Renewable energy is a term for any useable energy that is harnessed from natural resources that are either essentially inexhaustible (such as sunlight, or thermal energy generated and stored in the Earth) or naturally replenished in a timely manner on a human timescale (such as energy derived from wood). In contrast, non-renewable energy refers to energy derived from resources that are limited in amount and cannot be replenished in a a timely manner or feasibly and are essentially irreplaceable once extracted (such as energy derived from fossil fuels). The term renewable energy has not only been applied to the energy and forms of energy harnessed but also to the sources and technologies associated with that usable energy. Examples of renewable energy technologies include methods to harness water power, sunlight, thermal energy, or biomass for the generation of electricity.
The term renewable energy is often used interchangeable with alternative energy and green energy. While by most definitions there is substantial overlap between energy forms, sources, and technologies that fit into these three categories, the three terms also have been delineated differently. The term alternative energy generally references any nontraditional energy form, source, or technology differing from the current popular forms, sources, or technologies. Before natural gas gained popularity, this energy source could be classified under the category of alternative energy; however, it does not fit under renewable energy. Green energy references that subset of renewable energy that involves the least environmental harm.
The movement toward renewable energy reflects not just an awareness of the need to prepare for a future when fossil fuels are no longer in plentiful supply, but also the importance of balancing economic development with protection of the environment for future generations.
There are a multitude of definitions used for renewable energy. The various conceptions allow debate regarding what is included in this category, whether energy from peat, large hydropower plants, or biomass are counted as renewable energies, and whether one includes nuclear power.
A commonly cited definition is that provided by the International Energy Agency (IEA 2008):
"Renewable energy is derived from natural processes that are replenished constantly. In its various forms, it derives directly or indirectly from the sun, or from heat generated deep within the earth. Included in the definition is energy generated from solar, wind, biomass, geothermal, hydropower and ocean resources, and biofuels and hydrogen derived from renewable resources."
The IEA website references a similar but differently worded version: "Energy derived from natural processes (e.g. sunlight and wind) that are replenished at a faster rate than they are consumed. Solar, wind, geothermal, hydro, and some forms of biomass are common sources of renewable energy" (IEA 2019).
A definition used by the European Union utilizes this conception: "Renewable energy sources are defined as renewable non-fossil energy sources: wind, solar, geothermal, wave, tidal, hydropower, biomass, landfill gas, sewage treatment plant gas and biogases" (Gritsevskyi 2008).
The German Renewable Energy Act (Erneuerbare-Energien-Gesetz or EEG) delineates what fits into the renewable energy category as "hydropower including wave power, tidal power, salt gradient and flow energy, wind energy, solar radiation, geothermal energy, energy from biomass including biogas, landfill gas and sewage treatment plant gas, as well as the biodegradable fraction of municipal and industrial waste." Not included within this definition is biomass from non-biodegradable plants nor large hydropower plants (Jordan-Korte 2011).
Michael Hoexter offers this analysis: "Non-renewable energy sources are energy stores with zero or a minute rate of replenishment relative to its depletion by human beings" and "renewable energy sources are types of natural energy flux useful for human ends regularly occurring on or near Earth's surface and, additionally, useful natural energy stores that are replenished by natural flux within the time frame of conceivable human use." Hoexter further "all known renewable energy sources originate in, or are close derivatives of, electromagnetic radiation of our Sun, the Earth's and Moon's gravitational fields, and heat radiating from Earth's interior (Gritsevskyi 2008).
Whether a particular resources serves as a source of renewable energy is often a point of contention. Some countries, such as Sweden and Finland, label peat as a renewable source of energy, while the World Energy Council (WEC) suggests to not consider peat a renewable source of energy. (Gritsevskyi 2008). Peat has a notably slow and low renewable rate. Some US and UK politicians have termed nuclear power a renewable energy (Jordan-Koret 2011), and physicist Bernard Cohen has stated that uranium is "practically inexhaustible" and can be considered a renewable course of energy, with naturally-replenished uranium extracted from seawater able to supply energy as long as the sun's expected lifespan (Gritsevskyi 2008). Other have advocated against inclusion of nuclear energy ((Jordan-Koret 2011) and some has even questioned geothermal since it may lead to partial depletion at some locales (Gritsevskyi 2008).
