sustainable?
Renewable forms of energy are enduring and often emission free (or at least emission reduced). A sustainable energy system should meet humanity’s essential energy needs indefinitely, cleanly, and at a reasonable cost. This section provides an overview of the prospects for renewable energies in our quest for a sustainable energy system.

challenges
There are three main challenges to widespread development of renewable energies: low energy density, intermittency and inconvenient location. Intermittent, unpredictable energy sources make it difficult to match energy supply with the daily and seasonal cycles of human energy demand. Intermittent sources of energy require either connection with stand-by thermal generation facilities (fossil fuel, nuclear or biomass) or energy storage devices (battery, compressed air, hydrogen, etc.), if they are to play a major role in electricity supply. Transmission from source to consumption can provide for high initial capital costs. As well, some favorable sites may have land-use (or migratory pattern) conflicts.