Why should I use photovoltaics?

You should use a PV system if it operates better and costs less than alternatives.

The cost of energy produced by PV systems continues to drop. However, kilowatt-hour for kilowatt-hour, the cost of PV energy is still generally higher than energy bought from your local utility. Also, the initial cost of PV equipment is higher than an engine generator. Yet, there are many applications where a PV system is the most cost-effective long-term option. The number of installed PV systems increases each year because their many advantages make them the best option. Consider the following issues:

Site Access - A well-designed PV system will operate unattended and requires minimum periodic maintenance. The savings in labor costs and travel expense can be significant.

Modularity - A PV system can be designed for easy expansion. If the power demand might increase in future years, the ease and cost of increasing the power supply should be considered.

Fuel Supply - Supplying conventional fuel to the site and storing it can be much more expensive than the fuel itself. Solar energy is delivered free.

Environment - PV systems create no pollution and generate no waste products.

Maintenance - Any energy system requires maintenance but experience shows PV systems require less maintenance than other alternatives.

Durability - Most PV modules available today are based on proven technology that has shown little degradation in over 15 years of operation.

Cost - For many applications, the advantages of PV systems offset their relatively high initial cost. For a growing number of users, PV is the clear choice.

System designers know that every decision made during the design of a PV system affects the cost. If the system is oversized because the design was based on unrealistic requirements, the initial cost is increased unnecessarily. If less durable parts are specified, maintenance and replacement costs are increased. The overall system life-cycle cost (LCC) estimates can easily double if inappropriate choices are made during system design. Don't let unrealistic specifications or poor assumptions create unreasonable cost estimates and keep you from using this attractive power source. As you size your PV system, be realistic and flexible.

Can I afford photovoltaics?

That depends on your application. Generally, the cost of PV energy is higher than energy bought from your local utility. However, if you need power in a location not served by a utility, PV may be the cost-effective option. The number of PV system installations is increasing rapidly. As more people learn about this versatile and often cost-effective power option, this trend will accelerate.

You should consider your goals and distinguish between your wants and your needs. Reevaluate your ideas about having electric power available during all kinds of weather - 100 percent availability. Availability has a unique meaning for a PV system because it depends not only on reliable equipment but on the level and consistency of sunshine, and the capability of the energy storage system. Because the weather is unpredictable, designing a PV system to be available for all times and conditions is expensive, and often unnecessary. PV systems with long-term availabilities greater than 95 percent are routinely achieved at half the cost or less of systems designed to be available 99.99 percent of the time. Designing for lower availabilities decreases the size of the PV array and batteries and will save many dollars.

Another way to resolve the availability issues is to design a Hybrid system which will include another energy source.

Although saving money is important you should be determined to design and install a safe system that will last 25 years or more. Quality may cost more initially but will save money in the long run.

Tell me about Solar System

All life on earth is supported by the sun, which produces an amazing amount of energy. Only a very small percentage of this energy strikes the earth but that is still enough to provide all our needs. A nearly constant 1.36 kilowatts per square meter (the solar constant) of solar radiant power impinges on the earth's outer atmosphere. Approximately 70% of this extraterrestrial radiation makes it through our atmosphere on a clear day. In the southwestern United States, the solar irradiance at ground level regularly exceeds 1,000 w/m2. In some mountain areas, readings over 1,200 w/m2 are often recorded. Average values are lower for most other areas, but maximum instantaneous values as high as 1,500 w/m2 can be received on days when puffy-clouds are present to focus the sunshine. These high levels seldom last more than a few minutes. The atmosphere is a powerful absorber and reduces the solar power reaching the earth at certain wavelengths. The part of the spectrum used by silicon PV modules is from 0.3 to 0.6 micrometers, approximately the same wavelengths to which the human eye is sensitive. These wavelengths encompass the highest energy region of the solar spectrum.

