In a new study, Yale researcher Alison Sweeney found that giant clams in the western Pacific Ocean may represent the world's most efficient solar energy system.
Designers of solar panels and biorefineries can learn a lot from iridescent giant clams that live near tropical coral reefs, according to a new Yale-led study.
That’s because giant clams have a very precise geometry: dynamic, vertical columns of photosynthetic receptors covered by a thin, light-scattering layer. That could make them the most efficient solar energy systems on Earth.
“It’s counterintuitive to a lot of people, because mussels live in intense sunlight, but they’re actually very dark inside,” said Alison Sweeney, an associate professor of physics and ecology and evolutionary biology in the Yale Faculty of Arts and Sciences. “The truth is that mussels are more efficient at harnessing solar energy than any existing solar panel technology.”
In the new study, published in the journal PRX: Energy, a research team led by Sweeney presents an analytical model for determining the maximum efficiency of photosynthetic systems based on the geometry, motion and light scattering characteristics of giant clams. It’s the latest in a series of studies from Sweeney’s lab that highlight biological mechanisms in the natural world that could inspire new sustainable materials and designs.
In this case, the researchers looked specifically at the impressive solar energy potential of iridescent giant clams in the shallow waters of Palau in the western Pacific Ocean.
The mussels are photosymbiotic, with vertical cylinders of single-celled algae growing on their surface. The algae absorb sunlight — after the light is scattered by a layer of cells called iridocytes.
Both the geometry of the algae and the light scattering of the iridocytes are important, the researchers say. The arrangement of the algae in vertical columns — so that they are parallel to the incoming light — allows the algae to absorb sunlight as efficiently as possible. This is because the sunlight is filtered and scattered by the layer of iridocytes, and the light then wraps evenly around each vertical algal cylinder.
Based on the geometry of the giant clams, Sweeney and her colleagues developed a model to calculate quantum efficiency — the ability to convert photons into electrons. The researchers also accounted for variations in sunlight, based on a typical tropical day with sunrise, midday sunlight and sunset. The quantum efficiency was 42 percent.
But then the researchers added a new twist: the way giant clams stretch themselves in response to changes in sunlight. “Mussels like to move and groove throughout the day,” Sweeney said. “This stretching moves the vertical columns farther apart, essentially making them shorter and wider.”
With this new information, the quantum efficiency of the clam model jumped to 67 percent. For comparison, Sweeney said, the quantum efficiency of a green leaf system in a tropical environment is only about 14 percent.
One intriguing comparison, according to the study, would be northern spruce forests. The researchers said boreal spruce forests, surrounded by fluctuating layers of fog and clouds, share similar geometries and light scattering mechanisms with giant clams, but on a much larger scale. And their quantum efficiencies are nearly identical.
“One lesson here is how important it is to consider biodiversity as a whole,” Sweeney said. “My colleagues and I continue to brainstorm where else on Earth this level of solar efficiency might occur. It's also important to recognize that we can only study biodiversity where it's being sustained.”
She added: “We owe a lot to the Palauans, who place a vital cultural value on their mussels and reefs and ensure they remain in top condition.”
Such examples can provide inspiration and insights for more efficient, sustainable energy technology.
“You could think of a new generation of solar panels that grow algae, or cheap plastic solar panels made of stretchable material,” Sweeney said.
The study's first author is Amanda Holt, an associate research scientist in Sweeney's lab. The study's co-author is Lincoln Rehm, a Palauan-American and former doctoral student at Drexel University and researcher at the Palau International Coral Reef Center, now with the National Oceanography and Atmospheric Administration.
The research was funded by a grant from the Packard Foundation and the National Science Foundation.
