Voltage drop musings [RE-wrenches]
William Miller
wrmiller at slonet.org
Tue May 22 02:36:11 PDT 2001
Drake:
Thanks for your reply and the stimulating discussion. I finally had tome
to reply. I still can't seem to agree with all of your points. See my
comments below:
At 09:49 PM 5/4/01 -0600, you wrote:
>Hi William,
>
>Thanks for your input. It is fun to toss this stuff around. It seems that
>musings lead to more musings.
>
>Current is what charges batteries, not voltage or total power. If you have
>maximum power point tracking, then the voltage difference between the
>array voltage and battery voltage can be transformed into charging
>current. Without it, the extra voltage represents power that will be
>dissipated as heat.
>
The point of MPPT is to eke more watts out of a panel or panels by allowing
them to work at a more efficient voltage at the panel itself. If you
calculate the power into and out of a standard charge controller and
compare it to the power ratio of an MPPT controller, you generally find the
MPPT is more efficient, especially in colder weather. The extra POWER will
not be dissapated as heat, it will never be created because the PV panels
are operating less efficiently.
>The charge from the solar array is reversing a chemical reaction that took
>place in the batteries during discharge. It is moving electrons.
>
>In a standard deep cycle battery, the chemical reaction involves a transfer
>of valence electrons between plates of lead peroxide and sponge
>lead. Instead of the electrons transferring directly in the solution, they
>are conducted through a wire to complete the reaction. During charging,
>the electrons are forced back to their original position.
>
>It is the electrons that are forced through the battery that results in the
>charge. An ampere is a measure of the charge, also described as the number
>of electrons that pass a given point in a second. An "amp" is equal to
>one coulomb / second, or 6.242 X 10^18 electrons / second. It is the
>forcing of these electrons back to their original plates that results in
>the batteries charging.
>
>Voltage is essentially electrical pressure. You need sufficient potential
>to force electrons through the battery, but an excess amount is not helpful
>with a current limited device such as a PV module. You can charge a 6 volt
>battery with a 12 volt solar module. It will charge at the module's
>amperage, not the module's wattage.
You can force more chemical change in a battery for a given amperage if the
voltage is higher. To illustrate this, consider this "ASCII" diagram (I
hope it displays clearly):
DC----------------wire resistance-----------------------------Load
Source |
| |
|_______________________________________|
The DC source provides a finite amount of power. The power is dissipated
in the wire resistance and in the load. The power dissipated in the wire
plus the power dissipated in the load will equal the source power.
If the wire resistance is higher, the power available at the load is lower.
It is power that charges batteries. If you have amperage without voltage,
you are not going to charge your batteries. If you have voltage without
amperage, you will also not charge batteries. You need both, which is why
power = volts X amps.
>
>If you are using a standard charge controller, the voltage between battery
>voltage and array voltage represents power that is not utilized.
>
>A little higher voltage drop will slow the current down but a very small
>amount. The only time the voltage drop will have a negative effect is
>under conditions when the array is capable of putting out maximum
>amperage. That will only happen when the batteries can accept it, and the
>sun is at the best charging angle.
>
I disagree with the first sentence of the above paragraph. A voltage drop
in a series circuit will not reduce amperage one bit (Kirchhof's law).
The last sentence is the essence of my original question: What design
parameters should one use in specifying wire size? My long time colleague
in communications and now RE energy might have hit it on the head: One
must design for the AVERAGE photovoltaic source circuit power. There is no
sense in designing a system for parameters that happen only in the lab or
once in a great while. The next big question: How do we determine the
average system output?
In disagreement with the philosophy of designing for average operation is
the scenario of the homesteader with low batteries on the first clear
winter day. They don't care what the average system operating parameters
are, they just want the maximum power delivered to their batteries before
the sun goes down. If we saved a few bucks on buy wire too small, we may
have been doing them a disservice.
>A system doesn't lose a percent of its monetary worth for each percent of
>voltage drop in the feed from the array. A nominal 24 volt system may have
>an array voltage of 34 volts (Siemens SP 75s). A 3% voltage drop will take
>away 1.02 volts, leaving 32.98 volts to charge the battery bank.
In this case, lets assume two SP75s, or 150 watts. The current will be:
(P/E = I) 150W/34V = 4.41A. The power lost is also 3%: 1.02V X 4.41V =
4.5W. 4.5W/150W = .03 or 3%. When the panels are operating at rated
power, 3% of their capacity will not be charging batteries, they will be
heating wire.
Let's assume SP75s cost $400 each, or $800 for two. 3% of $800 is $24 of
the investment that will not charge batteries. Again, this is at rated
power, which we know will not happen every day.
Consider a line tie system. If you waste 4.5 watt for 6 hours every day
for a year, that is 9.86 kWh. At .30 per kWh, you are losing $2.95 per
year in revenue. This does not seem like much, but in reality, an average
system is 10 times as large. With a 1,500 watt system you are then losing
$29.50 per year in lost revenue, at rated power.
>
>32.98 volts is sufficient to push 1640 amps through 100 feet of #2 copper
>wire. The line loss in the feed from the array won't slow down the
>charging much. The current output of a PV module is limited by the
>module's current producing capability, and not directly subject to I = V/R.
>
This is only of you have an unlimited current source, which we do not have
with two, or with 200 SP75s.
>To fill in the equation for the total resistance, the resistance of the
>battery bank and charge control need to be taken into account, as well as
>the drop across fuses and switches.
>
>To figure what percent of loss to the system monetary value a given drop in
>charging represents, all inefficiencies must be taken into account,
>especially inverter losses.
>
Absolutely, and these losses add to wire losses, which is why we need to be
design carefully and not lose too much in wire loss.
>I try to stay under 3% VD. I saw one system operate satisfactorily with a
>10% VD. I sure wouldn't have gone that high. As I recall, it had high
>voltage modules, and coils of number 10 wire laying on the ground.
It all depends on how you define "works satisfactorily." My goal is to
make my clients investment work as well as possible. My sub-point would be
that as long as you are on site with your tools and vehicle, it does not
cost much more to pull the proper sized wire, especially considering the
long term economic losses if you do not.
My sub-sub point is that many manufacturers, even in the RE field, do not
provide large enough lugs or enclosures to allow over sizing wire. Many
times I have had to run 1/0 wire for less than 60 amps, yet PM60s and C60
charge controllers, for example, will not take much above #6 wire.
>
>With the advent of MPPT technology, lower voltage drops may become more
>important. I was very impressed with the output a Solar Boost unit we
>installed last year. If the meters were correct, the batteries were, at
>times, getting close to the array's rated wattage.
>
Actually, voltage drops may become less important. I am working a 12 volt
system right now. We are adding a Zomeworks rack some distance from the
battery bank. We are wiring the array at 24 volts and placing the MPPT
near the batteries. This will halve the current in the connecting wires.
>
>Drake
>
>
__________________________________________________________________
William Miller
SLO Communications: Communications and Power Systems Consulting
PO Box 50, Santa Margarita, CA 93453
Voice :805-438-5600 Fax: 805-438-4607 VMail: 805-546-4875
email: wrmiller at slonet.org
License No. C-10-773985
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