Author Topic: Off grid water heating project  (Read 25296 times)

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Offline dochubert

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Re: Off grid water heating project
« Reply #30 on: April 27, 2021, 07:12:25 pm »

3 days of dark clouds and rain.  Had to switch the 48v main bank back to grid overnight.  No problem with the water heating setup though.
The 24v water heater bank sailed right through 3 cloudy days/nights without much trouble.  It is performing very well.  I'm happy with the decision to make it a (mostly) separate system.
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Offline dochubert

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Re: Off grid water heating project
« Reply #31 on: December 02, 2021, 10:43:43 pm »

Been awhile since I've had something to post here.  Water heating has been working well for months.  Now that winter is here and sun is more scarce, added 4 winter-only 325w panels to my 24v bank (for water heating).  Helping a lot.

As the final winter-only addition, I just finished adding (4) 265w panels that are powering the lower element on my preheat tank.  Put in a transfer switch so I can go back to normal when I want.  Connected the panels to this DC-DC boost board, then to the element.

9185-0

The idea is that with this both the top and bottom elements can heat at the same time.
Only had it in for 2 days so far so the jury is still out.  Not sure the boost is doing any good. Might play with settings some more.   A guy on DIY Solar used it and said it worked well for him.  It's a 50gal tank so maybe I'm just expecting too much.
Will give it more time....
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Offline eidolon

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Re: Off grid water heating project
« Reply #32 on: December 03, 2021, 08:45:09 am »
I'm still around. Been trying to log in forever since my old computer died and couldn't remember my password.  "Forgot my password" routine hasn't worked even though it recognizes my email.  Finally found a scrap of paper with old password on it.

Im still heating water and this year I started desulfating old batteries I get at town recycling with the water heater as it conveniently produces 10A pulses. The garage system got two more panels added to it which face east. Greatest move ever.  I can now run the LG washer early in the morning with no batteries.  9187-0 Note the jumper cables.

So what boost converter is that? and what are you boosting to?

Offline dochubert

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Re: Off grid water heating project
« Reply #33 on: December 03, 2021, 07:07:47 pm »

Hi Eidolon,
Good to hear from you.  The boost controller is a
BST-900W DC-DC CNC Boost Converter 8-60V Step-up 10-120V Solar Charging CVCC

Here's a link for it if you're interested;

https://www.ebay.com/itm/224529024087?hash=item3446f90457:g:T7IAAOSwrolg6ZoK

There are no directions for setting it.(typical chinese doodad!)  Figured out I had to set the constant current to zero to get it to do anything.  I've only got constant voltage set for 60v at present.  Been pretty cloudy, but when there was some sun it was doing between 8 and 9 amps connected to the (4) 265w panels.  (two in series and those 2 sets in parallel)  Soon as I get a sunny day I plan to bump it up to 100v and see what I get there. 
Probably forgot to mention the tank elements are 120v/2000w elements.

No progress today as we were fogged in all day.
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Offline dochubert

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Re: Off grid water heating project
« Reply #34 on: December 13, 2021, 12:23:43 pm »

With all the gloomy weather, I can't really say if the DC-DC boost is really worth the expense/effort.  It puts up to 9a into the lower heating element so must be getting something out of it, but it's not much.  The best part is that it is independent of everything else, so works even when grid is powering everything else.

The real question is whether just connecting the solar panel output directly to the element would get me more water heating than using the boost controller.  I'll give it a few more days like this, then try the direct connect.
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Offline lighthunter