Talking about solar data requires some knowledge of terms because on any given day the solar radiation varies continuously from sunup to sundown and depends on cloud cover, sun position and content and turbidity of the atmosphere. The maximum irradiance is available at solar noon which is defined as the midpoint, in time, between sunrise and sunset. Irradiance is the amount of solar power striking a given area and is a measure of the intensity of the sunshine. PV engineers use units of watts (or kilowatts) per square meter (w/m2) for irradiance. Insolation (now commonly referred as irradation) differs from irradiance because of the inclusion of time. Insolation is the amount of solar energy received on a given area over time measured in kilowatt-hours per square meter (kwh/m2) - this value is equivalent to "peak sun hours". Peak sun hours is defined as the equivalent number of hours per day, with solar irradiance equaling 1,000 w/m2, that gives the same energy received from sunrise to sundown. In other words, six peak sun hours means that the energy received during total daylight hours equals the energy that would have been received had the sun shone for six hours with an irradiance of 1,000 w/m2. Therefore, peak sun hours corresponds directly to average daily insolation given in kwh/m2. Many tables of solar data are often presented as an average daily value of peak sun hours (kwh/m2) for each month.

Insolation varies seasonally because of the changing relation of the earth to the sun. This change, both daily and annually, is the reason some systems use tracking arrays to keep the array pointed at the sun. For any location on earth the sun's elevation will change about 47° from winter solstice to summer solstice. Another way to picture the sun's movement is to understand the sun moves from 23.5° north of the equator on the summer solstice to 23.5° south of the equator on the winter solstice. On the equinoxes, March 21 and September 21, the sun circumnavigates the equator. For any location the sun angle, at solar noon, will change 47° from winter to summer.

The power output of a PV array is maximized by keeping the array pointed at the sun. Single-axis tracking of the array will increase the energy production in some locations by up to 50 percent for some months and by as much as 35 percent over the course of a year. The most benefit comes in the early morning and late afternoon when the tracking array will be pointing more nearly at the sun than a fixed array. Generally, tracking is more beneficial at sites between 30° latitude North and 30° latitude South. For higher latitudes the benefit is less because the sun drops low on the horizon during winter months.

For tracking (structures that follow the sun across the sky by various mechanisms, thereby increasing the energy captured from the sun) or fixed arrays, the annual energy production is maximum when the array is tilted at the latitude angle; i.e., at 40°N latitude, the array should be tilted 40° up from horizontal. If a wintertime load is the most critical, the array tilt angle should be set at the latitude angle plus 15° degrees. To maximize summertime production, fix the array tilt angle at latitude minus 15° degrees.

Using inaccurate solar data will cause design errors, so you should try to find accurate, long-term solar data for your system location. These data are becoming more available, even for tilted and tracking surfaces. Check local sources such as solar system installers, universities, airports, or government agencies to see if they are collecting such data or know where you might obtain these values. If measured values on a tilted surface are not available, you may use the modeled data. Data for fixed and single-axis tracking surfaces at three tilt angles (latitude and latitude ±15°) are provided. Two-axis tracking data are given also, as well as a set of world maps that show seasonal values of total insolation at the three tilt angles. All data are in units of kilowatt-hours per square meter. This is equivalent to peak sun hours--the number of hours per day when the sun's intensity is one kilowatt per square meter. See the database on Solar Resource.

How long do PV systems last?

A well-designed and maintained PV system will operate for more than 20 years. The PV module, with no moving parts, has an expected lifetime exceeding 30 years.

Experience shows most system problems occur because of poor or sloppy installation. Failed connections, insufficient wire size, components not rated for dc application, and so on, are the main culprits. The next most common cause of problems is the failure of electronic parts included in the Balance of Systems (BOS) - the controller, inverter, and protection components.

Batteries will fail quickly if they are used outside their operating specification. In most applications, batteries are fully recharged shortly after use. In many PV systems the batteries are discharged AND recharged slowly, maybe over a period of days or weeks. Some batteries will fail quickly under these conditions. Be sure the batteries specified for your system are appropriate for the application.

Can I design and install a PV system myself?

Maybe. Sandia has many years of experience working with people who are new users of photovoltaics, much of which is now included at this website. The "Stand-Alone Photovoltaic Systems - A Handbook of Recommended Design Practices" has been used by thousands of people to design a PV system. This practical guide is written for beginners or professionals. It includes 16 sample system designs for practical applications.

The goal of a stand-alone system designer is to assure customer satisfaction by providing a well-designed, durable system with a 20+ year life expectancy. This depends on sound design, specification and procurement of quality components, good engineering and installation practices, and a consistent preventive maintenance program.

System sizing is perhaps the easiest part of achieving a durable PV power system. A good estimate of system size can be obtained with the worksheets provided and the latest component performance specifications. The resulting system sizes are consistent with computer-aided sizing methods. Photovoltaic systems sized using these worksheets are operating successfully in many countries.

