News and analysis about renewable energy in California.

How Much Would Hidden Hills Solar Threaten Birds?

A golden eagle spotted near the Hidden Hills site | Photo: Basin and Range Watch

Has BrightSource Energy understated the risk its proposed solar projects would pose to birds in the California Desert? The staff of the California Energy Commission (CEC) seems to be saying so, as that agency assesses the Hidden Hills Solar Electric Generating System prior to its likely approval. CEC staff and BrightSource disagree wildly over what level of concentrated solar energy is safe for the birds that would fly over Hidden Hills.

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Hidden Hills, which would generate up to 500 megawatts of electrical power on just under 3,100 acres of private lands near Tecopa, would consist of thousands of mirrored heliostats that would concentrate solar energy on boilers atop two 750-foot power towers. That concentrated solar "flux" is the issue. Much as a hand lens can concentrate solar energy to the detriment of living tissue receiving the focused sunlight, Hidden Hills' heliostats will pose a threat to wildlife -- mainly birds, though bats and insects are also of concern -- that stray too far into the zone of concentration.

Both BrightSource and CEC staff agree that there's a level of concentration of solar flux that will harm birds, and that discussion has been going on for some time. (It also tinged the assessment of the company's ill-fated Rio Mesa solar project, put on the back burner earlier this year.)

Where the disagreement arises is just what that level is.

We reported here on March 7 that BrightSource had conducted tests at a facility in Israel in which euthanized birds were hung near a power tower and subjected to different levels of solar flux. The test results apparently weren't pretty: BrightSource refused to allow photos of the test subjects to be viewable by the public. According to documents on the CEC's website, BrightSource claims that its tests prove that there's a safe threshold of solar flux exposure at around 50 kilowatts per square meter (kw/m2). Un-concentrated sunlight runs around 1 kw/m2, so BrightSource's recommended safe threshold is about 50 times stronger than normal sunlight.

With any exposure to radiation, the length the exposure is as important in determining the level of harm as would be the intensity of radiation. (Normal sunlight can burn human skin with a few hours of exposure, for instance, where 30 seconds is generally good for you.) BrightSource claimed that 50 kw/m2 was safe for birds flying across its solar field based on an estimate of 30 seconds travel time across the zone of concentrated solar flux.

As early as last month, CEC staff were challenging BrightSource's assumptions about both the safe level of solar flux at 50 kw/m2 and the notion that birds would cross the solar field quickly enough to get them out of the concentrated flux in 30 seconds. Staff pointed out in formal rebuttal documents posted to the CEC website that the carbonization shown in BrightSource's test photos -- and we'll have to take CEC's word for that, as you'll recall -- far exceeds the level of damage needed to have serious impacts on birds' health.

As CEC staff point out, bird feathers are composed of keratin, a type of protein that also makes up hair. Keratin denatures (breaks down) at temperatures significantly lower than those required to set it on fire, as anyone who regularly uses a curling iron can attest. If we accidentally denature the keratin on our heads it can be embarrassing, but we live. It's a much bigger deal for birds. Unlike hair, feathers are complexly engineered, relying on a system of interlocking barbules for their structural integrity. Feathers' structure is what allows birds not only to stay airborne, but to maneuver in the air and to land safely; feathers also provide insulation and protection from the elements. Mess with the structural integrity of a bird's feathers and you threaten its ability to survive.

Bird feather keratin typically starts to denature at around 160°C, about 340°F. Barbules, those critically important tiny structures that hold feathers together, start to break down at lower temperatures than that. At 400°C is when feathers carbonize, as reportedly happened in the BrightSource "test kitchen" in Israel. (400°C is a skosh over 750°F, hotter than the oven in most real kitchens can get.)

Thus birds can suffer deadly injuries from levels of concentrated solar flux far lower than those needed to actually char feathers. Melt the barbules that hold feathers together, and a bird may have trouble staying aloft. Damage the feathers on one side of a bird -- likely, given that of the myriad possible paths over a solar field, most won't approach the power tower dead center -- and that bird may suddenly find it harder to steer, in the way it's harder to steer when you've got one flat tire.

Again, no one involved in the discussion of Hidden Hills disputes the basic science here. The dispute is over what concentration of solar flux can be expected to raise the temperature of the keratin in the feathers to 160°C or above. A number of factors complicate that assessment: the reflectivity of the feathers, the degree to which sunlight passes through feathers without heating them, the speed and flight paths at which the birds actually traverse the solar field, and whether the birds' behavior will be affected by the bright "second sun" of the illuminated power tower.

