Since the 1970s, major progress has been achieved in the utilisation of renewable energies for the domestic sector. More and more single-family and multi-family houses are using solar energy, environmental heat or solid fuels such as wood logs and pellets to generate heating energy and hot water.
This trend is due not only to the government promotion programmes, which subsidise the installation of energy-efficient and environmentally compatible heating systems, but also to the fact that such systems meanwhile offer an operating comfort and performance that are comparable with those of oil and gas-fired heating systems. Moreover, regenerative energies help to cut current heating and hot water costs and to reduce pollutant emissions.

Solar thermal systems

The sun is an inexhaustible source of energy. Until well into the 1970s, however, there was only little interest in the utilisation of solar energy and the technology was underdeveloped. The solar heating of domestic water and solar heating support became a growth market only after the first oil crisis and as a result of growing environmental awareness. According to an IEA study, the global installed base of solar systems had an output of roughly 70 giga-watts in 2004; the IEA sees additional potential for solar systems. Thanks to technological progress, it has been possible for quite a while to cover a large portion of a private household ’s energy demand with solar energy also in less sunny regions such as Northern and Central Europe. The systems used today are very powerful; they use highly efficient collectors, respond quickly to changing weather thanks to modern control technology and switch to heating boiler operation whenever required.

Generally speaking, there are major regional differences with regard to the technology used. While solar thermal systems with a circulating pump in the solar cycle are primarily used in Northern and Central Europe, usually in combination with a heating boiler, gravity-controlled thermosiphon systems dominate in Southern Europe, Asia and South America.
In these more simple solar systems, which are used only to heat domestic water, the storage tank is mounted above the collector, so that gravity is used for the solar cycle. The sunlight heats up the solar liquid in the collector, which thus becomes lighter and moves up the pipes. An additional pump is not needed.
Even more simple and cheaper are single-circuit systems, in which natural water flows through the absorber, is heated and used as hot water in the domestic sector. This system may also be used to heat swimming pools. Given that these systems are very sensitive to frost, they are not suitable for use in Central Europe.
The special advantage of solar thermal systems is the fact that the availability of sunlight as an energy carrier is almost unlimited and that virtually no hazardous substances are produced in the generation of heat.
Given that energy is generated close to the place of consumption, the system losses are negligible.

In recent years, there has been growing interest in using solar energy also for the air-conditioning of buildings. This option is particularly interesting because of the obvious seasonal match between the energy requirements of such cooling systems and the availability of strong solar radiation. Given that the required systems technology is still in its infancy, however, only few such systems exist. Basically, advanced technologies could help reduce the consumption of primary energy substantially as compared to conventional air-conditioning plants running on electricity or gas.

This is how a solar thermal system in a private home works

The heart of a solar system is the solar collector, which converts the short-wave radiation from the sun into long-wave thermal radiation. The captured sunlight heats up a special heat transfer liquid in the collector absorber, which is transported through a cycle to the hot water storage tank, where the transfer liquid releases its heat. It is then cycled back to the collector.
If the solar radiation is insufficient, the water in the storage tank is additionally heated by a heating boiler.

Log and pellet boilers

Thanks to technological progress, wood has become an attractive regenerative source of energy in the past years. Especially in Western Europe, modern heating systems for wood logs, chips or pellets are becoming increasingly popular.
In addition to much greater operating comfort, modern wood and pellet boilers are low in pollutants and characterised by especially efficient combustion. Pellet boilers usually achieve a standard efficiency of over 90 percent.

Using wood for heating offers clear ecological advantages: wood is renewable, locally available and almost CO2 -neutral when burnt, as it releases only the carbon dioxide it has absorbed during its life. Moreover, the energy requirements for the production of wood logs and pellets are only 1 percent and 5 percent, respectively, of the energy content - less than . is needed to supply natural gas or heating oil. Also, wood is regionally available and thus reduces the dependence on oil and gas.

The range of log and pellet heatings includes everything from stoves used to heat individual rooms to central heatings installed in the cellar. Depending on the application, they can cover the entire heat requirements of a building on their own or in combination with other heating systems such as a solar thermal system.

Heat pumps

Heat pumps also use the power of the sun, i.e. solar heat stored in the air, the soil and the ground water. A heat pump takes heat from the soil, for example, so that it can be used by the heating and hot water system. The principle on which such a pump is based is not new, as it is the reverse principle of the refrigerator, which extracts heat from the food stored inside and releases it into the environment.
There are heat pumps that can also be used for both purposes - for heating in the winter and for cooling in the summer. Such heat pumps, which take their energy from the air, are especially popular in North America, whereas heat pumps for the heating of buildings and domestic water dominate in Europe. Here geothermal pumps are most common, which use the soil as a source of heat.

Heat pumps need only little energy. 1 kWh of electricity is sufficient to generate up to 5 kWh of heating energy, which means about 80 percent of the heating energy is drawn from the environment. Accordingly, current operating costs of a heat pump are low, but the upfront investment is usually higher than for a gas or oil-fired heating.

Although heat pumps are technologically mature, environmentally compatible and highly efficient, they hold only a relatively small share of the large European heating technology markets.
By contrast, this technology is installed in over 30 percent and over 90 percent of all new buildings in Switzerland and Sweden, respectively.

This is how a heat pump works

Depending on the source of heat, a distinction
is made between geothermal pumps, ground water pumps and air heat pumps. All these pumps work according to the same principle, though: A refrigerant, which is a mixture of water and environmentally compatible antifreeze, circulates in a cycle. This refrigerant extracts heat from the soil, the air or the water.
The thermal energy taken from the environment causes the originally liquid refrigerant to vaporise. It is then compressed in a condenser, which increases the pressure - the refrigerant vapour is "pumped" to a higher temperature. In the heat exchanger, the "heated" vapour transfers its heat to the heating water and liquefies. The pressure is then reduced, so that the refrigerant takes its original form and the cycle can start again.