2.3 COMPONENT OPERATION
|2.3 COMPONENT OPERATION|
Cells. At the present time, most commercial photovoltaic cells are
manufactured from silicon, the same material from which sand is made. In this case,
however, the silicon is extremely pure. Other, more exotic materials such as gallium
arsenide are just beginning to make their way into the field.
The four general types of silicon photovoltaic cells are:
Most photovoltaic cells are single-crystal types. To make them, silicon is purified, melted, and crystallized into ingots. The ingots are sliced into thin wafers to make individual cells. The cells have a uniform color, usually blue or black (Figure 2-11).
|Typically, most of the cell has a slight positive electrical
charge. A thin layer at the top has a slight negative charge.
The cell is attached to a base called a "backplane." This is usually a layer of metal used to physically reinforce the cell and to provide an electrical contact at the bottom.
Since the top of the cell must be open to sunlight, a thin grid of metal is applied to the top instead of a continuous layer. The grid must be thin enough to admit adequate amounts of sunlight, but wide enough to carry adequate amounts of electrical energy (Figure 2-12)
Operation of a Photovoltaic Cell
|Light, including sunlight, is sometimes described as
particles called "photons." As sunlight strikes a photovoltaic cell, photons
move into the cell.
When a photon strikes an electron, it dislodges it, leaving an empty "hole". The loose electron moves toward the top layer of the cell. As photons continue to enter the cell, electrons continue to be dislodged and move upwards (Figure 2-12)
If an electrical path exists outside the cell between the top grid and the backplane of the cell, a flow of electrons begins. Loose electrons move out the top of the cell and into the external electrical circuit. Electrons from further back in the circuit move up to fill the empty electron holes.
Most cells produce a voltage of about one-half volt, regardless of the surface area of the cell. However, the larger the cell, the more current it will produce.
Current and voltage are affected by the resistance of the circuit the cell is in. The amount of available light affects current production. The temperature of the cell affects its voltage. Knowing the electrical performance characteristics of a photovoltaic power supply is important, and is covered in the next section.
Polycrystalline cells are manufactured and operate in a similar manner. The difference is that a lower cost silicon is used. This usually results in slightly lower efficiency, but polycrystalline cell manufacturers assert that the cost benefits outweigh the efficiency losses.
Polycrystalline Silicon Cells
|The surface of polycrystalline cells has a random pattern of
crystal borders instead of the solid color of single crystal cells (Figure 2-1 3).
Ribbon-type photovoltaic cells are made by growing a ribbon from the molten silicon instead of an ingot. These cells operate the same as single and polycrystal cells.
The anti-reflective coating used on most ribbon silicon cells gives them a prismatic rainbow appearance.
Amorphous or thin film silicon
The previous three types of silicon used for photovoltaic cells have a distinct crystal structure. Amorphous silicon has no such structure. Amorphous silicon is sometimes abbreviated "aSi" and is also called thin film silicon.
Amorphous silicon units are made by depositing very thin layers of vaporized silicon in a vacuum onto a support of glass, plastic, or metal.
Amorphous silicon cells are produced in a variety of colors (Figure 2-1 4).
Since they can be made in sizes up to several square yards, they are made up in long rectangular "strip cells." These are connected in series to make up "modules." Modules of all kinds are described in Section 2.3.2.
An Amorphous Silicon Module
Photo Courtesy of Arco Solar, Inc.
|Because the layers of silicon allow some light to pass
through, multiple layers can be deposited. The added layers increase the amount of
electricity the photovoltaic cell can produce. Each layer can be "tuned" to
accept a particular band of light wavelength.
The performance of amorphous silicon cells can drop as much as 15% upon initial exposure to sunlight. This drop takes around six weeks. Manufacturers generally publish post-exposure performance data, so if the module has not been exposed to sunlight, its performance will exceed specifications at first.
The efficiency of amorphous silicon photovoltaic modules is less than half that of the other three technologies. This technology has the potential of being much less expensive to manufacture than crystalline silicon technology. For this reason, research is currently under way to improve amorphous silicon performance and manufacturing processes.
|2.3.2||Photovoltaic Modules. For
almost all applications, the one-half volt produced by a single cell is inadequate.
