Evaluating Operational Energy Performance of Buildings at the Design Stage - CIBSE TM54: 2013

There has been a growing awareness for some time that many 'low energy buildings' use more energy than the designers thought they would. As energy costs have risen, this awareness has started to spread to building owners, who hear much about low energy buildings and subscribe to programmes that rate the design of the building, only to find that their 'low energy design' turns out to be have a typical energy bill. The performance of low energy designs is often little better, and sometimes worse, than that of an older building they have replaced, or supplemented.This phenomenon is not restricted to the UK, but has been observed as far afield as the US and Australia. There is a mismatch between the expectations around the performance of new buildings and the reality of the utility bills. This difference between expected and realised energy performance has come to be known as the 'performance gap'.There are two main reasons for this performance gap. The first is that the method of calculating energy use for the purposes of compliance does not take into account all the energy uses in a building. In particular, it does not address energy used by lifts and escalators, for catering facilities, or for server rooms. This energy use can be substantial: in one case study, the National Trust HQ at Swindon, it was found that 60% of the energy use, that for the server room and the catering, was used in just 3% of the floor area, and more than doubled the operational energy use over the design estimates.The second reason for the performance gap is related to site practice. To deliver a building that uses as much energy as expected requires that the design is built as intended, the engineering systems are commissioned effectively and the operators and occupiers of the building understand how to operate and maintain the building so that it delivers the expected performance.This Technical Memorandum (TM) addresses the first reason. It provides building designers and owners with clear guidance on how to evaluate operational energy use more fully, and accurately, at the design stage. After a brief introduction, which explains the need for the guidance in more detail, it explains the importance of making an accurate estimate of the operating hours and likely occupancy of the building. It then sets out how theoperational energy required for the building can be estimated. As well as covering lighting, heating, ventilation and cooling and provision of hot water, it also considers lifts and escalators, small power loads, catering, server rooms and other plant and equipment.In each case, it provides guidance on how to make more accurate estimates based on the intended use and operation of the building. As well as enabling the designer to estimate energy use more accurately, it will highlight any areas where actual energy use may be higher than a typical building, allowing this to be considered at the design stage.This guidance will help to turn low energy designs into low energy buildings that achieve the design energy targets. It is one of several CIBSE actions to promote more effective assessment of energy performance. CIBSE, along with the RIBA and other partners, has been instrumental in the development of the Carbon Buzz website, which provides a practical tool for designers to compare their low energy designs against others on the website, as well as providing benchmarks of expected and realised performance. Users of this guidance will find that Carbon Buzz is based on similar principles and complements this guidance. For more details visit www.carbonbuzz.org.This TM seeks to provide designers with the means to better address operational energy at the design stage. Whilst political leaders in the UK continue to debate the future electricity supply arrangements needed to meet our future energy demands, this guidance offers practical guidance to enable the buildings we are designing, or redesigning, to deliver the energy performance that the owners and operators expect, and to reduce their demands for energy in the years to come. Front Matter 1 Foreword 3 Preface 5 References 6 Table of Contents 8 1. Introduction 9 2. Aims of This Document 10 3. Scope of This Document 11 4. Why Estimate Energy Use? 12 4.1 Current Situation 12 4.2 Evaluating Energy Use 13 5. Risks of Evaluating Energy Use at the Design Stage 15 6. Principles of Evaluating Energy Use 16 6.1 Principles of CIBSE TM22 16 6.2 Using Dynamic Simulation Models 17 6.3 Assumptions and Simplifications in Models 17 6.4 Using Benchmarks at Early Design Stages 17 7. The Methodology 18 7.1 Step 1: Establishing Floor Areas 19 7.2 Step 2: Estimating Operating Hours and Occupancy Factors 19 7.2.1 Operating Hours 19 7.2.2 Occupancy Factors 20 7.3 Step 3: Evaluating Lighting Energy Use 20 7.3.1 Interior Lighting 20 7.3.2 Other Lighting 20 7.3.3 Lighting Estimates for the Case Study Building 21 7.4 Step 4: Evaluating Energy Use for Lifts and Escalators 21 7.4.1 Lifts 21 7.4.2 Escalators 22 7.5 Step 5: Evaluating Energy Use for Small Power 22 7.5.1 Office Equipment 22 7.5.2 Other Equipment 23 7.5.3 Small Power Estimates for the Case Study Building 24 7.6 Step 6: Evaluating Energy Use for Catering 24 7.7 Step 7: Evaluating Energy Use for Server Rooms 24 7.7.1 Server Room Cooling 25 7.7.2 Server Room Estimates for the Case Study Building 25 7.8 Step 8: Evaluating Energy Use of other Equipment 26 7.9 Step 9: Evaluating Energy Use of Domestic Hot Water 26 7.10 Step 10: Evaluating Internal Heat Gains 27 7.10.1 Lighting Gains 27 7.10.2 Small Power Gains 27 7.10.3 Plant Equipment Gains 27 7.10.4 Server Gains 28 7.10.5 Catering and other Gains 28 7.10.6 Occupant Gains 28 7.11 Step 11: Evaluating Energy Use of Space Heating, Cooling, Fans and Pumps 28 7.11.1 Heating, Cooling, Fans and Pumps Estimates for the Case Study Building 28 7.12 Step 12: Evaluating Energy Use for Humidification and Dehumidification 29 7.13 Step 13: Estimating Management Factors 29 7.14 Step 14: Running Scenarios 29 7.15 Step 15: Sensitivity Analysis 30 7.15.1 The Impact of Weather 30 7.15.2 Occupant Density 31 7.16 Step 16: Review against Benchmarks 31 7.17 Step 17: Presenting the Results 32 8. Post Occupancy Evaluation 34