The term renewable energy—forms, technologies, and sources—is distinct from the term renewable natural resource in that this later term generally also includes resources from which useable energy is not obtained, such as fish stocks or wildlife (and non-renewable resources include minerals, such as gold, silver, diamonds, and copper). An energy source, according to IEA/Eurostat definition is "the kinetic (e.g. hydro, wind), thermal (eg. nuclear, geothermal), or combustible fuel used as the input to generate electricity or heat" (Gritsevskyi 2008). Energy inside a system is transformed from one energy form to another (thermal to electricity), and an energy technology is that which transforms, transports, or stores the energy form.
In 2010, renewable energy provided an estimated 16.7% of global final energy consumption, according to the Renewable Energy Policy Network for the 21st Century (REN21). About 8.5% of this came from traditional biomass and about 8.2% came from modern renewable energy (hydropower, solar, wind, geothermal, biofuels, modern biomass). Hydropower accounted for about 3.3% of global final energy consumption in 2010 (REN21 2012). Bloomberg (2011) reported that renewable sources, including large hydro, had a 12.6% share of total primary energy production in 2010.
At the national level, by 2013 at least 30 nations around the world had renewable energy contributing more than 20% of energy supply. National renewable energy markets are projected to continue to grow strongly in the coming decade and beyond (REN21 2013). Wind power, for example, is growing at the rate of 30% annually, with a worldwide installed capacity of 282,482 megawatts (MW) at the end of 2012.
In the United States, renewable energy sources were used in 2018 to provide abut 17% of total U.S. utility-scale electricity generation, mostly from hydroelectic power and wind, with smaller amounts from biomass, solar, and geothermal (EIA 2019).
Renewable energy resources and significant opportunities for energy efficiency exist over wide geographical areas, in contrast to other energy sources, which are concentrated in a limited number of countries. Renewable energy replaces conventional fuels in four distinct areas: electricity generation, hot water/space heating, motor fuels, and rural (off-grid) energy services.
Prior to the development of coal in the mid-19th century, nearly all energy used was renewable.
Almost without a doubt, the oldest known use of renewable energy was in the form of using wood (traditional biomass) to fuel fires. The discovery of how to make fire for the purpose of burning wood is regarded as one of humanity's most important advances. The use of wood as a fuel source for heating is much older than civilization and is assumed to have been used by Neanderthals. Although use of wood for fires has been dated to some 790,000 years ago, with evidence for controlled fire found at a Lower Paleolithic site in Israel, it possibly did not become commonplace until many hundreds of thousands of years later, sometime between 200,000 and 400,000 years ago (Hirst 2013; Roebroeks and Villa 2011; Twomey 2013). Historically, it was limited in use only by the distribution of technology required to make a spark. Wood heat is still common throughout much of the world.
Probably the second oldest usage of renewable energy is harnessing the wind in order to drive ships over water. This practice can be traced back some 7000 years, to ships on the Nile.
Overuse of forests lead to wood not being used in a renewable manner and this helped lead to the prominence of coal beginning in the late medieval period, moving humanity toward using this non-renewable energy source. Historian Norman F. Cantor describes how coal was then used as an alternative fuel to save the society from overuse of the dominant fuel, wood (Cantor 1993):
By 1873, concerns of running out of coal prompted experiments with using solar energy. Development of solar engines continued until the outbreak of World War I. The importance of solar energy was recognized in a 1911 Scientific American article: "in the far distant future, natural fuels having been exhausted, [solar power] will remain as the only means of existence of the human race" (Jones and Bouamane 2012).
In the early 19th century, whale oil was the dominant form of lubrication and fuel for lamps in the early 19th century, but the depletion of the whale stocks by mid century caused whale oil prices to skyrocket, setting the stage for the adoption of the non-renewable resource, petroleum.
