The search for energy from fossil fuels is often the search for heat. No matter where it comes from--oil, gas, coal, or even solar thermal--engineers usually turn heat into motion and then electricity with a generator.
Yet we waste an enormous amount of this heat every year. Between 20% to 50% of U.S. industrial energy input is lost as heat, much of it vented to the atmosphere in polluting exhaust gasses.
Now, a new use for an old technology, the Stirling engine, is converting this waste into usable electricity and boosting the efficiency of conventional power plants by making industrial exhaust into a power source. The physics are deceptively simple. Stirling engines, although vastly more efficient than when they were first invented in 1816, still work on the same basic principle: The difference between hot and cold fluids can generate mechanical motion. A heat source vaporizes and expands a fluid that pushes against a piston. When the fluid condenses as it cools, the cycle repeats.
For decades, the engines haven’t been used much because they don’t operate well at temperatures under about 1,200 degrees, making use cases few and far between. But a new company, CoolEnergy, backed by the National Science Foundation, the Department of Energy, and private equity investors, is deploying its first heat engines that can fire even at "very low" temperatures (about 200 to 500 degrees). That leaves a lot of temperature differential to tap. Composed of little more than heat-exchanging pipes, a storage tank, and a nitrogen-based fluid that transfers heat to an engine, it’s attracting interest from groups with high energy bills or special needs (e.g. the military). CoolEnergy figures its "fuel-free" power can pay for itself in as little as two years.
The technology is also starting to show up in the growing waste heat recovery market in everything from power plants to industrial boilers. Waste heat could be our next oil gusher.