Solar
energy does not live by cost alone. Although in many locations, photovoltaics
(PV) cost two to three times the price of on-grid electricity, consider
this: the cost has come down more than 80 percent in the last 25 years,
and continues to fall. Add to that equation a renewable source of non-polluting,
silently produced energy. A product analysis that includes environmental
considerations could make photovoltaic electricity a viable choice for
many areas of the country, especially when it is used in hybrid systems
with other sources of energy. The U.S. government and the AIA—among
many others—-currently are working to introduce photovoltaics to
the public: check out the Solar Decathlon projects up and running on the
National Mall between September 26 and October 6.
Following are some photovoltaic basics you should
know, adapted from the National Best
Practices Manual for High Performance Schools, offered by the U.S.
Department of Energy.
Recommendation:
Install photovoltaic (PV) arrays to convert radiant energy from the sun
to electricity. PV is ideal for isolated or stand-alone tasks and can
serve as an excellent teaching tool.
Description:
PV converts radiant energy from the sun into direct current electricity,
without any environmental costs (greenhouse or acid gas emissions) associated
with other methods of electricity generation. PV produces electricity
from an abundant, reliable, and clean source. In fact, the amount of solar
energy striking the earth is greater than the worldwide energy demand
each year.
What are they made
of?
The basic component of a PV system is a solar cell. Most solar cells are
made of specially treated silicon semiconductor materials. Sunlight striking
the cells generates a flow of electrons. Solar cells are laminated, most
have a tempered glass cover and a soft plastic backing sheet. This sealing
protects the lodged electrical circuits from the outside elements and
makes solar cells durable.
How much electricity
do they produce?
The flow is directly proportional to the surface area of the cells and
the intensity of the radiation (a cell area of 6.25 square inches will
produce 3 amperes in bright sunlight). Each solar cell produces approximately
.5 volts. Higher voltages are obtained by connecting the solar cells in
series. Modules may be connected in series for higher voltages and in
parallel for higher currents. The typical photovoltaic module uses 36
silicon solar cells connected in series to provide enough voltage to change
a 12-volt battery.
What
is a solar array?
Individual modules may be further combined into panels, sub-arrays, and
arrays. PV arrays with storage batteries are sources for uninterrupted
power supply. Schools requiring emergency back-up for communications systems
can use this type of standalone system with batteries. Batteries store
energy collected during the day for nighttime use. A battery charger controller
may be included to avoid overcharging the battery. In addition, all systems
include wire, connectors, switches, and electrical protective components.
If the load requires alternating current (AC), an inverter is used to
convert the direct current (DC) power to AC power.
Tying to the grid:
Most schools do not require battery storage and can use grid-tied PV systems.
A grid-tied system can provide electricity savings as well as provide
additional shading or cooling benefit. Most schools can switch to a net
metering schedule where utilities give credit for surplus electricity
produced by PV systems.
When can you use
a PV system in a school?
PV is ideal for climates where plenty of sunlight is available. PV is
also suitable for climates that may experience cloudy days periodically
but have sunlight available most days. For example, a 120-W PV module
system designed to operated a 72-W load for eight hours/day requires a
120-W PV module in southern Arizona, and a 240-W module in Wisconsin.
Solar intensity is measured as "insolation."
How do you design
a system?
Most PV dealers, including BP Solar, a major sponsor of the Solar Decathlon,
will work with an architect to design a system. To get a rough estimate
of the system size, you need to first estimate the requirements by estimating:
The daily load demand
Amount of solar energy available in your region
Battery size
Number of PV modules needed.
To estimate the array size, you need to estimate
how much power it can produce:
p = (solins + delta t) x A xEff,
where:
p = power generated
solins = incident solar radiation
delta t = difference between the
control and design temperatures (0 if the design temperature is between
50–60 degrees F; for control temperature, use 50 degrees F for colder
weather and 60 degrees F for warm weather)
A = array area (in square feet)
Eff = efficiency of the system (multiply cell efficiency by efficiency
of the storage unit).
PVWatts,
developed by the National Renewable Energy Research Labs, is a free software
program that can help you obtain quick estimates.
Other
design considerations:
The most important aspect of installing PVs is siting. Shading
can significantly reduce the output of solar cells. Mount PV arrays at
an elevation or on rooftops. Consider both summer and winter sun paths
and ensure that trees, neighboring buildings, or other obstructions do
not shade any portion of the array between 10 a.m. and 3 p.m.
Mount the system for maximum southern exposure. The exact mounting
angle will differ from site to site.
Flat, grassy sites work better than steep rocky sites
Use arrays as building components to economize on building materials
and for unobtrusive design solutions. Arrays can be used as a finishing
material on structures to create attractive roofs or skylights. Arrays
can be used to break up and add interest to a large, uniform surface.
They double effectively as shading devices because they both block the
sunlight and capture its radiant energy. Transparent arrays can be used
as structural glazing instead of glass. Arrays can also be part of a curtain-wall
system.
Copyright 2002 The American Institute of Architects.
All rights reserved.
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