Overview of different Battery Storage Technologies

The renewable energy industry is constantly evolving. One of the latest and greatest trends in the renewable energy industry is toward decentralization. Decentralization is the push for energy independence; it is the idea that it is beneficial, and often the least cost solution, to have distributed generation close to where the energy is consumed rather than over reliance on relatively few massive power generation plants and vast transmission lines across a country or region.

Solar PV has been the leading technology in this trend towards decentralization for many reasons. The cost of solar electricity has fallen dramatically over the last 15 years. Solar PV has a relatively fast construction time; it can be setup in days or weeks or months instead of several years as required by hydro or thermal plants. Solar PV is also extremely flexible in terms of suitable locations; most land area on earth has a suitable climate for solar pv to be effective, as long as there are no shadows or obstructions. This means solar pv can be put almost anywhere and so it is very versatile, especially when compared to wind, hydro or geothermal which each require a specific set of geographic conditions.

However the main obstacle for solar energy as the primary driver of energy independence is the fact that solar energy is intermittent; it cannot generate energy at night time. The use of solar energy for decentralization and energy independence relies upon the intelligent and strategic use of energy storage.

Especially in recent years, energy storage has been a sector with lots of innovation and investment. There are many energy storage technologies and manufacturers competing to gain traction in this exciting and still emerging market.

But choosing the right energy storage technology for your solar energy projects can be confusing. Which Energy storage technologies are reliable and which are not? Which energy storage technologies are financially viable and which are not?

This article is too short to answer those questions – batteries are a complex topic that could fill hundreds of pages. This article is not meant to be exhaustive, but it can at least give you some good information to assist in your decision making and lay the foundation for your further research into which energy storage technologies are a good fit for your needs. This article is meant to help you enter this discussion with confidence and with a solid foundation of knowledge how to assess battery storage technologies and to go further in your own research and experience.

We will look at

·??????Mechanical energy storage such as Pumped storage Hydro, Flywheels, Gravity Batteries and Compressed Air Energy Storage.

·??????Thermal storage such as molten salt commonly used in Concentrated Solar Power (CSP).

·??????Electrical means of storage such as supercapacitors.

·??????Chemical Storage such as Hydrogen

·??????Electro Chemical such as Lead Acid, Lithium Ion, Redox Flow batteries and others.

And we will also look at several of the main electro chemical battery chemistries and give the advantages and disadvantages of each one. At the end of this article you should have a broad understanding of different storage types and their best applications, you should have a good idea which battery types and chemistries might be right for you and your projects, and you should be able to do your own additional research to further your knowledge within the market context.

Definitions.

In order to discuss and evaluate battery storage technologies we should first define some terms that are relevant and specific to storage discussions.

The first term is CYCLE. When discussing batteries, a cycle means one process of charging the battery and discharging the battery. This charge and discharge is one cycle. The life of a battery is often expressed as?the number of charge and discharge cycles that can completed before the battery loses significant performance.

Another important concept is SPECIFIC ENERGY, also known as ENERGY DENSITY. This is a measurement of the amount of energy that can be stored in a certain mass or volume. When we speak of energy we are talking about kwh. So in relation to a battery, a high SPECIFIC ENERGY or a high ENERGY DENSITY means that the battery can store a lot of energy (kwh) in a relatively small mass or volume. If a battery has a poor SPECIFIC ENERGY or ENERGY DENSITY that means in order to store a lot of energy (kwh) you would need a very large space for the batteries. For example, Lithium ion batteries store 150–250 watt-hours per kilogram (kg) and this is 1.5–2 times more energy than Sodium batteries, 2-3 times more than redox flow batteries, and about 5 times more than lead storage batteries. This means that for the same amount of energy, the Lithium Ion batteries are much smaller than the sodium batteries or redox flow batteries.

Related to this concept, is SPECIFIC POWER, or POWER DENSITY. This is similar to specific energy, except that instead of energy (kwh) we are now talking about power or current (A). So if a battery has a high SPECIFIC POWER or POWER DENSITY this means that the battery can provide a high current from a small mass or volume. And if a battery has a poor SPECIFIC POWER or POWER DENSITY that means in order to provide a lot of power (high current) you will need a very large space for the battery.

