The technology that will be discussed in this series of papers will be flywheel energy storage. Flywheel energy storage consists of storing energy in the rotational kinetic energy of a spinning disk. This series of papers will focus on storing excess electrical energy from power generation by converting the electrical energy into the kinetic energy of the flywheel. The kinetic energy is then converted back into electrical energy on demand. The domain considered will be storage technology for electrical energy for on-grid and off-grid electrical power systems. Energy storage has been mentioned as a key element in increasing the use of renewable and clean energy sources which in turn will reduce carbon dioxide emissions and reduce the human contribution to global climate change. (Liu and Jiang, Ibrahim et al., Hall and Bain) Increased use of some renewable energy technologies, such as wind and solar power, for electricity generation results in a lack of predictability and control when compared to traditional power generation methods. Currently used power generation facilities can be controlled to meet the constantly changing demand for electricity. Less expensive sources of electricity make up the base supply of electricity while periods of increased demand, known as peak demand, are met by turning on additional facilities when necessary, usually at an increased cost. Energy storage systems can be used to level out the variable output of these renewable sources and match the energy supply to the energy demand. (Hall and Bain, Liu and Jiang) Energy storage systems are already used in uninterrupted power supply (UPS) applications in commercial and industrial facilities where temporary power outages will cause significant losses. In these installations, the UPS system provides temporary power between the loss of grid power and the start of on-site backup generation. Flywheel energy storage systems have been successful in this market. (US Department of Energy) At least one company, Beacon Power, is marketing flywheel energy storage as a technology to improve power quality on the grid. Their flywheel system is designed to replace frequency regulation on the power grid. (Lazarewicz and Rojas) There are several other potential applications for storing energy on the electrical grid. Some applications are leveling the balance between energy supply and demand and providing improved power quality. (Hall and Bain) Different grid storage applications require different performance specifications. More details on the specific applications and functions that flywheels will serve will be covered in a future paper. The key parameters for flywheel systems are separated between those related to energy storage systems in general and those related specifically to flywheel energy storage technology. Ibrahim, Ilinca, and Perron identified 16 characteristics important for grid energy storage. The most significant of those are listed here (Ibrahim et al.): 1. Cost: This includes initial, operation, and decommissioning costs in terms of power and energy capacity. 1
2. Power: This includes power density (power per unit volume), and specific power (power per unit mass). 3. Energy: including energy density (energy per unit volume) and specific energy (energy per unit mass). 4. Efficiency: the energy output as a fraction of the energy used to store and maintain the system. 5. Lifetime: the number of years, charge and discharge cycles, or other factors that limit the effectiveness of the technology. 6. Response time: how quickly the technology can respond to changing grid conditions. 7. Environmental and Safety: the risks associated with failure of the technology. How the equipment is decommissioned and disposed of at the end of life. For flywheel energy storage technology, the key parameters are: 1. Strength, density, and shape of the rotor and rotor material: The specific energy of the rotor is a function of the rotor material and shape. Strong and lightweight materials providing the highest specific energy rotors. The maximum speed of the rotor is dependent on material strength and shape (Liu and Jiang). 2. Bearing load limitations: The bearing specifications determine limits on how large a single rotor can be and may also place limits on how fast it can spin. Bearings are also a source of energy loss in the rotating system and affect the overall efficiency of the flywheel system. Wear on bearings may also affect the operating cost and life if the bearings need to be replaced. (Liu and Jiang) 3. Operating environment: High performance flywheels are housed in vacuum enclosures to minimize losses from wind resistance and to contain the flywheel safely in the event of rotor failure. (Hall and Bain) For energy storage systems in general, there is a performance envelope tradeoff between power and energy capacities. There is another tradeoff between efficiency and the power and/or energy capacity. A third tradeoff exists between cost and efficiency. The main performance envelope for flywheels is a tradeoff between minimizing the system cost and maximizing both the energy density and efficiency of the system. Figure 1 illustrates this tradeoff for several materials (Bolund, Bernhoff, and Leijund) along with a calculated material cost per kwh of energy storage. Note that figure 1 considers only the rotor and does not account for housing, bearing, motor/generator costs and weight contributions to the overall flywheel system so the values listed are good for comparison only to other flywheel materials and are not comparable to competing storage technologies. New high strength materials such as carbon fiber can provide significant improvements in energy density but have high cost relative to more traditional materials such as steel. Using high cost and performance material also increases the costs in bearings, housings, and the motor/generator to achieve high performance. Also, high 2