Therefore, cells are connected together in series to increase the voltage. Several of
these series strings of cells may be connected together in parallel to increase the
current as well.
These interconnected cells and their electrical connections are then sandwiched between a top layer of glass or clear plastic and a lower level of plastic or plastic and metal. An outer frame is attached to increase mechanical strength, and to provide a way to mount the unit. This package is called a "module" or "panel" (Figure 2-15). Typically, a module is the basic building block of photovoltaic systems. Table 2-1 is a summary of currently available modules.
A Photovoltaic Module
Photo Courtesy of Arco Solar, Inc.
|TABLE 2-1: Summary of Current Photovoltaic Technology
|Groups of modules can be interconnected in series and/or parallel to form an "array." By adding "balance of system" (BOS) components such as storage batteries, charge controllers, and power conditioning devices, we have a complete photovoltaic system.|
|2.3.3||Describing Photovoltaic Module
Performance. To insure compatibility with storage batteries or loads, it is
necessary to know the electrical characteristics of photovoltaic modules.
As a reminder, "I" is the abbreviation for current, expressed in amps. "V" is used for voltage in volts, and "R" is used for resistance in ohms.
A photovoltaic module will produce its maximum current when there is essentially no resistance in the circuit. This would be a short circuit between its positive and negative terminals.
This maximum current is called the short circuit current, abbreviated I(sc). When the module is shorted, the voltage in the circuit is zero.
Conversely, the maximum voltage is produced when there is a break in the circuit. This is called the open circuit voltage, abbreviated V(oc). Under this condition the resistance is infinitely high and there is no current, since the circuit is incomplete.
These two extremes in load resistance, and the whole range of conditions in between them, are depicted on a graph called a I-V (current-voltage) curve. Current, expressed in amps, is on the vertical Y-axis. Voltage, in volts, is on the horizontal X-axis (Figure 2-16).
A Typical Current-Voltage Curve
|As you can see in Figure 2-16, the short circuit current
occurs on a point on the curve where the voltage is zero. The open circuit voltage occurs
where the current is zero.
The power available from a photovoltaic module at any point along the curve is expressed in watts. Watts are calculated by multiplying the voltage times the current (watts = volts x amps, or W = VA).
At the short circuit current point, the power output is zero, since the voltage is zero.
At the open circuit voltage point, the power output is also zero, but this time it is because the current is zero.
There is a point on the "knee" of the curve where the maximum power output is located. This point on our example curve is where the voltage is 17 volts, and the current is 2.5 amps. Therefore the maximum power in watts is 17 volts times 2.5 amps, equaling 42.5 watts.
The power, expressed in watts, at the maximum power point is described as peak, maximum, or ideal, among other terms. Maximum power is generally abbreviated as "I (mp)." Various manufacturers call it maximum output power, output, peak power, rated power, or other terms.
The current-voltage (I-V) curve is based on the module being under standard conditions of sunlight and module temperature. It assumes there is no shading on the module.
Standard sunlight conditions on a clear day are assumed to be 1000 watts of solar energy per square meter (1000 W/m2or lkW/m2). This is sometimes called "one sun," or a "peak sun." Less than one sun will reduce the current output of the module by a proportional amount. For example, if only one-half sun (500 W/m2) is available, the amount of output current is roughly cut in half (Figure 2-17).
A Typical Current-Voltage Curve at One Sun and One-half Sun
|For maximum output, the face of the photovoltaic modules
should be pointed as straight toward the sun as possible. Section 2.3.5 contains
information on determining the correct direction and module tilt angle for various
locations and applications.
Because photovoltaic cells are electrical semiconductors, partial shading of the module will cause the shaded cells to heat up. They are now acting as inefficient conductors instead of electrical generators. Partial shading may ruin shaded cells.
Partial module shading has a serious effect on module power output. For a typical module, completely shading only one cell can reduce the module output by as much as 80% (Figure 2-18). One or more damaged cells in a module can have the same effect as shading.
