Water turbine

Water turbine


A water turbine is a rotary engine that takes energy from moving water.

Water turbines were developed in the nineteenth century and were widely used for industrial power prior to electrical grids. Now they are mostly used for electric power generation. They harness a clean and renewable energy source.

Kaplan turbine and electrical generator cut-away view.

History

Swirl

Water wheels have been used for thousands of years for industrial power. Their main shortcoming is size, which limits the flow rate and head that can be harnessed.

The migration from water wheels to modern turbines took about one hundred years. Development occurred during the Industrial revolution, using scientific principles and methods. They also made extensive use of new materials and manufacturing methods developed at the time.

The word turbine was coined by the French engineer Claude Bourdin in the early 19th century and is derived from the Latin word for "whirling" or a "vortex". The main difference between early water turbines and water wheels is a swirl component of the water which passes energy to a spinning rotor. This additional component of motion allowed the turbine to be smaller than a water wheel of the same power. They could process more water by spinning faster and could harness much greater heads. (Later, impulse turbines were developed which didn't use swirl).

The rotor of the small water turbine

Time line

Ján Andrej Segner developed a reactive water turbine in the mid-1700s. It had a horizontal axis and was a precursor to modern water turbines. It is a very simple machine that is still produced today for use in small hydro sites. Segner worked with Euler on some of the early mathematical theories of turbine design.

In 1820, Jean-Victor Poncelet developed an inward-flow turbine.

In 1826 Benoit Fourneyron developed an outward-flow turbine. This was an efficient machine (~80%) that sent water through a runner with blades curved in one dimension. The stationary outlet also had curved guides.

In 1844 Uriah A. Boyden developed an outward flow turbine that improved on the performance of the Fourneyron turbine. Its runner shape was similar to that of a Francis turbine.

A Francis turbine runner, rated at nearly one million hp (750 MW), being installed at the Grand Coulee Dam

In 1849, James B. Francis improved the inward flow reaction turbine to over 90% efficiency. He also conducted sophisticated tests and developed engineering methods for water turbine design. The Francis turbine, named for him, is the first modern water turbine. It is still the most widely used water turbine in the world today.

A propeller-type runner rated 28,000 hp (21 MW)

Inward flow water turbines have a better mechanical arrangement and all modern reaction water turbines are of this design. Also, as the swirling mass of water spins into a tighter rotation, it tries to speed up to conserve energy. This property acts on the runner, in addition to the water's falling weight and swirling motion. Water pressure decreases to zero as it passes through the turbine blades and gives up its energy.

Around 1890, the modern fluid bearing was invented, now universally used to support heavy water turbine spindles. As of 2002, fluid bearings appear to have a mean time between failures of more than 1300 years.

Around 1913, Victor Kaplan created the Kaplan turbine, a propeller-type machine. It was an evolution of the Francis turbine but revolutionized the ability to develop low-head hydro sites.

A new concept

All common water machines until the late 19th century (including water wheels) were reaction machines; water's pressure head acted on the machine and produced work. A reaction turbine needs to fully contain the water during energy transfer.

Figure from Pelton's original patent (October 1880)

In 1866, California millwright Samuel Knight invented a machine that worked off a completely different concept. Inspired by the high pressure jet systems used in hydraulic mining in the gold fields, Knight developed a bucketed wheel which captured the energy of a free jet, which had converted a high head (hundreds of vertical feet in a pipe or penstock) of water to kinetic energy. This is called an impulse or tangential turbine. The water's velocity, roughly twice the velocity of the bucket periphery, does a u-turn in the bucket and drops out of the runner at 0 velocity.

In 1879, Lester Pelton, experimenting with a Knight Wheel, developed a double bucket design, which exhausted the water to the side, eliminating some energy loss of the Knight wheel which exhausted some water back against the center of the wheel. In about 1895, William Doble improved on Pelton's half-cylindrical bucket form with an elliptical bucket that included a cut in it to allow the jet a cleaner bucket entry. This is the modern form of the Pelton turbine which today achieves up to 92% efficiency. Pelton had been quite an effective promoter of his design and although Doble took over the Pelton company he did not change the name to Doble because it had brand name recognition.

Turgo and Crossflow turbines were later impulse designs.

Theory of operation

Flowing water is directed on to the blades of a turbine runner, creating a force on the blades. Since the runner is spinning, the force acts through a distance (force acting through a distance is the definition of work). In this way, energy is transferred from the water flow to the turbine.

Water turbines are divided into two groups; reaction turbines and impulse turbines.

The precise shape of water turbine, whatever its design, is driven by the supply pressure of water.

Reaction turbines

Reaction turbines are acted on by water, which changes pressure as it moves through the turbine and gives up its energy. They must be encased to contain the water pressure (or suction), or they must be fully submerged in the water flow.

Newton's third law describes the transfer of energy for reaction turbines.

Most water turbines in use are reaction turbines. They are used in low and medium head applications.

Impulse turbines

Impulse turbines change the velocity of a water jet. The jet impinges on the turbine's curved blades which reverse the flow. The resulting change in momentum (impulse) causes a force on the turbine blades. Since the turbine is spinning, the force acts through a distance (work) and the diverted water flow is left with diminished energy.

Prior to hitting the turbine blades, the water's pressure (potential energy) is converted to kinetic energy by a nozzle and focused on the turbine. No pressure change occurs at the turbine blades, and the turbine doesn't require a housing for operation.

Newton's second law describes the transfer of energy for impulse turbines.

Impulse turbines are most often used in very high head applications.

Types of water turbines

Reaction turbines:
Francis
Kaplan, Propeller, Bulb, Tube, Straflo
Tyson
Water wheel

Impulse turbines:
Pelton
Turgo
Michell-Banki (also known as the Crossflow or Ossberger turbine)

Maintenance


A Francis turbine at the end of its life showing cavitation pitting, fatigue cracking and a catastrophic failure. Earlier repair jobs that used stainless steel weld rods are visible.

Turbines are designed to run for decades with very little maintenance of the main elements; overhaul intervals are on the order of several years. Maintenance of the runners and parts exposed to water include removal, inspection, and repair of worn parts.

Normal wear and tear is pitting from cavitation, fatigue cracking, and abrasion from suspended solids in the water. Steel elements are repaired by welding, usually with stainless steel rod. Damage areas are cut or ground out, then welded back up to their original or an improved profile. Old turbine runners may have a significant amount of stainless steel added this way by the end of their lifetime. Elaborate welding procedures may be used to achieve the highest quality repairs.

Other elements requiring inspection and repair during overhauls include bearings, packing box and shaft sleeves, servomotors, cooling systems for the bearings and generator coils, seal rings, wicket gate linkage elements and all surfaces.

Environmental impact

Water turbines have had both positive and negative impacts on the environment.

They are one of the cleanest producers of power, replacing the burning of fossil fuels and eliminating nuclear waste. They use a renewable energy source and are designed to operate for decades. They produce significant amounts of the world's electrical supply.

Historically there have also been negative consequences. Most water turbines require a dam that can interrupt the natural ecology of rivers, killing fish, stopping migrations, and disrupting peoples' livelihoods. For example, American Indian tribes in the Pacific Northwest had livelihoods built around salmon fishing, but aggressive dam-building destroyed their way of life. Since the late 20th century, it has been possible to construct hydropower systems that divert fish and other organisms away from turbine intakes without significant damage or loss of power; such systems require less cleaning but are substantially more expensive to construct. In the United States, it is now illegal to block the migration of fish so fish ladders must be provided by dam builders.

