Solar Thermal: Function, Costs, Advantages

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While photovoltaic systems are on everyone’s lips, solar thermal energy continues to eke out a shadowy existence. This form of the solar system also helps property owners to contribute to the energy transition and, ideally, to save money at the same time. Find out here how a solar thermal system converts solar radiation into heat, what you should consider when making your selection and what costs you have to reckon with.

How Does Solar Thermal Work?

How solar thermal energy works can be illustrated with a simple example: If you leave a dark water hose in the sun for a longer period of time, the water in it will inevitably heat up. In solar thermal systems, solar collectors on the roof are used to capture the sun’s rays. The natural heat heats the brine-water mixture circulating in the collectors. The heat is transported via the liquid into the insulated solar storage tank, where it can ultimately be used in the form of hot water or as solar energy for the heating system.

Solar drinking water heating and solar heating support

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

Solar drinking water heating

Here the solar thermal system is used exclusively for the production of hot water. On average over the year, the solar system can cover around 50 percent of the hot water requirement. In the summer months, 100 percent of the warm water can be obtained from solar thermal energy.

Solar heating support

If you opt for a combination system, the heat will not only be used to produce hot water. The heat is fed into the heating circuit via a buffer or combination storage tank, which relieves your regular heating system. The solar collectors can cover around 20 percent of the total heat demand.

Unfortunately, solar thermal systems are not efficient enough to heat the entire household. In addition, they are subject to seasonal fluctuations, which is why connection to conventional heating is always necessary. Solar thermal systems can be combined with the following heating systems:

  • Heat pump
  • Pellet heating
  • Oil heating
  • Gas heating
  • Block-type thermal power station (CHP)

Requirements for solar thermal systems

In order to be able to guarantee the highest possible efficiency for your solar thermal system, you should first check the structural requirements. The yield and performance of your system depend to a large extent on the following factors:

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

Advantages and Disadvantages

Before you get the first offers for your solar collectors, you should look again at the advantages and disadvantages of solar systems.

Solar thermal: Advantages and Disadvantages at a Glance

Advantages

  • Reduced energy consumption
  • Reduced heating costs
  • Reduced CO2 emissions
  • Increased independence from fossil fuels and their price development
  • The ideal addition to common oil and gas heating systems
  • Attractive funding opportunities

Disadvantages 

  • Ongoing maintenance costs
  • High acquisition costs
  • High dependence on solar radiation
  • Seasonal fluctuations in efficiency
  • High space requirements for solar, buffer or combination storage
  • High energy consumption in the manufacture of the collectors

What does a solar thermal system cost?

A solar thermal system is associated with relatively high acquisition costs. For example, if you want to supply a four-person household with hot water, you must expect an investment of between 5,000 and 6,000 U$D. It gets more expensive if you also want to relieve your heating with your solar panels. As a rule, this doubles the costs.

You also incur ongoing costs for maintenance and inspection. You should set aside around 150 to 200 USD per year for this. Trade associations recommend having the system serviced about once a year. After about three to five years, you should order a major inspection.

Funding opportunities for solar systems

The federal government has a great interest in promoting renewable energies, which is why the subsidies for solar thermal systems have been increased to 30 percent as part of the climate package. If you would like to get rid of your old oil heating system and instead switch to an environmentally friendly gas-solar hybrid heating system , the state will even subsidize your project with 40 percent.

You can apply for a low-cost loan for your solar system via the KfW Bank’s program 167 so that you do not have to raise the rest of the sum on your own.

Calculate size: How big does my solar thermal system have to be?

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

In contrast to the photovoltaic system, you cannot simply feed the surplus into the local grid with solar thermal systems. In order to ensure that your system is not only worthwhile from an ecological point of view, but also financially, you should not plan too much buffer. Also consider that solar collectors have a lifespan of around 20 to 25 years and therefore require long-term planning: If, for example, it is already foreseeable that your children will move out during this period and that the household size will be reduced as a result, you should include this in your calculation let it flow in.

In addition to the area of ​​the solar collectors, you also have to determine the volume 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 around 300 to 400 liters. If you also want to connect household appliances such as dishwashers or washing machines to the solar thermal system, you should plan an additional 30 to 50 liters per device. With additional heating support, a combination storage tank is required, which should hold around 600 to 800 liters under the same conditions.

Calculating the return: Is solar thermal worth it?

There is no general answer to whether solar thermal energy pays off financially. The possible yield depends on a number of individual factors. For example, it depends on how much of the energy you use in your household. For orientation: The consumer advice center estimates that a solar thermal system pays off in the long run from a hot water consumption of around 40 cubic meters per year.

Roughly, it can be said that you can reduce the hot water costs in a four-person household by around 120 to 160 USD with solar collectors. Depending on how old and inefficient your current system is, the savings can be higher. As far as the heating backup is concerned, experts assume savings of around 250 to 300 USD per year with a living space of 150 square meters.

You can also find out whether a system is worthwhile for you by dividing the acquisition costs minus government subsidies by 20 years of service life. If you add the annual maintenance costs of 100 to 120 USD to this value, you get the costs calculated over the year. 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 excessive savings: It is assumed that a solar thermal system will only pay for itself in the last third of its service life. Only then will the acquisition costs be amortized and you can enjoy free hot water.

With all of these calculations, keep in mind that there is one factor that is difficult to take into account: 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 predict: If CO2 taxes were introduced in 2021 – as planned by the federal government, this would also have a positive effect on your returns.

With all the return forecasts, however, you must not neglect one thing: With solar collectors, you not only protect your wallet, but also the environment. With solar hot water, you can save around 450 kg of CO2 per year. If you also use your system for heating support, the value increases to 750 kg.

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