[Technical article] Exploring the role of occupants in buildings' energy performance gap
Before new buildings are constructed, or existing buildings are retrofitted, their energy performance is generally estimated for various purposes, such as design optimisation, system configuration, compliance documentation, and the issuing of certificates.
To do this various calculation methods and simulation tools can be used. However, the actual energy use of buildings during the operational phase often deviates from expectations that are based on the results of such computational estimations. Various factors can contribute to this difference, which is broadly referred to as the Energy Performance Gap (EPG).
Occupants' control-oriented behaviour is a frequently cited item among such factors, as occupants make use of control opportunities to bring about favourable indoor-environmental conditions. Instances of such control opportunities include the opening and closing of operable windows, operating electrical lighting, and adjusting thermostat settings for space heating and cooling. However, review of the literature in this area suggests that the evidence regarding the role of occupants and their control-oriented behaviour in buildings' EPG is not conclusive.
In this context, this article explores the ways occupants' control-oriented behaviour (e.g., interactions with buildings' control components and systems) can influence buildings' EPG and revisits the empirical evidence for the extent of occupant-driven EPG. Moreover, strategies are discussed that can provide control opportunities to the occupants while reducing the risk of compromising energy efficiency targets.
Overview of the principal contributors to buildings' EPG
The main reasons behind the deviation of buildings' actual energy performance from the pre-construction estimates may be classified in terms of the following categories (Figure 1):
- There can be differences between as-designed versus as-realised construction features (e.g., building envelope, fabric, components).
- Specified building systems (for heating, cooling, ventilation, lighting) and their intended operation regime in the design phase may differ from the actually installed and operated system elements.
- Predictions of energy performance must make assumptions about buildings' contextual conditions, most importantly, the future weather conditions. However, long-term weather forecasts can be highly unreliable. Likewise, predicting future urban conditions (e.g., urban microclimate) is not a trivial task.
- Design-phase assumptions about buildings' future occupants cannot be expected to accurately capture occupants' control-oriented behaviour, that is their interactions with building environmental control elements and systems.
- Selected calculation methods and tools may involve limitations in view of validity, fidelity, and adequacy for the purpose. Moreover, errors could be made in the deployment of computational tools (mistaken input data, improper settings, etc.).
- Energy metering infrastructure may have insufficient resolution and errors may occur while monitoring and documenting the magnitude of consumed energy.