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Overview of Energy Storage Technologies
C H A P T E R
27 Overview of Energy Storage Technologies Le´onard Wagner Mora Associates Ltd, London, UK
27.1 IN A NUTSHELL Energy storage is the storage of some form of energy that can be drawn upon at a later time to perform some useful operation. A windup clock stores potential mechanical energy. A battery stores readily convertible chemical energy to keep a clock chip in a computer running even when the computer is turned off. A pumped-storage plant stores power in a reservoir as potential gravitational energy. ‘Rien ne se perd, rien ne se cre´e, tout se transforme’, proclaimed AntoineLaurent de Lavoisier (1743 1794). The French chemist, who became the father of modern chemistry, invented among others the system of chemical nomenclature still largely in use today and helped construct the metric system. His principle of mass conservation still holds and states that for any system closed to all transfers of matter and energy, the mass of the system must remain constant over time. This is the main concern and opportunity for energy storage technology. Phase changes the transformation of matter from one state to the other open up the possibility to transform electricity into different types of energy and storage media. More scientifically, the breaking of intermolecular attractions, such as found in fusion, vaporisation and sublimation, requires an input of energy to overcome the attractive forces between the particles of the substance. Phase changes involving the formation of intermolecular attractions, such as freezing,
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condensation and deposition, release energy as the particles adopt a lower-energy conformation. Figure 27.1 shows the names of the phase changes between solids, liquids and gases. The arrow to the right of the diagram demonstrates that these three phases have different enthalpies: gas has the highest enthalpy, liquid has an intermediate enthalpy and solid has the lowest enthalpy. Hence, each of the phase transitions shown in Figure 27.1 involves a change in the enthalpy of the substance. Electricity must be consumed as it is generated. It is transmitted in a closed circuit and, unlike common energy storage such as wood or coal, cannot be stored as electrical energy for any practical purpose. Consequently, increasing energy demand cannot be accommodated without either increasing or cutting supplies or arranging for storage techniques to buffer consumption swings. In modern electricity grids, the situation is somewhat more complex with the possibility of absorbing small variations through voltage variations. Energy storage solutions also allow the smoothing of these temporary imbalances in electricity consumption and production. On the one hand, energy storage produces flexibility gains with the prospect of benefiting consumers and producers. On the other hand, any mismatch between the supply and the demand results in either
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Deposition
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e Fre
Phase changes. From Ref. [1].
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energy or efficiency losses. Whenever energy is stored, a fraction is lost in transmission or during the storage period. To gain a better understanding of the technological viability of energy storage, we need to contrast residual energy and efficiency losses to the gains in terms of reliability, portability and flexibility. This is particularly relevant in regions of the world where energy production is intermittent and therefore not capable of providing stable electricity or heat production. Energy storage is a dominant factor in economic development, as was the case during the late 1900s with the widespread introduction of electricity and refined chemical fuels, such as gasoline, kerosene and natural gas [17].
27.2 ENERGY PRODUCTION AND TRANSMISSION Energy storage technologies provide grid operators with an alternative to traditional grid management, which has focussed on the ‘dispatchability’ of power plants, some of which can be regulated very quickly like gas turbines, others much more slowly like nuclear plants. The applications for long-term energy storage include counterbalancing the intermittency of renewable energy sources like wind and solar power, levelling the loads (‘load balancing’) and time-shifting periods of peak demand on the grid and avoiding or delaying construction of costly transmission and distribution (T&D) assets. In addition, policy-makers set ambitious targets attempting to reduce reliance on fossil and nuclear energy sources. For instance, Switzerland is determined to decommission gradually all nuclear power plants by 2034. At present, nuclear power accounts for 40 % of the total electricity produced in Switzerland. While radiation accidents at nuclear power plants remain relatively few, they cause widespread contamination, human hardship and havoc when they happen as evidenced by the incidents at Three Mile Island (1979), Chernobyl (1986) or Fukushima (2011). The share of sustainable but intermittent energy sources is likely to increase as new plants become operational. However, they cannot be used for baseload power generation as their output is relatively volatile and depends on the sun, water or wind. It must be noted that wind parks can be regulated by rotating the blades. Wind energy is maturing and innovation offers strong incentives to repower certain parks which may stretch electricity grids. Outdated infrastructure may struggle to handle increasingly large swings in production. This contrasts with geothermal, nuclear and coal-fired power plants whose advantage is their baseload production capacity [14].
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Where electricity is not immediately consumed, it risks causing a grid collapse. At times, the glut can be so great that utilities pay consumers to take the power and get rid of it. According to Czech grid operaˇ tor CEPS, a.s., the ageing power grids in former communist countries of Eastern Europe are ‘stretched to their limits’ and face potential blackouts when output surges from wind turbines in northern Germany or the Baltic Sea. The Czech Republic is planning to install security switches near borders to disconnect from Europe’s biggest economy to avoid critical overload. As a last resort, it may be necessary to reduce the demand by disconnecting some consumers (‘load shedding’). Technological obsolescence also translates into transmission losses. According to the US Energy Information Administration (2012), annual electricity T&D losses average 7 % [2]. It is estimated that the United States requires US$150 billion in capital expenditures to upgrade and maintain their electricity infrastructure. A study by the American Solar Energy Society (2007) shows that total electricity production can consist of 20 % intermittent energy sources with minimal impact on grid stability [3]. Denmark is a case in point and does not currently experience problems given its modern and wellmaintained energy infrastructure. Alternative solutions to deal with intermittency in wind energy production would be geographic dispersion to de-link weather system effects, and the ability of high-voltage, direct current cables to shift power from windy areas to non-windy areas. Energy storage could help alleviate supply shocks by storing peak production and thereby become a key enabler for intermittent energy sources such as wind and solar energy.
27.3 ENERGY CONSUMPTION Energy consumption follows regular patterns with a fairly large variation over the course of the day. It is the result of consumption habits, social trends, budgets and constraints. Superimposed on this daily pattern are smaller, longer term, seasonal variations with a greater demand for heating and lighting in winter months or in some regions for air conditioning in summer months. Demand from industrial users may also follow cyclic variations, which could affect aggregate demand. Operating experience and statistics backed by regional knowledge about economic growth trends allow reasonably accurate predictions. On top of this, the utility is expected to cater for unexpected emergency situations, such as accidents, natural disasters or breakdown of its own equipment.
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100 90
Demand/%
80 70 60 50 40 30
Japan 1:00
3:00
5:00
7:00
RWE
France
Italy
North Europe
PJM
9:00 11:00 13:00 15:00 17:00 19:00 21:00 23:00 Hours/h
FIGURE 27.2 Comparison of daily load curves. Data from Ref. [18].
Notwithstanding, actual demand needs to be constantly monitored in order for utilities to produce the appropriate amount required for a stable grid. Figure 27.2 shows a typical demand profile of an individual country over the course of a day. It would be difficult to match generating capacity to the peaky demand profile. The aggregate demand for all industrial and domestic consumers in a particular community tends to smooth out the overall demand profile, and although the aggregate demand varies during the day and also over the course of the year, it does so in reasonably predictable patterns.
