Tuesday, June 28, 2005

Passive Solar & Independence

Occasionally I go on a rant about how critical energy independence, energy decentralization and vernacular energy technology are to freedom. Or, how energy centralization creates empire. Every day I hear about how some new technology will save us from oil prices or pollution or whatever. What seems to be continually overlooked is how easily these problems -- and the danger of a solution that increases the centralization of energy production (nuclear...) -- can be solved: PASSIVE SOLAR.

I say this a lot, and I get the impression that the definition isn't really well known. There are those ugly solar photo-voltaic cells (ACTIVE solar) that you can put on your roof, but the energy required to make one (mainly derived from oil) is just about equal to the energy that they will produce over their lifetime, and they require centralized manufacturing and high-technology to produce. No, passive solar is simply the harnessing of the sun's power directly. Ever notice how a room with lots of windows that face the sun gets hot? OK, that's the whole concept... now let's explore how this can be applied effectively, simply, cheaply, etc.

Pretty much everyone can figure out how to heat a home with passive solar. But most people aren't aware at how effective passive solar can be for cooling, cooking, ventilation, waste disposal, etc. Let's take on the most difficult challenge: Cooling a home in Phoenix in the summer with only passive solar. Bill Mollison's "Permaculture: A Designer's Manual" outlines a passive solar cooling system which I have adapted into the graphic below.

For example, while in Phoenix it may never get below 90 at night during some points in the summer, the temperature of the earth at 10' underground is always a nice 55-65 degrees F. A simple solar chimney on your home (roughly, imagine a normal chimney x 50%, with a single-glazed window on the South side and a black-painted vent pipe inside) will heat up and pull air rapidly out of your home. Now, for air intake, lay a "radiator", a network of pipes 10' underground that acts as a heat-exchanger with the thermal mass of the earth. As the solar chimney draws air out, you get nice, cool air blowing in through vents in your floor. 0 energy cost, 0 moving parts, simple technology, and it keeps your (well insulated) home at a comfortable temperature and well ventilated, even in Phoenix in August. Similar technology has been in use in vernacular architecture in the Middle East for thousands of years.

Here's the catch: because it's vernacular technology, and can be easily implemented in a decentralized fashion, there isn't much money to be made off this through a centralized/industrialized economic mode. But it works... this is the very stuff of freedom.


shadoweyes said...

I hadn't heard that the photovoltaic cells taking so much oil to produce. I was wondering if this was documented somewhere ... when I tell someone about this and they do not believe me, I would like to point to a scientific report rather than this post =) I have been poking around but cannot find documentation for this anywhere - any ideas?

Jeff Vail said...

Start here:


This list of "emergy" ratios for various power sources lists PhotoVoltaic electricty generation as producing, over its life, 41% of the total embodied energy ("emergy") required to produce the PV cell.

There are basically three ways to look at this issue, so it is by no means simple:

1. EROEI (Energy Returned on Energy Invested). There really isn't a good measure of this term... some people look only at the diesel required to mine coal, for example. Others add the energy required to transport that coal. Others still add the energy required to build the mining equipment, the food energy to feed the miners, etc., etc.

2. Emergy. Basically, if you keep going on the above example until ALL embodied energy is represented, you get eMergy. The link listed above references several academic papers by Howard Odum (University of Florida, peer reviewed academic papers...)

3. An alternate method, but no less controversial, is to assess the economic opportunity cost. For example, purchasing PV cells that will generate X number of Kilowatt Hours over their life costs X dollars. How much crude oil could you purchase for that money? That's the opportunity cost, so evaluate the potential kilowatt hours from that quantity of oil, and that will give you a ratio that you can compare to values 1 and 2.

Ultimately all of these things are totally dependent on the assumptions we use. I'm of the belief that a really accurate calculation of this kind of ratio isn't really feasible. I prefer a simple thought experiment: could a society that uses EXCLUSIVELY source X for its power sustain itself in the long run? If so, then the the ratio (whatever you want to call it) is at least greater than 1. And the greater than 1 part seems to be what is really important...

Anonymous said...

Solar cells are evolving. Check out www.nanosolar.com .

Anonymous said...

I love the idea of using a network of underground pipes to cool air before it reaches the house. I actually had the same idea years ago before I heard of this technique. I have a few questions. First, how do you deal with the humidity issue? I realize the pipe is tilted for that reason, but it would seem to create mold problems in the rock bed or whatever is placed at the lower end. Secondly, how much pipe (also diameter) for how much temp difference? And thirdly, could the same goal as a solar chimney be accomplished with the use of ridge vents without soffit vents. Basically just let the entire roof be the chimney. I look forward to your reply.

Jeff Vail said...