Turgo turbine

Turgo turbine

The Turgo turbine is an impulse water turbine designed for medium head applications. Operational Turgo Turbines achieve efficiencies of about 87%. In factory and lab tests Turgo Turbines perform with efficiencies of up to 90%.

Turgo turbine and generator

Developed in 1919 by Gilkes as a modification of the Pelton wheel, the Turgo has some advantages over Francis and Pelton designs for certain applications.

First, the runner is less expensive to make than a Pelton wheel. Second, it doesn't need an airtight housing like the Francis. Third, it has higher specific speed and can handle a greater flow than the same diameter Pelton wheel, leading to reduced generator and installation cost.

Turgos operate in a head range where the Francis and Pelton overlap. While many large Turgo installations exist, they are also popular for small hydro where low cost is very important.

Like all turbines with nozzles, blockage by debris must be prevented for effective operation.

Theory of operation


The Turgo turbine is an impulse type turbine; water does not change pressure as it moves through the turbine blades. The water's potential energy is converted to kinetic energy with a nozzle. The high speed water jet is then directed on the turbine blades which deflect and reverse the flow. The resulting impulse spins the turbine runner, imparting energy to the turbine shaft. Water exits with very little energy. Turgo runners may have an efficiency of over 90%.

A Turgo runner looks like a Pelton runner split in half. For the same power, the Turgo runner is one half the diameter of the Pelton runner, and so twice the specific speed. The Turgo can handle a greater water flow than the Pelton because exiting water doesn't interfere with adjacent buckets.

The specific speed of Turgo runners is between the Francis and Pelton. Single or multiple nozzles can be used. Increasing the number of jets increases the specific speed of the runner by the square root of the number of jets (four jets yield twice the specific speed of one jet on the same turbine ).

Solar power tower

Solar power tower

Solar Two, a concentrating solar power plant.

Power towers (also know as 'central tower' power plants or 'heliostat' power plants) use an array of flat, moveable mirrors (called heliostats) to focus the sun's rays upon a collector tower (the target). The high energy at this point of concentrated sunlight is transferred to a substance that can store the heat for later use. The more recent heat transfer material that has been successfully demonstrated is liquid sodium. Sodium is a metal with a high heat capacity, allowing that energy to be stored and drawn off throughout the evening. That energy can, in turn, be used to boil water for use in steam turbines. Water had originally been used as a heat transfer medium in earlier power tower versions (where the resultant steam was used to power a turbine). This system did not allow for power generation during the evening. Examples of heliostat based power plants are the 10 MWe Solar One (later called Solar Two), and the 15 MW Solar Tres plants. Neither of these are currently used for active energy generation. In South Africa, a solar power plant is planned with 4000 to 5000 heliostat mirrors, each having an area of 140 m².Another design is a pyramid shaped structure - solar pyramid - which works by drawing in air, heating it with solar energy and moving it through turbines to generate electricity. Currently India is buiiding such pyramids.

Climate effect of solar radiation

Climate effect of solar radiation


Solar irradiance spectrum above atmosphere and at surface

On Earth, solar radiation is obvious as daylight when the sun is above the horizon. This is during daytime, and also in summer near the poles at night, but not at all in winter near the poles. When the direct radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright yellow light (sunlight in the strict sense) and heat. The heat on the body, on objects, etc., that is directly produced by the radiation should be distinguished from the increase in air temperature.

The amount of radiation intercepted by a planetary body varies as the square of the distance between the star and the planet. The Earth's orbit and obliquity change with time (over thousands of years), sometimes forming a nearly perfect circle, and at other times stretching out to an orbital eccentricity of 5% (currently 1.67%). The total insolation remains almost constant but the seasonal and latitudinal distribution and intensity of solar radiation received at the Earth's surface also varies . For example, at latitudes of 65 degrees the change in solar energy in summer & winter can vary by more than 25% as a result of the Earth's orbital variation. Because changes in winter and summer tend to offset, the change in the annual average insolation at any given location is near zero, but the redistribution of energy between summer and winter does strongly affect the intensity of seasonal cycles. Such changes associated with the redistribution of solar energy are considered a likely cause for the coming and going of recent ice ages.

Solar radiation

Solar radiation


The electromagnetic radiation and particles (electrons, protons, alpha particles, and rarer heavy atomic nuclei) emitted by the Sun. The electromagnetic radiation covers a wavelength range from x-rays to radio waves, that is, from about 0.01 nanometer to 30 km. The annual mean irradiance at Earth, integrated over the whole spectrum, amounts to 1365 W · m−2, and 99% of its energy is carried by radiation with wavelengths between 278 and 4600 nm, with the maximum at 472 nm. See also Electromagnetic radiation; Solar constant.

The Sun also emits a continuous stream of particles, the solar wind, which originates in coronal holes and the upper corona. Explosive events on the Sun, the solar flares and coronal mass ejections, emit particles that are much more energetic and numerous than those of the solar wind. Solar flares are produced by the most powerful explosions, releasing energies of up to 1025 joules in 100–1000 s and high-speed electrons that emit intense radiation at radio and x-ray wavelengths. They also produce nuclear reactions in the solar atmosphere with the emission of gamma rays and of neutrons that move nearly at the speed of light. Coronal mass ejections expand away from the Sun at speeds of hundreds of kilometers per second, becoming larger than the Sun and removing up to 5 × 1013 kg of coronal material. Both events are believed to be ignited by the reconnection of magnetic fields. If the emitted particles reach the Earth, they give rise to the aurora at high latitudes, and they can damage satellites, endanger humans in space, and on the Earth disturb telecommunications and even disrupt power systems. See also Aurora; Cosmic rays; Ionosphere; Magnetosphere; Solar wind; Sun.

The emission and diffusion of actinic rays from the sun. Overexposure may result in sunburn, keratosis, skin cancer, or lesions associated with photosensitivity.

The soldering process.

Solar furnace

Solar furnace


A solar furnace is a structure used to harness the rays of the sun in order to produce high temperatures. This is achieved by using a curved mirror (or an array of mirrors) acting as a parabolic reflector to concentrate light (Insolation) on to a focal point. The temperature at the focal point may reach up to 3,000 degrees Celsius, and this heat can be used to generate electricity, melt steel or make hydrogen fuel.

The solar furnace at Odeillo in the French Pyrenees can reach temperatures up to 3,000 degrees Celsius.

The solar furnace at Odeillo in the Pyrenees of France was opened in 1970 and is the largest in the world. It employs an array of plane mirrors to gather the rays of light from the sun and reflects them on to a larger curved mirror. The rays are focused on to an area the size of a cooking pot and can reach up to 3,000 degrees Celsius.

The first modern solar furnace is believed to have been built in France in 1949 by Professor Félix Trombe. It is still in place at Mont Louis, near to Odeillo. The Pyrenees were chosen as the site for these furnaces due to the weather being sunny for up to 300 days a year.

It has been suggested that solar furnaces could be used in space to provide energy for manufacturing purposes, although their reliance on sunny weather means that they are unlikely to be used as a major source of renewable energy on Earth.

The principle of the solar furnace may have been known in ancient times. During the Second Punic War (218 - 202 BC), the Greek scientist Archimedes is said to have repelled the attacking Roman ships by setting them on fire with a "burning glass" that may have been array of mirrors. An experiment to test this theory was carried out by a group at the Massachusetts Institute of Technology in 2005. It concluded that although the theory was sound, the mirrors would have been unlikely to produce sufficient power to set a ship on fire under battle conditions.

