Cairns Regional Council was seeking to improve reduce energy use and operating costs at within an existing automotive workshop.
Initially the brief was to consider electric lighting only, but this was extended to include consider daylighting – however there were staff concerns about potential glare and heat from skylights (based on adverse experiences in the past).
Whole System Design
A whole system design approach was taken to ensure that substantial energy and operating savings were achieved while maintaining – or improving – visual performance and thermal comfort.
Existing 250W metal halide ‘highbay’ lights were producing excessive illumination levels of >400 lux throughout – however lighting was considered ‘poor’ by the users. These were selected to be replaced with 162W T5 fluorescent highbay fittings with occupancy and daylight harvesting controls, to a more moderate 240 lux level.
Roof-lighting products were extensively researched, with a single-skin polycarbonate roof sheeting product being selected due to:
- Suitably low light transmittance (around 25%);
- Good thermal performance (more light transmitted than heat);
- Compatibility with existing roof sheeting;
- Low cost;
- Good durability (including storm/hail resistance) and warranty.
The roof-lighting layout was designed to provide even illumination with minimal glare, and to ensure that only as much daylight as needed was introduced – in this case a daylight factor of 2%, providing based on Cairns daylight availability:
- Minimum 240 lux for 90% of working hours; and
- Minimum 400 lux for 60% of working hours; but
- Not exceeding 600 lux for more than 25% of working hours.
The installation of the skylight sheeting was also specified with care to ensure maximum durability, lifespan, and cyclone resistance of the sheeting; as well as water-tightness of the roof.
In the tropical environment of Cairns, the workshop operates ‘open air’ throughout the year, so ambient heat build-up should not be a concern. However radiant heat gain from the skylights was a potential problem. Skylights were carefully laid out to ensure that localised radiant heat problems would not occur, however an increase in the Mean Radiant Temperature of the space was inevitable.
To offset the increase in Mean Radiant Temperature and thereby maintain comfort conditions, High Volume Low Speed fans (Big Ass Fans) were proposed. This approach is not particularly amenable to detailed simulation, so ‘first principles’ modelling was undertaken with the ASHRAE Thermal Comfort Tool used to determine comfort levels before and after the proposed works. This modelling considered both peak summer and annual average conditions, and also considered the work rate and clothing of occupants (these being particularly important when considering air movement in a workshop environment).
The modelling showed that, for the proposed design, the temperature as perceived by the workers should increase by not more than 1°C even in peak summer conditions.
Energy and Cost Savings
The proposed design used a whole system design approach to achieve substantial energy savings while maintaining comfort conditions by:
- Replacing less efficient metal halide luminaires with more efficient T5 luminaires;
- Providing more appropriate (reduced) general electric lighting levels;
- Providing appropriate levels of daylight with minimal glare;
- Providing daylight-harvesting controls to electric lighting;
- Offsetting radiant heat gain from the skylights via increased air movement from fans.
Capital costs, operational cost savings, and energy savings were all modelled. The proposed design was modelling as providing 29,000 kWh/y energy savings (about 8W/m² power reduction) with about 9 years simple payback.