What is energy storage?

This article aims to provide a concise overview of energy storage technologies and how they could be categorized and how energy storage can be utilized.

Energy storage – a definition

Energy storage is the capture of energy generated at a certain time for use at a different time in future. Energy storage is the means to decouple energy generation from energy demand. This is particularly important for electrical energy where demand always needs to match supply for stability assurance.

Most of us think of batteries when talking about energy storage yet there are many other forms energy can be stored besides batteries. For an overview of the different forms of storage regarding the type of energy used see the following picture.

Regardless of the form of energy storage, it always includes energy conversion from one type to another this unavoidably results in conversion losses.

Energy storage - categorization

A great variety of energy storage technologies exist and naturally there are several meaningful ways to categorize them depending on the purpose of the categorization. The following part briefly describes some typically used categorizations.

a)      Categorization by types of energy forms

As can be seen, there are different forms of energy that can be used to store energy. Mechanical storage could mean to convert electric energy into kinetic energy (flywheels), impose work on liquid (pumped hydro) or solid matter to increase potential energy and later convert it back to electrical energy.

Electrochemical storage plays an increasingly important role due to its flexibility and scalability, especially in mobile applications and with the integration of renewable energies into electrical grids.

b)      Categorization by discharge time versus storage capacity

 Not all storage technologies are equally well-suited for short-duration long-duration applications. It makes a big difference whether the energy must be stored for a couple of minutes and or used with high frequency versus seasonal storage.

c)      Categorization by energy and power density

This categorization usually uses Ragone diagrams, depicting energy density vs. power density. This view is important to discern between mobile and stationary technologies. Energy and power density also drive the footprint. It is important to stress that this figure depends on the depth of system integration, that is on cell level vs. system level. For end customers, the driving parameter should be the densities on system level (including power conversion, switchgear, safety distances and so on).

d)     Categorization by technology maturity

From a financial risk perspective, it may be interesting to look into the technology maturity of each technology (besides the track record of the storage supplier). It influences cost and availability of financing resources, insurance options and overall bankability (besides the robustness of the business case).

Some technologies are already massively deployed and engineered for decades. They tend to be proven, reliable and come with less technology risk exposure. Examples would be Lithium-Ion and lead-acid batteries and pumped hydro power plants.

Others are being commercialized currently but their track record is limited, and they are still in an early phase of mass deployment. Typical examples are all types of flow batteries (some are more developed than others, though) or thermal storage based on steel.

Yet some other technologies are still in early stage, promising in laboratory but with a long road to go to market readiness. Technology risk naturally is highest for this category, however from an investment point of view they have the highest chances of developing into gamechangers. Current examples are solid-state batteries.

You should bear in mind that the maturity factor may as well depend on the chosen application. That means that a proven technology in one application may not be well-proven in another application.

e)      Categorization by application

Of course, one can look at certain application and assess the suitability of storage technologies, most importantly between mobile and stationary applications.

Energy storage - Applications

Thinking of storage technologies, you may think of the road system. It doesn’t only consist of long interstates nor small roads. It is the right mix that makes the road network so efficient. Large highways for long transport compared to narrow roads to reach remote areas and serve city centers. The same holds true for storage: large plants like pumped hydro compared to small residential energy storage systems. We need solutions for all applications ranging from very large to small systems. Each has its own advantages and drawbacks.

A thorough analysis reveals the best application profile for each technology. And business case calculations are just a part of this analysis.

Why they could help the transformation towards sustainable energy supply? Simply because storage is the means to balance supply and demand. With energy storage, one can combine different sectors e.g. coupling electrical sector with the high-temperature processing industry by means of thermal storage. Various research projects are currently in progress or under consideration.

Energy storage already plays an important role in the grid balancing markets for example in the UK providing frequency response services. With the increasing integration of intermittent renewable energies more the need for balancing will rise.

What else is important?

Asking people what is the most important factor many would say price. Of course it’s important but it must not be the only decisive factor. Just think of other parameters such as recycling, raw material availability, sustainable sourcing of raw materials and last but not least safety and risk. Some of them are more difficult to count in then cost. Yet, that’s not an excuse to do so.

What’s more, even the correct calculation of the cost associated with a energy storage investment is far from trivial. This is mainly due to the fact, that is usually a long-term investment and the performance of the system usually depends (for most battery systems it does for sure) on the way the asset is used (depth of discharge, cycle frequency, ambient temperature, humidity….)

Summary

Energy storage is readily available, already deployed and features a wide range of technologies from ultra-short capacity to seasonal storage, that is from ultra capacitors to compressed air storage solutions and even hydrogen. Energy storage provides flexibility and can be tailored to meet exactly the demand and specifications required by the application. From small companies to industrial demand, utilities, regulation and microgrid applications in mining or hotels just to name a few examples.

Yet most people think that Li-ion batteries and pumped hydro power plants are the only option. The truth is that they are currently dominating the market for battery storage but there are many innovative companies that aim at developing alternative storage technologies for applications to make our world more sustainable.

I deeply believe that we should look together how to make our world more sustainable in our own interest. And I also believe that energy storage, in one way or the other, will play an important role to reach this goal. 

#energystorage #renewableenergy #energytransition

Notes: Picture 1,2 by Ilja Pawel, Picture 3 taken from "Power to Gas: The Case for Hydrogen White Paper", California Hydrogen Business Council, 2015