As we have learned in the previous three chapters, heat can be produced in various ways. Heating is needed in many different places, the most important of which is buildings. Today, many buildings are heated with electricity. However, other methods are also possible.

Wood is a very common fuel. Fuels are materials that are burned to produce heat energy. Wood can be burned in many forms, such as firewood, wood chips or pellets. Wood is a type of biomass. It contains chemical energy that has been harnessed by the tree from the light of the Sun. In this sense, all fuels are originally based on solar energy.

Peat is another type of biomass used in heating. It is a type of soil produced in bogs. The Sun's energy has been harnessed by bog plants, which have then been turned into peat in a slow decomposition process. Peat can be used as a fuel in electricity production, but it can also be burned to provide heat.

Coal is a fossil fuel. It is formed from peat that has been transformed by underground pressure. The formation of coal takes a very long time, which is why it is considered a non-renewable energy source. Coal is mainly used to produce electricity in power plants.

Peat and coal.

Oil is also a type of fossil fuel. It is made from the remains of ancient living organisms that have been transformed into liquid underground. Like the formation of coal, the formation of oil takes a very long time. Oil can be used as a fuel in household heating. 

Natural gas is another type of fossil fuel. It is burned in power plants in order to produce heat and electricity.

Oil and natural gas are fossil fuels.