Photovoltaic-Thermal (PVT) Systems for Combined Heat and Power: Technologies and Implementation

Abstract: Solar photovoltaic thermal technology (PVT) recovers waste heat from photovoltaic panels while generating electricity. This technology not only achieves combined solar heat and power generation but also reduces the temperature of the panels, improving power generation efficiency. This technology, especially when combined with low-temperature waste heat storage technology for seasonal storage, holds great promise for future applications.

Photovoltaic power generation technology is highly mature and widely used, offering promising applications. However, its efficiency is only around 20%, leaving a significant amount of solar energy lost to the environment.

Without PV generation, all solar heat is lost to the environment. In comparison, a 20% efficiency is sufficient. PV panels also have the effect of concentrating heat, reaching temperatures of up to 50 or 60 degrees Celsius. This concentrated heat reduces photovoltaic power generation efficiency, shortens its lifespan, and negatively impacts the panels. Heat that is dissipated into the environment in an uncontrolled manner is also not conducive to reuse.

Low-carbon energy sources are scarce in areas such as heating. Heat generation requires the consumption of primary energy, which consumes significant energy, is costly, and generates significant carbon emissions. Reusing the heat collected by PV panels can reduce panel damage, improve power generation efficiency, and provide a zero-carbon heat source, offering promising applications. With the increase in photovoltaic projects, this amount of heat is also very considerable.

Solar photovoltaic thermal technology (PVT) recovers waste heat from photovoltaic power generation, achieving solar combined heat and power.

PVT consists of two components: photovoltaic modules and heat sinks. PV modules are conventional technology, including photovoltaic glass, EVA film, solar cells, and a backsheet. The heat sinks consist of a heat-absorbing layer, heat transfer tubes, and insulation materials. These two components are assembled, and external components such as the frame and junction box are installed to form the PVT.

PVT Systems are available in two types: liquid-cooled and air-cooled. Liquid cooling typically uses water as the coolant, though antifreeze can also be used in cold regions. The structure is shown in the figure above. Air cooling uses gas, typically air, as shown in the figure below. The two products have different applications.

Air-cooled PVTs have wider cooling channels, and the outlet temperature is adjusted by controlling the gas flow rate. The hot air generated by this product is superheated, which can be used in two ways: as a direct drying heat source for products such as agricultural products and food that are not suitable for high-temperature drying; or as a low-temperature heat source in an air-source heat pump, generating high-temperature heat.

The cooling channel of water-cooled PVT is generally a thin tube. In order to ensure the cooling effect of the entire plate surface, the heat transfer tube needs to be tightly integrated with the heat transfer plate to achieve better heat recovery effect. The outlet water temperature of water-cooled PVT is low and difficult to use directly. It usually needs to be used in conjunction with a heat pump.

PVT Panel Systems with heat pumps

In general, the common application scenarios of PVT technology are as follows:

(1) Drying process: air-cooled PVT is used to generate superheated air to remove moisture from the dried material. Since the drying air temperature is low, it will not affect the quality. It is generally used in crop and food drying or other fields with high quality requirements.

(2) Distributed heating: PVT+heat pump is used to generate hot water for heating, domestic hot water, etc. Compared with the conventional air source heat pump route, the parameter optimization range of the PVT+heat pump process is large and the economy is very good.

(3) Inter-seasonal centralized heating: This is a scenario for large-scale application of PVT technology in the future. It is particularly suitable for occasions with huge space advantages such as rural heating. Rural heating systems have little usable waste heat, so directly using air-source heat pumps consumes significant energy. Leveraging the vast expanses of rural space, rooftop PVT systems can collect sunlight, while underground pipes can store heat in the soil during the off-season. Heat pumps can then utilize geothermal heat for heating in winter. This allows for the storage of low-temperature waste heat across seasons, raising the heat pump's waste heat temperature and significantly reducing operating costs.

In summary, PVT, as a low-cost improvement on photovoltaic technology, is a powerful complement to solar photovoltaic technology. If suitable waste heat utilization methods are available, such as drying, domestic hot water, or cross-season heating, PVT technology can significantly improve system efficiency, reduce operating costs, and possesses promising development prospects.