U.S. Department of Energy NCPV Link.
Download UCEP complete FAQ in a pdf.
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How is photovoltaics different than other solar energy conversion technologies?
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What is solar cell efficiency and why do numbers of efficiency appear to vary so widely?
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What is the difference between a solar cell and a photovoltaic panel or array?
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How much power is produced by a PV panel and what does the standard rating mean?
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How much photovoltaic power do I need for a given application?
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Do solar cells produce more energy than is used during their manufacture?
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How does the cost of PV electricity compare to electricity generated by other means?
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Is there enough sunlight to make a contribution to the world’s energy needs?
Photovoltaics is the direct conversion of sunlight into
electricity using the physical mechanism called the photovoltaic effect.
2. How is photovoltaics different than other solar
energy conversion technologies?
There are a variety of ways to convert sunlight into useful
energy. One method used for many centuries is to convert sunlight into heat,
which can then be used for building heating or water heating. Two common
examples of solar energy into heat are solar pool heating and solar water
heaters. There are also two ways to convert sunlight into electricity. One is
solar thermal electricity generation, which uses much of the technology from
conventional utility electricity generation. In most utility electricity
generation, heat is generated by burning a fuel such as coal or by a nuclear
reaction, and this heat is turned into electricity. In solar thermal generating
systems, the heat is created by focusing sunlight onto a spot rather than
burning fuels, but the remainder of the electricity generation process is the
same as conventional utility generation. Photovoltaics is another mechanisms for
converting sunlight into electricity.
Photovoltaics, (also called solar electricity, solar batteries
or solar cells) are fundamentally different in that they convert sunlight
directly into electricity without intermediate steps.
3. How
does a solar cell work?
Solar cells (or photovoltaic devices) directly convert light
into electricity, and usually use similar physics and technology as that used by
the microelectronics industry to make computer chips. The first step in the
conversion of sunlight into electricity is that light must be absorbed in the
solar cell. The absorbed light causes electrons in the material to increase in
energy, at the same time making them free to move around in the material.
However, the electrons remain at this higher energy for only a short time before
returning to their original lower energy position. To collect the carriers
before they lose the energy gained from the light, a pn junction is
typically used. A pn junction consists of two different regions of a
semiconductor material (usually silicon), with one side called the ptype region
and the other the n-type region. In p-type material, electrons can grain energy
when exposed to light but also readily return to their original low energy
position. However, if they move into the n-type region, then they can no longer
go back to their original low energy position and remain at a higher energy. The
process of moving a light-generated carrier from where it was originally
generated to the other side of the pn junction where it retains its higher
energy is called collection. Once a light generated carrier is collected, it can
be either extracted from the device to give a current, or it can remain in the
device and gives rise to a voltage. The generation of a voltage due to the light
generated carriers is called the photovoltaic effect. Typically, some of the
light generated carries are used to give a current, while others are used to
create a voltage. The combination of a current and voltage give rise to a power
output from the solar cell. The electrons that leave the solar cell as current
give up their energy to whatever is connected to the solar cell, and then
re-enter the solar (in the n-type region) at their original low energy level.
Once back in the solar cell, the process begins again: an electron absorbs light
and gains energy, the electron is collected by the pn junction, it leaves the
device to dissipate its energy in a load, and then re-enters the solar cell.
4. What is solar cell efficiency and why do numbers
of efficiency appear to vary so widely?
Solar cells are often characterized by the percentage of the
incident power that they convert into power, called the power conversion
efficiency or just efficiency. The efficiency is given by a percentage. The
efficiency of a solar cell is determined by the material from which it is made
and by the production technology used to make the solar cell. Efficiencies for
commercially available solar cells range from about 5% to about 17%. The bulk of
the commercial market consists of bulk silicon solar cells, and the research or
laboratory efficiency of these is close to 25%. Space applications, where
efficiency is more important, often use a different solar cell technology and
may consist of solar cells made from different materials stacked on top of one
another. The efficiency of these solar cells is up to 33%. The theoretical
efficiency limit of solar energy conversion given completely idealized
conditions and materials is 86%, but given present technology, solar cells that
can potentially be made have a theoretical conversion efficiency closer to 50%.
