On The Definition of Sustainable Energy

Let’s finally cover what I meant by truly sustainable energy production.

Listing electrical energy generation methods portrayed as sustainable, many would consider solutions such as solar power, wind turbines, hydroelectric dams, geothermal plants, sun farms, wave generators, nuclear power and nuclear fusion to fit the image of futuristic green power. But while all of the above may be green, not all of them are sustainable. To me at least, it seems that green energy is measured from the present as a bench point: anything that produces less greenhouse emissions or causes less ecological destruction than methods currently employed is considered "green." However, sustainable energy production is a more definite construct that can be described through thermodynamics.

From thermodynamics we know that every work-energy conversion in our universe is irreversible, meaning that energy is lost as heat every time something is done. Taking the planet earth as a giant engine, which is accurate in the sense that most life processes convert available energy into work and expend heat, we can conclude that the earth needs constant energy input in order to continue running. Aside from ambient space radiation produced by other sources, the sun is the only source of energy earth knows consistently from day to day. This is something many of us should experientially understand, contrasting the death of winter when days are short to the blossoming of spring when long sunlight returns.

Fig. 1: Boston sun angle during the seasons, affecting solar energy influx (Science Blogs)

Therefore, in order to be sustainable within the lifetime of our sun, an extremely high life expectancy for planet earth, our energy generation tactics should be geared towards the sun’s life-giving energy. Does this mean that solar is the only way to go? No, absolutely not. The sun’s energy hitting the earth enacts a grand cascade of events that grow wind, help generate tides and keep the water cycle flowing. This extends the definition of sustainable energy generation to include wind power, wave power, and hydroelectric generation.

Another side category of what I would consider sustainable energy sources includes those whose failure would correspond with the end of life on earth. One example is geothermal energy. Sure, eventually the earth’s core may cool down, but there would be bigger problems associated with this scenario than humans running out of electricity, such as the dissipation of the earth’s magnetic field letting a bombardment of solar radiation char the planet. Wind power may also be considered part of this category, as wind is in part generated by the Coriolis Effect that depends on the earth continuing to spin. If the earth stopped spinning, bad things would ensue.

So what is not covered under the definition of sustainable electricity generation that is commonly perceived as such? The top two that really get to me are nuclear energy and fusion energy. The selling point of nuclear energy is that it is clean in terms of greenhouse gas generation. However, clean does not directly translate into sustainable. The input into nuclear reactors is uranium, and lots of it. Uranium is not an uncommon element on earth, but a large 1000MWe nuclear power plant requires about 200 tons of refined uranium to operate for one year [1]. Refining uranium is itself a wasteful process, and mining large amounts of ore has an ecological impact as well. Nuclear fusion is often sold as the sustainable energy solution of the future, but in actuality nuclear fusion requires the same input as the sun does: hydrogen. With so much hydrogen on earth, why would this be a problem? Call me paranoid, but any time someone wants to convert something necessary for life into a luxury item (yes, electricity is a luxury item) and some useless helium, then I begin to worry. And relative to the sun’s supply of hydrogen, earth’s hydrogen bank is chump change. The hydrogen fusion reaction that generates the most energy is that of deuterium (₁H²) with tritium (₁H³) as described by this equation [2]:

                                                        1.  ₁H² + ₁H³ = ₂He⁴ + neutron

If we calculate the mass difference

       2.  m(₂He⁴) + m(neutron) – m(₁H²) – m(₁H³) = 4.002602 + 1.008665 – 2.01412 – 3.016050  =                   5.011267 – 5.030152= -0.018885 amu

then insert this mass difference into Einstein’s mass-energy equation

                  3.  ΔE=Δmc²

                  4.  Δm=0.018885 amu(1.66053892 x 10^-27 kg/amu)

                  5.  E=(3.135928 x 10^-29 kg)(3 x 10⁸ m/s)² = 2.82 x 10^-12 J/He nucleus formed

and convert the result into a meaningful number

       6.  2.82 x 10^-12 J/He(2.778 x 10^-7 kWh/J)(1 He nucleus formed/2H nuclei consumed)=
            3.92 x 10^-19 kWh/H nucleus consumed(6.022 x 10²³ H nuclei/mol H)(1 mol H/((2.014102 +                   3.0160492)/2) g) = 9.39 x 10⁴ kWh/g H

we get that 9.39 x 10⁴ kWh of power can be generated from one gram of hydrogen gas (assumption made that hydrogen composition is half deuterium, half tritium). Considering that the average American consumes 10,908 kWh annually [3] and that the population of America is about 321 million [4], approximately 37,300 kg of hydrogen gas would be consumed annually to support America alone by fusion energy. This is not zero input.

All in all, what I am trying to say is that science has laid out a very specific definition of what sustainable energy truly is, so it is the job of every person to evaluate whether what is being portrayed as sustainable energy generation is truly backed by the facts.

Does your definition of sustainable energy sourcing differ? Let me know your thoughts in the comments below!