A few more gallons? [RE-wrenches]

[Allan Sindelar]

Gary,
It would probably work as you describe, but it looks kinda iffy. At 20' head
the 9300 draws 1.5A and requires 58W minimum, per the manual. The pump would
run at about 25/32, or around 75%, of what it does now, using a rough ratio
of battery voltage/MPP voltage, so you'd have to increase daily run time by
around 1/3 to get equal gpd. The pump would see less stress because of
slower speed, but more stress from longer run time, and would probably fail
sooner. You would have to match the (additional runtime after dusk until a
voltage-controlled relay disengaged) to the (amount of marginal PV power
needed to charge the batteries over and above what is used by the pump
during the day (huh?)). One problem is that both of these setttings would
change with the length of the solar day. Add that to the changing static
levels and resultant additional amp draw (at 60' it's 2.1A and requires 78W
minimum) and it gets pretty touchy. Your pump would probably not start much
later unless you set a high hysteresis on the voltage-controlled relay
(advisable), as the moment the sun hit the array the voltage would rise,
just because of the voltage characteristics of batteries under charge and
discharge. The ratio of charging current to load current would probably be
high enough to not present voltage chatter problems in the morning, but it
might. Plus this isn't something you could set up without some
monitoring--put a TriMet on it unless your customer is way above average in
PV savvy. That adds to the cost, and a good adjustable voltage-sensitive
switch isn't cheap either. And the labor to set it all up, including a way
to run either PV-direct or battery, makes it pretty dear. Plus getting the
low-level sensors to work with both battery and PV-direct would be another
complication.

I think that unless your client is pretty sharp and attentive, or you add a
greater reserve of PV charging current, it would be lots of tweaking and
some callbacks to get it right--each month. I'd figger out a better way,
like get a better pump or something, like the others said. Good idea, but
the devil is in the details, as usual.

Sometimes it's just fun to wrap your head around an offbeat question like
this--sort of like mental masturbation. Let us know what solution you come
up with. Good luck.
Allan at +E


----- Original Message -----
From: "Gary Higbee" <gary at windstreamsolar.com>

> I installed a basic (SHURflo) submersible pump system for a client a while
> back. The system fit their budget and has worked fine. Their garden and
new
> orchard is expanding, though, and we'd like to squeeze more water from the
> system. I'm considering options for an economical solution.
>
> Here's the current configuration: SHURflo 9300 submersible pump and
> controller with low-water sensors, 2x75 watts PV modules, pump placement
at
> 80 feet. Static level around 20 feet much of the year, dropping somewhat
as
> summer progresses. Well tested at 7gpm. We've got plenty of storage, just
> not as much water out of the hole as we'll want in the summer.
>
> I need to go up the mountain and stick my clamp meter on the pump wire,
but
> believe that in peak sun the pump is using reasonably less than the PV
array
> could provide. We may only be using around 1/2-2/3 of the peak PV
potential,
> since the static water level is fairly high until late summer (we've got a
> nominal 150 watts and are likely using around 70). The idea here would be
to
> use a battery supplement to transfer the peak sun PV excess into later-day
> pumping--essentially broadening out the pumping curve.
>
> I'm contemplating adding deep-cycle batteries, a small charge controller,
> and a low-voltage disconnect. It seems that in principle this idea should
> work, though I know the system suddenly becomes more complex. It seems,
too,
> that the controller LCB wouldn't be doing anything, since the battery
would
> be the current source.
>
> In this scheme I think the system would start pumping later in the day,
> since we'd first be charging the battery. Then it should run at a fairly
> uniform rate until the low-voltage cutoff turned things off. Since there
> would be a lot of cycling in this I'd think we'd want to keep a relatively
> high battery voltage. To reduce annual cycling I'd also probably set up
> something to switch to normal PV-driven pumping in the many months when we
> don't have the demand (taking care to float the batteries). I could also
> consider adding a bit more PV.
>
> Would this be worth it?

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