Energy Density and Usable Energy Output

Are The Most Dense Energy Sources Also The Most Efficient?

Contents

1. Introduction

2. What Is Energy Density?

3. Usable Energy Output

4. Concluding Remarks

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Bitesize Edition

• In today’s post, I’ll explore the concept of energy density. Energy density determines how much energy a fuel or battery can store per unit of mass or volume. An important distinction here is between volumetric density and gravimetric density. Volumetric matters when space is limited, such as a fuel tank, whereas gravimetric density is crucial when weight is a factor. Some energy sources have both volumetric and gravimetric density, whereas others are only pertinent to one or the other.

• Consider the energy sources you know. Which do you think will be the most volumetrically energy-dense? Which do you believe will be the most geometrically dense?

• I’ll also then connect this discussion on energy density to energy efficiency. In making this connection, I return to a conclusion I frequently make in these posts discussing energy: the best energy source depends on the application and the environment. No single solution fits all, and understanding this key principle would aid decision-makers in ensuring they make the best decisions for our energy future.

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Introduction

In my final chapter discussing energy for a while, I’ll explore the idea of energy density. Is the highest-density energy source the best, or is there more nuance to this question? Let’s dive in.

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What Is Energy Density?

Energy density is the amount of energy stored in a given system per unit volume or mass. We can have volumetric energy density which proves useful in applications such as fuel tanks and batteries where we are limited by space. Gravimetric energy density is energy stored per unit mass, which is useful where weight matters, such as aerospace or electric vehicles.

From highest to lowest, the most volumetric energy-dense sources are as follows:

• Diesel

• Gasoline

• Kerosene

• Propane and Butane

• Ethanol

• Methanol

• Natural Gas

• Liquid Hydrogen

• Hydrogen Gas

• Lithium-Ion Battery

• Sodium-Ion Battery

• Lead-Acid Battery

Diesel is at the top of this list because it is a hydrocarbon with a higher carbon content than other hydrocarbons. This means diesel fuel is less volatile and, hence, has more energy per litre. This can be seen in diesel engines, which are more efficient than gasoline engines. High volumetric energy density sources are ideal for high-energy storage in a smaller space.

Photo by ALE SAT on Unsplash

The gravimetric energy-dense sources by ranking from highest to lowest are:

• Deuterium-Tritium Fusion

• Uranium-235

• Plutonium-239

• Hydrogen Gas

• Liquid Hydrogen

• Natural Gas

• Gasoline

• Diesel

• Propane and Butane

• Kerosene

• Ethanol

• Methanol

• Lithium-Ion Battery

• Sodium-Ion Battery

• Lead-Acid Battery

• Supercapacitors

• Pumped Hydro Storage

Fusion releases energy by combining two isotopes of hydrogen, deuterium and tritium, into helium. Deuterium has one proton and one neutron. Tritium has one proton and two neutrons. When these isotopes fuse, they form helium-4 (two protons, two neutrons), releasing energy and an additional neutron which sustains the chain reaction. Although this is the highest energy output per unit of mass, sustained fusion power isn’t yet possible.

Nuclear fission also requires a nuclear reactor to release energy. This highlights that an energy source can be dense, but it usually requires a method to extract and convert energy into a usable form. Energy rarely existing in its directly usable form is another key idea to consider when exploring energy density. Fuels need combustion and batteries need a circuit to deliver power.

How these energy systems converts energy is how efficient the system is. An energy source could be the most dense, but if its energy is lost in conversion through heat, sound, and friction, then a less dense source could produce more useful energy.

Photo by Garett Mizunaka on Unsplash

Now we’ve introduced energy efficiency into the picture, can we combine energy density and energy efficiency to understand which energy sources are the most energy-dense while also being the most energy-efficient?

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Usable Energy Output