Solar Thermal Power in the U.S.

What ever happened to solar thermal electricity generation?

As recently as around 2010, many experts thought the future of solar power would be huge arrays of mirrors, focusing sunlight to heat fluids to run steam engines. To replace fossil fuels we would need to carpet the world’s desert plains with these facilities, sending the power to less sunny places over thousand-mile transmission lines.

You can read all about solar thermal power (sometimes called concentrated solar power) at the SolarPACES web site, which has a map of solar thermal generating capacity by country, plus links to detailed data on individual projects. Wikipedia also provides a good overview and a convenient list. The United States and Spain have led the way in developing and deploying these technologies.

At least for the U.S., though, I think we can summarize much of the story in a single chart:

This chart (inspired by a similar chart from EIA) shows the annual electric energy produced by all large-scale solar thermal power plants in the U.S. To put the scale in perspective, the total energy generated by all these plants during any of the last few years—about 3,000 GWh—is comparable to that of a single medium-sized fossil power plant, or roughly a third of what’s produced by a typical nuclear power reactor.

Besides the relatively modest overall scale, I’m struck by the long stagnation during the 90s and 00s, followed by a burst of new construction in the early 2010s.

Here is a map of the locations of these facilities:

Most of these power plants use the parabolic trough design, shown in this old photo of one of the SEGS (Solar Energy Generating System) mirror arrays:

(This photo has been copied so widely that I haven’t been able to nail down its origin. It probably came from a SEGS owner or operator or contractor.) The trough structures are aligned north-south and pivot during the day to focus sunlight on a pipe full of fluid that runs along the focal line. The fluid then provides heat for a nearby steam turbine connected to a generator.

The Ivanpah and Crescent Dunes plants, on the other hand, use the “power tower” arrangement, with thousands of pivoting flat mirrors that reflect sunlight onto a fluid reservoir at the top of a tower:

(Photo of one of the three Ivanpah generators from EcoFlight.org.)

Two of the solar plants—Solana and Crescent Dunes—use molten salt to store some of the thermal energy for use during the hours after sunset.

Three of the plants—Genesis, Ivanpah, and Crescent Dunes—are located on federal (BLM) land. The rest are on private land.

As the map shows, all but one of these solar thermal power plants are located in the desert Southwest. The exception was Florida’s Martin Next Generation Solar Energy Center, which was also unique in being part of a “hybrid” plant, providing supplemental heat to a primarily gas-fired generator. Nevada Solar One and Ivanpah also burn small amounts of gas, as did the SEGS plants, for which the amount of gas wasn’t always small.

Details

Here are the plants’ nominal power capacities (in megawatts):

• SEGS (trough): 400 MW (total for 9 units at 3 sites)
• Nevada Solar One (trough): 75 MW
• Martin (trough): 75 MW
• Solana (trough): 280 MW
• Genesis (trough): 250 MW
• Ivanpah (tower): 392 MW
• Mojave (trough): 280 MW
• Crescent Dunes (tower): 125 MW

The total comes to 1877 MW, of which 1402 MW is still operating as of late 2024. Because the sun doesn’t always shine, the average total power over days and nights and seasons is currently about 340 MW. Again, that’s comparable to one medium-sized fossil plant or about a third of a typical nuclear power reactor.

The plants that are no longer operating are SEGS and Martin. The nine SEGS units had a good run, but began shutting down in 2015. The last of them, SEGS IX, was scheduled to retire in October 2024, according to information submitted to the Energy Information Administration. The sites of the other eight are all now occupied by photovoltaic solar farms: Sunray 2 and 3 at the site of SEGS I and II; Resurgence I and II at the site of SEGS III, IV, V, VI, and VII; and Lockhart Solar at the site of SEGS VIII.

Meanwhile, the solar portion of the Martin hybrid plant in Florida seems to have been a failed experiment. According to monthly generation data submitted to EIA, it worked pretty well for a few years, then much less well for a few more years, and was apparently shut down in October 2022.

The latest Google satellite images appear to show that Martin Solar’s mirrors have been dismantled, leaving only the support structures behind. I can’t find any news reports or other official announcement of Martin Solar’s demise, but I would imagine that operating a solar thermal plant in a non-desert climate was challenging—even in the Sunshine State.

