The words "sustainable" and "renewable" are often used to describe certain sources of primary energy, often interchangeably. However, these words have very different meanings. Not everything renewable is sustainable, and in turn not everything which is sustainable is necessarily renewable. This is closely related to the idea of reserve vs occurrence.
Literally 'to make new again', a renewable resource is one that is naturally replenished with time, like the growth of new organisms or natural recycling of materials. Renewable energy is any energy production which uses one of these resources. Renewable resources do not have a fixed quantity - more can always be generated. However, if the rate of use exceeds the rate of renewal - that is, the source is used more than it's being recreated - its continued use will become unsustainable.
Generally, renewable energy is taken to mean any of the following:
Resources are considered non-renewable if they take a very long time to be created (e.g. fossil fuels) or if their creation happened long ago and is not likely to happen again (e.g. uranium). Primary energy flows are almost always renewable. On the other hand, biofuels are renewable and definitely count as fuels.
Literally, that which can be maintained for a definable period of time, sustainable energy is energy production that can last for the foreseeable future. Sustainable energy practices must rely on resources which can continue to supply our needs. These sources must be used cautiously so that they will not be used up, run out, or otherwise become unusable.
Even renewable resources can become unsustainable. If a resource is used up faster than it can regenerate, it will eventually be entirely depleted despite its renewability. Conversely, a non-renewable resource can be sustainable if it's used in moderation. Again, if used without caution, these too may become be depleted in a short time.
For most people sustainable energy use means that the environment is not significantly damaged due to accumulated effects of an energy practice. This part of the definition of sustainable energy is quite politically charged with widely varying opinions. Often advocates for fossil fuels will claim that coal, oil and natural gas are sustainable because the reserves for these are so large, discounting the problems with climate change.
A good way of understanding the difference between "sustainable" and "renewable" is to put it in context of monetary income. If energy were money, a renewable source of income is one that would recur, like a paycheque, while a non-renewable source would be non-repeating, like receiving an inheritance.
A sustainable source of money might be one that could fund a desired standard of living for an extended period of time, while a non-sustainable source of income would be small and quickly used up.
|Renewable||A reliable yearly salary of $100,000||A meager stipend of $100 per month|
|Non-renewable||Winning $1,000,000,000 in the lottery||Receiving a one-time scholarship for $2500|
Sustainability is not a black and white issue. Just as a source of income (renewable or not) may become less sustainable when supporting a larger family or for a higher standard of living, energy sources become less sustainable with increased energy use due to world growth in population and, in fact, standard of living.
Differing values and perspectives lead people to having different views as to when a primary energy source is or is not sustainable.