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

Direct expansion (DX) cooling systems are commonly used in small to medium sized commercial buildings due to low capital investment and maintenance costs. Nowadays more DX units are equipped with variable-speed fans and capacity modulating techniques to enable higher efficiency and better comfort control. This complexity provides greater energy savings potential for control optimization compared to single stage DX units with constant air flow. This paper presents a generalized control heuristic for resetting supply air temperature setpoints for multi-stage DX units configured with variable-speed supply fans. The heuristic is shown to work well for a wide range of different DX systems with different compressor types and fan-duct combinations. Digital scroll, variable-speed and multi-cylinder reciprocating compressors are considered and modeled using curve fits to manufacturer’s data. Different evaporator and condenser characteristics are also considered using models constructed from catalog data of a representative split DX cooling system. The component models are coupled to provide an overall performance model 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 to consider different DX cooling systems and climates. Simulations were carried out for a 100-day cooling season indifferent locations of US and under different control strategies, including conventional control with a constant supply air temperature setpoint, the heuristic reset strategy and optimal supply air temperature setpoint control. It is shown that the heuristic control has performance almost as good as optimal control with significant cost savings relative to conventional control that depend on the system type and location