Beyond Batteries

A comparison of different methods of energy storage

13 min readJul 14, 2023

You’ve probably heard this a million times.

But it’s so important, I’m gonna tell you again. :)

Temperatures are rising, natural disasters are becoming more common, oceans are acidifying, and so much more. *Climate Change is real.*

We’ve got to make some drastic changes on how we use our resources or we’re going to get to a point where there is no un-doing the damage we’ve created.

Beyond that critical point, humans are kinda screwed.

And as you also might be aware, all these terrible events are caused by CO2 emissions, which have also been rising.

CO2 particles, along with other greenhouse gases, act like a blanket for the Earth and trap in some of the sun’s heat. And as a result, the planet heats up. This causes the ice at our poles to melt, and when this ice melts, there is less shiny surface to reflect the sun’s rays back off the Earth. So after a certain point, we can be emitting zero CO2 and still have the planet heating up.

Soooo yeah — we need to stop emitting CO2.

Let’s do a root cause analysis — what’s causing these deadly CO2 emissions?

In 2021, we emitted a total of 41.06 billion tonnes of CO2.

37.12 billion of those were from fossil fuels. [1]

Fossil fuels accounted for 84% of global energy in 2019. [2] That is wayyyyyy too much.

Let’s pretend for a minute that fossil fuels don’t cause any of this climate change stuff and are perfectly fine and safe to use. If we don’t find any more oil reserves, our oil is gonna be all gone by 2052. [3] That’s not all that far away.

It’s pretty clear that fossil fuels are an unsustainable source of energy. Fossil fuels are a non-renewable source of energy. One of the first things I learned about energy is the difference between renewable and non-renewable. If you missed that lesson at school, here’s the only important bit:

renewables are WAY better.

So why are we still using fossil fuels?

Given all that, you might be wondering why we’re even using fossil fuels. But the truth is, they’re working pretty well for us right now (minus the environmental damage.) Plus, it’s important to note that we’re probably not going to be able to completely eliminate our use of fossil fuels (not now at least), just reduce it. So, here are some reasons why we use fossil fuels:

⚡They have a crazy high energy density, making them efficient for energy production and transportation.
🏭Our infrastructure is already there, and it cost a lot of money to build.
🪙They’re comparatively cheap. Although the cost of solar and batteries has fallen dramatically in recent years, the cost of them combined needs to be less than the cost of fossil fuels.
⌚They’re consistent and reliable, meaning we can have energy whenever we need it.

⚡Energy Density
The energy density isn’t something we can really do much about. However, the potential for renewables like solar is overwhelming. Every year 5,455,728,000,000,000,000,000,000 watts of solar energy hit the surface of Earth. [4]

We only use 176431 terawatts, or 176,431,000,000,000,000 watts. [5]

As you can see, we still have a lot of extra energy. And that’s just solar. So energy density, while it is most certainly important, doesn’t have to be the deciding factor.

🏭Infrastructure
Yes, the infrastructure for fossil fuels is there, but as we already established they aren’t very sustainable. So we need to invest in new infrastructure. But interestingly, some of these renewable sources can be used with our existing infrastructure.

🪙Cost
Overtime, the cost of renewables will go down. This can already be seen. And the thing is, as we expand grid capacity, with each doubling of the cumulative installed capacity their price declines by the same fraction.

And remember how the cost of the source + the storage has to be less than fossil fuels? The cost of lithium ion batteries has fallen by 97% in the past 3 decades. [6]

⌚Intermittency
This is the biggest problem with renewables. Wind and solar plants can’t generate energy all the time, because the wind isn’t always blowing and the sun isn’t always shining. This means that we might not always have energy when we need it.

This is known as something called the duck curve: the imbalance between energy supply and demand. This can stress the grid, and is as a result something we’d like to avoid.

Energy demand is the highest in the mornings and evenings (people are getting ready for work or they are winding down for the evenings) and, coincidentally, that’s when solar output is lowest.

Fossil fuels don’t have this problem — we can generate energy whenever we want wherever we want.

So to make renewables more viable, we need something I’m particularly interested in — energy storage. This is basically us saving the energy for later.

By doing this, we can capture the sun’s energy when its at its highest (in the middle of the day) and save it for the evening and morning, where energy demand is the highest but solar produces the least energy.

There are a ton of different types of energy storage. I’ll list some of the common(-ish) ones along with their pros and cons.

Batteries

Most people already know about batteries. They power your phone, EV, computer, and so much more. The most common type of battery is the lithium ion battery, which has the highest energy density of all batteries and is very efficient. You can learn more here if you want. Batteries are a form of chemical energy storage.

