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Off-grid, Eskom as backup and batteries as last resort.


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Way back in 2008, when Eskom introduced us to load-shedding, I wanted a power solution that uses every watt the sun can provide, Eskom at night and when Eskom is off and there is no sun, only then use the batteries.
 
Here are the caveats:
a) the batteries can only be charge using solar power.
b ) System must be modular, for if a part breaks, or I want to expand, I need to be able to do so cheaply.
 
Searched and searched, nothing could do as I wanted for grid-tie was not an option (ever), off-grid is too expensive, hybrids did not exist as they do today (and today they are limited in some functions) and UPS'es are not designed to run 24/7/265.
 
So I made a plan to run all my computers (quite a few for we work from home and have kids), all the lights and the A++ fridge just because I can.
 
To do this:
Step 1: Had to reduced my load. Needs vs wants - HUGE price difference.
Step 2: Using:
  • 3 x 310w panels - R8370 back then.
  • Morningstar Tristar 45 Controller - +-R2500 - Now a Victron MPPT 150/35
  • 1600VA Victron Phoenix inverter - +-R8500
  • 4 x T105RE batteries (24v) - +-R8900
  • Victron Battery Monitor - +-R2800
  • And a magic box to swap automatically between power sources, based on battery SOC - +-R2500
  • Separate circuits for the selected equipment - +-R2000

I am now off-grid in good sunlight, on-grid at night and batteries as backup if both the sun and Eskom wants to see who can make it the darkest.

 

Ps. To run your lights off a UPS / solar inverter is easy: Get a electrician to put your lights on a extension lead, direct from the DB, that you plug into a UPS / solar inverter. Just make sure there are no halogen or incandescent bulbs anywhere. LED's only, and do not bypass the light trips in the DB.

 

Pss. It is a good idea to also run your alarm system off the inverter, in case of a power failure. Changing 7ah batteries are a waste of money if you have big batteries on the property.

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Victron battery monitor has a built in relay.

Setting the relay to trigger on a SOC at say 95% (5% of the batteries are used), it in turns triggers 220v AC relays that switches Eskom on and the inverter off.

At SOC 100%, or 98% in summer, it switches the relay again, Eskom off, inverter is on.

And, if Eskom is off, Inverter is off, the battery monitor relay is ignored, and inverter power is switched back on until Eskom comes back on.

 

A electronic engineer built if for me.

 

When we built the 2nd box, it was done with programmable electronic relays ... but I preferred the original, old style relays.

 

O yes, lest I forget, there must be at least a 5-8ms break between Eskom / Inverter power.

Original one I was over cautions and had a 1sec break, so for it I need a UPS ... or replace the timers ... one day.

 

I benefit from using as much solar as I can, keeping batteries maintained and having a Victron battery monitor to check batteries.

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  • 2 weeks later...

Re transfer switches, I wonder if there is anything special that needs to be done, like maybe switching on zero crossing or something. I should do some research, it appears that transfer switches are no longer rocket science.

 

As a programmer though, I would likely not use a microprocessor, or at least the switch bit will be such that it is physically impossible for both relays to close simultaneously... because have you seen what the output bank of an inverter looks like when it's seen mains? :-)

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Nothing special. The main trigger for the entire system is the relay in the Victron 702, based on SOC.

The new box, that one needs programming. Not sure what the engineer did, for he has to do it over.  :P

Using the older bigger relays, since May 2012, it has not given one days problem, and if they do one day, plug out the older one, plug in a new one.

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This is a what I look at to see what my system is doing:

No 1) Displays the Morningstar Tristar45 Controller.

             Connected to 3 x 310w Tenesol panels.

No 2) Is the readout from the BMW 702 reader.

             When it reaches as SOC of 95%, it goes back to Eskom.

             Monitors 4 x 225ah Trojan T105RE's, 24v configuration.

No 3) Is the Victron Phoenix 1600va Inverter.

             To power all the computers.

             Lights and small A++ fridge are 24/7 on solar only.

 

post-122-0-36608800-1441802099_thumb.jpg

 

Load is matched to the panels, not the panels to the load.  ;)

And in winter I take things off, and in summer I add a few things extra.

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As a programmer though, I would likely not use a microprocessor, or at least the switch bit will be such that it is physically impossible for both relays to close simultaneously... because have you seen what the output bank of an inverter looks like when it's seen mains? :-)

 

Therein the old style relays which I prefer ... no CHANCE of them getting inverter connected to Eskom, unless I go an fiddle off course.  :D

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  • 2 weeks later...