27.4 OVERVIEW OF STORAGE TECHNOLOGIES Energy storage technologies are segmented into those that can deliver precise amounts of electricity very rapidly for a short duration (capacitors, batteries and flywheels), as well as those that take longer to ramp up, but can supply tens or hundreds of megawatts for many hours (compressed air energy storage and pumped-storage hydropower). More recently, researchers have looked at ultracapacitors, which deliver high energy, high power density for easy to charge and
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discharge, and nano-materials that could significantly increase the capacity and lifetime of batteries. We distinguish between the following storage technology options and modes: • • • • • •
electrochemical energy (batteries, flow cells), electrostatic energy (capacitors), electromagnetic energy (superconducting magnets), chemical energy (hydrogen, methane, gasoline, coal, oil), kinetic energy (mechanical flywheels), potential energy (pumped-storage hydropower, compressed air, springs) • thermal energy (ice, molten salts, steam). The next step consists of benchmarking each storage technology against the following criteria: • • • • • •
energy capture rate and efficiency, discharge rate and efficiency, dispatchability and load following characteristics, scale flexibility, durability cycle lifetime, mass and volume requirements footprint of both weight and volume, • safety risks of fire, explosion, toxicity, • ease of materials recycling and recovery. Energy storage technologies may be broadly characterised by their ‘specific energy’ (energy stored per unit volume or mass) and by their ‘peak power’ (how fast that energy can be delivered from the device). For instance, batteries store a lot of energy, but they take a long time to charge and discharge. Capacitors can produce peak power but store only tiny amounts of energy. Supercapacitors offer a combination of high-power, high-energy properties, bridging the gap between batteries and capacitors. Fuel cells operate efficiently over a narrow range of parameters and at elevated temperature, rapidly becoming inefficient under high-power demands (Figure 27.3). Chemical energy is by far the most dominant form of energy storage, both in electricity generation and energy transportation. Chemical fuels in common use are coal, gasoline, diesel fuel, natural gas, liquefied petroleum gas, propane, butane, ethanol, biodiesel and hydrogen. These chemicals can be readily converted to mechanical energy and then to electrical energy. Despite the ecological footprint, liquid fuels are the most commonly used forms of energy storage in transportation. These fuels produce greenhouse gases when used in cars, trucks, trains, ships and aircraft.
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104 Advanced flywheels 5
2
Peak power/(W·kg−1)
Supercapacitors 10
3
Conventional flywheels
5
Ni/Zn
Lithium ion
Methanol H2 ICE
Gasoline
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Zn–Air
Pb–Acid
LiM/FeS2
2 H2 Fuel cell 10 5
10
2
5
102
2
5
103
2
3
−1
Specific energy/(W·h·kg )
FIGURE 27.3 Ragone plot showing energy density versus power density for various devices. From Ref. [4].
Carbon-free energy carriers, such as hydrogen and some forms of ethanol or biodiesel, are being sought in response to concerns about the consequences of greenhouse gas emissions.
27.4.1 Electrochemical Energy Storage An early solution to the problem of storing energy for electrical purposes was the development of the battery an electrochemical storage device that transforms chemical energy into electric energy. The battery has three basic components in each cell an anode, a cathode and an electrolyte and their properties relate directly to their individual chemistries. We distinguish between primary and secondary batteries. Primary batteries are the most common and designed for a single use, to be discarded or recycled after they run out. They have very high impedance which means long-life energy storage for low current loads. The most frequently used batteries are carbon zinc, alkaline, silver oxide, zinc air and some lithium batteries. Secondary batteries are
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designed to be recharged and can be recharged up to 1000 times depending on the usage and battery type. Deep discharges result in a shorter charging cycle, whereas shorter discharges result in long charging cycles for most of these batteries (‘memory effect’). The charge time varies from 1 to 12 hours, depending upon battery condition and ambient temperature among other factors. Commonly available secondary batteries are lead acid, nickel cadmium (NiCd), nickel metal hydride (NiMH) and lithium-ion (Li-ion) batteries. Some of the limitations posed by secondary batteries are limited life, limited power capability, low energy efficiency and disposal concerns. 27.4.1.1 Lead Acid Batteries (1859) Lead acid batteries, invented in 1859 by French physicist Gaston Plante´, are the oldest type of rechargeable battery. Despite having a very low energy-to-weight ratio and a low energy-to-volume ratio, their ability to supply high surge currents means that the cells maintain a relatively large power-to-weight ratio. These features, along with their low cost, make them attractive for use in vehicles to provide the high current required by automobile starter motors. 27.4.1.2 Nickel Cadmium Battery (1899) In 1899, the Swedish scientist Waldmar Jungner invented the nickel cadmium (NiCd) battery, a rechargeable battery that had nickel and cadmium electrodes in a potassium hydroxide solution the first battery to use an alkaline electrolyte. The first models were robust and had significantly better energy density than lead acid batteries, but were much more expensive. NiCd batteries may suffer from a ‘memory effect’ if they are discharged and recharged to the same state of charge hundreds of times. The apparent symptom is that the battery ‘remembers’ the point in its charge cycle where recharging began and during subsequent use suffers a sudden drop in voltage at that point, as if the battery had been discharged. 27.4.1.3 Alkaline Battery (1950s) Up until the late 1950s the zinc carbon battery continued to be a popular primary cell battery, but its relatively low battery life hampered sales. In 1955, Eveready (now known as Energizer) wanted to find a way to extend the life of zinc carbon batteries, but engineers at Eveready believed alkaline batteries held more promise despite the high price tag. They came up with a new alkaline battery that consisted of a manganese dioxide cathode and a powdered zinc anode with an alkaline electrolyte. Using powdered zinc gave the anode a greater surface area. Alkaline batteries hit the market in 1959 and still account for
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80 % of manufactured batteries and over ten billion (10 3 109) individual units produced worldwide in 2011. 27.4.1.4 Lithium Batteries (1970s) Lithium is the metal with lowest density and has the greatest electrochemical potential and energy-to-weight ratio, so in theory it would be an ideal material to manufacture batteries. Experimentation with lithium batteries began in 1912, and in the 1970s the first lithium batteries were sold. In the 1980s, an American chemist John B. Goodenough led a research team at Sony that would produce the lithium-ion battery, a rechargeable and more stable version of the lithium battery with the first ones sold in 1991. Yet, the risk of high temperatures and fire remains as of today as evidenced by Apple’s recall of 1.8 million notebook batteries in 2006, and the Boeing 787 Dreamliner battery problems which caused the emergency landing of an All Nippon Airways aircraft in January 2013. 27.4.1.5 Nickel Metal Hydride Battery (1980s) Towards the end of the 1980s, Stanford R. Ovshinsky invented the NiMH battery, a variant of the NiCd, which replaced the cadmium electrode with one made of a hydrogen-absorbing alloy. NiMH batteries tend to have longer lifespans than NiCd batteries (and their lifespans continue to increase as manufacturers experiment with new alloys), and since cadmium is toxic, NiMH batteries are less damaging to the environment. 27.4.1.6 Lithium-Ion Polymer Batteries (1990s) The lithium-ion polymer battery was released in 1996. These batteries hold their electrolyte in a solid polymer composite instead of a liquid solvent, and the electrodes and separators are laminated to each other. This difference allows the battery to be encased in a flexible wrapping instead of a rigid metal casing, which means such batteries can be specifically shaped to fit a particular device. They also have a higher energy density than normal lithium-ion batteries. These advantages have made it a choice battery for mobile phones, laptops and tablets.
27.4.2 Electrostatic Energy Storage 27.4.2.1 Capacitors Capacitors store energy in an electrostatic field rather than as a chemical state as in batteries. They use physical charge separation between two electrodes to store energy, for instance between the surfaces of metalised plastic film or metal electrodes. During charging, the
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electrically charged ions in the electrolyte migrate towards the electrodes of opposite polarity due to the electric field between the charged electrodes created by the applied voltage. Thus two separate charged layers are produced. Although similar to a battery, the double-layer capacitor depends on electrostatic action. Since no chemical action is involved the effect is easily reversible and the typical lifespan is hundreds of thousands of cycles. When compared to batteries, the energy density of capacitors is very low, but the power density is very high. This means that capacitors are able to deliver or accept high currents, but only for extremely short periods, due to their relatively low capacitance. As a consequence, applications are relatively limited. For instance, capacitors are used as power backup for memory functions in consumer products such as mobile phones, laptops and radio tuners. 27.4.2.2 Supercapacitors and Ultracapacitors The supercapacitor and ultracapacitor resemble regular capacitors except that they offer very high capacitance in a smaller form. Supercapacitors rely on the separation of charge at an electrified interface that is measured in fractions of a nanometre, compared with micrometres for most polymer film capacitors. Supercapacitors use a molecule-thin layer of electrolyte, rather than a manufactured sheet of material, as dielectric to separate the charge. Lifetime is virtually indefinite, and energy efficiency rarely falls below 90 % when they are kept within their design limits. The power density is higher than that of batteries, while their energy density is generally lower. However, unlike batteries, almost all of this energy is available in a reversible process.