And another important concept is DEPTH OF DISCHARGE. Depth of Discharge (DOD) is expressed as a percentage of the total capacity of the battery. In many battery technologies it is not advisable to discharge a battery completely, because this would dramatically shorten the useful life of the battery. The DEPTH OF DISCHARGE is the recommended level to discharge a battery and retain the useful life for as long as possible.

And ROUNDTRIP EFFICIENCY or charge/discharge efficiency is a performance scale that can be used to assess battery efficiency. It tells you how much of the energy you put into the battery will be available for use later on. Lithium ion batteries have the highest charge and discharge efficiency, at 95%, while lead storage batteries are at about 60%–70%, and redox flow batteries, at about 70%–75% and pumped storage hydro has approximately 80-90% roundtrip efficiency.

?Now we that we have defined these terms lets look at the broad range of energy storage technologies available. Hopefully this section will broaden your mind as to what a battery is and the different ways that energy can actually be stored.

Lets start with the oldest and most common batteries in the world: Mechanical Batteries.

?MECHANICAL

Mechanical batteries store energy without the use of chemical or electrical components. Common types of mechanical batteries store energy with gravity, or inertia or springs. These are the oldest types of energy storage in the world. Some common examples are springs in wind up clocks, or a pottery wheel that is spun by foot and then retains that energy in the form of inertia as the spool continues to spin.

But on a much larger scale and more relevant to our discussions is other types of mechanical storage. For example, pumped storage hydro power and gravity batteries.

Pumped Storage hydro power uses the same principles and construction as traditional hydropower based on the use of dams and reservoirs (not run of river hydro). With an upper reservoir of water at a relatively high elevation held in place by a dam, and mechanical turbines installed, a dam operator is able to control the flow of water out of the reservoir and the weight of the released water spins the mechanical turbines and generates electricity. Therefore, the water that is in the upper reservoir behind the dam represents stored energy because it can be released at will (dispatched) to generate electricity. The turbines are generally bi-directional, which means when water is released the turbines spin and generate electricity, but if we supply electricity to the turbine it will spin the opposite direction and pump water up into the upper reservoir. If we pump water into the upper reservoir, we are essentially recharging this massive battery. The more water we have behind the dam in the upper reservoir, the more energy we have stored and available to us. A pumped storage hydro plan typically has a lower reservoir as well as an upper reservoir so that the released water can be captured and pumped back up into the upper reservoir. In some cases the lower reservoir can be an existing body of water.

Pumped Storage hydro is the most common battery in the world in terms of installed capacity. It is also the oldest and most mature storage technology in common use, being over 100 years old. A majority of the projects operational today originate from the 1970s and 1980s and the concept originated long before that time.

You might be asking why we would want to use energy to pump water to up into an elevated reservoir only to release it and consume energy later? The answer is because you can pair the benefits and strengths of hydro power with the benefits and strengths of solar power and thereby get a very efficient and very effective power system. We already mentioned that solar energy is cheap and easily deployable in a wide range of locations, but that the problem with solar is the intermittent nature of sunlight. This problem can be solved by pairing solar PV with dammed hydro plants, this gives us the best of both technologies. The solar energy can provide the cheapest electricity available to us on the planet, so during the daytime when the sunlight is abundant and the energy is cheap we can pump water into the upper reservoir. And then at peak energy consumption times and at night when the solar energy is not available we can release the water and thereby release the stored solar energy. And with a round trip efficiency of between 80-90% this can be a very effective solution. This means that approximately 80-90% of the energy required can be recovered when the water is released back into the lower reservoir.

Pumped Storage hydro provides a great LCOE and all of the ancillary services you could ask for from a grid tied storage projects.

The main problem of pumped storage hydro is that it is mostly suitable for very big projects in the hundreds of megawatt range. These projects typically have a high construction cost, a long construction time, and long preparation and development time. They also have a considerable environmental impact that must be evaluated.

?Another type of mechanical energy is a flywheel. This is simply a wheel with a mass that spins around an axis, and the inertia of the wheel spinning represents stored energy that can be recovered to produce electricity. This is very similar to a potters wheel that continues to spin by inertia.

