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
PO Box 715 76 S. 2nd St. Carbondale, CO 81623 970-963-8855 fax: 970-963-8866
sei@solarenergy.org