The other big disappointment on the list is Crescent Dunes. It has never come close to generating its expected output for more than a few months in a row, and it has been shut down completely for 45 out of the 107 months since it began operating:

Still, Crescent Dunes has now operated for more than 12 consecutive months, and it sounds like its remaining problems might be solvable.

Impressions

Solar thermal electricity generation is clearly a technology that can work, at least in desert environments with plenty of days of full sunshine.

The challenges in non-desert environments are formidable. Even a thin cloud layer diffuses sunlight too much for mirrors to focus well. And because heating the fluid enough for efficient electricity generation takes time, even occasional cloud cover during a partly cloudy day takes a disproportionate bite out of the electricity output. The map of successful U.S. solar thermal plants confirms that this technology is proven only in desert settings.

For similar reasons, at the latitudes of U.S. deserts, solar thermal plants don’t work well in winter. This is especially true of parabolic trough systems, whose mirrors can’t be tipped toward the low winter sun. At southern California’s Genesis plant, for instance, the average generation in December has been only 18% of the average in June:

The good news, of course, is that we now have an alternative technology that’s better in almost every way: photovoltaic (PV) panels. They work fine under diffuse light. They don’t require any warm-up time before producing full power. They work great in deserts, but they also work pretty well almost anywhere else in the U.S. On average, across the U.S., they generate nearly half as much energy in December as in June. They require far less labor to maintain and operate, and somewhat less labor to install. They take up about the same amount of space to generate a given amount of electricity (substituting the quantum inefficiency of the photovoltaic process for the thermodynamic inefficiency of the steam engine). Best of all, they’re made in factories where steady improvements and economies of scale have brought their price down to almost miraculously low levels.

During the 2010s, after it became apparent that falling PV prices would make PV cheaper than solar thermal electricity, many experts still believed solar thermal would retain one advantage: its ability (when so designed) to store heat for several hours into the evening, generating when electricity demand tends to be highest. But over the last few years even this potential advantage has been erased by the falling prices of lithium batteries—a much more versatile (and lower maintenance) means of storing a few hours’ worth of energy.

And so it’s photovoltaic power, not solar thermal, that has now grown to become a significant component of U.S. (and worldwide) electricity generation. Here’s the same chart as at the top of this article, with the scale shrunk roughly 50-fold to make room to show PV:

In 2023 the U.S. got 3.9% of its electricity from utility-scale PV, 1.7% from small-scale PV, and 0.07% from solar thermal. As the chart shows, the first two are growing very rapidly while the third is in gradual decline.

Here’s a map that tries to show all utility-scale (1 MW and above) solar farms in the 48 states:

The dots are sized by the amount of electricity generated in 2023, but many of them are overlapping or too small to see (for a zoomable version click here). Of the more than 5000 dots shown (more or less) on the map, nine represent solar thermal power plants—all in the Southwest (can you find them?). The PV farms, meanwhile, are abundant not only in the Southwest but also in Texas and the Southeast. Florida alone now has more than 80 solar farms with capacities comparable to that of the defunct Martin Solar. And an increasing amount of PV is being installed in northern states.

There are no more solar thermal power plants currently planned or under construction in the U.S., and the list of those in development around the rest of the world is dwindling. Academic research into solar thermal technologies continues, but the topics of the latest articles on the SolarPACES site suggest that these efforts are shifting toward industrial heating applications rather than electricity generation. (This seems at least superficially promising, because when you use the heat directly there’s no thermodynamic efficiency penalty.)

Perhaps it’s unfortunate that the U.S. chose to invest several billion dollars in solar thermal power plants that came online in the mid-2010s, just as it was becoming clear that this technology had no future. But hindsight is always 20/20, and personally I’m glad that Americans (and others, of course) have been willing to make risky investments in new energy technologies. We need to plant the seeds and then wait to see what will bloom.

(Corrected 26 November 2024 to say that on the map of US solar farms, the number of dots representing solar thermal power plants is nine—not seven—because the dataset separates out the three Ivanpah plants.)