Just decades ago, with oil, coal and hydroelectric power lighting the world and driving its industries, alternate sources were dismissed by critics as utopian "wood chips and windmills."

Since then, alternatives, most lumped under the category of renewable energy, have benefited from an infusion of technology, the faith of investors and the increasing expense of conventional fuels. In particular, solar and wind-generated energy are popular because they generate electricity cleanly, quietly and relatively inexpensively.

Worldwide solar research and funding have doubled since 1990. Shipments of photovoltaic cells, which convert the sun's rays to electricity, have increased an average of 11% annually over the last decade, despite high manufacturing costs. According to some forecasts, the number of installed solar-energy units in the United States will quadruple to nearly 5 million by 2010.

Use of wind power to drive turbines and generate electricity, has also increased after a brief decline earlier in this decade. Several major projects are slated in Texas, Wyoming, and especially California, which along with Denmark operates 90% of the world's wind turbines. As with solar energy, operating expenses for wind systems are virtually nil after start-up.

The big industrial nations lead the world in conventional-power usage and also set the pace in renewable power, which is defined by the National Renewable Energy Lab as any source that can be used continuously without depletion--including solar, wind, hydroelecftric and geothermal energy.

WIND POWER

TURBINE SYSTEM

(1) The wind flows past airfoil-type blade, causing them to rotate and turn the shaft. The transmission maintains a constant speed in the shaft entering the generator, which produces electricity.

(2) The turbine assembly pivots to keep facing the wind as it changes direction. A wind turbine can stand alone or be installed with others to provide power.

Wind energy was first used in 7th-Century Persia when windmills used sails to develop power. Today, turbines employ advanced technology to increase efficiency.

(World wind energy gnerating capacity)

SOLAR POWER

CENTRAL RECEIVER SYSTEM

1) Computer-controlled concave mirrors called heliostats are arrayed in a circular pattern around a tower.

2) The mirrors reflect the sun's rays to a cylindrical receiver, a 46-foot-high steam boiler atop the tower.