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Photovoltaic Systems for Residences

Where PV Is a Good Choice

Residential PV systems are most cost effective in areas where there is not an existing utility power line, and where there is a good and accessible solar resource. For optimum system performance, photovoltaic modules should at least have full exposure to the sun from 10 am to 3 pm all year round. Prices are gradually falling for PV system components. This increases the types of applications and sites where PV systems make economic sense.

System Basics and Components

PV cells convert sunlight directly into electricity. PV cells are wired together to form modules, or panels, with a specific power output. There are various types of cell technologies. The most widely used PV cells are single-crystal silicon, multi-crystal silicon, and amorphous silicon. These cell technologies have operating lives of over twenty years.

Modules are specified on a peak-Watt (Wp) basis at standard test conditions (STC = 1,000 Watts/square meter solar radiation at 1.5 atmospheres and 25 degrees C ambient temperature). A module's Wp rating is approximately the amount of power it will generate at noon on a clear day. Modules produce the rated output at a specified voltage and current or amperage. Modules are available in capacities ranging from a few Wp to as high as 100 Wp. The sizes of modules vary, but they are about one square foot (0.09 square meters) in area for every 10 Wp capacity. Modules can be wired together in arrays to meet any power requirement. You can connect a module or array directly to an electrical appliance that operates on direct current (DC) electricity, such as a water pump (if you match the voltage and power requirements of the motor).

Most PV systems need a battery to store electricity to provide power when the sun is not shining, or when electricity demand exceeds the power output of the array. PV systems require a deep-cycle (or deep discharge) battery that can be repeatedly drained of much of its energy and recharged. Examples are marine and golf cart batteries. There are batteries being sold specifically for the remote home power market. (Automotive starting batteries are shallow-cycle batteries and are not designed for deep discharge.) A variety of battery technologies are available, but lead-acid is the most common. Batteries are sized based on their Amp-hour capacity (number of amperes supplied over time) and their charge/discharge rate. When you are planning a PV system, you should carefully research battery characteristics, availability, maintenance requirements, replaceability, and recycleability.

A charge controller is necessary to optimize battery life. The most basic controllers keep the battery from being overcharged. More sophisticated controllers provide a variety of other functions (including a low-voltage disconnect) to keep the battery from being fully discharged. This feature may significantly prolong lead-acid battery life. Other system components include junction and distribution boxes, wiring, and fuses, circuit breakers, safety disconnects, and a grounding circuit to protect the system components as well as anyone using the system. These components are common to any electrical system. Various types of meters are available to monitor system performance. Tracking devices are available that keep modules or arrays facing the sun to increase output, or to reduce the number of modules required in the system.

DC verses AC Systems

The voltage requirements of your system loads (appliances, motors, etc.) are an important consideration in PV system design. PV modules and batteries produce direct current (DC) electricity, but most appliances in the United States operate on 120-volt (V) alternating current (AC). Suppliers of recreational vehicles, marine equipment, and PV systems sell DC lights and appliances, but they tend to be more expensive than AC appliances.

The wiring in DC systems is also a consideration. DC systems are typically low-voltage (12, 24, or 48 V) and high amperage relative to conventional AC (120 V) power supply. Therefore, DC systems require much thicker wiring than standard AC wiring to operate safely and efficiently. Wiring costs are directly proportional to thickness and length: the farther away the module/array is from the battery, the more expensive the wiring will be. Also, the high amperage capacity fuses and circuit breakers necessary in DC systems are expensive, and may be more difficult to obtain than standard 120 V AC types. Thick wiring is also difficult to work with. However, for systems with a small energy demand and a short array-to-battery power transmission distance, DC systems are the least expensive.

Powering AC appliances with a PV system requires an inverter. An inverter converts DC power from the battery and/or array to AC. Some appliances that require high quality power, such as laser printers, may not run well or at all on the power output of some inverters. You should consult with inverter dealers or manufacturers to determine what types of appliances their inverter will or will not operate. Inverters for residential systems, though becoming more sophisticated, reliable, and less expensive, can still be costly. They may also be noisy, and they consume 5% to 10% of the PV system power to operate. The homeowner has to consider the various tradeoffs in energy needs, lifestyle, costs, and spare parts' availability before choosing whether to use DC or AC power.

System Sizing

The number of modules and the size of the battery bank, wires, controller, fuses, inverter, etc., mainly depends on the amount of power you plan to consume, and the amount of solar radiation available at your location on a daily and seasonal basis. System sizing is usually based on the maximum energy demand during the month of lowest solar radiation intensity. There are many books, manuals, and computer software packages available that detail system sizing procedures, and that provide general solar radiation data for many regions of the country.

High energy and power demands and/or a low solar resource increases system size and costs. Sizing a PV system to operate high wattage loads such as electric space and water heaters, stoves, and toaster ovens will make a PV system extremely expensive. It is important to use energy efficient appliances, such as compact fluorescent lights, to reduce the electrical loads and system costs. PV system owners must be aware of how they use electricity, and conserve it in order to keep within their system capacity and budget. Although sunlight is free, the equipment necessary to convert and use it is not.

In most parts of the United States, a basic DC system, consisting of one 58 Wp module, two 6 V, 220 amp-hour batteries (wired in series for 12 V), a charge controller, fuses, and safety disconnect is sufficient to operate a DC radio or stereo, a small black-and-white DC television, and a DC fluorescent light, for several hours a day throughout the year.

