Every year on Earth we consume more than 24 PWh of electricity: 24 000 000 000 000 000 Wh/year, i.e. the energy that a trillion 24 W energy-saving light bulbs left on for one hour would consume. To understand this, it is as if each of the seven and a half billion human beings who live on the planet kept 15 24 W light bulbs lit throughout the year, day and night. two thirds from fossil fuels, 10.7% from nuclear power and 23.9% from renewable sources. Even if the world averages say little, they range from France, which gets 75% of it from nuclear power plants, to China, which relies on coal for 78%, to Norway which gets 98% of its electricity from falling water. electricity.
However, let's go into the details of the three sources used to produce electricity:
– Among the renewable sources today, the lion's share is made up of hydroelectric turbines (16,6%). Followed by wind (3,7%) and biofuels (2,0%, especially first generation). At the tail comes solar photovoltaic 1,2% while concentrated solar thermal (CSP), geothermal, tidal and wave energy are all within the remaining 0,4% of the total 23,9% in renewables. We still need to invest in research so that the most technologically advanced sources (photovoltaic, biofuel from waste, tides, wind…) take over the less efficient ones that we have been exploiting for hundreds of years.
– A downsizing is expected for nuclear power, mainly for political reasons rather than for serious technical issues connected with plant safety or environmental protection (it is to all intents and purposes a fossil source, but it is among those with the lowest environmental impact among those available).
– To meet the growing demand for electricity, today and in the immediate future, we can therefore rely largely only on fossil fuels. Currently, 40,8%, 21,6% and 4,3% of electricity production derive respectively from coal, gas and oil. Returning to the previous example, four out of ten electric bulbs run on coal, two on gas, half lights up by burning oil, one exploits atom fission, two run on water, while the last half bulb is powered by a mix of all remaining renewable sources.
Basically, 66,7% of electricity produced globally (equal to 15,9 PWh) is obtained from fossil fuels (coal, gas, oil). These sources of energy, in solid, liquid or gaseous forms, are burned and transformed into heat. This evaporates water creating high pressure steam which sets in motion a turbine which is in turn connected to a generator which produces electricity. In the most modern plants, this transformation of the chemical bond energy stored in the fuel into thermal energy, then into mechanical energy and finally into electrical energy has an overall efficiency of about 40%. This means that 60% (or more) of the energy contained is lost to the environment, mainly as heat. The most virtuous systems try to reuse part of this heat to pre-heat the incoming flows or to heat surrounding systems or homes. While coal and fuel oil plants have a yield of around 40%, technological development has allowed combined cycle gas plants to reach yields of up to 55%.
Another important thing to know is that fossil fuels are not pure substances but contain variable quantities of other elements and compounds which are transformed into dust, fumes or gases when burned. The combustion process itself can produce toxic compounds. Coal is a particularly dirty fuel compared to the others already being extracted. Pollution from oil and gas exploration, drilling, and extraction has far less impact on the environment than the surface or underground mining required to extract coal. In addition, the health of extraction workers both deep inside and near the mine is much more at risk than that of oil and gas well technicians.
Furthermore, all fossil fuels burn by oxidizing carbon and producing carbon dioxide. While the various forms of coal produce from 350 to over 400 grams of CO2 per kWh, fuel oil produces emissions between 240 and 260 g/kWh and finally natural gas stops at just 200 g/kWh. The latter is mainly responsible for the greenhouse effect and global warming. The main advantage of using gas instead of coal for the production of electricity lies right here: while the chemical energy contained in coal is in the carbon-carbon chemical bond, in gas this is stored in the carbon-hydrogen bond. To release the energy trapped in those fuels millions of years ago, both carbon - producing CO2 - and hydrogen - producing water vapor H2O must be completely oxidized. For this reason, for the same amount of energy developed, the complete combustion of natural gas produces about half of the CO2 compared to that produced by burning coal. In this way, replacing coal-fired thermoelectric plants with gas-fired ones could save about half of the carbon dioxide emitted into the atmosphere, with clear advantages for global warming.
Nor is it easy to compare the average cost of electricity from different sources. In addition to the variable costs of raw materials, the construction and management costs of more or less advanced, more or less efficient and more or less polluting plants must be considered. A study by Fraunhofer ISE shows that in Germany the cost of one MWh of electricity ranges from 63-80 Euros for coal to 75-98 Euros for combined cycle gas plants. In the United States, the EIA calculates $95/MWh for coal and $75/MWh for gas. But if plants are equipped with carbon dioxide capture and storage systems, the costs rise to $144 and $100/MWh, respectively. Obviously, the less we think about purifying fuels and reducing fumes – and therefore the more polluting the plants are – the less they cost. For this reason, less developed countries and less aware of the risks of global warming tend to prefer coal-fired plants built without much scruples. The consequences were evident, for example, to the Chinese government when for the umpteenth consecutive winter Beijing was invaded by a pall of smog which made it impossible to see within a few meters and rendered the airports unusable. The Chinese Center for Prevention and Control of PM10 has published in the British Medical Journal an estimate of 1,2 million deaths caused by fine particles in just one year.
Bottom line, coal is a dirty fuel. Dirty when you extract it, dirty when you burn it and dirty when you have to deal with the dust it produces. Its only advantage is that it costs less. So what to do? Since it is not possible to replace it directly with green energy sources, on the one hand it is necessary to invest even more in the research and development of renewable sources, on the other to use gas as a "bridge" source towards a low carbon future. As? For example, launching a progressive replacement of coal-fired power plants with gas-fired ones would represent a decisive advantage in the process of decarbonisation and environmental protection envisaged by the Paris Agreement on climate already ratified by 195 countries. Not only. Even in the future, modern gas-fired plants will be able to be used in combination with renewable energies to compensate for the seasonal and daily fluctuations in energy demand with the discontinuity of energy production characteristic of renewable sources.
From the site Eniday.
