Solar mirror
Encyclopedia
A solar mirror is a reflective surface used for gathering and reflecting solar energy in a system being powered by solar energy. It comprises a substrate with a reflective layer for reflecting the solar energy, and in most cases an interference layer. This may be a planar mirror
or parabolic
arrays of solar mirrors used to achieve a substantially concentrated reflection factor for solar energy systems.
See the Heliostat
article for more information on solar mirrors used for terrestrial energy.
Glass may also be used as a protective layer to protect the other layers from abrasion and corrosion. Although glass is brittle, it is a good material for this purpose, because it is highly transparent (low optical losses), resistant to UV, fairly hard (abrasion resistant), chemically inert, and fairly easy to clean. It is composed of a float glass
with high optical transmission characteristics in the visible and infrared
ranges, and is configured to transmit visible light and infrared radiation. The top surface, known as the "first surface", will reflect some of the incident solar energy, due to the reflection coefficient
caused by its index of refraction being higher than air. Most of the solar energy is transmitted through the glass substrate to the lower layers of the mirror, possibly with some refraction
, depending on the angle of incidence
.
Metal substrates ("Metal Mirror Reflectors") may also be used in solar reflectors. NASA Glenn Research Center, for example, used a mirror comprising a reflective aluminum surface on a metallic honeycomb as a prototype reflector unit for a proposed power system for the International Space Station
. One technology uses aluminium composite reflector panels, achieving over 93% reflectivity and coated with a speciality coating for surface protection. Metal reflectors offer some advantages over glass reflectors, as they are lightweight and stronger than glass and relatively inexpensive. The ability to retain parabolic shape in reflectors is another advantage, and normally the subframe requirements are reduced by more than 300%. The top surface reflection coating allows for better efficiency.
or aluminum, but occasionally other metals. Because of sensitivity to abrasion and corrosion, the metal layer is usually protected by the (glass) substrate on top, and the bottom may be covered with a protective coating, such as a copper
layer and varnish
.
Despite the use of aluminum in generic mirrors, aluminum is not always used as the reflective layer for a solar mirror. The use of silver as the reflective layer is claimed to lead to higher efficiency levels, because it is the most reflective metal. This is because of aluminum's reflection factor in the UV
region of the spectrum
. Locating the aluminum layer on the first surface exposes it to weathering, which reduces the mirror's resistance to corrosion and makes it more susceptible to abrasion. Adding a protective layer to the aluminum would reduce its reflectivity.
.
from solar radiation at the surface of the earth
is about 1 kilowatt of energy
per square meter of area, normal
to the direction of the sun
, under clear-sky conditions. When solar energy is unconcentrated, the maximum collector temperature is about 80-100 deg C. This is useful for space heating and heating water. For higher temperature applications, such as cooking
, or supplying a heat engine
or turbine
-electrical generator
, this energy must be concentrated.
uses a Stirling engine
heated by a solar mirror concentrator
. Another configuration is the trough system.
applications, including solar power satellites, where a reflector focuses sunlight on to a heat engine
such as the Brayton cycle
type.
are quite expensive per unit area. Some types of PV cell, e.g. gallium arsenide, if cooled, are capable of converting efficiently up to 1000 times as much radiation as is normally provided by simple exposure to direct sunlight.
In tests done by Sewang Yoon and Vahan Garboushian, for Amonix Corp. silicon solar cell conversion efficiency is shown to increase at higher levels of concentration, proportional to the logarithm of the concentration, provided external cooling is available to the photocells. Similarly, higher efficiency multijunction cells also improve in performance with high concentration.
corporation. In their Fig. 4. captioned "Architecture 4. GEO Harris Wheel", the authors describe a system of solar mirrors used to augment the power of some nearby solar collectors, from which the power is then transmitted to receiver stations on earth.
, who wrote about systems called "Lunetta", "Soletta", "Biosoletta", "Powersoletta".
A preliminary series of experiments called Znamya
("Banner") was performed by Russia, using solar sail
prototypes that had been repurposed as mirrors. Znamya-1 was a ground test. Znamya-2 was launched aboard the Progress M-15
resupply mission to the Mir
space station
on 27 October 1992. After undocked from Mir, the Progress deployed the reflector this mission was successful in that the mirror deployed, although it did not illuminate the Earth. The next flight Znamya-2.5 failed. Znamya-3 never flew.
