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Batteries, Flywheels, Supercapacitors, Superconductors & Power Conditioning & Contol EquipmentBattery Technologies | Flywheel Technologies | Advantages of Flywheels | Disadvantages of Flywheels | Supercapacitors | Supercapacitor Applications | Superconductors | Superconductor Applications | Superconducting Magnetic Energy Storage Systems | Power Conditioning & Control Equipment | Inverters | Battery Charge Controllers (Regulators) | Further Information | References |Battery Storage TechnologiesEnergy storage technologies are used to provide power when there is insufficient power being generated, and to store excess production when there is more power being generated than can be used. Wind and sunshine are not always available when there is a demand for energy, so backup storage is generally required when using renewable inputs especially in Stand-alone Power Supply (SPS) systems. There are many types of battery technologies available. These include: lead-acid, alkaline, carbon-zinc, nickel metal hydride, nickel-cadmium, lithium-ion, vanadium-redox, zinc bromine, sodium sulphur, zinc-air, zinc chloride, silver-oxide, lithium manganese, mercury, thermal, molten salt etc. However the most common type of battery for renewable energy SPS systems are deep cycle lead-acid batteries. Deep-cycle lead-acid batteries are similar to car batteries, but are better suited to the heavy charging and discharging that is typical of SPS systems. When several batteries are connected, they are called a battery bank (see Figure 2). There are Australian Standards for battery installations and safety regulations that need to be followed in a SPS system installation. 
Figure 1 Schematic of a lead acid battery.
(©2005 EUROBAT) Battery storage can provide all the storage in renewable SPS systems that do not have a backup system. However, if a backup generator is available, less battery storage is required. Battery ratings are expressed using the voltage and the storage capacity of the battery. The storage capacity of a battery is the quantity of electricity that a fully charged battery can deliver under specified conditions. This capacity is expressed in Ampere-hours (Ah) and is usually specified at an operating temperature of 25°C. The capacity is determined by the time taken to discharge the battery at a constant current until a specified cut-off or final voltage is reached, which is dependent on the battery type and manufacturer. The capacity is stated as an Ampere-hour value at a discharge rate, which is noted by a "C" followed by a number indicating the rated hours. An example of a typical rating is 2-volt battery with a capacity of 100 Ah @ C100 rate.
 Figure 2 48V DC Battery Bank
Flywheel Storage TechnologiesA flywheel is a mass rotating about an axis, which can store energy mechanically in the form of kinetic energy. Energy is required to accelerate the flywheel so it is rotating. This is usually achieved by an electric motor when being used in an electrical system. Once it is rotating, it is in effect a mechanical battery, that has a certain amount of energy that can be stored depending on its rotational velocity and its moment of inertia. The faster a flywheel rotates, the more energy it stores. This stored energy can be retrieved by slowing down the flywheel and returning the kinetic energy to the electrical motor, which is used as a generator.
