Small HVAC Control Demonstrations in Larger Buildings Often Overestimate Savings

TL;DR

Controlling a small number of zones in large buildings often overestimates energy savings due to heat transfer effects, but a new measurement-based method can improve accuracy without baseline estimation.

Contribution

This paper mathematically characterizes overestimation errors in small-scale HVAC control experiments and proposes a new temperature measurement-based estimation method that avoids baseline energy estimation.

Findings

Overestimation errors can be significant in small control zones.

Controlling interior zones without outdoor contact leads to fictitious savings.

The proposed method accurately estimates savings using only zone temperature data.

Abstract

How much energy, money, and emissions can advanced control of heating and cooling equipment save in real buildings? To address this question, researchers sometimes control a small number of thermal zones within a larger multi-zone building, then report savings for the controlled zones only. That approach can overestimate savings by neglecting heat transfer between controlled zones and adjacent zones. This paper mathematically characterizes the overestimation error when the dynamics are linear and the objectives are linear in the thermal load, as usually holds when optimizing energy efficiency, energy costs, or emissions. Overestimation errors can be large even in seemingly innocuous situations. For example, when controlling only interior zones that have no direct thermal contact with the outdoors, all perceived savings are fictitious. This paper provides an alternative estimation method based on the controlled and adjacent zones' temperature measurements. The new method does not require estimating how much energy the building would have used under baseline operations, so it removes the additional measurement and verification challenge of accurate baseline estimation.