4 KW Grid Tie System in Off-Grid Mode

nthamizh · May 2015

I would like to design a 4KW Grid Tie System in Off-Grid Mode. This is for a resort which is in a remote place where Grid Connection is not available. All the loads in the resort are run through diesel generator.

Please find attached the diagram of the system designed. I would like to know the following

(a) Whether this system would work fine with the components as mentioned in the diagram
(b) How to charge the batteries, how the AC Input & AC Output of the inverter/charger be handled
(c) Will the system work with the small battery back-up system also (4 x 50 Ah 12V)
(d) When the battery is fully charged, then what will happen to the excess energy from the Grid Tie inverter
(e) Any errors in the system diagram

Please clarify these doubts that I have in my mind.

Thanks,
Arasan

BB. · May 2015

Do you have a link to the Axpert 4kWatt inverter with a spec. sheet/product manual? It may have its its own AC2 input/internal transfer switch.

Regarding your diagram, the generator transfer switch is in the wrong area (where it is in your drawing, I am not sure what you intend for it to do). The AC inverter output would be connected to the A input of the transfer switch. The Genset would be connected to the B input. The Loads would be connected to the Transfer Switch output. The Genset AC output should never be connected directly to the AC Inverter Output directly.

GEL batteries, in general, cannot be charged at very high rates--Generally only at 5% or C₂₀rate--Or ~2.5 amps for your 50 AH @ 48 volt battery bank.

The 4kWatt solar array, 4 kWatt AC Inverter, and a 50 AH battery bank (recommended around 500 Watt AC inverter output for normal off grid use and less than 200 Watt Solar Array) are very poor system design.

In general, GT inverters are designed to either be 100% of the array output power on, or 0% off. The SMA GT inverter does have a variable output power mode (the AC line frequency is slightly varied to tell the GT inverter how much output power to provide)--But I do not know is the Axpert inverter is capable of modulating the output frequency or not.

It all comes back to the loads--What are your needs? Do you want the pumps to operate only when the sun is up with AC generator backup power? Or do you want the pumps to cycle on/off any time during the 24 hour day?

I have great fears that you would end up with a bunch of destroyed equipment if you attempted to run this system as designed.

-Bill

nthamizh · May 2015

Hi Bill,
Thanks for your reply. Let me explain more on my requirements

My requirements are
1. To run 2 X 1.3KW Pumps via Solar only, during the daytime without Generator. The Generator may be used when it is cloudy or during rainy days
2. Would like to use more power from Solar PV directly and keep the battery back-up very little

Specifications of the Inverter
1. Off Grid Inverter : http://www.voltronicpower.com/oCart2/files/brochure/Axpert_KS_DS.pdf
2. Grid Tie Inverter : http://www.kinglongpower.com/userfiles/solartec%20brochure2_28.pdf

Could you suggest me a design, so that I can achieve my requirements with the above inverters.

Thanks,
Arasan

BB. · May 2015

I am on my phone, so I cannot type much right now.

But give this post a read about VFD powered solar pumping. In general, I think this is the path you want to take.

-Bill

BB. · May 2015

http://forum.solar-electric.com/forum/solar-news-reviews-product-announcements/solar-information-links-sources-event-announcements/4426-working-thread-for-solar-beginner-post-faq?p=191136#post191136

Try that link.

-Bill

BB. · May 2015

If you cannot find post 45 in the above link, here is the text:
:
Search for vfd, or look for post 45.... this phone stuff is not working.

-Bill

:

nthamizh · May 2015

I already tried with VFD based Pump Inverter powered by Solar PV directly, but it is a failure because the Pump is a Single Phase Pump. After this only I am trying a Grid Tied system in Off-Grid mode.

Please give me some ideas to implement Grid Tied system in Off-Grid mode, to run 2 X 1.3 KW Single Phase Swimming Pool Pump.

Thanks,
Arasan

Photowhit · May 2015

Well, you have no grid available, correct?

