The direct conversion of heat to electricity is extremely difficult, but would improve the electricity production of the world by a considerable margin. Most of the systems we have for generating electricity do capture the work done as heat flows from something really hot to something not as hot. In most cases, the amount of work we can extract from this flow of heat is directly related to the difference in temperature between the hot side and the cold side.
Basically, the hotter it is, then more efficient it is. Unfortunately, once that temperature difference drops below some insanely high value, we lose the ability to produce electricity and all that extra energy is basically wasted.
Just think about your car for a minute. The most efficient internal combustion engines only use about 33% of the energy generated. The rest is converted to heat (hence the massive cooling system in modern cars). More energy is lost to heat from friction in all the vehicles moving parts (hence the need for oil and other lubricants). All told, we’re lucky to 17-10% of the energy in the fuel converted into movement. The rest becomes an increase in temperature.
But wait, I just said we can convert a temperature difference into energy. And now thanks to a research team from the University of Minnesota, that ability may have gotten a little easier.
The material they developed (an alloy, Ni45Co5Mn40Sn10) has a unique phase change that helps it become a source of electricity.
A phase change, as you already know, is when the physical properties of a material change due to a change in the temperature of the material. For example, as the temperature increases, ice melts into water and then boils into steam. As the temperature drops, the steam condenses back into water and then freezes into ice.
This new allow (I really hope they come up with a snappy name for it) has a phase change, but not like melting or boiling. When the temperature hits a certain point, it becomes a strong magnet, then when the temperature drops, it stops being a magnet.
This is pretty cool when you think about it, but it’s even cooler because this happens at a relatively cool temperature. At 125°C the magnetization is 10 emu/cm3. But at just less than 150°C the magnetization is 1200 emu/cm3. This massive change can be used to induce electricity in a coil of copper wire.
How much electricity? Well, the sample used in the experiments resulted in a change from 0.6 mV to -0.6mV (millivolt, thousandths of a volt), which doesn’t sound like a lot. But keep in mind that modern electronics use not much more electricity.
The object and future work will be directed toward finding materials that will produce more electricity at cooler temperatures or wider temperature bands.
This is a really exciting development. We could attach tiny devices to our computers and harvest some waste heat. We could attach them to the exhaust of our cars and generate electricity to recharge hybrid batteries. Any place, there is a temperature difference, we could exploit that to produce a miniscule bit of electricity. Of course, miniscule bits times millions of devices can result in a significant amount of electricity production.
Vijay Srivastava , Yintao Song , Kanwal Bhatti , and R. D. James (2011). The Direct Conversion of Heat to Electricity Using Multiferroic Alloys Advanced Energy Materials, 97-104 : 10.1002/aenm.201000048