Solar thermal energy

Overview

Solar thermal collectors are classified by the United States Energy Information Administration as low-, medium-, or high-temperature collectors. Low-temperature collectors are unglazed and used to heat swimming pools or to heat ventilation air. Medium-temperature collectors are usually used for heating water or air for residential and commercial use. High-temperature collectors concentrate sunlight using mirrors or lensesand used for up to 300 deg C / 20 bar pressure in industries, and for electric power production. Two categories include Concentrated Solar Thermal (CST) for heat and energy, and Concentrated Solar Power (CSP) when heat is used for power generation. CST and CSP are not applicable. The largest facilities are located in the American Mojave Desert of California and Nevada. These plants employ a variety of different technologies. The largest examples include, Ivanpah Solar Power Facility (377 MW), Solar Energy Generating Systems Facility (354 MW), and Crescent Dunes(110 MW). Spain is the other major developer of solar thermal power plant. The largest examples include Solnova Solar Power Station(150 MW), Andasol Solar Power Station (150 MW) and Extresol Solar Power Station (100 MW).

History

Augustin Mouchot demonstrated a solar collector with a cooling engine making ice cream at the 1878 Universal Exhibition in Paris . The first installation of solar thermal energy occurred in the Sahara approximately in 1910 by Frank Shuman when a steam engine was run on steam produced by sunlight. Because liquid fuel engines were developed, the Sahara project was abandoned, only to be revisited several decades later. ^[1]

Low-temperature solar heating and cooling systems

Systems for utilizing low-temperature solar thermal energy usually heat storage, or short-term or interseasonal; and distribution within a structure or district heating network. In some cases, one of these functions is inherent to a single feature of the system (eg some types of solar collectors also store heat). Some systems are passive, others are active (requiring other external energy to function). ^[2]

Heating is the most obvious application, but solar cooling can be achieved for a building or district cooling network by using a heat-driven absorption or adsorption chiller (heat pump). There is a significant coincidence that the greater driving of insulation, the greater the cooling output. In 1878, Auguste Mouchout pioneered solar cooling by making use of a solar steam engine attached to a refrigeration device. ^[3]

In the United States, heating , ventilation , and air conditioning ( HVAC ) systems account for over 25% (4.75 EJ) of the energy used in commercial buildings (50% in northern cities) and nearly half (10.1 EJ) of the energy used in residential buildings. ^[4] [5] Solar heating, cooling, and ventilation technologies can be used to offset a portion of this energy. Reviews The most popular solar heating technology for heating buildings is the building integrated transpired solar air collection system qui connects to the building’s HVAC equipment. According to Solar Energy Industries Association over 500,000 m ² (5,000,000 square feet) of these panels are in operation in North America as of 2015.

In Europe, since the mid-1990s about 125 large solar-thermal district heating plants have been constructed, each with over 500 m ² (5400 ft ² ) of solar collectors. The largest are about 10,000 m ² , with capacities of 7 MW-thermal and solar heat costs around 4 Eurocents / kWh without subsidies. ^[6] 40 of them have nominal capacities of 1 MW-thermal or more. The Solar District Heating Program (SDH) has participation from 14 European Nations and the European Commission, and is working towards technical and market development, and holds annual conferences. ^[7]

Low-temperature collectors

Glazed solar collectors are designed primarily for space heating. They recirculate a solar air panel where the air is heated and then directed back into the building. These solar space heating systems require at least two penetrations in the building and only when the collector is warmer than the room temperature. Most glazed collectors are used in the residential sector.

Unglazed solar collectors are primarily used to pre-heat make-up ventilation in commercial, industrial and institutional buildings with a high ventilation load. They turn building walls or walls of low cost, high performance, unglazed solar collectors. Also called, “transpired solar panels” or ” solar wall “, they employ a painted perforated metal solar heat absorber that also serves as the exterior wall surface of the building. Heat transfer to the air takes place on the surface of the absorb, through the metal absorb and behind the absorb. The boundary layer of solar heated air is drawn into a hole before the heat can escape by convection to the outside air. The heated air is then drawn from behind the sink into the building’s ventilation system.

