Engineers at MIT and NREL have developed a heat engine with no moving parts that is as efficient as a steam turbine. Photo: Felice Frankel.
Engineers at MIT and NREL have developed a heat engine with no moving parts that is as efficient as a steam turbine. Photo: Felice Frankel.

Engineers at Massachusetts Institute of Technology (MIT) and the US National Renewable Energy Laboratory (NREL) have designed a heat engine with no moving parts. Their new demonstrations show that this novel heat engine can convert heat to electricity with over 40% efficiency – a better performance than traditional steam turbines.

The heat engine is a thermophotovoltaic (TPV) cell, similar to a solar panel’s photovoltaic cells, 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 of 1900–2400°C, or up to about 4300°F.

The researchers plan to incorporate the TPV cell into a grid-scale thermal battery. This system would absorb excess energy from renewable sources such as the Sun and store that energy in heavily insulated banks of hot graphite. When the energy is needed, such as on overcast days, TPV cells would convert the heat into electricity, and dispatch the energy to a power grid.

With the new TPV cell, the team has now successfully demonstrated the main parts of the system in separate, small-scale experiments. They are now working to integrate the parts to demonstrate a fully operational system. From there, they hope to scale up the system to replace fossil-fuel-driven power plants and enable a fully decarbonized power grid, supplied entirely by renewable energy.

“Thermophotovoltaic cells were the last key step toward demonstrating that thermal batteries are a viable concept,” says Asegun Henry, a professor in MIT’s Department of Mechanical Engineering. “This is an absolutely critical step on the path to proliferate renewable energy and get to a fully decarbonized grid.” Henry and his collaborators report their results in a paper in Nature.

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

On average, steam turbines reliably convert about 35% of a heat source into electricity – the highest efficiency of any heat engine to date is about 60%. But the machinery depends on moving parts that are temperature-limited. Heat sources with temperatures higher than 2000°C, such as Henry’s proposed thermal battery system, would be too hot for turbines.

In recent years, scientists have looked into solid-state alternatives – heat engines with no moving parts that could potentially work efficiently at higher temperatures.

“One of the advantages of solid-state energy converters are 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 cells offered one exploratory route toward solid-state heat engines. Much like solar cells, TPV cells can be made from semiconducting materials with a particular bandgap – the gap between a material’s valence band and its conduction band. If a photon with a high enough energy is absorbed by the material, it can kick an electron across this bandgap into the conduction band, thereby generating electricity – all without moving rotors or blades.

To date, most TPV cells have only reached efficiencies of around 20%, with the record at 32%. This is because they have been made from relatively low-bandgap materials that convert lower-temperature, low-energy photons, meaning they convert energy less efficiently.

In their new TPV design, Henry and his colleagues looked to capture higher-energy photons from a higher-temperature heat source, thereby converting energy more efficiently. The team’s new cell does this by using higher-bandgap materials and multiple junctions, or material layers, compared with existing TPV designs.

The cell comprises three main sections: a high-bandgap alloy, which sits over a slightly lower-bandgap alloy, underneath of which is a mirror-like layer of gold. The first layer captures a heat source’s highest-energy photons and converts them into electricity. Meanwhile, lower-energy photons that pass through the first layer are captured by the second layer and converted into electricity, thus adding to the generated voltage. Any photons that pass through this second layer are then reflected by the mirror, back to the heat source, rather than being absorbed as wasted heat.

The team tested the cell’s efficiency by placing it over a heat flux sensor – a device that can directly measure the heat absorbed by the cell. They exposed the cell to a high-temperature lamp and concentrated the light onto the cell. They then varied the bulb’s intensity, or temperature, and observed how the cell’s power efficiency – the amount of power it produced, compared with the heat it absorbed – changed with temperature. Over a range of 1900°C to 2400°C, the new TPV cell maintained an efficiency of around 40%.

“We can get a high efficiency over a broad range of temperatures relevant for thermal batteries,” Henry says.

The cell in the experiments is about 1cm2. For a grid-scale thermal battery system, Henry envisions the TPV cells would have to scale up to about 10,000 square feet (about a quarter of a football field), and would operate in climate-controlled warehouses to draw power from huge banks of stored solar energy. He points out that an infrastructure exists for making large-scale photovoltaic cells, which could also be adapted to manufacture TPVs.

“There’s definitely a huge net positive here in terms of sustainability,” Henry says. “The technology is safe, environmentally benign in its life cycle, and can have a tremendous impact on abating carbon dioxide emissions from electricity production.”

This story is adapted from material from MIT, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.