Solar mirror

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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.[1]

Contents

[edit] Components

[edit] 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.

[edit] 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[citation needed], wherein an aluminum layer would have to be placed on the top surface of the glass substrate, and not on the bottom surface.[citation needed] 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.

[edit] 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.

[edit] Solar thermal applications

The intensity of solar thermal energy from solar radiation at the surface of the earth is 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.

[edit] Terrestrial applications

Solar thermal systems have been constructed to produce "concentrated solar power" (CSP), for generating electricity.[2][3] The large Sandia Lab solar power tower uses a Stirling engine heated by a solar mirror concentrator.[4] Another configuration is the trough system.[5]

[edit] 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.[6]

[edit] 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.[7] photocell percent conversion efficiency actually increased at higher levels of concentration, often by significant amounts, provided external cooling is available to the photocells.

[edit] 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.

[edit] 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 corporation.[8] 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.

[edit] 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, who wrote about systems called "Lunetta", "Soletta", "Biosoletta", "Powersoletta".[9][10]

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[11][12] this mission was successful. The next flight Znamya-2.5 failed.[13] Znamya-3 never flew.

One interesting theoretical method to construct such an orbiting solar mirror is the "tension stabilized steerable orbiting mirror".[14]

[edit] See also

[edit] References

  1. ^ Solar mirror, process for its manufacture and its use (December 12, 1993). Retrieved on 2007-05-03.
  2. ^ Sandia Labs - CSP Technologies Overview
  3. ^ PowerTower The large design developed by Sandia National Labs
  4. ^ Sandia Lab - Solar Dish Engine
  5. ^ Sandia Lab - Trough System
  6. ^ Mason, Lee S.; Richard K. Shaltens, James L. Dolce, and Robert L. Cataldo (2002-01-XX). Status of Brayton Cycle Power Conversion Development at NASA GRC (PDF). NASA Glenn Research Center. Retrieved on 2007-02-25.
  7. ^ Yoon, Sewang; Vahan Garboushian (pub date unknown). Reduced Temperature Dependence of High-Concentration Photovoltaic Solar Cell Open-Circuit Voltage (Voc) at High Concentration Levels (HTML). Amonix Corp.. Retrieved on 2007-02-25.
  8. ^ Potter, Seth D.; Harvey J. Willenberg, Mark W. Henley, and Steven R. Kent (May 6, 1999). "Potter Architecture Options for Space Solar Power". High Frontier Conference XIV, Princeton, NJ, U.S.A.: Space Studies Institute. Retrieved on 2007-02-25. 
  9. ^ Ehricke, Krafft Arnold (September 1-4, 19). "Power Soletta: An industrial sun for Europe - Possibilities for an economically feasible supply with solar energy". Raumfahrtkongress, 26th: 85-87, Berlin, West Germany: Hermann-Oberth-Gesellschaft. vol. 14, no. 3. Retrieved on 2007-02-25. 
  10. ^ Ehricke, Krafft Arnold (January-February 1978). "The Extraterrestrial Imperative". Air University Review Vol. XXIX (No. 2). United States Air Force. 
  11. ^ McDowell, Jonathan (1993-02-10). Jonathan's Space Report - No 143 - Mir (HTML). Jonathan's Space Report. Jonathan McDowell. Retrieved on 2007-02-25.
  12. ^ Wade, Mark (pub date unknown). Mir EO-12 (HTML). Encyclopedia Astronautica. Mark Wade. Retrieved on 2007-02-25.
  13. ^ Wade, Mark (pub date unknown). Mir News 453: Znamya 2.5 (HTML). Encyclopedia Astronautica. Mark Wade. Retrieved on 2007-02-25.
  14. ^ Gould, Len. Tension Stabilized Steerable Orbiting Mirror (HTML). self published. Retrieved on 2005-01-03. “TSSOM” (no longer available February 25, 2007)