With the new TPV cell, the team has now succeeded in demonstrating the main parts of the system in separate small-scale experiments. They work to integrate the parts to demonstrate a fully operational system. From there, they hope to expand the system to replace fossil-fuel power plants and enable a fully carbon-free electricity grid, powered entirely by renewables.
“Thermophotovoltaic cells were the last key step to demonstrate that thermal batteries are a viable concept”, says Asegun Henry, Robert N. Noyce Professor of Career Development in MIT’s Department of Mechanical Engineering. “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. But machinery depends on moving parts that are temperature limited. Heat sources above 2,000 degrees Celsius, such as Henry’s proposed thermal battery system, would be too hot for the turbines.
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,” said Henry. “They just sit there and produce electricity reliably.”
Thermophotovoltaic cells offered an exploratory route to solid-state heat engines. Much like solar cells, TPV cells could be made from semiconductor materials with a particular band gap – the gap between a material’s valence band and its conduction band. If a photon with high enough energy is absorbed by the material, it can send an electron across the band gap, where the electron can then conduct, and thus generate electricity, without moving the rotors or blades.
To date, most TPV cells have only achieved efficiencies of around 20%, with a record high of 32%, because they are made of relatively low bandgap materials that convert photons at low temperatures and low energy, and therefore convert energy less efficiently. . In their new TPV design, Henry and his colleagues sought 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 material layers, compared to existing TPV designs.
The cell is made from three main regions: a high bandgap alloy, which sits on top of a slightly lower bandgap alloy, under 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 add to the generated voltage . Any photons that pass through this second layer are then reflected back by the mirror, back 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 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 – changed with the temperature. Over a range of 1,900 to 2,400 degrees Celsius, the new TPV cell maintained an efficiency of around 40%.
“We can achieve high efficiency over a wide range of temperatures relevant for thermal batteries”, said Henry.
The cell in the experiments is about one square centimeter. For a grid-scale thermal battery system, Henry envisions the TPV cells would need to expand to about 10,000 square feet (about a quarter of a football field) and operate in climate-controlled warehouses to fire energy from huge storage banks. solar energy. He points out that an infrastructure exists to manufacture photovoltaic cells on a large scale, which could also be adapted to manufacture TPVs.
“There is definitely a huge net benefit here in terms of sustainability,” said Henry. “The technology is safe, environmentally friendly in its lifecycle and can have a significant impact in reducing carbon dioxide emissions from power generation.”