Planning and design of an architectural energy supply system based on photovoltaic-thermal technology

China’s building energy consumption is growing and the energy consumption of the whole process of buildings accounts for more than 46.5% of the country’s total energy consumption, of which the proportion of energy consumption used for heating, ventilation and air conditioning is as high as 50%. Solar energy has the greatest potential to achieve energy conservation in buildings. Buildings are the best carrier for solar energy utilization because of the advantages of local collection and local application. The combination of solar energy and buildings can meet a variety of energy and health needs in buildings, among which solar photovoltaic and CSP building integration technology is an important way to reduce building energy consumption and improve indoor air quality. This article presents a PV/T-based thermal storage wall system that efficiently utilizes solar energy. It outlines the PV/T technology’s fundamental principles and provides a detailed description of the system’s construction and operation, highlighting critical components like the solid thermal storage medium and heat transfer via a heat pump. The analysis indicates improved energy utilization and favorable economic feasibility. This technology offers a practical solution for the construction industry, promoting sustainable energy practices and environmental conservation.


Introduction
The construction sector is a major energy consumer in China, driven by urbanization and the rising demand for electricity in residential, commercial, and industrial buildings.Data from the National Bureau of Statistics of China shows a continuous increase in the sector's energy consumption.It is estimated that the construction sector accounts for nearly 50% of the total national energy consumption.Additionally, heating and cooling in buildings make up over 50% of the construction sector's energy usage.Furthermore, operational emissions from buildings account for over 20% of China's total carbon emissions, highlighting their significance in China's efforts to reach carbon peak and carbon neutrality targets [1] .
Solar energy, as a natural resource, boasts unique advantages due to its abundant and clean nature.Photovoltaic-thermal (PV/T) technology is an efficient solar energy utilization method that combines both photovoltaic and solar-thermal conversion, increasing photovoltaic conversion efficiency while harnessing solar heat [2] .This technology not only enhances energy conversion efficiency but also maximizes solar energy utilization, reducing the demand for burning fossil fuels and mitigating carbon emissions.It holds significant potential for energy savings.Thermal storage materials store heat or cold, releasing it when needed, thereby increasing energy utilization efficiency.This paper presents an architectural energy supply system based on PV/T technology that caters to various regional energy demands.The system covers electricity, cooling, heating, and domestic hot water.By utilizing photovoltaic/thermal conversion, the PV/T thermal storage wall combines electricity generation with concurrent heat production.The heat generated is efficiently conveyed to the solid thermal storage material via a one-way heat pipe.This stored thermal energy supports continuous heating through a heat pump.For cooling, the building's internal heat is directed to the solid thermal storage material within the wall, serving as a heat source for domestic hot water, maximizing solar energy and utilizing building waste heat effectively, reducing the reliance of buildings on traditional energy sources, addressing the challenge of efficient solar energy utilization and meeting the energy needs of buildings.

Methods
The building energy supply system comprises PV/T thermal storage walls and a heat pump.PV/T thermal storage walls feature photovoltaic cells for direct solar energy conversion into electricity.This electricity satisfies the building's power needs with surplus power potentially fed back to the grid.Solar thermal technology within these walls harnesses solar radiation, converting it into thermal energy for hot water, heating, and cooling.The walls integrate a heat storage material for heat absorption during solar abundance and controlled release as needed, optimizing energy utilization.

PV/T (Photovoltaic/Thermal) technology
PV/T (Photovoltaic/Thermal) technology combines photovoltaic solar cells with solar thermal systems to convert sunlight into both electricity and thermal energy.The key innovation lies in the fact that PV/T systems simultaneously generate electrical and thermal energy, making more efficient use of solar energy resources.This technology enhances the overall solar energy utilization rate and meets users' needs for both high-quality electricity and lower-grade thermal energy.When compared to standalone photovoltaic or solar thermal systems, PV/T systems offer a more favorable return on investment, making them a compelling choice for sustainable energy solutions [3] .
The choice of heat transfer medium in a PV/T system significantly influences its solar thermal efficiency [4] .Air-based PV/T systems are relatively straightforward to design and manufacture, making them cost-effective.As a result, these systems were among the first to be researched and have matured for applications like air preheating, space heating, regional heating, grain drying and more.In 1999, Krauter and his team designed an integrated PV/T system that attached PV panels with air gaps to the external surface of conventional walls.The air gaps serve to lower the component temperature and boost electricity generation.Essentially, this design can be viewed as a vertically placed air-based PV/T system.Research findings have demonstrated that the working temperature of PV/T system panels can be reduced by over 20°C compared to standalone photovoltaic systems.This not only improves solar energy utilization but also minimizes the impact on a building's energy requirements [5] .
Compared to air-based PV/T systems, water-based PV/T systems have a more complex structure and higher processing difficulty.However, because water has a significantly higher heat capacity than air, it exhibits superior heat transfer characteristics, benefiting both the cooling of PV cells and the absorption of thermal energy.

