Exergy concept and its application to the built environment

This paper discusses how a built environmental control system such as space heating and cooling can be described by the concept of “exergy”, which quantifies what is consumed by any working systems from man-made systems such as heat engines or buildings to biological systems including human body. The reason for the intensive and extensive use of exergy concept is to deepen our understanding of the built environment and thereby to develop a variety of low-exergy systems for future buildings. First, the essence of exergy balance equations is reviewed and then some results obtained from the recent exergy research were presented. The important findings described in this paper are as follows: (1) a volume of indoor air contains both of “warm” or “cool” exergy and of “wet” or “dry” exergy, whose values are comparable to each other especially for a hot and humid summer condition; (2) an ordinary air-source heat pump is basically a device to separate exergy supplied by electricity into warm, cool and dry exergies by consuming more than 85% of the supplied exergy; (3) there is a set of a little higher mean radiant temperature and a little lower air temperature, which provides with the lowest human body exergy consumption rate in winter season; (4) availability of cool radiant exergy of 20–40 mW/m² seems to play a key role for thermal comfort in a naturally-ventilated room in summer season; and (5) “cool” radiant exergy available from the sky in hot and humid regions amounts to 1000 mW/m², which is not necessarily small if compared to the values of cool radiant exergy to be supplied indoors.