🔋Pack A Punch
Comparing the energy density of different fuels, storage, and conversion methods
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If you’ve ever played Call of Duty Zombies, you know what pack a punch is. If not it’s basically this magical machine that makes your weapon more powerful. The gun stays the exact same, it just does a lot more damage. Unfortunately, our energy is grounded in physics and there’s only so much energy to go around. In the real world this is something akin to a technological development or innovation that allows for a step change improvement over the previous iteration. You could say the pack a punch machine increases the energy density of your weapon by boosting the firepower.
Energy History
Humans’ primary energy sources have evolved over time in very significant ways. For much of our history we relied on our own physical energy through the consumption of food. Then, the discovery of fire allowed for food could be cooked, used strategically, construction of tools, warmth, safety, etc. Tools and domesticated animals were used as further energy sources to aid in an agricultural revolution. The use of coal began out of shortages of wood and charcoal and wound up becoming 50% of the energy mix by 1940. The steam engine was an invention heavily reliant on coal that boosted transportation capabilities. After 1940, oil and gas become dominant sources of energy which allowed for cars via internal combustion engines as well as gas heating and cooking. With increased emphasis on climate change, modern renewables and nuclear energy have been an increasing portion of the energy mix, but are still not close to eclipsing the share of coal or oil/gas.
Over time we have moved from lower energy dense generation to higher. These energy dense sources became cheaper as they overshadowed their predecessors over time in terms of output and cost. This has resulted in an increase in surplus energy. Surplus energy is what is not being consumed, used to stay warm, or perform day to day functions. More energy dense sources allowed for more energy to be spent on innovation and further quality of life rather than survival. With this came population growth and increases in wealth at the same time. Each energy transition increased the average standard of living as well.
Renewables (wind/solar) are often spoken of as the next energy transition. This lays in contrast to previous energy transitions which were significant boosters of energy density and cost per unit energy. I wanted to take a look at the numbers and try to compare the various types of energy sources in terms of energy density. This is often noted in energy per unit mass, volume, or area.
Energy Density
In order to compare different energy sources, the same units are required for each. This poses an immediate issue as certain types of energy are fuel themselves like oil or natural gas, but wind turbines, solar panels, and hydroelectric dams are tools to capture sources of energy. For this reason, comparing apples to apples is tricky, and perhaps why I could not find comparable data for every energy source I was interested in. Nonetheless, I gathered and/or calculated the gravimetric energy density (GED, Wh/kg) and volumetric energy density (VED, Wh/L) of various energy generating sources and a few energy storage technologies for comparison.
Since nuclear fusion and fission are so much more energy dense than any other technology, a logarithmic scale had to be used for both axis (energy density = 10^axis value). It is important to note that wind/solar were calculated as if running at their maximum power rating1.
Of the combustion reactions, hydrogen has by far the highest gravimetric energy density and does not emit any carbon dioxide. Since it is a gas though it suffers bigtime volumetrically. Natural gas is in a similar boat since its VED drops significantly from its liquified (LNG) form. My fellow battery researchers will clearly see the marked improvement that Li-ion has achieved over the earlier lead-acid battery which paved the way for cell phones, laptops, EVs, etc. For a similar diagram that shows more petroleum, coal, and biofuel sources see here.
Possibly the most noticeable thing in this figure is the stark difference between both nuclear energy sources and everything else on the list. The best realistic argument against nuclear currently is the cost. However, historically superior energy dense energy sources became more economical as they increased in scale. If nuclear was to be accepted, there is more than enough uranium and thorium to power the world for many generations. It is truly remarkable how energy dense nuclear power is.
One unfortunate observation from the graph is that the renewable energy sources such as wind, solar, and hydro are very low on the list in terms of GED and VED. Since renewables take up more mass and volume, for them to truly take over as energy sources they will have to undergo a step change improvement or two in cost. My graph obviously doesn’t take cost into account, and solar/wind have been decreasing in cost gradually over time. The issue is that cheapening energy as an input has definitely contributed to that decrease in cost, and we are seeing the cost of these renewables go back up currently with global energy and supply chain issues.
Batteries are batteries. They are storage mechanisms not energy sources or energy capturing technology. They aren’t designed to compete with petroleum, gas, or nuclear energy. No batteries with higher energy densities than traditional Li-ion have really broken out into the mainstream yet. Li-ion has been transformational in the modern life and of course I had to include them on the graph.
Capacity Factor
None of these energy sources are 100% efficient to take full advantage of their energy densities as some are intermittent sources and some lose energy as heat for example. The capacity factor (CP) is a measure of actual electrical energy output vs the theoretical electrical energy output over a certain period. Simply put it measures how much of its energy capability is utilized. This is really the crux of renewables, because battery storage is needed to overcome the poor CP. Natural gas, nuclear, and coal are sometimes termed “baseload” because they are able to generate energy virtually any time at a high CP. Below shows the common CPs for our energy sources.
I’m not trying to dunk on renewables, it is just the reality that they are not optimal sources of energy (where’s the pack a punch). They are the first energy transition that has a lower energy density + capacity factor and that is practically a giant hurdle. Wind/solar have tremendous potential and have many use cases don’t get me wrong, but nuclear is the only option that could be considered next-gen energy through the historical lens. The goal was to give an objective and fair comparison between energy sources in this piece and reflect on what I saw. I am looking forward to expanding more on this topic, as there are connections and trends in things like population growth, wealth, and emissions that can be seen through looking at this topic.
I hope you enjoyed today’s article. If you did consider leaving a like, sharing it with a friend, or sharing it on social media. I would greatly appreciate it!
-Grayson
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Wind GED was calculated using a 3MW turbine weighing 64.6tons. The VED value was calculated using data from the NREL - a class 7 1000W/m^2 turbine operating at maximum for 1h and an arbitrary “box volume” constructed to estimate the practical volume a wind turbine takes up. The dimensions used were 180m x 60m x 20m.
The solar calculations were for a 600W panel operating at maximum for 1h, mass of 31.5kg, and volume of 2.83 cubic meters which was estimated based off of the surface area of the panel.
Nuclear fusion is still a theoretical process and to my limited knowledge on the subject does not have a practical/experimental value, so I left it the way it was. For the volumetric calculation I just used the density of hydrogen gas.
Fission is usually 1% efficient as the uranium does not all come in the appropriate isotope. The density of uranium was used to convert to volumetric.
Many of the values used have been previously collected here.
I assume 500kg/cubic meter for wood.