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What is operational energy performance modelling?

To truly reach net zero-carbon buildings, we must gain a deeper understanding of their actual operational energy performance. In the UK there are several regulatory and voluntary measures that aim to improve the energy performance of buildings (Part L etc.). However, the current schemes do not measure actual performance outcomes which has led to a ‘Design for Compliance’ culture and the ‘Performance Gap’ where actual new building performance is considerably different from the designed performance.

To address this gap CIBSE have developed TM54 ‘Evaluating Operational Energy Use at the Design Stage’, which is a dynamic energy analysis using a tailored Part L model for the estimates of regulated and unregulated loads. This covers all areas of the development not covered by SBEM including unregulated energy loads, such as lifts, external lighting, small power plug in equipment etc. and uses more accurate profiles to represent plant operation and occupancy patterns specific to the end-user.

Figure 1 - Comparison of Part L, TM54, and actual energy performance (CIBSE TM54, 2022)

Applications of CIBSE TM54

London Plan and Regional Local Policy

Operational energy modelling now forms a crucial part of the planning process in London and new and emerging Local Plans across Local Authorities throughout the UK.

The Greater London Authority (GLA) and London Plan 2021 aims to address the performance gap within Policy SI 2 ‘Minimising greenhouse gas emissions’, by introducing a fourth stage to the energy hierarchy – the ‘be seen’ stage. This requires developments to undertake detailed predictive operational energy monitoring at the concept design stage, and report the Energy Use Intensity (EUI metric) to the GLA as part of the planning submission.

For non-domestic elements in any major development, the energy strategy will need to provide a comparison of Building Regulations Part L, accompanied by detailed energy modelling following the CIBSE TM54 methodology, which gets submitted to the GLA via the GLA ‘Be Seen Planning Stage webform’. Failure to do so will result in a non-compliant energy strategy submission, and will likely trigger pre-commencement planning conditions which will need to be discharged prior to starting on site.

There is also a responsibility for reporting estimated operational energy at the as-built stage and a five year commitment to submit in-use data. The GLA encourages Local Planning Authorities to secure this requirement through a legal agreement (Section 106 agreement) between the local authority and the responsible party.

Figure 2 – The London Plan 2021 Energy Hierarchy

BREEAM ENE01/ENE02

Credits are available for the prediction of operational energy consumption at RIBA stage 2 under BREEAM V6.1 (ENE01; 4 credits) and BREEAM V7 (ENE02; 3 credits). This requires an energy workshop with the design team to evaluate the likely operation of the building and to identify potential risks to the energy demand of the building.

Identifying energy risks allows energy assessors to undertake scenario and sensitivity analyses to gain a comprehensive picture of how the building will perform in operation. This identifies any weaknesses in the building design and energy strategy, whilst also testing climate change resilience and adaptation.

Data is presented for a range of likely energy demand scenarios, which are compared against industry leading benchmark values for EUI, such as LETI. This provides developers with a greater insight into the performance of the building, giving an opportunity to further refine the design, and convey the design intent to occupants.

Bespoke Assessments

Clients wanting to gain a better insight into how their buildings will perform and wanting to bridge the performance gap may choose to voluntarily undertake a CIBSE TM54 analysis. Sensitivity and scenario modelling can be used to identify areas of building design which can be optimised to reduce energy and carbon emissions in operation, and bridge the performance gap.

Methodology and Deliverables

In the UK, the National Calculation Methodology (NCM) was devised as an assessment tool to demonstrate compliance with Building Regulations Approved Document L2A. It provides a comparison between the carbon emissions of the proposed building and a ‘notional’ building.

The NCM template sets standard operational inputs. These standard inputs include: the hours of operation; the density of occupation; temperature set points; domestic hot water demand; fresh air flow rates; and lighting (lux) levels. There is a standard allowance for small power heat gains in order to calculate the heating and cooling demands. Other energy uses are not included, e.g. external lighting, lifts etc.

The CIBSE TM54 guidance document details the methodology which should be applied when evaluating operational energy performance of buildings at the design stage. There is a requirement to conduct a workshop with the building owner or intended occupant in order to establish realistic operation profiles, equipment schedules, small power and unregulated energy loads, as well as estimating how well the building will be managed.

A questionnaire is circulated ahead of a detailed energy workshop to gain a better idea of the intended operation of the building and allowing a more focused workshop to take place. The outcomes of the workshop are used to create bespoke operation profiles within the energy model and to include unregulated energy loads, such as lifts, external lighting and process equipment.

At AES we are IES specialists (approved dynamic modelling software) with a dedicated Building Physics Team, and CIBSE accredited energy assessors. We create bespoke energy models in accordance with the CIBSE TM54 methodology to provide key deliverables and provide a value-added service, giving developers a better understanding of the operation of their buildings and helping to bridge the energy performance gap.