  Flexible loads, especially heating, ventilation, and air-conditioning (HVAC)
systems can be used to provide a battery-like service to the power grid by
varying their demand up and down over a baseline. Recent work has reported that
providing virtual energy storage with HVAC systems lead to a net loss of
energy, akin to a low round-trip efficiency (RTE) of a battery. In this work we
rigorously analyze the RTE of a virtual battery through a simplified
physics-based model. We show that the low RTEs reported in recent experimental
and simulation work are an artifact of the experimental/simulation setup. When
the HVAC system is repeatedly used as a virtual battery, the asymptotic RTE is
1. Robustness of the result to assumptions made in the analysis is illustrated
through a simulation case study.