A trombe wall is a passive solar heating and ventilation system consisting of an air channel sandwiched between a window and a sun-facing thermal mass. During the ventilation cycle, sunlight stores heat in the heat and air circulation . During the heating cycle the Trombe wall radiates stored heat. ^[8]

Solar roof ponds are unique solar heating and cooling systems developed by Harold Hay in the 1960s. A basic system consists of a roof-mounted water bladder with a movable insulating cover. This system can control the heat between internal and external environments by covering and uncovering the bladder between night and day. When heating is a concern the bladder is uncovered during the day allowing sunlight to warm the water bladder and store heat for evening use. When cooling is a concern of the interior design of the interior of the day and is uncovered at night to radiate heat to the cooler atmosphere. The Skytherm house in Atascadero, California uses a prototype roof pond for heating and cooling. ^[9]

Solar space heating with solar air heat collectors is more popular in the United States and Canada than heating with solar liquid collectors. The two main types of solar panels are glazed and unglazed.

Of the 21,000,000 square feet (2,000,000 m ² ) of solar thermal collectors produced in the United States in 2007, 16,000,000 square feet (1,500,000 m ² ) were of the low-temperature variety. ^[10] Low-temperature collectors are generally installed to heat swimming pools, they can also be used for space heating. Collectors can use the air to their destination.

Heat storage in low temperature solar thermal systems

Interseasonal storage. Solar heat (or heat from other sources) can be effectively stored between opposing seasons in aquifers, underground geological strata, and especially large pits, and large tanks that are insulated and covered with earth.

Short-term storage. Thermal mass materials store solar energy during the day and release this energy during cooler periods. Common thermal mass materials include stone, concrete, and water. The proportion and placement of thermal mass should consider several factors such as climate, daylighting, and shading conditions. When properly incorporated, thermal mass can passively maintain comfortable temperatures while reducing energy consumption.

Solar-driven cooling

Worldwide, by 2011 there have been 750 cooling systems with solar-driven heat pumps, and annual market growth was 40 to 70% over the prior seven years. It is a niche market because the economics are challenging, with the annual number of cooling hours to limiting factor. Respectively, the annual cooling hours are roughly 1000 in the Mediterranean, 2500 in Southeast Asia, and only 50 to 200 in Central Europe. The International Energy Agency(IEA) Solar Heating and Cooling Program (IEA-SHC). ^[11]

Solar heat-driven ventilation

A solar chimney is a passive solar ventilation system composed of a hollow thermal mass connecting the interior and exterior of a building. As the chimney warms, the air inside is heated causing an updraft that pulls air through the building. These systems have been in use since Roman times and remain common in the Middle East.

Process heat

Solar process heating systems are designed to provide large quantities of hot water or space heating for nonresidential buildings. ^[12]

Evaporation ponds are shallow ponds that concentrate dissolved solids through evaporation . The use of evaporation is one of the oldest applications of solar energy. Modern uses include concentrating brine solutions used in leach mining and removing dissolved solids from waste streams. Altogether, evaporation ponds represent one of the largest commercial applications of solar energy in use today. ^[13]

Unglazed transpired collectors are perforated sun-facing walls used for air preheating ventilation. Transpired collectors can also be used for temperatures up to 22 ° C and deliver temperatures of 45-60 ° C. The short payback period of transpired collectors (3 to 12 years) make them more cost-effective alternative to glazed collection systems. As of 2015, over 4000 systems with a combined collector area of ​​500,000 m ² had been installed worldwide. Representatives include an 860 m ² collector in Costa Rica used for drying coffee beans and a 1300 m ² collector in Coimbatore, India used for drying marigolds. ^[14] [15]

A food processing facility in Modesto, California uses parabolic troughs to produce steam used in manufacturing process. The 5,000 m ² collector area is expected to provide 15 TJ per year. ^[16]

