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Off grid water heating project

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--- Quote from: eidolon on December 26, 2021, 01:09:58 pm ---My board has arc interrupt so that high voltage DC can be used with standard mechanical thermostats.  Water heating should be done with the highest PV voltage directly from the panels eliminating conversion losses and avoiding buying more expensive equipment for the increased load.

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I admit I don't completely understand the specifics of your system, but agree that eliminating the converter should be more efficient.  Also the idea that your board makes using panel voltage safe for water heater thermostats is a big plus.
Do you build your boards for sale?  I would like to use my four panels in series for an open circuit voltage of about 120vdc.  Would that be compatible?


--- Quote from: dochubert on December 26, 2021, 03:12:41 pm ---agree that eliminating the converter should be more efficient.

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The exception to this is where you are using an element that is not well matched.
Lets take a really BAD example...
You have a 5kW, 11.5 ohm resistive element (ie, 5kw/240V element), and 2kW of PV panels connected to give 60V output.
In full bright sun, you would think "I should be getting 2kW into my tank"... but at 60V, you're only going to convince 5.3A to flow for a total heating benefit of just 312 watts.
Using a dc-dc converter, or dc-ac inverter and boosting that to lets say 120V, even if your inverter is only 90% efficient, you're going to get 1125 watts into your tank.

1125 watts is almost 4 times more USEFUL POWER, even if you are "throwing away 10%".

You need to look at the big picture before deciding what is the "best" way to do most jobs.

Well, I do agree that most everyone just slaps a system together with whatever they have and just hopes for the best. 

What I want to get across is that every successful solar system has excess power by design in order to recover from bad days.  While many concentrate on getting that extra 2% efficiency by buying a better charge controller, they throw away thousands of WH not harvested.

Many with camp systems live like refugees when they could have ready hot water with minimal added expense.

Whatever system is used, monitoring PV voltage is an excellent way to divert excess power to water heating.

The easiest way to explain the board is to say it is designed to use four grid tie panels. These will be in the 7-10A range and the heating element which will match that, 4500-5500W 240V heating element.  This is what similar products are designed for. The capacitor bank is designed for a certain RC time constant. The capacitor bank must be able to supply nearly full power for 1ms. My camp system operates at 60V which is the extreme low voltage end region of operation if the array can supply 500W most of the day. I only talk about it because it is possible to get acceptable results with 120V heating elements in camp systems. Many tanks do not have replaceable elements. My board is designed to operate in parallel with charge controllers harvesting excess power. To my surprise there is no interest in this.  Everyone wants stand alone systems. This board is designed to use the natural power point of an array
and will not work well with single string partially shaded arrays. I would love to have a 90 or 120V system, but shading prevents that.

I had requests to create a board with high reliability and a high tolerance from lightning spikes. Techluck boards were failing and that company would not repair them. Proprietary microprocessors were avoided in favor of common discrete socketed IC's which would be
available for the next 20 years. The FET can also be replaced with a screwdriver.  The capacitor bank is far more robust than other products for long life.  Techluck uses only three 100uf capacitors which are rated at .58 amps. By contrast I have five 1200uF capacitors
rated at over 3A each and the design operates at lower frequency for less noise. This allows others to copy the design with simple perf board construction and still have high noise immunity. 

A limited number of the first run evaluation board and is available for purchase. It is unlikely there will ever be more made. It can be as low as $70 + shipping. Initial board only cost is $140 and $70 is refunded after you post an evaluation with a video.  Sorry I have to do this, but not one person has contacted me once they purchased a board with test results. I use these boards at two locations that have extensive shading and can not compare results to expected results of a solar calculator. These boards were never intended to be sold to the general public. Rather, thru installers who would take the time to learn how to set them up properly. You must also supply a photo of a power meter before I ship. Module can not be set up without this. Array / heater element information is also needed to insure proper matching. I assume this is just too much involvement for most people and I recommend for them the ACTii from Poland for about $200. 

Hi all!  I had two thoughts on the water heating thing. Doc mentioned a preheat tank. If that doesnt work for you, another approach that i use is a 100 gal tall electric. Not everyone has thought about this but water stratifies in an insane way. The water at the top can be 60C when the bottom is 20C and it seems to stay that way. By running top and bottom elements on different thermostats, you can use a priority heating scheme to meet your need. Also, i have a well shed with 80Gal pressure tank. Ive a 250w heat lamp plugged into a 3°C switch sitting beside a vulnerable pipe. So ive kinda the opposite scheme as doc where this water starts out at probably 10C drops to 3C and then gets piped to the house then heated in water heater.  So wouldnt it make sense to move that pressure tank to our basement where its around 25-27C?  Or would i create more problems like a pressure switch that cycles quickly because of delayed pressure response. Cant say ive ever seen anyone set up this way but then not sure anyone would have a reason to.

Hope everyone is getting plenty of sunshine! We got about 27kw today, yesterday was good too!


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