A Typical Current-Voltage Curve for an Unshaded Module and for a Module with One Shaded Cell
|This is why modules should be completely unshaded during
operation. A shadow across a module can almost stop electricity production. Thin film
modules are not as affected by this problem, but they should still be unshaded.
Module temperature affects the output voltage inversely. Higher module temperatures will reduce the voltage by 0.04 to 0.1 volts for every one Celsius degree rise in temperature (0.04V/0C to 0.1V/0C). In Fahrenheit degrees, the voltage loss is from 0.022 to 0.056 volts per degree of temperature rise (Figure 2-19).
This is why modules should not be installed flush against a surface. Air should be allowed to circulate behind the back of each module so it's temperature does not rise and reducing its output. An air space of 4-6 inches is usually required to provide proper ventilation.
A Typical Current-Voltage Curve for a Module at 25°C (77°F) and 85°C (185°F)
|The last significant factor which determines the power output
of a module is the resistance of the system to which it is connected. If the module is
charging a battery, it must supply a higher voltage than that of the battery.
If the battery is deeply discharged, the battery voltage is fairly low. The photovoltaic module can charge the battery with a low voltage, shown as point #1 in Figure 2-20. As the battery reaches a full charge, the module is forced to deliver a higher voltage, shown as point #2. The battery voltage drives module voltage.
Operating Voltages During a Battery Charging Cycle
|Eventually, the required voltage is higher than the voltage
at the module's maximum power point. At this operating point, the current production is
lower than the current at the maximum power point. The module's power output is also
To a lesser degree, when the operating voltage is lower than that of the maximum power point (point #1), the output power is lower than the maximum. Since the ability of the module to produce electricity is not being completely used whenever it is operating at a point fairly far from the maximum power point, photovoltaic modules should be carefully matched to the system load and storage.
Using a module with a maximum voltage which is too high should be avoided nearly as much as using one with a maximum voltage which is too low.
The output voltage of a module depends on the number of cells connected in series. Typical modules use either 30, 32, 33, 36, or 44 cells wired in series.
The modules with 30-32 cells are considered self regulating modules. 36 cell modules are the most common in the photovoltaic industry. Their slightly higher voltage rating, 16.7 volts, allows the modules to overcome the reduction in output voltage when the modules are operating at high temperatures.
Modules with 33 - 36 cells also have enough surplus voltage to effectively charge high antimony content deep cycle batteries. However, since these modules can overcharge batteries, they usually require a charge controller.
Finally, 44 cell modules are available with a rated output voltage of 20.3 volts. These modules are typically used only when a substantially higher voltage is required.
As an example, if the module is sometimes forced to operate at high temperatures, it can still supply enough voltage to charge 1 2 volt batteries.
Another application for 44 cell modules is a system with an extremely long wire run between the modules and the batteries or load. If the wire is not large enough, it will cause a significant voltage drop. Higher module voltage can overcome this problem.
It should be noted that this approach is similar to putting a larger engine in a car with locked brakes to make it move faster. It is almost always more cost effective to use an adequate wire size, rather than to overcome voltage drop problems with more costly 44 cell modules.
Section 2.5.5 discusses maximum power point trackers. These devices are used to bring the module to a point as close as possible to the maximum power point. They are used mostly in direct DC systems, particularly with DC motors for pumping.
|2.3.4||Photovoltaic Arrays. In
many applications the power available from one module is inadequate for the load.
Individual modules can be connected in series, parallel, or both to increase either output
voltage or current. This also increases the output power.
When modules are connected in parallel, the current increases. For example, three modules which produce 15 volts and 3 amps each, connected in parallel, will produce 15 volts and 9 amps (Figure 2-21).
Three Modules Connected in Parallel
|If the system includes a battery storage system, a reverse
flow of current from the batteries through the photovoltaic array can occur at night. This
flow will drain power from the batteries.
A diode is used to stop this reverse current flow. Diodes are electrical devices which only allow current to flow in one direction (Figure 2-22). A blocking diode is shown in the array in Figure 2-23.
Diodes with the least amount of voltage drop are called schottky diodes, typically dropping .3 volts instead of .7 volts as in silicon diodes.
