Passive Cooling

Sorry for the lack of posts. I’ve been busy doing summer things, seeing as it’s summer and all.

Last time we were talking about how to keep your house warm using only the sun. That’s called passive solar heating. Today, we’re going to talk about how we stay cool during the summer.

When we talked about keeping things warm, having good solar gain through south facing glass was a key part to that. However you might wonder that having all that sun exposure would make things really warm during the summer. The first thing we do to help with this is angle the glass that is facing the sun so it is perpendicular to it during the winter. This will give us the best solar gain when it is cold, but it will also reflect much more of the light away during the summer. The angle of the sun during the summer is much higher, so it will hit the glass at a much steeper angle, causing more light to be reflected away. Here are some simple diagrams.

WinterSunAngle

SummerSunAngle

So just by the fact that the sun is higher in the summer, means we have less solar gain. However, the sun is also stronger during the summer, so the angled glass alone will not suffice. To further help with this, modern Earthships make use of something referred to as a double greenhouse. That’s a bit of a misnomer, but I’ll explain as we go along.

If you’ve been following along and looked at the tour posts, you’ve seen that on all of the global model Earthships there is a long hallway that you come into first when entering the house. This hallway always has copious amounts of plants in it up against the glass. Examples of this can be found on the Global2 tour

Global2_002

… and the Suttan tour.

Suttan004

On the right side of the pictures you can see the angled glass that faces the sun. On the left side you can see another wall of glass. This creates a buffer zone in the main greenhouse. The temperature can fluctuate within the main greenhouse, but in the living areas, the temperature remains constantly comfortable. To give you some idea of where the sun is in relation to the green house, I have some pictures.

Here is a section drawing of where the sun falls during the summer.

EarthshipSummerSun

As you can see, the sun will only fall within the buffer zone created by the two glass walls. We can also further help reduce the solar gain buy using window shades or blinds. You can see those in picture of the Suttan house above.

Now in winter, it looks like this:

EarthshipWinterSun

Now the sun light goes all the way to the back of the room, covering a lot more area that will absorb the heat. Make sure your floors are made of something with a lot of thermal mass: brick, flag stone, tile, concrete or adobe are some options. You don’t want to use things like wood, carpet or vinyl as those will act as insulators and you will lose all of that heat coming into your house.

Back to cooling. With the sun beaming down in the front during the summer, it will make that greenhouse fairly warm. Unfortunately, I don’t have any pictures of this, but up in the ceiling of the greenhouse are openable skylights. This will naturally vent the hot air from the greenhouse out the roof, as we all know, heat rises. However, this isn’t all. There is one more feature used to help with overheating during the summer: cooling tubes.

Cooling tubes are 10″ diameter (25.4cm) steel culvert pipes that run into the back of the house from the north side. The north side of the house is all bermed up with earth right up to the roof, so the cooling tubes are buried deep in the ground. You can see the cross section of a cooling tube in the pictures of the sun angle during summer and winter above on the bottom left side.

Because the cooling tubes are buried in the ground, they tap directly into that constant 15 degrees C that the earth maintains. Any air moving through the tubes will be cooled to that temperature, so all we need to do is get it into the house for further ventilation. We don’t use any fans or other mechanical devices, we use pure convection. Take a look at this picture:

EarthshipCooling

Above the door in each room, an openable window is placed. By opening it, we tap into that natural draft happening in the greenhouse. Remember, it’s pretty warm in there with the sun shining down on it. We have those skylights open up top, letting out the hot air. With the venting windows open above the doors, this will naturally pull cool air from the tubes into the room. Warmer air will be pushed out the vent, into the greenhouse and up and out the skylight.

This maintains a constant cooling airflow through the rooms where the people are and it uses no mechanical or other powered systems to do it, just naturally occurring phenomenon. Pretty elegant when you think about it.

Now I’ve skipped over some details to try to keep from overwhelming you all at once. For instance, how do you choose what angle to put your front face glass at for optimal sun gain? Well, it depends on where you are in the world and a few other factors, but I’ll save that discussion for next time.

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