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Old 03-10-12, 10:27 AM   #1
aniraula
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Default Insights on a mini-AC grid design

Hey all,

I have been working on a design of a mini-AC grid that is a hybrid PV-diesel-battery system.
Since majority of the load is used during daylight hours, I was thinking of using large grid tied inverters to directly power the loads and reduce the size of the batteries which are very expensive and have a lesser lifetime. The batteries will store only the excess power and discharge during the night(when the loads are much lesser). I am thinking this would be a way to minimize the use of diesel generators and at the same time make use of the cheaper grid-tie inverters. Diesel Generators will only function as a back-up (for cloudy days).
I would love to hear some of your comments and feedback on the design and what are the issues that I might have to contend with here. I have attached a block diagram here as well.
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Regards,

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Old 03-12-12, 07:59 PM   #2
rhino 660
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will you be connected to the main grid or is this an offgrid system you are making
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Old 03-13-12, 12:37 AM   #3
aniraula
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I will be connected to the grid but the grid is very unreliable. The system could be subjected to around 16 hrs of power cuts per day in some winter months.
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Old 03-13-12, 08:47 AM   #4
rhino 660
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with a grid tied inverter as soon as you lose the grid power you lose power from that array
are you doing a 40 kw system or is that a drawing you found. a few months ago my boss finished a 108KW off grid battery system on the iron market in porta prince hatie.
will your power company pay you for power? this is i guess the deal breaker if they will then yes use grid tied if no then do energy efficient upgrades to reduce power consumption and size a battery system to match. btw our company travles to install.
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Old 03-13-12, 09:40 AM   #5
aniraula
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It is a 40Kw system. Since I require 300Kwh a day, I wanted 50% of that to be supplied by solar which brought me to that 40Kw mark. I considered 4.5 sun hours per day and with all the associated losses, I thought 40 Kw was logical. I wanted the rest of the load to be supported by the grid (when it is available) and the batteries were for the base load at night. The diesel generator only kicks in when all the above components fail to deliver(for eg. - when there is cloud cover and the batteries run out and there's no grid ). I am also thinking about using variable speed diesel generator to avoid frequent start/stop of the generators but that's a later concern.

My main reason for using the grid-tied inverters are to get an AC-coupled design where the loads are directly powered by the PV, so that I donot have to use large battery banks. Since my COE is still above where I want to be and the batteries are the most expensive part of my design, I wanted to use as less batteries as possible.

But, my primary concern is that by reducing the size of the battery bank too far i might end up cooking it eventually. So, i how do i find the right balance between reducing my costs and undersizing my batteries ?

And, In my country you cannot sell back electricity to the grid so that is out of the window.
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Old 03-13-12, 10:03 AM   #6
rhino 660
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the more you draw from the batt bank the faster they fail. you will have to decied which loads you want to be backed up by batt (lights refridgeration) and detrmine how many watts those loads require then size the bank accordingly. you will want an off grid inverter connected directly to the pannels otherwise in the event of grid failure you will be relying on diesel to charge the bank the solar will do nothing as it has a grid tied inv
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Old 03-13-12, 03:55 PM   #7
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As it has been pointed out already, grid tie inverters only work when the power grid is working and they sell back any extra power you produce, so in an area that has the grid go down a lot and that you can't sell back, it sounds like a poor choice to make, also if you already have to have a regular inverter then the cost of upgrading that to a 40kw shouldn't be that far from buying 40kw worth of grid tie inverters plus a 20kw stand alone inverter.
Also with Maximum Power Point Tracking (MPPT) you can get the rated output or even higher from the panels and there are MPPT charge controllers so you don't cook your batteries.
what kind of place are you powering? from the size of the system it sounds like it's either a huge building or a small village.

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