Passive Solar Energy is a neat way to collect the sun’s energy…

Passive Solar Energy is converting sunshine into usable heat without the assistance of electrical or mechanical devices. Surprisingly, it’s likely you’ve already experienced passive solar collectors. Wanna know where, read on…

Hey, wake up! Let’s recap solar a collectors function. A passive solar energy collector traps sunlight to produce heat. They can be frames, boxes, and open spaces in buildings. They have clear coverings to let in the sun’s rays.

Most if not all, have dark surfaces that soak up the sun’s heat. To contain the heat, insulation is used. And finally, they have vents or pipes to carry the heated air or water to where it’s used.

Okay, back to passive solar energy. The crudest form of passive solar hot water heating is a coiled dark colored garden hose lying in the sun.

Turn on the faucet and hot water comes out when you turn on the faucet, or at least until the cold-water replaces it.

Another good passive solar energy example is your car. Roll up all its windows and park it in the sun. We all know what happens next... Picture a VW bug.

Hot... HOT... HOT...

Wow! a mobile solar collector.

Sunlight heats the interior and can’t get out because the windows are closed. Sooo, either roll down the windows or move the car.

Yet, another form of passive solar energy is a storage water tank painted black. Sunlight heats the tank and water inside. Many a camper has had heated water for doing dishes and after swim showers. This kind of contraption faithfully served in summer campgrounds around the country and many are still in use.

A passive solar energy designed home will always use heat from the sun to the best advantage. It’s planning and construction utilizes dense materials wherever sunlight will enter. This material absorbs and stores the suns energy, providing free after sunset heating.

To increase storage further, perhaps large water containers or possibly rock storage bins conveniently arranged beneath the house will provide additional heating capacity. With ordinary backup systems, such as wood stoves, gas heaters, etc., cloudy days and cold weather will be of little consequence.

Advantages
Optimized passive solar energy systems make a very small dent in ones heating budget, maintenance is almost none existent and hydrocarbon releases are zero. Operating such a system is pure energy conservation.

As for overall costs, compared to conventional hot water and ventilation systems, passive solar is relatively inexpensive. This is always true because of the systems simplicity.

Disadvantages
The most important question facing would be passive solar energy consumers is heat storage. Collecting and storing heat are two different things. Unlike conventional energy systems where heat is instant, collected heat must be stored away for use later. Retrofit arrangements for the storing heat can often be difficult to achieve and maintain.

However such systems will still save considerable energy expenses. And don’t forget, a passive solar system captures solar energy without the assistance of pumps fans and such. It functions solely on the energy of the sun.

Okay, now let’s take a look at the most popular passive solar energy systems currently used. As you will discover, there are many sub-categories and variations. You will discover that passive solar energy includes direct, indirect and isolated solar gain for space heating and water heating.

These will incorporate some sort of thermal mass, operate as a thermosiphon collector, be used in solar cooking, even use earth sheltering and various types of solar chimneys.

Direct, Indirect and Isolated Gain information.

There are three main categories. Click on one for a look at its varieties and characteristics.

1. Air Based Solar Collectors
2. Batch Solar Collectors
3. Thermosiphon Solar Collectors

• Solar Cookers

Air Based Solar Collectors
The first passive solar energy system in this category is Air based (ventilation). We know that the mechanism used to transfer the sun’s energy from one medium to another is a Solar Collector. Well, air based systems do the same work as liquid based systems, only they do it without pipes, fluid and what not. In this case, they use solid surfaces, or some kind of mass to absorb the sun’s heat, then distribute it through ducting and air ways.
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1. Unglazed Perforated Plate
2. Glazed Flat Plate
3. Back Pass
4. Trombe Wall

Unglazed Perforated Plate
An unglazed flat-plate collector is a passive system used primarily for building ventilation/heating and crop drying. As you can see, this type produces excellent results when used for Air Heating and Ventilation and Crop Drying, but poor when used for Space Heating.

A construction grade corrugated type siding perforated with many small holes, through which air passes is used. As solar radiation heats the material, a ventilation fan sucks air through the perforations into the cavity between the solar wall and the buildings wall.

The cavity between the two walls allow the air’s movement upward. Negative pressure created by the air’s heated buoyancy transports it up the cavity to a ventilation air intake.

From there, a ventilation fan’s suction pulls it into the building’s air distribution ducting. Any heat lost through the wall returns back with incoming air in the cavity.
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Glazed Flat-Plate Collector
As shown in the chart, glazed flat-plate collectors are satisfactory for Air Heating and Ventilation, and Drying Crops. However, they produce poor results when used for Space Heating.

A glazed flat plate is a collector sandwiched between a transparent (glass or some other clear material) pane and insulation. Glazing allows maximum sunlight to reach the collector/absorber while minimizing heat loss.
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Back Pass
Back pass collector’s rate only fair when used for Air Heating and Ventilation, but somewhat better for Drying Crops. They are the simplest of the passive air based solar energy systems. The back pass and a perforated plate systems are basically the same except the back pass has no perforations.

They work when air flows across the back of the heated material (panel) and enters the room from the void space behind. The heating surface may be combined with the mass of a Trombe Wall.

