Energy sources
We distinguish between conventional energy and renewable energy sources. The heat generation can take place in both cases by means of heat exchange, i.e. transfer of energy due to temperature differences or by conversion from energy to heat, e.g. by means of combustion.

Heat generation through heat transfer
Heat generation by heat transfer takes place locally whilst the actual heat generation takes place in power plants.
In conventional systems coal, oil or gas are burned as fuels. So-called cogeneration generates electric power and hot water for local heat supply systems.
district heat, local heat or electric power
In schools solar technology is usually used only for the generation of domestic hot water using
solar heat, hydronic solar panel, vacuum hydronic solar panel, glass vacuum tube collector or heat pipe.

Apart from solar energy there are further renewable energy sources, which can be used for heat transfer. They include
- geothermal heat
- heat from bore-holes or watercourse
- solar generated electric power (PV)
- water and wind power

Heat generation through conversion
By heat generation through conversion we understand a heat generation, which is generated in boilers (with hot water storage), block type thermal power plants or other e.g. thermal-chemical processes of transformation in fuel cells.
Heat can be generated also by combustion of renewable materials. These are either in form of fermentation gas or liquid renewable fuel. Vegetable oil can also be used for combustion. Wood, in the form of wood chips or in form of pellets can be burnt CO2 neutrally.
- Renewable oil or gas fuels
- wood chips
- wood pellets