Solar panels

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File:Shaped Fixed Array.jpg
A Solar Panel Array Shaped to Catch the Most Energy During the Day

Photovoltaic devices (or PV cells) employ the photovoltaic effect to produce electricity directly from light. This phenomon is often employed in sensors and other solid state electronics. Certain types of photovoltaic device known as solar cells can be combined to create a sunlight powered electrical generator with no moving parts.

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A Solar Panel Array Powering a Moving Trailer

Photovoltaic devices are commonly used to power satellites and other remote applications, and to power many popular consumer electronics. As manufacturing techniques improve and costs decrease, the use of photovoltaic arrays as household powerplants is becoming more popular as well.

How PV Cells Work

File:Photocell Schematic.jpg
A Schematic of a Photovoltaic Cell

PV cells are made using materials which are electrically excited by absorbed light. The cells consist of at least four layers of different materials: a negative photoelectric layer (one that releases electrons when excited by light), a neutral or positive photoelectric layer (one that absorbs electrons when excited by light), and electrodes on both the lit face and shaded face of the cell. The electrode on the lit face must be either transparent or made of a fine enough mesh to pass light through to the cell, and must form an electrical circuit with the electrode on the rear face. If the photoelectric material has a relatively high resistance at low voltages (like a semiconductor), then the released electrons will find the circuit between the electrodes to be the easiest path from the positive face of the cell to the negative face, instead of simply flowing through the photoelectric material and short circuiting the cell. Electricity will flow in the circuit between the electrodes.

PV cells are typically manufactured as thin flat plates, in order to maximize their surface area exposed to light. They are often coated with other materials as well, such as protective substrates or translucent anti-glare layers.

Types of PV Cell

There are three basic structures used in PV Cells: homojunction cells, heterojunction cells, and multijunction cells.

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Schematic of an Amorphous Silicon Homojunction Cell

Homojunction cells use a single photoelectric material, which is doped with one impurity on one side to form a positive layer and doped with a different impurity on the other side to form a negative layer. These are among the thinnest, lightest, and most durable solar cells. They are also among the most popular and have the most well developed techniques for their manufacture. Silicon, gallium alloys, and other semiconductors are typically used in their construction. They are typically expensive and relatively delicate, but are among the more efficient PV cells and have a long operational lifetime.

Heterojunction cells use two different photoelectric materials for their positive and negative layers. They can also be relatively lightweight and efficient, and certain types are quite durable depending on the material. Heterojunction cells do not require semiconductors or doping in their manufacture, and some designs do not require complex equipment for in situ manufacture. Flexible and paint-on solar cells are of this type.

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Schematic of a Multijunction Photovoltaic Cell

Multijunction devices are piles of two or more PV cells in series, with the lower cells absorbing light that passes through the upper cells. The peak absorbtion wavelength of the lower cells must be matched to the bandpass of the upper cells. Diodes and other solid state circuitry can also be incorporated into the device. The component cells can be homojunction or heterojunction. This type of PV cell is difficult to construct. However, the most efficient PV devices known are of this type.

Manufacturing Techniques

A list of materials exhibiting the photoelectric effect is too long for this article. Simple silica sandpaper, covered with aluminum foil and immersed in a salt bath, will produce a feeble photovoltaic effect when illuminated through its paper backing. Titanium white house paint, pencil lead, anything “glow in the dark” that you’ve ever owned – all of these contain materials that can develop a feeble electrical charge under sunlight. The trick lies in selecting materials that can go from producing a barely measurable static voltage to generating useful amounts of electrical power. The most common materials used for this purpose are semiconductors like silicon or gallium arsenide. Cadmium sulfide, titanium oxide and other materials are also frequently employed. Finding materials for the manufacture of solar cells is likely to be less challenging than actually manufacturing them.

PV cells can be manufactured using the Czochralski process, the String Ribbon process, thin film methods, or paint-on techniques.

File:Czochralski Process.jpg
The Czochralksi Process for Manufacture of Crystalline Silicon Rods and Wafers

In the Czochralski process, a rod of semiconducting material is gradually drawn from a molten vat during a continuous cooling process. The rod is then sliced into semiconductor wafers. This method is the most well developed technique, with extensive industrial experience. The highest efficiency crystalline PV cells are produced using this method. However, it produces cells of only limited size.

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The String Ribon Process for the Manufacture of Amorphous Silicon Sheets

In the String Ribbon process, molten cell material is drawn into sheets by surface tension between two wires. This method is cheaper than the Czochralski process, and PV cells made this way can be as large as the length of wire used to draw them. This method produces an amorphous crystal structure in cells, reducing their maximum efficiency.

Thin film methods (chemical processed, vapor deposition, etc.) are used to deposit cell materials on a substrate. These have the lightest weight, but achieve lower efficiencies than cells made using other methods.

Paint-on techniques can be used to mechanically apply the layers of a heterojunction cell to a substrate. However, like thin film methods, these techniques produce cells of limited efficiency. Painting techniques can only be used with a limited range of materials.

Solar Panels

To produce useful amounts of electrical power, individual PV cells must be combined into modules. These models may then be frames to produce photovoltaic panels (solar panels).

There are three common types of panels: rigid, sheet, and flexible.

File:Rigid Solar Panels.jpg
Rigid Frame Solar Panels

Rigid panels have inflexible rigid frames and/or backing. They are generally the most durable type of solar panel, and are most readily protected from the elements. Because they support their own weight, these are the easiest type of panel to use in arrays that actively track the sun to produce maximum power.

