It is fairly safe to assume that most of us recognise the urgent need for energy storage, particularly for harnessing renewables. This dictum is readily found on page one of the well-known handbook from the ‘Ministry for the Blindingly Obvious’ (available from all good government book shops in any country).
In principle it is a very simple concept. An exact parallel would be that of a water reservoir to enable the all-day and every-day access to water for all of our requirements. Imagine life if we only had water when it rained for example, we would never have moved very far from rivers, lakes or water holes.
Our choice of renewable energy sources— mostly wind and solar— are variable providers of energy. Many wind farms are actually partially turned off at non-peak times because the generated excess energy cannot be accommodated in the grid.
Harnessing and storing that surplus energy is at the heart of dealing with a variable power source like wind turbines or solar power. Irrespective of the technology used, energy storage essentially provides a reservoir of the accumulated off-peak energy, to enable the provision of ‘on tap’ power, when we need it.
This article compares electrochemical battery storage with other prevailing methods currently under investigation or in commercial use.
ESS requirements
The uses of energy storage systems (ESS) go beyond harnessing variable energy sources. The ability to smooth out peak demands, prevent power outages, facilitate arbitrage contracts etc. are just a few examples; Table 1 gives a summary of most of the ESS applications currently being adopted by countries around the globe. For the majority of uses, including commercial, grid, community and residential purposes there is one common element— to store energy when plentiful, and to release it when it’s needed.
That said, there are requirements of the various methods that are designed to maximise the functional and economic efficiency of the operation. The ability to store energy is not the only requirement, there are other parameters such as round-trip efficiency, capital cost, lifetime and speed of response that determine the effectiveness of an ESS.
Given below is a summary of the most important features of an ESS:
• Storage capacity (autonomy)
• Round trip efficiency from charge to discharge (RTE)
• Response time
• Power output
• Financial return on investment (ROI)
• Specific and volumetric energy density
• Capital cost
• Lifetime and cycle life
• Stand loss
• Carbon footprint of manufacture and maintenance
Fig 1 is a schematic showing how a general grid distribution system with ESS would function. In all cases there are limitations as to the type of storage that is appropriate for a region or application. Considering grid distribution networks, the fundamental requirements are the amount of energy to be stored (capacity), the efficiency of the discharge/recharge cycle or round-trip efficiency (RTE) and the return on investment (ROI).
It is important that the storage system is able to absorb sufficient energy to last through the peak demand periods when required. It also needs to ensure minimum energy loss in the round trip of charge and discharge of a particular system, not only for the economic benefit, but also to maximise the harnessing of energy from variable renewable sources.
The method of energy capture and the interface that transforms one form of energy into another, followed by the release of the energy all combine to give a characteristic RTE for a particular system. The less energy that is lost, i.e. the more efficient the capture and release of energy, the more that is available, and the lower the unit and lifetime cost of the installation. This, combined with the capital and maintenance expenditure on the system, determine its economic attractiveness and commercial viability.
Although we can all agree on the necessity of ESSs, there is often a diversity of opinion as to what form the energy storage should take for individual installations. There are several reasons for this:
• Capital cost
• Local conditions
• Geopolitical factors
• Commercial considerations
• Technology suitability, including safety and ecology
In order to shed some light as to what factors are important to consider, the various technologies are divided into two main groups— electrochemical and physical methods. The technical and economic performance of commercial and upcoming technologies in these categories, are compared to assess their suitability and effectiveness in the transition to renewable energy generators.
Technology overview
Because this is a huge topic, it will be covered in more than one article. The first article is a straight comparison between batteries as a single category, i.e. electrochemical, and the other physical methods of storing something other than electrons, to include mechanical and heat-based storage systems.
Physical ESS methods
Included in this category are:
• Hydroelectric dam
• Flywheels
• Geothermal
• Compressed air
• Pumped hydro
Hydroelectric dam
This is a fairly straightforward method and often doubles as a water reservoir as well as a source of stored mechanical energy. This can be simply defined as:
Em = U + K
Where:
Em = mechanical energy
U = potential energy (height) = mgh (mass x gravitational constant x height)
K = kinetic energy (rate of flow) = ½ mv2 (mass x velocity squared)
Units joules: 1 Joule = 1 kg·m2/s2.
