Despite the fact that there’s rumours the upcoming budget is going to throw money at putting solar panels on people’s roofs, the conservatives are no fan of solar cells. And you know what? They’re dead right.
Solar panels are one of the most expensive forms of renewable energy out there, and, despite claims of cost reductions in the pipeline, they haven’t gotten any cheaper over the past five years. Particularly if you factor in the cost of batteries if you’re trying to completely replace grid electricity. For that reason, while solar panels may be the energy source of the future, right now, they’re one of the most expensive ways imaginable to reduce greenhouse gas emissions.
But the temptation of endless power from the sun is, well, tempting – there’s more than enough energy available, it’s just that a cost-effective means of tapping it is required. So it’s unsurprising that a less complicated but perhaps more affordable technology is getting a lot of attention at the moment – solar thermal energy.
Solar thermal is essentially a scaled-up version of frying ants with a magnifying glass. You use some kind of optical device – and, like telescopes, on a large scale mirrors are much cheaper than lenses- to collect the thermal radiation from the sun on a large area. You use your mirrors/lenses to focus all that thermal radiation on a much smaller area, which consequently gets nice and hot – potentially really, really hot. At that point, we can use the exact same technology we use in coal-fired power stations – the steam turbine – to turn that heat into electricity. Or a gas turbine. Or even something more exotic like a Sterling engine.
But there’s a problem. As well as not working at night, power plants like these won’t work at all if the sun is covered by cloud. So, if thermal solar power is to replace significant amounts of fossil fuel power, some method to store surplus energy when the sun’s shining, and release it again when it’s not, is required. While it’s certainly possible, like any other power source, to store the surplus electricity in batteries, there’s a more convenient, and cheaper, way to store the energy from the solar collector. Why not build something like a solar hot water system, where the heat energy is stored in an insulated tank for convenience?
A number of different projects around the world are proposing to do just that, though they mostly don’t use water as the storage medium as it boils at too low a temperature to be convenient. A number of different materials have been proposed, from molten salts to solid concrete, as energy storage – indeed, there have even been proposals to use molten salt as the working fluid in solar thermal plants, rather than water. But there’s a problem: molten salts, well, don’t stay molten if you let them cool to ambient temperatures.
So, to prevent seizing up your energy storage system, and allow a solar thermal plant to produce power all the time, many systems turn to an alternate heat source – natural gas. You can use natural gas in a hybrid solar thermal system right now, even without energy storage – instead of using the sun to make steam, you run a boiler with gas. This can, apparently, be cheaper than running separate natural gas fired power stations, as you don’t have to pay a second generator and turbine. However, while it’s running on natural gas, it’s just another fossil-fuel fired power station emitting greenhouse gases into the air.
The key question with this technology, is not so much the environmental goodness, it’s the cost. And that’s where the big unknowns come in. The US government’s renewable energy lab reports estimates that power from “parabolic trough” systems costs about 12 US cents per kilowatt hour right now, but projects costs going down to somewhere round 6 cents per kilowatt hour in a decade or so (big PDF document)). But there are hints that parabolic trough technology has been outrun by a new, Australian solar thermal technology. Ausra doesn’t have much to say on its website, but their chief investor, Vinod Khosla, claims that a gigawatt-scale plant, planned for “a few years from now”, will produce power at below than 7 US cents per kilowatt hour. The 4 Corners program I mentioned in an earlier post has an extended interview with Khosla in which he mentions the specific figure. The program also has a good illustration of the Ausra system, which uses less costly flat mirrors and simpler pipe technology than the hard-to-manufacture parabolic troughs.
Khosla is publicly confident that the technology he is investing in will be a dominant force in carbon-free power in the future. But, then, he is a venture capitalist. Keep in mind these numbers (which are anecdotal, but sound about right to me). Only maybe 2 out of 10 venture capital investments ever make serious money. More broadly, solar thermal remains a gamble. It’s a promising technology, sure. But ruling out geosequestration and nuclear on the basis that cheap solar thermal is just round the corner is a stretch not even Khosla was prepared to make.