Hot roofs

who needs a roof? Cover your house with solar cells instead. The idea may seem weird, but this is exactly what the architects of a us-based engineering and architectural firm have done while designing William and Deborah Lord's home in Kennebunkport, Maine, us. Now, when the morning sun breaks over the horizon, the Lords' home -- more precisely their roof -- begins to stir. Within moments, electric current begins trickling from the solar cells covering half of the sleek roof. The other half, made of thermal solar panels, begins warming the water that heats the house ( Popular Science , Vol 248, No 5).

Houses with solar cells or thermal panels or even both mounted on the roof are not uncommon to find. But what makes the Lords' house unique is that it does not have a roof beneath the solar panels. The panels constitute the roof. This frame has been designed by Solar Design Associates of Harvard, Massachusetts, us. Stevan Strong, head of the firm, says, "Building-integrated photovoltaics hold the promise to provide distributed electricity all across the country."

The 864-sq ft, south-facing roof is divided into two sections: one for heating water and the other for generating electricity. Sixteen photovoltaic panels generate an estimated six megawatt annually -- enough to meet the Lords' needs. Part of the heated water is pumped into a tubing embedded in the floor to warm the house. The rest circulates through water-to-water heat exchanger to provide domestic hot water.

The shiny blue panels are of the same type used in spacecrafts. They consist of poly-crystalline cells made from thin slices of semi-conducting silicon. When the photons from the sun's rays strike the silicon crystals, they free electrons which then travel through a circuit as electric current.

Soon, say manufacturers, home owners may have the option to buy a variety of roofing materials with built-in photovoltaics. These are also being embedded in claddings of exterior walls. Progress is also being made throughout the world in developing see-through photovoltaics that could replace windows and skylights. These products will not only help houses to satisfy their own energy requirements, they may even redefine the role of electric utilities.

Despite ardent pleas from photovoltaic cell protagonists, the exorbitant cost of such set-ups has deterred people in India from going for them. But now, not only are solar cells becoming more efficient at converting sunlight into electricity, they are about half as expensive as they were a decade ago. Researchers are now pushing hard to integrate solar cells into building material: by eliminating the need for shingles or other materials, solar products can become more affordable.

Researchers elsewhere in the us are taking a slightly different approach. They are focussing on the second class of solar cells called thin-film cells. These cells, which are less efficient but cheaper than crystalline cells, are commonly used in solar energy-based gadgets. They are made by applying a randomly ordered silicon alloy to a substrate, such as glass. From the customer's point of view, says Subhendu Guha of United Solar Systems Corporation, Michigan, us, "Dollars per watt is more important than actual efficiency of the solar cells. We are going to demonstrate that in terms of dollars per watt, thin films win over the crystalline ones."