Hybrid vs. OffGrid


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drbacke

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Apr 3, 2019
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Hey guys,
we are about to build a house and of course I am already thinking about the PV system with a DIY Battery Bank.
At the moment I have a 2x7s60p 9kWh 24V battery and would like to use it with a MPPSolar Inverter.
A few things I do not understand however yet.
In the Offgrid case:
1 How can I make sure that the battery is not overloaded?
E.g. I would charge the battery a maximum of 3kW, but the inverter can charge 4kW (4024MSP)?
2. or if the load is greater than 4kW, what happens then? Does the fuse blow? Or the bypass is enabled (how much power can you use with the bypass mode, all the grid power^^?)
3. Or just overdimension everything, so you can take all the load?

With the hybrid:
1. can I feed the inverter from the battery constantly (e.g. 1kW) into the house grid to cover the base load? And take the entire PV energy for load and battery during the day?
2. Is it possible to not feed anything into the public grid with the hybrid inverter, but use the internal house grid?

Shall I switch to 48V?

How do you all do it? This is much more complicated as I thought :s
 

Korishan

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drbacke said:
Hey guys,
we are about to build a house and of course I am already thinking about the PV system with a DIY Battery Bank.
At the moment I have a 2x7s60p 9kWh 24V battery and would like to use it with a MPPSolar Inverter.
A few things I do not understand however yet.
In the Offgrid case:
1 How can I make sure that the battery is not overloaded?
E.g. I would charge the battery a maximum of 3kW, but the inverter can charge 4kW (4024MSP)?
2. or if the load is greater than 4kW, what happens then? Does the fuse blow? Or the bypass is enabled (how much power can you use with the bypass mode, all the grid power^^?)
3. Or just overdimension everything, so you can take all the load?
1) By setting the appropriate cut off voltage. If set to 28V, that would be 4V/pack. But a BMS will make sure every pack is the same voltage.
2) If the load is higher than 4kW, your inverter goes into surge mode, this can only last a short period of time. The higher over the rating, the shorter the surge time. Your battery is capable of delivering about 120A in surge mode. I'm assuming 1A discharge/cell. So, 120A * 24V = 2,880W. So your battery will not be able to keep up with a full load on the inverter, even without going into surge. Going over 1A is not advisable unless using HighDrain cells or LiFePO4's.
Yes, if your inverter pulls 4kW and your batter can only do 2.8kW, fuses will be blown.
3) Over dimensioning is always a good thing in most places. Go larger on the wires, the bus bars, every connection except the fuses, obviously.


drbacke said:
With the hybrid:
1. can I feed the inverter from the battery constantly (e.g. 1kW) into the house grid to cover the base load? And take the entire PV energy for load and battery during the day?
2. Is it possible to not feed anything into the public grid with the hybrid inverter, but use the internal house grid?

Shall I switch to 48V?

How do you all do it? This is much more complicated as I thought :s

1) If the battery if full, then no charging will be done to the battery. Not sure what you mean feed the battery constantly. The charger portion will charge the batteries to full, splitting the power coming from solar between battery and house. When the batteries are full, then 100% solar will go to the house supply. If your house load is higher than solar input, then it will be a percentage split. If using 20kW of power, 15kW of solar, then it'll be a 75% from solar, 25% from grid. If you have 30kW of solar, then 100% will come from solar and 0% from grid.
2) And yes, most hybrid inverters can be set to not export. And if the inverter is not a Grid Tie, then it won't export anyways.

I, and most on the forum, would recommend 48V. Especially if you haven't bought the hardware yet. 48V based systems are easier to maintain, cheaper overall to install, more readily available hardware, more efficient.

How do you do it? Read through many threads here and there are many ways of construction. Please read the FAQ if you have not done so already. There are many threads on hybrid inverters alone, and then a bunch on solar as well.
 

OffGridInTheCity

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Dec 15, 2018
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I'm offgrid. In addition to the advice above (completely agree), the way I 'automatically' consume power is thru a combination of automatic transfer switches and manual transfer switches. I have a 14s (48v nominal) battery bank.

Here's a picture of the setup at my main panel:

image_palulb.jpg


In my case,'Inverter On' is at52v (3.71v/cell)and 'Inverter Off' at 49.5v (3.54v/cell). During the day the battery bank begins to charge and when it reaches 52v the inverter turns on which triggers the automatic transfer switch to take over for the grid and supply power to "Generator" circuits in the manual transfer switch. The house runs off solar and the PV array continues to poor power into the system. If PV power is greater than the load, the battery will continue to charge and if not, will gradually drain down to 49.5v. When the solar day ends and the inverter turns off (at 49.5v) the ATS goes to shore power automatically. All automatic - on/off each day.

