Homestead mailing list

[Bill Jones <billj AT harborside.com>]

Gene GeRue wrote:

> > Bill: Wouldn't the storage tank & 10W
> > pump combination be the more brilliant invention?
> >    
>
> I've extolled the virtues of an elevated tank delivering gravity  
> water for many years. Not everyone has enough elevation to make it  
> sensible. A tank on a wooden platform delivers very low pressure. It  
> takes 2.1 feet elevation to create one psi at the faucet or  
> showerhead. A wooden platform for a water tank must be very strong  
> and sit upon a good footing.
>  
When we go camping we usually use a bag that warms water in the sun.  
The low pressure's not a problem, if the nozzle's overhead.  It just 
doesn't hold much.

People routinely put the hot water storage tank for their solar water 
system up on the roof.  Why not the cold tank too?  We're installing the 
ultimate summer bathroom on the sunny side of the new shed (for bathing 
calves of course ;) ).  For agriculture, we're lucky to have a slope, 
with the well near the top.

There's something else really important to realize.  What's stopping you 
from pressurized the water a second time time to get the water into the 
shower?  Your existing pressure tank can still be used.  The price of 
the pump?  A pneumatic water pump made from PVC costs so little, and for 
such a short lift the "static" components of inefficiency are 
negligible.  Maybe I'll use the small model I just built for this.

> Regarding the compressed air pump, I always think of energy in and  
> energy out. What creates the compressed air? What is the source and  
> what is the cost? 
I'm going to build a windmill.  We have a unique summer wind pattern 
known as the "squamish".  A moderately strong north wind starts at noon 
and ends at sundown on every sunny summer day.  It's brought about by 
the same perpetual offshore high pressure that keeps the rain away in 
summer.  Fortunately the need to water is correlated with sunshine, and 
so is the wind.

Anyway, this all means that a windmill can always face north, may have 
no transmission, and will never accelerate out of control.  So the 
simplest old fan connected via a belt drive to a burnt-out engine with 
one good cylinder will do.  For those less meteorologically blessed, 
this is still the only difficult part, making a windmill that works well 
in all sorts of variable winds.

> I suppose to evaluate the efficiency one would need  
> to compare some other pump cost per thousand gallons, say, and the  
> compressed air pump energy source the same.
>  
It depends on whether you mean slow trickle into a storage tank, or 10 
gallons per minute.  It will be possible to run my new pump at the usual 
high speed, off an electric compressor, but at this rate some energy 
will be lost through frictional heat and heat from sudden compression.  
We could call this the "dynamic" component of inefficiency.  You can't 
calculate this, but most of it can be evaluated from the compressor 
specs (including the inherent "static" loss of the piston pump).  It's 
still hard to imagine that air will have more frictional loss than 
water, which is more viscous, and the amount of water squirting around 
through tubes is about the same as for any method.

This inherent loss of the piston pump depends on the pressure, not flow 
rate, so that's why I like to call it a "static" loss.  This loss would 
still happen even if the device only crept along at an imperceptible 
rate.  We need about 50 PSI for our 100' lift; at this pressure level 
even a bicycle pump is only about 55% efficient.  You may have noticed 
when filling a bike tire that the first part of every stroke is wasted, 
because air doesn't start to flow until the pressure in the cylinder 
exceeds the opposing pressure in the tire.  For a _really_ deep well, 
only the last tiny fraction of the piston stroke would do useful work, 
so for those situations you really need something else, like a 
multi-stage model with one or more reservoirs in the well shaft

The final component of inefficiency arises because of our pump's need to 
disconnect its air tube once per cycle to allow refilling, losing all 
the pressurized air in the tube.  It the valve is above ground, then we 
lose 100' of air tube's worth every cycle.  It just depends on the 
length of tube between the air valve and the pump, so it's a "static" 
source of inefficiency.  But if the pump chamber is big enough, it's 
negligible by comparison.  And none of this applies to pumps situated 
above ground.

So for low flow, which encounters only easily calculated "static" 
sources of inefficiency, we'd appear to be just about at 50%.  The other 
"dynamic" sources of inefficiency kick in only when the wind blows hard, 
and who would complain then?

> My cousin who built the elegant log cabin had as his first water  
> source a twelve-volt pump lifting water to a tank up the hill from a  
> small spring. The pump was energized by a battery charger. So it  
> still required electricity. I don't know if he ever calculated the  
> cost-per-gallon of pumping that way. I do know that he later had a  
> well dug and a normal deep pump installed.
>
>  
I really wanted to have enough solar panels to pump just enough 
agricultural water.  I set the limit for expenditure at $500 for the 
panels.  I think that would buy me about 50W worth.  If I were to try to 
run my existing 1000W impeller pump from a battery and inverter, it 
could still not be on for more than 1/20 (50/1000) of the time that the 
sun shines, assuming 100% battery efficiency.  So for about 3 minutes 
per hour, I'd get 10 gallons per minute from the pump, or about 30 
gallons per hour.  For the whole partly shady day I'd get a couple 
hundred gallons, but it's not enough.

My search for pumps turned up only models that suck water up (good only 
to 30 feet) or that cost $600 or more.  What kind of pump did your 
cousin have?

To me, we've been presented a false trichotomy.  Our apparent choices 
are 1) nothing at all; 2) an electric pump; or 3) a thousand-dollar 
windmill with a $400 rotary-linear motion converter from the Lehmann's 
catalog and a 100' long driveshaft ending in a $200+ solid brass deep 
well pump.

Bill
S. Oregon coast