Author Topic: Easy PV water heating  (Read 1201 times)

BruceM

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Re: Easy PV water heating
« Reply #15 on: May 22, 2022, 08:14:47 PM »
With those 4 panels in series, you could have as much as 1000W with perfect conditions cool and clear, ideal alignment, at roughly 122V.  Alas at around that voltage, about 300W.   (You can calculate the resistiance of the element and then it's wattage at some other voltage using Ohms law, which I did for your case.)  So yes, you can do it, you just won't get as much bang for your buck without switching to a better suited element at 120V. There are solid state relays for up to 250VDC which are well suited, since your open circuit PV voltage would be only 37.8x4= 151.2V.  Midnite Solar makes DC rated circuit breakers you could use for manual disconnect.

The practical way to drive the element at it's rated power is to use a 220VAC inverter; there are some all in one, off grid inverters that will operate without batteries.  In that case the panels will have to be configured to a voltage that suits the inverter. 

I would rather go the straight DC route myself, and just get a 120VAC rated element, which should be cheap enough.  It's a perfect match for your 4 panels, and should be quite efficient at a range of sun inputs with no other electronics than the 200V DC SSR  (solid state relay) connected through a thermostat to 3-12VDC.   An adjustable snap disk type thermostat that can be glued or attached to the water tank, that has contacts rated for low voltage DC is best for long term reliability.
 
https://www.amazon.com/SSR-25DD-3-32VDC-Output-5-240VDC-Plastic/dp/B08GNSPCND/ref=sr_1_4?crid=3B2GYWD419TQS&keywords=dc+solid+state+relay&qid=1653246620&sprefix=dc+solid+state+relay%2Caps%2C256&sr=8-4

Don't be shy if you need more info.
Bruce

« Last Edit: May 22, 2022, 08:21:48 PM by BruceM »

dieselspanner

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Re: Easy PV water heating
« Reply #16 on: May 23, 2022, 09:43:50 PM »
Hi Bruce

Thanks once again for the swift reply, you don't need to worry about my timidity, if the Olympics start an inter fora event for the brass necked I'll be representing Listerengine.com and going for a podium finish!

I've now looked up what a SSR does and which terminals do what, so if I understand your simple system I change the element to a 120v / 1000w, connect the four panels in series and then use the SSR to switch off the current if the water gets to 80 degrees or so.

The SSR you sent the link to is, I presume, normally closed and the thermostat (eBay UK item number:394041622609) closes at the set temperature thus disconnecting the supply.

My three heating element ports are 2 at the top, one at the bottom, is there such a thing as a SSR that, instead of switching the power off, diverts it? That would allow the upper element to raise the temperature of the top third and then use the lower one as a 'dump' for the rest of the day?

The barn is in a north / south valley, thus sun rise /set looses about an hour at each end of the day, 12 hours max at the summer solstice.

Cheers
Stef



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veggie

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Re: Easy PV water heating
« Reply #17 on: May 24, 2022, 12:04:34 AM »

Bruce,

I have several of those DC-DC relays ranging from 25 to 100 amps.
I have been reluctant to use them in any critical service because they only cost $12  ???
What do you think about grossly over sizing them?
For example, if there is a solar panel supply which max's out at 25 amps, then use the 100 amp SSR for switching the element load off when target temperature is achieved.
Do you think this would give the inexpensive SSR a fighting chance at staying alive more than a few months.
(I would also recommend to everyone to use the appropriate heat sink)
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BruceM

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Re: Easy PV water heating
« Reply #18 on: May 24, 2022, 01:39:51 AM »
Hi MikeNash.  I'll try to answer your questions first.
"The SSR you sent the link to is, I presume, normally closed and the thermostat (eBay UK item number:394041622609) closes at the set temperature thus disconnecting the supply."

No, all the higher voltage DC SSR's I've seen are normally open. You need a thermostat which opens on high temperature, closes on cold.  Just as a standard water heater thermostat does.  Of course with some simple electronics we can change a thermostat output to NO or NC (normally open or closed if you must, but keeping things simple is nice.

