Hybrid Car – More Fun with Less Gas

Is Efficiency dangerous ? - Page 14

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Posted by Greg Campbell on November 4, 2005, 1:39 am
Solar Flare wrote:

Yes, it's open, or at least vented, to allow for liquid expansion.  The
main tank, or heat exchanger, is sealed and slightly pressurized by the
static water head.  Without pumping, the water in both feed and return
pipes is at the level of the elevated reservior.  When the pump is
activated, water rises just a few feet in the feed pipe (apart from
resistive loss, the only work the pump has to do), traverses the panels,
and splashes down into the E.R.  With the return pipe constantly filled,
pumping effort should be greatly reduced.

Pump on level   >  ======
Pump off level  >  |    U  < Powered and unpowered return level
                    |    |   (drops slightly when pump is running)
                    |    |
                    |    |
                    |    |
Water pumps        |    |
clockwise          p    |

Posted by Solar Flare on November 4, 2005, 2:40 am
This doesn't appear to reduce the head level of the pump at all if I understand
that correctly.

It appears I may as well not have the elevated reservoir at all. It also appears
that the pump would fill the reservoir and spill it over the top.

What am I missing?

Posted by Greg Campbell on November 4, 2005, 4:00 am
 Solar Flare wrote:

The water is still being pumped up 10 feet (or whatever) from the
exchanger/pump, but it is being greatly helped along by the head of
water in the return line.  In effect, the pump's input is pressurized,
reducing the work it must do.

The effective head that the pump must drive is the pressure gradient
across it (Output - Input).  If the panels are 10 ft above the pump, and
the E.R. is 8 ft, the pump only has to generate enough pressure to raise
  the water 2 ft. (ignoring frictional/viscous loses)


It's draining as fast as it's being filled.  The bottom reservior or
heat exchangers are sealed and maintain a constant volume.  For every
gallon that gets pumped throught the panels and then splis/flows into
the E.R., a gallon is sucked from the E.R.

Posted by Iain McClatchie on November 4, 2005, 7:26 am

That's a nice solution.  So how big is this elevated tank, compared to
the reservoir?

Suppose the min to max temp in the store is 80 F (winter, after a week
of storms and the collectors have given up) to 180 F (10 F over max).
I get expansion of 0.016.  So for every 85 gallons in the reservoir I
need 1 gallon in the elevated tank.  Does that sound right to you?

So a 2000 gallon reservoir needs about a 25 gallon elevated tank.  That
seems like a pretty reasonable object to mount high in a closet

Posted by Christian Kaiser on November 4, 2005, 7:43 am
 BTW: nice explanation of drainback systems in


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