Solar collectors transform sunlight into thermal-energy - so use it
Did you know that cars are solar collectors? Surprised, well just roll up the windows in yours and park it under the sun. Sunlight - powerful stuff; I don’t have to tell you what happens next. Doing what it does inside your car, just think what else it can do.
Capturing sunlight is one thing; doing something with it is another... and that's where solar collectors come in. After capturing the sun’s energy, it needs to be stored if you plan to use it. First, you need an absorber. Yap, solar collectors have them too. With these, you can trap and store it. This is the most efficient way to convert it into usable energy.
A simple way to illustrate is by making a simple solar collector out of a small cardboard box. Open up the box and place a thermometer inside, then cover the opening with a piece of sheet metal, a cookie sheet or a flat pizza pan’ll work. Place the box under the sun and wait awhile. It won’t be long before the sheet metal gets blixy-hot. Remove the sheet metal and read the thermometer - interesting huh! Oh by the way, you just made a solar cooker.
You can see the absorbing material that traps the energy is simply a thin sheet of aluminum or copper. Then a covering of baked on black polyceram or epoxy paint is applied. This material maximizes heat absorption. The heat then transfers into another medium such as air, water, or a glycol solution, thus converting it into usable energy. Insulation helps the plate (absorber) hold the collected heat and glazing reduces heat lose even further.
Heat transfer takes place when either air or tubes filled with some kind of liquid come in contact with the hot absorber material. Glazing material is usually low iron tempered plate glass. The glazing allows sunlight to strike the absorber, but prevents the heat from escaping. Just as your car’s rolled-up windows kept the heat in.
Using plastic glazing will reduce the collector’s weight and cost, however over time, its performance may suffer and life serviceability may decrease. A well sealed collector will likely deteriorate due to out-gassing which results in a haze on the inside surface.
Regardless whether glass or plastic, the desirable radiation is 90% incoming and zero escape loses. From this point on, either gravity or electoral/mechanical devices process and store the heat for direct use or for further distribution.
Solar collectors are wonderful devices. They've been around in one shape or another for a long long time. The ancient Greeks, then later the Romans used passive solar collectors of a sort to capture sunlight to heat indoor spaces.
The Romans went a step further, placing glass (actually mica) on south facing building openings to capture and retain the winter sun’s heat. Moreover, both Greeks and Romans used solar energy to offset wood shortages. Sound familiar?!
In the seventh Century B.C., magnifying glasses were used to make fires and surprisingly – to burn ants. Come the first Century A.D., Roman bathhouses created large south facing windows to catch the sun’s warmth. Colorado Mesa Verde National Park’s Pueblo dwellings faced south to utilize seasonal sunlight.
Around the late eighteen hundreds; inventor Clarence Kemp received a patent for a solar-water heater. Then after World War II, applying passive solar techniques to buildings became popular because of the wartime energy shortages.
Then suddenly, energy from the sun began to get a lot more attention when NASA launched the first spacecraft equipped with a solar 470-watt solar array. The trend focused most of the attention on solar photovoltaics. But quiet development of solar collectors began to receive more and more attention.
Concentrating solar collectors have either a highly reflective surface or multiple Fresnel lens. Built up as a parabolic dish, or a trough. Both gadgets are made with reflective surfaces.
Imagine the trough as long live stock manger, and the dish like the familiar satellite TV receiver, but better yet, look at the illustrations. The Parabolic dish focuses sunlight into a concentrated area, usually an absorbent pipe where heat gets absorbed by some king of liquid. The absorber is always much smaller that the reflector otherwise it would block the sun’s light. This way, sun’s energy is highly concentrated into a small space.
The trough does the same thing as the dish, except the trough directs sun light on a focal point along the concentrators entire length.
Do you remember focusing sunlight through a magnifying glass when you were a kid? Remember that tiny spot of intensely hot sunlight – what it did to that piece of paper or dry leaf? Sure you do... an you remember what happened next… right?! Houston we have ignition!
Advantages
The sun is always there, serving an inexhaustible source of fuel.
Environmentally friendly
It is an excellent supplement to other sources
Versatile, can be used for hot water, heating, cooling, and power generation
Sunlight is absolutely free
Nearly maintenance free
Disadvantages
Some geographical areas may not be adequate to produce sufficient energy
Poor areas may require to produce sufficient energy
Initial installation initial cost may be prohibitive
IV. DESICCANTS and INSULATION Desiccants A desiccants is silica gel. The stuff comes in porous bags and used to keep a solar collectors insulation dry. Dry insulation keeps glazing clear. Stagnation temperatures will cause insulation to outgas. This will cause a rapid deterioration of its performance. Moreover, the air confined inside during assembly contains moisture, that will eventually condense and soak into the insulation.
Desiccant is an important part of a quality collector. Access to the desiccant is not required in properly sealed units, as they do not require rejuvenation. Where two glazing covers are used, placing desiccant between the panes is a good idea. In addition, providing small vent holes in the pane spacers to allow the trapped air access to the desiccant is necessary. Otherwise, condensation will appear on the inner surface of glass.
Insulation Polyisocyanurate and Polyurethane based foams are used extensively for collector insulation because of their high ‘R’ value. Both are closed cell foams available in spray-on, pour-in or in rigid foam boards. The ‘R’ value of course depends on the foam weight factor.
