While most research into photovoltaic technology focuses on mineral-based mechanisms, from crystalline silicon to the promising perovskite materials, there are other possibilities. One of these exploits the most successful type of solar energy generation, photosynthesis, which has been powering the planet’s plants for aeons.
Biological solar cells generally use single-celled plants — algae — to harvest solar energy. The Cambridge team, comprising chemists, biochemists and physicists, now claims to have overcome one of the biggest obstacles to developing this technology: the conflicting demands of generating electrons and converting them into useful electric current.
Previous biophotovoltaics (BPVs) have co-located these two functions in the same chamber; algae absorb sunlight, generate electrons, some of which are secreted outside the algae’s cell walls, and immediately inject these electrons into an electrical circuit. But this is not an efficient method, explained Kadi Liis Saar, of the Department of Chemistry. “The charging unit needs to be exposed to sunlight to allow efficient charging, whereas the power delivery part does not require exposure to light but should be effective at converting the electrons to current with minimal losses.”
The team designed a system where the two functions are separated into distinct chambers, using microfluidic technology in the power delivery chamber. “Separating out charging and power delivery meant we were able to enhance the performance of the power delivery unit through miniaturisation,” explained Professor Tuomas Knowles from the Department of Chemistry, also affiliated to the university’s Cavendish Laboratory. “At miniature scales, fluids behave very differently, enabling us to design cells that are more efficient, with lower internal resistance and decreased electrical losses.”
In a paper in Nature Energy, the researchers explain how the power conversion chamber uses laminar flow to separate fluid streams containing positive and negative charges, allowing it to work without membranes. Moreover, the algae used were genetically modified to minimise the amount of charge generated that could not be converted to current. Together, these innovations allowed the cells to generate power density of 0.5W/m2, five times that of previous designs.
This is still well below power densities of inorganic photovoltaics, the researchers admit. "While conventional silicon-based solar cells are more efficient than algae-powered cells in the fraction of the sun’s energy they turn to electrical energy, there are attractive possibilities with other types of materials," said Professor Christopher Howe from the Department of Biochemistry. “In particular, because algae grow and divide naturally, systems based on them may require less energy investment and can be produced in a decentralised fashion." This might be particularly useful in rural Africa and South Asia, where established grids may not exist and cells could be made in local communities without the need for the hjgh-tech factories required by inorganic PV technology.
Separating electron generation and power conversion also allows the energy to be stored, for example for transmission during hours of darkness. “This a big step forward in the search for alternative, greener fuels,” said Dr Paolo Bombelli, from the Department of Biochemistry. “We believe these developments will bring algal-based systems closer to practical implementation.”
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