[RE-wrenches] PV harvest in grid-tied systems

[Chris Mason]

William,
I must confess to being totally confused by your post.

I am currently at my office writing to you using electricity from our
off-grid solar system. We only have the solar power, no grid power.

We get exactly as much power as I predicted we would, in fact a little
more. But, there is an intrinsic issue with off-grid systems that many
people miss. Unless you have a use for the power, it won't be "harvested".
If you allow the inverter to charge the batteries from the grid, there will
be nowhere for the PV power to go.

To give you an example, our warehouse and office has 2,580 watts of PV on a
60 amp charge controller and a 3.5KW outback inverter, with 250Ah of VRLA
batteries (4 x 12V).

Each day, I read the amount of power the charge controller makes. It
averages about 12KWh, depending on how much we used that day and the state
of the batteries at the beginning of the day. We have found that the loads
did not adequately exploit the power product capabilities of the system, so
we moved our freezer from the house to the warehouse to use up some more
power.

As we are in the Caribbean, the insolation levels are pretty continuous, so
it does make calculations easier.


Chris



On Mon, Apr 16, 2012 at 1:50 AM, William Miller <william at millersolar.com>wrote:

> Friends:
>
> There has been recent discussion on this forum about PV production in
> grid-tied with battery systems.  I have been faced with this same problem
> in a 3 XW system installed in 2009.
>
> The customer's power bill never went down.  PV harvest was abysmal.  The
> system would create great short circuit current but would not harvest into
> the batteries.
>
> It is starting to appear the XW firmware was at fault.  The inverters
> would charge the batteries too high so the charge controllers would back
> off.  Upgraded firmware seems to be helping, but the data set is still
> limited.
>
> Remember this:  just because a PV system can produce does not mean it
> will.  The charge controller knows only one thing:  battery voltage.  If it
> is high, production is curtailed.
>
> Off-grid production will also be reduced is consumption is curtailed.
>
> Charge controllers that can be set for grid-tied applications help, but
> only if the inverter(s) do not keep the battery voltage too high.
>
> William Miller
>
>
> Miller Solar
> Voice :805-438-5600
> email: william at millersolar.com
> http://millersolar.com
> License No. C-10-773985
>
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-- 
Chris Mason
President, Comet Systems Ltd
masonc at cometsystems.co
www.cometsystems.co
Cell: 264.235.5670
Int: +1305.767.2094
Skype: netconcepts
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