Voltage drop musings [RE-wrenches]

[Phil Undercuffler]

William,

First, I often size for 3% max voltage drop, although I will shoot for 2% on
12 volt array inputs.  The main issue is that excessive voltage drop will
prevent the panels from being able to drive the batteries to an equalization
charge under all conditions.   We use an ElectriCalc Pro for calculating
wire sizing in the field and in the office.  Great product, highly
recommended.  Only problem is when the boss grabs the one out of the truck
and forgets to return it!  (Calculated Industries, 775-885-4975)

Remember, panels will produce rated or near-rated voltage under most
light--clear sun or haze, early morning or late evening.  Its the amperage
that changes as light intensity increases and angle of incidence decreases.

In summer, as the panel temperature increases its maximum power point
voltage drops, and the opposite happens as it cools.  Therefore, minimizing
voltage drop is most essential in the summer, so you can EQ with a full
battery and a hot panel.

Also, an array running through a conventional controller to a battery will
never put out its rated output--panels are rated at their maximum power
point, usually x amps at 17-18 volts.  As soon as you connnect them to a
battery operating at 12 volts, you lose a hunk of that "rated" output.

When I use a MPPT controller, whether gridtie batteryless or the Solar Boost
products I use the peak power  voltage rather than nominal voltage for wire
sizing.  When using conventional controllers I use the lowest voltage I
expect to see in normal conditions.

As a wholy unsolicited testimonial, Windy Dankoff has written a great sizing
program that takes this pv-battery mismatch as well as wiring losses,
inverter losses, etc into account when designing systems.  I think he offers
it free to folk who buy equipment from him (that will be two beers for the
plug, Windy).

Phil Undercuffler
Positive Energy

Injustice anywhere is a threat to justice everywhere--Martin Luther King

----- Original Message -----
From: William Miller <wrmiller at slonet.org>
To: <RE-wrenches at topica.com>
Sent: Tuesday, May 01, 2001 3:39 PM
Subject: Voltage drop musings [RE-wrenches]


I ran the
> calculations for a 1% drop and suggested a wire size to my client.  1% is
> my standard.
>
> The client is deciding when to replace the conduit and wire.  He was
musing
> out loud himself that he should do it before winter so that he can take
> full advantage of reduced solar energy in the winter.  This got me to
> thinking that in the winter, with reduced solar energy levels contacting
> the ground, the panels will probably not reach rated power.  So the winter
> is not when maximum wire sizing is needed.
>
> I pondered this question further.  How often will a given PV system
produce
> rated power?
>
> If the system is battery based, when the batteries are near full voltage,
> the PV arrays will be tapering the current output.  My conclusion is that
> this type of system will put out rated power only when battery voltage is
> low, during the mid-summer and near solar noon (for non-tracking arrays).
>
> If the system is a non-battery line tie, the inverters I am familiar with
> are MPPT based.  This means the system voltage will be at about 17 volts
> per panel, or 68 volts total.  Can I expect the PV system to put out rated
> power only if the sun is perpendicular to the panels?  I should calculate
> wire sizing based on 68 volts, not 48, right?
>
> If the array is fixed, this will be only at noon on two days of the year.
> Then how important is it to calculate for 1% voltage drop for those two
> moments in each year?
>
> PS:  I bought my self a palm pilot (my friend named her Rosie).  I can use
> spreadsheets in the filed now so I can calculate wire sizes, sun angles,
> shadow length, etc. without having to go back to the office.  Its great!
>
> WM


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