JDP correct. Back in 2008 there where no Axpert/Mecer/RCT branded hybrinds no. And none that I could find at the time bar really expensive ones like Outback, Victron with huge additional cost for the parts needed to swap sources. Today it is becoming the norm.

At the time (2008/09) I emailed every local and international supplier of branded systems asking for more cost effective methods of switching between utilities / solar. None replied. (Even a manufacturer or two in China.)

So yes, the last +-2 years quite a few hybrid units have come on the market, very cost effectively priced, packed with features. Time will tell if they last as long as their older brand name solar parts.

Also learned that running the batts down to a set voltage (I tried with the help of 2 engineers to trigger a relay, which was no problem at all) did not produce the desired results. As the load disconnects, volts climb, load engages, volts drop, disengages ... cycle continues until batteries where flat. Learned that using volts under load is never a good basis for my application.

Victron battery monitor solved my problem using the SOC relay quite nicely.

Been using my design since 2012 with only one flaw. Was way overcautious at the time to have a 1sec break instead of 8ms. Will sort that one day.

Also, if a part breaks in my design, I can substitute it quite quickly (and with different brands) cost effectively versus having a all in one unit which can be a burden if something fails and the unit must go in for weeks at a time for repairs. Jip, have had parts fail me.

Also learned that any savings made using solar power, the moment you charge them batteries using Eskom, those savings reduce very fast.

Will I ever install a Axpert / Mecer / RCT unit? Probably not for:
a) I prefer my MPPT charge controller separate from the inverter.
b ) Grid tied feature in my opinion is a waste.

Grid tie is just looking for drama - even if the feature is switched off - unless one does it with municipal approval and that makes just no sense ito fees levied by them for that approval,

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Will I ever install a Axpert / Mecer / RCT unit? Probably not for:

a) I prefer my MPPT charge controller separate from the inverter.

b ) Grid tied feature in my opinion is a waste.

 

Grid tie is just looking for drama - even if the feature is switched off - unless one does it with municipal approval and that makes just no sense ito fees levied by them for that approval,

 

These units cannot grid-tie.

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The one thing I must mention about the Axpert / Mecer / RCT units, is the hope that they will force the likes of Victron / Outback to reconsider their prices, for these guys have had a long run at very high prices.  ;)  

 

Especially if the Axpert / Mecer / RCT units last comparatively to the brands like Victron's etc. 

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TTT, I still use voltage as my switching criteria. You just have to use a voltage high enough so that it isn't reached unless there's a charger connected to the batteries. I set my setpoint at 26.5V (24V bank), and the go-back-to-grid to 24.2V. That works perfectly, at around 9PM (in summer) it falls back to the grid, and stays there until the sun lifts the voltage the next morning (which happens very quickly).

 

In winter I set the voltages a bit higher. This year, because of load shedding, my setpoint was at 28V.

 

I really need new batteries though.

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The magic words: "... unless there's a charger connected to the batteries."  :D 

 

Solar panels are connected so the problem came about that the system started yo-yo'ing early mornings and late afternoons.

 

Load on the inverter is +-600va, so it pulls a lot out of batteries, fast.

Early mornings the volts get quickly up to the set point, inverter starts, volts drop even faster. switches off again, charges again ... cycle starts.

Late afternoon inverter goes off, panels push the volts up, inverter starts, volts drop very fast, switches off again, charges again ... cycle started again.

 

Happens because batteries never got charged enough to handle the load early / late afternoon. Need more time to charge.

 

Solution for me was a SOC measurement device to stop the yo-yo'ing.

 

Then I discovered that if I set the SOC too low, say SOC of 80%, it takes too long to recharge the batteries, +-13h00 before fully charged, reducing the savings from running off solar from 9-18h00. Running off batteries are very inefficient due to all the losses and time it takes to recharge them, unless you throw a whole lot more panels on to charge them faster. Which is ok, but then the ROI becomes more complicated.

 

A SOC of 95% (DOD of 5%) seems to be, for me, the best balance for my system. To get a system to changeover on such small difference in voltages, is near impossible. I match my load to the panels, and the batteries are matched to the max the panels can charge per day..

 

Which means I have a lot of spare if Eskom is unavailable at night ... and my batteries will last for many many years due to light usage.

