Alt-BEAM Archive

Message #01571



To: John Mitchell johnm@magnet.com
From: Steven Bolt sbolt@xs4all.nl
Date: Wed, 17 Mar 1999 09:24:47 +0100 (CET)
Subject: [alt-beam] Re: `Green Thumb'


On Tue, 16 Mar 1999, John Mitchell wrote:

> Hmm. One possibility would be an Archimedes Screw Pump. That is, you
> have a long rod, with a spiral (helical) trough around the outside,
> covered by a tube. Incline the pump at ' or so, with the bottom in water.
> Turning the screw makes water travel up the inside of the spiral, over the
> top, thence toward your plant. This is mechanically simple.

The Archimedes pump did cross my mind. It has the advantage of
working without valves, so there is only one moving part. But
friction makes it rather inefficient, and of course all this
pumping and reservoiring must not visually disturb the window sill.

So far, the most appropriate pump appears to be the one which uses
two meshing gears to move the water (like a model aircraft fuel
pump).

> I dont know if this could be done beam-style, incrementally. Would the
> screw try to "unwind" and dump the water if not restrained?

As to depending on solar energy stored in caps, that wouldn't allow
reliable pumping. Too much water would leak back along the screw or
between the gears. A membrane pump might get you somewhere, but is
more complicated.

With nothing but caps to store energy, you'd probably be forced to
use a high reservoir or some other means to keep the water under
pressure. Opening and closing a valve is possible even without
large caps; one simple method involves an eccentric roller on the
minute hand shaft of an accelerated clock, to squeeze shut or open
a silicone tube. But it is impossible to guarantee a timely
cut-off. The Green Thumb might open the valve, and then try to
close it just as a cloud takes away the sunlight, resulting in a
drowned plant. The system doesn't fail safe.

So a valve needs long term stored energy to be reliable, and a pump
needs it to pump at all.

Best,

Steve

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# sbolt@xs4all.nl # Steven Bolt # popular science monthly KIJK #
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