Plane mirror
A plane mirror is a mirror with a plane reflective surface.For light rays striking a plane mirror, the angle of reflection equals the angle of incidence...
or parabolic
Parabola
In mathematics, the parabola is a conic section, the intersection of a right circular conical surface and a plane parallel to a generating straight line of that surface...
arrays of solar mirrors used to achieve a substantially concentrated reflection factor for solar energy systems.
See the Heliostat
Heliostat
A heliostat is a device that includes a mirror, usually a plane mirror, which turns so as to keep reflecting sunlight toward a predetermined target, compensating for the sun's apparent motions in the sky. The target may be a physical object, distant from the heliostat, or a direction in space...
article for more information on solar mirrors used for terrestrial energy.
Glass or Metal Substrate
The substrate is the mechanical layer which holds the mirror in shape.Glass may also be used as a protective layer to protect the other layers from abrasion and corrosion. Although glass is brittle, it is a good material for this purpose, because it is highly transparent (low optical losses), resistant to UV, fairly hard (abrasion resistant), chemically inert, and fairly easy to clean. It is composed of a float glass
Float glass
Float glass is a sheet of glass made by floating molten glass on a bed of molten metal, typically tin, although lead and various low melting point alloys were used in the past. This method gives the sheet uniform thickness and very flat surfaces. Modern windows are made from float glass...
with high optical transmission characteristics in the visible and infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
ranges, and is configured to transmit visible light and infrared radiation. The top surface, known as the "first surface", will reflect some of the incident solar energy, due to the reflection coefficient
Reflection coefficient
The reflection coefficient is used in physics and electrical engineering when wave propagation in a medium containing discontinuities is considered. A reflection coefficient describes either the amplitude or the intensity of a reflected wave relative to an incident wave...
caused by its index of refraction being higher than air. Most of the solar energy is transmitted through the glass substrate to the lower layers of the mirror, possibly with some refraction
Refraction
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...
, depending on the angle of incidence
Angle of incidence
Angle of incidence is a measure of deviation of something from "straight on", for example:* in the approach of a ray to a surface, or* the angle at which the wing or horizontal tail of an airplane is installed on the fuselage, measured relative to the axis of the fuselage.-Optics:In geometric...
.
Metal substrates ("Metal Mirror Reflectors") may also be used in solar reflectors. NASA Glenn Research Center, for example, used a mirror comprising a reflective aluminum surface on a metallic honeycomb as a prototype reflector unit for a proposed power system for the International Space Station
International Space Station
The International Space Station is a habitable, artificial satellite in low Earth orbit. The ISS follows the Salyut, Almaz, Cosmos, Skylab, and Mir space stations, as the 11th space station launched, not including the Genesis I and II prototypes...
. One technology uses aluminium composite reflector panels, achieving over 93% reflectivity and coated with a speciality coating for surface protection. Metal reflectors offer some advantages over glass reflectors, as they are lightweight and stronger than glass and relatively inexpensive. The ability to retain parabolic shape in reflectors is another advantage, and normally the subframe requirements are reduced by more than 300%. The top surface reflection coating allows for better efficiency.
Reflective Layer
The reflective layer is designed to reflect the maximum amount of solar energy incident upon it, back through the glass substrate. The layer comprises a highly reflective thin metal film, usually either silverSilver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
or aluminum, but occasionally other metals. Because of sensitivity to abrasion and corrosion, the metal layer is usually protected by the (glass) substrate on top, and the bottom may be covered with a protective coating, such as a copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
layer and varnish
Varnish
Varnish is a transparent, hard, protective finish or film primarily used in wood finishing but also for other materials. Varnish is traditionally a combination of a drying oil, a resin, and a thinner or solvent. Varnish finishes are usually glossy but may be designed to produce satin or semi-gloss...
.
