New Tandem Solar Cell Discovered Posing As Giant Clam

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With the end of the fossil energy era drawing near, researchers are turning renewed attention to natural systems that can help sustain the energy needs of the human population without killing the planet. One example is the use of algae to “grow” a new kind of solar cell. That may seem a little far fetched, but a newly published study of the superior light-harvesting abilities of algae-festooned giant clams supports the idea.

A Tandem Solar Cell Based On The Clam-Algae Connection

A tandem solar cell deploys two (or more) kinds of light-harvesting materials to improve overall solar conversion efficiency. The extra material means that tandem solar cells are typically more expensive than simple, single-junction solar cells, though a new perovskite-silicon formula could be the exception that proves the rule (see lots more perovskite background here).

In effect, giant clams are giant, living, and highly efficient tandem solar cells, as described by a team of researchers at Yale University.

“The truth is that clams are more efficient at solar energy conversion than any existing solar panel technology,” explains Alison Sweeney, associate professor of physics and of ecology and evolutionary biology in Yale’s Faculty of Arts and Sciences, for an article in Yale News.

The full study is available at the American Physical Society PRX Energy under the title, “Simple Mechanism for Optimal Light-Use Efficiency of Photosynthesis Inspired by Giant Clams.” It describes a symbiotic light-harvesting relationship between the iridescent giant clams found in Palau, in the Western Pacific, and the algae that grow on them.

The clams contribute cells on their surfaces, which create their iridescent appearance. Called iridocytes, these cells serve as a kind of light preparation mechanism. They filter and scatter light, enabling it to wrap around the algae uniformly.

The algae also contribute to the efficiency of the operation by growing in vertical columns, parallel to the incoming light. This geometry provides for maximum light absorption, explains Yale writer Jim Shelton.

The challenge was to develop an analytical model to quantify the efficiency of the operation. “In our model, photosynthesis-irradiance behavior obeys that of algal cells isolated from clams,” the researchers explain.

“Using a standard rate of eight photons of photosynthetically active radiation required to create one molecule of O2, we find that a fixed geometry of the “light-dilution” strategy employed by the clams can reach a quantum efficiency of 43% relative to the solar resource in intense tropical sunlight,” they add.

The team also found that the daily cycle of actual clam movements can boost quantum efficiency to 67%.

What Is Quantum Solar Cell Efficiency?

Before you get too excited, it’s important to note that the study refers to quantum efficiency, not solar conversion efficiency. Solar conversion efficiency refers to the ability of a solar cell to convert sunlight to electricity. Quantum efficiency is more of a census-taking calculation. Expressed as a percentage, it can easily surpass conversion efficiency.

Still, in terms of the emerging bioeconomy of the sparkling green future, a reasonably high quantum efficiency can indicate strategies for designing new, commercial-scale biomass operations that optimize land use.

“The general principles here also readily generalize to any photosynthetic cell type or organic photoconversion material and solar-irradiance regime,” the researchers explain, noting that spruce forests exhibit a similar potential.

Solar Cells From Algae

The Yale team is not the only one to notice that algae have natural properties that could be applied to human-engineered light harvesting systems.

Last year, a startup called the Swedish Algae Factory (SAF) reached the fifth and final year of an EU grant to develop a method for mass producing a low cost solar cell material made from silica shells. The silica is extracted from a single-celled class of microalgae called diatoms. The firm was also tasked with avoiding the environmental impacts of algae farming.

“One goal of the project was to produce the algae shells in an as environmentally friendly process as possible where water was cleaned, valuable nutrients recycled, carbon dioxide was absorbed, and a valuable sustainable organic biomass also was produced,” SAF explains.

SAF finished off the EU grant with a successful test of its “Algica” solar cell material. “The shells have unique light-manipulating properties that can potentially boost silicon solar panel efficiency by at least 4% and dye-sensitised solar cells by up to 36%,” SAF  concluded. The company also notes that the silica shells block UV light, helping to protect the solar cell from degradation.

Other researchers elsewhere in the world are also exploring the solar cell potential of algae. In April, a research team in India reported on its efforts to fabricate a new photovoltaic device with an assist from the freshwater macroalgae pithophora. As described in the journal Advanced Materials and Devices as a sort of sandwich. The top layer consists of carbon-coated copper and the bottom is tin oxide modified with tin and coated with titanium oxide, with the algae in between.

Beyond Algae Solar Cells

Aside from solar cells, algae is beginning to emerge as a key player in other areas of the bioeconomy of the future. Algae biofuel is an obvious example. ExxonMobile dropped the ball on a longstanding algae research project last year, but the field has been recovering momentum in recent months.

Another potentially widespread application is emerging in the field of building-integrated light harvesting systems. Back in 2013, the design and engineering firm Arup piloted a high tech facade system made with built-in algae bioreactors, enabling buildings to collect and deploy solar energy.

Moving forward to 2021, a researcher in Egypt modeled the energy-saving potential of buildings that sport an algae facade. Published in the open access platform Journal of Applied Science and Engineering, the study calculates electricity savings of 45 to 50%, along with a consequent drop in carbon emissions.

“The study concluded that using algae as an element of the building skin in densely populated cities as a biomimicry architecture strategy contributes to an innovative environmental approach,” concludes the author of the study, Walaa Hussein Hussein Hanafi of the Architectural Engineering Department, Pyramids Higher Institute for Engineering and Technology in Giza.

Also in 2021, the firm EcoLogicStudio demonstrated its “AirBubble” outdoor play area featuring an air purification system run by algae bioreactors.

That same year, the firm Cosign Group and researchers at British University in Egypt proposed a 3D-printed, algae-based rooftop system for cleaning and recycling the wastewater from building air conditioning equipment. The focus of that effort was preventing damage caused by runoff from air conditioning systems.

In 2022 a research team in Germany also proposed cultivating algae film on buildings, as a more efficient, low-maintenance means of natural cooling compared to ivy, moss, or other plantings.

Solar cells or not, if you’ve spotted any interesting news about algae this year, drop a note in the comment thread.

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Image (screenshot): An iridescent giant clam hooks up with algae to inspire a new bio-based approach to solar cell technology (courtesy of Yale University via YouTube).