DC Fan & Motor Battery Configuration
The maths is easy:

20 W x 30 s x 30 cycles = 18000 Ws (watt-seconds) and since there are 3600 s in one hour that's 18000 / 3600 = 5 Wh.

But since you are likely to find your batteries rated in mAh we can do it that way too:

The current, I = P/V = 20 W / 9 V = 2.2 A (or 2200 mA). You're going to run for 15 minutes or 0.25 h so you will need to supply 2.2 A x 0.25 h = 0.55 Ah. (Note that if we multiply this by the voltage we get 5 Wh as in the first calculation.)

Now lets pick an alkaline AA datasheet at random. First hit was the Energiser which is a respectable brand.



Figure 1. An extract from the Energizer AA datasheet.

At the bottom we can see that they don't even quote for 2200 mA and that there's an implied limit of 500 mA discharge current. Note also how the capacity drops to half capacity of the 25 mA discharge rate.

We'll have a look at another aspect too. The nominal internal resistance (IR) is 150 to 300 mΩ when fresh. (It will be worse as it discharges.) The internal resistance will mean that the voltage will droop as you increase the load. We can calculate this too: from Ohm's Law we know that V = IR so the drop will be 2.2 A * 0.25 Ω = 0.65 V per cell. That's 2.2 V from your 6 V pack leaving only 3.8 V.


  I'm trying to decide if 6 1.5 volt AA's are sufficient in series or would another configuration work better.


You could try it but it won't be good and you could chew through batteries fairly quickly. I won't recommend a battery but I think you have enough information to work out the performance of any other battery from the datasheet. You're looking for one with a rated discharge current of > 2 A and a capacity of about 1 Ah (giving a margin of nearly 100%).