significantly higher than in other months, while electricity consumption reduces during winter and summer vacation months, as opposed to when classes are in session. The direct heat water production mode showed decreasing electricity consumption with rising outdoor temperatures, whereas the circulation heat preservation mode demonstrated similar trends but with a notable increase in electricity consumption during winter and summer vacation months. This is attributed to reduced hot water demand, increased water storage in the pipeline, and heat loss, thereby increasing the operating time of the circulation heat preservation mode.

The operating time of both modes followed similar trends to the electricity consumption, with the circulation heat preservation mode recording its longest operating time in February, reaching 544 h, while other months’ operating times did not exceed 300 h. The direct heat water production mode allows for constant outlet water temperature, which facilitates efficient preparation of the required hot water. On the other hand, the circulation heat preservation mode operates efficiently at inlet and outlet water temperature differences below 45 °C. When operating in high-temperature water areas (48–60 °C), the compressor’s energy consumption increases due to the high condensing temperature of the unit, which significantly reduces the COP of the unit, thereby increasing the cost of hot water. Consequently, controlling the hot water temperature at 45 °C would be essential in enhancing the system’s energy efficiency.

2.4.4. Heating Seasonal Performance Factor (HSPF)

The performance parameter of the entire system is represented by the HSPF as the ratio of useful delivered energy (not power) over energy consumed [24]. The demand for hot water supply in university student dormitories exhibits strong regularity, with peak usage times concentrated after 9 p.m., followed by 6–9 a.m. and 6–9 p.m. During these peak water usage periods, the heating demand for hot water is substantial. Throughout 2022, the project operated normally, and actual measurements were taken to obtain monthly values of water consumption and electricity consumption from January 2022 to December 2022. As illustrated in Figure 10a, the total water consumption for the year was 9173 tons, and the total electricity consumption was 110,054 kW·h. The direct heat water production mode consumed 87,878 kW·h of electricity, and the circulation heat preservation mode consumed 22,176 kW·h of electricity. Combining the average system COP for the direct heat water production mode and the circulation heat preservation mode, the actual test results show that the total heat production for the year was 342,400 kW·h, including 290,900 kW·h for the direct heat water production mode and 51,500 kW·h for the circulation heat preservation mode.

The water usage during regular class months follows a clear pattern, with slightly lower usage during the summer transitional period, a significant increase during the winter months, and a sharp decrease in February and August during the holidays. This pattern is consistent with the building’s characteristics. In January and December, the low outdoor temperature leads to increased heat loss, resulting in higher electricity consumption and lower unit efficiency. In contrast, during July, the high outdoor temperature results in lower heat loss and lower electricity consumption, and higher unit efficiency. Thus, electricity consumption exhibits an inversely proportional relationship with the outdoor temperature trend.

Figure 10b illustrates the HSPF of the heat pump system, which was calculated for each month and compared with the system COP in the direct heat water production and circulation heat preservation modes. Apart from February and August, the HSPF trend is similar to that of the system COP, being lower than that of the system in direct heat water production mode and comparable to that of the system in circulation heat preservation mode. The seasonal environmental temperature has a significant impact on the system COP, with the maximum value observed in July and the lowest value in January and December. Conversely, HSPF is substantially affected by the winter and summer vacations in February and August, during which the hot water demand decreases due to a reduced number of students. This reduction leads to an increase in heat loss in the hot water pipeline network and insulation water tank, resulting in a higher operation time for the circulation heat preservation mode and an elevated cost of hot water. Therefore, by prolonging the operation time of the direct heat water production mode and reducing the operation time of the circulation heat preservation mode, the energy efficiency of the system can be effectively enhanced.

3. Modeling Activity

The original system was simulated using TRNSYS software to compare with the experimental results in order to infer the feasibility of the simulation. Furthermore, the control strategy of the hot water system was optimized from the original water level control to the time-sharing control to improve the energy-saving effect of the original system.

3.1. Model Establishment

The model is used to simulate the system’s energy consumption, and data such as the tank water level, water consumption, heating performance coefficient of each unit, heat pump outlet temperature, tank temperature, unit heating power, unit heat supply, and heat pump power are also used. The simulation results are compared with the actual measured data to verify whether the model meets the requirements.

