Lighting, Security and Health Applications
These designs are printed in the "Stand-Alone Photovoltaic Systems - a Handbook of Recomended Design Practices" available from Sandia. General information is provided in this section. For additional information select from the menu below.
PV powered systems for warning signals/lights, area lighting, and refrigeration are installed and operating in many locations. They have proved reliable and are the low cost alternative for many users. Recent improvements in lamp efficiency have allowed lighting system costs to decline. Security lighting, such as required for military applications, can be powered by PV systems at a fraction of the cost of extending utility lines to remote areas. Lamp control may be implemented with photocells, timers, switches, or sensors such as motion or infrared. The increasing demand for dc lamps has brought improvement in reliability of lamp fixtures and ballast performance. Outdoor lamps are packaged and sealed to prevent build up of dust and dirt on, or in the fixture, and inside reflectors. Starting gas lamps in cold weather can be a problem. The mounting position affects the performance and lifetime of gas bulbs; check specifications. Lamp efficiency generally increases with wattage so it is better to use fewer bulbs of higher wattage. Extended service lamps are generally less efficient than standard lamps but may prove to be the least cost option. Prepackaged systems with PV power supply, battery, ballast, lamp, and control are now available. Check with local PV system supply companies.
Most PV systems for these applications operate at 12 or 24 volts dc. Incandescent and halogen bulbs that operate at 12 volts have been used in automobiles for years but the bulbs are not energy efficient and must be incorporated into a lamp fixture that is suitable for the application. Their use is not recommended. Fluorescent lamps are recommended for their efficiency&emdash;up to four times the lumens per watt of incandescent lamps. The fluorescent bulb and any other gas bulb, such as mercury vapor or sodium, may be difficult to start in cold weather. The latter types, examples of high intensity discharge lamps, may require several minutes to light fully and must cool completely prior to relighting. A well-designed reflector and/or diffuser can be used to focus the available light and lower the total PV power required.
Warning signals are used for public safety and their operational performance is often mandated by law. The warning signal must be located where the need is and using a photovoltaic power system offers the mobility necessary to place the system at the optimum location. System availability requirements are near 100 percent for these systems that safeguard human life. Check with local authorities for applicable regulations. Lamps, sirens, and foghorns are common 12 volt dc loads. The power requirements for lamps are usually predictable but may vary seasonally if warning system operation is dependent on the number of nighttime hours. Power demand for highway and railroad signals will vary with traffic. Warning signals for dust or icy roads will vary with ambient conditions. Some warning systems use no controller because the battery storage is large relative to the array size and the probability of overcharging the batteries is low. A low-voltage disconnect should be used to prevent excessive battery discharge. A blocking diode should be used to protect against battery discharge through the array at night. Be sure that modules have sufficient voltage to charge the battery fully with the diode in the circuit.
The use of PV powered refrigerator/freezers (R/F) is increasing because of the high reliability they provide. The World Health Organization has specified PV power be used for medical vaccine refrigerators in numerous countries around the world. Many dollars worth of vaccine can be ruined if not maintained at the recommended temperature. This requires a reliable power source for the refrigerator. Also, the high-efficiency dc refrigerators are being used more in residential applications because their cost is decreasing as larger numbers are manufactured. In most instances, the efficiency of the dc refrigerator is higher than an equivalent size ac unit. This reduces the power requirements accordingly. For PV powered refrigerators, the mode of operation directly affects the total system cost. Factors such as number of users, door opening habits, seasonal use variations, time of loading, temperature of incoming material, and physical location of the unit will significantly affect the amount of power required. User training in proper operation and maintenance should be a part of any project to install R/F systems in remote areas.
Batteries are required for all PV lighting, signal, and refrigeration systems. Deep cycle lead acid and nickel cadmium batteries specifically designed for photovoltaic applications are recommended. Using sealed batteries minimizes the problem of ventilation and corrosion and lowers maintenance cost but the sealed battery may not last as long as an industrial grade deep-cycle battery. Batteries should be located in a weather resistant enclosure. Nonmetallic enclosures are recommended to prevent corrosion. Follow battery manufacturer's installation and maintenance requirements.
For systems installed in remote areas, the reliability of the charge controller is critical and directly affects life-cycle cost. Buy a high quality controller with plenty of safety factor on the current that can be handled. All connections should be made in water-tight junction boxes with strain relief connectors. See Article 310 of the NEC for wire types and approved usage. Conduit is often used to protect the wire from the lamp to the batteries. If conduit is not used, all wiring should be laced and attached to the support structure with sun resistant nylon or plastic wire ties. The array frame should be grounded. The array azimuth should be true south (north in southern hemisphere) with tilt angle as determined in sizing calculations. A switch is normally installed so the array can be disconnected. Use of a fuse in the battery lead is recommended to prevent damage in case of a short in the load. Use movistors or silicon oxide varistors for lightning and surge protection. Consider the possibility of vandalism when deciding how to install the system.
PV arrays may be ground mounted or pole mounted. Elevating the array above the structure may decrease the possibility of vandalism. Array frames should be anodized aluminum, galvanized, or stainless steel and the installation designed for maximum anticipated wind velocities. Stainless steel fasteners with lockwashers are recommended. Keep the wire length to a minimum. Fencing may be required to protect the array from animals. A steep array tilt angle increases snow shedding if there are no restrictions near the bottom of the array. Seaborne arrays may be subject to hurricane force winds up to 125 mph (60 meters per second) and salt spray environments. In marine conditions, all mounting structures should be constructed from non-corrosive materials such as stainless steel, aluminum, brass, or plastic. Protect wire and connections and keep wire runs short. A good ground must be provided, particularly for pole or tower mounted systems.
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