The foundation for alcohol to serve as an alternative to fossil fuels was laid in 1917, when Alexander Graham Bell advocated ethanol from corn, wheat, and other foods as an alternative to coal and oil, stating that the world was in measurable distance of depleting these fuels. Bell (1917) wrote: "In relation to coal and oil, the world's annual consumption has become so enormous that we are now actually within measurable distance of the end of the supply. What shall we do when we have no more coal or oil! .... There is, however, one other source of fuel supply which may perhaps this problem of the future. Alcohol makes a beautiful, clean and efficient fuel, and where not intended for human consumption can be manufactured very cheaply from corn stalks and in fact from almost any vegetable matter capable of fermentation.
Since the 1970s, Brazil has had an ethanol fuel program, which has allowed the country to become the world's second largest producer of ethanol (after the United States) and the world's largest exporter.
In the 1970s environmentalists promoted the development of renewable energy both as a replacement for the eventual depletion of oil, as well as for an escape from dependence on oil, and the first electricity generating wind turbines appeared.
The categorization into renewable energy sources versus nonrenewable probably came into general use around 1973 to 1975, relative to work on sustainability and energy security issues (Gritsevskyi 2008).
In general terms, geothermal energy is thermal energy (the energy that determines the temperature of matter) generated and stored in the Earth. The geothermal energy of the Earth's crust originates from the original formation of the planet and from radioactive decay of minerals, resulting in continual production of geothermal energy below the earth's surface. The geothermal gradient, which is the difference in temperature between the core of the planet and its surface, drives a continuous conduction of thermal energy in the form of heat from the core to the surface.
In terms of renewable energy, geothermal energy is the energy that is harnessed from the Earth's internal heat and used for practical purposes, such as heating buildings or generating electricity. It also refers to the technology for converting geothermal energy into useable energy. The term geothermal power is used synonymously as the conversion of the Earth's internal heat into a useful form of energy, or more specifically as the generation of electricity from this thermal energy (geothermal electricity).
The four basic means for capturing geothermal energy for practical use are geothermal power plants (dry steam, flash steam, binary cycle), geothermal heat pumps, direct use, and enhanced geothermal systems.
Geothermal provides a huge, reliable, renewable resource, unaffected by changing weather conditions. It reduces reliance on fossil fuels and their inherent price unpredictability, and when managed with sensitivity to the site capacity, it is sustainable. Furthermore, technological advances have dramatically expanded the range and size of viable resources.
However, geothermal also faces challenges in the need for significant capital investment, and a significant amount of time in terms of building geothermal plants. There are limitations in terms of placement of geothermal plants in regions with accessible deposits of high temperature ground water, and construction of power plants can adversely affect land stability. Geothermal power plants also can lead to undesirable emissions, with power plant emitting low levels of carbon dioxide, nitric oxide, sulfur, methane, and hot water from geothermal sources may hold in solution trace amount of toxic elements, such as mercury, boron, and arsenic.
From hot springs, geothermal energy has been used for bathing since Paleolithic times and for space heating since ancient Roman times, but it is now better known for electricity generation.
Generally speaking, wind energy is the form of energy created by wind. In terms of alternative energy, wind energy refers to the energy that is harnessed from wind for practical purposes. The term wind power is used synonymously as the conversion of wind energy into a useful form of energy, or more specifically as the generation of electricity from the wind. Among ways in which wind energy can be harnessed are wind turbines to make electrical power, windmills for mechanical power, windpumps for water pumping or drainage, or sails to propel ships.
Large wind farms consist of hundreds of individual wind turbines that are connected to the electric power transmission network. For new constructions, onshore wind is an relative inexpensive source of electricity, while small onshore wind farms provide electricity to isolated locations. Modern utility-scale wind turbines range from around 600 kW to 5 MW of rated power, although turbines with rated output of 1.5–3 MW have become the most common for commercial use; the power available from the wind is a function of the cube of the wind speed, so as wind speed increases, power output increases dramatically up to the maximum output for the particular turbine. Areas where winds are stronger and more constant, such as offshore and high altitude sites, are preferred locations for wind farms. As offshore wind speeds average ~90% greater than that of land, offshore resources can contribute substantially more energy than land stationed turbines.