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Re: Off grid water heating project
« Reply #35 on: December 13, 2021, 02:20:52 pm »
Hi Doc! Looks like your doing ok, seems better than i wouldve guessed. So you have two 30V panels in series= 60v + a little for the cold... for a current of 8A x2 = total of 60v times 16A = 960W available in full sun. You are boosting into a 125V element at 9 amps. Without knowing the voltage at the element, 16A x 125v. = 2000w = 7.8 ohm element.  E = IxR ... E = 9A x 7.8ohm = 70.2V.   P=IV so P = 9Ax70.2= 631W delivered with the booster. Direct connect the way they are will be P = I x V = (60/7.8)x(60) = 7.7A x 60V = 461W,  now if you change the element to a 1000W 15.6ohm element and connect all 4 in series, then you will deliver 960W+ cold bonus probably a bit over 1kw in full sun, you would then loose some in clouds when panels cant do imp.  It why i quit doing it when my grid tie capacity was capable as they automatically do this conversion you want. Ive not played with the boost or buck to do this. Both circuits can do the conversion but im not sure they can in stock form as they are designed to control the out not the in. The boost converter current limit perhaps offers a way to cheat to keep the panel volts up during clouds but you need a way to turn it up automatically when sunny.
The simplest most troublefree way is to match the 125v 1k element  with the 4 in series.

It looks like your only loosing 300-350w in full sun, if your panel volts are staying above 25 during cloudy weather, you are acheving some kind of crude mppt win. If not then matching the 1kw element with the series 4 will be superior.

 . I like wood heat and a plate heat xchgr in the winter. Those babys can make hot water as fast as you use it.

All the best!
Health Warning: May contain traces of nut!
LH

Offline eidolon

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Re: Off grid water heating project
« Reply #36 on: December 16, 2021, 08:54:24 am »
I played around with a boost converter to raise 60V to 80V using a China module modified to keep the panels at near power point. This was just an experiment to match the element better before I installed the new one.  The booster really cooked, efficiency dropped under 80% and the whole effort only gained a few extra watts. 

The main problem with buck or boost converters is they easily throw the panels into a death spiral. To meet their output, current from the panels has to increase and that drops voltage even lower.  I have a circuit that can be added to any module that will keep the panels from dropping below a minimum voltage if interested. 

Offline dochubert

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Re: Off grid water heating project
« Reply #37 on: December 16, 2021, 01:54:50 pm »
I have a circuit that can be added to any module that will keep the panels from dropping below a minimum voltage if interested.

Definitely interested!  Would this be instead of or added to the boost module?

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Offline dochubert

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Re: Off grid water heating project
« Reply #38 on: December 16, 2021, 02:06:13 pm »

Hi LH,
I have been thinking about what to try next.  Was next going to just bypass the boost module and see what difference it makes.  Then thought I would re-arrange the panels in series as you suggested and give that a go.   I know I'm getting something from that setup, it just doesn't seem like much.  Wish I could add more panels but no place to put any more for this application.  (Can't just stick panels out in the yard with all these high winds we've been getting! -- They have to be tied down to something solid.)
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Offline lighthunter

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Re: Off grid water heating project
« Reply #39 on: December 17, 2021, 08:04:45 am »
Hey Doc!
  Yeah your not kidding there about the wind, we had 100mph wednesday evening.
Ive no idea how i didnt have damage. Tore up some roofing and skylights where i work. Nothing that wasnt repairable but it was not just a breeze. I think the trees blocked most of it here though.

For several years i had 7 x 72 cell panels in series running a 1500w 240v element 38ohm. (Ran at about 1800w). It worked so well except two things, cloudy days and it killed heating elements sometimes 6 months only. So i got a 2000w 277v which is still 38ohms with lower power density. Never burned it out.

You are almost there with what you want. I could see you leaving your 60v parallel connection to the 125v element and adjusting your current limit for full sun vs clouds either with a switch and resistors or a relay. If your around home flipping a switch for clouds is a simple solution.


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Offline eidolon

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Re: Off grid water heating project
« Reply #40 on: December 20, 2021, 09:26:36 am »
I would like to start out saying that I think boost converters are generally a bad idea. I had someone from Europe inquiring with an array of 120V.  Their water heater had a 2,000W 230V element and there was no lower resistance replacement for it. In that case using a 48 power supply to increase the voltage was justified. It was not an issue of efficiency, but accomplishing a goal.

A boost converter can not be totally turned off.  Input voltage always flows from the input to the output thru the diode and inductor. Only the amount of boost is controlled. The lower panel power is just passed thru. One has to accept that the lower wattages are just lost.  I give this example which can control a boost or buck converter only for educational purposes.

