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, as explained by Andy Lewry of the BRE…

    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: 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 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.

    An operational rating gives information on actual energy usage and example is the Display Energy Certificate (DEC). Public buildings in the UK have to display a DEC; 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 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 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).

    The building quality (provided by the EPC) has a large impact on the total energy usage (from the DEC), but does not explain them all. 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 ratings 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 DECs, 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 interface for Simplified Building Energy Model (iSBEM).

    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. .

    A possible solution

    The tool supporting the UK Green Deal is the non-domestic GD tool 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. ■

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    Dr Andy Lewry

    Principal Consultant

    Building Research Establishment (BRE)

    Tel: 0333 321 88 11

    Dr Andy Lewry is a principal consultant at the Building Research Establishment 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 quoting reference “PerfGap” for a 20% discount.

    He will also be speaking on these topics at the Eco Technology Show in Brighton on 11th & 12th June 2015. More information on this event covering solutions for sustainable energy, build, transport, innovation and resource efficiency can be found here .


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