Solar Thermal Technology

Long before the invention of modern solar technologies, people drew upon the energy of the Sun to heat their dwellings. The ancient Greeks and the Anasazi people of North America, for example, were masters of passive solar use, designing their architecture to maximize solar energy. Early Ukrainian immigrants to Alberta drew upon similar strategies, using roof eaves and window patterns to draw warmth from the Sun in the winter while avoiding the worst excesses of heat during the summer. Today, people still draw on the Sun’s energy for use in their homes, though our methods of doing so have evolved considerably.

Today’s method is known as solar thermal, and its operation is very different from the photovoltaic technology used to generate electricity. Solar thermal systems require collectors, which consist of flat, metallic plates, usually coated in black to maximize heat absorption; a thin, transparent glass or

polycarbonate cover to help trap the heat; and embedded pipes that are filled with liquid (usually a mixture of water and glycol antifreeze). The collectors capture the heat from the sunlight and transfer it to the liquid in the pipes, which run into the building and are typically used for one of two purposes. The first is to produce hot water for the building—in such systems, the pipes carrying the heated liquid are connected to a water-heating system, and the heat from the pipes is used to heat up the water. The second is for space heating in the building—in this case, the heated liquid is passed through a heat exchanger which warms up the air circulating through the building. In both cases, the solar thermal system can dramatically reduce the amount of electricity used in buildings.

Experiments on solar thermal collection date back to the eighteenth century, but the specific technology for use in homes and other buildings was developed in the late nineteenth century. The first patent for a solar-powered device to heat water was issued in 1891, and the technology was very popular for several decades, particularly in places with significant year-round sunlight. By the 1920s, tens of thousands of systems had been installed in North America. Like other uses of solar power, however, this variety faced the huge problem of cost efficiency—as the price of coal and gas fell, there was little incentive to install a solar energy system. Like other sources of solar power, this system enjoyed a resurgence in popularity in the 1970s and beyond, and it is widely used today. Indeed, Alberta is home to a unique and innovate project drawing upon solar thermal technology, the Drake Landing Solar Community near Okotoks. Completed in 2007, the fifty-two houses in the community draw upon an integrated solar thermal system that

provides for up to 90% of the community’s space heating needs. Each house is also equipped with solar panels that provide energy for up to 60% of its hot water needs, and is designed and built to prioritize energy efficiency. By combining solar energy and energy efficiency, this award-winning project reduces greenhouse gas emissions by 236 tonnes (260 tons) per year.

Solar collectors can also be used to generate electricity, though in a very different way than photovoltaic technology. Photovoltaic cells use the sunlight to directly produce electricity in semiconductors. Thermal collectors can produce electricity by super-heating water and turning it into steam to drive an engine that generates electricity. To achieve this, solar collectors need to be capable of producing heat up to 600°C (1,112°F) (unlike thermal collectors for hot water use, which need to produce heat up to only 80°C (176°F)), and thus require a system of lenses and/or mirrors to concentrate the solar energy.