Humidity is something that must be designed for. Of course, this will differ based on whether you live in a dry climate or a humid climate--in some cases (as with classical Moorish architecture), it is desirable to increase the humidity of the incoming air, so the incoming air flows by a fountain or over a pool of water. If you want to avoid humidity (e.g. American South East), then you would want to have a gravity drain, perhaps with a removable bucket or something. I have also seen built-in chains that drag cleaning brushes through the tunnel in both directions. Certainly lots of options here, and there just isn't enough of this kind of design in place to say that one method is the best--room to innovate.

As far as using the entire roof as a solar chimney, at least one designer has already done this. Take a look at: http://www.greenershelter.com/index.php?pg=8

This designer used a metal roof with a space underneath on the south-facing side to create a massive thermal chimney. Also worth reading about the "annualized geo solar" concept on the same site--basically the recommendation that we use a massive heat bank to store heat from the summer all the way into the winter, and the reverse. Of course, a solar chimney is a great way to draw on such a thermal bank...

Anonymous said...

Hi Jeff,

Can you comment on the type of pipe best suited for the under ground cooling system? I would guess that a pipe with low thermal conductivity would be undesirable, as the air flowing through it would warm the inside surface without losing much heat to the surrounding cool earth. Would this make PVC drain pipe a poor choice? Single wall stove flue pipe would be good at conducting the heat out into the earth, but would probably not last long due to corrosion.

Jeff Vail said...

Despite the low conductivity (relatively), I think that PVC pice is the standard for modern "earth tubes" and other geothermal heat exchange pipes. It has the advantage of low corrosivity and easy to clean (mold can be an issue if the local weather patterns create continuously humid conditions in the tube or if they generate condensation as the humid and hot outside air cools). Cleaning can be accomplished by maintaining a cable loop in the tube by which a ball of rags (with some cleaining agent--bleach, vinegar, whatever) is pulled through. However, as you mentioned, PVC is not a great thermal conductor. Traditional geothermal heat exchanges (common in classical Islamic architecture) used stone passageways (often large enough to crawl through, facilitating cleaning and maintenance. Stone is an excellent thermal conductor, and has the added advantage of having a very high thermal mass, reducing the problem that in a very hot climate the short-term ability of the earth to continuously cool air would be reduced (earth surrounding a PVC tube could be temporarily heated up faster than the surrounding earth could cool it under extreme circumstances--far less of a problem with stone).

Anonymous said...

As far as using the entire roof as a solar chimney, at least one designer has already done this. Take a look at: http://www.greenershelter.com/index.php?pg=8

This designer used a metal roof with a space underneath on the south-facing side to create a massive thermal chimney. Also worth reading about the "annualized geo solar" concept on the same site--basically the recommendation that we use a massive heat bank to store heat from the summer all the way into the winter, and the reverse. Of course, a solar chimney is a great way to draw on such a thermal bank...

But there is no thermosiphoning in that graphic as far as I can see, Cold air enters the bottom of the metal roof, gets heated, and rises to the peak of the roof. Then the air is supposed to get conveyed through ducts and pipes into the soil beneath the house. How?

Hot air rises. In order to get it down below grade, one would have to use a pump or blower. But if one were going to go through all that trouble, then one might as wll use solar collectors and pump heated water down into the soil for thermal storage. Water requires much less energy to pump and has far greater heat transfer and heat stoarge capacity than air.

I guess I don't see role of the metal roof as a solar chimney in this AGS scheme.


Anonymous said...

I think it is cooling off the roof and there by helping to cool the building. good Idea.

A stove pipe with glass surrounding would work for a solar chiminey

Anonymous said...


Anonymous said...

Do you know of any specific existing buildings that employ this technology. i.e. ones I could go look at to see how it's done?

tonesmith said...

I was wondering if anyone thought of using Clay pipe for the solar chiminy underground.
I would think that would be a good thermal conductor. also comes in large sizes.

Anonymous said...

Hi do you know anybody who could help with designing earth tubes with solar chimney in Chicago area for new construction home .

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ecolo127 said...

A website is dedicated to the description of all the "solar towers" and solar chimneys". It's on:

The hole family is called The Meteorological Reactors:
Solar Chimney - Schlaich
Flotting - Papageorgiou
Tropical - Bonnelle
Polar - Bonnelle
Flying- Sorensen
Mountainside Chimney - Menard
Greentower - Stellenbosch South Africa 1.5km high
Super Cheminée - Pesochinsky 5km high

Energy Tower – Zaslavsky - Guetta

Bi-Directionnal - Rohatensky
Solar Nozzle - Williams

Vortex Michaud - Monrad
Vortex Nazare - Maugis-Sumatel
Self producing Chimney - Coustou-Alary
Vortex Mamulashvili

Other variants of Meteorological Reactors
James Bowery associated to algae biodiesel production
Tom Bosschaert for power co-generation
Tom Bosschaert in urban and city areas
Patrick Nicolas self standing
Solar Pyramid - MSC Power Corp
Nazar Hassen Solar Minaret
and many other solar towers, solar chimneys, energy towers...

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