While our recent look at residential solar may lead you to believe harnessing that power is a newer initiative, humans have been exploiting solar energy for thousands of years to heat their homes, cook, and produce hot water. Some of the earliest written references to technology consciously designed to capture the Sun’s rays come from ancient Greece. Socrates himself said, “in houses that look toward the south, the sun penetrates the portico in winter, while in summer the path of the sun is right over our heads and above the roof, so that there is shade.” He is describing how Greek architecture exploited the different paths of the Sun through the sky at different times of the year.
By the fifth century BCE, the Greeks were struggling with an energy crisis. Their predominant fuel, charcoal from trees, was scarce since they had stripped their forests in order to cook and heat their houses. Wood and charcoal were rationed, and olive groves needed protection from the citizenry. The Greeks addressed their energy shortage by carefully planning the layout of their cities to ensure that each house could take advantage of the sunshine in the way Socrates described. The combination of technology and enlightened government policy worked, and a crisis was avoided.
Technologies for harnessing the thermal energy in sunlight have only continued to grow over time. Colonists in New England borrowed the ancient Greek homebuilding techniques to keep warm in the harsh winters. Simple passive solar water heaters, little more than a black-painted barrel, were sold commercially in the United States in the late 19th century. And more elaborate solar heating systems were developed to pipe water through absorbing and/or focusing panels. The hot water is stored in an insulated tank until needed. In climates subject to freezing, a two-fluid system is used, where the Sun heats a water/antifreeze mixture that passes through coils embedded in the storage tank, which does double-duty as a heat exchanger.
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