Simplified Building Energy Model (SBEM)
SBEM stands for Simplified Building Energy Model and it is a computer program that provides an analysis of a building’s energy consumption.
It takes inputs from the software user and various databases, and by calculation, produces a result in terms of the annual CO2 emissions emanating from the energy used by the building’s systems and its occupants. Some of the inputs are standardised to allow consistent comparisons for building regulations and energy ratings purposes in new and existing buildings.
SBEM calculates the energy demands of each space in the building according to the activity within it. Different activities may have different temperatures, operating periods, lighting levels, etc. SBEM calculates the heating and cooling energy demands by carrying out an energy balance based on monthly average weather conditions (i.e. steady-state heat transfers, as opposed to Dynamic Simulation Modelling). This is combined with information about heating and cooling system efficiencies in order to determine the energy consumption. The energy used for lighting, ventilation, and hot water is also calculated.
iSBEM is the interface to the SBEM calculation engine. Both were developed for the UK Ministry of Housing, Communities and Local Government (MHCLG) in accordance with the EPBD, i.e. the Energy Performance of Buildings Directive (EPBD) 2002/91/EC of the European Parliament and Council (dated 16 December 2002). In more detail, the EPBD called on Member States to:
- develop a methodology of calculation of the integrated energy performance of buildings;
- set minimum requirements for the energy performance of new and existing buildings;
- ensure that those requirements for the energy performance are met in new buildings, and that the feasibility of certain alternative energy systems is checked for new buildings;
- ensure that those requirements for the energy performance are met in existing buildings that are subject to major renovation or extension;
- develop energy certification of buildings;
- set up regular inspection of boilers and of air-conditioning systems, and of the whole heating system where the boilers are more than 15 years old;
- ensure that certification and inspections are carried out by qualified and/or accredited experts.
This led to the National Calculation Methodology (NCM) and hence to SBEM for new construction, extensions, major refurbishment, and existing buildings.
An annex to the EPBD states that the calculation must be based on a general framework, which includes, at least, the following factors:
- Thermal characteristics of the building (shell and internal partitions, etc.); this may include air tightness.
- Heating installation and hot water supply, including their thermal characteristics.
- Air-conditioning installation.
- Natural and mechanical ventilation.
- Built-in lighting installation (mainly in the non-residential sector).
- Position and orientation of buildings, including outdoor climate.
- Passive solar systems and solar protection.
- Indoor climatic conditions, including the designed indoor climate.
The calculation should also deal with the influence of the following aspects on energy performance, where relevant:
- Active solar systems, and other heating and electricity systems based on renewable energy sources.
- Electricity produced by combined heat and power.
- District or block heating or cooling systems.
- Natural lighting.
The definition of “energy performance” in Article 2 of the EPBD refers to the estimation of energy needed for the “standardised use” of the building. This estimation is intended to enable comparisons made between buildings to be on the basis of their intrinsic properties rather than being dependent on the user’s choice of operating patterns which might exist in practice. Article 3 permits the use of CO2 emissions as a means of comparison, rather than
energy consumption, in the standard methodology.
The Building Act 1984 requires that all buildings constructed or refurbished should comply with the requirements of the current Building Regulations. As stated above, the EPBD calls for a calculation methodology on the energy performance of buildings to be established. The response to this by the UK’s government was to state in the 2006 Building Regulations Part L for England and Wales that:
- 17A – (1) The Secretary of State shall approve a methodology of calculation of the
energy performance of buildings.
- The methodology shall comply with the requirements of the Directive.
- The Secretary of State shall approve minimum energy performance requirements for
new buildings in the form of CO2 emission rates, which shall be based upon the
methodology approved pursuant to regulation 17A.
The NCM has been developed to provide this calculation. The EPBD permits the inclusion of a CO2 emission factor in the standard methodology. For Building Regulations compliance in the UK, it has been decided to base compliance on CO2 emissions, rather than on delivered or primary energy, in order to:
- avoid confusion over definitions of delivered and primary energy
- allow comparison of energy from disparate sources and of different costs
- avoid having to set different targets where there is the option of using electricity or other fuels for a given end use
- remind users that the overall objective for the UK is CO2 management in order to meet international treaty obligations.
Following on from the Article 3 requirement of the EPBD, the 2006 Building Regulations called for a proposed building to be assessed by comparing its expected annual carbon dioxide emissions with a target, on a consistent, calculated basis. This marks a change from the optional means of demonstrating compliance with previous Building Regulations which allowed either matching constructions with U-value requirements for particular elements, limits to glazing areas, etc., or achieving a calculated target. Previous calculation methods have been specified (e.g., CECM, as explained in CIBSE TM32) but these are not compliant with all the requirements of the EPBD, and there could potentially be difficulties in achieving consistent results.
To address these concerns, the National Calculation Methodology (NCM) has been established.
At the core of the NCM, the calculation process compares the CO2 emissions of the proposed building with target emissions which are based on those of a “Notional building”. This constitutes setting the standards in order to satisfy the requirements of Article 4 of the EPBD. The basis on a comparison minimises argument about how well the absolute CO2 emissions are predicted by different NCM-compliant methods because both the proposed and Notional buildings are subject to the same calculation approach. Instead, it concentrates on achieving improvements compared with the previous regulations. The NCM also requires the use of standard databases or information sources for:
- Environmental conditions and operating/occupation patterns in each part of each building
- Weather data
The reason for this is to encourage consistency between repeated evaluations of the proposals. Standard databases are also available for
- Heating and cooling system efficiencies
- Building component parameters
The NCM also requires that specific construction elements in the proposed building are checked for compliance with minimum performance standards specified in Part L in England (or equivalent in Wales, Scotland, and Northern Ireland). It also requires that the output report adopts a standard format so that building control officers will not have to interpret the way different tools present the results.
The performance requirement is for the proposed building to achieve carbon dioxide emission rate that is less than a “Target Emissions Rate” (TER), which is derived from the emissions of the Notional building introduced above. Briefly, the Notional building has the following characteristics:
- The same geometry, orientation, and usage as the evaluated building
- The amount of glazing in the Notional building is, however, not the same as that in the evaluated building. The area of glazing is a certain percentage of external walls and roofs and is dependent on the activity and building types.
- Standard operating patterns (to allow consistent comparison between buildings in the same sector)
- It is exposed to the same weather conditions as the evaluated building
- Standardised assumptions for the building fabric, glazing type, and HVAC plant efficiencies.
- Any service not covered by Part L (e.g., emergency escape lighting, specialist process lighting, etc.) is ignored in both the Actual and Notional buildings
While there are differences between the specifications of the Notional building, and the associated methodology for setting the target, in each of England, Wales, Scotland, and Northern Ireland, the philosophy is the same.
The NCM is used to calculate the energy consumption and, hence, carbon dioxide emissions of both the building being evaluated (its “Building Emissions Rate” or BER) and those of the Notional building (“Target Emissions Rate” or TER).