The DIY’er needs to know that different ‘R’ values vary by the foam’s weight. Obviously, two lb foam has a lower ‘R’ value than higher weights. Factories can afford costly application resources such as high-pressure duel-component spray machines and fusion guns. However, unless a DIY’er has access to this kind of equipment, the best bet is pour-in or rigid foam boards.
It is very important how case insulation is applied. Using binder free fiberglass wool to provide a thermal barrier blanket between the foam (if used), the absorber plate and connecting tubing is essential for hazard free medium to high temperature collectors. Should either the absorber or tubing come in contact with supporting case, severe heat losses will occur.
Ideal collector insulation should provide a tight vapor barrier. However, the use of polyurethane foam for insulation maybe prohibited in thermal hazard areas. DIY’ers should check with local building code for material restrictions. In addition, if a urethane is used, the toxic fumes may outgas when during temperature stagnation. Return to Collector Type List
Let’s do a quick comparison between the direct flow and heat pipe types: Direct Flow
Removing tubes may be a problem
New evacuated glass tube versions allow water pipes to slip in and out easily
More versatile
Will work on flat roof
Heat Pipes
Easily disconnected for removal
Effective at lower solar radiation levels
More expensive
Must be installed at an angle for gravity to work
Will not work on flat roofs
In summary, evacuated tube solar collectors will provide a credible money saving part of your heating, hot water and refrigeration budget. A twenty four-tube array will normally provide enough hot water for a family of four. Payback rewards may be modest at first, but eventually tight budgets will find relief. It all depends on the type of system and its extensiveness. Return to Collector Type List
Glass Tempered plate glass with low-iron content is the best glazing material. Low-iron glass permits more solar radiation to penetrate into the collector absorber and inhibits reflection losses.
In other words, short wave radiation from the sun passes through the collectors glazing and long wave (heat) infrared is blocked and therefore contained. The quality or grade of glass will determine the degree of the collector’s efficiency.
Plastic When cost and weight are important factors, using plastics such as ordinary fiberglass reinforced plastic, or polycarbonate plastic, acrylic or Tedlar coated PVC may be the way to go. However, there are drawbacks to using plastic. For example, according to NASA, weather-able vinyl PVC will deteriorate after approximately two years.
Tedlar coated PVC lasts eight years. Acrylics perform the best with a twenty-year service span. A well-sealed collector will likely deteriorate over time. During its deterioration, the plastic will outgas. Outgassing deposits an oily on the inside of the glazing. The result is an inefficient collector.
DIY’ers should consult local building codes should for material restrictions in high fire risk residential zones. Remember, shaving expenses may result in inefficient collector performance. Return to Collector Type List
A box cooker will cook a meal at moderate to high temperatures. If they are large enough, they can even accommodate several pans. A curved (parabolic) concentrator cooker focuses sunlight on a target pot. This variety cooks food very fast but require constant adjustment for optimum cooking. The panel cooker consists of a flat reflective panel that focuses solar radiation onto a cooking vessel. Return to Collector Type List
Ordinary south facing windows are solar collectors, as is tile or concrete, dwelling walls, solarium (sunroom), and green houses. The list of basic recommendations below provides insights into four of the five W’s of air based solar. The fifth is you should you decide to go solar.
The back pass and perforated plate systems are basically the same except the back pass has no perforations. Of the three, the back pass is the most basic of all air collector systems.
It is a sun facing wall consisting of material that acts as a thermal mass. For example, they generally made from concrete, masonry, adobe or tanks of water. Combined with an air space (void), insulated glazing and vents, they form very large solar thermal collectors.
Natural convection currents draw cool air through the perforations as heated airflow's up the wall to the top sill. The air space between the old wall and the collector plates provide a natural path for convection to take place. A blower or fan further assists the convection heat into the buildings ventilation system. Air flowing through this space is warmed by as much 40°,F.
The main benefit of Fresnel solar collectors is their space saving quality and cost of manufacturing. Lens efficiency depends upon the number of groves regardless of it’s flat construction. It’s focal point is close to the lens plate, therefore collector panels needn’t be very deep.
This type may use selective absorber fins or provide barium coating inside the tubes to collect sunlight. Heat-tube with attached absorber fins occupy the space inside a vacuum (thermos) sealed outer tube. This type of collector is among the most efficient of all solar collectors. They are well suited for cloudy cooler climates.
The design has a smaller tube (thermos style) inside a larger one. The outer tube is glass, the inner, either glass or metal. The metal of choice is copper. The inner copper tube has a flat or curved powder coated fin attached to it, which absorbs sunlight but inhibits refection eliminating heat loss.
Inside the glass tubes, rests a smaller copper heat pipe. Along with a vacuum, this pipe contains a small amount of distilled water or alcohol. Since liquid boils at lower temperatures, a change in the liquid when solar heat is applied will bring about an excellent heat source. Boiling occurs even in subdued sunlight.
During this step, the hot vapor cools, condenses and falls back to the bottom, in a liquid state. This cycle occurs continuously, or as long as solar radiation is present. This liquid may be potable water, alcohol or glycol.
Glazed flat-plate solar collectors are best suited for low to moderate temperature applications from 85 to 160°F during the colder seasons. The liquid-based collectors provide hot water for domestic and commercial uses and heating indoor swimming pools. Air-bases systems find use for general heating and ventilation and for drying crops.
