Green Building Forum - PV off grid direct connection to an immersion heater

PV off grid direct connection to an immersion heater

johnuready — Mon, 25 Jan 2016 02:12:34 +0000

Anybody moved forward with switching Immersions to accomate a variable PV source of power?

PV off grid direct connection to an immersion heater

fostertom — Mon, 25 Jan 2016 10:57:46 +0000

You mean to match load to available PV output (variable DC voltage)? Why wd that be necessary? Why can't an immersion element just accept whatever voltage is available (up to its rated max, 250v or whatever)?

PV off grid direct connection to an immersion heater

Ed Davies — Mon, 25 Jan 2016 11:08:14 +0000

Tom, see the second post in this thread - it's very inefficient at low light levels. (And a lot of the rest of the first page, for that matter.)

Johnuready, are you the John U who emailed me about this at the end of December? I did reply but didn't hear anything further so wondered if my reply went to whoever's spam folder or something. For background, here's my reply to them:

No, sorry, I don't know of anybody who's actually done this,
yet. At the time I wrote that page I was under the impression
that the system described here:

https://lifeattheendoftheroad.wordpress.com/2012/08/18/work-in-progress/

had multiple immersions but it turns out that it just has
a single custom 24 V one.

That came from http://www.tpfay.co.uk/ who seem like a good
possible source for other more complicated designs. E.g.,
two element immersions for PV in a single standard boss:

http://www.tpfay.co.uk/our-products/immersion-heaters/domestic-immersion-heaters/

I'd be interested to hear more about what you have in mind.
For just direct PV to multiple immersions then an Arduino
ought to be very suitable. Would you be using direct FET
switching or via relays/contactors or what?

For my own house I plan to have quite a few small 60 volt
immersions of maybe a bit less than 1 kW each for a nominally
48 volt system (16 3.2 V LiFePOâ‚„ cells in series) which when
properly balanced and fully charged to 3.6 V/cell would have
a voltage of 57.6 V. Overall management will be done by
something a bit more power (ARM & Linux) but low-level control
of the switching will be via an Arduino.

This is not quite the setup considered in that blog page, of
course, as there will be batteries present. Still, the idea
would be to match the solar (and, perhaps, wind) production
when the batteries are full. Also, there'd be the option to
prioritize at least some water heating when the batteries
are partially full (hence the higher-level Linux-base control).

As I say, I'd be interested to hear more about what you're
thinking,

PV off grid direct connection to an immersion heater

Ed Davies — Mon, 25 Jan 2016 11:20:47 +0000

But, I've been catching up on Ken Boak's blog of recent. He's done a series of posts on an Arduino based MPPT charge controller starting with:

http://sustburbia.blogspot.co.uk/2015/10/a-micro-solar-inverter-based-on-arduino.html

which is leading me to think that the way to go might be a buck mode MPPT feeding directly into a standard immersion.

PV off grid direct connection to an immersion heater

ringi — Mon, 25 Jan 2016 12:07:24 +0000

Assuming the system is sized for the DHW need and there is nothing else to use the output for in summer.

First thought is to design the system for when the sun is at 50% strength, and just except that you are not getting the highest possible output in summer, but the panel size would have been chosen to give enough hot water at other times of year anyway.

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11149 is anther thread about the same problem. Other links I found http://www.ata.org.au/forums/topic/12294

http://www.google.co.uk/patents/US5293447 and http://www.google.com/patents/US20150104160 may be of some use.

PV off grid direct connection to an immersion heater

Ed Davies — Mon, 25 Jan 2016 13:11:54 +0000

To save a bit of irritating copy-and-paste, Ringi's links as actual links:

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11149

http://www.ata.org.au/forums/topic/12294

http://www.google.co.uk/patents/US5293447

http://www.google.com/patents/US20150104160

(PS: I hope those patents get knocked down for being BLEEDIN' OBVIOUS.)

PV off grid direct connection to an immersion heater

johnuready — Mon, 25 Jan 2016 14:34:51 +0000

I'm gearing up to start on a project, look at Navitron under Johnuready "PV water heaters". I was thinking of using a 2kw Aztec Trianco system boiler bits adding the mosfets and using the pump etc to push water around a coil in a tank. Now thinking of building the mosfet load device to test the theory of monitoring a variable voltage and current from a bank of PV panels.

I have designed a switching matrix in my head allowing to switch between 3 No. Immersions in one unit in Parallel and or Series. It could be easy to run an Arduino device to monitor the voltage and current to switch between immersion configurations, hence as close to max transfer you can get by resistance.

In my case it's got to be off grid as DNO said no to any more panels on the grid and with cost of PV panels coming down at self install is real.

Any ideas?

PV off grid direct connection to an immersion heater

johnuready — Mon, 25 Jan 2016 14:42:01 +0000

Forgot to mention, spent weeks,one evening per week with help getting an SQL database running on a Raspberry Pi, that could run with data collection hopefully.

PV off grid direct connection to an immersion heater

Ed Davies — Mon, 25 Jan 2016 16:10:13 +0000

Thanks for that interesting Navitron reference Johnuready - I'd missed that thread. I like that Brian Drury design - particularly his first bipolar emitter-follower circuit.

