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

[William Miller]

Chris:

I would willingly clarify but I am confused about what you are confused 
about.  I think we are saying the same thing about off-grid systems:  If 
you don't consume power, battery voltage stays high and charge controllers 
reduce energy harvest.

William Miller



At 06:57 AM 4/16/2012, you wrote:
>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 
><<mailto:william at millersolar.com>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
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