resourceO5 news

CALCULATING THE ENERGY PERFORMANCE OF COMMERCIAL BUILDINGS

By Paul Davidson, BRE

Background

One of the first requirements set out in the text of the Energy Performance of Buildings Directive, known universally as the EPBD, is a 'calculation methodology' for determining the 'integrated energy performance of buildings'. All over Europe, administrators and their advisers have been working hard to come up with solutions to this demanding challenge. In the UK, such a methodology does exist for dwellings in the form of SAP - the Standard Assessment Procedure - and similar tools are in use in other member states. However, the problem of a methodology which will cope with all the other types of commercial and public buildings, in all their many and varied forms, is much more difficult. This article describes one possible solution developed by BRE for ODPM, which has recently been made available for public testing.

The EPBD was finally approved in 2003, and must be transposed into law in each member state with effect from January 2006. It can be summarised as seven different but interrelated requirements:

• A calculation methodology (Article 3)
• Minimum energy performance standards for all new buildings (Articles 4 & 5)
• Minimum energy performance standards for larger buildings undergoing major renovation (Articles 4 & 6)
• An energy performance certificate when a building is sold or leased out (Article 7.1)
• The public display of an energy performance certificate in certain public buildings (Article 7.3)
• Regular inspection of boilers and air-conditioning systems (Articles 8 & 9)
• Qualified and/or accredited experts to provide the certificates and carry out the inspections in an independent manner (Article 10)

In the summer of 2004, ODPM published a consultation paper on its plans for revising Part L of the Building Regulations and for implementing the EPBD. That paper proposed a National Calculation Methodology which would form the basis for four of the EPBD requirements listed above. In particular, it would be used:

• To demonstrate compliance with the minimum performance standards set by Part L2 of the Regulations (for non-dwellings), in accordance with Articles 5 and 6 of the EPBD
• To derive an Asset Rating in accordance with the energy performance certificate required by Article 7.1

National Calculation Methodology

The National Calculation Methodology (NCM) has now become a combination of the methodology defined in the EPBD, supplemented by a number of pragmatic elements as the process of developing a workable tool has evolved. The EPBD sets out a number of parameters which have to be taken into account - such as the building's fabric, its HVAC systems, its position and orientation, etc. It also specifies that these should be subjected to 'standardised' conditions of occupancy, and usage.

The predicted energy demand of any building, whether a shop, a hospital or an office block, is clearly determined in part by the thermal performance of its fabric - the walls, floor, roof and glazing - and by the resistance of that fabric to unwanted air infiltration. It will be influenced by the form, orientation and thermal mass of the building and by the extent to which daylight is able to penetrate. The energy needed to meet that demand will then depend on the type and efficiency of the heating, ventilation, cooling and lighting systems. But the energy demand is also driven by the activity going on in the different spaces within the building - in terms of, for example:

• The number of people within the space, and their density
• The activity level of those people, and therefore the amount of heat they produce
• The proportion of each day for which the space is occupied
• The temperature to which the space is heated in winter and cooled in summer
• The amount of fresh air needed
• The light level required to carry out the activity
• The amount of equipment within the space which gives off heat

This leads to the concept of different activity areas within buildings, each of which differs in one or more ways from the others by virtue of different values for these parameters. Examples of activity areas are spaces such as open-plan offices, operating theatres, classrooms, etc. In order to calculate the performance of a building under standardised conditions, it is necessary to define sets of standard values for each parameter for each different activity type. These standard values should ideally be based on an agreed consensus with building owners, users and designers.

The calculation core

The proposed NCM recognises that some buildings are so sophisticated in their design, that their energy performance can only be calculated by means of sophisticated energy simulation models. But simulation is expensive and time-consuming, and requires significant expertise to do successfully. During the design of a new building, particularly a complicated one, the design team may well use simulation as a design aid anyway. ODPM's proposal, therefore, is that simulation (using certain accredited tools) will be allowed as one means of deriving an asset rating. For existing buildings, however, and for new 'straightforward' buildings, a less demanding approach is required. ODPM therefore commissioned BRE, in November 2004, to develop a simplified calculation method, to be used for the majority of 'ordinary' buildings. This tool is then to be made publicly available by ODPM.

A second requirement is that the calculation method complies with a number of European standard procedures which have been defined by CEN working groups - with remarkable speed - in the last few months. These specify the means by which the energy performance should be calculated from first principles. BRE's answer to this is to offer a tool based on a Dutch public-domain methodology, which has been used in the Netherlands to set performance standards. Work had already started on this tool under a BRE-led European SAVE project called BESTCert.

The resulting calculation tool is known as SBEM - the Simplified Building Energy Method. It is based on monthly average calculations, rather than the hourly approach of more sophisticated models.

The Notional Building

The approach adopted by ODPM for demonstrating compliance with Building Regulations, and for calculating the Asset Rating, relies on the use of a 'notional' building. This is a building with the same geometry and activity data as the actual building, but built with fabric and service systems in accordance with the 2002 Part L2 regulations standards. The National Calculation Methodology then requires that two calculations are performed, one on the actual building with the actual fabric and actual HVAC and lighting systems, and one on the notional equivalent building. For Part L compliance purposes, the actual building has to perform better than the notional building by a specified percentage improvement.

This approach has to be used whether using simulation or SBEM to do the calculation. It requires access to a database of activity area data, which is kept the same in both buildings. At the design stage, the resulting relationship between the two buildings yields a Design Rating. Once the building is completed, the final Asset Rating is determined, based on the building 'as built'.

BRE's Calculation Tool

In order to carry out the calculation, BRE has also developed a basic user interface which captures all the required information about the building in terms of its geometry, its construction, the activities within it and the service systems. This interface, know as iSBEM, works with SBEM.

The interface has been developed using Microsoft Access in the short term, though ideally it should be independent of any specific software application. It asks the user a series of questions through Access forms, in order to produce the required input for the calculation core itself. Part of the process involves interacting with the Activity database described earlier, to deliver the relevant standard parameters to the tool depending on the activities present in the building. It can also interrogate a database of fabric details. This contains descriptions of many different wall, floor, roof and glazing systems and can be used in two ways. If the user knows the detail of the wall construction, for example, he or she can select it from the library of details and know that the U-value returned will be the agreed value for that construction. Alternatively, if details are not available (for example in an existing building), there is a crude but effective inference procedure which will provide a default construction system based on the age of the building. A similar process provides details on HVAC and lighting systems. 

The first version of the whole package is now available for what is optimistically called 'Beta test' from a special BRE website - www.ncm.bre.co.uk. It contains the calculation tool SBEM, plus the interface and fledgling databases. The data contained in the databases is subject to review and refinement in consultation with a range of industry representatives, but the basic framework should remain fixed. The interface requires the user to define his building in advance, and guidance on using the tool is also available.

It should be emphasised that the tool is not yet complete, and that the answers it produces are NOT suitable for use either in Building Regulation submissions or in calculating Asset Ratings. BRE and ODPM would, however, welcome feedback of the functionality of the tool and on suggestions for improvement, using the feedback form on the website. It is certainly far from perfect, but does seem to offer the promise of a flexible and relatively simple calculation method.

Return to news