Lets start from the basics.
The obvious test for sustainable energy is the energy source is replenished at the rate of consumption of the energy.
The second test for sustainability is that regardless of whether the energy will last for ever, will the using energy supply cause damage that cannot be sustained?
If we look at oil as an example, the first message that oil was not sustainable as an energy source was that the supply would run out. More recently, the argument has been that regardless of oil supply running out, using oil as fuel produces gasses that are sufficiently problematic that they make using oil unsustainable under the second test regardless of whether or not we run out of oil.
The reality is that oil is not really an energy ‘source’, but rather an energy storage. The source of the energy in oil is solar energy converted and stored by plants and the problem come from the fact that we are using the ‘storage’ faster than it is being replenished. If we used oil at the same rate new oil was being created then neither running out, nor the problematic gasses would be a problem. In fact instead of rapidly consuming ancient oil converted from sunlight to energy by plants over a period of period of millions of years, we do also use newer oil (and alcohol) directly from plants growing now which does avoid both problems of using a supply that is not being replenished and the problematic gasses.
Using oil or alcohol from plants we grow now means the supply is replenished at the rate we use the energy and the gasses that are produced using the energy should be being produced at the same rate the plants are consuming those gasses in making the oil (or alcohol) in the first place.
But we do get back to the fact that the source of energy is not the plants. The plants are merely the ‘factory’ to convert and store the energy into the form we can use. The same principle as a battery.
So what are the original sources of energy?
I believe it is worth thinking about the actual four sources of original energy. Everything else is just taking one of these energy sources and converting the energy to a form for us to use. For example, oil or alcohol from plants is really converted solar energy, so is it better to have the plants convert the energy or to use solar panels since in both cases we are accessing the same energy? Thinking about the actual energy source allows logical thinking about how to use the energy. The actual sources are :
1: Solar energy.
2: gravitational/kinetic energy.
3. Nuclear energy
4: Thermal energy from the earth.
None of these truly last forever, but the supply is extremely large.
Source 1: Solar.
If we start with solar energy, which itself is produced by nuclear reactions on the sun, then it is true that in around 4 billion years the sun will stop producing this enery as it does now, and progressively over that next 4 billion years the sun will gradually increase the supply causing the earth to slowly warm.
So solar does not last forever, but it does last for around as long as we can keep living on the earth anyway. Solar energy is converted by plants into hydorcarbons and carbohydrates that we can burn to retrieve the energy, as well as being the source or energy for hydo power and wind power and can be converted by humans directly into electrical power or stored as thermal energy.
Solar sounds ideal doesn’t it? Well, I will discuss the limitations in another a later post.
Source 2: gravitational/kinetic.
The moon and sun are both in motion relative to the earth. Yes, it can be said that it is the earth moving not the sun, but the result is still that relative to the earth the sun appears to move. In both cases this motion means that the gravitational fields from the sun and moon bring some of the energy of this motion to the earth. The resultant tides are a distinct source of energy. Yes, the energy transferred to the earth in the form of tides does consume a fraction of the motion energy causing both earth and moon to fractionally slow in their orbits, but us making use of the tidal energy does not have any impact as the slowing happens whether we use the energy or not.
Perhaps it is a waste not to use this energy? Could tidal/kinetic energy play a significant role in human energy needs? Again, I will explore in more depth in another post.
Source 3: Nuclear.
If the sun can produce so much energy form nuclear that at a distance of 150 million kilometres with only one quadrillionth of the energy reaching the earth the sun could supply all our energy needs, what if we were able to go straight to the source? Of courses, unlike the previous two energy sources, nuclear reactions on earth are not already happening so using nuclear energy is not tapping into energy already being produced. Is this good or bad? Yet again, I will make a specific post on nuclear at a future time.
Source 4: geothermal.
Just over 4 billion years ago gravitational forces pulled the material the makes up the earth into a ball floating in space. The formation of this ball created so much heat that now, over 4 billion years later, the centre of the earth is still hotter than the surface of the sun. That is a lot of heat and surely we could use a little of it without any significant effects? Once again, subject material for a future post!
As we can see, almost all sources of energy we normally discuss are not really the source. Electrical energy we have to make from another source of energy. Even fossil fuels are just solar energy previously stored by plants. Almost always our source of energy is solar, so the question becomes how best to utilise that original solar energy?