On the flight speed issue, CEC staff strongly dispute BrightSource's assumption of a 30-second maximum exposure time. In order for a bird to be exposed to the concentrated solar flux for 30 seconds and no longer, say CEC staff in the February rebuttal linked above, the birds would have to fly in a straight line at speeds above 30 miles per hour. That's well above the cruising speed of most birds, say the CEC staff in their February Rebuttal testimony:

The experiments are premised on a 30 second exposure duration being the time required for a bird traveling at 14 meters per second and the distance required for a bird to traverse a hypothetical air space with a constant flux density of 50 kW/m2.

Staff does not agree that such a flight pattern nor a 14 m/s flight speed (about 30 miles per hour) is plausible. To be protective of most species that could be exposed and considering the lower end of the range of flight speed that is plausible, a flight speed of 8 meters per second (about 18 miles per hour) is more appropriate to estimate potential impacts.

Of special interest in this context are migrating birds, which generally fly at speed much lower than those of which they're capable -- an important energy conservation strategy that would expose migrating birds to much longer doses of concentrated solar flux.

And of course if we decide that BrightSource's suggested safety threshold of 50 kw/m2 is too high, then the zone of damage the birds must fly through becomes larger and the exposure time proportionately longer.

What are those more likely safe exposure levels? Here's what CEC staff said in February:

The experimental results (i.e., photos of bird specimens exposed) display evidence of clear adverse effects at or above 50 kW/m2 observable with the naked eye. This experimental evidence indicates a level of feather damage exceeding what staff considers a Lowest Observed Adverse Effect Level (LOAEL) at 50 kW/m2

Most birds exposed to this level of flux would be expected to suffer adverse effects. Thus, this is not a protective exposure level (i.e. a threshold). As described above, many types of adverse effects would be expected to occur at lower temperatures (i.e. lower flux exposures). These effects could not be observed with the naked eye. This is precisely the reason that LOAELs are typically divided by an uncertainty factor of up to 10 to approximate a No Observed Adverse Effects Level (NOAEL) in establishing threshold or safe exposure criteria. In fact, the Cal EPA Department of Toxic Substances Control recommends that when converting an acute LOAEL to a chronic NOAEL for vertebrates, an Uncertainty Factor (UF) of 10 be applied and when adjusting from a chronic LOAEL to a chronic NOAEL a UF of 5 be applied.

If the experimental results from exposure of about 50 kW/m2 are divided by 10, the resultant approximate NOAEL is 5 kW/m2 which is essentially equivalent to staff's estimated threshold based on a NOAEL derived from results of thermo gravimetric analyses that are far more sensitive, far less subjective, more accurate, and more reliable than the visual observations made during the Applicant's experiments with dead specimens exposed to discrete flux levels for short durations.

Relative extents of solar flux density areas | Image: California Energy Commission

Once we're talking about more than 10 kw/m2 being potentially unsafe for birds, the area surrounding each tower at Hidden Hills that poses a risk to birds gets much larger. As shown in graphs provided by the CEC staff at hearings this month, a 50 kw/m2 damage zone -- which already has a diameter about the length of a football field -- is significantly smaller than one at 10 kw/m2. A 10 kw/m2 damage zone is about 2,000 feet in diameter, which works out to a 72-acre circle surrounding each power tower that could cause harm to birds.

Those are the basics of the issue of solar flux and the Hidden Hills plant. On March 14, an evidentiary hearing was held in Shoshone, California, in which CEC staff presented additional information on the risk the project would pose to birds. The transcript of that portion of the hearing is not yet online, though the previous days' transcripts were up by last week: CEC spokesperson Sandy Louey tells ReWire that the length of the testimony is impeding timely posting of those transcripts.

Whatever those transcripts will contain is apparently big news: shortly after CEC staff testimony, BrightSource filed a complaint-toned Motion to Supplement the Evidentiary Record in which the firm's attorneys allege "procedural unfairness and prejudice to the Applicant and all parties that resulted from Staff's surprise introduction of fundamentally new evidence on the issue of flux at the March 14, 2013 evidentiary hearing." Basin and Range Watch, whose members attended the hearings, characterizes staff testimony on the solar flux issue as follows, however: "Golden eagles and other birds will be killed by exposure to this flux field."

Basin and Range Watch also quote CEC staff as saying the number of birds killed in collisions with heliostats may run between 2,900-3,400 birds per year.

The CEC's Sandy Louey assures ReWire the transcript will be posted this week: We'll bring you Part Two of this story when that happens, with a description of the "fundamentally new evidence" that has BrightSource so upset.

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About the Author

Chris Clarke is a natural history writer and environmental journalist currently at work on a book about the Joshua tree. He lives in Joshua Tree.
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