A Solar-Power First

Published: July 27th, 2010

Michael D. Lemonick

When I hear term “solar power,” I immediately visualize arrays of solar cells. They could be on a rooftop, attached to the International Space Station, in a field somewhere — but it’s always an array of flat, dark, shiny panels, aimed up at the Sun.

It turns out that’s too narrow a view, though. Photovoltaic cells convert sunlight directly into an electric current, but there’s also another type of solar-electricity technology known as “Concentrating Solar Power” or, more informally, “solar thermal.” It uses mirrors to focus sunlight onto some sort of fluid, which then heats up and drives a turbine, either directly or indirectly, to generate power.

One disadvantage of solar thermal is that it isn’t efficient on the scale of individual households: you need to build a power plant to make it worthwhile. Another is that the technology isn’t as mature as photovoltaic cells are.

But there are some big potential advantages. It’s much lower-tech in the sense that mirrors are a lot simpler to make than solar cells, and the manufacturing process generates fewer toxic byproducts. And while solar cells go quiet at night, solar-thermal plants don’t necessarily. That sounds odd, since they use sunlight too, and there isn’t any at night (at least not the last time I checked).

The difference lies in the hot fluid. Some solar-thermal plants simply use water, which flashes into steam under concentrated sunlight. Other plants use mineral oil, though, which can heat up to higher temperatures than water can, and which cools off more slowly (you can’t use the oil to drive a turbine directly; the oil heats water into steam). The slower cool-down of mineral oil means you can keep generating electricity after the Sun sets.

Solar Two Power Tower Project The Solar Two power plant tower
in Daggett was demolished in 2009 in preparation for a larger
solar-thermal project. Solar Two used molten salt, a combination
of 60% sodium nitrate and 40% potassium nitrate, as an energy
storage medium and proved it could run continuously around
the clock producing power. (Credit: Sandia National Labs)

You can also use molten salt as the fluid — and that can get even hotter, and stay hot for longer. Which brings us, finally to the news: the world’s first molten-salt-based solar thermal power plant has just opened in Sicily, capable of generating five megawatts of power — about enough to power 5,000 homes.

So we can go off fossil fuels at last! Well, not quite. Molten salt is quite corrosive, and if you let it cool off too much it tends, unlike water or mineral oil, to solidify.

Beyond that, the electricity from the new molten-salt plant is dramatically more expensive than electricity from a new coal or natural-gas plant. If the technology ever takes off, so-called economies of scale could make the electricity a lot cheaper, but for now, it has to be heavily subsidized by a utility company or government, or, in this case, both, in order to be worthwhile.

Still, if it works out, this is an important test for an energy-generating technology that could be part of the mix as the world moves toward a lower-carbon future.