I think with some cool lights (CFLs) you could do decent. Any lights that make heat would probably be too close to your plants in the 18" vertical space. Try 2x26W CFLs, veg until ~10-12", and flower. Keep the lights as close as you can without burning the leaves and you should get some good results for that space.

Just noticed you have some room to work with width wise. Do the same thing I said before, but LST the plant so it grows more horizontally. Make it more like a bush so you can fit more in your area without getting taller. Also it helps by allowing your light to penetrate to the entire plant nicely.