Solar mirror

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 of abrasion and corrosion. Although it is highly transparent (low optical losses), resistant to ultraviolet light (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 transmitted visible light and infrared radiation. The top surface, known as the “first surface”, will reflect some of the solar energy incident, due to the reflection coefficient caused by its index of refractionbeing higher than air. Most of the solar energy is transmitted through the glass substrate to the lower layers of the mirror, possibly to some refraction , depending on the angle of incidence .

Metal substrates (“Metal Mirror Reflectors”) can also be used in solar reflectors. NASA Glenn Research Center , for example, used a reflective aluminum reflector on a surface of a metallic honeycomb ^[1] as a prototype reflector unit for a proposed power system for the International Space Station. One technology uses aluminum composite reflector panels, achieving over 93% reflectivity and coating with a special coating for surface protection. Metal reflectors offer some advantages over glass reflectors, they are lightweight and stronger than glass and relatively inexpensive. The ability to retain parabolic shape is another advantage, and the subframe requirements are reduced by more than 300%. The top surface coating 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 film includes a highly reflective thin metal film, commonly known as silver or aluminum , but sometimes other metals. Because of the sensitivity to abrasion and corrosion, the metal layer is usually protected by a glass layer and a protective coating, such as a copper layer and varnish .

Despite the use of aluminum in general mirrors, aluminum is not always used as a reflective layer for a solar mirror. The use of silver is the most reflective layer because it is the most reflective metal. This is because of aluminum’s reflection factor in the UV region of the spectrum . ^{[ citation needed ]} Locating the aluminum layer on the first surface exposes it to weathering, which reduces the 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 surface of the glass substrate. ^[2] It can be used to tailor the reflectance. It can 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 .

Solar thermal applications

The intensity of solar thermal energy from solar radiation at the surface of the earth is about 1 kilowatt per square meter (0.093 kW / sq ft), 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 ° C (176-212 ° F). 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.

Terrestrial applications

Solar thermal systems have been developed to produce solar power (CSP), for generating electricity. ^[3] [4] The wide Sandia Lab solar power tower uses a Stirling engine heated by a solar mirror concentrator . ^[5] Another configuration is the trough system. ^[6]

Space power application

“Solar dynamic” energy systems have been proposed for various spacecraft applications, including solar power satellites , where a reflector focuses on a heat engine such as the Brayton cycle type. ^[7]

Photovoltaic increase

Photovoltaic cells (PV) which can convert solar radiation directly into electricity . Some types of PV cell, eg gallium arsenide , if cooled, are able to be easily converted to direct sunlight.

In tests done by Sewang Yoon and Vahan Garboushian, for Amonix Corp. ^[8] Increased concentration of silicon solar cell conversion is shown at higher levels of concentration, proportional to the logarithm of concentration, provided to the photocells. Similarly, higher efficiency multijunction cells also improve performance with high concentration. ^[9]

Terrestrial application

To date no large scale testing has been performed on this concept. Presumably this is because of the cost of the reflectors and is not economically justified.

Solar power satellite application

Theoretically, for space-based solar power satellite designs, solar mirrors could reduce the cost of energy. Several options were studied by Boeing corporation. ^[10] In their Figs. 4. captioned “Architecture 4. GEO Harris Wheel”, the authors describe a system of solar mirrors used to augment the power of some solar collectors, from which the power is then transmitted to receiver stations on earth.

Space reflectors for night illumination

Another advanced space concept is the concept of Space Reflectors which reflect sunlight on the light of day to provide night time illumination. An early proponent of this concept was Dr. Krafft Arnold Ehricke , who wrote about systems called “Lunetta”, “Soletta”, “Biosoletta” and “Powersoletta”. ^[11] [12]

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 October 27, 1992. After undocked from Mir, the Progress deployed the reflector. ^[13] [14] This mission was successful in that the mirror deployed, did not illuminate the Earth. The next flight Znamya-2.5 failed. ^[15] [16] Znamya-3 never flew.

See also

• Parabolic trough
• Solar thermal collector
• Photovoltaics
• Passive Solar
• Solar tracker
• Sun gun

References