Medium-temperature collectors

These collectors could be used to produce 50% of the United States. ^[17]In the United States, a typical system costs $ 4000- $ 6000 retail ($ 1400 to $ 2200 wholesale for the materials) and 30% of the system qualifies for additional taxes. Labor for a simple open loop system in southern climates can take 3-5 hours for the installation and 4-6 hours in Northern areas. Northern system require more collector area and more complex plumbing to protect the collector from freezing. With this incentive, the payback time for a typical household is four years, depending on the state. Similar subsidies exist in parts of Europe. A crew of one solar plumber and two assistants with minimal training can install a system per day. Thermosiphon installation has negligible maintenance costs and reduces the operating costs by $ 6 per person per month. Solar water heating can reduce CO₂ emissions of a family of four by one ton / year (if replacing natural gas) or 3 ton / year (if replacing electricity). ^[18] Medium-temperature installations can be used for many different types of pressurized glycol, drain back, batch systems and newer low pressure freezing systems. European and International Standards are being reviewed to accommodate innovations in design and operation of medium temperature collectors. Operational innovations include “permanently wetted collector” operation. This innovation reduces or even eliminates the occurrence of no-flow high temperature stresses called stagnation which would otherwise reduce the life expectancy of collectors.

Solar drying

Solar thermal energy can be used for drying wood for combustion and wood chips for combustion. Solar is also used for food products such as fruits, grains, and fish. Crop drying by solar energy is cost effective while improving quality. The less money it takes to make a product, the less it can be sold for, pleasing both the buyers and the sellers. Technologies in solar drying include ultra low cost pumped transpired plate air collectors based on black fabrics. Solar thermal energy is useful in the process of drying up the environment. ^[19]

Cooking

Solar cookers use sunlight for cooking, drying and pasteurization . Solar cooking offsets fuel costs, reduces demand for fuel or firewood, and improves air quality by reducing or removing a source of smoke.

The simplest type of solar cooker is the box cooker first built by Horace de Saussure in 1767. A basic box cooker consists of an insulated container with a transparent lid. These cookers can be used effectively in some cases and will typically reach temperatures of 50-100 ° C. ^[20] [21]

Concentrating solar cookers use reflectors to concentrate solar energy onto a cooking container. The most common reflector is flat plate, disc and parabolic trough type. These designs require faster light at higher temperatures (up to 350 ° C) and require direct light to function properly.

The Solar Kitchen in Auroville , India uses a unique concentrating technology known as the solar bowl . Contrary to the flat mirror reflector / fixed receiver systems, the solar bowl uses a fixed spherical reflector with a receiver which tracks the focus of the sun. The solar bowl has a temperature of 150 ° C which is used to produce steam. ^[22]

Many other solar kitchens in India use another single concentrating technology known as Scheffler reflector. This technology was first developed by Wolfgang Scheffler in 1986. A Scheffler reflector is a parabolic dish that uses the Sun’s daily race. These reflectors have a flexible reflective surface that is able to change its curvature to adjust to seasonal variations in the incident angle of sunlight. Scheffler reflectors have the advantage of having a fixed focal point which improves the ease of cooking and can reach temperatures of 450-650 ° C. ^[23] Built in 1999 by the Brahma Kumaris , the world’s largest Scheffler reflector system in Abu Road, Rajasthan India is capable of cooking up to 35,000 meals a day.^[24] By early 2008, over 2000 large cookers of the Scheffler design had been built worldwide.

Distillation

Solar stills can be used to make water in areas where clean water is not common. Solar distillation is necessary in these situations to provide people with purified water. Solar energy heats the water in the still. The water then evaporates and condenses on the bottom of the covering glass. ^[19]

High-temperature collectors

Where: 95 ° C are sufficient, as for space heating, flat-plate collectors of the nonconcentrating type used. Because of the relatively high heat losses through the glazing, flat plate collectors will not reach temperatures much above 200 ° C even when the heat transfer fluid is stagnant. Such temperatures are too low for efficient conversion to electricity.

The efficiency of heat engines increases with the temperature of the heat source. To accomplish achieve this in thermal solar energy plants, solar radiation is Concentrated by mirrors or lenses to obtenir Higher temperature – a technology called Expired Concentrated Solar Power (CSP). The practical effect of high efficiencies is to reduce the plant’s size and total cost of production.

As the temperature increases, different forms of conversion become practical. Up to 600 ° C, steam turbines , standard technology, have an efficiency up to 41%. Above 600 ° C, gas turbines can be more efficient. Higher temperatures are problematic because different materials and techniques are needed. One proposal for very high temperatures is to use liquid fluoride salts operating between 700 ° C to 800 ° C, using multi-stage turbine systems to achieve 50% or more thermal efficiencies. ^[25] The higher operating temperaturesthe plant to use higher-temperature dry heat exchangers for its thermal exhaust, reducing the plant’s water use – critical in the deserts where large solar plants are practical. High temperatures also make heat storage more efficient, because they are stored per unit of fluid.

