If you follow renewable energy issues even when you're not reading ReWire, you might well have seen a rather inspiring photo making its way around social media in the last couple of weeks. In the image, Indian people are admiring a canopy built over an irrigation canal and covered with photovoltaic panels. A common caption accompanying the image claims that each kilometer of canal can produce a megawatt of power -- probably an error, as the figure would seem a serious underestimate -- and prevents the evaporation of "1 crore liters of water a year." "Crore" is a South Asian term for ten million.
Regardless of the accuracy of the figures involved, the image has already prompted quite a few Californians to wonder: Could we do something similar with our state's hundreds of miles of canals?
Let's take the California Aqueduct as a back-of-the-envelope case study. Running from the Sacramento Delta to three endpoints on the South Coast and the California Desert, the Governor Edmund G. Brown California Aqueduct totals 701 miles in length, of which around 444 miles are open canal.
A typical section of main stem Aqueduct is about 110 feet across at its widest point. Covering the Aqueduct with solar panels would require a trellis structure slightly wider than that, and we'd also have to account for breaks in the canopy to allow maintenance and repair access, so let's call it 100 feet of effective width to be conservative in our estimates and also because it makes our calculations easier.
A mile is 5,280 feet, so a 100-foot wide PV surface would total 528,000 square feet per mile. Polycrystalline solar PV cells have a real-world output of about 10 watts per square foot, so each mile of canal offers the potential for 5,280,000 watts of solar electrical capacity. Call it five megawatts per mile, again for the sake of being conservative in our estimates and making the math easier. If we covered 400 miles of California Aqueduct with PV, that works out to two gigawatts of solar generating capacity.
Capacity is one thing, of course. Actual output is another. As we've mentioned before, we use power in watt-hours rather than watts; what really matters isn't so much the capacity of a solar installation as its actual output. In California's Central Valley, where much of the Aqueduct runs, solar PV will produce about a fifth of its theoretical maximum output, which works out to about 24 megawatt hours of power generated each day by each mile of covered Aqueduct. Four hundred miles of Aqueduct could produce about 9,600 megawatt-hours each day.
Even given all our attempts to be conservative in our figuring, these are really optimistic numbers, not to mention rough. Polycrystalline PV is expensive, and any installation of the gargantuan scale we're talking about here would likely use less-expensive thin film PV cells, which can mean a cut of as much as 50% in capacity. The Central Valley's skies aren't always blue: the Valley has thick tule fogs in winter and a serious particulate matter pollution problem in summer. Both of those cut down on the amount of sunlight reaching the Aqueduct, and the dust that would likely settle on PV panels there wouldn't help.
And the biggest obstacle is that State Water Project (SWP) managers don't wanna. The idea has been floated before, as Todd Woody reported in the New York Times last year -- floated in a literal sense, with the theoretical PV panels placed directly on the surface of the water rather than on a trellis. According to Woody, the State Water Project's Deputy Director Ralph Torres said covering the Aqueduct in solar panels wouldn't work for them.
It's not that the SWP isn't interested: they're one of California's biggest consumers of electrical power. The old dictum about water in the West is that it flows uphill toward money: in the case of the California Aqueduct that's literally true. The Aqueduct's water has to get up and over the Tehachapi Mountains to get to Southern California, and it does so by way of the massive Edmonston Pumping Plant, which pumps the water from the Central Valley up 1,926 feet to the top of the Grapevine. Its pumps use 835 megawatts of power to make that water flow uphill. That's more than 20 gigawatt-hours for every 24 hours in which the pumps are running at full capacity. And Edmonston is merely the largest of half a dozen pumping stations on the Aqueduct. So the Aqueduct's managers are indeed interested in building solar capacity: they're just restricting their interest in solar power to adjacent parcels of dry land.
Even if the SWP was on board, there's no way the California Aqueduct could become energy self-sufficient by putting on a solar PV cap, even if we covered the entire length. But as California increasingly turns solar, every stretch of open industrial land that's anywhere near power demand will likely be eyed for solar development, and California's canals are no exception.
Irrigation canals in the deserts are a likely starting point. Not only is the rate of evaporation from uncovered canals significantly higher on, say, the Coachella and All-American Canals in the Salton Basin, but dozens of people drown in the All-American Canal every year while trying to cross the border, and catwalks built as part of a solar trellis could save many lives each year. The Imperial Irrigation District refuses to install safety ropes on the canal due to the potential for "encouraging" illegal immigration, but perhaps the prospect of selling more solar power to coastal cities could persuade them to overlook that side-benefit.
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