You’ve heard the advice not to run your oven or dryer on a hot day.
Why? Because much of the heat they generate gets out into the room. It’s wasted.
You can’t keep your laptop on your lap very long. It gets too hot. More waste heat
Imagine the heat wasted in industrial processes!
For just one example, when you see a smokestack, the smoke has passed through technology that burns nasty stuff like dioxins and volatile organic compounds. Which makes it even hotter than it was before. And it’s not only smokestacks that release waste heat into the atmosphere.
But what if we could capture that heat and put it to work? Turn waste heat to electric power?
We’ve used one technology to turn waste heat from large-scale industrial processes to electricity since the 1970s by using it to run a turbine. In other words, it uses waste heat instead of combustion or nuclear reactions, but otherwise operates turbines the same as traditional power plants. Traditional power plants can use their waste heat by installing an additional turbine before the steam reaches the condenser.
Converting waste heat to electricity uses no additional fuel. It releases nothing into the atmosphere that wouldn’t have been released anyway. It makes sense both economically and environmentally.
A newer technology, thermoelectrics, does not need a turbine. It converts waste heat to electricity directly.
Do you camp, grill in your back yard, or have a fireplace? You can buy and use one thermoelecrtric product right now. More consumer applications will probably follow quickly. Thermoelectrics makes possible a whole new kind of recycling.
Waste heat to power using a turbine or engine
There are multiple designs, but basically water boils, steam turns a rotor, and with or without a condenser the steam becomes liquid water and returns to the boiler.
Scotland-based Heliex Power distributes waste-to-power steam turbines in the UK and five other European countries. Heliex installations are well enough insulated that they can be installed indoors.
Steam turbines require high heat. Less than 10% of the heat generated by industry is hot enough to operate a steam turbine.
A newer technology, the organic Rankine cycle (ORC), vaporizes and condenses refrigerants, which boil at a much lower temperature than water. Some of them can work at temperatures as low as 200º Fahrenheit.
General Electric and Electratherm have installed ORC turbines in at least ten different countries. Ener-G-Rotors use ORC differently. Instead of operating a turbine, the refrigerant turns a simple rotor mounted on bearings, which make it nearly frictionless. MIT and the EPA have both described this new technology.
Echogen licensed a CO2 heat pump from NASA and after its first prototype figured out how to use supercritical CO2 to operate a generator instead of a heat pump. CO2 boils and vaporizes at a much lower temperature than even refrigerants.
Supercritical CO2 achieves a temperature where the CO2 isn’t quite a gas, but isn’t quite a liquid, either. The company introduced its first commercial product in December 2014.
Cool Energy, on the other hand, is resurrecting an old technology, the Stirling engine, that was invented in 1816 and ultimately proved unsuccessful.
It’s an external combustion engine that’s much more economical than steam engines. It’s a piston engine that operates on compressed air at a lower temperature than steam. It can use almost any heat source.
When Stirling engines failed (which all of the industrial scale engines did), the result was much less catastrophic than the explosion of a steam engine.
They worked well on a smaller scale, such as pumping water, until electric motors became available.
The technological and material roadblocks to the Stirling motor’s success were all solved by the end of the Second World War. Various companies tried without success to find a commercially viable use for them.
Cool Energy’s waste heat to power application is expected to become available commercially later this year. It will be interesting to see if the Stirling engine will finally be competitive with other technologies.
The junction of two dissimilar materials produces voltage if they are different temperatures.
To be a practical power source, thermoelectric materials must conduct electricity well, but conduct heat poorly.
That combination keeps the hot side hot and the cold side cold.
NASA developed a thermoelectric generator for the Voyagers 1 and 2, which launched in 1977. Thirty six years later, Voyager 1 left the solar system, with its thermoelectric generator still working.
Decay of radioactive material provides heat, and semiconductors made of rare and expensive materials turn it into electricity.
Nanotechnology and a much more common and inexpensive semiconductor, tetrahedrite, have made wider application of thermoelectrics practical. Scientists at Michigan State University developed the technology and licensed it to Alphabet Energy.
A circuit of thermoelectric materials that produce usable electricity is called a thermoelectric module. A thermoelectric module requires both a positively charged and a negatively charged semiconductor, configured to be in series electrically and in parallel thermally.
In order to be useful to turn waste heat to power, it must be able to withstand the stress of multiple thermal circuits in very large temperature gradients over time. Thermoelectric modules that are both engineered properly and made with economically feasible materials form the basis of a thermoelectric power generator.
A thermoelectric power generator requires a dependable heat source on the hot side and a suitable means of cooling the cold side. Various machines used both in industry and in personal activities provide an infinite variety of heat sources. Car exhaust can serve quite nicely. Air, water, or a refrigerant can provide the cooling.
The generator, like a solar array, produces direct current. Where alternating current is needed, the generator’s output goes through an inverter.
Alphabet Energy has designed and built the world’s largest thermoelectric power generator for use in the oil and gas industry, mining, defense, transportation, and manufacturing. It’s the size of a shipping container, but its construction comprises multiple thermoelectric modules. A single module would be useful for much smaller applications.
If Alphabet Energy claims the largest thermoelectric generator, LairdTech claims the smallest. It has developed a thin nanoscale thermoelectric film to create a device that puts out electricity measured in microwatts.
Such a small generator could capture heat within thermal environments that produce, and therefore waste, heat at much lower temperatures than industrial applications. The device is designed to provide power to distributed devices, such as remote sensors.
At least one consumer thermoelectric product, the PowerPot, is available already. More will surely follow. The PowerPot’s heat source can be a campfire, an outdoor grill, a fireplace. It looks like an ordinary cooking pot with a thermoelectric plate on the bottom. Filling the pot with water provides the coolant.
Even if the water boils, it still has a much lower temperature than the heat source. Melting snow, being much colder, allows the device to produce much more electricity.
The PowerPot produces enough electricity to charge a cell phone or any other device charged with a USB port. It begins to charge within seconds of being placed on the heat source. Its built-in regulator means that the variability of the temperature of the water doesn’t matter.
The electrical output is always the same: 5 volts and up to 1000 milliamps, the most any USB charged device can handle. The devices charge in the same amount of time with the PowerPot as they do plugged into a wall socket. It is therefore a very useful product on camping trips or in event of a power failure.
Choose which PowerPot is best for you.
Photo license statements:
Electric plant. Source unknown.
Waste heat to power diagram. US Air Force.
Stirling engine. Public domain from Wikimedia Commons.
Thermoelectric module. Pubic domain from Wikimedia Commons.