Renewable energy is usually understood as energy harnessed from continuously occurring natural phenomena. The International Energy Agency defines it as "energy derived from natural processes that are replenished at a faster rate than they are consumed". Solar power, wind power, hydroelectricity, geothermal energy, and biomass are widely agreed to be the main types of renewable energy.[21] Renewable energy often displaces conventional fuels in four areas: electricity generation, hot water/space heating, transportation, and rural (off-grid) energy services.[22] Although almost all forms of renewable energy cause much fewer carbon emissions than fossil fuels, the term is not synonymous with low-carbon energy. Some non-renewable sources of energy, such as nuclear power,[contradictory]generate almost no emissions, while some renewable energy sources can be very carbon-intensive, such as the burning of biomass if it is not offset by planting new plants.[12] Renewable energy is also distinct from sustainable energy, a more abstract concept that seeks to group energy sources based on their overall permanent impact on future generations of humans. For example, biomass is often associated with unsustainable deforestation.[23] As part of the global effort to limit climate change, most countries have committed to net zero greenhouse gas emissions.[24] In practice, this means phasing out fossil fuels and replacing them with low-emissions energy sources.[12] This much needed process, coined as "low-carbon substitutions"[25] in contrast to other transition processes including energy additions, needs to be accelerated multiple times in order to successfully mitigate climate change.[25] At the 2023 United Nations Climate Change Conference, around three-quarters of the world's countries set a goal of tripling renewable energy capacity by 2030.[26] The European Union aims to generate 40% of its electricity from renewables by the same year.[27] Renewable energy is more evenly distributed around the world than fossil fuels, which are concentrated in a limited number of countries.[28] It also brings health benefits by reducing air pollution caused by the burning of fossil fuels. The potential worldwide savings in health care costs have been estimated at trillions of dollars annually.[29] Main article: Variable renewable energy Energy from sunlight or other renewable energy is converted to potential energy for storage in devices such as electric batteries. The stored potential energy is later converted to electricity that is added to the power grid, even when the original energy source is not available. The two most important forms of renewable energy, solar and wind, are intermittent energy sources: they are not available constantly, resulting in lower capacity factors. In contrast, fossil fuel power plants, nuclear power plants and hydropower are usually able to produce precisely the amount of energy an electricity grid requires at a given time. Solar energy can only be captured during the day, and ideally in cloudless conditions. Wind power generation can vary significantly not only day-to-day, but even month-to-month.[30] This poses a challenge when transitioning away from fossil fuels: energy demand will often be higher or lower than what renewables can provide.[31] In the medium-term, this variability may require keeping some gas-fired power plants or other dispatchable generation on standby[32][33] until there is enough energy storage, demand response, grid improvement, or base load power from non-intermittent sources. In the long-term, energy storage is an important way of dealing with intermittency.[34] Using diversified renewable energy sources and smart grids can also help flatten supply and demand.[35] Electrical energy storage is a collection of methods used to store electrical energy. Electrical energy is stored during times when production (especially from intermittent sources such as wind power, tidal power, solar power) exceeds consumption, and returned to the grid when production falls below consumption. Pumped-storage hydroelectricity accounts for more than 85% of all grid power storage.[40] Batteries are increasingly being deployed for storage[41] and grid ancillary services[42] and for domestic storage.[43] Green hydrogen is a more economical means of long-term renewable energy storage, in terms of capital expenditures compared to pumped hydroelectric or batteries.[44][45] Two main renewable energy sources - solar power and wind power - are usually deployed in distributed generation architecture, which offers specific benefits and comes with specific risks.[46] Notable risks are associated with centralisation of 90% of the supply chains in a single country (China) in the photovoltaic sector.[47] Mass-scale installation of photovoltaic power inverters with remote control, security vulnerabilities and backdoors results in cyberattacks that can disable generation from millions of physically decentralised panels, resulting in disappearance of hundreds of gigawatts of installed power from the grid in one moment.[48][49] Similar attacks have targeted wind power farms through vulnerabilities in their remote control and monitoring systems.[50] The European NIS2 directive partially responds to these challenges by extending the scope of cybersecurity regulations to the energy generation market.[51]
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