Solar energy
Solar energy is renewable energy, obtained from the use of electromagnetic radiation from the Sun. The solar radiation that reaches the Earth has been used by humans since ancient times, through different technologies that have evolved. Today, the heat and light of the Sun can be used by means of various collectors such as photoelectric cells.
The different solar technologies can be classified as passive or active according to how they capture, convert and distribute solar energy. Active technologies include the use of photovoltaic panels and solar thermal collectors to collect energy. Among the passive techniques, there are different techniques framed in bioclimatic architecture: the orientation of buildings to the Sun, the selection of materials with a favorable thermal mass or that have light scattering properties, as well as the design of spaces through ventilation. natural.
In 2011, the International Energy Agency stated that “The development of clean, cheap and inexhaustible solar technologies will bring enormous long-term benefits. It will increase countries' energy security through the use of a local, inexhaustible energy source and, most importantly, regardless of imports, it will increase sustainability, reduce pollution, decrease the costs of climate change mitigation, and prevent the rise excessive prices of fossil fuels. These advantages are global. In this way, the costs for its incentive and development must be considered investments; they must be carried out correctly and widely disseminated.”
The most developed source of solar energy today is photovoltaic solar energy. According to reports from the environmental organization Greenpeace, photovoltaic solar energy could supply electricity to two thirds of the world's population by 2030.
Thanks to technological advances, sophistication, and economies of scale, the cost of solar photovoltaics has steadily decreased since the first commercial solar cells were manufactured, while increasing efficiency and cost. The means of electricity generation is already competitive with non-renewable energies in a growing number of geographic regions, reaching grid parity. Other solar technologies, such as solar thermal power, are also reducing their costs considerably.
Energy from the Sun
Earth receives 174 petawatts of incoming solar radiation (insolation) from the uppermost layer of the atmosphere. Approximately 30% returns to space, while clouds, oceans, and land masses absorb the remainder. The electromagnetic spectrum of sunlight on the Earth's surface is mainly occupied by visible light and the infrared ranges with a small part of ultraviolet radiation.
The power of radiation varies with time of day, damping weather conditions, and latitude. Under acceptable radiation conditions, the power is equivalent to approximately 1000 W/m² on the earth's surface. This power is called irradiance. Note that in global terms practically all the radiation received is re-emitted to space (otherwise there would be abrupt heating). However, there is a notable difference between the radiation received and that emitted.
Radiation can be used in its direct and diffuse components, or in the sum of both. Direct radiation is that which comes directly from the solar focus, without intermediate reflections or refractions. The diurnal celestial vault emits diffuse radiation due to the multiple phenomena of solar reflection and refraction in the atmosphere, in the clouds and the rest of the atmospheric and terrestrial elements. Direct radiation can be reflected and concentrated for use, while diffuse light coming from all directions cannot be concentrated.
The normal direct irradiance (or perpendicular to the sun's rays) outside the atmosphere is called the solar constant and has an average value of 1366 W/m² (corresponding to a maximum value at perihelion of 1395 W/ m² and a minimum value at aphelion of 1308 W/m²).
Radiation absorbed by oceans, clouds, air, and land masses increases their temperature. The heated air is the one that contains evaporated water that rises from the oceans, and also in part from the continents, causing atmospheric circulation or convection. When the air rises to the upper layers, where the temperature is low, its temperature decreases until the water vapor condenses, forming clouds. The latent heat of the condensation of the water amplifies the convection, producing phenomena such as wind, storms and anticyclones. The solar energy absorbed by the oceans and land masses maintains the surface at 14 °C. For the photosynthesis of green plants, solar energy is converted into chemical energy, which produces food, wood and biomass, from which are also derived fossil fuels.
| Annual Solar Flux and Human Energy Consumption | |
|---|---|
| Solar | 3 850 000 EJ |
| Wind energy | 2250 EJ |
| Biomass | 3000 EJ |
| Primary Energy Use (2005) | 487 EJ |
| Electricity (2005) | 56.7 EJ |
The total energy absorbed by the atmosphere, oceans, and continents is estimated to be 3,850,000 exajoules per year. In 2002, this energy in one hour was equivalent to the world's global energy consumption for one year. Photosynthesis captures approximately 3,000 EJ per year in biomass, which represents only 0.08% of the energy received by the Earth. The amount of solar energy received annually is so vast that it is approximately twice the amount of all energy produced. never by other non-renewable energy sources such as oil, coal, uranium and natural gas.
