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Tell me about the science of Photovoltaics.

           Photovoltaic science is the science of turning energy produced from the sun into electricity. Edmond Becquerel discovered the concept known as the photovoltaic effect in 1839. However, the first positive/negative (p/n) junction solar cell was not created until 1954 at Bell Labs.
          Photovoltaics are solid-state semiconductor devices that convert light directly into electricity. They are usually made of silicon with traces of other elements and are first cousins to transistors, LEDs and other electronic devices.
           Although making PV cells and modules requires advanced technology, they're very simple to use. PV modules are generally low-voltage DC devices (although arrays of PV modules can be wired for higher voltages) with no moving or wearing parts. Once installed, a PV array generally requires no maintenance other than an occasional cleaning, and even that is not imperative. Most PV systems do contain storage batteries which require some water and maintenance similar to that required by the battery in an automobile.
             A photovoltaic device (generally called a solar cell) consists of layers of semiconductor materials with different electronic properties. In a typical BP Solar crystalline silicon cell, the bulk of the material is silicon, doped with a small quantity of boron to give it a positive or p-type character. A thin layer on the front of the cell is doped with phosphorous to give it a negative or n-type character. The interface between these two layers contains an electric field and is called a junction. Light consists of particles called photons. When light hits the solar cell, some of the photons are absorbed in the region of the junction, freeing electrons in the silicon crystal. If the photons have enough energy, the electrons will be able to overcome the electric field at the junction and are free to move through the silicon and into an external circuit. As they flow through the external circuit they give up their energy as useful work (turning motors, lighting lamps, etc.) and return to the solar cell. The photovoltaic process is completely solid-state and self-contained. There are no moving parts and no materials are consumed or emitted.

 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.

            Everything is relative. Using a PV system can be more expensive than buying power from the local utility, through the electrical outlet in your wall. However, it is dramatically less expensive than running a power line to a site currently without service (off-grid homes, more than 0.25 mile [or 0.4 kilometer] away from power, or a mountain-top communications system). PV modules can cost less than US$5/watt, in quantity, but that is only one part of a system cost. A system could include design costs, land, support structure, batteries, an inverter, wiring, and lights/appliances. The total system cost could be as low as US$7/watt or as much as US$20/watt or more, depending on the complexity. Every application is unique, and generalizations on cost are difficult to make.

             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.

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.

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.

  • Set up a schedule for when you will do maintenance. 4 times a year is recommended.

  • Fix a dedicated test kit with needed tools, test equipment, and spare parts.

  • Keep documentation about the system in one place where you can find it when needed.

  • Buy a notebook to serve as a maintenance log, or use the System Maintenance Record given in this booklet. Record all visits and the action taken. This will help you identify recurring problems and show where improvements are needed.

           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.

How much PV do I need for my house?

          How much PV you need depends on your power loads and their duty cycles. If you wanted to completely replace your current electrical purchases from the utility with a PV system, you could look at your kWh usage on your electric bills for a year, calculate a daily average, and divide that by the number of average daily sun hours for your location. (3600 kWh/yr divided by 365 days/yr equals approximately 10 kWh/day, divided by 5 sun-hours per day (for locations in middle America), equals 2 kW. This would indicate that a 2-kW system would, over the course of an average year, produce enough energy to replace the power you are currently using.

         However, if you design an energy efficient home, you could cut the annual electricity usage dramatically, reducing the size of the system. In the real world, the majority of home systems range from 1 kW to 2 kW. Where you live, if you are on the grid or off, and how you live, will dictate the size of your system, and its ultimate cost and value.

What kinds of PV are available?

          The majority of power modules in use since 1955 are made of single- or multi crystalline silicon, though several manufacturers are producing large quantities of amorphous silicon power modules. Most solar-powered consumer products use thin-film amorphous silicon PV.

          Satellites and other space applications have used single-crystal silicon, single-crystal gallium arsenide, and test systems of thin-film materials.

          Several companies are manufacturing thin-film modules of cadmium telluride (CdTe) and copper indium diselenide (CuInSe2, or CIS), but these are mostly pilot production at this time and are not available in commercial quantities.

Does it takes more energy to manufacture PV than it will produce over its useful life?

            According to the article by J. Nijs, R. Mertens, R. Van Overstraeten, and J. Szlufcik (IMEC, Leuven, Belgium); D. Hukin (Oxford, UK); and L. Frisson, consulting engineer, in their paper "Energy Payback Time of Crystalline Silicon Solar Modules ("Advances in Solar Energy," ed. K. Boer, ASES, Boulder, CO USA; vol. 11, 1997. pp. 291-328), conservative calculations for the pay-back time of crystalline silicon PV modules varies from 2.58 years, for multicrystalline silicon and 2.66 years, for single-crystal silicon in the sunbelt, to 4.92 years and 5.07 years, respectively, for these same materials in less sunny areas. Projections for additional manufacturing improvements indicate improvements to 1.4 years (sunbelt) and 2.67 years (less-sunny areas) in the mid-term future, and 1.22 years and 2.33 years, respectively, for longer-term improvements.

            For other materials, estimates for amorphous silicon are just more than a year for making up their energy cost; we have not heard any numbers on the polycrystalline thin films (CdTe, CuInSe2), but their energy payback would also be quite short.

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