Thermal storage wall
Shown in Figure 1, the PV/T thermal storage wall consists of a front insulation layer, left insulation layer, rear insulation layer, right insulation layer, upper insulation layer, lower insulation layer, reflectance coating, photovoltaic panel, aluminum plate, unidirectional heat pipe, solid thermal storage medium, glass cover plate, Fresnel lens, and heat pump evaporator coil.Among these components, the aluminum plate features a serrated cross-section with an angle equal to twice the local latitude, and its outer surface is coated with a black selective coating to enhance solar radiation absorption efficiency [6] .
The photovoltaic panels are adhered to and pressed onto the front side with an acute angle to the horizontal plane.The adhesive material used is Ethylene-Vinyl Acetate (EVA).The reflective coating is sprayed on the front side with an obtuse angle to the horizontal plane and the angle between the reflective coating and the adjacent photovoltaic panel below does not exceed 90°.The installation angle of the photovoltaic panels is consistent with the local latitude, increasing the direct sunlight incidence rate, improving the photovoltaic-thermal efficiency and maximizing the photovoltaic panel area within a limited height.The photovoltaic panels are cross-placed with the reflective panels, allowing sunlight that falls on the reflective panels to be reflected onto the photovoltaic panels, making full use of solar energy.The one-way heat pipe is a flat, sealed copper heat pipe.The evaporative section is welded to the back of the aluminum panel and the condensing section slightly inclines upward as it passes through the solid heat storage material.The use of flat-shaped one-way heat pipes is advantageous for secure welding to the back of the aluminum panel, increasing the heat transfer surface area between the heat pipe and the aluminum panel.The condensing section of the heat pipe slightly inclines upwards and is positioned higher than the evaporative section.This design allows the medium inside the heat pipe to efficiently return to the evaporative section by gravity after releasing heat in the condensing section.Additionally, it prevents heat from transferring in reverse from the solid heat storage material to the aluminum panel due to lower nighttime temperatures, ensuring unidirectional heat transfer from the aluminum panel to the solid heat storage material.
The front insulating layer is adhered to the back of the aluminum panel and the evaporative section of the one-way heat pipe is tightly integrated with the front insulating layer.The cross-section of the front insulating layer is serrated.The front insulating layer, left insulating layer, rear insulating layer, right insulating layer, upper insulating layer and lower insulating layer enclose a sealed heat storage cavity that is filled with solid heat storage material.These insulating layers not only reduce heat loss but also compensate for the thermal expansion and contraction of the PV/T heat storage wall.
The glass cover plate uses tempered coated glass.There is an air gap between the glass cover plate and the aluminum panel.The Fresnel lens is adhered to the inside of the glass cover plate using a method involving vacuum suction with silicone gel.
The copper material of the heat pump evaporator coil is arranged in a serpentine fashion within the solid heat storage material and the connection between the heat pump evaporator coil and the PV/T heat storage wall is on the same side with the flow going in from the bottom and out from the top.
The PV/T heat storage wall can be installed on the outer edge of the building's exterior walls or can be arranged in an L-shape at the intersection of the building's exterior walls.The heat pump can be placed on the ground or an equipment platform attached to the exterior wall of the building.When the PV/T heat storage wall is positioned at the intersection of the building's exterior walls, it can reduce the adverse effects of thermal bridges.The PV/T heat storage wall consists of front and rear insulating layers, an air gap between the glass cover plate and the aluminum panel and solid heat storage material, which enhances the thermal insulation properties of the building envelope.This is shown in Figure 2.