Despite the use of aluminum in generic mirrors, aluminum is not always used as the reflective layer for a solar mirror. The use of silver as the reflective layer is claimed to lead to higher efficiency levels, because it is the most reflective metal. This is because of aluminum's reflection factor in the UV
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
region of the spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
. Locating the aluminum layer on the first surface exposes it to weathering, which reduces the mirror's resistance to corrosion and makes it more susceptible to abrasion. Adding a protective layer to the aluminum would reduce its reflectivity.
Interference layer
An interference layer may be located on the first surface of the glass substrate. It can be used to tailor the reflectance. It may also be designed for diffuse reflectance of near-ultraviolet radiation, in order to prevent it from passing through the glass substrate. This substantially enhances the overall reflection of near-ultraviolet radiation from the mirror. The interference layer may be made of several materials, depending on the desired refractive index, such as titanium dioxideTitanium dioxide
Titanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
.
Solar thermal applications
The intensity of solar thermal energySolar thermal energy
Solar thermal energy is a technology for harnessing solar energy for thermal energy . Solar thermal collectors are classified by the United States Energy Information Administration as low-, medium-, or high-temperature collectors. Low-temperature collectors are flat plates generally used to heat...
from solar radiation at the surface of the earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
is about 1 kilowatt of energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
per square meter of area, normal
Perpendicular
In geometry, two lines or planes are considered perpendicular to each other if they form congruent adjacent angles . The term may be used as a noun or adjective...
to the direction of the sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
, under clear-sky conditions. When solar energy is unconcentrated, the maximum collector temperature is about 80-100 deg C. This is useful for space heating and heating water. For higher temperature applications, such as cooking
Cooking
Cooking is the process of preparing food by use of heat. Cooking techniques and ingredients vary widely across the world, reflecting unique environmental, economic, and cultural traditions. Cooks themselves also vary widely in skill and training...
, or supplying a heat engine
Heat engine
In thermodynamics, a heat engine is a system that performs the conversion of heat or thermal energy to mechanical work. It does this by bringing a working substance from a high temperature state to a lower temperature state. A heat "source" generates thermal energy that brings the working substance...
or turbine
Turbine
A turbine is a rotary engine that extracts energy from a fluid flow and converts it into useful work.The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they move and...
-electrical generator
Electrical generator
In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...
, this energy must be concentrated.
Terrestrial applications
Solar thermal systems have been constructed to produce concentrated solar power (CSP), for generating electricity. The large Sandia Lab solar power towerSolar power tower
The solar power tower is a type of solar furnace using a tower to receive the focused sunlight. It uses an array of flat, movable mirrors to focus the sun's rays upon a collector tower...
uses a Stirling engine
Stirling engine
A Stirling engine is a heat engine operating by cyclic compression and expansion of air or other gas, the working fluid, at different temperature levels such that there is a net conversion of heat energy to mechanical work....
heated by a solar mirror concentrator
Concentrator
In telecommunication, the term concentrator has the following meanings:# In data transmission, a functional unit that permits a common path to handle more data sources than there are channels currently available within the path...
. Another configuration is the trough system.
Space power application
"Solar dynamic" energy systems have been proposed for various spacecraftSpacecraft
A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....
applications, including solar power satellites, where a reflector focuses sunlight on to a heat engine
Heat engine
In thermodynamics, a heat engine is a system that performs the conversion of heat or thermal energy to mechanical work. It does this by bringing a working substance from a high temperature state to a lower temperature state. A heat "source" generates thermal energy that brings the working substance...
such as the Brayton cycle
Brayton cycle
The Brayton cycle is a thermodynamic cycle that describes the workings of the gas turbine engine, basis of the airbreathing jet engine and others. It is named after George Brayton , the American engineer who developed it, although it was originally proposed and patented by Englishman John Barber...
type.
Photovoltaic augmentation
Photovoltaic cells (PV) which can convert solar radiation directly into electricityElectricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
are quite expensive per unit area. Some types of PV cell, e.g. gallium arsenide, if cooled, are capable of converting efficiently up to 1000 times as much radiation as is normally provided by simple exposure to direct sunlight.