The max (absorb/float)is set to56.4v (4.03v/cell) but I rarely reach this as my goal is to adjustthings toconsume as much PV power during the day as I can to lighten the load on the battery bank. I adjust (increase of descrease)the load on the inverter buy setting circuits to "Gen" (generator) or "Line" on the manual transfer switch. In an emergency I can change my mind and use the battery more by setting some circuits to "Line" (such as the cooktop) and saving power for lights.

Finally, the ATS switch twice a day will causecomputers (sensitive electrical equipment)to shutdown - butUPSs easilycover that.

One side-affect of this approach is that its easy to turn off the solar system to work on it. The ATS simply stays on grid till I'm done This means that the house is not affected while I work on things :)
 

completelycharged

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If your considering 4kW and starting then 48V.

1. "How can I make sure that the battery is not overloaded?"
A. Build the battery far larger than the inverter, or prevent anyone else switching appliances on other than yourself. Visitors have zero load coordination and the wife may have her moments (ok, not ment to be sexist!!!!!)

2. or if the load is greater than 4kW, what happens then?
A. UPS line active / off-grid the inverter either blows magic moke or gracefully shuts down after sweating for a short duration (short term overload capability). Grid tied the grid just draws the excess.

3. Or just overdimension everything, so you can take all the load?
Battery yes, inverter yes/no. Over size the inverter means lots of cash sitting providing capacity that is normally never used and the larger capacity would just end up "ah, it can take it" as you switch on the kettle.

For your location off-grid all year would not really be an issue (lack of sun in winter) so your system ends up having to have grid support one way or another (hybrid).

Regular ATS I would not recoomend as an approach as you will eventually destroy appliances through repetative degredation on high switching transients (out of phase switching) and needing to resort to UPS units for computers/electronics seems counter to the whole setup (and adds to idle losses / vampire drain).

My own setup is errr.... slightly non standard, with grid tie, hybrid and off-grid all mixed in part as an experiment. Off-grid when you have grid available just menas you will always have an excess or deficit in part of your system at different times of the year.

Standby (low load) power use typically consumes over 80% of your energy so if your just trying to be green don't try to match the peak. Base is where the real energy is and a small 500W hybrid/grid tie inverter may well provide over 80% of your actual energy needs (not peak demand !) Balance the energy with the battery and over size the battery 10x the inverter capacity (500W inverter 5kWh battery).

https://secondlifestorage.com/showthread.php?tid=8053
 

OffGridInTheCity

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completelycharged said:
Regular ATS I would not recoomend as an approach as you will eventually destroy appliances through repetative degredation on high switching transients (out of phase switching) and needing to resort to UPS units for computers/electronics seems counter to the whole setup (and adds to idle losses / vampire drain).
The ATS I use has time delay on detection of alternate power(fast) switching and "switching transients" (momentary burst of energy)don't come into play.

Appliances have electronics and/or motors. Motors (AC compressor, Furnace Motor, Refrigerator, etc)are not bothered by ATS as when they are spinning they may do a split-second pause but continuespinning after the split second gap - they don't 'stop' and 'restart'. Appliances such as the Hybrid Water Heater,Cooktop, Ovendon'tskip a beat. Other appliances such as the microwave can stop if its actually running during the transfer, but is not harmed (and we don't use the clock).

I do agree that UPS draw idle power - quite a bit of it - but they are needed for grid power as I have lost more than 1 expensive printer/computers to grid snafus. A pair of master UPSs that powera set of 'sensative equipment'outlets around the house cut down on the idle power - and can be bypassed in an emergency. The manual transfer switch is part of the design and enables easy on/off of what's ATS'ed.

System is working well for 1 1/2 years and counting, with no damage to anything...
 

completelycharged

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It's things like the fridge compressor, if it stalls through a transient the thermal cutout will kick in to allow the pressure to reduce before restarting again and only intended for rare incidents. This is one type of hidden operation that when repeated is not going to do the applaince much good over time and where transient issues will show over time.

It is the out of phase re-syncing with an ATS that is the main cause for concern with the added surge on the inverter, which may or may not be up to handling the transient. If your inverter is sized to deal with them then it may well create no issue.