The amount of power produced by renewable energy devices such as photovoltaic cells and wind turbines varies significantly on an hourly, daily and seasonal basis due to the variation in the availability of the sun, wind and other renewable resources. Even when conventional technologies are generating electricity at a constant rate, there are demand fluctuations throughout the day. This mismatch of load to electrical supply means that power is not always available when it is required and on other occasions, there is excess power. Flywheel technologies can used to provide power when there is insufficient power being generated, and to store excess production. Another important application for flywheel technologies is for power conditioning and for providing power when there are durations of total power loss as a result of electricity grid failure.  Figure 3 A G2 Flywheel Module, courtesy of NASA
Flywheels are a developing technology that may replace conventional batteries in such applications as stand-alone power systems, vehicles and uninterruptible power systems (UPS) and are also are playing a growing role in power conditioning in conventional grid systems. Disadvantages of FlywheelsHowever, use of flywheel accumulators is currently hampered by the danger of explosive shattering of the massive wheel due to overload. One of the primary limits to flywheel design is the tensile strength of the material used for the rotor. Generally speaking, the stronger the disc, the faster it may be spun and the more energy the system can store. When the tensile strength of a flywheel is exceeded the flywheel will shatter, releasing all of its stored energy at once; this is commonly referred to as "flywheel explosion" since wheel fragments can reach kinetic energy comparable to that of a bullet. Consequently, traditional flywheel systems require strong containment vessels as a safety precaution, which increases the total mass of the device. Fortunately, composite materials tend to disintegrate quickly once broken, and instead of large chunks of high-velocity shrapnel, we obtain a containment vessel filled with red-hot sand. Many customers of modern flywheel power storage systems prefer to have them embedded in the ground to halt any material that might escape the containment vessel. (Wikipedia, 2007). Advantages of FlywheelsSome of the advantages of flywheels are that they can store and release large amounts of power very quickly and efficiently when compared to conventional batteries. Also, the lifetime and maintenance of flywheel technologies are around 20 to 30 years and some can operate with no maintenance in that time. Batteries often need strict environmental conditions to operate correctly, such as operating temperatures below 40 degrees. Flywheels also do not suffer from the memory effect, which plagues some types of batteries. Flywheels can operate under higher temperatures and a wider range of environmental conditions. Flywheels are also less potentially damaging to the environment, being made of largely inert or benign materials. Another advantage of flywheels is that by a simple measurement of the rotation speed, it is possible to know the exact amount of energy stored. When used in vehicles, flywheels also act as gyroscopes, since their angular momentum is typically of a similar order of magnitude as the forces acting on the moving vehicle. However this property may be detrimental to the vehicle's handling characteristics while turning. On the other hand, this property could be utilised to improve stability in curves. (Wikipedia, 2007).  Figure 4 A power and energy storage comparison diagram.
(courtesy of NREL) Many electrical applications need high quality power to ensure they function properly and do not lose valuable information or communication links. Flywheels are used as Uninterruptible Power Supply (UPS) systems to deliver power protection for critical operations. A growing use for flywheel technology involves frequency regulation on the electricity grid.  Figure 5 Beacon Power's Smart Energy 25 system produces 100 kW of maximum power and stores 25 kWh
(© 2008 Beacon Power)
 Figure 6 Active Power’s CleanSource® single module systems provide 130 – 240 kW of output, while their multi-module systems can provide up to 8,400 kVA.
(© 2008 Active Power, Inc) SupercapacitorsWith characteristics of both batteries and capacitors, supercapacitors (also called electrochemical capacitors or ultracapacitors) could be used by utilities to regulate power quality. A capacitor is a device that stores energy in the electric field created between a pair of conductors on which electric charges of equal magnitude, but opposite sign, have been placed. A supercapacitor is an electrochemical capacitor that has an unusually large amount of energy storage capability relative to its size, when compared to common capacitors. These are of particular interest in automotive applications for hybrid vehicles and as supplemental storage for battery electric vehicles, as well as power electronics applications such as in wind turbines. When a supercapacitor is charged, the energy is stored as a charge or concentration of electrons on the surface of a material. This means a supercapacitor is capable of very fast charges and discharges which can achieve a very large number of cycles without degradation, even at 100% depth of discharge (DOD). Capacitors are made from various materials in many ways, from multilayer ceramics, ceramic disc, multilayer polyester film, tubular ceramic, axial and radial polystyrene, to carbon nanotubes (Wikipedia, 2006a). Supercapacitors found their first application in military projects such as starting the engines of battle tanks and submarines or replacing batteries in missiles. Common applications today include starting diesel trucks and railroad locomotives, actuators, and in electric/hybrid-electric