Solar thermal energy: function, costs, advantages

While photovoltaic systems are the talk of the town, solar thermal energy is still in the shadows. In the process, this form of solar system also helps property owners contribute to the energy transition and, ideally, save money along the way. Find out here how a solar thermal system converts solar radiation into heat, what you should consider when choosing one, and what costs you should expect to pay.

How does solar thermal work?

A solar thermal system is installed on the roof

The functioning of solar thermal energy can be illustrated with a simple example: if you leave a dark water hose in the sun for a long period of time, the water inside it will inevitably heat up. In solar thermal systems, solar collectors on the roof are used to capture the sun's rays. The brine-water mixture circulating in the collectors heats up due to natural heat. The heat is transported via the fluid to the insulated solar storage tank, where it can eventually be used in the form of hot water or as solar energy for the heating system.

Solar domestic hot water and solar heating support

If you are toying with the idea of a solar thermal system, you must first decide how you want to use the heat. You have two options for this:

  • Solar domestic hot water: in this case, the solar thermal system is used exclusively for the production of hot water. On an annual average, the solar system can cover about 50 percent of the hot water demand. In the summer months, hot water can be obtained 100 percent via solar thermal energy.
  • Solar heating support: if you decide for a combined system, the heat is not only used for the production of hot water. The heat is fed into the heating circuit via a buffer or combination storage tank, thus relieving the burden on your regular heating system. Solar panels can cover about 20 percent of the total heat demand.

Unfortunately, solar thermal systems are not efficient enough to heat the entire household. Moreover, they are subject to seasonal fluctuations, which is why it is always necessary to connect them to a common heating system. Solar thermal systems can be combined with the following heating systems:

Requirements for solar thermal systems

To ensure the highest possible efficiency for your solar thermal system, you should first check the structural requirements. Yield and performance of your system depend largely on the following factors:

  • Orientation of the roof (ideally, the orientation is to the south or southwest)
  • Available roof area (minus shadows cast by other houses or trees)
  • Regional solar radiation
  • Roof pitch (ideal for hot water production: 30 to 50 degrees, ideal for backup heating: 45 to 60 degrees)

Advantages and disadvantages

Before you get the first quotes for your solar panels, it's a good idea to review the advantages and disadvantages of solar thermal systems.

Solar thermal: advantages and disadvantages at a glance

What does a solar thermal system cost??

A solar thermal system comes with relatively high initial costs. For example, if you want to supply a four-person household with hot water, you will need to make an investment in the amount of 4.000 to 5.000 euros. It will be more expensive if you also want to relieve your heating with your solar panels. As a rule, the costs for this are doubled.

You will also incur ongoing costs for maintenance and inspection. You should set aside about 80 to 100 euros per year for this purpose. Professional associations recommend having the system serviced about once a year. After about three to five years, you should commission a major inspection.

Funding opportunities for solar systems

The federal government has a vested interest in promoting renewable energy, which is why subsidies for solar thermal systems were increased to 30 percent as part of the climate package. If you want to get rid of your old oil heating system and switch to an environmentally friendly gas-solar hybrid heating system instead, the state will subsidize your project by as much as 40 percent.

So that you don't have to shoulder the remaining sum entirely on your own, you can apply for a favorable loan for your solar system through the kfw bank's program 167.

Calculate size: how big does my solar thermal system need to be?

Before you can calculate the size of your solar thermal system, you need to decide on the type of solar panels: in germany, either flat plate collectors or evacuated tube collectors are usually installed. Although the latter are more expensive, they are also characterized by higher performance due to better insulation. They are therefore particularly suitable for small roofs. If you have enough roof space available, you can confidently choose the cheaper flat plate collectors.

Furthermore, when determining the size, it also depends on whether you only want to generate hot water or also heat through solar thermal energy. Depending on the case, you can follow the following rules of thumb:

Flat plate collectors evacuated tube collectors
hot water 1,2-1,5 sqm per person 0,8-1 sqm per person
heating support 0,8-1,1 sqm per 10 sqm living area 0.5-0.8 sqm per 10 sqm of living space

Unlike the photovoltaic system, you cannot simply feed the surplus from solar thermal systems into the local grid. In order to ensure that your system is not only ecologically but also financially worthwhile, you should not plan too much buffer. Also keep in mind that solar panels come with a lifespan of about 20 to 25 years and accordingly require long-term planning: if, for example, it is already foreseeable that your children will move out in this period and that the household size will be reduced as a result, you should include this in your calculation.

In addition to the area of the solar collectors, you must also determine the capacity of the associated storage tank. To supply a four-person household with hot water, you should opt for a solar storage tank with a volume of about 300 to 400 liters. If you also want to connect household appliances such as dishwasher or washing machine to the solar thermal system, you should plan for another 30 to 50 liters per appliance. With additional heating support, a combination storage tank is necessary, which should hold about 600 to 800 liters under the same conditions.

Calculate return on investment: is solar thermal worth it?

Whether solar thermal energy pays off financially cannot be answered in a generalized way. The possible return depends on several individual factors. For example, it depends on how much of the energy produced you use in your household. For orientation: the consumer center estimates that a solar thermal system pays off in the long run from a hot water consumption of about 40 cubic meters per year.

Roughly, it can be said that with solar panels you can reduce hot water costs in a four-person household by about 100 to 140 euros. Depending on how old and inefficient your current system is, the savings can also be higher. As far as the heating support is concerned, experts assume a saving of about 200 to 250 euro per year for a living area of 150 sqm.

You can also find out whether a plant is worthwhile for you by dividing the acquisition costs minus the government subsidies by 20 years of operation. If you now add the annual maintenance costs of 80 to 100 euros to this value, you get the costs calculated on an annual basis. Now compare this value with your current annual oil and gas costs: the greater the difference, the greater the potential savings. However, you should not expect too much savings: it is assumed that a solar thermal system pays for itself only in the last third of your lifetime. Only then will the purchase costs have been amortized and you will be able to enjoy free hot water.

Consider with all these calculations that you can calculate one factor only with difficulty: the future energy costs. The more the oil and gas prices rise, the more you save by switching to solar thermal energy. Further developments are also difficult to foresee: if CO2 taxes are introduced in 2021, as planned by the german government, this would also have a positive impact on your return on investment.

With all the yield forecasts, however, you must not neglect one thing: with solar collectors, you not only protect your wallet, but also the environment. With solar water heating you can save about 450 kg CO2 per year. If you also use your system for heating support, the value increases to 750 kg. This is roughly equivalent to a medium-haul flight from munich to athens.

Conclusion: gentle heat energy thanks to solar thermal energy

With a solar thermal system, you can easily turn solar energy into heat that you can use in your home. It is possible to use solar thermal energy to generate hot water or additionally to relieve the regular heating system. In both cases you can relieve your wallet and make yourself independent of the rising prices for fossil fuels to a certain extent. Incidentally, you also make an important contribution to the environment, which is why you can look forward to attractive government subsidies.