Theoretical Evaluation of Photovoltaic Thermal Water Source Heat Pump, Application Potential and Policy Implications: Evidence from Yangtze River Economic Belt, China

Abstract

In the context of carbon neutrality, making full use of renewable energy is the key to further improve China’s development index. Within China’s Yangtze River Economic Belt, solar energy and river water, as clean and abundant sources of renewable energy, have garnered increasing. In this paper, a solar energy and surface water driven cogeneration system model is developed by TRNSYS to provide users with heat, cold and electricity. Six representative cities located along the upper, middle, and lower reaches of the Yangtze River Economic Belt were selected to evaluate and analyze the energy-saving, emission reducing, and economic and environmental benefits of solar energy and river water heat utilization in the aspects of energy, economy, and environment. The results shows that the annual power output of PV/T-GSHP system, from the west to the east of the Yangtze River, shows a phase growth trend, which is related to the light intensity. The annual heat output of PV/T plate gradually decreases from the lower reaches of the Yangtze River to the upper reaches. The research findings confirm the application potential of new energy sources in the Yangtze River Economic Belt and quantify the emission reduction effects of new environmental protection actions such as solar energy and river water heat sources. It provides valuable guidance for the utilization of new energy sources, including solar energy and surface water heat energy in the Yangtze River Economic Belt, as well as for optimizing energy policies.

1. Introduction

Global warming, environmental degradation, and energy consumption have emerged as significant concerns worldwide. These issues pose major challenges to the survival and development of human beings [1]. Cooling, heating, and electricity are the most basic demand of human beings. At present, the consumption of building cooling and heating power accounts for more than 30% of social energy consumption and more than 55% of power demand [2]. As industrial and economic development progresses, and living standards improve, the energy demand of buildings continues to increase, bringing more and more pressure to the energy and environmental systems [3].

To achieve the long-term mitigation goal set forth in the Paris Agreement, which aims to limit the global temperature rise to below 2 °C and strive for 1.5 °C above pre industrial level [4,5]. Due to the growing energy demand, global buildings account for an increasing proportion of the energy consumer and may become a significant source of carbon emissions [6,7,8]. According to statistics, the energy consumption of building cooling and heating accounts for a very high proportion of global energy demand, about 30–45% [9]. If the global construction industry does not decarbonize, the 2 °C and 1.5 °C climate targets of the Paris Agreement may not be achieved. In order to achieve the goals of the Paris Agreement, the building energy system must rapidly decarburize and transition to renewable energy sources [10]. Since the economic reform and opening up in 1978, China has experienced rapid economic growth and significant urbanization [11]. This growth has resulted in a significant increase in energy consumption in China, with the country’s total energy consumption reaching the highest in the world since 2017 [12]. In the same year, China’s total floor area reached 59.1 billion square meters, leading to high energy consumption in building operation, which accounts for 21% of China’s total energy consumption, equivalent 963 million tons of standard coal. The huge energy demand and coal based energy structure puts enormous pressure on China’s sustainable economic growth [13]. Consequently, promoting green development and the use of renewable energy have become crucial national strategy in China [14]. The Chinese government promises to reach the peak of carbon dioxide emissions by 2030 and achieve carbon neutrality by 2060 [15]. According to the latest research, China’s construction industry should reduce carbon dioxide emissions by at least 17–41% by 2035 [16]. This shows that China should combine more renewable energy technologies with buildings and optimize its energy structure to reduce pollution emissions [11].

Solar energy is an abundant, inexhaustible, widely distributed and pollution-free renewable energy source that has garnered global attention [17,18,19]. By integrating solar energy into daily life, production processes, and buildings, significant energy conservation and emission reduction effects can be achieved. China has become the largest photovoltaic production capacity country and application country. In fact, a separate solar energy utilization system also has some defects. For instance, solar photovoltaic power generation capacity is reduced during rainy weather and at night. For cold and hot supply, it requires a complex system, which is an expensive application. In addition, solar thermal cooling systems have limited usability in temperate climates, as their effectiveness is constrained by external temperature and solar radiation [20].

Heat pumps are highly efficient systems that can transfer a large amount of heat/cold energy while consuming less energy, thereby potentially reducing the energy consumption and greenhouse gas emissions of buildings [21]. Heat pump is expected to become a possible tool to stabilize the international energy market in the future [22]. Heat pump technology is recognized as a typical clean residential heating system and is promoted in China to respond to the national energy conservation and emission reduction policies [9]. Heat pump systems are almost distributed in all provinces of China [9]. Therefore, expanding the use of renewable energy is not the sole aspect of transforming the energy system. Another key factor is to realize energy-saving transportation of cold and hot through heat pumps [23,24,25,26,27,28].

