Perhaps we are naïve. We want to believe there are technological fixes for major problems. For example, we know we have to find ways of replacing oil (petroleum) as a major source of energy. We know, sooner or later, we will run out of oil. One alternative is to use less energy; unfortunately that ultimately means a drop in living-standard worldwide. So we really want to believe that science and technology will find alternative sources of energy.
With this kind of motivation and for the most part adequate money, scientists and their engineering brethren are hard at work on scientific and technological fixes for the looming energy crisis. The work takes many paths, not all of which lead anywhere, but they don’t know that until they try them.
That said; here are two relatively ‘out there’ stories of work in progress on solar energy. They both take their cue from plant life. One is about creating an ‘artificial leaf’ to produce electricity, the other is about solar cells that regenerate – just as leaves do when sunlight breaks down the chemistry of photosynthesis.
Water-gel solar device like a synthetic leaf
The concept is not hard to describe: Take a water-based gel, mix in some light-sensitive molecules, add electrodes coated with carbon nanotubes. When the light-sensitive molecules become excited (electrically charged) by sunlight, the electrodes tap the charge for use elsewhere. Something like this happens when plants capture sunlight for the production of glucose in photosynthesis, which is how studying plant biology inspired the approach.
A team of researchers from the United States and South Korea led by Dr. Orlin Velev from North Carolina State University (Raleigh, USA) have reported their proof of concept for mixing water-based gels (in the jargon, aqueous soft gels) with photosensitive compounds (including chlorophyll) in the Journal of Physical Chemistry Aqueous soft matter based photovoltaic devices. The photovoltaic devices created by the team are made with inexpensive materials and can be flexibly packaged in both a literal and figurative sense. Sounds good, but this is work in the very early stages.
“The next step is to mimic the self-regenerating mechanisms found in plants,” Velev says. “The other challenge is to change the water-based gel and light-sensitive molecules to improve the efficiency of the solar cells.”
“We do not want to overpromise at this stage, as the devices are still of relatively low efficiency and there is a long way to go before this can become a practical technology,” Velev says. “However, we believe that the concept of biologically inspired ‘soft’ devices for generating electricity may in the future provide an alternative for the present-day solid-state technologies.”
[Source: North Carolina State University]
Self-regenerating solar cells
We’ve all seen how sunlight can make fabric colors fade. Ever wonder why the color of leaves doesn’t fade? (Discounting, of course, seasonal changes.) Actually, they would fade as the sunlight used for photosynthesis also breaks down the plant’s chemistry, but nature has developed ways for the chemistry to automatically regenerate. Scientists would like to do that too, regenerate as people no doubt, but this is about imitating nature for regenerating solar cells.
Solar cells degrade under the power of the sun. This is especially true for the new organic solar cells. To get around the problem, Michael Strano and a team of colleagues at the Massachusetts Institute of Technology (MIT, Boston, USA) studied how plants regenerated their photosynthesis chemistry. Published in Nature [Photoelectrochemical complexes for solar energy conversion that chemically and autonomously regenerate] the approach they developed is not difficult to describe: A solution is created containing carbon nanotubes, bacterial light-harvesting proteins, tiny discs made from the lipid molecules that form the membrane of cells (phospholipids), and a surfactant. The surfactant, a chemical that makes the molecules of a liquid spread, prevents the mixture from interacting.
Remove the surfactant with a filter and the proteins bind to the lipids, which attach to the nanotubes – all self-assembling. Expose this mixture to sunlight and the light-harvesting proteins generate a charge that is carried to the electrical contacts of the solar cell by the carbon nanotubes. Eventually the proteins break down. Pour in surfactant, the molecules disperse, add some new protein molecules, take out the surfactant again – and the solution regenerates. Repeat as needed for efficiency.
Crude as this is, the team assembled cells that produced electricity for 32 hours and required 8 hours for regeneration. Even including the hours of regeneration, this cycle was at least 300% more efficient than non-regenerative organic solar cells. At the molecular level, this new solar cell is operating at about 40% efficient; roughly double that of the best commercial solar cells. The difficulty will be to continuing this level of efficiency in a device that can survive mass production and commercial competition. There are plenty of practical hurdles, for example, how are the protein molecules to be replenished for the regeneration cycle? How, physically, is the removal and injection of surfactant to be handled?
Theoretically, the efficiency of the structures could be close to 100 percent, Strano says. But in the initial work, the concentration of the structures in the solution was low, so the overall efficiency of the device — the amount of electricity produced for a given surface area — was very low. They are working now to find ways to greatly increase the concentration.