In addition to the power conversion efficiency, other methods to characterise
solar cells also contain the word efficiency and are also given by a percentage.
For example, the quantum efficiency measures, at a given wavelength of light,
how much of the incident light is turned into current – not power.
Quantum efficiency is a chiefly a method of analyzing devices used by
specialists in the area and does not simply or directly relate its power
conversion efficiency. For solar cells that have power conversion
efficiencies of 15%, the quantum efficiencies may routinely reach
over 90%. For newer or experimental solar cells, the quantum efficiency is often
much lower, about 30%, and the power conversion efficiency is often less than
10%. The quantum efficiency and power conversion efficiency are sometimes
confused in press or non-specialist articles, leading to apparent claims of very
high solar cell efficiencies.
5. What are the different solar cell technologies?
Solar cell technologies differ from one another based firstly on
the material used to make the solar cell and secondly based on the processing
technology used to fabricate the solar cells. The material used to make the
solar cell determines the basic properties of the solar cell, including the
typical range of efficiencies.
Most commercial solar cells for use in terrestrial applications
(i.e., for use on earth) are made from wafers of silicon. Silicon wafer solar
cells account for about 85% of the photovoltaic market. Silicon is a
semiconductor used extensively to make computer chips. The silicon wafers can
either consist of one large singe crystal, in which case they called single
crystalline wafers, or can consist of multiple crystals in a singe wafer, in
which case they are called multicrystalline silicon wafers. Single crystalline
wafers will in general have a higher efficiency than multicrystalline wafers.
Silicon wafers used in commercial production allow power conversion efficiencies
of close to 20%, although the fabrication technologies at present limit them to
about 17 to 18%. Multicrystalline silicon wafers allow power conversion
efficiencies of up to 17%, with present fabrication achieving between 13 to 15%.
The efficiency achieved by a solar cell depends on the processing technology
used to make the solar cell. The most commonly used technology to make
wafer-based silicon solar cells is screen-printed technology, which achieves
efficiencies of 11-15%. Higher efficiency technologies are the buried contact or
buried grid technology, which achieves efficiencies op up to 18% and has been in
production for about a decade.
Although silicon solar cells are the dominant material, some
applications – particularly space applications – require higher efficiency
than is possible from silicon or other solar cell technologies. Solar cells made
from GaAs or related materials (called III-V materials since they are ingeneral
made from groups III and V of the periodic table) have a higher efficiency than
silicon solar cells, particularly for the spectrum of light that exists in
space. GaAs solar cells have efficiencies of up to 25% measured under
terrestrial conditions. To further increase these efficiencies, solar cells made
from different kinds of materials are stacked on top of one another. Such
devices are called tandem or multijunction solar cells (the term multijunction
applies to other types of structures as well). Such solar cells have
efficiencies of up to 33% (under concentration, see below).
A final class of solar cell materials is called thin film solar
cells. These solar cells can be made from a variety of materials, with the key
characteristic being that the thickness of the devices is a fraction of other
types of solar cells. Thin film solar cells may be made either from amorphous
silicon, cadmium telluride, copper indium diselenide or thin layers of silicon.
The efficiencies of thin film solar cells tend to be lower than those of other
devices, but to compensate for this the production cost can also be
significantly lower. Of these technologies, amorphous silicon is the best
developed, and laboratory efficiencies are between 10 to 12%, with commercial
efficiencies just over half these efficiencies. The other thin film technologies
are still the subject of development, although commercial products exist. The
efficiency of these devices is about 6% to 10% efficient.
Most solar cells will theoretically operate with a higher
efficiency under intense sunlight than under the conditions encountered on
earth. Concentrator solar systems exploit this effect, by focusing sunlight into
a concentrated spot or line. Concentrator systems exist for both silicon and
III-V solar cells. Silicon concentrator systems have reached efficiencies of 28%
while III-V based systems have reached about 33%.
6. What is the difference between a solar cell and a
photovoltaic panel or array?