Given China’s vast territory, diverse climate zones, and specific constraints related to solar energy resources, water resources and environmental policies, China has its own characteristics in the research and application of solar energy and water source heat pumps. Solar energy systems have been well adapted to the peak demand for hot water, heating and cooling in southern provinces. These provinces have higher peak demand in summer, but there are also problems of power distribution and shortage. Different types of heat pump systems are used in different climatic regions. Such as the groundwater heat pump (GWHP) system is widely used in Beijing, Liaoning, Hebei and Shandong provinces, which are mainly located in cold climate regions. At the same time, the application of surface water heat pump (GSHP) system is mainly expanded in Beijing, Chongqing, Anhui, Hubei and Jiangsu, which are mainly in the “cold” and “hot summer and cold winter” climate regions. In addition, surface water heat pump (SWHP) system is mainly used in areas with rich groundwater and surface water resources [9].

Moreover, the cost of replacing conventional energy sources with renewable energy technologies is a significant consideration. Due to the high capital and operating costs of renewable energy technologies, most alternative projects are less willing to pay for relatively low-income areas. Heat pump projects are mainly distributed in the Middle East, which has a relatively large population and is wealthier than the West [9]. Therefore, an important issue to address is how to better promote the wide application of solar energy surface water and other clean energy in appropriate areas in view of resources, geography, policies, environment, economy and other factors [29]. Coupling heat pumps with solar energy can improve the intermittent characteristics of new energy utilization, enhance stability, improve comprehensive efficiency and reduce unit cost [30,31,32,33,34].

Based on the above analysis, this study developed a TRNSYS-based model for a multi-source heat pump/cooling/power generation system model driven by solar and surface water. This system can simultaneously utilize solar power generation and surface water heat pump cooling/heating. The research focused on the demand for hot water, heating, and cooling in southern provinces in the “cold” and “hot summer and cold winter” climate regions of the Yangtze River Economic Belt, where the multi-source heat pump system is more suitable. Taking a building in Wuhan, a typical city in the Yangtze River Economic Zone, as an example, key system parameters such as radiation control strategy, tank volume, and photovoltaic power generation area were discussed and optimized. Using the optimized and validated model, six typical cities along the Yangtze River Economic Belt were selected to evaluate and analyze the energy efficiency, emission reduction, and environmental effects of solar energy and hot water utilization in typical areas of the upper, middle, and lower reaches of the Yangtze River. The feasibility and potential of regional solar energy and river water heat utilization were studied using the energy balance method. Furthermore, through the analysis of energy, economy, and environment, its benefits can be quantitatively analyzed, and the emission reduction effects of new environmental protection actions such as solar energy and surface water use in the Yangtze River Economic Zone were quantified. The study also explored the long-term performance of urban solar energy systems and water source heat pump systems in the hot summer and cold winter climate zones of the Yangtze River Economic Zone in China, providing technical, economic and environmental guidance for southern provinces in these climate regions to implement energy conservation designs using solar energy, surface water and other renewable energy.

Section 1 of the article introduces the current demand for energy conservation and emission reduction in China, as well as the current situation of renewable energy utilization, such as solar surface water sources in various regions. Section 2 presents our proposed new solar and surface water multi source heat pump system, including the related thermal and electrical models, as along with relevant economic, environmental, and energy efficiency evaluation indicators and calculation methods. In Section 3, the feasibility of the corresponding new solar and surface water multi heat source heat pump system is modeled and verified using TRNSYS software. Section 4 conducts a comprehensive evaluation of the potential benefits of solar and surface water sources in selected typical cities of the Yangtze River Economic Belt, based on the validated model. This evaluation encompasses energy efficiency, economy, and environmental aspects. Section 5 and Section 6 provide corresponding conclusions and policy guidance. Please refer to the following text for detailed information.

2. Modeling

2.1. Description of System and Control Strategy

The Yangtze River Economic Belt has the most abundant river in China—the Yangtze River, with an annual total water resources of 996 billion cubic meters. The forest coverage rate is more than 40%, and the area of river and lake wetlands accounts for about 20% of the country. And the Yangtze River Economic Zone is rich in water resources with good macro thermal energy characteristics, so it has a great prospect to use it as a heat pump cold and heat source for building cooling and heating. The river water source heat pump uses the Yangtze River water as the cold and heat source of the system, which is highly efficient, does not require cooling towers, boilers and other equipment, occupies a small area of the machine room, does not discharge pollutants and heat to the atmosphere, and improves the indoor environment and urban environment. Photovoltaic cells can convert 6% to 25% of the solar energy into electric energy. At the same time, they can reduce the temperature of the cells in normal operation through river water cooling panels to increase the photovoltaic power generation, and extract heat from the PV panels by using water pumps for water circulation. The combination of solar energy and heat pump can further promote energy conservation and efficiency. As shown in Figure 1, we built a solar photovoltaic heat and water source heat pump cogeneration system based on an actual building near the Yangtze River. The schematic diagram of system is shown in Figure 2, the system includes PV/T panels, storage tank, DHW tank, water pump, WSHP and other equipment. The heat including heating and hot water, supplied by PV/T and WSHP, the cooling is supplied by WSHP, and the electricity is generated by PV/T. The detailed operation process of the novel system is described as follows.

