Solar thermal energy


For anyone looking for a sustainable way to provide heating or cooling for buildings or low temperature processes, solar thermal energy is one good option. Go ahead and find out more about solar collectors, their usage and fields of application!

Image Copyright: Fraunhofer ISE

Solar heating collector

© Fraunhofer ISE

While photovoltaic technology converts sunlight into electricity, solar thermal technology, on the other hand, turns sunlight into heat.

Solar collectors are used to harvest solar energy and use it to heat a carrier fluid, which is conducted into a heat store. Solar thermal energy can be used to heat water and to support heating in private homes. Commercially, it is used to produce process heat or cooling energy or to support drying processes in industry, agriculture and forestry.

The advantage of this technology is that the heated liquid can be stored. As a result, energy is also available at night when there is no sunshine.

Solar thermal energy is an extremely reliable and climate-friendly technology. However, its market development is dependent on changes in the price of electricity and gas and on funding opportunities.
In 2016, the total capacity of solar thermal technology installed worldwide amounted to more than 450 gigawatts-thermal (see IEA SHC).

Companies in the German solar thermal sector count among the world leaders in this field. Currently, they are investing in particular in research into new materials and technologies.


  • Secure heat supply in comparison to fire places or gas stove
  • Reduced fossil fuel use, emissions and costs
  • Quiet electricity generation
  • Tried-and-tested technology
  • Low maintenance requirementss

How it works: Solar thermal energy for heating and cooling


At the heart of the heat generation process is the collector. Usually mounted on rooftops, the installation absorbs the thermal energy provided by the sunlight, which is in turn used to heat a thermal agent, most commonly a fluid. Via a pipe system, the heated fluid can circulate through the building in question and be utilised for space heating, hot water generation or other purposes that require thermal energy.

Image Copyright: Bundesverband der Deutschen Heizungsindustrie (BDH)

© Bundesverband der Deutschen Heizungsindustrie (BDH)


Solar thermal systems are also used for cooling. In this application form open and closed solar thermal systems are common. In closed systems, an absorption cooling process is initiated using solar energy. Here the enclosed liquid makes no contact with the outside air. In open systems on the other hand, the water used as a coolant is in direct contact with the outside air which is to be cooled. Open systems normally use a combination of sorptive dehumidification and evaporation cooling.

A closer look: Different types of solar collectors

Solar thermal systems use different types of solar collectors with varying characteristics, especially in terms of the temperature level they can achieve and their cost intensity. They can be categorised as unglazed absorbers, flat plate collectors, air heater solar collectors and evacuated tube collectors.

Unglazed absorbers are the simplest type of solar collector, in which water flows through pipe- or pad-shaped installations, slowly heating up over time by absorbing the heat of solar radiation. The main advantage of this technology is its modest price, being less expensive than fossil fuel boilers. However, due to their simple design, unglazed absorbers achieve temperature levels of only 30–40°C and offer relatively low efficiency. They are frequently used for heating swimming pools or powering heat pumps.

Accounting for almost 90% of solar collectors installed, the flat plate collector is the most common type of collector in Germany. Beneath a robust glass pane, a metal solar absorber is used to absorb as much solar thermal energy as possible. These collectors and operate in a temperature range of 60– 90°C but are more expensive than unglazed absorbers.

Air heater solar collectors are similar to flat plate collectors. Yet, unlike other technologies, they use air instead of fluids as a heating agent. Air is heated and normally used to heat buildings immediately, without having to be stored in the interim. The heated air can also be used to dry agricultural products. The use of air-water heat exchangers enables the heating of water, e.g. tap water. While being relatively inexpensive in terms of purchase and maintenance, air collectors are less efficient than flat plate technology.

Evacuated tube collectors can be used to achieve high temperatures and efficiency levels. Isolated in a vacuum, the individual tubes form a closed system that transfers thermal energy through a frost-proof heat cycle to water or spaces to be heated. Tube collectors can achieve temperatures of 120°C. They are also the most cost-intensive technology option.

Concentration technologies such as parabolic troughs or linear fresnel collectors track the sun with moving mirrors or a reflective coating and can reach temperatures up to 400 degrees Celsius to produce either steam for industrial purposes or to evaporate water for driving turbines for electricity production. Further information on technologies is available in the section Concentrated Solar Power.

Fields of application: Domestic, commercial and industrial use

Image Copyright: KBB Kollektorbau GmbH

© KBB Kollektorbau GmbH

Domestic use

In domestic use, heating water for detached houses is the most common application for solar thermal energy systems worldwide. In Europe, these systems are designed to provide 100% of the warm water required in summer and 50–70% in winter. They consist of large collectors with a surface area of 3 to 6 m2 and a hot water storage capacity of 200 to 400 litres. With such a capacity, the average amount of heated water necessary for of a family of four can be stored. In colder months, the hot water can be heated mainly via a heat generator, such as a boiler or a heat pump, which is supported by the solar thermal energy system on sunny days. The boiler is usually operated with gas, oil or wood.

Commercial and industrial use

Solar thermal energy also has a variety of industrial and commercial applications. The energy obtained through a collector can, for example, be used to operate an air-conditioning system. The advantage of this technology is that its energy supply is highest when the need for cooling is the greatest – when the sun’s rays are most intense. In addition to the immediate savings in fossil fuels, this also reduces the peak period power loads in summer since less electricity-based cooling is needed. Furthermore, it can be used to heat fresh water and buildings, dehumidify the air, provide process heat for drying or washing purposes and for seawater desalination, or as direct steam production for processes up to 400°C.