Energy for use in buildings can be produced locally at the building or at a remote location. Local production is often a furnace burning oil, natural gas, wood and so on, or it can be a geothermal
resource utilised by e.g. a heat pump. Furnaces are mainly used for heating and hot water. Other local supplies are renewable sources such as solar collectors or photovoltaic panels (PV).
Remote production of electricity is based mainly on the combustion of fossil fuels, biomass or waste, or by nuclear power. Heat can also be produced in a
remote location in the form of district heating. This can be generated in combination with electricity plants (combined heat and power, CHP) making it a more energy-efficient method. In recent years, central solar heating plants have been built in connection with district heating systems. Generally speaking, there is a great interest in renewable energy sources but most of the world’s energy demand is still met by fossil fuels.
Fossil fuels emit CO2 when converted into heat or electricity. The CO2 causes climate change (IPCC, 2007) and reserves are on their way to depletion. Renewable sources (wind energy, hydro power, solar power, etc.) are all powered by the sun, a virtually unlimited source of energy.