1 Kw = 1000J/s
It is easy to see how a water dam would provide the potential energy from the height of the water behind the dam and also the kinetic energy when the water is released to drive the turbines generating the electricity. But what are the pros and cons?
Clearly, a river dam has the potential and kinetic energy already built in and, apart from the construction effort, will give low carbon free energy. Estimates of round-trip efficiency are difficult to find, chiefly due to the fact that dams are usually a barrier for rivers. The energy comes from the river flow and the height of the dam. The river flow can vary due to rainfall and, depending on the dam design, the water level often reflects that of the river.
Since the energy source is essentially free and has been created by natural forces, the conversion from potential energy to electrical energy is the best guide. This is a standard value for turbines and alternators and can peak at around 90%, but could drop to 60% depending on the variability of water flow.
The downside is that rivers and lakes are not always where you need them, nor might they be the right size and shape for your needs. The tallest dam in China, which is also the tallest dam in the world, is the Jinping-I Dam at 305m (1,001ft). The accolade for the largest reservoir goes to the Three Gorges Dam (main image), which stores 39.3 billion m3 of water and has a surface area of 1,045km2 (403 sq mi).
Looking at these statistics, there is a growing concern for their environmental impact due to the drastic changes that hydro-electric dams can bring to the local ecology.
One dystopian argument that is emerging is that the number of dams, coupled with their size in the northern hemisphere, is so great that they are having an effect on the rotation and axial tilt of the Earth. It is logical to acknowledge that raising the height of all that water and preventing river flows from completing their courses has to be felt somewhere.
The analogy would be an ice skater spinning freely, but when they put their arms out, they slow down. Dams are effectively raising huge masses of water to a higher point on the surface, mimicking the pushing out of the skater’s arms. However, it’s not yet time to join Elon Musk on the next rocket to Mars, as the best estimates (from respectable authorities), show that the rotational slowing is only fractions of a millisecond per day. We have nearly 100,000 years before one Earth Day becomes 25 hours.
Pumped hydro
This is an emerging technology that is gaining popularity as a credible, grid-scale energy storage approach. Unlike the straightforward river dam, it needs energy to pump water up to a higher reservoir in order to use the potential energy from pushing the mass of water to an increased height. These stations use electricity to pump the water from the lower reservoir to the upper reservoir. When demand is very high, the water flows out of the upper reservoir and activates the turbines to generate high-value electricity for peak hours. It is a mature and well-known technology.
The amount of water and the height of the reservoir required for a 6 MWh installation is given in Fig 3. It is clear from this that very high water-storage-tanks are required, capable of holding tens of thousands of cubic metres. This method is often found where there are hills containing easily accessible natural lakes.
The water pumping should be carried out by renewable energy generators such as wind turbines or solar arrays, which typically are not the most reliable sources of renewable energy. Unless there is a purpose built, high-level reservoir, then it will also have geographical restrictions. Finding suitable land masses with an appropriate height difference is not straightforward. There is also the question of the huge amounts of water needed to provide several hours of flow to generate megawatt hours of electricity. On the plus side, the response time is reasonable, and modern systems claim to have efficiencies of up to 85% (current standard is 65 – 80%).
The main use is in peak demand levelling and emergency backup power applications. Another benefit is that unless there is a leak, it does not lose charge, its cycle-life is near limitless (with some maintenance of course), and it provides energy at around 50 to 100 $/kWh. However, at an embarrassingly low energy density of 2–6kWh/m3 you do need some space as well as the right geographical location.
Flywheels
This is an up-and-coming technology revitalised by the use of modern engineering advances. The inefficiencies of the past related to the bearings of the wheel and the wheel materials. Current advances in magnetic and cryogenic bearings have virtually eliminated the frictional forces, whilst use of fibre composite materials have removed the problems of the higher rotational speeds, giving higher energy storage capacity. The energy is stored as rotational energy and can be expressed as:
Ef = ½ lω2
Where:
Ef = flywheel kinetic energy (Nm, Joule, ft lb)
I = moment of inertia (kg m2, lb ft2)
ω = angular velocity (rad/s)
From this it is clear that rotational speed (ω2) has a far greater impact on the stored energy than the mass of the flywheel. Fig 4 shows the principle of operation. Commercially available systems generally use magnetic bearings and fibre composite or steel flywheel. With these advances, claimed efficiencies are as high as 90%. Typically, flywheels are designed to provide power for up to 30 minutes and are principally used for load levelling, peak shaving or frequency control.
