2023-12-14 07:45:26
Deprived of energy, a city loses its means of communication, signaling, transport and heating. The progress made during the 20th century was achieved through the use of abundant, highly flexible and relatively cheap energy: oil. However, a transition is necessary. On the one hand, fossil fuel resources, and particularly hydrocarbons, are limited. On the other hand, the impact of their consumption on the environment is becoming increasingly worrying.
An extract of Energy transition – Challenges and perspectives by Alexandre ROJEY
The carbon content of energy can be reduced by substituting natural gas for coal, as well as by increasing the share of nuclear and renewables in the energy mix. But there are still many obstacles. Nuclear power, for example, is faced with the high level of necessary investments and public reluctance in many countries, notably following the Chernobyl and Fukushima accidents. As a result, the share of nuclear power in global electricity supply has declined sharply, from 17.5% in 1996 to just over 10% in 2019. However, the need to increase the share of Carbon-free electricity might lead to its renaissance. In the case of renewable energies, economic profitability often remains insufficient in the absence of subsidies. In addition, their intermittency requires either a modular electricity generation system in back-upor the use of expensive electricity storage devices that generate energy loss.
Different types of renewable energy
The first renewable energy to be exploited was hydraulics, which was used since the Middle Ages to drive water mills. Hydropower has many advantages. It leads to the construction of relatively large installations, of the order of 300 MW on average, mainly used at around 50% of full power. It is easily modular and can even be used to store electricity. The availability of hydroelectric resources nevertheless remains limited. At the global level, its relative share in the supply of electricity has fallen from around 20% around thirty years ago to 16% today.
Biomass has the advantage of being able to easily replace fossil resources, both as fuel and as raw material. Carbon from biomass is considered recycled, as it is captured from the atmosphere. As a result, the combustion of biomass is supposedly carbon neutral. This, however, assumes that the biomass is entirely regenerated, which is not always the case.
Geothermal energy has the advantage of high availability. It is also one of the few renewable energies that can be used on a continuous basis. However, the geothermal heat flow is low. As a result, a power plant operates with a finite amount of stored heat, which is depleted within a few decades, leading to a decline in performance. This stock can only be renewed following a very long time. Geothermal energy can therefore only play an important role in volcanic areas where the geothermal gradient is high, such as in Iceland (where it can exceed 20°C/100 m).
The kinetic power of the wind varies as the cube of its speed. It is approximately 612 W/m2 for a wind speed of 10 m/s. The average wind power available varies between 50 and 400 W/m2, depending on the region. Wind energy production is intermittent and cannot be modulated according to demand. Economic profitability is all the better assured if the wind speed is regular. This condition is generally best achieved at sea or on high ground. Due to the intermittency of production, it is necessary to associate it with an energy storage system or back-up electricity production by a power plant operating with a fossil fuel.
Finally, the energy supplied by solar irradiation in the world varies between 500 and 2,500 kWh/m2 × year, or by a factor of 5. In France, it is between 1,200 and 1,800 kWh/m2 × year, from north to south. This radiant solar energy can then be converted into heat (thermal sensors) or electricity (photovoltaic sensors).
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