1. During the heating season and when the building needs heating, the system has three heating modes according to the water temperature of the heat storage tank and solar radiation. Figure 3 depicts the flow chart of the system control strategy.

Mode 1 (direct supply mode): When $T_{cs}\ge 45$ °C, the hot water at the outlet of the storage tank directly supplies heating. Here $T_{cs}$ is the outlet temperature of the storage tank.

Mode 2 (auxiliary mode): When the low solar irradiation condition is satisfied, the water temperature on the WSHP source side is greater than the river water temperature and $T_{cs}<40$ °C, the auxiliary pump and the load side water pump are turned on. The river water is directly heated by the PVT to evaporator, and the WSHP provides heating.

Mode 3 (single mode): When mode 1 and Mode 2 are both off, both the source side pump and load side pump are turned on, WSHP separately provides heating.

2. During the cooling season and when the building needs refrigeration, like mode 3, the WSHP separately provides cooling.

It is worth mentioning that in the non-heating period and the auxiliary pump is turned on, its function is to supply river water to the PV/T, so as to supply DHW and electricity simultaneously. The opening conditions of the auxiliary pump here will be introduced in detail in the optimization of irradiation control strategy.

2.2. Therml and Electrical Efficiency Analysis

Based on energy conservation law energy balance on the collector surface at any point along the surface applies the following relationship:

(1)$0=S-h_{outer}\left(T_{PV}-T_{amb}\right)-h_{rad}\left(T_{PV}-T_{sky}\right)-\frac{\left(T_{PV}-T_{abs}\right)}{R_T}$

where $h_{outer}$ is convective heat transfer coefficient, $T_{PV}$ is PV cells temperature, $T_{amb}$ is ambient temperature, $h_{rad}$ is radiation heat transfer coefficient, $T_{sky}$ is sky temperature, $T_{abs}$ is absorber plate temperature, $R_T$ is thermal internal resistance between PV cells and absorber plate.

S is the net absorbed solar radiation per unit area and accounts for the absorbed solar radiation minus the PV power production. The heat absorbed by water is highly dependent on the solar radiation. The S value can be determined using Equation (2).

(2)$S={\left(\tau \alpha \right)}_n{G}_t(1-{\eta }_{PV})$

where $\tau \alpha$ is the transmittance-absorptance product for the solar collector, $G_t$ is the total solar radiation on inclined surfaces of PV/T panels.

The efficiency of the PV cells (${\eta }_{PV}$) as a function of the cell temperature and the incident solar radiation can be computed by Equation (3) [35,36].

(3)${\eta }_{PV}={\eta }_{ref}\left[1-\Psi (T_{PV}-T_{ref})+\kappa \ln (\frac{\tau \alpha G_t}{G_{ref}}\right]$

where ${\eta }_{ref}$, $T_{ref}$, $G_{ref}$ are the efficiency of the PV cells, PV cells temperature and solar radiation under standard test conditions, respectively, $\Psi$ is the temperature coefficient of PV cell, $\kappa$ is solar irradiance coefficient.

After computing the new efficiency of PV cells based on the instantaneous values of the incident solar radiation and the PV temperature, the output power ($Power$) can be computed using Equation (4).

(4)$Power={\left(\tau \alpha \right)}_n{G}_{t}Area{\eta }_{PV}$

where $Area$ is the PV/T panels area, which can be determined using Equation (5).

(5)$Area=\frac{\mathrm{86,400}{Q}_{J}f}{J_T{\eta }_{cd}(1-{\eta }_L)}$

where $Q_J$ is the design load of the solar collector system ($W$). $J_T$ is the average daily solar irradiance in December on the lighting surface of the local collector, which is 9404,000 $J/(m^2·d)$. $f$ is the solar energy guarantee rate, which is 20%, ${\eta }_{cd}$ is the average heat collection efficiency of the collector based on the total area, which is 40%. ${\eta }_L$ is the heat loss rate of the pipeline and the heat storage devices, which is 15%.

When water passes through the pipes underneath the absorber plate, so the useful energy gain ($Q_u$) can be calculated using Equation (6).

(6)$Q_u=\dot{m}{C}_p(T_{fluid,out}-T_{fluid,in})$

where $\dot{m}$ is the mass flow of water, $C_p$ is the specific heat capacity of water, $T_{fluid,out}$ and $T_{fluid,in}$ are the outlet water temperature and inlet water temperature of PV/T panels respectively.

The pump power consumption $W$ can be computed as [37,38]:

(7)$W=\frac{\Delta P{\dot{m}}_{out}}{(\rho \cdot {\eta }_P)}$

where $\Delta P$ is the pressure drop across the pump, ${\dot{m}}_{out}$ is the actual fluid mass flow rate, $\rho$ is the density of the fluid, ${\eta }_P$ is the pump efficiency.

The outlet fluid temperature $T_{P,out}$ can be obtained by:

(8)$T_{P,out}=T_{P,in}+\Delta P(\frac{(1/{\eta }_P)-1}{\rho \cdot c_P})$

where $T_{P,in}$ is the inlet fluid temperature. $c_P$ is the specific heat of fluid.

