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A Short Course on PV

The heart of most PV systems is a PV Cell, made of a thin slice of silicon, the same material used in computer and communication chips. In order to produce a PV cell, the silicon is intentionally doped with impurity atoms, usually phosphorous and boron which produce holes and electrons respectively. This doping is realized in two thin adjacent layers, which results in a p-n junction between them. When light strikes this junction, electron-hole pairs are liberated and swept away from the junction by the built-in field. The electron is picked up by metal contacts on the front and back of the wafer and supplied to an outside circuit.

There is a potential difference or voltage produced by the built-in field, which in silicon is ~0.6 volts. Because this voltage is usually too small to be useful, 20-80 cells are combined in series to produce voltages ranging from 12 V to 48 V.

Good cells produce convert around 14% of the incident light into electricity. The reason that this efficiency is not 100% is due to the fact that a given cell is tuned to high efficiency only over a small range of wavelengths in the Sun's spectrum. Multiple junction cells have a number of junctions, stacked on top of one another, each tuned to a different part of the spectrum.

 
Image Cells can be fabricated from a single crystal boule or cast polycrystalline ingot. Some cells are even fabricated from amorphous silicon which is created by deposition onto a smooth substrate. Although amorphous silicon is less efficient

than single- or poly-crystalline cells, they are cheaper to make.

 

Figure 2. Operation of a PV cell

An anti-reflective coating or surface treatment is also used to reduce the fraction of photons that get reflected off the surface of the silicon instead of penetrating the silicon to and possibly producing an electron-hole pair.

 
What Does a PV Module Look Like?
Image

A typical PV module is composed of a series string of PV cells, environmentally sealed and enclosed in a rugged frame. After the cells are processed and electrically connected, they are embedded in PVA/EVA polymer between a front glass and a multilayer backskin of Tedlar. PVA/EVA is translucent and allows light to enter the cells and lamination in PVA/EVA facilitates thermal expansion, provides moisture protection and ensures UV stability and electrical insulation.

The front glass has a low Fe content (high transmissivity), is tempered (offering high impact resistance) and is approximately 1/8" thick. Tedlar also provides considerable moisture resistance. After these components are laminated under high heat and vacuum, the module is fitted with an anodized aluminum frame, capable of withstanding wind loads of up to 120 km/hour. A thin layer of silicon sealant is applied between the frame and the module, providing a further barrier against humidity, as well as a cushion against impact and thermal expansion. The sandwich cross-section and frame are shown in the accompanying figures.

A good example of a commercially-available PV module is shown at the left, the newly-introduced Shell Solar SQ150PC, has 72 nearly square cells. The module is 32"x64"x1.6" and weighs 38 lbs.

Other PV Cell Materials

Single crystal silicon isn't the only material used in PV cells. Polycrystalline silicon is also used in an attempt to cut manufacturing costs, although resulting cells aren't as efficient as single crystal silicon. Amorphous silicon, which has no crystalline structure, is also used, again in an attempt to reduce production costs. Other materials used include gallium arsenide, copper indium diselenide and cadmium telluride. Since different materials have different band gaps, they seem to be "tuned" to different wavelengths, or photons of different energies. One way efficiency has been improved is to use two or more layers of different materials with different band gaps. The higher band gap material is on the surface, absorbing high energy photons while allowing lower energy photons to be absorbed by the lower band gap material beneath. This technique can result in much higher efficiencies. Such cells, called multi-junction cells, can have more than one electric field.

The Sun as an Energy Source

The Sun provides a broad range of electromagnetic energy, with its greatest output in the visible portion of the spectrum (4,000 to 7,000 A). It also provides infrared and ultraviolet light. The output of the Sun is a spectrum, as shown in the figure below:

Image

The dashed line indicates a smoothed spectrum peaking near 4,500 A. The deviations from the smooth spectrum are due to elements in the atmosphere of the Sun which absorb energy at characteristic wavelengths. The Sun's spectrum is approximated by a "black body" with a temperature of 5,800 K. The atmosphere of our Earth attenuates this solar spectrum due to the presence of gases and particulate matter. At the Earth's surface, the Sun's irradiance is approximately 1,000 Watts per square meter.

