How can I convert color photos, as a large batch, into black and white?

You need a program that supports batch processing. The better programs, such as Adobe Photoshop and Lightroom and Apple Aperture do. I do not know of any others, though GIMP might.

I have to ask though, you say you are wanting to do this to "save space"? Save space where? Are you making the BAD mistake of using your computer as a storage place for your photos? NEVER a wise thing to do. Never. You should always use an external device, (or even more than one method), for photo storage. Put the photos on CD, or flashdrive, or external hard drive, or even a "cloud" site is better than depending on your computer to safeguard them. One simple hard drive malfunction and your photos are gone.

So if you are only wanting to go to b/w to save space, why bother? Get the photos as they are OFF your computer onto a more secure storage medium.

steve

It is easy to convert photo into black and white with simple apps, like Color Splash Pro

http://goo.gl/TT5WnM