Renewable energy resources
Summary: Renewable energy is the type of energy that comes from regenerative resources, namely: biomass, hydro, solar, wind and geothermal power.
Renewable energy is energy derived from regenerative resources—that is, resources that can be fully replenished in a short time period. Renewable energy is generated from elements found in nature and includes the use of biomass in fuels such as ethanol, biodiesel, charcoal, and biogas; the use of water or hydropower by means of waterwheels, water mills, run-of-the-river hydroelectricity facilities, hydroelectric dams, tidal power, and wave power; the use of solar energy in solar heating, photovoltaic (PV) systems, and concentrating solar power; the use of wind energy to drive turbines via windmills; and the use of geothermal energy, the heat energy that emanates from deep inside the Earth. These renewable energy resources, if well explored and developed, can be inexhaustible.
By contrast, nonrenewable energy resources, such as fossil fuels and nuclear power, are limited, and their use leads to the depletion of those reserves. For much of human history, the energy used to power societies came from renewable resources; however, with the rise of the Industrial Revolution and its use of coal, and later oil, nonrenewable fossil fuels became predominant.
Until the nineteenth century, civilization survived using essentially renewable energy resources, based on burning firewood and biomass for cooking, heating, and building materials. In modern times, the least developed countries still use essentially these kinds of energy resources.
The use of fossil fuels became dominant in the twentieth century and continued to increase at the beginning of the twenty-first. During the Industrial Revolution, the demand for energy increased drastically, and the high use of coal, petroleum, and natural gas led to the large-scale generation of electricity, heat, and fuel. Today, fossil fuels account for more than three-fourths of the energy used in the world. Fossil fuels have higher energy density when compared to raw biomass. They are easy to extract because huge amounts of these materials are found in a single place, which make the raw materials relatively low cost. The use of fossil fuels has had a large impact on civilization’s industrial development. However, the reduction of fossil-fuel usage has become an important issue for two main. reasons. The first is related to the depletion of reserves, cost instability, and irregular distribution of these resources around the world, conditions that have led to many conflicts and wars; the second reason is related to climate change due to the greenhouse gases emitted by the burning of these fuels for energy.
The 2014 Renewable Energy Data Book, issued by the US Department of Energy, notes that 23.6 percent of energy consumed worldwide comes from renewable sources, with 16.3 percent coming from hydropower, 1.0 percent from solar power, 1.9 percent from biomass, 4.2 percent from wind energy, and 0.3 percent from geothermal energy. In 2009, during the worldwide economic recession, there was a boom in renewable energy; in mid-2010, more than a hundred countries focused their energy production on renewable sources, an increase of almost 100 percent from 2005.
There has thus been a major shift in the global energy scenario with regard to renewable fuels. Between 2004 and 2014, new installations for wind power and solar power, in the form of both photovoltaic (PV) power and concentrated solar power (CSP), showed significant worldwide growth, with electricity capacity increasing at compound annual rates of 22.7 percent (wind), 26.5 percent (CSP), and 46.1 percent (PV). In 2014, renewable energy accounted for an estimated 58.5 percent of net additions to global power-generating capacity; by the end of the year, renewable-energy facilities represented 27.7 percent of that capacity. China was the world leader in cumulative wind capacity and hydropower capacity, as well as total overall renewable electricity capacity; the United States led in both geothermal and biomass capacity; and Germany had the greatest PV capacity.
In 1990, wind power was used in only a few countries; by 2015, it played a role in the energy sources of more than one hundred countries. That year, China added more than 30 gigawatts (GW) of wind-power capacity, representing almost half of all new installations and bringing the nation's total capacity to more than 145 GW, according to the Global Wind Energy Council. However, the United States was the world leader in actual wind-powered electricity production in 2015, generating about 190 million megawatt-hours (MWh) compared to China's 185.1 MWh. Meanwhile, Argentina, Brazil, Colombia, Costa Rica, and Paraguay have become Latin America’s largest biofuel producers, and other renewable technologies are being expanded as well. In 2014, the total investment in renewable energy by developing economies—including China, Brazil, and India—amounted to US$131.3 billion combined, an increase of 36 percent over the previous year. All these changes in the world energy scenario have helped to increase confidence in renewable energy. By the end of 2014, the United States had 15.5 percent of total installed capacity and 13.5 percent of total generation focused on renewable sources.
The first application of biomass as a source of energy occurred in the early days of humankind, when people learned to use fire to produce heat and cook food. The first evidence that humans cooked food over controlled fires, based on the evolution of human molars, dates to 1.9 million years ago.