Wind power offers a number of benefits as an alternative to fossil fuels. It is plentiful, renewable, widely distributed, clean, produces no greenhouse gas emissions during operation, and uses little land. The effects on the environment are generally less problematic than those from other power sources. Costs are relatively low (Daniels 2015).
The main disadvantage of wind power is the fact that wind is unpredictable, inconsistent, and unsteady, as well as the concern that the full costs of harnessing wind power are not cheap and thus rely on government subsidies to be set up and be competitive. There also are aesthetic concerns, with wind farms being considered by some to be an eyesore, whether restricting the normally picturesque view offshore or in rural areas. Furthermore, there are complaints of noise from turbines, and some communities have been required to shut off their turbines during certain times because of the noise. Older type wind farms have turbines that spin at high speeds and can thus kill wild birds and bats, although this design has changed so newer wind farms largely avoid such a problem (Daniels 2015).
As of 2011, Denmark is generating more than a quarter of its electricity from wind and 83 countries around the world are using wind power to supply the electricity grid (REN21 2011). In 2010 wind energy production was over 2.5% of total worldwide electricity usage, and growing rapidly at more than 25% per annum. The long-term technical potential of wind energy is believed to be five times total current global energy production, or 40 times current electricity demand, assuming all practical barriers are overcome. This would require wind turbines to be installed over large areas, particularly in areas of higher wind resources, such as offshore.
The energy of falling water and running water can be utilized to provide water power or hydropower—the form of renewable energy derived from the gravitational force of falling or flowing water harnessed for useful purposes. Since water is about 800 times denser than air, even a slow flowing stream of water, or moderate sea swell, can yield considerable amounts of energy. Since ancient times, hydropower has been used for irrigation and the operation of various mechanical devices, such as watermills, sawmills, textile mills, dock cranes, domestic lifts, and power houses.
Since the early 20th century, the term hydropower has been used almost exclusively in conjunction with the modern development of hydroelectric power, which allowed use of distant energy sources. Hydroelectricity is the term referring to electricity generated by hydropower; the production of electrical power through the use of the gravitational force of falling or flowing water.
There are many forms of water power:
Hydroelectricity today is the most widely used form of renewable energy (unless all biomass categories, such as wood and biofuels, are lumped together), accounting for 16 percent of global electricity generation—3,427 terawatt-hours of electricity production in 2010. Hydropower is produced in at least 150 countries, with five countries (China, Brazil, United States, Canada, and Russia) accounting for about 52 percent of the world’s installed hydropower capacity in 2010 (Worldwatch 2013). The Asia-Pacific region generated 32 percent of global hydropower in 2010, with China being the largest hydropower producer, producing 721 terawatt-hours in 2010 and having the highest installed hydropower capacity, with 213 gigawatts (GW) at the end of 2010. The Three Gorges Dam, spanning China's Yangtze River, is the world's largest hydroelectric power station in terms of installed capacity, followed in second place by the Itaipu Dam in Brazil/Paraguay. The Three Gorges Dam is operated jointly with the much smaller Gezhouba Dam.
The cost of hydroelectricity is relatively low, making it a competitive source of renewable electricity. The average cost of electricity from a hydro plant larger than 10 megawatts is 3 to 5 U.S. cents per kilowatt-hour (Worldwatch 2013). Hydro is also a flexible source of electricity since plants can be ramped up and down very quickly to adapt to changing energy demands. Once a hydroelectric complex is constructed, the project produces no direct waste, and has a considerably lower output level of the greenhouse gas carbon dioxide (CO2) than fossil fuel powered energy plants (REN21 2011). However, damming interrupts the flow of rivers and can harm local ecosystems, and building large dams and reservoirs often involves displacing people and wildlife. Given such concerns, in some nations building new dams on major rivers to capture hydroelectric energy meets a lot of resistance and further expansion of hydropower in the United States is unlikely. On the other hand, China's Three Gorges Dam became fully functional in just 2012.
