cost per kWh [RE-wrenches]

Daryl Thayer daryl_solar at yahoo.com
Thu May 29 10:04:35 PDT 2003


Hi Hugh and others
In regards to performance predictions VS actual
performance, my local study found all methods, sun
hours via the Sandia method, Retscreen, Maui Solar
(known as Solar Pro), and PV-Fchart are all about the
same for annual and seasonal performance.   In my
local study I found the error of over-predicting in
the summer of about 10% was due to  the error in the
solar radiation, using the measured radiation, the
performance was in excellent agreement.  The second
largest area of disagreement was the performance of
the MPPT which worked much better in bright sun than
in dim sun and its performance was less reliable in
the dimmer light of the winter.   By the way I have
been keeping track of an MX60 in the winter and it is
spot on.  

Another problem also looked at was the loss of load
probability.  The old estimates used in the Sandia
method were not adequate in my area.  The Periods of
very poor sun were longer than expected by the Sandia
assumptions, which greatly increased the loss of load
probability.     This is exasperated by the on-off
performance of the MPPT.  Low sun never once confused
the new MX60.  (only 9 months experience to date on
the MX60)  My study found very dark days for up to
seven days duration, (periods where the solar on the
AC bus was less than 10% of the monthly average). This
only occurred in the winter, never in the summer where
one or two days were more likely.   I am sure this is
local specific, and here we are hit by several one and
two day storms in a row.  

In reinforcement of these finding I have monitored
some of my stand-alone DC lighting installations and
found loss of load occurring in systems with
5-day-storage to loss-of-load.  My load is of course
fixed and my array is sized to exceed the average load
in the worst month by 25% to 50%.  If this is relevant
I could reexamine the data and be more specific. 
However, I think this is regionally dependent problem,
your area may be better or worse.  I am concerned as
to battery life in these systems because the
time-to-full-recharge is then exceeding 15 days.

Daryl

--- Hugh Piggott <hugh at scoraigwind.co.uk> wrote:
> Hi PV guys
> 
> I am more into wind and hydro.  But I am called upon
> to do PV, so I do.
> 
> I wonder if anyone can help me with solar pv kWh
> performance 
> prediction?  I want to know about kWh into the
> battery from a pv 
> array.
> 
> My approach has been to find the kWh/sqm/day (same
> as 'sun hours'?) 
> and multiply by the array watts-peak.  But I feel
> uneasy with this. 
> Of course you will not see that peak watts in a
> system unless it has 
> MPPT.  And even MPPT is not perfect is it?
> 
> Would it be better to look at the output in amps and
> multiply by a 
> typical system voltage and then multiply by sun
> hours?  I have used 
> this approach too but it seems a bit vague.
> 
> Now we have discussion of module performance
> degradation over time...
> 
> I have been using the retscreen software and it is
> very good for 
> predicting kWh/sqm/day for different azimuths and
> angles in different 
> locations.  Then it has loads of other stuff, but it
> is not explicit, 
> so I cannot tell what variables are controlling
> which other ones and 
> how.  Without some sort of flow chart describing
> what it is doing 
> with the numbers I cannot use the answers reliably.
> 
> What do you guys do and are there any databases of
> actual performance 
> relative to 'sun hours' that I can use to calibrate
> my assumptions?
> 
> thanks
> -- 
> Hugh
> 
> hugh at scoraigwind.co.uk
> http://www.scoraigwind.co.uk/
> 
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