Page 49 - Alternative Energy Systems in Building Design

26 SOLAR POWER SYSTEM PHYSICS AND TECHNOLOGIES

TABLE 3.1 AC AND DC DEVICE APPLICATIONS

AC DEVICES—1950 DC DEVICES—2000

Electric typewriters Computers, printers, CRTs, scanners
Adding machines CD-ROMs, photocopiers
Wired, rotary telephones Wired, cordless, and touch-tone phones
Teletypes Answering machines, modems, faxes
Early fluorescent lighting Advanced fluorescent lighting with electronic ballasts
Radios, early TVs Electronic ballast, gas-discharge lighting
Record players HDTVs, CD players, videocassettes
Electric ranges Microwave ovens
Fans, furnaces Electronically controlled HVAC systems

power and requiring more and more expensive solutions for the conversion and regula-
tion of incoming ac supply. Table 3.1 lists some ac and dc device applications of the
mid-twentieth and twenty-first centuries.
As seen from this table, over the last 50 years we have moved steadily from an electro-
mechanical to an electronic world, a world where most of our electric devices are driven
by direct current and most of our non–fossil fuel energy sources (such as photovoltaic cells
and batteries) deliver their power as a dc supply.
Despite these changes, the vast majority of today’s electricity is still generated, trans-
ported, and delivered as alternating current. Converting alternating current to direct
current and integrating alternative dc sources within the mainstream ac supply are inef-
ficient and expensive activities that add significantly to capital costs and lock us all into
archaic and uncompetitive utility pricing structures. With the advent of progress in solar
power technology, the world that Thomas Edison envisioned (one with clean, efficient,
and less costly power) is now, after a century of dismissal, becoming a reality. The
following exemplify the significance of dc energy applications from solar photovoltaic
systems: (1) on-site power using direct current to the end source is the most efficient
use of power, and (2) there are no conversion losses resulting from the use of dc power,
which allows maximum harvest of solar irradiance energy potential.

Solar Cell Physics

Most solar cells are constructed from semiconductor material, such as silicon (the
fourteenth element in the Mendeleyev table of elements). Silicon is a semiconductor
that has the combined properties of a conductor and an insulator.