Regardless of the method used to size a system, a thorough knowledge of the availability, performance, and cost of components is the key to good system design. Price/performance tradeoffs should be made and reevaluated throughout the design process. When you start your design, obtain as much information as you can about the components you might use.

After studying all the issues, you should do an initial sizing of the PV system and get some ideas about specifying system components.

Sizing examples for specific applications are provided in the categories in the table below.

Water Pumping

Residential

Lighting/Security

Communications

Hybrid Systems

How much system maintenance is required?

Preventive maintenance is the least costly of all maintenance! After more than 20 years of experience with PV systems, it is clear that the amount and type of maintenance performed directly affects performance and lifetime of a system. PV systems require much less maintenance than conventional power generators. Anyone considering a PV power system must answer two questions about maintenance: how to and how often? The technical procedures, the how to, are similar for systems large or small. Instructions and suggestions are provided here. The 'how often' is just as important and may have a bigger effect on your system's life cycle cost. Many PV systems are located in remote areas where frequent visits are impractical. Yet, experience shows a clear relation between too little maintenance and a short-lived system with too much downtime. Provide too much maintenance and your cost per kilowatt-hour may be doubled or tripled. These maintenance issues must be an integral part of the system design. Component selection should be based, in part, on the type and frequency of maintenance that will be performed.

How do you decide on a maintenance plan? Estimate maintenance cost? There are three interrelated factors: personnel, access and level of maintenance. PV power systems are simple; most are just battery chargers. Routine maintenance can be performed with common tools and common sense. Checking connections, fluid level in batteries, shading of modules, etc., can be accomplished in a few minutes on-site. Follow these suggestions regularly and you will extend your system's life and lower your costs.

Taking care of a small PV system is mostly common sense. The likely failures are connections, fuses, switches--the kind of things you can fix--or better yet, keep from failing with regular preventive maintenance. Check your system several times a year.

One last reminder: take a first aid kit and a partner. Never test an electrical system alone. Many people think that because PV systems are low voltage (many are 12-24 volts), they can't get hurt. That's not true. Practice safety first, and always.

Where can I buy a PV system or components?

This is a simple question that has no easy answer! You can buy a turn-key system or components for installing yourself. You can buy from local dealers (look in the yellow pages under Solar) or from mail-order catalogs. The best advice is to be an educated consumer. Know what you want to accomplish. Learn to ask the right questions. Then talk to several vendors. However, be courteous about how much time you demand of their salesperson. They aren't in business to design your system for you, then have you buy hardware from someone else at a lower price.

 

What about system availability?

System availability is defined as the percentage of time that a power system is capable of meeting load requirements. The number of hours the system is available, divided by 8,760 hours, will give the annual system availability. A system with availability of 95 percent would be expected to meet the load requirements 8,322 hours during an average year for the useful life of the system. Annual availability of 99 percent would mean the system could operate the load for 8,672 of 8,760 hours.

Failures and maintenance time are the primary contributors to lowering system availability for any energy system. However, for PV systems, availability takes on added uncertainty because of the variability of the system's fuel source. PV system design requires an estimate of the average amount of sunlight available. Using these average values means that in a year with above average solar insolation, the system may not experience any downtime. However, in a year with much cloudy weather the system may be unavailable more than the expected number of the hours per year. A PV system designed to have 95 percent availability will, on average, provide power to the load 95 percent of the time. The most likely time for periods of unavailability is when the solar resource is at a minimum, such as in winter.

A study of weather distribution patterns in the United States shows that for a system with 95 percent availability, the downtime will be distributed over the system lifetime as follows: 1.2 years will have fewer than 24 hours downtime per year, 2.3 years will have 25-240 hours, 11.3 years will have 241-538 hours, 5.6 years will have 539-912 hours, and 2.7 years will have over 913 hours.

The system designer should understand the relationship between cost and availability. Experience shows that PV system customers have a tendency to over-specify the requirements and thereby drive the initial system cost unreasonably high. They should keep in mind that no energy-producing system is available 100 percent of the time. Utilities obtain high system availability by using multiple and redundant power sources. There are few single generators, coal fired, nuclear, or hydropower, that achieve 90 percent availabilities. Many PV systems exceed this figure even when component reliability, maintenance, and solar variability are accounted for.

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