![Solar Mirror - GPN-2000-001455](https://upload.wikimedia.org/wikipedia/commons/4/43/Solar_Mirror_-_GPN-2000-001455.jpg)
A solar mirror contains 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 article "
Heliostat" for more information on solar mirrors used for terrestrial energy.
Components
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
ultraviolet light
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
(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
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
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
In physics and electrical engineering the reflection coefficient is a parameter that describes how much of a wave is reflected by an impedance discontinuity in the transmission medium. It is equal to the ratio of the amplitude of the reflected w ...
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
In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenome ...
, depending on the
angle of incidence as light enters the mirror.
Metal substrates ("Metal Mirror Reflectors") may also be used in solar reflectors.
NASA Glenn Research Center
NASA John H. Glenn Research Center at Lewis Field is a NASA center within the cities of Brook Park and Cleveland between Cleveland Hopkins International Airport and the Rocky River Reservation of Cleveland Metroparks, with a subsidiary facilit ...
, 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
The International Space Station (ISS) is the largest modular space station currently in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos (Russia), JAXA ( ...
. One technology uses aluminum 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
silver
Silver is a chemical element with the symbol Ag (from the Latin ', derived from the Proto-Indo-European ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical ...
or
aluminum
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...
, 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 is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
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
A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors ...
. 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 dioxide
Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. It is a white solid that is insolub ...
.
Passive mirror cooling applications
The use of solar mirrors as a form of
passive daytime radiative cooling
Passive daytime radiative cooling (PDRC) is a renewable cooling method proposed as a solution to global warming of enhancing terrestrial heat flow to outer space through the installation of thermally-emissive surfaces on Earth that require zer ...
for
solar radiation management has been proposed to address local temperature increases as well as to decrease
global warming
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
.
Propositions have focused on the usage of solar mirrors both on the Earth's surface and in space.
Terrestrial applications
Passive mirror cooling systems reduce temperatures by reflecting solar radiation while shielding the base of the mirrors from heat penetration. The effectiveness of such systems may be reduced with the accumulation of dust on mirrors, with maximum dust accumulation reducing mirror effectiveness by 63%. However, mirrors may be "self-cleaned" by rain (reducing the soiling rate to 18.6%) or cleaned by humans.
On a local scale, passive mirror cooling systems have been implemented to lower the energy consumption used to cool residential and commercial buildings and thus offset the need for
air-conditioning.
When passive mirrored surfaces are placed on roofs, they have been shown to reduce electricity consumption and costs for cooling, with one case study reducing costs by 15%.
While the use of solar mirrors as a form of
solar radiation management on a global scale has been proposed, more data and funding is required. Increasing awareness of passive radiative cooling's potential to lower costs as well as its role in reducing solar radiation may increase applications.
Researchers who support passive mirror cooling applications on a mass scale, such as Ye Tao of MEER, argue that
carbon dioxide removal
Carbon dioxide removal (CDR), also known as negative emissions, is a process in which carbon dioxide gas () is removed from the atmosphere and sequestered for long periods of time. Similarly, greenhouse gas removal (GGR) or negative greenho ...
alone will not work fast enough to prevent global temperature increases from surpassing life-threatening levels.
Space-based applications
Solar thermal applications
The intensity of
solar thermal energy
Solar thermal energy (STE) is a form of energy and a technology for harnessing solar energy to generate thermal energy for use in industry, and in the residential and commercial sectors.
Solar thermal collectors are classified by the United St ...
from
solar radiation
Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument.
Solar irradiance is measured in watts per square metre ( ...
at the surface of the
earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
is about , of area
normal Normal(s) or The Normal(s) may refer to:
Film and television
* ''Normal'' (2003 film), starring Jessica Lange and Tom Wilkinson
* ''Normal'' (2007 film), starring Carrie-Anne Moss, Kevin Zegers, Callum Keith Rennie, and Andrew Airlie
* ''Norma ...
to the direction of the
sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
, under clear-sky conditions. When solar energy is unconcentrated, the maximum collector temperature is about . This is useful for space heating and heating water. For higher temperature applications, such as
cooking, or supplying a
heat engine
In thermodynamics and engineering, a heat engine is a system that converts heat to mechanical energy, which can then be used to do mechanical work. It does this by bringing a working substance from a higher state temperature to a lower state ...
or
turbine
A turbine ( or ) (from the Greek , ''tyrbē'', or Latin ''turbo'', meaning vortex) is a rotary mechanical device that extracts energy from a fluid flow and converts it into useful work. The work produced by a turbine can be used for generating ...
-
electrical generator
In electricity generation, a generator is a device that converts motive power (mechanical energy) or fuel-based power ( chemical energy) into electric power for use in an external circuit. Sources of mechanical energy include steam turbines, g ...
