Center for the Built Environment

There is an increasing focus on the time at which energy is used in buildings both to reduce utility costs and carbon emissions in response to time-dependent grid signals. One method to shift electrical load out of peak pricing hours is to use batteries, but they have high first costs and also incur an energy penalty due to round trip efficiency and other losses. Another method is to use thermal storage to offset heating and cooling. Similarly, mechanical ventilation systems can also be controlled to shift energy use to periods of the day with lower energy, cost, and environmental impacts by varying the ventilation rate while still meeting ventilation code requirements. Mechanical ventilation systems in large multi-family residential buildings are mostly central air systems with either manually balanced dampers or constant airflow regulator (CAR) dampers that aim to provide a constant ventilation airflow rate to each apartment. ASHRAE 62.2 allows for dynamic ventilation rate systems in these buildings as long as the average relative exposure rate and the peak relative exposure rate during occupied periods are no more than 1 and 5, respectively, for any time interval that cannot exceed an hour. In this study, we used EnergyPlus simulations to examine energy end-use profiles for a large multi-family building under design in San Jose, California. We considered a balanced ventilation system using a central dedicated outdoor air supply (DOAS) system. We tested different load-shifting scenarios with multiple parameters to explore how the ventilation airflow rate can be varied to shift load, while also assessing energy and utility cost impacts. The parameters we assessed in each scenario were: the presence of a centralized ERV system or not; ventilation design sizing; and length of load shifting time period. All dynamic ventilation cases, with and without ERV systems, resulted in energy and operational cost savings relative to the constant ventilation cases when compared to providing the same amount of load shifting using batteries, and all tested strategies met ASHRAE 62.2 requirements. The results show that after accounting for the battery penalty typically associated with load shifting, all dynamic ventilation cases reviewed result in improved energy savings when compared to the constant ventilation strategy.