Combating greenhouse gas emissions is now a priority for many countries. In addition to energy conservation and energy efficiency, one of the avenues to be explored is “CO₂-free energies”. But do they really exist? The answer to that question is much more complex than meets the eye.

The opinion of Jean-Marc Jancovici

Graduate of École Polytechnique and École Nationale Supérieure des Télécommunications of Paris. He specializes in the environment, and more specifically in energy uses and greenhouse gas emissions. He is one of the founders of Carbone 4, a “carbon strategy” consulting and auditing firm.

Though the media sometimes explain that “clean” energies do exist, this question is much more complex than it appears on the surface. In fact, to “make” energy – though in reality, energy can only be converted, not “made” – not only must one have a source of primary energy (a source found “as is” in nature), one must also build the system that will allow this primary energy source to be converted into usable “final” energy, such as refined oil products, electricity or municipal gas lines. Examples of such systems include refineries, dams, coal-fired plants, nuclear power plants, and so on.

What sources of primary energy are available to us? There are many: coal, oil, natural gas, uranium, the wind (wind energy), the sun (solar energy), moving water (hydropower or mechanical power, such as in the old-fashioned windmills), the tide, geothermal energy (heat from deep in the earth), and biomass (plants), all of which supply compounds whose primary components are carbon, hydrogen and oxygen. Some forms of primary energy are renewable; others are not. All renewable sources are more or less derived from solar energy – even the tides, subject as they are to the pull of the sun. The only exception is geothermal energy, which is produced by the natural radioactivity of earthen rocks and therefore does not depend directly on the sun. But considerable quantities of heat have been accumulating for a very long time deep within the earth. Although technically not renewable, the supply of geothermal energy is so great that it could almost be considered one of the renewable energies. This reasoning holds true for two forms of nuclear energy as well. Though not renewable, they have tremendous sources of supply. The first form is “breeding”*, such as the Superphénix reactor in France, which can use all uranium or thorium from supplies that will last for thousands or even tens of thousands of years. The second is fusion – assuming we are able to master it someday – which, according to experts, will not be available for 50 to 100 years.

Energy and greenhouse gases

Some of the available energy sources emit greenhouse gases when used, while others emit very few. The primary energy sources that emit greenhouse gases are coal, oil, and gas. If trees are not replanted – a situation seen in tropical countries –burning their wood also emits greenhouse gases. Processes in which the primary energy source does not produce CO₂ include all forms of nuclear, solar, wind, tidal and geothermal energy as well as hydropower and, if plants are replaced, biomass. For some processes, even though the primary energy source does not produce greenhouse gases, in particular CO₂, some of the related activities do: front-end mining, construction of the system designed to use the energy itself (dam, power plant, etc.), dismantling or waste treatment. Building a dam or an electric power plant that runs on coal, nuclear, natural gas or oil requires materials such as cement and steel; their production generates CO₂ emissions. Manufacturing a wind turbine or a solar panel requires base materials such as aluminum and glass or more elaborate materials such as semiconductors, and greenhouse gases are released during their production. Adding these “intermediate emissions” to the total, taking into account the system’s service life or “life cycle”, gives the “total emissions” sent into the atmosphere for a given amount of final energy, i.e. energy available for use: gasoline, electricity, cooking gas, etc.

This calculation proves that there are “fossil” energies, which emit a lot of carbon dioxide when used, and what are sometimes called “CO₂-free” energies, which emit less CO₂, although they always emit some amount. Energies that emit few greenhouse gases, once the complete life cycle has been factored in, are nuclear power, hydropower, solar thermal energy, replanted biomass, wind energy – discounted for fluctuations in their supply to the grid, which makes backup electricity from somewhere else necessary – and, in the future, photovoltaic solar energy, once the panels themselves are manufactured with electricity generated by nuclear power, solar energy or hydropower. The big question for these “CO₂-free energies” is whether or not they can supply as much in terms of price and volume as fossil fuel energies, which currently account for 80% of the world's energy supply. Such an occurrence is very unlikely over the next 50 years. During the half-century to come, CO₂ emissions will be cut in half, and most of the “energy transition” will come about largely through conservation and, to a much lesser extent, through substitution of primary energy sources.

* Breeders are reactors that produce more fissile material than they consume. They operate with fuel made of plutonium (fissile) and uranium 238 (non-fissile). They are also called “fast neutron reactors” because the neutrons are not slowed by a “moderator” as in light water reactors.