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Author Topic: Easy PV water heating  (Read 412 times)

dieselspanner

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Easy PV water heating
« on: March 25, 2022, 06:08:00 PM »
Hi All
As posted else where I'm well into the off grid barn project, anything mechanical isn't a problem, but if it has anything electrically challenging I get to my limits in very short order!

Here's the first situation I could do with some input on.

At present I have a wood fired oven with a back boiler (a converted 1980's oil burning Rayburn) which supplies a 20mm (3/4'') coil in a very well insulated 110 litre (20 gallon) domestic hot water tank, before going on to feed 2 towel radiators a large cast iron radiator and underfloor heating. It works surprisingly well with thermo syphon, tho' I have built in a pump, just in case.

It gets the water up to 45 / 50 degrees c (110 / 125f)  from around 25c (left over from the night before) with no trouble and warms the rest of the barn too.

The cylinder has 3 standard (2 1/4 BSP) 1000w 220v immersion heaters fitted - as I had the tank custom made I had three immersion heater fittings and a few extra 3/4 BSP fittings for temperature gauges ect put in, so that I have the choice of heating water by PV panels, the generator or a hydro genny without having to have any change over switch gear. It also gives a large amount of redundancy.

 I did mention that I try to avoid anything complicated in the electronic department, didn't I? (It was only by being a member here that I discovered Mosfet wasn't a resort on the Red Sea)

A couple of weeks ago I did a bit of a trial and an hours run with a 3kva genny supplying one 1000w heater, it raised the temperature in the tank (at the very top) from 25 to 35 degrees c in an hour.

As the days are getting longer and warmer I'll be using the stove less and don't have time to start on a little hydro plant yet, I wondered if there is an easy way of connecting solar panels to one of the immersion heaters and dumping everything into heating the water. A system where a temperature gauge in the hot water tank switched the output into the batteries would be good. As of today we're getting in excess of eight hours sun a day, The barn is in a north / south valley and we loose a bit at each end of the day behind the ridges either side, we get quite a lot of sunny days and the air up here is clear too.

For 220v power I have an HGI remote work station genset I picked up (unused) off eBay which has a 3 kw Hatz single cylinder diesel charging four deep cycle batteries (in parallel) through a Victron Multiplus 3000 inverter charger, which all works automatically, as soon as the voltage in the batteries drop below the preset.

No, I didn't design, programme or set it up in any way, I just plugged all the connections in, added batteries and filled the tank, honest!

It would be nice to get a little  solar power into the batteries but I'd rather heat the water first (I'm only burning around 1.2 litres / 3 pints of diesel (with 20% wvo) a day, depending on power tool usage)

The one small issue I have encountered so far is that the Victron is so sensitive that it doesn't like to power the immersion elements, all three have a very tiny earth leak and and this causes the Victron to shut down automatically. I did the trial with a 1960's Villers powered genset that didn't object at all. I checked with the heater manufactures and the tech guy was most helpful, and said he's not surprised and that under mains fed circumstances the 'leak' would not be enough to trip an RCD. This leaves me to believe a totally separate circuit would be better.

I don't object to changing the immersion heater to a 12v model, I made a few enquiries and they seem to be available, with adaptors to 2 1/4'' BSP

So, as you can see I'm trying to keep everything simple, Any suggestions on size of panels,  controlers, size of cables and anything else I don't know about is more than welcome, I can probably stand a little abuse too!

Thanks in expectation....

Cheers
Stef

Tighten 'til it strips, weld nut to chassis, peen stud, adjust with angle grinder.

BruceM

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Re: Easy PV water heating
« Reply #1 on: March 26, 2022, 11:08:58 PM »
Hi Stef,
It seems unlikely to me that the Victron is more sensitive than an RCD breaker (50 or 100 ma of current to earth as trip point) , and more likely that you have some bad or cracked heating elements.  A brief overheat (air exposed element or sediment encasement) can cause this, or it could have been a mfg screwup.

https://jestineyong.com/earth-leaking-heating-element/

So something is off with your setup, or the Victron has a problem, which shouldn't be ignored either.  You didn't mention a model number so I couldn't search the manual for the specified RCD/GFCI trip current.

You can get a clamp on amp meters that will read milliamps, so could directly capture the peak leakage current. In this case it would go around both hot wires 2together, so the element current is self canceled and you only read the leakage current.  It's a useful tool to have.  I have a $50 chinesium clamp on amp meter  that does both AC and DC (true RMS) current sensing and I find that doubly useful, as you can shift to measuring DC current your PV, charge controller, or battery draw.

https://www.amazon.com/gp/product/B07P5QKQ5L/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

If you do decide to go 12V, it will be a challenge to get large enough cable and wire to the elements,  The usual  snap disk thermostats are NOT rated for DC current, but some are.  If you use an AC snap disk for DC, yous must keep DC current to microamps, which is readily doable via transistor.  I can help with this but I'm not a fan of high current 12V due to the fire hose wire/terminations required.