vehicles for transient load levelling and regenerating the energy of braking. NASA has used 30 large supercapacitors in its turbo-electric city bus (EERE, 2006). Carbon nanotubes and polymers, or carbon aerogels, are practical for supercapacitor designs. Carbon nanotubes have excellent nanoporosity properties, allowing tiny spaces for the polymer to sit in the tube and act as a dielectric. Polymers have a redox (reduction-oxidation) storage mechanism along with a high surface area. MIT's Laboratory of Electromagnetic and Electronic Systems (LEES) is researching using carbon nanotubes (LEES, 2006). Supercapacitors are also being made of carbon aerogel. Carbon aerogel is a unique material providing extremely high surface area of about 400-1000 m2/g. Capacitances of up to 104F/g and 77 F/cm3 have been achieved. Some corporations, such as Cooper Electronic Technologies, are already producing aerogel-based supercapacitors. Their maximum voltage is 2.5V, but they can achieve an energy density of 325 kJ/kg (disputed as 10.6 kJ/kg, see Discussion), which is about 70% of that provided by the state-of-the-art lithium polymer batteries. Power densities achieved are even higher, up to 20 kW/kg, orders of magnitude higher than what Li-poly offers. Small aerogel supercapacitors are being used as backup batteries in microelectronics, but applications for electric vehicles are increasing (Wikipedia, 2006b).  Figure 7 Superconductors. Courtesy of Harald Sattler’s Modellbau und Elektronik Supercapacitor Tuning Page
Supercapacitor ApplicationsThe newly developed Honda Fuel Cell Stack and ultra-capacitor combine to power the motor, with onboard high-pressure hydrogen tanks for fuel storage for the new Honda FCX hydrogen fuel cell car. The fuel cell vehicle is powered by an electric motor running on electricity generated by a fuel stack which uses hydrogen as its energy source. Considering factors such as energy efficiency during power generation and driving, overall system weight, and packaging efficiency, Honda has equipped the FCX with a system that combines a fuel cell stack and ultra-capacitor with onboard high-pressure hydrogen tanks (Honda Worldwide, 2006). Renewable energy technologies feature in the applications for supercapacitors with Alain Riedo, vice president and general manager of Maxwell’s Swiss subsidiary, Maxwell Technologies SA, said that Enercon currently uses BOOSTCAP® ultracapacitors for backup energy storage and power delivery in wind turbine models ranging in output from 300 kW to 6 MW. “In addition to being one of the world’s largest wind turbine producers, Enercon is recognized as a leading innovator in the design and manufacture of megawatt class turbines,” Riedo said. “To optimize energy output and enhance system reliability and longevity, each of Enercon’s turbines’ three blades has an independent braking and pitch adjustment mechanism with backup power to ensure continuous operation in the event of a power failure. Each turbine incorporates from 200 to 700 BOOSTCAP ultracapacitors for backup power.” (Maxwell Technologies, 2006). Ulrich Neundlinger, Enercon’s managing director of switching units, said that the company is expanding its use of ultracapacitors for blade pitch system backup power after initial deployments confirmed their significant advantages over traditional battery solutions. “Ultracapacitors enabled us to overcome a number of battery-related design challenges, including poor low temperature performance and limited operational life,” Neundlinger said. “Maxwell’s products emerged as the clear choice for this application on the basis of their robust construction, long operating life and cost-effectiveness. Wind turbine operators need low-maintenance systems that operate reliably for many years, and BOOSTCAP products have proven that they can help us to continue to meet our customers’ expectations.” (Maxwell Technologies, 2006).  Figure 8 Photo copyright of Maxwell Technologies.
SuperconductorsThe capacity of superconducting materials to handle large currents with no resistance and extremely low energy losses is a huge benefit over competing technologies. The relatively recent developments in this field are impressive and will play a part in more efficient energy systems.  Figure 9 A superconducting metal. (courtesy of Tom & Robs Superconducting Website)
Superconductors are generally divided into two categories; Low-Temperature Superconductors (LTS or type I), and High-Temperature Superconductors (HTS or type II). LTS must be cooled to just above absolute zero (-269°C) and can be utilised as storage devices that provide power conditioning and power backup and are used by some electricity utilities. HTS only have to be cooled to -173°C are preferred to LTS technologies as they do not have problems of maintaining such low temperatures (EERE, 2005). The applications of superconductors are diverse. They are being used to improve the efficiency of motors, generators, transmission lines, transformers, and energy storage technologies. Using an integrated power system incorporating superconductor technologies can reduce the amount of power that is needed to be generated to supply the same demand, as well as allow many new applications, such as magnetic levitating trains (See Figure 2). Superconducting Magnetic Energy Storage (SMES) is a new technology that is used to regulate power fluctuations and maintain the stability of the grid when large changes in load occur. SMES systems store energy in a magnetic field created by the flow of direct current in a coil of superconducting material that has been cryogenically cooled. A superconducting material enhances storage capacity. In low-temperature superconducting materials, electric currents encounter almost no resistance. The challenge is to maintain that characteristic without having to keep the systems quite so cold (NREL, 2006).