Basic Operation of a Diode
|Because diodes create a voltage drop, some systems use a
controller which opens the circuit instead of using a blocking diode.
If the same three modules are connected in series, the output voltage will be 45 volts, and the current will be 3 amps.
If one module in a series string fails, it provides so much resistance that other modules in the string may not be able to operate either. A bypass path around the disabled module will eliminate this problem (Figure 2-23). The bypass diode allows the current from the other modules to flow through in the "right" direction.
Many modules are supplied with a bypass diode right at their electrical terminals. Larger modules may consist of three groups of cells, each with its own bypass diode.
Built in bypass diodes are usually adequate unless the series string produces 48 volts or higher, or serious shading occurs regularly.
Combinations of series and parallel connections are also used in arrays (Figure 2-24). If parallel groups of modules are connected in a series string, large bypass diodes are usually required.
Three Modules Connected in Series with a Blocking Diode and Bypass Diodes
|Isolation diodes are used to prevent the power from the rest of an array from flowing through a damaged series string of modules. They operate like a blocking diode. They are normally required when the array produces 48 volts or more. If isolation diodes are used on every series string, a blocking diode is normally not required.|
Twelve Modules in a Parallel-Series Array with Bypass Diodes and Isolation Diodes
|Flat-plate stationary arrays
Stationary arrays are the most common. Some allow adjustments in their tilt angle from the horizontal. These changes can be made any number of times throughout the year, although they are normally changed only twice a year. The modules in the array do not move throughout the day (Figure 2-25).
Adjustable Array Tilted for Summer and Winter Solar Angles
|Although a stationary array does not capture as much energy
as a tracking array that follows the sun across the sky, and more modules may be required,
there are no moving parts to fail. This reliability is why a stationary array is often
used for remote or dangerous locations. Section 2.3.5 contains information on determining
the correct tilt angle and orientation for different photovoltaic applications.
A portable array may be as small as a one square foot module easily carried by one person to recharge batteries for communications or flashlights. They can be mounted on vehicles to maintain the engine battery during long periods of inactivity. Larger ones can be installed on trailers or truck beds to provide a portable power supply for field operations (Figures 2-26 and 2-27)
Personal Photovoltaic Array
Portable Power Supply
Photo Courtesy of Integrated Power Corp
Arrays that track, or follow the sun across the sky, can follow the sun in one axis or in two (Figure 2-28). Tracking arrays perform best in areas with very clear climates. This is because following the sun yields significantly greater amounts of energy when the sun's energy is predominantly direct. Direct radiation comes straight from the sun, rather than the entire sky.
Normally, one axis trackers follow the sun from the east to the west throughout the day. The angle between the modules and the ground does not change. The modules face in the "compass" direction of the sun, but may not point exactly up at the sun at all times.
Two axis trackers change both their east-west direction and the angle from the ground during the day. The modules face straight at the sun all through the day. Two axis trackers are considerably more complicated than one axis types.
One Axis and Two Axis Tracking Arrays
Three basic tracking methods are used. The first uses simple motor, gear, and chain systems to move the array. The system is designed to mechanically point the modules in the direction the sun should be. No
sensors or devices actually confirm that the modules are facing the right way.
The second method uses photovoltaic cells as sensors to orient the larger modules in the array. This can be done by placing a cell on each side of a small divider, and mounting the package so it is facing the same way as the modules (Figure 2-29).
Used as Solar
|An electronic device constantly compares the small current
flow from both cells. If one is shaded, the device triggers a motor to move the array
until both cells are exposed to equal amounts of sunlight.
At night or during cloudy weather, the output of both sensor cells is equally low, so no adjustments are made. When the sun comes back up in the morning, the array will move back to the east to follow the sun again.
Although both methods of tracking with motors are quite accurate, there is a "parasitic" power consumption. The motors take up some of the energy the photovoltaic system produces.
A method which has no parasitic consumption uses two small photovoltaic modules to power a reversible gear motor directly. If both modules are in equal sunlight, as shown in Figure 2-30, current flows through the modules and none flows through the motor.