Commercial concentrating solar thermal power (CSP) plants were first developed in the 1980s. The world’s largest solar thermal power plants are now the 370 MW Ivanpah Solar Power Facility , commissioned in 2014, and the 354 MW SEGS CSP installation, both located in the Mojave Desert of California, where several other solar projects have been made. With the exception of the Shams solar power station , built in 2013 near Abu Dhabi , the United Arab Emirates, all other 100 MW or larger CSP plants are located in the United States or in Spain.

The main advantage of CSP is the ability to efficiently add thermal storage, allowing the dispatching of electricity over a 24-hour period. Since peak electricity demand Typically OCCURS entre about 4 and 8 pm, ^[26] Many CSP power plants use 3 to 5 hours of thermal storage. With current technology, storage of heat is much cheaper and more efficient than storage of electricity. In this way, the CSP plant can produce electricity day and night. If the CSP site has predictable solar radiation, then the CSP plant becomes a reliable power plant. Reliability can be further improved by installing a back-up combustion system. The back-up system can be used most of the CSP plant, which decreases the cost of the back-up system.

CSP facilities utilize high electrical conductivity materials, such as copper , in field power cables , grounding networks, and motors for tracking and pumping fluids, as well as in the main generator and high voltage transformers . (See: Copper in Concentrating Solar Thermal Power Facilities .)

With reliability, unused desert, no pollution, and no fuel costs, the obstacles for large deployment for CSP are cost, aesthetics, land use and similar factors. Although a small percentage of the world is necessary to meet global electricity demand, a large area still needs to be covered with mirrors or lenses to obtain a significant amount of energy. An important way to decrease cost is the use of a simple design.

When Considering land use impacts associated with the exploration and extraction through to transportation and conversion of fossil fuels , qui are used for MOST of our electrical power, utility-scale solar power COMPARED as One of The Most land-efficient energy resources available: ^{[27 ]}

The federal government has devoted nearly 2,000 times more than just solar development. In 2010 the Bureau of Land Management approved nine large-scale solar projects, with a total generating capacity of 3,682 megawatts, representing approximately 40,000 acres. In contrast, in 2010, the Bureau of Land Management handled more than 5,200 applications and released 1,308 leases, for a total of 3.2 million acres. Currently, 38.2 million acres of onshore public lands and an additional 36.9 million acres of offshore exploration in the Gulf of Mexico are under lease for oil and gas development, exploration and production. ^[27]

System designs

During the day the sun has different positions. For low concentration systems (and low temperatures) tracking can be avoided if or notimaging optics are used. ^[28] [29] For higher concentrations, however, if the mirrors or lenses do not move, then the focus of the mirrors or lenses changes (but also in these cases nonimaging optics provides widest acceptance angles for a given concentration). Therefore, unavoidable It Seems That There needs to be a tracking system That follows the position of the sun (for solar photovoltaic ‘s solar trackeris only optional). The tracking system increases the cost and complexity. With this in mind, different designs can be distinguished in the light of the sun.

Parabolic trough designs

Parabolic trough power plants use a curved, mirrored trough which reflects the radiation on a glass tube containing a fluid (also called a receiver, absorb or collector) running the length of the trough, at the focal point of the reflectors. The trough is parabolic along one axis and linear in the orthogonal axis. For change of the daily position of the sun perpendicular to the receiver, the trough tilts east to west so that the direct radiation remains focused on the receiver. However, seasonal changes in the angle of sunlight parallelto the trough does not require adjustment of the mirrors, since the light is simply collected elsewhere on the receiver. Thus the trough design does not require tracking on a second axis. The receiver may be enclosed in a glass vacuum chamber. The vacuum significantly reduces convective heat loss.

A fluid (also called heat transfer fluid) passes through the receiver and becomes very hot. Common fluids are synthetic oil, molten salt and pressurized steam. The fluid containing the heat is transported to a heat engine where a third of the heat is converted to electricity.