, this energy must be concentrated.
Terrestrial applications
Solar thermal
Solar thermal energy (STE) is a form of energy and a technology for harnessing solar energy to generate thermal energy for use in industry, and in the residential and commercial sectors.
Solar thermal collectors are classified by the United S ...
systems have been constructed to produce
concentrated solar power (CSP), for generating electricity. The large Sandia Lab
solar power tower
A solar power tower, also known as 'central tower' power plant or ' heliostat' power plant, is a type of solar furnace using a tower to receive focused sunlight. It uses an array of flat, movable mirrors (called heliostats) to focus the sun's r ...
uses a
Stirling engine
A Stirling engine is a heat engine that is operated by the cyclic compression and expansion of air or other gas (the ''working fluid'') between different temperatures, resulting in a net conversion of heat energy to mechanical work.
More specif ...
heated by a solar mirror
concentrator In the evolution of modern telecommunications systems there was a requirement to connect large numbers of low-speed access devices with large telephone company 'central office' switches over common paths. During the first generations of digital netw ...
. Another configuration is the trough system.
Space power application
"Solar dynamic" energy systems have been proposed for various
spacecraft
A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, p ...
applications, including
solar power satellites, where a reflector focuses sunlight on to a
heat engine
In thermodynamics and engineering, a heat engine is a system that converts heat to mechanical energy, which can then be used to do mechanical work. It does this by bringing a working substance from a higher state temperature to a lower state ...
such as the
Brayton cycle
The Brayton cycle is a thermodynamic cycle that describes the operation of certain heat engines that have air or some other gas as their working fluid. The original Brayton engines used a piston compressor and piston expander, but modern gas tu ...
type.
Photovoltaic augmentation
Photovoltaic cells
A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon. (PV) which can convert solar radiation directly into
electricity
Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as describ ...
are quite expensive per unit area. Some types of PV cell, e.g.
gallium arsenide, if cooled, are capable of converting efficiently up to 1,000 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
Boeing
The Boeing Company () is an American multinational corporation that designs, manufactures, and sells airplanes, rotorcraft, rockets, satellites, telecommunications equipment, and missiles worldwide. The company also provides leasing and p ...
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 Ehricke
Krafft Arnold Ehricke (March 24, 1917 – December 11, 1984) was a German rocket-propulsion engineer and advocate for space colonization.
Biography
Born in Berlin, Ehricke believed in the feasibility of space travel from a very young age, ...
, who wrote about systems called "Lunetta", "Soletta", "Biosoletta" and "Powersoletta".
A preliminary series of experiments called
Znamya
''Znamya'' ( rus, Знамя, p=ˈznamʲə, a=Ru-знамя.ogg, lit. "The Banner") is a Russian monthly literary magazine, which was established in Moscow in 1931. In 1931–1932, the magazine was published under the name of ''Lokaf'' ("Лок ...
("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
Progress M-15 (russian: Прогресс М-15, italic=yes) was a Russian uncrewed 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 ...
resupply mission to the
Mir
''Mir'' (russian: Мир, ; ) was a space station that operated in low Earth orbit from 1986 to 2001, operated by the Soviet Union and later by Russia. ''Mir'' was the first modular space station and was assembled in orbit from 1986 to&n ...
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.
In 2018,
Chengdu
Chengdu (, ; simplified Chinese: 成都; pinyin: ''Chéngdū''; Sichuanese pronunciation: , Standard Chinese pronunciation: ), alternatively romanized as Chengtu, is a sub-provincial city which serves as the capital of the Chinese pro ...
, China, announced plans to place three solar reflectors in orbit around the Earth in hopes of reducing the amount of electricity required to power streetlights. Skepticism has been voiced regarding the technological feasibility of the plan.
See also
*
Parabolic trough
A parabolic trough is a type of solar thermal collector that is straight in one dimension and curved as a parabola in the other two, lined with a polished metal mirror. The sunlight which enters the mirror parallel to its plane of symmetry is foc ...
*
Solar thermal collector
A solar thermal collector collects heat by absorbing sunlight. The term "solar collector" commonly refers to a device for solar hot water heating, but may refer to large power generating installations such as solar parabolic troughs and so ...
*
Photovoltaics
Photovoltaics (PV) is the conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect, a phenomenon studied in physics, photochemistry, and electrochemistry. The photovoltaic effect is commercially ...
*
Passive Solar
In passive solar building design, windows, walls, and floors are made to collect, store, reflect, and distribute solar energy, in the form of heat in the winter and reject solar heat in the summer. This is called passive solar design because, unli ...
*
Solar tracker
*
Sun gun
References
{{Photovoltaics
Mirrors
Solar thermal energy
Solar-powered devices
Photovoltaics