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

[Kent Osterberg]

Mick,

In the first case you are selecting a resistor that matches the IV curve 
peak power point at 1000 W/sq meter irradiance. You will indeed get 2000 
watts of power into the resistor. For the second case, at 500 W/sq meter 
irradiance, the PV module's IV curve has about the same peak power 
voltage, but only half as much current. The resistance that would get 
the maximum power changes from 29 ohms to 58 ohms. With a 29-ohm 
resistor connected, the power into the resistor will be a little more 
than half of the 1000 watts the PV module could deliver. The linear 
current booster is designed to boost the current when the voltage (and 
resistance) are less than optimum, so I think it would improve the power 
transfer.

Kent Osterberg
Blue Mountain Solar, Inc.
www.bluemountainsolar.com


On 5/19/2012 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>
>
> Voice: 970-731-4675
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