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

PV off grid direct connection to an immersion heater

bxman — Wed, 01 Feb 2017 21:07:45 +0000

Hi Ed

So you are saying that when the cloud came over heavy and it dropped to a 30 watt yield .

I should have been able to have got 260 watt instead with a MPPT controller in circuit.

I have got another set of panels with the same orientation so I can hopefully extend the leads and try and observe the readings on both sets at the same time .

cheers Patrick

PV off grid direct connection to an immersion heater

Ed Davies — Wed, 01 Feb 2017 21:25:57 +0000

Yes, if you can compare two identical sets of panels, one into an MPPT and the other into a fixed resistance that should give a good illustration.

Since my previous post I've been wondering what I have as a resistor which I could put on my little toy amorphous panels to do a similar experiment.

PV off grid direct connection to an immersion heater

bxman — Sun, 05 Feb 2017 11:31:38 +0000

Good Morning all Ed in particular

The Night storage heater I am using has 3 element .

I have managed to obtain some spare elements

and proved yield is substantially increased when connected to a single element .

But still clearly very inefficient except when in full sun

looks like I need a switching arrangement along the lines @johnuready and Ed suggest .

But do they make 60 or 110 v elements that I could use ?

PV off grid direct connection to an immersion heater

Ed Davies — Sun, 05 Feb 2017 12:23:52 +0000

Excellent, thanks Bxman.

I was going to connect my little â€œ60 Wâ€ [Â¹] Maplin panels to a 2 kW heater this morning. Wiring the four panels in series/parallel (2S2P [Â²]) and switching the heater between the 750 W element, the 1250 W element and both together would allow not-terrible matches over a reasonable range of brightnesses. I had a junction box set up for 2S2P to give nominal 24 V to feed my MPPT controller with a 12 V battery but I can't find it now - got all the other bits but it must be in some mystery box in my spare bedroom. Your confirmation saves me bodging it.

Posted By: bxmanBut still clearly very inefficient except when in full sun [â€¦] But do they make 60 or 110 v elements that I could use ?

Are you sure that's what you want? If they're approximately matched in full sun then for lower sun you'd want more resistance (same voltage, less current) but 60 or 110 V elements of the same power would have higher current and therefore lower resistance.

Presumably your heater presently has the elements in parallel. Maybe arranging them in series would be more appropriate.

[Â¹] Actually about 45 W, I think.

[Â²] Or 2P2S if you follow the opposite convention. Where's the headslap emoticon when you need it?

PV off grid direct connection to an immersion heater

johnuready — Sun, 05 Feb 2017 19:05:36 +0000

I was thinking that the closer you can get to Max Power Point voltage on a fixed resistance is best. ie The lowest resistance you can get to a perfect resistance match will generate max power using V * V / R

PV off grid direct connection to an immersion heater

johnuready — Sun, 05 Feb 2017 19:22:34 +0000

I find the following a great help to work out DC variables:

PV off grid direct connection to an immersion heater

johnuready — Sun, 05 Feb 2017 22:34:41 +0000

If you have 2kW max @ 240v DC into 70 ohms the max power from the immersion is 822 watts. To achieve more power the resistance must be less.

Or have I got it wrong?

John

PV off grid direct connection to an immersion heater

Ed Davies — Sun, 05 Feb 2017 22:48:09 +0000

Yes, you're right. But he has three 70 Î© ohm elements in parallel so can absorb 3Ã—822 = 2466â€¯W at most.

PV off grid direct connection to an immersion heater

johnuready — Sun, 05 Feb 2017 22:58:00 +0000

Ed

Understand, pleased as all my calculations I have worked out using the above.

I hope to switch coil combinations when the voltage approaches the max power point on the highest resistance then switching progressively to the lowest resistance setup.

Probably 4 weeks away from trying the software with the Solid State Relay Switching and a set of working numbers.

PV off grid direct connection to an immersion heater

bxman — Tue, 07 Feb 2017 12:23:14 +0000

n anyone advise or help with tips on preserving format thanks

Keith can you help I seem to have lost what I had half an hour ago

thanks Patrick

PV off grid direct connection to an immersion heater

johnuready — Tue, 07 Feb 2017 12:57:57 +0000

Interesting readings, your 3 No. coils in parallel look to taking max power for the resistance.

I'm pleased to see that the single element was max power at 240v as I'm going to track voltage to shift coils and that coil would have reached max.

Anybody, am I right in my thinking on the above?

PV off grid direct connection to an immersion heater

bxman — Tue, 07 Feb 2017 13:39:55 +0000

@johnmuready
you clearly saw it before it disappeared I hope Keith can find it again If not I will have to have another go this evening
cheers Patrick

PV off grid direct connection to an immersion heater

bxman — Tue, 07 Feb 2017 13:48:41 +0000

`I have now managed to get some readings hope they will be useful

Single string of 8 JA Solar (JAM6 R 270)

directly into a Night Storage Heater.