A solar cell is a single device. A photovoltaic or solar panel
consists of multiple solar cells connected together into a single unit to
protect the solar cells and increase the voltage and power above that of a
single solar cell. Typically, you cannot buy solar cells, only photovoltaic
panels. “Photovoltaic panel” and “photovoltaic array” are
sometimes used interchangeably, but a photovoltaic array refers to all of the
photovoltaic panels in particular systems that are connected together.
7. What type of electricity is produced by a PV
panel?
PV panels produce DC power, which stands for direct current.
This is the same type of power as in a battery, but is diffe rent to that
produced by the utility company, which is AC power. “AC” stands for
“alternating current”. DC power is converted into AC power via an
inverter, which may be incorporated into some types of PV modules, such that
these modules produce AC power.
8. How much power is produced by a PV panel and what
does the standard rating mean?
A PV panel is rated in terms of the power it would produce under
standard light intensity conditions called AM1.5 and at room temperature. For
most locations, the standard light intensity rating is about the amount of light
produced at noon in summer on a sunny day. (Locations close the equator or at
higher altitudes may exceed this at certain times of the year, while locations
far away from the equator will not reach this level). For climates at latitudes
of about 30° above or below the equator, you can multiply the rating of the
panel by 5 to get the amount of kWhr produced per day to get a rough estimate of
the energy produced. For higher latitudes, multiply the rated power of the panel
by about 3.
9. How much photovoltaic power do I need for a given
application?
Detailed calculations and system designs are often calculated
using computer programs, but rough estimates can be determined by a simple rule
of thumb. The rule of thumb for locations around 30° above or below the equator
is:
PV power needed = Total daily load in kWhr / 4
The total daily load in kWhr can be determined from either your
utility bill (which will usually lists your daily energy consumption in kWhr),
or by finding the power used by the appliance in kilowatts ( 1 kW = 1,000 Watts)
and multiplying by the number of hours used. The power used by an appliance is
often listed on either the box or somewhere on the appliance. An apartment will
usually have a load of about 10 kWhr per day. Large variations from this number
can be experienced in the daily load if the dwelling or the water heater uses
electric heating. Heating loads are very energy intensive, and in a system using
PV-generated electricity, such heating loads would be switched to solar (ie.,
not solar electric), gas or oil heating. For locations at higher latitudes, the
load in the above equation should be divided by a lower number (3 is often a
reasonable estimate), while locations closer to the equator or in high sunlight
des ert regions can use higher numbers (5 to 6).
10. What are common PV applications?
PV products are used in many different applications, covering a
power range from 0.0001 Watts to 2,000,000 Watts. Traditionally, the most common
application of PV has been for electrical loads that cannot be easily plugged
into the electricity grid, either because they should be transportable – such
as solar calculators, watches etc – or because the electricity grid does not
exist at a particular location. When the grid is located far away from a
particular application, PV is being used to provide “remote power”.
Examples of these applications are houses not connected to grid power,
telecommunications, remote villages, water pumping and space. However, a recent
and rapidly growing application for photovoltaics is for residential or building
integrated which are connected to the electricity grid. During the day, power
is used from photovoltaics, and at night power is used from the
electricity grid. A final application is utility-scale photovoltaics, in which a
utility company installs a large amount of photovoltaic power. These larger
systems, which are far less common than other applications, are typically
installed to achieve a specific technical goal.
11. Do solar cells produce more energy than is used
during their manufacture?
Yes. The amount of time it takes for a technology to produce
more energy than was used in their manufacture is called the energy payback
time. Solar cells have an energy payback time ranging from a few months to 6
years, depending on the type of materials, the type of solar cell and where it
is used. Solar cells have warranties well in excess of these numbers, typically
20 years. The origin of the popular myth that solar cells do not produce enough
energy in their lifetime to recover the energy in making them is unknown, as
every published study has shown that solar cells produce more energy in their
lifetime than the energy used in production.
