Collection
You may think you don’t know anything about solar energy collection, but you’ve probably already experienced its basic effects. If you have ever left your car in the sun with the windows rolled up for an hour or more, you returned to a car that was warmer inside than the air outside.
This happens because the sunlight passes through the car’s windows to heat the interior parts of the car, such as the dashboard and seats, which in turn pass their heat to the air in the car. Because the windows are closed, the air is trapped and can't escape so it becomes warmer and warmer.
Essentially, this is the basis for solar collection systems, which are constructed to absorb and trap as much of the sun’s heat as possible.
Air outside the car heats up in the same way, but it is free to rise as it warms, and is replaced by cooler air from high above the ground.
Collection systems use the principle that dark colors absorb heat and light colors reflect it. You have experienced this principle first-hand if you’ve ever worn a light-colored shirt in summer, or a dark-colored coat in winter. Some sunny winter day when the snow is piled up, try this simple experiment: Put a piece of black cloth and a piece of white cloth next to each other on a snow-bank. Soon the black cloth becomes wet—it is absorbing the sun’s heat, and melting the snow beneath it. However, the white cloth stays dry—in fact, it may even insulate the snow against the sun and keep it from melting, depending on such factors as temperature and wind.
Solar energy collectors use this principle when they are painted or coated a dark color. A dark-colored collector will receive and absorb the sun’s southerly rays. The combination of a dark-colored absorptive material and glass to trap the heat make up the basic elements of solar heat collectors.
Storage
If solar heat is to be used when the sun is not shining, excess heat must be stored. A storage unit must have sufficient mass to store the collected heat. The dark cloth on the snowbank absorbed the sun’s heat, but if you continued the experiment until sundown, you would see that it also cooled down rapidly. This is because it has very little mass in which to store the collected heat.
In some systems, called storage heaters, collection and storage take place in the same space. (The black cloth is an example of this principle.) In other systems, the collector is completely separate from the storage unit, and the collected heat is transported from one to the other.
In either case, the essential feature of a solar storage unit is the heat-retaining ability of the material it is made of. While the mass is subjected to direct sunlight, or while the air surrounding it is warmer than the mass itself, the mass will continue to absorb heat. But as soon as the surrounding air becomes cooler than the mass, it begins to lose its heat to those surroundings by conduction, convection, or radiation (see ‘How Heat Moves”). Therefore the most effective way of storing heat is in massive (that is, heavy and dense) objects that will retain the heat even in the absence of direct sunlight. Examples include an internal wall or floor made of masonry, brick, or stone, especially if painted a flat, dark color; dark-colored cylinders, drums, or tanks filled with water; thermal walls such as the Trombé wall and the tube wall; or bins of rocks.
The Greenhouse Effect Sunlight is able to enter a greenhouse because it can penetrate glass. The sun’s rays warm solid objects in the greenhouse, which radiate energy in their turn. But these objects radiate energy at a longer wavelength, which can't penetrate the glass. So these rays are reflected and absorbed, and a good deal of their energy stays in the greenhouse. For many years people believed that the ability of glass to trap the longer infra red waves was responsible for the heat gain in greenhouses during the day, so they called this phenomenon the “green house effect.” However, we now know that the greenhouse effect is responsible for only a small part of the heat trapped in a greenhouse. Most of the heat gain comes from keeping the warmed air in an enclosed space so that it is unable to escape. If the greenhouse effect actually were the primary cause of heat gain in green houses, polyethylene greenhouses would remain cool in the sun, because the longer waves can penetrate polyethylene. But these greenhouses have the same heat gain as glass ones do. However, the greenhouse effect is most important in heating the earth. Be cause our atmosphere is surrounded by a vacuum, and because convection cur rents and conduction can't occur in a vacuum, the earth’s atmosphere can lose heat only through radiation. If there were nothing to slow down the heat loss through radiation of infrared waves, the temperature of the atmosphere would drop dramatically, completely altering our climate. However, water vapor in the atmosphere, like glass in the greenhouse, can be penetrated by infrared waves. The infrared waves are trapped in the atmosphere in just the same way that glass traps them in a greenhouse. The difference is that this trapped heat is crucial to the earth’s atmosphere but not to the heat gain in a greenhouse.
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How Heat Moves The natural movement of heat occurs in three different ways: conduction, convection, and radiation. Conduction is the movement of heat through materials that are in direct physical contact, or within two parts of the same material. An iron pot, warmed by the fire beneath it, conducts heat to the stew cooking in it, to the sides and handle of the pan itself, and to the air immediately surrounding it. Convection is the transfer of heat by the physical movement of a fluid (gas or liquid) medium which has been itself heated. Hot stew from the bottom of the pot rises to the surface to be replaced by the cooler surface stew. This movement is convection. The heat from the stew simmering in a pot then warms the surrounding air. The air itself rises after it is heated, and falls after it has given up its heat to the cooler ceiling, and is thereby cooled itself. Radiation is the transfer of energy by ultraviolet, infrared, or visible waves. Ultraviolet and infrared waves are part of the light spectrum beyond what the human eye can see, and the infrared waves are primarily what we feel as heat. Infrared cameras work by photo graphic heat emanations; radiant heat is what you feel when you stand next to the stove flame, or next to a fireplace— one side of you is warmed, while the other side remains cool. Radiation is the only mode of heat transfer that can take place in a vacuum, and it is the way the earth receives heat through millions of miles of space from the sun.
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Distribution
Once the heat has been collected and stored, it must be distributed. For systems in which collection and storage are combined, the heat is distributed only from that point to wherever it is wanted. For systems in which collection and storage are separate, the heat must be distributed first from the collector to the storage unit and then to the desired location. The heat can be distributed immediately into the area surrounding the storage unit by the principles of natural heat movement, or by means of vents and ducts. Or the heat can be forced into places it would not naturally go, by means of pumps and fans. If the distribution is accomplished with the aid of architectural design but without mechanical devices, it is called a passive system. If mechanical devices such as fans or pumps are used to transport the heat, the system is said to be an active one.
Passive Distribution; Active Distribution: Mass for storage
Next: Passive Solar Heating Systems