Full-scale parabolic trough systems consist of many such troughs laid out in parallel over a large area of ​​land. Since 1985 a solar thermal system using California in the United States . It is called the Solar Energy Generating Systems (SEGS) system. ^[30] Other CSP designs lack this kind of long experience and it can be said that the parabolic trough design is the most thoroughly proven CSP technology.

The SEGS is a collection of nine plants with a total capacity of 354 MW and has been the world’s largest solar power plant, both thermal and non-thermal, for many years. A new plant is Nevada Solar One plant with a capacity of 64 MW. The 150 MW Andasol solar power stationsare in Spain with each site having a capacity of 50 MW. Note however, that those plants have heat storage which requires a larger field of solar collectors relative to the size of the steam turbine-generator to store heat and send heat to the steam turbine at the same time. Heat storage enables better use of the steam turbine. Andasol 1 at 50 MW peak capacity produces more energy than Nevada Solar One at 64 MW peak capacity, due to the plant’s thermal energy storage system and larger solar field. The 280MW Solana Generating Station came online in Arizona in 2013 with 6 hours of power storage. Hassi R’Mel integrated solar combined cycle power station in AlgeriaMartin Next Generation Solar Energy Center both uses parabolic troughs in a combined cycle with natural gas.

Enclosed trough

The enclosed trough architecture encapsulates the solar thermal system within a greenhouse-like glasshouse. The glasshouse creates a protected environment and the elements that can negatively impact the reliability and efficiency of the solar thermal system. ^[31]

Lightweight curved solar-reflecting mirrors are suspended within the glasshouse structure. A single-axis tracking system positions the mirrors to track the sun and focus its light onto a network of stationary steel pipes, also suspended from the glasshouse structure. ^[32] Steam is generated directly, using oil field-quality water, as it flows through the length of the pipes, without heat exchangers or intermediate working fluids.

The steam produced is then fed directly to the field’s existing steam distribution network, where the steam is continuously injected into the oil reservoir. Sheltering the mirrors from the wind allows them to achieve higher temperatures and prevents dust from building up. ^[31] GlassPoint Solar , the company that created the Enclosed Trough design, states its technology for EORfor about $ 5 per million. ^[33]

Power tower designs

Power towers (also known as’ central tower ‘power plants or’ heliostat’power plants) capture and focus the sun’s thermal energy with thousands of tracking mirrors (called heliostats) in a two square mile field. A tower resides in the center of the heliostat field. The heliostats focus concentrated sunlight on the top of the tower. Within the receiver the sunlight heats up to over 1,000 ° F (538 ° C). The heated molten salt then flows into a thermal storage tank where it is stored, maintaining 98% thermal efficiency, and eventually pumped to a steam generator. The steam drives has a standard turbine to generate electricity. This process, also known as the “Rankine cycle”, is similar to a standard coal-fired power plant, except that it is fueled by clean and free solar energy.

The advantage of this design above the parabolic trough design is the higher temperature. Thermal energy at higher temperatures can be more easily achieved. Furthermore, there is less need to flatten the ground area. In principle a power tower can be built on the side of a hill. Mirrors can be flat and plumbing is concentrated in the tower. The disadvantage is that each mirror must have its own dual-axis control, while in the parabolic trough design single axis tracking can be shared for a large array of mirrors.

A cost / performance comparison between power tower and parabolic trough concentrators was made by the NREL which estimated that by 2020 electricity could be produced from power towers for 5.47 ¢ / kWh and for 6.21 ¢ / kWh from parabolic troughs. The capacity factor for power towers is estimated to be 72.9% and 56.2% for parabolic troughs. ^[34] There is some hope that the development of cheap, sustainable, mass producible heliostat power plant components could bring this cost down. ^[35]

The first trade tower power plant Was PS10 in Spain with a capacity of 11 MW, completed in 2007, since then a number of plants-have beens Proposed, SEVERAL-have-been built were number of countries (Spain, Germany, US, Turkey, China , India) but several proposed plants were canceled as photovoltaic solar prices plummeted. A solar power tower is expected to come online in South Africa in 2014. ^{[36] The} Ivanpah Solar Power Facility in California provides 392 MW of electricity from three towers, making it the largest solar power tower.