The principle of the solar furnace has also been used to make solar powered barbecues.

Solar tracker

Solar tracker


A solar tracker is a device for orienting a solar photovoltaic panel or concentrating solar reflector or lens toward the sun. Concentrators, especially in solar cell applications, require a high degree of accuracy to ensure that the concentrated sunlight is directed precisely to the powered device, which is at (or near) the focal point of the reflector or lens. Non-concentrating applications require less accuracy, and a tracker is not necessary, but can substantially improve the amount of power produced by a system by enhancing morning and afternoon performance. Strong afternoon performance is particularly desirable for grid-tied photovoltaic systems, as production at this time will match the peak demand time for summer season air-conditioning. A fixed system oriented to optimize this limited time performance will have a relatively low annual production.

A backyard installation of passive single–axis trackers, DC rated at 2340 watts. Seen here in winter position, tilted toward the south. The tall poles allow walk-under and use of the ground space underneath the panels for plantings that thrive on protection from the severe summer midday sun at this location.

For low-temperature solar thermal applications, trackers are not usually used, owing to the relatively high expense of trackers compared to adding more collector area and the more restricted solar angles required for winter performance, which influence the average year-round system capacity. Compared to photovoltaics, trackers can be relatively inexpensive. This makes them especially effective for photovoltaic systems using high-efficiency panels. Some solar trackers may operate most effectively with seasonal position adjustment and most will need inspection and lubrication on an annual basis.

Tracker mount types

Solar trackers may be active or passive and may be single axis or dual axis. Single axis trackers usually use a polar mount for maximum solar efficiency. Single axis trackers will usually have a manual elevation (axis tilt) adjustment on a second axis which is adjusted on regular intervals throughout the year. There are two types of dual axis trackers, polar and altitude-azimuth.

Polar

Polar trackers have one axis aligned close to the axis of rotation of the earth, hence the name polar. By this definition, only high accuracy astronomical telescope mounts rotate on an axis parallel to the earth's axis. For solar trackers, so called "polar" trackers have their axis aligned perpendicular to the "ecliptic" (an imaginary disc containing the apparent path of the sun).

Simple solar trackers are manually adjusted to compensate for the shift of the ecliptic through the seasons. Adjustment is usually at least twice a year at the equinoxes; once to establish a position for autumn and winter, and a second adjustment for spring and summer. Such trackers are also referred to as "single axis", because only one drive mechanism is needed for daily operation. This reduces the cost and allows the use of passive tracking methods (described below).

Horizontal axle

Wattsun HZ-Series Linear Axis Tracker in South Korea

Several manufactures can deliver single axis horizontal axis trackers which may be oriented by either passive or active mechanisms, depending upon manufacturer. In these, a long horizontal tube is supported on bearings mounted upon pylons or frames. The axis of the tube is on a North-South line. Panels are mounted upon the tube, and the tube will rotate on its axis to track the apparent motion of the sun through the day. Since these do not tilt toward the equator they are not especially effective during winter mid day (unless located near the equator), but add a substantial amount of productivity during the spring and summer seasons when the solar path is high in the sky. These devices are less effective at higher lattitudes. The principal advantage is the inherent robustness of the suporting structure and the simplicity of the mechanism. Since the panels are horizontal, they can be compactly placed on the axle tube without danger of self-shading and are also readily accessible for cleaning. For active mechanisms, a single control and motor may be used to actuate multiple rows of panels. Manufacturers include Array Technologies, Inc. Wattsun Solar Trackers, (gear driven active), Zomeworks (passive) and Powerlight.

Vertical axle

Gemini House rotates in its entirety and the solar panels rotate independently, allowing control of the natural heating from the sun. The inventor stands in the middle of the group

A single axis tracker may be constructed that pivots only about a vertical axle, with the panels either vertical or at a fixed elevation angle. Such trackers are suitable for high lattitudes, where the apparent solar path is not especially high, but which leads to long days in Summer, with the sun traveling through a long arc. This method has been used in the construction of a cylindrical house in Austria (lattitude above 45 degrees north) that rotates in its entirety to track the sun, with vertical panels mounted on one side of the building.

Altitude-azimuth

Two-axis mount

Point focus parabolic dish with Stirling system. The horizontally rotating azimuth table mounts the vertical frames on each side which hold the elevation trunions for the dish and its integral engine/generator mount.

Restricted to active trackers, this mount is also becoming popular as a large telescope mount owing to its structural simplicity and compact dimensions. One axis is a vertical pivot shaft or horizontal ring mount, that allows the device to be swung to a compass point. The second axis is a horizontal elevation pivot mounted upon the azimuth platform. By using combinations of the two axis, any location in the upward hemisphere may be pointed. Such systems may be operated under computer control according to the expected solar orientation, or may use a tracking sensor to control motor drives that orient the panels toward the sun. This type of mount is also used to orient parabolic reflectors that mount a Stirling engine to produce electricity at the device.

Multi-mirror reflective unit

Energy Innovations test units

A recent development, this device uses multiple mirrors in a horizontal plane to reflect sunlight upward to a high temperature photovoltaic or other system requiring concentrated solar power. Structural problems and expense are greatly reduced since the mirrors are not significantly exposed to wind loads. Through the employment of a patented mechanism, only two drive systems are required for each device. Because of the configuration of the device it is especially suited for use on flat roofs and at lower lattitudes. While imited commercial availability was expected in 2007 the company has removed the descriptive web page from their site and is now promoting a two-axis clustered fresnel lens device. The units illustrated each produce approximately 200 peak DC watts.

Drive types

Active trackers

Active trackers use motors and gear trains to direct the tracker as commanded by a controller responding to the solar direction. Devices of this type are manufactured by Wattsun and others.

Active two-axis trackers are also used to orient heliostats - movable mirrors that reflect sunlight toward the absorber of a central power station. As each mirror in a large field will have an individual orientation these are controlled programmatically through a central computer system, which also allows the system to be shut down when necessary.

Passive trackers

Zomeworks passive tracker head in Spring/Summer tilt position with panels on light blue rack pivoted to morning position against stop. Dark blue objects are hydraulic dampers. Click image for additional information.

Passive trackers use a low boiling point compressed gas fluid that is driven to one side or the other (by solar heat creating gas pressure) to cause the tracker to move in response to an imbalance. As this is a non-precision orientation it is unsuitable for certain types of concentrating photovoltaic collectors but works fine for common PV panel types. These will have viscous dampers to prevent excessive motion in response to wind gusts. Shader/reflectors are used to reflect early morning sunlight to "wake up" the panel and tilt it toward the sun, which can take nearly an hour. The time to do this can be greatly reduced by adding a self-releasing tiedown that positions the panel slightly past the zenith (so that the fluid does not have to overcome gravity) and using the tiedown in the evening. (A slack-pulling spring will prevent release in windy overnight conditions.) This type of tracker is supplied by Zomeworks.

Solar thermal collector

Solar thermal collector


Solar Thermal Collector Dish

A solar thermal collector is a solar collector specifically intended to collect heat: that is, to absorb sunlight to provide heat. Although the term may be applied to simple solar hot water panels, it is usually used to denote more complex installations. There are various types of thermal collectors, such as solar parabolic, solar trough and solar towers. These type of collectors are generally used in solar power plants where solar heat is used to generate electricity by heating water to produce steam and driving a turbine connected to the electrical generator.

