Tuesday, June 9, 2009
Solar Air Conditioning
This goes back to something the Romans actually did. It is called a solar chimney. It turns out that the soil about 5 feet underground remains a constant temperature year round. For instance, on the southern tip of Vancouver Island it is about 52 F year round. We don't really need air conditioning though. But a lot of furnaces bring in fresh air, so why not preheat that air from 30 F up to 52 F? So, we could use part of this system as well.
With the solar chimney, the sun heats the air in a black metal pipe or a black box acting like a solar oven. When the air gets hot it rises, but it is connected through the attic to a ceiling vent. If the windows are all closed, then this causes a suction in the house. That suction causes air to come into a lower vent that connects to a pipe or pipes that run underground like in the picture above. The pipe isn't under the house, it is just buried in the yard about 5 to 6 feet deep.
There is a lot of math involved, but you could assume certain things. First, use the 4 inch pvc pipe. Secondly, you will need about 50 feet of it per 60 - 80 cfm of air flow you need. And you would want to use a solar chimney for each pvc pipe you use. Thirdly, use one pipe for every main room in your house. If you had a dining room, kitchen, 3 bedrooms, living room and 2 bathrooms (just ignore the bathrooms) then you would need 6 pvc pipes and 6 solar chimneys on the roof. The pvc pipe would join up in a common place when they enter the house and the solar chimneys would all be running parallel. You don't have 6 seperate ceiling vents and 6 seperate cold air vents coming in.
And it is better to run 2 lengths of 50 feet than to run one at 100 feet. You get 120 - 160 cfm, plus twice the surface area for cooling.
This page talks about using the pipes with your current AC fan.
Warning: the above page is very long but full of some great info.
Also, if you live in a place with dry soil or if your ground temp is closer to 60 F than 50, then you will want a few more pipes. Even more if your ground temp is closer to 70 F.
But I would like to point out something because it may not hit home until you read the above link. A 50 foot pipe that is 4 inches in diameter touches over 50 square feet of soil. At 52 F soil temp, then that is about 15,000 BTU/hr cooling. After about an hour the soil temp starts to rise and eventually you are down to about 5,000 BTU/hr. But if you were just cooling one room, then it is perfect. Not because 5,000 BTU/hr is perfect for a small room, but because if you are cooling a small room, then it gets cool and you shut the system down. This gives the soil time to cool off again around the pipe. If you try to cool a whole house with the one pipe, it never would cool enough and it would run all the time and the soil temp rises and it stops cooling completely.
I am sad that all new houses weren't built with this kind of thing in mind. It can save over 100 kWh per day in the summer per house that uses this type of setup. Even if it isn't solar chimney but underground piping and existing air handler in the home. A 200 to 600 watt fan sure beats a 5,000 to 10,000 watt compressor. Imagine if the housing boom just didn't create big Mcmansions that cost more (for no good reason) but it created new houses that could add value to society. All that energy and money wasted and nothing to show for it except houses that will use up energy like nobody's business and will fall apart (due to shoddy construction and cheap parts) within 10 years. And what is really sad, a system like this could be installed by the builder for less than $1,000. But instead, that builder chose to pay more than that for a conventional air conditioner. And the system talked about on the above link uses your existing air conditioner minus the compressor. So, once it is designed and built correctly, it acts just like a conventional AC, but almost energy free. Or you could use solar and batteries for the AC fan at that point.