Breakthrough TPV cell makes more power from heat than a steam turbine

The majority of humanity's electricity comes from heat – burning coal or natural gas, nuclear fission, concentrating solar – that's used to boil water and spin steam turbines. This method of power generation has been around since Charles Parsons first hooked a steam turbine up to a dynamo in 1884, and licensed the patent to George Westinghouse. Over the last century and a half, it's become ubiquitous all over the world as a mature and well-optimized technology with known strengths and limitations.

One of those limitations is efficiency. While some turbines have managed to convert up to 60 percent of a heat source's energy into electricity, the average turbine operates at closer to 35 percent efficiency. Another limitation is heat – the moving parts in turbines prevent them from operating, for example, at temperatures over 2,000 °C (3,600 °F).

These figures come from an MIT research team that's been working on an alternative: a heat engine with no moving parts, a thermophotovoltaic (TPV) device that the team has now demonstrated in a small 1 x 1-cm (0.4 x 0.4-in) prototype, maintaining an efficiency over 40 percent across a temperature range between 1,900 - 2,400 °C (3,450 - 4,300 °F).

That's a significant advance, says the team, from typical TPV heat engines, most of which operate at about 20-percent efficiency, with the previous record being 32 percent, and offers a more effective way of harvesting energy than turbines under the right circumstances.

Thermophotovoltaic heat engines boil down to something like this: heat arrives and is collected by an absorber/emitter material, which takes in the heat and kicks out photons on the other side. These photons are harvested by a regular photovoltaic cell in close proximity, which converts them into usable electricity.

The MIT team made its impressive efficiency advance by tweaking a few variables. Firstly, the input heat temperature – this device is designed specifically to work at hot temperatures above the range where turbines can no longer function. This allows the team to use higher-bandgap absorber/emitter materials, which take in more energy and release higher-energy infrared photons on the emitter side – as well as photovoltaic cells designed to take maximum advantage of those high-energy photons.

Then, the team layered the photovoltaic cells – the first layer being designed to harvest the highest-energy photons at transmission-efficient higher voltages, and the second layer being there to mop up lower-energy photons. Photons that make it through both layers are reflected back onto the absorber/emitter with a mirror, so that any photons outside the optimal ranges can feed back into the start of the process and help to keep the emitter temperature up.