Active Solar Power

Creative ways of tapping into the sun's energy

We currently use three different ways to harness the sun's power: passive solar, using buildings to absorb the sun's energy; photovoltaic, which uses light to generate electricity; and active solar, collecting the sun's heat through solar-thermal concentration systems.

How does active solar technology work?

If you've ever gone on a camping trip, you may have been grateful for active solar technology. At its most basic, a solar collector is a black bag filled with water, as used by campers for a hot shower. Although most solar-thermal concentration systems are more complex, the principles tend to remain simple.

Many active solar water heaters consist of a flat plate mounted on a wall or roof, which receive a lot of solar radiation (north-facing in the southern hemisphere and south-facing in the northern hemisphere).

The plate cover will be transparent, usually made of glass, to admit as much thermal energy as possible. Inside the plate is a series of tubes containing fluid (usually water, but sometimes oil) on a black background (to capture as much thermal energy as possible). The liquid is heated by the sun and transported by a pump to a heat exchanger in an insulated water tank.

Why aren't active solar systems more widely used?

In summer, such systems offer benefits to the environment and savings on utility bills. Unfortunately in winter, when it is needed most, the output of the solar thermal system is at its lowest level, and unable to meet the demand of most modern buildings.

One solution is to improve the building's insulation, thus saving energy and easing pressure on the system. In examples like this, active and passive solar can overlap to good effect. Energy efficiency is maximized when you use an active solar thermal system in conjunction with a passive solar building. Active and passive solar also continue to function (albeit less efficiently) when the sky clouds over, where as photovoltaic cells need direct sunlight to work effectively.

Can active solar technology generate electricity?

When sufficiently high temperatures are reached, active solar power can generate steam that can be converted to electricity.

At the end of the eighteenth century, the pioneering French chemist Anton Lavoiser built a solar furnace that reached temperatures of 1,750 degrees Centigrade. He achieved this by using mirrors and lenses to focus light energy on a small area, much as - on a camping expedition - you might use a magnifying glass on a sunny day to light a fire.

A modern day example of a high temperature solar system is Solar One, a "solar farm" in the Mojave Desert. Solar One uses an array of parabolic mirrors, aimed at a central column containing oil. As the sun changes position during the day, computers track its movements, in order to absorb as many rays as possible. The column is heated to temperatures of up to 400 degrees Celsius. This generates steam for turbines and, eventually, electricity.

The Solar One station generates 275 Megawatts but, at present, systems like this are only feasible in the Mojave Desert, where irradiation levels remain very high all year round.