Using Solar Energy
To understand how a solar home functions it is first necessary to review some basic information about the sun. We all know that it provides heat and light. We also tend to take that for granted without thinking about what that really means. According to William H. Kemp, in his book The Renewable Energy Handbook, all energy is solar. The sun's rays enter windows and create heat. It miraculously turns into electricity when striking a photovoltaic cell. Air that is heated by the sun begins to rise and wind is formed which causes a wind turbine to spin eventually creating more electricity. The heat of the sun causes water to evaporate and form rain clouds which will supply streams and rivers with flowing water. That water might flow through a water turbine creating more electricity. These are all renewable sources of energy which come to us from the sun [3]. The fossil fuels upon which our society depends so heavily were products of the sun's energy through photosynthesis.
From their science classes, my students will know that the earth tilts on its axis as it orbits the sun. In the Northern Hemisphere, the sun is lower in the sky during the winter and higher in the sky during the summer (see Figure 1). For this reason, a home with south facing windows will receive direct solar gain in the winter as the sun's rays extend directly into the windows. If there is thermal mass in the home, it absorbs the sun's heat and releases it at night when the sun goes down. Thermal mass is a term for any mass used to hold or contain temperature. During the summer, the rays of the sun approach the house from a higher angle and are deflected off of the front of the home. Without the direct gain from the sun, the house remains cool. "The amount of energy received by vertical south-facing glass in December or January is almost triple the amount received in June [4]."
Figure 1. Reprinted with permission of the publisher from Power With Nature by Rex A. Ewing: PixyJack Press.
Thermodynamics is the movement of heat. Heat tends to move from the hotter place to the colder. "The hotter will lose energy and the colder will gain energy until a state of equilibrium is attained. Cool mass walls will absorb the sun's heat, but when the sun goes down and the air in the room cools, the heat will slowly be drawn back out of the walls [5]." A good example of this phenomenon would be the way a roadway gives off the heat it has collected all day into the cool nighttime air [6]. Conduction is the process by which heat energy moves through a material. For example, the sun heats the south side of a mass wall and the heat moves through that wall to the room on the north side of the wall [7]. These terms are essential to understanding how passive solar energy can heat a home.
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