Is solar thermal better than PV

For a long time, photovoltaics performed excellently for private households in this comparison because solar power was heavily subsidized through a high feed-in tariff. This subsidy was cut significantly, which is why the earlier calculations no longer work out today. But electricity storage and heat pumps open up new uses for self-generated electricity.

Comparison of the degrees of efficiency

The collectors of a solar thermal system convert around 80 percent of the radiated solar energy into heat. Some losses occur during the subsequent circulation of the heated water, which is why the overall system typically has an efficiency of around 50 percent. In contrast, crystalline photovoltaic modules only convert around 20 percent of sunlight into electricity. Because the further losses of a photovoltaic system are relatively small, these 20 percent can also be viewed as a useful estimate of the overall efficiency of the photovoltaic system. It is not surprising that the manufacturers of solar collectors like to point out the significantly higher efficiency. This comparison is not really helpful. The conversion of sunlight into heat is a natural process that happens on its own everywhere. And even with efficiencies close to 100 percent. The conversion of sunlight into electricity, on the other hand, is much more complicated and inevitably involves considerable losses.

Comparison of storage technologies

Both solar thermal and photovoltaic reach their maximum output at midday on sunny summer days. In both cases, however, the energy is used more in the evening. The energy must therefore be temporarily stored. The buffer storage of the heating, to which the solar thermal system is connected, achieves this much cheaper than an electricity storage for a photovoltaic system.

Heating with solar power?

Heating with electricity did not enjoy a good reputation for a long time. Two reasons have led to a rethink in this regard. The first reason is the now affordable heat pumps. Even the particularly simple and inexpensive air heat pumps achieve annual performance factors of 2.5 or more. This means that they generate at least 2.5 kilowatt hours of heat from one kilowatt hour of electricity. This significantly improves the ecological and economic balance of heating with electricity. The second reason is that heat is cheaper and easier to store than electricity. Photovoltaic systems are volatile power generators whose short-term fluctuating power feeds pose a problem for the power grids. Until sufficient electricity storage capacity is available, it makes perfect sense as an interim solution to smooth out power peaks in electricity generation by temporarily storing the electricity in the form of thermal heat. This means that the areas of application of photovoltaics and solar thermal energy overlap. However, it would be less than convincing to install a photovoltaic system exclusively for heating purposes. Solar thermal is clearly the better choice for this.

Solar thermal in the new building

For new buildings, the Energy Saving Ordinance contains strict requirements with regard to the primary energy requirement, which can hardly be met with exclusively fossil heating systems. Solar thermal energy is ideal for this. As far as the fulfillment of these legal requirements is concerned, photovoltaics is not an alternative, as it is not about the energy turnaround, but about the climate-neutral building stock. However, pellet heating systems or heat pumps, which are the most important competitors of solar thermal energy in this segment, are very well considered. In addition to heat pumps or pellet heating systems, photovoltaics can of course be considered, but not as a substitute for them.

Do photovoltaics and solar thermal also work together?

Both systems can definitely be combined. For this purpose, the roof area can be divided between collectors and solar modules or both can be combined in so-called hybrid collectors. In a hybrid collector, the collector is located behind the solar module and uses the radiation that cannot be used by photovoltaics. Theoretically, this concept is convincing because in this way the heat is dissipated into the heating circuit behind the solar module. This increases the efficiency of the solar modules because they work more efficiently at lower temperatures. In practice, however, hybrid collectors have never been able to establish themselves.

Photovoltaic or solar thermal? In this case, electricity was generated using a photovoltaic system and heat was generated using a solar thermal system.

Expensive kilowatt hours, cheap kilowatt hours

Roughly simplified, the comparison between photovoltaics and solar thermal systems is as follows: The solar thermal system delivers 2.5 times as many kilowatt hours of heat per square meter of roof area as the photovoltaic system delivers electricity. In addition, the photovoltaic system is more expensive to purchase. On the other hand, every kilowatt hour of household electricity costs around 3.5 times as much a kilowatt hour of heat. The higher the self-consumption, the better the photovoltaic scores compared to solar thermal. Without any measures to increase self-consumption (electricity storage, heat pump), the higher costs of photovoltaics compared to solar thermal systems usually no longer pay off.