Dish designs

CSP-Stirling is known to have the highest efficiency of all solar technologies (around 30%, compared to solar photovoltaic’s approximately 15%), and is predicted to be able to produce all energy sources in high-scale production. hot areas, semi-deserts, etc. A dish Stirling system uses a large, reflective, parabolic dish (similar in shape to a satellite television dish). It focuses on the subject of a dish, where a receiver catches the heat and transforms it into a useful form. Typically the dish is combined with a Stirling engine in a Dish-Stirling System, but also sometimes a steam engine is used. ^[37]These create a rotational kinetic energy that can be converted to electricity using an electric generator. ^[38]

In 2005 Southern California Edison announced an agreement to purchase solar powered Stirling engines from Stirling Energy Systems over twenty-year period and in quantities (20,000 units) sufficient to generate 500 megawatts of electricity. In January 2010, Stirling Energy Systems and Tessera Solar commissioned the first demonstration 1.5-megawatt power plant (“Maricopa Solar”) using Stirling technology in Peoria, Arizona. ^[39] At the beginning of 2011 Stirling Energy’s development arm, Tessera Solar, sold off its two large projects, the 709 MW Imperial project and the 850 MW Calico project to AES Solar and K.Road, respectively. ^[40] [41] In 2012 the Maricopa plant was bought and dismantled byUnited Sun Systems . ^[42] United Sun Systems released a new generation system , based on a V-shaped Stirling engine and peak production of 33 kW. The new CSP-Stirling technology brings down LCOE to USD 0.02 in utility scale. ^{[ quote needed ]}

According to its developer, Rispasso Energy , a Swedish firm, in 2015 its Dish Sterling System being tested in the Kalahari Desert in South Africa showed 34% efficiency. ^[43]

Fresnel technologies

A linear Fresnel reflectorpower plant uses a series of long, shallow-curvature (or even flat) mirrors to focus on one or more linear receivers above the mirrors. A small parabolic mirror can be attached to further light. These systems are multiplexed by a multi-mirrored system, while still using the single line-focus geometry with one axis for tracking. This is similar to the trough design (and different from central towers and dishes with dual-axis). The receiver is stationary and the fluid couplings are not required (as in troughs and dishes). The mirrors also do not need to support the receiver, so they are structurally simpler.

Rival single axis tracking technologies include the relatively new linear Fresnel reflector (LFR) and compact-LFR (CLFR) technologies. The LFR differs from that of the parabolic trough in that the absorb is fixed in space above the mirror field. Also, the reflector is composed of many low-ranged segments, which focus collectively on an elevated long tower receiver running parallel to the reflector rotational axis. ^[44]

Prototypes of Fresnel lens concentrators have been produced by International Automated Systems . ^[45] No full-scale thermal systems using Fresnel lenses are already available. ^[46]

MicroCSP

MicroCSP is used for community-sized power plants (1 MW to 50 MW), for industrial, agricultural and manufacturing ‘process heat’ applications, and when large amounts of water are needed, such as resort swimming pools, water parks, large laundry facilities, sterilization, distillation and other such uses.

Enclosed parabolic trough

The enclosed parabolic trough solar thermal system encapsulates the components within an off-the-shelf greenhouse type of glasshouse. The glasshouse protects the components from the elements that can negatively impact system reliability and efficiency. This protection importantly includes nightly glass-roof washing with optimized water-efficient off-the-shelf automated washing systems. ^[31] Lightweight curved solar-reflecting mirrors are suspended from the ceiling of the glasshouse by wires. A single-axis tracking system positions the mirrors to retrieve the optimal amount of sunlight. The mirrors concentrate the sunlight and focus on a network of stationary steel pipes, also suspended from the glasshouse structure. ^[32]Water is pumped through the pipes and boiled to generate steam when intense sun radiation is applied. The steam is available for heat. Sheltering the mirrors from the wind allows them to achieve higher temperatures and prevents them from falling into the atmosphere. ^[31]

Heat collection and exchange

More energy is contained in higher frequency light {\ displaystyle E = h \ nu}where is the Planck constant and{\ displaystyle \ nu}is frequency. Metal collectors downconvert to higher frequency light by producing a series of compton shifts into an abundance of lower frequency light. Glass or ceramic coatings with high transmission in the visible and UV and effective absorption in the IR (heat blocking) trap metal absorbed low frequency light from radiation loss. Convection insulation against mechanical losses. Once collected as heat, thermos containment efficiency improves significantly with increased size. Unlike Photovoltaic technologies, which are often used in the light of solar energy, Solar Thermal depends on the environment.