Pros:

compact — variety of applications
lithium ion batteries only lose 5% of charge (relatively efficient)
high energy density

Cons:

require special materials (lithium is mined in unethical and environmentally damaging ways)
degrade overtime and have a limited lifespan
too expensive for large scale storage
can only store energy for a few hours

Supercapacitors

Batteries and capacitors both store electricity, just in very different ways. Batteries use chemistry (in an electrolyte solution) to store electricity, and capacitors use static electricity. There are two conducting metal plates with an insulating material between them. Positive and negative charges build up on the plates, and the separator that prevents them from touching is what stores the energy. A supercapacitor is a high-voltage capacitor.

Pros:

very fast charge and discharge rate, able to absorb lots of energy at once
long cycle life
generally safe

Cons:

lower energy density (store less energy per unit of volume/weight)
energy can dissipate over time
limited energy capacity, better suited to short term storage
more expensive than batteries

Hydrogen

Hydrogen is something that’s gained a more recent interest, and is cited to have huge potential. It’s kind of like a Swiss army knife: you’re able to store energy, fuel various things, and use in power generation/buffering.

Pros:

wide range of potential applications
high energy density (energy per unit of weight)
potential for long term storage without significant degradation

Cons

hard to transport
low energy efficiency (~50–70% of energy input translates to output)
very flammable
carbon emissions from production of hydrogen

Pumped Hydro

This is basically running water up a hill and then when you want to re-harvest the energy, letting the water flow back down the hill. It’s a gravity-based energy storage device. In all honesty, this is probably one of the best sources we have.

Pros

high efficiency
long lifespan
rapid response and grid stability

Cons

basically everywhere we can make this, we have. It’s geographically bound
environmental concerns (habitat destruction)
high capital costs

Flywheels

Flywheels use kinetic energy. Basically, this wheel is spinning, and to use the energy the wheel will slow down, and to absorb/store energy, the wheel will speed up. However, overtime it can slowly lose its energy to friction. To overcome this, flywheels can be used with magnetic bearings and in a vacuum to reduce the drag and friction.

Pros:

high energy output
quick response time
high energy efficiency (80-90%)
long lifespan

Cons:

limited energy storage — it depends on the rotational speed and mass of the flywheel
expensive compared to other options
somewhat unsafe (it’s spinning really fast)

Thermal Energy Storage

Thermal storage systems store energy in the form of heat or cold using materials with high heat capacity. The stored energy can then be used for heating/cooling buildings, or can be converted into electricity. This can be done with salt, or potentially sand (its way cheaper).

Pros:

high energy density
can be used either to heat/cool buildings or to generate electricity — variety of applications
much cheaper than battery storage
long term energy storage

Cons:

energy is lost when it’s converted from one form to another, so the efficiency could be limited
slow charge and discharge rates
may need frequent maintenance

Compressed Air Energy Storage (CAES)

Basically, compressing air and storing it underground in tanks. When you want to use this energy, you can release the air and use it to power turbines to generate electricity.

Pros:

long duration of energy storage
high energy efficiency
compatible with existing infrastructure
rapid response time

Cons:

geographically limited (may require underground caverns)
high capital costs
energy is lost during the compression and expansion processes

Superconductors

A superconductor is essentially a material with zero electrical resistance. Superconductors are not generally used for energy storage by themselves, however they still present potential. They can be used for superconducting magnetic energy storage (SMES), or in a superconducting flywheel. SMES stores energy in the form of a magnetic field — a current is passed through the superconducting coil creating a persistent current that generates a magnetic field. And superconducting flywheels are beneficial because using superconducting materials in the construction of the flywheel can minimize energy losses due to electrical resistance.

Pros:

high energy efficiency
high current-carrying capacity
fast response time
potential for long term energy storage

Cons:

expensive
have to operate at cryogenic temperatures
are quite complex
currently more feasible with short term energy storage

Which one?

I just listed a whole bunch of energy storage methods, and as I’m sure you can guess, there are a ton more. I picked the ones that I find most interesting, common, and applicable.

So to decide which one to focus on, we have to define the requirements it’s going to be filling.

(also just so we’re clear, it’s likely going to require a combination of various energy storage techniques and energy sources to help reduce our reliance on fossil fuels. I’m looking specifically for the one that has the most potential to help with the intermittent nature of renewables -> so as a long term energy storage system.)

We need a system that is:

Cost competitive with fossil fuels. The generation + storage has to be <10$/kwh to be cost competitive with fossil fuels (won’t be factoring this in too much at the moment because costs can be reduced)
Requires no special materials. For this to become widespread, we can’t be dependent on materials that are limited or only available under certain conditions.
Energy efficient. Why would we waste energy?
Not geographically bound. This would limit the scaling up of the storage system.
Capable of long-term storage. It needs to be able to store energy for days with minimum loses to be viable for grid use.