Types

Flat plate and box-type collectors are typically used in domestic and light industry applications. Parabolic troughs, dishes and towers are used almost exclusively in solar power generating stations or for research purposes.

Flat plate


Solar thermal system for water heating - these are deployed on flat roof.

This is the most common type of solar thermal collector, and is usually used as a solar hot water panel to generate solar hot water. A weatherproofed, insulated box containing a black metal absorber sheet with built in pipes is placed in the path of sunlight. Solar energy heats up water in the pipes causing it to circulate through the system by natural convection. The water is usually passed to a storage tank located above the collector. This passive solar water heating system is generally used in hotels and homes in sunny climates such as those found in southern Europe.

For these purposes, the general practice is to use flat-plate solar energy or evacuated tube collectors with a fixed orientation (position). The highest efficiency with a fixed flat-plate collector or evacuated tube collector is obtained if it faces toward the sun and slopes at an angle to the horizon equal to the latitude plus about 10 degrees. Solar collectors fall into two general categories: nonconcentrating and concentrating.

In the nonconcentrating type, the collector area (i.e. the area that intercepts the solar radiation) is the same as the absorber area (i.e., the area absorbing the radiation).

There are many flat-plate collector designs but generally all consist of (1) a flat-plate absorber, which intercepts and absorbs the solar energy, (2) a transparent cover(s) that allows solar energy to pass through but reduces heat loss from the absorber, (3) a heat-transport fluid (air or water) flowing through tubes to remove heat from the absorber, and (4) a heat insulating backing. One flat plate collector is designed to be evacuated, to prevent heat loss.

The most effective use of collectors is with a sealed heat exchange system, rather than having the potable water flow through the collectors. A mixture of water and propylene glycol (which is used in the food industry) can be used as a heat exchange fluid to protect against freeze damage, up to a temperature that depends on the proportion of propylene glycol in the mixture.

The first accurate model of flat plate solar collectors were developed by Hottel and Whillier in the 1950's.

Evacuated Tube

Evacuated (or vacuum) tubes panel.

These collectors have multiple evacuated glass tubes which heat up solar absorbers and, ultimately, solar working fluid (water or an antifreeze mix -- typically propylene glycol) in order to heat domestic hot water, or for hydronic space heating. The evacuated tubes minimize the re-radiation of infrared energy from the collectors, allowing them to reach considerably higher temperatures than most flat-plate collectors. For this reason they can perform well in colder conditions. The advantage is largely lost in warmer climates, except in those cases where very hot water is desirable, for example commercial process water. The high temperatures that can occur may require special system design to avoid or mitigate overheating conditions. A further advantage this design has over the flat-plate type is that the constant profile of the round tube means that the collector is always perpendicular to the sun's rays and therefore the energy absorbed is approximately constant over the course of a day.

Pool or Unglazed

This type of collector is much like a flat-plate collector, except that it has no glazing/transparent cover. It is used extensively for pool heating, as it works quite well when the desired output temperature is near the ambient temperature (that is, when it's warm outside). As the ambient temperature gets cooler, these collectors become extremely ineffective.

Air

These collectors heat air directly, almost always for space heating. They are also used for pre-heating make-up air in commercial and industrial HVAC systems

Box type

A common solar cooker is a box type collector. It is a metal box open from top, and insulated from sides with an equally sized mirror hinged to it (like a simple box with a mirror attached to the underside of the cover).

Parabolic trough

Solar Parabolic dish

This type of collector is generally used in solar power plants. A trough-shaped parabolic reflector is used to concentrate sunlight on an insulated tube (Dewar tube) or heat pipe, placed at the focal point, containing coolant which transfers heat from the collectors to the boilers in the power station.

Parabolic dish

It is the most powerful type of collector which concentrates sunlight at a single, focal point, via one or more parabolic dishes -- arranged in a similar fashion to a reflecting telescope focuses starlight, or a dish antenna focuses radio waves. This geometry may be used in solar furnaces and solar power plants.

There are two key phenomenena to understand in order to comprehend the design of a parabolic dish. One is that the shape of a parabola is defined such that incoming rays which are parallel to the dish's axis will be reflected toward the focus, no matter where on the dish they arrive. The second key is that the light rays from the sun arriving at the earth's surface are almost completely parallel. So if dish can be aligned with its axis pointing at the sun, the incoming radiation will almost all be reflected towards the focal point of the dish -- most losses are due to imperfections in the parabolic shape and imperfect reflection.

Losses due to atmosphere between the dish and its focal point are minimal, as the dish is generally designed specifically to be small enough that this factor is insignificant on a clear, sunny day. Compare this though with some other designs, and you will see that this could be an important factor, and if the local weather is hazy, or foggy, it may reduce the efficiency of a parabolic dish significantly.

In some power plant designs, a stirling engine coupled to a dynamo, is placed at the focus of the dish, which absorbs the heat of the incident solar radiation, and converts it into electricity.

Power tower

Power Tower

A power tower is a large tower surrounded by small rotating (tracking) mirrors called heliostats. These mirrors align themselves and focus sunlight on the receiver at the top of tower, collected heat is transferred to a power station below.

Solar pyramids

Another design is a pyramid shaped structure, which works by drawing in air, heating it with solar energy and moving it through turbines to generate electricity. Solar pyramids have been built in places like Australia. Currently India is building such pyramids.

Advantages
Very high temperatures reached. High temperatures are suitable for electricity generation using conventional methods like steam turbine or some direct high temperature chemical reaction.
Good efficiency. By concentrating sunlight current systems can get better efficiency than simple solar cells.
A larger area can be covered by using relatively inexpensive mirrors rather than using expensive solar cells.
Concentrated light can be redirected to a suitable location via optical fiber cable. For example illuminating buildings, like here (Hybrid Solar Lighting).

Disadvantages
Concentrating systems require dual axis sun tracking to maintain Sunlight focus at the collector.
Inability to provide power in diffused light conditions. Solar Cells are able to provide some output even if the sky becomes little bit cloudy, but power output from concentrating systems drop drastically in cloudy conditions as diffused light cannot be concentrated passively.

Solar thermal energy

Solar thermal energy


Solar thermal energy is a technology for harnessing solar energy for practical applications from solar heating to electrical power generation. Solar thermal oncentrated solar pocollectors, such as solar hot water panels, are commonly used to generate solar hot water for domestic and light industrial applications. Solar thermal energy is used in architecture and building design to control heating and ventilation in both active solar and passive solar designs. This article is devoted primarily to solar thermal electric power plants; that is, solar power plants that generate electricity by converting solar energy to heat, to drive a thermal power plant. These plants include the Solar Energy Generating Systems, Nevada Solar One, and Solar Tres. The article on photovoltaics reviews solar power generation by means of solar electric panels.

Concentrated solar power (CSP) plants

Where temperatures below about 95°C are sufficient, as for space heating, flat-plate collectors of the nonconcentrating type are generally used. The fluid-filled pipes can reach temperatures of 150 to 220 degrees Celsius when the fluid is not circulating.

A concentrating collector intercepts the same amount of solar radiation as a flat-plate collector of the same area, but contains a parabolic reflector that focuses the energy onto the surface of an absorber of much smaller area. This concentration of energy heats the absorber to a much higher temperature than that produced in the flat-plate type. Whilst the amount of energy remains the same, the higher temperature enables the system to generate electrical or mechanical energy more efficiently. This is because the maximum theoretical efficiency of any heat engine increases as the temperature of its heat source increases.

