Author Topic: UK PV water heater DIY MPPT booster  (Read 484 times)

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Offline fourtytwo

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UK PV water heater DIY MPPT booster
« on: April 23, 2017, 07:41:12 AM »
Hi All this is my 2nd year of PV production having dipped my toe in the water last year with a 520W system to trial the performance in my location, it has been so successful (e.g. no need to run an oil boiler in the summer) I have added two more panels to increase the power to 1040W. I must confess to not hand building the panels though I did investigate thoroughly but got a good deal on some panels at about 40pence/watt. Part of the reason for using PV is the startup cost to me as an electronics engineer is far lower than an equivalent solar/water heating system and I accept the consequential loss of efficiency.

The interesting part is converting the maximum solar energy into heating the water, as everybody knows to extract the maximum output from the panels they must be run at there maximum power point. I had already decided to use the existing electric water heater often fitted in our house water cylinders here in the UK as a backup in case of boiler failure (a long story but I have what some may say is an old fashioned indirect water heating system) this being a 240Vrms 3Kw element with a cold resistance of appx 19 Ohms.

After quite a bit of spreadsheeting with various panel and converter schemes the power loss of a simple boost converter outweighed the complexity of a full monty sepix converter (allowing both boost & buck), basically the boost converter only operates when it is possible to extract more power from the panels than the direct connection of the heater alone, this occurs at above about 50% insolation for these 60 cell panels with a 19R load.

A serious problem with boost converters is input ripple current, if this is to high a percentage of the nominal current the PV efficiency suffers accordingly, there are several solutions, lots of expensive capacitors, multiple converter phases or high frequency operation, I chose the latter but constrained by breadboard construction, switching losses and the chosen micro-controller (PIC) pwm frequency range and is in fact 125Khz.

The inductor is constructed using hand made Litz wire on an ETD44 core, the mosfet and schottky diode are both good for 200V output (2.1kW), the only capacitors required at this frequency being film rather than electrolytic. One word of warning about film capacitors they can have a surprisingly large ESR and it is often unspecified!

Cooling was originally by conduction to the steel enclosure but at continuous high powers the rather thin aluminum U channel used to conduct the heat was inadequate and caused thermal shutdowns. In redesigning the system for 1kW this has been replaced with a heavy extrusion force air cooled (when required).

Another issue with PV is generating the operating voltage for the control system in an efficient manner with a low start up voltage, originally this was done using a discrete linear regulator (not many chips for 80V) but with the advent of the fan this became a semi-discreet switching regulator that handles inputs as high as 150V (1kW system).

Apart from it's other duties the PIC processor sends out the power generated periodically and this is used by an indoor display/logger unit that shows present production and averages for the last week, month and year the data base can also be dumped to a PC.

I should mention also EMI was a nuisance when trying to use a scope so the mosfet was slowed down and more power dissipated in the snubber to combat it. Also this is not the end of the project and a complimentary GTI is presently under construction to use the surplus power once the water is hot.

Ohh I almost forgot to mention the thermostat! without this the tank would boil but switching 1kW at high voltage DC will destroy mechanical contacts so a mosfet is used controlled by a tank sensor, this raises the cut-in insolation slightly as the mosfet is in turn protected from low gate voltages. Note the converter keeps running to power the forthcoming GTI after the tank reaches it's setpoint.

I have attached some pictures often with weird file names to illustrate the parts of the system.
I would like to thank this website for its inspiration and those who helped me procure some of the more esoteric parts :)

Offline Pete

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Re: UK PV water heater DIY MPPT booster
« Reply #1 on: May 02, 2017, 03:40:33 AM »
Hello 42, looking at your photos it appears that the tank temperature is not getting very high. Do you boost the tank with mains power at night to bring it up above 60 degrees. The reason I ask is that if the water is stored below 60 degrees for too long it can breed Legionaires bacteria.
Looks good though, pretty comprehensive data logging setup.
Cheerio
Pete

Offline fourtytwo

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Re: UK PV water heater DIY MPPT booster
« Reply #2 on: May 02, 2017, 03:39:21 PM »
Hi Pete many thanks for the tip! I guess that pic is quite old and when it was just a 2 panel system, these days I have the opposite problem, to much heat! Seriously though if I have a bad day and the temps are a bit low I set the oil boiler to come on the following morning and boost it, this very rarely happens now.
What a wonderful place you live :)
Roger

Offline MadScientist267

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Re: UK PV water heater DIY MPPT booster
« Reply #3 on: May 02, 2017, 09:30:59 PM »
Fourtytwo - Welcome to the forum :)

Looks like a fun project... A couple of things came to mind reading the original post... With the frequency you're running at, and schottky diodes, I'm imagining that's where a fair bit of loss is coming in...?

Wondering if you've considered sync rectification for a further mod to this version or possibly as part of a down the road revision? A little more involved to bring together but would generate a lot less heat at those speeds.

Either way, looks pretty good. Looking forward to hearing more on it as it develops.

Steve
Wanted: Schrödinger's cat, dead and alive.

