# Confusion regarding solar cell?

This below passage is taken from chap 9 of book Renewable and Efficient Electric Power Systems

I have almost understood this whole passage and its key take away concept (LCB) except the line I bold out. Can someone please explain the bold out segment especially its first segment Out by the knee of the curve. What this means at knee or before knee or after knee?

There is a device, called a linear current booster (LCB), that is designed to help overcome this loss of potentially usable insolation when current delivered to the motor is insufficient to overcome friction (Fig. 9.10). Notice from the I –V curves of Fig. 9.9 that the operating point in the morning is nowhere near the knee of the insolation curve where maximum power is available. Out by the knee of the curve, the PVs may be able to supply enough power to overcome friction, but without some clever electronics, this power would be delivered with relatively low current and relatively high voltage and still wouldn’t start the motor. What an LCB does is to shift this relationship around. By converting low-current, high-voltage power into high-current, low-voltage power, they can get the motor started earlier in the morning. The lower voltage, however, means that the motor will spin at a slower rate, but at least it is working. In addition, the motor with an LCB will not stall as early in the afternoon, though it will slow down. So there are additional gains.

Fig 9.9

Fig 9.10

• Sounds like snakeoil. No linear circuit can boost voltage. The MPPT is already invented for the very purpose. If you need to overcome some intertia on the motor, capacitors/batteries exists already. Commented Mar 19 at 7:36
• The "knee" is the part of the curve where the slope changes. Commented Mar 19 at 7:49
• @winny a linear current booster, MTTP and a buck booster are basically the same. The differ in the feedback and therefore regulate the power consumption via current and voltage different. I think a linear current booster tries to achieve a linear current, hence the name. And its absolutely not a linear circuit. Commented Mar 19 at 7:51

## 1 Answer

Out by the knee of the curve, the PVs may be able to supply enough power to overcome friction, but without some clever electronics, this power would be delivered with relatively low current and relatively high voltage and still wouldn’t start the motor.

This is drawing the distinction between power, which the solar PV has enough of, and voltage to current ratio, or operating impedance, at the knee of the curve. The knee is where the voltage is still high - the energy of the individual photons is still the same in high and low light, but the available current has dropped - low light means fewer photons.

A stalled motor needs very little voltage to actually spin, but it draws a large current. The PV cannot supply that current.

The answer is to reduce the impedance, that is reduce the voltage, increase the current, keeping the same power (less some small losses). 'Linear Current Booster' is a name I've not met before. Electronic engineers would generally call it a buck converter, or a switch mode power supply - SMPS.

• Please also explain " Out by the knee of the curve. " Commented Mar 19 at 8:57
• @Engr The curve is shaped like a knee joint in a bent leg at that point. It's a common description of a curve that's flat(ish) for some way, then drops like a knee. 'Off a cliff' is another description, but usually of more extreme 'dropping off' behaviour. We try to operate devices 'up to the knee', but not beyond where they fail abruptly. Commented Mar 19 at 11:58
• "Out by the knee" means away from knee?? Commented Mar 21 at 6:55
• @Engr 'out by the knee' means at the knee voltage, and above, where the current rapidly decreases, and the device stops working well. Commented Mar 21 at 8:02