Current Flow with Both Modules in Equal Sunlight
|If the right module is shaded, it acts as a resistor (Figure 2-31). Now the current will flow through the motor, turning it in one direction.|
Current Flow with One Module Shaded
|If the other module, shown in Figure 2-32 on the left, is shaded, the current from the right module flows in the opposite direction. The motor will turn in the opposite direction as well.|
Current Flow with the Other Module Shaded
|The motor must be able to turn in both directions.|
|A third tracking method uses the expansion and contraction of
fluids to move the array. Generally, a container is filled with a fluid that vaporizes and
expands considerably whenever it is in the sun. It condenses and contracts similarly when
in the shade. These "passive" tracking methods have proven to be reliable and
durable, even in high wind situations.
One system, the 9'SUN SEEKER" TM from Robbins Engineering, uses the pressure of the expansion and contraction to operate a hydraulic cylinder. Flexible piping from two containers filled with freon goes to opposite sides of a piston in the cylinder (Figure 2-33).
Sun Seeker System without Modules
Photo Courtesy of Robbins Engineering, Inc.
|If the array is facing the sun, the pressure in both
containers stays the same, and the piston will not move in the cylinder. However, when the
sun moves the shading on the containers changes, placing them under different pressures.
The pressure difference, brought to the cylinder by the piping, will move the piston. The shaft from the piston will move the array. When the array is pointed back at the sun, the pressure stops increasing in the cylinder, and the piston and rod stop moving.
Another way to move the array with an expansive fluid is to use the change in fluid weight when it vaporizes. The Solar Track Rack TM by Zomeworks uses this method (Figures 2-34 and 2-35).
Solar Track Rack without Modules
Photo Courtesy of Zomeworks Corp.
|The fluid-filled containers are integrated into the sides of
the array mounting structure. They are connected together flexible piping, which is
protected in the mounting structure. As long as the array is facing directly at the sun,
the shades cover each container equally.
When the array is no longer facing directly at the sun, one container is exposed to more heat from the sun. This causes the fluid in that container to boil out of that container into the other one. Now the shaded container has more fluid in it and is heavier. The array will drop down like a "teeter-totter" in the direction of the shaded container until the shading equalizes on the two containers again.
Solar Track Rack without Modules Mounted
Photo Courtesy of Zomeworks Corp.
|Since this method is more sensitive, wind can move the array.
A shock absorber is included in the system to absorb such rapidly applied forces.
Reflectors are sometimes used to increase the amount of solar energy striking the modules (Figure 2-36). Since reflectors cost less than photovoltaic modules, this method may be used for some applications. There are several problems with reflectors, however.
Not all photovoltaic modules are designed for the higher temperatures reflectors cause. The performance and physical structure of many modules will suffer if reflectors are used with them. Remember that higher module temperatures mean lower output voltages.
Reflectors on a Fixed Photovoltaic Array
|Another problem is that reflectors work mostly with sunlight
coming directly from the sun. Since a great deal of the sun's energy in cloudy climates
comes to the earth's surface from all parts of the sky, reflectors are most effective in
In all but the clearest of climates, the amount of direct solar energy is rarely high enough to justify the use of reflectors all year.
By increasing the overall surface area of the array, reflectors also increase the array's wind loading characteristics.
Finally, some type of tracking system may be required. This increases the system cost, may add a parasitic power loss, and can reduce the system reliability. Poorly designed or improperly installed reflectors have been known to shade modules.
Concentrators use lenses or parabolic reflectors to focus light from a larger area onto a photovoltaic cell of smaller area. The cells are spread out more than a typical module, and must be a high temperature type. They may have a heat removal system to keep module temperatures down and output voltages up. These systems have the same disadvantages of reflectors, and are higher in cost. As a consequence, large systems feeding a utility grid are usually the only ones using reflectors or concentrators.
Small arrays of one or two modules can use simple brackets to secure the modules individually to a secure surface (Figure 2-25). The surface may be a roof, wall, post, pole, or vehicle. Brackets can include some method to adjust the tilt angle of the module.
