The evaluation of the energy utilisation in buildings is currently based on considering the primary energy. Calculations of the primary energy demand are based on the application of energy balances that take into account all energy conversion steps and the resulting losses. However, this is a purely quantitative approach. Although different forms of energy are assessed differently based on the primary energy factors, there is no comprehensive consideration of the thermodynamic qualities of the required energy flows. This is where so-called low-exergy concepts come into play. The intention is to not only reduce the respective quantities for the demand and supply but also to harmonise the respectively deployed energy qualities with one another. It is only when the quality is taken into consideration that the use of adapted heat sources and sinks is able to take full effect.
The exergy-based optimisation of supply concepts with the corresponding system components is aimed at minimising both the exergy destruction within a component or system as well as the external exergy losses. This not only reduces the exergy requirement owing to the lower demand for energy but also improves the use of the supplied exergy.
So-called low-exergy systems are distinguished by the fact that their energy efficiency performance depends decisively on system temperatures – the lower the better. However, high temperatures of up to 75 °C are particularly used in existing building stock to prevent radiators from being too large. Here there are also new solutions for transferring heat to the room with low temperature differences.