Energy storage and conversion : summary and outlook

The closing lecture of the Focus on was intended to remind us of the problematic aspects of energy storage and conversion, but above all to provide a synthetic analysis of i) the intrinsic problems linked to the efficiency of the various energy chains, ii) the approaches and scientific trends pursued, and iii) the structuring of research to estimate the chances of meeting our 2050 energy challenge, which calls for minimal consumption of fossil fuels.

A simple exercise first consisted, on the basis of the efficiencies displayed during the lecture, for i) the conversion of solar energy into energy carriers (electricity, hydrogen, biofuels) and ii) the conversion of chemical energy into electricity, in establishing the final energy balances of different energy scenarios ranging from the well (solar energy) to the wheel (electric vehicles). Such considerations, although approximate, indicate that the energy efficiency of the scenario [sun → photovoltaic cell → power grid → → battery electric vehicle] is ≈ 10%, compared with ≈ 2.8% and ≈ 0.05% for the respective scenarios [solar-hydrogen-electric vehicle] and [solar-bio-electric vehicle], while the current scenario [gasoline → thermal vehicle] has an energy efficiency of 18%. In this context, given the falling price of solar energy (equal to that of nuclear power in 2035) and the kWh battery cost, the winning trio for the future would appear to be photovoltaic → battery → electric vehicle.

Whatever storage or conversion systems are considered, their energy efficiency limits are constantly linked to the lack of suitable materials. Materials are definitely at the heart of every technological innovation and revolution: we have a periodic table full of combinations, but how can we quickly find the winning combination in the limited time available? The chemist's intuition is certainly invaluable, but it will no longer suffice. It will have to be backed up by the contribution of predictive theoretical Combinatorics chemistry, so as to do for materials what genomics is trying to do for biology. We would then have to rely on chemists' green thumbs to synthesize the potentially interesting fictitious materials predicted by theorists.

In the current context of sustainable development, in addition to preparing for a new phase, a special effort will also have to be made to simplify the processes used to produce these materials. Technological breakthroughs rather than incremental improvements are needed. To achieve this, we need to think outside the box and encourage chemists to explore new avenues, or even develop new, eco-efficient synthesis concepts. It's a safe bet that chemists will continue to draw their inspiration from the chemistry of the living world, integrating to a greater extent the bio-inspired, biomimetic and even bio-sourced approaches we've already mentioned in the fields of batteries, photovoltaics and artificial photosynthesis. What's more, there's no doubt that the recycling chemistry we've already mentioned for batteries, or the chemistry used to treat nuclear waste, will experience considerable growth. The thoughtful integration of several functions in a single system is now seen as an opportunity to prepare intelligent systems. For example, the possibility of creating a single autonomous solar cell system by combining battery and solar cell functions is just one example of the many possible combinations.

Apart from these scientific considerations, a legitimate question is what else is needed to increase our chances of meeting our 2050 challenge (producing an additional 130,000 TWh of clean energy). First and foremost, we need to change our scientific landscape, in particular through substantial scientific and financial investment combined with a courageous, proactive policy on the part of our institutions. The risks of failing in this race towards clean energy are minimal, apart from that of not being able to do it in the time allotted to us. We should even be optimistic, because the principles of the processes involved in converting solar energy into electricity, or chemical energy into electricity, are already known. All we need to do is find a way to make them profitable and efficient on a large scale by 2050. On the other hand, if we don't act today, we're certainly committing ourselves to changing life on our planet for the worse for centuries to come.

The final solution to the energy problem that surrounds us will not be a single one, but a combination of solutions, often referred to as the "energy mix". In the current context, scientists have a number of responsibilities: i) to ensure credible analyses of the various energy options by conducting serious life-cycle analyses; ii) to promote transdisciplinary approaches encompassing the expertise of many scientific fields: the quality of these interactions will determine our chances of success; iii) to strike a balance between research into new materials and systems research; iv) to mobilize and federate all the players in society concerned, and to act as messengers to our institutions.

Energy concerns our planet, hence the need for a federative and concerted global approach. The reality, however, is different, reflecting the energy paradox. Each country creates its own program, its own agenda, even its own infrastructure, whereas it would be better to build bridges than walls. The fact that the kWh is seen by some as our next monetary unit is probably at the root of this individualistic behavior in the face of a major planetary problem that calls for union. However, every country wants to take advantage of this green economy to reconcile environmental protection, job creation and competitiveness, so as to create new economic growth.

In conclusion, I'd like to stress once again that managing-controlling-managing energy is the biggest scientific and technological problem our planet will have to solve in the next fifty years. As scientists, it's up to us to alert those around us, and especially young people, to this formidable challenge and to the opportunities that energy management represents for the years to come. That was the aim of these lectures.