Supplemental Power and Hybrid Systems

It is usually too expensive for PV to supply all household energy and power requirements. Most PV system owners use another energy source for cooking, space and water heating, and to run large loads such as air conditioners, power tools, and washing machines. Propane, natural gas, gasoline, or diesel fuel generators (gensets) are often used to run major electric appliances, and to recharge or equalize the battery when necessary. Systems incorporating gensets are often called "hybrid systems," since they are not solely PV powered.

Utility-Connected Systems

Some residential PV systems are connected (intertied) to an electric utility's power transmission and distribution system (commonly referred to as the "grid"). The system owners sell their excess electricity to the utility company, and buy electricity when their PV system cannot meet their electricity demand. In effect, they use the utility grid for storage. Inverters are available that produce electricity of the quality acceptable to electric utilities. They incorporate safety features to automatically disconnect the PV system from the grid in case of a power outage on the grid. Some intertied systems have a small-capacity battery for emergency power supply if the grid goes down.

The Public Utility Regulatory Policy Act of 1978 (PURPA) requires utilities to purchase excess power from small power producers that use renewable energy at a rate equal to what it costs the utility to produce the power itself. Most utilities' power purchase rates are much lower than the costs of PV-generated electricity. At current module prices, it may be difficult to justify a utility-integrated system on the basis of electricity sales to the utility alone. Several utilities are demonstrating grid-integrated residential PV systems. Such systems are attractive in areas where there is a high summer air conditioning demand, because the PV array is generally producing maximum power when air conditioning loads are highest. These systems can help to reduce a utility's peak power demand, which is generally the most expensive to meet.

System Costs and Considerations

It is difficult to predict the cost of a PV system because of the site and owner-specific variables involved in system sizing and installation. PV module purchase prices are currently about $4-7/ Wp. Depending on the size and sophistication of the system, other component prices and installation costs may add $3-$5/Wp for a total system cost of $7-$12/Wp. Advances in cell and module technologies continue to reduce costs, increase output, and extend operating life. Component prices are decreasing due to increasing competition and larger, economy-of-scale production. You should consult as many PV equipment installers or dealers as possible to get the most favorable system and component prices.

One of the major impediments to widespread use of PV systems is the high initial cost. Large intermittent costs, for example, battery replacement, are also an impediment. In contrast, electric utilities bill their customers every month. The utility recovers the cost of its power plants and distribution network through its rate structure, and is able to spread this cost out among its many customers over a long period of time. One way to reduce the impact of the initial purchase is to gradually increase the size and capacity (adding modules and batteries) of the system. (You must carefully consider wire size, controller and inverter specifications beforehand to avoid having to replace expensive wiring and components to meet increases in system capacity.) Another possibility is to take out a loan, which, although increasing total system cost, may make the system more "affordable" on a month-by-month basis. Some PV system suppliers are now offering financing for their systems.

The operating life of the PV modules and other system components is another important factor when considering a PV system. Many module manufacturers now provide performance warranties of ten years or more, and you should expect modules to last at least 20 years. Also consider the estimated operating lives of other components, especially batteries and inverters (if used). You will probably have to replace batteries before modules. Although PV modules are usually the largest part of the initial system purchase cost, the battery may be the largest part of a system's life-cycle cost.

Even though PV systems may be expensive to purchase, they may have lower overall life-cycle costs than other power supply alternatives. For example, extending electric utility service to a "remote" location may also be expensive; even before paying monthly utility bills.

Safety Considerations and the National Electrical Code

Low-voltage, high-amperage DC-based PV systems require thicker wire than high-voltage, low-amperage AC systems. Therefore, proper DC wire sizing is necessary to avoid causing a fire and/or system failure. Some types of batteries, when recharging, can give off explosive hydrogen gas. They must be placed in a well-ventilated area. You should have a fire extinguisher and container of baking soda, to neutralize the battery electrolyte, in easy reach near the battery cabinet. Fuses and circuit breakers, which can spark, should not be placed in the same area as the battery bank. Most controllers and inverters also should not be located near the battery bank. Safety disconnects, fuses, and circuit breakers are required to isolate the array, battery, and loads from each other. The system and all components, especially the array, must be properly grounded.

Article 690 of the National Electrical Code addresses PV systems. However, the system design also will have to conform with all local electrical codes. In many areas, local code officials may be unfamiliar with PV system components and DC wiring. The system owner or installer may have to inform or educate the inspecting official about PV systems. Besides the code itself, the book Photovoltaic Power Systems and the National Electrical Code-Suggested Practices by John Wiles, Southwest Technology Development Institute (STDI), is a good reference on this subject. The book is available for free from STDI, P.O. Box 30001, Dept. 3SOL, Las Cruces, NM 88003-0001.

Exercise care with any electrical system, whether low or high voltage, DC or AC. If you are thinking about investing in a PV system, seek professional assistance and learn all about the safety aspects.

U.S. Department of Energy

Energy Efficiency and Renewable Energy Clearinghouse (EREC)
P.O. Box 3048 Merrifield, VA 22116
Voice: 1-800-DOE-EREC
E-mail: doe.erec@nciinc.com

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