Solar Power Short Course
What are photovoltaics?
Photovoltaics are solid-state semiconductor devices that convert light directly
into electricity. They are usually made of silicon with traces of other elements and are
first cousins to transistors, LEDs and other electronic devices.
How does it work?
A photovoltaic device (generally called a solar cell) consists of layers of
semiconductor materials with different electronic properties. In a typical Solarex
polycrystalline cell, the bulk of the material is silicon, doped with a small quantity of
boron to give it a positive or p-type character. A thin layer on the front of the cell is
doped with phosphorous to give it a negative or n-type character. The interface between
these two layers contains an electric field and is called a junction.
Light consists of particles called photons. When light hits the solar cell, some of the photons are absorbed in the region of the junction, freeing electrons in the silicon crystal. If the photons have enough energy, the electrons will be able to over come the electric field at the junction and are free to move through the silicon and into an external circuit. As they flow through the external circuit they give up their energy as useful work (turning motors, lighting lamps, etc.) and return to the solar cell.
The photovoltaic process is completely solid-state and self- contained. There are no moving parts and no materials are consumed or emitted.
What can photovoltaics do?
Virtually any electric power need can be met by an appropriately designed PV power
system. This includes power for lighting, pumping, refrigeration, radio transmission, etc.
The only limitation is the cost of the equipment and occasionally the size of the PV
array, although this is rarely a factor.
What does PV cost?
Although this depends greatly on the application, some general guidelines can be
given. Systems containing 100 watts or more of PV generally cost between $10 and $30 per
watt of PV. Smaller systems are more expensive on a per-watt basis. The cost of the PV
modules is typically 1/3 to 1/2 of the total system cost. Each watt of PV array typically
produces between 2 and 6 watt- hours of energy per day, depending on the season and
location. Very dark conditions (e.g., December in Alaska) and very bright conditions will
produce energy outside this range.
Using typical borrowing costs and equipment life, the life-cycle cost of PV- generated energy generally ranges from $0.30 to $1.00/Kwh. This cost generally limits the current application of PV to areas which are not served by an existing utility grid, although low-power applications may be cost-effective close to (sometimes only a few feet from) a power line.
Is PV difficult to use?
In a word, no. Although making PV cells and modules requires advanced technology,
they're very simple to use. PV modules are generally low-voltage DC devices (although
arrays of PV modules can be wired for higher voltages) with no moving or wearing parts.
Once installed, a PV array generally requires no maintenance other than an occasional
cleaning, and even that is not imperative. Most PV systems do contain storage batteries
which can require some watering and maintenance similar to that required by the battery in
an automobile.
How are modules rated/certified?
PV modules are rated at a well- defined set of conditions known as Standard Test
Conditions (STC). These conditions include the temperature of the PV cells (25 C or 77
F.), the intensity of radiation (1 kW/square meter), and the spectral distribution of the
light (air mass 1.5 or AM 1.5, which is the spectrum of sunlight that has been filtered by
passing through 1.5 thicknesses of the earth's atmosphere). These conditions correspond to
noon on a clear sunny day with the sun about 60 degrees above the horizon, the PV module
directly facing the sun, and an air temperature of 0 C (32 F). In production, PV modules
are tested in a chamber known as a flash simulator. This device contains a flash bulb and
filter designed to mimic sunlight as closely as possible. It is accurate within about 1%.
Because the flash takes place in only 50 milliseconds, the cells do not heat up
appreciably. This allows the electrical characteristics of the module to be measured at a
single temperature, the ambient temperature of the module/factory. Since this temperature
is usually close to 25 C, a minor adjustment corrects output characteristics to the
25-degree standard temperature.
Most manufacturers give only nominal power ratings and a tolerance (usually plus/minus 10%) for a given type of module. Solarex is unique in labelling every module with its tested output, eliminating uncertainty in module performance (within measurement tolerances). Solarex also gives, in addition to its STC rating, a rating at operating conditions of 80% sun and a cell temperature of 47 C, which represents conditions more common in actual operation.