Heat pump
A heat pump is an energy conversion device used in heating and cooling systems that moves thermal energy from one location to another to provide heating, cooling and hot water, among other purposes.Heat pump systems operate on basic thermodynamic principles, utilizing processes such as compression, expansion, evaporation and condensation to transfer thermal energy from a low-temperature area to a high-temperature area, thereby achieving temperature control.
The operation of a heat pump system can be reversed, meaning it can be used for heating as well as cooling, making it a very versatile energy system.Due to their ability to efficiently transfer heat and reduce energy consumption, heat pumps are commonly used for residential heating, air conditioning, hot water supply and in some industrial and commercial applications.
The heat pump evaporator coil is connected through a refrigerant pipeline to a sequence of components including a compressor, condenser and expansion valve.These components, including the evaporator coil, compressor, condenser, and expansion valve, form a closed heat pump cycle path.The condenser is externally equipped with a thermal storage tank with the condenser submerged in the thermal storage tank.One side of the PV/T thermal storage wall is connected to the heat pump, while the other side of the PV/T thermal storage wall is connected to the evaporator/condenser coil.The other end of the evaporator/condenser coil is connected to the heat pump unit.The hot water side of the thermal storage tank is connected to both the hot water supply pipe and the makeup water pipe.

Dual-purpose unit
A dual-purpose unit typically refers to an air conditioning or heat pump system that has both heating and cooling capabilities.These units switch between heating (thermal production) and cooling based on seasonal and temperature requirements to provide indoor comfort.
Dual-purpose units typically include a compressor, condenser, evaporator and air delivery system.The dual-purpose unit mentioned in this paper consists of a four-way reversing valve, a heat exchanger, and a thermal storage tank.The four-way reversing valve facilitates the switching between the heating and cooling modes of the unit.The heat exchanger is immersed in the thermal storage tank, which is connected to both heating/cooling supply pipes and heating/cooling return pipes.
In the cold season, dual-purpose units extract heat from the outdoor low-temperature environment using heat pump technology and transfer it indoors to warm and heat the indoor space.During hot summer months, dual-purpose units can absorb heat from the hot air, expel it through the cooling cycle and thus lower the indoor temperature, providing a comfortable indoor environment.

Principle
The solar radiation penetrates through the outer glass cover and is concentrated on the photovoltaic panel by the Fresnel lens on the inside of the glass cover.A portion of the light is reflected by the reflective coating to the adjacent photovoltaic panel underneath, allowing the photovoltaic panel to convert the sunlight into electricity.The Fresnel lens focuses sunlight from a relatively large area onto a more concentrated area, further improving the photovoltaic panel's electricity generation efficiency.Additionally, installing the Fresnel lens can enhance the strength of the glass cover.
The evaporator section of the one-way heat pipe absorbs the solar heat from the aluminum plate and the photovoltaic panel.The heat transfer medium inside the one-way heat pipe absorbs heat and evaporates, then flows to the condenser section of the one-way heat pipe, where it transfers heat to the solid heat storage material and condenses.The condensate in the condenser section of the one-way heat pipe flows back to the evaporator section through gravity.
In the heating scenario, the refrigerant inside the heat pump evaporator coil absorbs heat from the solid thermal storage material, evaporates, gets compressed by the compressor, heats the water in the thermal storage tank through the condenser, and then returns to the heat pump evaporator coil through the expansion valve to provide heating.In the cooling scenario, the internal heat of the building is effectively transferred to the PV/T thermal storage wall, serving as the heat source for the heat pump, thereby efficiently utilizing the waste heat generated within the building.
In the cold/hot supply scenario (Figure 3), the heat pump provides hot water throughout the year.During winter, the refrigerant in the evaporator/condenser coil absorbs heat from the solid thermal storage material, compresses in the compressor within the cold and hot unit after passing through the four-way reversing valve, and then enters the heat exchanger to release heat to the water in the thermal storage tank.The hot water in the thermal storage tank is used for heating through the heating/cooling supply pipe.In summer, the heating/cooling return pipe transfers heat from inside the building to the thermal storage tank.The refrigerant in the heat exchanger absorbs heat, evaporates and then compresses in the compressor within the cold and hot unit after passing through the four-way reversing valve.It then enters the evaporator/condenser coil, transfers heat to the solid thermal storage material and returns to the heat exchanger through the expansion valve in the cold and hot unit, absorbing heat from the thermal storage tank.The low-temperature water in the thermal storage tank is used for cooling inside the building, transferring heat from inside the building to the solid thermal storage material, which supplies heat to the heat pump's evaporator coil.

