The 20% Barrier, or Long-Distance Transmission Again
A few years ago, I jumped on compressed air energy storage (CAES) as a means of shifting solar electricity to nighttime.
More recently, I figured out that as long as wind was less expensive than PV, wind would be stored first in CAES.
Under the circumstances that wind is cheaper than PV and wind blows more at night, we won’t be seeing much PV stored for shifting to nighttime.
It’s going to be hard for PV to get cheaper than on-shore wind. Wind today seems to have about 50% more output per installed watt than PV does. So that means PV must be 2/3 of the cost of wind per watt to be equivalent to it. If on-shore wind is about $2/W, then PV would have to reach $1.33/W – maybe $1.5/W because wind has higher O&M costs. This is tough but eventually likely about 2020. Even when it is achieved, however, it only means PV becomes about the same price as on-shore wind is now. (Of course, solar is produced during the day, when it is more valuable than wind. And PV is already about the same price as off-shore wind.)
According to recent DOE studies, up to deployments of about 20% PV or wind, we can handle the variability. But above something like 20% electricity each, there will start to be too much PV electricity during the Spring and Fall days, and too much wind on many nights.
So here we have it – a world where we push the limits of the grid with 20% PV and 20% wind, at something up to 40% of our electricity. And sometimes, on the days or nights of the lowest demand or the highest wind or PV, we have too much wind at night or too much PV during the day. Does this limit us? Is 20% each the end?
That’s the point where people start talking about storage. But storage about doubles the cost for the electricity that is stored (roughly!), and it isn’t even all that proven at scale.
What about moving electricity as an alternative to storage? Move PV electricity from midday to evening by sending it east from the west coast. Or move it from daytime to nighttime by sending it from the Sahara to New York City under the Atlantic with high voltage DC. Or move wind from nighttime to daytime. You can build more and more PV and wind as long as you can send it further and further away where conditions are different.
Surprisingly, the economics of shifting electricity are about the same as storage. 10% loss for three thousand miles is like batteries; 20% for 6000 miles is like pumped hydro; 40% loss is about 12,000 miles, half the earth away – and is like compressed air energy storage (CAES). Then you have to compare the capital costs, and interestingly, they tend to favor transmission, except for the very longest distances (and depend sensitively on whether you can use the transmission both ways, i.e., fully use their capacity to offset their cost).
It comes down to caverns and transmission on land for CAES; or transmission on land and underwater for shifting wind and sun to where it’s needed. It’s transmission either way, but shifting is more proven at cost and scale than storage. Yet it is rarely compared to storage.
Transmission isn’t as much a technical problem (it is already way further along than storage) as a societal one (i.e., NIMBY); and political, since it connects different parts of the Earth. So it doesn’t support the idea of energy independence; and it is vulnerable to culture clashes and isolated revolutionaries. But perhaps proper redundancy and keeping the fossil fuel power plants ready and storing some fossil fuels needed to overcome an emergency might be enough (I want to acknowledge that I heard something like this idea for backup fossil fuels first from Arnold Goldman; and the idea of international transmission goes back at least to Buckminster Fuller). After all, as everyone knows, it’s a lot easier to store fuel than store electricity.
It isn’t time to get too squeamish about solar and wind beyond 40% of our electricity. As we know from other blogs, this is enough to eliminate all our imported oil, if we had electric transport. So it’s quite a good amount. But it is time to think a bit beyond the box about alternatives.