Power tower solar has been under a cloud — in the U.S., anyway — after 321 birds or bats were killed in the first 6 months of operation by flying through solar flux above the Ivanpah Concentrated Solar Power (CSP) plant, the Google-funded colossus in California’s Mojave desert.
Opponents then greatly exaggerated the numbers, making it a challenge for California regulators to maintain a level-headed approach to permitting future power tower projects.
Regulators do have access to the actual facts as an independent tally and mortality estimate algorithms are typically required for permitting renewable energy. But battling public opinion, even if misinformed, takes courage.
Now there is good news coming from SolarReserve. It has been testing the standby procedures at the Crescent Dunes Solar Project, a 110-megawatt (MW) power tower CSP plant located in the Nevada desert near the town of Tonopah.
Power Tower: Bird Deaths Zero
The first power tower with storage in the U.S. seems to have solved the problem of avian mortality altogether. However, Crescent Dunes initially experienced the same issues as Ivanpah, but just for one day until plant operators cracked the code.
For four or five hours on one day in January, the team focused 3,000 mirrors on one tight donut of air space above the tower for several hours in standby mode, and a reported 115 birds were killed.
When the mirrors are in standby mode — not aimed at the tower receiver to make power — they are just aimed at a point in the sky. Standby is the high risk position for birds. There is a right way and, now they know, a wrong way to aim them during standby.
“As soon as we realized this, we halted testing,” CEO Kevin Smith told me this week. “And our engineering team came up with a correction within a day or two.”
No More than Four Suns Anywhere
The solution the SolarReserve engineers devised was to extend multiple standby points out across a huge “pancake” area hundreds of meters from the tower “so that no single point in the sky has a concentration of more than four suns.”
“We’ve seen birds fly in and out of the standby positioning and there’s no harm at all,” Smith said.
As of mid-April, SolarReserve had zero bird deaths for three months since that mid-January day when they discovered the problem and how to solve it.
Solar flux, or concentrated sunlight that is a measurement of light energy radiated in a given area, is not heat. It’s effects matter only when light energy is absorbed by an object that it hits, which during operation is the receiver at the top of the tower. Birds recognize the receiver and the tower as a solid object, so they are unlikely to fly into it.
Only when multiple reflections are not focused on the tower but instead focused in a concentrated area of clear space, there is a risk that birds will fly through that space.
However, by focusing no more than four suns at any one standby point in the sky, the level of solar flux is safer for birds.
“Birds can fly in and out of this zone without injury,” said Smith.
Of course, birds can and do fly into most manmade structures — cell phone towers, houses, skyscrapers, industrial buildings, and coal plants and natural gas plants — so the risk of flying into the tower itself can’t be eliminated.
When Heliostats Are Put in Standby
Like computers, the heliostats are asleep at night — just laying flat to avoid wind. Each sunrise, they are woken up and pointed “to a place where we know their exact location” so that when aimed at the receiver for the day, they are traveling from a known position.
However, unlike Ivanpah, during normal operation at Crescent Dunes the heliostats do not need to go into standby after the initial standby position at sunrise due to energy storage.
“In normal operation the mirrors go into standby position in the morning and they are only there for few minutes, after which they are in operation mode and pointed on the receiver all day long,” Smith explained. “During testing, we actually have the mirrors in standby for several hours. Even though in normal operation, they’re in standby only for a few minutes at daybreak.”
Storage Removes Need for Standby
With storage, the mirrors remain focused on the receiver all day because it is beneficial to keep heating the molten salts. Each cycling through the receiver adds more heat to the molten salts and all excess heat is stored. So moving mirrors to standby during the day would actually reduce its selling point — the dispatchability supplied by storage. So, because it has storage, Crescent Dunes doesn’t need to go into and out of standby during normal operation.
Ivanpah’s design is different. It does not have storage, and it does need to go into standby during the day to protect the turbine power block, which is run on steam by directly boiling water in the receiver with the sun’s energy. If clouds pass, steam can cool to create partial water droplets in the system, which would damage the turbine that generates the electricity. To prevent this, the steam turbine is switched off and the mirrors are put in standby during those lower solar periods throughout the day.
Ivanpah owners have now also developed a similar algorithm specific to their system to reduce standby solar flux, and they also use bird, bat and insect deterrents under the oversight of the California Energy Commission.
But the SolarReserve answer to reduce the risks to birds now provides another option to provide clean, safe and reliable energy, which with storage provides solar power even after the sun goes down.
Solar tower developers have been working together to solve this issue once and for all, and SolarReserve’s 4 sun standby solution has been made available to be used by other power tower developers.
