MIT and National Renewable Energy Laboratory (NREL) engineers have created a heat engine with no moving parts. Their new demonstrations indicate that it converts heat to electricity with greater than 40% efficiency, outperforming typical steam turbines.
The heat engine is a thermophotovoltaic (TPV) cell, similar to the photovoltaic cells in a solar panel, that passively collects and transforms high-energy photons from a white-hot heat source into electricity. The team’s invention can produce power from a heat source ranging from 1,900 to 2,400 degrees Celsius, or up to 4,300 degrees Fahrenheit.
The researchers intend to incorporate the TPV cell into a grid-scale thermal battery. The device would take excess energy from renewable sources such as the sun and store it in strongly insulated banks of heated graphite. When energy is required, such as on cloudy days, TPV cells turn the heat into electricity and send it to the power grid.
The team has now successfully shown the key components of the system in separate, small-scale trials using the novel TPV cell. They are attempting to integrate the components in order to display a fully operational system. They intend to build up the system from there in order to replace fossil-fuel-powered power plants and enable a fully decarbonized power grid powered exclusively by renewable energy.
Asegun Henry, the Robert N. Noyce Career Development Professor in the Department of Mechanical Engineering at MIT, says, “Thermophotovoltaic cells were the final critical step in demonstrating that thermal batteries are a realistic concept.” “This is a crucial step toward expanding renewable energy and achieving a fully decarbonized system.”
“Thermophotovoltaic cells were the last key step toward demonstrating that thermal batteries are a viable concept. This is an absolutely critical step on the path to proliferate renewable energy and get to a fully decarbonized grid.”
says Asegun Henry, the Robert N. Noyce Career Development Professor in MIT’s Department of Mechanical Engineering.
Henry and his colleagues published their findings in the journal Nature today. Alina LaPotin, Kevin Schulte, Kyle Buznitsky, Colin Kelsall, Andrew Rohskopf, and Evelyn Wang, the Ford Professor of Engineering and chair of the Department of Mechanical Engineering, are among the MIT co-authors, as are collaborators from NREL in Golden, Colorado.
Jumping the gap
More than 90% of the world’s electricity is generated by thermal sources such as coal, natural gas, nuclear energy, and concentrated solar energy. For over a century, steam turbines have been the industrial norm for converting such heat sources into power.
On average, steam turbines reliably convert approximately 35% of a heat supply into electricity, with approximately 60% marking the best efficiency of any heat engine to date. However, the machinery is dependent on temperature-sensitive moving parts. Heat sources with temperatures of more than 2,000 degrees Celsius, such as Henry’s planned thermal battery system, would be too hot for turbines.
In recent years, scientists have investigated solid-state alternatives, which are heat engines with no moving parts that may be able to perform efficiently at higher temperatures.
One of the benefits of solid-state energy converters is that they can function at higher temperatures while requiring less maintenance, Henry explains. “They just sit there and generate electricity on a consistent basis.”
One exploration path toward solid-state heat engines was provided by thermophotovoltaic cells. TPV cells, like solar cells, might be produced from semiconducting materials having a specific bandgap – the difference between a material’s valence band and its conduction band. If a high-energy photon is absorbed by the material, it can kick an electron across the bandgap, where the electron can then conduct and generate electricity – all without moving rotors or blades.
Most TPV cells have only achieved efficiencies of approximately 20%, with the record being 32%, due to the use of relatively low-bandgap materials that convert lower-temperature, lower-energy photons and hence transfer energy less effectively.
Detecting light
Henry and his colleagues aimed to capture higher-energy photons from a higher-temperature heat source in their novel TPV design, transforming energy more effectively. In comparison to prior TPV designs, the team’s novel cell uses higher-bandgap materials and many junctions, or material layers.
The cell is made up of three major regions: a high-bandgap alloy that sits on top of a slightly lower-bandgap alloy that sits on top of a mirror-like layer of gold. The first layer absorbs and transforms the highest-energy photons from a heat source into electricity, while lower-energy photons that pass through the first layer are captured and converted to add to the created voltage by the second layer. Any photons that travel through this second layer are reflected back to the observer by the mirror.
The cell is made up of three major regions: a high-bandgap alloy that sits on top of a slightly lower-bandgap alloy that sits on top of a mirror-like layer of gold. The first layer absorbs and transforms the highest-energy photons from a heat source into electricity, while lower-energy photons that pass through the first layer are captured and converted to add to the created voltage by the second layer. Any photons that pass through this second layer are reflected back to the heat source via the mirror, rather than being absorbed as wasted heat.
The effectiveness of the cell was tested by placing it over a heat flux sensor, which directly monitors the heat absorbed by the cell. They subjected the cell to a high-temperature bulb, focusing the light on it. They next altered the intensity, or temperature, of the light bulb and observed how the cell’s power efficiency (the amount of power produced compared to the heat absorbed) fluctuated with temperature. The new TPV cell maintained an efficiency of roughly 40% over a temperature range of 1,900 to 2,400 degrees Celsius.
Henry explains that “we can achieve great efficiency over a wide temperature range relevant for thermal batteries.”
In the experiments, the cells were around a square centimeter in size. Henry anticipates TPV cells scaling up to 10,000 square feet (approximately a fifth of a football field) for a grid-scale thermal battery system and operating in climate-controlled warehouses to draw power from massive banks of stored solar energy. He points out that there is an infrastructure in place for producing large-scale solar cells, which could be converted to produce TPVs.
“In terms of sustainability, there’s obviously a significant net benefit here,” Henry says. “The technology is safe and environmentally friendly throughout its life cycle, and it has the potential to significantly reduce carbon dioxide emissions from energy production.”
The US Department of Energy helped fund some of this study.