3.1.1. Mathematical Model Establishment

(1)

Mathematical Model of the Water Tank

This system adopts a variable-volume water level tank model, which facilitates the simulation of the direct heat water production mode and circulation heat preservation mode of the air-source heat pump hot water unit controlled by changes in the water level of the tank. The mathematical model of the variable-volume water tank can be simplified to a completely mixed variable-mass water, and the calculation method for describing the changes in the mass and internal energy of the water in the tank is as follows [25,26,27]:

$$\frac{dM}{dt}=m_i-m_O$$

(5)

$$C_{pf}\frac{d(MT)}{dt}=m_i{C}_p{T}_h-m_O{C}_{p}T-(UA{)}_t(T-T_{env})$$

(6)

where M is the mass of liquid in the water tank; T is the temperature of the liquid in the water tank; $m_i$ is the net inflow rate of the water tank; $m_O$ is the net outflow rate of the water tank; $C_{pf}$ is the specific heat capacity of the liquid in the water tank; $T_h$ is the temperature of the liquid entering the water tank; ${(UA)}_t$ is the total thermal conductance of the water tank heat loss; and $T_{env}$ is the environmental temperature of the water tank loss.

The calculation methods for the final mass and average mass within a given time variation are as follows:

$$M_{\tau }=M_{\tau -△t}+(m_i-m_O)∆t$$

(7)

$$\overline{M}=\frac{M_{\tau }+M_{\tau -∆t}}{2}$$

(8)

where $M_{\tau }$ represents the final mass of the liquid in the water tank after the given time interval; $M_{\tau -∆t}$ represents the initial mass of the liquid in the water tank before the given time interval; △t is the time interval; $\overline{M}$ represents the average mass of the liquid in the water tank over the given time interval.

(2)

The Control Model of the Heat Source Side

The direct heat water production mode is used by the unit to heat cold water to high-temperature hot water and supply it to the storage and insulation water tank. The main control parameter is the water level $L_s$ of the tank. When the water level switch detects $L_s$ < 75%, the direct heat water production mode starts and continues until the water level switch detects the water level reaching the 100% set value. The mathematical model of the water level switch control can be calculated based on the on-off principle [28]. It is calculated as follows:

$$\left\{\begin{array}{c}{\beta }_{\mathrm{c},\mathrm{t}-1}=0 and L_{s,i}<L_{s,1},{\beta }_{\mathrm{c},\mathrm{t}}=1\\ {\beta }_{\mathrm{c},\mathrm{t}-1}=0 and L_{s,i}\ge L_{s,1},{\beta }_{\mathrm{c},\mathrm{t}}=0\\ {\beta }_{\mathrm{c},\mathrm{t}-1}=1 and L_{s,i}<L_{s,0},{\beta }_{\mathrm{c},\mathrm{t}}=1\\ {\beta }_{\mathrm{c},\mathrm{t}-1}=1 and L_{s,i}\ge L_{s,0},{\beta }_{\mathrm{c},\mathrm{t}}=0\end{array}\right.$$

(9)

where ${\beta }_{\mathrm{c},\mathrm{t}-1}$ represents the output value of the water level switch control at the previous moment, where 1 indicates “running” and 0 indicates “stopped”; ${\beta }_{\mathrm{c},\mathrm{t}}$ represents the current output value of the water level switch control, where 1 indicates “running” and 0 indicates “stopped”; $L_{s,0}$ represents the set water level value of the storage and insulation water tank, expressed as a percentage; $L_{s,1}$ represents the next set water level value of the storage and insulation water tank, expressed as a percentage; $L_{s,i}$ represents the instantaneous water level value of the storage and insulation water tank, expressed as a percentage.

The circulation insulation mode is only activated when the water level in the storage and insulation water tank $L_s$ is greater than or equal to 75%, and it mainly relies on temperature sensors to control the start and stop of the circulating hot water pump. When the temperature sensor detects that the water temperature $T_s$ in the tank is lower than 40 °C, the circulation mode starts and stops only when the temperature reaches the set temperature. The mathematical model of the water temperature switch control is obtained based on its start-stop control principle, and the calculation method is as follows:

$$\left\{\begin{array}{c}{\beta }_{x,t-1}=0 and T_{s,i}<T_{s,1},{\beta }_{\mathrm{c},\mathrm{t}}=1\\ {\beta }_{x,t-1}=0 and T_{s,i}\ge T_{s,1},{\beta }_{\mathrm{c},\mathrm{t}}=0\\ {\beta }_{x,t-1}=1 and T_{s,i}<T_{s,0},{\beta }_{\mathrm{c},\mathrm{t}}=1\\ {\beta }_{x,t-1}=1 and T_{s,i}\ge T_{s,0},{\beta }_{\mathrm{c},\mathrm{t}}=0\end{array}\right.$$