Parabolic trough designs


Sketch of a Parabolic Trough Collector

Parabolic trough power plants are the most successful and cost-effective CSP system design at present. They use a curved trough which reflects the direct solar radiation onto a hollow tube running along above the trough. The whole trough tilts through the course of the day so that direct radiation remains focused on the hollow tube for as long as the sun shines. A fluid, normally thermal oil, passes through the tube and becomes hot. Full-scale parabolic trough systems consist of many such troughs laid out in parallel over a large area of land. A solar thermal system using this principle is in operation in California in the United States, called the SEGS system.At 350 MW, it is currently not only the largest operational solar thermal energy system, but the largest solar power system of any kind. SEGS uses oil to take the heat away: the oil then passes through a heat exchanger, creating steam which runs a steam turbine. The 64MW Nevada Solar One plant also uses this design. Other parabolic trough systems, which create steam directly in the tubes, are under development; this concept is thought to lead to cheaper overall designs, but the concept is yet to be commercialized.

Power tower designs


Solar Two, a concentrating solar power plant.

Power towers (also know as 'central tower' power plants or 'heliostat' power plants) use an array of flat, moveable mirrors (called heliostats) to focus the sun's rays upon a collector tower (the target). The high energy at this point of concentrated sunlight is transferred to a substance that can store the heat for later use. The more recent heat transfer material that has been successfully demonstrated is liquid sodium. Sodium is a metal with a high heat capacity, allowing that energy to be stored and drawn off throughout the evening. That energy can, in turn, be used to boil water for use in steam turbines. Water had originally been used as a heat transfer medium in earlier power tower versions (where the resultant steam was used to power a turbine). This system did not allow for power generation during the evening. Examples of heliostat based power plants are the 10 MWe Solar One (later called Solar Two), and the 15 MW Solar Tres plants. Neither of these are currently used for active energy generation. In South Africa, a solar power plant is planned with 4000 to 5000 heliostat mirrors, each having an area of 140 m².Another design is a pyramid shaped structure - solar pyramid - which works by drawing in air, heating it with solar energy and moving it through turbines to generate electricity. Currently India is buiiding such pyramids.

Dish designs

A dish system uses a large, reflective, parabolic dish (similar in shape to satellite television dish). It focuses all the sunlight that strikes the dish up onto to a single point above the dish, where a thermal collector is used to capture the heat and transform it into a useful form. Dish systems, like power towers, can achieve much higher temperatures due to the higher concentration of light which they receive. Typically the dish is coupled with a Stirling engine in a Dish-Stirling System, but also sometimes a steam engine is used. These create rotational kinetic energy that can be converted to electricity using an electric generator.

Energy storage

Energy storage

Rammed earth trombe wall uses thermal mass to store solar energy.

Solar energy has traditionally been stored as heat in thermal storage systems or chemically in batteries. Solar energy has been experimentally stored thermochemically in phase change materials and at high temperatures using molten salts. The storage of excess solar energy allows for the availability of this energy during hours of darkness or cloud cover.

Thermal mass systems use various methods and materials (adobe, earth, concrete, water) to store solar energy for short or long durations (Seasonal thermal store). Thermal mass can be used to lower peak demand, shift time-of-use to off-peak hours and reduced overall heating and cooling requirements.

Solar energy can be stored thermochemically with phase change materials (PCM). Devices of this type which store latent heat can be thought of as heat batteries. Phase change materials are classified as organic (paraffins, fatty acids) and inorganic (salts, metals,alloys).

A Paraffin wax thermal storage system consists of a solar hot water loop connected to a paraffin wax tank. During the storage cycle, hot water flows through the storage tank melting the paraffin wax. The enthalpy of fusion for paraffin wax is 210-230 kJ/kg. During the heating cycle, stored heat is extracted from the tank as the wax resolidifies. These systems heat air and water up to 64°C and can reduce conventional energy use by 50%-70%.

Eutectic salts such as Glauber's salt can also be employed in thermal storage systems. Glauber's salt are relatively inexpensive and readily available and can store 347 kJ/kg and deliver heat at 64°C. The Dover house in Dover, Massachusetts was the first to use a Glauber's salt heating system in 1948.

Solar energy can be stored at high temperatures using molten salts. Salts are an effective storage medium because they are non-flammable, non-toxic, low-cost, have a high specific heat capacity and can deliver heat at temperatures compatible with conventional power systems. A molten salt storage system consists of a salt loop connected to a insulated storage tank. During the heating cycle the salt mixture is heated from 290°C to 565°C. During the power cycle the salt is used to make steam for a steam-electric power plant. The Solar Two used this method of energy storage. The Solar Two could store 1.44 TJ (400,000 kWh) in its 68 m³ storage tank and had an annual storage efficiency of about 99%.

Rechargeable batteries can be used to store excess electricity from a photovoltaic (PV) system. This type of storage system consists of a PV power source connected to a battery bank via a charge controller and inverter. Lead acid batteries are the most common type of battery associated with PV systems because of their relatively low upfront costs and high availability. Lead acid batteries have an energy density of 110-140 kJ/kg, a charge/discharge efficiency of 70-92% and cost $150-200 per kWh ($45-55 per MJ). Batteries used in off-grid applications should be sized for 3-5 days of capacity and should limit depth of discharge to 50% to minimize cycling and prolong battery life.

Excess electricity from PV systems can also be sent to the transmission grid. This electricity can be used to fill existing demand or temporarily stored for later use. Net metering (Grid-tied electrical system) policies give PV system owners a credit for the electricity they deliver to the grid. This credit is used to off-set electricity provided from the grid when the PV system cannot fill demand.

Desalination and disinfection

Desalination and disinfection

Solar still built into a pit in the ground

A solar still uses solar energy to distill water. A few basic types of solar stills are cone shaped, boxlike, and pit. For cone solar stills, impure water is inserted into the container, where it is evaporated by the sun through clear plastic. The pure water vapor condenses on top and drips down to the side, where it is collected and removed. The most sophisticated of these are the box shaped types. The least sophisticated are the pit types.

Solar water pasteurization uses solar energy to disinfect water. The basic pasteurization process consists of heating water up to 60-70°C and holding the temperature steady for a time period. The most heat resistant organisms will be rendered inert by a temperatures of 70° in ten minutes, 75°C in one minute, and 80°C in five seconds.

Solar water disinfection (SODIS) is another method of disinfecting water using sunlight. The basic process involves filling a clear container 3/4 with water, shaking the container vigorously for 20 seconds, topping off the container and placing it in the sun. Shaking the container allows for the water to become aerated which encourages disinfection. As sunlight shines on the container the UV-A radiation causes the dissolved oxygen to become highly reactive. This reactive form of oxygen kills microorganism directly and interferes with the reproduction cycle of bacteria. As the container warms harmful organisms are also destroyed by heat treatment. Although endorsed by the World Health Organization, SODIS is not as effective as pasteurization and the completeness of disinfection is not easily measurable.

Solar vehicles

Solar vehicles

The solar powered car The Nuna 3 built by the Dutch Nuna team

Development of a practical solar powered car has been an engineering goal for 20 years. The center of this development is the World Solar Challenge, a biannual solar powered car race over 3021 km (1877mi) through central Australia from Darwin to Adelaide. The race's stated objective is to promote research into solar-powered cars. Teams from universities and enterprises participate. In 1987 when it was founded, the winner's average speed was 67 km/h (42 mph).By the 2005 race this had increased to an average speed of greater than 100 km/h (62 mph), even though the cars were faced with the 110 km/h (68 mph) South Australia speed limit.


