Solid-state motor efficiently converts heat into electricity

A motor with no moving parts that converts heat into electricity with over 40% efficiency has been developed by MIT engineers.

The performances obtained are superior to traditional steam turbines. The heat engine is a thermophotovoltaic cell (TPV), similar to the photovoltaic cells of a solar panel, which 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°C and 2,400°C and can hopefully be integrated 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 hot graphite banks. When power is needed, such as on an overcast day, TPV cells convert heat into electricity and send the power to an electrical grid.

With the new TPV cell, the team has now succeeded in demonstrating the main parts of the system in separate small-scale experiments.

They are working to integrate the pieces to demonstrate a fully operational system with plans to scale it to replace fossil fuel power plants and enable a fully carbon-free electricity grid powered entirely by renewables.

The prototype device

Image credit: Felice Frankel

“Thermophotovoltaic cells were the last key step in demonstrating that thermal batteries are a viable concept,” said MIT professor Asegun Henry. “This is an absolutely critical step on the path to proliferating renewables and achieving a fully carbon-free grid.”

More than 90% 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 these heat sources into electricity.

On average, steam turbines reliably convert around 35% of a heat source into electricity, with around 60% representing the highest efficiency of any heat engine to date.

In recent years, scientists have been looking at solid-state alternatives – heat engines with no moving parts, which could potentially operate efficiently at higher temperatures.

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

The team tested the efficiency of the TPV cell by placing it on 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 focused the light on the cell.

They then varied the intensity or temperature of the bulb and observed how the energy efficiency of the cell (the amount of energy it produced, compared to the heat it absorbed) varied with the temperature. Over a range of 1900°C to 2400°C, the new TPV cell maintained an efficiency of approximately 40%.

“We can achieve high efficiency over a wide range of temperatures relevant to thermal batteries,” Henry said.

The cell in the experiments is about one square centimeter. For a grid-scale thermal battery system, the TPV cells would need to be scaled to around 1,000 square meters and operated in temperature-controlled warehouses to draw power from huge stored solar power banks. .

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