Liquid Air Energy Storage (LAES) is a form of storing excess energy just as CAES (Compressed Air Energy Storage) or other battery storage systems. The system is based on separating carbon dioxide and water vapour from the air to produce a higher concentration of nitrogen. This nitrogen can then be liquefied for storage and expanded back to a gas when we need to make electricity. Liquid nitrogen is a commonly used substance in current industrial processes and can be stored and transported in large volumes at atmospheric pressure. This is different to the CAES process, which requires high-pressure storage chambers. Liquid nitrogen is also a good substance because it has a high expansion rate. In fact, it expands 700 times in volume when turning back into a gas from a liquid.
The LAES form of storage works well with intermittent power sources like wind and solar power, because the additional electricity created can be used to help filter the air into nitrogen, and then turn it into a liquid form. The electricity is recreated when the liquid is expanded to turn into its gas form.
How does the Liquid Air Storage process actually work?
The process depends on using liquefied air or liquid nitrogen (78% of air), which can be stored in large volumes at atmospheric pressure. The air is taken through an inlet and then into a compressor. On entering the compressor air is made up of a percentage of oxygen, water vapour, carbon dioxide and mostly nitrogen. When too much electricity is generated, the excess electricity is used to power the compressor and the chilling unit. Here the nitrogen component is separated and further cooled so it forms into a liquid, at precisely -196°C. The liquid nitrogen is then stored in a chilled vacuum until it is needed to drive power recovery. If there is a peak in demand or if the grid is struggling to meet demand, the liquid nitrogen can help with this. Then first step is to transfer the liquid into an ambient temperature chamber, usually into a heat exchanger. Since liquid air boils at -196°C, any small change to ambient temperature will superheat it. This creates expansion as the liquid once again becomes a gas. The expanding steam then drives a turbine (cryogenic engine), which drives the standard electric generator. Upon creating the electricity, it can then be reconnected and integrated back into the grid.
Can Liquid Air Storage energy be a commercially viable solution?
According to the Institute for Mechanical Engineers (IMechE), the process is anywhere between 25% and 70% efficient. This means that up to 70% of the electricity can be recreated from the cryogenic air compression process explained above. The process becomes more efficient if the cryogenic chambers, air vacuums and generator are located near a factory or an existing power station. This is because excess heat is usually vented, but it can be used efficiently in the LAES process. However despite a possible 70% efficiency, it is worth noting that batteries have an efficiency of 80%. Therefore, to be commercially viable, a cryogenic LAES plant needs to be able to achieve even more efficiency. Ideally the cryogenic LAES plant should also be located close to an intermittent renewable source of energy such as solar PV, wind power or marine energy, so that it can minimise the electricity and heat loss during the transmission process.
How can Liquid Air Energy Storage combat volatilities in electricity demand?
The main challenge we all have is how to deal with erratic demand for electricity and somewhat intermittent supply from non-traditional sources like wind, solar and marine energy. One of the answers could be to use a solution like LEAS to store energy when it is plentiful and then use the storage to convert it back to electricity to meet demand. Therefore current power grids need to be upgraded so they are able to work smarter in order that energy can be provided when it is needed. Many argue that wind, solar and marine energy generation are tied to local environmental conditions, and there is no point transporting this energy around the country via the grid because much of it is lost through the transformation process. Therefore having LAES solutions sitting alongside more responsive local grids is something that should also be considered rather than just applying storage to an already inefficient national grid.
How does Liquid Air Storage compare to Compressed Air Energy Storage?
The energy density of cryogenically frozen fluids, such as liquid nitrogen compares very well against current alternatives such as compressed air, mainly because it can be used and transported under atmospheric pressure. Compressed air needs to be stored in special pressurised tanks, which are liable to leaks. Also compressed air requires special storage space underground, like mining shafts, which are limited in availability.
Liquid Air Energy Storage and the UK
Cooling air using a cryogenic process to create liquid nitrogen is not new, but the current efficiency levels are low because of the energy required to produce the storage fuel, and then again to create electricity. Highview Power in Buckinghamshire operate a heat recover and energy storage system with this process at 25% efficiency. They could increase this efficiency to 70% if they were located next to a conventional power station where excess heat could be recycled. The DECC is currently looking at additional measures the boost research and development in this area, and it has recently announced that it is soon to launch a scheme to get more companies involved. This will be in addition to the grants available through OFGEM.