PV cells are designed to make use of the most abundant portion of the Sun's spectrum, but cannot be uniformly efficient at all wavelengths. Commercial silicon-based PV cells are most efficient for photons of 1.1 eV (electron volts) corresponding to 11,000 A (1.1 micron), which is in the infrared.

Path of the Sun through the Sky

The Sun provides the photons that energize a PV system. In order for the PV module to produce full power, the Sun must uniformly illuminate the module at right angles to the surface of the module. On a clear day, the Sun provides approximately 1,000 Watts per square meter (W/m2). If a PV module is oriented South and tilted away from the zenith to an angle T equal to the local latitude, the module will receive the 1,000 (W/m2) at Noon on the Vernal (Spring) and Autumnal (Fall) Equinoxes. Earlier and later in the day, the Sun's rays will strike the PV module at more acute angles, and consequently the module will produce less electricity.

At other times of the year, the Sun's rays will strike the module at different angles. In the figures below, we show how the path of the Sun through the sky varies over a day and how this path changes with the time of year.

This first figure shows the Sun's path through the sky on the Spring or Fall Equinox. We have the Spring Equinox about March 21st. and the Fall Equinox about September 21st. Notice that the Sun rise due East and sets due West. The maximum position above the horizon, the maximum elevation, occurs at Noon and is equal to (90 -T).

 
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After the Fall Equinox, the Sun will follow a lower and lower path through the southern sky and the days will grow shorter and shorter until it reaches its lowest path on the Winter Solstice. The graphic below shows the Sun's path through the sky on the Winter Solstice, which occurs on December 21st. During the short winter days the Sun does not rise exactly in the east, but instead rises south of east and it sets south of west.

 
Image

Each day after the Winter Solstice, the Sun's path becomes a little higher in the southern sky. The Sun also begins to rise closer to the east and set closer to the west until we reach the day when it rises exactly east and sets exactly west. This day is the Spring Equinox.

After the Spring Equinox, the Sun continues its ascent in the sky, until the Summer Solstice, which occurs on June 21. Now the Sun takes its highest path through the sky and the day is the longest. Because the day is so long the Sun does not rise exactly in the east, but rises to the north of east and sets to the north of west allowing it to be in the sky for a longer period of time.

 
Image

After the summer solstice the Sun follows a lower and lower path through the sky each day until it reaches the point where it is in the sky for exactly 12 hours again. This is the Fall Equinox. Just like the Spring Equinox, the Sun will rise exactly east and set exactly west on this day and everyone in the world will experience a 12 hour day.

The PV System Hierarchy

As mentioned earlier, a PV module is a collection of PV cells. The BP Solar module BP3150, for example, is composed of 72 series-connected cells. Each group of 24 cells is provided with a bypass diode [more about this later]. Each cell has an open circuit voltage of ~0.6 V and a short circuit current of 4.55 A.

The BP3150 module has an open circuit voltage of ~43 V, 72 times the open circuit voltage of a single cell. On the other hand, the short circuit current of the string is only 4.55 A, the same as an individual cell, because all of the cells are in series.

A collection of series connected PV modules is called a string. open in small separate window For a grid-tie application, typically 8-12 PV modules are wired together in this fashion and connected to an inverter, which converts the DC power into 60 Hz AC power (usually 120/240 V single phase). A string of ten BP3150 modules would produce an open circuit voltage of 430 V, and a short circuit current of 4.55 A.

One or more strings can be connected in parallel into the same inverter, up to the limit of the inverter input power. The collection of PV modules connected to one inverter is referred to as a PV sub-array. In systems with a large number of strings, each string can be fitted with a series blocking diode. This blocking diode is designed to prevent power from fully-illuminated sub-arrays from dumping power into a sub-array that is being shaded.

Several inverters can feed the same electrical service. The entire collection of PV modules is called the PV array.

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