27.4.3 Electromagnetic Energy Storage 27.4.3.1 Superconducting Magnetic Energy Storage In a superconducting magnetic energy storage (SMES) system, the energy is stored within a magnet that is capable of releasing megawatts of power within a fraction of a cycle to replace a sudden loss in line power. It stores energy in the magnetic field created by the flow of direct current (DC) power in a coil of superconducting material that has been cryogenically cooled. The stored energy can be released back to the network by discharging the coil. The power conditioning system uses an inverter/rectifier to transform alternating current (AC) power to DC or convert DC back to AC power. The inverter/rectifier accounts for about 23 % energy loss in each direction [16]. In comparison to other storage methods, SMES
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systems lose the least amount of electricity during the storage process with a round-trip efficiency greater than 95 %. Due to the energy requirements of refrigeration and the high cost of superconducting wire, SMES technology is currently used for short duration energy storage. These systems have been in use for several years to improve industrial power quality and to provide a high-quality service for individual customers vulnerable to voltage fluctuations. Typically, SMES systems are installed on the exit of the power plants to stabilise output or on industrial sites where they can be used to accommodate peaks in energy consumption (e.g. steel plants or rapid transit railway) in a highly efficient manner.
27.4.4 Chemical Energy 27.4.4.1 Hydrogen and Fuel Cell Technologies Fuel cells were invented about the same time as the battery. However, fuel cells were not well developed until the advent of spacecraft when lightweight, non-thermal sources of electricity were required. Fuel cell development has increased in recent years to an attempt to increase conversion efficiency of chemical energy stored in hydrocarbon or hydrogen fuels into electricity. Like a battery, a fuel cell uses stored chemical energy to generate power. Unlike batteries, its energy storage system is separate from the power generator. It produces electricity from an external fuel supply as opposed to the limited internal energy storage capacity of a battery. Hydrogen is a chemical energy carrier similar to petroleum, ethanol and natural gas with the unique characteristic that it is the only carbonfree or zero-emission chemical energy carrier. It is a widely used industrial chemical that can be produced from any primary energy source. The topic is the subject of Chapter 23 of this volume. Fuel cells are very useful as power sources in remote locations, such as spacecraft, remote weather stations, large parks, rural locations and in certain military applications. Fuel cell systems running on hydrogen can be compact and lightweight. Because they have no moving parts, and do not involve combustion, they can achieve up to 99.9999 % reliability in ideal conditions. This equates to less than 1 min of downtime in a 6-year period [5]. Hydrogen production in quantities sufficient to replace existing hydrocarbon fuels is not possible at present. So far, the significant capital investment in hydrogen production plants has limited widespread use. If production costs were to be reduced, hydrogen fuels may become more attractive commercially, providing clean, efficient power for our homes, businesses and vehicles.
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27.4.5 Kinetic Energy Storage 27.4.5.1 Flywheels Flywheels are rotating mechanical devices to store kinetic energy. They capture the momentum in a rotating mass and release the energy by applying torque to a mechanical load. The potter’s wheel is often cited as the earliest use of a flywheel. Advanced flywheel energy storage systems have rotors made of high-strength carbon filaments, suspended by magnetic bearings and spinning at speeds from 20 000 to over 50 000 rpm in a vacuum enclosure [6]. Such flywheels can come up to speed in a matter of minutes, much faster than most energy storage technologies. The benefits of this storage technology include: not being constrained by tight temperature limits, no development of a charge memory and no lifetime degradation. Large-sized flywheels exist and operate on the same principle but store more energy with a higher mass and physical size.
27.4.6 Potential Energy Storage Compressed air energy storage uses wind turbines to drive compressed air into underground aquifers. The air is released to generate electricity when needed. This is a new twist on the idea of using wind energy in a way that removes the variability to a large extent and increases the dispatchability. At the moment, there are only two operational compressed air storage plants. One is in Huntdorf, Germany and the other one in McIntosh, the United States. The Huntdorf plant is located on a 300 000 m3 salt dome, in which compressed air is stored, originally to capture excess nuclear power production [7]. For rapid responses to power shortages, the air is channelled to a conventional gas turbine, at a capacity of up to 290 MW. Smaller, even mobile compressed air batteries are currently in deployment as well. They are designed to take advantage of variations in the price of electricity. When power is cheap, it is used to run their compressors. When it is expensive, the valves are opened and the generators turn on. Compressed air storage plants are inefficient, and so they are commercially viable only in places where the price of power varies dramatically. But the intermittent nature of wind power can cause just that sort of variability. 27.4.6.1 Cryogenic Energy Storage Cryogenic energy storage is a variant of the compressed air energy storage and uses low-temperature (cryogenic) liquids such as liquid air or liquid nitrogen as energy storage.
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27.4.6.2 Pumped-Storage Hydropower Some areas of the world have used geographic features to store large quantities of water in elevated reservoirs, using excess electricity at times of low demand to pump water up to the reservoirs, then letting the water fall through turbine generators to retrieve the energy when demand peaks. Most of the global pumped-storage hydropower capacity caters for applications such as energy management, frequency control and provision of reserve. These are usually connected to the high-voltage transmission grid and designed to buffer fluctuations originating from a large number of sources on both supply and demand side. From a business perspective, the beauty of providing negative balance energy is that the pumped-storage can use surplus energy taken from the grid at night time, for example (for which the facility is paid) to generate additional revenues by selling it on the power market at peak prices 12 h later. Worldwide pumped-storage capacity stands at roughly 100 GW in 2012, which represents 3 % of global generation capacity [8]. In 2000, the United States had 19.5 GW of pumped-storage capacity, accounting for 2.5 % of its baseload capacity. In 1999, the European Union had 32 GW capacity of pumped-storage, representing 5.5 % of total electrical capacity in the EU. Pumped-storage hydropower presents major advantages, viz. efficiency between 70 % and 85 %, low running costs and scalability with discharge times ranging from several hours to a few days. The major drawbacks are the long construction times and high capital expenditure.
27.4.7 Thermal Energy Storage 27.4.7.1 Molten Salt Batteries Molten salt batteries are a class of primary and secondary electric batteries that use molten salts as an electrolyte. They offer both a higher energy density through the proper selection of reactant pairs as well as a higher power density by means of a high conductivity molten salt electrolyte. They are used in services where high energy density and high power density are required. These features make rechargeable molten salt batteries a promising technology for powering electric vehicles. However, operating temperatures of 400 to 700 C create thermal management and safety issues, which place more stringent requirements on the remaining battery components.
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27.4.7.2 Solar Ponds A solar pond is simply a pool of water, which collects and stores solar energy. It contains layers of salt solutions with increasing concentration and therefore density to a certain depth, below which the solution has a uniform high salt concentration. When sunlight is absorbed, the density gradient prevents heat in the lower layers from moving upwards by convection and leaving the pond. As a result, the temperature at the bottom of the pond will rise to over 90 C while the temperature at the top of the pond is usually around 30 C. The heat trapped in the salty bottom layer can be used for many different purposes, such as the heating of buildings or industrial hot water or to drive a turbine for generating electricity. Examples of solar pond installations include a 150 kW solar pond built by Ormat Technologies in Israel near the Dead Sea (1980), a salinity gradient solar pond at El Paso, the United States (1986), the Bhuj solar pond in India (1993) and a demonstration solar pond and associated heating system developed in collaboration with RMIT University, Melbourne, Australia (2001). 27.4.7.3 Seasonal Thermal Storage (Heat Pumps) Seasonal thermal storage can be divided into two broad categories. In both cases, very effective insulation of the building structure is required to minimise heat loss from the building, and hence the amount of heat that needs to be stored and used for space heating. Low-temperature systems use the soil adjoining the building as a low-temperature seasonal heat store (reaching temperatures similar to average annual air temperature), drawing upon the stored heat for space heating. Such systems can also be seen as an extension to the building design (normally passive solar building design). High-temperature seasonal heat stores are essentially an extension of the building’s heating, ventilation, air conditioning and water heating systems. Water is normally the storage medium, stored in tanks at temperatures that can approach boiling point.
