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Author Topic: A simple Idea ?  (Read 5331 times)
biff
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« on: November 26, 2012, 11:24:37 AM »

I wonder would  this work,
                    I already have a couple of 1kw x 120vdc arrays which dump there surplus energy in to our H/W/C and water heater via 138/9 vdc x 2kw immersion heaters.This works very well.However,I have been wondering if it would be possible to bypass the battery completly and just connect the solar array straight into the 2kw x 138/9 dc immersion without even going near the controller.Perhaps a couple of blocking diodes on the - and + coming from the array.The voltage could go from zero to 144vdc and the immersion should be able to handle anything that the array could throw at it.
  What could possibly go wrong with such a simple idea,?
                                                                      Biff
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jonesy
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« Reply #1 on: November 26, 2012, 11:41:26 AM »

I spose by going direct, you take stress off the controller, and can't discharge the battery.  So why not?
You won't need any blocking diodes on an immersion directly connected to a panel.
If you controller is mppt I could imagine that the direct connected panel would not optimally pull power from the panel, but that's only normally a problem with say a 36V panel on a 24V battery.
I used to connect my 240V kettle straight to the 120V panels.  Took a bit longer to boil, but where's the hurry.
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biff
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« Reply #2 on: November 26, 2012, 11:48:26 AM »

Thanks Jonesy,
                  I had the immersions specially ordered in 138/9 vdc.The panels can get up to 144vdc +,just wondering if it might burn the element out on the immersion,?
               Biff
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pj
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« Reply #3 on: November 26, 2012, 11:50:20 AM »

Biff,
I did suggest this once before:
http://www.navitron.org.uk/forum/index.php/topic,17421.msg199275.html#msg199275

Although it would work, the main problem is efficiency - the resistive load needs to vary to match the output of the panels at varying light levels. So the downside is you only get some fraction of the power available.

I've been looking at Power Optimisers to overcome this - they sem to be DC-DC convertors, varying the output volts to maximise power transfer. If this works, then a very simple system.

regards PJ
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biff
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« Reply #4 on: November 26, 2012, 12:10:01 PM »

I have  72volt and a 48 volt forklift battery chargers.  each 80amps.
                                                    I could pull the caps out of these and string them in series to 120volt,This would take the place of the battery and it could have its own controller. It would be just like the normal battery system but could be allowed to dip below the permissable voltage.The caps would only have to come to life when the current begins to flow,
                  Would I need anything else,?
                                             Biff
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jonesy
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« Reply #5 on: November 26, 2012, 01:12:06 PM »

I have to say I'd be tempted to run a GTI on the panels (assuming you can connect a GTI to your inverter) and pull the excess off at AC.  That way you can use normal AC switches and the standard immersion thermostat. You lose a bit with the GTI efficiency, but that will be much less than a cheapo DC/DC converter.
Can you not re-purpose the motor controller out of a fork lift to drive the immersion.
I doubt you'll burn the elements out as the panels will nominally sit at Vmpp.  I did observe my panel voltage drifting up on lighter loads, but you'll have a permanent load.
I'd be tempted to try a direct connection and see how it goes.  If you find efficiency is poor then try the controller/caps, but unless they are in the millifarad range I don't think they'll have much affect.
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stannn
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« Reply #6 on: November 26, 2012, 02:32:54 PM »

This is a strange turnout. I'd always feared that Biff's idea risked damaging the PV panels.
Stan
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biff
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« Reply #7 on: November 26, 2012, 03:15:06 PM »

The current is quite low,
                    Less than 9amps in full sunshine delivers 1kw.I wonder would installing a blocking diode take away from the performance.?That would do away with the worry of the panels cooking.Its not a bad idea and its certainly a short cut but then short cuts have a habit of being painfull.I dont think I could connect a GTI to our chinese 2kw inverter.I would love to give this a go,I have a spare set of 120vdc panels sitting ready to be fitted.Did not need them during the summer,we had loads of power and was afraid of perhaps frying and drying out the battery.The sun is not as strong now and this would be an idea time to experiment. My main aim,if I got it to work sucessfully was to fit it to a willis heater on our triple coil solar cylinder.This would circulate the water in the cylinder and stop the overheating at the top which became a problem during the summer.The cylinder is 1800 x 550.I thought that this would take care of the dump load from the turbine but the top of the cylinder where the dc immersion sits,is 600mm from the coil and expansion pipe stat and the stat could not trigger the central heating circulation pump to cool the water at the head of the tank and as a consequence,the water became scalding hot. If push comes to a shove,I can make up another pack from a lot of old powersafe agms,and use the controller dump load to heat the willis.Still,I like the idea of the panels going direct in to immersion. banghead
                                                                Biff
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pj
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« Reply #8 on: November 26, 2012, 03:52:23 PM »

You lose a bit with the GTI efficiency, but that will be much less than a cheapo DC/DC converter.

I just looked at the Solar Edge Optimisers, they are claiming 98.8% weighted efficiency - you'd be hard pushed to beat that with a GTI  Huh
They are full MPPT as well.
« Last Edit: November 26, 2012, 03:54:24 PM by pj » Logged

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biff
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« Reply #9 on: November 26, 2012, 04:06:19 PM »

I will look into that,
                A bargain @ 50euros,? fight
                                biff
       
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pj
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« Reply #10 on: November 26, 2012, 04:17:37 PM »

Biff,
You'll need to get the standalone version, not the ones that expect to talk to a GTI.
Also, the normal thermostat in an immersion expects to switch AC, and will not be able to switch off the DC load (arc-ing and all that) - you'll need to drive an intermediate high current dc relay.
regards PJ
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biff
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« Reply #11 on: November 26, 2012, 04:35:26 PM »

Thanks for the help PJ,
                             I would be bypassing the thermostat and using the expansion pipe stat to trigger the circulation pump on the C/H ststem to keep the Tank from overheating.
 I had a look at solaredge,s blether,,very impressive but not a word about costs which means it is extremely pricy.
     A standalone version sounds good,,
                                   Biff
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