Evaluation of Demand Shifting Strategies with Thermal Mass in Two Large Commercial Buildings

Building thermal mass can be used to reduce the peak cooling load. For example, in summer, the building mass can be pre-cooled during non-peak hours in order to reduce the cooling load in the peak hours. As a result, the cooling load is shifted in time and the peak demand is reduced. The building mass can be cooled most effectively during unoccupied hours because it is possible to relax the comfort constraints. While the benefits of demand shift are certain, different thermal mass discharge strategies result in different cooling load reduction and savings. The goal of an optimized discharge strategy is to maximize the thermal mass discharge and minimize the possibility of rebounds before the shed period ends. A series of filed tests were carefully planned and conducted in two commercial buildings in Northern California to investigate the effects of various precooling and demand shed strategies. Field tests demonstrated the potential of cooling load reduction in peak hours and importance of discharge strategies to avoid rebounds. EnergyPlus simulation models were constructed and calibrated to investigate different kind of recovery strategies. The results indicate the value of pre-cooling in maximizing the electrical shed in the on-peak period. The results also indicate that the dynamics of the shed need to be managed in order to avoid discharging the thermal capacity of the building too quickly, resulting in high cooling load and electric demand before the end of the shed period. An exponential trajectory for the zone set-point during the discharge period yielded good results and is recommended for practical implementation.