A thermal resistance-based method for the optimal design of central variable water/air volume chiller systems

Varying water/air volume (VWV/VAV) favors the performance improvement of central chiller systems. However, due to the lack of an intrinsic relation connecting these design parameters to the requirements, both the operating parameters of variable speed pumps/fans (VSP/VSF) and the structural parameters of heat exchangers are usually designed empirically or semi-empirically, which offers a huge potential for energy conservation. Therefore, through analyzing the heat transfer processes in each heat exchanger of a central VWV/VAV chiller system by the entransy-dissipation-based thermal resistance (EDTR) and studying the fluid flow processes by the characteristics of VSPs/VSFs and pipeline pressure drops, respectively, we yield the formulas connecting the areas of each heat exchanger and the rotating speeds of each VSP/VSF directly to the design requirements. Combining these formulas with the conditional extremum method gives several optimization equations corresponded to different optimization objectives. Simultaneously solving these equations directly gives the optimal area allocation of each heat exchanger and the operating parameters of VSPs and VSFs, which successfully avoid the limitation of the trial-and-error method. Finally, a simple central VWV/VAV chiller system is taken as an example to illustrate the applications of the newly proposed optimization method.