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PV FAQ
The following is a listing of Frequently Asked Questions about
PV.
Question:
What is PV?
Answer:
PV is short for photovoltaics (photo=light, voltaics=electricity).
PV is a semiconductor-based technology used to convert light energy
into direct current (dc) electricity, using no moving parts, consuming
no conventional fuels, and creating no pollution.
Question:
What does PV cost; aren't PV systems expensive?
Answer:
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.
Question:
What is BOS?
Answer:
BOS stands for "Balance of Systems." This refers to everything,
all costs, in a PV system except for the PV modules (design, land,
wiring, electronics, loads/appliances).
Question:
How much PV do I need for my house?
Answer:
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.
Question:
How long will PV last?
Answer:
PV modules have been tested in controlled settings and in the field,
with results showing module lifetimes in excess of 20 years. Other
system components have varied lifetimes (batteries can last 2-15
years, and power electronics are the most sensitive components).
Question:
What kinds of PV are available?
Answer:
The majority of power modules in use since 1955 are made of single-
or multicrystalline 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.
Question:
What makes up a PV system?
Answer:
A PV system comprises the PV modules and the balance of systems
(support structure, wiring, storage, power electronics).
Question:
Where is PV used?
Answer:
PV is used in space, in consumer products, in remote communications,
in village power systems, in traffic signs and lights, in cathodic
protection systems, in road-side emergency call boxes, in grid-connected
systems (residential or grid support), in remote homes, and many
other applications.
Question:
What are the markets for PV?
Answer:
The largest market for PV today is in developing countries, in village
power and remote communications systems (estimates indicate that
more than 2 billion people world wide have no access to conventional
electric power; if they have electricity, they use batteries or
diesel generators). There are projections of large markets for utility
grid support and for building-integrated PV systems in developed
countries.
Question:
How long has PV been around?
Answer:
The photovoltaic effect was first recognized by Edmund Bacquerel,
in France, in 1839. Scientists made solar cells of selenium in the
1880s. And, modern PV technologies were developed at Bell Labs and
RCA Labs in the mid 1950s.
Question:
I've heard that it takes more energy to manufacture PV than it will
produce over its useful life. Is that true?
Answer:
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; I've not heard any
numbers on the polycrystalline thin films (CdTe, CuInSe2), but their
energy payback would also be quite short; and I am unsure of the
payback for concentrator systems.
Question:
Can photovoltaic systems operate normally in grid-connected mode,
and still operate critical loads when utility service is disrupted?
Answer:
Yes, however battery storage must be used. This type of system is
extremely popular for homeowners and small businesses where critical
backup power supply is required for critical loads such as refrigeration,
water pumps, lighting and other necessities. Under normal circumstances,
the system operates in grid-connected mode, serving the on-site
loads or sending excess power back onto the grid while keeping the
battery fully charged. In the event the grid becomes de-energized,
control circuitry in the inverter opens the connection with the
utility through a bus transfer mechanism, and operates the inverter
from the battery to supply power to the dedicated loads only. In
this configuration, the critical loads must be supplied from a dedicated
sub panel. Figure 8 shows how a PV system might be configured to
operate normally in grid-connected mode and also power critical
loads from a batterybank when the grid is de-energized
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