Page 42 - Energize May 2022

TECHNICAL

uncertainties in design and operation,
as well as limitations of measurements
system, explaining its magnitude and
underlying causes are necessary to
confidently forecast and understand
energy use in buildings.
Instances of EPG have been reported
in relation to existing buildings, building
retrofit projects and new constructions.
Findings from studies between 1995 and
2002 have revealed that actual energy
consumption in buildings is often twice
as much as predicted. More recent
studies have suggested that in-use energy
consumption can be 5 to 10 times higher
Figure 3: The 4755 MW PV installation at Mall of Africa (Clean Technica) than compliance calculations carried out
during the design stage. 2
development goals, such as addressing climate change, creating sustainable and thriving
communities, and driving economic growth. Causes of the EPG
Studies suggest that factors contributing to
The energy performance gap the performance gap include:
To design and operate more efficient buildings, many classification schemes have been • A lack of monitoring and feedback
established, providing a means to communicate a building’s relative energy efficiency and following occupancy, meaning that
carbon emissions. These assessment schemes are related to the energy consumption of a problems are rarely identified, user
building and can be quantified using different methods, both in the design stage and once behaviour is not corrected and lessons
operational. are not learned for future projects.
Accredited performance assessment tools, ranging from steady-state calculations to • Design assumptions do not properly
dynamic simulation methods are used to predict the energy consumption of a building. reflect the in-use performance of
Currently, all classification schemes (such as South Africa’s EPC) and standard calculation buildings.
procedures for quantifying the energy use of a building, show significant discrepancies • Calculations for regulatory compliance
between predicted and measured energy use during occupation. do not account for all energy uses
This phenomenon has been named the “energy performance gap” (EPG). Although in buildings. These calculations
a margin of error between predicted and measured energy use is inevitable due to are commonly misinterpreted
as predictions of in-use energy
consumption, when in fact they are
simply mechanisms for complying with
the building regulations.
• Unregulated sources of energy
consumption such as small power loads,
server rooms, external lighting, and so
on are rarely considered at the design
stage. Yet these loads typically account
for more than 30% of the energy
consumption in office buildings.
• Occupant behaviour differs from that
modelled in the design stage.

The recent inclusion of own generation,
such as rooftop PV and storage,
complicates building design energy
calculations, as PV is a component with
both long- and short-term variations.
Often average values of PV energy
production are used, which leads to
Figure 4: A review of the EPG and its underlying causes in non-domestic buildings (Frontiers in
mechanical engineering) inaccurate assumptions.

energize | May 2022 | 40