solar cells

Solar Cells - building blocks

Solar cells, also called photovoltaic cells, are the building blocks of solar panels.  Solar cells are made of semiconductor materials like silicon that produce a "photoelectric effect" when exposed to sunlight.  The photoelectric effect occurs when sunlight knocks electrons loose from their atoms.  The electrons then travel along a circuit built into the solar cell, creating a current of electricity.  The electricity can then be used immediately or stored in batteries.

Solar cells are produced from silicon or are printed inexpensively on thin film

Solar cells - built for modularity

The basic solar cell produces only a small amount of power. To produce more power, solar cells (about 40) can be interconnected to form panels or modules. Solar cell modules range in output from 10 to 300 watts. If more power is needed, several modules can be installed on a building or at ground level in a rack to form a solar cell array.

Solar cell arrays can be mounted at a fixed angle facing south, or they can be mounted on a tracking device that follows the sun, allowing them to capture the most sunlight over the course of a day.

Because of their modularity, solar cell systems can be designed to meet any electrical requirement, no matter how large or how small. You can also connect them to an electric distribution system (grid-connected), or they can stand alone (off grid).

What are the components of a solar cell system?

These include solar cell panels; one or more batteries; a charge regulator or controller for a stand-alone system; an inverter for a utility-grid-connected system and when alternating current (ac) rather than direct current (dc) is required; wiring; and mounting hardware or a framework.

How do I know if I have enough sunlight for a solar cell system?

A solar cell system needs unobstructed access to the sun's rays for most or all of the day. Climate is not really a concern, because solar cell systems are not affected by severe weather. In fact, some solar cell modules actually work better in colder weather. Most solar cell modules are angled to catch the sun's rays, so any snow that collects on them usually melts quickly. There is enough sunlight to make solar energy systems useful and effective nearly everywhere in the United States. ??Even hail won't harm most solar cell systems. Most homes have adequate roof space for a solar cell system, but you will have to size your system first to discover how much space is required. If you don't have adequate roof space, look at other options such as integrating the system into a wall or putting the system in the backyard. You could also use the system to cover a porch or patio in the backyard or mount the system on the roof or wall of a garage. Remember: an energy-efficient building requires a smaller solar cell system.

How long do solar cells last?

A well-designed and maintained solar cell system will operate for more than 20 years. The solar cell module, with no moving parts, has an expected lifetime exceeding 30 years. Most system problems occur because of poor or sloppy installation. Failed connections, insufficient wire size, components not rated for dc application are the main reasons. 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 solar cell 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 solar cell system are appropriate for the application.

Power your home with solar cells

Solar cells can be used to power your entire home's electrical systems, including lights, cooling systems, and appliances. Solar cell systems can be blended easily into both traditional and nontraditional homes. The most common practice is to mount solar cell modules onto a south-facing roof or wall. For an additional aesthetic appeal, some solar cell modules resemble traditional roof shingles or can be built right into glass skylights and walls. These building-integrated solar cells, as a dual-use building material, reduce solar cell system costs by using the building as the mounting or support structure, and reduce utility bills with on-site power production.

Why should I use solar cells?

You should consider using a solar cell system if it operates better and costs less than the alternatives. The cost of energy produced by solar cell systems continues to drop. However, kilowatt-hour for kilowatt-hour, depending on where you live, solar cell energy still usually costs more than energy from your local utility. Also, the initial cost of solar cell equipment is higher than that of an engine generator. But there are many applications for which a solar cell system is the most cost-effective long-term option, such as for power in remote areas.

Advantages of solar cells:

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

Modularity - A solar cell system can be designed for easy expansion. If your power demand could increase in future years, the ease and cost of increasing the solar cell 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 of charge.

Environment - Solar cells create no pollution and generate no waste products when operating.

Maintenance - Solar cells have no moving parts and require no maintenance other than to be regularly cleaned.

Durability - Most of today's solar cells are based on proven technology that has experienced little degradation in more than 15 years of operation.

Cost - For many applications, the advantages of solar cells offset their relatively high initial cost.  Federal energy tax credits, new feed-in tariffs, net metering and lower prices are combining to make solar cells and solar panels an attractive energy alternative.

System designers know that every decision made during the design of a solar cell system affects the cost. If the system is oversized because the design was based on unrealistic requirements, the initial cost is unnecessarily high. If less durable parts are specified, maintenance and replacement costs will increase. The overall system life cycle cost 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 clean power source. As you size your solar cell system, be realistic and flexible, and select an experienced designer to assist you.

What is the energy payback period for solar cells?

Typically, the energy payback time (i.e., the time it takes for solar cells to generate the same amount of energy that it took to manufacture the system) for solar cells is 2 to 5 years. Since a well-designed and maintained solar cell system will operate for more than 20 years, and a system without moving parts will operate for close to 30 years, solar cells produce far more energy over their useful life than we use to manufacture them.

Financial incentives for installing solar cells

Solar cell systems placed in service by December 31, 2016 are eligible for a 30% federal tax credit, with no upper limit, for existing homes and new construction.

Check the DSIRE (Database of State incentives for Renewable Energy) for information on state programs.

What is an average break-even point for a solar cell system?

It depends. The break-even point for a system depends on financing and incentives, which vary from place to place, and it depends on your solar resources and what you would pay for another source of energy. A system designer that has information about your location, the amount of energy you typically use, how much land or roof area you have for the system, etc., could give you a more accurate answer. You can obtain a very good estimate by contacting a qualified solar panel installer.

What is the latest in solar cell technology?

Nanosolar, in California, is one of the new "thin film" solar companies. Nanosolar manufactures panels not from fragile silicon wafers, but from ink. It's a delicate and complex industrial process. Nanosolar starts with nanoparticle ink. It is made up of Copper Indium Gallium diSelenide, or CIGS, for short. The liquid ink is deposited in micron-thin layers onto metal foil then heated to convert it into a semiconductor.

The result is a solar cell that's nearly as efficient as silicon and a lot cheaper. It's like printing a very long newspaper. It is very thin and flexible and it creates electricity when sunlight hits it. The company can produce many rolls each day that are one mile long. A mile long roll of “thin film” solar could generate a gigawatt of power, the same amount of power a nuclear power plant can produce in a year.

This thin film is covered with thick glass sheets. The company has to guarantee buyers that this new material will last as long as silicon panels. The glass adds expense but protects the film from the elements. As a result, these panels aren't as cheap as they could be. Still, they're sold out for the next three years.

Google, located in Silicon Valley, has invested in Nanosolar’s “thin film” solar cells to power its large campus, save money on electric bills, and help the environment. The panels will actually pay for themselves in about seven and a half years. After that Google will be getting free electricity from these panels. And they'll continue to generate electricity for 25 to 30 years.

Sources: Department of Energy, Solar Energy Technologies Program Nanosolar.com

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Solar Cells using thin film
Thin Film Photovoltaics in action
Solar Panel history lesson