Researchers from the University of Cambridge have developed a new design of algae-powered fuel cells that is five times more efficient than existing plant and algal models. These fuel cells are potentially more cost-effective to produce and practical to use.
As the global population and global energy demand are correlated, the threat of climate change means that there is an urgent need to find cleaner, renewable alternatives to fossil fuels. Solar power is one of the most attractive energy sources because it receives around 10,000 times more energy from the sun, in a given time, that is required by human consumption.
In recent years, in addition to synthetic photovoltaic devices, biophotovoltaics (BPVs, also called biological solar-cells) have emerged as an environmentally friendly and low-cost approach to harvesting solar energy. These solar cells use photosynthetic properties of microorganisms, like algae, to convert light into electric currents.
During photosynthesis, algae produce electrons, some of which are exported outside the cell where they can provide electric current to power devices. All the BPVs demonstrated have located charging and power delivery in a single compartment; the electrons generate current as soon as they have been secreted.
Using a new technique, researchers from the departments of biochemistry, chemistry and physics have collaborated to develop a two-chamber BPV system where the two core processes involved in the operation of a solar cell — generation of electrons and their conversion to power — are separated.
"Charging and power delivery often have conflicting requirements," explains Kadi Liis Saar, of the department of chemistry. "For example, 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 researchers build a two-chamber system that allowed them to design the two units independently and to optimize the performance of the processes simultaneously.
"Separating out charging and power delivery meant we were able to enhance the performance of the power delivery unit through miniaturization," explains Professor Tuomas Knowles from the department of chemistry and the 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."
The researchers used algae that have been genetically modified to carry mutations that enable the cells to minimize the amount of electric charge dissipated non-productively during photosynthesis. Together with the new design, it enabled the researchers to build a photovoltaic cell with a power density of 0.5 W/m2, five times that of the previous design. This is still only a tenth of the power density provided by conventional solar fuel cells.
The new BPVs have several attractive features, according to the research team.
"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," says 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 decentralized fashion."
Separating energy generation and storage components has other advantages. The charge can be stored, rather than having to be used immediately. This means the charge could be generated during daylight and used at nighttime.
Algae-powered fuel cells are unlikely to generate enough electricity to power a grid system, but they might be particularly useful in areas, like rural Africa, where sunlight is in abundance but there is no existing electric grid system. In addition, the production of BPVs could be carried out directly by the local community, according to researchers.
"This a big step forward in the search for an alternative, greener fuels," says Dr. Paolo Bombelli, from the department of biochemistry. "We believe these developments will bring algal-based systems closer to practical implementation."
The paper on this research was published in Nature Energy.