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Solar Systems
Photovoltaics
 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.
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.
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.
.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.
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. |
How to Size Your PV System
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
| Load | Daily Use (hrs) | | Wattage | | Total Energy Consumption (watt-hrs) |
| Radio | 2 | x | 25 | = | 50 |
| Lamps (fluorescent) | 3 | x | 27 | = | 81 |
| 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).
Let's examine two locations: Albuquerque, New Mexico, and Pittsburgh, Pennsylvania. Albuquerque is a fairly sunny area. In Albuquerque, for each peak watt that a PV module is rated, it will produce a yearly average of 6.2 watt-hrs* of electricity daily. In Pittsburgh, a cloudier area, the same module will produce an average of 2.4 watt-hrs* of electricity daily.
If you wanted to use a PV system in Albuquerque for the appliances listed in the table, you would divide 506 watt-hrs by 6.2, divide that by 0.8 to account for inefficiency of the batteries and, finally, multiply by 1.2 to cover anything that may have been overlooked. You find that you would need a PV system rated at 124 peak watts. If you were buying 50-watt modules, you would need three modules, because you round up to the next highest number.
506 ÷ 6.2 = 82
82 ÷ 0.8 = 103
103 x 1.2 = 124
124 ÷ 50 = 3 modules
For Pittsburgh, you would divide 506 watt-hrs by 2.4, divide by 0.8, and multiply by 1.2, which yields 317 peak watts, or seven modules at 50 watts each.
506 ÷ 2.4 = 211
211 ÷ 0.8 = 264
264 x 1.2 = 317
317 ÷ 50 = 7 modules
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 (see Source List) for more precise ways to make your calculations.
*This is based on the winter average. For more precise calculations, consult month-by-month averages and use the lowest monthly average.
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Module temperature also affects output. The conversion efficiency of crystalline-silicon modules falls significantly at elevated module temperatures.
When designing a PV system, be sure your PV installer obtains data specific to your area, rather than relying on general data. The National Oceanic and Atmospheric Administration began collecting solar data nearly 20 years ago. The National Renewable Energy Laboratory's Renewable Resource Data Center (see Source List) can provide solar radiation information, or you can Ask an Energy Expert for help. Some state energy offices also have solar data-collection programs to assist solar designers. Finally, books are available that contain solar data on most major cities in the United States, and a few of these are listed in the Reading List.
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. The sidebar illustrates the steps involved in sizing a PV system. You can also Ask an Energy Expert for more specific information on sizing.
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, a grounding circuit, fuses, safety disconnects, 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.
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 generally cost between $20 and $400, 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.
Deep-cycle batteries cost from about $65 up to $3,000. 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 National Electric Code (NEC), 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.
Inverters for most stand-alone applications (i.e., those systems not connected to the utility grid) cost less than $1 per rated output watt. 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.
The National Electric Code
The National Electric Code (NEC) was established in 1897 to ensure safety in all systems that generate, store, transport, and consume electricity. You or the dealer who installs your PV system should be careful to follow NEC's equipment requirements so that the PV system can be approved by local electric code officials. Be aware that many states require all electrical equipment to be installed by a licensed electrician.
However, many local code officials are not familiar with PV systems. Though you follow the provisions of NEC, you may have difficulty convincing a code official that you have installed a code-approved system. Contact and educate (if necessary) local code officials before you purchase and install the system. Throughout the installation process, invite them to observe what you or your dealer have done before you build any enclosure around wiring, connections, or other components. This will help ensure that your system receives approval and will also help future PV installers to get code approval.
Local insurance providers and lenders may also need to be educated about the safety, reliability, and cost-effectiveness of PV systems. Obtaining insurance will be easier in states where PV systems are more common.
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 determine 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.
Source List
The following are just a few of the many organizations that can help you with locating PV equipment dealers in your area and designing and installing PV systems.
Ask an Energy Expert
Energy Efficiency and Renewable Energy Clearinghouse (EREC)
P.O. Box 3048
Merrifield, VA 22116
(800) 363-3732
Fax (703) 893-0400
E-mail: doe.erec@nciinc.com
Consumer Energy Information Web site
Energy experts at EREC provide free general and technical information to the public on the many topics and technologies pertaining to energy efficiency and renewable energy, including PV systems, solar energy, and solar radiation data.