However, newer models with higher rotation speeds (NASA has a flywheel that rotates at twice the speed of sound), give much higher energy outputs. Reference to Table 2 shows that the energy density is not great but better than some electrochemical methods. The big advantages are the extremely long life (related to the ability to repair any malfunction and extend the calendar life indefinitely), whilst also being made of easily recycled components with no hazardous chemicals. On the downside the operational losses are quite high due to systems losing their ‘charge’ within a few days, if the angular momentum is not kept topped up.
Heat storage
There are many variations on this theme, but all basically use generated heat to raise the temperature of a medium, and then store that for use in heating when required. SES is the simplest storage method and can be as crude as a black water pipe running through a roof space in an apartment block to raise the temperature of the water inside the pipe. The amount of heat stored depends on the specific heat of the medium (usually water for domestic applications), the temperature change and the mass of the medium. This can be expressed as:
Qs = mcpdt
Where:
Qs = sensible heat capacity
m is the mass of the stored medium
cp is the specific heat capacity
dt is the temperature difference
Other variants utilise the latent heat generated by a material phase change (PCM) or the thermo-chemical heat of an exothermic reaction.
Compressed air energy storage
Very simply put, this technology relies on electricity from a variable or off-peak supply, to power a compressor in order to store air under high pressure. When the power is needed, the compressed air is released to power generators to provide or supplement an electricity supply during peak demand periods. Efficiency is quite low, although there are two forms: diabatic and adiabatic (Fig 5a and 5b), which differ in their utilisation of the waste heat generated during air compression.
The pros and cons are fairly straight forward. Compressed air occupies a high volume and is normally stored underground e.g. in convenient sealed salt mines or other suitable geological caverns. The available locations are therefore quite limited. Use of high-pressure containers to store the compressed air is also a feasible option, but adds to the initial cost of installation, which is relatively high. On the plus side, it has a very high energy storage capacity, and a long lifetime which can ameliorate the relatively low operational efficiency of less than 80% to give respectable ROI values.
Operating costs can be low and are reported to be around 50 to 100 $/kWh. The latest, and world’s largest CAES, is planned to be built by Hydrostar in California. This will have a storage capacity of 4GWh and an output of 500MW. It is planned to be an adiabatic system with an efficiency of around 60%. Existing installations include the McIntosh plant in Alabama, built in 1991 it stores 2.86 GWh with a 110MW output.
From basic thermodynamics you can guess that the adiabatic system reuses the heat stored from the compression phase to warm up the expanding compressed air to give the gas as much energy as possible to turn the turbines of the generator. The diabatic method needs additional heat from an external fuel source, making the process a lot less efficient, around 40% compared to 55-70% for an adiabatic construction.
Electrochemical methods
In this category there are many chemistries and designs for consideration. Because there is near-frenetic activity in developing improved battery chemistries, it is difficult to sift out the truly commercial from the wannabes currently touting for funding. In commercial terms, the current landscape can be summarised as current commercial chemistry, including the ‘newly commercialised’, and the “it’s been around so long that it should be commercialised” categories.
For this reason, the complex task of sifting through technical and commercially available information for state-of-play battery technology will be covered within the next couple of articles later this year. However, the situation is progressing at such a pace that we may very well see a changed picture before the end of this year. For the present comparison of batteries vs. mechanical, we can list the following as the prominent technologies and chemistries for inclusion into this article:
• Lithium-ion (Li-ion)
• Nickel Metal Hydride (NiMH)
• Lead-acid (PbA)
• Zinc (Zn)
• Redox flow batteries (RFB)
Currently several forms of the lithium-ion chemistry are dominating BESS installations worldwide. Its relatively high cycle life, decent energy efficiency, and the benefit of a low physical footprint, are the key factors for its popularity; although you could also add marketing offensive to the popularity reasons. The downsides are the initial cost, lack of recyclability, safety concerns and geopolitical resource pressures.
However, regarding recycling, a major mitigating factor is the ‘second life’ route for EV battery packs. Use of these 80%-capacity-cells drops the capital cost considerably and puts a better gloss on BESS purchases. On the other hand, BESS battery installations may end up with a lower cycle life, which negatively affects the ROI.
