Renewable energy is energy obtained from continuous or repetitive currents of energy recurring in the natural environment. In order to be considered “renewable,” an energy flow must be replenished at the same rate at which it is used. We often think of wind energy, solar energy, hydropower, and geothermal energy as renewable energies, as each of these energy flows can be used at the same rate at which it is replenished, unlike fossil fuels, which cannot be replaced.  Renewable energies have a host of benefits, most notably that they do not emit greenhouse gas emissions during operation and because they rely on “free” fuel are often less expensive to operate than fossil fueled power.  However, just because an energy can be replenished at the same rate at which it is used does not mean it is free of negative environmental, social or economic impacts. For instance, large hydroelectric dams can require the relocation of residents, wind farms can negatively affect bird and bat populations, and large solar farms can negatively impact biodiversity.  Wind and solar energy in particular are intermittent, meaning they do not generate power all the time.

It’s often helpful to distinguish between “renewable” and “sustainable” energy, even though the terms are often used interchangeably, as well as between non-renewable or fossil-fueled energy.  Sustainable energies, rather than renewables, are defined as those energy sources that are not significantly depleted by continued use and can support our present and future needs without entailing the emission of pollutants or hazards to human, ecological, or climate systems on a significant scale and do not perpetuate social justices.  Fossil-fueled energies do not fit this definition as they are being rapidly depleted, emit greenhouse gas emissions and air particulate emissions like nitrous and sulfur oxides and contribute to significant environmental and energy injustice.