Excess PV Power Diverted At Power Point To Heat Water Efficiently

efficientPV

Solar Addict
To many it will just be entertainment. To the few that can understand it will be quite valuable.

This picture was handy and it had an expanded scale covering a little more than one day. It is the primary 6 gallon ECOsmart heater and typical of
daily cycle. This data taken last summer,time and date were not corrected after battery change. I hadn't saved it as a data file, just a pdf. Using
a caliper to determine time, it takes 2 hours to come up to temperature. The start of heating generally begins a little after 7am. Remember that
this array also charges the battery and is running the fridge. Even at this recharge time there is some excess energy available. Over nite it drops
20F from 127F to 107F. Daily repeat cycle time is about every 1.5hrs for a 3F drop. That gives an indication of expected heat loss. Tank is at
temperature 8 hours a day. Heating stops for 14 hours. Literature states tank can be set up to 150F, but max setting is actually 127F. This was
probably to avoid scalding water lawsuits.

The current 9 gallon preheat tank in series has more than double (>40F) the heat loss even with additional fiberglass blanket overnight. It still
shuts off mid day and is set to 140F. Tank was used when I obtained it. So old that it still uses fiberglass as insulation.

ECOsmart 6 water heater was chosen as a replacement for a leaking 10 gallon tank as it was available new on craigslist at a super cheap price. It is
also typical of a tank that many would use for a small camp with a 120V heating element that can not be changed. This overall design was intended
to supply some free hot water using existing panels of a small camp where ready hot water would be a welcome luxury. Even in a minimal configuration, one day could be used as a pre heat for the tank with some washing and a shower the following day when up to full temperature.
 

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gnubie

Photon Sorcerer
That's a little different again, 60V and 10.8 ohms, maybe we can fix it there. That gives a maximum rate into the tank of approx 330 watts. There has been no mention of actual water flow out of the 6 gal tank, nor the inlet temperature, nor the hours / day the array is capable of holding 60v while 330 watts is being drawn from it so who knows what is going on.

It's one thing to hold a tank at a given temperature with nothing coming out of it, it's another thing entirely if we are taking water in at 20c (made up because it hasn't been mentioned) and raising it to over 50. Posting graphs of an idle tank holding temperature with isn't all that useful.

Either way I'm not debating that heating water with PV is not possible. It certainly is. All I'm interested in is what efficientPV is really doing in terms of circuit (it'd be really nice if a complete diagram was posted), in terms of solar array capabilities, in terms of volume of water and in terms of temperature differential - ie, water in temperature vs water out temperature.
 

efficientPV

Solar Addict
Every design is different depending on the application. These are the building blocks of a system. Is there one element or more, multiple tanks, is stratification of the tank used to increase efficiency, mechanical or electronic thermostat, operating voltage and is a charge controller used in parallel. A simple box to connect any panel to any heater would be too complex. This is design for purpose at the lowest cost. This requires serious understanding of the problem. How much power is needed for water use is a lookup and for serious solar enthusiasts only. Motivated people can and have figures it out. I've done the heavy lifting and will answer any serious questions.

These pictures are from a system video that was built in northern Europe which is a IR2153 design mentioned before. I was notified a long time but could never view the until recently when it was switched from a private video to an unlisted one which I could view. The current array is four grid tie panels totaling 1200W. This water heating system first ran at 60V for a short while till he gaind confidence. The the board was later converted to 120V DC to better match the heating element with a couple more FET and a heatsink. The builder choose to hand etch the circuit board in a very spacious layout. Except for the FET, it would easily fit on a small perforated board like the ones from China that are 5 for $2. This was his first electronics build.
Pretty impressive.
 

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Capt Bill

Sailing Options
I've been diverting excess PV power to heat water at my camp for years. It is the only source of hot water as I got rid of my propane heater. A simple method is used to determine when there is excess power. At the power point voltage, the maximum power is obtained from the panels. Use less power and the voltage will rise up to the open circuit voltage depending on the current. Light intensity only changes the panels current, not the power point voltage. This circuit diverts enough current to drop the panels voltage back to the power point. It can be used in parallel with PWM and MPPT controllers. No battery is needed and it can operate in stand alone mode.

The principal of operation is energy is stored in a capacitor bank and is PWMed to a heating element. Here is a simple example. Think of a 10A heating element which is ideally matched to the solar in best conditions. In less than ideal sun that panel can produce 5A. For 50% of the time the panels 5A charges the capacitor bank when the heating element is turned off. In the next 50% the heating element turns on and receives a full 10A, 5A from the solar panel and 5A from the capacitor bank. The panel always sees the same ideal voltage so it operates at maximum power. The duty cycle is changed by sensing thevoltage on the capacitor bank and is kept within a fraction of a volt. Updates are 120 times a second.

