On the round-trip efficiency of an HVAC-based virtual battery

TL;DR

This paper rigorously analyzes the round-trip efficiency of HVAC systems used as virtual batteries, revealing that previous low efficiency reports are artifacts and that the true asymptotic efficiency approaches 1.

Contribution

It provides a physics-based model demonstrating that HVAC virtual batteries have an asymptotic RTE of 1, challenging prior findings of low efficiency.

Findings

Previous low RTE reports are artifacts of experimental setups.

The asymptotic RTE of HVAC virtual batteries is 1.

Simulation confirms robustness of the theoretical result.

Abstract

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.