1. What is photovoltaics?

Photovoltaic (fo-to-vol-ta-ik) systems are solar systems that produce electricity directly from sunlight. The term "photo" comes from the Greek "phos," meaning light. "Voltaic" is named for Alessandro Volta (1745-1827), a pioneer in the study of electricity for whom the term "volt" was named. Photovoltaics, then, means "light electricity." Photovoltaic systems produce clean, reliable electricity without consuming any fossil fuels. They are being used in a wide variety of applications, from providing power for watches, highway signs, and space stations, to providing for a household's electrical needs.

2. What is the difference between "solar energy" and "photovoltaics?"

Photovoltaics is one form of solar energy. The term solar energy can refer to something as simple the energy gathered in your parked, sealed car (your solar collector) and converted into heat. Solar energy is often used to heat houses directly through passive means (sun enters window, room warms). Solar energy is also often used to heat water (a solar collector is mounted in direct sunlight, which warms a heat transfer fluid, which in turn heats the water in your hot water tank).

Photovoltaics refers specifically to the practice of converting the sun's energy directly into electricity using photovoltaic cells. Photovoltaic cells are often referred to as PV cells or solar cells.

3. What is solar thermal energy?

Solar thermal energy refers to harnessing the sun's light to produce heat. Heat results when photons, packets of light energy, strike the atoms composing a substance (water, your body, asphalt), exciting them. Solar thermal technologies include passive solar systems for heating (or cooling!) buildings; flat plate solar collectors, often used for providing households with hot water; and solar concentrator power systems. These systems, also known as solar thermal power plants, use the sun's heat to create steam, which then turns a turbine and produces electricity. (Fossil fuel burning power plants also produce electricity by first creating steam in order to turn a turbine.)

4. Can I heat my house with photovoltaics?

Using electricity to heat a house, as anyone who uses electric heat and pays monthly bills to the utility knows, is very inefficient and costly. Theoretically, one could heat one's home with photovoltaics (electricity is electricity, whether it comes from PV panels or from a coal burning power plant). Practically, though, this would be costly, as producing electricity from a PV system is more expensive than purchasing it from the utility. One can, however, heat one's house very effectively and cheaply by harnessing the sun's energy in other ways.

5. What are the components of a PV system?

• Photovoltaic Cell -- Thin squares, discs, or films of semiconductor material that generate voltage and current when exposed to sunlight.
• Module -- Photovoltaic cells wired together and laminated between a clear superstrate (glazing) and encapsulating substrate.
• Array -- One or more modules with mounting hardware and wired together at a specific voltage.
• Charge Controller -- Power conditioning equipment to regulate battery voltage.
• Battery Storage -- A medium that stores direct current (DC) electrical energy.
• Inverter -- An electrical device that changes direct current to alternating current (AC) to operate loads that require alternating current.
• DC Loads -- Appliances, motors and equipment powered by direct current.
• AC Loads -- Appliances, motors and equipment powered by alternating current.

6. How do the panels work?

A solar panel (module) is made up a number of solar cells. Solar cells are generally made from thin wafers of silicon, the second most abundant substance on earth, the same substance that makes up sand. To make the wafers, the silicon is heated to extreme temperatures, and chemicals, usually boron and phosphorous, are added. The addition of these chemicals makes the silicon atoms unstable (their electrons less tightly held). When photons of sunlight hit a solar panel, some are absorbed into the solar cells, where their energy knocks loose some of the modified silicon's electrons. These loose electrons are forced by electric fields in the PV panel to flow along wires that have been placed within the cells. This flow of electrons through the wires is electricity, and will provide power for whatever load we attach (a calculator, a light bulb, a satellite, etc.)

Because solar cells are modular, a system's size can be increased (or decreased) over time, according to need.

For more details on the workings of solar cells, check out the following web sites:
http://www.howstuffworks.com/solar-cell1.htm
http://www.eren.doe.gov/pv/howworks.html

7. Are there any applications for photovoltaics where I don't need batteries?

The simplest and least expensive PV systems are designed for day use only. These systems consist of modules wired directly to a DC appliance, with no storage device. When the sun shines on the modules, the electricity generated is used directly by the appliance.  Higher insolation (sunshine) levels result in increased power output and greater load capacity. And when the sun stops shining, your appliance stops working.

These simple systems are an appropriate, cost-effective option for loads operated only during the daytime. Examples of day use systems include:

• Remote water pumping with a storage tank.
• Operation of fans, blowers, or circulators to distribute thermal energy during the day for solar water heating systems or ventilation systems.
• Stand-alone, solar-powered appliances such as calculators and toys.

It is also possible, in a utility grid interconnected system (see below), to do without batteries, as such a system is essentially using the grid as its storage device.

8. What if I want electricity at night or on cloudy days?

Introducing batteries to a PV system allows electricity to be stored when the sun is shining. This electricity can then be used to provide power after the sun goes down.

9. What is a utility grid interconnected system?

Utility-connected systems, also called "grid-connected" or "grid-tied" systems, are for homes or commercial buildings that are connected to an electric utility. They are designed to provide a modest part to all of the building's total electricity needs. Advances in solar power electronics make it relatively easy to connect a solar electric system to the utility. Energy generated by such a system is first used within the home, and surplus power is "pushed" onto the utility's wires. In many states of the U.S., local utilities have "net-metering," which allows a homeowner's meter to spin backwards when his or her electricity is pushed back onto the grid. When this happens, the utility buys electricity from the homeowner, instead of the other way around! (And the utility pays the homeowner for the retail value of the electricity. Without net-metering, the utility would be forced by law to buy electricity from the homeowner, (an independent producer), at wholesale.)

