Solar mirror

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 glass substrate, a reflective layer for reflecting the solar energy, and an interference layer. The purpose of the solar mirror is to achieve a substantially concentrated reflection factor for solar energy systems. [cite web
title =Solar mirror, process for its manufacture and its use
date =December 12, 1993
url =http://www.freepatentsonline.com/5253105.html
accessdate =2007-05-03]

Components

Glass Substrate

The glass substrate is the top layer of the mirror in which solar energy is transmitted. Its purpose is 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), relatively inexpensive, 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.

Reflective Layer

The reflective layer is designed to reflect the maximum amount of solar energy incident upon it, back through the glass substrate. The reflective layer comprises a highly reflective layer of thin silver plating. The use of silver as the reflective layer leads to higher efficiency levels, because it is the most reflective metal. Despite being relatively sensitive to abrasion and corrosion, the silver layer is protected by the (glass) substrate on top, and the bottom is covered with a protective coating which usually comprises a copper layer and varnish.

Despite the use of aluminum in generic mirrors, aluminum is often not used as the reflective layer for a solar mirror. This is because of aluminum's reflection factor in the UV region of the spectrum Fact|date=March 2008, wherein an aluminum layer would have to be placed on the top surface of the glass substrate, and not on the bottom surface. Fact|date=March 2008 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 (i.e. scratching). Adding a protective layer to the aluminum would reduce its reflectivity. For this reason silver is a higher performance reflector material, and (presumably) its higher cost is justified due to higher efficiency and longevity.

The reflective layer has a high refractive index (see dielectric). In order to enhance reflection in the near-UV region of the spectrum, the thickness of this layer may be optimized for its interference effects.

Interference Layer

An interference layer is located on the first surface of the glass substrate. It is designed for diffuse?-reflectance of near-ultraviolet radiation, in order to prevent it from passing through the glass substrate. Were this interference layer not present, it would allow near-ultraviolet radiation to pass into the glass substrate and through to the reflective layer. This would substantially enhance the overall reflection of near-ultraviolet radiation from the mirror. The interference layer is composed predominantly of titanium dioxide.

Solar thermal applications

The intensity of solar thermal energy 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, a task normally assigned to flat or parabolic arrays of solar mirrors.

Terrestrial applications

Solar thermal systems have been constructed to produce "concentrated solar power" (CSP), for generating electricity. [ [http://www.energylan.sandia.gov/sunlab/overview.htm#tower Sandia Labs - CSP Technologies Overview] ] [ [http://www.energylan.sandia.gov/sunlab/PDFs/solar_tower.pdf PowerTower The large design developed by Sandia National Labs] ] The large Sandia Lab solar power tower uses a Stirling engine heated by a solar mirror concentrator. [ [http://www.energylan.sandia.gov/sunlab/PDFs/solar_dish.pdf Sandia Lab - Solar Dish Engine] ] Another configuration is the trough system. [ [http://www.energylan.sandia.gov/sunlab/Snapshot/TROUGHS.HTM Sandia Lab - Trough System] ]

Space power application

"Solar dynamic" energy systems have been proposed for various spacecraft applications, including solar power satellites, where a reflector focuses sunlight on to a heat engine such as the Brayton cycle type. [cite web
last =Mason
first =Lee S.
authorlink =
coauthors =Richard K. Shaltens, James L. Dolce, and Robert L. Cataldo
title =Status of Brayton Cycle Power Conversion Development at NASA GRC
work =
id=NASA TM-2002-211304
publisher =NASA Glenn Research Center
date =2002-01-XX
url =http://gltrs.grc.nasa.gov/reports/2002/TM-2002-211304.pdf
format =PDF
accessdate =2007-02-25

]

Photovoltaic augmentation

Photovoltaic cells (PV) which can convert solar radiation directly into electricity are quite expensive per unit area. Some types of PV cell, e.g. gallium arsenide, if cooled, are capable of converting efficiently up to 250 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. [cite web
last =Yoon
first =Sewang
authorlink =
coauthors =Vahan Garboushian
title =Reduced Temperature Dependence of High-Concentration Photovoltaic Solar Cell Open-Circuit Voltage (Voc) at High Concentration Levels
work =
publisher =Amonix Corp.
date =pub date unknown
url =http://www.amonix.com/Tech_Papers/Temp_Depend.htm
format =HTML
accessdate =2007-02-25] photocell percent conversion efficiency actually increased at higher levels of concentration, often by significant amounts, provided external cooling is available to the photocells.

