Solar Power Towers Aren’t the Avian Annihilators Once Thought

Solar power towers have had a reputation as alleged avian vaporizers since preliminary reports emerged in 2014 of birds being burned in mid-air as they flew through the intense photonic flux at California’s Ivanpah solar thermal plant. Their reputation was muddied even more during tests early this year at SolarReserve’s Crescent Dunes power tower in Nevada; the solar thermal plant just recently began producing power. California public radio station KCET reported that as many as 150 birds were killed during one six-hour test in January.

It is obviously upsetting to imagine birds ignited in the name of renewable energy. (KCET reporter Chris Clarke, who has tracked the issue since BrightSource Energy began building Ivanpah in the Mojave Desert, described burning birds as “beyond the pale” in a recent article suggesting that power towers may be finished in California.)

But, upsetting as any killing of birds is, avian mortality is a downside common to many modern human creations—including buildings, highways, and powerlines. The best data on bird mortality at Ivanpah, macabre as it might be, shows the death rate to be small and likely of little ecological significance.

Meanwhile, operational adjustments at both Ivanpah and Crescent Dunes are pushing avian impacts even further below levels that could threaten local bird populations. “The data does support a low level of avian mortalities and hopefully, through adaptive management and deterrence, it will go even lower,” says Magdalena Rodriguez, a senior environmental scientist with the California Department of Fish and Wildlife. Continue reading

Return of the Solar Power Tower

Last week Spectrum Online ran my profile of Andasol 1, a solar thermal power plant that’s set to startup in Andalucia with the largest installation built expressly for storing renewable energy: a set of molten salt storage tanks that will hold enough heat energy to run its 50 MW steam turbine for 7.5 hours after dark. This week brought decisive evidence that another solar thermal design that makes even better use of energy storage — a so-called ‘power tower’ whereby sunlight is focused on a central tower — will also have its moment in the Andalucian sun.

The project, dubbed Gemasolar, will employ sun-tracking mirrors covering an area equal to 40 soccer fields to focus light at the top of a roughly 120-meter-high tower. There the sunlight will heat a solar receiver full of molten salt. In contrast, Andasol 1 (like most of the solar thermal plants under construction in the U.S., Spain, North Africa and the Gulf) uses thousands of square meters of trough-shaped mirrors to focus light on a synthetic oil; energy is stored via heat exchangers that transfer the synthetic oil’s heat to a molten salt.

One advantage of the power tower is thus obvious: heating salt directly eliminates the need for heat exchangers, reducing installation and operating costs. Another lies in the fortuitous thermodynamics of heating molten salts, whose maximum safe temperature of 565 C is about 165 C higher than the synthetic oil’s.

Sandia National Lab researchers verified these power tower advantages in the second half of the 90s, but also suffered through a series of operational difficulties. Five years ago the European Commission provided funding for the Gemasolar project (then known as the Solar Tres) to demonstrate that the difficulties could be overcome, but the project foundered on legal issues and changes in Spain’s renewable energy law. But engineering continued and this March the project sprung back to life when its lead proponent, Spanish engineering firm Sener, clinched a solar thermal joint venture with Abu Dabi’s alternative energy program.

With Abu Dabi’s deep pockets Gemasolar’s financing just might survive the current financial crisis. Siemens confirmed that the tower was moving forward this week by disclosing that it would supply the steam turbine to convert the tower’s solar-generated heat into up to 19 MW of electricity for the Spanish grid. 

For further details on Gemasolar, see this frank telling of its origins, design and goals on Sener’s website. For details on a competing power tower design that directly produces steam, see this white paper from Spains’ Abengoa Solar.

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This post was created for Tech Talk – Insights into tomorrow’s technology from the editors of IEEE Spectrum.