Development of solar energy
Dawn of solar technology
Development of solar energy Capacity in GW for technology 100 200 300 400 500 600 700 800 900 1000 2006 2009 2012 2015 2018 2021 |
| Global solar energy development between 2006 and 2021.
Photovoltaic solar energy Thermoelectric solar energy (CSP) |
The early development of solar technologies, beginning in the 1860s, was motivated by the expectation that coal would soon become scarce. However, the development of solar energy stalled at the turn of the 20th century due to the increasing availability and economy of scale from non-renewable sources such as coal and oil. In 1974, it was estimated that only six private homes in all of North America were powered by solar systems. However, the 1973 oil crisis and the 1979 oil crisis caused a major change in energy policy around the world and put new focus on emerging solar technologies. The first development strategies were developed, centered on incentive programs such as the Federal Photovoltaic Utilization Program in the United States and the Sunshine Program in Japan. Other efforts included the creation of research organizations in the United States (NREL), Japan (NEDO), and Germany (Fraunhofer–ISE). Between 1970 and 1983, installations of photovoltaic systems grew rapidly, but the fall in the price of oil in the 1980s moderated the growth of solar energy between 1984 and 1996.
From 1998 to today
In the mid-1990s, the development of rooftop photovoltaics, both residential and commercial, as well as grid-tie plants, began to accelerate due to growing concerns over oil and natural gas supplies., the Kyoto protocol and the concern for climate change, as well as the improvement in the competitiveness of the costs of photovoltaic energy compared to other energy sources. At the beginning of the century XXI, the adoption of subsidy mechanisms and support policies for renewable energies, which gave them priority access to the grid, exponentially increased the development of photovoltaic energy, first in Europe and then in the rest of the world. Solar thermal power (CSP), however, although it has also progressed in recent decades, still accounts for a small fraction of the global contribution of solar energy to energy supply.
Technology and uses of solar energy
Classification by technology and its corresponding more general use:
- Active solar energy: for low temperature (between 35 °C and 60 °C), is used in houses; medium temperature, reaches 300 °C; and high temperature, reaches 2000 °C. The latter is achieved by inciding the solar rays in mirrors, which are directed to a reflector that leads to the rays to a specific point. It can also be by tower centers and by parabolic mirrors.
- Passive solar energy: Take advantage of the heat of the sun without the need for mechanical mechanisms or systems.
- Thermal solar energy: It is used to produce low temperature hot water for sanitary use and heating.
- Photovoltaic solar energy: It is used to produce electricity through semiconductor plates that are altered with solar radiation.
- Thermal concentration energy: It is used to produce electricity with a conventional thermodynamic cycle from a high temperature heated fluid (thermal oil).
- Hybrid Solar Energy: Combines solar energy with other energy. According to the energy you combine with is a hybridization:
- Renewable: biomass, wind energy.
- Non-renewable: fossil fuel.
- Solar wind energy: It works with the air warmed by the sun, which rises through a fireplace where the generators are.
A parabolic solar disk that concentrates solar radiation on a heating element of a Stirling engine. The whole unit acts as a solar follower.
Cylinder-parabolic mirrors used in a solar thermal plant located in the United States.
The central solar tower of the "Solar Two", belonging to the Solar Project.
Ecological solar house, located on the island of Santa Helena (Montreal, Canada), designed in the framework of the international Solar Decathlon competition.
Marquesina fotovoltaic located in the car park of the Universidad Autónoma de Madrid (Madrid, Spain).
The Hubble Space Telescope, equipped with solar panels, is placed in orbit from the Discovery ferry cellar in 1990.
The 2009 edition winner Global Green Challenge, the Tokai Challenger, of the Solar Car Team of the University of Tokai (Japan).
The Helios, unmanned solar plane prototype developed by NASA in flight.
The manned solar plane Solar Impulseprepared for takeoff.
