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

Sun May 20 07:20:08 PDT 2012

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, 29Ω 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 58Ω. 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 29Ω load.   

Counterpoint? Inquiring minds want to know!

Larry Crutcher
Starlight Solar Power Systems
(928) 342-9103
www.starlightsolar.com

On May 19, 2012, at 10:16 PM, boB at 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
> 
> 
> 
> 
> 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
> 
> 
> 
> 
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