The HP heating/cooling capacity, power and other parameters are determined by the building load and performance data of the HP manufacturer [39,40,41].

The COP of the HP in heating mode (${COP}_{heating}$) is calculated by Equation (9) [41].

(9)${COP}_{heating}=\frac{{Cap}_{heating}}{{\dot{P}}_{heating}}$

where ${Cap}_{heating}$ is the HP heating capacity at current conditions and ${\dot{P}}_{heating}$ is the power drawn by the HP in the heating mode.

The amount of absorbed energy from the source fluid (${\dot{Q}}_{absorbed}$) is calculated using Equation (10).

(10)${\dot{Q}}_{absorbed}={Cap}_{heating}-{\dot{P}}_{heating}$

The outlet temperatures of two liquid streams can be calculated through using Equations (11) and (12).

(11)$T_{source,out}=T_{source,in}-\frac{{\dot{Q}}_{absorbed}}{{\dot{m}}_{source}C{p}_{source}}$

(12)$T_{load,out}=T_{load,in}+\frac{{Cap}_{heating}}{{\dot{m}}_{load}C{p}_{load}}$

where $T$,$\dot{Q}$,$\dot{m}$ and $Cp$ are temperature, heat flux, flowrate and specific heat respectively. The subscript source, load, in and out refer to the water source side, load side, inlet and outlet of the cycle respectively.

The COP of the HP in cooling mode (${COP}_{cooling}$) is calculated using Equation (13).

(13)${COP}_{cooling}=\frac{{Cap}_{cooling}}{{\dot{P}}_{cooling}}$

where ${Cap}_{cooling}$ is the HP cooling capacity at current conditions and ${\dot{P}}_{cooling}$ is the power drawn by the HP in the cooling mode.

The amount of rejected energy by the source fluid in cooling mode (${\dot{Q}}_{rejected}$) is calculated using Equation (14).

(14)${\dot{Q}}_{rejected}={Cap}_{cooling}+{\dot{P}}_{cooling}$

The outlet temperatures of the two liquid streams are calculated using Equations (15) and (16).

(15)$T_{sour,out}=T_{sour,in}+\frac{{\dot{Q}}_{rejected}}{{\dot{m}}_{source}C{p}_{source}}$

(16)$T_{load,out}=T_{load,in}-\frac{{Cap}_{cooling}}{{\dot{m}}_{load}C{p}_{load}}$

2.3. Energy Balance Analysis

The energy analysis is based on the first law of thermodynamics, which concludes the total input energy, output energy and energy loss from the system. The general energy balance is shown in Equation (17). The input energy is the sum of the solar energy striking on the PV/T panels, $Q_{sol}$, and the total power consumption of the PV/T-WSHP system, $\Sigma W$. The output energy is the sum of the heat and cool output of the HP unit, $Q_H$ and $Q_C$, the heat output of the PV/T panels of the system, $Q_P$, and the output electric power of the PV/T panels,$\Sigma W_P$. According to the energy balance of the PV/T-WSHP system, the total energy loss, $\Sigma W_{loss}$, is the difference between the input energy and the output energy.

(17)$Q_{sol}+\Sigma W=Q_H+Q_C+Q_P+\Sigma W_p+\Sigma Q_{loss}$

$Q_{sol}$ can be obtained in Equation (18).

(18)$Q_{sol}=G_{sol}\cdot Area$

where $G_{sol}$ is the solar intensity, MJ/m².

$\Sigma W_P$ can be obtained in Equation (19) [42,43].

(19)$\Sigma W_P=\Sigma (U\cdot I)$

where $U$ is the voltage of the output electric power, V; $I$ is the current of the output electric power, $A$.

$Q_P$ can be obtained in Equation (20).

(20)$Q_P={\eta }_{ex}\cdot Q_J$

where ${\eta }_{ex}$ is heat exchanger efficiency, %; $Q_J$ is the heat collection of PV/T panels.

$Q_J$ can be obtained in Equation (21).

(21)$Q_J=m_{e}c(T_{fluent,out}-T_{fluent,in})$

where $c$ is the specific heat capacity of water, 4187 $J/(kg\cdot K)$; $m_e$ is the water flowrate of the cooling circulating water, kg/s.

$Q_H$ and $Q_C$ can be obtained in Equation (22).

(22)$Q_{H(C)}=m_{L}c|T_{o,ex}-T_{i,ex}|Q_{H(C)}=m_{L}c$

where $m_L$ is the water flowrate of the condensing water, kg/s; $T_{o,ex}$ and $T_{i,ex}$ are the outlet and inlet water temperature of condenser or evaporator, °C, respectively.

The Energy Efficiency Ratio (EER) of the PV/T-WSHP system can be obtained in Equation (23).

(23)$EER=\frac{Q_{H(C)}+Q_P}{\Sigma W}$

2.4. Economic Analysis

2.4.1. Annual Investment Cost

The annual cost method is used to convert the investment and annual operation cost of different systems into equivalent cost at the end of each year. The system with the lowest annual cost is the best choice. The annual cost including the annual investment and operation costs are shown in Equation (24) [43].

