Another renewable adventure

Enough sunlight in 1 hour for the world’s annual needs

Photo by Clay Banks on Unsplash

It’s very easy for despondency to creep in when news constantly reflects an underwhelming response from governments and the fossil fuel industry to the imminent threat of global warming. This week it’s been revealed that £400k has been donated to the Conservative party this year alone by fossil fuel interested concerns, at a time when the government is considering new exploration licences for North Sea oil and gas. 

Good news green stories are few and far between, which is why I am probably attracted to them. Science and discovery is an important thread for the ultimate survival of this planet, so I was delighted when I found another green energy story.

A few weeks ago I talked about a promising nuclear energy future which cannot end in a holocaust, but could result in energy for everyone. Such a democratisation of power could radically transform the world’s prospects. Another energy source with similar promise is harnessing solar power but not using the more conventional systems we’ve seen populating farmer’s fields. 

The power of the sun

In an average hour, enough sunlight falls on our planet to fulfil the world’s energy needs for a year. This explains why solar panels, photovoltaic cells, have become a significant, clean, alternative energy source. Solar currently accounts for about 4% of Britain’s electricity generation, a 0.9% increase since 2016. In the renewables contribution table it’s third after wind energy and biomass.

The downside of solar panels

Advances have been made in recent years, which has made panels cheaper and more efficient. The latest development is to make them even thinner, extending their opportunities for use. But a photovoltaic cell can only provide electricity and has to be connected to the local grid. There is no opportunity for the energy generated to be stored as a liquid fuel, for instance, and if not hooked up to the grid, it would be lost unless stored in bulky batteries. 

The conversion process is also inefficient. A solar cell can only capture around 20% of the sun’s available energy.

Nature had the answer all along

Erwin Reisner, the energy and sustainability professor at Cambridge University has some interesting observations.

“The sun produces more than enough energy for human activities, but we still can't capture enough of it. Plants are a huge inspiration, because they have learned over millions of years how to take up sunlight and store the energy in energy carriers.”

The idea is to develop a form of artificial photosynthesis. The blueprint has been around for billions of years and only needs a few key components to work.

Reisner believes that it will become a significant contributor to the world’s energy portfolio in the next 20 years.

What is photosynthesis

Photosynthesis is the process by which the green leaves of plants suck up lots of carbon dioxide, mostly overnight, then use sunlight to synthesise the gas and water molecules into usable energy in the form of glucose. To do this they use a pigment called chlorophyll, as well as proteins, enzymes and metals.

The cleverness comes from the way the energy from the sun is locked up within the chemical bonds of the glucose. Not only does the plant produce energy, it also has a method of storing it.

Plants aren’t very good at it.

A typical crop plant is only 1-2% efficient, enough to survive, but nowhere near efficient enough when you’re looking to synthesise fuels for power. Carbohydrates are also not great when your focus is energy production. 

Nathan Lewis, a chemist at the California Institute of Technology and principal investigator at the Joint Centre for Artificial Photosynthesis comments, 

“plants are not optimal energy conversion machines. Plants should be black, not green, to absorb most of the colours in the solar spectrum.” 

The promise

The aim is to improve on natural photosynthesis creating a technology at least ten times more efficient.

Yulia Puskhar, a biophysicist and professor of physics in Purdue's College of Science, is more optimistic.

"With artificial photosynthesis, there are not fundamental physical limitations. You can very easily imagine a system that is 60% efficient because we already have a precedent in natural photosynthesis.”

What’s Cambridge been up to?

Reisner’s team have looked at a number of processes. One is to split water and harvest hydrogen, but it’s difficult to store.

More promising is their artificial leaf, a very thin device of photocatalysts, similar to a sheet of paper. Submerged in water and carbon dioxide and exposed to sunlight, a chemical reaction occurs producing oxygen and formic acid, a liquid fuel with a higher energy density. The same acid which bees and ants produce in their stings and venom.

The formic acid acts as a carrier for hydrogen. One litre of acid is equivalent to 590 litres of hydrogen and it doesn’t need to be stored under pressure at low temperatures.

The US invests $100m in two research groups

Chase, led by the University of North Carolina is working in similar areas to Cambridge. They’re developing systems that use semiconductors to capture light, using different catalysts to convert the carbon dioxide to fuel. Particular focus is the idea of a cascade of catalysts because turning CO2 into fuel requires more than one chemical process.

Lisa, the Liquid Sunlight Alliance, has taken a more theoretical approach focusing on every stage and component of artificial photosynthesis. The conclusion so far is don’t expect fields of photosynthesis panels in the near future. Bringing all the components together into one technology is complex and problematic.

One view of the future

Artificial leaves could look more like photosynthetic covers that can be rolled out. Imagine the covers at Wimbledon being a bit smarter and potentially producing enough energy for the whole event. The solar cells in the cover would split water into more reactive compounds via a tank full of catalysts, converting CO2 into a carbon neutral chemical fuel. Unfortunately, the covers would have to come out, whether it rained or not.

On a more serious note, it is the lack of infrastructure requirement which makes this energy future appealing. It becomes instantly deployable in developing countries where communities are spread out and not connected by significant government investment in expensive power grids.

Final thought

In November, Cop26, a UN summit on climate change takes place in Glasgow. It’s viewed as one of the last chances to put the world back on track regarding climate change, where countries will be asked to commit to stringent targets shifting dependency from fossil fuels to renewables.

I wonder whether the government has looked in the diary recently - sticky situation approaching?