The other downsides to the use of this chemistry, apart from the initial capital outlay, are the safety measures employed due to the fire risk, the availability of increasingly scarce transition and alkali metals used in the cathodes, its dependency on transistors in the BMS and the initial carbon footprint of manufacture. The pros and cons of the different battery types will be covered in a separate article as promised earlier.
The BESS categories can be subdivided into three broad categories:
1. Reaction based, sometimes referred to as conversion chemistry, e.g. NiMH, PbA and some developing lithium types
2. Intercalation: the ion group of chemistries, li-ion, Na-ion etc
3. Flow: Zn-Br and other variations
Conversion or reaction-based
The general reaction for electrolyte based, electrode reaction batteries can be written as:
Ma + MbQ2 + 2AP ↔ MaP + MbP + 2AQ
In this case the electrolyte AP, can be an acid or alkali with A being either a metal or non-metal, and P a radical to form acids or alkalis. Ma and Mb are electrode materials and Q is a non-metal.
The most prolific and widely sold battery chemistry on the planet is lead-acid (PbA). This uses lead in both the tetra and di-valent forms to provide the electron imbalance to electrochemically store electricity in the cell. The cell discharge reaction is:
Anode
Pb → Pb2+
Pb + SO42- → PbSO4 + 2e–Cathode
PbO2 → Pb2+
PbO2 + SO42- + 4H+ + 2e– → PbSO4 + 2e– + 2H2O
Overall
Pb + PbO2 + 4H+ + 2SO42- → PbSO4 + 2H2O
The fully charged anode consists of lead (IV) oxide (PO2), and the cathode of sponge metallic lead (Pb). When the electrodes are connected through a resistance, the battery discharges and electrons flow in a direction opposite to that during charging. On discharge the sulphuric acid molecules break up into (2H+) and (SO42–) ions. At the same time, positively charged hydrogen ions move to the anode, where they receive one electron via the external circuit, to form a hydrogen atom. Since it is directly in contact with the anode of PbO2 it reacts and forms lead sulphate (PbSO4) and water. Negatively charged sulphate ions (SO42–) move to the cathode and give up two electrons, again via the external circuit connecting the two electrodes. The sulphate ion then becomes the SO42– radical and reacts with the metallic lead cathode to form lead sulphate.
Intercalation
In this electrochemical system, it is not the valence electrons taking part in a chemical reaction that are utilised in providing an electric current; it is instead, a straightforward transfer of electrons to and from a mobile charged ion that enables a current flow. These batteries have a simple principle of shuttling charged ions such as lithium, Li+ from one electrode to the other on charge and discharge. These ions are intercalated into the atomic and molecular interstices of a transition metal or metal oxide cathode, or to an anode receptacle (mostly carbon or graphite), via an electrolyte. The electrodes act as hosts for the shuttling ion and are chosen (partly) for their ability to provide a low resistance and capacious repository for the particles (Fig 7).
Typical overall reaction (li example) is:
LiMO2 + C ⇄ LixC + Li1-xMO2
The electrode half reactions tell the electron transfer story:
Positive electrode
LiMO2 ⇄ Li1-xMO2 + xLi+ + xe–
Negative electrode
C + xLi+ + xe– ⇄ LixC + Li1-xMO2
The most abundant example of a metal ion battery is the now familiar lithium-ion chemistry, used in many applications. For the purposes of this article, the lithium-ion standard technology, LiFePO4 is the most commonly used comparator to show the relative merits of battery energy storage against its non-electrochemical counterparts.
Flow batteries
A flow battery is a fully rechargeable electrical energy storage device where fluids containing the active materials are pumped through a cell, promoting reduction/oxidation on both sides of an ion-exchange membrane, resulting in an electrical potential. In a battery without bulk flow of the electrolyte, the electro-active material is stored internally in the electrodes. However, for flow batteries, the energy component is dissolved in the electrolyte itself. The electrolyte is stored in external tanks, usually one corresponding to the negative electrode and one to the positive electrode.
The chemical process can be generalised to the following half reactions during discharge:
Anode Compartment:
An+1 – e– → An
Cathode Compartment:
Cn+1 + e– → Cn
The main advantages of flow batteries are that they can be very cheap, have a very long cycle life and are safe. The main disadvantages are the low energy and power densities. These aspects, however, are less important for ESS applications. Examples are vanadium flow and zinc bromine. Vanadium flow batteries are becoming more prevalent in large scale energy storage installations.
