Flywheel Cooling Offers a Passive, Sustainable Way to Reduce Energy Consumption


The evaporative roof cooling system on Patterson Fan Company, Blythewood, South Carolina. Photo credit: Cody Patterson

Now more than ever, facility managers are being asked to use less and spend less. This edict has put air conditioning directly in the crosshairs, as it accounts for more than 50% of a building’s energy consumption. In 2011, a study by the University of Stellenbosch in South Africa investigated a number of different 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%).

However, the study went a step further. Researchers 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.

Anatomy of a Flywheel Cooling System

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. Night flushing 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. Each facility should be analyzed individually to determine its air flow needs.

A typical flywheel cooling system consists of three major components

A typical flywheel cooling system consists of three major components: a wall-mounted supply fan (1), a roof- or wall-mounted exhaust fan (2) and an evaporative roof cooling system (3).  

An evaporative roof spray system intermittently sprays a thin film of water on the roof’s surface, and then allows the water to evaporate. This process, when performed in regular cycles, prevents the roof from getting hot and transferring that heat into the interior of the building. The result is lower air temperatures, and thus a reduced cooling load on the facility HVAC system. The entire system is usually operated by means of a programmable control box or software based control scheme. A thermostat is also integrated into the system; ensuring proper conditions exist for evaporation to occur.

How to Run Flywheel Cooling

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 pm to 9 pm). The fans would then circulate air through the facility until around 7 am 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 and a more comfortable working environment, since nearly 50% of the generated heat load inside a given structure emanates from its roof.
The concept of flywheel cooling may seem counterintuitive to some facility managers. It is common practice at most industrial sites to open doors and windows, or turn on the large supply and exhaust fans once ambient temperatures start to rise. However, doing so introduces warm daytime air into the building, causing elevated interior temperatures. Therefore, a culture of vigilance should be fostered by facility management to ensure the treated area stays reasonably closed during the workday. To capture the most benefit from these tandem systems, conventional thinking must be abandoned and replaced by a commonsense approach to passive, sustainable cooling.

Tangible Results

For sites that use large HVAC systems, incorporating flywheel cooling into the daily operations plan translates into direct energy savings and reduced electricity consumption. Night flushing and evaporative roof spray greatly diminish the building’s cooling load, decreasing HVAC cycle frequency and duration. Depending on the application, the return on investment (ROI) period can be two to three years or less. 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.”


Solar Shield Effect Graph

The effectiveness of evaporative cooling is shown by hourly data readings taken on the roof of Patterson Fan Company, Blythewood, South Carolina on June 15 and 16, 2011. June 15 (daytime high 91°F) readings were taken with the system off, while the system was active on the 16th (daytime high 88°F).

The university study highlights another important point regarding air conditioning capacity. In many cases, cooling load reductions may be large enough to put 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.”

While researchers at Stellenbosch focused mainly on using flywheel cooling to remove load from air conditioners, it should be noted that existing facilities without HVAC capacity also stand to benefit from its application. Night flushing and evaporative roof spray, when properly designed and utilized, provide tons of cooling equivalent at a fraction of the operational and maintenance cost.

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. Working with an established expert in the design and application of passive cooling systems becomes the logical next step. 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: [2009, October 16].


More from Sustainable Plant

Add a comment

You cannot post comments until you have logged in, and have an appropriate permission level. Login here or register for a new account.