Let’s go through the above storage systems and see if they meet the criteria.

Batteries — ✅(lithium ion batteries require special materials BUT the supply chains are quickly evolving to make that not a significant problem)

Supercapacitors — ❌ (more suitable for short-term storage)

Hydrogen — ❌ (low energy efficiency + dangerous)

Pumped hydro — ❌ (geographically bound)

Flywheels — ✅(just might be too expensive)

Thermal energy storage — ✅ (only thing is that energy can be lost when its converted between forms)

Compressed Air Energy Storage (CAES) — ❌(high capital costs + may be geographically limited)

Superconductors — ✅ (expensive because of cryogenic temperatures though)

Ok, so flywheels, thermal energy storage, and superconductors have potential. However, they still have some limitations, which I listed above.

So:

Can we make the materials used in flywheels cheaper?

Can we make converting between thermal and electrical energy more efficient?

Can we make superconductors that don’t have to operate at cryogenic temperatures?

I mean yeah in theory we could do all those things. 💃

What I’m gonna end up with here is just because of personal preference: what I find most interesting to work on and most intriguing to me personally. So that’s not to say these energy sources don’t have potential (or any of the others listed above for that matter!)

Cheaper Flywheel materials:

Materials used for flywheels have to meet many requirements, and that’s one of the reasons flywheel technology is not where we want it to be. Material science is limiting it. (😢)

Achieving cheaper flywheel materials often requires a trade-off between performance and cost-effectiveness.

The materials used in flywheels must have high strength, low density, good fatigue resistance, and heat dissipation properties. A potential material that could be a good fit is carbon fiber, however the complicated synthesis process results in this being a very expensive material.

That brings me to the next issue, flywheels need specialized manufacturing processes, which can be expensive as well.

This would also be an interesting problem to work on. Basically, to solve it, you need to make a cheaper version of carbon fiber (improve the synthesis method). There are already people working on this though, so who knows?

Increasing thermal conversion efficiency:

There is a maximum conversion efficiency, known as the Carnot efficiency. It’s defined by thermodynamics and physics. The equation is as follows:

η_carnot = 1 — (T_cold / T_hot)

Where T_cold is the absolute temperature of the colder reservoir and T_hot is the absolute temperature of the hotter reservoir. However, this is not the only limiting factor, as there are also energy losses depending on the specific device or system used.

There are many different systems that can be used to convert heat energy to electrical. Here are some examples.

Steam turbines (combined heat and power systems can achieve 70–85% efficiency)
Stirling engines (can achieve 15–30% efficiency)
Thermoelectric conversion (uses the Seebeck effect, can achieve efficiencies of 5–15%)
Concentrated solar power (efficiency of 20–30%)

The last three have potential for higher efficiencies given advanced materials or designs, which are also interesting.

So in summary, there is still room for improvement through advanced materials and designs, however there is a physics-based limit.

Room-temperature superconductors:

So I’m already like really really biased here (being a material science geek 🙃) but I find this to be very fascinating. But just think: if we could make these operable at room temperature we’d make this form of energy storage so much more feasible — along with so many other cool technologies such as magnetic levitation trains and MRI machines and quantum computing and so much more. Also, superconducting materials can be used to create a superconducting flywheel. 🤩

Energy storage in superconductors is currently more aimed towards short term. However, with advancements in room temperature superconductor technology, the method would become more efficient, with high energy densities, and long cycle lives. In addition, they could also produce more compact energy storage.

Personally, I find the potential of thermoelectric devices and room temperature superconductors to be the most interesting.

There is a class of materials called topological materials that I recently learned about, and these materials have very unique electronic properties. Not only do they have high potential for the devices I mentioned above, but also for computing!

They are insulating on the inside but conductive on the outside. How cool is that?! This basically means that electrons can flow smoothly on the surfaces of these materials, even if there are defects or imperfections inside.

I hope you enjoyed the read— I have been interested in energy for a while, and I think combining material science with energy storage has interesting applications. In the coming weeks, this is what I will be looking into!

Thank you so much for reading! Have a wonderful rest of your day :)

Beyond Batteries

A comparison of different methods of energy storage

So why are we still using fossil fuels?

Batteries

Supercapacitors

Hydrogen

Pumped Hydro

Flywheels

Thermal Energy Storage

Compressed Air Energy Storage (CAES)

Superconductors

Which one?

Cheaper Flywheel materials:

Increasing thermal conversion efficiency:

Room-temperature superconductors:

Citations

Written by Sasha Przybylski