Start from the operating voltage of 1 cell. Say, 0,50-0,70 V for a PEM FC. If your stack voltage output should be 360-720V, then you will need 504/0,7 = 720 cells connected in series to generate this voltage. You may choose to have 1 stack of 720 cells, but it might be better to split it into two or four stacks. A PEM FC would reasonably generate around 1 - 1.5 A/cm2 of active area at 0.5-0.7 V cell voltage. Assuming 1 A/cm2 at 0.7V, you will get 220A per cell if active cell area is 220 cm2, or 10 by 22 cm. This is a reasonable size, and you can choose it (assuming that you are designing a FC stack from ground up). If you have some other constraints or cell designs, however, you might settle for lower or higher cell active area. If you choose 110 cm2 per cell, for example, you would need to connect two stacks in parallel to generate 220+ A, or use a DC/DC converter.

For a 500 kW system, however, I would choose something like 500 cm2 active area per cell, and end up with 2 stacks connected in parallel, 720 cells each. Don't forget to account for degradation over the lifetime (add some more cells). You will probably need some DC/DC convertors anyway, depending on the load requirements.

As for PAFC, the cell voltages and current densities will be a bit different, but the procedure is the same.

Ask a supplier, such as Nedstack. In addition to the stacks, external equipment will be required such as cooling. Not so many suppliers to choose from today.

With the HP stacks (designed for intermittent use), it seems you would need about 2000 kg of 10 kWe stacks, 5 lines of 10 stacks each, to get the required power.

This would output up to 1150 A at about 430 - 730 V, so over voltage could be a risk.

Depending on your exact voltage and current requirements, 6 x 9 or 7 x 7 or some other configuration might be better.

The XXL range is designed for continuous operation and so has a little lower power rating per stack.

You may start with the following datasheet and extrapolate:

http://ballard.com/files/PDF/Material_Handling/9SSL.pdf

This is below 1 kg/kW, but only the stack without peripherals and coolant.

Another brochure with data:

http://w3app.siemens.com/mcms/infocenter/dokumentencenter/cc/InfocenterLanguagePacks/SINAVY%20PEM%20Fuel%20Cells/sinavy-pem-fuel-cells.pdf

These are systems that are available commercially. 'Scaling' power is more or less a question of connecting the modules in series/parallel.

I d'not understand your problem. So I have not idea to give a right answer to you.

Missed the link.

http://www.nedstack.com/products/product-specifications-of-hp-stacks