A news item to cheer up any of our readers who worry about the intermittency of wind and solar energy. (Tidal and geothermal, not so much).
The Gemasolar thermal solar tower plant near Seville achieved 24-hour operation on a day in July, using molten salt heat storage. At 20MW, it’s a real pocket-sized power station, not a tiny pilot. It achieved this only one month after commissioning, so the design clearly works. The average capacity factor will not of course be 100%, especially in winter. It doesn’t have to be, and every power plant is offline part of the time for maintenance. The molten salt technology gets solar thermal close to the availability of the conventional or nuclear pack, so problem apparently solved, technically.
Costs? They are not telling. and solar energy in Spain still relies on public subsidy through a generous feed-in tariff.
This was reduced in 2008 causing an investment stall, but things seem to be picking up again. Since CSP is a young technology, the current costs don’t mean very much looking forward. Economies of scale – mass-produced heliostats (steerable mirrors) and standardised receivers and turbines – will bring them down. The long-term problem is the land take; it’s not like wind where cows graze up to the pylon base. I don’t myself see why you couldn’t raise the heliostat supports and intercrop with vegetables or parked cars – the partial shade isn’t a big drawback in climates good for CSP.
Molten salt isn’t the only scheme around. Gemasolar’s steam plant needs water, a constraint in hot countries. If you use air as the working fluid instead, you can today get it up to 900ºC or more in the receiver. This is hot enough to drive a gas turbine, aka known as a Brayton cycle. Even higher temperatures of 1200ºC or so are more efficient thermodynamically. This is not directly relevant to a free resource, but it’s desirable to reduce the number and area of mirrors. Pilot towers are being built in Spain, Israel and Australia that combine hot air with gas or biofuel. You can drive up the temperature, and above all have a 24-hour combined cycle that doesn’t need water, though it’s higher carbon than the hot-salt system. There are ideas for heat storage too but 1000ºC is a hard target. One plus here is modularity – you can build small as well as well as large units for remote mines and city rooftops.
Go, go, go. We need all of this, urgently.
How serious is the intermittency worry anyway? It’s not the killer objection some dinner-table conservatives would make it. For 15 years, electricity professionals have been saying that integration of intermittent sources up to 20% of supply is manageable without reducing service standards. The integration has costs, but also benefits – a more diverse and fine-grained generating park, linked to the required better grid, can be more reliable and flexible overall. Multiple sources cited here, including 2006 UK estimates for grid integration costs. Denmark is now close to 20% wind supply, and SFIK manages fine.
This chart for SE Australia in 2010, taken from here, tells the story.
(Sorry about the poor resolution, better pdf here.)
The variation in wind supply (about 4% of mean demand) is dwarfed by the 20% daily variation in demand, so it’s lost in the overall balancing. But what would happen if you scaled up? This chart from the same source plots total demand against wind’s capacity utilisation factor, on incommensurate scales.
If you scaled wind up to match 100% of demand at the average utilisation factor of 30%, superimposing the blue and red, you would need almost as much conventional capacity in reserve. Not a tragedy since you have it already.
My calculations for a sample month (April 2011) suggest that the balancing capacity (coal, oil, gas, hydro, imports, or storage) on this thought experiment would need to be 98% of mean demand, or 30% of nominal wind capacity, and it would supply about 40% of total power summed over the month. The true carbon intensity of a wind-heavy power system includes this smoothing overhead, so it’s low but far from zero. If you look at marginal impact, the story is different: you are starting, unless you are France, with a mainly fossil mix, so the marginal impact of wind is a near-total displacement of fossil carbon emissions.
Note: My calculations are from the data set supplied here, see my spreadsheet. (Warning: 5-minute sampling of 24 wind farms, so big file.) I can’t be fagged to repeat them for all 12 months, but anyway the numbers won’t generalise to other countries or continents. It’s only an approximate simulation anyway – a real expansion would be more dispersed and smoother in output than the grainy current installed base. A lot depends on how wide the reference area is; on the scale of a continent with a good long-distance grid, there’s natural smoothing from the cycle of weather systems. So my 42% balancing is an illustrative upper bound, not a central estimate.