History of Solar Panel Development

The history of our utilisation of solar energy is as long as mankind has existed. Many tribal peoples used caves that faced south and the Greeks and Romans after them understood the value in constructing buildings that were open to the sun in winter, spring and autumn for heat, but remained shaded from the heat of the summer sun. This passive use of solar energy has once again come to the fore, as architects seek to maximise this 'free energy' gain by the designing buildings that have larger glazing areas to the south but fairly minimal areas to the north side. By arranging rooms that require most heat to the south side and cooler rooms to the north of the building, a larger proportion of heating demand can be met through passive use of solar, particularly if the building is well insulated.

Horace de Saussure and his Hot Boxes of the 1700's

The increased use of glass during the eighteenth century made many people aware of its ability to trap solar heat. In 1767, de Saussure set out to determine how effectively glass heat traps could collect the energy of the sun.

De Saussure first constructed 5 miniature greenhouses one inside the other decreasing in size from 12 inches. The bases of the glass boxes were laid on a black wooden table. The boxes were rotated so that the sun struck the glass perpendicularly for several hours, during which de Saussure measured the temperature inside. The outermost box was the coolest, and the temperature increased in each succeeding smaller box. The bottom of the innermost box registered the highest temperature—189.5º F. "Fruits. . . exposed to this heat were cooked and became juicy," he wrote.

De Saussure lacked modern physics to understand what was going on but today we can better explain what went on in de Saussure’s glass boxes or what will occur in any glass container or glass-walled building exposed to the sun. Sunshine penetrated the glass covers of the boxes, and was absorbed by the black surface of the table on which the boxes rested. In the process, the light energy was convened into heat. Clear glass has a peculiar property: it easily allows sunshine to pass through, but inhibits thermal radiation from doing the same. Therefore this trapped energy heated the air inside the box and blocked most of it from escaping. Eventually the glass boxes became so hot that an equal amount of heat was lost by conduction through the glass as entered by the sunlight and the temperature stabalised. In solar panels when this happens on a hot day, it is normally referred to as stagnation.

Later Hotboxes

A later version of hot box helped de Saussure ascertain why it is cooler in the mountains than in lower-lying regions. He reasoned that the same amount of sunlight strikes the mountains as low down, but because the air in the mountains is more transparent it cannot trap as much solar energy. To test the theory, de Saussure brought a hot box to the top of Mt. Cramont in the Swiss Alps. The thermometer in the hot box reached 190 °F, while the temperature outside was 43 °F. The following day he descended to the Plains o f Cournier, 4,800 feet below, and repeated the experiment. Although the air temperature was 34 ° F hotter than on the mountain, the temperature inside the hot box was almost the same as in the previous experiment.

Thus de Saussure’s theory was confirmed: the sun shines with almost equal force at higher and lower elevations—as proved by the equal temperatures in the hot box on the mountain and on the plains. At lower elevations there are greater amounts of carbon dioxide and water vapor in the air. This denser atmosphere holds in the solar heat more effectively, retarding its escape into space; so it gets hotter lower down. The glass covers of a hot box present an equally effective barrier to solar radiation trying to escape from the box whether it is located in the mountains or at sea level; so it registers the same temperature in both places.

In the 1890s solar water heaters were being used all over the United States. They proved to be a big improvement over wood and coal-burning stoves, particularily for heating water in the summer. Coal-gas was available to heat water, but was very expensive.

Once survey found that in 1897, 30 percent of the homes in Pasadena (east of Los Angeles) were equipped with solar water heaters. By 1920, hundreds of thousands of solar water heaters were installed throughout the southern U.S. Unfortnately, with the advent of cheap oil and gas were discovered solar water systems began to be replaced with heaters burning fossil fuels.