(10)

where ${\beta }_{x,t-1}$ represents the output value of the temperature switch control at the previous moment, where 1 means running and 0 means stopping; ${\beta }_{x,\mathrm{t}}$ represents the instant output value of the temperature switch control, where 1 means running and 0 means stopping; $T_{s,0}$ is the set temperature value of the thermal insulation water tank, in °C; $T_{s,1}$ is the next set temperature value of the thermal insulation water tank, in °C; $T_{s,i}$ is the instant temperature value of the thermal insulation water tank, in °C.

3.1.2. Model Establishment of the Hot Water System

After completing the module call, parameter setting, calculation formula preparation input, and external file development for this dormitory hot water system according to the actual test content, five air-source heat pump hot water units of model RSJ-380 were set up in conjunction with the actual situation. The modules were correlated according to temperature, flow rate and control relationship, etc. After continuous debugging [29,30,31], the system model was obtained, as illustrated in Figure 11.

3.2. Model Validation of the Hot Water System

(1)

Water Production

The system model was employed to simulate the annual energy consumption of the system, and the accuracy of the model was verified by comparing the simulation results with the measured data. Figure 12 demonstrates that the annual water production of the simulated system was 9215 tons and the measured annual water consumption was 9173 tons, resulting in a simulation error of only 0.46%, indicating good consistency between the model and the actual data. During winter and summer vacations in February and August, the hot water usage was significantly reduced, whereas during the remaining months, the hot water consumption increased with decreasing outdoor temperature. Moreover, the monthly water consumption values obtained from the simulation were in close agreement with the actual measurement results.

(2)

Energy Consumption

Figure 13 illustrated the system’s total annual power consumption at 114,397 kW·h, with the direct heat water production mode accounting for 91,434 kWh and the circulation heat preservation mode accounting for 22,963 kW·h. Although slightly higher than the actual test results of the total annual power consumption at 110,054 kW·h, with the direct heat water production mode consuming 87,878 kW·h and the circulation heat preservation mode consuming 22,176 kW·h, the error remains within 3.95%. The simulated results align with the measured data values, and the trend remains consistent.

According to the actual test analysis, the direct heat water production mode energy consumption accounts for 79.85% of the annual energy consumption, while the simulation results show that the direct heat water production mode energy consumption accounts for 79.93% of the energy consumption, which is not significantly different. The direct heat water production mode and circulation heat preservation mode energy consumption increase as seasonal temperature decreases, with direct heat water production mode showing greater variation in energy consumption. Furthermore, the energy consumption of the direct heat water production mode decreases notably in February and August due to the influence of students leaving school during winter and summer vacations, aligning with the actual results. Figure 14 illustrates the monthly operating hours of the direct heat water production and circulation heat preservation modes, which exhibit similar trends and values to measured results.

(3)

COP

In order to verify the heat production capacity of the system, we will mainly analyze the heat production performance of a single unit and the total heat production capacity hour by hour (month by month) throughout the year. As illustrated in Figure 15, the simulation results demonstrate that the heat production performance of a single unit increases linearly with the seasonal temperature, and the COP also increases accordingly. The highest heating capacity is observed in July, with an average COP of 4.62, while the worst heating capacity is observed in January, with an average COP of 2.86.

The simulation results were compared with the actual measurement results, as illustrated in Figure 16. The maximum monthly error was found to be less than 2.09%, with most months falling around 1%, resulting in an average error of 1.14%. Overall, the TRNSYS simulation software successfully simulated the modified air-source heat pump hot water system’s hourly heat supply and hourly heat production performance coefficients of a single unit in the direct heat water production mode throughout the year, showing consistency in values and change trends with the experimental test results.

