Researchers at the Massachusetts Institute of Technology (MIT) have developed a new type of heat engine that could revolutionize power generation. The device, known as a solid-state thermophotovoltaic (TPV) heat engine, is capable of harvesting more energy from heat than the average steam turbine.
In addition, the TPV engine is much cheaper to build and operate, and it has no moving parts. These advantages could have huge implications for future power stations, as well as grid-level energy storage. The key to the TPV engine's success is its unique design, which uses a stack of nanoscale layers to absorb and convert heat into electricity. The researchers believe that this technology could one day be used to generate electricity from waste heat, or even from the sun. The TPV engine is still in its early stages of development, but the potential applications are vast. With further research, this technology could help to solve the world's energy problems.
The majority of humanity's energy comes from heat - burning coal or natural gas, nuclear fission, concentrating solar - which is used to boil water and turn steam turbines. This type of power production has been in use since Charles Parsons connected a steam turbine to a dynamo in 1884 and licensed the invention to George Westinghouse. It has become widespread all around the world since then as a mature and well-optimized technology with established advantages and limits.
Turbines are a critical part of the modern world, providing clean energy to homes and businesses around the globe. However, turbines have several limitations that prevent them from being even more efficient and widespread. One of those limitations is efficiency. While some turbines have managed to convert up to 60 percent of a heat source's energy into electricity, the average turbine operates at closer to 35 percent efficiency. Another limitation is heat – the moving parts in turbines prevent them from operating, for example, at temperatures over 2,000 °C (3,600 °F).
The figures come from an MIT research team that's been working on an alternative: a heat engine with no moving parts, a thermophotovoltaic (TPV) device that the team has now demonstrated in a small 1 x 1-cm (0.4 x 0.4-in) prototype, maintaining an efficiency over 40 percent across a temperature range between 1,900 - 2,400 °C (3,450 - 4,300 °F). The beauty of the TPV design is that it can be made with existing materials and fabrication methods, meaning that it could be scaled up relatively easily and cheaply. The TPV could also be used to power everything from cars to portable electronics.
That's a significant improvement from typical TPV heat engines, which have an efficiency of around 20%, with the previous record standing at 32%, and it may be more energy-efficient than turbines in specific circumstances.
Thermophotovoltaic (TPV) heat engines are a promising new technology for converting heat into electricity. TPV systems work by collecting heat from a variety of sources, such as solar radiation or waste heat from industrial processes. This heat is then used to generate photons, which are captured by photovoltaic cells and converted into electricity. TPV systems offer a number of advantages over traditional power generation technologies, including higher efficiency and lower environmental impact. In addition, TPV systems can be easily integrated into existing infrastructure, making them a highly versatile and scalable power generation solution. With further development, TPV technology has the potential to revolutionize the way we generate electricity, providing a cleaner, more efficient alternative to traditional power plants.
The MIT team made its impressive efficiency advance by tweaking a few variables. Firstly, the input heat temperature – this device is designed specifically to work at hot temperatures above the range where turbines can no longer function. This allows the team to use higher-bandgap absorber/emitter materials, which take in more energy and release higher-energy infrared photons on the emitter side – as well as photovoltaic cells designed to take maximum advantage of those high-energy photons. The second variable the team tweaked was the output heat temperature. By engineering their device to extract energy at lower temperatures, they were able to increase efficiency even further. Then, the team layered the photovoltaic cells – the first layer being designed to harvest the highest-energy photons at transmission-efficient higher voltages, and the second layer being there to mop up lower-energy photons. Photons that make it through both layers are reflected back onto the absorber/emitter with a mirror, so that any photons outside the optimal ranges can feed back into the start of the process and help to keep the emitter temperature up.
In a paper published in Nature, the research team discussed its record-breaking experimental results, noting that "reaching 40 percent efficiency with TPVs is notable from the standpoint that it now renders TPV as a heat engine technology that can compete with turbines. An efficiency of 40 percent is already greater than the average turbine-based heat engine efficiency in the United States, but what could make TPVs even more attractive than a turbine is the potential for lower cost, faster response times, lower maintenance, ease of integration with external heat sources and fuel flexibility." The research team's findings could have major implications for the future of energy production, providing a more efficient and cost-effective alternative to traditional turbine-based power plants.
This novel heat engine technology works in a temperature range that's "applicable for natural gas or hydrogen combustion," implying it has the potential to generate more power, cheaper, from a combustion source. IIn the case of green hydrogen, if not nitrous oxide emissions are eliminated, a plant could be carbon-free.
But the research team also points out that at this level of efficiency, these TPV heat engines can instantly make thermal energy grid storage (TEGS) projects an economically viable way to store and release renewable energy at the grid scale. The team's proposed design would use excess electricity to heat up "heavily insulated banks of hot graphite," which would act as an energy storage medium. When that energy is needed, large banks of TPV heat engines would convert it back into electricity for the grid.
The team calculates that this heat battery would operate at a round-trip efficiency somewhere around 40-55%. That's somewhat wasteful compared with lithium batteries, which the team estimates operate at closer to 70% efficiency. But the researchers believe it'll be so much cheaper – with a capital cost per unit energy projected at just one-tenth that of lithium batteries – that TEGS will compete favorably wherever the TPV heat engine can perform over about 35% efficiency. What's more, unlike lithium batteries, TEGS can be easily scaled up or down to meet changing energy demands, making it a flexible and versatile option for grid storage.
“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 technology is safe, environmentally benign in its life cycle, and can have a tremendous impact on abating carbon dioxide emissions from electricity production.”
What's more, this team's record efficiency figure will surely soon be eclipsed.
"Turbine costs and performance have already reached full maturity," reads the study, "so there are limited prospects for future improvement, as they are at the end of their development curve. TPVs, on the other hand, are very early in their progress down a fundamentally different development curve. Consequently, TPVs have numerous prospects for both improved efficiency (for example, by improving reflectivity and lowering series resistance) and lowering cost (for example, by reusing substrates and cheaper feedstocks)."
Despite all of the progress that has been made in renewable energy, the vast majority of the world still relies on fossil fuels for its energy needs. In order to transition to a clean energy future, it is essential to find ways to make existing technologies more efficient. One promising area of research is solid-state heat engines. These devices are able to convert heat into electricity with unprecedented efficiency, and they have the potential to revolutionize the way we power our homes and businesses. As the world continues to grapple with the climate crisis, it is crucial that we explore every opportunity to reduce our carbon footprint. Solid-state heat engines could be a major step in the right direction, and we should be doing everything we can to support this important area of research.. We hope to hear a lot more about these solid-state heat engines in the next few years.
The paper is open access at the journal Nature.