'Thermally activated cooling system' puts waste heat to use
By Ben Coxworth
June 13, 2011
Automobiles, appliances, power plants, factories and electrical utilities all waste one thing: heat. More specifically, they produce heat as a by-product of their normal operations, but that heat is just dispersed into the air instead of being put to use. Researchers from Oregon State University, however, have created a prototype system that harnesses waste heat to (rather ironically) cool the device that's creating the heat in the first place. While it isn't the first system to do so, it is claimed to be unusually efficient ... and, it can generate electricity.
The "thermally activated cooling system" combines two systems that have previously been used for the harnessing and dispersion of waste heat - a vapor compression cycle and an organic Rankine cycle.
A vapor compression cycle is what's at work in a refrigerator. It incorporates a recirculating liquid refrigerant, that (in this case) travels through microchannel heat exchangers, absorbing and carrying heat away from hot surfaces, to be released elsewhere.
An organic Rankine cycle, on the other hand, utilizes an organic liquid with a lower liquid-vapor phase change point than that of water. This means that it doesn't take as high a temperature to get it to boil, and once it boils, it can generate electricity.
By combining the two cycles, heat is both drawn away, and put to work powering cooling systems. The prototype at OSU has already been shown to be capable of turning 80 percent of every kilowatt of waste heat into one kilowatt of cooling capability.
When it comes to pure electricity production, the thermally activated cooling system isn't quite as impressive, coming out at 15-20 percent efficiency. Not great, admit the researchers, but still considerably better than nothing.
They envision the system being used to cool electronics, factories, alternative energy systems, and perhaps even to use the heat from hybrid cars' combustion engines to charge their batteries.
The research was recently published in the journal Applied Thermal Engineering.