Potential energy savings using optimal economizer cycle/heat reclaim operation

This paper investigates the potential savings in building heating, ventilating and air-conditioning (HVAC) system energy and costs from the optimal operation of a heat reclaim and/or air-economizer cycle. Hour-by-hour, PC-based energy-calculation programs (in C language) were developed and verified for commonly-used building HVAC systems. The programs use U.S. Weather Bureau hourly data (Test Reference Year (TRY) format). An hourly building-loads subprogram was first developed to calculate heating and cooling loads for a multizone building. This subprogram was incorporated into system-simulation programs for the following types of terminal systems: variable air-volume systems with reheat, four-pipe fan coil units, constant volume dual duct (or multizone) systems, and single zone reheat systems. For each of these, the primary system consisted of a heat reclaim chiller or heat pump with a double bundle condenser, a supplementary heater, and a heat rejection device. Simulation of an individual or combined heat reclaim-economizer operation, with priority given to the operation of either heat reclaim or the economizer, was then added. A simple method of life-cyde costing was also included which enabled identification of the optimal (most economically viable) economizer cycle and/or heat reclaim operation. Simulations were carried out for the systems mentioned above and for four U.S. locations representative of cold, temperate, hot-humid, and hot-dry climates. The results show that the choice of HVAC system and the building's geographical location strongly influence potential energy and cost savings using optimal operation. Results show that for most building locations and HVAC terminal systems, the combined operation of heat reclaim and economizer cycle with heat reclaim priority will result in the least annual total HVAC energy and is identified as the optimal operation scheme. Potential annual HVAC energy savings are estimated to range from 18% to 64%, and cost savings from $0.21 to $2.96 per square foot of building floor area in the optimal operation scheme. Parametric studies that consider the effects of including thermal storage capability, the effects of internal loads, and the use of an enthalpy economizer cycle are ongoing. Recommendations for further study include the use of a more sophisticated economic analysis and simulation of other types of terminal systems.