Flywheels are generally used to store energy for very short periods of time. The most common applications are in most rotating engines and machines for very short-term energy storage, for example to smooth the torque pulses in internal combustion engines.

Another type of mechanical battery is a gravity battery. This simply uses energy to lift weights to a high elevation, and then when electricity is needed, the weights can be released the weights slowly back down to earth. Similar to the pumped storage hydro, this can make sense when there is an abundant cheap resource like the sun. Solar energy is often produced in excess during peak hours, and that energy can be used to lift weights to store energy which can be recovered by releasing the weights back to the ground, thereby generating electricity when the solar resource is not abundant, i.e. at night.

Another type of mechanical battery is Compressed Air Energy Storage (CAES). This typically uses underground caves or abandoned mine shafts, and high powered pumps push air into the caverns and create high pressure. When electricity is needed the pressure from the air can be released to spin turbines and create energy.

These types of mechanical energy are usually only practical at very large scales or very specific applications. They often require many years of development and high construction costs. And also they often require some unique geographical locations. These are not a great solution to residential and small scale projects.

Now lets look at thermal energy storage.

THERMAL

Thermal batteries store energy in the form of heat, and the heat can be used to create steam which spins a turbine to create electricity.

One significant form of thermal battery and perhaps the most relevant to our discussion is a Molten salt battery. Sodium–Sulfur (Na–S) Battery?liquid metal, or molten salt battery is a type of molten metal battery constructed from sodium and sulfur. It exhibits a high energy density, high efficiency of charge and discharge (89%–92%), and a long cycle life, and is fabricated from inexpensive materials. However, because of its high operating temperatures of 300°C–350°C and the highly corrosive nature of sodium polysulfides, there must be construction of complicated and dangerous heat transfer systems and fluids and pipes.

These batteries are commonly used in Concentrated Solar Power (CSP) plants. CSP plants normally build a high tower, and at the top of the tower is a kind of heat transfer fluid that is often molten salt. Around this tower would be installed hundred of solar reflectors, and these reflectors would be precisely calibrated to reflect the sunlight up to the heat transfer fluid at the top of this central tower. In this way the molten salt becomes very hot (500 to 1000 degrees Celsius) and through a complicated piping system this ultra hot molten salt can be used to generate steam and electricity. The molten salt can retain its heat for a long time and is able to provide steam power and electricity all through the night, therefore the daily solar energy is stored in the thermal salt to be used over night.

Now lets look at electrical forms of energy storage.

ELECTRICAL

Electrical forms of energy storage do not use any chemical reactions to store energy. Instead, the energy is stored in the electromagnetic field around the battery. If you are an electrical engineer then you are familiar with capacitors, which fit this definition. For our discussion we will be looking at Super Capacitors, which work on basically the same principle but on a much bigger scale

Supercapacitors don’t rely on a chemical play to function. Instead, they store potential energy electrostatically within them. Supercapacitors use dielectric or insulator between their plates to separate the collection of positive and negative charges building on each side’s plates. It is this separation that allows the device to store energy and quickly release it.

It basically captures static electricity for future use.

The most significant advantage of this type of storage is that it has an extremely long useful life, perhaps as much as 1 million cycles! A 3V capacitor now will still be a 3V capacitor in 15-20 years. In contrast a battery may lose voltage capacity over time and repeated usage.

Also it can charge and discharge in a fraction of the time other batteries require. Supercapacitors are best suited for very small bursts of power.

A great example of creative use of supercapacitors can be seen in Switzerland, where there is a fleet of buses running on supercapacitors. The buses are exposed to charging stations at various stops along their daily commutation route. Just 15 seconds can provide a quick refill of the stored energy, and only a few minutes would suffice for a full charge. And because Supercapacitors draw a lower current for a few minutes at a time, this puts less stress on the grid?