Best Wishes,
Bruce M






mihit

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Re: Easy PV water heating
« Reply #2 on: March 27, 2022, 06:54:45 AM »
The most efficient solar water heating is vacuum tubes. These can run on thermosiphon, or pumped systems. The biggest thing would be either running an open system, or having a suitable over-pressure relief (they get seriously hot - boiling off hot)

Generally found more in charge controllers for wind turbines but "dump" or "diversion" loads/modes are an option. (Usually same thing: a heating element, which provides a resistive load and also brakes the wind turbine so it doesn't overspeed)

You could probably find a controller that would take your panels' input, wire the heater via a thermostat as the "main/ battery" and your batteries as the "diversion load"
(Though personally I'd do it the other way around)
I'd go for higher DC voltage, 24 or 48 or even more. This will mean you can use smaller gauge wiring for the same cable lengths, compared to 12V.

mike90045

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Re: Easy PV water heating
« Reply #3 on: March 27, 2022, 06:26:30 PM »
The most efficient solar water heating is vacuum tubes. ........

Yes, but the 5+ year failure rate on the tubes is abysmal.  I see way too many folks looking for replacement tubes, developing schemes to recharge the tubes with acetone or something ( I love sweat soldering a tube with acetone in it ) to even want to bother with any of the vacuum tube systems.   Often, many tubes have failed, but the rest of them keep the system going for a while.

dieselspanner

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Re: Easy PV water heating
« Reply #4 on: March 28, 2022, 10:51:31 PM »
Evening Gents

Thanks for the input, it's been helpful certainly.

I'm not sure how to reply, so I'll just womble through it all....

Bruce, I hadn't considered the ramifications of 12v dc, so that's out.

The Tech guy from Tesla UK (nothing at all to do with Elon) who supplied the elements, admitted that the earth leakage was common but liveable with, I didn't put power on them until they were in the cylinder and the it was full of water. prior to that they were in the original packaging and dry stored. Any damage, as you say, must me a manufacturing issue.

The Victron unit is a 12 volt 3000 va 120 amp, well that's what it says on the box.

I've got a clamp meter somewhere, I'll try and look it out, it's either in my other barn, about a mile down the hill or in my other Land Rover, in the UK ( I abused it really badly last year, setting this place up, so I ran it back to the UK where a mate with a dedicated Land Rover attitude is going through it so I can overwork it again)

The clip you recommended was quite apposite, i I'm a bit wiser now.

I've sent off for the Victron interface which a mate assures me he can make sense of, I've looked at a couple of YouTube clips and it looks like a worthwhile exercise.

Mike and Mihit,

The layout or the barn doesn't suit the use of a diy solar water heater, the wet stuff is on the north side and the sun isn't so i'd have to pump the hot water around the long way, maybe if it had been built with a bit more forethought i could have fitted it in, but the way the project has grown its a bit late.

I don't need the amount of hot water a vacuum system would generate, I'm trying to cut down my power consumption and support it with 50% efficient 'redneck' engineering that I can manage myself rather than build an expensive off grid system that will run an all electric twenty first century palace, a bit of a flowery turn of phrase but I'm sure you get the idea...

So....

What would be the simplest system that would take the power from PV panels / a wind turbine / a pico hydro plant and convert it to 220v ac so I can feed a 1000w heating element directly?

A friend of mine, a Basque lady who lectures in physics at the university in Bordeaux, reckons it would be almost impossible for a 1000w element to boil 110 litres of water even if you ran it constantly, The heat loss as it got closer to boiling point  would get greater, I'm not sure if this is correct but as there's not a full days sun in the valley her opinion is that the need to divert excess power shouldn't arise.

All further thoughts and suggestions welcome!!

Cheers
Stef

 







Tighten 'til it strips, weld nut to chassis, peen stud, adjust with angle grinder.

BruceM

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Re: Easy PV water heating
« Reply #5 on: March 28, 2022, 11:53:55 PM »
"What would be the simplest system that would take the power from PV panels / a wind turbine / a pico hydro plant and convert it to 220v ac so I can feed a 1000w heating element directly?"

