Hydro and PV diversion [RE-wrenches]

[Gary Higbee]

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Wrenches,

I'd appreciate some sage advice on a project. Here's the info:

I'm working an upgrade on an existing PV system, adding another 1200 watts
to the existing 1200, bringing a Harris PM online, and converting from a
24-volt DR-based system to a 48-volt Outback (FX2548, PS2DC, MX60), 16
T-105's. The system is off-grid.

We'd like to be able to divert everything--hydro and PV into a hot water
tank, but if we're maxing out the PV array and adding 400 watts of hydro
we're looking at about 2800 watts maximum. It's unlikely, of course, that
we'll see full power out of the array--but we've got to design for these
things, right? I do have two water heater elements rated at 20 amps at 48
volts, but this falls short of what would be needed. I see that AEE has an
element rated at 30 amps at 58 volts. Two of these, then, should be able to
handle up to about 3400 watts. My plan would be to use the MX60 with a relay
to run the elements, dumped into a 50 gallon hot water tank--yes, with a
temp/pressure relief routed to a place we wouldn't mind dumping hot water
now and then. Hmm, how about a thermostat that would change PV array
controller setpoints...

Questions include:

1) Whether the switching of this much current will pull down the modest
battery bank so fast that we'll cycle really fast (even with hysteresis).
Would it be better to go with a C-60, so we regulate the output?

2) If the controller failed this could be bad (boiled batteries followed by
hydro damage from over rpm, I would expect). It seems that it would make
sense to set the MX60 to limit the PV just a bit above the diversion
setpoint, so at least we'd turn off the PV if something failed between the
MX60 and diversion elements. I don't suppose this is really redundancy, but
rather a safety net so long as the MX60 didn't fail full-on.

3) Does this configuration make sense? Is there a better way to do this?

We'd really like to be able to divert all the power we harvest to do
meaningful work, so I'm hopeful some of you have thoughts on this. Why just
turn off the PV when we need hot water? I expect numerous folks would
benefit by a clean, safe design that let us make use of all the power we can
tap.

Thoughts???

Thanks!

Gary


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                     ~ Windstream Solar ~
     Gary Higbee  (gary at windstreamsolar.com)
                     (541 ) 607-1818 (Eugene)
                        (541) 954-3881 (Cell)
Solar, wind, and hydro site analysis and system design
    Components dealer and installation assistance
 Energy Trust of Oregon contracted system inspector
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


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