What is chemical energy storage?
An example of chemical energy storage is the common battery. By using the liquid inside it to store electricity it can then release it as required. Large batteries can act as chemical energy storage for industry and could make future energy generation solutions more efficient and profitable. This will be achieved by storing energy generated when demand on the grid is low and releasing this as required to help meet peak demand.
How does the battery chemical energy storage process actually work?
Batteries are portable devices that can be used in many different areas. The way a battery works is very simple and based on three components:
The anode (the negative pole),
The cathode (the positive pole),
The electrolyte (the liquid chemical that produces the flow of energy).
The anode and the cathode are also known as ‘terminals’ and are made of a metal, which are then separated by the electrolyte.
Converting stored energy to electricity
If you take a device like a light bulb or a simple electrical circuit and connect it to the battery terminals, the chemical on the anode causes a release of electrons to the negative pole and ions in the electrolyte. This is the chemical oxidation reaction.
On the positive pole the cathode accepts the flow of electrons, which completes the circuit for the flow of electrons. These two reactions happen simultaneously: the ions transport current through the electrolyte while the electrons flow in the external circuit. This then generates the electric current.
Storing electrical energy in a chemical store
The process for battery energy storage works in reverse, transforming electrical energy into chemical energy. When excess electricity is produced in the grid, it can be channelled into a battery system, and then be stored in the chemical system.
The mobile phone and electric car both take advantage of a rechargeable battery system. The hope is that in the future this process could be ‘up-scaled’: when electricity is produced by intermittent renewable sources like wind or a solar PV it can be stored in big industrial sized battery systems.
What types of batteries can be used for mass energy storage?
There are a number of different battery solutions that are currently being used in industry and under consideration for mass scale national grid use. This section briefly considers each type for these ambitious future requirements.
Lithium-ion batteries are the fastest growing battery type in the consumer market today. They have many uses, including powering laptops, mobile phones and hybrid vehicles due to the high amount of energy they can store. They also have high energy-efficiency, operate well under a wide range of temperatures, can be recycled and also have a low level of self-discharge.
However to be used as a grid storage solution this type of battery will require some refinement. They will need to operate with improved lifespan (number of charging and discharging cycles that can be achieved) and improved safety. Most importantly the cost to produce the lithium-ion batteries needs to come down – a storage solution these days needs to be cost effective.
Lithium-ion polymer batteries
Like the lithium-ion battery, the lithium-ion polymer batteries not only have a high-energy output, but also have a good safety record and a longer life span. However these are also uneconomical to produce, so the production costs would need to come down to make these viable as the mass-produced storage solution.
Lead-acid batteries can be designed to power large applications and are relatively cheap, safe, and reliable. They are already being used in large storage and uninterrupted power supply solutions (e.g. emergency lighting and powering back-up generators), which means they can be increased further in size to power grids. They can also be easily recycled and an infrastructure around this process already exists.
The problem they have is that they are rather large, heavy and immobile. They also have poor cold temperature performance and a short life cycle.
There are several characteristics of a flow battery system that will enable them to provide very high power and very high capacity on a grid type system. For example, unlike a conventional battery system, the energy output is independent of the energy storage capacity. While output depends on the fuel cell stack, the energy storage depends on the size of the electrolyte tanks and these are independent from one another. This operating capability is very useful when large current flows need to be transported to a national electricity grid system.
Energy output ratio to weight can be up to three times better than lead-acid batteries, but they do have lower energy efficiency.
At the moment, there are only experimental flow battery schemes in operation and, since they haven’t been around as long as the lithium ion battery, it is taking longer for electricity distribution industries to adopt them.
A sodium–sulphur battery is a type of molten-salt battery constructed from liquid sodium and sulphur. The sodium-sulphur battery has a very high energy and power density as a result of sodium being a highly reactive alkali metal. This type of battery has a high energy density, high efficiency of charge/discharge (89–92%) and long cycle life, and is fabricated from inexpensive materials.
