“Leave the algae alone”. Low-tech Magazine. 4 Apr. 2008 – Most algae are grown in brackish or salt water. That sounds as if water is no issue, since our planet has not a shortage of salt water. However, just like solar energy plants, algae plants are best located in very sunny regions, like deserts. But, in deserts, and in very sunny places in general, there is not much water to find. That’s not a problem for solar plants, because they don’t need it. But, how are you going to get seawater to your desert algae plant? Check the websites of all these companies: not a word about it.
There are not that many possibilities. You can transport seawater to the desert, but that’s going to cost you an awful lot of energy, probably more than what can be produced by the algae. You can also take freshwater from more nearby regions or underground aquifers and turn it into artificial seawater. But, you promised that algal fuel would not compete with food production. A third option is to put your algae plant next to the sea.