27.5 RESEARCH AND DEVELOPMENTS 27.5.1 Smart Grids and Networked Energy Storage Utilities and grid operators are increasingly deploying networked energy storage solutions. This trend is similar to developments in the IT sector, where data centres distinguish between computing and storage clusters. The development has been spearheaded by faster virtualisation
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technology and cheaper storage costs. In a similar fashion, smart grids begin to morph into a network-attached storage. New kinds of electricity grids will be self-balancing, self-healing networks, which smooth and optimise energy production using demand response algorithms and sophisticated prediction models. Smart grids and connected grid-energy storage will allow electricity producers to send excess supply to temporary storage sites that become energy producers when electricity demand is greater, optimising the production by storing off-peak power for use during peak times. Intermittent energy producers would not need any local storage devices, but would be connected directly to the grid, which effectively becomes a giant battery. Solar energy could be stored for the night’s use, while wind power could be stored for calm times. On the demand side, grid operators and utilities in numerous countries plan to roll out smart metering devices in every household. This development would allow the harnessing of valuable information regarding the consumption habits of end-users and consequently optimise the flows between baseload and intermittent power plants, as well as smart grids and connected storage facilities (Figure 27.4).
27.5.2 Vehicle-to-Grid The vehicle-to-grid (V2G) concept aims to optimise the way we transport, use and produce electricity by turning electric cars into ‘virtual power plants’. Under this relatively new concept, electric cars would store and dispatch electrical energy stored in networked vehicle batteries which together act as one collective battery fleet for ‘peak shaving’ (sending power back to the grid when demand is high) and ‘valley filling’ (charging at night when demand is low) [13]. V2G would allow consumers to charge electric vehicles and monitor their energy costs, using mobile devices. This information helps utilities to better manage grid loads during peak times. Pilot projects include applications for smartphones and a black box with cellular data modem collecting information on the car’s state of charge, the vehicle location and the type of power source it is connected to [15]. Collected data is sent to the cloud where computers calculate, depending on the grid load, the optimal time to recharge [3]. When the electric utility would like to buy power from the V2G network, it holds an auction. The car owners or leasing companies would be able to define the parameters under which they will sell energy from their battery pack. This has led to the emergence of a new term ‘carbitrage’, a fusion of car and arbitrage, coined by the Rocky Mountain Institute in 2008 [10]. The roll out of a ‘fast recharge’ infrastructure is
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SMRT GRID A vision for the future – a network of integrated microgrids that can monitor and heal itself
Smart appliances Can shut off in response to frequency fluctuations.
Demand management Use can be shifted to offpeak times to save money.
Solar panels
Houses Offices
Processors
Sensors
Execute special protection schemes in microseconds.
Detect fluctuations and disturbances, and can signal for areas to be isolated.
Disturbance in the grid
Storage Energy generated at offpeak times could be stored in batteries for later use.
Wind farm
Isolated microgrid
Generators Energy from small generators and solar panels can reduce overall demand on the grid
FIGURE 27.4 Smart grid: a vision for the future. From Ref. [9].
Central power plant Industrial plant
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currently in nascent stages and would need to be extended further into nationwide systems to allow these projects to take off. According to Peter Franken, head of the Energy Distribution department of EKZ in Switzerland, ‘electric vehicles can be used to buffer the irregular production of electricity from future renewable sources, which will contribute to the overall stability of the electrical network’ [11].
27.6 CONCLUSIONS Against the backdrop of technological advances in energy storage, the corner to commercial viability is being turned, with advanced battery systems, flywheel energy systems and other technologies coming of age in terms of pricing and technology. New technologies as well as variations on ‘old’ technologies such as compressed air and pumpedstorage hydropower are being deployed at a rapid pace around the world. The market is ripe with opportunities. Utilities, grid service providers and equipment suppliers are intensifying their efforts in the energy storage arena. The investments in energy storage have shifted away from demand for portable energy to energy efficiency, transmission congestion and levelling solutions for intermittent energy sources. The research firm Navigant Research predicts global investment in energy storage projects to reach US$122 billion, or 56 GW in capacity, between 2012 and 2022 [12]. Widespread adoption of energy storage presents significant benefits which include the following: • protection from long outages, voltage sags and surges; • effective on-site generation for peak shaving customers; • streamlining supply during peak periods by coalescing storage capabilities with renewable resources; • complementary optimisation of photovoltaic and wind-generated electricity; • favourable life-cycle cost, including capital and installation cost, operation and maintenance cost and disposal cost; • versatility for transitioning to micro-grids and decentralisation. The uncertainty around the pricing of storage systems that allows both developers and customers to profit is one of the major hurdles. However, the industry has been working with governments, regulators, utilities and operators to overcome the challenges to facilitate the proliferation of energy storage. Governments, municipalities and cities are eager to create incentives and lift regulatory constraints and participate in demonstration projects that encourage further investment in R&D.
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New developments such as smart grid technologies and increased energy storage efficiency hold the premise to revolutionise the way we produce, use and store energy. ‘Everything flows’, the aphorism attributed to Greek philosopher Heraclitus, should be the objective of a modern energy ecosystem, in which energy storage is one of the building blocks to optimise energy flows and deliver stability. Simultaneously, energy storage solutions lift certain obstacles to the mass development of intermittent energy sources, especially solar and wind power.
References [1] R. Casiday, R. Frey, Phase Changes and Refrigeration: Thermochemistry of Heat Engines, Department of Chemistry, Washington University, 2001. Available at: ,http://www.chemistry.wustl.edu/Bedudev/LabTutorials/Thermochem/Fridge. html. (accessed 11.01.13) (online). [2] US Energy Information Administration, Frequently asked questions. Available at: ,http://www.eia.gov/tools/faqs/faq.cfm?id5105&t53., 2012 (accessed 24.03.13) (online). [3] American Solar Energy Society, Tackling climate change in the U.S. Available at: ,http://ases.org/images/stories/file/ASES/climate_change.pdf., 2007 (accessed 05.09.12) (online). [4] M. Golay, R. Field, W. Green, J.C. Wright, Introduction to Sustainable Energy, Massachusetts Institute of Technology, MIT Open Courseware, Undergraduate course taught in Fall 2010, 2010 Available at: ,http://ocw.mit.edu/courses/nuclearengineering/22-081j-introduction-to-sustainable-energy-fall-2010/lectures-andreadings/., (accessed 03.05.13) (online). [5] R.R. Prabhu, Stationary Fuel Cells Market size to reach 350,000 Shipments by 2022. Renew India Campaign. Available at: ,http://www.renewindians.com/2013/01/stationary-fuel-cells-market-size-to-reach-350000-shipments-by-2022.html., 2013 (accessed 14.01.13) (online). [6] D. Castelvecchi, Spinning into control: High-tech reincarnations of an ancient way of storing energy, Sci. News 171 (20) (2007) 312 313. Available at: ,http://sciencewriter.org/flywheels-spinning-into-control/., 2007 (accessed 20.02.13) (online). [7] L. Herman, International trends in storage: Why? What? How? University of Ljubljana, Faculty of Electrical Engineering, 2013. [8] J.W. Tester, E.M. Drake, M.J. Driscoll, M.W. Golay, W.A. Peters, Sustainable Energy: Choosing Among Options, second ed., MIT Press, 2012. ISBN: 9780262017473. [9] K. Wanda, Smart Grid: a transformative vision, IEEE Smart Grid Taskforce. Available at: ,http://www.fiercesmartgrid.com/story/smart-grid-transformativevision/2013-04-30., 2013 (accessed 02.05.13) (online). [10] Rocky Mountain Institute, Smart Garage Charrette Report. Available at: ,http:// www.rmi.org., 2008 (accessed 14.02.13) (online). [11] Alpstore, National frameworks. Available at: ,http://www.alpstore.info., 2013 (in ‘Downloads’ section) (accessed 08.01.13) (online). [12] R. Martin, Nearly 56 Gigawatts of New Long-Duration Energy Storage to be Installed From 2012 to 2022, Navigant Research (formerly Pike Research), 2013. Available at: ,http://www.navigantresearch.com/newsroom/nearly-56-gigawattsof-new-long-duration-energy-storage-to-be-installed-from-2012-to-2022. (accessed 03.05.13) (online)
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[13] Alpstore, The project: who, why, what? Available at: ,http://www.alpstore.info., 2013 (in ‘Downloads’ section) (accessed 04.05.13) (online). [14] European Commission, DG ENER Working Paper The future role and challenges of Energy Storage. Available at: ,http://ec.europa.eu/energy/infrastructure/doc/ energy-storage/2013/energy_storage.pdf., 2012 (accessed 16.01.13) (online). [15] Freshmile, Electric vehicle as energy storage, Paper presented at Alpstore Kick-Off Conference, City of Grafing b. Mu¨nchen, 25 26 February 2013. [16] C. Pieper, H. Rubel, Revisiting Energy Storage (There Is a Business Case), The Boston Consulting Group (BCG), 2011. Available at: ,http://www.bcg.com/expertise_ impact/publicationdetails.aspx?id5tcm:12-72094. (accessed 14.02.13) (online). [17] L. Wagner, Overview of energy storage methods. Research report published in December 2007, Mora Associates Ltd. Available at: ,http://www.moraassociates. com/publications/., 2007 (accessed 26.05.13) (online). [18] Electrical Energy Storage, White paper released by the International Electrotechnical Commission (IEC). Originally published in The Institute of Energy Economics, Japan, 2005. Available at: , http://www.iec.ch/whitepaper/energystorage/ . , 2011.
