Engineers at MIT and the National Renewable Energy Laboratory (NREL) have
designed a heat engine with no moving parts. Their new demonstrations show
that it converts heat to electricity with over 40 percent efficiency—a
performance better than that of traditional steam turbines.
The heat engine is a thermophotovoltaic (TPV) cell, similar to a solar
panel's photovoltaic cells, that passively captures high-energy photons from
a white-hot heat source and converts them into electricity. The team's
design can generate electricity from a heat source of between 1,900 to 2,400
degrees Celsius, or up to about 4,300 degrees Fahrenheit.
The researchers plan to incorporate the TPV cell into a grid-scale thermal
battery. The system 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, TPV cells would convert
the heat into electricity, and dispatch the energy to a power grid.
With the new TPV cell, the team has now successfully demonstrated the main
parts of the system in separate, small-scale experiments. They are working
to integrate the parts to demonstrate a fully operational system. From
there, they hope to scale up the system to replace fossil-fuel-driven power
plants and enable a fully decarbonized power grid, supplied entirely by
renewable energy.
"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."
Henry and his collaborators have published their results today in the
journal Nature. Co-authors at MIT include Alina LaPotin, Kevin Schulte, Kyle
Buznitsky, Colin Kelsall, Andrew Rohskopf, and Evelyn Wang, the Ford
Professor of Engineering and head of the Department of Mechanical
Engineering, along with collaborators at NREL in Golden, Colorado.
Jumping the gap
More than 90 percent of the world's electricity comes from sources of heat
such as coal, natural gas, nuclear energy, and concentrated solar energy.
For a century, steam turbines have been the industrial standard for
converting such heat sources into electricity.
On average, steam turbines reliably convert about 35 percent of a heat
source into electricity, with about 60 percent representing the highest
efficiency of any heat engine to date. But the machinery depends on moving
parts that are temperature- limited. Heat sources higher than 2,000 degrees
Celsius, such as Henry's proposed thermal battery system, would be too hot
for turbines.
In recent years, scientists have looked into solid-state alternatives—heat
engines with no moving parts, that could potentially work efficiently at
higher temperatures.
"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."
Thermophotovoltaic cells offered one exploratory route toward solid-state
heat engines. Much like solar cells, TPV cells could be made from
semiconducting materials with a particular bandgap—the gap between a
material's valence band and its conduction band. If a photon with a high
enough energy is absorbed by the material, it can kick an electron across
the bandgap, where the electron can then conduct, and thereby generate
electricity—doing so without moving rotors or blades.
To date, most TPV cells have only reached efficiencies of around 20 percent,
with the record at 32 percent, as they have been made of relatively
low-bandgap materials that convert lower-temperature, low-energy photons,
and therefore convert energy less efficiently.
Catching light
In their new TPV design, Henry and his colleagues looked to capture
higher-energy photons from a higher-temperature heat source, thereby
converting energy more efficiently. The team's new cell does so with
higher-bandgap materials and multiple junctions, or material layers,
compared with existing TPV designs.
The cell is fabricated from three main regions: a high-bandgap alloy, which
sits over a slightly lower-bandgap alloy, underneath which is a mirror-like
layer of gold. 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. Any photons that pass through this second layer
are then reflected by the mirror, back to the heat source, rather than being
absorbed as wasted heat.
The team tested the cell's efficiency by placing it over a heat flux
sensor—a device that directly measures the heat absorbed from the cell. They
exposed the cell to a high-temperature lamp and concentrated the light onto
the cell. They then varied the bulb's intensity, or temperature, and
observed how the cell's power efficiency—the amount of power it produced,
compared with the heat it absorbed—changed with temperature. Over a range of
1,900 to 2,400 degrees Celsius, the new TPV cell maintained an efficiency of
around 40 percent.
"We can get a high efficiency over a broad range of temperatures relevant
for thermal batteries," Henry says.
The cell in the experiments is about a square centimeter. For a grid-scale
thermal battery system, Henry envisions the TPV cells would have to scale up
to about 10,000 square feet (about a quarter of a football field), and would
operate in climate-controlled warehouses to draw power from huge banks of
stored solar energy. He points out that an infrastructure exists for making
large-scale photovoltaic cells, which could also be adapted to manufacture
TPVs.
"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."
Reference:
LaPotin, A., Schulte, K.L., Steiner, M.A. et al. Thermophotovoltaic efficiency
of 40%. Nature 604, 287–291 (2022).
DOI: 10.1038/s41586-022-04473-y