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If you are thinking of generating your own electricity, you should consider a photovoltaic (PV) system—a way to generate electricity by using energy from the sun. These systems have several advantages: they are cost-effective alternatives in areas where extending a utility power line is very expensive; they have no moving parts and require little maintenance they produce electricity without polluting the environment. This publication will introduce you to the basic design principles and components of PV systems. It will also help you discuss these systems knowledgeably with an equipment supplier or system installer. Single PV cells (also known as “solar cells”) are connected electrically to form PV modules, which are the building blocks of PV systems. The module is the smallest PV unit that can be used to generate substantial amounts of PV power. Although individual PV cells produce only small amounts of electricity, PV modules are manufactured with varying electrical out-puts ranging from a few watts to more than 100 watts of direct current (DC) electricity. The modules can be connected into PV arrays for powering a wide variety of electrical equipment. Two primary types of PV technologies available commercially are crystalline silicon and thin film. In crystalline-silicon technologies, individual PV cells are cut from large single crystals or from ingots of crystalline silicon. In thin-film PV technologies, the PV material is deposited on glass or thin metal that mechanically supports the cell or module. Thin-film-based modules are produced in sheets that are sized for specified electrical outputs. In addition to PV modules, the components needed to complete a PV system may include a battery charge controller, batteries, an inverter or power control unit (for alternating-current loads), safety cut-offs and fuses, an earthing circuit, and wiring. (See Balance-of-System Equipment section.)
PV
System Applications Many people are familiar with PV-powered calculators and watches, the most common small-scale applications of PV. However, there are numerous large-scale, cost-effective PV applications, including: Water pumping for small-scale remote irrigation, stock watering, residential uses, remote villages, and marine sump pumps; Lighting for residential needs, bill-boards, security, highway signs, streets and parking lots, pathways, recreational vehicles, remote villages and schools, and marine navigational buoys; Communications by remote relay stations, emergency radios, orbiting satellites, and cellular telephones; Refrigeration for medical and recreational uses; Corrosion protection for pipelines and docks, petroleum and water wells, and underground tanks; Utility grids that produce utility- or commercial-scale electricity; and Household appliances such as ventilation fans, coolers, televisions, blenders, stereos, and other appliances. The decreasing cost of PV systems and the increasing number of manufacturers and dealers for PV equipment have contributed to widespread use of the technology. In PV’s early days, do-it-yourselfers had to search for companies that manufactured PV modules and often had to adapt or reconfigure components from other non-PV systems. Today, dealers offer ready-to-use systems and state-of-the-art equipment designed specifically for PV systems. Many dealers have computer software that helps to design systems and specify appropriate components. As PV markets expand, dealers are gaining greater experience with PV applications, making it cheaper and easier to purchase PV systems.
When
Are PV Systems Appropriate? People select PV systems for a variety of reasons. Some common reasons for selecting a PV system include: Cost—When the cost is high for extending the utility power line or using another electricity-generating system in a remote location, a PV system is often the most cost-effective source of electricity. Reliability—PV modules have no moving parts and require little maintenance compared to other electricity-generating systems. Modularity—PV systems can be expanded to meet increased power requirements by adding more modules to an existing system. Environment—PV systems generate electricity without polluting the environment and without creating noise. Ability to combine systems—PV systems can be combined with other types of electric generators (wind, hydro, and diesel, for example) to charge batteries and provide power on demand. PV systems are not cost-effective for all applications. The following discussion gives some general guidelines to consider when deciding whether a PV system is appropriate for your situation. First, if your site is already connected to a utility grid, or within one-quarter mile of the grid, a PV system will probably not be cost-effective. Each utility company spreads the cost of its power plants and fuel costs among all its customers. When you install a PV system, you are essentially installing your own mini-utility system. You pay all the costs of generating the electricity you consume. Although the sun’s energy is free, the PV equipment is not free. Small PV systems are not practical for powering space-heating systems, water heaters, air conditioners, electric stoves, or electric clothes dryers. These loads require a large amount of energy to operate, which will increase the size and cost of your PV system. Therefore, select the most energy-efficient loads available. For example, if your PV system will power lights, look for the most energy-efficient light bulbs. If your system will pump water for toilets and showers, look for the most water-conserving fixtures.
Is
My Site Adequate for PV? A PV system designer can conduct a detailed site assessment for you. To save the dealer time (and possibly save yourself some money), you can conduct a preliminary assessment to determine whether your site has potential for a PV system. There are three factors to consider when determining whether your site is appropriate. First, systems installed in the UK must have a southern exposure. For maximum daily power output, PV modules should be exposed to the sun for as much of the day as possible, especially during the peak sun hours of 10 a.m. to 3 p.m. Second, the southern exposure must be free of obstructions such as trees, mountains, and buildings that might shade the modules. Consider both summer and winter paths of the sun, as well as the growth of trees and future construction that may cause shading problems. Finally, the unobstructed southern exposure must also have appropriate terrain and sufficient space to install the PV system. A flat, grassy site is appropriate terrain, whereas a steep, rocky hillside is not.
How
Does Weather Affect PV Module Output? Unlike utility power plants, which produce electricity constantly despite the time of day and year or the weather, the output of PV modules is directly related to these two factors. Where you live will affect the number of PV modules you will need for power, because different geographic regions experience different weather patterns. Seasonal variations affect the amount of sunlight available to power a PV system Module temperature also affects output. The conversion efficiency of crystalline-silicon modules falls significantly at elevated module temperatures.