Superconductor ApplicationsThe following table shows a list of (potential) applications for superconducting technologies;
Table 1 Table derived from EERE.
 Figure 10 Maglev trains in Japan (© Maglev R&D at the Railway Technical Research Institute)
Superconducting Magnetic Energy Storage SystemsA typical SMES system includes three parts: superconducting coil, power conditioning system and cryogenically cooled refrigerator. Once the superconducting coil is charged, the current will not decay and the magnetic energy can be stored indefinitely. 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 direct current or convert DC back to AC power. The inverter/rectifier accounts for about 2-3% energy loss in each direction. SMES loses the least amount of electricity in the energy storage process compared to other methods of storing energy. SMES systems are highly efficient; the round-trip efficiency is greater than 95%. Due to the energy requirements of refrigeration, and the limits in the total energy able to be stored, SMES is currently used for short duration energy storage. Therefore, SMES is most commonly devoted to improving power quality. If SMES were to be used for utilities it would be a diurnal storage device, charged from baseload power at night and meeting peak loads during the day. The high cost of superconductors is the primary limitation for commercial use of this energy storage method (Wikipedia, 2006). One of the applications for SMES systems are with renewable energy technologies that produce large fluctuations in power generation. There are electricity utility applications for SMES for MW sized systems that are being used in Northern Wisconsin. This installation uses a string of distributed SMES units, deployed to enhance stability of a transmission loop. The transmission line is subject to large, sudden load changes due to the operation of a paper mill, with the potential for uncontrolled fluctuations and voltage collapse. Besides stabilising the grid, the six SMES units also provide increased power quality to customers served by connected feeders. Several 1 MW units are used for power quality control in installations around the world. (EERE, 2005). SMES power is available almost instantaneously and very high power outputs are provided for a brief period of time, with no loss of power, and there are no moving parts. However the energy content of SMES is short-lived and the technology used to cool the SMES system down can cause issues (NREL, 2006). This technology is used in the high-speed magnetic-levitated trains. In addition to this application, superconductors are also being developed for use in microelectronics and communications (NREL, 2006). In addition to storage, http://www.newscientist.com/blog/technology/2007/07/levitating-in-wind.html Power Conditioning & Control EquipmentThe variable output from renewable energy devices means
that power conditioning and control equipment is required to transform output
into a form (voltage, current & frequency) that can be used by electrical appliances. In systems with a number of power sources, careful design and matching of
system components are required to ensure the system functions correctly. Most
components have control functions inbuilt for the specific task for which they are
designed. In some cases the inverter has inputs indicating the state of the
system, and it can change the system's operation if necessary. The overall
system control is automatically monitored by key components, such as the
inverter (see Figure 1).
InvertersRenewable energy systems often provide low voltage, direct current (DC) from batteries, solar panels or wind generators. To use this DC power directly requires special non-standard appliances that may be available for camping and other portable or low power applications. Large DC, such as fridges, are relatively expensive. Electricity available from the main electricity grid is provided as alternating current (AC) at 240V in Australia, so most appliances are manufactured to suit this supply. The electrical energy used by the appliance is referred to as the load on the system. An inverter is an electrical device that changes direct current (DC) into alternating current (AC). The inverter enables standard appliances designed for the main electricity grid to be used in SPS systems. Inverters often incorporate extra electronic circuits that control battery charging and load management. Generally, inverters used in most household systems now produce power of a similar quality to that in the main electricity grids. These are referred to as true sinewave inverters. Earlier model inverters produced lower quality power, which was adequate for most appliances. These are now only used in very small, inexpensive systems. They are often referred to as modified square wave inverters and sometimes as modified sinewave inverters.
