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Newby researching hybrid system


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Good day

I'm busy looking into grid-tied hybrid systems to help reduce my energy consumption as well as provide backup in case of Eskom loadshedding. I keep and breed reptiles and fish as a hobby with their heating consuming around the same if not more electricity than my 3kW 150l geyser... My average electricity use is around 30kWh/day in summer up to around 50kWh/day in winter.

I've been doing a lot of reading on various hybrid systems with the Victron Multiplus II inverter in a ESS setup being very appealing due to its modularity and expandibility. Some questions I currently have:

1. I've been playing around with the Victron MPPT selection spreadsheet using parameters of various Canadian Solar panel sizes. Using the STC parameters I find that the MPPT's always bum out 2-5V over max. at 0'C with trying to fit that 1 extra series panel in a string. Can the NOCT parameters be used instead or must I stick to STC?

2. I've read that the law restricts grid-tied inverters to max. 25% of your main breakers rating, i.e. 0.25x60A = 15A (3450W). The datasheet for the Multiplus II 5000VA states that it has a max. apparent feed-in of 4000VA (3200W) which means it is within the legal limit?

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Hi ewertb,

In order to be able to manually calculate the voltages regarding your first question, what is the lowest temperature you can expect in your location? What is the highest voltage this MPPT can handle (Voc)? And what is panel temp. coeff. of Voc and Voc itself? Use STC, never NOCT.

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12 hours ago, ewertb said:

4000VA (3200W)

4000VA isn't necessarily always 3200W. That 80% thing is a rule of thumb used by IT people and generator salesmen ūüôā

The actual power (P) is equal to the apparent power (VA) multiplied by the power factor (cos theta), where the power factor is determined by what kind of loads you're powering.

Grid tied regulations require embedded generators to have a power factor of close to unity, better than 0.9, so for all practical purposes you have to assume that 4000VA is 4000W.

So no... you can't get away with the 5kva inverter. It makes 4000W and that is more than the allowed 3500W.

In Cape Town there is a new thing I've been hearing, which is that a software power limit is allowed. If that is true, then you can install the 5kva and software limit it to 3.5kw. @Rautenk might know.

Edited by plonkster
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4 minutes ago, plonkster said:

In Cape Town there is a new thing I've been hearing, which is that a software power limit is allowed

true.

Another option (at least here in Cape Town) is to prove that your actual systems output based on PV generation is within the 3.5kVA limit. Because efficiency is about 80% you can install up to +- 4.4kWp of panels irrespective of inverter theoretical max output.

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35 minutes ago, plonkster said:

In Cape Town there is a new thing I've been hearing, which is that a software power limit is allowed. If that is true, then you can install the 5kva and software limit it to 3.5kw. @Rautenk might know.

From the webinar on Nov 29 2019 it was stated that hardware/software limiting can be used to throttle it within spec:

https://arepenergy.co.za/city-of-cape-town-webinar-nov-2019/

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41 minutes ago, plonkster said:

4000VA isn't necessarily always 3200W. That 80% thing is a rule of thumb used by IT people and generator salesmen ūüôā

The actual power (P) is equal to the apparent power (VA) multiplied by the power factor (cos theta), where the power factor is determined by what kind of loads you're powering.

Grid tied regulations require embedded generators to have a power factor of close to unity, better than 0.9, so for all practical purposes you have to assume that 4000VA is 4000W.

So no... you can't get away with the 5kva inverter. It makes 4000W and that is more than the allowed 3500W.

In Cape Town there is a new thing I've been hearing, which is that a software power limit is allowed. If that is true, then you can install the 5kva and software limit it to 3.5kw. @Rautenk might know.

That sucks a bit... But it is what it is I guess, try and up my breaker to 80A or see how I can best optimize my situation using a Multiplus II 3000VA unit, i.e. solar geyser.

Just to make sure I understand the "software power limit" part, that is the ability to for example disable the feed-in of excess solarcharger power in the Multiplus II setup menu?

39 minutes ago, RikH said:

Hi ewertb,

In order to be able to manually calculate the voltages regarding your first question, what is the lowest temperature you can expect in your location? What is the highest voltage this MPPT can handle (Voc)? And what is panel temp. coeff. of Voc and Voc itself? Use STC, never NOCT.

I live near Secunda and it can get in the negatives here. Using the Canadian Solar panel data will be able to fit max. 2 panels in series on a Victron MPPT 150/x and 4 on an MPPT 250/x.

Edited by ewertb
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18 minutes ago, ewertb said:

Just to make sure I understand the "software power limit" part, that is the ability to for example disable the feed-in of excess

no. You have to limit the maximum output power of the inverter (even if it is all self consumption). In my opinion it is complete nonsense and as far as I know only applied here in South Africa. The reason given to us when we queried it with the CoCT was that if all inverters would suddenly stop generating the grid would have to take up the slack.