A good way to demonstrate this is by placing a thermometer in an open cardboard box, then place a piece of sheet metal over the opening. Now position the box in the direct sunlight and wait. After a bit, quite naturally you’ll find the metal to be very hot. Next, remove the metal and read the air temperature inside the box.

A factory-quilted metal panel fitted to a building as a stand off wall will produce heat in the space behind. Painting the panel a dark color or black will increases its heat transfer efficiency.

Picture example of quilting here

Positioned correctly, the quilted coated surface will get very hot (like what you felt when you put your arm out the car’s window). As the metal is heated, a convection current of cool air rises across its heated surface. Placing vents at the top and bottom allows the air to circulate freely. Heated air may then heat the inner surface a mass, possibly trombe wall.

On the other hand, you may direct the hot air into a ductwork where a furnace type blower circulates it throughout the building. Okay, now you know about glazed, unglazed, back pass and perforated plate solar collector/absorbers, let’s see, what’s next – ah yes, the trombe wall.

Trombe Wall
Trombe wall is an effective passive solar energy system. However it’s collectors will not provide decent Air Heating and Ventilation or Crop Drying efficiency. Why? because thermal losses can be significant at night or with overcast conditions. However, it works fairly well for Space Heating. See the chart above.

Each of these four systems can readily integrate into a buildings design in order to form an integral part of the structure’s envelope. Solar energy used to dry agricultural products is inexpensive and efficient.

In fact, it is one of the oldest passive solar energy applications around. And it ranks first, among the alternate energy sources for such application. Farmers down through the ages have known that air is a fast reactor and therefore a satisfactory heat transfer medium.

Advantages
Absence of energy transfer fluid, piping and associated control equipment. Little maintenance required. Expect long service life. Low system cost. Good return on initial investment.

Disadvantages
Air Collectors are not as common because of low heat storage capacity. Temperatures may be difficult to regulate. Additional thermal mass may be required. Extra insulation to maintain inside temperatures during dark hours may also be required.
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Disadvantages
Batch systems are subject to freeze-ups in cold weather. If placed on the roof, it is aesthetically unpleasing because of the boxes profile. The structural integrity of the roof or supporting structure must be adequate to support the box tank’s weight.
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Thermosiphon Systems
Everyone knows that heat rises. Soooo, it makes sense that heated water rises too. Why? because heated water has a lower density than cold water. In fact, the hotter the water, the faster it rises. Actually, the principal of natural convection is what makes a thermosiphon work. This system requires the tank be located above the solar collector/absorber panel.

The water in the collector/absorber heats expands and becomes lighter. The hotter it gets, the lighter it gets until it flows into the tank above. Gravity pulls the displaced heavier cold water downward into the collector/absorber inlet. The cold water further forces the heated water out the collector/absorber outlet into the top of the tank. This process continues until hot water completely fills the tank. The hot cold temperature differential, thus maintains circulation.

As the upper tank is heated, water flows from it into an auxiliary tank located at ground level. As persons use water from this tank, solar heated water from the upper tank flows down replacing it.

Cold water feeds into the bottom inlet of the roof tank. It then circulates through its outlet to the collector/absorber. Once heated, it flows into top of this tank, flows out and down to the ground level tank for the occupant’s use.

Both flat plate and evacuated tube collectors/absorbers are well adapted for thermosiphon systems.

Thermosiphon passive solar energy systems are ingenious contraptions. They provide a-lot of hot water without much ado and other technical stuff… And they will save you money.

Advantages
A thermosiphon system is easy to install. Pumps and controllers are not required. They require very little maintenance. This system will provide it’s owner a positive money saving cost benefit.

Disadvantages
The roof’s structural integrity may require additional reinforcement to support the upper tank and solar collector/absorber’s weight. Thermosiphon tanks disrupt the building skylines aesthetics.

Last Considerations
Installing a thermally operated valve and an isolation valve will protect the system during cold weather. The thermal valve will protect the collector/absorber from freezing. An isolation valve allows manual drainage of the entire system.
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Solar Cooking Devices
I thought, before we got any further, I should take us on a short junket into the realm of solar cooking. That’s right, I said Solar Cooking. Though not listed in the reflectors list, I believe it’s appropriate to mention it anyway.

A solar cooking device is one of the best and useful examples of passive solar energy. Found throughout the world making short work of quick hot meals. Cooking temperatures commonly reach 200 to 300 F while some can reach 450 F.

One version may use a Parabolic Dish, and another, a Heat-Trap Box. The last is a Panel Cooker. All three share at least two attributes: a reflector lens or concentrator, and insulation to curb heat loss.

Photograph here

A panel cooker is actually a combination of a curved (modified parabolic) reflector and a heat-trap box. Remember the example cardboard box with the thermometer inside? It’s something like that.

A curved reflector or a modified parabolic reflector cooks food very fast, and at extremely high temperatures. Millions in third world and developing countries use them. Many large institutions use them for large scale meal production.

Solar box cookers (heat-trap boxes) can reach moderate to high temperatures. These are the most popular cookers used in both China and India. Give the system a try, the kids will love solar cooked hot dogs. Okay, so much for cooking with solar energy.
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Well that raps up passive solar energy devices, hope you discovered something you can use. I certainly enjoyed writing it. Remember, use the sun it’s free, and it’ll save you many many $$.

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