Broadly speaking, solar energy is energy from the Sun. In terms of renewable energy, solar energy refers to the energy that is harnessed from solar radiation, using the radiant light and heat from the Sun for practical purposes. This energy is harnessed using a range of ever-evolving technologies such as solar photovoltaics, solar heating, solar thermal electricity, solar architecture, and artificial photosynthesis (IEA 2011; RSC 2014). Solar energy can be harnessed at different levels around the world, mostly depending on distance from the equator.
The term solar power either is used synonymously with solar energy or is used more specifically to refer to the conversion of sunlight into electricity. The conversion into electricity is done either directly using photovoltaics (PV), or indirectly using concentrated solar power (CSP). Concentrated solar power systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. Commercial concentrated solar power plants were first developed in the 1980s. Photovoltaics convert light into electric current using the photoelectric effect. Photovoltaics are an important and relatively inexpensive source of electrical energy where grid power is inconvenient, unreasonably expensive to connect, or simply unavailable. However, as the cost of solar electricity is falling, solar power is also increasingly being used even in grid-connected situations as a way to feed low-carbon energy into the grid.
Solar technologies are broadly characterized as either passive solar or active solar depending on the way they capture, convert and distribute solar energy. Active solar techniques include the use of photovoltaic panels and solar thermal collectors to harness the energy. Passive solar techniques include orienting a building to the Sun, selecting materials with favorable thermal mass or light dispersing properties, and designing spaces that naturally circulate air. Other broad classifications are direct solar, which generally refers to technologies or effects that involve a single-step conversion of sunlight that results in a usable form of energy, and indirect solar, which generally refers to technologies or effects that involve multiple-step transformations of sunlight.
Benefits of solar energy system include the huge potential in terms of energy hitting the earth, the low environmental impact, and the lack of producing carbon dioxide and air pollutants. Limitations preventing the large scale implementation of solar powered energy generation is the inefficiency of current solar technology and the cost. In addition, the amount of sunlight varies depending on weather conditions, location, time of day, and time of year, and the need for a large surface to collect the energy, since it does not deliver concentrated energy at any one place.
Biomass refers to biological material derived from living or recently living organisms. It most often refers to plants or plant-derived materials, which are specifically called lignocellulosic biomass. As an energy source, biomass can either be used directly via combustion to produce heat, or indirectly after converting it to various forms of biofuel. Conversion of biomass to biofuel can be achieved by different methods, which are broadly classified into: thermal, chemical, and biochemical methods. This biomass conversion can result in fuel in solid, liquid, or gas form.
Biofuel, wood, and waste are the three main categories of the use of biomass as an energy source.
Wood remains the largest biomass energy source today (Scheck and Dugan 2012); examples include forest residues (such as dead trees, branches, and tree stumps), yard clippings, wood chips, and even municipal solid waste. Industrial biomass can be grown from numerous types of plants, including miscanthus, switchgrass, hemp, corn, poplar, willow, sorghum, sugarcane, bamboo, and a variety of tree species, ranging from eucalyptus to oil palm (palm oil).
Plant energy is produced by crops specifically grown for use as fuel that offer high biomass output per hectare with low input energy. Some examples of these plants are wheat and straw. The grain can be used for liquid transportation fuels, while the straw can be burned to produce heat or electricity. Plant biomass can also be degraded from cellulose to glucose through a series of chemical treatments, and the resulting sugar can then be used as a first generation biofuel.
Biomass can be converted to other usable forms of energy like methane gas or transportation fuels like ethanol and biodiesel. Rotting garbage, and agricultural and human waste, all release methane gas—also called "landfill gas" or "biogas." Crops, such as corn and sugar cane, can be fermented to produce the transportation fuel, ethanol. Biodiesel, another transportation fuel, can be produced from left-over food products like vegetable oils and animal fats. Also, biomass to liquids (BTLs) and cellulosic ethanol are still under research.
There is a great deal of research involving algal, or algae-derived, biomass due to the fact that it’s a non-food resource and can be produced at rates 5 to 10 times those of other types of land-based agriculture, such as corn and soy. Once harvested, it can be fermented to produce biofuels such as ethanol, butanol, and methane, as well as biodiesel and hydrogen.
