nonconsistant power conversion 4

[greycloud]

Hi

various renewable energy methods for producing electricity like wind turbine,solar panels, or wave power buoys produce power at inconsistent Hz and voltage so how is it converted into use friendly power?

Thanks for helping

 

[greycloud]

great post JPTS. There is one point you mentioned that I was wandering about lately as well, that of generators operating at low speeds. some renewable energy sources provide lots of energy but are slow moving ,although they have lots of force stored in them, in nature as in the case of ocean waves. whether we are talking about linear or regular generators I'm interested in how generators can produce power from low speed motion that is combined with high force.

 

[davidbeach]

Gear up to higher speed, generate low frequency AC and rectify to DC, or generate DC. Any DC then gets inverted back to system frequency.

 

[greycloud]

Lets arguably say that for some reason I don't want to use a gearbox or that I'm using a linear generator so a gearbox is not an option.

 

[davidbeach]

So you're left with choices 2 and 3.

 

[greycloud]

Let me clarify my question. what I meant to ask is can you generate power efficiently from slow motion sources of renewable energy given that they pack a lot of force as in the example of ocean waves using linear or ordinary generators. This question came to my mind because what JPTS mentioned in his answer.

Some turbine designs use so-called direct-drive concept, where turbine is directly rotating the generator. In this case, no gear-box is needed, and the only suitable generator type is synchronous machine either with electrical or permanent magnet excitation (induction machines cannot be designed to operate at such low speeds).

I need some elaboration on this matter

 

[waross]

A lot of people have been wondering for a long time how to extract energy from slow sources.
As a poor analogy, but the best that I can come up with, consider an induction motor. A motor with a synchronous speed of 1800 RPM may run at 1750 RPM while delivering rated torque. That 50 RPM difference is the slip frequency. If an inverter is used to drop the frequency to 30 Hz and the synchronous speed to 900 RPM, the motor will run at 850 RPM. Still 50 RPM slip.
If you drop the frequency to 3 1/3 Hz, the synchronous speed will be 100 RPM but the slip speed will still be 50 RPM and the motor will run at 50 RPM.
So at 1800 RPM and 50 RPM slip, the slip or convesrsion loss may be 50/1800 or 2.8% but at 100 RPM the loss will be 50/100 or 50%.
To extact energy we need different energy levels. These may be temperature differences, heads of water, currents, winds or other sources of energy.
While this is not a perfect anology, we often find that when extracting energy from weak sources, be they small temperature differences, small heads of water, slow currents, slow winds or other small differences in energy levels, the losses become a much greater percent of the theoretical energy available than when there is a larger difference in energy levels.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jpts]

Regarding using direct-drive (gearless) concept in renewables: If you for example think about megawatt -class wind turbine, their blade aerodynamics is optimized so that they operate at 10...15 RPM at rated wind speed. The most natural way would be to match the generator speed to this low-speed, in which case no gear-box would be needed. The main drawback is the physical size of the generator, since the physical size of electric machines is always proportional to torque (for given power, low speed means high torque and therefore physically big generator). Like also waross mentioned, (slip) losses of induction machine would be far too high at such low speeds, and the only suitable machine type is synchronous one (either with electrical- or permanent magnet excitation). Permanent magnet concept is much more common nowadays due to better efficiency and brushless operation.

Big physical size of a direct-drive generator is the main reason that the majority of the megawatt class turbines are equipped with a gear-box, which reduces the size of the generator. For example, 3 MW gearless generator rotating at 10..15RPM weighs maybe around 50...80 tons, while geared generator running at e.g. 1500RPM weighs less than 10 tons. But of course the latter one needs a gear-box, which weighs tens of tons and brings lots of potential for technical failure and costly downtime.

But nevertheless, both concepts are widely used. For example Siemens has invested a lot on direct-drive technology lately (they are using geared concept too), if you are interested, here is some more info:

http://www.controleng.com/single-ar...muscles/4be132ffb053a53fc4ab10b2c9c57340.html

Regarding the question can you generator power efficiently from low speed sources like wind, I would say definitely yes (altough it is a relative term), but the drawback is that low speed means high torque (or force), which makes the power train components bigger and more expensive, as the size of mechanical components (gear-box, generator, shaft etc) grows with the torque. Also it should be mentioned that due to need for gear-boxes and frequency converters and all the auxiliary devices, total efficiency (of a wind turbine) is not even near for example to directly-grid connected hydro-generator system.

 

[greycloud]

Thanks for answering my question guys.
what about wave energy power generation, I think it is a better source since you always have waves and they pack more force. what is interesting is that they usually use linear generators, so do the same rules that u talked about apply to these generators.