Author Topic: Matching solar output to consumption.  (Read 512 times)

glort

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Matching solar output to consumption.
« on: April 11, 2018, 05:10:37 AM »

This one will be right up you alley Bruce both in knowledge and understanding.

I am wanting to match my solar output to my grid consumption so as to neither draw ( when sufficient solar is available) nor back feed to the grid.
There is a roll out of Smart meters here which I am able to refuse ( supposedly) But I am concerned they will install one Illegally as they have done to hundreds of people and once they are there, you are stuck with the things.

I'd like to prepare for that possibility so if it does happen, I'm not totally up ship creek.
There is another forum this has been discussed on but the solution's were for electronic engineers that probably build a Plasma TV reciever for fun in their lunch hour not plebs that haven't got a clue like me.

What I would like is a devise or assembly thereof which would monitor the incoming power to the property ( probably with a CT clamp) and monitor the solar output  and match the 2 so there was neither power being drawn or pushed back. Witht he new smart meters, pushing power back results in being charged for it as well.

I would Imagine I'm going to need some PWM controller(s) to regulate the back feed but wondering how they will get on with a GTI?
 There are units like this available commercially but as usual they are priced so the ROI is years and I always feel ripped off with this sort of thing because I know how cheap electronics can be made.

After the outstanding success of the Voltage monitoring relays you put me onto which do everything I want and these expensive commercial units do, I'm wondering if you know of any other components I could wire together to do the current matching between the grid and the solar output?

The other thing I'd like to do for hecks sake now more than anything is run the water heater direct off some panels.  I don't need to with the Voltage relays but I would still like to come up with a SIMPLE way of getting round the DC problem with switching  and not burning the crap out of the thermostat.
I could Go to an inverter but that seems a clunky and inefficient way of doing a simple job. I did see a diagram somewhere that I think was 2 double pole relays with the wiring crossed over from memory that allowed the DC to be switched. 

I have also seen setups that used the heater thermostat to act as a switch for relays.
I have got some DC SSR's but there seems to be some question as to the longevity and robustness of these things as well.

My wife is very supportive of my Solar efforts with the hot water. When I put the Voltage monitor on I also turned up the thermostat. She tells everyone how much hotter the water is now on solar and how much more effective it is.  May be some small thing to it. The thing tends to heat up closer to when we use it in the evenings and through the day than in the early mornings.

Any suggestions and input for these things would be greatly appreciated.


BruceM

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Re: Matching solar output to consumption.
« Reply #1 on: April 11, 2018, 05:41:11 AM »
Glort, I'm confused about your goal re: smart meter.  Why not just get a meter that allows you to legally backfeed.  Sure, it's not as good as your bootleg setup but you would offset your own use completely on sunny days so it's not so bad.  Or is that a problem with licensing, inspections, etc? 

For the direct DC water heater, some custom electronics may be required that may be beyond your comfort zone. Let me sleep on it and see if I can think of something simple.  Using the existing heating elements means needing close to but not over 230VDC.
Wish you had a schematic or specs for the PWM unit; no telling how that would fair with HV DC input otherwise.  I am not aware of high voltage and  high current DC-DC converters but I suppose they could be out there.  I know some big UPS units had battery voltages up to 144V nominal.  Take a look and see what's out there. 


glort

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Re: Matching solar output to consumption.
« Reply #2 on: April 11, 2018, 05:58:21 AM »
Why not just get a meter that allows you to legally backfeed.   Or is that a problem with licensing, inspections, etc? 

Yes!

That's the very reason i am able to buy these panels sop cheap. Everything has to be on an " Approved" list now supposedly for " safety concern's. I'd sure like to see an explanation how a 5 yo panel is any less safe than a current one on the list. I have both, there is NO difference at all.
Of course then you also have it the inverters have to be on the list, the switchgear and so it goes.
A complete load of Cock all designed to increase the revenue of the solar industry and the fees and charges the gubbermints and power companies impose.

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  I know some big UPS units had battery voltages up to 144V nominal.  Take a look and see what's out there.

Brilliant suggestion yet again!
I KNOW I gave aay some brand new high power UPS units when I moved because they had an input of 90V from memory.  I have a vauge recollection I  -may- have kept one. I'll have to go root through the mess up there and see.
That's the next thing on the list once the weather cools, get up and organise the garages. Amazing how much crap has been put up there and we have only been here 6 months. I can say hand on heart though, 95% of it is the mrs crap.  Ship just keeps coming!
New furniture is due to be delivered next week so I'll have enough cardboard and rubbish to build another house with.  Luckily plenty of cardboard Bins around the shops and factorys here.

For the water heater, All I really need is a way to switch the high power DC.
I bought some little transformers the otehr week and already built a setup using them with a low powered switch to control a relay for my inverter cooling fan.  I could do the same sort of thing, just need to switch the DC.  i'll have to see if I can find that diagram again.  Pretty sure it was a couple of Double pole relays kind of cross wired. One went to the other so as to speed up the switching time and change both polaritys at once. 
Simple but looked effective.

