Waste solar to hot water tank

[admiralslater]

I have heard some discussion of using waste solar to heat water , also some rumours of dual voltage elements. I wnder if it would be better to run the hot water off the batteries through the inverter. This assumes that inverters etc are the correct size .
What is the latest thinking on this

[mdenize]

I'm doing exactly that right now... beautiful sunny day. 3.5kW of Solar Array 1800W hot water element plenty to burn so none from the battery. We turn it on and off manually because we have way more load than the inverter can handle (3.8kW continuous) but don't bother just for a short term load like a kettle as it can do 10kW peak.

Go for it!

Mark

[mdenize]

P.s.the reason we don't muck around with a low voltage DC element like some have done is that our system can handle the AC load without breaking a sweat and even our 275l tank would overheat without a good thermostat which is so much easier to do at 230v

[OldGreyB]

Our boat has 1.4kW of solar, 840Ah of AGM and 3kW Victron inverter. Our Solar Hot Water cycle system is activated by a manual impulse switch located on our auxiliary electrical panel. This action turns on a digital timer set for 54 minutes to allow heating the water via the inverter and battery system. The time period is more than sufficient for our thermostatically controlled 1.2kW element to heat the 40L of water, and means we don't have to think about battery status or water once we toggle it on. We typically toggle the circuit on when batteries are greater than 85%SOC, as by then the MTTP solar controllers are backing off their ampere feed as the batteries head towards float. The "Solar Hot Water cycle on action" could be made automatic, but as we are not full time liveaboards it is not required in our situation.

The control circuit also has a 12Vdc Victron battery under voltage protection module switching a relay to disconnect the 230Vac to the Hot Water cylinder if battery under-voltage is sensed during the Solar Hot Water cycle period. This automatically resets to on again when the battery voltage returns during the Solar Hot Water cycle period. This protection will only trigger if the galley 230Vac usage via the inverter is high (causing House Battery Volt-drop) during the period the Solar Hot Water cycle is operating. The whole system works wonderfully well - no need for any noisy generator.

[chris mac]

I swapped the heating element out for a 300watt 12 volt. Which means a 25 amp draw on the batteries.
When our mppt (victron) hits absorbtion mode, we turn on the water heater for a 2-3 hours.
We use golf cart batteries, so I didn't want to be pulling high amp draws.
I do have the relay needed to set it up thru the mppt, I just haven't got around to it yet. Too many more important things to fix[emoji16]

[LoudMusic]

Originally Posted by mdenize

I'm doing exactly that right now... beautiful sunny day. 3.5kW of Solar Array 1800W hot water element plenty to burn so none from the battery. We turn it on and off manually because we have way more load than the inverter can handle (3.8kW continuous) but don't bother just for a short term load like a kettle as it can do 10kW peak.

Go for it!

Mark

I did it manually as well. We had too many systems on board competing for battery amps or inverter watts to allow that to be automated. But on a bright sunny day it's pretty easy to keep an eye on it and flick on the water heater as needed.

The thing I'd change is having multiple elements in the water heater tank. That way I could vary the consumption if needed. Likely not frequently different, but having the option would have been good. Perhaps on a future boat with 48vDC I could do a 15A DC element and a 15A AC element. Needs more research, but seems like fun, and a good way to use that excess solar without putting more cycles in the battery.

[moseriw]

I would not waste my current for a water heater or electric cooking. (Turn on the engine for 10 Minutes and your boiler is hot)

OK, if one is able to mount 10 m² of solar panels it will be OK but it may not even possible on a trawler and certainly not on a sailing boat. Maybe in 2-3 years we will get panels with 45+ % efficiency. By then we can discuss it,

[dougweibel]

Originally Posted by moseriw

I would not waste my current for a water heater or electric cooking. (Turn on the engine for 10 Minutes and your boiler is hot)

OK, if one is able to mount 10 m² of solar panels it will be OK but it may not even possible on a trawler and certainly not on a sailing boat. Maybe in 2-3 years we will get panels with 45+ % efficiency. By then we can discuss it,

The point is that many have excess solar capacity on days with full sun, without 10 m2 of solar panels. Our panels are more like 3 m2 and are sized to meet our everyday needs with partially cloudy skies. Unless we have done something unusual overnight our house bank is usually back at 100% by noon on a clear sunny day. The rest of that day our solar capacity goes to waste. If there were an easy way to dump that available power into the hot water tank then why not. We need to do some rewiring to do that. It is on my to do list. In the mean time we use sunny days to bake bread with the convection oven instead of propane, or run the laundry machine.

[moseriw]

The point is that many have excess solar capacity on days with full sun, without 10 m2 of solar panels. Our panels are more like 3 m2 and are sized to meet our everyday needs with partially cloudy skies.


I have 2,8 m² 420 Wp monocrystallines with 21% effectivity and my harvest is max 19A x 10 hrs. That is usually a minimum of 110Ah a day. My load is around 12A while sailing that is 240Ah a day, Sooo?

[rotrax]

Originally Posted by moseriw

Turn on the engine for 10 Minutes and your boiler is hot

Not with our boat. At least an hour at idle on the hook or alongside. Half an hour if under load, motoring or against the springs.


Ten minutes is wildly optimistic in my direct experience of living on the hook five months each year in a temperate climate.

[JustMurph]

I do this. 1.7kW of solar, 5kW inverter and a 750W 240V 40L hot water system. This is all controlled by my Victron Cerbo. It waits until the batteries are 90% full and then turns the HW on if there is more than 750W going into the batteries (that way I'm not drawing from the batteries at all). If I turn any larger loads on the HW turns off automatically.

I have an override so that I can turn it on at any time, e.g. it's been a cloudy day, batteries are below 90%, but I know it'll be sunny the next day, so I'm happy to run the batteries down.

One day I'll learn how to use Node Red on the Cerbo so I can program in some more complex control.

Btw, if I could get a 24V DC element for my isotemp HW system I would go that way...

[sailorboy1]

I just turn on the inverter and water heater for 20 minutes. That "wastes" solar without messing with anything else.

[moseriw]

Depends a lot on the heater. There are 40 Litre heaters mit 500 W or even 4200W so
500 W / 12V = 42A and 4200 W / 12V = 350A


at 20 Minutes it is 14A vs. 117A and yes one can calculate it this way as you run off the inverter.
14A is certainly no problem but 117A is an overkill.

btw. I don't think one can feed the heater with waste energy of 1 to X Amps as the heating coil needs a certain amount of current which the inverter will draw from the battery and nowhere else from.

[gregabbiss]

We have about 2.4kw of solar and also, during summer , manually turn on the water heater around midday most days.

We do it manually as we don't break so often and are hugely programmable. We haven't bothered with the 12V option because it just adds complication and we didn't need it.

P.S. we had it automatically engaged at a certain % of battery charge but found that this was not always ideal, i.e. don't really want heater coming on when your at 95% SOC and its 4pm and sun is starting to fade :-). Again, we are a much more programmable option and its not exactly difficult!!!

[JustMurph]

That works well if you're at home during the day (and remember!).

I'm back at work, so I'm not at home during the day. It's nice to have an automated system that just looks after itself. It's been absolutely faultless the last two years since I installed it.