A Device for Grid Tied PV System

System

Hello experts,

I am writing from Bangladesh, a country in south asia. In our country the government has no policy to feed back to the utility grid from privately owned PV systems.

In this circumstance, i wish to install a PV system in my house (suppose 5KW load driving capacity). Moreover, there will be utility connection in my house. In case of driving the total load present in my house, I wish to draw the energy generated from PV source instantly. If PV source lacks the power to drive total load that time, then the required excess power will be drawn from the utility grid. (since same amount of energy won't be produced by pv panels thorough out the day time).

Is there any device which will instantly measure the power generated from PV source and instant requirement in the load side? Then completely utilizes the energy generated from PV source and draw rest amount from utility?

Waiting for your expert opinion.
Regards,
Mamun

BB.

Re: A Device for Grid Tied PV System

More or less, the Schneider/Xantrex XW Hybrid inverter will do that (not 100%, but, from what I have read from others here, something like 80-90% of the load from PV array and the rest from the utility).

The XW 6048 (6kW and, a minimum of, 600 AH 48 volt battery bank) with whatever sized solar array you need + charge controllers to recharge the battery bank. Supports a backup genset.

The XW will also take over running your home if the utility power fails and power your home from battery, solar panels, and backup genset.

This system will do roughly what you want, but because of the battery bank, the system is not inexpensive (you will need to replace the battery bank roughly every 8-15 years or so, and a new inverter+charge controllers every ~10+ years, etc.).

Are you looking to reduce electric costs or for reliable backup power?

Grid Tied Solar, in the US, is pretty close to equal cost with utiltity power (depending on how much sun and how much electric power costs). Off-Grid / Hybrid power is still quite expensive and only makes sense if emergency power is needed or the home is located far from any utility power.

-Bill

niel

Re: A Device for Grid Tied PV System

if i'm reading the op correctly here he is not looking for a whole home back ups, but rather he wants solar to be the primary power source for his home and if it won't supply enough then the grid will take up the slack. this describes exactly what a grid tied system does. if you generate more than your needs it gets exported to the grid. if your pvs generate the same as your needs you import/export no power as the pvs are running the loads. if you use more power than the pvs can supply then the excess is run by the grid. no batteries are involved in a gt system, but the grid itself acts like a battery if your meter would run backwards during excess pv power production and that is a big if because many people have run into meters that will essentially add your excess power you exported to the grid to the power the grid supplied to your home as the meters only add and cannot subtract. these types are not all meters, but you would need to know what your meter would do. your utility may not actually know what the meter would do in reverse as they probably aren't used to people generating power to their grid yet.

System2

Re: A Device for Grid Tied PV System

@BB
Thanks a lot for your suggestion. Your suggestion is what that i had before writing this post. as i didn't get any better solution than schenider upto this moment, it's the solution to me of this problem.

@niel
Thank you very much for your clear understanding. In my country i have not experienced any grid tied pv system yet, that's why i have no idea whether it would meet my necessity or not. i will try to study the user manuals/catalogs/articles on grid tied inverter to better understand their operation.

Thanks guys.

Seven

Re: A Device for Grid Tied PV System

Bill,
Why the 600ah bank requirement?

BB.

Re: A Device for Grid Tied PV System

My way of explaining... On the DC side of a Hybrid (and off grid) power system, the battery bank is responsible for regulating the DC voltage (not the inverter/charger or the MPPT charge controller).

Using the 6kW @ 48 volt XW system as the example, the battery bank not only needs to output 12 kW surge (10 seconds?), it needs to be able to absorb 6+kW from the MPPT charge controller for a few seconds as it does the MPPT sweep/test--even if the battery bank is "full".

For a flooded cell battery bank, normally we assume C/2.5 maximum surge current (40% of bank rated amp*hour capacity). Running the math:

• 12,000 watts * 1/48 volts * 2.5 surge =625 AH minimum

There is no specification, that I know of, which suggests a battery sizing calculation for the battery to absorb current when charging if full. But it appears that there have been reports of the XW "faulting" with high battery bank voltage.

6kW "charging surge" (from solar array) into a 48 volt battery bank with 600 AH battery bank:

• 600 AH / (6,000 watts * 1/48 volts) = 4.8x surge current

So, we see ~C/5 as the maximum charging current surge that a 6kW array would feed a 48 volt @ 600 AH battery bank. This seems like a reasonable charging surge current into, an already charged, flooded cell battery bank.

Now, an AGM battery bank would be able to support a much greater than 12kW surge current with a smaller battery bank... However, again from reading what a few people have said here, that a small capacity AGM battery bank would not be capable of absorbing a large surge current if fully charged (Concorde batteries, for example, seem to simply increase Vbatt instead--which can cause a battery bank voltage fault for the XW--greater than 72 VDC if I recall correctly?).

In any case, the 600 AH @ 48 volts for a 6kW XW Hybrid inverter system came from early design and testing done by Solar Guppy...

Could you use a smaller battery bank--probably yes. But you would have to limit the maximum supported surge power (on output) and/or limit the maximum array (input charging current) in proportion to the 6kW/600 AH ratio.

My two cents... Hope that make sense. Again, this is my understanding at a distance. I have never laid hands on such a system or performed any testing myself.

Yes, Xantrex/Schneider does list smaller supported battery bank capacity, but from the comments we have had here, it does sound like like a problem (either new, or when the battery bank ages).

-Bill

Seven

Re: A Device for Grid Tied PV System

That does make sense. Thanks.