Hot idea to cool down

Boeing engineer is getting national attention for his efforts to turn diesel exhaust into cooling agent

BY BOB HOWARD

Imagine getting energy from the diesel exhaust of 18-wheelers, or an environment free of diesel or heat pollution. For most mortals, this would seem hard to fathom—but not for Craig Christy.

A Boeing engineer based at Huntington Beach, Calif., Christy spends his "free" time hot on the trail of a new way to cool things down. Christy is working toward a way to refrigerate the hundreds of thousands of truck trailers that carry food goods and other precious essentials, crisscrossing the world's highways day and night.

Christy recently won an award from the U.S. Department of Transportation for his efforts to capture diesel exhaust, run it through a passive, charcoal-adsorption compressor that would take out the heat and then provide refrigeration or air conditioning out the other end with no added emissions. (Adsorption refers to molecules accumulating on the surface of a solid or liquid; Absorption is to take in, or as if in, through pores.)

The idea has a number of other applications, from satellites to ship containers. Basically, wherever heat is generated, it can be reused to create cold air via a passive ammonia-adsorption system that requires no mechanical compressor, engine or fuel.

At Boeing, Christy is project lead engineer for Fluids Systems and Propulsion Systems for various spacecraft projects. He has received special recognition from NASA for his contributions to the X-43 Hyper-X program. The X-43 Hyper-scramjet vehicle was designed and built to explore Mach 7 to 10 regions of flight.

Christy is currently pursuing a Ph.D. in Engineering and Applied Mathematics through a joint program between Claremount Graduate University and California State University Long Beach.

Christy's findings and research laboratory at California State University Long Beach have been the object of two federal grants, of $50,000 each. His current research involves improving efficiency and sizing the units for truck trailers and containers. Already he has achieved constant cold air, flowing at 40 degrees Fahrenheit, from the lab version.

So how does it all work? The units would be built under a sub-floor inside 40-, 50- and 60-foot-long refrigerated truck trailers. This new vapor compression concept takes advantage of the ability of certain materials such as activated carbon, or charcoal, to soak up a relatively large quantity of ammonia refrigerant at a low temperature and low pressure.

The activated charcoal material would be stored in a canister or sorbent bed. A minimum of four sorbent beds allows a continuous flow of compressed refrigerant through the system. Of the four beds, one is always kept hot and one always cold; the other two are either becoming hot or becoming cold.

This temperature cycling replaces the work a mechanical compressor does. Like today's mechanical compressor, Christy's thermal compressor will create high-temperature, high-pressure gas. That gas is acted on through another part of the system that releases the pressure, which drops the temperature and creates cold air.

"The technology has a unique potential for applications generating substantial waste heat," Christy says. "A modular sorption system would thrive on the plentiful high-temperature waste heat available from most transportation systems compared to the costly work input required of conventional mechanical air conditioning compressors.

"In hot and desert climates," Christy notes, "where cooling is most desired, this system is particularly attractive, because energy required for heating the refrigerant can be accomplished entirely by solar energy. The higher the intensity of solar heating, the more cooling can be made available."

Future experimental work on this system will include a detailed characterization of the adsorption beds, and optimization of temperature limits and cycle times.