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| What You Will Find In This Chapter 2.2 BASIC SYSTEM CONFIGURATIONS 2.4 TYPICAL APPLICATIONS |
| 2.4.1 | DC
Loads. Effect of loads on the system Loads directly influence the performance of the entire photovoltaic system. Oversize or extra loads can cause a system to fail if the loads require more power than the modules can generate or than the battery can store. Likewise, the efficiency of the load influences the photovoltaic system's performance. All loads should be as efficient as oossible. Lighting and other resistive loads Incandescent or quartz halogen lighting for general purposes, security, or navigational aids is available in DC versions, and can be supplied with power by a photovoltaic system. Timers, motion detectors, or photocells (to determine dusk and dawn) should be used whenever possible, to eliminate leaving the lights on when they are not needed. The lamp, fixture, and general system design should be as efficient as possible. The use of DC lighting equipment is a good way to avoid the inefficiencies of the inverter needed to convert DC to AC. DC fluorescent and low pressure sodium systems are available, are much more efficient, and are discussed below under inductive loads. "Heating" loads are a poor use of PV generated electricity. These include resistive heating appliances and tools such as toasters, coffee makers, soldering irons, space and water heaters. Because of the high amount of energy they consume, these loads should be used only when there is no other option, or if the load will only be used occasionally. Oversized or inappropriate loads often result in system failure. Inductiveloads Inductive loads are those involving a motor or an electromagnet. Many photovoltaic systems supply energy to DC motors driving power tools, fans, pumps, and appliances. Inductive loads also include solenoids, which use electricity to create a magnetic field to open or close valves or perform other operations in a variety of mechanical systems. Again, the efficiency of the load should be as high as possible. One advantage to DC motors is that they are more efficient than AC motors. DC lighting systems using a ballast, such as low pressure sodium and fluorescent systems, are also inductive loads (Figure 2-43). They should be used whenever possible, as they are considerably more efficient than incandescent or quartz halogen systems.
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| FIGURE 2-43 Low Pressure Sodium Light Photo Courtesy of |
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| 2.4.2 | AC Loads. AC loads can be used if the photovoltaic system includes an inverter. In general, it is best to try to limit AC loads because of the energy lost in the conversion of DC to AC in an inverter. Inverters are discussed in detail in Section 2.5.3. Lighting and other resistive loads Sometimes, the availability of AC power makes the use of small amounts of AC incandescent lighting reasonably appropriate. With photovoltaic systems, incandescent lighting should be minimized because of its poor efficiency. AC fluorescent and low pressure sodium lighting systems are more efficient. Using DC power directly from the battery to operate DC versions of these lighting systems is an even better choice. The use of AC appliances and tools such as toasters, dryers, soldering irons, and heat guns (which are primarily resistance heaters) should be minimized. Inductive loads Many appliances and power tools are only available with AC motors. These motors generally require a "clean" source of AC power, and thus a more sophisticated inverter. More information on inverters can be found in Section 2.5.3. Motors operating on a power source which is not clean enough will waste electrical energy. The wasted energy is dissipated through the motor as heat. This can shorten the lifetime of the motor. Combination 120V AC/DC motors are available which can operate on either power source. These can be used as a portable DC device in the field with a photovoltaic system, and "plugged in" to a utility's AC supply at other times. It should be noted, however, that the majority of the DC systems installed are rarely 1 20 volts. Microwave ovens require a fairly clean AC power supply, and are an inductive load. The power supply requires the peak voltage of a sine wave (clean power to operate properly). For lighting, high pressure sodium lighting systems offer the most efficiency, followed in order by low pressure sodium, metal halide, mercury vapor, and fluorescent. For indoor lighting, fluorescent is probably the best choice, since the others supply light with poor color rendering. Electronic loads Some electronic devices, including communications devices and small computers, will operate satisfactorily on the output of simple inverters. Other devices, such as video and audio equipment, require the cleaner output of more sophisticated inverters. Again, more information on inverters can be found in Section 2.5.3. A good example of the difficulty of categorizing electronic loads is a personal computer. The computer itself may run without any problems on the output of a simple inverter. The video monitor and printer, however, will not. Therefore, the entire package should be run with the cleaner power supply of a more sophisticated inverter. Clocks may run fast on an inverter that produces a square or quai-sine wave. Another way to categorize electronic loads is their sensitivity to RFI (Radio Frequency Interference) or EMI (ElectroMagnetic Interference). Simpler inverters can create significant RFI or EMI "noise," which may interfere with the operation of some equipment. This is particularly true of two-way communication equipment, televisions, and radios.
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