New Heat Engine With No Moving Parts Could Fully Decarbonize the Power Grid

Thermophotovoltaic Cell

A thermophotovoltaic (TPV) cell (size 1 cm x 1 cm) mounted on a heat sink designed to measure the TPV cell efficiency. To measure the efficiency, the cell is exposed to an emitter and simultaneous measurements of electric power and heat flow through the device are taken. Credit: Felice Frankel

A New Heat Engine With No Moving Parts Is As Efficient as a Steam Turbine

The novel design could someday enable a fully decarbonized power grid, researchers say.

A heat engine with no moving parts has been developed by engineers at MIT and the National Renewable Energy Laboratory (NREL). Their new demonstrations show that it converts heat to electricity with over 40% efficiency — a performance better than that of 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 between 1,900 to 2,400 degrees Celsius, or up to about 4,300 degrees Fahrenheit.

The TPV cell will be used in a grid-scale thermal battery, according to the researchers. Excess energy from renewable sources, such as the sun, would be absorbed by the system and stored in heavily insulated banks of hot graphite. When the energy is needed, such as on cloudy days, TPV cells would convert the heat into electricity and send it to the 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 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, the Robert N. Noyce Career Development 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 have published their results on April 13, 2022, in the journal Nature. Co-authors at MIT include Alina LaPotin, Kyle Buznitsky, Colin Kelsall, Andrew Rohskopf, and Evelyn Wang, the Ford Professor of Engineering and head of the Department of Mechanical Engineering, along with Kevin Schulte and collaborators at NREL in Golden, Colorado.

Jumping the gap

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

On average, steam turbines reliably convert about 35 percent of a heat source into electricity, with about 60 percent representing the highest efficiency of any heat engine to date. But the machinery depends on moving parts that are temperature-limited. Heat sources higher than 2,000 degrees Celsius (~3,600 degrees Fahrenheit), 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 could 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 the bandgap, where the electron can then conduct, and thereby generate electricity — doing so without moving rotors or blades.

To date, most TPV cells have only reached efficiencies of around 20 percent, with the record at 32 percent, as they have been made of relatively low-bandgap materials that convert lower-temperature, low-energy photons, and therefore convert energy less efficiently.

Catching light

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 so with higher-bandgap materials and multiple junctions, or material layers, compared with existing TPV designs.

The cell is fabricated from three main regions: a high-bandgap alloy, which sits over a slightly lower-bandgap alloy, underneath which is a mirror-like layer of gold. The first layer captures a heat source’s highest-energy photons and converts them into electricity, while lower-energy photons that pass through the first layer are captured by the second and converted to add 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 directly measures the heat absorbed from 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 1,900 to 2,400 degrees Celsius, the new TPV cell maintained an efficiency of around 40 percent.

“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 a square centimeter. 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.

Reference: “Thermophotovoltaic efficiency of 40%” by Alina LaPotin, Kevin L. Schulte, Myles A. Steiner, Kyle Buznitsky, Colin C. Kelsall, Daniel J. Friedman, Eric J. Tervo, Ryan M. France, Michelle R. Young, Andrew Rohskopf, Shomik Verma, Evelyn N. Wang and Asegun Henry, 13 April 2022, Nature.
DOI: 10.1038/s41586-022-04473-y

This research was supported, in part, by the U.S. Department of Energy.

8 Comments on "New Heat Engine With No Moving Parts Could Fully Decarbonize the Power Grid"

  1. FAKE LIES.Steam turbines have an efficiency of over 70%

  2. That’s very interesting but you won’t find any renewable source at 2000 °C very soon, unless you consider solar concentration. As far as I know, at those temperatures, steam turbines are much more efficient, maybe something like 80%.

  3. Cherlyn Kirk | April 16, 2022 at 1:54 pm | Reply

    Great article, well done MIT, a sane and rational university.

  4. I still think of Tesla’s fascination with lightning energy, and his tenet that higher frequency wireless electrical transmission enables more usable power at the target. I think of lightning rods located at areas of frequent strikes, the energy moves along a cable into a super capacitor bank, and when the capacitors are charged, inverters that feed the grid are activated and the grid is replenished. If the riddle of high frequency wireless transmission gets solved, there will be little need for batteries, and vehicles and other machinery will be monitored for their wireless energy usage, by modern methods that would please even J.P. Morgan, who cut Tesla’s funding when the inventor couldn’t explain how his high frequency energy transmission could be tallied and charged for. Like with modern electrical meters, the amount of energy used from the grid will be automatically added to each user’s account balance, which would include one’s residence as well. Also, there will be no need to strip the earth of millions of tons of soil, and to have to leech out the infinitesimal tiny amount of lithium in the soil by using thousands of gallons of sulphuric acid or some other noxious material, See: https://www.theguardian.com/news/2020/dec/08/the-curse-of-white-oil-electric-vehicles-dirty-secret-lithium

  5. You may call me a dreamer, but I’m not the only one (John Lennon). If we’re going to improve the way we harness and use energy, let’s not jump from the frying pan into the fire pit of further environmental destruction. We could start by stopping the enormous energy waste, which is the COMMUTE, the elephant on the planet that uses more energy than any other daily phenomenon. Why not wed our workers and their housing to the locations where they work? Most of their needs could be satisfied by high efficiency super light rail, that could service workers and their work spaces right at their residence areas which could be located along with residential lands for fresh food production. The energy saved daily would be almost unimaginable. Have a look at the 20 lane freeways in China, and imagine the unnecessary pollution and wasted energy that could be put to better use.

  6. Where did you get your data for steam turbines, from what I’ve learned, they start at around 40% efficiency and can be up to as high as 90% efficient.

  7. thanks alot of information

  8. I am wondering how long the integrated circuit would last at 1800 degrees Celsius – I don’t think it would last for very long ( days? 1 mo maybe?)

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