I just don't like the idea of ATS switching every day because you have to have a secondary UPS(s) for sensitive stuff. I always find resetting the clock on the coooker annoying, stupid cooker (Home Simpson style).
 

drbacke

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Messages
60
Thanks for the answers, that was very helpful. I think I understood the off grid solution now.
Just one question to off-grid: The most off-grid inverter (i'm intersted in mppsolar devices, or maybe there are better) have a bypass mode, so just connect the grid to the output if th battery goes low. Do you know how much load this bypass can take? Is it the same as the inverter power or more?
I will convert the packs to a 48V system, since I have 2x7s60p, this should be relatively easy. First measure the capacity and then add a few cells if necessary.

Hybrid:
Unfortunately it is a huge bureaucracy in Germany to legally connect a DIY system to the grid. I'm not even sure if it's possible.
Apart from that, I still don't fullyunderstand thehybrid solution.
Let's say the following:
10:00-19:00: Good sun, the battery charges via the PV system and the rest goes either into the public grid or asload into some rooms of thehouse.
19:00- ...: No more sun. The inverter can only pull from the battery. Now there is a load and a grid connection at the inverter. Why should I still use the load connection at all, the most reasonable would be to feed in the base load constantly and thus operate refrigerators, heating (heat pump) etc. with it or not?
So you could also best conserve the battery by feeding a smaller constant load in the evening and using it yourself or not?

It would be best if the inverter only provided what was needed. As far as I've been able to do that so far, the MPPSolar devices can't do that, can they?

How do you solve this problem?
 

100kwh-hunter

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Over dimension...do it, up to 500 kwh battery, king winter is coming..... :cool:
i would when i have a chance :p ....
 

completelycharged

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Bypass mode is usually a mechanical relay(s) rated a lot higher than normal loading, but the issue is if there is say 6kW attached to a 4kW unit and the grid goes out, shutdown or magic smoke.


This is one potential extreme to expect from solaroutput for any system

image_tdygkz.jpg


If you had 500W load attached, above 500W goes to the battery, blow 500W the battery supplies the balance or the grid...
 

OffGridInTheCity

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completelycharged said:
It's things like the fridge compressor, if it stalls through a transient the thermal cutout will kick in to allow the pressure to reduce before restarting again and only intended for rare incidents. This is one type of hidden operation that when repeated is not going to do the applaince much good over time and where transient issues will show over time.
Seem like you have read or encountered 'trasient' (power surge) situations? but they are not occurring for me. I've read that ATS equipment can be hurt by 'transient' events but they don't create a 'transient' event. I bought higher end ATS with surge protection to protect the ATS. Maybe you could link to an article or source of info to explain more? - For example, this article https://www.powerelectronicsnews.co...ormers-during-power-outages-or-engine-testing
If so, this article does not define 'large' motors and I don't believe I'm seeing this on my AC Compressor (largest motor I have), Furnace Fan, or Refrigerator.

In my case, there is no noticeable 'transient event'. There is no 'thermal cutout'. Itsa fastswitch fromgrid 60hz to (powered up/waiting)inverter 60hz and back. The motorcontinues to run smoothlyacross the millisecond(s) gapwith no damage - because the momentum of the motor easily carries it for the small time involved.

To help advance the discussion, this is the type of ATS I'm using - PD52DCSD 240 VAC 50-Amp Automatic Surge Protected Transfer Switch The Silent ATS https://www.progressivedyn.com/rv/a...rge-protected-transfer-switch-the-silent-ats/


completelycharged said:
It is the out of phase re-syncing with an ATS that is the main cause for concern with the added surge on the inverter, which may or may not be up to handling the transient. If your inverter is sized to deal with them then it may well create no issue.
In US case, the grid is single-phase 60hz, the inverter is single-phase 60hz and its a simple switch from one source to the other. The slight difference in actual hz (say 60.0 to 59.999) is not significant except clocks run a little bit off-time over many hours on the inverter.


completelycharged said:
I just don't like the idea of ATS switching every day because you have to have a secondary UPS(s) for sensitive stuff. I always find resetting the clock on the coooker annoying,stupid cooker (Home Simpson style).
As I mentioned, I use UPS(s) for grid - so its not a solar 'thing' but it solves the ATS switch nicely. I cannot imagine running sensitive electronics without one as we have routine (once a month) power flickers and occasional surges (once a year) just from the grid - so for me this is not an issue. But I get what you are saying.
 

completelycharged

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I was searching through my previous posts as this question arrose previously and I did a chart to show what I was trying to explain. The issue mainly has an impact on syncronous or inductive equipment (transformers / motors / compressors) and a lot of other equipment is just exposed to high voltage transients caused by the disconnection (when you disconnect a coil/inductor the coil generates a back EMF spike). With a lot plugged in the voltage spikes are absorbed and may have no issue.