The biomass used for electricity generation varies by region. Forest by-products, such as wood residues, are common in the United States. Agricultural waste is common in Mauritius (sugar cane residue) and Southeast Asia (rice husks). Animal husbandry residues, such as poultry litter, are common in the UK.
The generation of renewable energy from biomass on the scale needed to replace fossil energy would present serious environmental challenges. For example, biomass energy generation would have to increase 7-fold to supply current primary energy demand, and up to 40-fold by 2100 given economic and energy growth projections (Huesemann and Huesemann 2011). Humans already appropriate 30 to 40% of all photosynthetically fixed carbon worldwide, indicating that expansion of additional biomass harvesting is likely to stress ecosystems, in some cases precipitating collapse and extinction of animal species that have been deprived of vital food sources (Rojstaczer et al. 2001; Vitousek et al. 1986). The total amount of energy capture by vegetation in the United States each year is around 58 quads (61.5 EJ), about half of which is already harvested as agricultural crops and forest products. The remaining biomass is needed to maintain ecosystem functions and diversity (Pimentel et al. 1994). Since annual energy use in the United States is ca. 100 quads, biomass energy could supply only a very small fraction. To supply the current worldwide energy demand solely with biomass would require more than 10% of the Earth’s land surface, which is comparable to the area use for all of world agriculture (i.e., ca. 1500 million hectares), indicating that further expansion of biomass energy generation will be difficult without precipitating an ethical conflict, given current world hunger statistics, over growing plants for biofuel versus food (Hoffert et al. 2002; Nakicenovic et al. 1998). Of course, developing productive means to produce energy from the parts of vegetation not utilized in food production, such as discarded, inedible cellulose components, would help address some of these difficulties.
Biofuel is one of the three main categories of the use of biomass as an energy source (the others being wood and waste).
A biofuel is a solid, liquid, or gaseous fuel made from biomass. In other words, these fuels contain energy from geologically recent carbon fixation of living or recently living organisms. As noted by Wilkie (2013), "Any combustible fuel derived from recent (non-fossil) living matter (biomass) may be considered a biofuel, including ethanol derived from plant products, biodiesel from plant or animal oils, as well as, biogas from biomass.
Liquid biofuels include bioalcohols, such as bioethanol, and oils, such as biodiesel. Gaseous biofuels include biogas, landfill gas, and synthetic gas.
Bioalcohols. Biologically produced alcohols, most commonly ethanol, and less commonly propanol and butanol, are produced by the action of microorganisms and enzymes through the fermentation of sugars or starches (easiest), or cellulose (which is more difficult). Alcohol fuels are produced by fermentation of sugars derived from wheat, corn, sugar beets, sugar cane, molasses, and any sugar or starch from which alcoholic beverages can be made (such as potato and fruit waste, etc.). These alcohols are also produced by chemical means. When obtained from biological materials and/or biological processes, they are known as bioalcohols (e.g. "bioethanol"). There is no chemical difference between biologically produced and chemically produced alcohols.
Ethanol fuel, or bioethanol, is the most common biofuel worldwide, particularly in Brazil, but also in the United States and elsewhere. Ethanol can be used as a fuel for vehicles in its pure form, but it is usually used as a gasoline additive to increase octane and improve vehicle emissions. With advanced technology being developed, cellulosic biomass, such as trees and grasses, are also used as feedstocks for ethanol production. Cellulosic biofuels, such as cellulosic ethanol, began to be produced in commercial-scale plants in 2013. Most methanol (the simplest alcohol) is produced from natural gas, a nonrenewable fossil fuel, and modern methanol also is produced in a catalytic industrial process directly from carbon monoxide, carbon dioxide, and hydrogen. However, methanol also can be produced from biomass (as biomethanol) using very similar chemical processes. Butanol can be produced from biomass (as "biobutanol")as well as fossil fuels (as "petrobutanol"); but biobutanol and petrobutanol have the same chemical properties.
Biodiesel. Biodiesel is made from vegetable oils, animal fats or recycled greases. Biodiesel can be used as a fuel for vehicles in its pure form, but it is usually used as a diesel additive to reduce levels of particulates, carbon monoxide, and hydrocarbons from diesel-powered vehicles. Biodiesel is produced from oils or fats using transesterification and is the most common biofuel in Europe.