In tests done by Sewang Yoon and Vahan Garboushian, for Amonix Corp. silicon solar cell conversion efficiency is shown to increase at higher levels of concentration, proportional to the logarithm of the concentration, provided external cooling is available to the photocells. Similarly, higher efficiency multijunction cells also improve in performance with high concentration.
Terrestrial application
To date no large scale testing has been performed on this concept. Presumably this is because the increased cost of the reflectors and cooling generally is not economically justified.Solar power satellite application
Theoretically, for space-based solar power satellite designs, solar mirrors could reduce PV cell costs and launch costs since they are expected to be both lighter and cheaper than equivalent large areas of PV cells. Several options were studied by BoeingBoeing
The Boeing Company is an American multinational aerospace and defense corporation, founded in 1916 by William E. Boeing in Seattle, Washington. Boeing has expanded over the years, merging with McDonnell Douglas in 1997. Boeing Corporate headquarters has been in Chicago, Illinois since 2001...
corporation. In their Fig. 4. captioned "Architecture 4. GEO Harris Wheel", the authors describe a system of solar mirrors used to augment the power of some nearby solar collectors, from which the power is then transmitted to receiver stations on earth.
Space reflectors for night illumination
Another advanced space concept proposal is the notion of Space Reflectors which reflect sunlight on to small spots on the night side of the Earth to provide night time illumination. An early proponent of this concept was Dr. Krafft Arnold EhrickeKrafft Arnold Ehricke
-External links:* * *...
, who wrote about systems called "Lunetta", "Soletta", "Biosoletta", "Powersoletta".
A preliminary series of experiments called Znamya
Znamya (space mirror)
The Znamya project was a series of experimental orbital mirrors, designed to beam solar power to Earth by reflecting sunlight. It consisted of two experiments - the Znamya 2 experiment, and the failed Znamya 2.5 - and the proposed Znamya 3...
("Banner") was performed by Russia, using solar sail
Solar sail
Solar sails are a form of spacecraft propulsion using the radiation pressure of light from a star or laser to push enormous ultra-thin mirrors to high speeds....
prototypes that had been repurposed as mirrors. Znamya-1 was a ground test. Znamya-2 was launched aboard the Progress M-15
Progress M-15
Progress M-15 was a Russian unmanned cargo spacecraft which was launched in 1992 to resupply the Mir space station. The thirty-third of sixty-four Progress spacecraft to visit Mir, it used the Progress-M 11F615A55 configuration, and had the serial number 215...
resupply mission to the Mir
Mir
Mir was a space station operated in low Earth orbit from 1986 to 2001, at first by the Soviet Union and then by Russia. Assembled in orbit from 1986 to 1996, Mir was the first modular space station and had a greater mass than that of any previous spacecraft, holding the record for the...
space station
Space station
A space station is a spacecraft capable of supporting a crew which is designed to remain in space for an extended period of time, and to which other spacecraft can dock. A space station is distinguished from other spacecraft used for human spaceflight by its lack of major propulsion or landing...
on 27 October 1992. After undocked from Mir, the Progress deployed the reflector this mission was successful in that the mirror deployed, although it did not illuminate the Earth. The next flight Znamya-2.5 failed. Znamya-3 never flew.
See also
- Parabolic troughParabolic troughA parabolic trough is a type of solar thermal energy collector. It is constructed as a long parabolic mirror with a Dewar tube running its length at the focal point. Sunlight is reflected by the mirror and concentrated on the Dewar tube...
- Solar thermal collectorSolar thermal collectorA solar thermal collector is a solar collector designed to collect heat by absorbing sunlight. The term is applied to solar hot water panels, but may also be used to denote more complex installations such as solar parabolic, solar trough and solar towers or simpler installations such as solar air...
- PhotovoltaicsPhotovoltaicsPhotovoltaics is a method of generating electrical power by converting solar radiation into direct current electricity using semiconductors that exhibit the photovoltaic effect. Photovoltaic power generation employs solar panels composed of a number of solar cells containing a photovoltaic material...
- Passive Solar
- Solar trackerSolar trackerA solar tracker is a generic term used to describe devices that orient various payloads toward the sun. Payloads can be photovoltaic panels, reflectors, lenses or other optical devices....