[RE-wrenches] PV into a resistor: please advise

[boB at midnitesolar.com]

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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