Though the press sometimes labels an energy “clean”, this question is not as simple as it may seem. Indeed, to “produce” energy (actually energy can’t be “produced”, but just converted, as physicists know well !) it is not only necessary to have a source of primary energy (this term designates a source that we find “as is” in nature, such as coal, oil, wind, sun rays, a fissile element…), but also to build a device that will transform this primary energy into something useful for us (heat, electricity, movement). Such devices are for exemple a coal fired powerplant, a windmill, a nuclear reactor, etc.
What are the sources of primary energy that we have on our planet ?
- coal: it is a mixture of mineral elements, sulfur, and mostly carbon, and when it burns it produces CO2,
- oil: it is a mixture of molecules that mainly contain carbon and hydrogen. When it burns it also produces CO2, but 25% less than coal for the same amount of energy (because the combustion of hydrogen gives energy but generates only water),
- Natural gas: it is also an hydrocarbon, as oil, but it is the least “carbonated” of all hydrocarbons. Yet it still produces CO2 when it burns (40% to 50% ss than coal for the same energy, but it is not zero !).
- Atomic disintegration (that can be exploited in a nuclear reactor),
- Sun rays),
- Water in motion,
- Geothermal energy, which designates the heat coming from the entrails of earth
- Biomass (plants), that give us compounds basically containing carbon, hydrogen and oxygen. Hence burning biomass produces CO2. Biomass is generally considered to be a renewable resource, because it can be planted again, and it is also considered as “climate neutral”, because the CO2 emitted when the biomass (wood, biofuel, straw…) burns is compensated by the photosynthesis of other plants that grow. Such a statement can prove right or wrong:
- the resource is indeed renewable, in the sense that it can be renewed, but the good question is to know whether it is renewed !
- if the fraction of the biomass which is burnt represents exactely the annual growth of the remaining biomass, the effect on the climate is indeed neutral (we have the absorption on one side that balances the emissions on the other side),
- But when timber is burnt without new wood being planted (which is the case for a large fraction of the wood burnt in the world, corresponding to deforestation in the developping countries) then it stops to be a renewable energy, since it is not renewed ! In such a case it can’t be labelled “CO2 free” either, since its use generates on the contrary additionnal emissions. One might better understand, while reading this, all the difficulties that forests arouse in the international negociations…
Some primary energies are renewable, which means that they get renewed permanently, and others are not (they are then subject to depletion, like oil). All renewable sources are more or less derivates of the existence of the sun (even tides: it is the attraction of the sun that predominates !), except geothermal energy:
- the wind is generated by temperature differences between air masses, that come from the fact that some places are more insolated than others,
- water in motion is a consequence of rain, itself a consequence of evaporation under the effect of the sun,
- biomass is a result of photosynthesis (sun again).
- and even coal, oil and natural gas are renewable….if we can wait several million years ! But as we are burning all these resources in a couple of centuries, for such a time horizon these energies are not renewable, of course. And anyway we also face here a concentrated derivate of past solar energy, very ancient it is true.
Geothermal energy is a consequence of the natural radioactivity of terrestrial rocks (where atomic disintegrations produce heat), and therefore is not induced by the presence of the sun. The annual flux is very low: about the equivalent of the world annual energy consumption (but distributed all over the surface of earth), or 1/10,000th of the energy we receive from the sun. Nevertheless, this flux, being very ancient, has accumulated considerable quantities of heat in the deep layers of the earth’s crust, and this heat stock could be exploited for very long times without a significant depletion (meaning without a significant temperature decrease of the crust as a whole).
Though geothermal energy, if exploited as the low cooling of the earth crust, is not a true renewable, the stock is so huge that it can be considered just “as if”. This way of considering things is also valid for two forms of nuclear energy, that are not renewable but that would draw on such a huge stock that we can consider that it is just “as if”:
- breeding (of which Superphénix in France was an example), that can “burn” all uranium isotopes (the classical line, fission, uses only 0,7% of uranium ; the very minor 235 isotope) or thorium,and the available stocks are such that we could draw for tenths of thousands of years on these elements.
- <lifusion, if we can ever manage to perfect it one day (not before 50 to 100 years say the experts, so we will have to do without for the coming century).
