‘Sustainable energy’: what is it? What are the actual sources of energy?

  • Sources Of Energy, and sustainability.
    • 1: Solar energy.
    • 2: gravitational/kinetic energy.
    • 3. Nuclear energy
    • 4: Thermal energy from the earth.
  • Energy Usage
    • Heat, Light, Electronics, Kinetic
  • Power Storage
    • Chemical potential, electrical potential, gravitational potential
  • Renewable & Sustainable
    • Renewable
    • Sustainable
  • Examples

Original sources of energy, and their sustainability.

There are few actual original sources of power. Everything we use as fuel or an energy source, has one of these sources as the origin of that energy. For example, oil or alcohol from plants, is sourced from solar energy and converted to chemical potential energy by photosynthesis. We use the chemical energy, but the original source was solar energy. For a given area of land, there is a certain amount of solar energy available. We need to choose, do we use plants or solar cells to ‘farm’ than energy? Each have advantages that means the best choice will not always be the same, but thinking about the actual energy source allows logical thinking about how to harvest the energy.

The actual sources are :

  • 1: Solar energy.
  • 2: gravitational/kinetic energy.
  • 3. Nuclear energy.
  • 4: Thermal energy from the earth.

While none of these is a genuinely infinite source of energy, each will last as long as we can live on Earth.

1: Solar.


Solar, wind, hydro, biofuels, and fossil fuels all have solar energy as their original source.


Wind energy, form of solar energy that is produced by the movement of air relative to Earth’s surface. This form of energy is generated by the uneven heating of Earth’s surface by the Sun and is modified by Earth’s rotation and surface topography.

Wind energy (Britannica)


Hydropower is the power of moving water. To move water you can either let it move ‘downhill’ under gravity, or you can pump the water. Pumping the water in putting energy into the water, letting it flow downhill allows converting the gravitational potential energy into electricity, and this is what we know has hydro-electricity. But where does the gravitational potential energy come from? What moves the water to the higher location? The sun. Evaporation using solar energy provides the energy to form clouds, with generate the rain. This allows capturing rain water at locations above sea level. So again, the original source of energy for hydro power is also solar energy.

Biofuels + Fossil .

CO2 + water + sunlight ->  O2 + Sugar/Carbohydrate/Oil/Fat

Photosynthesis, from the equation above, generates not just biofuels, but the ingredients of fossil fuels.


For 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 energy in the current form. Also, progressively over that next 4 billion years the sun will continue to gradually increase the energy supply, which without counter measures, will causing temperatures on Earth to increase.


So solar does not last forever, but it does last for at least as long as we can keep living on the earth anyway. Solar energy:

  • can be converted by humans directly into electrical power or stored as thermal energy
  • is also the source or energy for hydo power and wind power
  • is converted by plants into chemical potential energy as hydrocarbons, and carbohydrates, that we can burn to retrieve the energy, either as biofuels or fossil fuels

Solar sounds ideal doesn’t it? There are limitations, but that requires a separate exploration.

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. With both Sun and Moon the motion means that the gravitational fields 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 that tidal energy does not have any impact on the rate of slowing, which will take place whether we use the energy or not. Those ‘tidal drags’ will not result in significant slowing of either orbit for over 100 million years, and given the otherwise fragile environment for life on Earth, that allows us to consider the energy supply as never expiring.

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 other posts.

Source 3: Nuclear.

If the sun can produce so much energy from nuclear that at a distance of 150 million kilometres from us, and as a result only one quadrillionth of that energy reaching the earth, 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? Clearly, while there is almost unlimited energy available, there is a limit to how much extra energy we should produce on Earth.

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 even 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?

Energy Usage.

We have uses for energy only in very specific forms. Regardless of the original form of energy, to use energy we must transform the energy in to relevant form.


We use heat for cooking, and to keep warm in cold climates. To generate heat we may use electrical or chemical energy.


To generate light, we use either electrical or chemical energy.


For electronics, we require electrical energy.


To produce kinetic energy on demand for transport, or milling or other use of movement, we use either chemical (Internal combustion, or steam or rocket) energy, or electrical energy. We can use turbines powered by gravitational energy for milling, or other movement, however most often we use turbines to generate electricity.

Combination: Eg, Cooling.

Air conditioners and refrigeration require heat pumps, both heating and the movement of fluids.

Power Storage: Potential Energy.

As we can see, almost none of the energy we use is ‘direct from the source’ of one the original sources.

We mostly use batteries, or fuels, or power from generators that in turn run on fuels or hydro-electricity. What we mostly use is energy from that although originally from one of the energy sources, has first been converted into a form of ‘potential’ energy. Potential energy is energy that we can use whenever we need, while most original energy sources are not available ‘on demand’.

Chemical Potential Energy.

Fossil fuels are chemical potential energy converted from solar energy over long periods of time. Biofuels are chemical potential energy converted from solar energy over much shorter periods of time. Most batteries are also chemical potential energy, ready for conversion to electrical energy on demand. As fossil fuels demonstrate, chemical potential energy can be kept for extremely long time periods.

Electrical Potential Energy.

Although electrical energy is itself ‘potential’, capacitance is our only way of keeping electrical energy to ready on demand. Almost all electrical energy is available only for use at the time another source, wind, solar, coal or hydro generators, or chemical batteries, produce the electrical energy.

Gravitational Potential Energy.

Almost entirely used with Hydro, stored gravitational potential energy is normally used to produce electrical energy on demand.

Renewable Vs Sustainable

Although the terms can be used interchangeably, the literal meanings are quite different. Renewable means what you use can be replaced, sustainable means “able to continue at the same level for a period of time“, or ‘to maintain its own viability’.

But maintain for how long?  The implication is to maintain forever but in reality nothing in the universe, that we are aware, of is sustainable forever.  The earth will be consumed by the sun one day, and life on Earth doesn’t even have that long. This means the ‘forever’ test is not meaningful.  Certainly something sustainable for 1 billion years should be more than sufficient.  Could something sustainable for 1 million years be regarded as sustainable? For one thousand?  I will work with 1 billion years as effectively ‘infinitely sustainable’ unless stated, but sustainable or not is not always a case of ‘yes’ or ‘no’ but for how long.

Strictly speaking, renewable could apply only to “power storage”, while “sustainable” is a test we need to apply to everything we do, now that the world being finite is our reality.

Our first fuel was timber. Timber is renewable, because for every tree we burn, we could at least in theory, grow a replacement. Then we started using coal, and eventually oil. Civilisations did not think of coal as renewable, even before we understood coal and oil to be the long buried remains of living organisms. Turns out fossil fuels are in theory renewable, if you have millions, or even billions, of years to wait while renewal takes place. We don’t, so for us, we felt the fact that coal and oil could ‘run out’ was our main problem.

Reality is, that thought was always very wrong, and still causes confusion today. We do not have to worry about running out of coal and oil, but we do have to worry that burning coal and oil is not sustainable.

Coal and oil deposits are effectively storage of carbon dioxide underground as it is replaced in the atmosphere with oxygen. It turns out we are only alive today because of all that CO2 being underground. If we burn all the oil, gas and coal, not only would their be no Oxygen to breath, we would also fry!

Running out of coal oil and gas was never a real risk, but becoming extinct trying was!

Our only real issue ever, was sustainability.

The reality is that oil is not really an energy ‘source’, but rather a store of solar energy that has been built by plants. The source of the energy in oil, is solar energy converted and stored by plants. The limitations come from the fact that we are using the ‘storage’ faster than it is being replenished. If we use oil at the same rate new oil is 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.

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