 

Batteries for me are the weakest link in the chain due to their costs and life expectancy, therefor I decided years ago to use them lightly, necessitating to move away from volts to changeover, as it is rather inaccurate.  ;)

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Case point:

13h37 in the afternoon, cloudy day.

Set to 95% SOC changeover, running 451va (fridge and all lights are off).

Volts are at 24.73 for batts, incoming amps are 8.69 and outgoing is -6.42a

System will swap over shortly, back to Eskom, saving the batteries just in case there is a Eskom outage.

 

 

post-122-0-77858200-1442835578_thumb.jpg

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It depends for me. On a sunny day, by the time it's reached the switch-off point there isn't enough sun left to get it high enough again. My bank is large enough (apparently). But on a semi-cloudy day, with the sun going in and out... then I definitely get a bit of yoyo-ing. Nothing too bad.

 

Re size. From what I've seen, proper sizing sets the batteries to about twice as much as you can generate on a day. So I SHOULD have 10kwh of storage in place. I have only about 4. So I do the same thing you do. Go down to about 80% SoC... just so that tomorrow the self-consumption and battery use just more or less works out to a full day's generation. Of course you don't want to get this right too precisely... you want to be a little inefficient and charge those batteries properly :-)

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... you want to be a little inefficient and charge those batteries properly :-)

 

Every week or two, I equalize, for using solar like this in off-grid scenario, you have to fully charge the batteries every now and then.

 

Battery sizing, is it not linked to panel ability to recharge in 5.5 hours?

Go too big a bank and batteries never gets fully charge.

Too small and you run the risk of charging them too fast.

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Every week or two, I equalize, for using solar like this in off-grid scenario, you have to fully charge the batteries every now and then.

 

Battery sizing, is it not linked to panel ability to recharge in 5.5 hours?

Go too big a bank and batteries never gets fully charge.

Too small and you run the risk of charging them too fast.

 

Of course it doesn't have to be exact. There's a certain range within which it must fall. The one limit is the maximum allowed charge current, that limits the array size relative to the battery, and conversely limits the size of the battery on the lower end if you have a large solar array. On the other end, your battery bank can be much larger than your daily generation, several days as far as I know, though hardly anybody does this due to cost. Most of the kits I see seem to use a 2:1 ratio, battery storage of around twice the daily generation. Now that makes sense if you think about it: You can do 50% DoD and still get them full again the next day. Larger systems use even higher ratios, because they attempt to cover days with little sunshine, so they go 4:1.

 

So as long as you know the limits... ;-) My dad had a 1000Ah 36V bank with less than 1kwp of panels on it for a while... it worked fine :-)

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  • 2 months later...

I see what your system is now, and how it works. So yes you are correct, for your system it is not worth discharging the batteries overnight because it would take too long to charge them next day.

 

I am trying to do something very similar, I work from home and we try to spend all solar energy as is is produced and when there is no sun I go in a low consumption mode in order to conserve battery but still have enough juice left to last the whole night.

 

 

 

 

Swapped over, now it will quickly climb in volts, but not in SOC. So if I had no SOC measurement, the yo-yo would have started.

 

attachicon.gif2015-09-21_13-53-19.jpg

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I am trying to do something very similar, I work from home and we try to spend all solar energy as is is produced and when there is no sun I go in a low consumption mode in order to conserve battery but still have enough juice left to last the whole night.

 

Exactly our case also. Also work from home. Grid tie costs just made no sense at all. And charging the batts using Eskom, never.

 

There are devices that run 24/7 off solar like the fridge, routers, switches and DSTV decoder. But the heavy loads like computers, once SOC is 95%, goes back to Eskom. Not cost effective long term ... yet.

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I am pushing that ratio to the opposite side now, I will have 30kwh production potential and only 10kwh of storage... As I said before I can use up all the power during the day, from water heater (gaysers you call them?), pool pump, and cooking. The whole house is electric, no gas/propane or wood...

 

Time will tell, but even on the worst cloudy day I will have a potential of making over 10kwh from 6kwp of panels.

 

In a future, I could have to buy another 260ah at 48V, but I think I would still invest that money(

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We went the other way. First we reduced consumption over time ... that costed a fortune in its own right.

 

The geyser is heated using evacuated tubes so in summer it uses no electricity. In winter about 2 hours per day. Jaaa, the family had to adapt.  :P

Pool I had off for 8 months of the year, under a cover. Today it is being closed up. Enough is enough.

 

Still reducing consumption before I go off grid daytime. Using about 24kwh per day, which I need down to +-15kwh before I make the next move.

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