However, they operate at a temperature of about 300-3500C, and therefore they require energy to keep them operational. And, due to the highly corrosive nature of sodium polysulphides, such cells must be kept stationary. Therefore they are ideal for for energy arbitrage, which is when the grid system fluctuates between peak demand and supply, so the battery can help manage the load.
Could chemical energy storage be a commercially viable solution?
We are still far from producing batteries that are a viable and cost-effective solution to managing the variation in grid systems. It would be incredibly expensive to make batteries capable of storing excess energy on the grid. So if this energy storage solution was implemented it may significantly increase the cost of electricity to consumers, which would be highly unpopular in the current economic climate.
People use electricity around the clock, so electricity transmission and storage must be kept in mind as key players. Smart grids will require advanced utility-scale batteries to store electricity so it can be delivered when needed.
How does chemical energy storage compare to other storage technologies?
In the UK, the DECC has been running a programme funded by public money that looks at various energy-storage solutions that could be used in the national grid. At the moment there is no obvious solution as everything from compressed air storage to molten salt and battery power is being considered.
The UK does have pumped storage (hydroelectric), but not on a scale seen in countries like Norway and Canada, which make use of their natural topography.
What is hydrogen?
Hydrogen is the most abundant element in the universe but naturally occurring hydrogen on earth is rare. This is because it is highly reactive, and so it reacts with most elements to form compounds that we see around us today. Hydrogen is found in pretty much all the food we eat, in fossil fuels to power our cars and also in water, which is made up of two hydrogen atoms combined with one oxygen atom.
If hydrogen is isolated and ignited with a flame in air, it will combust fiercely and produce heat and water as a by-product. There are already cars that run solely on hydrogen, which means that these cars are 100% emission free. So hydrogen is abundant and combusts with zero dangerous emissions, which makes it a perfect candidate for energy storage.
How can hydrogen combat volatilities in electricity demand?
Many countries have electricity demand volatilities, where a large amount of electricity is needed at rush hour, and again when people come home and use appliances during the evening. Conversely, while people are asleep at night, demand is much lower. This is a rather simplistic model, but it demonstrates the electricity demands of the modern world.
Unfortunately electricity cannot be stored, and at the moment most is either used or wasted as it is produced. However, if we intervene correctly we can transfer the excess electricity into another form of potential energy such as hydrogen. It can then be used to create electricity when required, thereby reducing the volatility of demand on the grid.
Producing hydrogen for grid storage
The main commercial process used for creating pure hydrogen is steam reforming, which involves breaking down a hydrocarbon into hydrogen and carbon monoxide.
However, as an energy storage measure for the grid, the most suitable method of creating hydrogen is through a process called electrolysis, which is simply using electricity and water. Importantly, hydrogen can be produced using electricity sourced from renewable sources such as wind and solar. Therefore when there is excess supply capacity, such as during the night when demand is also low, this excess electricity can be used to produce the hydrogen.
The process of the electrolysis of water involves passing an electrical current through water, which then produces pure hydrogen gas and oxygen. Two electrodes are positioned in the water, and when an electrical current is passed down the cathode (the negatively charged electrode) hydrogen bubbles out, while at the anode (the positively charged electrode) oxygen is released.
Another potential mechanism for making hydrogen is using algae or cyanobacteria, which use the sun to split water into hydrogen and oxygen. Under normal conditions, however, hydrogen production is secondary to the production of compounds that the organisms use to support their growth. However specialised enzymes can be introduced to suppress sugar production in the organism so it produces much more hydrogen gas. This research is still at an early stage, but the paper that describes the process can be found here.
What do we do with the hydrogen sas produced?
This hydrogen gas can then be stored and used in a fuel cell to create electricity (or it can be burnt to power a traditional turbine/ generator system). In a fuel cell, the hydrogen is combined with oxygen to create electricity without producing any heat. There are several advantages of producing electricity with a fuel cell rather than combusting the hydrogen. The fuel cell operates very efficiently, is very reliable and the only by-product is water – an obvious environmental bonus.