(24)$\stackrel{\cdot }{Z}=\frac{{\stackrel{\cdot }{Z}}_{CL}\times d}{1-{(1+d)}^{-n}}+{\stackrel{\cdot }{Z}}_{OM}$

where $\stackrel{\cdot }{Z}$ is the annual cost, RMB, ${\stackrel{\cdot }{Z}}_{CL}$ is the initial investment cost of the system, RMB, ${\stackrel{\cdot }{Z}}_{OM}$ is the operation cost, RMB, d is the discount rate, n is equipment lifetime.

Then, the annual power generation of the PV/T is considerable, so it is necessary to consider the annual power generation revenue into the operating cost, in addition to annual electricity cost and maintenance cost. So ${\stackrel{\cdot }{Z}}_{OM}$ can be determined using Equation (25).

(25)${\stackrel{\cdot }{Z}}_{OM}={\stackrel{\cdot }{Z}}_e{\stackrel{\cdot }{+Z}}_M-{\stackrel{\cdot }{Z}}_P$

where ${\stackrel{\cdot }{Z}}_e$ is the annual electricity cost, RMB, ${\stackrel{\cdot }{Z}}_M$ is the annual maintenance cost, RMB, ${\stackrel{\cdot }{Z}}_P$ is the annual power generation revenue, RMB. The electricity price, maintenance cost ratio and power generation revenue ratio are regarded as the calculation coefficients of the annual electricity cost, maintenance cost and power generation revenue, respectively.

2.4.2. Additional Investment Payback Period (PBP)

PBP is the time to recover the additional investment cost using the annual saving on operating of the larger investment project between two systems, which can be obtained by Equation (26).

(26)$\alpha =\frac{\Delta {\stackrel{\cdot }{Z}}_{CL}}{{\stackrel{\cdot }{Z}}_{OM}}$

where $\alpha$ is the additional investment payback period, year, ${\stackrel{\cdot }{Z}}_{CL}$ is the additional investment for system 2 compared with system 1, RMB, ${\stackrel{\cdot }{Z}}_{OM}$ is the operating cost saving for system 2 compared with system 1, RMB.

2.5. Environmental Analysis

The PV/T-WSHP system makes a great contribution to environmental protection by utilizing solar energy. Therefore, taking coal-fired power generation as a reference, the energy saving and emission reduction benefits of the system are calculated. The pollutant emission factors [44,45,46] are presented in Table 1. Since cooling capacity indirectly affects carbon emissions through electricity consumption, the annual cooling capacity of the system is converted into power generation (The average ${COP}_{cooling}$ of the WSHP is 6.37.) to calculate carbon emission reductions. In addition, the heat exchanger is used in the DHW tank, its heat exchange efficiency is 90% [46], ignoring piping losses, the heat supply in the non-heating season can be calculated by the PV/T heat collection. The pollutant emissions reduction (PER) was calculated according to reference [47]

3. Model

3.1. TRNSYS Model

In order to facilitate the simulation calculation, the following assumptions are made for the TRNSYS model of the PVT-WSHP system [35,36].

1. It is considered that water is a single-phase, homogeneous, constant physical property and incompressible fluid, which flows in a steady, one-dimensional and steady state;

2. The storage tank is filled with water during heat collection. The heat collection circulation system is powered by circulating pump and carries out heat collection circulation with fixed flow;

3. The temperature in the storage tank is stratified, and there is a temperature difference between layers, and the temperature of each layer is evenly distributed;

4. Ignore the pipe heat loss, the heat loss of the water tank is constant, and the water tank does not age with time.

The TRNSYS model is shown in Figure 4.

3.2. Model Verification

The case study is conducted in Wuhan (30°55′ N, 114°45′ E) [48,49]. The total solar horizontal radiation as well as air temperature in Wuhan city and meteorological data of typical years is available on the Energyplus website [50]. The maximum incident radiation is about 979 W/m², The annual solar radiation is 3329.08 GJ/m². The average annual air temperature is 17.3 °C, the hottest month is July with 29.7 °C average air temperature, and the coolest month is January with average air temperature of 4.7 °C.

In order to verify the proposed TRNSYS simulation model, compare the subsystem (PVT and WSHP) under three operation modes in the operating parameters of TRNSYS model of PVT-WSHP system with the actual operating parameters of existing systems [51]. Figure 5 shows the annual profile of the heating and cooling loads. Imposed by legislation for the hot-summer and cold-winter zone in China, the building should be maintained at 18 °C all day in the heating season (from December 1 to February 28 of the following year) and 26 °C all day during the cooling season (from June 15 to August 31). Figure 6 shows the annual operating performance of the PVT-WSHP system when the indoor temperature is set to 18 °C in winter and 26 °C in summer respectively.