Heat gain in heat gain; heat transfer ; heat storage ; heat transport ; and heat insulation . ^[47] Here, heat is the measure of the amount of thermal energy and is determined by the temperature, mass and specific heat of the object. Solar thermal power plants use heat exchangers that are designed for constant working conditions, to provide heat exchange. Copper heat exchangersare important in solar thermal heating and cooling systems because of their high thermal conductivity, resistance to atmospheric and water corrosion, sealing and joining by soldering, and mechanical strength. Copper is used both in primary and secondary circuits (pipes and heat exchangers for water tanks) of solar thermal water systems. ^[48]

Heat gain is the heat accumulated from the sun in the system. Solar thermal heat is trapped using the greenhouse effect; the greenhouse effect in this case is the ability of a reflective surface to transmit short wave radiation and reflect long wave radiation. Heat and infrared radiation (IR) are produced when the wave is absorbed by the light of the light, which is then trapped inside the collector. Fluid, usually water, in the tubes absorbs the heat and transfer it to a heat storage vault.

Heat is transferred by conduction or convection. When water is heated, it is conductive to water molecules throughout the medium. These molecules spread their thermal energy by conduction and occupy more space than the cold slow moving molecules above them. The distribution of energy from the rising hot water to the sinking water supply to the convection process. Heat is transferred from the absorbing plates of the collector in the fluid by conduction. The fluid collector is circulated through the carrier pipes to the heat transfer vault. Inside the vault, heat is transmitted throughout the medium through convection.

Heat storage allows solar thermal plants to produce electricity during hours without sunlight. Heat is transferred to a thermal storage medium in an insulated reservoir during sunlight, and is withdrawn for power generation during lacking sunlight. Thermal storage mediums will be discussed in a heat storage section. Rate of heat transfer is related to the conductive and convection medium and temperature differences. With higher temperature differences.

Heat transport refers to the activity in which heat from a solar collector is transported to the heat storage vault. Heat insulation is vital in both heat transport tubing and vault storage. It prevents heat loss, which in turn relates to energy loss, or decrease in the efficiency of the system.

Heat storage for space heating

A collection of mature technologies called seasonal thermal energy storage (STES) is capable of storing heat for months at a time, so it can be used for all-year heating. Solar-supplied STES technology has been developed primarily in Denmark, ^[49] Germany, ^[50] and Canada, ^[51] and applications include individual buildings and district heating networks. Drake Landing Solar Community in Alberta, Canada has a record of providing 97% of the community’s all-year space heating needs from the sun. ^[52]STES thermal storage mediums include deep aquifers; native rock surrounding clusters of small-diameter, heat exchanger equipped boreholes; wide, shallow, lined pits that are filled with gravel and top-insulated; and large, insulated and buried surface water tanks.

Heat storage to stabilize solar electric power generation

Heat storage allows a solar thermal plant to produce electricity at night and overcast days. This Allows the use of solar power for baseload generation as well as peak power generation , with the potential of Displacing Both coal- and natural gas-fired power plants. Additionally, the use of the generator is higher. Even short term storage can help by smoothing out the ” duck curve ” of rapid changes in generation requirements at sunset When A grid includes wide water equivalent of solar capacity.

Heat is transferred to a thermal storage medium in an insulated reservoir during the day, and withdrawn for power generation at night. Thermal storage media include pressurized steam, concrete, a variety of materials phase changes, and molten salts Such as calcium, sodium and potassium nitrate. ^[53] [54]

Steam accumulator

The PS10 solar power tower stores heat in tanks as pressurized steam at 50 bar and 285 ° C. The steam condenses and flashes back to steam, when pressure is lowered. Storage is for one hour. It is suggested that it is possible, but that it has not been possible in an existing power plant. ^[55]

Molten salt storage

A variety of fluids have been tested to transport the sun’s heat, including water, air, oil, and sodium, Rockwell International’s goal ^[57] selected molten salt as best. ^[58] Molten salt is used in solar power tower systems because of its low temperature, low temperature, high temperature, low temperature, high temperature, low temperature, high temperature, high temperature, high temperature and high temperature. Molten salt is used in the chemical and metals industries to transport heat, so the industry has experience with it.