Metal air rechargeable
This category of battery, if successfully developed, has very desirable properties. The use of air as a cathode, instead of another material such as a metal or carbon, means that it is almost half the weight of a conventional reaction or ion type of cell. The general principle is shown in Fig 9.
General metal air reactions:
Anode
M ⇄ Mn+ + ne–
Cathode
O2 + 2H2O + 4e– ⇄ 4OH–
The absence of a second electrode can give mouth-watering specific energies of thousands of Wh/Kg and at a far lower cost than its binary electrode counterpart. The downsides are mostly related to inability to electrically recharge, and excessive corrosion of the metal anode. To appreciate the difficulty level of solving these problems, think how long aluminium air has been “near commercial” status.
Comparisons
Table 1 gives a summary of the different uses for energy storage and their requirements for an ESS. There are several points that emerge from the comparison; they are centred around the size of the installations in terms of the volumetric energy density, initial cost, the flexibility to increase the capacity, and the suitability for different applications. We need to compare each of the ESSs included in the table against the ESS requirements to identify the most suitable for the different applications already mentioned.
These tables clearly show that there are differing requirements for the various energy markets, and that there are also very different properties of the storage methods that can align with the requirements. For example, the applications of energy-shifting arbitrage along with supplemental reserve, require response times in minutes or hours. This compares with voltage stability and UPS applications where millisecond responses are mandatory requirements.
Looking at Table 3, it is evident that batteries are well positioned to supply the larger output requirements across the whole table as well as the response time needed for any application. The largest installation in the UK is the Minety battery located in Wiltshire, currently at 100MW output but earmarked to expand as demand grows. Alongside this, Vistra Corp’s Moss Landing Energy Storage Facility in California was the worlds largest lithium-ion BESS in 2021. The installation produces 400MW of power and 1.4 GWh of energy, supplied by LG-manufactured T1300 lithium-ion packs. According to Vistra Corp, the site could support a 1,500MW/6GWh energy storage facility.
The parameters of response time, capacity and power delivery are powerful vectors when deciding the technology adoption for a particular ESS installation. However, capital cost, ROI and round-trip-efficiency refine the application-suitability of these technologies. These additional properties can seriously change the positions of the pieces on the decision-making chessboard.
Table 4 is an estimate of some of the leading ESS contender’s credentials to consider when deciding on the most appropriate technology for an installation. In this table, the cost of energy and power, the energy densities (if weight or volume are considerations) and the expected life are important considerations. Other factors of local geography such as the proximity of a river for a dam, nearby cliffs for pumped hydro or deep holes for compressed air systems will all play a role.
Table 4: Commercial properties of a variety of ESS methods. Energy storage technology comparisonOn looking at all the data provided, it does appear that the electrochemical option provides the fastest response time, best round-trip-efficiencies and highest power outputs of all the systems considered here. These factors have a significant influence on the decision-making process for the technology adoption by energy providers or arbitrage operators. It would appear that the battery performance characteristics ensure that this group of ESSs are the most versatile of all the systems.
In particular, the ROI is boosted by the high round-trip-efficiency of up to 99%, and the millisecond response time, coupled with high power output, cannot be matched by any other technology. Other factors such as the portability of the equipment (mostly containerised), the lack of expensive maintenance, the flexibility of transport to, and storage in, many areas and terrains, give the BESS technology a very high flexibility. Table 5 (below) is a chart showing a list of applications and the suitability of the various storage technologies to meet their requirements. The green dots representing the electrochemical battery systems make up almost 50% of the suitability matrix. This compares with under 20% for the rest.
As noted earlier, there are many different types of electrochemical systems already commercialised, and many more are near commercial or in the development stage. A detailed comparison of these existing and emerging technologies will be made in the upcoming editions of BEST. The importance of materials sustainability and supporting a circular economy with full recycling are other key factors that will be included, along with cost and performance considerations. However, for the purposes of this article, it seems quite clear that BESS solutions are the most likely option to provide a fast and low-cost solution to providing energy storage to manage our transition from fossil fuel dependency to cleaner sustainable power generation.
