Heating Water Efficiently With PV Diversion Controller

[OperaHouse]

Thought it was time to post a project again.  This is free hot water from energy you normally waste.

This is just a beginning explanation of the high efficiency power point tracking water heater board. It is good
up to 200V open circuit and about 12A. Actual use is limited by the FET and capacitors chosen. there really isn't
any need to go over that though it could. Additional power should be accomplished by switching in additional heaters
based on total current.  This can be used in parallel with any MPPT or PWM charge controller to divert any excess
power not being used for charging.  It does not take any power from a CC need any battery or put extra load on your
inverter. It is capable of tracking power point as panels heat up and cool.  The device runs very cool and produces
no EMI.  Arc Interrupt capability allows standard mechanical thermostats to be used without damage. I've ben running
this technology for many years at my camp it is flawless and built to last. The principals will be explained in future
posts. All IC's are generic and will be available for more than 20 years. FET screw int terminals and can be replaced
by the user.

In the simplest use, the solar array connects to the left side of the board and the water heater to the right. Lower
connectors are for options of external voltage control, heater LED, remote temperature sensor, external power and
electronic thermostat/on off switch. Voltage can be set with on board pot.

This is the display of a Hidance/Atorch 300V 100A bluetooth power meter on my phone. The time is 11am to 12:40pm
when there is partial shading. Note the PV voltage in green remains constant as the current in blue changes. The
green dies change as the temperature of the panels causes the power point to change.  Yellow on the chart
is the power which has no relative scale.  Entertaining, but many things keep this from being a useful device. It
comes with no instructions even though it states it does. You are le to a video of a phone taken from about 3
feet away which only barely hows the steps to load the software. I have been not able to find operating instructions
on the internet.  If you have found some, please share. Fooling around I have been able to figure some things out.
It definitely requires someone who is a maestro with their fingers. There seems to be many things yet to be found which
this software can do. Startup is going into apps on your phone and searching for E-test. The bluetooth device will
be DT3010 with 3 letters on the end which define the device if you have multiple units. Bluetooth icon must be
tapped and unit selected. Location must also be turned on I guess for spying. What other use can this be? Even when
you turn this app off Chinese characters will sometimes pop up on your phone and foster paranoia.  Might want to
disable app when not used. This would be a good use for your ol phone collecting dust. Oh, and the meter doesn't
store data and you have to keep the phone nearby all day.  It does keep totals though. Going to English doesn't
correct all screens or the pop up comments from being Chinese.   I constantly wonder what goes thru peoples minds
who sell this stuff.

Even things that are simple are complicated.  There is a recording time  9:562:34  maybe that is days, minutes and
seconds. Skipping hours for easier calculations.  A short time later it is 10:605:59. Not much longer the microwave
is used and it shows up as 001:90:32.  Probably some new metric time I have to learn.

There is no consistency in the colors. Yellow is used for power in the graph and green in the readout. The graph of power
doesn't relate to any scale and whatever it is sometimes changes. The voltage and current scales can change with a double
tap to reduce the scale by about 2/3 each time. But, they both change. If you have high voltage and low current it becomes
useless. It is about impossible to recover when you go too far. A two finger swipe can expand the scale and the time
by going horizontal or vertical.  When you go from normal 60A, this is a 100A meter, it recalculates the scale to eight
digits further reducing graphing space. No more than three digits are useful. A single finger moving on the graph can
move it up and down at times.

Getting the bluetooth to start communicating can be a challenge at times.  Any interference like a microwave can stop
charting and your data will be lost.  This software can be used on their other products.  AC and USB will be in grey
boxes when not applicable on DC.

There is a SETUP at the bottom of the screen that seems to have no purpose.  The back light on the actual meter can be
set to a timed period up to 59 seconds or on/off constantly. AH, total KWH, carbon dioxide and value of energy saved
along with the time the graph has been on. + - are used to enter energy costs. There is also the temperature of the
unit which might be useful in an overheating cabinet. And for me, I now don't have to go downstairs just to see
how things are going. Data updates are about every second so not very useful for motor startup monitoring like a
fridge. There is no way I can tell to save the screen to a file to save or share with others. Far from professional,
still a fun item for only $15. 

