Questions and answers about solar thermal energy
When it comes to buying, installing and operating a solar thermal system, a lot of questions arise. Especially with regard to funding: because many renovators do not know how they can use the state aid for themselves. We have compiled the most frequently asked questions in the FAQ and answered them together with our experts.
# 1: Is a solar thermal system on every house worthwhile?
If the roof orientation and roof pitch allow it, then a solar thermal system is a sensible addition to an existing heating system (standard condensing technology). It saves 5 to 20 percent of annual energy consumption and is often planned for when a heating system is replaced. For those who prefer to switch off their heating completely in summer and still do not want to do without hot water comfort, there is no alternative to a solar thermal system.
#2: What subsidies are available for solar thermal energy?
Renovators receive funding from the Federal Funding for Efficient Buildings (BEG). Through the "Individual Measures" part of the programme, the state awards grants or loans with repayment subsidies of 30 per cent. Anyone who also installs a new gas heating system with the solar thermal system will receive 30 percent of the total costs (solar + heating) subsidized. For the combination with an environmental heating system (heat pump, wood heating), there is even a subsidy of 35 percent. In addition, the following bonuses are possible:
In total, there are subsidies of 20 to 50 percent for solar thermal energy. However, it is important that renovators apply for this before awarding contracts. Those who have already missed the time can use the tax bonus for renovation and claim 20 percent of the costs for tax purposes.
We provide an overview of current possibilities in the article on solar thermal subsidies.
#3: What are the different types of solar thermal systems?
A solar thermal system is available in two variants: Most often, it is installed for solar heating of hot water. It can cover up to 60 percent of the annual requirement. A drinking water storage tank or solar storage tank absorbs the heat from the solar fluid via a heat exchanger and stores it for use in the kitchen or bathroom. If the solar thermal system is also to serve as a heating support, it must be larger. With today's technology, this means that 20 percent of the total heating energy consumption can be saved. It is important to have a sufficiently dimensioned storage tank for heating drinking water and heating water. Space-saving combination storage systems are often used for this purpose.
# 4: What requirements should my roof meet for a solar thermal system?
The ideal pitched roof faces south, but a deviation of plus/minus 30 degrees is still within reason. When facing east or west, the solar heat yield is reduced by about 20 percent. This disadvantage can be compensated for by a larger collector area.
Trade associations recommend a roof pitch of 30 to 50 degrees for a solar system for hot water preparation, and 45 to 60 degrees for a solar system for additional heating support. In winter, more sunlight is captured when the sun is low.
On flat roofs, the collectors can be erected in the optimal possible orientation and inclination. It must be clarified in advance whether the roof is also structurally suitable to support the additional collectors (weight/wind load).
Caution with a listed house: In this case, a permit from the responsible building authority is required in advance.
#5: Is the roof area large enough to mount the solar collectors?
Just five to six square metres, i.e. about 3 collectors, are sufficient to partially cover the hot water needs of a four-person household. With a combined solar system for hot water preparation and heating support, almost twice the area must be planned. This space requirement can be reduced by using more powerful vacuum tube collectors.
Important: The selected roof area should not be shaded by neighboring buildings or trees.
#6: What Types of Solar Collectors Are There?
Flat plate collectors are most commonly used. These combine good performance values as well as cost-effective technology that has been tried and tested over many years with a compact design. A flat plate collector consists – from the outside to the inside – of a glass that allows as much sun as possible to pass through to the highly conductive aluminium absorber surface. This converts the radiation into heat, which is then transferred to the heat transfer medium. The so-called solar fluid flows in pipelines past the collector and transports the heat absorbed there directly to the storage tank.
Vacuum tube collectors are an alternative. These are more expensive than flat plate collectors, but harvest heat at a higher temperature level. They consist of glass tubes arranged next to each other in which there is a vacuum. The absorber is located in the core of the tubes and converts the sun's rays into heat without major losses.
# 7: When is which collector type used?
Flat plate collectors are the inexpensive all-rounders. Vacuum tube collectors are preferably used when high efficiency is required, for example with small roof areas, unfavorable building orientations or when higher temperatures are required. These collectors often provide usable heat yields even with less solar radiation, which makes them particularly suitable for use in a solar thermal system for solar heating support.
# 8: Where and how can solar collectors be installed?
In the case of a pitched roof, the solar collectors are usually mounted on the roof covering. Also possible here is the visually more appealing, but also more cost-intensive in-roof installation. There are suitable, elevated systems for flat roof installation. Even mounting solar collectors on the façade can be an efficient alternative.
#9: Does a solar thermal system also work in winter?
A solar thermal system always works when solar radiation hits the collector. Of course, this can also be in winter. However, the sun is then flatter on the horizon and the sun's rays are less likely to be captured. The duration of exposure to sunlight is also shorter in winter. Vacuum tube collectors are installed at a steeper angle and are thus able to generate high temperatures even from the short irradiation in winter. Solar thermal systems on vertical facades can also achieve good yields even in the cold season.
#10: Does a solar thermal system need to be maintained?
Trade associations recommend an annual inspection of the solar system as well as detailed maintenance every three to five years. In addition to the visual inspection of all components and the thermal insulation, the setting of the control system will be checked to see whether the solar fluid still has sufficient frost and corrosion protection.
#11: What is the lifespan of a solar thermal system?
A solar thermal system can be expected to have a service life of around 20 to 25 years. Many manufacturers give a 10-year warranty on the function of the collectors.
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