OK, so while you wrote 120 amps I assume you meant your Victron makes 120VAC.  If you pitch your electrically leaky elements (which should never have been sold to you or anyone) for some decent 120V elements, then you could manually switch on the element direct from the Victron. Or if you like to live dangerously, add a 1000W toroidal transfomer and use your leaky ones.  It will take hours, as your physicist friend noted correctly.  The standard element in the US is 4500W, and while dual elements are used, only one is on at a time.  So multiply the normaly recovery time times 4.5.  For automation, two things can be done to make it idiot proof (something I increasingly find I must have for myself).  One, you could add a  relay to the element and thermostat on the tank so it turns off when hot.  Second, you could add some electronics to turn off the element if the battery voltage dropped below full float voltage, or even better, based on  both PV voltage and battery voltage. You'd like to not have the water being heated when the battery is being charged with all the available PV.  So turn off when PV voltage is too low (measure it when the battery is below and after bulk charging). A time delay before on relay could prevent rapid cycling if that is warranted.

You didn't mention your PV configuration regarding nominal (marketing) string voltage and amps.  Since you wrote about 1000W heating element I am assuming you have more than that for PV power.  I could find an appropriate voltage sensing relay if I knew the PV voltage.  Many systems today use higher voltage series strings of PV to reduce wiring losses, which the PV charge controller buck converts down to your nominal  12V battery voltages.  Depending on the actual voltage you are seeing on the PV side of the charge controller while charging and once the batteries are near or after full, I can help pick some appropriate hardware.  Voltage sensing relays seem to be in the $14-30 range.

Other possibilies are to use dedicated PV panels for water heating, with a DC solid state switch rated above the maximum open circuit voltage of the panels.  4- 24V, 240-270W panels will operate a 1000W, 120V element nicely.  Only a thermostat would be needed to make it fool proof, self regulating.

Best Wishes,
Bruce







mihit

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Re: Easy PV water heating
« Reply #6 on: March 30, 2022, 07:15:48 AM »
If you're sticking with AC elements, I'll chip in with "micro inverter" - these clip/bolt/glue on the back of the panels and invert panel output to your line voltage AC, then you run AC wiring from the panels to the load.

Simplest I think. Probably not cheapest.
Also knocks your panels out for battery charging unless you either plug in a battery charger, or otherwise rectify your AC and through a controller of some ilk.

mihit

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Re: Easy PV water heating
« Reply #7 on: March 30, 2022, 07:38:49 AM »
The most efficient solar water heating is vacuum tubes. ........

Yes, but the 5+ year failure rate on the tubes is abysmal.  I see way too many folks looking for replacement tubes, developing schemes to recharge the tubes with acetone or something ( I love sweat soldering a tube with acetone in it ) to even want to bother with any of the vacuum tube systems.   Often, many tubes have failed, but the rest of them keep the system going for a while.

Really? Interesting... I think here they're warranted for 10-15 years.
I haven't gone there yet, but it was definitely my plan...you've got me thinking twice now. What kind of climate are you talking? What are the failures?

BruceM

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Re: Easy PV water heating
« Reply #8 on: March 30, 2022, 08:52:09 AM »
Alas, the microinvetrters I'm aware of are for grid tie only and not stand alone.  But I like the idea! 
There are a couple all in one inverterr/PV charge controller units that will operate without batteries that could do the job, but not cheaply.

One thing I forgot to mention is that a (US) standard 4500W/230V water heater element is about 1200W on 120V.  So perhaps the leaky element can be replaced cheaply.




veggie

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Re: Easy PV water heating
« Reply #9 on: March 31, 2022, 03:43:06 PM »
dieselspanner,

Have you looked into the CyboEnergy hot water inverter.
Might work well in your case.
https://www.cyboenergy.com/products/offgridHmodeloverview.html
« Last Edit: March 31, 2022, 03:45:06 PM by veggie »
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dieselspanner

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Re: Easy PV water heating
« Reply #10 on: March 31, 2022, 04:32:13 PM »
Hi once again Gents

Sorry for the slow reply, I'm off on a bit of a trip on Sunday, Greece, Holland, UK and back and trying to get caught up with things before departure, it's not going to happen!

Thanks once more for further food for thought

Bruce,

A couple of points....
 The Victron is a Multiplus 12v/3000va/120amp, it supplies a nominal 240v dc, not 120.

As for solar panels, I don't have any yet!

When I do I'll have to try and hide them from the Mayor, he refused my mate permission to have a pair of small arrays, about 8 feet long by 5 feet high in a field behind his house because the land is 'red zoned' for agricultural use only and anyway it would spoil the view, even though you can't see the site from anyone else's land. he's now got permission to mount them on the slate roof of his 300 year old farmhouse, clearly visible from the main road, the Mayor don't like that but has no power to  stop it as the regs for that come from a higher level of administration, apparently....

