ewarnerusa wrote:
To me, the cool part about the diversion load idea is that 100% of solar harvest capacity is used throughout the entire solar day.
Our setup is significantly different from yours, but you might be interested in what a fellow traveler did to use excess solar to make hot water. We have 1,050 watts of panels and six 6-volt golf cart batteries. Not huge by today's standards, but it was considered a lot of solar when we installed it in 2005. And since we still have a Norcold and not a residential refrigerator, it's still enough.
Our motorhome came with a gas/electric water heater that was installed on a non-inverter circuit. We moved it to the inverter circuit, so it would work on the batteries.
(And actually, we tested everything before moving the water heater breaker by necking down the shore power cord to a 15-amp plug, and plugging it into one of the 15-amp plugs on the coach. That meant the water heater "saw" shore power and would work, but the shore power was actually coming from the batteries. It looked very strange, having the shore power cord go to an outlet on the side of the coach (I can only imagine what our neighbors thought), but was easier than moving the breaker only to find out the whole plan was bogus.)
The basic idea was to rewire the AC power to the water heater through a solid state relay that comes ON when the battery voltage is at float level, and turns OFF when it drops below that, which happens almost instantaneously once the water heater begins to draw current. So the water heater is on for just a fraction of a second, and the battery voltage recovers just as quickly, and then it does the ON/OFF thing again.
This happens dozens, even hundreds, of times a second. The fraction of time ON versus OFF tracks the amount of incoming solar power available constantly, via a process called pulse width modulation ("PWM"). And since the water heater is a simple resistive load, it doesn't mind being rapidly pulsed like that.
Best of all, we put in a little LED light that flashes when the water heater is going on and off, and you'll probably appreciate how much fun it is to watch that light flash like mad. And it really does heat the water, to the point that we've boondocked for weeks at a time not running the generator and not using any propane for the water heater, taking daily showers.
We currently have a Midnite Classic controller, which has an AUX output and a high-speed FLOAT-tracking PWM mode that works great for this purpose, by connecting the controller's AUX output to the high-current solid state relay that switches the water heater's AC on or off.
We originally had an Outback MX-60 controller. We did get this scheme to work with that controller, but it was kind of tricky and took some monitoring that's not needed with the Midnight, which is basically "set and forget." However, with the Midnight's "set and forget," it does it only when the batteries are in float mode, and not when they're in absorb, when there's also excess solar that could be used.
So with the current controller, we're not using 100% of the possible solar, because it doesn't get diverted while the batteries are in absorb stage. Wringing out every electron with the old controller had its charms, but really, this is all just icing on the cake, so I've decided I'm okay with sacrificing some output.
