# Two AC power sources - connection?

1. Jun 2, 2009

### EEintern

two AC power sources -- connection?

i am working on a control system for a compressed air energy storage system for a wind turbine (generates ac). the goal is to shave peak power consumption, so at night the compressor will charge (either by the wind or by the grid), and during peak hours, it would release, generate AC power (turbine->inverter), and feed into a meter. however, we are looking to optimize power delivered to the meter, so during peak times when the compressor is discharging, we want to hook the wind turbine up to the meter also. however, this is two ac sources we are trying to hook into the meter--can we add these? the easiest solution would be to connect them to separate meters, i know. but it would be great to shave more peak consumption.

2. Jun 2, 2009

### vk6kro

Re: two AC power sources -- connection?

You might have to explain your system a bit more.

But, basically, you can't connect two AC sources in parallel unless they have the same voltage, frequency and phase. This is hard to do.

Sounds like you are using a wind generator.... to drive a motor ....to drive a compressor ...to put compressed air into a holding tank. This stores the power.

Then you use the compressed air to drive a fan which drives a generator to drive an inverter to give AC again.

You want to parallel this AC with the AC coming from the wind generator. No chance.

Then it gets even more confusing. Why are you driving a meter? Do you mean a motor?

3. Jun 3, 2009

### EEintern

Re: two AC power sources -- connection?

Sounds like you are using a wind generator.... to drive a motor ....to drive a compressor ...to put compressed air into a holding tank. This stores the power.

Then you use the compressed air to drive a fan which drives a generator to drive an inverter to give AC again.

correct! we are hooking them up to a meter because we want to subsidize some of the power the loads on that meter are consuming so they don't have to buy from the utilities. this gets into peak demand charging, which is for another thread. there are multiple meters for farmers (one for the house, one for irrigation pumps, etc), so that's what i mean when i say we could hook the turbine up to a meter and the CAES (compressed air energy storage) to another--this would be no problem. but the real power hogs are the irrigation pumps which is what we are trying to shave, hence the desire to put both the wind turbine AND CAES into that meter. but unless the connection to the meter allows for two power supplies, this is not possible?

4. Jun 3, 2009

### vk6kro

Re: two AC power sources -- connection?

basically, you can't connect two AC sources in parallel unless they have the same voltage, frequency and phase.

Sorry, but the above statement is absolutely true.

I have never heard of using compressed air like this.
There do seem to be an awful lot of steps though. Every step drops your overall efficiency.
I'd guess you are only getting about 30% efficiency overall.
Could you just put the compressor up the tower and let it generate compressed air down a hose to the tank directly from the propellor rotation?

I suppose you could have the compressor pushing air into the tank while your turbine-inverter was taking it out. You would need two outlets on the tank (and some expert advice! I'm only guessing here.)

5. Jun 3, 2009

### EEintern

Re: two AC power sources -- connection?

thanks for the advice! and just for my own knowledge, what would happen if you connect them in series? i guess as i envision it, in parallel, the currents would add but they could very well cancel each other out because they're alternating. what would happen in series? i know this isn't really possible in this system, but just for my own knowledge...

and yes, our system is very inefficient at the moment! and at every step you do loose a lot of efficiency. we are working on developing a better turbine for our compressor output, because that's where we loose the most efficiency. but this is only a prototype system, and would require a lot more expert/professional design to make an actual finished product. and that's a good idea about hooking the turbine up to the tower itself, but that would require more set up time/expertise/. we are trying to optimize the system to the common, if you will--so that we can use a wide variety of wind turbines that are already in place at the farms we'll be using this system on.

this is a growing field (wind turbines powering compressors to store energy at night and release during peak hours), and there has been a lot of talking and planning but no real implementation.

6. Jun 3, 2009

### vk6kro

Re: two AC power sources -- connection?

Putting AC generators in series would not be good.

If they were not in phase, they would produce a beat effect where they alternately moved into phase and out of phase with each other. This would sometimes give the sum of the individual voltages and sometimes give the difference.

It would be safe enough, but pretty useless for any practical purposes.

7. Jun 3, 2009

### EEintern

Re: two AC power sources -- connection?

gotcha. thanks!

8. Jun 11, 2009

### agabrielauto

Re: two AC power sources -- connection?

But why in your house you can have ac current conected in series and parallel??

9. Jun 11, 2009

### vk6kro

Re: two AC power sources -- connection?

It was about putting two sources of power in series or in parallel.

As it was unlikely they would be in phase or even have the same frequency, there was no chance it would work properly or even be safe to do it in some cases.

In your house, you only have one source of AC and all current is derived from that.

Putting parallel or series loads on a supply is different to connecting different supplies together.

10. Jun 17, 2009

### m.s.j

Re: two AC power sources -- connection?

Fundamentally the operation of two ac power supply in series is possible but it dont have any advantage except power supply voltage increasing (as applied in DC systems) which there are better solution for it in AC systems (power transformers).

-----------------------------------------------------------------
Creative thinking is enjoyable, Then think about your surrounding things and other thought products. http://electrical-riddles.com

Last edited: Jun 18, 2009
11. Jun 17, 2009

### fleem

Re: two AC power sources -- connection?

On a side note, storing energy as compressed air pretty much requires a container with a REALLY good insulating lining inside, so that the process is adiabatic. It'll have to keep it close to the same temp for several hours.

As far as the AC phase problem, solar arrays solve this by keeping everything in the form of DC until it needs to be connected to the grid. Maybe you could just rectify the AC from the turbines (both wind and pressure driven) and then use a COTS inverter. Then you can put those DC sources in series, or even parallel if the voltage is the same. And also those rectifiers will keep one generator from spinning the other!

Last edited: Jun 17, 2009
12. Jun 17, 2009

### Bob S

Re: two AC power sources -- connection?

You absolutely need separate grid-tie inverters for each system, but you can use both in parallel on one meter. What kind of compressor are you using (for both charging and discharging the CAES), and what do you think the round trip efficiency is?