The first commercial salt solution , 60% sodium nitrate and 40% potassium nitrate . Saltpeter melts at 220 ° C (430 ° F) and is kept liquid at 290 ° C (550 ° F) in an insulated storage tank. Calcium nitrate can reduce the melting point to 131 ° C, permitting more energy to be extracted before the salt freezes. There are several technical calcium nitrate grades stable at more than 500 ° C.

This solar power system can generate power in a cloudy weather or at night using the heat in the hot salt tank. The tanks are insulated, able to store heat for a week. Tanks that power a 100-megawatt turbine for four hours would be about 9 m (30 ft) tall and 24 m (80 ft) in diameter.

The Andasol power plant in Spain is the first commercial solar thermal power plant using molten salt for heat storage and nighttime generation. It came on line March 2009. ^[59] On July 4, 2011, a company in Spain celebrated a historic moment for the solar industry: Torresol’s 19.9 MW concentrating solar power plant became the first ever to generate uninterrupted electricity for 24 hours straight, using a molten salt heat storage. ^[60]

In 2016 SolarReserve proposed a 2 GW, $ 5 billion concentrated solar plant in Nevada.

Phase-change materials for storage

Phase Change Material (PCMs) offers an alternative solution in energy storage. ^[61] PCMs have the potential of providing a more efficient means of storage. PCMs can be organic or inorganic materials. Advantages of organic PCMs include no corrosive, low or no undercooling, and chemical and thermal stability. Disadvantages include low phase-change enthalpy, low thermal conductivity, and flammability. Inorganics are advantageous with greater phase-change enthalpy, but exhibit disadvantages with undercooling, corrosion, phase separation, and lack of thermal stability. The greater phase-change enthalpy in inorganic PCMs make hydrates a strong candidate in the solar energy storage field. ^[62]

Use of water

A design which requires water for condensation or cooling with solar radiation. The conflict is illustrated by plans of Solar Millennium , a German company, to build a plant in the Amargosa Valley of Nevada which would require 20% of the water available in the area. Some other targets in the Mojave Desert of California may also be affected by difficulty in obtaining adequate water supplies. California water law currently prohibits the use of drinking water for cooling. ^[63]

Other designs require less water. The Ivanpah Solar Power Facility in south-eastern California canned water by using air-cooling to convert the steam back into water. Compared to conventional wet-cooling, this results in a 90% reduction in water use at the cost of some loss of efficiency. The water is then returned to the boiler in a closed process which is environmentally friendly. ^[64]

Conversion rates from solar energy to electrical energy

Of all of these technologies the solar dish / Stirling engine has the highest energy efficiency. A single solar dish- Stirling Engine Installed at Sandia National Laboratories National Solar Thermal Test Facility (NSTTF) produces as much as 25 kW of electricity, with a conversion efficiency of 31.25%. ^[65]

Solar parabolic trough plants have been built with efficiencies of about 20%. ^{[ citation needed ]} Fresnel reflectors have an efficiency that is slightly lower (but this is compensated by denser packing).

The gross conversion efficiencies (taking into account that the solar dishes or troughs occupy only a fraction of the total area of ​​the power plant) are determined by net generating capacity over the solar energy that falls on the total area of ​​the solar plant. The 500-megawatt (MW) SCE / SES plant would extract about 2.75% of the radiation (1 kW / m²; see Solar power for a discussion) that falls on its 4,500 acres (18.2 km²). ^[66] For the 50 MW AndaSol Power Plant ^[67] that is being built in Spain (total area of ​​1,300 × 1,500 m = 1.95 km²) gross conversion efficiency comes out at 2.6%.

Furthermore, efficiency is not directly related to cost: it is calculated on the basis of total cost, both efficiency and cost of construction and maintenance should be taken into account.

Standards

• EN 12975 (efficiency test)

See also

Notes

References