Balance-of-System
Equipment In addition to the PV modules, you must purchase balance-of-system (BOS) equipment. This includes battery charge controllers, batteries, inverters (for loads requiring alternating current), wires, conduit, an earthing circuit, fuses, safety cut-offs, outlets, metal structures for supporting the modules, and any additional components that are part of the PV system. Below, we’ll discuss PV and BOS configurations first for loads requiring direct current, then for loads needing alternating current. Note that, in many systems, the cost of BOS equipment can equal or exceed the cost of the PV modules. When examining the costs of PV modules, remember that these costs do not include the cost of BOS equipment. Ask your PV dealer about the BOS equipment required by your system. Sizing
the System to Meet Your Needs After you have assessed the appropriateness of your site, you need to determine how much electricity your PV system must generate. This depends on how much electricity your loads require. Again, your dealer can help you with sizing a system that will meet your needs. To size your PV system, you must first know your energy needs, which you figure by listing all your daily loads. A load includes anything that uses electricity from your power source, such as lights, televisions, radios, or batteries. Some loads need electricity all the time, such as refrigerators, whereas others use electricity less often, such as power saws. To determine your total energy consumption, multiply the wattage of the appliance by the number of hours it is used in a day. Some appliances do not give the wattage, so you may have to calculate the wattage by multiplying the amperes times the volts. After adding the totals for each appliance, you can decide what power output you need for your PV system.
Example Daily Use Total Energy Load (hrs) Wattage Consumption (watt-hrs) Radio 2 x 25 = 50 Lamps 3 x 27 = 81 (fluorescent) VCR 0.5 x 30 = 15 Television 6 x 60 = 360 Total Daily Energy Consumption 506 watt-hrs For the items listed above, you would need a system that produces an average daily energy output of 506 watt-hours. Obviously, different parts of the country receive varying amounts of sunlight. Because sunlight is the source of power for PV, you must determine the daily amount of sunlight in your region. Remember that PV systems are rated by peak watt, which is the amount of power produced when the module receives 1,000 watts per square meter of exposure to the sun (insolation). Determining your daily energy consumption can be done through simple calculations like the example above or with the aid of sophisticated computer programs. If you are seriously considering purchasing a PV system, there are also other factors to consider. You may want to refer to other sources for more precise ways to make your calculations.
System
Components Direct-Current System Equipment Charge Controller. The charge controller regulates the flow of electricity from the PV modules to the battery and the load. The controller keeps the battery fully charged without overcharging it. When the load is drawing power, the controller allows charge to flow from the modules into the battery, the load, or both. When the controller senses that the battery is fully charged, it stops the flow of charge from the modules. Many controllers will also sense when loads have taken too much electricity from batteries and will stop the flow until sufficient charge is restored to the batteries. This last feature can greatly extend the battery’s lifetime. Controllers cost varies depending on the ampere capacity at which your PV system will operate and the monitoring features you want. When selecting a controller, make sure it has the features you need; cost should be a secondary consideration. Battery The battery stores electricity for use at night or for meeting loads during the day when the modules are not generating sufficient power to meet load requirements. To provide electricity over long periods, PV systems require deep-cycle batteries. These batteries, usually lead-acid, are designed to gradually discharge and recharge 80% of their capacity hundreds of times. Automotive batteries are shallow-cycle batteries and should not be used in PV systems because they are designed to discharge only about 20% of their capacity. If drawn much below 20% capacity more than a few dozen times, the battery will be damaged and will no longer be able to take a charge. The cost depends on the type, capacity (ampere-hours), the climatic conditions in which it will operate, how frequently it will receive maintenance, and the types of chemicals it uses to store and release electricity. A PV system may have to be sized to store a sufficient amount of power in the batteries to meet power demand during several days of cloudy weather. This is known as “days of autonomy.” Consult with your PV dealer before selecting batteries for your system. Most types of batteries contain toxic materials that may pose serious health and safety problems. The battery companies, and PV system designers recommend that lead-acid and wet cell batteries, which give off explosive hydrogen gas when recharging, be located in a well-ventilated space isolated from the other electrical components of the system and away from living spaces. Allow enough space for easy access during maintenance, repair, and replacement. Most important, maintain the battery according to the manufacturer’s instructions, and recycle the batteries properly when they wear out. Alternating-Current System Equipment Inverter AC systems also require an inverter, which changes the DC electricity produced by PV modules and stored in batteries into AC electricity. Different types of inverters produce a different “quality” of electricity. For example, lights, televisions, and power tools can operate on lower-quality electricity, but computers, laser printers, and other sophisticated electronic equipment require the highest-quality electricity. So, you must match the power quality required by your loads with the power quality produced by the inverter. The cost is affected by several factors, including the quality of the electricity it needs to produce; whether the incoming DC voltage is 12, 24, 36, or 48 volts; the number of AC watts your loads require when they are operating normally; the amount of extra surge power your AC loads need for short periods; and whether the inverter has any additional features such as meters and indicator lights. Tell your PV dealer if you plan to add additional AC loads in the future. If you are considering building another room onto your house or adding electrical loads, consider purchasing an inverter with a larger input and output rating than you currently need. This may be less costly than replacing it with a larger one later.
What
Else Do I Need to Consider? No PV system is maintenance-free. Schedule regular inspections of your system to ensure that the wiring and contacts are free from corrosion, the modules are clear of debris, and the mounting equipment has tight fasteners. You should also monitor the power output of your PV modules, the state-of-charge and electrolyte level of your batteries, and the actual amount of power that your loads use. Writing this information in a notebook is a good way to track your system’s performance and help you deter-mine whether your system is operating as designed. Monitoring will also help you understand the relationships between your system’s power production, storage capability, and load requirements.
PV
Can Power Your Future PV systems can be cost-effective options for providing electricity to your home or remote site. However, they are not appropriate for all situations. Deciding whether a PV system is right for you depends on many factors. Therefore, conduct careful research and consult with PV equipment dealers and others who have installed these systems. If you then decide that a PV system is right for you, the power of the sun will take on a new meaning in your life.
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