Snippets on Energy #1

Energy Density

Dense hubs of energy. Image credit: NASA, via Unsplash

Let’s start our journey towards understanding energy with the imperative of high energy density, which you can think of as the Watts being captured and/or used up in a given area (like a building, or a town). Power is what every civilization needs, and as overall energy use goes up, so does power density. Starting with a 19th century farm with a barn and small house, then continuing to 20th century city, up until the Burj Khalifa today, with one of the highest energy use per footprint area of any building in the world, energy use has continued to go up and up.

What is this? This is a mini-series on the energy transition dilemma. If you are new to the topic, please read the kick-off article for the sequence titled: The crux of the energy transition. ..and now, back to the show:

In order to power such energy hungry structures, you would need a similarly high energy density in capturing, then transferring the required energy. For example: on a 19th century farm you needed many acres of land to produce the food for yourself, your family and your animals. Not much energy in terms of calories, but a huge land area nevertheless. In other words: a relatively large area was needed to support a low energy lifestyle (lighting with candles, heating with wood, but no TV or pick-up truck). Low energy density in — low energy out.

A mid-20th century city in contrast consumed food produced by using lot’s of fertilizers, pesticides, and oil burning machinery, plus electricity and gas. As a result both the amount of food produced on a given land, and the amount of energy embedded in chemicals and burned by harvesters went up exponentially. As the power and population density of a city grew, so did the power density of its hinterland .

Fast forward 70 years: today’s super-metropolises use the resources of an entire planet and thus need high power transportation of food and other materials from all around the world. Imagine feeding all of today’s Los Angeles with food produced by small family farms using oxen, and delivered on horse carriages to the market. For a big city, you need big productivity — combined with fast long distance transport. It is not by chance, that food itself is now produced with higher energy inputs than it provides (8 kcal of fossil fuels go into each and every food kcal we eat). High intensity in — high intensity out.

In addition to food, these cities operate on 24/7 electricity to power 24/7 air conditioning, hot water, sewage, lights, televisions and lots of gadgets… Not to mention the inflow of gasoline and diesel to power all those cars and trucks. Despite the fact that these technologies (cars, trucks, air cons, computers, farming etc.) are getting more and more energy efficient, we are using more and more of them, which has resulted in ever higher energy use per square mile — higher than ever in human history. Just take a look at the photo above and imagine all those engines humming in traffic jams, air cons blowing cool air into offices, city lights, cafes, restaurants, and trucks carrying fresh groceries to a nearby supermarket.

Enter wind & solar. They both take up enormous land areas to produce very little energy in comparison. In other words: they have a very low energy density. Let’s take solar for example: the current industry best panels have 26% efficiency when it comes to converting sunlight into electricity. Since the Sun doesn’t shine at night, you can immediately halve this nameplate efficiency. Now we are at 13%. Clouds are another problem, especially in Europe (1), where they block the Sun at least half of the time — which itself doesn't shine always in perfect alignment with your panels. That takes us down to 6–7% efficiency. You also need to take into account, that you cannot cover 100% of a given area: you need to leave room for maintenance, or streets in case you are planning to put photovoltaics on roofs in cities. This effectively halves any area you plan to cover with them. Now we are close to 3% overall, average efficiency.

In mid latitudes you get a theoretical maximum of about 1000 Watts of solar energy from the Sun per square meter — multiply it with this 3% efficiency and you get 30 W per square meter (averaged on a 24 hours basis). A 1000 MW nuclear power plant (that is 1 000 000 000 W) in comparison takes up a little more than a square mile (about 2 600 000 sq meters). This translates to 385 W per square meter on average — more than twelve times than solar. A container sized gas engine on the other hand, can produce an electric output of 2000 kW in a footprint of a mere 45 sq meters… that is 44 444 W per square meter, 24/7 day in, day out — almost one and a half thousand times solar. Fossil fuels are hard to beat when it comes to energy density.

Why is this so important? Well, for a number of reasons. For starters it explains why cars (and more importantly trucks), and as a result large cities exist: vehicles with gas or diesel engines can travel a long distance, carrying tons of metal and plastic plus people and their payload. Diesel is one of the most energy dense commercially available fuels on Earth, only Uranium-235 is produces more energy per unit of volume. In the later case however you need a large supporting infrastructure (reactor, heat exchanger, turbines, generators, cooling units etc.) which makes the entire setup much less energy dense (and much more costly), compared to a diesel engine.

Back to solar. Due to its extremely low energy density, transportation would be impossible with solar panels on the roofs of vehicles. Yes, you can build ultralight vehicles carrying one person, but they certainly won’t be able to supply grocery stores... To circumvent this issue, you would need to electrify or convert transportation to use H2 and as a result increase grid power generation capacity significantly. This would come above and beyond what we currently have, just to power road transportation (and we haven’t mentioned industry or heating buildings).

Replacing oil here with “renewables” would end up mining enormous amounts of new raw materials (causing additional habitat destruction amidst an ongoing ecological crisis), and then covering further swaths of land or water with wind turbines or panels — dealing further damage to the living world. You cannot replace a system designed to operate on high energy density fuel, long distance transport, industrialized farming, 24/7 electricity etc. — with a low energy density system, and hope to have the same results. If it was this high intensity which have made modern civilization possible, then the lack of it will make it disappear.

This leads us to the question: How does energy density relates to resource intensity? And why is that a major issue? You will find it out in the next installment on Snippets on Energy.

Until next time,



(1) Of course, you can move your panels into the Sahara-desert, but then you will need thousands of miles of high voltage copper wires to transmit electricity. At such a large distance you will experience significant losses due to electric resistance, not to mention the extra cost incurred at purchasing then maintaining the transmission lines. In addition to that, photovoltaics under-perform under high external temperatures (they perform best at around 20°C). These numbers and calculations are not trade secrets. It is no wonder that no one has proposed this to build this structure seriously so far. Desperation might drive us to do this, but this will not alter the facts.