5.4 Energy use in buildings

The VELUX Group’s current terminology on energy use and windows includes two concepts: Energy Performance and Energy Balance (VELUX Group 2009).

Most of the energy used in buildings is used to maintain a comfortable indoor environment in terms of thermal comfort (heating or cooling) and air quality (ventilation). Other energy uses are electric light, domestic hot water and household appliances or other electrical equipment (refrigerators, computers, TVs etc.).

​Figure 5.4.1 Illustration of the flow of energy through a building on an annual basis. The amount of energy supplied from an external source is less than the total heat loss of the building, because occupants, electrical devices and especially windows add "free" energy. 

While energy consumption for heating in Denmark has been reduced during the last four decades due to efforts in legislation, electricity consumption has risen (Marsh et al., 2006). Similar trends in electricity consumption are expected to be seen in the rest of the western world. The reason is an increased number of consumer electronics, such as TVs, computers, stereos, portable music players, etc., which apart from stand-by consumption, are not covered by legislative requirements for energy efficiency.

When designing a building or planning for refurbishing, it is important to use energy-efficient solutions, and perhaps even more important to do so without compromising the quality of the indoor environment. In the end, buildings are built to protect us from the weather and keep us comfortable and healthy. However, considerate design can reduce energy demand significantly.

5.4.1 Primary energy vs net energy

Net energy (or final energy) is often the result of energy performance calculations. Different energy sources have different utilisation factors and different impact on the environment, and should, therefore, be weighted differently. The concept of “primary energy” is that a factor for each energy source is used to weigh each source with regard to environmental impact. The factor is multiplied by the energy demand and can be different for different types of energy.

​Figure 5.4.2 Energy demand of an existing Danish house for heating and electricity (cooling, ventilation fans and lighting) compared with the primary energy (factor = 2.5). 

In Norway and Sweden, a considerable proportion of electricity production is hydro powered and thus has no great impact on the environment; the primary energy factor for electricity in Sweden is 2.35 (Smeds and Larsen, 2007). In Germany, the main energy source for electricity production is still coal, which has a much greater impact; the primary energy factor for electricity in Germany is 2.7 (Reiser, 2008). For some types of energy use, the conversion factor may become less than 1. An example is district heating in Denmark, where the factor is 0.8 due to the increasing amount of renewable energy in the supply of district heating.

In the UK, the primary energy factor for natural gas is 1.02 and 2.92 for electricity, (British Research Establishment, 2009). Figure 5.4.2 illustrates the difference between net energy and primary energy; the net heating demand is substantially higher than the net electricity demand, whereas the primary energy demand for heating and electricity is approximately the same.