A Heuristic Supply Air Temperature Reset Strategy for VAV Systems Employing Variable-Capacity DX Cooling Equipment

This paper presents a generalized control heuristic to reset supply air temperature setpoints for DX units with capacity modulation and variable air flow. The heuristic is shown to work well for different DX systems with different compressor types and fan-duct combinations. Digital scroll and variable-speed compressors were considered and modeled using curve fits to manufacturer’s data. Evaporator and condenser models were also constructed using the catalog data of a representative split DX cooling system. The component models were coupled to provide an overall performance map for each DX system. Fan power is another critical portion of the total HVAC power consumption. In this study, a simple method was used to characterize the performance of a duct system based on typical relationships between pressure drop and air flow rate. Different duct characteristics were coupled to different types of supply fans to provide a set of fan power maps. Optimization was then performed for each combination to find the optimal supply air temperature that minimizes total fan and compressor power. This provided a baseline for developing and validating results of the heuristic control strategy. To assess the integrated energy savings potential of the heuristic strategy, a simulation platform was developed for a medium sized commercial building. Simulations were carried out for a 100-day cooling season in different U.S. locations and under different control strategies, including a conventional control with a constant supply air temperature setpoint and the heuristic reset strategy. It is shown that the heuristic control can provide significant cost savings relative to conventional control depending on the system type and location.