My place is a barn conversion and it says in the conditions of conversion that PV panels on the (slate, has to be slate, nailed, like Grandaddy did, not clipped even though you can't tell the difference from the road, 300 yards away) roof and my land is also red zoned......


Bruce / Mi,

I like the idea of direct from the panels straight into an element, is that something that would be self regulating, ie under less than optimal cloudy conditions or at night, it would simply render the element less efficient, or 'off'? any links to the inverters would be appreciated.

I'm, hopefully, going to build a small shed for the hydro plant later in the summer so I guess I could put the CS in there with an automatic timed start stop on it and run it 3 hours a day, I've got loads of WVO......

Veggie,

As I clicked to post this reply your post came up.....

Looks very promising, I'll get onto it, thanks very much.

Cheers
Stef


Tighten 'til it strips, weld nut to chassis, peen stud, adjust with angle grinder.

BruceM

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Re: Easy PV water heating
« Reply #11 on: March 31, 2022, 08:02:52 PM »
Some mistake there with 120A of 230V; that's 27,600 VA. That's why I assumed you meant 120VAC even though I thought Euro inverters would all be 230V.

Direct DC operation of water heater elements is as old as PV.  Still quite effective with cloudy weather, MPPT is mostly marketing hype over a small improvement in efficiency when there is too little to worry about (at very low PV power levels). Direct PV to the heating element requires matching the PV voltage  to the load voltage(s). It will self regulate is designed properly; the heater element will hold down voltage and keep making some useful heat with clouds/overcast. The converter Veggie found will free you from that one time design effort, somewhat, and may not require a dedicated element for DC only PV heating. 

It all seems academic if you can't have any PV. 

dieselspanner

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Re: Easy PV water heating
« Reply #12 on: April 01, 2022, 12:58:49 PM »
Hi Bruce

I'm reasonably sure I'll be ok with four panels, the big thing is not to seek permission!

It's a bit off topic but....

There's still a lot of stuff left over from WWll up here, people were denounced by neighbours and deported ect. It's only 3/4 generations back, and it's Left a bit of a legacy.

Should your neighbour build a shed, put up PV panels or whatever and you squeal to the Mayor he has to do something about it, if you get on great with those around you - and I do, nothing better than  a bloke keeping old tractors running up here! - no one will say anything. if the do, the offender has the right to ask who put the poison in (not something that happened when the 'Bosche' were in charge) and yet another feud starts.

This is mountain folk mentality, there are families that have born a grudge for generations, particularly over land boundaries and grazing rights. I've only been here 11 years and have a good name with all. The last Mayor brought one of his favoured contractors around when I found it impossible to get a 100m drive resurfaced and managed not to notice a three car carport I'd put up. The wooden structure was mounted on adjustable feet and open fronted so could in theory be counted as a temporary agricultural building, good enough for him!

When we sold the property a clause was included to the effect that if there was an issue the new owner would be responsible for it's removal, it's still there.....

As a Brit I'm not regarded a 'foreign' unlike the rest of the population who live outside 'The Valley' despite the fact that this area was under English rule for almost 300 years after Eleanore of Aquitane married a bloke who became the king of England and we've been fighting them on and off for centuries.....

You got to love 'em!

Cheers
Stef
Tighten 'til it strips, weld nut to chassis, peen stud, adjust with angle grinder.

BruceM

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Re: Easy PV water heating
« Reply #13 on: April 01, 2022, 04:47:10 PM »
Sounds not much different than rural northern NY where I grew up.  New neighbors were ignored for a few winters to see if they'd stick. They would be grudgingly accepted after 5 years if well behaved and/or generally useful.  One of the things I liked about Arizona in general and rural mountain AZ in particular was the friendliness.  People actually wave on the road and would come over to meet and greet if they saw you working outside. 

Four 24V nominal, 260W-300W panels in series will work nicely for a  direct  DC driven 120V, 1000W element.  They would have a peak open circuit voltage of under 176V , so easy to find a cheap solid state DC relay (SSR) rated for 200V. An adjustable thermostat for water heaters epoxy'd to the tank could drive the SSR via your battery 12V with a 3K ohm resistor to reduce current through the thermostat to about 3.5 ma.  A small switch in that path could turn off the PV heating.

Edit:  PS, I found that the 120A in Victron marketing parlance is the maximum battery charge rate.  Mystery solved.
Looks like a capable all in one unit.  Popular here in the US, too, though I haven't seen one myself.







« Last Edit: April 01, 2022, 07:52:15 PM by BruceM »