Diagram shows the idealized arrangement of a vat of molten salt used to store solar heat, located at the base of a gently sloping hillside that could be covered with an array of steerable mirrors all guided to focus sunlight down onto the vat. Image: Courtesy of Alexander Slocum et al.
biggest hurdle to widespread implementation of solar power is the fact that the
sun doesn’t shine constantly in any given place, so backup power systems are
needed for nights and cloudy days. But a novel system designed by researchers
at the Massachusetts Institute of Technology (MIT) could finally overcome that
problem, delivering steady power 24/7.
basic concept is one that has been the subject of much research: using a large
array of mirrors to focus sunlight on a central tower. This approach delivers high
temperatures to heat a substance such as molten salt, which could then heat
water and turn a generating turbine. But such tower-based concentrated solar
power (CSP) systems require expensive pumps and plumbing to transport molten
salt and transfer heat, making them difficult to successfully commercialize—and
they generally only work when the sun is shining.
Alexander Slocum and a team of researchers at MIT have created a system that
combines heating and storage in a single tank, which would be mounted on the
ground instead of in a tower. The heavily insulated tank would admit
concentrated sunlight through a narrow opening at its top, and would feature a
movable horizontal plate to separate the heated salt on top from the colder
salt below. (Salts are generally used in such systems because of their high
capacity for absorbing heat and their wide range of useful operating
temperatures.) As the salt heated over the course of a sunny day, this barrier
would gradually move lower in the tank, accommodating the increasing volume of
hot salt. Water circulating around the tank would get heated by the salt,
turning to steam to drive a turbine whenever the power is needed.
plan, detailed in a paper published in Solar Energy, would use an array
of mirrors spread across a hillside, aimed to focus sunlight on the top of the
tank of salt below. The system could be “cheap, with a minimum number of
parts,” says Slocum, the Pappalardo Professor of Mechanical Engineering at
MIT and lead author of the paper. Reflecting the system’s 24/7 power
capability, it is called CSPonD (for Concentrated Solar Power on Demand).
new system could also be more durable than existing CSP systems whose
heat-absorbing receivers cool down at night or on cloudy days. “It’s the
swings in temperature that cause [metal] fatigue and failure,” Slocum
says. The traditional way to address temperature swings, he says: “You
have to way oversize” the system’s components. “That adds cost and
Small-scale laboratory setup was used by the team to test the ability of a container of molten salt to absorb and store heat from concentrated sunlight, simulated using powerful spotlights. Photo: Courtesy of Alexander Slocum et al.
team analyzed two potential sites for CSPonD on hillsides near White Sands,
N.M., and China Lake, Calif. By beaming concentrated sunlight
toward large tanks of sodium-potassium nitrate salt—each measuring 25 m across
and 5 m deep—two installations could each provide 20 mW of electricity 24/7,
which is enough to supply about 20,000 homes. The systems could store enough
heat, accumulated over 10 sunny days, to continue generating power through one
full cloudy day.
exact costs are difficult to estimate at this early stage of research, an
analysis using standard software developed by the U.S. Department of Energy
suggests costs between seven and 33 cents per kilowatt-hour. At the lower end,
that rate could be competitive with conventional power sources.
team has carried out small-scale tests of CSPonD’s performance, but its members
say larger tests will be needed to refine the engineering design for a
full-scale powerplant. They hope to produce a 20 to 100 kW demonstration system
to test the performance of their tank, which in operation would reach temperatures
in excess of 500 C.
biggest challenge, Slocum says, is that “it’s going to take a company with
long-term vision to say, ‘Let’s try something really different and
fundamentally simple that really could make a difference.'”
of the individual elements of the proposed system—with the exception of mirror
arrays positioned on hillsides—have been suggested or tested before, Slocum
says. What this team has done is essentially an “assemblage and
simplification of known elements,” Slocum says. “We did not have to
invent any new physics, and we’re not using anything that’s not already proven”
in other applications.
emphasizes that this approach is not intended to replace other ways of
harvesting solar energy, but rather to provide another alternative that may be
best in certain situations and locations. Playing on the familiar saying about
rising tides, he adds, “A rising sun can illuminate all energy harvesters.”