Solid-state engine efficiently converts heat to electricity

The performance achieved is superior to 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 at between 1,900°C and 2,400°C and can hopefully be incorporated into a grid-scale thermal battery.

The 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 working to integrate the parts to demonstrate a fully operational system with plans to scale it up to replace fossil-fuel-driven power plants and enable a fully decarbonised power grid supplied entirely by renewable energy.

“Thermophotovoltaic cells were the last key step toward demonstrating that thermal batteries are a viable concept,” said MIT professor Asegun Henry. “This is an absolutely critical step on the path to proliferate renewable energy and get to a fully decarbonised grid.”

More than 90 per cent of the world’s electricity 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 per cent of a heat source into electricity, with about 60 per cent representing the highest efficiency of any heat engine to date.

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 said. “They just sit there and reliably generate electricity.”

The team tested the TPV 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) varied with temperature. Over a range of 1,900°C to 2,400°C, the new TPV cell maintained an efficiency of around 40 per cent.

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

The cell in the experiments is about a square centimetre. For a grid-scale thermal battery system, the TPV cells would have to scaled up to about 1,000 square metres and would operate in climate-controlled warehouses to draw power from huge banks of stored solar energy.