Unbalancing mean radiant temperature and air temperature

Abstract

Radiant heating and cooling systems promise to provide heat transfer over large surface areas to enable lower temperature supply improving both supply efficiency and user comfort. Radiant panels provide both radiant and convective heat transfer. We investigate the balance of the heat transfer occurring via radiation and convection. A commonly referenced benefit of radiant panels is that the radiant heating or cooling delivered to occupants allows thermal perception to remain neutral while lowering or raising air temperatures, which in turn reduces heat losses or gains through the facade. In our previous work on radiant systems, we have observed that very rarely do air temperatures actually get set to more ideal temperatures alongside the radiant panels. Using the framework of a recent study of an office space in Singapore we present a simulation showing the breakdown of radiant and convective heat and the resulting air temperature in a simple heat balance model. We then use a computational fluid dynamics (CFD) model and analytical model to observe the relationship between air temperature and mean radiant temperature (MRT) through space. We find that it can take up to 40 kW of additional heat gain to successfully decouple the air temperature from the MRT generated by the panel by more than 2 °C. We also show that in the CFD model, while the air temperature and MRT remain relatively tightly coupled, the vertical spatial variation of MRT is significant, changing by more than 2 °C across the space from floor to ceiling.