Energy transition is a significant structural change in an energy system. Historically, there is a correlation between an increasing demand for energy and availability of different energy sources. The current transition to renewable energy, and perhaps other types of sustainable energy, differs as it is largely driven by a recognition that global carbon emissions must be brought to zero, and since fossil fuels are the largest single source of carbon emissions, the quantity of fossil fuels that can be produced is limited by the COP21 Paris Agreement of 2015 to keep global warming below 1.5 °C. In recent years, the term energy transition has been coined in the framework of a move towards sustainability through increased integration of renewable energy in the realm of daily life.
An example of transition toward sustainable energy is the shift by Germany (Energiewende) and Switzerland, to decentralized renewable energy, and energy efficiency. Although so far these shifts have been replacing nuclear energy, their declared goal was the coal phase-out, reducing non-renewable energy sources and the creation of an energy system based on 60% renewable energy by 2050. As of 2018, the 2030 coalition goals are to achieve 65% renewables in electricity production until 2030 in Germany. Another such example is the drive to transition from internal combustion engine powered vehicles to electric vehicles as a way to reduce the global reliance on fossil fuels and reduce greenhouse gas emissions. This transition in particular however has begun to stimulate debate considering it requires a tenfold increase in mineral extraction and therefore will lead to an increase of the mining processes themselves and of the associated environmental and societal impacts. A potential solution that has arisen for this energy transition dilemma is to explore collection of minerals from new sources like polymetallic nodules lying on the seabed. Ongoing research is exploring this as a way to facilitate the energy transition in a more sustainable manner.
An "energy transition" designates a significant change for an energy system that could be related to one or a combination of resource use, system structure, scale, economics, end use behaviour and energy policy. An 'energy transition' is usefully defined as a change in the state of an energy system as opposed to a change in an individual energy technology or fuel source. A prime example is the change from a pre-industrial system relying on traditional biomass and other renewable power sources (wind, water, and muscle power) to an industrial system characterized by pervasive mechanization (steam power) and the use of coal. Market shares reaching pre-specified thresholds are typically used to characterize the speed of transition (e.g. coal versus traditional biomass) and typical market share thresholds in the literature are 1%, 10% for the initial shares and 50%, 90% and 99% for outcome shares following a transition.
However, since the adoption of the COP21 Paris Agreement in 2015, the Energy Transition to net zero greenhouse gas emissions is defined as the downshift of fossil fuel production to stay within the carbon emissions budget to limit global warming to 1.5 °C. The term "Net Zero" includes recognition that some atmospheric CO2 is sequestered in the growth of plants and animals, and that this natural sequestration could be enhanced through soil conservation, reforestation and protection of peatland, wetland and marine environments.
The term 'energy transition' could also encompasses a reorientation of policy and this is often the case in public debate about energy policy. For example, this could imply a rebalance of demand to supply and a shift from centralized to distributed generation (for example, producing heat and power in very small cogeneration units), which should replace overproduction and avoidable energy consumption with energy-saving measures and increased efficiency. In a broader sense the energy transition could also entail a democratization of energy or a move towards increased sustainability.
Public and academic debates on ‘energy transition’ and its policy implications increasingly take Co-benefits of climate change mitigation into account. Co-benefits describe the positive side-effects that occur from an energy transition and can be defined as: “simultaneously meeting several interests or objectives resulting from a political intervention, private sector investment or a mix thereof. Opportunistic co-benefits appear as auxiliary or side effect while focusing on a central objective or interest. Strategic co-benefits result from a deliberate effort to seizing several opportunities (e.g., economic, business, social, environmental) with a single purposeful intervention.”  Especially the deployment of renewable energies can have positive socio-economic effects on employment, industrial development, health and energy access. Depending on the country and the deployment scenario, replacing coal power plants with renewable energy can more than double the number of jobs per average MW capacity. In non-electrified rural areas, the deployment of solar mini-grids can significantly improve electricity access. Additionally, the replacement of coal-based energy with renewables can lower the number of premature deaths caused by air pollution and reduce health costs.
Historic approaches to past energy transitions are shaped by two main discourses. One argues that humankind experienced several energy transitions in its past, while the other suggests the term "energy additions" as better reflecting the changes in global energy supply in the last three centuries.
The chronologically first discourse was most broadly described by Vaclav Smil. It underlines the change in the energy mix of countries and the global economy. By looking at data in percentages of the primary energy source used in a given context, it paints a picture of the world's energy systems as having changed significantly over time, going from biomass to coal, to oil, and now a mix of mostly coal, oil and natural gas. Until the 1950s, the economic mechanism behind energy systems was local rather than global.
The second discourse was most broadly described by Jean-Baptiste Fressoz. It emphasises that the term "energy transition" was first used by politicians, not historians, to describe a goal to achieve in the future – not as a concept to analyse past trends. When looking at the sheer amount of energy being used by humankind, the picture is one of an ever-increasing energy consumption that is met by an ever-increasing consumption of all the main energy sources available to humankind. For instance, the increased use of coal in the 19th century indeed did not replace wood consumption, but came on top of increased wood consumption. Another example is the deployment of passenger cars in the 20th century. This evolution triggered an increase in both oil consumption (to drive the car) and coal consumption (to make the steel needed for the car). In other words, according to this approach, humankind never performed a single energy transition in its history but performed several energy additions.
Contemporary energy transitions differ in terms of motivation and objectives, drivers and governance. As development progressed, different national systems became more and more integrated becoming the large, international systems seen today. Historical changes of energy systems have been extensively studied. While historical energy changes were generally protracted affairs, unfolding over many decades, this does not necessarily hold true for the present energy transition, which is unfolding under very different policy and technological conditions.
For current energy systems, many lessons can be learned from history. The need for large amounts of firewood in early industrial processes in combination with prohibitive costs for overland transportation led to a scarcity of accessible (e.g. affordable) wood and it has been found that eighteenth century glass-works “operated like a forest clearing enterprise. When Britain had to resort to coal after largely having run out of wood, the resulting fuel crisis triggered a chain of events that two centuries later culminated in the Industrial Revolution. Similarly, increased use of peat and coal was vital elements paving the way for the Dutch Golden Age roughly spanning the entire 17th century. Another example where resource depletion triggered technological innovation and a shift to new energy sources in 19th Century whaling and how whale oil eventually became replaced by kerosene and other petroleum-derived products. With the success of a rapid energy transition it is also conceivable that there will be government buyouts or bailouts of coal mining regions.
The term "energy transition" has had a moving definition of the few decades of its life. It was first coined by US politicians and media after the 1973 first oil shock. It was popularised by US President Jimmy Carter in his 18 April 1977 televised speech from the Oval Office, calling to "look back into history to understand our energy problem. Twice in the last several hundred years, there has been a transition in the way people use energy ... Because we are now running out of gas and oil, we must prepare quickly for a third change--to strict conservation and to the renewed use of coal and to permanent renewable energy sources like solar power." As historian Duccio Basosi underlines, the term was later globalised after the 1979 second oil shock, during the 1981 United Nations in Nairobi, on new and renewable sources of energy.
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