On 5/20/2012 7:20 AM, Larry Crutcher, Starlight Solar Power Systems wrote:
boB,
I know I'm getting out of my league here but I'm so intrigued. Doesn't
a LCB device work on a feedback loop that looks at the output voltage
and current to a load and compares it to input voltage? Since we have
two fixed values here, 29? resistance and 4.15A PV current, how can
there be any excess voltage at the circuit input?
I can see that the PV modules should produce 1kW @ 500W/m2 IF the load
allowed it. As Kent said, the resistance would have to change to
58?. On the other hand...if I don't understand the function of a
LCB...operating the PV module at 170 Vdc would result in 5.87 amps
while feeding a 29? load.
Counterpoint? Inquiring minds want to know!
I don't know exactly how they work either... There was not quite
enough depth on the web site I went to.
It just seemed sorta (to me) like what I described... An MPPT without
batteries running on the input (PV) side.
i.e. Higher input voltage at a lower current transforms into lower
voltage at higher current at the output side.
I did find some supposed' LCB schematics online that were pretty much
was what I expected.
To get max current and therefore max power across an output resistive
load, the input voltage must be
at MPP voltage of the PV which (to me) means it would have to be an MPPT
type device. Without a
battery, the output voltage would not matter so I think of it as a
Another use for a circuit like this would be a cathodic protection device.
So, if that is the case, it could be correct as is, OR, may be with SMOP
(Simple Matter Of Programming) which isn't
necessarily all that simple. Looks like the voltage that Mick is
looking for may be too high for the application
he is wanting though.
Please let me know if I'm wrong here as I'd love to know.
boB
On May 19, 2012, at 10:16 PM, b...@midnitesolar.com
<mailto:b...@midnitesolar.com> wrote:
On 5/19/2012 5:47 PM, Larry Crutcher, Starlight Solar Power Systems
wrote:
Good one, Mick. Here's the way I see it:
For question one, I say yes, other than the obvious wire and
connection losses, because you are operating the cell(s) at the peak
power point.
Since a PV solar cell is a constant current device and your 29 ohm
load did not change, reducing your Lab Light source to 500 W/m2 can
only affect the voltage and resulting power. This means your load
forces the voltage down to 120 volts resulting in 500 watts of power
dissipated. You have forced an IV curve shift of current AND voltage.
The LCB boosts current based on IV curve voltage. I don't think an
LCB can change anything in this scenario since you have forced the
circuit to operate at 120 volts and 4.15 amps.
Realizing that these LCB things run off the PV input because there is
no battery.... If an LCB outputs
maximum current into a fixed resistance, (and therefore maximum
power), kind of like an MPPT
charge controller does, then it ~should~ adjust its voltage "gear
ratio" to maximize the output
current and power.
At least I think the idea is correct, if the LCB was large enough and
high enough voltage operation
for your application. But then again, I may be missing something
here too.
boB
Keep in mind that I'm a solar dummy. My logic may be faulty.
Larry Crutcher
Starlight Solar Power Systems
(928) 342-9103
_www.starlightsolar.com <http://www.starlightsolar.com/>_
_
_
_
_
On May 19, 2012, at 12:35 PM, Mick Abraham wrote:
Hi, Wrenchies~
Kindly educate me regarding PV behavior when the load is a resistor.
Here's a hypothetical situation:
* Eight 250 watt PV modules (60 cells per module), all connected in
series for "peak" ratings of 240 volts DC & 8.3 amps
* Lab type cell temperature & illumination so that the eight would
truly pump 2,000 watts */into an ideal load/*
* A 240 volt AC heating element designed for 2,000 watt heat
dissipation at 240 volts AC...that's about 29 ohms resistance for the
heat element
* Connect the PV string to the heat element, with nothing in
between except a fused disconnect.
In the above situation, would the resistance of the heating element
be all that's needed to force the PV array to operate near the "peak"
wattage?
Would the heater actually get 2,000 watts to turn into heat?
+++++++++++++++++++++++++++
Now consider the same cell temperature but half the illumination.
That's similar voltage but half the amps at peak wattage. If this is
sent into the same 29 ohm
resistor--again with no intervening electronics, could we count on
1000 watts of heat?
If the answers come up "no", would the power throughput be helped
by a SolarConverters style MPPT pump controller (Linear Current
Booster kinda thing), assuming that one could be found to operate in
the 240 volt range?
Thanks & Jolliness,
Mick Abraham, Proprietor
www.abrahamsolar.com <http://www.abrahamsolar.com/>
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