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27 Overview of Energy Storage Technologies Le´onard Wagner Mora Associates Ltd, London, UK
27.1 IN A NUTSHELL Energy storage is the storage of some form of energy that can be drawn upon at a later time to perform some useful operation. A windup clock stores potential mechanical energy. A battery stores readily convertible chemical energy to keep a clock chip in a computer running even when the computer is turned off. A pumped-storage plant stores power in a reservoir as potential gravitational energy. ‘Rien ne se perd, rien ne se cre´e, tout se transforme’, proclaimed AntoineLaurent de Lavoisier (1743 1794). The French chemist, who became the father of modern chemistry, invented among others the system of chemical nomenclature still largely in use today and helped construct the metric system. His principle of mass conservation still holds and states that for any system closed to all transfers of matter and energy, the mass of the system must remain constant over time. This is the main concern and opportunity for energy storage technology. Phase changes the transformation of matter from one state to the other open up the possibility to transform electricity into different types of energy and storage media. More scientifically, the breaking of intermolecular attractions, such as found in fusion, vaporisation and sublimation, requires an input of energy to overcome the attractive forces between the particles of the substance. Phase changes involving the formation of intermolecular attractions, such as freezing,
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condensation and deposition, release energy as the particles adopt a lower-energy conformation. Figure 27.1 shows the names of the phase changes between solids, liquids and gases. The arrow to the right of the diagram demonstrates that these three phases have different enthalpies: gas has the highest enthalpy, liquid has an intermediate enthalpy and solid has the lowest enthalpy. Hence, each of the phase transitions shown in Figure 27.1 involves a change in the enthalpy of the substance. Electricity must be consumed as it is generated. It is transmitted in a closed circuit and, unlike common energy storage such as wood or coal, cannot be stored as electrical energy for any practical purpose. Consequently, increasing energy demand cannot be accommodated without either increasing or cutting supplies or arranging for storage techniques to buffer consumption swings. In modern electricity grids, the situation is somewhat more complex with the possibility of absorbing small variations through voltage variations. Energy storage solutions also allow the smoothing of these temporary imbalances in electricity consumption and production. On the one hand, energy storage produces flexibility gains with the prospect of benefiting consumers and producers. On the other hand, any mismatch between the supply and the demand results in either
Gas
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ori
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Sublimation
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en
sa
sa
tio
n
tio
n
Liquid n sio ) Fu lting e (M ing
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e Fre
Phase changes. From Ref. [1].
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energy or efficiency losses. Whenever energy is stored, a fraction is lost in transmission or during the storage period. To gain a better understanding of the technological viability of energy storage, we need to contrast residual energy and efficiency losses to the gains in terms of reliability, portability and flexibility. This is particularly relevant in regions of the world where energy production is intermittent and therefore not capable of providing stable electricity or heat production. Energy storage is a dominant factor in economic development, as was the case during the late 1900s with the widespread introduction of electricity and refined chemical fuels, such as gasoline, kerosene and natural gas [17].
27.2 ENERGY PRODUCTION AND TRANSMISSION Energy storage technologies provide grid operators with an alternative to traditional grid management, which has focussed on the ‘dispatchability’ of power plants, some of which can be regulated very quickly like gas turbines, others much more slowly like nuclear plants. The applications for long-term energy storage include counterbalancing the intermittency of renewable energy sources like wind and solar power, levelling the loads (‘load balancing’) and time-shifting periods of peak demand on the grid and avoiding or delaying construction of costly transmission and distribution (T&D) assets. In addition, policy-makers set ambitious targets attempting to reduce reliance on fossil and nuclear energy sources. For instance, Switzerland is determined to decommission gradually all nuclear power plants by 2034. At present, nuclear power accounts for 40 % of the total electricity produced in Switzerland. While radiation accidents at nuclear power plants remain relatively few, they cause widespread contamination, human hardship and havoc when they happen as evidenced by the incidents at Three Mile Island (1979), Chernobyl (1986) or Fukushima (2011). The share of sustainable but intermittent energy sources is likely to increase as new plants become operational. However, they cannot be used for baseload power generation as their output is relatively volatile and depends on the sun, water or wind. It must be noted that wind parks can be regulated by rotating the blades. Wind energy is maturing and innovation offers strong incentives to repower certain parks which may stretch electricity grids. Outdated infrastructure may struggle to handle increasingly large swings in production. This contrasts with geothermal, nuclear and coal-fired power plants whose advantage is their baseload production capacity [14].
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Where electricity is not immediately consumed, it risks causing a grid collapse. At times, the glut can be so great that utilities pay consumers to take the power and get rid of it. According to Czech grid operaˇ tor CEPS, a.s., the ageing power grids in former communist countries of Eastern Europe are ‘stretched to their limits’ and face potential blackouts when output surges from wind turbines in northern Germany or the Baltic Sea. The Czech Republic is planning to install security switches near borders to disconnect from Europe’s biggest economy to avoid critical overload. As a last resort, it may be necessary to reduce the demand by disconnecting some consumers (‘load shedding’). Technological obsolescence also translates into transmission losses. According to the US Energy Information Administration (2012), annual electricity T&D losses average 7 % [2]. It is estimated that the United States requires US$150 billion in capital expenditures to upgrade and maintain their electricity infrastructure. A study by the American Solar Energy Society (2007) shows that total electricity production can consist of 20 % intermittent energy sources with minimal impact on grid stability [3]. Denmark is a case in point and does not currently experience problems given its modern and wellmaintained energy infrastructure. Alternative solutions to deal with intermittency in wind energy production would be geographic dispersion to de-link weather system effects, and the ability of high-voltage, direct current cables to shift power from windy areas to non-windy areas. Energy storage could help alleviate supply shocks by storing peak production and thereby become a key enabler for intermittent energy sources such as wind and solar energy.
27.3 ENERGY CONSUMPTION Energy consumption follows regular patterns with a fairly large variation over the course of the day. It is the result of consumption habits, social trends, budgets and constraints. Superimposed on this daily pattern are smaller, longer term, seasonal variations with a greater demand for heating and lighting in winter months or in some regions for air conditioning in summer months. Demand from industrial users may also follow cyclic variations, which could affect aggregate demand. Operating experience and statistics backed by regional knowledge about economic growth trends allow reasonably accurate predictions. On top of this, the utility is expected to cater for unexpected emergency situations, such as accidents, natural disasters or breakdown of its own equipment.
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100 90
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FIGURE 27.2 Comparison of daily load curves. Data from Ref. [18].