Equipment, Dealers, and Installers
Renewable Energy & Efficiency Training Institute (RETI)
1800 M Street, NW
Suite 300
Washington, DC 20036
(202) 496-1417
Fax: (202) 496-1494
RETI offers customized PV design, installation, and maintenance programs to meet the needs of a wide range of customer groups.
Solar Energy Industries Association (SEIA)
1616 H Street, N.W. 8th floor
Washington, D.C. 20006
(202) 628-7745
E-mail: info@seia.org
SEIA is the national trade organization of PV and solar thermal manufacturers and component suppliers.
Training Programs
Florida Solar Energy Center (FSEC)
Photovoltaic System Design Assistance and Training Center
1679 Clearlake Road
Cocoa, FL 32922
(321) 638-1000
Fax: (321) 638-1010
E-mail: info@fsec.ucf.edu
FSEC offers workshops on a variety of topics related to PV system design and use.
Siemens Solar
Photovoltaic Technology and System Design Training Course
4650 Adohr Lane
Camarillo, CA 93012
(800) 272-6765 or (805) 388-6561
Fax: (805) 388-6395
Siemens offers training programs on PV technology and system design.
Solar Energy International (SEI)
P.O. Box 715
Carbondale, CO 81623
(970) 963-8855
Fax: (970) 963-8866
E-mail: sei@solarenergy.org
SEI offers training programs on PV system design and installation, as well as on wind energy, mini-hydro systems, and solar home design. SEI also sells books on a variety of renewable energy topics.
Online Renewable Energy Information
DOE Office of Energy Efficiency
The EERE Web site is DOE's premier resource for information about renewable energy and energy efficiency technologies, including solar radiation and photovoltaic data.
National Renewable Energy Laboratory (NREL)
NREL Renewable Resource Data Center
NREL, one of the Department of Energy's national laboratories, leads the nation toward a sustainable energy future by developing renewable energy technologies. Its Web site includes information on many renewable energy topics. NREL's Renewable Resource Data Center (RREDC) site provides publications, data, and maps on biomass, geothermal, solar radiation, and wind resources in the United States.
Reading List
- Periodicals, Books, Pamphlets, and Reports
Consumer Guide to Solar Energy, S. Sklar, Bonus Books, Chicago, 1991.
- Home Power Magazine: The Hands-On Journal of Home-Made Power, Home Power, Inc., P.O. Box 520, Ashland, OR 97520; (916) 475-3179; www.homepower.com.
- Photovoltaic Fundamentals, National Renewable Energy Laboratory, Document No. DE-91015001, available from National Technical Information Service, U.S. Department of Commerce, 5285 Port Royal Road, Springfield, VA 22161, 1991.
- The Solar Electric House, S. Strong, Sustainability Press, Still River, MA, 1993.
- Solar Electricity: A Practical Guide to Designing and Installing Small Photovoltaic Systems, S. Roberts, Prentice Hall, NJ, 1991.
- Stand-Alone Photovoltaic Systems: Handbook of Recommended Design Practices, Sandia National Laboratory, Document No. SAND87-7023, available from National Technical Information Service, U.S. Department of Commerce, 5285 Port Royal Road, Springfield, VA 22161, 1991 (revised).
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Solar Heating Collectors
 Solar collectors are the heart of most solar energy systems. The collector absorbs the sun's light energy and changes it into heat energy. Solar collectors heat a fluid, either air or liquid. This fluid then is used to heat—directly or indirectly—the following.
1. Water for household use.
2. Indoor spaces
3. Water for swimming pools
4. Water or air for commercial use
5. Air to regenerate desiccant (drying) material in a desiccant cooling system.
There are several types of solar collectors used for residences. These are flat-plate, evacuated-tube, and concentrating collectors.
Flat-Plate Collectors
Flat-plate collectors are the most common collector for residential water-heating and space-heating installations. A typical flat-plate collector is an insulated metal box with a glass or plastic cover—called the glazing—and a dark-colored absorber plate. The glazing can be transparent or translucent. Translucent (transmitting light only), low-iron glass is a common glazing material for flat-plate collectors because low-iron glass transmits a high percentage of the total available solar energy. The glazing allows the light to strike the absorber plate but reduces the amount of heat that can escape. The sides and bottom of the collector are usually insulated, further minimizing heat loss.
