# Best efficiency to convert Kinetic into Electric

1. Jan 29, 2009

### Bacat

I am seeking help with the theory of energy conversion from kinetic energy to electric energy. Is it possible to convert kinetic energy to electric energy at an efficiency higher than 75% If so, how?

Specifically, I mean not using heat (cogeneration power plants) or potential energy (hydroelectric plants). But just turning motion into electricity (faraday effect, brakes on a Prius, etc.).

Can someone point to a source that gives a percentage conversion higher than 75%?

2. Jan 29, 2009

### Staff: Mentor

Sure, most modern electric generators are well over 75% efficient. They are up to 95% efficient.

3. Jan 29, 2009

### jambaugh

Indeed an electric generator can in principle approach 100% efficiency. The sources of loss are all "correctable". Resistance in the conductors, friction in the moving parts, and electromagnetic radiation.

4. Jan 29, 2009

### Bacat

Yes, cogeneration and hydroelectric power plants are easily over 90%. But the cogeneration requires the input of thermal energy and the hydro requires the exploitation of potential energy. What I'm asking about is the conversion of kinetic energy without the aid of thermal, chemical, or potential energies. Wind would be one example, but I am not limiting the question to a particular field (such as large scale energy production).

5. Jan 29, 2009

### Staff: Mentor

Neither cogen nor hydroelectric plants are above 90%. No power plant is. But you didn't ask about power plants, you only asked about the device that converts the mechanical energy to electrical energy. The efficiency of the rest of the plant is irrelevant to that question.
The way you asked the question, you assume we have some kinetic energy to utilize. That means a spinning shaft somewhere that can be attached to a device to generate power. And that means attaching an electric generator. How that shaft got to be spinning in the first place (whether connected to a hydro turbine, a steam turbine, a diesel engine, etc.) and the efficiency of the creation of that mechanical energy doesn't appear to enter into the question anywhere.

So perhaps you could rephrase what you are asking...

6. Jan 29, 2009

### Bacat

Sure, Russ. Thanks for the clarification on what the problem is.

I am seeking theory or experimental data that could reasonably be called the most efficient way to convert kinetic energy into electrical energy that does not include the following:

1) Adding heat to the system in the form of fuels (combustion disallowed, solar allowed, geothermal allowed)
2) Exploiting gravitational potential energy of natural features (hydropower disallowed, wind, waves, tides allowed)

Does this make it more clear? Or have I still missed something?

7. Jan 29, 2009

### Staff: Mentor

No, it doesn't make it any more clear. If you have some kinetic energy to use (a rotating shaft) - and you are only asking about how to get that rotating shaft to make electricity, then the answer is still just an electric generator.

1. What is the best way to get electricity from a rotating shaft?
or:
2. What is the best way to make a shaft rotate?

Question #1 (the one you appeared to ask) really only has one answer. Question #2 is very complicated.

8. Jan 30, 2009