Passive solar energy
Passive solar technology is the set of techniques designed to use solar energy directly, without transforming it into another type of energy, for immediate use or storage without the need for mechanical systems or external input of energy, although they can be complemented by them, for example for their regulation.
Passive solar technology includes direct and indirect gain systems for space heating, thermosiphon-based water heating systems, the use of thermal mass and phase change materials to smooth out fluctuations in air temperature, solar cookers, solar chimneys to improve natural ventilation and the shelter of the earth itself.
Bioclimatic architecture is the application of this principle to the design of buildings. The energy is not used by means of industrialized collectors, but rather it is the constructive elements themselves that absorb the energy during the day and redistribute it at night.
Solar thermal energy
Solar thermal energy (or thermosolar energy) consists of the use of the Sun's energy to produce heat that can be used to cook food or for the production of hot water for domestic water consumption, be it sanitary hot water, heating, or for the production of mechanical energy and, from it, electrical energy. Additionally, it can be used to feed an absorption refrigeration machine, which uses heat instead of electricity to produce cold with which the air in the premises can be conditioned.
Solar thermal energy collectors are classified as low, medium and high temperature collectors:
- Low temperature collectors. They provide useful heat at temperatures below 65 °C by means of metal or non-metallic absorbers for applications such as pool heating, domestic heating of water for bath and, in general, for all those industrial activities in which process heat is not greater than 60 °C, for example pasteurization, textile washing, etc.
- Average temperature collectors. They are devices that concentrate solar radiation to deliver useful heat at higher temperature, usually between 100 and 300 °C. In this category, the stationary concentrators and the parabolic channels are held, all of them are concentrated by mirrors directed towards a smaller receiver. They have the inconvenience of working only with the direct component of solar radiation so their use is restricted to high insolation areas.
- High temperature collectors. They were invented by Frank Shuman and today exist in three different types: parabolic plate collectors, the new generation of parabolic channel and central tower systems. They operate at temperatures above 500 °C and are used to generate electricity (thermal electricity) and transmit it to the electricity grid; in some countries these systems are operated by independent producers and installed in regions where the possibilities of cloudy days are remote or scarce.
Low temperature solar thermal energy
A low-temperature solar thermal installation is made up of solar collectors, a primary and secondary circuit, heat exchanger, accumulator, expansion vessel and pipes. If the system works by means of a thermosiphon, it will be the difference in density due to a change in temperature that will move the fluid. If the system is forced, then it will also be necessary to provide the system with a circulation pump and a control system.
Solar collectors are the elements that capture solar radiation and convert it into thermal energy, into heat. Flat plate solar collectors, vacuum tube collectors and unprotected or uninsulated absorber collectors are known as solar collectors. Flat collection systems (or flat plate) with a glass cover are the most common in the production of DHW domestic hot water. The glass lets the sun's rays through, these heat up metal tubes that transmit the heat to the liquid inside. The tubes are dark in color, since dark surfaces heat up more.
The glass that covers the collector not only protects the installation but also allows heat to be conserved, producing a greenhouse effect that improves the performance of the collector.
They are made up of a closed aluminum casing resistant to marine environments, an aluminum frame, a silicone-free perimeter gasket, thermal insulation (usually rock wool), a highly transparent solar glass cover, and finally tubes welded that conduct the heat carrier fluid to the inside and outside of the collector.
Solar collectors are made up of the following elements:
- Cover: It is transparent, may be present or not. It is usually glass although they are also used as plastic as it is less expensive and manageable, but it must be a special plastic. Its function is to minimize convection and radiation losses and therefore must have a solar transmittal as high as possible.
- Air channel: It is a space (empty or not) that separates the cover from the absorbing plate. Its thickness will be calculated taking into account to balance the convection losses and the high temperatures that can be produced if it is too narrow.
- Absorbent Plate: The absorbing plate is the element that absorbs solar energy and transmits it to the liquid that circulates through the pipes. The main characteristic of the plate is that it has to have a large solar absorption and a reduced thermal emission. Since common materials do not meet this requirement, combined materials are used to obtain the best absorption / emission ratio.
- Tubes or ducts: Tubes are touching (sometimes welded) the absorbing plate to make the energy exchange as large as possible. Through the tubes circulate the liquid that will heat and go to the accumulation tank.