[OperaHouse]

The theory will be posted later. I will start with my current camp system. This is the ECOsmart 6 gallon water heater.
Found on craigslist for $75 new in box. This is a good example of what could be used at a small camp. With a 1500W 120V
heating element it can produce 400W of heat at 60V. It replaced a 9 gallon tank I was given and is fed by another 10 gallon
preheat tank. Six gallons can be heated 60F with less than 1KWH. That is pretty easy to do even with a small PV system.
Look at the graph of two days of tank temperature. Time and date are incorrect, but heating begins about 7:30am and fully
recovers in three hours. Thermal cycling is then about every 1 1/2 hours. Remember this PV array is also recharging the
battery and the refrigerator is recovering at this same time.  The refrigerator does not run all night and must recover
in the morning. This picture is of the primary hot water controller which has automatic transfer to the dishwasher
providing additional water heating and heated dry. Next controller is for a pre heat water tank. It is  another design
variant which operates on a legacy 50V array.  Far from ideal for a 2KW 120V heating element. Typical daily PV diversion
is about 2.5KWH a day.

The last picture is my first water heater controller which was mounted outside with the tank on the side of the house
under bubble wrap to protect it from weather. The camp runs on a microprocessor and I just added a few lines of code
to heat water. Just about anyone can can make something look better than this. Don't sell yourself short, you can build
something to heat water. Note, the ECOsmart tank has to be connected with laundry hoses because the only way to drain
it for winter is to turn it upside down.

[OperaHouse]

I was given this 40 gallon hot water heater after it failed to provide sufficient hot water. It was filled  with lime
scale, filled a 5 gallon bucket with it, though the real problem was the plastic tube that sent the cold water to the
bottom of the tank. Now cold water is fed to the prior drain port. The two heater elements are in parallel for about
6.5 ohms resistance powered by the house 60V array. Not to say I am cheap, but I bought this two year old washer off
craigslist for $150 delivered. The sale label was still attached and it spend most of its time in storage. He regretted
that he hadn't bought the extended warranty with it.  I was hopeing for more of a challenge. A plastic trim piece
caused one of the buttons to stick down preventing it to start.  These washers are great when they work and many find
they are a nightmare when they don't. All circuit boards are potted making repairs impossible and replacement boards
are expensive. I was given another LG before this for free and that repair proved to be cost prohibited.

I stored the tank for several till I bought the LG direct drive washer with operates on less than 400W if the internal
heating elements are not used. Hot water is fed to the cold water inlet so all cycles use hot water. This same washer
at home has to have the soap dispenser cleaned regularly cleaned because of film buildup.  At camp this soap dispenser
stays spotless because the hot water washes away everything. The washer also operates off the PV array voltage with no
battery. Array voltage is chopped with a modified MSW inverter using only the H bridge section. This chart shows five
days of heating. The really big dips are from two laundry loads that day. Even so, the water only dropped to 95F. This
40 gallon tank is highly stratified and the temperature is sensed 15 inches from the top. In short, this tank is only
heated when all house needs are satisfied and it still provides reliable results. The cloths come out steaming.

I have a fairly small system and the majority of my panels are shaded at any time. I live here for four months of the
year without issue. I remember one day when the refrigerator was down to temperature, both house water heater tanks
had shut off, a load of dishes was done in the dishwasher and two loads of laundry were done. All before noon! Any small
camp can have hot water using only the existing panels. It may not sound like much but you can take a shower with that
and ready hot water for hands and dish washing is a real luxury. You don't need to have dedicated panels to heat water.

[SparWeb]

That's really neat- especially getting the washer running off of it.
Do you have a guess at the amount of water used by washer for every load?

[Bruce S]

OperaHouse;
With spring in the air, I've gone to redirecting the solar from my still to the seed sprouting heating pads. I now redirect my "excess" to those. I learned that wood pallets are better insulators than thin plastic and almost any open-cell foam will sprout seeds.

I'm still learning the ins and outs of programming and still take inspiration to your posts.

Cheers
Bruce S 

[DamonHD]

You gave me inspriation to try something at a much smaller scale to make better use of my off-grid solar when the batteries are full or nearly so.

https://www.earth.org.uk/note-on-site-technicals-46.html#2021-03-23

I've shown that there is plenty of extra available once the battery is full, and had a nice cup of tea from my proof of concept.  B^>

I discover that the thing I bought is only rated up to almost exactly Vmp for my panels (27V), so I'll get something simpler that gets connected to the raw panel DC somewhere over 30V (Voc is ~33V), ie when the controller is letting the panels float above the MPPT.

