There is a ‘performance gap’ between building design theory and real energy in-use. To achieve net zero-carbon buildings, a better understanding is required of actual operational energy performance. This understanding needs to inform the design theory, thus creating a ‘feedback loop’.
The Mayor of London has declared a climate emergency and has stated his ambition for London to be net zero carbon city by 2030. Recent Planning submissions show that developers are committing to reducing overall carbon emissions by 40.6% beyond the 2013 Building Regulations. However, the operational energy of the buildings once in use, is not expected to correlate with proposed targets.
The draft London Plan Policy SI 2 sets out the ‘be seen’ requirement for all major development proposals to monitor and report on their actual operational energy performance. The ‘be seen’ policy will help us to understand the performance gap and identify ways of closing it while ensuring compliance with London’s net zero-carbon target.
The recently published TM61: Operational performance of buildings sheds some light onto this problem. The technical memorandum explores contributing factors to the energy performance gap by investigating energy usage in four contemporary case studies:
The contributing factors to the energy performance gap found in these four case studies were:
Compliance modelling (represented by software output documents BRUKL and EPC for non-domestic buildings and SAP and EPC for domestic properties) has been very useful in introducing energy efficiency in the construction industry. It created straightforward targets for energy efficiency, however research has shown that compliance model energy outputs are not necessarily suitable for use as design estimates for operational performance. The difference between compliance energy estimates and operational energy is called the regulatory performance gap (as demonstrated in the graph above)
The graph above (taken from TM54 – Estimating operational energy performance of buildings at the design stage) gives an example of a case study comparing predicted energy, the Part L calculation and TM 54 estimates and actual building usage. In the part L model, the purple, grey and orange stacks are completely missing, these represent unregulated energy like small power equipment, lifts, external lighting and IT servers. Depending on the type of the building the unregulated energy can represent a large proportion of electrical consumption (e.g. data centre equipment, radiology equipment)
Another problem when evaluating operational energy, stems from conflict between indoor environmental quality and energy usage. As per Best Practice – Recommendation 2 in TM61, a holistic approach is needed when designing energy efficient buildings in order to provide an indoor environment which stimulates comfort, productivity and health and wellbeing. As seen in the graph above, this conflict between thermal comfort and predicted energy consumption can lead to increased heating and cooling consumption. Introducing post occupancy surveys provides a positive feedback loop within the built environment sector.
To understand and address the issue further BS ISO 17772-1:2017: Energy performance of buildings – specifies requirements for indoor environmental parameters for thermal environment, indoor air quality, lighting and acoustics and specifies how to establish these parameters for building system design and energy performance calculations. It includes design criteria for the local thermal discomfort factors, draught, radiant temperature asymmetry, vertical air temperature differences and floor surface temperature.
Increased lighting consumption was another issue observed between predicted and operational energy. In the case study this was attributed to incorrectly set lighting sensors. While lighting commissioning is time consuming during hand-over, the post occupancy evaluation (POE) proves importance of seasonal and enhanced commissioning.
One of the major factors that has been reported to have a large influence on the discrepancy between predicted and measured energy use is the issue of unexpected night-time energy use, caused by factors such as leaving office equipment on, denser occupancy or extended hours of operation.
To guide engineers/developers through the process of accurately assessing operational energy, TM61 is accompanied by three additional TM’s dealing with various aspects of operational performance:
In order to successfully deliver a low-energy building which provides occupational comfort, it is crucial to align all aspects of building design process. This should start at the inception of the building design with correctly chosen building services and a building envelope which accommodates occupant’s needs whilst simultaneously delivering energy efficient measures; continuing through the construction phase ensuring that correct commissioning and high efficiency services are installed. Finally, closing the loop with a high level of testing and feedback during occupancy.