"My three heating element ports are 2 at the top, one at the bottom, is there such a thing as a SSR that, instead of switching the power off, diverts it? That would allow the upper element to raise the temperature of the top third and then use the lower one as a 'dump' for the rest of the day?"

This is easy if you have a standard electric water heater thermostat which already has just such an dual temperature element diversion from upper to lower elements (at least here in the US). You'll have to add a second SSR, one for each element, and use the water heater thermostat to just switch the  12V to the SSR control.  Replacement thermostats of any rating will be fine, since you will only be swiching about 6 ma of 12V via the contacts. 

Again, many ways to do this electronically, like a couple thermisters, a zener reference and a single quad op amp plus "glue" (resistors and capacitors) but I'm trying to avoid custom electronics.  If you can't find what you need, I can help with plan B.

Veggie,
It is wise to oversize the ampacity of a DC SSR and there is no penalty for doing so.  The higher rated once will have lower on resistance, and thus for small currents will not need much heat sinking, if any.  I agree that some skepticism in the rated ampacity should be applied, as they are made of Chinesium and specification were written by the marketing department.  I have used them well under the rating and they do seem to work and hold up fine.





mike90045

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Re: Easy PV water heating
« Reply #19 on: May 28, 2022, 02:01:13 PM »
SSR's most often fail from :

  being a counterfeit clone

or not having a Snubber circuit in the IN & OUT terminals. 
  When you get high power flowing thru a cable and the thermostat clicks it OFF, the entire circuit acts like a ignition coil and points, you get a high voltage spike.   Some configurations never have a problem, some eat SSR's weekly.  All depends on the randomness and inductance of the cables.

https://www.omron-ap.com/service_support/FAQ/FAQ02088/index.asp

veggie

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Re: Easy PV water heating
« Reply #20 on: May 28, 2022, 02:39:35 PM »
In his book "Take back the power", the author puts forth a case for switching to a full DC grid.
Partly as a solution for to coming social issues and energy scarcity, but also as a solution for off-grid and personal grids.
One interesting project he describes (along with circuit diagrams for self-builders) is a unit which takes direct PV power and converts it to high frequency pulsed DC.
When DC is pulsed at frequencies above 120 hz, the sparking at contact points is virtually eliminated. (Less than AC at 60 hz).
He points out that this allows the direct connection of PV panels to a water tank heating element while keeping the original water tank temperature switches in the circuit.
He wires his panels in series to get ~120 volts, then passes the power through his PWM DC unit, then on to the hot water heater. (120 volt heating element)
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veggie

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Re: Easy PV water heating
« Reply #21 on: May 28, 2022, 02:54:00 PM »


I see some DIY installers using Insulated Gate Bipolar Transistor or IGBT Transistor relays.
Are these inherently better for switching higher amperage DC (say 50 amps) ?
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BruceM

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Re: Easy PV water heating
« Reply #22 on: May 28, 2022, 03:53:44 PM »
Regarding the need for a snubber; this is Omron's guidance for an AC SSR switching a large inductive load like a motor. For MikeNash's water heater application using a DC SSR for a resistive heating element, a snubber is likely not needed.

I use 120VDC (and 12V) (nominal) for my home and computer/electronics.  No inverter losses, and ideal (unregulated) for incandescent lighting.  I view the hours of lighting in fall through spring as efficient supplemental heating, which it is.
I only run my inverter for running AC motors.  My night time loads are less than the 24/7 power loss of the typical inverter, idling. 

Pulsing DC to gain compatibility with AC switches such as bimetal thermostat contacts is clever but to me ruins the beauty of clean DC;  you will have EMI generated and conducted on the wiring from the switching, and AC magnetic fields from the attached loads.  It is simple enough to add solid state switching to the appliances which must have a thermostat; I have done that for 120VDC toaster ovens; the stock switches and thermostats control only the transistor gate current.  My rice cooker has a similar mod; I added a transistor turns on/off the element which normally would be switched by magnetic pan scorch sensing magnet switch and a timer.  Note that there is no need for fancy electronics or a snubber as I have chosen a transistor with sufficient voltage headroom and with the ability to handle some slow switching.  For my neighbor's larger toaster oven, I went with one transistor per element, same approach. 