Here is a video of a commercial product AC7931 that does this. I have not personally used it. It does miss design targets because it requires an external isolated power supply, has to use an internal fan, and capacitor bank may not be large enough for long life.


There are many circuits that can accomplish this from discrete to micro and I've built most of them. This is the heater control I use at home. It is an unconventional use based on a IR2153 half bridge driver chip which provides many of the functions needed at very low current. It is highly stable with built in voltage regulator, FET driver, oscillator and under voltage protection to prevent FET damage. Pictured in case is a low cost board from China which was modified. I can no longer recommend these because which board you get now totally random regardless of what the listing says. Some have the correct 8 pin chip and some the 16 pin SG3525 chip. Both can be made into controllers. However, they now come with 55V FET which makes them only suitable for 36V arrays max. Poor trace layout also makes them unsuitable for more than 60V. The very small number of components make it suitable for a perf board construction and I recommend using four 110N15 which are 110A 150V FET.

Reasons to divert power from panels instead of battery

1. Lower current and lower resistance losses with higher voltage
2. Smaller charge controller
3. Smaller battery bank
4. No double or tripple conversion loss from charge controller, battery, inverter
5. Battery left as fully charged as possible
6. Does not interfere with charge cycles
7. Captures energy when charge controller is not transfering maximum
power to battery during charge stages
8. Inability to actually know charge state of battery
9. Can capture even small amounts of extra power
10.Often uses standard heater elements instead of battery types

An interesting feature of this circuit is mechanical AC thermostats can be used in switching primary DC supply without arcing when the array is less than 100V. A capacitor bank of about 6,000uF is required made of multiple capacitors to handle the high surge currents. The front panel consists of a power meter and a pot to set the voltage. A switch also allows direct connect to the heater elements for performance comparison. Here are some typical values. Due to low tilt, maximum panel power is about 395W into 7.5 ohms. 44W may not seem like much, but that is what is needed to replace lost heat. The blue wire out of the power meter is a modification to provide external power. Low power levels of direct connect often do not provide enough voltage to self power the meter. Also shown is a breadboard build.

POWER POINT DIRECT
44W 5W
65W 9W
117W 47W
118W 52W
205W 103W
265W 195W
302W 226W
Wow: Looks like the way to get more hot water from my Solar Panels when my batteries become full (or whenever I dirvert solar panel current to a water heating element). I will come back and study this option more later. I understand basic electricity cicuits way better than the electronic circuits/ and this strikes me as interesting! :+)
 

Henrysangret

New Member
I've been diverting excess PV power to heat water at my camp for years. It is the only source of hot water as I got rid of my propane heater. A simple method is used to determine when there is excess power. At the power point voltage, the maximum power is obtained from the panels. Use less power and the voltage will rise up to the open circuit voltage depending on the current. Light intensity only changes the panels current, not the power point voltage. This circuit diverts enough current to drop the panels voltage back to the power point. It can be used in parallel with PWM and MPPT controllers. No battery is needed and it can operate in stand alone mode.

The principal of operation is energy is stored in a capacitor bank and is PWMed to a heating element. Here is a simple example. Think of a 10A heating element which is ideally matched to the solar in best conditions. In less than ideal sun that panel can produce 5A. For 50% of the time the panels 5A charges the capacitor bank when the heating element is turned off. In the next 50% the heating element turns on and receives a full 10A, 5A from the solar panel and 5A from the capacitor bank. The panel always sees the same ideal voltage so it operates at maximum power. The duty cycle is changed by sensing thevoltage on the capacitor bank and is kept within a fraction of a volt. Updates are 120 times a second.

Here is a video of a commercial product AC7931 that does this. I have not personally used it. It does miss design targets because it requires an external isolated power supply, has to use an internal fan, and capacitor bank may not be large enough for long life.


There are many circuits that can accomplish this from discrete to micro and I've built most of them. This is the heater control I use at home. It is an unconventional use based on a IR2153 half bridge driver chip which provides many of the functions needed at very low current. It is highly stable with built in voltage regulator, FET driver, oscillator and under voltage protection to prevent FET damage. Pictured in case is a low cost board from China which was modified. I can no longer recommend these because which board you get now totally random regardless of what the listing says. Some have the correct 8 pin chip and some the 16 pin SG3525 chip. Both can be made into controllers. However, they now come with 55V FET which makes them only suitable for 36V arrays max. Poor trace layout also makes them unsuitable for more than 60V. The very small number of components make it suitable for a perf board construction and I recommend using four 110N15 which are 110A 150V FET.

