Solar Energy Harvesting Technology

Trace almost any energy we use back far enough and you arrive at the Sun. Wind blows because the Sun heats the ground unevenly, the water cycle runs on the Sun's heat, and plants make their food from sunlight. Even the coal and petroleum under the ground are the remains of living things that stored the Sun's energy millions of years ago. That is why the Sun is called the chief source of energy.

Technology lets us change the Sun's energy into the form we actually need. A solar panel changes solar energy into electricity. Many families fix a solar water heater on the roof, where the Sun's heat warms the water they bathe and wash with. Small solar lamps, solar street lights, the solar dryer that dries food and the solar cooker that cooks it are all everyday examples of solar energy harvesting technology.

A solar panel converts solar energy into electricity, and that electricity can run lights, radios, televisions and many other electrical appliances. A single panel is really a group of many small photovoltaic cells working together.

When sunlight lands on the cell, electrons in the silicon atoms are set free and begin to move, and a current is produced between the two ends of the cell. Joining many small cells in series increases the amount of current. The stronger the light falling on the cells, the greater the flow of electricity. That is why a solar panel must be fixed in a spot the Sun reaches well, so it can give electricity at its full capacity.

Solar lamps and solar street lights use the same photovoltaic cells. A small solar lamp carries an LED bulb, which glows on a very small current, so the lamp needs only a few photovoltaic cells grouped together. Charged in the daytime sun, it gives enough light to read by at night.

A solar street light has four main components: the solar panel, the battery, the LED light and the controller. Each has its own job.

• Solar panel: makes electricity from sunlight during the day.
• Battery: stores that electricity for the evening.
• LED light: gives plenty of light even on a small current.
• Controller: charges the battery by day and switches the lamp on by itself at dusk.

The electronic circuit inside the controller charges the battery from the panel's electricity through the day. When evening falls, the same controller switches the LED on by itself and sends the battery's electricity to the lamp. Nobody has to walk along the street pressing switches; the light manages its own day and night.

Radish, potato, carrot, tomato, cauliflower, cabbage and leafy greens have always been dried by spreading them in the open sun, the way dried vegetables and fermented leaf foods are prepared at home. Open drying is slow, and while the food lies uncovered, dust, flies and insects can settle on it. A solar dryer saves time, and because the food dries inside a closed box, it is protected from being contaminated by the outside air.

The dryer is a box with heat insulating walls on three sides and a slanting transparent sheet, usually glass, laid across the top. The inside is painted black, because a black surface absorbs heat well. A vent is cut low at the front for cool air to enter, and another vent is cut high at the back for hot air to leave. Sunlight passes through the glass and heats the black surface and the food. The heated air becomes lighter, rises, carries the moisture from the food out through the upper vent, and fresh cool air keeps entering through the lower vent to take its place.

In a solar water heater, a row of evacuated tubes, glass tubes built to absorb the Sun's radiation, is joined to a water tank so that the tubes lie parallel to one another. Cold water fills the tubes from the lower part of the tank. As the Sun heats the water inside the tubes, its density decreases. The lighter hot water rises and collects in the upper part of the tank, and denser cold water sinks down to fill the tubes again. The water keeps circulating by itself all day, with no pump anywhere, and the tank grows steadily hotter.

The tank itself is wrapped in a heat insulating layer that stops the heat escaping. Because of it, the water stays hot even at night, and there is warm water for washing early the next morning.

A solar cooker gathers the Sun's heat onto the cooking pot using a large shiny curved surface that works like a concave mirror. The curve reflects all the sunlight falling on it toward one place, and the cooker is built so the pot sits exactly where that heat is concentrated. The outside of the pot is made black so that it absorbs as much of the heat as possible. Food cooks on the Sun's heat alone, without burning any fuel.

All of these technologies start from the same sunshine and end at different forms of energy. In some, photovoltaic cells make electricity; in others, a black surface or a shiny mirror puts the heat straight to work. The table below sets the five side by side.