- Insulating layer: The purpose of the insulating layer is to coat the system to avoid and minimize losses. In order for the isolation to be the best possible, the insulating material must have a low thermal conductivity.
Medium temperature solar thermal energy
Medium temperature installations can use various designs, the most common designs are: glycol pressurized, back drain, batch systems and newer freeze tolerant low pressure systems using photovoltaic pumped water containing polymer pipes. European and international standards are being revised to include innovations in the design and operation of medium temperature collectors. Operational innovations include the operation of "permanently wet collectors". This technique reduces or even eliminates the occurrence of high temperature non-flow stresses known as stagnation, which reduce the expected life of these collectors.
High temperature solar thermal energy
Temperatures below 95 degrees Celsius are sufficient for space heating, in which case non-concentrating flat collectors are generally used. Due to the relatively high heat losses through the glass, flat plate collectors fail to reach much above 200 °C even when the transfer fluid is stagnant. Such temperatures are too low to be used for efficient conversion to electricity.
The efficiency of heat engines increases with the temperature of the heat source. To achieve this in thermal power plants, solar radiation is concentrated by means of mirrors or lenses to achieve high temperatures using a technology called Concentrated Solar Power (CSP).. The practical effect of the higher efficiencies is to reduce the size of the plant's collectors and the use of land per unit of energy generated, reducing the environmental impact of a power plant as well as its cost.
As the temperature rises, different forms of conversion become practical. Up to 600 °C, steam turbines, the standard technology, have an efficiency of up to 41%. Above 600 °C, gas turbines can be more efficient. Higher temperatures are problematic and different materials and techniques are needed. One approach for very high temperatures is to use liquid fluoride salts operating at temperatures between 700 °C to 800 °C, using multi-stage turbine systems to achieve thermal efficiencies of 50% or more. Higher operating temperatures they allow the plant to use high-temperature dry heat exchangers for its thermal exhaust, reducing the plant's water use, which is critical for plants located in deserts to be practical. Higher temperatures also make heat storage more efficient, since more watt-hours are stored per unit of fluid.
Since a concentrated solar power (CSP) plant first generates heat, it can store that heat before converting it into electricity. With today's technology, heat storage is much cheaper than electricity storage. In this way, a CSP plant can produce electricity during the day and at night. If the CSP plant location has predictable solar radiation, then the plant becomes a reliable power generation plant.
Accumulation and exchange of heat
Heat storage allows solar thermal power plants to produce electricity during daylight hours without sunlight or at night. This enables the use of solar energy for baseload generation as well as peak power generation, with the potential to replace fossil fuel-fired power plants. Additionally, the use of accumulators reduces the cost of electricity generated with this type of solar power plant.
The heat is transferred to a thermal storage medium in an insulated tank during daylight hours and is recovered for electricity generation at night. Thermal storage media include pressurized steam, concrete, a variety of phase change materials, and molten salts such as calcium, sodium, and potassium nitrate.
Photovoltaic solar energy
Photovoltaic solar energy consists of obtaining electricity obtained directly from solar radiation through a semiconductor device called a photovoltaic cell, or through a metal deposition on a substrate called a thin-film solar cell.
Photovoltaic solar panels
A photovoltaic panel consists of an association of cells, encapsulated in two layers of EVA (ethylene-vinyl-acetate), between a front sheet of glass and a back layer of a thermoplastic polymer (usually tedlar). This set is framed in an aluminum structure with the aim of increasing the mechanical resistance of the assembly and facilitating the anchoring of the module to the support structures.
The cells most commonly used in photovoltaic panels are made of silicon, and can be divided into three subcategories:
- Monocrystalline silicon cells are made up of a single silicon crystal, usually manufactured by the Czochralski process. This type of cells has a uniform dark blue color.
- Polycrystalline silicon cells (also called multicrystalline) are made up of a set of silicon crystals, which explains that their performance is somewhat lower than that of monocrystalline cells. They are characterized by a more intense blue color.
- The amorphous silicon cells. They are less efficient than crystalline silicon cells but also less expensive. This type of cells is, for example, the one used in solar applications such as clocks or calculators.