PV modules are certified for a number of characteristics including safety, durability, and output by a number of agencies around the world. The most significant rating agencies are U.L., F.M., and the Commission of the European Communities (C.E.C). Solarex is the only manufacturer to have approvals from these three agencies.
Who uses PV?
PV is used by individuals, businesses, governments, and non- profit organizations.
Anyone requiring electricity without connection to the existing grid is a potential PV
user.
Does PV work in the cold?
Yes, very well in fact. Contrary to most peoples' intuition, PVs actually generate
more power at lower temperatures, other factors being equal. This is because PVs are
really electronic devices and generate electricity from light, not heat. Like most
electronic devices, PVs operate more efficiently at cooler temperature. In temperate
climates, PVs will generate less energy in the winter than in the summer, but this is due
to the shorter days, lower sun angles and greater cloud cover, not the cooler
temperatures.
Does it work in cloudy weather?
PVs do generate electricity in cloudy weather although their output is diminished.
In general, the output varies linearly down to about 10% of the normal full sun intensity.
Since flat-plate PVs respond to a 180-degree window, they do not need direct sun and can
even generate 50- 70% of their rated output under a bright overcast. A dark overcast might
correspond to only 5-10% of full sun intensity so output could be diminished
proportionately.
Aside from PV modules, what else do I need in my PV
system?
Although a PV system can be as simple as a module and a load (such as a
direct-driven fan), most PV systems are designed to supply power whenever it is needed and
so must include batteries to store the energy generated by the PV array. Systems with
batteries also need electronic devices to control their charging or limit the discharging
of the batteries. Since PVs and batteries are inherently DC devices, larger systems
usually include DC/AC inverters to supply AC power in standard voltages and frequencies.
This enables the use of standard appliances in the system. Otherwise special DC appliances
(usually from the RV or marine industry) must be used.
On the electrical side, protective devices such as diodes, fuses, circuit breakers, safety switches and grounds are required to meet electric code safety standards. In general, PV systems also require mounting hardware to support and elevate the PV modules and wiring to connect the PV modules and other components together.
How long will my PV system last?
Do PV modules lose power over time?
In general, the PV modules are the longest-lived component of a PV system.
Top-quality modules such as Solarex's MEGA series are designed to last at least 30 years
and carry a 20- year warranty. They are designed to withstand all of the rigors of the
environment including arctic cold, desert heat, tropical humidity, winds in excess of 125
mph (200 kph), and 1 inch (25mm) hail at terminal velocity.
High-quality industrial batteries will at best last about 7 years. Smaller sealed units will typically last 3 to 5 years. Automotive batteries are poorly matched to the characteristics of PV systems and will generally only last 12 to 18 months in PV service.
Some types of PV module (using thin-film silicon) have a predictable falloff in output in the first few months of operation which slows down and stops after some time. The modules' output from then on is relatively stable. This is a comparatively small effect in current Solarex thin-film modules, which carry an 80% power warranty for 5 years. Polycrystalline modules such as Solarex's MEGA series do not experience this kind of degradation and in fact are warranted to produce 80% of their original minimum power rating for 20 years.
What about breakage?
Don't most modules contain glass? The most reliable, longest-lived PV modules use
a glass superstrate (top layer). Solarex's MEGA series uses low-iron tempered glass with
its circuitry laminated under the glass with layers of plastics. This construction is very
durable but given a strong enough impact, it will break. If the glass is shattered or
punctured the module will eventually fail due to water getting into the solar cells and
causing corrosion. It may take years for the module to completely fail (produce no power).
On the other hand, if the module is damaged in such a way that the two electrical
connections between any given pair of cells are both severed there will be no path for the
current and the module will have no output.
Solarex makes a series of products called Lite modules which use an aluminum substrate rather than a glass superstrate. These modules are designed for light weight and ruggedness in applications such as camping and are shatterproof. In a permanent installation, however, they will not last as long as equivalent glass-front modules. This is because the plastic covering used is not as inert as glass and the aluminum's thermal expansion characteristics do not match the silicon solar cells as well as glass.
In summary, given enough force, anything will break. The most effective protection against vandalism, theft and other catastrophes is property/casualty insurance.