Rgds

Damon

[Markcw]

Thanks for posting, I am planning on putting in some solar later in the year and this would be fantastic as I have to heat my water even in the summer in sunny Wales with kerosene which is not good

Mark

[MattM]

I would think, maybe I'm wrong, that heating elements are more limited by overall wattage and are less fussy about how you many volts are used to get to those watts.

[Bruce S]

MattM;
This might help.
My heating mats seem to be self-limiting, they are 120Vac. They heat up to a "warm" (I know , what is considered "warm"  . Now that I know this works I'll grab my temp gauge and check this evening. ) and keep it there so the flats of seedlings I have on top stay nice and warm too. There is both the regular seed starting soil and Hydroponic (rock wool) units.
I allow the insulation to keep them warm throughout the night instead of having a switch on the system that will kick them over to mains powers when the panel voltage drops off.
That's is in the works as yet another test. . For this I will probably borrow from how OperaHouse's programming  that switches from fridge to water heater to accomplish this.

Cheers
Bruce S

[OperaHouse]

Supplemental PV water heating is probably the most cost effective use of solar panels.  This is based on the fact the
size of the array will never be enough to raise water temperature above 140F. Therefore all the energy the panels
produce will serve a useful purpose. Also, if home brew the cost will not be much above the panels themselves. Compare
that to the often talked about home office where the additional cost of a charge controller, inverter and battery much
be purchased. The KWH lifetime cost of the battery has been said to be close to that of utility Power. Array voltages
of 90-130V are suitable for using the existing 240V water heating element and that is another cost savings.

Supplemental PV water heating is a great way to enter solar. Some want to have some emergency backup power, but an idle
system can't be justified. Heating water is a priority system. Adding a charge controller to keep a boat battery or an
old battery charged will present almost no load till the need arises and then charging takes priority. The emergency
system pays for itself in the meantime.

Many think adding a preheat tank is the way to go. Any preheat tank going over 80F usually just wastes that extra
temperature in loss unless there is a high use of water. A 20 gallon tank almost costs as much as a large one. Again,
this added cost reduces your payback. It is very appealing because it does keep the PV system totally isolated from
the existing home system. There are three phase 400V elements that can be isolated into three heating elements. One
can be used on existing utility power and two remaining in parallel for the solar array.

Common water tanks just don't have enough ports for heating elements.A thermal siphon can be another way to add an
element to an existing tank as in the picture shown. The heating element is installed in piping as low as possible.
Cold water is sourced from the bottom of the tank via the drain port and the rising hot goes to the side or top port
used for the pressure/temperature relief valve. Care must be taken to insure this hot water does not exceed the valve
setting. That valve can also be moved to the hot water exit port. Almost all tanks have a check valve and feeding
heated water to the hot port will soon be mixing cold from the bottom of the tank.

Raising tank temperatures is a way to keep utility power from being used overnight and making the tank operate as if
it was much larger. It is necessary to add a mixing/tempering valve to keep the exit water safe at about 120F which
prevents scalding.   


 

The theory is simple,

1. Hold the panel at power point voltage to get maximum energy transfer.
2. When a MPPT charge controller isn't using all the power the panel can produce, panel voltage increases. When
   that voltage increases, enough average current is sent to the the heating element to reduce the array voltage
   back to power point
3. A capacitor bank stores energy from the panel when the heating element is off.
4. The capacitor bank provides energy along with the panel when the heating element is on. The heating element
   always sees the same current when on. This capacitor bank is an absolute necessity and needs to be large enough
   to store all the panels power during the minimum off time. 
5. A comparator circuit keeps the capacitor bank voltage in a narrow voltage range for maximum efficiency.
6. The power sent to the heating element is a function of the duty cycle. This is the same principal as a light dimmer.
7. Temperature can be used to calculate the array power point voltage. This is necessary because two MPPT circuits
   can not operate together on the same array.
8. The power point voltage of the panel remains relatively constant regardless of light intensity. 

