Surplus pv power use for heating water?

[gramos]

I have just installed my PV system and am now trying to figure out the most efficient way to utilise excess power to heat water,.
As we are in a sunny place I calculated there will be surplus energy after the 3x 200 life p04 A batteries are charged .
I dont want a solar thermal system for heating water because of the various issues with these , I have seen burst pipes/ tanks/ rusty panels etc , etc.
So want use the inverted power direct to a 230v hot water tank element .
II hope to have the batteries charged a.m. and use the remaining solar to heat water and run A/C if needed .

Is it simply a case of running straight from the house 230v circuit board ?
Would it be better to run a separate feed / surplus , direct to the tank element ?

28x 400 w panels run in 4x strings of 7 in series to 2x Growatt 5000 inverters and 3x 200ah batteries .

[Ampster]

I have just installed my PV system and am now trying to figure out the most efficient way to utilise excess power to heat water,.
As we are in a sunny place I calculated there will be surplus energy after the 3x 200 life p04 A batteries are charged .
.....
Is it simply a case of running straight from the house 230v circuit board ?
Would it be better to run a separate feed / surplus , direct to the tank element ?

The most efficient way to heat water with electricity is with a heat pump water heater. It would require a separate circuit. Are you grid tied or off grid? Where are you located?

[gramos]

I looked at a ground loop but the substrate is solid rock , not enough depth , less than 500 mm .

I looked at ASHP too but the electric consumption is too high for this off grid system .
there is no grid near us , which is why the system is on the large side .

There is a 6.5kva generator in the back ground if needed,
We are in Kefalonia , Ionian Islands , Greece , plenty of sun hours throughout the year ,
this points me toward using the solar surplus for water heating .
The house is close to Passiv Haus insulation standards , so heating demand is negligible,
so its just the water heating that I am searching for the best solution.

[LucMan]

A sanco2 air to water HP only draws 5 amps look into them.

[Ampster]

......

I looked at ASHP too but the electric consumption is too high for this off grid system
.......
so its just the water heating that I am searching for the best solution.

The best and most efficient solution is ASHP. It is at least 3 to 4 times more efficient to move heat into water than to heat it with a 4kW electric water heater. The initial cost is more is more but the payback is worth saving wear and tear on your batteries and the rest of the system. In addition you will not have to wait for your batteries to be charged in order to heat water.

[Mike90250]

Solar panels produce DC voltage, and the thermostats and switches in water heaters are designed to use AC voltage. DC at 200V will nearly instantly arc weld the AC contacts upon the first switching event. It is difficult to locate DC rated thermostats.

[gramos]

Mike I was looking at using my surplus AC power from the Growatt inverters , not DC from the PV array .
This will be fed from the main house 230v board .

Ampster , thanks , I have looked at ASHP too , but as I have a large PV array , wanted to use the surplus to heat water ,
this saves the cost of an ASHP unit , both purchase and kw.

Another thought is to set the DHW tank timer to run only during peak sun hours , thereby acting as a power minimiser ?

[Mike90250]

If running from AC, then the thermostats should be fine. A timer for the "good solar" hours in your area would work OK.

[PNPmacnab]

The eternal question of how to heat water from the array with DC. It is beyond me why this isn't common. Here is my garage laundry which takes any excess power the charge controller doesn't use, be it 5W or 500W. The house has its own tank and this only gets the excess. Enough power so I can run all laundry with hot water going into the cold inlet. Resistance heating isn't so bad when only using power that would not be used and the cost is so darn cheap. This has arc interrupt so existing thermostats can be used with pulsed DC. This is the only water heater you will see with jumper cables. As a bonus the pulsed power works pretty good reviving sulfated batteries. This graph demonstrates how momentary peaks of power can be diverted to heating water. The LG washer works off a modified MSW inverter with no battery from raw PV voltage.

[J.P.M.]

The eternal question of how to heat water from the array with DC. It is beyond me why this isn't common. Here is my garage laundry which takes any excess power the charge controller doesn't use, be it 5W or 500W. The house has its own tank and this only gets the excess. Enough power so I can run all laundry with hot water going into the cold inlet. Resistance heating isn't so bad when only using power that would not be used and the cost is so darn cheap. This has arc interrupt so existing thermostats can be used with pulsed DC. This is the only water heater you will see with jumper cables. As a bonus the pulsed power works pretty good reviving sulfated batteries. This graph demonstrates how momentary peaks of power can be diverted to heating water. The LG washer works off a modified MSW inverter with no battery from raw PV voltage.

