Engineers from MIT and NREL designed a heat engine with no moving parts. Its new tests show that it converts heat into electricity with an efficiency greater than 40%, an efficiency superior to that of traditional steam turbines.
The heat engine is a thermophotovoltaic cell, similar to photovoltaic cells in solar panels, which passively captures high-energy photons from an incandescent heat source and converts them into electricity.
The design of the equipment can generate electricity from a heat source between 1900 and 2400 ºC.
Researchers plan to integrate the TPV cell into a grid-scale thermal battery. The system would absorb excess energy from renewable sources, such as the sun, and store it in heavily insulated hot graphite banks. When electricity is needed, TPV cells convert heat into electricity and send the electricity to a power grid.
With the new TPV cell, the team succeeded in demonstrating the main parts of the system in separate small-scale experiments. They are working on integrating parts and demonstrating a fully operational system. From there, they hope to expand the system to replace fossil fuel-powered power plants and make possible a fully carbon-free electricity grid, powered entirely by renewable energy.
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The challenge.
On average, steam turbines reliably convert about 35% of a heat source into electricity, with 60% representing the highest efficiency of any heat engine to date. But machines rely on moving parts that are limited by temperature. Heat sources above 2000 ºC, such as the proposed thermal battery system, would be too hot for the turbines.
In recent years, scientists have investigated alternatives to solid-state heat engines with no moving parts, which could operate efficiently at higher temperatures.
Thermophotovoltaic cells offer an exploration route to solid-state heat engines. Like solar cells, TPV cells could be made from semiconductor materials with a certain bandgap. If the material absorbs a photon with high enough energy, it can kick an electron across the band gap, where the electron can conduct itself, and thus generate electricity, without the need to move any rotors or blades.
To date, most TPV cells have achieved only around 20% efficiency, with a record high of 32%, because they have been fabricated with low-bandgap materials that convert photons at low temperatures and low energy and therefore, they convert energy less efficiently.
catch the light
In their new TPV design, Henry and his colleagues attempted 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 higher bandgap materials and multiple junctions, or layers of material, compared to existing TPV designs.
The cell is made up of three main regions: a high band alloy, which sits on top of a slightly lower band alloy, below which is a mirror-like layer of gold. The first layer captures higher energy photons from a heat source and converts them into electricity, while lower energy photons passing through the first layer are captured by the second and converted to increase the voltage generated. Photons passing through this second layer are reflected by the mirror and returned to the heat source, rather than being absorbed as waste heat.
The team tested the cell’s efficiency 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 it. They then varied the bulb’s intensity, or temperature, and observed how the cell’s energy efficiency, the amount of energy it produces relative to the heat it absorbs, changes with the temperature. In a range of 1900 to 2400 ºC, the new TPV cell maintained an efficiency of approximately 40%.
They can achieve high efficiency in a wide range of temperatures relevant for thermal batteries.
The experiment cell is about one square centimeter. For a grid-scale thermal battery system, it is expected that the TPV cells would need to be sized at around 3,000 m2, operating in temperature-controlled warehouses to draw power from huge banks of stored solar energy. He points out that there is an infrastructure to manufacture photovoltaic cells on a large scale, which could also be adapted to manufacture TPVs.
More information: www.nature.com (English text).