I'm wondering about something like it with multiple transistors in series. It'd be slightly less appealing as you'd have to insulate the cases of the transistors but would allow the whole thing to run at a bit higher voltage. It's all a compromise but I'd think there's a lot to be said for getting the voltage from the PV as high as possible (to keep the amount of cable required down) while at the same time staying within the ELV voltage limits for safety.

Johnuready, do you want to say more about the switching matrix you have in mind?

ELV = extra-low voltage (where mains is considered low voltage). Less than 60 V AC or 120 V DC.

PV off grid direct connection to an immersion heater

johnuready — Mon, 25 Jan 2016 18:38:08 +0000

I'll get the matrix down on paper. Any electronic engineers out there, how difficult would it be to take Brian's mosfet load circuit to take 120 volt DC. With 4 panels that could be approx 9 amps at 120 volt DC on full sun?

PV off grid direct connection to an immersion heater

johnuready — Tue, 26 Jan 2016 00:37:05 +0000

I was thinking of using an Aztec 2kw boiler by taking out the heating section and using the mosfets on a pipe type device and heating water that way. I use one of these for my only heating on towel rails and 2 radiators they work very well as designed. On the hacked version you could push the water round a tank coil. It comes complete with expansion tank and pump in the case. Look on eBay: 371279987503

You see on Ebay for <Â£100.

Now though the switching of Immersions is beginning to sound more exciting.

PV off grid direct connection to an immersion heater

johnuready — Mon, 15 Feb 2016 17:59:09 +0000

Ed

Been a bit busy but i have at last attached the first draft of a switch matrix for 3 Immersions in 1 boss.

PV off grid direct connection to an immersion heater

johnuready — Mon, 15 Feb 2016 18:03:56 +0000

My thoughts

Immersions in Parallel

I1+I2+I3 = A+D+F
I1+I2 = A+D
I1+I3 = A+F

Immersions Series

I1+I2+I3 = A+C+E
i1+i3 = E
I1+I2=C+D

Immersion & Series

I1+I2 = C+D
I3 = F

PV off grid direct connection to an immersion heater

johnuready — Mon, 15 Feb 2016 18:07:52 +0000

Thoughts on switching, you can get cheap Solid State Relays with the switching from an arduino after sensing the current and voltage from the panels to give the power then matched to a combination of immersion elements to match the resistance required for max power transfer.

Immersion design another day

Any ideas?

PV off grid direct connection to an immersion heater

Ed Davies — Mon, 15 Feb 2016 18:58:23 +0000

Personally, I get confused by diagrams which don't have (conventional) current flowing downhill. Here's my attempt to redraw like that:

PV off grid direct connection to an immersion heater

Ed Davies — Mon, 15 Feb 2016 19:03:10 +0000

Immediate thoughts are:

1) that switches B & C are in parallel and therefore one is redundant and

2) that if A&B (or A&C) or D,E&F are switched on it'll cause a short. Not the end of the world from PV which is current limited anyway so long as switches can deal with breaking that current again (which'll only be a little more than the normal operating current, so they should).

I'll need to think about this a bit more.

PV off grid direct connection to an immersion heater

SteamyTea — Mon, 15 Feb 2016 20:21:40 +0000

If it is DC, you really need DP switching don't you. You can't risk a gas build up if there is a fault.

PV off grid direct connection to an immersion heater

Ed Davies — Mon, 15 Feb 2016 20:48:57 +0000

You might want a double-pole isolator somewhere in the path from the PV panels. I don't see any point in DP switching after that. What do you mean by â€œgas build upâ€?

PV off grid direct connection to an immersion heater

Ed Davies — Mon, 15 Feb 2016 22:29:33 +0000

How about?

Diode orientation is on the assumption that â€œLiveâ€ is positive. Obviously reverse if the other way round.

I've renamed the switches to avoid confusion with your naming.

Any immersion on its own by just closing the switches above and/or below it. Any combination in parallel by again switching on the required individual immersion's switches.

I1 and I2 in series by just closing N2.
I2 and I3 in series by just closing L2.
I1 and I2 in series with I3 in parallel by closing N2 and L3.
I2 and I3 in series with I1 in parallel by closing N1 and L2.

With all switches open it's I1, I2 and I3 in series which'll cause a trickle of heat into the tank. Maybe having a separate thermostat-driven all-off switch separate from the power-point tracking function would be a good idea anyway. Or there could be a switch in series with D1 or D2 to stop this.

I1 and I3 in series is not possible as drawn; it'd need another switch (from the bottom of I1 to the top of I3, maybe via one or both diodes or another diode).

Assuming the diodes keep dioding I don't think you can make a short circuit.

PV off grid direct connection to an immersion heater

johnuready — Mon, 15 Feb 2016 22:54:22 +0000

I assumed that a double pole switch isolator was a must. Also the thermostat with the immersions and the over temperature breaker on the tank would be an override signal back to the Arduino.

I was thinking that the software would look at the power available and switch a suitable configuration. If it was then checked every 10 secs to see if a change was required.

