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.