Heat to electricity with no moving parts
Engineers develop a cell like those used in solar panels that turns heat into electricity more efficiently than steam turbines
Last updated 4/19/2022 at 12:34pm
Since its invention nearly 140 years ago, the modern steam turbine has represented the apex of efficiency when it comes to converting thermal energy into electricity. Now, engineers from the Massachusetts Institute of Technology and National Renewable Energy Laboratory have created a heat engine with no moving parts that exceeds the efficiency of the turbines used in today's coal, natural gas, and nuclear power plants.
Reminiscent of the photovoltaic solar cells that convert light into electricity, the thermophotovoltaic cell created by the MIT and NREL researchers captures high-energy photons from a white-hot heat source and converts them into electricity. The team has demonstrated that this cell can convert heat into electricity at 40% efficiency, which exceeds the roughly 35% efficiency of today's steam turbines.
The engineers of this most efficient TPV cell to date are not aiming to retrofit legacy steam turbine power plants with this new solid-state electrical generating technology. Instead, they plan to incorporate the TPV cell into a grid-scale thermal battery system that 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, the TPV cells would convert this stored heat into electricity for delivery to the power grid.
"Thermophotovoltaic cells were the last key step toward demonstrating that thermal batteries are a viable concept," says Asegun Henry, the Robert N. Noyce Career Development Professor in MIT's Department of Mechanical Engineering. "This is an absolutely critical step on the path to proliferate renewable energy and get to a fully decarbonized grid."
The idea of thermal batteries as a means of storing energy, especially from intermittent power sources such as solar, is not a new one. Most renditions, however, have relied on the same steam turbine technology that has delivered 90% of the world's electricity from coal, natural gas, and nuclear power plants over the past century.
These classic thermal batteries store heat generated from concentrated solar or some other source in sand or some other such medium, which is used to create steam to turn a turbine and generate electricity as needed.
On average, steam turbines reliably convert about 35% of a heat source into electricity. These mechanical systems, however, need to be maintained and cannot operate at the higher than 2,000 degrees Celsius (3,600 degrees Fahrenheit) temperatures Henry and his team propose for their thermal battery system.
"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 says. "They just sit there and reliably generate electricity."
Much like solar cells, the solid-state TPV cells developed by the MIT and NREL engineers utilize semiconducting materials with a particular bandgap – the minimum amount of energy required for an electron to break free of its bound state. If a high-energy photon is absorbed, the semiconductive material in the TPV cell can kick an electron across the bandgap, which excites the electron into a free state and generates electricity without moving rotors or blades.
While many semiconductive materials have been tested in the past, the TPV engineered by Henry's team uses two layers of materials with gallium, indium, aluminum, and arsenic to turn the heat produced photon into electricity.
The first layer captures a heat source's highest-energy photons and converts them into electricity, while lower-energy photons that pass through the first layer are captured by the second and converted to add to the generated voltage. A highly reflective gold film was added to the back of the cell to bounce photons back up through the semiconductive materials to the heat source, rather than being absorbed as wasted heat.
In lab testing, the new TPV cell maintained an efficiency of around 40% over a temperature range of 1,900C to 2,400C (3,450F to 4,350F).
"We can get a high efficiency over a broad range of temperatures relevant for thermal batteries," Henry says.
While such a device made with critical metals and gold sounds expensive, a paper published in the science journal "Nature," indicates the costs to build and operate thermal energy grid storage systems utilizing the new TPV cell would compete with traditional steam turbine power plants.
Using graphite to store the heat, which was the most efficient thermal storage medium identified, and the TPV cell developed by the MIT and NREL team would cost less than US$10 per kilowatt-hour of energy storage capacity to build. This would enable long-term thermal energy grid storage systems to be developed below the US$20 per kWh cost target that allows renewable energy with storage to be cost-competitive with fossil fuels.
"There's definitely a huge net positive here in terms of sustainability," Henry says. "The technology is safe, environmentally benign in its life cycle, and can have a tremendous impact on abating carbon dioxide emissions from electricity production."
The next step for the MIT and NREL research team is to scale up their experiment, which utilized a cell that was only about one square centimeter.
For a grid-scale thermal battery system, Henry envisions the TPV cells would need to be about 10,000 square feet, or about a quarter of the size of a football field, which would be installed in climate-controlled warehouses to draw power from huge banks of stored solar energy.
Given the thermal converting technology's similarities to photovoltaic cells used in traditional solar panels, the MIT professor says the infrastructure already exists to produce the TPV cells his team developed.
This research was supported, in part, by the U.S. Department of Energy.
The Nature article on this technology "Thermophotovoltaic efficiency of 40%" can be read at https://www.nature.com/articles/s41586-022-04473-y#Sec4.