However, supercapacitors have a very low specific energy as compared to batteries. This means the space required by supercapacitors is relatively high. And also supercapacitors have a relatively high up front cost, which can possibly be recovered over the very long lifetime of the battery, but is still an obstacle in many cases

CHEMICAL

Chemical forms of energy storage don’t rely on electrical input, they rely entirely on chemical reactions for charging and discharging. A good example of this is Hydrogen, which is also a very new and interesting source of power that you may have heard of recently. Hydrogen is a gas, and is in fact the most abundant element in the universe. When you expose hydrogen atoms to oxygen atoms and apply the right conditions, the hydrogen and oxygen will form a bond and become water (H2O) and when this happens energy is released. This is the basis of hydrogen energy storage—that if we can capture and store pure hydrogen we have the potential to expose the hydrogen to oxygen and utilize the released energy.

There is also the possibility of separating the Hydrogen and Oxygen from water molecules by a process called electrolysis, and so we can expend energy to separate the hydrogen and oxygen and then we can store the hydrogen and we are essentially storing a source of power because of the possibility of exposing the hydrogen to oxygen to create water and releasing the stored energy.

Hydrogen fuel is a topic of great discussion these days as a fuel to help decarbonize away from fossil fuels. There is rapid development of portable hydrogen fuel engines for use in residences, small business, vehicles and large grid scale projects as well.

The main disadvantages of hydrogen are that it requires a mature and expensive supply chain to source the gas and capture it and store it and transport it to the generator. Also there are environmental concerns with collecting the gas and consuming the fuel as well.

ELECTRO CHEMICAL

Now lets look at electro chemical batteries. This is the type of battery that most people are commonly referring to when they use the word “battery”. An electrochemical battery can create electrical power by means of chemical reactions within the battery, and it can also make use of electrical power supplied into the battery to cause a chemical reaction that will allow energy to be stored and released at a later time.

This category includes Lead Acid batteries, Lithium Ion batteries, Redox Flow batteries, and others.?

Each of the types of batteries we have discussed have strengths and weaknesses. Each of them is suited to a particular range of applications, each technology is suited for a different duration of storage, a different scale, and different geographic and size constraints.

Below is an image that shows how different batteries compare in terms of suitable project size and duration of storage.?

You can see that pumped storage hydro, for example is suitable for utility scales above 100MW, and can store energy from minutes up to seasons.

You can see supercapacitors can store energy from 1kw to around 10MW, but the duration of storage is much less, mostly suitable for seconds and minutes (although some manufacturers have made developments to allow them to store energy for days.)

And you can see that Electrochemical batteries can operate from very small sizes below 1kw up to 100s of MW, and can store energy from seconds and minutes up to several days.

This is why for solar projects typically electrochemical batteries are used. In fact for residential and C&I solar projects, electro chemical batteries are used almost exclusively, and they are also used very often in utility scale storage as well.

Within the designation of Electro Chemical Batteries are several different battery chemistries that have gained market share. These include Lead Acid, Lithium Ion, Nickel-Cadmium, Redox Flow and others. Here is another graphic similar to the previous one that shows some of the different electro chemical batteries construction and how they compare in regards to power ratings and discharge time.

This shows that Lead Acid batteries (including advanced lead acid batteries), and Lithium Ion Batteries have the most versatility in terms of size and duration of storage. It should be no surprise that these are the two most common battery chemistries on the market for solar storage projects.

The global rechargeable battery market is growing, from an estimated $90 billion in 2020 to an estimated $150 billion in 2030.

And of this global battery market, electro chemical batteries are the vast majority

As you can see below, Lead Acid batteries have been the leader in this segment, and it is expected to remain the leader, although Lithium Ion is a relatively new technology and is also growing rapidly.

The main reason for the continued growth of electro chemical batteries is the increased use distributed storage for residential, commercial and grid scale, and also due to the use of batteries in electric vehicles. Both lead acid and lithium ion are expected to be used in electric vehicles in the next decade. Lithium Ion batteries generally perform better and have a greater energy density than lead acid batteries, but lead acid batteries are cheaper and the manufacturing processes and supply chains are more mature and developed to allow for more widespread access.

LEAD ACID BATTERIES

Lead acid batteries have been around for a very long time and are in almost every corner of the globe. Car batteries have traditionally been lead acid batteries. Here you can see the construction of a typical lead acid battery.

There is an outer casing and a positive terminal and a negative terminal. These batteries can be connected in series and parallel to give the desired voltage and current, which means a typical battery back for a residential storage system could be 8 or more batteries, and this can take up a significant space in a residence.

