Harnessing Heat for Work | Green Energy News

April 9, 2009 – Vol.14 No.3

HARNESSING HEAT FOR WORK.
by Bruce Mulliken, Green Energy News

The heat that exits the exhaust pipe of our cars, the heat from the Sun that’s vented from our home’s attic, and the heat that leaves industrial smoke stacks all represent considerable energy that is expelled into the atmosphere as waste. We shouldn’t let this heat go. We should be putting it to work.

The most obvious example of work-worthy waste heat is from our car’s highly inefficient internal combustion engines. If we could tap the heat leaving by the exhaust pipe or via the vehicle’s radiator, the more energy efficient the engine and thus vehicle would be. But tapping the waste heat of combustion is not easy. Hot exhaust gases are moving rapidly and engine heat in the radiator is blown quickly away by a fan. It’s hard to grab heat that’s running away.

Nor is it easy to transfer the heat in toxic exhaust to another more benign medium. The radiator in a car removes engine heat by transferring heat from the engine’s coolant to the air and thus regulating the engine’s operating temperature. But there is no similar heat exchanger to remove heat from the exhaust and do something with it.

However, with technology being developed jointly by Oregon State University (OSU) and the Pacific Northwest National Laboratory, it may soon be possible to grab that waste exhaust heat and use it for air conditioning or to generate electricity. To capture that waste heat much improved, smaller, lighter, much more efficient heat exchangers are being developed that include microchannel heat transfer components that provide high heat transfer rates. Along with a more efficient vapor expander the researchers will be able to turn exhaust heat into cold air. So efficient is the thermodynamic cycle, that it could also be used to drive a generator to provide electricity, but not do both at the same time.

The joint venture of OSU and the Pacific Northwest National Laboratory called the Microproducts Breakthrough Institute.

In the first waste-heat-to-work project, engineers are using exhaust heat from a diesel generator to drive an expander-compressor cycle to provide cooling. The system will work like a heat pump found in many homes but will use waste heat instead of electricity to chill air. The prototype, which the U.S. Army is interested in for cooling heat-sensitive operations, will be a small five-kilowatt system that's a little larger than an automobile air conditioner in cooling capacity. The waste-heat-driven air conditioner will improve the fuel efficiency of the diesel generator by 20 - 30 percent. The Army is interested in fuel efficiency to reduce fuel consumption in field operations.

Eventually the technology could make its way into our civilian cars and trucks. Using waste engine heat to provide air conditioning would relieve the engine of the energy-robbing duties of running its conventional air conditioner. Then, if the waste heat can also be used to run a generator, the vehicle’s alternator would no longer be needed, again improving fuel economy or perhaps running an electric assist motor.

There are likely more uses for the technology as well.

Instead of solar photovoltaics to power our homes, solar thermal energy could be moderately concentrated to provide provide heat, air conditioning, or to operate a generator to provide electricity.

Maybe there’s enough heat in the flue of a wood or pellet stove to generate electricity.

Basically anything that is hot, including industrial processes, could be used to provide air conditioning or electric power. Hot emissions from metal producing operations could generate electricity. All varieties of thermal power plants – coal, nuclear, biomass, oil, natural gas – could become more efficient while providing air conditioning to nearby buildings.

Two companies that offer commercially available waste heat to electricity machines are ElectraTherm with its Green Machine and Ener-G-Rotors with its waste heat appliances.

The Green Machine uses waste heat as low as 200 F degrees to boil refrigerant (R-245fa). The resulting vapor is used to drive a patented Twin Screw Expander that generates usable electricity.

The Ener-G-Rotors waste heat appliances convert low temperature heat sources of 150 - 400 degrees F into electricity at various scales from less than 1 kilowatt capacity to eventually as large as 250 kilowatts. The company has taken its first order for a 50 kilowatt TGE50 that will be available next year. That device will be going to Harbec Plastics, an injection molding company located in upstate New York.

The Microproducts Breakthrough Institute appears to closing in on the next generation of waste-heat-to-energy technology. They can already produce the microchannel devices needed to make these systems operational, but work will continue in order to develop improved manufacturing efficiencies and create less expensive devices at higher production volumes.

The OSU engineers should have a working prototype ready for demonstration by this summer.

Links:

Microproducts Breakthrough Institute
http://www.pnl.gov/microproducts

Ener-G-Rotors
http://www.ener-g-rotors.com

ElectraTherm
http://www.electratherm.com

Harbec Plastics
http://www.harbec.com