So let's get some definitions and roles straight. The Energy Performance Indicator Method (EPIM) looks at the energy performance of a building design in terms of the ratio of its expected energy use to a benchmark figure. This expected energy use takes into account the size of building services (installed electrical power per m2), plus system types and controls. It also uses occupancy hours, but this is taken as constant for a particular building type.
The benchmarks used by the EPIM are derived from a publication called Energy Consumption Guide 19, or Econ 19 for short.
Econ 19 is essentially a dataset of building energy consumption figures taken from studies of real buildings. Energy consumption is disaggregated for end use, such as ventilation, pumping, lighting etc., and classified for building type: air conditioned, mechanically ventilated or naturally ventilated.
Econ 19 is an excellent way to benchmark building energy consumption, even if the benchmarks are not absolute and more a reflection of current UK practice.
Breaking down a building's energy consumption by end use requires a robust method of analysis, and that is where the Energy Analysis Reporting Method (EARM) comes in. Enshrined in the recently published CIBSE Technical Memorandum TM22, the EARM enables the energy performance of an existing occupied building to be assessed by comparing its actual energy performance and characteristics against benchmarks. This is done either at a simple overall level of energy use per m2 or at a detailed system level.
Both approaches can be applied to a wide range of uses. For example, the EPIM can either be used by a design team or for regulating building design. The more detailed EARM has already been used as in the PROBE post- occupancy reviews, and has reached a point where it could be used by engineers to demonstrate compliance with a future regulation covering building energy consumption.
What this implies, of course, is a degree of overlap. Could the overlap be helpful? EARM and EPIM are two tools doing different jobs, although they are part of a bigger scheme. The bigger scheme allows "closing the loop" between operation and design. EARM provides a time-efficient way of looking at buildings in use. EPIM, on the other hand, provides a simple but effective assessment of a building at the design stage.
Both approaches are underpinned by the 'tree diagram' system performance analysis. This synergy means that data from design and operating buildings will build up reliable and useful benchmarks for a wide range of systems during design and use.
How can this benefit be realised? One way would be for the Building Regulations to require an EPIM to be carried out at the design stage. This would record details so that an EARM could be easily carried out during operation. This study would provide crucial information about the performance of the building against expectations and national benchmarks.
A further way would be for businesses to require initial and operational analysis in order to develop procurement and operational requirements. If the government is true to its word and brings the energy performance of existing buildings into Part L of the Building Regulations, then both the EPIM and EARM will be useful methods of assessment and benchmarking.
Lastly, a simple depository for the information collected would provide valuable dataset for future benchmarking and regulation.
The Carbon Performance Index
In the context of Part L, a Carbon Performance Index (CPI) has been proposed as a way of assessing air conditioning and mechanical ventilation services in offices at the design stage. The CPI would reveal a building's potential to emit CO2 through energy use.
The performance potential of a design is assessed using basic information about the fixed servicing hardware the designer intends to install. This design assessment would use the EARM.
This means the ability to share common information is a fundamental requirement for both the design CPI assessment, and the performance assessment conducted using the EARM, to be used consistently with Econ 19 benchmarks. The consistency extends to:
Other issues relate to sports or leisure facilities, mainframe computer suites, and energy used by ground floor retail units.
Assessment can be made consistently throughout the stages of procuring and operating a building. In the case of offices, the Econ 19 benchmarks for plant energy use can inform the design process by providing typical and good practice targets for the operation of heating, cooling, ventilation and lighting services.
This energy use can be influenced by the design of the building fabric and servicing systems. CPI compares the likely efficiency of the services under typical operating circumstances with benchmarks from Econ 19.
Unusual factors that could influence energy use, but which are not included in Econ 19, must be separated and excluded before comparison.
The CPI and TM22 assessments also make use of more detailed benchmarks to describe the operation of particular services – heating, refrigeration or ventilation. These can refer to the installed capacity of plant, typical hours of operation, load factors or specific efficiencies of systems. They may be used to aid the design process, or help estimate achieved energy use when direct measurement or observation is not possible.
Improving the benchmarks
At present, benchmarks for the realised performance of plant and systems are difficult to update as practice develops. Ideally, they should be derived from extensive and detailed observation and monitoring.
Financial and practical constraints, however, dictate that they be produced from fairly brief examination and observation – using estimation and apportioning methods, as in the most detailed application of the EARM as described in TM22 – to deduce energy consumption and hours of use.
Performance measurement, and opportunities to gain feedback to improve benchmarks and inform the design process, could be made much more straightforward if many more energy meters were used. Proposals being considered for possible future revisions of Part L might include installing energy meters – at least at the level of the individual building or business, and perhaps for each major item of plant.
Together with hours-run metering, such a development would provide an unprecedented improvement in closing the feedback loop between building operation and design.
Further reading
CIBSE, 'Energy assessment and reporting methodology: office assessment method', Technical Memorandum TM22, CIBSE, 1999. Grigg P F and Birtles A B, 'Rating the energy efficiency of air conditioned buildings', CIBSE/ASHRAE National Conference 1996, Volume 1, p214-221. Grigg P F, Moss S A and Birtles A B, 'Assessing non-domestic building design using an Energy Performance Index Method', CIBSE National Conference 1997, Volume 1, p134-140. Grigg P F and Birtles A B, 'Energy efficiency of buildings: simple appraisal method', BSER&T 18(2), p109-114, 1997. Energy Consumption Guide 19 (Econ 19): energy use in offices, Department of the Environment, Transport and the Regions, February 1998. Field J W, Jones P G, Bordass W T and Grigg P F, 'Comparing non-domestic buildings and their energy use', CIBSE National Conference 1994, Volume 1, p203-207. Field J W, Soper J, Jones P G, Bordass W T and Grigg P F, Energy performance of occupied non-domestic buildings: Assessment by analysing end-use energy consumptions, BSER&T 18(1), p39-46, 1997Source
Building Sustainable Design
Postscript
John Field MA CEng MInstE MInstP is an energy specialist with Target Energy Services and was principal author of Technical Memorandum TM22. Peter Grigg MInstR is principal scientist in the Environmental Engineering Centre at the BRE.
