Multiple choice

When you’re about to splash out £23 million, you want to be sure you’re getting value for money. But how can you when the product to be purchased has yet to be made? Glasgow City Council, which recently embarked on large-scale reform of its schools estate including a £23 million construction contract for four new schools, decided simulation modelling was the answer.

The council wanted the schools to provide the best environment for learning through excellent use of natural daylight, superlative air quality and energy efficient buildings. It tendered the work as a design-and-build contract and three bidders responded.

Rather than take the bidders’ word, the council sought independent verification of the energy performance of the designs and the creation of an effective education environment. The aim of using simulation modelling was to find out how each scheme would actually perform, to give an indication of the best design and to ensure equality of comparison.

To carry out the assessment, the council turned to Glasgow-based building performance simulation consultant IES Consulting. IES first modelled the schools’ geometry using AutoCAD drawings submitted by the bidders. It then ran simulations on this model. It scored each of the tender designs on a series of key factors specified by the council that impact on the classroom environment: heating and electrical loads, natural daylight, natural ventilation, temperatures and glare.

These scores were entered on a matrix along with proof of compliance with Building Bulletin 87 (this has now been superseded by Building Bulletin 101 – Ventilation in School Buildings).

Each school was modelled using operational data provided by the council including set point temperatures, ventilation requirements and casual gains. A responsive lighting control strategy was used to maximise the use of daylight and minimise solar gain.

To calculate annual heating load normalised performance indicators (NPIs) were used.

The process involved using the calculated annual heating load and electrical consumption load being multiplied by a number of factors and then divided by the floor area. Both “typical” and "worst case" NPIs were calculated, where typical is for normal school period use and worst case included evening community use.

Bidder 1

• School area: 7725m2
• Volume: 28,739m3
• Ventilation strategy: open windows when zone temperature reaches 23ºC, close them when it falls below this. Windows open 100mm.
• Space heating: 83kWh/m2, worst; 75kWh/m2, typical
• Electrical: 30.8kWh/m2, worst; 15.1kWh/m2, typical

Bidder 2

• School area: 6,575m2
• Volume: 20,360m3
• Ventilation strategy: open windows when zone temperature reaches 25ºC and keep open until end of school day. Windows open 100mm.
• Space heating: 89.9kWh/m2, worst; 80.9kWh/m2, typical
• Electrical: 24.7kWh/m2, worst; 16.9kWh/m2, typical

Bidder 3

• School area: 5,395m2
• Volume: 15,344m3
• Ventilation strategy: gradually open windows on a “ramping profile” when zone reaches 20ºC, windows reaching maximum opening of 100mm at 24ºC. Strategy assisted by stack ventilation to draw out “dirty” air.
• Space heating: 85.5kWh/m2, worst; 72.7kWh/m2, typical
• Electrical: 21.4kWh/m2, worst; 15.8kWh/m2, typical

For all bids the space heating NPI is dominant over the electrical NPI because of daylight control. This is beneficial because low-grade energy is being used to heat the space rather than high-grade energy for lighting. The above figures can be used for an emissions comparison to give an environmental footprint for the area.

Bidder 3 performs well with only a small average number of hours above 26ºC.

The assessors said that overall the three schools are of a high standard, requiring only tweaks to give a very good environment. Their main concern was CO2 levels, particularly in winter.

When contractor Barr was appointed preferred bidder, IES worked with it on the areas the software had shown could be improved. This mostly focused on a cross-flow natural ventilation solution to reduce overheating.