Figure 17 illustrates that the heat production of the system is low during February and August due to the winter and summer vacation periods. In January, half of the time is also affected by the winter vacation period, which has some influence on the heat supply value. From March to July, heat production decreases linearly as the temperature rises, and from September to December, it increases linearly. The simulation results show that the system has an annual heat production capacity of 351,200 kW·h, with 290,900 kW·h produced in the direct heat water production mode and 53,300 kWh in the circulation heat preservation mode. This is only a 2.57% error compared to the actual test result of 342,400 kW·h of heat production for the whole year, with 290,900 kW·h in the direct heat water production mode and 51,500 kW·h in the circulation heat preservation mode. Therefore, the simulated annual heat production capacity of the system is consistent with the actual test results in terms of value and change trend.

Through a comparison between simulation and experimental results, it was found that the water yield error was only 0.46%, indicating that the water yield values were almost identical. The total annual energy consumption, as well as the energy consumption of the direct heat water production mode and the circulation heat preservation mode, had an error of approximately 3.95%, indicating that energy consumption was consistent. The annual average COP error of the unit was 2.49%, and the variation trend of the unit COP month by month was almost identical. Therefore, the simulation model was successful in replicating the actual system operation process based on the analysis of water yield, energy consumption, and COP results.

3.3. Improvement of Control Methods

The original hot water system control model is only the water level control and water temperature control of the water tank, and the user can obtain hot water all day long. As a result, the heat loss of the water tank and the pipeline is large, and the energy waste is serious. Therefore, through the increased time-sharing control method, the running time of the hot water system is reduced, and the time proportion of the direct heat water production mode and the circulation heat preservation mode is controlled at the same time, thereby maximizing the COP of the hot water system.

The control methods are improved with a focus on initial investment control, by adopting a time-sharing control strategy for tank water level control in the original air-source heat pump hot water system [32,33]. This improvement of control methods aims to improve the energy efficiency of the heat production unit. The simulation results of the optimized model are compared with the original model to analyze the energy-saving potential after optimization. The conclusions drawn from this analysis can provide valuable references for the design and renovation of air-source heat pump hot water systems in student dormitories [34].

The direct heat water production mode of the air-source heat pump hot water system is the main energy-consuming part, accounting for 80.84% of the system’s total energy consumption. The proportion of energy consumption of the direct heat water production mode and the circulation heat preservation mode in each month of the year is illustrated in Figure 18. It is evident that, in normal class months, the direct heat water production mode energy consumption accounts for 87% and above. During the summer and winter months, the figures stand at 16% and 33%, respectively. Therefore, improving the heat production performance coefficient of the direct heat water production mode is of the utmost importance. Additionally, the heating performance coefficient during the operation of the circulation heat preservation mode is low, and in order to reduce the overall energy consumption of the system, the operation time of the circulation heat preservation mode should be minimized.

The characteristics of the building heat demand of the university student dormitory were combined with the hot water demand time and the demand comparison rule to ensure year-round hot water use. A specific timing control method was devised and presented in

Table 2. Average water production and operation time of a single unit.

Time (Month)	Daily Demand (Ton)	Daily Water Production (m3/h)	Running Time (h)	Single Unit Runtime (h)	Set Duration (h)	Set Time
1	50.54	0.60	83.93	16.79	18	0:00–2:00, 8:00–24:00
2	2.10	0.66	3.19	0.64	1	15:00–16:00
3	29.91	0.83	36.01	7.20	9	11:00–20:00
4	29.70	1.05	28.40	5.68	6.5	13:00–19:30
5	27.63	1.23	22.48	4.50	5.5	13:30–19:00
6	25.42	1.47	17.31	3.46	4	14:00–18:00
7	23.27	1.63	14.31	2.86	3.5	14:00–17:30
8	2.67	1.71	1.56	0.31	0.5	15:00–15:30
9	28.62	1.56	18.39	3.68	4.5	14:00–18:30
10	29.89	1.09	27.44	5.49	6.5	13:00–18:30
11	34.41	0.89	38.68	7.74	10	11:00–21:00
12	41.57	0.66	62.72	12.54	15	9:00–24:00

to ensure the air-source heat pump hot water unit runs efficiently at regular intervals. To improve the accuracy of data on the impact of holidays, the daily demand and running time calculation only considered normal school period water parameters, with December having the largest monthly hot water demand of 41.57 tons. As the winter temperature is low, the heat production performance coefficient of the unit is not high, with a single unit producing only 0.66 tons of direct heat per hour. Furthermore, the highest demand for the units occurs during the non-holiday time in January, requiring an average of 16.79 h of operation per day. During the transitional seasons of March–May and October–November, the daily demand decreases, and the energy efficiency of the unit increases, resulting in a calculated average daily running time of 5–8 h for a single unit.