Notwithstanding, actual demand needs to be constantly monitored in order for utilities to produce the appropriate amount required for a stable grid. Figure 27.2 shows a typical demand profile of an individual country over the course of a day. It would be difficult to match generating capacity to the peaky demand profile. The aggregate demand for all industrial and domestic consumers in a particular community tends to smooth out the overall demand profile, and although the aggregate demand varies during the day and also over the course of the year, it does so in reasonably predictable patterns.
27.4 OVERVIEW OF STORAGE TECHNOLOGIES Energy storage technologies are segmented into those that can deliver precise amounts of electricity very rapidly for a short duration (capacitors, batteries and flywheels), as well as those that take longer to ramp up, but can supply tens or hundreds of megawatts for many hours (compressed air energy storage and pumped-storage hydropower). More recently, researchers have looked at ultracapacitors, which deliver high energy, high power density for easy to charge and
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discharge, and nano-materials that could significantly increase the capacity and lifetime of batteries. We distinguish between the following storage technology options and modes: • • • • • •
electrochemical energy (batteries, flow cells), electrostatic energy (capacitors), electromagnetic energy (superconducting magnets), chemical energy (hydrogen, methane, gasoline, coal, oil), kinetic energy (mechanical flywheels), potential energy (pumped-storage hydropower, compressed air, springs) • thermal energy (ice, molten salts, steam). The next step consists of benchmarking each storage technology against the following criteria: • • • • • •
energy capture rate and efficiency, discharge rate and efficiency, dispatchability and load following characteristics, scale flexibility, durability cycle lifetime, mass and volume requirements footprint of both weight and volume, • safety risks of fire, explosion, toxicity, • ease of materials recycling and recovery. Energy storage technologies may be broadly characterised by their ‘specific energy’ (energy stored per unit volume or mass) and by their ‘peak power’ (how fast that energy can be delivered from the device). For instance, batteries store a lot of energy, but they take a long time to charge and discharge. Capacitors can produce peak power but store only tiny amounts of energy. Supercapacitors offer a combination of high-power, high-energy properties, bridging the gap between batteries and capacitors. Fuel cells operate efficiently over a narrow range of parameters and at elevated temperature, rapidly becoming inefficient under high-power demands (Figure 27.3). Chemical energy is by far the most dominant form of energy storage, both in electricity generation and energy transportation. Chemical fuels in common use are coal, gasoline, diesel fuel, natural gas, liquefied petroleum gas, propane, butane, ethanol, biodiesel and hydrogen. These chemicals can be readily converted to mechanical energy and then to electrical energy. Despite the ecological footprint, liquid fuels are the most commonly used forms of energy storage in transportation. These fuels produce greenhouse gases when used in cars, trucks, trains, ships and aircraft.
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104 Advanced flywheels 5
2
Peak power/(W·kg−1)
Supercapacitors 10
3
Conventional flywheels
5
Ni/Zn
Lithium ion
Methanol H2 ICE
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Zn–Air
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LiM/FeS2
2 H2 Fuel cell 10 5
10
2
5
102
2
5
103
2
3
−1
Specific energy/(W·h·kg )
FIGURE 27.3 Ragone plot showing energy density versus power density for various devices. From Ref. [4].
Carbon-free energy carriers, such as hydrogen and some forms of ethanol or biodiesel, are being sought in response to concerns about the consequences of greenhouse gas emissions.
27.4.1 Electrochemical Energy Storage An early solution to the problem of storing energy for electrical purposes was the development of the battery an electrochemical storage device that transforms chemical energy into electric energy. The battery has three basic components in each cell an anode, a cathode and an electrolyte and their properties relate directly to their individual chemistries. We distinguish between primary and secondary batteries. Primary batteries are the most common and designed for a single use, to be discarded or recycled after they run out. They have very high impedance which means long-life energy storage for low current loads. The most frequently used batteries are carbon zinc, alkaline, silver oxide, zinc air and some lithium batteries. Secondary batteries are
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designed to be recharged and can be recharged up to 1000 times depending on the usage and battery type. Deep discharges result in a shorter charging cycle, whereas shorter discharges result in long charging cycles for most of these batteries (‘memory effect’). The charge time varies from 1 to 12 hours, depending upon battery condition and ambient temperature among other factors. Commonly available secondary batteries are lead acid, nickel cadmium (NiCd), nickel metal hydride (NiMH) and lithium-ion (Li-ion) batteries. Some of the limitations posed by secondary batteries are limited life, limited power capability, low energy efficiency and disposal concerns. 27.4.1.1 Lead Acid Batteries (1859) Lead acid batteries, invented in 1859 by French physicist Gaston Plante´, are the oldest type of rechargeable battery. Despite having a very low energy-to-weight ratio and a low energy-to-volume ratio, their ability to supply high surge currents means that the cells maintain a relatively large power-to-weight ratio. These features, along with their low cost, make them attractive for use in vehicles to provide the high current required by automobile starter motors. 27.4.1.2 Nickel Cadmium Battery (1899) In 1899, the Swedish scientist Waldmar Jungner invented the nickel cadmium (NiCd) battery, a rechargeable battery that had nickel and cadmium electrodes in a potassium hydroxide solution the first battery to use an alkaline electrolyte. The first models were robust and had significantly better energy density than lead acid batteries, but were much more expensive. NiCd batteries may suffer from a ‘memory effect’ if they are discharged and recharged to the same state of charge hundreds of times. The apparent symptom is that the battery ‘remembers’ the point in its charge cycle where recharging began and during subsequent use suffers a sudden drop in voltage at that point, as if the battery had been discharged. 27.4.1.3 Alkaline Battery (1950s) Up until the late 1950s the zinc carbon battery continued to be a popular primary cell battery, but its relatively low battery life hampered sales. In 1955, Eveready (now known as Energizer) wanted to find a way to extend the life of zinc carbon batteries, but engineers at Eveready believed alkaline batteries held more promise despite the high price tag. They came up with a new alkaline battery that consisted of a manganese dioxide cathode and a powdered zinc anode with an alkaline electrolyte. Using powdered zinc gave the anode a greater surface area. Alkaline batteries hit the market in 1959 and still account for
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80 % of manufactured batteries and over ten billion (10 3 109) individual units produced worldwide in 2011. 27.4.1.4 Lithium Batteries (1970s) Lithium is the metal with lowest density and has the greatest electrochemical potential and energy-to-weight ratio, so in theory it would be an ideal material to manufacture batteries. Experimentation with lithium batteries began in 1912, and in the 1970s the first lithium batteries were sold. In the 1980s, an American chemist John B. Goodenough led a research team at Sony that would produce the lithium-ion battery, a rechargeable and more stable version of the lithium battery with the first ones sold in 1991. Yet, the risk of high temperatures and fire remains as of today as evidenced by Apple’s recall of 1.8 million notebook batteries in 2006, and the Boeing 787 Dreamliner battery problems which caused the emergency landing of an All Nippon Airways aircraft in January 2013. 27.4.1.5 Nickel Metal Hydride Battery (1980s) Towards the end of the 1980s, Stanford R. Ovshinsky invented the NiMH battery, a variant of the NiCd, which replaced the cadmium electrode with one made of a hydrogen-absorbing alloy. NiMH batteries tend to have longer lifespans than NiCd batteries (and their lifespans continue to increase as manufacturers experiment with new alloys), and since cadmium is toxic, NiMH batteries are less damaging to the environment. 27.4.1.6 Lithium-Ion Polymer Batteries (1990s) The lithium-ion polymer battery was released in 1996. These batteries hold their electrolyte in a solid polymer composite instead of a liquid solvent, and the electrodes and separators are laminated to each other. This difference allows the battery to be encased in a flexible wrapping instead of a rigid metal casing, which means such batteries can be specifically shaped to fit a particular device. They also have a higher energy density than normal lithium-ion batteries. These advantages have made it a choice battery for mobile phones, laptops and tablets.
