Photovoltaics: Basic Design Principles and Components

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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; and 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. Because this publication is not intended to cover everything about designing and installing a PV system, a list of additional PV resources is provided at the end.

Introduction to PV Technology

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 outputs 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 disconnects and fuses, a grounding 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, billboards, 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, swamp 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.

How Do I Select a PV Dealer?

Choosing a PV professional will be one of your most important decisions. If you choose a competent dealer, you won't need to know all the details of designing, purchasing, and installing your PV system. Instead, you can rely on the dealer's expertise to design and install a system that meets your needs. However, just like buying a car or a television, you must have confidence in the dealer's products and services and be an informed consumer. With the growth of the PV industry, the number of regional dealers, mail-order businesses, and local distributors has expanded rapidly. Many telephone directories contain listings for PV dealers under the 'Solar' heading.

Professional credentials are one indication of a PV dealer's knowledge and qualifications. Ask dealers what PV-related courses they have taken, certifications they have earned, and licenses they have received.

A second consideration is the dealer's experience in the field. How long has the company been in business? The local Better Business Bureau can advise you whether any customers have registered complaints about the dealer. You should also ask the dealer how many systems like yours he or she has designed and installed. Ask to see installations, and talk with owners of systems similar to the one you want to purchase.

A third consideration in selecting a system installer is the variety and quality of products offered for each component of the system. Because PV systems are often designed for a specific site, one company's products may not be appropriate for all applications. Competent dealers will stock components manufactured by several companies. A variety of product options will help ensure that the most appropriate components are available for your system. When a dealer recommends a product, ask what the recommendation is based on, whether there are consumer or independent testing facility reports you can read, and whether the products are listed with Underwriters Laboratories (UL).

Fourth, consider the service agreements and performance guarantees the dealer provides and the warranties given by the product manufacturers. No system is maintenance-free, nor will all components function flawlessly forever. When problems emerge with your system, what services will the dealer provide? What warranties do the manufacturers provide? What costs should you expect to pay, and which costs will be assumed by the dealer and/or the manufacturer?

Finally, you should compare prices from different dealers. Because distribution channels and dealer networks have expanded dramatically, the opportunity to 'shop around' is much greater today. If possible, approach more than one dealer about a draft design and cost estimate for your system.

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. Most utilities can provide electricity to consumers for about 6 cents to 14 cents per kilowatt-hour. 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. The electricity generated by PV systems at current module and balance-of-system prices can cost 20 cents to 40 cents per kilowatt-hour, depending on the installation cost and intensity and duration of the sunlight at the site.

Second, 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. Contact the Energy Efficiency and Renewable Energy Clearinghouse (EREC) for more information on conducting a detailed site feasibility assessment.

There are three factors to consider when determining whether your site is appropriate.

First, systems installed in the United States 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.

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