The sun has colossal energy potential, yet it is barely exploited. It radiates 10,000 times more energy than man now uses. Plants only recover one thousandth of this immense energy source. A closer look at the potential of processes for using solar energy, from photovoltaics to biofuels and biomass to the “promise” of artificial photosynthesis.

The opinion of Pierre Joliot

Honorary professor at the prestigious Collège de France and former chair of cellular bioenergetics, Pierre Joliot is also a member of France’s Académie des sciences and the United States’ National Academy of Sciences. From an early age, research was very close to an artistic endeavor or an adventure for Pierre Joliot. As an expert in plant biology, his research focused on the energy conversion and transfer mechanisms involved in photosynthesis.

The sun represents an inexhaustible source of energy. It is the well from which most of the energy sources available on the earth’s surface spring, excepting nuclear and geothermal energy. So what are we doing with solar energy today, and where is research headed?

Diverse energy recovery techniques

Among the methods capable of recovering solar energy directly, three of them are currently in use :
- Producing hot water with solar collectors. This technology supplies supplemental energy for home heating and domestic hot water production. This is the least expensive method currently available.
- High-temperature thermal power plants (1,000 °C or more). These plants concentrate solar energywith large mirrors that must be installed in very sunny and preferably uninhabited regions, thus requiring a means of transmission.
- Photovoltaics. This technology holds the most promise. Its development is currently hampered by economics rather than for technical reasons. It is reasonable to believe that technological progress will significantly lower production costs. Photovoltaic electricity already represents a significant source of energy in areas with low population density. Generating such power can also help meet the basic needs of poor countries. The last two methods nevertheless require a means for storing the energy produced, a function provided by the hotwater tank in the first method.

Photosynthesis, a renewable energy source

What about photosynthetic reactions that convert solar energy into chemical energy? Photosynthesis not only synthesizes organic products, it also recycles carbon dioxide and regenerates oxygen, all ofwhich are vital tomaintaining life on earth. We currently expect to produce renewable energy – mainly biofuels – by converting biomass produced by photosynthesis. Plants generally store less than 1% of the sun’s energy in their organic matter. This feeble energy balance can become negativewhen the energy spent for sowing, harvesting and processing is taken into account, especially if only a fraction of the organic matter is actually collected, as in the case of corn or rapeseed oil. The substantial amount of land and water needed to produce large quantities of biofuels would put toomuch strain on food crops, especially in light of the large and evergrowing numbers of people suffering from malnutrition in the world. The only reasonable energy policywould therefore involve improving the collection and use ofwood and food crop waste, which already account for 10% of the world’s energy production.

Replicating the natural process of photosynthesis

process of photosynthesis Last in line comes artificial photosynthesis, which is often described as an energy source that competes with photovoltaic energy. First, it’s worth recalling that the photosynthetic apparatus is the product of billions of years of evolution. François Jacob¹ said that, like any othermajor biological process, photosynthesis is the result of “clever handiwork” by Mother Nature rather than of engineering acumen. The process brings into play a series of energy transfer and conversion mechanisms that are all subtly interlinked, and itwould be useless to try to imitate it. The term “artificial photosynthesis” is devoid of any technical meaning. What it really comes down to is communication. However, it does seem possible to imitate certain components of the photosynthetic apparatus, such as those involved in decomposing water into hydrogen and oxygen. Most pigments capable of trapping light have a short lifespan because the repeated absorption of photons is accompanied by gradual photodestruction. This iswhat causes posters to fade from repeated exposure to light.

Research should continue

These types of processes also occur in the photosynthetic apparatus, but it is constantly regenerated by its cellular machinery, which requires a large amount of energy. That is why it is important to continue researching the photochemical conversion of solar energy. However, it seems highly unlikely that this technology could compete with photovoltaic power generation, which involves a relatively simple physical process whose feasibility has already been demonstrated.

1. French biologist and 1965 Nobel Prize Laureate with André Lwoff and Jacques Monod for his research in genetics.