The strengths of lead acid batteries and the reasons it is the leading type of electrochemical battery is mainly due to price and availability. These batteries are relatively cheap and have been around for a long time, with the first lead acid batteries invented in the 1800s..

In the 1970s the first “maintenance free” lead acid batteries were used. These are sometimes called “Sealed” or VRLA (valve regulated lead acid batteries).

Because of the maturity of the technology and the manufacture process and the supply chains these batteries are usually easily available even in very remote areas. The are capable of providing high load current, and they have a good specific power, however their energy density is low and they can occupy a large space for medium sized storage projects.

The weaknesses of this battery is the performance and life span. They typically have a Depth of Discharge around 50% which means you need to oversize the battery bank in order to get good performance. And even with this DoD, the life span of around 1000 cycles in perfect conditions. So if we assume one cycle of charging and discharging per day, 1000 cycles means approximately 2.7 years of life.

NICKEL BASED BATTERIES

Nickel–Cadmium (Ni–Cd) Battery A nickel-cadmium battery (Ni-Cd) is a rechargeable battery used for portable computers, drills, camcorders, and other small battery-operated devices requiring an even power discharge. Nickel–Metal Hydride (Ni–MH) ?The Ni–MH battery combines the proven positive electrode chemistry of the sealed Ni–Cd battery with the energy storage features of metal alloys. Ni-MH batteries currently finds widespread application in high-end portable electronic products, where battery performance parameters, notably run time, are major considerations in the purchase decision.

Some analysts think High-nickel battery chemistries will dominate the passenger-EV market over the next 10 years, but others think that Lithium-iron phosphate will remain in high demand.

Nickel Cadmium is impacted by the fact that Cadmium is a toxic metal which cannot be disposed of in landfills. Also this battery does not hold storage well for ling periods of time; the stored energy will leak in what is called a high self-discharge rate. Also these batteries have a low cell voltage, which means many cells are required to achieve a high voltage.

Nickel metal hydride (NIMH) batteries typically characterized by higher energy density, superior capacity, and longer run time between charges. NIMH batteries accept a high depth of discharge rate, longer life span, and wide operating temperature of -50oC to +85oCNi–MH has a?higher specific energy with fewer toxic metals, less effect on memory and generates high peak power. It also has good deep discharge and is environmentally friendly..

NIMH batteries are easy to transport. Leading vendors such as Panasonic offer NiMH batteries without air restrictions or regulations.

However, NiMH is more expensive, has higher self-discharge and has lower efficiency than the lead–acid and Nickel-Cadmium batteries.

REDOX FLOW BATTERIES

A Redox Flow Battery (RFB) is an electrochemical energy storage device that converts chemical energy into electrical energy through reversible oxidation and reduction of working fluids. Redox Flow Battery (RFB) are charged and discharged by means of the oxidation–reduction reaction of ions of vanadium or the like. Redox Flow batteries have a very long life time of up to 20 years without maintenance and unlimited charge cycles with almost no degradation of electrodes and electrolytes, Redox Flow batteries are also very safe due to their being free of combustible materials, and availability of operation under normal temperatures.

Another unique advantage of the Redox Flow batteries is that the energy capacity and power capacity can be designed independently. This means that you can get determine the current that you need from the battery without increasing the size of the battery, resulting in a high power density.

However Redox flow batteries have a relatively low roundtrip efficiency at around 60%. They also have a low power density resulting in very large batteries for medium and large projects. The price of Redox Flow batteries is typically less that Lithium Ion but more than Lead Acid. However their complexity makes the installation and accessories overall quite costly. Currently the best potential application is on large grid scale projects where safety is of utmost important, not for residential or medium sized projects or normal grid projects,

LITHIUM ION

Lithium Ion are the batteries most often in our consumer electronic devices such as our cell phones, tablets and laptops. Now Lithium Ion batteries are also the preferred energy storage technology for residential, C&I and even utility scale projects.

According to a Wood Mackenzie Battery Report 2021: The US’ installations of advanced energy storage — almost entirely lithium-ion battery systems — went beyond the 1GW mark in 2020, while in capacity terms the figure was close to 3.5GWh.

