Hydro and PV diversion [RE-wrenches]

[Christopher Freitas --- OutBack Power]

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Windstream Solar-Gary wrote:
> 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?

The Brand X C60 is only 12/24 vdc - so its a no go - but the old C40 
will do 48 vdc systems. 

Using a relay with a 60 amp DC load would be pretty severe - but there 
are other options - such as using a solid state relay instead of the 
relay and setting the time delays on the MX60 to zero - which puts it 
into PWM mode (like the C40 diversion mode).  This of course still 
allows the MX60 to MPPT control the PV array at the same time as it PWMs 
the diversion load since the control of the solid state relay is via the 
AUX OUTPUT.  

I have ordered one of the following solid state relays to test out with 
the MX60's AUX OUTPUT - it looks like just the ticket for adding PWM 
diversion to a system controlled by the MX60.  
 
http://www.hellroaring.com/common.htm  (yes that is the correct link)

> 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.

The MX60 won't fail full-on (or at least we can't imagine a way for it 
to do this without several failures at the same time).  The diversion 
mode for the AUX output is adjustable in tenths of a volt either above 
or below the MX60s regulation setpoint - it then automatically stays 
sync'd with the tempcomp changes, bulk-absorb-float modes and 
equalization.  Pretty nice to have everything work together.  

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

I would suggest an additional back-up control for the diversion load 
system - in case the water heater element or the control failed - this 
can be done with the AUX OUTPUT from the FX inverter and a plain old 
relay (the FX can not be set to PWM the diversion loads like the MX60 - 
but it does have a on-off type diversion mode).  

Also - be sure to have the PT valve on the water heater dump to a safe 
but visible location - I have seen them leak when operated frequently.  
I also know people who put in redundant PT valves with situations like 
this... or use a non pressurized tank with a good sized vent pipe out 
the top.  
 
> 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.

Yes - I am running a MX60 with a Hydro to maximize its DC power output 
and am setting it up to dump the excess to a hot-tub... all in the name 
of science of course - testing can be tough some times... 

Christopher Freitas
OutBack Power Systems, Inc.
cfreitas @ outbackpower.com  
www.outbackpower.com
Arlington WA USA
Tel 360 435 6030  

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