Technologies and applications

There are two types of solar thermal energy systems for residential buildings: those used solely to heat water and those that also provide heating, the latter are so-called ‘combi-systems’. Around half of all newly installed solar thermal energy systems in Germany are used exclusively to heat water. They are typically designed to meet all hot water needs during the warmer half of the year. During the colder half of the year, a boiler fired by gas, oil or wood, or a heat pump, supported by the solar thermal energy system on sunny days, provides water heating. This means that around 60 % of water heating requirements can be met by solar thermal energy over the entire year.

In 2009, solar combi-systems, which also provide heating, had a market share of more than 60 % of newly installed systems in Germany. These have larger solar collector surface areas and help to heat the building in the spring and autumn months. Detached houses usually have installed systems with collector surfaces of 10 to 18 m². Solar energy typically supplies 10 – 30 % of a building’s total heating needs, depending on how well insulated the house is and on the extent of its heating requirements. There are also special solar houses that obtain more than 50 % and up to 100 % of their total heating requirements from solar thermal energy.

Solar collectors
There are different types of solar collectors.

The simplest form of collector is the unglazed plastic absorber; these black plastic mats, through which water is pumped, are typically used to heat swimming pools, reaching temperatures of 30 – 50 °C. About three quarters of the collectors used in Germany are flat plate collectors, the rest are evacuated tube collectors.

In flat plate collectors the solar absorber, which converts solar radiation into heat, is installed in an insulated glazed box to reduce heat loss. Flat plate collectors usually operate at temperatures ranging from 60 °C to 90 °C.

Air collectors are a special type of flat collector, which heat air and are usually used to heat buildings directly, without intermediate storage.
The heated air can also be used to dry agricultural produce. Integrating an air-water-heat exchanger can also enable these systems to heat water for domestic use.

Evacuated tube collectors achieve even higher temperatures and efficiency factors, because the strong negative pressure in the glass tubes further reduces heat loss. A collector is comprised of several glass tubes. The rotatable mounting of the individual tubes enables the flat absorber in the glass receiver to be optimally positioned towards the sun. For this reason, evacuated tube collectors can also be installed nearly horizontally on flat roofs. The individual tubes form a closed system that transfers the heat to the water via a frost-resistant thermal circuit.