Discussion and results
Through the simulation of the building energy supply system based on PV/T technology, we obtained s o m e r e l e v a n t d a t a a n d e v a l u a t e d t h e s y s t e m ' s p e r f o r m a n c e i n t e r m s o f C O P ( C o e f f i c i e n t o f Performance), overall average efficiency and economic benefits.

The coefficient of performance (COP)
The Coefficient of Performance (COP) is an essential parameter used to evaluate the performance of heating, cooling, air conditioning, or heat pump systems.It represents the ratio of the energy consumed by the system to the heat or cooling it provides per unit.A higher COP indicates greater efficiency, as the system can deliver more energy during the heating or cooling process, reducing energy consumption.It is a crucial performance metric for comparing the efficiency of different systems and determining which system is more economical and environmentally friendly.
Traditional heating systems typically have thermal efficiencies ranging from 70% to 85%.COP is not always used to measure traditional heating systems but it is useful for heat pump systems.For traditional heating systems, COP values are generally lower, which is often below 2.0, indicating that the system consumes much more electrical energy than it generates in terms of heating energy.
Table 1.Average COP of PV/T system.
In Table 1, the data indicates that during the four months of the heating season when the system provides full heating, the COP of the system remains stable at around 2.5.This demonstrates that the system is highly efficient in energy conversion, which is very advantageous for applications like heating and hot water supply.

The comprehensive average efficiency
The comprehensive average efficiency in a photovoltaic-thermal (PV/T) system considers both photovoltaic cell and heat collection efficiency.It indicates the ratio between the total energy extracted from solar sources and the energy input into the system, covering the production of electrical and thermal energy.This metric is crucial for assessing the overall performance of PV/T systems, providing a comprehensive evaluation of energy utilization under diverse environmental conditions.Table 2.The comprehensive average efficiency of the PV/T system.
In Table 2, the data indicates that during the four months of full heating in the heating season, the system's comprehensive average efficiency consistently exceeds 55%.The system demonstrates relatively high energy utilization efficiency, effectively converting energy into useful heat.This means that the system efficiently utilizes the supplied energy, reducing waste and providing the required heat with greater efficiency.This is a positive sign for heating systems because highly efficient systems

COMBINED AVERAGE EFFICIENCY
Combined average efficiency typically have lower operating costs and reduced environmental impact, making a beneficial contribution to energy sustainability and environmental protection.

Economic analysis
The purchase of electricity is one of the important operational costs for the system.Through the evaluation of electricity purchase costs, it is determined whether the system is economical.Lower electricity purchase costs typically indicate a more economical system because it can provide the required electricity at a lower cost.PV/T systems, due to their combined photovoltaic and thermal energy technologies, can generate electricity, which means that PV/T systems can not only meet the building's electrical needs but also generate additional electricity.Table 3. Economic analysis of PV/T system.
In Table 3, the data indicates that during the heating season, systems using PV/T technology can reduce energy costs, increase revenue and lower maintenance expenses, thereby demonstrating economic benefits in overall costs.These benefits contribute to improving the sustainability of buildings, reducing reliance on traditional energy sources, minimizing environmental impacts and providing a long-term return on investment for the system.

