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High pressure pump for water storage
There are products on the market by Dankoff
and Grundfos for
pumping water at high elevations and targeted for solar.
I have a friend with a need for such a pump and thought
I'd take on the challenge. In his case there will be a
mile of line and about 550 of elevation to deal with. To
keep cost down it is desirable to use 1/2" line. As long
as we stay below a gallon a minute there is no real
issue here. From this
calculator
Liquid Friction Pressure Loss
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Pressure Loss
(psi): 27.17 Head
Loss (ft): 62.8
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Line Number:
Date: 7/7/2016
Nominal Pipe Size: 0.5
Pipe Schedule: SCH 40
Flow Rate (gpm): 1
Viscosity (cP): 1
Specific Gravity (water=1): 1
Temperature (F): 70
Pipe Roughness (ft): 0.000016
Actual Pipe ID (in.): 0.622
Fluid Velocity (ft/sec): 1.06
Reynolds Number: 5085
Flow Region: Turbulent
Friction Factor: 0.038
Overall K: 3623.12
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Piping Length (ft): 5000
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About a 10% cost in energy over the cost to pump the
water. And the pump that would just squeeze by from
Dankoff is a model #1408. The specs are at this
link. What is interesting is that at 560 feet and
1.67 gpm this pump demands 435 watts. A foot-pound of
energy per minute is equivalent to 0.0226 watts.
560 ft. * 8.35 lbs. * .0226 = 106 watts. That makes
this pump ~40% efficient. Not great. I don't know
if they just way under spec the pump or there is really
that much friction. I don't know what style the pump is,
they don't say. I'd guess rotary vane from the looks of
it. If we want to keep the flow down to 1 gpm, then we
would have to pulse width modulate the Dankoff pump. The
loss goes up at about the square of the flow and one
gpm meets our tolerable loss.
But what really hurts with this kind of efficiency is
the added cost of solar panels. If our pump really turns
out to be more than twice as efficient it will mean as
much as a $1000 savings in required panels. It works out
that way because there may only be some 25% utilization
of the panels in the winter. A battery will be charged
and would handle a majority of the demand.
Our Pump
Yes, this is a PM treadmill motor and a pressure washer
pump, belt driven. The dark fitting is a pressure relief
valve that is set at 500 lbs/in2. That is
equivalent to 1160 feet of head. I cranked it up until it
was delivering a gallon per minute, pressure meter steady
at 500 lbs/in2. The motor required 40 volts and
loaded the source at 6.75 amps, 270 watts. These
measurements were taken with a pure DC supply.
1160 ft. * 8.35 lbs. * .0226 = 218 watts.
80% efficient, that is very satisfying. And it ran like
there was no load at all, very smooth and quiet. The
transmission losses and other friction won't decline
linearly with a lower head, so the efficiency won't be
that good at 550 feet of head.
OK, so we can't drive this from solar panels and/or
batteries. At these power levels 12 volts would be nice.
So we need power conversion...
The Power Supply
On the right is a salvaged 12 volt DC to 120 volt AC 1000
watt converter. I got it because it would shut down with
just a few watts of load. The primary, (above the red
outline), was fine putting out some 140 volts DC. It was
the secondary that was faulting. I rewired the secondary
so that the MOSFETs could pulse width modulate the motor.
The circuit on the lower left of the inverter, inside the
red outline, was the output driver and no longer part of
the circuit. The two boards to the left are the new power
and driver circuit for the output. The breadboard to the
far left is supporting an MPS430F2012, the new brain for
the output. With a microprocessor, the likes of
acceleration, rate of flow, possible faults, etc., can all
be easily dealt with. I've tested this with a load on the
pump and it works great. I didn't take numbers yet to see
what kind of efficiency I was getting, but soon to be
done.
Expected MTBF and/or regular maintenance, unknown. But
when you can replace the pump for less than $100, there
may be no concern. Another acquaintance recently bought a
Dankoff #1408. It was over $1200 in the door. And it
didn't have a brain.
That is it for now. I'll keep posting as we progress.
Everything I'm doing here will be open source.
Thanks, Dan.
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Electronics All Rights Reserved.
Page created: 7
jul 16 rev: 8 jul 16
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