In the Know: Recycling Heat Waste

Willow Strey

Have you ever wondered where the heat from your oven goes after you are finished using it? The answer is that it is dissipated into the oven surroundings or vented away. No longer is this heat energy useful — it simply becomes waste.

Because heat is difficult to convert to electricity, there are very few devices designed to recycle it. Additionally, low temperature thermal energy has proven difficult to both harvest and harness.

Despite this, the technology exists. But is expensive, inefficient and fickle— Imagine if such technologies were developed to be as reliable and effective as, say, solar power. Similar to sunlight, heat energy is all around us: in the road during a hot Arizona summer, in the bottom of your laptop after playing too many video games and even in your own body. Its potential is undeniable.

So what are our current options for thermal energy harvesters, and what are their weaknesses? There are two promising types of generators: thermoelectric generators (TEG) and thermionic converters (TIC). 

The former works by creating a thermal gradient across the TEG that induces the Seebeck effect. Named after physicist Thomas Johann Seebeck, this phenomena is the induction of electric current (electricity) across two dissimilar materials via a temperature gradient. The higher kinetic energy of the hot material next to a different cold material of a lower kinetic energy creates a potential difference and allows for the flow of electrons. While exciting in theory, TEGs struggle to maintain an energy conversion efficiency (electric output/heat input) of even 1%, and extracting the converted electrical energy can be strenuous.

Alternatively, one could consider thermionic converters. Thermal gradients are similarly used to induce an electric current, but while this current is generated in TEGs by applying the gradient across two dissimilar nodes, TICs induce the transfer of electrons from a heat sink to a cold sink between disjointed materials (visualize a capacitor!). The space between said materials (called the inter-electrode gap) generates a vacuum that allows the thermionic electron emission process to manifest along the hot surface. So while TEGs utilize contrasting kinetic energies to induce current, TICs use suction.

Although TICs require extremely high temperatures (starting around 1328) to work effectively, they exhibit an energy conversion efficiency of around 15% (for comparison, common solar power systems have an energy efficiency of around 16%). This makes them immediately relevant as industrial thermal energy harvesters such as those used in a nuclear power plant, but they are not as practical for everyday life as TEGs.

As of now, nor TEGs or TICs are economic solutions to the renewable energy problem. They are not only costly, but difficult to implement into pre-existing structures. However, their development demonstrates our positive progression towards innovative energy waste solutions. Perhaps in the near future, heat energy recycling could look as simple as a portable device for your oven, car engine, or even your wrist.

If you are interested in reading more about the current state of thermal energy harvesting, check out the free scientific journal article Solid state generators and energy harvesters for waste heat recovery and thermal energy harvesting online at Elsevier.