By Harald Desing
Energy is conserved. Counter to common language, energy can neither be "produced" nor "consumed", but only transformed. However, there are more and less useful forms of energy: electricity, for example, is extremely versatile and can be transformed into any other form of energy with close to 100% efficiency. In contrast, low temperature heat cannot do much work anymore; it is the energy "waste" with no work potential. The work potential is called exergy: electricity has 100% exergy content, whereas heat at the same temperature than the surroundings has 0%. Society is driven by useful work provided through different energy resources. So, different energy forms should be compared with the useful work they are able to do.
Commonly, this is not the case. Energy statistics, such as the IEA or from national statistic offices, do compare apples with oranges: "Primary" energy is accounted on different levels: calorific energy content of fuels (that is the heat potential that can be generated by burning the fuel) alongside with solar electricity and geothermal heat at different temperature levels. All of them have different work potentials, and they are later used as different forms of energy: heat for buildings and industry, electricity, or motion for mobility.
When we defossilize the energy system, most of the fossil energy applications, which are not yet electricity, have to be replaced by renewable electricity. For example, instead of internal combustion engines in cars and trucks, we need battery electric vehicles and electric trains; gas boilers can be replaced by heat pumps powered by electricity; and high temperature heat for industry with either direct electric heating (such as an electric arc furnace) or hydrogen produced with renewable electricity (such for hydrogen-reduced steel). Low temperature heat could also be provided by solar thermal collectors, with a similar efficiency than converting to solar electricity first and to heat with heat pumps later. High temperature heat could be provided by concentrated solar systems, however, this is currently not very practical for most applications in industry. In particular cases, it may make sense to provide heat directly from solar, biomass or geothermal. However generally, electricity is the universal and versatile intermediary form of energy for all sectors.
We do not have to replace "primary" fossil energy, but only the useful work they provide to society. Fictively converting all primary energy to electric energy equivalents using state-of-the-art conversion technologies, provides a more reasonable estimate for what really needs to be replaced. It reduces the energy supply to society from almost 19 terawatt (TW) in 2019—as counted by IEA as "primary" energy—to 7.3TW. Renewable energies already provide 15% of this, so "only" 6.3TW needs to be replaced during the energy transition with renewable electricity.
Sometimes, this reduction is labeled "gigantic efficiency improvements" when switching to RE systems, but actually it is merely counting energies on the basis of usefulness to society. The efficiency of the subsequent energy services remain the same. The efficiency of the energy provisioning system, in contrast, could be measured by tracing energy conversions all the way back to their origin. For most energy forms, this is our sun. Hydropower is nothing but converted sunlight: sunshine on oceans evaporates water, forms clouds and generates winds that carry vapor over land where it falls as rain. The height difference of the runoff back to the oceans is what can be used as hydropower. This description makes it clear already that from the original solar energy, only a tiny fraction can be converted to hydroelectricity. The same applies to wind and biomass. All of them are much less efficient than direct solar energy conversion. Fossil fuels are also nothing but (ancient) sunlight. They had been slowly built over many million years by buried biomass; now we burn them at a rate more than ten thousand times faster than they were built. The solar energy that created coal, oil and gas deposits is again much more than solar energy stored in recent biomass, reducing the conversion efficiency from sunlight to useful work even further. Nuclear, geothermal and tidal energies do not originate from our sun. They originate from exploding stars and need to be traced back all the way to the big bang. The energy from our sun can be traced back to the big bang too, which would be truly primary energy: all the useful work at our disposal originates from there.
Due to all the additional conversion steps for other energy forms, direct solar energy conversion is the most efficient way to provide useful work to society. And electric energy is the embodiment of useful work (100% exergy), which is why it is ideal for comparisons and modelling substitutions among different energy provisioning systems.
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