The Biomass engines mentioned elsewhere on the site can be powered with stored heat. This would allow them to operate when no Sun or Wind are available, and would further reduce the time when biomass would actually have to be burned.
The process of collecting, storing, reclaiming, and converting heat to electricity is somewhat less efficient (at present) than direct use of the latest PV Cells. For that reason, a mix of both solar thermal and PV best meets our needs overall. However, using stored heat is better than burning Biomass, so thus we must include both PV and Solar Heat systems in our overall energy mix.
Collecting the energy from the Sun in the form of heat is another valuable source of energy than can easily be converted to electricity. Using Concentrated Solar Energy, such as that obtained from a solar trough for example, is even more effective as the heat is collected at a higher temperature that is more readily used.
Stored solar heat, say from an array of collectors, can be stored in a number of ways, including in a tank filled with a storage media such as a phase change salt. Then it can be reclaimed when needed.