There are a variety of alternatives to solar energy. The one currently being used the most is petrochemical energy. Petrochemical energy is based on breaking of chemical bonds in hydrocarbon chains by combustion. There are several problems with continued reliance on petrochemical energy. One is that the source for the hydrocarbon chains is fossil fuels.
Fossil fuels are a limited, nonrenewable resource.
Another obvious problem with fossil fuels is the pollution that results from combusting them. There are also political issues associated with fossil fuels that make them an undesirable for some countries. The energy crisis of the seventies on the United States by is one example. At the nuclear level, fusion and fission could both be the source of energy. Fission has the problem of relying on non-renewable resources like Uranium, and producing radioactive waste.