12. How much does PV power cost?
To buy a photovoltaic panel in small consumer quantities
presently costs about $5/Watt. This number can vary widely depending on the
amount of photovoltaic panels bought. Furthermore, installation and other
component costs can up to double this number. An estimate for the installed
price of a residential system is about $7/Watt, for a remote system up to
$10/Watt. Although less common, a PV panel may also be priced in $/m². When
priced in this way, it is difficult to compare to other panels priced in $/Watt,
since the conversion factors depend on the panel efficiency, which is usually
not given. It can however, be possibly determined by the power produced and the
module area.
13. Is photovoltaics economically viable?
This question depends completely on the application for which
you are trying to use PV – PV is clearly the lowest cost power source in some
cases but in other it may be one of the more expensive options. In general, PV
becomes more economic as the size of the load becomes smaller and farther away
from grid power. If grid power is not available and the load is that of a
typical household or less, a PV system is usually the lowest cost option.
Similarly, for consumer appliances, PV is usually a factor of 10 to 100 cheaper
than battery power. For cases where reliable grid power is readily available, PV
is usually not the lowest cost option, unless other considerations such as
environmental impact or different financing schemes are factored in. However,
even in these cases, the economic viability of PV varies widely. PV is typically
not technically or economically suited to the provision of large base-load power
for utilities, but may be suited to power production for individual houses in
locations with high peak electricity prices occurring during the day. For
grid-connected applications, the costing of a PV system for a particular
location and application needs to be considered on a case-by-case basis or at
least region-by-region basis.
14. How does the cost of PV electricity compare to
electricity generated by other means?
This is a complex question, and requires a fairly lengthy
answer. To ignore the explanation and reasons why it is difficult, skip to the
last paragraph in this section. Comparing the cost of renewable energy
technologies to conventional electricity sources is inherently difficult. PV
panels, in common with other renewable energy technologies, are most commonly
sold or prices quoted in terms of power (Watts W or kilo-Watts kW)–
which does not have a time component – whereas utility companies or batteries
usually quote electricity prices in terms of energy (kilo-Watt-hours or kWhr),
which is power ´ time.
This difference is inherent due to the fundamental differences between renewable
and conventional energy technologies. In renewable energy technologies, the
major cost is incurred at the initial purchase of the system. Since there are no
fuel costs, the price essentially stays the same regardless of the time over
which it is used. Hence power, which has no time component, makes most sense in
quoting prices. However, for conventional generating systems, where the time of
operation is a major cost component, electricity prices must include the time
over which power is used and hence utility companies quote prices for energy
(power ´ time), not
just power. When comparing electricity costs for renewable and conventional
electricity generation, this time component must be accounted for.
The energy costs of a renewable energy system can calculated by
determining the energy it will produce in its 20+ year lifetime life and the
total cost over its lifetime. However, comparing these costs to conventional
electricity prices can be tricky. One issue is that this comparison is for the
cost of PV over the next 20 years, while the electric prices are present
prices, usually based on a plant that has been existence for a long time.
Correcting for this factor involves estimating what conventional electricity
prices will do over the next 20 years, which is notoriously difficult. Even
determining the true present conventional electricity costs can be
difficult due to the debate on the level of subsidies provided to electricity
generation and hence its true cost. An additional problem is that the costs tend
to be highly sensitive to the assumptions made in the analysis. For example,
costs associated with borrowing money for the photovoltaic system can double the
cost of the PV system. Also, the costs are sensitive to the assumptions about
the amount of sunlight and the location of the PV system. Locations with high
sunlight will have a lower PV electricity cost, although the cost of the system
is the same. Finally, renewable energy systems may also receive subsidies when
they are installed, but these vary from country to country and these effects
cannot be easily generalized.
With the above disclaimers, estimates of photovoltaic system
energy cost that include all the costs of borrowing money but assume a
reasonably optimum location (such as the desert regions of Southern California
or other high sunlight regions) and do not include any rebates or subsidies
usually arrive at numbers in the range of 20 to 40¢/kWhr. As a rough
comparison, many customers in the US pay about 8 ¢/kWhr for their electricity.
Recently, prices in California have reached 22 ¢/kWhr. Overseas, electricity
prices tend to be higher. In Japan and Europe, electricity prices have
historically been in the range of 15 to 25 ¢/kWhr (note that this number is
complicated by exchange rate variations among currencies and is presently
decreased by the high US dollar).
