Why LED Bulb Has Energy Efficiency Rating of ‘F’ – Explained

For Problem 1:
A string of LEDs hooked up in series with a current regulator like SuperTex CL220 would do the trick: The component is a simple a 2-terminal device (like a diode) and needs no additional components or configuration. It allows 20 mA (+/-10%) current to pass through as long as there is sufficient voltage headroom: 5 volts above the total forward voltage of the LEDs is sufficient. This current regulation is stable up to 160 volts, enough for your purposes.

Note that LEDs are current-dependent rather than voltage dependent devices. They glow at essentially the same brightness as long as the current is constant.

For your application, if the LEDs need to start glowing from around 60 Volts, half the 120 Volts your students generate, then a string of 25-30 standard 5-mm red LEDs would be optimal. They would glow without intensity change till your maximum voltage.

Too many LEDs = they won't glow till a higher voltage.
Too few LEDs = the CL220 device would overheat in dissipating the surplus voltage.

For Problem 2: The extent of energy storage required to keep a string of around 25 LEDs (from above section) glowing for even half a minute, is pretty high. Capacitors would not be the way to go, unless you have access to big power-line capacitors through surplus channels.

• Your capacitor bank would need to provide 20 mA at a minimum of 60 volts (again from above section), for "a few" seconds.
• Capacitor needs to be rated for a voltage higher than the highest the generator could conceivably generate.
• Though "supercapacitor" is a popular term these days, typical supercaps are rated for 5.5 Volts or 12 Volts, not hundreds of Volts.
• Adding in buck/boost generator trickery to make this work would result in complexity far beyond using a battery and off-the-shelf charger.

I hope this helped.

The LED power may be limited by thermal considerations. Both the power supply and the LED are prone to failure. The former because of possible shoddy transient protection and inevitable death of filter capacitors (exacerbated by the typical high temperatures). LED life will also be greatly foreshortened by high junction temperature.

These things are a trade-off. My suspicion is that the LED rating of 3W you mention is not really achievable (with any reasonable lifetime) in the environment of the light bulb, and everything (driver and LED) is really close to the line. Here's a snippet from a Chinese "3W" LED datasheet:

The Power Dissipation is absolute maximum 3W at \$T_A = 25°C \$. It will have to be derated above 25°C at the bulb, so I think you can see that calling that thing a "3W" LED is somewhere between irrationally optimistic and an outright lie. While the bulb maker might be perfectly happy to run it right at 3W, if they die so fast that they are getting 40' container loads of lamps dumped back at them, they'll not be so happy.

The only thing I really expect to be true of the bulb is the (initial) lumen output (you can expect them to fade over time). If the lumen output falls short, especially out of the box, that is false advertising and I would return them. So far, from reputable makers such as GE, I have not been disappointed. I can't directly measure lumens, but I use lux meter to check light levels on work surfaces and so on, and can reasonably well compare halogen and LED bulbs with similar angles.