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.

olar 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 corporation. [cite conference
first=Seth D.
last=Potter
authorlink=
coauthors = Harvey J. Willenberg, Mark W. Henley, and Steven R. Kent
title = Architecture Options for Space Solar Power
booktitle = High Frontier Conference XIV
pages =
publisher = Space Studies Institute
date = May 6, 1999
location =Princeton, NJ, U.S.A.
url =http://www.ssi.org/Potter_SSP_99_SSI.pdf Potter
accessdate =2007-02-25
id = ] 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.

pace 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, who wrote about systems called "Lunetta", "Soletta", "Biosoletta", "Powersoletta". [cite conference
last =Ehricke
first =Krafft Arnold
authorlink =Krafft Arnold Ehricke
coauthors =
title =Power Soletta: An industrial sun for Europe - Possibilities for an economically feasible supply with solar energy
booktitle =Raumfahrtkongress, 26th
language=German
pages =85-87
publisher =Hermann-Oberth-Gesellschaft
date =September 1-4, 19
location =Berlin, West Germany
url =http://adsabs.harvard.edu/abs/1977hogr...14...85E
accessdate =2007-02-25
id =vol. 14, no. 3 ] [cite journal
last =Ehricke
first =Krafft Arnold
authorlink =Krafft Arnold Ehricke
coauthors =
title =The Extraterrestrial Imperative
journal =Air University Review
volume =Vol. XXIX
issue =No. 2
pages =
publisher =United States Air Force
date =January-February 1978
url =http://www.airpower.maxwell.af.mil/airchronicles/aureview/1978/jan-feb/ehricke.html
doi =
id =
accessdate =2007-02-25 ]

A preliminary series of experiments called Znamya was performed by Russia. The first, designated Znamya-2, was launched aboard Progress-TM-15 on 27 October 1992. After visiting the EO-12 crew aboard the Mir space station the Progress-TM-15 then undocked and deployed the reflector [cite web
last =McDowell
first =Jonathan
authorlink =
title =Jonathan's Space Report - No 143 - Mir
work =Jonathan's Space Report
publisher =Jonathan McDowell
date =1993-02-10
url =http://www.planet4589.org/space/jsr/back/news.143
format =HTML
accessdate =2007-02-25] [cite web
last =Wade
first =Mark
authorlink =
title =Mir EO-12
work =Encyclopedia Astronautica
publisher =Mark Wade
date =pub date unknown
url =http://www.astronautix.com/flights/mireo12.htm
format =HTML
accessdate =2007-02-25] this mission was successful. The next flight Znamya-2.5 failed. [cite web
last =Wade
first =Mark
authorlink =
title =Mir News 453: Znamya 2.5
work =Encyclopedia Astronautica
publisher =Mark Wade
date =pub date unknown
url =http://www.astronautix.com/details/mir50838.htm
format =HTML
accessdate =2007-02-25] Znamya-3 never flew.

One interesting theoretical method to construct such an orbiting solar mirror is the "tension stabilized steerable orbiting mirror". [ cite web
url =http://www.ecologen.com/page_TSSOM2-75.html
title =Tension Stabilized Steerable Orbiting Mirror"
accessdate = 2005-01-03
last =Gould
first =Len
authorlink =
format =HTML
publisher=self published
quote =TSSOM (no longer available February 25 2007)]

See also

* Parabolic trough
* Solar thermal collector
* Photovoltaics
* Passive Solar
* Solar tracker

References