Engineer pursues biological solar power

The new designs don’t make biological solar cells practical, yet.

Here’s the challenge:

Current photovoltaic cells generate watts of energy per square centimetre. A solar chip about the size of your fingernail can power a simple handheld calculator. Existing biological cells, which use photosynthesis to generate electricity, produce picowatts per square centimeter, a trillionth of a watt. To power that same calculator, the cells would stretch 20 metres wide and from Binghamton to Ireland.

Choi’s first biological solar cell produces a million times more energy (microwatts per square centimetre), allowing the calculator to operate with a solar panel measuring 20x5m. His findings were recently published in the Royal Society of Chemistry’s journal Lab on a Chip.

Choi’s latest experiment produces milliwatts per square centimetre, reducing the calculator’s solar panel to 0.2032x0.506m.

That brings it into the range of practical application, says Hongseok ‘Moses’ Noh, an engineer and professor at Drexel University who specialises in lab-on-a-chip technology and applications. “Milliwatt power should be sufficient to meet those needs. However, the device, so far, is too big for hand-held systems.”

If Choi can reduce the cell to a tenth of its size while maintaining milliwatt power density, it would be enough to power hand-held blood analysis devices or air-testing machines. “This is one of very few miniaturised bio-solar products,” says Noh.

What makes Choi’s approach different? Existing biological solar cells use a thin strip of gold or indium tin oxide as an anode between the bacteria and an air cathode. This is not very efficient, and the bacteria eventually die because they lack air.

Choi uses a carbon anode immersed in the bacteria-laden fluid. This is more efficient, and because the solution has access to air, it’s self-sustaining. It also uses the plant’s natural respiration to draw energy from the sugars in the cells to keep power up even if light is low.

Choi, an assistant professor of electrical and computer engineering, says he doesn’t understand why one form of cyanobacteria works better than another, or why a mixture of cyanobacteria and heterotrophic bacteria work even better than a single variety.

Choi might work with bioengineers to develop a bacterium with its photosynthetic engine on the cell’s surface instead of deep in its heart. He has received seed funding from Binghamton’s Transdisciplinary Area of Excellence in smart energy to continue this work.