Controlling load of the generator

[Roadam]

I am looking into possibility of constructing a small hydro turbine behind my house which was a mill a way back.

 

Anyway, while I was struggling with drawing some kind of a transmission as the turbine would rotate at the same speed at different power levels, an Idea struck me. :)

 

So, as far as I know generators only do something when the load is applied. And for a set rotation speed they have some voltage as induction induces voltage difference.

 

What if some kind of a buffer would be used between the generator and the rest of the grid, so that generator wouldn't feel its load. Say a big capacitor and you would be able to change the rate of its filling, or in the other words, the load on the generator.

 

That way it may be possible to set how much power would it make without changing its rotational speed.

 

Or did I miss something again?:shrug:

[Turtle]

Not sure how a large capacitor affects the load, but I have heard/read talk of using capacitors rather than batteries for storing solar/wind made electricity. Here's a top link that popped up on a search of "large scale capacitor storage from generators". :)

 

ScienceDirect - Electric Power Systems Research : Super-capacitor based energy storage system for improved load frequency control

 

Maybe put your generator load into an electrolytic cell? Give it something to do and collect some Hydrogen and/or Oxygen if you want at the same time. :shrug: :)

[Thunderbird]

I am looking into possibility of constructing a small hydro turbine behind my house which was a mill a way back.

 

Anyway, while I was struggling with drawing some kind of a transmission as the turbine would rotate at the same speed at different power levels, an Idea struck me. :)

 

So, as far as I know generators only do something when the load is applied. And for a set rotation speed they have some voltage as induction induces voltage difference.

 

What if some kind of a buffer would be used between the generator and the rest of the grid, so that generator wouldn't feel its load. Say a big capacitor and you would be able to change the rate of its filling, or in the other words, the load on the generator.

 

That way it may be possible to set how much power would it make without changing its rotational speed.

 

Or did I miss something again?:shrug:

 

 

 

Just add water and gravity.

 

What you need is large water pumping station that would use the power during least usage, at night to fill a large reservoir. Then during peak hours the water helps the turbines with extra load.

[Thunderbird]

So you need a water tower or a steep hill.:shrug::):)

[engineerdude]

Hey Roadam,

 

I am an engineer, and my degree was in power systems design, so I have a little knowledge about this issue.

 

In water-turbine electrical generation, a variable pitch rotor is generally used to provide for different torques. So, when the load is very light, the turbine does not allow much water through, when the load is heavy a bunch of water is used. Electrical generators are synchronous machines, and are tied into the electrical grid, and so always have to spin at a constant speed. Once you hook your generator to the grid, the generator will always spin synchronized at exactly the same speed as every other generator that is tied to the system. If you do not apply force to spin the generator, it will turn into a motor and pull power off the grid instead of putting it in. If you are going to use this generator to power your house, and if your house is connected to the electric grid, you have to take this into consideration.

 

As far as capacitors go, they are not for energy storage. You cannot store any practical amount of energy in a capacitor. There have been theoretical designs to store energy in an inductive loop with superconductors, but that is still a long way off, if ever. Batteries are the only viable way directly store electrical power currently, and they can't store much.

 

One of the replies mentioned pumping water up hill during off hours. This is called pumped hydro storage, and is commonly used by utilities with hydroelectric plants to use up excess power in the middle of the night. Nuclear Power Plants in particular can't easily lower their power outputs, so when the power consumption on the grid is very low and there no buyers off grid for the power they have to do something. It is very inefficient to do pumped hydro storage - you end up getting maybe 10-20% of your electricity back once all the losses are figured in.

[Roadam]

Well I do know most of what was proposed here. I admit that I didnt explain myself very well.

 

Usually when you have a grit tied generator, you have a turbine, a generator then recitifier and lastly a grid tie inverter. Say there is a turbine that is most efficient at just right speed, no matter how much power would it produce. So I want to directly connect a permanent magnet alternator to it and somehow controll the load on the generator, or control the power drain on it. Just as like havning a constant voltage battery where you change the load on it trough changing resistance of the loop.

 

I think that may be possible trough having two sets of capacitors, so that a generator is only connected to a capacitor and not the grid.

 

Basically I want to control the power output with load, so that the system would drain as much power from the turbine as it can.

[Roadam]

:confused: I was designing this and then I realised that most better inverters have built-in function to sense the outpout of photovoltaics or generator. So I guess the inverter would do just fine without any big caps and switches.

 

Now I am asking myself how efficient would a generator be at high rpm and little load.

[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?

[DFINITLYDISTRUBD]

As far as capacitors go, they are not for energy storage. You cannot store any practical amount of energy in a capacitor.

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.

[Roadam]

Does no one know what am I blabbering about or doesn't know what to say???

 

If everyone would keep so silent in all those quasi scientific topics about paradoxes and perpetual machines, the thread starters would lose their will quite fast. Well it is making me feeling strange at least.

[engineerdude]

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]

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 :teeth:, 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]

So you need a water tower or a steep hill.B):eek2:

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.