My Micro grid system

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81

I understand it as follows: An MPPT has an output limit, say 45A in my case. So doesn’t matter what is on the roof, that MPPT would only ever supply 45AxSystem Voltage (say 52.4V) =~ 2.36kW max. If the batteries are sitting at 50V, then only 2.25kW etc.

If the batteries are flatter, I’ve noticed that the MPPT can slightly exceed the battery voltage, but I presume this is similar to try and shift the grid frequency with your generator… probably only possible because I have relatively little battery storage compared to PV generation? Anyways, doesn’t matter, the MPPT would never exceed 45A on the output.

Now, I understand that oversizing on the input is fine (in order to hit your 45A limit for longer periods of time), but there’re limits. The bad-wine, music-stealing cat, @plonkster (did I get your nickname correct, Izak?), explained this once somewhere on this forum…

82

Well, it cannot really… but also it can… it is complicated.

As an analogy, if you put ice in a glass, and half the ice melts… what is the temperature of the water around the ice? The answer, of course, is 0°C (or 32°F if you are some kind of heathen :slight_smile: ). The temperature cannot rise until the remaining ice has absorbed enough energy to melt.

But if you go in on a microscopic scale, you may well find that the water at the edge of the container is a tiny tiny bit warmer than the water right next to the ice. And of course the way we know that, is that in very large containers (like the ocean), some parts can be quite warm, even though we have (not yet) melted all the ice in that container.

Similarly, the MPPT cannot really raise the voltage any more than the (flat) battery will allow it. But since there is a little resistance in the cable, and the energy flow is from the MPPT towards the battery, you will see a slightly elevated voltage at the source, and this will be more pronounced the larger that distance (cable length, size) becomes. There is nothing really weird being done here, that’s just how the laws of nature is playing out :slight_smile:

You are mostly correct about the oversizing. You can oversize on the input. It will run at no more than 45A (using your example MPPT) on the output though, unless it gets very hot… then it may derate slightly.

The caveats is 1) never exceed the input current limit, there is one on the specification sheet, and 2) never exceed maximum open circuit voltage, also on the spec sheet.

The input current, is simply your Imax (maximum current) or Imp (current at max power, usually a smidgen less than Imax) on the back of the panel, multiplied by the number of parallel strings. If you oersize such that either the input voltage or the maximum input current exceeds these limits… well then you’re going to blow up the hardware of course.

83

Thank you for that reminder!!!

84

@Sarel.Wagner On your VRM I noticed you have BatCable temperature, is that a BYD thing or how do you measure it?

85

I used a sensor that feeds into one of the Cerbo GX analog temp inputs.

Groetnis

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86

Need to consume more PV energy. So Aircons it is. I need 4 of em but only have funding for 2, so one 24 000 Btu and one 12 000 Btu Samsung AR9500T Wind Free (Wi-Fi) Wall Split Inverter Air Conditioner, for the time being. The WiFi part is for my home automation integration via HA. So now I need to install them 2 DIY :slight_smile: Fun it will be as I know zip about installing aircons.

Buying them tools, I need the Vacuum pump for another project anyhow. And the cost for tools is only a bit more than the commercial install costs by 2 and less than the costs for 4 x install.

Groetnis

87

My system can generate 1 500kWh per month, (not day) and currently I only consume about 900kWh per 24 hours. So there is room for more consumption :man_shrugging:t3:

Groetnis

88

Hey, you do not have to work them to death 24/7 (or10/7 rather if we go by daylight hours)

I suspect your system will last longer if it only works on half tilt most of the time. All electronic components have a lifetime and I’d like to prolong mine as much as reasonably possible.

Good luck, and hats off to you as far as installing the aircons are concerned. I will not attempt such a thing alone. Make sure this does not invalidate your warranty!

89

If your house is reasonably well insulated, those aircons are not going to help you much with your consumption problem… I recently installed a bunch of them - download the manual; all the installation instructions and tools required are in there. Make sure you get the eccentric flaring tool (available at Adendorff).

