Conventional energy supply systems use fossil energy to heat and cool while we have sources of heat and cold all around us to supply the demand for energy, either in part or in full. Examples include the residual heat from production processes, the heat contained in groundwater, ventilation air or the outside air. In order to use these sources of energy to heat or cool, they first have to be brought to the correct temperature using heat pumps or held, for instance, in seasonal storage in aquifers (subterranean water basins) enabling the energy to be used at a later date.