Figure 11 Photo of an inverter Battery Charge Controllers (Regulators)A battery charge controller should always be used to protect the battery bank from over-charging and over-discharging. The simplest method used for charge control will turn off the energy source as the battery voltage reaches a maximum and will turn off the load when the battery voltage reaches a preset minimum. The battery charge controller for a system is more commonly referred to as a regulator. There are 3 main types of regulator; Shunt, Series and Pulse Width Modulated (PWM) regulators.
Figure 12 Plasmatronics series regulator To stop the battery from being over discharged a load disconnector can be used. Quite often this can be through an auxiliary circuit in the battery charge controller or through a low voltage disconnect function built into the inverter. Maximum Power Point TrackerAnother type of regulator is the Maximum Power Point Tracker (MPPT). This is a DC to DC converter that allows the array voltage to be different to the battery voltage used with photovoltaic modules to optimise the match between the panels and the battery bank. MPPT battery charge controllers measure the incoming power from the array and adjusts the arrays, so that the maximum power is being sent to the battery bank independent of the battery bank voltage.  Figure 13 Photo courtesy of Industrial
Electronics Research and Development.
Advancements in power conditioning technologies in recent decades is providing many new methods of providing quality power from sub optimal electricity supplies. The decreasing costs of many of these technologies, especially low power inverters is increasing access to power conditioning solutions to low voltage DC generating systems such as photovoltaics.
Further InformationRISE Information Portal - Information regarding renewable energy resources, technologies, applications, systems designs and case studies.SEDO - RE RAPS Brochure (PDF) SEDO - RAPS User Guide and Maintenance Advice (PDF) Battery Electricity - Wikipedia Beacon Power Flywheels - Wikipedia Mpower – Supercapacitors Mpower - Cells Honda’s FCX Hydrogen Powered Vehicle with Supercapacitors Maxwell Technologies Ultracapacitors Maglev R&D - Railway Technical Research Institute Wikipedia - Superconductivity Inverters – Solar Panel Info
ReferencesEERE, 2006. “Supercapacitors” (Online) http://www.eere.energy.gov/de/supercapacitors.html (Accessed 27 February 2007). EERE, 2005. “Super Conductivity” (Online) http://www.eere.energy.gov/EE/power_superconductivity.html (Accessed 28 February 2007). Honda Worldwide, 2006. “Overview – The Honda FCX” (Online) http://world.honda.com/FuelCell/FCX/overview/ (Accessed 27 February 2007). LEES, 2006. “Researchers Fired Up over New Battery” (Online) http://lees.mit.edu/lees/battery_001.htm (Accessed 27 February 2007). Maxwell Technologies, 2006. “Maxwell Technologies Receives 1.5 Million-Unit Ultracapacitor Purchase Order From Enercon For Wind Energy Systems” (Online) http://www.altenergystocks.com/archives/2006/02/maxwell_technologies_receives_15_millionunit_ultracapacitor_purchase_order_from_enercon_for_wind_energy_systems.html (Accessed 27 February 2007). NREL, 2006. “Superconducting Magnetic Energy Storage” (Online) http://www.eere.energy.gov/de/supercon_magnetic.html (Accessed May 24 2006). Wikipedia, 2006a. “Capacitor” (Online) http://en.wikipedia.org/wiki/Capacitor (Accessed 27 February 2007). Wikipedia, 2006. “Grid Energy Storage” (Online) http://en.wikipedia.org/wiki/Grid_energy_storage (Accessed 28 February 2007). Wikipedia, 2007. “Flywheel Energy Storage” (Online) http://en.wikipedia.org/wiki/Flywheel_power_storage (Accessed 21 February 2007). Wikipedia, 2006b. “Supercapacitor” (Online) http://en.wikipedia.org/wiki/Supercapacitor (Accessed 27 February 2007). |
 
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