The storage heater has 3 elements

I have 3 additional identical elements which can switch in and out as 1,2 or ,3 in series or the other 3 in parallel

Good Sun now ------------------------------------------------- + earlier on

3 in parallel ---- 6.5 amps @155v

to a single element 3.5 amps @240v ------------- 1amp @24v

to 2 elements 1.9 amps @ 265v ------------------ 1 amp @42v

and 3 elements 1.4 amps @ 285v ------------------ 1 amp @ 215v

I tried putting 4 in parallel and it appeared that the current did not increase bot it did drop the voltage

seems like the 210 ohm load maximises the current to 1.4 amps and the 140 ohm at about 2 amps

open circuit voltage is around 285v

Edit
SORRY ABOUT THE LOSS OF FORMAT tried saving both ways and makes no difference
better now but not how I had hoped

PV off grid direct connection to an immersion heater

Ed Davies — Tue, 07 Feb 2017 13:51:24 +0000

Yep, shame it's not easy to do tables in this software AFAIK. Trying HTML table elements doesn't seem to work, either (cjard will, no doubt, take that as a challenge ).

Still, I'm very confused by your â€œearlier onâ€ readings:

1 element: 1â€¯A at 215â€¯V
2 elements: 1â€¯A at 42â€¯V
3 elements: 1â€¯A at 24â€¯V

Are you sure that's the right way round? I'd expect the voltage to be higher with more elements than with fewer. Or is that the voltage across a single element? In which case, I think it's inconsistent with the full sun readings.

PV off grid direct connection to an immersion heater

bxman — Tue, 07 Feb 2017 14:01:47 +0000

@ Ed

Quite right

corrected now I hope !

PV off grid direct connection to an immersion heater

Ed Davies — Tue, 07 Feb 2017 16:05:21 +0000

Thanks, so:

number of elements, bright-sun power, dim-young-sun power:
3P | 1007W | - 1S | 840W | 24W 2S | 503W | 42W 3S | 399W | 215W

(PS, ha! using HTML and tags (â€œteletypeâ€) gives a fixed-pitch font which at least makes tables a bit more reproducible. Also <pre> and tags preserve whitespace (rather than packing runs of two or more spaces down to one) which also helps. Though it does add some vertical whitespace underneath the table for some reason - I seem to remember trying this before and the vertical whitespace causing some sort of mayhem.)

PV off grid direct connection to an immersion heater

bxman — Tue, 07 Feb 2017 17:57:10 +0000

Ed
I will try again tomorrow

I am almost sure the higher resistance gave the best yield in the lower light levels.

do you mean Tab's by ?

old age is no help I am afraid

PV off grid direct connection to an immersion heater

Ed Davies — Tue, 07 Feb 2017 19:47:02 +0000

Posted By: bxmanI am almost sure the higher resistance gave the best yield in the lower light levels.

It did: 215 watts vs 24 watts for the low resistance.

That's what you'd expect. To get maximum power you want to operate at the maximum-power-point voltage. For your panels the Vmp for each panel is a tad over 30 volts in full sun, a tad under in dimmer light. So for the whole lot you want to be operating at around 240 volts for best power. In low sunlight you'll only get a low current out so need a high resistance to make use of all that voltage.

(I don't understand how it could be 24 volts and 1 amp simultaneously into a single 70 Î© resistor. My assumption is that it wasn't actually simultaneous - that you have had to swap the meter round or something - and the sun changed a bit in the mean time.)

No, doesn't act as a tab - it changes the font. The start tag () goes before the table and the end tag () goes after. As input the whole table looked like this: number of elements, bright-sun power, dim-young-sun power: 3P | 1007W | - 1S | 840W | 24W 2S | 503W | 42W 3S | 399W | 215W