27.4.2 Electrostatic Energy Storage 27.4.2.1 Capacitors Capacitors store energy in an electrostatic field rather than as a chemical state as in batteries. They use physical charge separation between two electrodes to store energy, for instance between the surfaces of metalised plastic film or metal electrodes. During charging, the
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electrically charged ions in the electrolyte migrate towards the electrodes of opposite polarity due to the electric field between the charged electrodes created by the applied voltage. Thus two separate charged layers are produced. Although similar to a battery, the double-layer capacitor depends on electrostatic action. Since no chemical action is involved the effect is easily reversible and the typical lifespan is hundreds of thousands of cycles. When compared to batteries, the energy density of capacitors is very low, but the power density is very high. This means that capacitors are able to deliver or accept high currents, but only for extremely short periods, due to their relatively low capacitance. As a consequence, applications are relatively limited. For instance, capacitors are used as power backup for memory functions in consumer products such as mobile phones, laptops and radio tuners. 27.4.2.2 Supercapacitors and Ultracapacitors The supercapacitor and ultracapacitor resemble regular capacitors except that they offer very high capacitance in a smaller form. Supercapacitors rely on the separation of charge at an electrified interface that is measured in fractions of a nanometre, compared with micrometres for most polymer film capacitors. Supercapacitors use a molecule-thin layer of electrolyte, rather than a manufactured sheet of material, as dielectric to separate the charge. Lifetime is virtually indefinite, and energy efficiency rarely falls below 90 % when they are kept within their design limits. The power density is higher than that of batteries, while their energy density is generally lower. However, unlike batteries, almost all of this energy is available in a reversible process.
27.4.3 Electromagnetic Energy Storage 27.4.3.1 Superconducting Magnetic Energy Storage In a superconducting magnetic energy storage (SMES) system, the energy is stored within a magnet that is capable of releasing megawatts of power within a fraction of a cycle to replace a sudden loss in line power. It stores energy in the magnetic field created by the flow of direct current (DC) power in a coil of superconducting material that has been cryogenically cooled. The stored energy can be released back to the network by discharging the coil. The power conditioning system uses an inverter/rectifier to transform alternating current (AC) power to DC or convert DC back to AC power. The inverter/rectifier accounts for about 23 % energy loss in each direction [16]. In comparison to other storage methods, SMES
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systems lose the least amount of electricity during the storage process with a round-trip efficiency greater than 95 %. Due to the energy requirements of refrigeration and the high cost of superconducting wire, SMES technology is currently used for short duration energy storage. These systems have been in use for several years to improve industrial power quality and to provide a high-quality service for individual customers vulnerable to voltage fluctuations. Typically, SMES systems are installed on the exit of the power plants to stabilise output or on industrial sites where they can be used to accommodate peaks in energy consumption (e.g. steel plants or rapid transit railway) in a highly efficient manner.
27.4.4 Chemical Energy 27.4.4.1 Hydrogen and Fuel Cell Technologies Fuel cells were invented about the same time as the battery. However, fuel cells were not well developed until the advent of spacecraft when lightweight, non-thermal sources of electricity were required. Fuel cell development has increased in recent years to an attempt to increase conversion efficiency of chemical energy stored in hydrocarbon or hydrogen fuels into electricity. Like a battery, a fuel cell uses stored chemical energy to generate power. Unlike batteries, its energy storage system is separate from the power generator. It produces electricity from an external fuel supply as opposed to the limited internal energy storage capacity of a battery. Hydrogen is a chemical energy carrier similar to petroleum, ethanol and natural gas with the unique characteristic that it is the only carbonfree or zero-emission chemical energy carrier. It is a widely used industrial chemical that can be produced from any primary energy source. The topic is the subject of Chapter 23 of this volume. Fuel cells are very useful as power sources in remote locations, such as spacecraft, remote weather stations, large parks, rural locations and in certain military applications. Fuel cell systems running on hydrogen can be compact and lightweight. Because they have no moving parts, and do not involve combustion, they can achieve up to 99.9999 % reliability in ideal conditions. This equates to less than 1 min of downtime in a 6-year period [5]. Hydrogen production in quantities sufficient to replace existing hydrocarbon fuels is not possible at present. So far, the significant capital investment in hydrogen production plants has limited widespread use. If production costs were to be reduced, hydrogen fuels may become more attractive commercially, providing clean, efficient power for our homes, businesses and vehicles.
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27.4.5 Kinetic Energy Storage 27.4.5.1 Flywheels Flywheels are rotating mechanical devices to store kinetic energy. They capture the momentum in a rotating mass and release the energy by applying torque to a mechanical load. The potter’s wheel is often cited as the earliest use of a flywheel. Advanced flywheel energy storage systems have rotors made of high-strength carbon filaments, suspended by magnetic bearings and spinning at speeds from 20 000 to over 50 000 rpm in a vacuum enclosure [6]. Such flywheels can come up to speed in a matter of minutes, much faster than most energy storage technologies. The benefits of this storage technology include: not being constrained by tight temperature limits, no development of a charge memory and no lifetime degradation. Large-sized flywheels exist and operate on the same principle but store more energy with a higher mass and physical size.
27.4.6 Potential Energy Storage Compressed air energy storage uses wind turbines to drive compressed air into underground aquifers. The air is released to generate electricity when needed. This is a new twist on the idea of using wind energy in a way that removes the variability to a large extent and increases the dispatchability. At the moment, there are only two operational compressed air storage plants. One is in Huntdorf, Germany and the other one in McIntosh, the United States. The Huntdorf plant is located on a 300 000 m3 salt dome, in which compressed air is stored, originally to capture excess nuclear power production [7]. For rapid responses to power shortages, the air is channelled to a conventional gas turbine, at a capacity of up to 290 MW. Smaller, even mobile compressed air batteries are currently in deployment as well. They are designed to take advantage of variations in the price of electricity. When power is cheap, it is used to run their compressors. When it is expensive, the valves are opened and the generators turn on. Compressed air storage plants are inefficient, and so they are commercially viable only in places where the price of power varies dramatically. But the intermittent nature of wind power can cause just that sort of variability. 27.4.6.1 Cryogenic Energy Storage Cryogenic energy storage is a variant of the compressed air energy storage and uses low-temperature (cryogenic) liquids such as liquid air or liquid nitrogen as energy storage.
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27.4.6.2 Pumped-Storage Hydropower Some areas of the world have used geographic features to store large quantities of water in elevated reservoirs, using excess electricity at times of low demand to pump water up to the reservoirs, then letting the water fall through turbine generators to retrieve the energy when demand peaks. Most of the global pumped-storage hydropower capacity caters for applications such as energy management, frequency control and provision of reserve. These are usually connected to the high-voltage transmission grid and designed to buffer fluctuations originating from a large number of sources on both supply and demand side. From a business perspective, the beauty of providing negative balance energy is that the pumped-storage can use surplus energy taken from the grid at night time, for example (for which the facility is paid) to generate additional revenues by selling it on the power market at peak prices 12 h later. Worldwide pumped-storage capacity stands at roughly 100 GW in 2012, which represents 3 % of global generation capacity [8]. In 2000, the United States had 19.5 GW of pumped-storage capacity, accounting for 2.5 % of its baseload capacity. In 1999, the European Union had 32 GW capacity of pumped-storage, representing 5.5 % of total electrical capacity in the EU. Pumped-storage hydropower presents major advantages, viz. efficiency between 70 % and 85 %, low running costs and scalability with discharge times ranging from several hours to a few days. The major drawbacks are the long construction times and high capital expenditure.
27.4.7 Thermal Energy Storage 27.4.7.1 Molten Salt Batteries Molten salt batteries are a class of primary and secondary electric batteries that use molten salts as an electrolyte. They offer both a higher energy density through the proper selection of reactant pairs as well as a higher power density by means of a high conductivity molten salt electrolyte. They are used in services where high energy density and high power density are required. These features make rechargeable molten salt batteries a promising technology for powering electric vehicles. However, operating temperatures of 400 to 700 C create thermal management and safety issues, which place more stringent requirements on the remaining battery components.