I didn't suggest the direct transistor method for MikeNash as electronics are not his forte and the 250V DC SSR's are now
fairly cheap and readily available. 

I'm attaching a schematic so you can see the circuit for my toaster oven.









veggie

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Re: Easy PV water heating
« Reply #23 on: May 28, 2022, 07:33:26 PM »
That's interesting Bruce.
There certainly is a simplicity and elegance to DC home grid systems.
In the book I noted above, the author runs many appliances (laptop, lighting, etc) on 120 VDC without modifications. No inverter.
Many of today's appliance power adapters and chargers can rectify AC and/or handle 100 to 250 volts DC. Of course one must investigate the appliance before plugging in, to be sure that the power adapter does not use a transformer.
For the food freezers and such he uses cheap small dedicated square wave inverters at each location.
After years of use, no issues
« Last Edit: May 28, 2022, 07:39:53 PM by veggie »
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BruceM

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Re: Easy PV water heating
« Reply #24 on: May 28, 2022, 08:11:49 PM »
Pulsed DC is not a panacea of compatibility, but it is an interesting solution.  The method of doing  PWM of the raw PV input to get RMS 120VDC is clever and minimalistic, since no filtration is required.  I appreciate learning of this method, Veggie, it's something I've never read about before. 

Induction motors will not run on pulsed DC.    Some switchers have bootstrap aka startup circuits that are AC dependent, and they will not start on pure DC.  About 80% work fine.  Pulsed DC should work for many of the ones that don't, depending on the bootstrap circuit.  Near 95% of switchers should be fine with pulsed DC.

LED lights/bulbs with switching supplies in the base that I've tested work fine on 120VDC.  I don/'t use them because I find the light spectrum and EMI bothersome.  I have a lifetime supply of soft white incandescent bulbs instead.






veggie

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Re: Easy PV water heating
« Reply #25 on: May 29, 2022, 04:00:22 PM »

While I may not venture down the pulsed DC path because of my lack of electronic skills and inability to build circuits from schematics, I did find the concept very interesting.
The author does state that resistive loads are best suited to the pulsed DC. Inductive applications do not work. That's where the cheap square wave inverters are used.
He tested square wave and sine wave inverters on several appliances and concluded that there was nothing to be gained by going with the more expensive sine units.
The test results are documented the book.
The book I noted above advocates pulsed DC and includes schematics/circuit diagrams to build small (up to 800 watt) modules for powering DC loads
While not an engineering manual per say, it it more about the author's journey to bring Solar DC power to a village in Africa. However the second half of the book does get more technical where schematics and charts are discussed along with the theory and application.
https://www.amazon.com/Take-Back-Power-Marvin-Motsenbocker/dp/B08P2C6HB4/ref=sr_1_1?crid=3BCV82L13YWEU&keywords=take+back+the+power&qid=1653836393&s=books&sprefix=take+back+the+power%2Cstripbooks-intl-ship%2C99&sr=1-1
Paperback. $25
« Last Edit: May 29, 2022, 04:07:08 PM by veggie »
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BruceM

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Re: Easy PV water heating
« Reply #26 on: May 29, 2022, 06:21:13 PM »
One of the issues that is swept under the rug is the power consumption of an inverter, even when there is no load. It's not unusual to have 50W or or more for just running the inverter, all night long.  That affects you DOD which affects you lead acid battery life, and increases your charging time the next day.  Straight 120VDC (or pulsed DC) gets around that inverter power loss.

One issue of modified sine and even worse, square wave inverters when used for induction motors is the alleged 20% loss of efficiency due to heating (something I have not confirmed personally). I wanted to get around that so when designing my own inverter for 120VDC input, I opted for a 5 step sine (originally 7 step but later settled on 5 after much testing).  It has a measured THD of 12%, though I could do better with better software for waveform timing.  I only used the "that looks about right" method, using a small Basic program to generate my timing tables.  The 5 step keeps even my washer's timer motor happy (it gets warm and makes noise on ST-3 power, measured at  15% THD.   My inverter is small, designed for 1500W continuous, but since it uses two 1000W toroidal trransformers with secondaries in series, it can start a 1.5 hp air compressor or my well pump (3500W starting surge) as if they were on grid power.  The inverter idle current is 15W, but I only turn it on when needed.  A single twisted pair allows for start/stop control of the inverter at multiple points of use. 