Helios UAV in flight

Helios, named after the Greek sun god of the same name, was a prototype solar powered unmanned aircraft. AeroVironment, Inc. developed the vehicle under NASA's Environmental Research Aircraft and Sensor Technology (ERAST) program.

On 13 August, 2001, it set an unofficial world record for sustained altitude by a winged aircraft. It sustained flight at above 96,000 feet (29,250 m) for forty minutes, and at one time it flew as high as 96,863 feet (29,524 m). Later, in June 2003, the prototype broke up and fell into the Pacific Ocean about ten miles (16 km) west of the Hawaiian Island Kauai.

The first practical solar boat was probably constructed in 1975 in England (see Electrical Review Vol 201 No 7 12 August 1977). By 1995 passenger boats began appearing and are now used extensively.Solar powered boats have advanced sufficiently to cross the Atlantic. The first crossing of the Atlantic Ocean was achieved in the winter of 2006/2007 by the solar catamaran sun21.

A solar balloon is a black balloon that is filled with air. As sunlight shines on the balloon the air inside is heated and expands. This causes an upward buoyancy force. As such, the balloon functions like a hot air balloon. Some solar balloons are large enough for human flight but usage at the moment is restricted to the toy market.

Solar chemical

Solar chemical


Solar chemical processes convert solar energy into chemical energy. These processes use both light (photochemical) and heat (endothermic) to drive chemical, thermochemical or thermoelectric reactions. Solar chemical reactions can be used to store solar energy or replace energy that would otherwise be required from an alternate source.

Electrochemical cells, commonly known as batteries, convert electrical energy into chemical energy. Solar energy can indirectly be converted into chemical energy in a system involving a photovoltaic to electrochemical cell exchange. A more direct approach involves the use of photoelectrochemical cells which use light to produce hydrogen in a process similar to the electrolysis of water. A third approach involves the use of thermoelectic devices which convert a temperature difference between dissimilar metals into an electric current between those metals. This current can be use to produce hydrogen and oxygen through the electrolysis of water. The solar pioneer Mochout envisioned using the thermoelectric effect to store solar energy for later use during darkness; however, his experiments toward this end never progressed beyond primitive devices.

Concentrating solar thermal technologies can be used to drive high temperature chemical processes.
Ammonia can be decomposed into nitrogen and hydrogen at high temperatures (650-700°C). The stored gases can be subsequently recombined to generate heat or electricity via a fuel cell. A prototype system was constructed at the Australian National University.

Zinc Oxide (ZnO) can be decomposed at high temperatures (1200-1750°C). The resulting pure zinc can be marketed directly. Alternatively, the zinc can be reacted with water at (350°C) to produce ZnO and hydrogen.

Water can be directly dissociated at high temperatures (2300-2600°C). These process have so far been limited due to their high level of complexity and low solar to hydrogen efficiency (1-2%).An alternate path of research is investigating solar thermochemical cycles that can be used to dissociate water at lower temperatures. Thermochemical cycles are currently at the prototype stage.

Concentrating solar thermal has also been investigated as a direct thermal method of producing aluminum.

While not a technology, photosynthesis is arguably the most important photochemical interaction. A diverse biology has developed capable of photosynthesizing light in the visible, ultraviolet, near infrared and far infrared regions of the electromagnetic spectrum.

Salt evaporation ponds are shallow man-made ponds designed to extract salt from sea water. The seawater is fed into large ponds and water is drawn out through natural evaporation. After the sun and winds have evaporated the water the salt is harvested.

Solar cooker

Solar cooker


Solar Cookers use sunshine as a source of heat for cooking as an alternative to fire.

Solar cookers (or solar ovens) use sunlight for cooking, drying and pasteurization. Solar cookers offset fuel costs and reduce demand for local firewood. Solar cookers also improve local air quality by removing a source of smoke. The most common designs are box cookers, concentrating cookers and panel cookers.
Solar box cookers consist of an insulated container with a transparent lid. Horace de Saussure developed this design in 1767 after observing: "It is a known fact, and a fact that has probably been known for a long time, that a room, a carriage, or any other place is hotter when the rays of the sun pass through glass." These cookers can be effectively used with partially overcast skies and can typically reach temperatures of 50-100°C. These are the cheapest and most widely used cooker design.

Concentrating solar cookers use a parabolic reflector to concentrate light on a container positioned at the reflector's focal point. These designs cook faster and at higher temperatures (up to 315°C). As with other concentrating technologies these cookers require direct light and must be repositioned to track the sun.

Solar Panel cookers (SPC) use flat reflectors to concentrate sunlight on a container within a transparent covering. Roger Bernard is credited with introducing panel cookers in 1994. This design uses partial concentration and will maintain effective operation with limited repositioning.

Solar power plants

Solar power plants

Solar Two power tower surrounded by a field of heliostats.

Solar power plants use a variety of methods to collect sunlight and convert this energy into electricity, distill water or provide heat for industrial processes. Concentrating solar thermal power plants have traditionally been the most common type of solar power plant; however, multi-megawatt photovoltaic sites have seen recent rapid deployment.

Concentrating Solar Thermal (CST) systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. CST technologies require direct insolation to perform properly. This requirement makes them inappropriate for significantly overcast locations.

The three basic CST technologies are the solar trough, solar power tower and parabolic dish. Each technology is capable of producing high temperatures and correspondingly high thermodynamic efficiencies but they vary in the way they track the sun and focus light.
Line focus/Single-axis
A solar trough consists of a linear parabolic reflector which concentrates light on a receiver positioned along the reflector's focal line. These systems use single-axis tracking to follow the sun. A working fluid (oil, water) flows through the receiver and is heated up to 400 °C before transferring its heat to a distillation or power generation system.ugh systems are the most developed CST technology. The Solar Electric Generating System (SEGS) plants in California and Plataforma Solar de Almería's SSPS-DCS plant in Spain are representatives of this technology.

Point focus/Dual-axis
A power tower consists of an array of flat reflectors (heliostats) which concentrate light on a central receiver located on a tower. These systems use dual-axis tracking to follow the sun. A working fluid (air, water, molten salt) flows through the receiver where it is heated up to 1000 °C before transferring its heat to a power generation or energy storage system. Power towers are less advanced than trough systems but they offer higher efficiency and energy storage capability.The Solar Two in Daggett, California and the Planta Solar 10 (PS10) in Sanlucar la Mayor, Spain are representatives of this technology.
A parabolic dish or dish/engine system consists of a stand-alone parabolic reflector which concentrates light on a receiver positioned at the reflector's focal point. These systems use dual-axis tracking to follow the sun. A working fluid (hydrogen, helium, air, water) flows through the receiver where it is heated up to 1500 °C before transferring its heat to a sterling engine for power generation. Parabolic dish systems display the highest solar-to-electric efficiency among CST technologies and their modular nature offers scalability. The Stirling Energy Systems (SES) and Science Applications International Corporation (SAIC) dishes at UNLV and the Big Dish in Canberra, Australia are representatives of this technology.

A solar updraft tower (also known as a solar chimney or solar tower) consists of a large greenhouse which funnels into a central tower. As sunlight shines on the greenhouse the air inside is heated and expands. The expanding air flows toward the central tower where a turbine converts the air flow into electricity. A 50 kW prototype was constructed in Ciudad Real, Spain and operated for eight years before decommissioning in 1989.