PV off grid direct connection to an immersion heater bxman — Mon, 13 Feb 2017 20:27:22 +0000 Not been a lot of sun about until today and I was not that well organised However I have located a 4 th element so now have the option of a 280 ohm load so options are now 3 in // , 4 in series , 3 in series, 2in series and a single element. so this PM I had an OC Voltage of 290 volts 140 ohms 1100ma 150volt 210 ohms 950ma 200volt 280 ohms 780ma 225volt last week 290v OC 20 ohms 1700ma 38 volt 70 ohm 1700ma 230 volt 210 ohms 1400ma 270 volt 280 ohm 1300ma 280 volt PV off grid direct connection to an immersion heater Ed Davies — Mon, 13 Feb 2017 21:14:33 +0000 If there's any reasonable amount of sun the open-circuit voltage will be pretty much the same. The short-circuit current is a better indication of the light level. Your power outputs this week weren't a lot different from each other (165, 190, 175.5 watts) but still the 2S case seemed best compared with the 1S and 3S in dim and bright sun last week so presumably an intermediate amount of sun. PV off grid direct connection to an immersion heater johnuready — Mon, 13 Feb 2017 23:05:04 +0000 bxman I'm also at max for my grid tie system. I'm working with a DC immersion with three coils. All 3 No. coils are 36 volt - 400 watts each. An immersion like this you can buy on Amazon for Â£30 compared to a special build to the correct resistances that will cost Â£700 + My new PV install, I have 4 panels in parallel, approx 30 volts at 8 amps each I proposing to use 6 combinations of the coils, switching coil combinations as the sun level increases and the max voltage is reached. PV off grid direct connection to an immersion heater johnuready — Fri, 03 Mar 2017 17:50:46 +0000 bxman - the notes below follow my thoughts and parts used. I should be live testing in the next two weeks: The configuration of my 4 panels to produce the voltage / current combination was worked back from the immersion. It was very expensive to have a bespoke DC multi coil immersion built. I went for a 3 coil at 36 volt / 400 w per coil, approx Â£30 from Amazon. My panels are in parallel, 30 volts @ 32 amps. This also fits the cheaper Solid State Relays, DC @ 40 amp. Max power transfer will be achieved by using the 3 coils in 6 configurations to provide the best path to max power transfer using a switching matrix of 5 solid stare relays and two diode to switch between the coil configurations. The Solid State Relays are connected to the raspberry Pi through an 8 channel optical isolated relay board. Using the PV voltage to establish the max voltage point per coil, switch configurations and tank temperatures is achieved by using an analogue / digital converter on the Pi supplied by A B Electronics. As the Pi has no real time clock a clock board was supplied by A B Electronics and spare memory on that board to store kWh. Tank over heat protection is provided by a mechanical sensor on the tank to cut off low voltage power to switching solid state relays. To overcome tank stratification an external low voltage circulating pump to be fitted, required as my DC immersion is located in the top 1/3 of the tank. System screen display ( using Nextion screen connected to Pi) showing voltage, amp, matrix configuration and total kWh to date diverted, date and time etc. The Pi, Screen, Analogue to Digital and Real Time Clock programmed using Python. PV off grid direct connection to an immersion heater johnuready — Thu, 23 Mar 2017 09:02:30 +0000 Pleased to say itâ€™s working, steps through coil matrix 1 to 6 to find the best resistance for power transfer every 6 minutes. It has a log file that I write to every time it moves matrix, changes display or stores kWh, date time etc. that holds enough data to chase any bugs, probably small bugs still there. Next step: Give the hardware and software a long term monitored testing. Optimise the sleeping mode on no sun and scanning times during the day Get remote web page working to monitor, hopefully replicating the current display screen plus one day the graphical displays on historical data etc. Get the tank temperature sensors working on display and think about stratification pump. Perhaps a remote screen using bluetooth. PV off grid direct connection to an immersion heater SteamyTea — Thu, 23 Mar 2017 09:06:44 +0000 Any chance of a a wiring diagram and the code you have written. May be helpful to others that want to try the same thing. PV off grid direct connection to an immersion heater johnuready — Thu, 23 Mar 2017 15:00:07 +0000 Coil configuration PV off grid direct connection to an immersion heater johnuready — Thu, 23 Mar 2017 15:09:19 +0000 Looking at the data from the running logs I now think it would lead to very little power loss in my case to reduce the number of coil configuration from the current 6. Perhaps another exercise to crunch the data one day and find out. Less coil configurations could play apart if the current Solid Sate Relays and mini optical isolation relays were replaced with mosfet type switching from a direct connection to the Pi GPIO PV off grid direct connection to an immersion heater johnuready — Fri, 24 Mar 2017 13:30:22 +0000 All 3 coils are approx 3.5 ohms. Configuration: Matrix 1 = 3 coils in series = 10.5 ohms / 92w Matrix 2 = 2 coils in series = 7 ohms / 138w Matrix 3 = 2 coils in parallel and 1 coil in series = 5.25 ohms / 178w Matrix 4 = 1 coil in series = 3.5 ohms / 277w Matrix 5 = 2 coils in parallel = 1.75 ohms / 555w Matrix 6 = 3 coils in parallel = 1.17 ohms / 833w PV off grid direct connection to an immersion heater SeanKettle — Sat, 23 May 2020 15:07:04 +0100 Wondered if anyone had come up with an idiot proof solution for this? Started a similar thread on BuildHub listing a few devices that supposedly do this job - https://forum.buildhub.org.uk/topic/14863-pv-to-thermal-store/