The future for hydrogen mass storage
The future seems bright for hydrogen as a mass storage technique. The ability to create hydrogen by applying an electric current to water when there is excess supply in the grid is a very simple process. ITM Power, based in the UK, are currently building an infrastructure to create hydrogen on a large scale by using electrolysis to power industry and road vehicles.
The electrolysis units are then positioned where the gas is required. This removes the cost of implementing the infrastructure required to pump the hydrogen between locations, or to carry the hydrogen gas in lorries.
The test units they currently have in operation create 5kg of hydrogen gas over a 24-hour timeframe. It takes roughly 60kWh of electricity to produce 1kg of hydrogen and that would cost about £7.20 at today’s electricity prices. If this was used in car it would work out at about £0.12 / mile, where as a traditional petrochemical engine would work out at about £0.18 / mile.
However if the electricity required to make the hydrogen was sourced 100% from renewable technologies, then the whole process would be 100% emission free.
The major issue with hydrogen gas
Whilst hydrogen does sound like a genuine mass storage contender, there is one main issue with it, which is storage. It is difficult to store because it has very low volumetric energy density. It is 3.2 times less dense than natural gas and 2,700 times less dense than gasoline. Therefore to store it, it needs to be compressed, liquefied or chemically combined prior to storing. A standard method to do this on a large scale needs to be formalised to ensure that it becomes economical to store the hydrogen.
Could hydrogen be the answer to grid energy storage
In a word, yes! Producing hydrogen can be done now and hydrogen has a very high specific energy (energy per unit mass). However the mechanism for producing the gas has to be standardised so economies of scale are introduced into its production, helping to bring the cost of production down.
The major drawback with hydrogen is that it takes up such a large space when it is stored. Therefore new, more effective storage processes will need to be introduced, to make its storage economically viable. Rather than turning off wind turbines to prevent excess electricity going into the grid, hydrogen storage in fuel cells is an excellent way to conserve energy.
What is a capacitor?
A capacitor is much like a battery, but instead of using a chemical reaction to create a flow of electrons and therefore provide power for an appliance, the capacitor stores electrons.
As such a capacitor is far simpler than a battery, and is used in all sorts of electronics today, to provide a small amount of electricity storage.
What is inside a capacitor?
A basic capacitor consists of two metal plates separated by a non-conducting substance known as a ‘dielectric’. In theory the dielectric substance used can be any non-conducting substance, but in practise specific materials are used such as porcelain or mica to give the capacitor particular features to fulfil its role.
How does a capacitor work?
When a capacitor is in an electric circuit, the electrical source pushes negatively charged electrons on to one of the metal plates. The dielectric substance in the middle prevents the movement of these electrons across the circuit, so this plate becomes highly negatively charged and a positive charge builds on the other plate.
When the repulsive force of the negatively charged plate becomes large enough, it repels further electrons from approaching the plate, and at this point the capacitor is charged. The capacitor can then discharged, which involves allowing the electrons to complete the circuit, providing electricity that can be used for anything.
Using capacitors in commercial scale processes
Capacitors are most commonly used in many everyday electric circuits and appliances, for example in mobile phones where they help maintain the power supply while the main battery is dead to prevent losing information stored in the memory.
Recent advances in capacitor technology have lead to super capacitors that may have the potential to store energy on a commercial scale.
These super capacitors do not have the conventional dielectric separating the two plates; instead they consist of two plates made up of a sponge like substance known as activated carbon. These plates are separated by a nanometer thick separator and then are immersed in a liquid electrolyte where a current is applied to them. Electrons then build up on the negatively charged plate (as per a traditional capacitor), but as there are free positive ions in the electrolyte solution, a layer of positive ions builds next to this negatively charge plate.
The same occurs at the positive plate, where free negative ions are attracted to this plate and build up around the positive charge. Therefore in an super capacitor each carbon electrode ends up having two layers of charge coating its surface, hence these are sometimes referred to as double layered capacitors.
As the super capacitor is discharged, the negative charge on the plate weakens, which releases the positive ions back into the electrolyte solution and the same happens at the positively charge plate, allowing it to be recharged once again.