Not a knock as you seem pretty knowledgeable, but one reason might be that it sounds a little like redneck engineering that most folks doing laundry are clueless about and even if they did, probably don't have high on their bucket list.

Another reason might be that not everyone heats water with electricity.

But for those who heat water with electricity under a T.O.U. tariff, I do kind of like Gramos' idea of a timer as sort of a back door method to game the system. Still, for warm(er) climates, a HPWH and a timer might be workable as well. Both would be better yet.

[nomadh]

It's crazy to heat water w electricity....... unless it's FREE ELECTRICITY! That's a great idea. So you're pulling it from the daytime solar production after the inverter but before the batteries? Sounds perfect. The only trick is guessing how much to pull for water heating but leaving enough to make sure you max out the batteries during the day. Maybe a basic daytime timer in line with a cutoff switch than can sense your few cloudy days?

[Ampster]

It's crazy to heat water w electricity.......

Yes it is much more efficient to use electricity to move heat into a tank of water. As mentioned earlier, in the right climates HPWHs can make sense. Then there should be no worries about heating the water and charging the batteries.

[PNPmacnab]

Although not intended that way, I'll take "redneck" as a complement. The goal of any designer should be to make technical things seem simple and obvious. I seamlessly move energy around to where it can be used automatically. The whole washer thing was to demonstrate the concept of using energy at the time `it is created with minimal cost. So for $500 I got 1KW and everything needed to do laundry free including the hot water. I even included the cost of the two year old LG washer in that since I hauled off a perfectly good washing machine so I could use the LG with electronic drive without large current surges. I have three hours in the morning when I can do up to three loads of wash from the east facing panels. Since I air dry we only do laundry on sunny days. I suppose I should include the money I paid a neighbor to have his rotted tree cut down professionally. That was the best money I ever spent. Extreme shade forces me to even put panels not on my property.

I take power from the PV array DC which eliminates the need for larger chare controller, inverter and battery. Hust sent a module to MN where a camp has a 700W array and 12V battery. He will now be able to heat a standard 6 gallon tank with excess power without expanding his current system.

[J.P.M.]

Although not intended that way, I'll take "redneck" as a complement. The goal of any designer should be to make technical things seem simple and obvious. I seamlessly move energy around to where it can be used automatically. The whole washer thing was to demonstrate the concept of using energy at the time `it is created with minimal cost. So for $500 I got 1KW and everything needed to do laundry free including the hot water. I even included the cost of the two year old LG washer in that since I hauled off a perfectly good washing machine so I could use the LG with electronic drive without large current surges. I have three hours in the morning when I can do up to three loads of wash from the east facing panels. Since I air dry we only do laundry on sunny days. I suppose I should include the money I paid a neighbor to have his rotted tree cut down professionally. That was the best money I ever spent. Extreme shade forces me to even put panels not on my property.

I take power from the PV array DC which eliminates the need for larger chare controller, inverter and battery. Hust sent a module to MN where a camp has a 700W array and 12V battery. He will now be able to heat a standard 6 gallon tank with excess power without expanding his current system.

Not meant as a criticism. Truth to tell, I practice it myself around the house when/where safety isn't an issue. I've known the term as long as I've practiced engineering. It's a term of semi-respect for a style of engineering used by folks who can do what you describe as a goal. The "semi" part comes in when safety, good design and long term fit for purpose goals can and often do take a back seat to expedience. Another way I've had it explained to me (by a mustang P.E. engineer BTW) was that redneck engineering is often better than the mostly european method than stresses form over results.

Hell, if it weren't for redneck engineering, engineering and building safety codes wouldn't need to exist.

[LucMan]

Nothing wrong with redneck engineering as long as safety precautions are adhered to. Dangerous situations arise quickly enough with water and electricity even when safety requirements are followed. But looking at those pictures curls my toes. Just had a friend get out of the hospital after getting scalded over 20% of his body from lifting the blow off with his shoulder while standing up from the floor. No tube extension on the blowoff, scalding water hit him in the face, neck and side. His face and neck look like cooked bacon! **** happens!