Nowadays, energy systems in buildings are designed based solely on the energy conservation principle. However, this principle alone does not provide a full understanding of important aspects of energy use in buildings, e.g. matching the quality levels of energy supply and end-use; describing how the human body experiences temperature differences between indoor air and surfaces (e.g. wall, ceilings, etc.); fully expressing the advantages of using passive (e.g. thermal insulation, window design) and ambient energy (e.g. heat pumps) in buildings.
The exergy analysis method is well known for optimisation of energy conversion in large industrial and power plants. It is also applied to quantify material flows (e.g. plastics, metals) involved in the manufacturing and recycling of industrial products (e.g. cars). However, it is not popular in the building sector, and needs to be adapted to the needs of the building profession.
The effective use of energy is determined with both the first and second laws of thermodynamics. The first law of thermodynamics states that energy is conserved and cannot be destroyed. The second law of thermodynamics introduces the useful concept of exergy, which is a measure of the quality of energy. Exergy can be destroyed when energy is transferred or converted.
The classical exergy analysis enables to pinpoint the location, to understand the cause, and to establish the true magnitude of waste and loss. Exergy analysis is therefore an important tool for the design of systems since it provides the designer with answers to two important questions of where and why the losses occur. The designer can then proceed forward and work on how to improve the system.
The quality of energy represents its capacity to cause change. The fact that there is an energy quality is evident from our experience in everyday life, as illustrated in the Figure. It is obvious that 100 kJ of electricity stored in a 12 V and 2.3 Ah car battery is more useful (i.e., easier to transform into something useful) than the same amount of energy stored in 1 kg water at a temperature of 43 °C, if the ambient temperature is 20 °C. The first energy form is good for running a machine (e.g., a computer), or for operating a 40 W light bulb for 42 min or at least for warming 1 kg of water by 23 °C. The energy in the second example is only useful to wash our hands or a few dishes.