The standardized parameter to classify the power of a photovoltaic panel is called peak power, and corresponds to the maximum power that the module can deliver under standardized conditions, which are:
- Radiation of 1000 W/m2
- Cell temperature of 25 °C (no ambient temperature).
Typical efficiencies of a polycrystalline silicon photovoltaic cell range from 14%-20%. For monocrystalline silicon cells, the values range from 15%-21%. The highest are achieved with low-temperature solar thermal collectors (which can reach 70% efficiency in transferring solar to thermal energy)..
Photovoltaic solar panels do not produce heat that can be reused -although there are lines of research on hybrid panels that allow the generation of electrical and thermal energy simultaneously. However, they are very appropriate for rural electrification projects in areas that do not have a power grid, simple rooftop installations, and photovoltaic self-consumption.
Development of photovoltaic solar energy in the world
Due to the growing demand for renewable energy, the manufacture of solar cells and photovoltaic installations has advanced considerably in recent years. Photovoltaic solar energy has been traditionally used since its popularization at the end of the 1970s to power countless autonomous devices, to supply shelters or houses isolated from the electricity grid, but above all, increasingly in recent years, to produce electricity on a large scale through distribution networks, either by injection into the network or for domestic self-consumption.
Germany is, along with Japan, China and the United States, one of the countries where photovoltaics is experiencing the fastest growth. By the end of 2015, nearly 230 GW of PV power had been installed worldwide, making PV the third largest renewable energy source in terms of global installed capacity, after hydropower and wind power., and already accounts for a significant fraction of the electricity mix in the European Union, covering an average of 3.5% of electricity demand and reaching 7% in periods of greatest production.
The considerable installed capacity in Germany (38 GW in 2014) has set several records in recent years. In June 2014, it produced up to 50.6% of the entire electricity demand in the country during a single day, reaching an instantaneous power of over 24 GW, which is equivalent to the generating power of almost 25 nuclear power plants. working at full capacity.
Photovoltaic self-consumption and grid parity
| Network parity reached before 2014 Network parity reached only for peak prices Network parity reached after 2014 States of the United States. America that will reach network parity soon |
Photovoltaic self-consumption consists of small-scale individual production of electricity for own consumption, through solar panels. This can be complemented with the net balance. This production scheme, which allows electricity consumption to be offset by what is generated by a photovoltaic installation at times of lower consumption, has already been successfully implemented in many countries. It was proposed in Spain by the ASIF photovoltaic association to promote renewable electricity without the need for additional financial support. The net balance was in the project phase by IDAE and has been included in the 2011-2020 Renewable Energy Plan and Royal Decree 1699/2011, of November 18, which regulates the connection to the grid of small power electricity production facilities.
To encourage the development of technology with a view to achieving grid parity -equating the price of obtaining energy to that of other cheaper sources today-, there are premiums for production, which guarantee a fixed price of purchase by the electricity grid. This is the case of Germany, Italy or Spain. This incentive scheme has already borne fruit, bringing the costs of photovoltaics to below the selling price of traditional electricity in a growing number of regions.
The energy of the future
According to reports from Greenpeace, photovoltaics will be able to supply electricity to two thirds of the world's population in 2030. And according to a study published in 2007 by the World Energy Council, by the year 2100, 70% of the energy consumed will be of solar origin.
On the other hand, some countries, such as Tokelau, an archipelago located in the Pacific Ocean, do not have an electrical mix, since they obtain all the electricity they need from the sun. The country is made up of about 125 islets They cover an area of 10 km² and have about 1,500 inhabitants. The geographical situation of the archipelago makes the use of fossil fuels comparatively much more expensive and difficult to maintain than a photovoltaic system.
The Tokelau facility is an example that other countries in Oceania have already taken note of. In fact, the neighboring Cook Islands and the Tuvalu archipelago also aim to be fully powered by renewable energy by 2020.
Pérgola Fotovoltaica del Fórum de las Culturas de Barcelona (2004).
45 MW solar plant in the Philippines.
Fine-layer solar modules, on a plant at the U.S. National Renewable Energy Laboratory (NREL).
Broken Hill Solar Plant, New South Wales.
321 MW solar plant in Brazil.
Solar floating power project in Florida.