Biogas. Biogas, which is also known as biomethane, landfill gas, swamp gas, and digester gas, is a collection of gases (largely methane and carbon dioxide) produced by the anaerobic degradation of biomass (non-fossil organic matter) by various bacteria. The primary component of biogas is methane gas, which comprises 50-90% by volume of biogas. More than half of the gas used in Sweden to power the natural gas vehicles is biogas (Wilkie 2013).
Biofuels have increased in popularity because of rising oil prices and the need for energy security. In 2010, worldwide biofuel production reached 105 billion liters (28 billion gallons US), up 17% from 2009 (Worldwatch 2011). Global ethanol fuel production reached 86 billion liters (23 billion gallons US) in 2010, with the United States and Brazil as the world's top producers, accounting together for 90% of global production. The world's largest biodiesel producer is the European Union, accounting for 53% of all biodiesel production in 2010 (Worldwatch 2011). Biofuels provided 2.7% of the world's transport fuel in 2010 (REN21 2011).
Other renewable energy technologies are still under development, including ocean energy, as well as the above-mentioned cellulosic ethanol and enhanced geothermal systems. These technologies are not yet widely demonstrated or have limited commercialization. Many are on the horizon and may have potential comparable to other renewable energy technologies, but still depend on attracting sufficient attention and research, development, and demonstration (RD&D) funding.
There are numerous organizations within the academic, federal, and commercial sectors conducting large scale advanced research in the field of renewable energy. This research spans several areas of focus across the renewable energy spectrum. Most of the research is targeted at improving efficiency and increasing overall energy yields (Jupe et al. 2007). Two of the most prominent of these labs are Sandia National Laboratories and the National Renewable Energy Laboratory (NREL), both of which are funded by the United States Department of Energy and supported by various corporate partners.
Marine energy (also sometimes referred to as ocean energy) refers to the energy carried by ocean waves, tides, salinity, and ocean temperature differences. The movement of water in the world’s oceans creates a vast store of kinetic energy, or energy in motion. This energy can be harnessed to generate electricity to power homes, transport, and industries.
The term marine energy encompasses both wave energy and tidal energy. Offshore wind power is not a form of marine energy, as wind power is derived directly from the wind, even if the wind turbines are placed over water.
Wave energy is the transport of energy by ocean surface waves. Waves are generated by wind passing over the surface of the sea. As a renewable energy, wave energy is the capture of the energy of waves for a useful purpose, such as electricity generation, water desalination, or the pumping of water (into reservoirs). The term wave power is used synonymously with wave energy, or references the generation of electricity from the energy of waves. Wave power may be considered a form of hydropower whereby the definition of hydropower is expanded to encompass any type of energy gained from the movement of water. Wave power is distinct from the diurnal flux of tidal power and the steady gyre of ocean currents. Wave-power generation is not currently a widely employed commercial technology, although there have been attempts to use it since at least 1890 (Miller 2004). In 2008, the first experimental wave farm was opened in Portugal, at the Aguçadoura Wave Park (Khalip 2008).
Tidal energy is the form of energy created by movement of tides. In terms of renewable energy, tidal energy refers to the energy that is harnessed from the tides for practical purposes. The term tidal power is used synonymously as the conversion of tidal energy into a useful form of energy, or more specifically as the generation of electricity from the tides. Tidal power is the only technology that draws on energy inherent in the orbital characteristics of the Earth–Moon system, and to a lesser extent in the Earth–Sun system. As with wave power, tidal power may be considered a form of hydropower whereby the definition of hydropower is expanded to encompass any type of energy gained from the movement of water. Although not yet widely used, tidal power has potential for future electricity generation. Tidal power has traditionally suffered from relatively high cost and limited availability of sites with sufficiently high tidal ranges or flow velocities, thus constricting its total availability. However, many recent technological developments and improvements, both in design (e.g. dynamic tidal power, tidal lagoons), and turbine technology (e.g. new axial turbines, cross flow turbines), indicate that the total availability of tidal power may be much higher than previously assumed, and that economic and environmental costs may be brought down to competitive levels. The world's first large-scale tidal power plant (the Rance Tidal Power Station) became operational in 1966.