A draw-back of connecting your PV system to the grid (and using the grid as "storage") is that when your area suffers a power outage, your PV system automatically shuts off. (This is done intentionally, in order to protect people working on the lines from live electricity.) To avoid this problem, many people introduce batteries to their grid-tied system, which provide power in the event of a utility power outage.

10. How many PV panels do I need for my house?

This depends on how much electricity you use in your home, and where your house is located. The average American household uses 600 Kilowatt-hours of electricity per month.  However, an energy efficient home may use only half that. In a sunny climate, a 2 kilowatt PV system can produce 300 kilowatt-hours of electricity per month. (To generate 2 kilowatts of power you need about 240 square feet of solar panels.) Therefore, the first step in planning a solar system is reducing electricity consumption. It is always more cost-effective to invest in energy efficiency than to install a larger PV system. Planning, mindfulness and some initial investment can result in a dramatic reduction in electricity use, without sacrificing the comforts to which we've become accustomed. As SEI alum Cari Spring says in her book When the Light Goes On: "You don't have to sit in a dark, cold room to save energy!"

11. How can I conserve energy? Electricity?

It is critical that heating and cooling systems, (which account for 40% of the energy budget of the average American household), be highly efficient. Electric heaters and air conditioners are tremendous energy hogs; fortunately, more efficient options abound. In addition, it is important that once your house feels comfortable to you, it stays that way--good insulation is crucial. (Preventing air leakage by caulking and sealing is the most cost-effective way of reducing heating and cooling costs.)

A household can save electricity a number of ways, including: purchasing energy efficient appliances and fixtures (e.g. compact fluorescent lights); using solar thermal energy (e.g. drying clothes in the sun, using a solar hot water system); investing in propane or natural gas-powered major appliances (such as refrigerators, stoves, and clothes dryers); and cutting back on appliance use (e.g. turning off lights, abandoning the electric can opener).

For more ideas (including a list of top-rated, energy efficient appliances, cars, and trucks)
see: http://www.aceee.org
Be sure to see their home energy saving checklist, too:
  http://www.aceee.org/consumerguide/chklst.htm

And for more energy-saving appliances, check out http://www.energystar.gov

12. What is solar cooking?

Solar cookers use no electricity or gas, require no fire wood, and produce no air pollution. The simplest type of solar cooker is a box cooker: an insulated box painted black on the inside and covered with glass or plastic. Sunlight enters the box and heats the food inside. Reflectors can be added to increase the solar insolation captured. An inexpensive cooker can be made out of cardboard, crumpled-up newspaper for insulation, and aluminum foil for reflectors, and can reach temperatures over 250° F. Higher-quality cookers can reach temperatures of up to 425° F.

In many countries of the world, burning wood and animal dung for cooking is wreaking havoc on the environment: contributing to deforestation, desertification, air pollution, and global warming. In addition, cooking over smoky fires contributes to respiratory illnesses, and in many parts of the world, women and children spend over half their waking hours gathering firewood (which, in many places, is becoming more and more scarce). Besides ameliorating these problems, solar cookers can also be used to purify drinking water, sanitize medical instruments, and heat water for laundry. Their potential for bettering lives is tremendous.

And, in this country, cooking outside in a solar cooker can dramatically reduce your home cooling bills in the summer!

The best solar cooking web site we've seen is www.solarcooking.org

13. What can be cooked in a solar oven?

Anything you can cook in a conventional oven--the limit is your imagination. Dishes often require less water when cooked in a solar oven, as well as less salt and sugar (due to the gentle cooking process). Just remember to use a dark colored pot, and use potholders! Solar ovens get hot!

14. Are there solar energy power plants?

Yes. Many utility companies have recently installed large photovoltaic arrays to provide consumers with solar generated electricity or as backup systems for "critical" equipment. Solar thermal power plants produce electricity more cheaply than photovoltaic plants, at least in regions where there is little to no cloud cover. (Solar thermal systems need direct sunlight; photovoltaic systems will still function in cloudy conditions, though their output is diminished.) The first commercial solar thermal plant was erected in California's Mojave Desert in 1984. Despite the success of this project, and the great potential of solar thermal plants in general, only a handful have been built worldwide in the past decade, though there are a number in the planning stages.

For an interesting article comparing photovoltaic and solar thermal power plants, see:  http://www.volker-quaschning.de/downloads/VGB2001.pdf

15. How much of the world's energy does the United States use?

Though we make up just 6% of the world's population, we, the citizens of the United States, consume 25-30% of the energy produced in the world today. We consume twice as much energy as the average British citizen, two and-a-half times as much as the average Japanese citizen, and 106 times that of the average Bangladeshi. Consequently, we Americans produce, per capita, the most greenhouse gases on the planet. As of 1996, each of us here in the US produced, on average, almost twice the greenhouse gases of the average German, and 80 times that of the average Indian.

But don't despair! Think of all the room we have to improve! According to  www.energystar.gov, if, over the next ten years, everyone in the U.S. chose  energy-efficient appliances, "we would cut the nation's utility bills by up to $100 billion and make major reductions in greenhouse gas emissions at the same time."

More Links:
For another (more sophisticated) take on Frequently Asked Questions about PV, see:
www.pvpower.com/pvfaq.html

For information on the history of solar energy, see:
www.abc.net.au/rn/science/earth/stories/s225110.htm
www.pvpower.com/pvhistory.html

For information on how solar cells work see:
www.science.howstuffworks.com/solar-cell