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Sheet Solar Panels

Sheet panels consist of PV cells which are either wired together or fixed to a flexible backing to form a panel that will roll or flex about at least one axis. These are easy to store and deploy, and can achieve the same efficiencies and outputs as a rigid panel with less weight. They are far less durable, though. They are relatively common in spacecraft applications, such as the International Space Station.

File:Flexible Solar Panels.jpg
Flexible Solar Panels

Flexible panels consist of PV cells which are themselves flexible. They are as easy to store and deploy as sheet panels, and are more durable, but not generally as efficient.

Less common types of solar panels include thin film and paint-on solar panels, which can be shaped to match whatever substrate is used.

Photovoltaic Arrays

Solar panels are combined into arrays for power production.

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A Heliostatic (Sun Tracking) solar Array

Heliostatic arrays track the sun to produce the closest to maximum power at all times. These require motors and control systems.

File:Fixed Array.jpg
A Fixed Array of Solar Panels

Fixed arrays do not track the sun. Because they require no moving parts, they are more reliable and versatile. However, their power depends on the angle of incidence with which they are illuminated. While a fixed array can be shaped to maximize the amount of power produced over the course of a day, they must be larger than a heliostatic array to produce the same amount of power.

File:Fresnel Lens Concentrator.jpg
A Fresnel Lens Solar Concentrator
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A Mirror Solar Concentrator

Lenses and mirrors can be used as concentrators to increase the amount of light absorbed by the array where direct sunlight is available. Fresnel lenses can be constructed of flat plastic panels, and mirrors can be made of aluminized plastic.

Martian Conditions

Most semiconductor solar cells function more efficiently at Martian temperatures than they do at room temperature.

The solar radiation constant in Mars orbit is only 590 W/m2, compared to 1370 W/m2 in Earth orbit. The martian atmosphere is thin, but so dusty that it cuts an average 25% of the incident sunlight before reaching the surface, leaving only an average 440 W/m2 at the Martian equatorial surface. The reduction of sunlight can reach 75% during a dust storm, further reducing the surface insolation to only 150 W/m2.

A typical amorphous silicon solar cell can convert only about 7% to 10% of this surface sunlight into electricity, depending on its design, yielding less than 44 W/m2 under best case conditions and as little as 10 W/m2 during an ongoing dust storm. This requires large arrays to provide even modest amounts of electrical power. The use of concentrators would need to be restricted to storm-free conditions, because they cease to function effectively when incident light becomes diffused by scattering dust. Because large dust storms are a seasonal phenomenon on Mars, this could make any type of concentrator functionally useless for weeks at a time each year.

Airborne dust will also slowly accumulate on any deployed solar panels. Horizontal flat arrays can lose power as fast as 0.5% to 1% per day due to dust accumulation, and must be cleaned to operate more than a few months. Vertical arrays will accumulate dust almost as rapidly due to the nature of typical glare-reduction coatings, although they should have an upper limit to immediate power loss. Solar panels deployed under these conditions will require dust protection and/or provision for cleaning.

Heliostatic Array Example

The power per unit area produced by a solar array is given by the equation:

<math>{dP \over dA} = \epsilon C cos(\phi) </math>

where:
<math>dP \over dA</math> is the power per unit area
<math>\epsilon</math> is the collector efficiency
C is the solar radiation constant
<math>\phi</math> is the angle of incidence

A 10% efficient, rigid flat solar panel array producing 1 kW electrical power on Mars under average conditions would require 22.75m2 of collector area if illuminated at an angle exactly perpendicular to its surface (<math>\phi</math> = 0o). That would be equivalent to a square array 4.8m on each side.

Because the sun moves across the sky during the course of each sol, changing its angle of incidence, maintaining a constant power output would require that the array in question either be heliostatic, or be larger to accomodate sunlight at low angles. The array must also tilt seasonally, to track the change in the sun's elevation. If the array received 8 hours useable sunlight per day, a heliostatic array could produce 28.8 MJ of electrical energy per day, or an average of about 330 W over a 24 hour day. Thus, a 1kW heliostatic array can only power a 330 W system, including power for its own motors, control systems and losses due to battery charging and other inefficiencies.

Flat Fixed Array Example

Fixed arrays must be still larger, because their angle of incidence varies over time. If the array is flat, then the daily energy absorbed per unit area is:

<math>{dE \over dA} = {\epsilon C \Delta t_{day} \over \pi} ( sin( \phi_{1} ) - sin( \phi_{2} ) )</math>

where:
<math>dE \over dA</math> is the energy absorbed over the course of an entire day
<math>\Delta t_{day}</math> is the useful length of the day in seconds
<math>\Delta \phi</math> is the change in sun angle over the course of the day.

(Note that the above equation was derived by integrating the power with respect to time, not angle.)

Assuming that the useful angle of variation is from -67.5o to 67.5o, the angular velocity of the sun across the sky is 15o/hr, and the insolation is constant (440 W/m2 at 0o angle of incidence), then the energy per unit area absorbed over the course of the day is is only 745000 J/m2. This is only 62% of the energy intake of a heliostatic array, which means that the flat fixed array must be more than 1.6 times larger than a heliostatic array to get the same power.

A fixed array with a cylindrical collector, rather than a flat one, can have the same cross-sectional area as a heliostatic array, but must have a collector area up to three times as large.

These arrays do have two advantages over a heliostatic array, though. The 22.75mm2 required for a heliostatic array is already quite large for a single array, and would be best supported by a multiple array - requiring still more power to operate it. A fixed array requires no power to operate. It can also function in multiple arrangements, including cylindrical or double sided vertical. The fixed array can be shaped to both maximize power and minimize dust buildup. Fixed arrays are also easier to store. An inflatable cylindrical fixed array need only have the same height and diameter as a square heliostatic array in order to produce the same power without a single moving part.

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