Solar Steam-Power Just Got A Big Boost

Anyone who has watched a road steaming away in the sunny aftermath of a
rainstorm knows that sunshine is a powerful evaporator. Using the sun’s
heat to warm or evaporate water is a far older form of gathering solar
energy than using photovoltaic cells to convert sunlight directly into
electricity.

Solar-powered
steam can make electricity too, by driving a turbine. But it has many
other uses–some of which are extremely handy in parts of the world
where the sun is the only readily available source of energy. These
include running desalination plants, refrigeration, sterilization,
chemical purification and numerous kinds of waste treatment. So there is
a big incentive to make it more efficient.

The main problem is that harnessing enough solar energy to put steam
to work is both hard and costly. Some existing solar-power plants use
vast arrays of movable mirrors, or heliostats, to track the sun through
the day and concentrate its energy more than 1,000 times onto a
“receiver” that generates steam to drive turbines. Others use rotating
“parabolic troughs” to concentrate the sun’s energy 60-80 times.

One way to get sunshine to boil water more efficiently is to mix the
water with something. A recent effort involves adding gold nanoparticles
that swiftly get hot under the sun. But this also requires an intense
concentration of solar power–which means using costly heliostats. And
the efficiency with which the nanoparticles help the sun turn water into
steam is only 24%, so a great deal of energy is lost.

Now a group of researchers at the Massachusetts Institute of
Technology (MIT) has come up with an alternative approach that borrows
from the wet road. Steam is generated at the surface of water, but the
mass of liquid below acts as a heat-sink that conspires against steam
generation. This is why sunshine can readily turn a thin layer of water
on a road into steam but cannot do the same for a lake.

The MIT researchers sought to address this in a laboratory set-up
that consists of a double-layered black disc floating on the surface of
water in an insulated beaker. The disc’s top layer consists of graphite
flakes that were treated by placing them in a domestic microwave oven
and heating them up, “just like making popcorn,” says Gang Chen, head of
the MIT research team. The resulting “exfoliated” graphite forms a
5mm-thick porous matrix that absorbs and concentrates the heat from
sunshine.

The lower layer is a 10mm-thick porous carbon foam that floats on the
water and prevents the heat in the top layer from being lost to the
water below. The heat in the top layer creates a pressure gradient that
slowly and continuously draws water up through the disc, where the
popcorned graphite easily turns the thin layer into steam.

This simple disc turns out to be a very efficient steam generator.
For one thing, it produces steam when sunlight is magnified by a factor
of just ten. This requires little more than cheap lenses and it
increases the efficiency of using solar energy to make steam to 85%.

With some refinement of the graphite layer, thinks Dr. Chen, the
technique could be made to work with sunlight that is concentrated as
little as three times. It might also be possible to make the top layer
from even cheaper materials. One is “carbon black”, a widely available
by-product of hydrocarbons that have not been completely burned.

The approach by Dr Chen and his colleagues has yet to be scaled up to
demonstrate its commercial potential to make solar-steam processes more
efficient. But the graphite and carbon sponges have other possible uses
too. One idea is laying them like a carpet to dry out waterlogged areas
for farmers. One way or another, it appears to be a new technology that
is set to earn its place in the sun.