How are solar panels constructed

Get heat from the sun with solar collectors

Solar collectors absorb solar energy and convert it into heat. As the main part of a thermal solar system, they produce heat for hot water generation and heating support in the home. Here at you can read more about the structure and function of solar collectors and get tips for buying them.

Structure and function of solar panels

Solar panels, also called solar panels or thermal solar panels, represent the main component of one Solar thermal system In contrast to solar modules, which convert solar energy into electricity, the collectors produce heat or thermal energy that can be used in private households for hot water preparation and heating. In this way, up to 40% of the heating and up to 70% of the domestic hot water can be provided by solar energy. Furthermore, solar collectors are used to heat swimming pools, in solar thermal power plants for power generation and for the production of process energy in the industrial sector.

This is how a solar collector is constructed:

Inside the collector is the absorberthat collects solar radiation, heats up and transfers the heat to a heat transfer medium. The heat transfer medium is a liquid that usually consists of water and propylene glycol - one also speaks of the Solar fluid. Propylene glycol is said to prevent the water from freezing in winter and getting too hot in summer. The heated liquid is pumped through the absorber via pipes and then transported to the solar storage tank. This supplies the household with warm water.

So that the collector is protected from heat loss to the outside, the absorber is insulated at the side and below. Insulation is only omitted in low-temperature collectors for heating swimming pools. The collector housing and a transparent one protect against the weather and mechanical loads such as snow, hail or wind Glass cover. The latter also prevents heat from the absorber from being extracted by cold wind, but also prevents heat from being radiated from the absorber.

For more information on how a solar panel works, see this video:

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These types of solar panels exist

A distinction is made between the following collectors for private use:

Flat plate collector:

Flat-plate collectors work according to the principle described above. The absorber is a black coated sheet made of copper or aluminum. The pipes for transporting the heat-conducting solar fluid are attached to its underside. The thermal insulation on the rear and the side walls of the absorber usually consists of polyurethane foam, mineral wool or foam glass. The glass cover is usually a low-iron solar safety glass. Flat-plate collectors are among the most common collectors in Germany, as they are relatively cheap to buy with 300 to 400 € / m² collector surface. Their disadvantage: Due to the mediocre insulation, they are neither suitable for the very high water temperatures required nor for low outside temperatures - in both cases the heat loss is high.

Evacuated tube collector:

This collector shape has a more complex insulation: Here the absorber surface is installed in evacuated (evacuated) glass tubes. Metal reflectors surround these vacuum tubes to direct sunlight into the tubes and heat the absorber. The heat from the absorber is conducted via a heat transfer medium through the tubes to a collecting tube on the frame of the collector. This is connected to the collector circuit.

A distinction is made between the following types of vacuum tube collector:

  • Vacuum tube collector with direct flow: The heat transfer medium is a liquid that flows directly through the tube into the collecting tube.
  • Heat pipe tube collector: The liquid in the heat pipe evaporates even if it is only slightly heated. The steam rises in the tubes and transfers the heat to a heat transfer fluid in the collecting tube via a heat exchanger. The steam then cools down and flows back to the end of the tube. In order for this evaporation and condensation process to work, the tubes must be installed at a certain angle of inclination.

Since with a vacuum tube collector the conduction of heat only takes place at the end, i.e. in the collecting pipe, heat losses can be significantly reduced. This is a great advantage, especially when the outside temperature is cold. Higher temperatures can also be reached. The efficient construction, however, is associated with higher acquisition costs: calculate with 700 to 900 € / m² collector surface.


The construction or expansion of a solar thermal system for hot water and heating is subsidized by the Federal Office of Economics and Export Control (BAFA) with up to € 200 / m² gross collector area. You can find out more about this on this page.

Other designs of the solar collector are:

  • Vacuum flat-plate collectors (a hybrid of flat and vacuum tube collectors)
  • Absorber mats (made of black plastic for heating the water in swimming pools)
  • Hybrid collectors (combination of photovoltaic modules and solar collectors)
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Solar collectors achieve a very high level of efficiency (60-75% compared to only around 20% for photovoltaic modules). In addition to generating hot water, they are therefore also used to generate electricity. This is done in Solar thermal power plantsthat work according to the burning mirror principle and use special parabolic internal collectors. The heat conductor can be heated to temperatures of up to 400 ° C. The resulting steam is used to drive turbines, which in turn generate electricity. However, this procedure is only worthwhile if there is very strong direct sunlight.

Which solar collector is the right one?

When choosing the type of collector you should ask yourself the following questions:

  • Should the solar thermal system provide the highest possible solar coverage or is maximum economy and money saving more important to you?

    In the first case, tube collectors are profitable because they have a higher degree of efficiency and produce a lot of power even with little solar radiation (e.g. in winter). For a system that is as profitable as possible, you should opt for flat-plate collectors: they are much cheaper to buy.

  • How big must or may the system be?

    The size of the system and the number of collectors depend on the one hand on your power consumption from: Have your consumption calculated in advance so that the size of the system can be planned correctly. A system that is too large leads to high investment costs, while a system that is too small reduces the energy gain. On the other hand, the system size depends on the available area influenced: If there is little space available for the system, tube collectors are more advantageous because they are more powerful than flat-plate collectors.

  • What is the required temperature range?
  • While a simple absorber with a maximum temperature of 40 ° is usually required for swimming pools, the temperature range of flat-plate collectors is 20 to 80 ° - ideal for heating water in the household. In contrast, vacuum tube collectors deliver temperatures of up to 120 °.
Further tips for making a purchase decision:

In order for solar collectors to achieve their maximum heat yield, in addition to the system area and the type of collector, the following factors are decisive:

  • Global solar radiation: Good locations in Germany can deliver over 1,000 kWh / m² annually.
  • Orientation of the system: an orientation to the south and an angle of inclination of around 45 ° are ideal. The incline of the collectors can be optimized with the help of an elevation.
  • Shading or soiling of the system: Neighboring houses, trees or other objects that cast a shadow over the system can significantly reduce its efficiency.
  • Weather conditions: Especially when snow covers the collectors, they can no longer deliver any yield.
  • Performance and efficiency of the heat exchanger
  • Transmission losses
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