Imagine switching a single phase over to another single phase supply which is 180 degrees out of sync, i.e. switching from +220V to -220V. With the ATS switching from grid to inverter output regardless of timing / syncronisation. Ideal would be to have the ATS switch only when the two grids are in alignment (zero voltage difference between live) as this would at least then just appear as a transient outage the same as occurs on the grid when switching at times.

Agree the UPS option is a brilliant way to fence off and safeguard expensive kit, I went that route and still put things behind a UPS if I'm in doubt but that is typically for short durations while I'm playing trying not to burn the house down, lol.

Just some thoughts.
 

OffGridInTheCity

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completelycharged said:
I was searching through my previous posts as this question arrose previously and I did a chart to show what I was trying to explain. The issue mainly has an impact on syncronous or inductive equipment (transformers / motors / compressors) and a lot of other equipment is just exposed to high voltage transients caused by the disconnection (when you disconnect a coil/inductor the coil generates a back EMF spike). With a lot plugged in the voltage spikes are absorbed and may have no issue.

Imagine switching a single phase over to another single phase supply which is 180 degrees out of sync, i.e. switching from +220V to -220V. With the ATS switching from grid to inverter output regardless of timing / syncronisation. Ideal would be to have the ATS switch only when the two grids are in alignment (zero voltage difference between live) as this would at least then just appear as a transient outage the same as occurs on the grid when switching at times.

Agree the UPS option is a brilliant way to fence off and safeguard expensive kit, I went that route and still put things behind a UPS if I'm in doubt but that is typically for short durations while I'm playing trying not to burn the house down, lol.

Just some thoughts.

I've been reading and I now see what you are talking about. Even single phase US - the single phase could be out of sync / directly opposite in the wave form. I had read previously that it would not hurt motors, but deeper reading today says that's a myth.. (don't believe what you read on internet). Its also a matter of the motor (power) size and design as to how at risk it is.

The ATS(s) I'm using have a printed circuit boards with mechanical relays - so maybe they are smart enough to do somesyncing and that's why I haven't had problems (i.e. dumb luck in buying a good ATS). In any case, I've put a direct questions to the manufacturers and will try to get to the bottom of this in my case.

Thank you!
 

OffGridInTheCity

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I use Progressive Dynamics and Go-Power ATSs.

Here's a question/answer from Progressive Dynamics:
Question: I'm in a discussion that using automatic transfer switch between grid and inverter can cause my refrigerator motor to burn out during switch over because of out-of-sync 60hz wave from (from grid to inverter power) causing inductive load on the motor. Does your electronics match the wave form to avoid this? or is this true?

Answer:
From: Tech Support [mailto:techsupport@gpelectric.com]
Sent: Thursday, October 17, 2019 1:17 PM
Cc: Tech Support
Subject: RE: Tech Support Notification #GP00049023

Hi,

Thank you for your inquiry.

I have never come across a situation where this would be the case. If it was I am sure I would have heard about it before now

There is usually a delay before the switch, but there is no frequency matching hardware involved.

Regards, Bob
 

LPG

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Take a look at this video for a close up of what happens when your ATS switches mid wave

 

OffGridInTheCity

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The youtube was targeted to MPPSolar which appears to 'sync' the wave form. So it doesn't answer the non-sync case specifically.

After this excellent discussion I get the concern. The Progressive Dynamics ATS technical answer verified that they are *not* doing any wave form matching - but they also report that they have not heard of any issues. The articles I read today on ATS motor generated transient (surge) all say 'large motor' but hey don't specify large. For example, my:
- 50gallon Hybrid Hot Water Heater - is 240v and draws between 380 and 450watts. Small motor?
- Refigerators (2 of them) each draw 2-3a @ 120v = 360w. Small motor?
- Furnace Fan draws 6a @ 120v = 720w. Medium motor?
- Portable AC draws 11a @ 120v = 1320w. Medium motor?
- Next year - adding the AC compressor which draws 22a @ 240v = 5280w. Maybe this will meet the definition of a large motor?

Sometimes there are perfectly valid concerns but the fact details may be too specific to be taken as a general rule. I want to know more about the definition of a 'large motor' and perhaps they types of motors. Perhaps my newer appliances are not as susceptible as older models or something.

However, for me, things are working great and I'm nearing 700 switch overs... so I will continue operations and continue to research/learn about this. The AC compressor is interesting as it may actually be in the range of a 'large motor' and I will watch that very carefully.
 
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