Hot Air Solar Heating


Solar heated air is sometimes used to heat a dwelling in a cold climate. In a not so cold climate, a thick walled adobe house, like the one on the left, may be be sufficient to moderate temperature swings... BUT in colder areas glazing are secured to south facing masonry wall to slow down heat losses. The garage on the right is an example of a glazed wall used to trap heat. in 1964 Felix Trombe, a French inventor, helped to popularize this method of solar heating.

The original Trombe Wall  was later modified by placing holes above and below the massive heat storage wall. This allowed heated convection currents to circulate through the dwelling and increase the heat transfer during the day. The thermal mass of the interior walls as well as the furniture also increased the heat storage capacity.

This simple passive solar hot air heating system, uses the excess heat, not stored in the Caption Wall to directly heat the living space and everything in that space. How do you think this modified Trombe Wall can be further improved?

Notice any difference between this hot air collection system and the Modified Trombe Wall?

Yes this hot air collector appears to be active rather than passive, because a fan is used to push solar heated air out of the cavity behind the glazing.

Very good but there's another difference that's even more important.

Are you talking about the insulation that's used instead of the thermal mass wall?

Very good again. Isolating the solar heat gain area from the solar heat collection area is an important method of conserving energy and preventing heat loss. We'll take a closer look at this later but now let's look at some front views of hot air collectors. 

I call this the up/Down solar hot air collector because air is forced across a black metal plate by using a network of baffles. Without baffles forced air would follow the path of least resistance and collect little heat... BUT with the baffles household air circulates across the entire surface area of the collector. It works fine and Scott D. should be congratulated for the research he did on this design.

My main concern has to do with heated air's natural tendency to rise. In other words extra fan power would be needed to overcome the natural convection currents that would exist inside the up/down collector, but there may be other advantages of collection heat in this manner.

An alternative to the up/down collector is the horizontal flow

By allowing air to flow horizontally additional fan power would not be required to push heated air down.

Of course I have no way of testing these assumptions without a comparative evaluation so I could be wrong, but then I could be right. The best solution to a problem may not always be the most practical solution, but it's still helpful to understand the trade offs in cost, efficiency and aesthetic appeal.




How would it be if we maximized the convection current effect by using vertical columns? 

Notice that the illustrated solar hot air system to the left has no baffles to channel air. This is a front view of our first solar hot air collector. Do you think rising hot air in this collector could be used to create a simple passive solar heating system?

Unfortunately the heat exchange efficiency of a passive system like this is not very high... However with a tiny computer muffin fan at the bottom of each section the solar heat harvest could be greatly improved. So here is the question: Are six tiny fans in a system like this as good or better that a system that uses one large fan? 

I don't know. If you run some tests please let me know what you discover. There is no right way or wrong ways. There are only alternative ways.  




Let's take a quick look at a simple hot air collector that uses a fiberglass screen as an absorber plate. Notice that this screen is placed on a diagonal with the top part of the screen below the out fan. The reason for terminating the screen below the fan is to encourage the unrestricted flow of air away from the glazing material where I would expect heat to be lost. The placement of the screen in this position also guarantees that all household air will pass through the screen before being blown into the house. 

The drawback of this hot air system is that each bay would require it's own fan. The good news is that the fan is so small that the cost 10 of these fans is less than the cost of one large squirrel cage fan. Now let's take a look at this type of hot air collection with some heat storage.

The heat storage chamber may be made from bricks or bottles of water or any other substance capable of storing heat.

There would be 2 fans for this system. One fan would be used to circulate heated air around the thermal mass and the other fan would be used to extract heat from the thermal mass and blow it into the house as needed.

Although this system gives the home owner greater control of temperature fluctuations it may not be worth the added expense to say nothing about the difficulty associated with transferring air heat into the solid or liquid materials inside the heat storage chamber. 

An alternative to the insulated vault is the cement block wall. Hot air from the hot collector is pumped through the hollow spaces inside the cement blocks. The nice thing about a block wall like this is that the blocks do not have to be mortared in place. Just tying some blocks to the inside wall in a few places should be sufficient.

Heat stored inside the block walls during the day may be released gradually at night. The main drawback of a system like this has to do with the wall spaced needed to make it practical. 

Remember this system would need to be installed on the south side of a house and the user would have to give up south facing windows to make it work.

If the walls and furnishings of a house are used to store heat a sunspace may be used instead of a hot air collector to gather heat. As long as the sun space is isolated from the rest of the house a net solar heat gain is possible.

A fan may be used to extract solar heat from the sun space as soon as it forms. The nice thing about a sun space is that it may be used for things other than gathering heat.

The user should understand that sunspaces will undergo large temperature variations that may exceed 120* F during the day and drop below 20* F at night. 




Heat from a sunspace may also be stored in an attic by taking advantage of the natural tendency of hot air to rise and cold air to fall. 

Since 3 or more tons of water are commonly used for the heat storage system the celing above a sunspace must be well supported and the heat exchanfe system between the air an the wayer must be carefully planned.


Once heat is collected and stored inside an attic it may be extracted with a fan and blown into the living quarters of the house.











 Now let's take a look at our last alternative a solar greenhouse.

A solar greenhouse is a special kind of greenhouse used to heat a house as well as get a head start on germinating seedlings. The funneling effect of the greenhouse concentrates heat near the apex. When the heat is sufficient the top flap opens into the living quarters and the bottom flap opens into the greenhouse area. At night when no sunlight is available for heating both flaps close as the convection currents reverse direction.

A solar greenhouse may be active or passive, but in most cases solar greenhouses are designed to transfer heat to a house. Heat stored in a greenhouse is rapidly lost through the glazing. There are many uses for sunlight. if you ever get lost just...

Follow the Sun

If sufficient thermal mass is available inside the greenhouse plant life expectancy may be increased, but remember more heat stored in the greenhouse means less heat stored in the the house. 

For this reason a home owner with a solar greenhouse would adjust the ventilation system to favor either the plants or the people in the house.

I built this greenhouse for a friend who lived on the side of a steep hill in Saranac, NY where temperatures dropped below -30*F in the dead of winter and fire wood was the only other source of heat.


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