round-trip efficiency, demand response, ancillary service, virtual energy storage, HVAC system
The flexibility in power consumption of heating, ventilation, and air-conditioning (HVAC) systems in buildings can be utilized to provide a battery-like service to the power grid. Recent work has reported that using HVAC systems in such a manner may lead to a net increase in energy consumption compared to normal operation, similar to a low round-trip efficiency (RTE) of a battery. In our previous work we showed that the low RTEs reported were due to the way the experiments/simulations were performed, and that using an HVAC system as a virtual battery repeatedly leads to an asymptotic RTE of one. In this work we show that when an additional constraint is imposed---that the mean temperature of the building must remain at its baseline value---the asymptotic RTE can be lower than one. We numerically investigate dependence on parameters such as building size and time period of charging/discharging.