MIT’s new heat engine beats a steam turbine in efficiency

Since the Industrial Revolution, steam has been a mainstay of the modern world. Even today, more of the world’s electricity is produced by steam turbines in coal, gas and nuclear power plants.

But now another type of heat engine — developed by researchers at MIT and the National Renewable Energy Laboratory (NREL) — exceeds steam turbine efficiency, potentially unlocking a transformation in how we make and store energy.

The challenge: The production cost of renewable energies has fell dramatically over the past decade and into 2020, the International Energy Agency reported that solar energy had become the cheapest source of electricity in history.

Heat engines are devices that convert heat into electricity – steam turbines are the most common example.

Renewables are also better for the environment than fossil fuels, but we still depend on coal, oil and natural gas to produce two-thirds of our electricity, largely because they are more reliable – we can always burn more fuel, but we can’t do the sunshine or the Gale.

We box temporarily store excess renewable energy in batteries, but because batteries discharge over time, storage only lasts for days or weeks. That means we can’t store a ton of solar power from the summer to use during the overcast winter.

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hot topic: Electric batteries that don’t lose their charge so quickly are under development, but there is another option that could make a renewable grid more reliable: thermal battery systems.

More … than 90% of the world’s electricity is generated from heat in one way or another, and heat engines are the devices that manage the conversion process.

Steam turbines are the most common example of this – we create heat (usually by burning coal or gas), boil water into steam which turns a turbine and this mechanical energy is converted into electricity .

“This is the first time that thermophotovoltaic cells have achieved truly promising levels of efficiency.”

ANDREJ LENERT

Thermophotovoltaic (TPV) cells are another type of heat engine – they use semiconductor materials to directly convert photons from a heat source into electricity.

Like photovoltaic solar cells, TPV cells have no moving parts, making them cheaper to maintain than steam turbines. They can also convert heat at higher temperatures than turbines, increasing their efficiency.

However, TPV cells have traditionally not been as efficient as turbines, converting only 20% of thermal energy into electricity, compared to 35% for steam turbines.

from MIT thermal motor: In terms of how this relates to renewables, we could store energy generated by the wind or the sun as heat, by heating tanks of liquid metal in heavily insulated thermal batteries that can store it long-term.

To convert energy back into electricity, we could use TPV cells to generate electricity from heat on demand.

In 2019, researchers from MIT calculated that achieving TPV cell efficiency rates of up to 35% would make thermal battery systems commercially viable, and with the help of NREL scientists, they have now designed one that can convert heat up to 4,350 degrees Fahrenheit in electricity with approximately 40% efficiency

“Thermophotovoltaic cells were the last key step in demonstrating that thermal batteries are a viable concept.”

ASEGUN HENRI

The key was to use many layers of different semiconductor materials – some absorb photons in the primarily visible and ultraviolet wavelengths, while others absorb infrared. A gold-plated mirror in the cell reflects all unabsorbed photons back to the heat source to minimize waste.

“This is something very exciting,” said Andrej Lenert, a materials engineer at the University of Michigan, who was not involved in the study. told science. “It’s the first time [TPVs have] reaches really promising efficiency ranges, which is ultimately what counts for many applications. »

Look forward: The team’s TPV heat engine measures approximately one square centimeter. MIT’s Asegun Henry imagines network-supporting TPV cells 10,000 square feet big and claims that the systems used to create large photovoltaic cells could be adapted to this manufacturing process.

“Thermophotovoltaic cells were the last key step in demonstrating that thermal batteries are a viable concept,” Henry said. “This is an absolutely critical step on the path to proliferating renewables and achieving a fully carbon-free grid.”

This article was originally published by our sister site, Freethink.

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