As the great spectre of climate change continues to loom large over the future, the search for viable, renewable energy sources is becoming ever more important.
Solar power has long been seen as a vital ingredient in our clean energy future. With a little inspiration from nature, solar power might just have become an even more promising prospect than was first thought.
Sunlight is a nearly unlimited energy source that is made up of many different wavelengths. The wavelengths from 400 to 700 nanometres (one nanometre = one billionth of a metre) make up the so-called “visible range” – the light our eyes can detect. These wavelengths in the visible spectrum are the colours that form a rainbow, from red (longest wavelength) to violet (shortest wavelength). Visible light makes up roughly 40% of the sunlight reaching the Earth’s surface every day.
All light is made up of photons, tiny, massless particles that come in a variety of “energies”. The energy of a particular photon is dependent on the wavelength of light it makes up – shorter wavelength light (ultraviolet light for example) is comprised of higher energy photons than longer wavelength light (infrared light, for example).
Apart from providing the light that enables us to see, sunlight plays another vital role in the natural world.
Photosynthesis is the solar energy storage process in which plants take sunlight, carbon dioxide and water and convert these into energy (in the form of sugar) and oxygen.
Over millions of years, photosynthesis in plants has led to the creation of energy resources we now take for granted, including as oil and coal (when plants become fossilised and compressed into liquid form). Photosynthesis is also responsible for the oxygen in the atmosphere that we breathe every day.
The traditional view of photosynthesis is that long-wavelength light (far-red and infrared light, with wavelengths longer than 700 nanometres) contains low energy photons. We used to think that light at these long wavelengths wasn’t “energetic” enough to produce oxygen. In other words, we thought photosynthesis could only occur with the light we humans can see.
But the discovery of a new type of chlorophyll – called Chl f – changes the way we think about photosynthesis.
Chlorophylls are essential molecules that absorb and convert light energy into chemical energy in photosynthetic organisms. There are five known chlorophylls: chlorophyll a, b and c were identified in the 19th century and chlorophyll d was first reported in 1943.
Chlorophyll f, the fifth chlorophyll, was reported in Science in 2010. This new green molecule has the greatest ability of any cholorophyll to absorb red-shifted light – that is, light with wavelengths longer than the human eye can detect.
The red-shifted chlorophylls, chlorophyll d and chlorophyll f, can absorb the light at the red edge of the visible spectrum or beyond.
This discovery of chlorophyll f challenges traditional views about the physical limits of photosynthesis.
When the discovery was reported in Science, it immediately attracted a great deal of attention due to its potential applications.
One of the most promising potential applications for chlorophyll f is in the development of new solar cells.
In existing solar cells, visible light can provide enough energy to produce a current within the cell. Short wavelength light with too much energy (UV light, for example) will simply pass through, and longer wavelength light with lower energy (such as infrared light) does not provide enough energy to create a useful current.
But the discovery of red-shifted chlorophylls, such as chlorophyll f, may provide a solution for improving the efficiency of solar cells by expanding its solar spectrum input.
That is, if light from a greater portion of the spectrum can be used in solar cells, we might be able to generate more energy from sunlight, more efficiently.
In a world that will be relying on renewable energy more and more in future, the more efficiently we can generate such energy, the better.