Renewable Energy Questions/Discussion > Solar (heating or electric)

UK PV water heater DIY MPPT booster

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eidolon:
I'm surprised that water heater tanks exist at all in the UK. Must admit it is a little unnerving at first stepping into a shower that has a 240V line going into it and a dodgy installation at that. You are at a disadvantage that 120V heating elements are not prevalent.  I also have been heating water with PV at power point for years.

I use simple PWM to feed two 2000W 120V heating elements in two tanks, a 9 and 19 gallon. These common size heaters work pretty well with a PV voltage of about 52V.  When the first gets up to normal temperature, the second tank starts heating.  If there is enough power, both heaters will be on and allowed to go to higher temperatures.  The "storage" is with a capacitor bank of about a dozen capacitors, hardly expensive.  As one heater is usually fully on the bank doesn't have to be that large. My feeling is to keep each capacitors current to less than a half amp.  My array is 36V just out of convenience and 900W. It also serves to power the rest of the camp as it is off grid.  So, water heating is really just a dump load. I can usually divert 2700WH to heating a day which is more than enough for showers.  Each heater has two parallel FET taken out of old UPS which are plentiful and I use the supplied heat sink.  They don't heat up at all. Micro is a 328 and pwm at 240Hz. If you hold a radio 3 inches from the wire, you can just pick up the noise.  FET drivers are just an opto isolator, slow but sufficient at these speeds and no RF. It also allows me put the control electronics 50 feet away from the FET at the heater.

I did try the boost route at home just to get some data on panel performance in the area using a single 12V panel.

fourtytwo:

--- Quote from: eidolon on May 03, 2017, 09:09:14 am ---I'm surprised that water heater tanks exist at all in the UK
--- End quote ---
Well I am fortunate to have an elderly oil boiler with gravity hot water heating, more "modern" installations use a puny gas boiler with tin heat exchangers doing direct on demand contact heating, useless IMOP The only other houses with hot water cylinders are those built or converted for renewable heating.


--- Quote from: eidolon on May 03, 2017, 09:09:14 am ---I use simple PWM to feed two 2000W 120V heating elements in two tanks, a 9 and 19 gallon. These common size heaters work pretty well with a PV voltage of about 52V. My array is 36V just out of convenience and 900W.
--- End quote ---

I confess to being confused, your heater elements have a resistance of ~7.2Ohms so at 52V you get 375W and at 36V you get 180 watts yet you say your pwm operates at 240hZ far to low for a ferrite boost inductor so how does your system operate, do you have a 120V AC inverter somewhere thats feeding your heaters ?


--- Quote from: eidolon on May 03, 2017, 09:09:14 am ---The "storage" is with a capacitor bank of about a dozen capacitors, hardly expensive.
--- End quote ---
May I ask what the capacitors do I am intrigued ?

Maybe we need to exchange diagrams :)
Many thanks for the comments :)

Pete:
Eidolon, in Australia most houses have storage hot water heaters. The Heater tank is Earthed and so are the water pipes (in houses with copper pipe). So unless both these earth connections fail, if the heater element fails (which they do with age and corrosion), the fuse or circuit breaker trips.
There have been a couple of cases that I have heard of where the earth connection has been severed accidentally or neglected. And people have suffered shocks or death, but very few. The Neutral conductor here is also earthed so there are three layers of protection. It takes a cascade of faults to cause electric shocks caused by water heaters.

eidolon:
The two heaters can be on at the same time, so I can dump 750W. That is close enough to the practical limit of 900W of panels in the summer heat.  Admit it is slightly undersized although that condition can only exist for about an hour.  I wish these small tanks had the ports for two heaters each. I do prefer low density heaters.  If the water is allowed to boil on the element calcium scale will develop on the element.

No transformer is needed.  The PV panels are connected to a capacitor bank.  At 50% duty cycle, 5A will come from the PV panel and 5A from the capacitor bank for half the time.  While the heater is turned off, the PV panel recharges the capacitor.  An over simplification but that is the basic idea.  The PWM duty cycle increases until the voltage on the capacitors drops to the power point.  It is fun to watch it track from 5W up to full power. It only takes a wisp of a cloud to drop he power by half.  To prevent FET heating with narrow pulses a PWM of 5 becomes zero and over 250 it becomes full power.  Then it just sorta bumps. 

When I traveled in Europe, it seemed that everywhere had point of use on demand water heaterswhich are more efficient.  In the US 40 gallon hot water tanks are common and they can take 70-100W of continuous heat just to overcome thermal losses.  At home I have a heat pump connected to the tank.  A single 300W panel connected to the auxiliary resistive element could easily reduce the need for the heat pump to turn on.  The trouble with most PV systems is they have to be oversized.  Using a small panel guarantees 100% of possible PV power is used, the best payback you can get with almost only panel costs.  Everyone can find a place for one or two panels.  With the ease of install I wonder why these supplemental systems aren't commonplace.  People spend $1800+ for the HPWH just to save half in electric. Two PV panels would be less than this.

eidolon:
Here is a guy that sells a similar product, I'll let him explain. techluck.com

I do not recommend it as it has two flaws.  He uses an IGBT instead of a FET so it has to waste 20W in heat because of the saturation voltage. Notice the tiny three caps. If they don't short in a couple years from high ripple currents, they will just dry out from heat open up. The poor sods will never understand why their system has lost performance.

Here is a working module made from a small old computer UPS.  At home I have lots of resources.  At camp I just go out to the garage and sort through boxes of crap. Easy enough to trace out just from the picture.

If you haven't tried it, most electronic wall warts will work at 50VDC at reduced current making it easy to power boards or even high side FET.

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