Bridging the performance gap
Buildings rarely perform as well as their designers predicted – energy consumption can be as much as double what was expected, so annual energy costs can also be doubled. This difference has become known as the performance gap
Report from – Dr Andy Lewry
Operators of commercial and public buildings need clear and realistic guidance on targeting energy running costs for their properties and on the potential savings available. At their disposal are two seemingly irreconcilable indicators of performance available: energy performance certificates (EPCs) provide a theoretical assessment of their asset but under standardised 'driving conditions', while operational ratings based on energy bills give no indication of how much lower those bills could be. The operational rating is nearly always higher due to non-standard hours of operation, occupancy patterns and unregulated loads, such as IT and office equipment.
To truly understand how a building uses energy it is necessary to know something about the building itself and about how it is used; this requires both an asset rating and an operational energy rating.
What are the ratings?
The asset rating is a measure of building quality in that it assesses designed performance: the higher the rating the worse the building is, and the greater the opportunity to reduce carbon emissions and improve the building itself. An asset rating models the theoretical, as-designed energy efficiency of a particular building, based on the performance potential of the building itself (the fabric) and its services (such as heating, ventilation and lighting). However, the asset rating provides no information about how the building is operated in practice.
The operational rating records the actual energy use in a building over the course of a year, and benchmarks it against buildings of a similar type. Therefore, to understand and manage the energy use in a building, both ratings are required as they show different aspects of a building’s total energy performance.
Two offices with the same asset rating could have very different operational ratings even if they are working under the same operating schedule – a building with a low rating can be used well by its occupants, a building with a high rating can be used wastefully. In the latter case, measures to change the behaviour of the occupants would be the best option for reducing energy use and carbon emissions.
What do they do?
The asset rating is intended to inform people on first occupancy, i.e. at the point of construction, sale or rent, in order to help purchasers or tenants in selecting the right building. At this point in time, any previous metered information is either unavailable or not very helpful as the previous occupants’ operation of the building, unregulated energy use, etc., could be quite different to that of the new occupants. The software is also used when designing new buildings, for demonstrating compliance with the UK Building Regulations for buildings other than dwellings. This is done by calculating the annual energy use for a proposed building and comparing it with the energy use of a comparable 'notional' building.
For example, in England, Approved Document L2A uses a holistic approach where a building must achieve a better predicted performance than a Target CO2 Emission Rate (TER) which is calculated at both the design and as built stage by tools such as the Simplified Building Energy Model (SBEM). L2A contains limiting fabric parameters and uses the standards laid out in the Non-Domestic Building Services Compliance Guide (NDBSCG) as design limits (or backstop values).
An example of an operational rating is the Display Energy Certificate (DEC). Public buildings in the UK have to display a Display Energy Certificate, which is an Operational Rating based on measured energy use. There is pressure to extend DECs into the commercial sector - initially on a voluntary basis.
What are the differences?
The two ratings show different aspects of a building’s total energy performance. A Display Energy Certificate (DEC), or operational rating, records the actual energy usage from a building over the course of a year, and benchmarks them against buildings of similar use. An Energy Performance Certificate (EPC), or asset rating, models the theoretical, as designed, energy efficiency of a particular building, based on the performance potential of the building itself (the fabric) and its services (such as heating, ventilation and lighting), compared to a benchmark.
The building quality (provided by the EPC) has a large impact on the total emissions (from the DEC), but does not explain all emissions. Other factors such as unregulated loads or building user behaviour also use energy, which is reflected in the DEC. However, in order to understand what is driving these emissions, the EPC plays a critical role in separating the influence of building quality from other influences such as end user behaviours.
To truly understand the energy performance and the factors driving consumption within a building you need both certificates and the ability to tailor them.
The scenario where a difference is found between the EPC and DEC – is the so called ‘performance gap’. This is where the real operation of the building is different to that predicted by design due to the conditions of use being totally different from those standard assumptions.
Resolving the performance gap
The poor performance of buildings in use compared with their design predictions has been much discussed and various approaches to resolving the problem have been suggested, including:
- Whole building energy benchmarking – modelling the energy use at the design stage and comparing this directly with the in-use performance. This requires realistic whole building energy calculations at the design stage, which has been infrequent and, as stated earlier, is not straightforward, as it depends so much on occupancy. In common with Display Energy Certificates, it also requires benchmarks which are robust and applicable to specific building types. Such benchmarks were researched and produced by the Energy Efficiency Best Practice Programme in the 1990s but have not been maintained.
- More complex modelling such as dynamic simulation models. The extra detail may provide more accuracy in the comparison process, but it does not necessarily resolve the underlying problems: modelling all building energy uses and addressing actual occupancy and services operation.
- Practical analysis of building energy as the sum of all end uses such as lighting, ventilation and small power at the design stage and in use. TM54 Evaluating operational energy use of buildings at the design stage, published by the Chartered Institute of Building Services Engineers (CIBSE), covers the analysis of the energy use of systems, so that this can be done at the design stage with the likely building occupancy and use, ready for later assessment when in use. This provides one process for resolving the problems identified above.
- EPCs/DECs - use existing data and tools – can be used as a first cut to target issues or prioritise a portfolio of buildings. We will now consider how this can be achieved.
Using existing data and tools
One of the software tools used to create a Non Domestic EPC (NDEPC) is the Simplified Building Energy Model (SBEM). This was produced by BRE in 2006 for the Department for Communities and Local Government (DCLG) in England and Wales as a mechanism for calculating the energy used by buildings. It forms part of the department’s process for implementing the EU’s Energy Performance of Buildings Directive (EPBD). An NDEPC can be generated using the tool iSBEM, the free downloadable user interface for SBEM that was developed by BRE for DCLG.
iSBEM captures all the details of the building (geometry, constructions and building services) in a series of zones. This data is then used to produce an asset rating for the purposes of showing compliance to Approved Document L of the UK’s building regulations and to produce an Energy Performance Certificate (EPC) and its recommendation report.
The Operational Rating Calculation (ORCalc) is the software used to calculate the operational rating of a building from annual utility consumption and to produce the DEC and an advisory report providing advice on energy efficiency measures.
The issue with both rating is that they are designed for compliance purposes and as a result are calculated using standard “driving conditions” for each building type and activity.
A possible solution
The tool supporting UK Green Deal is the non-domestic GD tool, which is based on iSBEM and has the ability to link NDEPCs and DECs.
The Green Deal tool brings the two assessments together by unlocking the “standard driving conditions” and allowing assessors to tailor the model to real life occupancy. Actual data on how the building is being run and used can now be entered. This allows the asset performance to be compared to the performance in use.
The tool also identifies potential operational measures and quantifies savings from improved management, to the benefit of owners and occupiers. In addition scenarios for asset improvement can be input and the tool calculates the cost / benefits. As a result this tool can be used to provide data to underpin business cases.
In my next article I'll be explaining how to building the business case for investing in energy efficiency.
Dr Andy Lewry is a principal consultant at BRE and author of its new guide, “Bridging the performance gap” – Understanding predicted and actual building operational energy” on which this article is based. The full publication is available from www.brebookshop.com. Quote reference “PerfGap” for a 20% discount.
He will also be speaking on these topics at the Eco Technology Show in Brighton on 11-12June, 2015. More information on this event covering solutions for sustainable energy, build, transport, innovation and resource efficiency can be found here www.ecotechnologyshow.co.uk.