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28 minutes ago, ewertb said:

Just to make sure I understand the "software power limit" part, that is the ability to for example disable the feed-in of excess solarcharger power in the Multiplus II setup menu?

In the ESS menu it's the "Maximum inverter power" option.

The point of the 25% limit is that they want to limit the pick-up. If your inverter trips out for whatever reason, the generation capacity that is lost has to be picked up by the grid (it is like a load the equivalent of whatever power you were producing suddenly switched on). They want to limit this to 25%. So it's not only about what is being exported, it is about the total being produced, even if your own loads use it.

So yeah... for people with consumption like yours (geyser being on permanently)... it sucks.

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Thank you for the information. Based on this discussion it seems my optimum solution will be to replace my geyser with a solar vacuum tube version and install the smaller Multiplus II 3000VA system to power my the rest of the house (split between critical and non-critical loads).

On a different point, my roof is oriented less than ideal, i.e. facing NE with some smaller sections where I might fit 4 or so smaller panels on facing NW. I figured it's better to split the aray and use smaller MMPT chargers to better utilize the roof oreintation, i.e. one larger MPPT managing NE facing panels and one smaller one managing NW facin panels. How many MPPT chargers can be managed by the CCGX? Is it recommended to split the bigger NE array into smaller arrays as well to better buffer against potential shading?

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1 hour ago, ewertb said:

On a different point, my roof is oriented less than ideal, i.e. facing NE with some smaller sections where I might fit 4 or so smaller panels on facing NW. I figured it's better to split the aray and use smaller MMPT chargers to better utilize the roof oreintation, i.e. one larger MPPT managing NE facing panels and one smaller one managing NW facin panels. How many MPPT chargers can be managed by the CCGX? Is it recommended to split the bigger NE array into smaller arrays as well to better buffer against potential shading?

 

@ewertb, This whole answer isn't necessarily to address your set up, but to rather address panel positioning in general. Your " less than ideal" orientation and numerous different aspects is not a disadvantage, and no you don't need numerous smaller MPPT's if you do things right.

I'll explain:

The maximum voltage threshold that an MPPT can deal with, will dictate the number of panels you can have in a string.

Every panel in this string should be of identical spec and pointing in the same direction at the same tilt.

Now the current capability of your MPPT will dictate the number of strings you can deal with.

So you can parallel your similar strings to match this current capability.

Now, I want you to be aware of something, a solar panel with very little light still basically outputs full voltage. It may not be capable of supplying current at low light but its operational curve is such that it is very close to full voltage. This in turn means that it wont drag down the voltage of a panel with good light that can output a decent current. Which means a shaded panel can be paralleled with an un-shaded panel with impunity.

The common misunderstanding I am trying to address, is that it is often thought that all strings to a single MPPT have to all be at the same tilt and direction as each other.

As long as each string contains the same number of identical panels, and within each different string the panels are the same tilt and direction, then the strings can be paralleled.

Between the strings themselves, they can be at a different tilt and direction to each other on the same MPPT, without an operational detriment to speak of, in fact this has a considerable upside.

The tilt and direction of a string determines when in the day a string has its peak output. If all the strings pointed in the same direction they would peak together, and that peak current versus your MPPT's capability would determine the number of panels you could have. Your MPPT will have to be sized to deal with a big peak of current for a short period of the day.

This is inefficient, because the other 92% of the solar day, that MPPT is idling. The cost of that MPPT is a production overhead.

Paralleling strings with different directions means you can match waxing and waning strings together and spread their peaks. This softens the noon peak.

This in turns means you could either use a smaller (cheaper) MPPT, or alternatively increase the number of panels and get more useable units of power throughout the day. 

The MPPT still isn't overstretched at noon, but at least it is working at a reasonable pace throughout the day.

I mix and match strings like this, and you'll be surprised how many more strings of panels you can add before you reach your MPPT's capability. Paralleling low tilt E and W strings, anecdotally, I estimate it is about twice the number of panels, of an ideal tilt N - facing array. My loads are such that I can use every watt I make in a day, in other words my MPPT's do not back off when then batteries are charged.

You might argue that this is just trading panel efficiency for MPPT efficiency.

So, I'd like to point out two further advantages that I have noticed:

1.Getting the power throughout a longer solar day also makes the power more useable than having schedule my loads to match a massive noon peak.

2. On an overcast day, when you really need the power, a panel will still produce say 10% of its output, largely regardless of it's tilt and direction. Having twice as many panels under those circumstances makes a welcome difference. 

 

 

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