There are other considerations needed to keep the circuit stable and prevent self destruction in all conditions. Adding
arc interrupt capability is also needed to allow the use of standard mechanical thermostats without arcing.



A video worth watching is adiabatic capacitor charging which is similar. How losses are incurred is explained.
This professor has many excellent videos on electronics for those more than a beginner.
https://www.youtube.com/watch?v=ZtgOboxMNoU

An easy calculator for calculating how much energy is needed is needed to heat water this.
https://gettopics.com/en/calc/water-heating-energy     Remember once the tank is up to temperature the energy
needed will be the losses and only the replacement water.

Selecting the  proper heating element is critical for best performance. The ideal resistance in ohms is the MPPT
voltage of the array divided by the MPPT current.  This is much like the calculation for direct connect of the
array to the heating element. There is a difference. In direct connect you benefit y having the resistance slightly
higher.  In fact, that resistance can almost double before a real penalty in monthly power totals.  In contrast,
with a capacitor bank duty cycle circuit the resistance will be slightly less for maximum power transfer.

There are many charts for element resistance.  Ohms can be calculated by the rated voltage squared divided by the
rated wattage of the element.  Example 240 X 240 = 57600    57600 / 4500W =  12.8 ohms

The scope display shows the on and off periods.  The pyramid waveform is the 2 volt difference on top of the 60V of
the array capacitor bank.

[OperaHouse]

The only concern is the fusing current of a heating element. Considering that a #20 wire is more than 20A, there is little chance of that. So, the only real concern is that of wattage or losing what you transfer to like water.  Most heating elements are too high density to start with.  They operate at a higher temperature in water than they really should.  This causes scale buildup on the element which further impedes heat transfer.   One concern is that not all heating elements have a metal shield that is grounded.  These tend to be grey looking and are just potted in epoxy at the base fully insulated.  I would prefer not to use those on DC because some leakage is allowed by most cert agency. Electrolysis involves extremely small currents.

Microprocessor is the way to go for most flexibility.  However as many people found with techluck, They are left with no repair options once they fail.  Pulling more than 1KW out of a capacitor bank is electron inappropriate.  You can do it but I prefer to switch in additional heating elements so only a portion is PWM.  An arduino design is quite easy and my first two water heaters ran inside my refrigerator program with just a few lines of code.

 

[Fredrik]

@opera
Is the board you show available for purchase, or are you sharing the layout details?

[OperaHouse]

Just got around to installing the new board last week. Here is a picture and one of the designs someone in Lithuania built for heating with 4 grid tie panels in series. This picture was sometime last winter 978W and about 5KWH a day.  Just got my manual transfer switch yesterday and now I can switch from 60VDC, my camp array, to 120VDC and do further testing at higher voltage.  What is your array voltage.  This can work in parallel with CC.  Let me know the details of V, wattage and what you have for heater elements.  I have these evaluation boards available in US till they run out.  First board has numerous layout changes due to parts used.  I may not make one with corrections due to low interest.  Private email you later about y0or options.

[OperaHouse]

Another one of the 10 gallon water tanks started leaking and pretty startled at the price of new tanks.  I decided to try one of these 50L (13 gal) tanks from China, only $150 shipped.  The nice thing is it has a digital temp readout which I converted to work on 12V.  The insulation is not as good as my ECOsmart and I built it into an insulated semi box.  Advertized as three heater temp ranges but that three position switch was phony with no contacts.  There are some wich are 800W, 1200W 2000W which would be nice for direct connect solar and an automated relay switching.  This one was only 1500W 110V with two elements welded together.  I run it at 62C, whatever that is, for the dishwasher and haven't  needed to add in the extra 6 gallon tank.  The 1/2 inch fittings are funky.  Some metal NPT fittings fit and others don't.  All sealing is supposed to be compression fit on flat washer.  Plastic fitting can fit with pipe dope and a lot of thread.  Overall I'm pretty happy with it.

Next picture is the garage water heater with the new control.  Finally learned how to get a screen shot with my new phone.  My phone carrier switched networks on me and never told me I needed a new phone because they went to a different standard.  Happened just when I was going to camp.  Thought it was just a bad reception area.  Anyway, this shows about 15 minutes of clouds in and out.  Note the voltage remains relatively constant. This is the same voltage the charge controller sees.