A solar power tower facility proposed for Riverside County would kill or injure more than twice as many birds with its concentrated solar radiation as compared to an existing solar project in San Bernardino County, according to a document published by the California Energy Commission on Friday.
The document, which contains testimony from commission staff submitted as part of upcoming hearings on the proposed Palen Solar Electric Generating System, estimates that the dangerous zone of concentrated solar "flux" around each of the project's two 750-foot power towers would be twice as large as those around the three towers at the Ivanpah Solar Electric Generating System near the Mojave National Preserve.
That would make each of the Palen towers almost four times as dangerous as their counterparts at Ivanpah, say commission staff, so that even with just two towers compared to Ivanpah's three, Palen would likely pose a 2.5 times greater risk to birds flying near the facility.
Bird deaths have been a mounting concern for wildlife advocates and agencies monitoring Ivanpah. Opinions differ as to the actual number of birds the plant injures or kills, but monthly compliance reports that document the carcasses recovered at the plant each month report between 50 and 100 birds dead in a typical month, with numbers apparently peaking during migration seasons. In April and May of this year, in which 97 and 80 bird carcasses were recovered respectively, about half showed clear signs of overexposure to the plant's solar flux, in other words singed, scorched, or melted feathers, eye damage and/or burns.
Only about a fifth of the Ivanpah plant is systematically surveyed for bird carcasses. Considering that fact, and given disagreements over the degree to which small birds might be consumed by scavengers before biologists can detect them, some observers suggest the actual death toll may be considerably higher.
Increasing concern over solar flux injuries to wildlife at Ivanpah played a large role in a tentative December 2013 decision by the CEC to reject a request by the owners of Palen to change the plant's design from solar trough technology to the power tower proposal, in which hundreds of thousands of independently targetable mirrored heliostats would be arrayed around two 750-foot towers with boilers on top. The heliostats reflect sunlight at the boilers, which heat steam to drive turbines.
But that same concentrated energy that can create superheated steam can cause serious problems for birds and other animals that fly through the zone of concentrated solar flux. And according to one particularly worrisome note in Friday's testimony, those problems may not be limited to obvious injuries such as burns and melted feathers.
According to the testimony, bird carcasses with no obvious signs of thermal injury tend to be found more often the closer surveyors get to Ivanpah's power towers. That indicates to commission staff that solar flux likely played a role in those deaths, despite lack of burned feathers.
"it is likely that solar flux is the source of mortality for a significant amount of
non-singed birds," write commission staff engineers Geoff Lesh and Brett Fooks, continuing:
All avian species are impaired by flux exposure once the birds enter the heliostat fields around [Ivanpah], but the heliostat field is less toxic on a per-area basis the farther from the tower. The risk of collision or other non-flux sources of death would be expected to be equal across the project site. However, the data illustrates that non-singed bird mortality increases closer to the tower, just as singed bird mortality increases.
Lesh and Fooks suggest that simple increased thermal stress, already an issue for birds flying in the California desert, could well be increased enough by exposure to even relatively low levels of concentrated solar flux to cause serious health problems in birds passing through the flux field.
Palen's proponents, Palen Solar Holdings, a joint venture of BrightSource Energy and Abengoa Solar, maintain that there's a safe "threshold" level of solar flux exposure -- about 25 kilowatts per square meter, or about 25 times normal sunlight -- below which birds suffer no ill effects from exposure. Lesh and Fooks' figures, however, show that the distribution of bird carcasses without visible flux injuries increases in an almost linear fashion the closer you get to the power tower. Here's their graph:
If there were a safe level of exposure to elevated solar flux, you'd expect to see a curve in the graph at the distance that corresponded with that safe level, left of which the line indicating the number of mortalities would rise more steeply.
And if these mortalities were caused by something other than solar flux, they wouldn't increase the closer you get to the towers. Which would seem to indicate that solar flux can indeed injure birds as far as 1,000 meters (a kilometer, or about .6 miles) away from the power towers at Ivanpah, with the only "threshold" being the one at about 300 meters where feathers start to melt faster, as shown in this graph of singed bird mortalities:
That 300-meter distance coincides roughly with the 25 kilowatts per square meter flux level suggested by Palen Solar Holdings as a safe exposure threshold. But based on Lesh and Fooks' data, says the commission staff testimony, "staff's current analysis does not indicate that there is a safe threshold for flux (above ambient conditions) that does not produce some risk of avian mortality."
Which means that Palen's significantly larger solar flux field will pose a significantly larger risk to birds and other wildlife than does Ivanpah. And to underscore the relative difference between the two projects' solar flux fields, Lesh and Fooks helpfully came up with a diagram:
For perspective, here's a detail of the photo we used at the top of this article, showing one of the towers at Ivanpah. There's a little orange dot to the left of the base of the tower: it's a standard passenger sedan we turned orange in Photoshop to help you see it better.
Hearings leading to a final California Energy Commission decision on Palen will begin in Blythe on Tuesday, June 29.