How do you make super capacitors hold enough charge for commercial uses?
The capacitance, measured in Farads, is how much electric energy a capacitor will hold given a certain voltage and is based on a few factors which are described below.
The surface area of the plate coating. In super capacitors the coating they use on the plates is activated carbon, this has an incredibly large surface area, helping to maximise the number of electrons that it can hold. Graphene is now being also being used in supercapacitors; it is made from pure carbon, with atoms arranged in a regular hexagonal pattern but the sheets are only one atom thick. One gram of Graphene has a 1520m2 surface area, so this characteristic obviously helps the super capacitor to hold an incredible amount of charge.
The voltage the capacitor can handle. The higher the voltage the capacitor can handle, the more charge it can hold, so it is key to pick a material that can handle very high voltages.
The distance by which the two plates are separated. The size of the electrical field generated within the capacitor is inversely proportional to the separation distance. I.e. the smaller the distance the larger the electrical field, hence in supercapacitors the space between the two plates is nanometers thick.
The future of super capacitors
Tesla Motors chief executive Elon Musk has recently come out and said that he believes super capacitors will supercede batteries in the not too distant future. They can be charged in a matter of seconds, and discharged and recharged millions of times without any degradation in performance.
Compared to old style capacitors, a super capacitor has the potential to store about 1500x the amount of electricity, but only about 5% that of a lithium ion battery. However, the ability to release the power is useful, for example if a particular job requires a very short, strong burst of energy, using a capacitor would be better than using a battery.
The hope is that in the coming years, capacitors will become comparable to batteries, so they can hold more charge. Imagine for example, the ability to charge your mobile phone in seconds.
For long-term commercial energy storage, capacitors are still someway off, although even now they can provide a mechanism for dealing with spikes in demand across the grid.
Compressed Air Energy Storage
What is compressed air energy storage?
The purpose of compressed air energy storage is to help manage the supply of electricity in the grid. For example when the wind blows a wind turbine will produce power, but this power may be produced when there is no demand for it. At this time it becomes necessary for us to be able to store the electricity, so we can use it when there is a peak in demand. Compressed air energy storage is the second biggest form of energy storage currently behind pumped storage.
Compressed air energy storage involves converting electrical energy into high-pressure compressed air that can be released at a later time to drive a turbine generator to produce electricity. This means it can work along side technologies such as wind turbines to provide and store electricity 24/7. Ideally the compressed air is stored in an existing geographical formation such as a disused hard-rock or salt mine (keeps cost down), rather than producing specialist surface piping, which can be expensive.
How does compressed air energy storage work?
The first compressed air energy storage facility was the E.ON-Kraftwerk’s
290MW plant built in Huntorf, Germany in 1978. This plant was built to help manage grid loads, by storing the electricity as pressurised air when demand was low during the night. When there was peak demand, the compressed air was released to create the electricity, in an effort to lower peak electricity costs.
This plant, which is still maintained as a power back-up installation today, compresses air during times of low demand and stores it in two underground salt caverns. It takes 8 hours to fill both of the caverns at a rate of 108 kg / sec. When electricity is needed, the compressed air is released and heated by combusting natural gas to get the air to expand. This drives a 320MW turbine producing electricity for 2 hours, before the caverns need to be refilled.
Most other compressed air energy storage plants operate along the same principle, although to increase efficiency they are more focussed on retaining the heat associated with compression, which is discussed below.
Increasing the efficiency of compressed air energy storage
One of the major issues with compressed air energy storage is that when you compress air it heats up. When the electricity is required it needs to be expanded, which requires heat. In addition the cooler the air, the more you can store. Companies are therefore trying to find ways to best store the heat generated during compression, so it can then be used to heat the air for the expansion helping drive more efficiency in the overall process.
There are three types of solutions that can deal with heat build up when the air is initially compressed and are discussed below:
Adiabatic storage – retains the heat from compression and re-uses this when the air is expanded to produce the power – expected efficiency around 70% (although theoretically 100%)
Diabetic storage – takes the heat and dissipates it into the atmosphere via heat intercoolers. When the air is released to go through the turbines it needs to be heated – expected efficiency around 70%.
