This post is long overdue. Even before I wrote about nuclear fusion, I knew that I wanted to write a post about energy storage. It’s such a relevant topic today, given the (possibly counterproductive and definitely premature) push for electrification, and also one that is poorly understood and poorly communicated. However, when I first planned the post, it was going to be specifically about battery technology, and in that guise it was quickly becoming more technical and more complicated than I wanted for the site. Writing a post that is just as complicated as the scientific literature on energy storage wouldn’t really accomplish my goal. Only when I reframed the post as being about energy storage did the discussion click into place.
I got to thinking in-depth about energy storage recently because of a NASA challenge to create a system of energy storage and transfer for the Moon. While I don’t know if I will ultimately end up submitting to the challenge, it did get me thinking about electricity, and especially energy storage. If I wanted to make this post extremely short, I could probably leave it at this: we humans are absolutely terrible at energy storage.
Energy storage can come in a multitude of forms. You might default to batteries, but there are other forms of energy storage all around you, every day: chemical, mechanical, even informational (although that one gets a little complicated). Each of them have their own traits and characteristics, and we will get to many of the possibilities, but before we do we need to establish a few things regarding terminology and relevance. First, and foremost for a post on energy storage, we need to establish what energy actually is.
If you look up “energy units,” you will see energy measured in everything from joules to barrels of oil. There are units of energy like “tons of TNT,” “British thermal units,” “US thermal units,” and the extremely annoying “foot-pounds.” Most useful to us are joules, calories, and watt-hours. We will first look at joules, which are the standard unit of energy (or work – they’re the same thing, mathematically). Breaking out the component units of a joule, we get kilogram meters squared per second squared. We can derive this with some elementary physics. Work (or energy) in Newtonian physics can be defined as the force multiplied by a displacement. Force is equivalent to mass times acceleration, and acceleration is equal to velocity divided by time. So, we have the quantity of velocity divided by time, multiplied by mass, multiplied by displacement, equals work or energy. Velocity is meters per second, which divided by seconds gives us meters per second squared. Multiply that by kilograms for the mass, and meters for the displacement, and you get the unit we originally had: kilogram meters squared per seconds squared. In words, this means that a joule is the amount of work done by one newton acting through one meter.
More broadly, it, and every other unit of energy, is a measurement of the amount of energy needed to accomplish some task. The calorie, for instance, is traditionally defined as the amount of heat required to raise the temperature of one gram of water by one degree Celsius. This gets complicated, so the calorie has been redefined in modern times in terms of the joule, at one calorie being equivalent to 4.1868 joules. It started, though, as a way of quantifying the energy needed to accomplish some task, just like all of these other units, and that quantification helped lay the groundwork for the entire industrial revolution (as discussed in Einstein’s Fridge).
It is also worth examining the watt-hour, in part because it will enable us to explore the difference between energy and power. Those terms are used interchangeably in colloquial settings, but the difference between them is hugely important to keep in mind when we start talking about energy storage. A watt, you see, is a unit of electrical power. In base SI units, it is kilograms meters squared per seconds cubed – we have divided by time once more, so power is just energy per unit of time. When some appliance gives you a number of watts it requires, it is saying that it requires that amount of electrical energy per unit of time, not that it requires that amount of electrical energy, total. A watt is furthermore defined electrically as the power from one ampere of current across one volt of potential difference. A watt-hour, then, is just multiplying the watt by time so that we end up with energy again. So you would measure how much energy your appliance needs each second in watts, but you might measure the total amount of energy it uses over the course of a day in watt-hours.
Now that we’ve established what we mean when we say “energy,” we can establish why storing that energy is relevant, and why it is especially relevant today. While I have deep reservations about the current push for electrification, as well as the embrace of photovoltaics and wind turbines for electricity generation, it is a fact that both trends are presently happening. Both of these trends are based upon, enabled by, and potentially hamstrung by, energy storage technology and its limitations. With that, let’s turn to energy storage itself, the main topic of this post.
