Solar bypass load to hot water heater using Victron Cerbo GX

[jeffatoms2]

I am seeking advice and possibly first hand experience designing our new solar installation.  A key feature that excess solar should be automatically bypassed, following battery charging, directly to a 24 volt DC / 600 watt electrical heating element on our Torrid MVS18-2IX2EL Explorer series hot water heater.  I believe the key component is the ability to program a Victron Cerbo GX as a relay controller.

FULL DISCLOSURE: MY WIFE SUE AND I OWN AND OPERATE TORRID MARINE. 

We designed the MVS18-2IX2EL for our personal boat, a 1998 Bavaria 38 Ocean.  It fits wonderfully on top of the centerline black water seacock and to the rear of the saildrive.  It has two independent electrical circuits-ours has a 120 VAC, 1500 watt element powered by shore power, generator or possibly inverter.  It has two independent coolant coils.  The lower one is connected to engine coolant.  The upper one is connected to our Espar hydronic heaters coolant (connected to a "summer loop" that shares the cabin air heating system).

Our solar config consists (so far) of two LG390W NeON panels,  a Victron 150/70 SmarSolar MPPT controller and 2 Lifeline 210AH AGM batteries (core components).

I want to have the system automatically bypass the charging cycle when SOC reaches 100% t which point 24ish volts will be "dumped" into dedicated hot water element (the rest of the boat is 12Vdc.

Thought, ideas, experience?

[jeffatoms2]

Enclosed are a few drawing I have made of our set-up.  Everything is installed and working with the exception of the solar set-up.  We have the panels and the Victron MPPT controller but not yet sure on the relay.  Currently we are thinking the.Victron Cerbo GX has two relay controls that can be programming opposite of each other to act as a switch controlling the on/off of two high amp solenoids.  Another option is to dump Victron and get a Xantrex C60 solar controller that has a DC diversion dump built in.

Again, any experience or advice is greatly appreciated.  We have time to get a good solution as the solar arch is still on the drawing board.

[jeffatoms2]

OK, here are our seven ways to up to 18 gallons of heat hot water on SV Zephyrus, our 38 Ocean...well six for now but we would like to add solar.  Here are the drawings.

[SYJetzt]

No need for an expensive cerbo. A BMV712 Smart can monitor SOC and has a programmable relais for switching. A cerbo alone isn't enough, you need a battery monitoring equipment as well  and a Connection between both.
Topping up a 18gallon hot water tank from 20 to 60 degrees celsius will take approx 3kWh, and based on a 24V system will take 130Ah. This means discharging your AGM's down to a SOC of 40%, which will harm the batteries in the long run. I think the installed battery capacity and solar power is (far) to less or the water tank far too big  for this application .
 

 

[jeffatoms2]

Thank you!!  Geat info that's really helpful. 

One of my stated goals is to never heat water from battery, though possible.  I have found that I can literally "top off" the hot water, thanks to the convective property of hot water, hot water generated at the coil or at the element tends to rise upward towards the outlets location.  I suspect there is some stratification as well.  The only time I might be heating the whole 18 gallons is from pre-departure dock/shore power, while running the engine or from the generator (which I have not needed to do so far). 

My guess is that 30 minutes on the Espar produces somewhere in the range of 3-5 gallons of 125 degree water.  The feedback I have received from other customers with much larger solar arrays is that the 18 gallon unit gets heated sufficiently during a sunny blue water day to provide daily showers for four plus water for cleaning dishes satisfactorily.

Much thanks!