Reasons to divert power from panels instead of battery

1. Lower current and lower resistance losses with higher voltage
2. Smaller charge controller
3. Smaller battery bank
4. No double or tripple conversion loss from charge controller, battery, inverter
5. Battery left as fully charged as possible
6. Does not interfere with charge cycles
7. Captures energy when charge controller is not transfering maximum
power to battery during charge stages
8. Inability to actually know charge state of battery
9. Can capture even small amounts of extra power
10.Often uses standard heater elements instead of battery types

An interesting feature of this circuit is mechanical AC thermostats can be used in switching primary DC supply without arcing when the array is less than 100V. A capacitor bank of about 6,000uF is required made of multiple capacitors to handle the high surge currents. The front panel consists of a power meter and a pot to set the voltage. A switch also allows direct connect to the heater elements for performance comparison. Here are some typical values. Due to low tilt, maximum panel power is about 395W into 7.5 ohms. 44W may not seem like much, but that is what is needed to replace lost heat. The blue wire out of the power meter is a modification to provide external power. Low power levels of direct connect often do not provide enough voltage to self power the meter. Also shown is a breadboard build.

POWER POINT DIRECT
44W 5W
65W 9W
117W 47W
118W 52W
205W 103W
265W 195W
302W 226W
You are all doing a interesting study , but a easier way is first off using excess power out of your inverter and do a little work with your controller supplier with the power to grid feature which does all the shunting when the batteries are charged and directs the excess to the water heater.
 

Henrysangret

New Member
An interesting study in thermodynamics lol. There is a easier method which I am doing now. I have 2 water heaters 1 propane and the other electric. I use the electric heater to preheat water to the propane heater, I changed out the the 220v elements to 120v and use the one with the thermostat so I don’t overtemp. I use a solar controller charger and use the inverter to drive the hot water heater using the power to the grid feature. You can set up your priority usage and everything works.
 

efficientPV

Solar Addict
It is amazing what can be done if you approach things as a system. For example, one day at my camp before noon I heated two small tanks of water (16 gallons total), did two loads of laundry using hot water for all cycles (it is fed into the cold water inlet from its own 40 gallon tank). That hot water keeps the dispenser spotless with no buildup. Refrigerator was down to temperature. And ran the dishwasher with heated dry. Of course the battery was also charged. It is only a single car battery. You can't do anything like that with conventional solar equipment.
 

Henrysangret

New Member
It is amazing what can be done if you approach things as a system. For example, one day at my camp before noon I heated two small tanks of water (16 gallons total), did two loads of laundry using hot water for all cycles (it is fed into the cold water inlet from its own 40 gallon tank). That hot water keeps the dispenser spotless with no buildup. Refrigerator was down to temperature. And ran the dishwasher with heated dry. Of course the battery was also charged. It is only a single car battery. You can't do anything like that with conventional solar equipment.
Sounds like you have everything down to a science...
 

Capt Bill

Sailing Options
This circuit diverts enough current to drop the panels voltage back to the power point. It can be used in parallel with PWM and MPPT controllers. No battery is needed
Wow; I think this is really an ingenious use of your options. Your posts is spiking my interest to study more about electronics (as I have not yet built a DIY board w electronic components). I am heading for auto controls to heat up water via 120 - 240vac options first; as my my 6000 watt solar system is currently generating excess power I am lining options options to use (plus has me exploring what my least expensive option might be for a legal grid tie). I would like to experiment with your PV direct to water heating way at a later date. I marked this thread to come back to; ... while wondering what kind of efficiency difference would I see between Solar PV power direct to Water Heating; contrasted to an efficient 240ac circuit to water heat coil(s)??? Does Solar PV Direct heat water more efficiently than running Solar PV through an MPPT converstion, and then the inverter to 120 or 240vac to water heating coil(s)? What kind of difference would I see ??? ... I like the automation impression I have have your setup from what I have taken in so far. Thanks for this post :+)
 
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efficientPV

Solar Addict
Didn't you get that one from Poland? In a few weeks someone here may do a post on the two systems he will be installing with my board. This will be a first. One will be stand alone and the other in parallel with a charging system. I have been sent several private videos and pictures of systems build. None of them have wanted to go public because of the comments they will get.
 
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