When I said energy storage, you probably first thought of batteries, but that is far from the most ubiquitous form of energy storage, and that term applies to several sub-technologies, besides. Probably the most common form of energy storage with which you interact on a daily basis is chemical, in the form of both food and fuel. Whether it’s your steak, a log for your fireplace, or a gallon of gasoline, it’s all chemical energy storage (note: we are differentiating between chemical energy storage, which refers to energy stored in molecular and atomic bonds, and the energy stored in chemical batteries through such phenomena as redox reactions). Energy can be stored electrically, thermally, chemically, even mechanically, and each specific form has advantages and disadvantages. We couldn’t possibly discuss, or even list, all of the specific types and their characteristics, advantages, and disadvantages, but we will attempt to cover the broad categories.
Thermodynamics tells us that energy can be neither created nor destroyed. However, it also tells us that energy will tend towards less useful forms, through entropy. The energy in a gallon of gas you burn in your car isn’t gone, but recovering it from the heating of the pavement through friction, the disturbances to the atmosphere, and a dozen other sources is all but impossible. A large part of the utility of a specific energy storage system is, therefore, how easily the stored energy can be made to accomplish the desired tasks, and how rapidly the energy will deteriorate from that form into a less useful one.
If you remember high school physics and conversations of potential and kinetic energy, you know how at least one form of energy storage works. Whether it’s lifting and storing a mass at some height, extending a spring, starting a pendulum swinging, or twisting up a rubber band, these are all ways of storing energy in a mechanical way. It can be argued that every form of energy storage is in some sense mechanical, but using this as a specific category referring to large scale forms of energy storage, such as the spring in an old wind-up watch or music box, is useful to us for differentiating, and for not getting too deep into quantum and particle physics.
Storing energy chemically may not be as intuitive as storing it mechanically, but it is arguably the most ubiquitous, most stable, and most readily useful form of energy storage. It is in, well, pretty much everything, although how easy it is to release it can vary widely. Organic compounds, for instance, readily convert from chemical to heat energy (think of wood burning), while other molecules with tighter bonds, like water, can be quite jealous of their energy, not giving it up readily. At a very elementary level, chemical energy can be thought of as the energy contained in chemical bonds, the bonds within and between molecules. Slightly more scientifically, these bonds are not physical things, but rather interactions of forces, especially the electromagnetic force, and what is being released is the energy it took to form those bonds.
Thermal energy storage is almost as intuitive as mechanical energy storage: it involves using a heat sink as a store of energy. Certain material have a high specific heat capacity, meaning that it takes a lot of energy to change their temperature (which also means that it will take a long time for them to passively cool). We can therefore dump heat from, say, the sun into a substance like salt, and then using the superheated salt later to generate steam to turn a turbine to generate electricity.
Speaking of electricity, I don’t think that I can put off the discussion of batteries any longer. There are a lot of reasons why batteries are terrible for energy storage: they’re heavy, they’re complicated, they require unusual and difficult to obtain and sometimes dangerous materials, they’re inefficient, they degrade rapidly, they’re very sensitive to usage patterns and ambient conditions…unfortunately, they’re also incredibly useful, mostly because of how we tend to use energy. If the energy that you want to use is electrical, and the energy that you want to store is electrical, then you probably want to store it electrically to minimize conversion losses, and that means batteries (it can also mean capacitors for the short term, but that’s beyond the scope of this post, as is the difference between a battery and a capacitor).
At the most basic level, batteries are all about charge separation and mediation, and are composed of an anode (a negative electrode, the negative terminal, the lead that undergoes oxidation during the discharge of the cell’s electricity), a cathode (the positive electrode, counterpart to the anode, the positive terminal, the lead that will go through the reduction component of the redox reaction when the cell is discharging), and an electrolyte, which is just a medium through which ions can flow. There is also usually a separator, which helps ensure that the battery does not develop a short.
