A Proper Understanding of Convection Improves Passive Solar Design
- Make the heat move itself to where you want it, when you need it.
- Power a fresh air system to keep you comfortable.
- Make that fresh air warm in the winter and cool in the summer.
Convective Heat Flow in the Passive Solar Home
Radiant sunshine comes into the passive solar home, with the home itself being the solar collector. But much of this heat will not go directly into the heat saving earth. It must be carried there by convective heat flow.
“Hot air rises,” is the way most people put it. To be more precise, it floats. If all of the air in a passive solar house is heated to the same temperature, it simply gets hot and goes nowhere. The hot air that rises is actually any air that is warmer than the surrounding air and the cool air which descends must only be cooler than the warm air which displaces it. Convection takes place within what we would normally call hot air or cold air. In fact, air masses of different temperatures will tend to seek their own levels, hot air on top, warm air in the middle and cool air at the bottom.
If the cool air is not allowed to descend, then the hot air cannot rise to replace it. A close examination of many passive solar plans for homes will reveal that a proper place has been made for hot air to go, but no provision has been made for the cool air. The usual result is stagnation with places that are too hot or too cold to be pleasant. Many passive solar homes have been built with the expectation of passive solar energy storage by convection which, in fact, run cool air into the storage bins.
Conventional Passive Solar Architecture
An improperly designed passive solar envelope house, whether underground or not, will fail to store more passive solar energy than it would with a correct layout of windows and envelope.
Here (above) is a typical double envelope passive solar design that has appeared in a number of magazines, basic passive solar architecture that has often been applied to underground houses. The claim is made that the passive solar energy is stored in the crawl space underneath the passive solar house. Is it? Look closely at its convective loop. Where is the cold air going? Isn’t the so called storage area actually the coolest part of the loop? How effective can such an arrangement be?
The convective loop requires a heat input and a heat output in order to keep working. To force such an arrangement to work at least a little bit, fans are used to make happen what would have happened via passive solar energy if it had been designed right in the first place. Even with fans, the entire body of air, must be heated up warmer than the storage zone by an appreciable amount before any heat will be stored at all.
When the sun goes down and the outdoor temperature cools off, the windows become the heat sink (a cool place for heat to go). So the interior and the crawl space (if there is any heat stored at all), become the source. Now the convective loop reverses direction, pumping the heat back out. This reverse flow, unless prevented by closing off the air flow passages, or insulating the glass at night, is worsened by the addition of, what would ordinarily be considered a good idea, a super insulated north wall.
Why would a super insulated north wall make it work even worse? Because the convective loop requires both an input and an output in order to keep working. If the daytime output is shut off by super insulating the back wall, the loop will stop. When the loop stops, so does the storing of what little heat was moving into the crawl space. However, this occurs only during the daytime when the sun is shining. At night an output path is readily available and, because the sink is now above, or at the very least, level with the new source, what stored passive solar energy there is, is pumped outside much more rapidly than it was pumped in. It is, in effect, a reverse thermal siphon.
Notice why these actual problems have not been so obvious. A great number of things are usually included in each design that clouds the basics of convective heat flow. Insulation on the windows at night, fans, conduction and the sun shining all the way inside, and overall super insulation makes them work better than many conventional homes. But so called conventional above ground homes are really a poor standard. The only viable standard is one that requires no commercial energy at all.
A well thought out convective loop should be the first consideration in designing any passive solar home and the concept of using a double envelope should not be discarded off hand. Convection is very efficient. However, it may be working for you or against you.
These passive solar plans (above) have a heat input and a heat storage sink (a heat output when the sun is shining). There is a place for the hot air to go and a place for the cold air to go. Everywhere we want heat to move in and out of storage is located in the warm parts of the loop. The cold part of the loop is in the “cold sump” and it must be large enough to contain the expected amount of cold air that will be gathered there. Also, it must be insulated from the warm conductive surroundings. With this arrangement we will actually be storing heat under the floor.
The interior of the passive solar home would have very little effect on the loop during the daytime, summertime or whenever you are collecting those golden rays. Therefore, at input time, the internal envelope is really of little value unless the air temperature is made uncomfortably high.
At night however, if any of the basic principles that make a convective loop work are removed, the passive solar energy flow will stop. Cold air will settle into the cold sump and prevent reverse flow. However, extreme cold can over power this and turn the loop on anyway. The passive solar energy output may be prevented by covering the windows with insulation or by having the air flow stopped and then the loop will be broken. There is, however, some heat loss that will be sustained by the interior. Therefore, heat must be allowed to enter it. Also, the windows may be placed below the level of storage. The sinking cold air will then fill these collectors and trap all the heat above it as with a thermal siphon.
Since these methods of preventing night and cloudy day losses work even without the interior envelope, good air circulation can accomplish the same job much more cheaply and easily. However, the raised floor has additional comfort advantages so you may wish to retain it.
These particular passive solar plans (above, left) also give us an idea of how the passive solar energy storage principles discussed earlier may be applied to passive solar architecture, and are by no means confined to the full earth shelter.
Passive Solar Design with an Open Convection Loop
Consider what can be done if the convective loop is modified as in this diagram (above). This is called the open loop. Note that it requires the source, sink and connecting pipes to be separate. An open loop system will not work in a big open room or if a door or window is open. As before, the heat input (sunshine) heats the air on the left and the heat sink removes the heat allowing the cool air to fall through the right hand pipe. Rather than recycling the same old air through the system, the open loop takes a new breath of air as long as heat is moving in the loop, either into or out of storage. If part of the loop is a living space, then we will have a continuous supply of fresh air whenever heat is moving into or out of the passive solar house. Now we have a passive solar powered ventilation system that works even when the sun isn’t shining, since it is also powered by stored solar heat.
In order to isolate the system from the outdoor weather the heat exchanger will warm the winter air, cool the summer air and allow the convective loop to function at any temperature we like. The operating temperatures naturally will be in the range where we feel comfortable.
Adapted from chapter 6, “What Goes Up,” Passive Annual Heat Storage Improving the Design of Earth Shelters, by John Hait