Isothermal storage – involves using heat exchangers to try to always keep the internal and external temperatures the same, so as the air is compressed heat dissipates into the atmosphere. Once the air is released to drive the turbine and produce the electricity, heat is bought in from the external environment.
New technology is being developed by companies to try and increase the heat retained from the compression process. For example, SustainX from the USA have working on a process to remove the heat by injecting water vapour into the compressed air. The water absorbs the heat which then gets stored and reapplied to the air during the expansion process.
Keeping the pressure up
One of the final issues to overcome for successful CAES revolves around the pressure of the compressed air as it comes out. If the storage facility is full of compressed air, the cavern pressure is higher. Visa versa if cavern is almost empty then the cavern pressure will be low. Therefore there are two operating modes when storing compressed air in a fixed volume cavern, which are demonstrated in detail below:
Allow the pressure to change naturally as the air is released, which will mean the turbine creates less electricity as time goes on.
Control the flow of air out of the cavern, so it releases more of the compressed air at the end to counter the lower pressure. This will ensure constant electricity supply from the turbine / generator.
The latter option is normally used, despite ‘throttling loses’ (the speed at which the air is released from the cavern). This is so there is always a constant store of air driving the turbine, so you can accurately reflect the amount of electricity that the compressed air energy storage facility will be producing.
The future of compressed air energy storage
In the UK, Seamus Garvey, a professor of Dynamics at Nottingham University, is looking at using deep ocean bags known as ‘energy bags’ to fulfil the role of the natural cavities that are used at the moment. The deep water acts as the pressure vessel, and no matter how full the energy bags are, energy should always be the same. In terms of energy storage, Garvey says that with this proposed technology the cost per unit of energy stored is in the order of £1-£10 / kWh, where as comparators such as pumped storage come in at £50 / kWh and electrochemical stores are about £500 / kWh, therefore make this solution very attractive.
It is expected that the UK will need to be able to store about 200GWh of electricity by 2020, to help support the grid that becomes more dependant on intermittent renewable energy sources. Compressed air energy storage could be a valuable tool in allowing us to hit these ambitious targets.
Flywheel Energy Storage
Flywheels as mechanical batteries
Flywheel Energy Storage (FES) is a relatively new concept that is being used to overcome the limitations of intermittent energy supplies, such as Solar PV or Wind Turbines that do not produce electricity 24/7.
A flywheel energy storage system can be described as a mechanical battery, in that it does not create electricity, it simply converts and stores the energy as kinetic energy until it is needed. In a matter of seconds, the electricity can be created from the spinning flywheel making it the ideal solution to help regulate supply in the electrical grid.
It is based on a really old concept and is very similar to an old-fashioned pottery wheel where the potter moves his feet to make the wheel spin. As the potter works, he removes energy from the system, so to keep the wheel spinning; he needs to keep moving his feet.
So how exactly does it work?
A flywheel is a heavy shaft-mounted rotating disc that speeds up when electrical energy is applied to it. When energy is needed, the flywheel is slowed and the kinetic energy is converted back to electrical energy, where it can be transmitted to where it is required.
The energy a flywheel contains is a function of the speed that it is spinning multiplied by the moment of inertia.
The moment of inertia states that the effective mass of a spinning object is not dependant on how much actual mass the spinning object contains. Instead, it is dependant on where the mass is located in relation to the central point that it is rotating around.
For example, if spinning at the same speed, a solid flywheel will store less energy than a flywheel of the same mass that has spokes and its weight situated around the rim of the wheel.
A high moment of inertia is good, but speed of rotation is better!
The speed that the flywheel rotates has a larger effect on the energy stored within it compared to the moment of inertia. If you have a flywheel with a rim weighing 1kg and replace it with a flywheel with a 2kg rim, it has the potential to store double the energy. If you take the original flywheel and double the speed at which it spins, you quadruple the potential energy that it can store.