Offline fourtytwo

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Re: UK PV water heater DIY MPPT booster
« Reply #4 on: May 03, 2017, 01:28:38 AM »
Hi Steve, yup fun is always 1st priority :)

Well being a boost converter the diode is high side and operating at nearly 200V above ground so a mosfet would be a pain to drive up there! As for efficiency the simulated loss of the diode is around 5W at over 1Kw output (0.5%) so IMOP not worth the cost/complexity.
I am loosing more in the mosfet/snubber deliberately to reduce EMI, I know a much better way, construct it on a pcb to reduce the switching node loop length but once again cost comes in to play as a pcb of that size would be ~£50/$65USD.

Like all designs it's a compromise :) fortunately I had a big block of aluminum kicking around in the junk-box and that provides so much thermal lag that with typical British weather (sunny periods) the fan doesn't even come on, but it may do in July/August (hope) :)

The GTI portion is much harder, apart from having problems compensating the output LCL filter I think I have blown yet another unfolder mosfet and when integrated with the booster exposed a latent problem in the boosters aux psu that it shares causing the boosters thermostat mosfet to blow! All in a day's work and everything is at least 10 times more complicated than you first thought hahaha

Many thanks for your comments and I hope to keep posting progress all be it slow :)
Roger

Offline eidolon

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Re: UK PV water heater DIY MPPT booster
« Reply #5 on: May 03, 2017, 09:09:14 AM »
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.

Offline fourtytwo

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Re: UK PV water heater DIY MPPT booster
« Reply #6 on: May 03, 2017, 11:52:44 AM »
I'm surprised that water heater tanks exist at all in the UK
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.

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.

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 ?

The "storage" is with a capacitor bank of about a dozen capacitors, hardly expensive.
May I ask what the capacitors do I am intrigued ?

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

Offline Pete

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Re: UK PV water heater DIY MPPT booster
« Reply #7 on: May 03, 2017, 05:02:47 PM »
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.

Offline eidolon

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Re: UK PV water heater DIY MPPT booster
« Reply #8 on: May 03, 2017, 05:33:37 PM »
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.

Offline eidolon

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Re: UK PV water heater DIY MPPT booster
« Reply #9 on: May 04, 2017, 05:25:47 AM »
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.

Offline fourtytwo

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Re: UK PV water heater DIY MPPT booster
« Reply #10 on: May 04, 2017, 11:49:33 AM »
Here is a guy that sells a similar product, I'll let him explain. techluck.com

Thanks for the link, I got it, its a charge pump wanabe voltage doubler, like you say capacitor killer! In theory with standard 10 Amp panels in a series string the ripple current is a hefty 20A and the ripple current on the panels horrendous reducing there efficiency enormously.  I recognize this guys site he used to peddle power factor correction for reducing your electric bills till someone finally shut him down!

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.
Umm not without rewinding the trafo I think and anyway its a PV system, why would I want a wall wart in it :)

Offline eidolon

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Re: UK PV water heater DIY MPPT booster
« Reply #11 on: May 04, 2017, 04:04:43 PM »
No, it is not a voltage doubler nor do you want it to be. It is like any other power
supply storing a charge between cycle peaks. If you add some external capacitors for
added current capacity it will be just fine. Just like all those little converter boards
on ebay.  The caps are just big enough to make them stable.  Run any current nad they
will pop he top.  Odd thing is, a FET could replace the IGBT in that design with practically
no circuit change and loose that 20W of heat.  Probably a copy of an old design before there
were good cheap FET in abundance.  IGBT were the semiconductor of the month, still great for
VFD motor drives. His design just needs a revisit.

The techluck unit also does not have any temperature control.  It relies on the existing mechanical snap switches of the water heater. Breaking DC above 35V is problematic.  They may never allow the PWM to get to 100%, thus allowing some arc extinguishing.  I know others who have used water heaters as DC dump loads have had switch problems.  I would just like to see an electronic switch off as the first line of defense for overheating.

Wall warts are nifty voltage sources for those panel meters that measure 100V but the
supply can't be powered with more than 25V. Cell phone chargers are 5V at about an amp.
they can easily supply 100ma for a micro or whatever. Wouldn't want to use them as the
voltage reference of the board.  Transformer does not need to be rewound.  Some won't
operate because of the low voltage detect circuit.  This can easily be fooled if opened
up.  Even the TNY series chip can operate with as low as 12V by putting a 100K from power
to the bypass pin.  I have a half dozen operating in my system and even my laptop will
charge from 50V DC. I have a thrift store I go to that sells them for 29 cents.  If you
open them up, like the HP printer ones, you can change the output voltage or eliminate the
diode drops and up to 100 ohms of resistance on the line side for noise filtering.


Offline fourtytwo

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Re: UK PV water heater DIY MPPT booster
« Reply #12 on: May 06, 2017, 09:56:39 AM »
I am sorry this seems to have gotten way off the thread topic, if you want to submit a schematic that makes your system comprehensible that might help. In no way would I condone the modification or breaking open of safety isolated products that are UL approved for a reason (fire), it is unlikely they are as safe to users for shock hazard after re-assembly. Please do not pollute this thread with such hazardous exploits.