Unsteady-state exergy analysis on two types of building envelopes under time-varying boundary condition

In the built environment, the thermal exergy behavior is very sensitive to the change of environmental temperature, because the temperature difference between the reservoir and a system of interest is small. Moreover, the transient characteristics become very important for the building envelope, which is primarily affected by the environmental temperature changes and has a relatively large heat capacity. Most of the exergy analyses have been performed under steady-state assumption. However, it may miss some important details of the transient process. Thus, when the transient transfer process becomes important, the unsteady-state exergy analysis should be conducted. In this study, we propose complete energy, entropy, and exergy equations in their partial differential forms. By solving them numerically, we examined the transient exergy process inside the building envelope composed of concrete and insulation layers under time-varying boundary condition. Using this new methodology, we can improve the temporal and spatial resolution of the exergy analysis and thus provide more complete information about exergetic behavior

Exergy performance of different space heating systems: A theoretical study

Three space heating systems (floor heating with different floor covering resistances, radiator heating with different working temperatures, warm-air heating with and without heat recovery) were compared using a natural gas fired condensing boiler as the heat source. For the floor heating systems, the effects offloor covering resistance on the whole system performance were studied using two heat sources; a natural gas fired condensing boiler and an air-source heat pump. The heating systems were also compared in terms of auxiliary exergy use for pumps and fans.The low temperature floor heating system performed better than other systems in terms of exergy demand. The use of boiler as a heat source for a low-exergy floor heating system creates a mismatch in the exergy supply and demand. Although an air-source heat pump could be a better heat source, this depends on the origin of the electricity supplied to the heat pump. The coefficient of performance (COP) of the heat pump has a critical value (2.57 in this study); it is beneficial to use a heat pump instead of a boiler only when the COP is above this critical value.The floor covering resistance should be kept to a minimum, in order not to hinder the performance of the floor heating and the whole system. The exergy input to auxiliary components plays a significant role in the overall exergy performance of systems, and its effects become even more significant for low temperature heating systems

Evaluating assumptions of scales for subjective assessment of thermal environments – Do laypersons perceive them the way, we researchers believe?

International audienc