David Wright: Passive Solar Design
(Page 8 of 16)
September/October 1977
By Travis Brock
Ah, but there's another-and much simpler-way to do the same job. Just set the collector up so that its highest point is at least 18" lower than the lowest point in the exchanger tank.
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Now we've got a built-in, automatic thermosiphon working for us. As the water/ antifreeze mixture is warmed in the collector, it rises. And as it naturally rises, it flows up into the exchanger tank. And as it does so, the colder mixture of water and antifreeze already up in the tank flows down another pipe into the bottom of the collector . . . where it, in turn, is warmed . . . which, in turn, causes it to rise up to the exchanger.
And as long as the collector is hotter than the exchanger tank, this thermosiphon will continue and the water in the whole system will get warmer and warmer until it reaches the maximum temperature that the system is capable of generating. But as soon as the sun goes behind a cloud or the sun goes down and the collector becomes colder than the exchanger tank . . . the automatic siphon will just as automatically shut itself off. The heavier mixture of cold water and antifreeze at the bottom of the collector will stay down there and the lighter hot mixture at the top will stay up at the top and keep the water up there in the heater tank warm.
PLOWBOY: So we've used what we know about the physical properties of hot and cold fluids to design ourselves a very simple, low-cost, passive water heater . . . a water heater that does just what we want it to without the addition of any complicated pumps or valves . . . a solarpowered water heater that works just fine without the addition of any commercial energy.
WRIGHT: Right. And that's exactly what the passive conditioning of a living space is all about. It's about the design and construction of a space so that the natural elements-the natural forces-of the microclimate surrounding it are all that's ever needed to make that space a comfortable place to be.
Now this is kind of a revolutionary idea to most modern designers because we've gotten into the lazy habit of just slapping together any kind of structure and putting it up almost any old place and then pumping in enough commercial energy to run enough air conditioners and space heaters and humidifiers and dehumidifiers to make it a comfortable space to live in. Commercial energy has been cheap and we've gotten into the habit of letting that fact do our thinking for us.
But this idea of passive systems wasn't revolutionary to the more primitive and traditional cultures of this planet at all. Those people didn't have electricity available at the flip of a switch. They didn't have natural gas piped into their homes or fuel oil delivered regularly to their doors. All they had to work with was their local micro-climate-a certain amount of solar fall each year, a certain amount of rain, a certain amount of wind, and so on-and that was it. If you guessed wrong, nobody came around with a tanker full of OPEC oil-at any price-to bail you out.
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