System design for charging battery from turbine alternator

[yahastu]

I am designing a hydro power system that will tie into an off grid solar system. I will be using a 48v carbon AGM battery bank. Will be using the Sol-Ark 12kw inverter/charger, which has 2 built in MPPT charge controllers for solar. Assume the hydro power will be around 5 kw potential, and the turbine will be located ~3000 feet away from the battery bank. The turbine will use an electromagnetic alternator (not permanent magnet).

Based on my understanding/research so far, it seems that there are 3 basic approaches that could be used to tie in the hydro power, and I'm a little confused about which approach would be best...

1) The conventional automotive approach. Use a three-phase alternator with built-in bridge rectifiers to convert the 3-phase AC signal into a bumpy DC signal, then the built int AVR help to smooth it out to your target voltage of around 12V, which is then directly connected to the batteries. I could basically do a similar thing but for 48V and connect directly to the batteries. It seems a big disadvantage of this approach would be the voltage drop...if the integrated AVR outputs 48V at the turbine, it's going to be too low to charge the battery due to resistive losses due to the long line length. Additionally, my understanding (which I'm not sure if correct) is that the AVR increases or decreases the strength of the electromagnet in order to maintain a desired maximum AC voltage under variable current load. Therefore, if the battery is fully charged and the current demand drops, then the alternator would spin freely at a higher velocity, potentially reducing wear on the turbine.

2) Same as #1 above, but instead of 48V, design it to output a higher DC voltage, which I run up to the battery bank. Being a higher voltage, the current will be less, and resistive losses will be less. Then route through an MPPT charge controller (eg, one of the 2x MPPT charge controllers built into the Sol-Ark, or an auxiliary MPPT) located nearby to the battery bank.

Note: I have heard that it makes sense to add a diversion load (ie, hot water heater) here to the MPPT controller to keep the turbine RPM in check. However, I am a little confused in the mental model here: if the AVR senses how much load and only produces enough current to meet demand, then how does that work when you add a diversion load, that can soak up an infinite demand?

3) Design the alternator to output 240v AC voltage, run this AC voltage up to the battery bank, then through an AC voltage stabilizer to correct for resistive losses...and then connect the output AC to the "aux generator"input of the Sol-Ark (normally intended for, eg, propane generators).

Related question: I understand that the turbine should be spinning at 50% of the water velocity for maximum efficiency (if it's free flowing with the water, then it provides no resistance and no power, and if it's 0% then it is stopped, also no power). Would the MPPT charge controller have the effect of essentially setting the turbine velocity to this most efficient point? If so, can someone explain how that works? I am a bit confused because if the AVR increases resistance in response to increased demand, what is to stop it from effectively creating too much resistance and stopping the turbine in response to huge demand?

 

[hutchphd]

Do you have essentially a unlimited supply of water at head or does that need to be conserved?

 

[anorlunda]

Your 3 scenarios are confused. Turbines don't make electricity, alternators do.

Option 1 does not work well with 3000 feet because of the voltage drop as you said.

I don't know how you would make 400-600 VDC. I'm skeptical that the charger/inverter could accept such high voltage.

You could generate single phase AC using an AC generator. You could do that with electronics and a BLDC motor/generator, or you could use a PMG or induction generator. The PMG or induction choice will generate variable frequency as turbine speed varies. AC can be boosted in voltage by a transformer, sent over the 3000 feet then stepped down and rectified to DC at the other end, with DC feeding the charger. The advantage of all that complexity is the ability to use less copper in the 3000 foot cable. The design of the whole thing is non trivial. You may need a professional engineer to design it.

Your best bet is to move the turbine and the battery bank closer together; 3 feet instead of 3000 feet, and stay with DC voltages within the capability of the charger/inverter. What is the reason why you can't locate the turbine closer to the house?

 

[yahastu]

Your 3 scenarios are confused. Turbines don't make electricity, alternators do.

