Active Passive

| 12/1/2009 9:45:32 AM

Tags: passive solar, renewable energy, architecture,

Passive SolarIf wind-power is a no-brainer, what about passive solar power? It’s ubiquitous, clean, renewable and free. Our homes could cut their heating requirements dramatically simply by capturing the solar energy that lands on the house naturally. Imagine if all new homes had a row of tall, insulated windows on their solar exposure (the south side, if the home is north of the equator). Then, we add a heat-absorbing concrete floor in the room with the windows. All day long the sunlight warms the house and heats up the concrete. At night the concrete radiates warmth.

The basics of passive solar architecture are simple.[1] First, the building needs what the engineers call an “aperture” through which the sun can shine – in other words, a window. If the sun shines into a window in your home or office, then you have a passive solar collector.

Second, if you want to store some of that solar energy you need an “absorber,” some “thermal mass” that can be heated by the sun. Dense substances like stone, concrete or adobe are very good for storing thermal energy. Sometimes a water tank is used to store thermal energy with the added benefit of being able to distribute the warmth in pipes. Less dense materials – like wood or carpet – can’t effectively store heat.

Third, for a building to be effectively warmed by passive solar energy there needs to be some way of distributing the warmth through the house. In an Earthship, nearly every room has its own solar aperture and thermal mass, so there’s no distribution problem. In more complex structures, fans, ducts and blowers may be used to circulate warm air.

And passive solar buildings need some control mechanism to regulate their temperature – open windows, broad eaves that block the summer sun, insulated shades.

It’s fun to think about homes heated entirely by the sun, but the larger benefit today could come from a tiny alteration in the way we think about site design and construction. Very minor changes can orient buildings more efficiently and place some thermal mass in the sunlight. Every calorie of solar energy captured on a cool day preserves a calorie of fossil fuel. If all our new construction was designed with solar energy in mind, imagine the global benefit. The reductions in energy consumption and carbon emissions are incalculable – but they would be enormous.

11/2/2010 3:58:46 AM

To Scott, who has a problem with this article: I don't want this to sound too oversimplified, but I'm sure if you presented a well-written article to Mother Earth News, outlining the details you believe the article should have, they would consider publishing it.

1/5/2010 9:56:17 AM

It can get complicated, Scott, but it doesn't have to. I've been in dozens of passive solar homes that were comfortable, efficient and easy to maintain. To discuss all the variables of architecture, materials and climate is the province of books, not blogs.

scott raney
1/4/2010 8:20:13 PM

The phrase "no-brainer" certainly applies to this article. Rather than an enlightened discussion of the pros and cons of passive solar technology, it provides a ridiculously oversimplified description of the concepts, no data on actual projects that work, and a brainless, feel-good conclusion about how wonderful the world would be if we all believed this drivel. Where is the discussion of the poor temperature regulation of passive solar houses? Or any mention of the expensive, complex, and intrusive movable insulation systems that are required to prevent the building from losing more heat during nights and cloudy days than it gains during periods of full sun? In my opinion, passive solar is the "Hybrid SUV" of energy technology, and articles like this have no more benefit to anyone than having them watch commercials for 20mpg hyrid SUVs when what the world needs (and should expect) are vehicles that get 3, 4, or 5 times that.

john abert_2
1/4/2010 11:21:09 AM

Rachel- The same principals as far as insulation apply, except in the floor. We lived in the Phoenix Valley for ten years, and we know that even in the winter, when it can get to low 30's overnight, it is usually back up to 60 in the afternoon. For this application, I would leave out the insulation under the slab, but everything else would remain the same. Natural ground temp is 58 degrees, so make that work for you. The lack of insulation in the floor out there in the winter would be minimal, and you could always put down a room size rug in the winter, if you need it. You could install some 6-inch PVC pipe under the floor (3-4 feet deep) with ends that return back to floor level on each side of a room. Six feet between runs of pipe should work well for any size room. Add a small duct fan at one end of each run (on a theromstat), and they can pull the cooler air from ground level to the room. You can close off the ducts in the winter to keep the cool air from interfering with the heating from above. Since cooler air drops anyway, you could even take the incoming side all the way up through a wall and put the registers up near the ceiling. Also, a ceiling fan would help distribute the air in the room to balance it out. Of course, what was said about shielding the sun during the hot months is just as important. Use awnings on your windows, or an overhang to block the sun from coming in when it is high in the sky, while letting it in during the winter months when it is low.

john abert_2
1/4/2010 10:48:09 AM

Besides the walls, another thing that needs to be mentioned is the fllors themselves. Too many builders simply build a slab and footing in one piece, and that's not enough. It's better to build the footing and foundation separately, and then insulate at least one or both sides of it before backfilling. Then the slab that's poured inside of that has to be insulated from the outer wall (not touching it). Also, a proper vapor barrier and insulation must be laid under the slab to prevent moisture and ground temperature from taking away all the heat. If you don't do that, you've lost most of what you've gained by letting the sun shine on it. John Abert -

rachel _2
1/4/2010 10:40:07 AM

I'd be interested in how this plan would be changed when the issue is not heating but cooling for folks in hot desert climates.

12/17/2009 5:40:19 AM

Good point, Ron.

ron rancourt
12/8/2009 8:34:42 PM

Some good points here about passive solar, but one missing feature of a passive house is the building envelope itself. Insulation is absolutely paramount to retain the collected solar energy. The German Passivhaus standard advocates thick walls, efficient windows and doors to keep the interior comfortable year round.

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