Every switching converter has an IC pin which is a reference which is connected to the output to regulate voltage. The reference is typically 1.2 or 2.5 volts.  That connection is easiest to find by either measuring the lowest voltage or resistance. Pot pins are easiest to solder to. When the reference pin goes above the reference voltage the switching regulator will turn off. A 12K resistor can usually supply enough current thru the diode to that pin. Less than 1mv increase is more than enough. The diode is there only to affect the circuit in turn off mode. When the FET, TL431, or opamp pulls the 12K resistor to - common, the added control circuit has no effect, and the switching regulator operates normally. A FET is used in this example for simplicity, and it is a power FET because these are common. It is also not linear having a sharp turn on between 2.5 and 4V with a high impedance that will not load down a voltage divider. There is some drift with temperature. Everything to the right of the diode are switching module components. The 12V can be obtained from the regulator of the switching module.


As PV voltage drops below the power point voltage, added current feeds to the reference pin. This adds to the current from the output voltage divider. This action does not completely turn off the switching controller. Rather it reduces the amount of boost till the PV voltage stabilizes near power point.

This circuit helps a little preventing a boost converter from putting the panels in a death spiral by drawing more and more panel current. It turns off the boost converter and makes it the same as direct connect. Boost converters are just a bad bad bad idea.  To be efficient the panel should feed into a capacitor bank and be pulsed from that maintain a constant voltage from the panel near power point.

Offline eidolon

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Re: Off grid water heating project
« Reply #41 on: December 21, 2021, 11:57:35 am »
Here is a version of one of my designs in northern Europe. I'm quite impressed as he had no prior electronics experience and even etched his own circuit board. He states he is getting over 5KWH a day in winter from four panels. Please respect his privacy and do not comment. 

 Lithuania video

This next pre shipment video is of a board I sent to NM operating on four 255W panels facing east. This is on the 40 gallon tank in the garage I just use for laundry hot water.  It is
connected to only the lower 5500W element for the 120V test. I normally connect upper and lower in parallel with my 60V house array. Setting priority on the basis of PV voltage works well at only diverting excess house power to the garage only when house water heater finally turns off. The garage has a transfer switch so I can run the washer off 120V DC garage panels without using a battery. Totally free laundry now. The generator is only used now for the wood saws.

120V first test

Offline eidolon

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Re: Off grid water heating project
« Reply #42 on: December 22, 2021, 07:21:04 am »
Here is an example of constant voltage as represented by the green line. The small higher peaks are when the panel goes over power point due to heating element being higher resistance than optimal.  Power is the yellow line.
9204-0

This other graph is when the board is operated in diversion mode. This scan is only a half hour and demonstrates how varied power can be.  Besides clouds, as devices in the house turns on diversion drops to zero.  At the very end the voltage drops to 44V, likely refrigerator turning on. Manel panels are shaded reflecting the lower voltage of the charge controller.

9206-1

Offline Pete

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Re: Off grid water heating project
« Reply #43 on: December 22, 2021, 03:05:40 pm »
You guys take a pretty high tech approach, but it seems to be working out fine.
I have a evacuated tube solar hot water system. It is backed up in winter with a wet back on our wood heater.
Then when the wood heater is not running and the sun not quite strong enough, I use an element in the tank and power it by my inverter.
It is a low tech approach where I look at a guage I have to decide if the water temp is high enough. Then I switch the element on. The power is regulated by both a thermal switch on the gauge and also by the battery level.
The Victron Battery monitor I have has a settable relay, so i have the relay switching a higher power relay to turn the water heater element on. It allows the element to run only when the batteries are fully charged and turns the element off if the batteries go below 97%.
I know it is a low tech approach but it suits me.
I am guessing that your systems boost DC up and use DC to run the elements, Does this damage the over temp thermostats in the tanks?
Cheers
Pete

Offline eidolon

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Re: Off grid water heating project
« Reply #44 on: December 26, 2021, 01:09:58 pm »
Many focus on a single issue. Heating water can be complex having many issues.  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. My method allows using existing array to heat water and supply charging needs which have priority.  These methods will be common in another 10-15 years. At this time almost no one understands the science.  For now the solar world goes for easy.