Biomass is organic raw material derived, directly or indirectly, from plants as a result of photosynthesis. It includes crop residues for cogeneration, forest residues, animal wastes, municipal solid waste, and energy crops such as sugarcane, switchgrass, jatropha, and corn. Photosynthesis is the chemical process that plants use to convert energy from sunlight, carbon dioxide, and water into oxygen and organic compounds, especially sugars (stored energy). The use of biomass instead of fossil fuels to generate energy mitigates the greenhouse effect, because even though the carbon present in the biomass will be emitted when used as fuel, it comes from plants that previously removed the carbon (as carbon dioxide) from the atmosphere; hence, the “carbon sink” role of the plant has balanced the later combustion of the fuel and emissions from the plant.
Biomass can provide raw material for multiple fuel uses, for the production of heat, electricity, and both liquid and gaseous fuels for transport. Energy is extracted from these various sources of biomass through biochemical processes, chemical reactions, and mechanical technologies to convert biomass into liquid or gaseous fuel. However, a considerable disadvantage of biomass is related to its low energy density when compared with that of fossil fuels. The processing of biomass can require significant energy inputs, which should be minimized to maximize the conversion of biomass and energy recovery.
There are three basic uses of biomass as fuel. These include biofuels, biogas, and thermochemical energy.
Biodiesel and bioethanol are the best-known biofuels available for use in automobiles and other motor vehicles. Biodiesel is produced by transesterification, which is a reversible chemical reaction of vegetable oils or animal fats with alcohols, producing esters (biodiesel) and glycerin. As of 2015, the most commonly used raw materials to produce biodiesel are rapeseed (predominantly in Europe) and soybean (in the United States, Brazil, and Argentina) oils. Biodiesel can be mixed with traditional diesel fuel and used in compression-ignition engines without engine adaptations. Sugarcane, maize, wheat, sugar beets, and sweet sorghum can be used as raw materials for the production of bioethanol. These products are rich in sugars or starches, which are converted to alcohol by means of fermentation. Bioethanol production using sugarcane fermentation techniques has been commercially undertaken in Brazil since the 1980s.
Biogas can be produced from any kind of biomass by means of anaerobic microbes (bacteria that live in the absence of oxygen). Pigs, cattle, and chickens reared in confined areas produce a considerable concentration of organic waste matter with high moisture content, which can be use for biogas production. Biogas contains mainly methane and carbon dioxide, along with small amounts of other gases, such as hydrogen sulfide, ammonia, hydrogen, and carbon monoxide, giving it a very bad smell. The wet biomass is fed into an enclosed digestion tank together with a source containing anaerobic microbes; in the tank, anaerobic reactions occur. The remaining solid and liquid residues can be used as fertilizers. The period of time that biomass should remain in the digestion tank can range from a single day to several months. In 1630, it was discovered that decomposing organic matter is capable of producing a flammable gas, and in 1808 it was discovered that the gas contained methane. Biogas can be used as a low-cost fuel for heating and cooking; it can also be converted to electricity and heat or purified and compressed, much like natural gas, to create fuel to power motor vehicles.
Finally, gasification and pyrolysis are thermochemical processes in which organic matter is degraded by thermal reactions in the presence of limited amounts of air or oxygen. The major products are biochar (charcoal), bio-oil, or a gaseous product, which can also be burned as fuel. The amount of each of the three products formed is dependent on the type and nature of the biomass input, the type of facility used, and the particular process adopted. Pyrolysis aims to obtain solid and liquid products, whereas gasification produces a gaseous product, composed mostly of hydrogen, carbon monoxide, methane, carbon dioxide, and water vapor.
Hydropower refers to the energy generated through the use of flowing water. From millennia, hydropower has been used for irrigation; notable engineering of water channels has been found in the ancient remains of Egyptian and Mayan civilizations, and the engineering feats of the Roman civilization are well documented. There are many ways to harness the potential and kinetic energy of water to perform work; some examples include the use of waterwheels, water mills, run-of-the-river hydropower plants, hydroelectric dams, tidal power, and wave power.
Hydroelectric power is the electricity generated when water flows through a turbine to a lower level. These turbines, which are flow-controlling blades mounted on rotating shafts, are usually located within the dams. The potential energy is stored by dams as a volume of water located behind the dam. As long as water is being released through the dam, it rotates the turbines, which are coupled to generators that then supply electricity to transmission lines. Hydropower plants play a major role in the world’s capacity to generate electricity. For example, the Three Gorges Dam in China—the greatest hydropower plant in the world—has more than twice the capacity of the Kashiwazaki-Kariwa Nuclear Power Plant in Japan, which is the nuclear plant with greatest capacity in the world.
Tidal power is the energy that can be extracted from the rise and fall of ocean tides. Extraction of tidal power is simple in theory. A tidal dam, termed a “barrage,” is built across an estuary, creating an enclosed basin for storingwater at high tide. Turbines in the barrage are used to convert the potential energy, resulting from the difference in water levels, into electrical energy.
Less common types of hydroelectricity include wave power, run-of-the-river hydropower, and marine (or ocean) current power.