Innovation of the flywheel
Historically, flywheels have been huge steel structures with the majority of the weight distributed towards the rim of the wheel. However, over the last 30 years, scientific innovation has meant that flywheels can store more energy in less weight and volume, increasing their potential for energy storage. Newer flywheels are made from very strong composite materials and are operated on a bed of near frictionless magnetic bearings housed in a vacuum enclosure. This allows the flywheels to be spun at incredible speeds helping maximise the energy that they can store. In fact NASA scientists have managed to get flywheels to spin in excess of 60,000 revolutions per minute, which is nearly 2.5 times the speed of sound. The amount of kinetic energy that can be stored at this speed makes them ideal for replacing chemical batteries in the future.
There is also potential to use magnetic levitation as a way of prolonging the life of the flywheel energy storage systems. Since there is no friction on a system that is magnetically levitated there will be no wear on the system, so it is thought that these systems could last fifteen years or more as opposed to a chemical battery that may only last five years.
Flywheel energy storage in action
In June 2011, the Beacon Power Corporation completed the company’s first flywheel energy storage plant in Stephentown, New York at a cost of $60m. The plant utilises 200 flywheels spinning at a maximum speed of 16000 rpm to store excess energy and help regulate the supply to the local grid.
On 7th March 2012, Rockland Capital acquired the assets of the Beacon Power Corporation and put up funding to develop a second 20 MW flywheel regulation plant in Pennsylvania.
Flywheel could be one of the solutions to provide mass scale storage of electricity during excess supply and provide the release of energy during excess demand.
What is pumped storage?
The principles of pumped storage hydropower have been around for many years. Pumped storage accounts for more than 99% of bulk storage capacity worldwide, approximately 127,000MW according to the Electric Power Research Institute (EPRI). In the UK, for example, one of the biggest hydroelectric power stations is at Dinorwig (Wales) which has provided pumped storage since 1984.
How does pumped storage work?
The process requires two reservoirs: one at high altitude and one at low altitude. When there is low demand for electricity, water is pumped up into the top reservoir through bi-directional turbines, via large water pumps, where it is stored.
When electricity is needed in the grid to meet increased demand, the water is allowed to flow back down through the turbines, which power the generators to create the electricity that can be fed back into the grid.
In the case of Dinorwig, the pump motors are powered by off-peak electricity from the National Grid when it is cheaper overnight because demand is so low. Storage generation therefore offers a critical back-up facility during periods of excessive demand, whilst being efficient in storing energy during periods of low demand.
Getting around the pumped storage topography issue
The problem with traditional hydroelectric pumped storage is that is has limited scope for expansion. You need specific sites that are few and far between. Therefore new forms of pumped storage have to be devised.
One example is the Green Power Island concept devised by the Danish architecture firm Gottlieb Paludan. This concept involves creating a man-made island with a deep reservoir in the middle of it. Wind turbines produce the electricity to drive the pumps that pump the water out of the reservoir in to the sea. When electricity is needed, the water is allowed to re-enter this reservoir driving turbines that power the electricity generators.
The American firm Advanced Rail Energy Storage (ARES) has designed another potential grid-scale pumped storage solution. ARES is the first pumped storage company that does not rely on water. Instead it is a rail-based technology that stores energy by moving a heavy mass ‘train’ against the force of gravity. When the electricity is required, the train uses the stored gravitational potential energy to return to its original position. This motion drives turbines that produce energy via generators.
ARES is aiming to produce a wide range of energy storage facilities from 100MW to 3GW. A 3 GW system has the potential to store 24GWh of energy. In real terms, if it were implemented in the UK it would be able to power over five million homes for eight hours. Another advantage of this water-free pumped storage solution is that it is highly efficient at 85% (the ratio of energy in to energy out).
Could pumped storage be a viable energy storage technique?
In conclusion, pumped storage has already a proved to be a solution to helping countries meet peak demand. The Dinorwig power plant in Wales has been in operation now for over 25 years helping manage demand in a highly reactive way; in just 12 seconds the plant can switch from being fully shut down to operating at full capacity. To take this a step further, building wind turbines, solar PV farms and tidal energy near Dinorwig is one way to replace traditional generation technologies and connect renewables with energy storage.
New concepts are constantly being developed to remove the need to find particular topological features for pumped storage, thereby helping to open this energy storage solution to all corners of the globe.