The Secrets Of Air Jets In Airplanes
How do they do that? How can they afford to keep so many people cool in a confined space with the doors open? The answers can be found in PCA (preconditioned air) and TES (thermal energy storage).
In the past, an airplane's auxiliary power unit (APU) was called upon to supply cooling. The APU is a small jet engine designed to provide cooling, electrical power and, in some cases, power to start the main engines. These engines consume between 35 and 120 gallons of jet fuel per hour. In addition to the high fuel costs, the APU is subject to costly FAA-approved maintenance based on hours of operation.
The economic and practical necessities of shifting air conditioning loads away from costly APU engines led to development of the PCA system. The economics of electrical power to run PCA chillers led to the addition of thermal energy storage.
As compared to typical air conditioning systems, PCA/TES installations are designed for the most extreme demands. The most obvious extremes are those associated with high, short-term loads imposed on the system while the aircraft is parked followed by no-load conditions when the plane departs. In addition to the "peaky" intermittent demands for cooling, PCA systems normally use 100% outside air for cooling and ventilation. Of course, this means high temperature air (and often very humid air) must be cooled without the benefit of recirculation — remember, the doors are open.
To accomplish the task of managing such extremes in an economical manner, water-cooled industrial grade chillers are combined with thermal energy storage in a low temperature glycol loop. The system supplies an aircraft with a large volume of relatively high-pressure air (22 in. of water) at the low temperature of approximately 30 degrees F. Cold fluid from the glycol-based system is delivered at about 20 degrees. Thermal storage flattens the peak loads inherent with such an application. It also allows the use of lower cost off-peak electricity to charge the system at night.
Exact details of the low temperature process are proprietary and the key system components and Cryogel Ice Ball thermal storage media are protected by patents.
Since 1995, 10 PCA systems using the ice balls have been installed at various airports. Miami International Airport has two PCA systems and is preparing to install a third, 12,000-ton hour TES system for standard air conditioning service.
Ott is president of Cryogel, San Diego, CA. Phone is 858-457-1837 and the website is www.cryogel.com.
Publication date: 03/18/2002