The average inlet and outlet circulating water temperatures of the water source are 7.1 °C and 9.0 °C respectively, and the minimum inlet and outlet temperatures in the heating season (February 28) are 13.5 °C and 16.3 °C respectively. In the cooling season, the IOTG is 28.3 °C and 24.2 °C respectively, and the maximum inlet and outlet temperatures are 24.9 °C and 23.8 °C respectively (August 28). During the actual operation of PVT-WSHP system, the temperature difference between the inlet and outlet of water source ranges from 0.7 to 1.4 °C [33]. The temperature difference in the proposed WSHP subsystem model of PVT-WSHP is 1.3 °C It can be concluded that the simulation results are within the scope of actual operation, thus verifying the reliability of the model. It is worth noting that the current simulation results are based on the operation of heat pump units under three modes, and the experimental data are collected under the operation of an independent water source heat pump or PVT. When the three modes are operated according to the meteorological conditions during the actual operation, the operation performance of the unit will be improved.

4. Case Study

The Yangtze River Economic Belt is the most urbanized area in China, with a subtropical monsoon climate. Therefore, there is a great demand for heating, ventilation and air conditioning (HVAC) systems. At the same time, the Yangtze River is also the most abundant river in China, with an annual total water resources of 996 billion cubic meters. The forest coverage rate of the Yangtze River basin is more than 40%, and the area of river and lake wetlands accounts for about 20% of the country, as shown in the Figure 7. Cities in the Yangtze River Economic Belt have rich surface water resources. This section selects 6 typical cities along the Yangtze River Economic Belt (Shanghai (121°47′ E, 31°23′ N), Nanjing (118°76′ E, 32°04′ N), Anqing (117°05′ E, 30°53′ N), Wuhan (30°55′ N, 114°45′ E), Chongqing (106°50′ E, 29°53′ N), Yibin (104°62′ E, 28°77′ N). Based on the urban meteorological data and local economic data [46,49,50,52,53,54,55], the dynamic cooling load and heating load are calculated using the new hybrid photovoltaic heat and river water source heat pump system model established by TRNSYS. In this system, solar irradiance condition, the volume of storage tank and the PV/T area have a great impact on system performance, so they are analyzed as optimization variables with a base case of 100–1000 $\mathrm{W}/{\mathrm{m}}^2$, 80 ${\mathrm{m}}^3$ and 800 ${\mathrm{m}}^2$, respectively. Here, 100 $\mathrm{W}/{\mathrm{m}}^2$ and 500 $\mathrm{W}/{\mathrm{m}}^2$ are referred to as the low irradiance threshold (LIT) and high irradiance threshold (HIT), respectively. That is, satisfying the corresponding threshold is one of the conditions for pumps to start and between the LIT and HIT is low irradiance condition.

4.1. Energy Analysis Results

PV/T-WSHP system takes electric power and solar radiation as input and outputs electric energy, thermal energy and cold energy. Figure 8 shows the annual energy balance of PV/T-WSHP system. Table 2 lists the energy input and output of the six cities. It can be seen from Figure 9 that the annual power output of the system is related to the solar radiation. From the west to the east of the Yangtze River, the annual solar radiation of the city increases gradually (as shown in Figure 10), resulting in the same trend in the annual power generation of the system (as shown in Figure 11). In addition, the annual heat output of PV/T plate gradually decreases from the lower reaches of the Yangtze River to the upper reaches, but Wuhan is higher than Anqing. According to the control strategy of the system, it can be analyzed that the annual solar radiation in Wuhan higher than 200 W/m² is better than Anqing.

Table 3 shows the average COP of water source heat pumps in heating and cooling periods of each city. The heating COP is related to the water temperature of the Yangtze River and the PV/T auxiliary mode of the system, while the cooling COP is only related to the water temperature of the Yangtze River because the water source heat pump operates independently. Table 4 shows the monthly average water temperature data of the upper, middle and lower reaches of the Yangtze River. Different cities import different water temperature data according to different basins of the Yangtze River. As shown in Figure 12, during the heating period, Wuhan’s heating COP reached 3.42 due to its relatively high average water temperature, while Shanghai, Nanjing and Anqing, three cities located in the lower reaches of the Yangtze River, had their heating COP gradually reduced, because Shanghai could meet the best solar irradiation conditions for PVT auxiliary mode. Obviously, according to the change of water temperature, the cooling COP increases gradually from the downstream to the upstream, as shown in Figure 13.

4.2. Economic Analysis Results

In this study, the investment cost includes equipment cost, auxiliary material cost and installation cost. The cost details and parameters of PV/T-WSHP system economic calculation are shown in Table 5.

Table 6 shows the annual cost statistics of PV/T-WSHP system. Among them, the selected load data are consistent, so the heat pump model selection is the same, the initial investment and annual maintenance cost are the same, but the annual electricity charge and annual generation income are related to the local average annual electricity charge in each region, and the corresponding data are shown in Table 7.

