Solar Boost / C40 test [RE-wrenches]

[Bill Brooks]

Allan and wrenches,

Nominal Operating Cell Temperature (NOCT) is a temperature given by most
manufacturers. This represents very closely the temperature at which most PV
modules in the lower 48 states operate at on an annual basis. It is a
statistically verified set of conditions based on the average daytime
temperature at dozens of cities geographically dispersed throughout the
lower 48 states.

We find that in no case does the average daytime temperature exceed 24C
(Miami, FL) or dip below 16C (International Falls, MN). This means that the
conditions of 20C for NOCT is statistically valid for annual operating
conditions in our lower 48 states.

Having said this, the typical temperature that a PV module in an open rack
installation will operate at ranges from 42C up to 49C under NOCT conditions
(20C ambient temp, 800 W/sq.M., calm wind). Looking at the available modules
on the market today, none that I know of have max power voltages at this
temperature above 16 Volts. Most are quite a bit less, so 30 Volts is a safe
average max power voltage for a 24-Volt PV array.

In the winter, 32-34 Volts is achievable on cold days (5C), and this is
where MPPT controllers on off-grid system are clearly beneficial. In the
midst of the summer, our max power voltage can dip to 26-28 Volts depending
on how hot it gets. I measured a max power voltage of 13.4 Volts (26.8 V for
a 24V system)on my PV modules on the hottest day of the year. That was
before going though any voltage drops in wiring or controllers.

Bill.

> -----Original Message-----
> From: Allan Sindelar, Positive Energy, Inc.
> [mailto:allan at positiveenergysolar.com]
> Sent: Monday, May 07, 2001 2:00 PM
> To: RE-wrenches at topica.com
> Subject: Re: Solar Boost / C40 test [RE-wrenches]
>
>
> Bill,
> I would expect to see 35V (but have not yet checked) because that is the
> approximate MPP of a 24V array under cool ambient conditions. HP's TtW!
> measured a range of 30.1-35.95, so I may be a little high; depends on
> modules. Now I'm curious. In the next few days I'll check my
> input voltage,
> let you know. BtW, HP measured an average efficiency of 97.2% O/I.
> Allan @+E
> ----- Original Message -----
> From: "Bill Brooks" <billbrooks7 at earthlink.net>
> > Why would you expect to see 35 volts under any sunlight
> conditions (except
> > below zero ambient temperatures)? You are quoting Standard Test
> Condition
> > numbers that should only be used as a starting condition to then convert
> to
> > real conditions.
> >
> > Again, MPPT controllers are good for off-grid systems. The jury is still
> out
> > on whether they provide much benefit to on-grid systems. It
> will probably
> > depend very heavily on climate-specific issues. In California, with our
> > rainy winters and sunny summers, this makes MPPT controllers on
> > grid-connected systems produce the same or less power than a system
> without
> > an MPPT controller. Remember, the controller can take around 5% of the
> power
> > just to operate. Our industry needs these controllers, but we have to be
> > clear on when and how to use them. Hopefully RV will get their
> ETL listing
> > soon so we can use them in inspected systems.
>
>
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