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90

Inverter units, their current draw is low, 6A and 9.5A at most but that is startup, after at most 30min that will drop by a lot. Since they heat pumps, there are no resistive heating elements either.

My home is badly insulated, more than 60 years old and lots of windows and about half that surface are are louvers. So no them units will work a bit. Should have enough capacity to run them for heating during winter nights as well.

Groetnis

91

I think there is a rounding error here.

92

Where please?

93

If you click on your name in the top right of his comment, you see the post he replied to. It would be quite impressive if your system can generate 1500kWh per day. Mine does not even come close to that in an entire month!

94

Ta thank you, ja per month… I am an idiot hahahah…

Groetnis

95

Best full month;

Screenshot 2022-01-27 at 15.19.31
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Month to date;

Screenshot 2022-01-27 at 13.44.59
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96

HI Sarel, this is not really the case. If the ambient temp is high, the inverter aircon needs to work almost as hard as an non inverter aircon. In Phalaborwa for example we see an dramatic drop in current draw after 22H00 at night. In Gauteng that should be earlier.

For example, an inverter aircon set at 18deg on a 35plus degree summers day, will take quite a bit longer than 30min to start reducing power… And running at full power for extended periods of time on a very hot day
is not unusual…

Also one thing to remember, don’t undersized the inverter unit, using a 9000btu aircon where a 12000btu unit is needed, will result the unit running at full power basically all of the time because it can’t reach temp.

On the other hand, if it’s sized correctly and you chose a setpoint of 23 or 24deg, it’s easier to reach setpoint, resulting in much bigger savings on power consumption.

97

Maybe I should be more spesific with the info :white_check_mark::thinking: provided, otherwise its assume and we all know what that means. My statements was not meant as absolutes, this I will have to measure so will throw a Shelly 3EM on them and chart the actual consumption and performance and report that over time.

Both units are oversized for the room sizes and the amount of leakage those rooms suffer from. Small bedroom is 16m2 and getting a 12 000Btu unit, one size up. Second room is 28m2 and getting a 24 000Btu unit, also one size larger.

So on setpoint, summer would be around 23 or 24 degrees and winter on 22 or 23 degrees, wifi will ensure that the manual override will be reset via the HA system. The past winter, I programmed my HA to manage the heating temps between 22 and 23 degrees actual room temps, it was super comfy for us at those temps. It also reduced the IR heaters power consumption drastically. Now with the heat-exchanger AC units the loads should be more inverter friendly and the heating cycle much more efficient than those IR heaters, time will tell.

Once setpoint is reached, power consumption will be reduced. To maintain that volume of air at setpoint with the inverter units will reduce power consumption as the compressor motor speed is reduced to maintain the setpoint.

Having these 2 consuming extra power will surely get the self consumption up…. So in actual fact the more they consume the better, to an extent. The inverter models will reduce the startup current demands by ramping up gradually, good thing on the inverter so no sudden big load changes.

But first, I need to get everything to be able to install and evacuate those systems. Shortages on AC tool are at an all time high currently, almost no stock of tools anywhere……

Groetnis

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98

Oh and Jaco, a lot, used in my previous post is not a technical term, its more akin to pulling it out me arse :laughing:

Groetnis

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99

Just one note: the Samsung units don’t go into max power draw immediately, it depends on the temperature differential. The only time mine ever draw near the rated wattage is when you put it in “turbo” mode. The rest of the time it won’t go over 70%.

Another thing is there is a “max current” spec that is maybe 1.6x the “rated current” spec - but that is only the current drawn to charge up the caps in the inverter, so it is nothing like the 5x current draw for ~1second that non-inverter types have. (So you don’t need D-rated breakers. In fact with C rated you can probably ignore max and go by rated current).

100

Thank you Sir, duly noted.

Groetnis