I did not invent the concept of adding a step; there was a single research paper proposing before I did it which I had found showing a dramatic reduction in THD by adding just one more step to the typical MSW inverter (a 3 step waveform).  I used the now expired patent, Trace SW series method of multiple transformers with secondaries in series as a means of doing a true, all low frequency, single conversion design in order to use very slow, soft switching and thus dramatically reduce the EMI on both AC and DC.  I used high efficiency toroidal transformers and got a measured 92% efficiency.

I'm attaching a Picoscope image of what my 5 step sine looks like, using a couple surplus transformers for 120VAC ouput.  The final version uses Antek transformers, with rewound secondaries for 230VAC for my well pump and electric air compressor.  I step it down and have both 230 and 120VAC at my shop/laundry.

dieselspanner

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Re: Easy PV water heating
« Reply #27 on: May 31, 2022, 06:14:08 PM »
Hi Veggie

Take Back The Power! is a nice find, if it helps anyone else out it's free to those with Amazon Prime (UK Accounts, anyway)

The first part, helping out in a African village with very low tech is great, however he then continues to support the idea of a micro community after the fall of civilisation as we know it, without explaining how the panels ect. needed are going to get from the big five manufacturers in China. Let alone the dishwashers aircon and all the other 21st century stuff that the western world wants......

It's also nice that he is happy to provide all the information as 'open source' to all and sundry, asking only that he is credited as the author.

Anyway I'd happily recommend it, if only for it's low tech approach and the way it explains how a home built system can be massively cheaper than a turn key solution from a major enterprise.

Cheers
Stef
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dieselspanner

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Re: Easy PV water heating
« Reply #28 on: July 12, 2022, 09:49:04 PM »
Hi All

Things have progressed, and as Ed Dee has said, school fees have been paid!

I've been doing other things and awaiting deliveries of adaptors and elements etc.

I've fitted the the solar panels to the roof, they are at around 45 degrees and 20 degrees to the west of south, perhapsnot ideal, but that's what I have. The mayor has yet to notice them.....

They start producing around 11 am and the sun drops behind the ridge just before 7 pm, 'noon' is 2 pm here (France is on Berlin time with a 1 hour advance for summer time)

I have the panels directly wired into a 220v 1000w element and from a 35 degree start point in the morning they get the 110 litre (22 gallons UK, 30 US) up around 55 /60 degrees with the extremely sunny weather we've has lately . I'm more than happy

I thought I was burning out elements but whilst changing them over realised that the DC had burned out the contacts for the integral thermostats, after checking with the manufacturers (Tesla UK, they were Tesla long before Elon Musk and ain't changing, they said!) I pulled the contacts out and for some reason there's a fitting for the larger Lucar connector (yellow) in the right place, happy days.

My biggest problem has been that the standard UK immersion element had a 2 1/4'' BSP fitting. NOTHING else has 2 1/4'' so I've had to have some adaptors made - big mention for Newark Copper Cylinders who made my hot water tank, great guys and mega helpful

With the old 20 / 20 hindsight I could have had any standard size fitting built in for a tenner each, just my luck that after 2" BSP pipe fittings increase in 1/2'' increments, I'll know next time!

When I've time Ill drain the tank and try a 120v 1000w element (as per Bruce's suggestion) now I have adaptors to 1" BSP, and report further.

I'm also considering using a solar charger to produce 12 volts for my battery bank and use some of that with a 12 v element, however the largest unit I can find is 600w and I'm not certain that will be large enough to be of any use.

As an aside, I have found that five well seasoned 30cm / 12" beech logs in the Rayburn will heat the the tank to 55 degrees or so in less than 2 hours, this enough for 2 days showering ect. and you can cook supper at the same time, so maybe that's a better use of energy.

Cheers
Stef



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