The oceans have a tremendous amount of energy and are close to many if not most concentrated populations. Ocean energy has the potential of providing a substantial amount of new renewable energy around the world.
Since the late 1980s, there have been several attempts to investigate the possibility of harvesting energy from lightning. While a single bolt of lightning carries a relatively large amount of energy (approximately 5 billion joules (IOP 2014), this energy is concentrated in a small location and is passed during an extremely short period of time (milliseconds); therefore, extremely high electrical power is involved (Williams 1988). It has been proposed that the energy contained in lightning be used to generate hydrogen from water, or to harness the energy from rapid heating of water due to lightning (Knowledge 2007), or to use inductors spaced far enough away so that a safe fraction of the energy might be captured (Helman 2011).
A technology capable of harvesting lightning energy would need to be able to rapidly capture the high power involved in a lightning bolt. Several schemes have been proposed, but the ever-changing energy involved in each lightning bolt have rendered lightning power harvesting from ground based rods impractical. Additionally, lightning is sporadic, and therefore energy would have to be collected and stored; it is difficult to convert high-voltage electrical power to the lower-voltage power that can be stored. Another major challenge when attempting to harvest energy from lightning is the impossibility of predicting when and where thunderstorms will occur. Even during a storm, it is very difficult to tell where exactly lightning will strike (IOP 2014).
Carbon-neutral fuels are synthetic fuels produced by hydrogenating waste carbon dioxide recycled from power plant flue-gas emissions, recovered from automotive exhaust gas, or derived from carbonic acid in seawater (Graves et al. 2011). Among such fuels are included methane, gasoline, diesel fuel, jet fuel or ammonia (Leighty and Holbrook 2012). Production of such fuels is considered to be carbon-neutral because it does not result in a net increase in atmospheric greenhouse gases (Lackner et al. 2012). To the extent that synthetic fuels displace fossil fuels, or if they are produced from waste carbon or seawater carbonic acid (Eisaman et al. 2012), and their combustion is subject to carbon capture at the flue or exhaust pipe, they result in negative carbon dioxide emission and net carbon dioxide removal from the atmosphere, and thus constitute a form of greenhouse gas remediation (Goeppert et al. 2012).
Such renewable fuels alleviate the costs and dependency issues of imported fossil fuels without requiring either electrification of the vehicle fleet or conversion to hydrogen or other fuels, enabling continued compatible and affordable vehicles (Pearson et. al. 2012). Carbon-neutral fuels offer relatively low cost energy storage, alleviating the problems of wind and solar intermittency, and they enable distribution of wind, water, and solar power through existing natural gas pipelines (Pearson et al. 2012). Nighttime wind power is considered the most economical form of electrical power with which to synthesize fuel, because the load curve for electricity peaks sharply during the warmest hours of the day, but wind tends to blow slightly more at night than during the day, so, the price of nighttime wind power is often much less expensive than any alternative (Pearson 2012). Germany has built a 250 kilowatt synthetic methane plant which they are scaling up to 10 megawatts (Fraunhofer-Gesellschaft 2010).
The George Olah carbon dioxide recycling plant in Grindavík, Iceland has been producing 2 million liters of methanol transportation fuel per year from flue exhaust of the Svartsengi Power Station since 2011 (CT 2014).
Artificial photosynthesis uses techniques include nanotechnology to store solar electromagnetic energy in chemical bonds by splitting water to produce hydrogen and then using carbon dioxide to make methanol (Collins and Critchley 2005). Researchers in this field are striving to design molecular mimics of photosynthesis that utilize a wider region of the solar spectrum and employ catalytic systems made from abundant, inexpensive materials that are robust, readily repaired, non-toxic, stable in a variety of environmental conditions, and perform more efficiently, allowing a greater proportion of photon energy to end up in the storage compounds, i.e., carbohydrates (rather than building and sustaining living cells) (Faunce et al. 2013).
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