The absorber plate is usually black because dark colors absorb more solar energy than light colors. Sunlight passes through the glazing and strikes the absorber plate, which heats up, changing solar radiation into heat energy. The heat is transferred to the air or liquid passing through the collector. Absorber plates are commonly covered with "selective coatings," which retain the absorbed sunlight better and are more durable than ordinary black paint.
Absorber plates are often made of metal- usually copper or aluminum—because they are both good heat conductors. Copper is more expensive, but is a better conductor and is less prone to corrosion than aluminum.
Flat-plate collectors fall into two basic categories: liquid and air. And both types can be either glazed or unglazed.
Liquid Collectors
In a liquid collector, solar energy heats a liquid as it flows through tubes in or adjacent to the absorber plate. For this type of collector, the flow tubes are attached to the absorber plate so the heat absorbed by the absorber plate is readily conducted to the liquid.
The flow tubes can be routed in parallel, using inlet and outlet headers, or in a serpentine pattern. A serpentine pattern eliminates the possibility of header leaks and ensures uniform flow. A serpentine pattern is not appropriate, however, for systems that must drain for freeze protection because the curved flow passages will not drain completely.
The simplest liquid systems use potable household water, which is heated as it passes directly through the collector and then flows to the house to be used for bathing, laundry, etc. This design is known as an "open-loop" (or "direct") system. In areas where freezing temperatures are common, however, liquid collectors must either drain the water when the temperature drops or use an antifreeze type of heat-transfer fluid.
In systems with heat-transfer fluids, the transfer fluid absorbs heat from the collector and then passes through a heat exchanger. The heat exchanger, which generally is in the water storage tank inside the house, transfers heat to the water. Such designs are called "closed-loop" (or "indirect") systems.
Glazed liquid collectors are used for heating household water and sometimes for space heating. Unglazed liquid collectors are commonly used to heat water for swimming pools. Because these collectors need not withstand high temperatures, they can use less expensive materials such as plastic or rubber. They also do not require freeze-proofing because swimming pools are generally used only in warm weather.
Air Collectors
Air collectors are simple, flat-plate collectors used primarily for space heating. The absorber plates in air collectors can be metal sheets, layers of screen, or non-metallic materials. The air flows past the absorber by natural convection or when forced by a fan. Because air conducts heat much less readily than liquid does, less heat is transferred between the air and the absorber than in a liquid collector.
In some solar air-heating systems, fins or corrugations on the absorber are used to increase air turbulence and improve heat transfer. The disadvantage of this strategy is that it can also increase the amount of power needed for fans and, thus, increase the costs of operating the system. In colder climates, the air is routed between the absorber plate and the back insulation to reduce heat loss through the glazing. However, if the air will not be heated more than 30°F (17°C) above the outdoor temperature, the air can flow on both sides of the absorber plate without sacrificing efficiency.
Air systems have the advantage of eliminating the freezing and boiling problems associated with liquid systems. Although leaks are harder to detect and plug in an air system, they are also less troublesome than leaks in a liquid system. Air systems can often use less-expensive materials, such as plastic glazing, because their operating temperatures are usually lower than those of liquid collectors.
Evacuated-Tube Collectors
Evacuated-tube collectors heat water in residential applications that require higher temperatures. In an evacuated-tube collector, sunlight enters through the outer glass tube, strikes the absorber tube, and changes to heat. The heat is transferred to the liquid flowing through the absorber tube. The collector consists of rows of parallel transparent glass tubes, each of which contains an absorber tube (in place of the absorber plate in a flat-plate collector) covered with a selective coating. Evacuated-tube collectors are modular—tubes can be added or removed as hot-water needs change.
When evacuated tubes are manufactured, air is evacuated from the space between the two tubes, forming a vacuum. Conductive and convective heat losses are eliminated because there is no air to conduct heat or to circulate and cause convective losses. There can still be some radiant heat loss (heat energy will move through space from a warmer to a cooler surface, even across a vacuum). However, this loss is small and of little consequence compared with the amount of heat transferred to the liquid in the absorber tube.
Evacuated-tube collectors are available in a number of designs. Some use a third glass tube inside the absorber tube or other configurations of heat-transfer fins and fluid tubes. One commercially available evacuated-tube collector stores 5 gallons (19 liters) of water in each tube, eliminating the need for a separate solar storage tank. Reflectors placed behind the evacuated tubes can help to focus additional sunlight on the collector.
