Controlling load of the generator

[engineerdude]

Originally Posted by DFINITLYDISTRUBD

ummmmm...yeah...where'd you come up with this lil gem....guess you haven't heard of capacitive storage systems being used in busses and for other automotive uses.

I have indeed heard of the double-layer type capacitors that are being tested on some buses. The very best of commercially available of these can store around 6W-hours per Kilogram. These capacitors are used only for very short duration power supply, or to supply additional power for acceleration. Even experimental capacitors that can store 30 W-h or more per kilogram are not commercially viable for bulk power storage.

To put things in perspective, the largest of these capacitors I could find were from Maxwell technologies, and weighed 58 kg. See link below. This capacitor could store 146 W-hours of power. This would store about 9 minutes worth of power output from a tiny 1000W generator, and the power from this would power an average home for a few minutes or so.

This post was not about supplying peak power in a bus it was about a generator and storage system. And in that context, capacitors simply cannot store any practical amount of energy.

http://www.maxwell.com/pdf/uc/Maxwell_UC_comparison.pdf

[engineerdude]

Originally Posted by Roadam

Any electrical engineers out there?

I just want to know if I am thinking right. As I think that permanent magnet alternator running at high rpm would induce high voltage. (Induced voltage is proportional to the speed that magnetic flux changes. )
Loses are mainly heat which rises with square of a current running trough the windings. So at high voltage and low current, loses would be smaller compared to power produced.

Although it would be a waste of a big alternator, running it at high speed and low power would be energy efficient. I think.

To be more specific, I recently saw an alternator which can produce 1900W at 425V at 500rpm. I digged its efficiency at 85%. So its internal resistance is probably about 17 Ohms. If I would set the current so that it would produce only 500W at the same voltage and speed, the efficiency would be about 95%. But I am not sure how much of that 15% original losses come from bearings.

Anyone care to comment?

85% efficiency is very good for a small generator. The published ratings, in this case 1900W at 425V and 500 RPM is probably the "sweet spot" on the curves for the particular generator, which is why the manufacturer publishes those numbers. You'd have to look at the performance curves to be sure, but if you tried to increase or decrease rpms, or mess with the operational voltage, you will almost certainly lose efficiency.

As far as producing lower amperage output to increase efficiency, you could easily have the generator produce less amperage, but the efficiency would only go up very slightly. As you noted base inefficiencies like bearing loss,flux leakage, and other things are always there. No matter what you did, I would not expect you to get efficiencies above 85%.

[Roadam]

Thing is that I am looking at permanent magnet alternators for a generator in my project, so I am not sure where to fit flux leakage.

And I am looking at crossflow turbine which has the best efficiency at same rpm no matter how much water comes trough it. Thats why I meant that I have constant voltage at diffrent power levels.

I didnt really mean to store the energy in the cap, only transfer it trough. As far as I am guessing the cap connected directly to a small generator running at some rpm would only charge with the current that the generator is able to supply, although it would want to with the current that is directly proportional to the voltage divided by resistance of the loop.

[gregdevid]

I am not sure how you have this system set up.

[Roadam]

I dont have it set up yet, I am just trying to figure out the easiest way around it.

I found the efficiency curve of this kind of a turbine. Its from a book dated some 60 years back when american team done some testing on it.



So it seems it has quite good performance at partial flows, but has narrow rpm sweet spot.

And since I want everything I can get from my water , I would like to run it at that speed for all the different flows I get. And I figure that making load onto the turbine would be easier electronically than to build a transmission.

In the end I will probably figure out that it is the best to just contact a firm that makes these and order the whole setup in one package.

[Michaelangelica]

Originally Posted by Thunderbird

So you need a water tower or a steep hill.

And a Hydraulic Ram?

[gushovard]

If you only have a small generator why do you want to control the load? If your water supply is constant then charge a grid connect inverter using the generator and put power to the grid and or your house. This way the power is always connected to the grid and you get to collect all the power available and store it as a credit on the gird if you are not using it, for later use. Very few people have power to spare however so not many installations supply power to their house and have a surplus as well. If your turbine is an enclosed case type with an archemedes spiral to run the turbine then you need to run this type of turbine at about half the inlet speed of the water. If you dont know this then convert the pressure at the inlet to head in meters and use the following G (9.8) x Head ( in meters) = 1/2 x V^2 to determine the inlet velocity. Measure the diameter of the turbine and work out the circumference to determine the RPM then divide it by 2. Your turbine will be most efficient at this speed. The faster the turbine spins the more centrifugal force will have to be overcome which is a real cost in turbines so the larger the diameter of the turbine the slower it spins (since centirugal force is 4xpi^2xrxm/t^2) and the less centrifugal force has to be overcome. Capacitors can store a small amount of energy for a short time but are impractical for storage or varying the load. If you put one in the circuit it will generally increase the current and cause losses unless you are using it for power factor correction. Forget batteries if you are connected to the grid since they are not required and also the power companies wont let you connect to the grid if you have batteries in your system as they are worried you might be charging them off peak and then using the power during peak period. Good luck with your turbine project.