Why do you want all the extra's to have a grid tied type system in off grid mode?

Just buy an off grid system.

Your array size might just power the pumps during solar noon on sunny days, but likely you will need additional array as well.

You might find a small lithium battery bank a good buffer to start the pumps as an alternative to a pretty large lead acid battery bank, since storage won't be much if any concern. lithium has the advantage of being able to deliver high amounts of current without a voltage drop.

mike95490 · May 2015

Generally, Grid Tie inverters dump all possible power, into the grid, which is your Generator. That will cook it in about 1 second. A Grid Tie inverter wants to see an infinite grid, and when it does a low impedance "ping" of your mini-AC grid, it will likely fail.

Your off grid inverter choice, does not list if it contains Auto Generator Start feature. That would be required if you want to automate the cloudy day generator usage. Your generator must support auto-start functions too.

With an off-grid system, the inverter wants to have a large battery bank to supply the surge power needed to produce each AC sine wave, and for short term surge current, A 50ah battery will not be sufficient.

Have you done a load measurement of your pumps, both starting load and running load ? You will HAVE to stagger the starting sequence, it's unlikely any inverter would be able to start 2 large motors simultaneously.

My 1/2 hp pump, consumes 1Kw of power as logged by the inverter, your pumps are likely to give you a surprise too.

I'm guessing about 400Ah battery @ 48V would be the very smallest battery you could get by with, to run 2 pumps. You may want to consider variable speed pumps, which have softer starting surges and may be less demanding on the electrical system.

nthamizh · May 2015

In case if I go for Solar Off-Grid system, what would be the minimum battery capacity required. You can suggest the sizing of both Lead Acid Vs Lithium. Please let me know the design criteria for the battery back-up required for only starting the pump and after starting, consumption from Solar PV. With the Off-Grid Inverter, is the power from the Solar PV based on Battery Charging or on Load requirement. Please let me know.

BB. · May 2015

How much do you know about AC electrical systems in general, and specifically about Grid Tie and Off Grid/Hybrid AC inverter/AC generator systems?

We can go into detail about how you can configure a GT + OG inverter system--But it is going to be a bit confusing (at first) to understand the why and details of such a design... And, from what I can see, I do not believe that a GT inverter to OG Inverter system is going to be of any great advantage to you--And probably lots of disadvantages (cost of installation and maintenance, debugging, etc.).

For a pure off grid system--The only time a GT system may make sense is if you have a long distance from the Solar Array to the Battery bank/charge controllers. And you have a substantial battery bank/night time/cloudy day loads.

For day time/sunny weather loads and a small battery bank, GT Solar, usually makes no economic sense. And even Off Grid solar + Battery bank + maintenance + new batteries every 3-8 years or so, can be difficult to justify too.

Basically the problem with GT solar back feeding an OG AC inverter is that the power from the solar array has to go somewhere--Charging the battery bank (generally ~100 AH @ 48 volt battery bank per 1,000 Watt array) has to be large enough to "absorb" the GT solar output--Plus once the battery bank is full, you have to "turn off" the GT inverter or "reduce GT inverter output wattage" or install a "dump controller+Load bank" to do something with the excess solar array output.

A 50 AH @ 48 volt battery bank is not near large enough to absorb a 4k kWatt array (buffer the energy)... And is not really enough to power a few kWatts of AC pumps--perhaps maybe for 30 minutes or less with GEL batteries (and GEL Batteries cannot charge quickly--around 5% of their rated AH is their limit--You would need ~1,500 AH @ 48 volt GEL battery to properly "buffer" a 4kWatt GT solar array.

Next, it appears that the Axpert 4kWatt Off Grid AC inverter may be a good start for you. The maximum solar array it can support natively (with its internal PWM solar charge controller) is, roughly:

50 amps * 72 Volt Vmp-array = 3,600 Watt array (just a guess)

So--Connect your solar array to the PWM solar charger input to your Axpert OG AC Inverter. Connect a minimum of 360 AH @ 48 volt Flooded Cell (or AGM) batteries (I would be very worried about how well GELs would last--You need to research the brand/model of GELs you wan to use).

