Energy modelling is not commonly practiced other than for the purpose of meeting the requirements for green building certification. The benefits of good energy modelling include reduced capital cost of construction as the tendency to oversize equipment and systems is reduced, as well as improved energy efficiency.

Energy Use Assessment

These are generally based on i) performance, ii) features and iii) provisions for management, operation and maintenance.

i) performance - an assessment of design improvements for the assessed building compared to a benchmark building, based on detailed calculations and/or computer simulations.

ii)features - that improve energy efficiency or provide for energy conservation, e.g. heat reclaim, or features that help with maintaining performance in use, e.g. monitoring/metering provisions.

iii) provisions for better operation, maintenance and use, e.g. good O&M manuals, tent fitting-out guide.

Energy Modelling

Energy modelling for a whole building is undertaken to demonstrate how a DESIGN building compares with a BASELINE building. The software used should comply with ASHRAE Standard 140.

The BASELINE is usually a building that barely conforms to the applicable energy regulations, code or standard. For example Singapore building regulations refer to Singapore standards to define the baseline performance for use in Green Mark. In Hong Kong, BEAM makes reference to the requirements of EMSD’s energy codes. LEED energy performance assessments are based on both mandatory and performance-based requirements of ASHRAE 90.1.

The following diagram illustrate the procedure used in BEAM Version 4/04 ‘New buildings’.

Simulation Outcomes

Simulations can provide a reasonable estimate of percentage performance gain of an assessed (as designed) building compared to a code compliant building, but usually underestimates (sometimes by a significant margin) actual annual energy consumption. There are a number of reasons for the discrepancies.

The software used for the simulation, the experience of the user, the data and assumptions made, etc. can impact significantly on the outcome (number of points or credits achieved). Software should be validated against a standard such as ASHRAE 140.

The assumed set-points (e.g. temperature settings) and equipment operating schedules used in the simulation may be (are often) changed by building operators or building users.

Additional equipment ‘hot spots’ like computer data centres may add significant loads that are not covered by the simulation (not covered by code or defaults used in the assessment method).

Energy modelling software generally assumes perfect control of HVAC and lighting systems, which can be a major ‘error’, as actual systems are unlikely to operate so perfectly.

Simulations may not take into account the impact of oversized main plant and equipment or the deterioration in performance over time or because of inadequate maintenance.

Simulations assume as-designed or as-built performance specifications are met, but this is often not the case when testing and commissioning is inadequate.

The impact of adjacent buildings (shading, overshadowing, etc) is not usually taken into account.

Simulations often do not include energy consuming systems such as lifts, exterior and public area lighting and ventilation, water heating, car park lighting and ventilation, etc which are often accounted for separately in BEAM assessments. Clearly, unless there is adequate metering in a building separating landlord energy used by different loads to reveal the HVAC energy use, it will be difficult to compare estimated annual energy use with actual energy use. There is much evidence to show that there is often poor correlation between the two!