Also the high end Electric Vehicles like Tesla and others most often use Lithium Ion batteries in their vehicles. This technology has several advantages over the traditional lead acid batteries.

The first advantage over lead acid is the life span. Lithium Ion batteries can commonly have a life span of 5000 cycles or more. This means the design life can be 10 to 12 years, more than three times longer than most lead acid batteries.

Another advantage is the Depth of Discharge, Lithium Ion batteries commonly have a DoD of 80% up to even 100%. This means you will be able to use (almost) all of the energy stored without negatively affecting the life of the battery.

Lithium Ion battery also has a better Energy Density than Lead Acid batteries, which means you can store more energy in a smaller mass than with Lead Acid batteries.

Below you can see the internal construction of a Lithium Ion battery, and also some pictures of different Lithium Ion Battery products.

The main drawback of Lithium Ion batteries is the upfront cost. This is the main reason that Lead Acid is currently and is expected to remain in much greater demand than Lithium Ion batteries.

The cost of Lithium Ion batteries has fallen dramatically over the last few years as shown below.

This is mainly due to massive investments into the manufacturing process by electric vehicle companies.

Lithium-ion battery costs are falling the fastest due to the increased investment in the supply chain and manufacturing process due to the demand from electric vehicles. Rapid scaling of new giga-factories is leading to falling battery costs at about 18% each year.?

However the raw materials required to make lithium ion batteries are not found everywhere, and the price of these raw materials is quite volatile and subject to geo political influences that can affect the prices. The graph below shows that recently the price of lithium ion batteries has been going up, and this will likely continue for the short term until raw materials can be sourced more efficiently. This has also affected the manufacture lead time on lithium ion batteries which again affects their availability and their price.

Another drawback of Lithium Ion batteries is the threat of thermal runway which can cause fires and explosions. When a lithium ion battery catches fire, there is almost nothing that can be done to extinguish the fire until it burns itself out. Firefighters typically have to just draw a perimeter and leave the lithium ion battery to burn for a few days. This is obviously a big fire hazard and has occasionally caused great damage to homes. There was also a massive high profile fire in Australia at a 300Mwh lithium ion battery in 2021. Lithium Ion batteries are also subject to transportation regulations and challenges to ship these batteries via air freight. Despite this, however, lithium ion batteries are considered quite safe.

And finally lets look at a new type of Lead Acid battery that is quickly filling the gap between the traditional lead acid and lithium ion batteries. This can be considered an Advanced Lead Acid Battery.

This new type of Lead Acid battery is called Lead Carbon. This is because the construction has added a plate of carbon to the negative side of the battery as shown below.

The result is two fold: one is that the charging speed of this battery is improved over the traditional lead acid battery, thus incorporating a small part of the advantages of a supercapacitor.

The second is that the life span of the battery is greatly extended from around 1000 cycles to more than 3000 cycles, thus approaching the life span similar to the lithium ion batteries.

This is partially due to significantly reduced sulfation in the batteries due to the changes in internal construction. Sulfation is the leading cause of failure in a typical lead acid battery.

?

In conclusion, there are far too many battery technologies to discuss all of them, and too much complexity to look at them all in depth. Lead Acid and Lithium Ion are the two battery chemistries that are dominating the solar plus storage market. Pumped storage hydro has some great benefits, but the high cost and large size and long construction times are difficult to overcome. And other batteries have some specific advantages and disadvantages that may make them ideal for certain specific applications.

?

I hope that this article has served as an introduction to the concept and application of energy storage in the renewable energy industry. I attempted to give you relevant and current information about the markets and the advantages and disadvantages of several technologies, but also to try to give a broad context so that you can assimilate this information easily and it can be useful to you in your decision making rather than only providing confusing facts and figures and jargon.

If nothing else I hope that at least you are more prepared to dive further into these researching technologies and products with a solid foundation gained from this article.

?

In my opinion, energy storage is the next frontier of the solar energy industry, and great leaps have already been made, and I believe we will witness the massive growth of storage in residential, C&I, and utility scale projects driven largely by the progress that has been made and continues to be made in the battery technology