Energy and greenhouse gases
Among all these energy sources that we can draw on, some free greenhouse gases when used and others don’t, or more exactely very little. The primary energies that free greenhouse gases (essentially carbon dioxide – CO2) when used are:
- natural gas,
- wood if not replanted (what happens in most tropical countries)
Among the processes for which the primary energy source does not free any CO2 we will have :
- all the forms of nuclear energy,
- hydroelectricity (though dam reservoirs in the tropics might free a little methane, which is a greenhouse gas, because of the decaying of biological debris in the water),
- sun rays,
- tidal energy,
- geothermal energy,
- biomass if replanted.
What is the life cycle ?
Albeit the primary energy source does not free any CO2 (or any other greenhouse gas), the construction of the device that exploits it might have freed some:
- in order to build a power plant (coal, oil or gas fired, of nuclear, or a dam), it is necessary to produce construction materials (cement, steel, etc) and that generates greenhouse gases emissions, particularly CO2,
- to produce a windmill or a solar panel it is also necessary to use basic materials (aluminium, glass, etc), or more elaborate ones (semi-conductors) that generate greenhouse gases when manufactured.
If we take these “intermediary emissions” into account, we will get all the emissions we have to produce in order to get a given quantity of final energy (final energy is the energy that is available in a form suitable for our needs: refined oil, electricity, gas delivered to our stoven etc). This operation is called a life cycle analysis (LCA in short), and is of current use for any pollutant. We can then see that, opposed to “fossil” fuels that free a lot of CO2 when used, there are energies much less “carbon intensive” (but never totally “clean”) that we designate sometimes, in short, as “CO2 free” energies (chart below).
GHGs emissions (in Kg carbon equivalent emitted by tonne oil equivalent) for various energies.
(1 tonne oil equivalent = 11.600 kWh = 42 billion Joules).
For purely electric forms (hydroelectricity, eolian, nuclear) the indicated values refer to 11.600 kWh of electricity delivered.
The question marks mean that I have a rough idea of the value (corresponding to the height of the bar) but I could not get a precise figure.
For the particular case of electricity, we also have very different values depending on the primary source used.
|Source of production of electricity||Emissions of CO2 in g/kWh (ACV)|
|coal||800 to 1050|
|gas turbines||430 (*)|
|wood||1500 without replantation|
|photovoltaïc||60 to 150 (**)|
|wind generation||3 to 22 (***)|
(*) I copies the chart from the original article, but I should have say “from 400 to 500” : every technology does not emit the same amount of CO2
(**) The CO2 mainly comes from the manufacturing of the solar panel, but also of the battery that stores the electricity at night. Depending on the fact that these panels are manufactured in Denmark (electricity mostly coal produced) or in Switzerland (electricity mostly nuclear and hydro produced), the “CO2 content” of the panel is very different. The paying off is done in 20 to 30 years. But if “CO2 free” energies were used all along the cycle (manufacturing, transport, etc), and with “sober” technologies for the production (like thin layers) we would probably get to much lower values.
(***) same remark as above for the production country.
Source : Jean-Pierre BOURDIER, La Jaune et La Rouge de Mai 2000
In short, the energies that free very little or almost no CO2 for all the life cycle are:
- nuclear and hydro power today,
- Thermal solar today, be it used for producing hot water at home or steam in a power plant (concentration solar),
- Photovoltaïc solar tomorrow, when the panels will be themselves produced with electricity coming from nuclear, solar or hydro electricity). A little calculation on the potential of this energy is available here,
- Wind energy, but it has a very limited potential in regard of our present energy consumption,
- Biomass, but there are some limitations:
- biofuels may present a weak performance regarding greenhouse gases, because producing such fuels require fossil fuels spendings in the first place (gasoline for the tractor ; manufacturing of fertilizers and pesticides), then there are emissions of nitrous protoxyde when the fertilizers are spread, and then there are methane and nitrous oxydes emissions when burning this biofuel that can be superior to those obtained when burning oil.
- biofuels require a lot of available agricultural soils, because their net output per hectare is pretty low,
- using wood for heating is definitely interesting when living close to a forest, but I don’t know how it compares to other energies when it has to be transported over long distances.