And connect your pumps to the AC output of the inverter... And if you want a back up AC genset, connect the genset to the AC1 input on the Axpert inverter.

That is it... If you need a larger solar array and/or higher Vmp-array voltage---You could go with a MPPT Solar Charge controller connected to the battery bank and not use the Axpert's own PWM solar charge controller (several electrical/solar panel cost advantages, but a large 80 amp MPPT solar charge controller can cost $600 USD or more--Of course there are other MPPT controllers from China/elsewhere that may meet your needs too).

You need to design the support hardware (wire diameter from solar array to Axpert), copper wiring to the battery bank, monitoring system (if you are going to have one), etc... And plan on replacing batteries every 5 years, and the AC inverter every 10+ years. Replace solar array every 20+ years. See if the system is cost effective for you.

An alternative system--Don't use swimming pool pumps--Use pumps + motors designed for solar power applications. That may be PM (permanent magnet motors) with high efficinecy pumps designed to run directly from Solar panels (a 1,000 Watt DC motor+pump may cost $2,000 USD or more). Some solar direct pumps can be run from Solar panels, battery banks, or even AC gensets directly (no extra hardware needed).

Other option is to pick a 3 phase motor to drive the pump and use a VFD designed (or hacked) for use with solar panels... Again, you don't need a battery bank+charge controller, and can use a backup genset when needed.

I highly recommend that you do several paper designs---Complete with parts lists and pricing. This is not a trivial project and solar power has significant limitations and is not cheap.

-Bill

Photowhit · May 2015

BB. wrote: »

A 50 AH @ 48 volt battery bank is not near large enough to absorb a 4k kWatt array (buffer the energy)... And is not really enough to power a few kWatts of AC pumps--perhaps maybe for 30 minutes or less with GEL batteries (and GEL Batteries cannot charge quickly--around 5% of their rated AH is their limit--You would need ~1,500 AH @ 48 volt GEL battery to properly "buffer" a 4kWatt GT solar array.

While a 50 AH Lead Acid, would fail pretty much instantly. It has 50AH X 48 volts or about 2.4 Kwh of storage and a load of 2.6 Kw would be more than any Lead acid battery is designed to deliver in this ratio. I suspect you would need to use agm batteries designed for high draw rates and still want to have a battery bank around 1:4.

Lithium ion batteries can deliver 1:1 for most of the capacity and Lithium/Iron/Phosphate can run about 1:2, if I recall correctly. also they have a relatively flat voltage range, so would be less likely to have the inverter shut down from low input voltage. So long as they are used primarily just to start your pool pumps I suspect a 50ah would work until both pumps tried to start at the same time. this even might get a bit of help from the array.

If you intend to run during the day, direct from solar, understand that the panels only produce about 75% of their panel rating when hot, and only produce this when in a somewhat direct angle to the sun. So only if setup directly facing south and you have increased your array a good bit (6-8kwh) If solar noon is 1:00pm your array is unlikely to produce enough energy until 9:00-9:30 and would need to be shut down by 5:00 or so... The alternative is to have enough storage to run from the battery bank, and I would guess looking at around 800ah 48v lead acid bank, if you wanted to try to run sun up to sun down when sunny without running the genny. you would also want to further increase your array so you would have excess energy to charge your battery banks on top of the load....

nthamizh · May 2015

Is there a possibility for the below design
- Use battery ONLY for starting 2 X 1.3 KW Pumps
- Once started, the power from Solar PV directly support the pump running

Please let me know.

- Arasan

Photowhit · May 2015

Read my previous post... if you don't understand it, ask questions.

In truth the answer is NO, but it can become a neutral loss system. if you never have excess energy to charge the battery, it will fail. The pumps are A/C so must run off the inverter, but the array may generate enough energy to supply the needs.

4 KW Grid Tie System in Off-Grid Mode

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