15. What
companies make PV cells and products?
The following is a partial list of large PV manufacturers. In
addition to other large PV companies, there are also numerous smaller local
retailers do not fabricate solar cells, but rather sell PV system components and
also provide design assistance, installation or maintenance. Check your local
telephone listings for such services. A good site that lists manufacturers is:
http://www.solarbuzz.com/CellManufacturers.htm
16. What are the advantages and disadvantages of
photovoltaics?
Photovoltaic systems have many advantages. In many types of
applications, PV systems have several important technical advantages
that make them the best choice for electricity generation. PV panels are extreme
ly reliable and require low maintenance, they can operate for long periods
unattended, they are suitable for both large and small loads and additional
generating capacity can be readily added. These characteristics make
photovoltaics an ideal technical choice for both remote power and remote
residential electricity applications. For such remote applications, a PV-based
system is also usually the lowest cost system. There are a number of additional
technical advantages, such as the distributed nature of PV power production and
the low lead times to installation, which may be beneficial in gridconnected
installations. In addition to its technical advantages, photovoltaics
electricity generation is also environmentally benign, with arguably the lowest
environmental impact of any of the electricity generating technologies.
The key disadvantage of photovoltaics is its relatively high
cost compared to many other large-scale electricity generating sources. This
disadvantage applies mainly to the use of PV for applications that are already
tied to the electricity grid. Another disadvantage is that the power density of
sunlight is relatively low. This means PV tends to be less suited to
applications that are physically small compared to the amount of power they
require. This affects primarily transport applications. Although solar cars,
solar trains, solar planes and solar boats have all been made and used, in
general these applications are difficult for PV or other solar-based systems.
17. What do you do for power at night?
A photovoltaic stand-alone system (i.e., no other generating
components) will include some storage system – usually batteries – is power
is needed at night. For residential systems that are connected to the utility
grid, the power is used from the electricity grid at night.
18. How long to does a photovoltaic system last?
Photovoltaic systems are very robust and reliable, since there
are no moving parts. A photovoltaic system would be expected to last in excess
of 20 years. Many manufacturers have 20-year warranties on the photovoltaic
modules. The electronic components can also be made reliable, since again there
are no moving parts, but the warranties on these systems tend to be lower, about
5 years. If the photovoltaic system contains batteries (most stand-alone systems
to and residential grid-connected do not), then the batteries will need to be
replaced every 5 to 10 years.
19. What are the components of a photovoltaic
system?
The possible components of a PV system are a power conditioning
sub-system, a storage mechanism, and other general components called
“balance of system” components. The power conditioning sub-system
serves of two basic functions. One component of a power sub-system is often
called a charge controller, which ensures that a battery in the system is
correctly charged from the photovoltaic array. The second component is typically
an inverter, which usually converts the low DC voltage of the photovoltaic
system into the same type of power (higher voltage AC) produced by the utility
company. In the US, the utility company produces 120 V at 60 Hz. Depending on
the type of application, the inverter may also serve several other functions,
such as battery charging or may disconnect the system from the utility when
necessary. Another possible component of a photovoltaic system is the storage
system. When included, this is usually battery storage, consisting of lead-acid
batteries modified from those in cars in order to allow large amounts of energy
to be drawn from them. Other system components are usually grouped under the
term “balance of system (or BOS) components” and include the wiring of
the photovoltaic array, the array mounting, battery housing, etc. The actual
components of a photovoltaic system depend on what the system will be used to
power. For example, if the load is DC, then the inverter (which converts AC to
DC) is not needed. Similarly, if the system is connected to the utility grid,
the storage (and hence a charge controller) is not needed, while the inverter
is.
20. Is there enough sunlight to make a contribution
to the world’s energy needs?
Yes. The earth receives more energy from the sun in just one
hour than the world uses in a whole year.
21. How can I participate in renewable energy
programs?
In addition to the installation of photovoltaics on your roof,
each country typically has a variety of “green energy” programs in
place, by which you but electricity generated from renewable energy sources.
Copyright 2004.