As shown in Figure 14 and Figure 15, the annual investment and payback period of PVT-WSHP system in each region are respectively. It can be seen that Shanghai has the best economic performance, with an annual investment of 131,960.58 yuan. Compared with a single WSHP system, the PBP of its PV/T-WSHP system is about 8.4 years. In addition, the investment payback period of cities in the middle and lower reaches of the Yangtze River is about 9 years, which is very considerable compared with the 20 years of the system life. The investment payback periods of Chongqing and Yibin are 13.2 and 13.3 years, respectively, with relatively poor economic performance.

4.3. Environmental Analysis Results

PV/T-WSHP system has made great contributions to environmental protection through the use of solar energy. Therefore, taking coal-fired power generation as a reference, the energy-saving and emission reduction benefits of the system are calculated. See Table 8 for pollutant emission coefficient. Since cooling indirectly affects carbon emissions through power consumption, the annual cooling capacity of the system is converted into power generation to calculate carbon emission reduction. The pollutant emission reduction (PER) is calculated in this paper. The system energy output is converted into standard coal equivalent according to Table 2 and Table 8.

Figure 16 shows the annual carbon emission reduction of PV/T-WSHP system in each region. Among them, PV/T-WSHP system has the largest standard coal equivalent in Shanghai, reaching 99.11 tons; The standard coal equivalent of Chongqing and Yibin, located in the upper reaches of the Yangtze River, is relatively low, 61.03, 50.97 tons. In addition, as shown in Figure 17, taking Wuhan as an example, the average annual carbon dioxide emission reduction of PV/T-WSHP system is 229.98 tons. And the annual average SO₂ and NOX emission reductions of PV/T-WSHP system are 6.92 tons and 3.46 tons respectively. The reduction of CO₂ emissions in the system is significantly greater than that of SO₂ and NOX emissions, which is crucial for achieving the carbon peak and carbon neutralization goals. Therefore, the application of PV/T-WSHP system with environmental benefits will help promote the transition to green and low-carbon development.

5. Conclusions

The use of renewable energy is one of the promising efforts to reduce fossil fuel consumption and carbon dioxide emissions. In recent years, the utilization of solar energy and surface water heat energy has been paid more and more attention in China, especially in cities. However, regional differences are large. The large-scale promotion of solar energy and surface water heat energy still faces several challenges. In this paper, the corresponding conclusions and suggestions to overcome existing and potential obstacles are put forward through simulation and research. The typical achievements in research are shown as following:

• A new photovoltaic thermal-water source heat pump system, PV/T-GSHP, is proposed for the first time.

• A new hybrid system (GHE) photovoltaic river water source heat pump system, the TRNSYS model of PV/T-GSHP, is developed. The model considers three modes of regulation for heating season and building needs to improve energy efficiency and economy.

• The annual power output of PV/T-GSHP system, from the west to the east of the Yangtze River, shows a phase growth trend, which is related to the light intensity. In addition, the annual heat output of PV/T plate gradually decreases from the lower reaches of the Yangtze River to the upper reaches.

• During the heating period, due to the relatively high average water temperature in Wuhan, its heating COP reached 3.42, while the heating COP in Shanghai, Nanjing and Anqing, which are located in the lower reaches of the Yangtze River, gradually decreased, because Shanghai can meet the optimal solar radiation conditions of the PVT auxiliary mode. According to the change of water temperature, the cooling COP gradually increases from the downstream to the upstream.

6. Policy Implications

The following suggestions are provided in this paper:

• The results show that compared with the traditional system (single WSHP system), it has a high application potential in integrating higher energy returns, lower operating costs and high pollutant emissions. It is suggested to construct and popularize the composite system in areas with rich water resources and solar radiation.

• The benefits of comprehensive utilization of solar energy and surface water and heat energy in the lower reaches of the Yangtze River are the largest, and the amount of carbon reduction is also the largest. In the middle and upper reaches of the Yangtze River, the utilization of surface water should be emphasized.

• The optimal size, operation and policy support of PV/T-WSHP system under different conditions will be the subject of future research to make the configuration more feasible and commercial.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to [The project undertaken has not been completed].

Conflicts of Interest

The author declares no conflict of interest.

Nomenclature

c	Specific thermal capacity, $J/(kg\cdot °\mathrm{C})$
d	Discount rate
h	Heat transfer coefficient, W m−2 K−1
$\dot{m}$	Mass flow, kg/s
A	Area, m2
G	Solar radiation, W/m2
P	Power, W
Q	Energy, J
R	Thermal resistance, m2 K/W
S	Net absorbed solar radiation per unit area, W/m2
T	Temperature, K
W	Power output, W
Z	Cost, Yuan
Greek symbols
$\tau \alpha$	transmittance-absorptance
η	Efficiency
Ψ	Temperature coefficient of PV cell
κ	Solar irradiance coefficient
ρ	Density of the fluid

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Conceptual framework of River water PV/T-WSHP applied system.

Figure 2. Schematic system of the river water PV/T heat pump system.

Figure 3. The control strategy of the system.

Figure 4. The TRNSYS model of the PV/T-WSHP system.

Figure 5. The annual profile of the heating and cooling loads.

Figure 6. The monthly energy output of the PV/T-WSHP system for year.

