advances in heat-activated cooling systems

Advances in Heat-Activated Cooling Systems
An open-cycle desiccant system dehumidifies and cools air directly, providing greater efficiency and less pollution.

By William M. Worek, Davor Novosel, and R. Kirk Collier

Regulatory pressures are imposing new requirements on air conditioners, but recent advances show promise, particularly an open-cycle desiccant system that has begun to enter the market.

In the Pennington cycle, widely used in desiccant cooling systems, a desiccant removes moisture from the air. The desiccant is regenerated by the application of heat, which releases the moisture from the desiccant surface. The moist, warm air is then exhausted to the outdoors. The air passes through a sensible heat exchanger, where it is prechilled, and reaches its final temperature by passing through an evaporative cooler where water is added back into the air.

On the regeneration side, air is first evaporatively cooled to create a cold sink for the sensible heat exchange with the process air. Passing through the sensible heat exchanger, the regeneration air is preheated. It reaches the desired regeneration temperature in the gas-fired heater. The hot air drives the moisture out of the desiccant wheel and exhausts it to the outdoors.

Conventional desiccants cannot contain the temperature and moisture wavefronts within the desiccant bed, as required for high-efficiency systems. Studies have revealed that a desiccant material having a slightly strong adsorption characteristic (isotherm), called a Type 1M material, is needed. Type 1M desiccant materials also can take advantage of the higher regeneration temperatures available from a fossil fuel source. The equipment designer can either achieve more dehumidification capacity in the same equipment volume or achieve the same dehumidification capacity in a smaller package.

William M. Worek is a professor of mechanical engineering at the University of Illinois at Chicago; Davor Novosel, the marketing manager for air-quality products at Semco Inc. in Columbia, Mo., and R. Kirk Collier is the president of Enerscope Inc. in Reno, Nev.

The above was adapted from an article in the August issue of Mechanical Engineering magazine. To obtain a copy of this issue, click here.