In the summer, the daily demand for hot water is similar to that of the transitional season, but the heating performance coefficient of the unit increases significantly, reducing the running time of the unit. Additionally, the average daily running time of a single unit is also significantly reduced in the summer and winter because the demand for hot water decreases due to a decrease in the number of people. Typical days in Chongqing were analyzed in each quarter, and it was observed that the daily temperature was high from 11:00 am to 10:00 p.m. Taking into consideration the actual monthly operating hours and a certain amount of water production design surplus to ensure a normal hot water supply, the monthly set hours are presented in Table 2 to improve the energy efficiency of the system.

On the basis of the original system model, according to Table 2, to establish the average water production and operating hours of a single unit to establish the building user hot water demand time file, while establishing the hot water demand module, to obtain the system air-source heat pump unit direct heat water production mode daily opening time, the simulation of the improved system, the improved system schematic diagram is illustrated in Figure 19.

4. Results

The improved hot water system has been analyzed with regard to heat production, energy consumption, and COP, yielding the following analytical results.

4.1. Heat Production of Improved System

The results of the study indicated that the heat production of the centralized hot water system is linearly related to the hot water demand, while the end water demand remains constant and the hot water temperature is affected by the heat production, as illustrated in Figure 20. The simulation results showed that the annual heat production of the system was 351,200 kW·h before improvement, with 297,900 kW·h from the direct heat water production mode and 53,300 kW·h from the circulation heat preservation mode. However, after the improvement of the control strategy, the annual heat production was reduced to 342,100 kW·h, with 297,900 kW·h from the direct heat water production mode and 44,200 kW·h from the circulation heat preservation mode. Furthermore, the water temperature of the tank remained at 40–45 °C throughout the year, with an average annual temperature of 42.34 °C before improvement and 42.62 °C after improvement, representing a significant improvement of 0.28 °C.

Based on the findings, it can be inferred that increasing the time-sharing control strategy can greatly reduce the system heat loss by extending the time of the direct heat water production mode and reducing the time of the circulation heat preservation mode. This can result in enhanced user-side water satisfaction and energy savings.

4.2. Energy Consumption of Improved System

As illustrated in Figure 21, after optimizing the air-source heat pump hot water system with time-sharing control, the annual energy consumption decreased from 114,400 kW·h to 104,400 kW·h, resulting in an annual energy saving of 10,027 kW·h, which corresponds to an energy-saving rate of 8.77%. The direct heat water production mode yielded the highest energy-saving rates in February and August, with respective energy-saving rates of 29.82% and 24.51%. However, the energy-saving effect was less significant in January and December due to weather conditions and lower hot water demand, with an energy-saving rate of only 0.89% and 2.74%, respectively. The energy-saving rate was good for the other months, ranging from 5.41% to 9.93%.

Improving the control methods of the air-source heat pump hot water system resulted in a reduction in energy consumption from 91,433 kW·h to 86,255 kW·h, achieving an annual energy-saving rate of 5.66% in the direct heat production mode. The circulation heat preservation mode demonstrated even greater energy-saving rates, ranging from 18.16% to 33.75% in January, March–May, and October–December. The highest energy-saving rate was observed in December (33.75%), while the remaining months had lower energy-saving rates (below 6%). By improving the system with time-sharing control, the annual energy consumption decreased from 22,963 kW·h to 18,906 kW·h, resulting in an annual energy-saving rate of 17.66% in the circulation heat preservation mode. The significant energy-saving effect of this mode was due to the time-sharing control, which reduced the insulation tank’s heat loss during non-peak hot water use hours and decreased the circulation heat preservation mode operating hours.

4.3. COP of Improved System

In this study, the results showed that after the improvement of the time-sharing control strategy, the COP of the air-source heat pump unit in the direct heat water production mode was improved in all months of the year, with an average improvement of 6%. The COP improvement was higher in spring and autumn, ranging from 4.14% to 8.48%, and less in summer and winter, with strong seasonal characteristics. These findings suggest that adjusting the operation time of the unit can ensure its efficient operation and improve the system COP, which has stable and applicable effects in different seasons, as illustrated in Figure 22. These results provide valuable insights into the potential of time-sharing control strategy improvement to achieve energy savings and promote sustainable and environmentally friendly heating solutions.

