Diversion / Opportunity Load

[kitestrings]

Greetings,

Haven't had a chance to contribute much in recent times, but I've enjoyed following many of the stories posted.  Our wind project has gotten delayed due to other priorities some of which are self-evident in the photo below (that & raisin' young'ns).

A couple years ago I bought a small Marathon water heater through a silent auction where I work.  I finally got around to changing the port bushings and completing the wiring for use as an "opportunity load", as described more in the link below:

http://www.outbackpower.com/forum/viewtopic.php?f=29&t=2865&st=0&sk=t&sd=a&sid=d



cd929c9eb4286ca7289c42405b0267b&start=25

It is a slightly different approach to dump load charging for wind.  The main difference is it really is looking at leveraging more PV potential.  The controller connects the opportunity load to the battery bank and sustains it when it can by increasing array current, but the charge controller is still operating as the main charge regulation for the bank.





The basic concept is to send surplus power, once the batteries are satisfied, to a diversion load via a quasi-PWM signal.  In this case the signal is 12VDC supplied by the Outback (MX60).  The elements are 48VDC, interlocked and controlled via solid state relays.  I'd been experimenting with it with a timer, and some manual intervention for a spell.  We definitely record much higher kWh production with the diversion load.  It's connected like this.









The two SS relays share one large heat sink.  The left one controls the upper element; the right one is the lower.





The green indicator light on the side of the controller is an LED that I added to the "aux diversion" output of the MX60.  When it is on, we're diverting energy to the water heater assuming the thermostat(s) is calling for heat.  The relays also have LEDs, but they're out of site in the basement.  In an effort to compensate for the tanks modest size, we're heating the tank to ~170F, and then supplying preheated water via a mixing valve to our instantaneous dhw heater (which modulates to meet the outlet setting).

Last night our water tank was ~160 degF.  I've left the mixing valve at the factory setting, which is fairly low (~106F I think).  So after our normal activities last night (dishes, clean up ,etc.), and two showers this AM, the in-coming water was still 90F.  I've tweaked up the element temperature settings a bit more today, and anxiously looking to see how things go this week.

On a day like today (clear, sunny, and no one's home), it should allow us to squirrel away the equivalent of another 6-8 kWh, or roughly 25,000 Btus.





What I've also noticed is that there are conditions where the solar output is less than the diversion load, but the wind - we're still operating our trusty Scencenbaugh - is contributing enough to pulse the relay(s), so indirectly we're getting a bit more usable power from that as well.

Kind regards,

~kitestrings

[dbcollen]

Not sure how you have the thermostats wired, or if you are still using them. I use the seperated thermostats to cut power in the signal to the SSRs (so the snapdisks are switching 12vdc low current, not the 20A 59VDC load)I have a 1200W air diversion load set to trigger at .2V higher than the water heater. that way I don't trigger the T&P valve on the water heater due to overheat, and always have at least 1200 watts of diversion capacity.

Dustin

[dbcollen]

Now that I looked at your schematic I see you are switching the High current DC. That is a really bad Idea, the snapdisks are rated for Ac current which has a zero crossing every 120th of a second so arcs are by nature self quenching, DC has no zero crossing, and if you happen to sustain an arc you will likely burn the house down.  Other than that small detail the system looks real nice. I like the way you hung the heatsink from the cold water pipe.

Dustin

[tecker]

That's a radical pitch on the roof . There was some clinching of teeth on that install .Very nice . Are there restrictions on tower installations there ?

[BigBreaker]

SSRs use MOSFETs not contactors to switch current.  There is no arc.

[Ungrounded Lightning Rod]

If you don't have a fan forcing the circulation the fins on the heatsink should be vertical.  That way convection is encouraged and the cooling is MUCH greater.

There also shouldn't be anything blocking the airflow at the ends of the fins.

[neilho]

Dustin wrote:

-the snapdisks are rated for Ac current...-

It's not evident at first glance (It took me a while to figure it out), but the shot with the schematics shows the standard connections for the water heater on the left and the as-connected diagram on the right. The thermostats are switching a 12V control signal from the charge controller and the SSRs are switching current to the elements.

Just curious- Mad River as in Mad River, VT? (I'm in Vershire doing wind and Kitestrings is in the Northeast Kingdom.)

Neil

[kitestrings]

Thanks for comments.

Yes, Neil's got it right - the water heater discs are only switching the control circuit to the relays.  The relays are rated for 40A at 0-200VDC:

 http://www.futurlec.com/Relays/SSRDC200V40A.shtml

The heatsink is finned on two sides, and the crawl space is cool year-round, but URL's comment is good.  I should consider rotating the fins 90-degrees.

I'm still fiddling with the MX settings, currently:

Relative volts 0

Hysteresis 0

Hold 7.5 seconds (time below target V before controller turns off aux-div)

Delay 3 seconds (time at, or above target V before controller turns on aux-div)

We've only had things connected for about a week now, but so far so good.  Our peak days are usually 12-15 kWh.  April is looking good.

It took three flights of staging to reach the highest sections (and roof brackets beyond that), but the shingles had to be done anyway so there was some efficiency with doing both at once.  It also made the mounts much cleaner.

There are no height restrictions (or zoning for that matter) in our town as yet.  The Scencenbaugh is on a 100' Rohn 25G.

kitestrings

[cjbotkin]

Hello,

This is a very nice setup with useful and practical solutions.  Love it!  I also have an MX-60 and 2200 watts of PV with 'opportunity' to harvest excess.  In your schematic of the water heater - I'm sure you are connecting the 12 vdc MX-60 AUX to the SSRs...  But, what is the Orange + and - at the top of the water heater thermostat?  Is this also the 12 vdc control line?  If so - is that working OK on the 240 vac relay?  Thanks for the great info.

[kitestrings]

CJ,

I realized after my post that I created some confusion by not labeling my sketch better.  The left side is the stock 240 VAC conventional wiring diagram as it appeared in the water heater manual.  The left side (with all the colors) is my "as-wired" diagram.  The orange is the 12V(+) control signal from the MX-60.  The stock thermosatats switch the control to the DC SSRs and they in turn switch the 48V elements.

I'm happy to report it is working very well.  Our April production was actually higher then last July since we installed the water pre-heater.  Here's my crib notes of the physical connections if that helps:

regards,  ~kitestrings