Picking up steam

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, sterilisation, 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.