The revolutionary new engine is more efficient than a steam turbine and has no moving parts.

MIT and NREL engineers have developed a heat engine with no moving parts that is as efficient as a steam turbine. (CREDIT: Felice Frankel)

Engineers at the Massachusetts Institute of Technology and the National Renewable Energy Laboratory (NREL) have developed a heat engine with no moving parts. Their new demonstrations show it converts heat to electricity at over 40 percent efficiency, better than traditional steam turbines.

A heat engine is a thermophotovoltaic (TPV) cell, similar to solar panel photovoltaics, that passively captures high-energy photons from a white-hot heat source and converts them into electricity. The team’s design can generate electricity from a heat source between 1,900 and 2,400 degrees Celsius, or up to about 4,300 degrees Fahrenheit.

The researchers plan to include a TPV element in a grid-scale thermal battery. The system will absorb excess energy from renewable sources such as the sun and store that energy in highly insulated banks of hot graphite. When energy is needed, such as on cloudy days, TPV cells will convert heat into electricity and transfer the energy to the power grid.

With the new TPV cell, the team successfully demonstrated the main parts of the system in separate small experiments. They are working on integrating parts to demonstrate a fully working system. From there, they hope to scale the system to replace fossil fuel power plants and create a fully decarbonized power grid powered entirely by renewable energy.

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“Thermophotovoltaics have been the latest key step in demonstrating that thermal batteries are a viable concept,” says Asegun Henry, Robert N. Noyce Career Professor in the MIT Department of Mechanical Engineering. “This is an absolutely important step towards the spread of renewable energy and the transition to a completely decarbonized grid.”

Henry and his collaborators published their results today in the journal Nature. The co-authors at MIT are Alina LaPotine, Kevin Schulte, Kyle Buznitsky, Colin Kelsall, Andrew Roskopf, and Evelyn Wang, Ford professor of engineering and head of the mechanical engineering department, and NREL staff in Golden, Colorado.

Jump over the gap

More than 90 percent of the world’s electricity comes from heat sources such as coal, natural gas, nuclear power and concentrated solar power. For a century, steam turbines have been the industry standard for converting such heat sources into electricity.

On average, steam turbines reliably convert about 35 percent of heat to electricity, with about 60 percent representing the highest efficiency of any heat engine to date. But the mechanism depends on moving parts whose temperature is limited. Heat sources with temperatures above 2000 degrees Celsius, such as Henry’s proposed thermal battery system, would be too hot for the turbines.

In recent years, scientists have been exploring solid-state alternatives—thermal engines with no moving parts that can potentially operate efficiently at higher temperatures.

“One of the benefits of solid state power converters is that they can operate at higher temperatures with lower maintenance costs because they have no moving parts,” Henry says. “They just sit there and reliably generate electricity.”

Thermophotovoltaic elements have offered one of the ways to study solid-state heat engines. Like solar cells, TPV cells can be fabricated from semiconductor materials with a specific band gap—the gap between the material’s valence band and its conduction band. If a photon of high enough energy is absorbed by a material, it can push an electron through the bandgap, where the electron can then conduct, and thus generate electricity—without the rotors or blades moving.

The thermal energy storage system consists of blocks of graphite for storing heat (left) and a tower of heat engines (center) that work by absorbing high-energy photons (right). (CREDIT: Alina Lapotin)

To date, most TPV cells have only achieved an efficiency of about 20 percent, with a record of 32 percent, because they were made from relatively narrow bandgap materials that convert lower temperature, lower energy photons and therefore convert energy less efficiently. .

Catching the light

In their new TPV design, Henry and colleagues sought to capture higher energy photons from a higher temperature heat source, thereby converting the energy more efficiently. The team’s new cell does this with materials with a larger bandgap and multiple compounds or layers of material compared to existing TPV designs.

The cell is made up of three main regions: a high bandgap alloy that sits on top of a slightly smaller bandgap alloy, underneath which is a specular layer of gold. The first layer captures the highest energy photons of the heat source and converts them into electricity, while the lower energy photons passing through the first layer are captured by the second and converted to add to the voltage generated. Any photons that pass through this second layer are then reflected back to the heat source by the mirror rather than being absorbed as wasted heat.

The team tested the cell’s efficiency by placing it above a heat flux sensor, a device that directly measures the heat absorbed by the cell. They exposed the cell to a high-temperature lamp and concentrated the light on the cell. They then varied the lamp’s intensity, or temperature, and observed how the cell’s energy efficiency—the amount of energy it produced compared to the heat it absorbed—changed with temperature. Between 1900 and 2400 degrees Celsius, the new TPV element maintained an efficiency of about 40 percent.

“We can achieve high efficiency over a wide range of temperatures that are typical for thermal batteries,” Henry says.

The cell in the experiments has an area of ​​about a square centimeter. For a grid-scale thermal battery system, Henry envisions, TPV cells will need to scale to about 10,000 square feet (about a quarter of a football field) and run in climate-controlled warehouses to draw power from huge banks of stored data. solar energy. He points out that there is an infrastructure for the production of large-scale photovoltaic cells, which can also be adapted for the production of TPVs.

“There is definitely a huge positive value here in terms of sustainability,” says Henry. “This technology is safe, environmentally friendly throughout its entire life cycle, and can have a huge impact on reducing carbon emissions from power generation.”

For more science news, visit our New Innovations section at The bright side of the news.

Note. Materials provided above by the Massachusetts Institute of Technology. Content can be edited for style and length.

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