"viable" probably means some else then just "technically possible". I would also see as requirements: It must be cost effective, it must be scalable in several

"viable" probably means some else then just "technically possible". I would also see as requirements: It must be cost effective, it must be scalable in several dimensions: fast to plan and approve, fast to build, able to be build in massive numbers, able to be financed in large numbers, able to be operated in large numbers, able to cool in large numbers, able to operate in a secure environment, ...