I am curious as to how much power can be made from a liter of urine. Urine does have nitrogen, chloride, and potassium in it, but they are present in only milligram to gram quantities per kilogram of urine. Consider the average person urinates about two liters (with a specific gravity of 1.003-1.030, weighs about 2 kg), so it will likely take a huge amount of urine to yield much energy. I have a hunch that's why the authors listed this as possibly suitable for situations like music concerts where there are a lot of people urinating in one place.