These collectors are more efficient than flat-plate collectors for a couple of reasons. First, they perform well in both direct and diffuse solar radiation. This characteristic, combined with the fact that the vacuum minimizes heat losses to the outdoors, makes these collectors particularly useful in areas with cold, cloudy winters. Second, because of the circular shape of the evacuated tube, sunlight is perpendicular to the absorber for most of the day. For comparison, in a flat-plate collector that is in a fixed position, the sun is only perpendicular to the collector at noon. While evacuated-tube collectors achieve both higher temperatures and higher efficiencies than flat-plate collectors, they are also more expensive.
Concentrating Collectors
Concentrating collectors use mirrored surfaces to concentrate the sun's energy on an absorber called a receiver. Concentrating collectors also achieve high temperatures, but unlike evacuated-tube collectors, they can do so only when direct sunlight is available. The mirrored surface focuses sunlight collected over a large area onto a smaller absorber area to achieve high temperatures. Some designs concentrate solar energy onto a focal point, while others concentrate the sun's rays along a thin line called the focal line. The receiver is located at the focal point or along the focal line. A heat-transfer fluid flows through the receiver and absorbs heat.
These collectors reach much higher temperatures than flat-plate collectors. However, concentrators can only focus direct solar radiation, with the result being that their performance is poor on hazy or cloudy days. Concentrators are most practical in areas of high insolation (exposure to the sun's rays), such as those close to the equator and in the desert southwest United States.
Concentrators perform best when pointed directly at the sun. To do this, these systems use tracking mechanisms to move the collectors during the day to keep them focused on the sun. Single-axis trackers move east to west; dual-axis trackers move east and west and north and south (to follow the sun throughout the year). In addition to these mechanical trackers, there are passive trackers that use freon to supply the movement. While not widely used, they do provide a low-maintenance alternative to mechanical systems.
Concentrators are used mostly in commercial applications because they are expensive and because the trackers need frequent maintenance. Some residential solar energy systems use parabolic-trough concentrating systems. These installations can provide hot water, space heating, and water purification. Most residential systems use single-axis trackers, which are less expensive and simpler than dual-axis trackers.
Technological Improvements
The efficiency of solar heating systems and collectors has improved from the early 1970s and costs have dropped somewhat. The efficiencies can be attributed to the use of low-iron, tempered glass for glazing (low-iron glass allows the transmission of more solar energy than conventional glass), improved insulation, and the development of durable selective coatings.
Also, a new solar air collector, formerly used primarily for commercial buildings, is now available for homes. Called a transpired collector, it eliminates the cost of the glazing, the metal box, and the insulation. This collector is made of black, perforated metal. The sun heats the metal, and a fan pulls air through the holes in the metal, which heats the air. For residential installations, these collectors are available in 8-foot by 2.5-foot (2.4-meter by 0.8-meter) panels capable of heating 40 cubic feet per minute (0.002 cubic meters per second) of outside air. On a sunny winter day, the panel can produce temperatures up to 50°F (28°C) higher than the outdoor air temperature. Transpired air collectors not only heat air, but also improve indoor air quality by directly preheating fresh outdoor air.
These collectors have achieved very high efficiencies—more than 70% in some commercial applications. Plus, because the collectors require no glazing or insulation, they are inexpensive to manufacture. All these factors make transpired air collectors a very cost-effective source of solar heat.
There are other prototype cooling systems operating today. Some use heat from solar collectors for absorption cooling. Others are being used to renew the desiccant material in desiccant cooling systems. Desiccants, such as silica gel, naturally attract moisture. They are used to reduce humidity and the resulting cooling loads in hot, humid climates.
Collector Performance Ratings
When you are shopping for solar collectors, you can compare their performance. Look for a Solar Rating & Certification Corporation (SRCC) or Florida Solar Energy Center (FSEC) sticker on the equipment you are considering to check their comparative performance ratings.
A Bright Future
Solar collectors can be used for nearly any process that requires heat. As environmental laws become stricter and the price of conventional power increases, it is likely that solar collectors will be integrated into many applications.
Low-Tech Solar Collectors
Several inexpensive, "low-tech" solar collectors with specific functions are also available commercially. Batch heaters are simple, effective solar water heaters; solar box cookers are used for cooking and for purifying water; and solar stills produce inexpensive distilled water from virtually any water source.
