Page 48 - Alternative Energy Systems in Building Design

BRIEF HISTORY OF THE PHOTOELECTRIC PHENOMENON 25

Tesla: “A few large AC generating plants, such as my hydroelectric station at Niagara Falls, are
all you need: from these, power can be distributed easily wherever it is required.”
Edison: “Small DC generating plants, as many as are required, should be built according to
local needs, after the model of my power station in New York City.”

EARLY AC DOMINANCE

After Edison introduced his dc power stations, the first of their kind in the world, the
demand for electricity became overwhelming. Soon, the need to send power over long
distances in rural and suburban America was paramount. How did the two power
systems compare in meeting this need? Alternating current could be carried over long
distances, via a relatively small line given an extremely high transmission voltage of
50,000 V or above. The high voltage then could be transformed down to lower levels
for residential, office, and industrial use.
While higher in quality and more efficient than alternating current, dc power could
not be transformed or transmitted over distances via small cables without suffering
significant losses through resistance. AC power became the standard of all public util-
ities, overshadowing issues of safety and efficiency and forcing manufacturers to pro-
duce appliances and motors compatible with the national grid.

THE 100-YEAR-OLD POWER SCHEME
With ac power the only option available from power utilities, the world came to rely
almost exclusively on ac-based motors and other appliances, and the efficiencies and
disadvantages of ac power became accepted as unavoidable. Nicola Tesla’s develop-
ment of the polyphase induction ac motor was a key step in the evolution of ac power
applications. His discoveries contributed greatly to the development of dynamos, vac-
uum bulbs, and transformers, strengthening the existing ac power scheme 100 years
ago. Compared with direct current and Edison’s findings, ac power is inefficient
because of the energy lost with the rapid reversals of the current’s polarity. We often
hear these reversals as the familiar 60 cycles per second (60 Hz) hum of an appliance.
AC power is also prone to harmonic distortion, which occurs when there is a disrup-
tion in the ideal ac sinusoidal power wave shape. Since most of today’s technologi-
cally advanced on-site power devices use direct current, there is a need to use inverters
to produce alternating current through the system and then convert it back to direct
current at the end source of power. These inverters are inefficient; energy is lost (up to
50 percent) when these devices are used. This characteristic is evident in many of
today’s electronic devices that have internal converters, such as fluorescent lighting.

ALTERNATING AND DIRECT CURRENT: 1950–2000
The discovery of semiconductors and the invention of the transistor, along with the
growth of the American economy, triggered a quiet but profound revolution in how we
use electricity. Changes over the last half-century have brought the world into the era
of electronics, with more and more machines and appliances operating internally on dc