Biological photovoltaics, biophotovoltaics, or BPV, is a renewable energy technology that uses oxygenic photoautotrophic organisms (or parts) to generate electricity from solar power.
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Biological photovoltaic systems generate electrons through the photolysis of water, which are then transferred to an anode. A high-potential reaction occurs at the cathode, and the potential difference between the anode and cathode creates a current through an external circuit.
The technology behind these systems is still in its infancy, and there is not yet a widespread commercial application. However, it is hoped that the use of a living organism for light harvesting will result in photovoltaic devices that are cheaper and easier to maintain than synthetic alternatives such as the silicon-based photovoltaics that form the basis for most commercial and domestic solar panel installations. This is because living organisms have evolved to self-assemble and self-repair extremely efficiently through organic processes.
Types of Biological Photovoltaics
Biophotovoltaic systems are categorized by their light-harvesting material and the mode of electron transfer between biological material and anode.
Simple and complex materials are both under consideration. The advantage of simple materials is that they tend to be more efficient than complex materials, but the trade-off is that they are generally less robust.
Isolated photosystems are the simplest available, offering a direct connection between anode reduction and water photolysis. These systems are typically isolated and absorbed to a conductive surface. However, these tend to have short lifetimes (just a few hours) and requirements for low temperatures to improve stability.
Slightly more complex are sub-cellular fractions. These devices use fractions of photosynthetic organisms such as purified thylakoid membranes.
Some biological photovoltaic systems, such as cyanobacteria, have been developed to take advantage of entire biological organisms. The system grows cyanobacteria in suspension with an anode made from indium tin oxide.
These are the most robust type of biological photovoltaic system, with lifetimes spanning months so far observed in the literature. The whole cells’ insulating outer membranes reduce electron transfer, resulting in devices with poorer energy conversion efficiency.
Developing Biological Photovoltaic Systems
The working principle behind biological photovoltaics has only recently been introduced in research settings. The use of natural photosynthesis for direct energy production has only been seriously put forward in the last few decades. While there is considerable promise for the technology, there are still several gaps in knowledge before it can come to fruition (or be found to be unworkable).
Specifically, the light-to-electricity conversion efficiency is still very limiting for the devices. Recently, scientists have discussed the possibility (in theory) of genetically engineering cyanobacteria to improve photocurrent production.
As with any emerging technology, standardization remains a challenge going forward. As a result, it is currently difficult to compare results from different studies. Biomass generation, the growth method employed, and platform architecture vary widely from one biological photovoltaics application to another.
So far, no observed currents have come close even to traditional microbial fuel cells, let alone to synthetic photovoltaics. Future applications of these systems will have to be markedly more powerful to make further development viable.
The systems proposed also depend on artificial light sources and constant illumination. So far, no system has been put forward that could reasonably work in natural settings to exploit power transfer from the sun’s light. Specifically, the heterogeneity of natural light (from night to day, in shade, and throughout changing seasons) is a limiting factor in current biological photovoltaic systems.
However, ensuring the systems can output enough current to be viable is a precursor to developing systems that react well in variable, natural light conditions.
There is an encouragement within the field, however, with studies laying the basis for more research into biological photovoltaics.
Developing a fundamental understanding of microbes involved in biological photovoltaic systems and directing targeted optimization could lead to higher power outputs in future devices.
Solutions proposed to date include synthetic biology approaches like the introduction of alternative electron transfer routes.
Targeted optimization also relies on standardization. For scientists to effectively develop and test new systems, a series of standards must be adopted across the field against which new and proposed systems can be measured.
Finally, a more robust understanding of the underlying principles involved with biological photosynthesis needs to be reached. For example, the electron transfer from photosystems to electrodes is not currently well understood.
Will We See Biological Photovoltaics in the Future?
The theoretical advantage of biological photovoltaic systems – efficient self-assembly and self-repair – continues to motivate research and development efforts. Theoretically, these organisms can also store energy through biological processes, which could make future biological photovoltaic systems capable of generating power in the dark.
Using photosynthetic organisms to assemble biological photovoltaics may also point to another key advantage: carbon-negative energy. Theoretically, it could be possible to create systems for renewable power generation that result in less carbon dioxide in the atmosphere due to their production.
Biological photovoltaics are an emerging and highly interesting possible solution to humanity’s pressing energy and carbon emissions concerns. While not viable, the next few years of research and development may yield promising results.
References and Further Reading
Bradley, R.W., et al (2012) Biological photovoltaics: intra- and extra-cellular electron transport by cyanobacteria. Biochemical Society Transactions. doi.org/10.1042/BST20120118.
Tschörtner, J., B. Lai, and J.O. Krömer (2019) Biophotovoltaics: Green Power Generation From Sunlight and Water. Frontiers in Microbiology. doi.org/10.3389%2Ffmicb.2019.00866.
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