A study by the University of Stellenbosch in South Africa investigated a number of cooling alternatives and their effect on a one-room building. The results were decisive: evaporative roof spray showed the largest reduction in cooling load (59%) and the largest overall reduction in net energy transferred (72%). Researchers also found that by continuously cycling cool ambient air through the building at night, and then using an evaporative roof spray system during the day, cooling load and energy transfer reductions improved an additional 5% and 8%, respectively. This application is known as flywheel cooling.
FLYWHEEL COOLING SYSTEMS AT A GLANCE
Night flushing is simply the movement of cool night air through a building by means of a ventilation system. Generally, this system consists of at least one wall-mounted supply fan in conjunction with one or more wall- or roof-mounted exhaust fans. Cross ventilation is important, so fans should be placed in such a manner as to allow air flow from one side of the facility to another. Effectiveness is primarily due to air flow rate, expressed as air changes per hour. Studies vary widely on the optimum number of air changes per hour, ranging from 10 to 30.
An evaporative roof spray system intermittently sprays a thin film of water on the roof surface, and then allows the water to evaporate. When performed in regular cycles, this prevents the roof from getting hot and transferring that heat into the building. The result is lower air temperatures, and a reduced cooling load on the facility HVAC system.
Most industrial sites can benefit from the use of flywheel cooling. The ideal schedule would call for the supply and exhaust fans to be turned on at the end of the workday (or for a 24 hour operation, around 8 – 9 pm). The fans would then circulate air through the facility until around 7 or 8 am the following morning. At that time, the area should be “buttoned up” to retain as much of the cool night air as possible. Then, the evaporative roof cooling system would be activated, preventing the sun’s radiant heat from penetrating the building. The end result would be lower interior temperatures, since nearly 50% of the generated heat load inside a given structure emanates from its roof.
For sites that use large HVAC systems, incorporating flywheel cooling into the daily operations plan translates into direct energy savings and reduced electrical consumption. Night flushing and evaporative roof spray greatly diminish the building’s cooling load, decreasing HVAC cycle frequency and duration. Large consumers of electricity will also see an additional benefit of lower peak demand charges and ratchets often imposed by power companies. The University of Stellenbosch study agrees, stating these methods “…not only constituted a saving in the energy consumed by a conventional air conditioner but also decreased the required size of the air conditioner.”
The university study highlights another important point regarding air conditioning capacity. In many cases, cooling load reductions may be dramatic enough to pull unneeded A/C tonnage offline entirely, bringing down yearly HVAC maintenance costs. Facilities located in more moderate parts of the globe could take things even further: “…in milder climate conditions the necessity of a conventional air conditioner may be averted.”
Regardless of climate, HVAC capacity, or industry, environmentally and safety conscious organizations would do well to explore the prospect of adding flywheel cooling to their facilities. The rewards of lower energy bills, more productive employees, and a carbon footprint reduction await.
Artmann, N., Manz, H. & Heiselberg, P., (2008). Parameter study on performance of building cooling by night-time ventilation, Renewable Energy, Vol. 33, pp. 2589-2598
Dobson, R. & Vorster, J., (2011). Sustainable cooling alternatives for buildings, Journal of Energy in South Africa, Vol. 22, No. 4, pp. 48-66
Finn, D., Connolly, D. & Kenny, P., (2007). Sensitivity analysis of a maritime located night ventilated library building, Solar Energy, Vol. 81, pp. 697-710
Geros, V., Santamouris, M., Tsangrasoulis, A. & Guarracino, G., (1999). Experimental evaluation of night ventilation phenomena, Energy and Buildings, pp. 141-154
World business council for sustainability, [Online]. Available: http://www.wikipedia.com/theinnovationchain. [2009, October 16].