Hand-cranked vs hydro-turbine generator for single potential energy (no continuos flow)

Question

Let's say you have a certain amount of water at a height; think a bucket with 3 liters of water in it on a 4 meters tall wall. By letting this water to fall, we'd have an opportunity to convert some of this potential energy into electricity.

By using cheap commercial gadgets, one could achieve this by:

• Hand-cranked generator, using the falling mass of the full bucket to do the cranking (using pulleys to get the right torque). example
• Micro water turbine, letting the water flow from top to bottom, where the turbine would be located. example

There are many options on the market for each strategy. Some are more expensive, some have very specific limitations in terms of working pressure, minimum rpm, etc.

My question is: is there a preferred method between these two from a purely conceptual point of view? Should one of these methods achieve higher efficiency in most cases under similar limitations?

Answer 1

With 3 kg of water falling 4m, you're talking about a very small amount of energy, 120 Joules, or 20 watts for 6 seconds.

While both hydro and hand-cranked generators can be very efficient, the question is how well they keep their efficiency when scaled down to this very low power.

My money is on a mechanical solution scaling better, the thought of 4m of pipe and all that turbulence just feels very lossy indeed. Most machines get better as they get bigger.

Answer 2

Hydroelectric power generation is by far the most efficient method of large scale electric power generation. See Comparison Chart. Energy flows are concentrated and can be controlled. The conversion process captures kinetic energy and converts it directly into electric energy. There are no inefficient intermediate thermodynamic or chemical processes and no heat losses. The overall efficiency can never be 100% however since extracting 100% of the flowing water's kinetic energy means the flow would have to stop.

The conversion efficiency of a hydroelectric power plant depends mainly on the type of water turbine employed and can be as high as 95% for large installations. Smaller plants with output powers less than 5 MW may have efficiencies between 80 and 85 %.

Source: https://www.mpoweruk.com/hydro_power.htm

A typical pulley system with friction can range in the 95% to 70% but could be made efficient by reducing the friction with special materials.

Assuming you had the same motor and torque input, it would probably be a wash and would depend on the actual turbine or block and tackle system.