installation

Photovoltaic Modules

A photovoltaic module is the smallest replaceable unit in a PV array. The module is an integral unit that provides support for a number of PV cells connected electrically and protected from the elements. The electrical output of the module depends on the size and number of cells, their electrical interconnection, and, of course, on the environmental conditions to which the module is exposed. PV modules come in all shapes and sizes, and may be made from different materials. However,the most commonly used module is a "glass-plate-sandwich" that has 36 PV cells connected in series to produce enough voltage to charge a 12 volt battery. The purpose of the structure is to provide a rigid package and protect the inter-cell connections from the environment. Plus (+) and minus (-) connectors are located on the back of the module for interconnection. The modules may have an individual metal frame or be protected by a rubber gasket and intended for installation in a larger frame designed to hold several modules.

There are four factors that determine any photovoltaic module's output - efficiency of the photovoltaic cells, the load resistance, solar irradiance, and cell temperature. The solar cell efficiency is set by the manufacturing process - today's commercially available modules are from 3% to 15% efficient at converting the solar energy to electrical energy. The load resistance determines where, on the current and voltage (I-V) curve, the module will operate. The obvious preferred operating point is where maximum power is generated - the peak power point. Study the I-V curve shown below. This curve represents the output of any PV generator -from a cell to the largest array.

For a given solar cell area, the current generated is directly proportional to solar irradiance (S) and is almost independent of temperature (T). Thus, as the sun's brightness increases the output Voltage and power decrease as temperature increases. The voltage of crystalline cells decreases about 0.5 percent per degree centigrade temperature increase. Therefore, arrays should be mounted in the sunniest place (no shading) and kept as cool as possible by ensuring air can move over and behind the array. No part of a PV array can be shaded. Unlike solar thermal collectors, the shading of small portions of a PV module may greatly reduce output from the entire array. PV modules connected in series must carry the same current. If some of the PV cells are shaded, they cannot produce current and will become reverse biased. This means the shaded cells will dissipate power as heat, and over a period of time failure will occur.
However, since it is impossible to prevent occasional shading, the use of bypass diodes around series-connected modules is recommended. You do not need bypass diodes if all the modules are in parallel, i.e., a 12-volt array using 12-volt modules and many designers do not use them on 24-volt arrays. However for array voltages higher than 24 volts, bypass diodes should be used around each module to provide an alternative current path in case of shading. Figure 10 shows the use of bypass diodes on a 48-volt series string. Note the bypass diodes are reverse biased if all modules are operating properly. Many module manufacturers will provide modules with the bypass diodes integrated into the module junction box. If you need to connect modules in series, ask the supplier for this feature. Using bypass diodes may postpone failure, but it does not prevent the loss of energy production from the shading. It is important to check for potential shading before installing the PV array. Consider the seasonal changes in foliage and sun angle. After installation, the area must be maintained to prevent weeds or tree branches from shading the array.

PV arrays include panels and source circuits. A panel is a group of PV modules packaged in a single frame. Each panel should be sized for easy handling and mounting. A source circuit, sometimes called a string, may include any number of PV modules and panels connected in series to produce the system voltage.

All PV modules should have durable connectors on the module. The connectors should be sturdy, and the method of attaching the wire should be simple, yet provide a secure connection. Most modules have sealed junction boxes to protect the connections. Field testing experience shows that PV cells and connections between cells within the module laminate rarely fail. Most problems occur in the module junction box where the interconnections between modules are made. These can often be repaired in the field without replacing the module. Before buying a PV module, look at the junction box and see if it is easy to make the connections. Are the terminals rugged and is there a place to connect bypass diodes? Is the junction box of good quality?

Blocking diodes are used to control current flow within a PV system. Any stand-alone PV system should have a method to prevent reverse current flow from the battery to the array and/or to protect weak or failed strings. Individual blocking diodes are sometimes used for this purpose if the controller used does not contain this feature. Figure 11 shows the location of blocking diodes that can be installed in each parallel-connected string or in the main wire connecting the array to the controller. When multiple strings are connected in parallel, as in larger systems, it is recommended that blocking diodes be used in each string as shown on the left to prevent current flow from strong strings into weak strings (due to failures or shading). In small systems, a single diode in the main connection wire is sufficient. Do not use both. The voltage drop across each diode, 0.4-0.7 volt, represents about a 6 percent drop in a 12-volt system.

A switch or circuit breaker should be installed to isolate the PV array during maintenance. This same recommendation applies to the battery circuit so another switch or circuit breaker is required. Also circuit breakers are normally installed to isolate each load. Fuses are used to protect any current carrying conductor. Fuses and cables in the array circuit should be sized to carry the maximum current that could be produced by short-term "cloud focusing" of the sunlight--up to 1.5 times the short circuit current at 1,000 w/m2 irradiance. Slow-blow fuses are recommended. Only fuses rated for dc current should be used. (Auto-motive fuses should not be used.) All metal in a PV array should be grounded to help protect the array against lightning surges, and as an added safety feature for personnel working on the system. The negative conductor on most PV systems is also grounded to the same grounding electrode used for the equipment ground. Other disconnect and grounding requirements are given in the National Electrical Code® (NEC). This code is intended to ensure that safe, durable PV systems are installed.

Orientation

A photovoltaic array can be mounted at a fixed angle from the horizontal or on a sun-tracking mechanism. The preferred azimuth for arrays in the northern hemisphere is true south. The decrease in energy production for off-south arrays roughly follows a cosine function, so if the azimuth of the array is kept to ±20° of true south, annual energy production is not reduced significantly. Some arrays are sited west of south to skew the production toward an afternoon peak load demand. The effect of array tilt angle on annual energy production is shown in Figure 12. For most locations, a tilt angle near the latitude angle will provide the most energy over a full year. Tilt angles of latitude ±15° will skew energy production toward winter or summer, respectively.

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