From MIT News
Machine Heart Compilation
The Massachusetts Institute of Technology (MIT) and the National Renewable Energy Laboratory (NREL) have designed a new thermal photovoltaic (TPV) battery that converts heat into electricity at more than 40 percent efficiency, outperforming conventional steam turbines. The study was featured in Nature.
This thermal photovoltaic cell, which resembles a solar panel, passively captures high-energy photons from a white-hot heat source and converts them into electrical energy, which can be used to generate electricity at temperatures as high as 1900 to 2400 degrees Celsius.
The researchers plan to integrate such TPV cells into grid-scale thermal batteries. The system will absorb excess energy from renewable sources such as solar energy and store this energy in highly insulated hot graphite libraries. When energy is needed, such as on a cloudy day, the TPV battery converts heat into electrical energy and distributes the energy to the grid.
With the new TPV battery, the team has now successfully demonstrated the main parts of the system in a separate small-scale experiment. They are currently working on bringing these components together and then showcasing a fully operational system. They hope to expand the system in the future to replace fossil fuel-powered power plants and achieve a fully decarbonized grid powered entirely by renewables.
Asegun Henry, a professor in the Department of Mechanical Engineering at the Massachusetts Institute of Technology, said: "Thermal photovoltaic cells are the final and critical step in proving that 'thermal cells are a viable concept.'" "This is an absolutely critical step on the road to promoting renewable energy and achieving a fully decarbonised grid."
Henry and his collaborators published their findings in the journal Nature. MIT co-authors include Alina LaPotin, Kyle Buznitsky, Colin Kelsall, Andrew Rohskopf and Evelyn Wang, professor of engineering and chair of the Department of Mechanical Engineering at Ford, as well as Kevin Schulte of NREL in Golden, Colorado, and other members.
Cross the chasm
More than 90% of the world's electricity comes from heat sources such as coal, natural gas, nuclear and concentrated solar. For a century, steam turbines have been the industry-standard method for converting such heat sources into electrical energy.
On average, steam turbines reliably convert around 35 percent of heat sources into electrical energy, and the highest efficiency of all heat engines to date is around 60 percent. But these machines rely on temperature-limited moving parts. Heat sources above 2000 degrees Celsius, such as the thermal battery system proposed by Henry et al., are too hot for turbines.
In recent years, scientists have been working on solid-state alternatives — power generation devices with no moving parts — to work efficiently at higher temperatures.
Henry says, "One of the advantages of solid-state energy converters is that they can operate at higher temperatures and with lower maintenance costs because they have no moving parts." "They're just placed there to generate electricity reliably."
Thermal photovoltaic cells provide an exploration path for solid-state power generation equipment. Just like solar cells, TPV cells can be made from semiconductor materials that have a specific band gap (the gap between the valence band and the conduction band of the material). If a photon with enough energy is absorbed by the material, it can kick the electrons through the band gap, and then the electrons can conduct in the band gap, generating electricity. No rotors or blades are required to do so.
To date, most TPV batteries have achieved an efficiency of only about 20%, with a record high of 32%. These batteries are made of relatively low band gap materials that convert low-temperature, low-energy photons, so the efficiency of converting energy is lower.
Capture light
In their new TPV design, Henry and his colleagues hope to capture higher-energy photons from a heat source at higher temperatures, thereby converting energy more efficiently. Compared to existing TPV designs, the team's new battery uses materials with higher band gaps and multiple junctions or layers of material.
The battery is made of three main areas: a high band gap alloy is located on top of an alloy with a slightly lower band gap, and the lowest layer is a mirror-like layer of gold. The first layer captures the highest-energy photons in the heat source and converts them into electrical energy, while the low-energy photons that pass through the first layer are captured and converted by the second layer to increase the voltage generated. Any photons that pass through the second layer are mirror-reflected back to the heat source, rather than being absorbed as waste heat.
The team tested the efficiency of the battery by placing it on top of a heat flux sensor that directly measures the heat absorbed from the battery. They exposed the battery to a high-temperature lamp and concentrated the light on the battery. They then changed the intensity or temperature of the bulb and observed the battery's power efficiency (how the amount of electricity it produces changes with temperature compared to the heat it absorbs). In the temperature range of 1900 to 2400 degrees Celsius, the efficiency of the new TPV battery remains at around 40%.
"We can achieve high efficiency in the temperature range related to thermal batteries." Henry says.
The battery in the experiment is about one square centimeter. For grid-scale thermal battery systems, Henry envisions TPV batteries having to expand to about 10,000 square feet (about a quarter of a football field) and will operate in temperature-controlled warehouses to get power from huge solar storage repositories. He noted that there is already an infrastructure for making large photovoltaic cells, which can also be used to make TPVs.
"There's definitely a huge net positive here in terms of sustainability," Henry said. "The technology is safe, environmentally sound throughout its life cycle, and can have a huge impact on reducing CO2 emissions from electricity production."