Solar energy

Solar energy


Solar energy is energy from the sun. It supports life on Earth and drives the Earth's weather. Solar energy predominantly arrives in the form of infrared, visible and ultraviolet light, and is either returned back to space or is absorbed. Nearly all of the absorbed energy is converted directly to heat, with a small but important fraction converted to chemical energy, such as in ozone production, photosynthesis or photovoltaic energy production.

Solar energy also broadly describes technologies that utilize sunlight. These technologies are diverse and date back millennia. The Greeks, Native Americans and Chinese warmed their buildings by orienting them toward the sun. In Europe, farmers used elaborate field orientation and thermal mass to increase crop yields during the Little Ice Age. Modern solar technologies continue to harness the sun to provide water heating, daylighting and even flight.

Solar power from a parabolic reflector.


The amount of solar energy available to the Earth in one minute exceeds global energy demand for a year.

Solar power generally describes technologies that convert sunlight into electricity and in some cases thermal or mechanical power. In 1866, the French engineer Auguste Mouchout successfully powered a steam engine with sunlight. This is the first known example of a solar powered mechanical device. Over the next 50 years inventors such as John Ericsson, Charles Tellier and Frank Shuman developed solar powered devices for irrigation, refrigeration and locomotion. The progeny of these early developments are concentrating solar power plants.

The modern age of solar power arrived in 1954 when researchers at Bell Laboratories developed a photovoltaic cell capable of effectively converting light into electricity. This breakthrough marked a fundamental change in how power is generated. Since then solar cells efficiencies have improved from 6% to 15% with experimental cells reaching efficiencies over 40%. Prices on the other hand have fallen from $300 per watt to less than $3 per watt.

The utilization of solar energy and solar power spans from traditional technologies that provide food, heat and light to electricity which is uniquely modern. Solar energy is used in a wide variety of applications, including:
Heat (hot water, building heat, cooking, process heat)
Lighting (daylighting, hybrid lighting, daylight savings time)
Electricity generation (photovoltaics, heat engines)
Transportation (solar car, solar plane, solar boat)
Desalination
Biomass (wood, biofuel)
Clothes drying

Energy from the Sun


Solar power as it is dispersed on the planet and radiated back to space.

The Earth receives 174 petawatts (PW) of solar radiation at the upper atmosphere. While traveling through the atmosphere, 6% of the incoming solar radiation (insolation) is reflected and 16% is absorbed. Average atmospheric conditions (clouds, dust, pollutants) further reduce insolation by 20% through reflection and 3% through absorption. The absorption of solar energy by atmospheric convection (sensible heat transport) and by the evaporation and condensation of water vapor (latent heat transport) drive the winds and the water cycle.

Atmospheric conditions not only reduce the quantity of light reaching the Earth's surface but also affect the qualities of light by altering its spectrum and diffusing approximately 20% of the incoming light.After passing through the Earth's atmosphere approximately half the insolation is in the visible electromagnetic spectrum with the other half mostly in the infrared spectrum, and a small part of ultraviolet radiation.Upon reaching the surface, sunlight is absorbed by the oceans, earth and plants. The energy captured in the oceans drives the thermohaline cycle. As such, solar energy is ultimately responsible for temperature driven ocean currents such as the thermohaline cycle and wind driven currents such as the Gulf Stream. The energy absorbed by the earth in conjunction with that recycled by the Greenhouse effect warms the surface to an average temperature of approximately 14°C.The solar energy captured by plants and other phototrophs is converted to chemical energy via photosynthesis. All the food we eat, wood we build with, and fossil fuels we use are products of photosynthesis.


Annual average insolation at the top of Earth's atmosphere (top) and at the surface (bottom). The black dots represent the land area required to replace the total primary energy supply with electricity from solar cells.

The map on the top shows how solar radiation at the top of the earth's atmosphere varies with latitude. The bottom map shows annual average ground level insolation. For example, in North America the average insolation at ground level over an entire year (including nights and periods of cloudy weather) lies between 125 and 375 W/m² (3 to 9 kWh/m²/day).At present, photovoltaic panels typically convert about 15% of incident sunlight into electricity; therefore, a solar panel in the contiguous United States on average delivers 19 to 56 W/m² or 0.45 - 1.35 kWh/m²/day.

Types of technologies

Many technologies use solar energy. Some classifications of solar technology are active, passive, direct and indirect.
Active solar systems use electrical and mechanical components such as tracking mechanisms, pumps and fans to process sunlight into usable outputs such as heating, lighting or electricity.
Passive solar systems use non-mechanical techniques of controlling, converting and distributing sunlight into usable outputs such as heating, lighting, cooling or ventilation. These techniques include selecting materials with favorable thermal properties, designing spaces that naturally circulate air, and referencing the position of a building to the sun.
Direct solar generally refers to technologies or effects that involve a single conversion of sunlight which results in a usable form of energy.
Indirect solar generally refers to technologies or effects that involves multiple transformations of sunlight which result in a usable form of energy.

Architecture and Urban planning


The Zion National Park Visitor's Center incorporates several aspects of solar design.

Solar architecture controls the use of the sun to provide comfortable temperatures, lighting and air quality. The basic elements of solar architecture are building orientation, proportion, thermal mass and window placement. The solar architecture and design process tailors these elements to the local climate and environment.

The oldest principle of solar architecture is building orientation. The entire building can be positioned and angled to be oriented towards or away from the sun, overshadowing from other structures or natural features can be avoided or used, and the building can be set into the ground using earth sheltering techniques.
As a general rule, a solar building's axis should run lengthwise east to west and the structure should be twice as long as wide.
Windows facing the equator should be equal to 5-7% of the building's floor space.If heating is a concern, window area facing away from the equator should be minimized.
The thermal mass in the building should be sized to smooth out temperature swings.
Spaces can be designed to naturally circulate air. Cooling elements such as a solar chimney can be incorporated to help with ventillation.
Lighting quality and energy use are strongly influenced by window design. In cold climates insulated glazing with low-emissivity coatings can maximize solar gain and reduce heat losses by 30-50%. In hot climates low-emissivity coatings on the outside of window panes can be used to reduce and control solar gain.

The albedo of an object indicates the percentage of light it reflects. Asphalt has an albedo of around 10% while the average albedo of the Earth is 30%.Urban heat islands (UHI) are metropolitan areas with significantly higher temperatures than the surrounding environment. These higher temperatures are the result of urban materials such as concrete and asphalt which have lower albedos and higher heat capacities than the natural environment. A straightforward method of counteracting the UHI effect is to paint buildings and roads white and plant trees. A hypothetical "cool communities" program in Los Angeles, California called for the planting of ten million trees, the reroofing of almost 5 million homes and painting one-quarter of the roads. These measures are estimated to reduce urban temperatures by approximately 3°C. The projected costs of such a program are approximately $1 billion. The annual savings from reduced air-conditioning costs are estimated at $170 million with an additional yearly health benefit of $360 million in smog-reduction savings.

