Photovoltaic (PV), or solar electric cells, convert sunlight directly into electricity. While the manufacture of PV may involve the use of fossil fuels, its operation relies solely on sunlight and does not produce any smog, acid rain, or global warming emissions. PV is modular and easily expandable. A well-designed PV system requires minimal maintenance and has a long service life. Applications range from solar cells that are used to power calculators, to modules (containing multiple cells) which can power a home, to large arrays of modules capable of providing commercial or industrial-scale power.

How do PV cells work?
A photovoltaic solar cell consists of ultra-thin layers of semiconductor material with different electronic properties -- a top layer of phosphorus-doped (N-type) silicon and a bottom layer of boron-doped (P-type) silicon. When sunlight hits the surface of the PV cell some of the photons are absorbed in the P-N junction, freeing electrons in the silicon crystal. The result is electrical voltage which becomes electrical current when the cell is connected to an electric load.

Residential PV and “Net-Metering”
Residential PV systems can be either off-the-grid or grid-interconnected with net-metering. Off-the-grid systems utilize batteries to store PV-generated electricity and must have enough capacity to meet all electrical needs even after many cloudy days. Typically, off-the-grid homes maximize energy efficiency and minimize energy use in order to reduce PV system size and cost to a minimum. Grid-interconnected systems can be any size because they can receive electricity from the grid whenever PV system output is less than home electricity needs. Net-metering allows a grid-interconnected system to run the electric meter backwards or forwards, depending on whether PV system output is greater or smaller than the amount of electricity being consumed within the household at any given moment. Net metering reduces PV system costs by eliminating the need for batteries.

PV System Components
In addition to PV modules, PV systems typically include a voltage regulator and batteries or an inverter to convert the direct current (DC) output of the solar cells into alternating current (AC).

Factors Affecting PV Power Output
Power output depends on the solar cell efficiency, surface area, and intensity of sunlight striking the cell. PV cell efficiency ranges from 6% for some amorphous silicon-based solar cells to 30% or more efficient research grade cells made of exotic materials. A 15% efficient solar module will produce about 15 watts of electric power per square foot of solar cells under full sunlight conditions. In the summer, there are more daylight hours and sunlight is more intense. The summer sun also follows a path more directly overhead, thus maximizing the energy output of horizontally mounted PV arrays – like UB’s. In the winter, days are shorter and the sun is low in the sky, significantly reducing PV output. However, solar cells operate at a higher efficiency when cold.

Benefits of PV
•Produces power from sunlight which is free and renewable
•Zero emissions -- pollution-free in operation
•Reliable
•No maintenance
•Long life time (30 years or more)
•Maximum power production coincides with times when grid power is strained and most expensive, e.g. mid afternoon on a hot sunny summer day
•Can be integrated into building design

Drawbacks
•Still expensive with long payback -- though costs steadily falling
•Efficiencies low – though steadily improving
•Large surface areas needed to generate significant power
•Fossil fuels used in manufacturing

Building Integrated PV (BiPV)
PV can be integrated into the design of buildings and into common building materials. PV shingles can substitute for conventional shingles. Vision glass and glass spandrel units can be impregnated with PV. These BiPV products are expensive and have lower efficiencies than more conventional PV panels but they open up interesting architectural possibilities and may have aesthetic advantages. In the future, the entire skin of buildings may generate electricity.