PV off grid direct connection to an immersion heater

johnuready — Mon, 15 Feb 2016 22:57:32 +0000

Thinking of running 4 panels - 120 volt @ 9 amp

PV off grid direct connection to an immersion heater

djh — Tue, 16 Feb 2016 09:40:37 +0000

Posted By: johnureadyAlso the thermostat with the immersions and the over temperature breaker on the tank would be an override signal back to the Arduino.

I'd want at least one limit thermostat that cut the DC power circuit directly, without having to rely on the Arduino being functional.

PV off grid direct connection to an immersion heater

WillInAberdeen — Tue, 16 Feb 2016 10:51:16 +0000

For this kind of project, how do you go about: fuses, RCDs, earthing, and separating the high-power DC from the logic DC and the pipes?
Is it notifiable Part P etc?

PV off grid direct connection to an immersion heater

johnuready — Tue, 16 Feb 2016 13:44:13 +0000

I would imagine that breakers and fuses are as per any DC connected PV to an invertor, to include the labelling etc. As the DC is isolated from any mains connection ie the DC to be switched via Solid Sate Relays from the low voltage Arduino, isolation would be OK. Labelling and documentation as be PV install would be important. One person on this threat mentioned that 120volt DC is classified as low voltage. Probably much less then a string on a normal grid connected invertor

As there is no connection to the grid and it's a self install, Part P, probably not required.

PV off grid direct connection to an immersion heater

Ed Davies — Tue, 16 Feb 2016 14:20:55 +0000

Posted By: johnuready120volt DC is classified as low voltage

From a regulation point of view â€œlowâ€ voltage is up to about 1000 V or so.

ELV (extra-low voltage) has various definitions for different circumstances but is generally less than 50 V AC or 120 V DC. Lot to be said for keeping the Voc of your panels on a cold sunny day to less than that.

Edit: looked low voltage up; it's up to 1000 V AC or 1500 V DC between conductors, 600 V AC or 900 V DC from any conductor to earth.

PV off grid direct connection to an immersion heater

Ed Davies — Tue, 16 Feb 2016 15:44:13 +0000

Posted By: johnureadyAs there is no connection to the grid and it's a self install, Part P, probably not required.

Part P definitely applies. Whether it's notifiable under Part P is another question. Personally, I can't see why cat-5 network cabling pinned to the skirting board isn't notifiable but maybe I'm missing something but thankfully I live in a country where Part P doesn't apply (Scotland) so I haven't looked too hard.

PV off grid direct connection to an immersion heater

chuckey — Fri, 19 Feb 2016 18:33:53 +0000

I am considering this sort of thing. Batteries are a no no, doubles the cost of the installation. I am of the opinion that a copper tank filled with water would absorb heat very readily from an external heater (remove the foam first!). The nicest way would be to use stainless steel banding with a layer of mylar tape between it and the tank, then it could easily be tensioned with external springs. If the resistors are selected to have a binary value (R, 2R, 4R, 8R. . ) then by switching them in via suitable logic 15 of resistance between 8R and about .5R can be used.
To find the best value you need either a multiplier IC (V X I = W), so you measure the volts and the current, look at the output of the IC, remember it. Alter the resistors, see if the new W is greater then the first. repeat ad nauseum. Or just calibrate the resistor switching with a voltage sensing IC and a load of comparison circuits. So you go out read the voltage see what the current is, fiddle with the resistors until V X I is a max, then set one comparator to select this combination, repeat for different levels of sunlight. I would not use a microprocessor unless you like programming them and are prepared to keep the thing going for at least 10 years.
Frank

PV off grid direct connection to an immersion heater

Ed Davies — Fri, 19 Feb 2016 19:21:26 +0000

Yes - just sticking the resistors on the outside of the tank could work well. Lots of room so could have quite a few. You'd want good insulation which is quite heat resistant over them, of course. Thermal paste would help couple to the curved tank surface. Not sure what the cheapest stuff is that can be got in â€œbulkâ€ - i.e., lots of more than a few grams.

Using a microprocessor (e.g., Arduino) would probably be simpler and cheaper. Just measure the PV voltage and switch resistors on and off as required to keep it around a target Vmpp - knowing that when the current is low the target Vmpp could be a little lower. That'd be plenty accurate enough, I'd think.

PV off grid direct connection to an immersion heater

djh — Fri, 19 Feb 2016 20:33:55 +0000

Posted By: chuckeyswitching them in via suitable logic

I suspect you might find gray codes useful in this application.

I would not use a microprocessor unless you like programming them and are prepared to keep the thing going for at least 10 years.

Indeed, best to choose a popular design that will be available for a long time. Pi would be my pick at the moment for that reason.

Posted By: chuckeyjust calibrate the resistor switching with a voltage sensing IC and a load of comparison circuits

But the same question of longevity applies to this arrangement, not to mention the logic circuits to do the encoding and all the possible dry solder joints.

PV off grid direct connection to an immersion heater

johnuready — Mon, 22 Feb 2016 20:54:01 +0000

Chuckey

You mentioned reading the voltage, can you explain further using the example of 4 Panels @ 250w per panel.