Understanding Solar Flux: Science vs. Science Fiction

Understanding Solar Flux: Science vs. Science Fiction

by Joe Desmond

An August 18 Associated Press article discussing avian mortality at solar power facilities noted that at Ivanpah, “estimates per year now range from a low of about a thousand by BrightSource to 28,000 by an expert for the Center for Biological Diversity environmental group.” How can there be such wildly divergent estimates for one facility? 

Well, let’s start with the facts. The Ivanpah project owners are now implementing its Avian and Bat Monitoring and Management Plan approved by state and federal agencies and required by permit. Under the approved plan, Ivanpah reported 321 avian fatalities between January and June 2014, of which 133 were related to flux.

Click to Enlarge

That’s correct – 321 reported bird deaths that occurred during the plant’s first six months of operation. That is far from adding up to the 28,000 annual bird deaths estimated by the Center for Biological Diversity (CBD) expert K. Shawn Smallwood, Ph.D. In fact, Dr. Smallwood questioned the veracity of his assumptions when he testified under oath to the California Energy Commission that, “The calculations I just made of fatality rates at Ivanpah were back-of-the-napkin-level, and were based on assumptions that I cannot at this time verify as correct i (emphasis added).

So, how did Dr. Smallwood arrive at his “back-of-the-napkin-level” estimate? His estimates were only based on two months of reported data from Ivanpah and he used a flawed scale-up approachii, that in his own words he “cannot at this time verify as correct.”

While Dr. Smallwood’s assumptions are an extreme example, it can be difficult to accurately assess avian impacts from solar flux. Solar power tower is a newer technology and solar flux is not widely understood. In its examination of the environmental impact of the Palen Solar Electric Generating System, the California Energy Commission staff conflated solar flux with heat flux when estimating avian mortality.iii

Science vs. Science Fiction

Concentrated sunlight from our mirrors is not super heating the air in the solar field or causing birds to “vaporize into thin air” as has been theorized by some project opponents. There is simply no scientific evidence to support such claims. In fact, the air temperature above the heliostat field at the height of the tower is the same as the temperature on the ground, as explained below.

The following is a detailed description of solar flux and thermal flux, as paraphrased from BrightSource’s Binyamin Koretz, who presented this information at the Palen evidentiary hearingsiv last month.

Solar (light) flux

Solar flux, or concentrated sunlight, is a measure of how much light energy is being radiated in a given area. Solar flux can be characterized by the familiar W/m² or kW/m².

Thermal (heat) flux

Thermal energy, or thermal flux is what we call heat. It is a different form of energy, easier to understand physically – it is mostly from atoms or subatomic particles moving excitedly. Thermal energy is a form of energy that is internal to an object. Thermal energy – heat – can be transferred from one object to another in one of three ways: by conduction, convection and radiation. Conduction is heat transfer directly from one object to another in contact with it (e.g. touching a hot stove), and convection is when thermal energy is conducted to fluids such as liquids or gases that then carry the heat away (e.g. blowing on hot soup).

Electromagnetic radiation

Lastly, there is electromagnetic radiation, something that travels from point A to point B, or from one object to another, like from the sun to the earth or from a heliostat to a boiler. This is the type of thermal energy that is relevant to our discussion of solar flux.

Click to Enlarge

When objects get hot, they radiate electromagnetic energy in the infrared part of the spectrum. All objects above absolute zero do this. The bigger and hotter the object, the more energy it radiates. This is how we get our heat from the sun. It is also how so-called “thermal imaging” works. Infrared cameras can’t really read temperatures: they see radiant energy – infrared light – emitted by your body, and translate its intensity and wavelength to temperature, based on built-in software.

There is no super-heated air in the solar field

Light energy, and even infrared light energy, is not heat. Only when the light energy is absorbed by an object that it hits, is it converted to thermal energy. As we all learn as kids, dark colors absorb more and light colors absorb less. This is why light-colored clothes are more comfortable in a sunny environment and transparent objects absorb basically nothing.

The glass in BrightSource’s heliostat mirrors, is perfectly transparent. As such, it absorbs very little, just a few percent of light, mostly on the low or UV end of the solar spectrum. Air absorbs, for all practical purposes nothing, but small particles in the air can scatter or absorb depending on their color or reflectivity. That is why the air in the solar field does not get hot from solar flux: it cannot absorb the flux and convert it to thermal energy.

When properly considered, solar (light) flux, not heat (thermal radiation) flux, provides the scientific explanation for effects reported by the US Fish & Wildlife Office of Law Enforcement Avian Mortality Report.

For NRG Energy's perspective on the Associated Press story, read Ivanpah: The Facts Behind the Headlines. As an organization, we will continue to rely on rigorous, objective scientific observations to inform our decision-making process.

i CEC Docket Number 09-AFC-07C, Palen Solar Power Project – Compliance, TN# 202736: Exh. 3128 Testimony of Smallwood and CV

ii CEC Docket Number 09-AFC-07C, Palen Solar Power Project – Compliance, TN# 202736: Palen Solar Holdings, LLC's Opening Brief

iii CEC Docket Number 09-AFC-07C, Palen Solar Power Project – Compliance, TN# 202736: Palen Solar Holdings, LLC's Opening Brief

iv CEC Docket Number 09-AFC-07C, Palen Solar Power Project – Compliance, TN# 202736: Transcript of the July 30, 2014 Evidentiary Hearing, pages 243-250