4.4. Environmental Economic Benefits of Improved System

The improvement of the control method has resulted in an annual energy saving of 10,027 kW·h, which is a significant achievement in terms of sustainability. The technical and economic environmental indicators of the system have been analyzed in conjunction with this study. The simulation results show that the annual average COP of the air-source heat pump unit before and after optimization is 3.54 and 3.75, respectively. Additionally, the primary energy utilization rate before and after improvement has been calculated as 1.15 and 1.21. Furthermore, the annual standard coal consumption per unit heating area before and after improvement has been determined to be 39.25 kg/(${\mathrm{m}}^2\cdot \mathrm{a}$) and 36.08 kg/(${\mathrm{m}}^2\cdot \mathrm{a}$), respectively, which corresponds to an 8.77% reduction in primary energy consumption [35].

The improved control strategy has resulted in an 8.21% reduction in $C{O}_2$ emissions, a 7.88% reduction in ${SO}_2$ emissions, and an 8.11% reduction in dust emissions. These reductions are an important step towards achieving sustainability goals. Overall, the improvement of the control strategy has significantly improved the energy efficiency of the system and reduced its environmental impact. The results of this study demonstrate the importance of adopting sustainable practices in the design and operation of heating systems, as presented in

Table 3. Calculation results of environmental protection indicators before and after improvement.

Project	${\mathit{C}\mathit{O}}_2$ Emission	${\mathit{S}\mathit{O}}_2$ Emission (kg)	Dust Emission
Before improvement (kg)	104,368.94	753.57	376.78
After improvement (kg)	95,800.25	694.19	346.22
Reduction rate (%)	8.21	7.88	8.11

.

The air-source heat pump hot water system is equipped with a digital display on the main control panel of the external unit, which allows for real-time monitoring of the water level, temperature, power, ambient temperature, and humidity, among other variables. By adopting a time-sharing control strategy, the system achieved significant energy savings, with an annual energy savings of 10,027 kW·h. Moreover, the improvement of the system resulted in an annual economic savings of 5314 CNY, highlighting the potential economic benefits of sustainable energy solutions. Importantly, these savings were achieved without requiring any additional investment in the system, as the improvement was achieved solely through the use of the digital control panel.

5. Discussion and Perspective

The results of our study suggest that air-source heat pump systems are a promising solution for achieving sustainable and environmentally friendly heating in college dormitories. By improving the original hot water system and implementing a time-sharing control strategy, we were able to increase the unit annual average COP by 6% and achieve an energy-saving rate of 8.77%. These improvements have significant economic and environmental benefits, as energy consumption and associated emissions are reduced. Furthermore, our simulations indicate that air-source heat pump systems have great potential for energy savings in student dormitories across the country. This is particularly important in the context of China’s goal of achieving a “carbon peak” by 2030 and being “carbon neutral” by 2060, as the widespread adoption of sustainable energy solutions will be crucial for meeting these targets.

While our study focused specifically on college dormitories, the findings have broader implications for the adoption of sustainable energy solutions in educational institutions across the country. As institutions of learning and innovation, educational institutions have a unique opportunity to lead the way in the adoption of sustainable practices and technologies. By promoting the adoption of air-source heat pump systems and other sustainable energy solutions, educational institutions can serve as models for sustainable development and contribute to a more sustainable future for all.

6. Conclusions

Based on the experimental tests and simulation analysis, it is evident that the energy consumption and efficiency of the hot water system are influenced by the climatic conditions, with hot water consumption being the highest during winter. The unit energy efficiency is proportional to the outdoor temperature, and the HSPF of the system is lower than the COP of the direct heat water production mode but similar to the COP of the circulation heat preservation mode.

Improving the hot water system by implementing time-sharing control of the water tank results in a remarkable improvement in energy efficiency, with the annual energy saving rate being 8.77% and the unit annual average COP increasing by 6%. The adjustment of the proportion of the direct heat water production mode and the circulation heat preservation operation time leads to a reduction of 17% in the heat production of the circulation heat preservation mode and an increase in the annual average water tank temperature by 0.28 °C, leading to a reduction in the heat dissipation loss and an increase in the satisfaction degree of hot water consumption at the user end. Overall, these findings highlight the significant economic and environmental benefits of implementing energy-efficient hot water systems in university dormitories.