Conclusion
Under the background of China's current energy conservation and emission reduction and the realization of the "double carbon" goal, the field of building heating and cooling has great development prospects.The building energy supply system based on PV/T proposed in this paper has unique characteristics, combined with photovoltaic solar thermal technology, to achieve the efficient and comprehensive utilization of solar energy.Features include thermal energy storage, optimized photovoltaic panel angles, application of reflector and concentrating technologies, and efficiency enhancement of insulation, which makes the system excellent for power generation, heating and hot water.
Building energy supply systems that are based on photovoltaic technology have problems such as low power generation efficiency, indoor overheating in summer, single function and spot pollution, which hinder its large-scale application.Based on solar thermal technology, solar water heating technology and passive heating technology, there are also problems such as indoor overheating, single function and large temperature fluctuation in summer, low utilization rate throughout the year and poor indoor comfort.Due to the use of a heat pump system, the system can realize a variety of functions required by modern buildings, including year-round power supply, solar auxiliary heating in winter, single heating of heat pump in winter, hot water supply in winter, single cooling and cooling heating in summer.The system gives full play to the advantages of photovoltaic technology and solar thermal technology and complements each other's shortcomings.
Through the analysis of COP, comprehensive average efficiency and economy of the building energy supply system proposed in this paper, we find that compared with the single photovoltaic utilization technology and the single solar thermal utilization technology, the application of photovoltaic solar

PV/T economics analysis
Electricity purchase cost (RMB) Electricity sales fee (RMB) Total cost thermal technology has a significant increase in system performance.Therefore, by complementing photovoltaic utilization technology and solar thermal utilization technology, the overall utilization efficiency of solar energy can be improved.At the same time, PV/T systems have the potential economic benefits of reducing electricity bills and increasing revenues because the energy supply system can generate electricity on its own and sell it with electricity.
In the construction industry, energy efficiency and sustainability are top priorities.The PV/T system provides a viable solution for green buildings, reducing not only energy consumption but also less reliance on traditional power supplies.This study proves the potential and application of PV/T systems in the field of green building.The introduction of this technology will help improve the energy efficiency of buildings, reduce environmental impact and support the future development of the sustainable building industry.

Figure 1 .
Figure 1.-PV/T thermal storage wall.The one-way heat pipe is a flat, sealed copper heat pipe.The evaporative section is welded to the back of the aluminum panel and the condensing section slightly inclines upward as it passes through the solid heat storage material.The use of flat-shaped one-way heat pipes is advantageous for secure welding to the back of the aluminum panel, increasing the heat transfer surface area between the heat pipe and the aluminum panel.The condensing section of the heat pipe slightly inclines upwards and is positioned higher than the evaporative section.This design allows the medium inside the heat pipe to efficiently return to the evaporative section by gravity after releasing heat in the condensing section.Additionally, it prevents heat from transferring in reverse from the solid heat storage material to the aluminum panel due to lower nighttime temperatures, ensuring unidirectional heat transfer from the aluminum panel to the solid heat storage material.The front insulating layer is adhered to the back of the aluminum panel and the evaporative section of the one-way heat pipe is tightly integrated with the front insulating layer.The cross-section of the front insulating layer is serrated.The front insulating layer, left insulating layer, rear insulating layer, right insulating layer, upper insulating layer and lower insulating layer enclose a sealed heat storage cavity that is filled with solid heat storage material.These insulating layers not only reduce heat loss but also compensate for the thermal expansion and contraction of the PV/T heat storage wall.The glass cover plate uses tempered coated glass.There is an air gap between the glass cover plate and the aluminum panel.The Fresnel lens is adhered to the inside of the glass cover plate using a method involving vacuum suction with silicone gel.The copper material of the heat pump evaporator coil is arranged in a serpentine fashion within the solid heat storage material and the connection between the heat pump evaporator coil and the PV/T heat storage wall is on the same side with the flow going in from the bottom and out from the top.The PV/T heat storage wall can be installed on the outer edge of the building's exterior walls or can be arranged in an L-shape at the intersection of the building's exterior walls.The heat pump can be placed on the ground or an equipment platform attached to the exterior wall of the building.When the PV/T heat storage wall is positioned at the intersection of the building's exterior walls, it can reduce the adverse effects of thermal bridges.The PV/T heat storage wall consists of front and rear insulating layers,

Figure 2 .
Figure 2. PV/T heat storage wall layout at the intersection of exterior walls with heat pump connection schematic.

Figure 3 .
Figure 3. Schematic diagram of PVT thermal storage wall and energy supply system in cold/hot supply scenario.