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27.4.7.2 Solar Ponds A solar pond is simply a pool of water, which collects and stores solar energy. It contains layers of salt solutions with increasing concentration and therefore density to a certain depth, below which the solution has a uniform high salt concentration. When sunlight is absorbed, the density gradient prevents heat in the lower layers from moving upwards by convection and leaving the pond. As a result, the temperature at the bottom of the pond will rise to over 90 C while the temperature at the top of the pond is usually around 30 C. The heat trapped in the salty bottom layer can be used for many different purposes, such as the heating of buildings or industrial hot water or to drive a turbine for generating electricity. Examples of solar pond installations include a 150 kW solar pond built by Ormat Technologies in Israel near the Dead Sea (1980), a salinity gradient solar pond at El Paso, the United States (1986), the Bhuj solar pond in India (1993) and a demonstration solar pond and associated heating system developed in collaboration with RMIT University, Melbourne, Australia (2001). 27.4.7.3 Seasonal Thermal Storage (Heat Pumps) Seasonal thermal storage can be divided into two broad categories. In both cases, very effective insulation of the building structure is required to minimise heat loss from the building, and hence the amount of heat that needs to be stored and used for space heating. Low-temperature systems use the soil adjoining the building as a low-temperature seasonal heat store (reaching temperatures similar to average annual air temperature), drawing upon the stored heat for space heating. Such systems can also be seen as an extension to the building design (normally passive solar building design). High-temperature seasonal heat stores are essentially an extension of the building’s heating, ventilation, air conditioning and water heating systems. Water is normally the storage medium, stored in tanks at temperatures that can approach boiling point.
27.5 RESEARCH AND DEVELOPMENTS 27.5.1 Smart Grids and Networked Energy Storage Utilities and grid operators are increasingly deploying networked energy storage solutions. This trend is similar to developments in the IT sector, where data centres distinguish between computing and storage clusters. The development has been spearheaded by faster virtualisation
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technology and cheaper storage costs. In a similar fashion, smart grids begin to morph into a network-attached storage. New kinds of electricity grids will be self-balancing, self-healing networks, which smooth and optimise energy production using demand response algorithms and sophisticated prediction models. Smart grids and connected grid-energy storage will allow electricity producers to send excess supply to temporary storage sites that become energy producers when electricity demand is greater, optimising the production by storing off-peak power for use during peak times. Intermittent energy producers would not need any local storage devices, but would be connected directly to the grid, which effectively becomes a giant battery. Solar energy could be stored for the night’s use, while wind power could be stored for calm times. On the demand side, grid operators and utilities in numerous countries plan to roll out smart metering devices in every household. This development would allow the harnessing of valuable information regarding the consumption habits of end-users and consequently optimise the flows between baseload and intermittent power plants, as well as smart grids and connected storage facilities (Figure 27.4).
27.5.2 Vehicle-to-Grid The vehicle-to-grid (V2G) concept aims to optimise the way we transport, use and produce electricity by turning electric cars into ‘virtual power plants’. Under this relatively new concept, electric cars would store and dispatch electrical energy stored in networked vehicle batteries which together act as one collective battery fleet for ‘peak shaving’ (sending power back to the grid when demand is high) and ‘valley filling’ (charging at night when demand is low) [13]. V2G would allow consumers to charge electric vehicles and monitor their energy costs, using mobile devices. This information helps utilities to better manage grid loads during peak times. Pilot projects include applications for smartphones and a black box with cellular data modem collecting information on the car’s state of charge, the vehicle location and the type of power source it is connected to [15]. Collected data is sent to the cloud where computers calculate, depending on the grid load, the optimal time to recharge [3]. When the electric utility would like to buy power from the V2G network, it holds an auction. The car owners or leasing companies would be able to define the parameters under which they will sell energy from their battery pack. This has led to the emergence of a new term ‘carbitrage’, a fusion of car and arbitrage, coined by the Rocky Mountain Institute in 2008 [10]. The roll out of a ‘fast recharge’ infrastructure is
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SMRT GRID A vision for the future – a network of integrated microgrids that can monitor and heal itself
Smart appliances Can shut off in response to frequency fluctuations.
Demand management Use can be shifted to offpeak times to save money.
Solar panels
Houses Offices
Processors
Sensors
Execute special protection schemes in microseconds.
Detect fluctuations and disturbances, and can signal for areas to be isolated.
Disturbance in the grid
Storage Energy generated at offpeak times could be stored in batteries for later use.
Wind farm
Isolated microgrid
Generators Energy from small generators and solar panels can reduce overall demand on the grid
FIGURE 27.4 Smart grid: a vision for the future. From Ref. [9].
Central power plant Industrial plant
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currently in nascent stages and would need to be extended further into nationwide systems to allow these projects to take off. According to Peter Franken, head of the Energy Distribution department of EKZ in Switzerland, ‘electric vehicles can be used to buffer the irregular production of electricity from future renewable sources, which will contribute to the overall stability of the electrical network’ [11].
27.6 CONCLUSIONS Against the backdrop of technological advances in energy storage, the corner to commercial viability is being turned, with advanced battery systems, flywheel energy systems and other technologies coming of age in terms of pricing and technology. New technologies as well as variations on ‘old’ technologies such as compressed air and pumpedstorage hydropower are being deployed at a rapid pace around the world. The market is ripe with opportunities. Utilities, grid service providers and equipment suppliers are intensifying their efforts in the energy storage arena. The investments in energy storage have shifted away from demand for portable energy to energy efficiency, transmission congestion and levelling solutions for intermittent energy sources. The research firm Navigant Research predicts global investment in energy storage projects to reach US$122 billion, or 56 GW in capacity, between 2012 and 2022 [12]. Widespread adoption of energy storage presents significant benefits which include the following: • protection from long outages, voltage sags and surges; • effective on-site generation for peak shaving customers; • streamlining supply during peak periods by coalescing storage capabilities with renewable resources; • complementary optimisation of photovoltaic and wind-generated electricity; • favourable life-cycle cost, including capital and installation cost, operation and maintenance cost and disposal cost; • versatility for transitioning to micro-grids and decentralisation. The uncertainty around the pricing of storage systems that allows both developers and customers to profit is one of the major hurdles. However, the industry has been working with governments, regulators, utilities and operators to overcome the challenges to facilitate the proliferation of energy storage. Governments, municipalities and cities are eager to create incentives and lift regulatory constraints and participate in demonstration projects that encourage further investment in R&D.
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27. OVERVIEW OF ENERGY STORAGE TECHNOLOGIES
New developments such as smart grid technologies and increased energy storage efficiency hold the premise to revolutionise the way we produce, use and store energy. ‘Everything flows’, the aphorism attributed to Greek philosopher Heraclitus, should be the objective of a modern energy ecosystem, in which energy storage is one of the building blocks to optimise energy flows and deliver stability. Simultaneously, energy storage solutions lift certain obstacles to the mass development of intermittent energy sources, especially solar and wind power.
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[13] Alpstore, The project: who, why, what? Available at: ,http://www.alpstore.info., 2013 (in ‘Downloads’ section) (accessed 04.05.13) (online). [14] European Commission, DG ENER Working Paper The future role and challenges of Energy Storage. Available at: ,http://ec.europa.eu/energy/infrastructure/doc/ energy-storage/2013/energy_storage.pdf., 2012 (accessed 16.01.13) (online). [15] Freshmile, Electric vehicle as energy storage, Paper presented at Alpstore Kick-Off Conference, City of Grafing b. Mu¨nchen, 25 26 February 2013. [16] C. Pieper, H. Rubel, Revisiting Energy Storage (There Is a Business Case), The Boston Consulting Group (BCG), 2011. Available at: ,http://www.bcg.com/expertise_ impact/publicationdetails.aspx?id5tcm:12-72094. (accessed 14.02.13) (online). [17] L. Wagner, Overview of energy storage methods. Research report published in December 2007, Mora Associates Ltd. Available at: ,http://www.moraassociates. com/publications/., 2007 (accessed 26.05.13) (online). [18] Electrical Energy Storage, White paper released by the International Electrotechnical Commission (IEC). Originally published in The Institute of Energy Economics, Japan, 2005. Available at: , http://www.iec.ch/whitepaper/energystorage/ . , 2011.
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