Here’s how any battery works. When the circuit is complete, it causes a chemical reaction that leads to cell discharge through a redox (reduction-oxidation) reaction. The anode material will shed electrons, which will flow through the circuit (this is current, but is represented as flowing in the opposite direction of current, for reasons that have to do with the early history of electricity and the fact that we didn’t really understand what was going on at a subatomic level when we were defining some of these conventions). The electrons will return to the cathode once they’ve passed around the whole circuit. The electrolyte enables the necessary ion transfer. Chemically, the reduction reaction at the cathode leads to a net positive charge, which will draw electrons through the circuit from the anode, which is undergoing an oxidation reaction that leads to a net negative charge.
The above will apply, in its essence, to any battery type: nickel-cadmium, nickel-hydrogen, lead acid, and the now-ubiquitous lithium-ion. We could digress into a discussion of the advantages and disadvantages of different types of batteries, but this is a post about energy storage, not about batteries, specifically. Suffice to say that, depending on the choice of anode material, cathode material, and electrolyte material, the resulting battery will have very different properties, which may or may not be beneficial. Lead-acid batteries, for instance, are used in cars because they can provide a very large amount of current in a short period of time to start the car, and can endure a large number of charge-discharge cycles without significantly degrading in performance.
Batteries are characterized by certain performance metrics, like memory effect, energy capacity, power output, and voltage. Mass and composition are also important design factors to consider when choosing a battery, especially if you’re concerned about SWaP (size, weight, and power – it’s a spacecraft tradeoff acronym). Instead of examining all of the different possible types of batteries (which are probably nigh infinite, although they are certainly not all going to be useful), we’ll look specifically at lithium ion batteries, which are the batteries upon which the somewhat irrational hopes for electrification are founded.
Lithium-ion batteries have become the default for all manner of electronics because they are relatively light, have relatively high energy density, and, perhaps most importantly, can undergo many, many charge-discharge cycles without significant degradation of other performance metrics. Plus, the degradation tends to be steady, instead of the rapid drop-off that some other types can experience. Capacity is usually measured in ampere-hours, and plotting that versus the number of cycles the battery has undergone will give insight into how the battery will behave over its lifespan.
There are disadvantages to lithium-ion battery technology, however, like their tendency to undergo thermal runaway (which is a nice way of saying that they are inclined to explode). Look up what happens when you put pure lithium, or any of the other alkaline metals (sodium, potassium, rubidium, cesium, francium) into water, and you will see both why lithium-ion batteries are so energy-dense, and also what happens if something goes wrong with them. That makes their use more complicated, because they require additional electronics to ensure that they maintain the proper conditions, and they are very sensitive to temperature. I’d be very curious to know how much range these EV drivers get out of their cars when it’s not seventy degrees and sunny in California. We’ve probably all experienced our phone battery dropping rapidly in the cold: I’ve even gone on runs when it’s so cold that I’ll lose my music halfway through. Furthermore, lithium and the other materials involved in making lithium-ion battery technology functional can be difficult to obtain.
I realize that we have only scratched the surface of batteries, their chemistry, and the technology that surrounds them. That is a deliberate choice, because this post was not working when it was all about batteries, and needs to be about energy storage. That being said, if you would like to discuss batteries further, or if you have specific questions about them, I would be happy to engage with you in the comments.
We’ve covered energy storage chemically, mechanically, thermally, and electrically, but this is not an exhaustive list, and it is not meant as one. There are even more techniques out there, and doubtless some that we haven’t imagined yet. Despite being twenty five hundred words, this post should be more thought of as an introduction to energy storage. That concept of energy storage, though, is all around us, and I don’t just meant in things that involve electricity. Understanding what’s really happening when we’re storing energy is almost as important as understanding transformation of energy.
This is a rapidly evolving field, especially when it comes to batteries. I’ll try to post some articles when I come across interesting ones about new developments. In the meantime, let me know what you thought of this effort in the comments below, and I’d be happy to discuss anything we brought up in this post further there, as well. Now, I’m going to go back to trying to figure out how to store energy on the Moon.