PVT collectors for the regeneration of geothermal fields (probes) in heat networks
PVT modules represent a combination of photovoltaic (PV) and thermal solar systems for the generation of electricity and heat. These systems utilize both solar radiation for electricity generation and heat energy. When PVT collectors are integrated into a heating network, they can also be used to regenerate geothermal probes in the summer, thereby increasing their efficiency in the winter.
How is a PVT collector structured, and what types are there?
A PVT module is essentially a hybrid collector consisting of a combination of a photovoltaic module and a solar thermal module. PVT collectors can be classified into different types depending on whether the focus is on electricity generation or heat production. PVT systems with an emphasis on heat production include covered PVT collectors. Due to the glazing, they have an enclosed air layer that acts as insulation, allowing heat to accumulate better in the collector and achieve higher heating outputs. However, the higher collector temperatures can have adverse effects on the PV cells, as increasing cell temperature reduces cell efficiency.
What are the advantages of a PVT collector?
By using combined energy sources, solar energy can be utilized more efficiently, leading to a reduction in the necessary collector area. The solar collector can also contribute to an increase in the efficiency of the PV module, as electrical performance decreases with rising temperature. Additionally, the PVT system, through the solar thermal collectors, prevents snow from covering the PV collectors, as the snow melts away, thereby improving the electricity generation of the PV collectors even in the winter. In combination with geothermal fields, heat production in the summer can be meaningfully utilized for regeneration.
What are the disadvantages of a PVT collector?
PVT collectors face some technical challenges that limit their widespread use. Similar to solar thermal collectors, there is a significant disparity between heat production in the summer and the heat demand in the winter. Due to the combination of different technologies, PVT collectors may have additional maintenance requirements to keep both the photovoltaic cells and the thermal components in good condition.
Why do geothermal probes need regeneration, and how can PVT collectors contribute?
Although geothermal energy is considered a renewable heat source, it's important to note that ground probes do not have an infinite heat potential. Geothermal probes extract a significant amount of heat from the ground during the winter. To avoid overcooling of the ground probes and to ensure a nearly constant heat extraction rate at the beginning of each heating season, it's advantageous to regenerate the ground probe fields with the supply of heat energy during the summer. This is often achieved through passive cooling of buildings. In conjunction with a heating network and geothermal probes, the excess heat energy from a PVT collector can be used to regenerate the ground probe fields while simultaneously keeping the PV cells at an optimal temperature level to maximize electricity production.
What roles do PVT collectors and geothermal fields play in heat networks?
Depending on the temperature level of the heating network, different heat sources are required to supply the network. Due to the limitation of the solar collector temperature by the operating temperature of the PV modules, the operating temperature of the PVT combination is typically between 20-50°C (depending on the design). Therefore, the use of PVT collectors is primarily suitable for heating networks with lower temperature levels, such as LowEx heating networks (35-60°C) or cold district heating networks (below 35°C), also known as anergy networks. Ground probe fields typically have an extraction temperature that fluctuates between 0-20°C over the year, serving as a heat source for cold district heating networks. However, it is also possible to provide heat from shallow geothermal sources for higher temperature levels through a central network heat pump. Over the years, the extraction temperature of geothermal sources decreases because more heat is extracted from the ground over the year than is coming from the ground. To prevent a decline in the extraction temperature over several years, geothermal fields should be regenerated. The larger the geothermal field, the more important regeneration becomes. Regeneration can be achieved using waste heat from cooling processes in the summer, conventional solar thermal collectors, or PVT collectors. By using PVT collectors, the number of probes or drilling meters in the ground probe field can be reduced, which can offer significant economic advantages. This should be considered in a comprehensive economic analysis for PVT collectors.
Sources
- Lämmle et al.: "PVT collector technologies in solar thermal systems: A systematic assessment of electrical and thermal yields with the novel characteristic temperature approach", Solar Energy, 155, pp. 867-879, 2017, DOI: 10.1016/j.solener.2017.07.015
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