The minimum capacity of HVAC secondary systems (with capacity reduction by interzonal airflow) (RP-1049)

To date, the performance of alternative HVAC secondary systems has been compared using either the systems’ energy use directly or by a life-cycle cost analysis. This paper introduces the concept of rating a system’s performance by comparing its capacity at a particular operating point to the thermodynamic minimum capacity. A simple ratio, termed the “system effectiveness,” is also introduced to indicate the extent to which the system operates with the minimum possible capacity. This paper describes the calculation of the minimum system capacity as a nonlinear, single- criterion, constrained optimization problem. In particular, it describes the case for the minimization of the system capacity by use of interzonal airflow (the interzonal airflow rates and zone thermal conditions being variables of the optimization). This optimization problem is multi-modal in that the same system capacity can result from more than one interzonal airflow configuration. The optimization problem has been solved here using a genetic algorithm (GA) search method. This paper illustrates the use of the minimum system capacity as a benchmark for the performance of a typical multizone heating, ventilating, and air-conditioning (HVAC) system. The example also illustrates the potential benefit of using interzonal airflow to reduce the required capacity of a system. It can be concluded from the example that the arrangement of the system components is a significant factor in determining the thermal effectiveness of HVAC systems.