PVT – Solar energy for electricity and heat usage
What is a PVT module?
First, a brief summary of the concept "PVT module": PVT stands for photovoltaic + thermal. Photovoltaic systems convert sunlight into electrical energy using solar cells.
A PVT collector generates electrical and thermal energy. At the back of the PV module is a heat exchanger that transfers the heat generated to a carrier medium - usually a brine mixture that prevents the liquid from freezing in winter. The recovered heat can then be used to heat the building and supply hot water via a heat pump. This causes the PV module to cool down, which in turn leads to a higher electrical yield. This is because the warmer the PV module, the poorer its efficiency.
So, what is the electrical yield?
In addition to the location, the irradiation also depends on the orientation and the inclination of the collector. In Germany, the optimal orientation for the annual yield is south, with an angle of inclination of about 30°. This results in a radiation energy amount of about 1,000 kWh per m² per year, depending on the location. So that would be the amount of energy available. With a modern PV module with an efficiency of 20%, one could thus generate a roughly estimated electrical energy quantity of 200 kWh per m² per year.
By the way, the maximum theoretical efficiency of polycrystalline silicon cells is 29.1%. This is because only 29.1% of the light that reaches our photovoltaic modules is energetic enough to excite electrons to flow in the crystalline silicon.
Each substance can only process certain wavelengths of light. Development is therefore moving in the direction of so-called tandem solar cells. These combine several materials that can use different wavelengths to generate energy. In this way, significantly higher efficiencies may also be possible in the future.
And what is the thermal yield?
To get to the bottom of this question, we performed simulations in TRNSYS, a software for the dynamic calculation of building energy systems. The following table shows the thermal yield normalized to the maximum value of a south-facing PVT collector with 35° inclination at the Augsburg site:
The PVT yield was determined for each month as a function of the return temperature. This is the temperature that the heat transfer medium has at the collector inlet. The very large influence on the yield can be clearly seen. The warmer the return flow, the lower the yield.
Through the simulation it becomes clear that the yield of the collector depends not only on its efficiency, location and orientation, but also very much on the return temperature, and thus on the consumers. In reality, the return temperature is never constant, but dynamically dependent on the overall system. The more heat can be taken from the consumer side, the cooler the brine flows back to the collector. A general statement about the collector's yield is therefore not possible. Therefore, it is not enough to simulate the collector at the site alone. We need a system simulation, including the consumers.
For this purpose, we use the simulation program Polysun, a software for simulation-based planning, design and optimization of holistic energy systems. We have investigated different scenarios and compared the annual thermal yield per m² collector area.
Here again, the strong dependence on the consumer side becomes clear. If the PVT system is used to preheat hot water for the heating system, we are below 300 kWh/m²/a in this example. If, on the other hand, the system is used for heating a swimming pool or regenerating a geothermal probe, values above 350 kWh/m²/a can be achieved.
When is PVT particularly efficient?
Whereas the electricity generated can always be fed into the grid and thus remunerated, this is not possible with heat. The generated heat must always be consumed directly or stored temporarily in a heat storage tank.
When the solar supply is highest in summer, the heating demand is usually lowest. Thus, only part of the energy provided by a thermal collector in summer can be used.
If there are large thermal consumers in the system, such as a swimming pool or a geothermal probe, the heat generated can be used to a greater extent. This increases the annual yield immensely. So the more consumers I have that also take heat energy in the summer, the more efficient my PVT system is.
See also our earlier blog post Miracle weapon PVT - what are collectors capable of? And what aren't they?
Summary of the most important points:
- PVT collectors generate electricity and heat from sunlight.
- The electrical yield can be predicted well via efficiency, orientation and location.
- The thermal yield is very strongly dependent on the overall system, i.e. also on the consumers. Therefore, a system simulation is necessary for a yield prediction.
Are you thinking about installing a PVT system for the simultaneous generation of heat and electricity? We will be happy to advise you and can determine the expected yield and profitability of the system with a system simulation.
While the electrical power generated by pure PV modules can be calculated directly from the module efficiency and the solar radiation, the overall system plays a decisive role in the thermal yield of PVT modules. And that means: It is not only the system and its location that matter, but also what the energy is to be used for.