The brackets are usually aluminum. If steel is used, it should be painted or treated to prevent corrosion. Galvanized steel is normally avoided, because the continuous grounding used on arrays aggravates the galvanic corrosion that occurs between galvanized steel and almost all other metals.
Fastener hardware should be stainless steel or cadmium plated to prevent corrosion. Identical metals should be used for components and fasteners whenever possible.
Typically, up to four modules can be connected together and mounted on a pole (Figure 2-37). Typically, 2 1/2" nominal steel pipe (O.D. of 3") is used.
Black iron or steel pipe can be used, if painted. Galvanized pipe, rarely available in this size, can be used if compatible fasteners are used. Larger arrays can be pole mounted, if hardware sizes are appropriately increased.
The same types of materials used for bracket mounting should be used for pole mounting.
Pole Mount of Photovoltaic Array
For arrays of eight or more modules, ground mounting is usually the most appropriate technique. The greatest concern is often the uplifting force of wind on the array. This is why most ground mounted arrays are on some kind of sturdy base, usually concrete.
Concrete bases are either piers, a slab with thicker edges, or footings at the front and rear of the array (Figure 2-38). All three usually include a steel reinforcement bar.
In some remote sites it may be more desirable to use concrete block instead of poured concrete. The best way to do this is to use two-web bond-beam block, reinforce it with steel, and fill the space between the webs with concrete or mortar.
Pressure-treated wood of adequate size is sometimes used for ground mounting. This can work well in fairly dry climates, but only if the beams are securely anchored to the ground, and regular inspection and maintenance is provided.
|The array's mounting hardware can be bolted to an existing
extensive shimming, some mountaintop arrays are bolted to exposed rock. In either case, adequately sized expansion-type anchor bolts are used. The heads of the bolts should be covered with some type of weatherproof sealant. Silicone sealant is the best choice.
Forces on a Photovoltaic Array
Photovoltaic modules mounted on buildings or other structures are subjected to downward force when the wind hits their front surfaces. When the wind strikes the back of the modules, upward force is generated (Figure 2-39).
For this reason, the attachment to the building of modules with exposed backs is designed to resist both directions of force.
Another consideration when modules are mounted to a structure is the trapped heat between the module and the structure. Remember that module voltage drops with increased temperature.
Generally, photovoltaic arrays are mounted on structures in such a way that air can maturely circulate under the modules. This keeps the modules operating at the lowest possible temperature and highest possible output voltage. Access to the back of the modules also simplifies service operations.
|2.3.5||Module Tilt and Orientation.
Permanently mounted modules should be tilted up from the horizontal (Figure 2-40
and Table 2-2). The correct tilt angle varies with the times of year the system is used,
and the latitude of the site. The tilt angle is measured from the horizontal, not from a
pitched roof or hillside.
The tilt should be within 10 degrees of the listed angle. For example, a system used throughout the year at a latitude of 350 can have a tilt angle of 250 to 450 without a noticeable decrease in annual performance.
Module Tilt Measured form the Horizontal on Level and Tilted Surfaces
|Table 2-2 Photovoltaic Module Tilt Angles|
|For proper operation, the modules must be oriented as close
as possible toward the equator. In the Northern Hemisphere, this direction is true south.
In most areas, this varies from the magnetic south given by a compass. A simple correction
must be made.
First, find the magnetic variation from an isogonic map. This is given in degrees east or west from magnetic south (Figure 2-41).
Figure 2-41: Isogonic Map of the United States
|For example, a site in Montana has a magnetic variation of 200 east. This means that trne south is 200 east of magnetic south. On a compass oriented so the north needle is at 3600, true south is in the direction indicated by 1600 (Figure 2-42).|
Directions on a Compass at 20° East Magnetic Variation
|The modules should be installed within 200 of true south. In
areas with morning fog, the array can be oriented up to 200 toward the west to compensate.
Conversely, arrays in areas with a high incidence of afternoon storms can be oriented
toward the east.
If the array is located in the Southern Hemisphere, the array must face true north.
Small portable arrays are usually just pointed at the sun, and moved every hour or so to follow the sun across the sky.