This device uses a variable excitation alternator followed by low power scavenging to allow underpowered (weak) energy sources to generate useful electricity.
In the case of wind farms it would be used in times of low wind power density.
It could even be used on the dreaded cliché - exercise bike
Many alternators (automotive, wind turbine nacelles, motor-generator sets) already use a variable exciter field. It is tightly controlled to regulate the voltage output.
My idea is to vary the exciter field to ease the magneto-resistance through which the alternator rotor must turn. So a mild wind (or a gym user who is taking it easy on the bike) could still drive the alternator. Admittedly this results in an output that varies in both voltage and frequency, but that is not a problem to my low power scavenger.
As with all alternators, the first part of any generated electricity goes to providing the field current. Therefore there will still be a lower limit of energy input below which no useful output can be generated. However, the lower limit is vastly decreased by allowing the field current to be reduced.
The exciter field would be increased when the wind strength increased, or when the gym user wanted to increase pedal resistance. In the case of the wind turbine, this could be achieved using closed loop control where the measured-value is the speed (rather than voltage output). When the turbine slows, the excitation field strength reduces (to lower resistance), and vice versa. In the case of the gym user, the excitation field strength could be manually controlled.
The low power scavenger consists of a voltage multiplier and a rechargeable battery. A voltage multiplier is a common electrical circuit. [See the wikipedia link].
The voltage multiplier would take the low voltage ac source and convert is to a higher voltage DC output. When the alternator excitation is low (because there is only a mild wind, or the gym user only pedals lightly) there is a low voltage input to the multiplier, and therefore it takes more cycles (no pun intended) to convert to the higher voltage DC.
The voltage multiplier output is connected to a rechargeable battery. Once the multiplier output voltage rises high enough, it would trickle-charge the battery via a current limiter (simple resistor). While the output is lower than the battery terminal voltage, the multipliers diodes would prevent backfeeding.
The battery power can be utilized later, possibly via a synchronous inverter.
Someone pedalling for an hour might only charge the battery enough for 5-10 minutes worth of useful discharge. Well that is the price to be paid for taking it easy on the exercise bike. [It is a well known phenomenon in mechanics what is gained in mechanical advantage is lost in velocity ratio.] However, this is not about efficiency. It is about utilizing an energy source that would otherwise go unused.
This method could be applied to lots of different low energy sources. Small streams could be used for hydraulic power. We might even be able to develop piezoelectric speed bumps (no alternator, just the scavenger).
The world is our bi-valve mollusc.