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Net balance and costs
Photovoltaic self-consumption consists of small-scale individual production of electricity for own consumption, through renewable electricity equipment (photovoltaic solar panels, wind turbine) some of them self-installable. It can be complemented with the net balance in autonomous installations or facilitate energy independence (disconnected installations).
The net balance allows the excess produced by a self-consumption system to be poured into the electrical network in order to be able to make use of that excess at another time. In this way, the electric company that provides the electricity when the demand is higher than the production of the self-consumption system, will discount the excess discharged to it from the consumption of the network on the bill.
In recent years, due to the growing boom in small renewable energy installations, self-consumption with a net balance has begun to be regulated in various countries around the world, being a reality in countries such as Germany, Italy, Denmark, Japan, Australia, United States, Canada and Mexico, among others, due in part to the constant drop in the cost of photovoltaic modules. To help achieve this goal, many countries are also launching grants, subsidies, or tax breaks to help citizens and businesses finance these types of facilities.
In 2013, the price of solar modules had been reduced by 80% in 5 years, putting solar for the first time in a competitive position with the price of electricity paid by the consumer in a good number of countries. sunny countries. The average cost of electricity generation from solar photovoltaics is already competitive with that of conventional energy sources in a growing list of countries, particularly when considering the hour of generation of such energy, since electricity is usually more expensive. during the day. There has been stiff competition in the production chain, and further declines in the cost of photovoltaics are expected in the coming years, posing a growing threat to the dominance of solar-based generation sources. fossil fuels. As time goes by, renewable generation technologies are generally cheaper, while fossil fuels become more expensive:
The more the cost of photovoltaic solar energy decreases, the more favorable it competes with conventional energy sources, and the more attractive it is for electricity users worldwide. Small-scale photovoltaic can be used in California at $100/MWh ($0.10/kWh) prices below most other types of generation, even those that work with low-cost natural gas. Reduced costs in photovoltaic modules also represent a stimulus in the demand of private consumers, for which the cost of photovoltaic is already compared favorable to that of the final prices of conventional electricity.
By 2011, the cost of photovoltaics had fallen well below that of nuclear power, and is expected to continue to fall:
For large-scale installations, prices have already been reached below $1/watt. For example, in April 2012, a price of photovoltaic modules was published at 0.60 Euros/watt ($0.78/watt) in a 5-year framework agreement. In some regions, photovoltaic energy has reached the network parity, which is defined when the costs of photovoltaic production are at the same level, or below, of the electricity prices that the final consumer pays (although in most of the occasions still above the costs of generation in coal or gas plants, without having the distribution and other costs induced). Photovoltaic energy is generated during a period of the day very close to demand peak (previously) in electrical systems that make great use of air conditioning. More generally, it is clear that, with a carbon price of $50/tonned, which raises the price of coal plants to 5 cent./kWh, photovoltaic energy will be competitive in most countries. The downward price of the photovoltaic modules has been quickly reflected in a growing number of installations, accumulating in all 2011 about 23 GW installed that year. Although some consolidation is expected in 2012, due to cuts in economic support in the major markets in Germany and Italy, strong growth will most likely continue for the rest of the decade. In fact, a study already mentioned that total investment in renewable energy in 2011 had exceeded investments in coal-based electricity generation.
The trend is for prices to decline further over time once PV components have entered a clear and direct industrial phase.
Solar energy research centers
- Centre for Energy, Environment and Technology Research (CIEMAT).
- Almeria Solar Platform (PSA)
- Instituto de Energía Solar, Universidad Politécnica de Madrid.
- Photovoltaic Institute Berlin in Germany.
- Institut für Solare Energiesysteme ISE in Germany.
- Renewable Energy Laboratory NREL in the United States.
- Centro de Estudios de las Energías Renovables en México.
- Procédés, Matériaux et Énergie Solaire PROMES en Francia.
Associations
- ISES – International Solar Energy Society.
- ASADES – Asociación Argentina de Energías Renovables y Ambiente.
- Unión Española Fotovoltaica, the main association of the photovoltaic sector in Spain.
- Educational teaching units for schoolchildren on solar energy.
- Mexican National Solar Energy Association.
- European Association of Photovoltaic Industry (EPIA).