Water heating

Solar water heaters, on a rooftop in Jerusalem, Israel

Solar hot water systems use sunlight to heat water. Commercial solar water heaters began appearing in the United States in the 1890s. These systems saw increasing use until the 1920s but were thereafter gradually replaced by relatively cheap and more reliable conventional heating fuels. The economic advantage of conventional heating fuels has varied over time resulting in periodic interest in solar hot water; however, solar hot water technologies have yet to show the sustained momentum they lost in the 1920s. That being said, the recent price spikes and erratic availability of conventional fuels is renewing interest in solar heating technologies.Solar water heating is highly efficient (up to 86%) and is particularly appropriate for low temperature (25-65 °C) applications such as domestic hot water, heating swimming pools and space heating. The oldest and simplest type of solar water heater is a black water tank which is exposed to the sun. These are called batch systems but there are many other configurations. Some configurations are designed to heat water to high temperatures while other are designed for economy. A solar pond is a pool of salt water that collects and stores solar energy. Solar ponds were first proposed by Dr. Rudolph Bloch in 1948 after coming across reports of a lake in Hungary in which the temperature increased with depth. This effect was due to salts in the lake's waters which created a "density gradient" that prevented convection currents. A prototype was constructed in 1958 on the shores of the Dead Sea near Jerusalem.The pond consisted of layers of water that successively increased from a weak salt solution at the top to a high salt solution at the bottom. This solar pond was capable of producing temperatures of 90 °C in its bottom layer and had an estimated solar to electric efficiency of 2%. Current, representatives of this technology include a 150 KW pond in En Boqeq, Israel, and another used for industrial process heat at the University of Texas El Paso.

Photovoltaic module

Photovoltaic module


In the field of photovoltaics, a photovoltaic module is a packaged interconnected assembly of photovoltaic cells, also known as solar cells. An installation of photovoltaic modules or panels is known as a photovoltaic array. Photovoltaic cells typically require protection from the environment. For cost and practicality reasons a number of cells are connected electrically and packaged in a photovoltaic module, while a collection of these modules that are mechanically fastened together, wired, and designed to be a field-installable unit, usually with a glass covering and a frame and backing made of metal, plastic or fiberglass, are known as a photovoltaic panel or simply solar panel. A photovoltaic installation typically includes an array of photovoltaic modules or panels, an inverter, batteries (for off grid) and interconnection wiring.


A photovoltaic module is composed of individual PV cells. This crystalline-silicon module has an aluminium frame and glass on the front.

Theory and construction


The majority of modules use water based Crystalline silicon cells or a thin film cell based on cadmium telluride or silicon (see photovoltaic cells for details).

In order to use the cells in practical applications, they must be:
connected electrically to one another and to the rest of the system
protected from mechanical damage during manufacture, transport and installation and use (in particular against hail impact, wind and snow loads). This is especially important for water based silicon cells which are brittle.
protected from moisture, which corrodes metal contacts and interconnects, (and for thin film cells the transparent conductive oxide layer) thus decreasing performance and lifetime.
electrically insulated including under rainy conditions
mountable on a substructure

Most modules are rigid, but there are some flexible modules available, based on thin film cells.

Electrical connections are made in series to achieve a desired output voltage and/or in parallel to provide a desired amount of current source capability. Diodes are included to avoid overheating of cells in case of partial shading.

Since cell heating reduces the operating efficiency it is desirable to minimize the heating. Very few modules incorporate any design features to decrease temperature, however installers try to provide good ventilation behind the module,

New designs of module include concentrator modules in which the light is concentrated by an array of lenses or mirrors onto an array of small cells. This allows the use of cells with a very high cost per unit area (such as gallium arsenide) in a cost competive way.

Depending on construction the photovoltaic can cover a range of frequencies of light and can produce electricity from them, but cannot cover the entire solar spectrum. Hence much of incident sunlight energy is wasted when used for solar panels, although they can give far higher efficiencies if illuminated with monochromatic light. Another design concept is to split the light into different wavelength ranges and direct the beams onto different cells tuned to the appropriate wavelength ranges.This is projected to raise efficiency to 50%. Sunlight conversion rates (module efficiencies) can vary from 5-18% in commercial production. It normally cost $1,000,000.

Crystalline silicon modules

The most common design of modules contains cells made from silicon wafers. The wafers and cells are manufactured separately from the modules, sometimes in different factories and by different companies.

These cells are connected using conductive ribbons into one or more 'strings'. The module consists of a transparent top surface, an encapsulant, the strings of cells, another layer of encapsulant and a rear layer, and a frame around the outer edge. Typically, the top surface is low iron solar glass, the encapsulant is crosslinkable Ethylene-vinyl acetate (EVA), and the rear layer is a Tedlar- PET-Tedlar laminate.Glass

The front glass must have a high transmission in the wavelengths used by the cells, ie in 350 to 1200 nm range. Tempered, low-iron glass is the material of choice. Starting in 1990, cerium was added to absorb UV light and thus protect the encapsulant from degradation. Today, due to improvements in encapsulant stability, Ce is rarely used.

Typically, 8% of light is reflected by the outer surface of the glass i.e. at the air-glass interface. This reflective loss can be reduced to allow more light to reach the cell and more electrical power to be generated. Antireflective coatings (ref) or texturing the surface will reduce the reflective loss. However in some cases the textured modules collect dust and the increased light transmission through lower reflective loss is outweighed by light lost due to soiling.

The glass also serves the purpose of keeping out water and rigidifying the module, protecting the cells from damage from hail impact and bending and impact during manufacture, transport and installation. It must be stable under long-term exposure to ultraviolet radiation.

Encapsulant

The encapsulant serves two main purposes. The first purpose is to mechanically bond the strings of cells to the glass and thus maintain their positions over the life of the module. The second purpose is to provide an optical bridge between the glass and the cells. Otherwise there would be another air-glass interface and an air silicon interface each causing >=8% loss. The material must, like the glass, have excellent light transmission and be durable enough to last at least 20 years without degrading or debonding from the glass or the silicon. The material of choice is crosslinkable EVA, although Polyvinyl butyral (PVB) is used in modules with a glass backsheet.

Backsheet

The material on the back side of the module provides protection from UV degradation, electrical resistance, and moisture penetration. The majority of modules use a Tedlar-PET-Tedlar sheet. Glass and coated PET are also used.



Process

The encapsulant is melted and crosslinked in a vacuum laminator (except for glass front-glass back modules which are made in an autoclave. The glass, strings and backsheet are now attached to one another and form a 'laminate'. A frame made of aluminium profile is fitted around the edges of the laminate with an elastomeric seal (e.g. silicone) to seal the edges against moisture. The frame provides rigidity and the means to attach the module to a supporting structure. Finally, the strings are electrically terminated into a junction box usually glued to the back of the module.

Crystalline silicon modules have an efficiency of 13-18%.

1. Cells connected to make string


2. encapsulant film ready


3. ready for lamination. Note ribbons terminating 2 strings


4. after lamination


5. Aluminium profiles added to make the frame

Rigid thin-film modules

In rigid thin film modules, the cell and the module are manufactured in the same production line.

The cell is created directly on a glass substrate or superstrate, and the electrical connections are created in situ, a so called "monolithic integration". The substrate or superstrate is laminated with an encapsulant to a front or back sheet, usually another sheet of glass.

The main cell technologies in this category are CdTe, amorphous silicon, micromorphous silicon (alone or tandem), or CIGS (or variant). Amorphous silicon has a sunlight conversion rate of 5-9%.

Flexible thin-film modules

Flexible thin film cells and modules are manufactured in the same production line. They are created by depositing the photoactive layer and other necessary layers on a flexible substrate. If the substrate is an insulator (e.g. polyester or polyimide film) then monolithic integration can be used. If a conductor then monolithic integration cannot be used, and another technique for electrical connection used. The cells are converted to a module by lamination to a transparent colourless fluoropolymer on the front side (typically ETFE or FEP) and a polymer suitable for bonding to the final substrate on the other side. The only commercially available (in MW quantities) in a flexible module is amorphous silicon triple junction (from Unisolar).