Most of the energy used on Earth has its origin in the electromagnetic radiation from the sun, including biomass, hydropower, and wind power. However, the term “solar power” in the context of energy generation is used to refer to the direct conversion of solar radiation to a useful form of energy. Forms of solar power include photovoltaic electricity, solar power tower plants, and solar thermal heating, among other forms.
Although only a very small fraction of the radiation from the sun reaches the Earth, sunlight represents a tremendous source of renewable, greenhouse-gas-free energy. Passage through the atmosphere splits the radiation reaching the surface into direct and diffuse components, reducing the total energy through selective absorption by dry air, water molecules, dust, and cloud layers, while heavy cloud coverage eliminates all the direct radiation. Sunlight is intermittent, varying diurnally from day to night over a twenty-four-hour period as the Earth rotates. Thus, storage of the energy is a very important factor if it is to be used efficiently and economically.
A typical procedure is to use a solar collector to absorb the solar energy and convert it to thermal energy, which is transferred by heat pipes carrying pumped fluids for low-temperature (less than 100 degrees Celsius) heating or storage. Therefore, the collector should be made of materials with high thermal conductivity and low thermal capacity, such as metals (copper, steel, and aluminum) and some thermal-conducting plastics. The most common collectors are flat, blackened plates, since they convert both direct and diffuse (cloud-mitigated) radiation into heat.
Direct solar radiation can also be focused by a range of concentrating solar power technologies and collected to provide medium- to high-temperature heating. These technologies for concentrating solar power are of three types: parabolic troughs, power towers, and heat engines. The heat generated by the radiation is then used to operate a conventional power cycle, generally by steam-generating techniques similar to those used in conventional power plants. Solar thermal power plants designed to use direct sunlight must be sited in regions with high direct solar radiation.
Another way to use solar power is by means of photovoltaic (PV) conversion. The PV effect is the production of electric potential and current when a system is exposed to light. The sun serves as the light source, and photovoltaic cells, also called solar cells, convert that light to energy. A PV cell consists of a semiconductor electrical junction device, which absorbs and converts the radiant energy of sunlight directly into electrical energy. Solar cells may be connected in series and/or parallel to obtain the required values of current and voltage for electric power generation needs. Most PV cells are made from single-crystal silicon and have been expensive for generating electricity, but they have found applications. Research has emphasized lowering the cost of PV cells by improving performance and by reducing the costs of materials and manufacturing. Besides their low efficiency (relative to the percentage of the incident sunlight that is converted into electrical output power) and high costs, PV cells’ power generation is limited by the presence or absence of solar radiation. For some applications, the electricity can be stored (in batteries, for example) to supply electricity on cloudy days and during the night.
Windmills have been used to pump water and perform other kinds of mechanical work for centuries, but they were not used to produce electric power until the late 1800s. A wind power station consists of rotating blades attached to a generator, which is connected to transmission lines.
Wind power does not emit polluting emissions and does not produce unwanted substances that require careful disposal. There appear to be minor environmental impacts associated with the installation of wind turbines, aside from the possible disturbance of wildlife habitat and farming, which for some include the visual impact of a large multi-turbine wind farm on the natural beauty of an area. Wind turbines are not considered to be noisy machines; however, some noise is generated in their operation, and this has led to negative reactions of the public in some areas. Another issue is the coincident location of wind turbines in areas along the migration routes of birds; there have been reports of birds dying after colliding with the rotating blades of wind turbines. However, perhaps the greatest obstacles to wind farms have been that the areas where there is wind are often heavily populated and that the kind of equipment wind farms require is still too expensive.
Geothermal energy is heat energy from the depths of the Earth. It originates from the Earth’s molten interior and from the radioactive decay of isotopes in underground rocks. The heat is brought near the surface by crustal plate movements, deep circulation of groundwater, and intrusions of molten magma from a great depth into the Earth’s crust. In some places, the heat rises to the surface in natural streams of steam or hot water, which have been used since prehistoric times for bathing and cooking. Wells can be drilled to trap this heat in supply pools, greenhouses, and power plants. The reservoirs developed to harness geothermal power to generate electricity are termed "hydrothermal convection systems" and are characterized by circulation of water to depth. The driving force is convection, via the density difference between cold, downward-moving recharge water and heated, upward-moving thermal water. Hot water from a reservoir is flashed partly to steam at the surface, and this steam is used to drive a conventional turbine-generator set.
Geothermal energy tends to be relatively diffuse, which makes it difficult to trap. If it were not for the fact that the Earth itself concentrates geothermal heat in certain regions—typically regions associated with the boundaries of tectonic plates—geothermal energy would be essentially useless.
Geothermal resources are renewable within the limits of equilibrium between offtake of reservoir water and natural or artificial recharge. Within this equilibrium, the energy source is renewable for a long period of time. Although geothermal energy may not be technically “renewable,” the global geothermal potential represents a practically inexhaustible energy resource. The issue is not the finite size of the resource but the availability of technologies able to trap this kind of energy.
