Particular balloons: Tethered-balloon tests insure safety of new solar-power technology

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Three tethered balloons were deployed both upwind and downwind of the Sandia National Laboratories National Solar Thermal Testing Facility during a falling particle receptor test. The team, led by Cliff Ho, found that the concentration of tiny particles, finer than talcum powder, escaping from the receptor was well below Environmental Protection Agency limits. (Randy Montoya / Sandia National Laboratories)

What do tiny dust particles, 22-foot-wide red balloons, and “focused” sunlight have in common?

Researchers at Sandia National Laboratories recently used 22-foot-wide tethered balloons to collect airborne dust particle samples to help keep emerging solar energy technology safe. The study determined that the dust created by the new technology is well below dangerous levels, said Cliff Ho, the project’s principal investigator. Ho’s team has just received $ 25 million from the Department of Energy to build a pilot plant that will incorporate this technology.

This next generation renewable energy technology is called a high temperature falling particle receiver to concentrate solar energy. Concentrating solar power, while not as common as solar panels or wind turbines, has several advantages over these renewable energy sources, including the ability to store energy as heat. before converting it into electricity for the power grid.

One concentrating solar power plant in Arizona uses molten salt to store that heat for six hours, while others, in theory, could store heat for days or weeks, solar expert Ho said. to concentration of Sandia. This would help power companies equalize the daily and seasonal variation in the energy produced by solar panels and wind turbines.

A team of researchers from Sandia National Laboratories recently used tethered balloons to collect samples of airborne dust particles to ensure the safety of a falling particle receiver for concentrating solar energy, a technology emerging solar energy.

The Falling Particle Receiver works by dropping dark, sand-like ceramic particles through a concentrated beam of sunlight and then storing the particles as heated. These round particles cost around $ 1 for 2.2 pounds and can get much hotter than conventional molten salt concentrating solar power systems, increasing efficiency and lowering costs. The Sandia team also evaluated other particles like sand, which costs just pennies per pound, but they determined that due to the ceramic particles’ ability to absorb more solar energy and provide a more fluid flow, ceramic particles were the best solution. The Department of Energy’s goal is to reduce the cost of electricity from concentrating solar power to five cents per kilowatt hour, which is comparable to that of conventional fossil fuels.

However, reused particles can eventually decompose into fine dust. The Environmental Protection Agency and the Occupational Safety and Health Administration regulate tiny dust particles, finer than talcum powder, which are known to pose a risk of lung damage.

“The motivation for particle sampling was to make sure that this new renewable energy technology did not create any problems for the environment or the safety of workers,” Ho said. “Particles are emitted from the receptor of particles that fall, but the amounts are well below the standards set by the EPA and the National Institute for Occupational Safety and Health. ”

Use captive balloons to catch dust

Last fall, the research team used sensors located a few meters from the falling particle receptor on the solar tower platform, or National Solar Thermal Test Facility, and sensors suspended from helium balloons. 22-foot-wide, attached, to measure particles that were released while operating at temperatures above 1,300 degrees Fahrenheit.

Sandia’s tethered balloon expert Dari Dexheimer and her team deployed a ball a little less than a soccer field near the solar tower and two leeward balloons to detect dust particles away from the receiver. One leeward ball was a little more than one football field and the other more than two football fields. The leeward balloons were floating about 22 stories – a little higher than the solar tower itself – and the upwind balloon was a little lower than that.

The balloons and their attachments were fitted with a variety of sensors to count the number of dust particles in the air around them, as well as their altitude and precise location. The captive balloons remained at their specified altitude for three hours, which allowed the team to collect a lot of data. They also operated a small, remote-controlled balloon that was much more mobile in terms of altitude and position, Dexheimer said.

Sandia National Laboratories tethered balloon expert Dari Dexheimer and her team prepare the 22-foot-wide tethered helium balloons for launch on a beautiful fall morning. (Randy Montoya / Sandia National Laboratories)

“This allowed us to collect data every second for three hours over the entire area,” said Dexheimer, who typically flies tethered balloons over northern Alaska to collect data for monitoring and modeling. of the arctic climate. “Since we got the data in real time, we were able to move the tethered balloons to measure in the region of highest intensity of the plume, identify where the edges of the plume were, or follow the entire movement of the plume over time. . “

The team also placed a variety of sensors on the solar tower platform, a few feet from the falling particle receiver. These sensors could count the number of dust particles as well as determine their size and characteristics.

Andres Sanchez, a Sandia expert on airborne fine particulate matter measurement, conducted these and similar tests two years ago, along with his colleague Andrew Glen.

For the most recent tests, the researchers built special saw-shaped tilting bucket collectors to measure both the amount of particles and their size. Much like a tilting bucket rain gauge, particles in the air would descend into a funnel and land on the swing-like platform. Once a certain weight of dust particles accumulated on the platform, it would tip over and send an electrical signal to the researchers. The number of tipping signals in a certain amount of time told the researchers the frequency of the particle emission events, and after the test they were able to weigh the particles at the bottom of the buckets to determine how much was collected.

Computer modeling and dust mitigation

Comparing the results of the sensors near the falling particle receptor and those further away on the balloons, they found that the concentration of tiny particles, finer than talc, was well below EPA limits.

They found that the concentration of dust particles depended on the weather conditions at the time. They detected dust particles farther from the solar tower on windy days, and higher concentrations of dust particles near the solar tower on calm days, Sanchez said. Ho added that when the wind blew into the receiver from the north or northwest, it produced the most dust particles.

“We did some computer modeling using the EPA’s particle dispersion model,” Ho said. “Basically it would take a particle emission 400 times higher than what we found in the tests. precedents to start moving closer to EPA standards. Based on our measurements and models, I do not foresee any conditions where we will actually reach these thresholds. “

From computer modeling tests and simulations, the team was able to develop several different methods to reduce the emission of fine dust particles. First, they optimized the shape and geometry of the falling particle receptor to reduce particle loss, Ho said. They developed a staircase-like system that slows down particles in the receptor as they fall and a ” muzzle ”which helps mitigate the impact of wind on falling particles.

They also explored and ultimately discarded two other ideas. The first was to have a window on the falling particles, as it would get too hot from the concentrated sunlight and was not easy to adapt to large sizes. The other was to protect particles with an air curtain, like those used at store entrances to keep the air warm or cool inside the store.

Ho and his team have just received funding to build a falling particle reception pilot plant that will incorporate the improvements developed from these tests.

“I normally focus on atmospheric measurements and modeling how the atmosphere would react if carbon dioxide emissions were reduced by a certain amount,” Dexheimer said. “With this work, I was able to participate in the active reduction of these emissions. I think we all really enjoyed seeing the other side of the coin, finding out how to make renewables more efficient and more achievable. “

The balloon tests were funded by the DOE’s Office of Solar Energy Technologies, one of three teams testing different high-temperature concentrating solar power systems with built-in heat storage.

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