Joe is trying to find out how solar energy can replace the functioning of certain industrial needs. With this research, which phenomenon is Joe trying to curb?

Solar energy is a form of renewable energy, namely energy that cannot be depleted. Other forms of energy like carbon based sources of energy cannot be renewed in a reasonable span after being used (forests take time to grow and oil is created in geological timescales). Joe is trying to deal with the phenomenon of resource depletion. The way he goes about it is making a renewable source of energy more efficient and usable.