Alternative energy sources to fossil fuels are numerous, but have their drawbacks and limitations. Hydropower has been used on a large scale for over a hundred years, but the installation of new capacity is becoming more limited because the best sites are already developed and environmental and socio-economic considerations must increasingly be taken into account before flooding large areas for reservoirs. Energy from geothermal wells can play an important role on a local scale for some countries, but resources are limited on the global scale. Solar energy, photovoltaic and thermal is still too expensive and intermittent. Suitable energy storage or supplementary other energy sources are needed as, there is only limited solar energy production under cloudy conditions, and none during the night. Wind power is also reliant on the intermittent power and speed of winds, but is much cheaper than present solar energy and is therefore of great potential. Biomass can provide a significant but nevertheless limited amount of energy that is inadequate to sustain our modern society's needs. Ocean power in the form of tides, waves and thermal energy is unlikely to represent a significant share of the global energy production in the foreseeable future. Taken together, these renewable energy sources must - and will - certainly play an increasing role in our future global energy mix. However, they will be unable to replace by themselves the energy obtained from non-renewable fossil fuels. With decreasing petroleum and gas reserves, we could rely for some time more heavily on the larger resources of coal. This, nevertheless is only a temporary solution until coal production eventually also starts to decline. It would also imply much larger air pollutant and greenhouse gases emissions, with severe health consequences and contributions to global climate change. By considering environmental, energy security and long-term stability viewpoints, it is clear that nuclear power - albeit made even safer and with problems of radioactive byproduct reprocessing and storage solved - is the major energy source of choice based on current knowledge. Indeed, for the foreseeable future it may produce the vast and increasing amounts of energy needed by humanity. Advanced breeder reactors and new fusion technologies could provide our energy needs for centuries or millennia to come. Eventually, it may be possible to find more efficient, as-yet unknown, ways of using the Sun's energy, but this is for future generations.
Nuclear reactions should not be viewed as purely human inventions. Rather, they are naturally occurring events throughout the Universe, and humankind has only relatively recently succeeded in controlling and harnessing the energy of the atom. Without thermonuclear reactions in the Sun and nuclear decay inside our own Earth, there would be no renewable energies such as solar, wind, biomass, geothermal or hydro energy, or even fossil fuels on our planet. Once we produce energy, it still must be stored, transported and provided in suitable form for subsequent use. This is a major unresolved challenge. We must also identify new solutions to provide convenient hydrocarbon-based fuels for transportation and household needs, as well as the variety of products and materials derived from renewable and sustainable sources. These points are discussed in the following chapters.
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