How Solar Power works Part II

Passive Solar Design for Buildings

One simple, obvious use of sunlight is to light our buildings. If properly designed, buildings can capture the sun's heat in the winter and minimize it in the summer, while using daylight year-round. Buildings designed in such a way are utilizing passive solar energy—a resource that can be tapped without mechanical means to help heat, cool, or light a building. South-facing windows, skylights, awnings, and shade trees are all techniques for exploiting passive solar energy. Buildings constructed with the sun in mind can be comfortable and beautiful places to live and work.

Residential and commercial buildings account for more than one-third of U.S. energy use.[1] Solar design, better insulation, and more efficient appliances could reduce this demand by 60 to 80 percent. There are several hundred thousand passive solar homes in the United States, but there should be many more. Simple design features such as properly orienting a house toward the south, putting most windows on the south side of the building, and taking advantage of cooling breezes in the summer are inexpensive yet improve the comfort and efficiency of a home.

Solar Heat Collectors

Besides using design features to maximize their use of the sun, some buildings have systems that actively gather and store solar energy. Solar collectors, for example, sit on the rooftops of buildings to collect solar energy for space heating, water heating, and space cooling. Most are large, flat boxes painted black on the inside and covered with glass. In the most common design, pipes in the box carry liquids that transfer the heat from the box into the building. This heated liquid—usually a water-alcohol mixture to prevent freezing—is used to heat water in a tank or is passed through radiators that heat the air.

Oddly enough, solar heat can also power a cooling system. In desiccant evaporators, heat from a solar collector is used to pull moisture out of the air. When the air becomes drier, it also becomes cooler. The hot moist air is separated from the cooler air and vented to the outside. Another approach is an absorption chiller. Solar energy is used to heat a refrigerant under pressure; when the pressure is released, it expands, cooling the air around it. This is how conventional refrigerators and air conditioners work, and it’s a particularly efficient approach for home or office cooling since buildings need cooling during the hottest part of the day. These systems are currently at work in humid southeastern climates such as Florida.

Solar collectors were quite popular in the early 1980s, in the aftermath of the energy crisis. Federal tax credits for residential solar collectors also helped. In 1984, for example, 16 million square feet of collectors were sold in the United States, but when fossil fuel prices dropped and tax credits expired in the mid-1980s, demand for solar collectors plummeted. By 1987, sales were down to only four million square feet. Most of the more than one million solar collectors sold in the 1980s were used for heating hot tubs and swimming pools.

Today, about 1.5 million U.S. homes and businesses use solar water heaters—still less than one percent nationwide.[2] In other countries, solar collectors are much more common; Israel requires all new homes and apartments to use solar water heating, and 92 percent of the existing homes in Cyprus already have solar water heaters.[3] But the number of Americans choosing solar hot water could rise dramatically in the next few years. With natural gas prices at historically high levels, solar water and space heaters have become much more economic.

According to the U.S. Department of Energy, water heating accounts for about 15 percent of the average household’s energy use.[4] As natural gas and electricity prices continue to rise, the costs of maintaining a constant hot water supply will increase as well. Homes and businesses that heat their water through solar collectors could end up saving as much as $250 to $500 per year depending on the type of system being replaced.

Part III will come this week!
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Tags: solar water heaters, Department of Energy, absorption chiller