The turbine will come pre-designed with a built in alternator, so I was just referring to the whole thing as a turbine...but yes I understand that the turbine converts head pressure into rotational kinetic energy, and the alternator component then converts the rotational energy into electrical energy

I don't know how you would make 400-600 VDC. I'm skeptical that the charger/inverter could accept such high voltage.

I mentioned 600 V because that is a common AC voltage...although I meant to say 300 V DC, because 600V AC would be +/- 300 V, and after passing through the rectifier and AVR, you'd get a 300V DC signal. I understand what whatever voltage I choose, would need to be less than or equal to the max voltage of the MPPT. The Sol-Ark integrated MPPT charge controllers accept up to 425 V DC so 300V wouldn't be a problem.

You could generate single phase AC using an AC generator. You could do that with electronics and a BLDC motor/generator, or you could use a PMG or induction generator. The PMG or induction choice will generate variable frequency as turbine speed varies. AC can be boosted in voltage by a transformer, sent over the 3000 feet then stepped down and rectified to DC at the other end, with DC feeding the charger. The advantage of all that complexity is the ability to use less copper in the 3000 foot cable. The design of the whole thing is non trivial. You may need a professional engineer to design it.

The turbine and alternator are a package deal, which would be engineered to my specifications. The purpose of this post is for me to figure out what the best specifications will be. Everything I have read indicates that three-phase alternators are much more efficient and better in almost every way than single phase alternators. The only advantage I can think of for single-phase alternator over three-phase would be if I decide to run the wir from powerhouse to the battery bank as AC, then single phase would require fewer conductors..

Your best bet is to move the turbine and the battery bank closer together; 3 feet instead of 3000 feet, and stay with DC voltages within the capability of the charger/inverter. What is the reason why you can't locate the turbine closer to the house?

The house and battery bank is located at the top of the stream, the turbine and powerhouse must be located at the bottom of the stream. If I moved the turbine closer to the house, I would not be able to generate enough head pressure for the hydro system to be worthwhile. It is preferrable that the house be located far away from the turbine anyway, due to the loud sound the turbine will make..and from my initial calculations, I don't think resistive losses will be prohibitive in terms of energy loss, just more of an inconvenience factor that must be handled

 

[yahastu]

Do you have essentially a unlimited supply of water at head or does that need to be conserved?

A minimum flow rate must be maintained at all times to prevent freezing. Flow rate through the turbine will be manually set by a spear valve at the location of the turbine.

 

[anorlunda]

OK. It sounds like you already researched it beyond my ability to advise.

I'm curious. Do you have a link to that turbine+alternator package?

 

[berkeman]

although I meant to say 300 V DC, because 600V AC would be +/- 300 V, and after passing through the rectifier and AVR, you'd get a 300V DC signal.

Not quite; you are mixing Vrms and Vpp...

 

[yahastu]

OK. It sounds like you already researched it beyond my ability to advise.
I'm curious. Do you have a link to that turbine+alternator package?

here it is
https://www.alibaba.com/product-det...43.html?spm=a2700.12243863.0.0.38403e5fy0zvkg

They are asking me what voltage and frequency I want it manufactured to. eg, 240v/60 hz. If it really does output 240v/60hz, then I could just plug it in directly to the inverter as a generator/grid source...but I'm not really sure what circuitry they could possibly be installing that makes it operate that way, so I'm suspecting it may actually be "wild power" that just happens to be tuned to output 240v/60 hz at the particular head/flow rate that I specified...

 

[anorlunda]

That offering on Alibaba is impressive. Note that they say 20-50 m head. The page also mentions a different model they sell for 60-180m head. Note their invitation:

2. How to choose a right turbine? There are so many models offering on your website. And how to calculate the turbine capacity?

Just tell me the water head, flow rate, our senior engineer will work out the solution for you. The turbine capacity: P=Flow rate(cubic meter/second) * Water head(m) * 9.8(G) * 0.8(efficiency).

Also note that the page shows that you need a "smart governor" and "control panel" it's not clear if those are included.

Maybe you should just start corresponding with their senior engineer.