Figure 7. Distribution of the Yangtze River Economic Belt in China.

Figure 8. Annual energy balance of PV/T-WSHP system.

Figure 9. Relationship between annual power generation and solar radiation.

Figure 10. Annual solar radiation of each city.

Figure 11. Annual system power generation of each city.

Figure 12. Average heating COP of water source heat pump in each city.

Figure 13. Average cooling COP of water source heat pump in each city.

Figure 14. Annual investment in PVT-WSHP system in various regions.

Figure 15. Investment payback period of PVT-WSHP system in each region.

Figure 16. Annual carbon emission reduction of each regional system.

Figure 17. Annual PER of the system (taking Wuhan as an example).

Table 1

Pollutant emission factor [48].

Items	Emission Conversion Factor of Standard Coal, C
Electricity	0.404 kg/kWh
Heat	0.03412 kg/MJ
CO2	2.493 kg/kg
SO2	0.075 kg/kg
NOX	0.0375 kg/kg
PM2.5	5.77 g/kg

Table 2

Statistics of energy input and output of each city [46,50,51,52,53,54,55].

Energy Balance (GJ)
	Annual Solar Energy	Annual Power Consumption	Annual Cool Output of the HP Unit	Annual Heat Output of the HP Unit	Annual Heat Output of the PV/T Panels	Annual Output Electric Power
Shanghai	3479.85	419.92	911.29	254.8	1521.93	604.65
Nanjing	3326.4	438.96	911.3	294.03	1483.73	579.39
Anqing	3296.49	444.33	911.44	302.24	1421	573.21
Wuhan	3229.08	414.59	911.54	286.1	1461.53	562.17
Chongqing	2410.32	432.93	911.46	364.18	1055.48	427.78
Yibin	2290.74	431.36	911.43	367.05	827.96	404.94

Table 3

Average COP of water source heat pump in heating and cooling periods of each city.

	COP of the WSHP in Heating Season	COP of the WSHP in Cooling Season
Shanghai	3.35	5.76
Nanjing	3.34	5.76
Anqing	3.30	5.76
Wuhan	3.42	6.37
Chongqing	3.13	7.77
Yibin	3.14	7.77

Table 4

Water temperature data of Yangtze River in different basins (°C).

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sept	Oct	Nov	Dec
Upward	2.1	3.7	7.2	9	12	14.1	16	16.2	15.4	11.8	6.1	3.3
Middle	14.7	13.7	13.45	16.2	19.07	23.04	22.89	24.89	23.55	20.3	18.55	15.5
Backward	11.2	10.3	13.8	17.2	22.6	26.4	26.4	30	27.6	21.6	18.7	12.4

Table 5

Economic parameters of the system [56].

Parameter	Value	Unit
Investment cost of water source heat pump	500	Yuan/Kw
Investment cost of source side pump	3000	Yuan
Investment cost of load side pump	2000	Yuan
Investment cost of circulating pump	2000	Yuan
Investment cost of auxiliary pump	3000	Yuan
Investment cost of direct supply pump	3000	Yuan
Investment cost of PV/T	800	Yuan/m2
Investment cost of water storage tank	1000	Yuan/m3
Control equipment investment cost	35,000	Yuan
Investment cost of pipeline, valve, auxiliary materials and installation	30	%
Inflation rate, i	4.5	%
Discount rate, d	7	%
Equipment life, n	20	Year
Maintenance cost ratio	2	%
Generation revenue ratio	Average tariff	KWh
Electricity price	Average tariff	KWh

Table 6

Annual cost statistics.

	Initial Investment(Yuan)	Annual Maintenance Cost(Yuan)	Annual Electricity Charge(Yuan)	Annual Generation Income(Yuan)
Shanghai	1,294,800	25,896	106,962.95	122,609.58
Nanjing	1,294,800	25,896	100,997.38	103,807.38
Anqing	1,294,800	25,896	109,601.4	112,200.6
Wuhan	1,294,800	25,896	100,539.04	107,749.71
Chongqing	1,294,800	25,896	98,611.83	75,693.3
Yibin	1,294,800	25,896	98,542.44	74,883.65

Table 7

Electricity charge statistics by region.

	First Level(Yuan/Kwh)	Second Level(Yuan/Kwh)	ThirdLevel(Yuan/Kwh)	Average(Yuan/Kwh)
Wuhan	0.573	0.623	0.873	0.689
Nanjing	0.528	0.578	0.828	0.645
Shanghai	0.617	0.667	0.917	0.734
Chongqing	0.520	0.570	0.820	0.637
Yibin	0.552	0.622	0.822	0.666
Anqing	0.588	0.638	0.888	0.705

Table 8

Pollutant emission coefficient.

	Emission Conversion Coefficient of Standard Coal, C
Electricity	0.404 kg/kWh
Heat	0.03412 kg/MJ
CO2	2.493 kg/kg
SO2	0.075 kg/kg
NOX	0.0375 kg/kg
PM2.5	5.77 g/kg

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