I'll also wire in the PWM controller I'm running now so when we get the inevitable fortnight of wet weather, I can just run the heater off the normal power output at the 1000W I am driving it at now.

BruceM

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Re: Matching solar output to consumption.
« Reply #3 on: April 11, 2018, 06:29:51 PM »
I haven't had any great ideas yet on your DC switching.
Be wary of bargain DC solid state relays-  I bought a couple on ebay and their on resistance was pretty high so that had to have a huge heatsink for high currents.  You need to look for specifications for either on resistance or forward voltage drop at various currents.  When these aren't provided it's often for good reason.  Ohms law then tells you how much heat you'll have to dissipate.

For myself I'd probably roll my own with a few IGBT transistors and a optically isolated gate driver IC.  That will need a 12VDC power supply to operate but it is pretty simple for custom electronics.  I could give you a schematic and parts but I know you're looking for something you can just plug in and I haven't seen that; switching 200V+ DC at currents of 10 amps or so just isn't done much commercially. 

The other thing I looked at was VFDs-  some are now designed for PV  DC input for operating well pumps.  Could be sweet if you found a bargain unit that was suitable. 

If going for the direct PV to DC water heater element drive, you'd have to match your series string voltage to the load. The sort of thing that a jury rigged test setup would be useful for to find the sweet spot.  The open circuit voltage is important to watch, your DC load switch must be well above that; in fact I prefer double the actual open circuit voltage so that inductive spikes on switching will not cause the transistors to fail over time. 

Driving with AC has the advantage of no mods to thermostat/power switches. 


buickanddeere

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Re: Matching solar output to consumption.
« Reply #4 on: April 11, 2018, 09:33:36 PM »
Can not Net Meter anymore in most of Ontario. I have been considering a two array PV system aimed east and west with a grid tie inverter . Then over sizing the system by a few hundred watts over average demand to net out to near zero draw from the grid while the sun shines .
    Weekday daytime electrical prices are high and are going to become more expensive to pay for subsidized wind, solar , natural gas and hydraulic after the election.

BruceM

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Re: Matching solar output to consumption.
« Reply #5 on: April 11, 2018, 10:59:01 PM »
Odd, I read Ontario did have net metering, though I can't find the details at the energy ministry website. What happened?

Planning your PV locations to distribute power generation throughout the day makes good sense if you are getting only a small rebate or none for what your provide.   Off grid it helps keep your battery costs down, which is now the big ticket item to watch closely in planning your loads and system. 

I vaguely recall that Glort's phase with a digital meter BILLS him for backfed power, so he's got to avoid that.  Alas, the GT inverters aren't set up to handle that nasty situation where you want to offset loads but not backfeed.  It could be sensed by two CTs and some circuitry for computing net current and then the DC (PV) input to the inverter throttled via PWM or a load could be added but it's a fair amount of custom electronics and not trivial. 

I'm glad I'm off grid and only have a $18/mo ongoing battery replacement cost.  Most have 3-4 times that. 





« Last Edit: April 12, 2018, 08:51:31 AM by BruceM »

glort

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Re: Matching solar output to consumption.
« Reply #6 on: April 12, 2018, 07:46:29 AM »
I vaguely recall that Glort's phase with a digital meter BILLS him for backfed power, so he's got to avoid that.  Alas, the GT inverters aren't set up to handle that nasty situation where you want to offset loads but not backfeed.

I am reading that the latest and higher specced ( very expensive) GTI's do in fact have this feature. Predictably, they cost far more than a similar output unit that dose not have the limiting feature.


 
Quote
It could be sensed by two CTs and some circuitry for computing net current and then the DC (PV) input to the inverter throttled via PWM or a load could be added but it's a fair amount of custom electronics and not trivial. 

That's what the commercial units do. The electronics are fairly simple looking and can be purchased in kit form. I am not advanced enough to assemble on however.
I'm equally amazed at the amount of boards around these days and what they do as well as how cheap they are just as much as i'm surprised at the lack of things to do simple jobs which I would think would have a large market and application.

For my electronic metered phase I have now that only has the AC on it, I am thinking about using one of the voltage monitoring switches and a relay to control a GTI to supplement that phase.  When the voltage level drops under load, the voltage relay switches a higher power relay from the GTI and adds a limited amount of power to that leg.

I'm wondering if I can keep the GTI powered up by connecting it to the leg with something like a lightbulb that will feed the GTI the power it wants to see for voltage and frequency and then switching in the main output when the compressor from the AC kicks in.
If I leave the GTI in all the time, I'll be getting billed when the compressor is not running therefore have to sync the output with demand.
If this is feasable, It will be OK for the AC but not the normal household loads which will be the real issue.

Quote

I'm glad I'm off grid and only have a $18/mo ongoing battery replacement cost.  Most have 3-4 times that.

Even at 3-4 times that expense, you are still country miles ahead of what anyone I know is paying for power!