Batch heaters, also known as "breadbox" or integrated collector systems, use one or more black tanks filled with water and placed in an insulated, glazed box. Some boxes include reflectors to increase the solar radiation. Solar energy passes through the glazing and heats the water in the tanks. These devices are inexpensive solar water heaters but must be drained or protected from freezing when temperatures drop below freezing.
A batch heater is a simple solar water heater that uses one or more black tanks filled with water and placed in an insulated, glazed box. Solar box cookers are inexpensive to buy and easy to build and use. They consist of a roomy, insulated box lined with reflective material, covered with glazing, and fitted with an external reflector. Black cooking pots serve as absorbers, heating up more quickly than shiny aluminum or stainless steel cookware. Box cookers can also be used to kill bacteria in water if the temperature can reach the boiling point.
Solar stills provide inexpensive distilled water from even salty or badly contaminated water. They work on the principle that water in an open container will evaporate. A solar still uses solar energy to speed up the evaporation process. The stills consist of an insulated, dark-colored container covered with glazing that is tilted so the condensing fresh water can trickle into a collection trough. A small solar still, which is about the size of your kitchen stove, can produce two gallons of distilled water on a sunny day.
Source List
The following organizations can provide you with information to help you find the solar water heater that is right for you.
American Solar Energy Society (ASES)
2400 Central Avenue, Unit G-1
Boulder, CO 80301
(303) 443-3130
Fax: (303) 443-3212
E-mail: ases@ases.org
ASES is a nonprofit educational organization founded in 1954 to encourage the use of solar energy technologies. ASES publishes a bimonthly magazine, Solar Today, and offers a variety of solar publications through its catalogue.
Florida Solar Energy Center (FSEC)
1679 Clearlake Road
Cocoa, FL 32922-5703
(407) 638-1000
Fax: (407) 638-1010
E-mail: info@fsec.ucf.edu
FSEC is an alternative energy center. The FSEC staff conducts research on a range of solar technologies, offers solar energy workshops, and distributes many free publications to the public.
Solar Energy Industries Association (SEIA)
1616 H Street, N.W. 8th floor
Washington, D.C. 20006-4999
Phone: 202-628-7745
E-mail: Solarsklar@aol.com
SEIA provides lists of solar-equipment manufacturers and dealers.
Solar Rating & Certification Corporation (SRCC)
c/o FSEC
1679 Clearlake Road
Cocoa, FL 32922-5703
(407) 638-1537
Fax: (407) 638-1010
E-mail: srcc@fsec.ucf.edu
SRCC publishes the thermal-performance ratings of solar energy equipment. The SRCC offers a directory of certified solar systems and collectors as well as a document (OG-300-91) that details the operating guidelines and minimum standards for certifying solar hot-water systems.
For information about many kinds of energy efficiency and renewable energy topics, contact:
Ask an Energy Expert
The Energy Efficiency and Renewable Energy Clearinghouse (EREC)
P.O. Box 3048
Merrifield, VA 22116
(800) DOE-EREC (363-3732)
Fax: (703) 893-0400
E-mail: doe.erec@nciinc.com
Consumer Energy Information Web site
Energy experts at EREC provide free general and technical information to the public on the many topics and technologies pertaining to energy efficiency and renewable energy.
You may also contact your state and local energy offices for region-specific information on solar water heaters.
Reading List
The following publications provide further information about solar collectors. The list is not exhaustive, nor does the mention of any publication constitute a recommendation or endorsement.
Books, Pamphlets, and Reports
- Consumer Guide to Solar Energy, S. Sklar and K. Sheinkopf, Bonus Books, Inc., 160 East Illinois Street, Chicago, IL 60611, 1991.
- The Fuel Savers, B. Anderson, Morning Sun Press, Lafayette, CA, 1991.
- The New Solar Home Book, B. Anderson and M. Riorden, Brick House, Amherst, NH, 1987.
Periodicals
- Home Energy Magazine, 2124 Kittredge Street, No. 95, Berkeley, CA 94704-9942, (510) 524-5405. Home Energy Magazine is a source of information on reducing energy consumption.
- Solar Today, 2400 Central Avenue, Unit G-1, Boulder, CO 80301. Solar Today covers all the solar technologies, both mature and emerging, in a general-interest format. Each issue includes a solar building case study.
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