performance flywheels require precision engineering and manufacturing to limit the dynamic forces that are created by high speed rotating bodies. While flywheel technology is very old for mechanical applications, the use of flywheels as a technology to store electrical energy is a developing field. Using flywheels as an alternative to batteries in UPS systems has been commercially available since 1998 (US Department of Energy). Current actual flywheel specific energies are in the 10-20 W hr/kg. With advances in material technologies, one source predicts flywheel energy densities of 200 W hr/ Kg are possible in the near future. (Liu and Jiang) Advances are being made in bearing technology, with high speed applications using magnetic bearings. Additional advances in high temperature superconducting bearings will improve the overall speed and efficiencies possible. Materials with higher tensile strength and lower density will also contribute to increased specific energies. Since many of these technologies are newer, costs should also come down over time. (Liu and Jiang) There are several competing technologies for grid energy storage applications. The first is pumped hydro storage. Pumped Hydro technology consists of maintaining two lakes or reservoirs separated by a vertical distance. Energy is stored by pumping water from the lower level reservoir to the higher level reservoir. Energy is released back to the grid by running water from the upper lake through a generator back to the lower reservoir. (Dell, Ibrahim et al.) Compressed air energy storage technology is being used as another alternative. Large capacity compressed air storage uses naturally occurring underground formations as air reservoirs. Energy is stored by compressing air, thereby generating a pressurized volume of air that can be converted back to electricity on demand. The capacity of a system depends on the size of the available reservoir. (Dell, Ibrahim et al.) 3
The advantage of pumped hydro and compressed air energy storage is the large energy storage capacity that is achieved at relatively low cost. A disadvantage of the technologies is a limitation on where facilities can be located, since they require naturally occurring land formations for the reservoirs to be cost effective. The upper limit of efficiency is reported to be 80%, below that of some battery, supercapacitor, and flywheel systems. (Dell, Ibrahim et. Al.) Battery storage technologies are another major competitive technology. The best battery technologies have better specific energies and lower capital costs than flywheel technologies. However, they have a shorter lifetime and increased operating cost over flywheel technologies. (Dell, US Department of Energy, Ibrahim et al.) However, there has been significant research and improvement on battery technology in recent years. In 2002, flywheels were considered superior to Lead-acid batteries (US Department of Energy) for large UPS applications. That same comparison needs to be made between current flywheel technology and current battery technologies of lithium-ion and sodium sulfate batteries. Figure 2 shows the improvements in specific energy and specific power from Lead-acid to Lithium-ion, and Sodium-Sulfure batteries. (Dell) Costs for Lithium-ion batteries were estimated by a 2008 US Department of energy report to be $3400 for a usable range of 11.4 kwh or approximately $300/kWh. (Howell) Their 2002 analysis of Lead-Acid batteries in comparison to flywheels estimated a cost of $13/kWm or $780/kWh. (US Department of Energy) This is a significant improvement in cost of 61% over a 6 to 7 year period and shows the significant rate of improvement of battery technology in the last several years. Flow battery technology is a chemical storage technique similar to a conventional battery but where the materials are liquid and stored in process tanks. This arrangement allows for larger energy capacity in a single system. The main advantage of flow batteries would seem to be their long lifetime. They can last over 10,000 cycles as compared to 1,000 cycles for conventional batteries. (Dell) However, reported efficiencies are around 75%. (Ibrahim et al.) Figure 2 compares the specific energy of flow batteries against conventional battery types. Supercapacitors are a high power density and high efficiency energy storage solution. They currently suffer from reliability issues, but the technology is still being developed. Supercapacitors have excellent specific power capability but do not offer as high a specific energy (< 10 W h/kg) as other technologies and as a result will likely not compete directly for the same applications in this domain as flywheel technologies. (Dell, Ibrahim et al.) Hydrogen fuel cell technology is still under development. Current literature suggests that significant improvement in efficiency is needed before this will be a viable option and that these improvements are not immediately forthcoming from current research. (Dell) It is not considered to be a legitimate competitor to flywheel energy storage in the near future. 4
Of all the competing technologies, the current state of pumped hydro and compressed air energy storage will see only small incremental improvements in the near future. These improvements could come in the areas of efficiency and the start-up costs for new facilities. The cost of new facilities should be evaluated against the cost and benefits of using large scale implementations of other technologies as they develop. Eventually battery or flywheel technology could compete against these technologies as costs come down for large capacity systems. The future of the other technologies in this domain will be significantly impacted by the improvement of battery, superconductor, and flywheel technology. These technologies should all see continuing improvements in the near future. (Hall and Bain) The rate of these technological improvements as well as any breakthrough developments will need to be considered with respect to the strategy involved in developing and marketing flywheel technology. All of these technologies are benefitting from development of new materials, composites for flywheel technology, and sodium sulfate for batteries, and carbon nanotubes for supercapacitors. (Hall and Bain) For this reason it is unlikely that any of these technologies will approach a natural technological limit in the near future. The potential rapid rate of improvements of the competing technologies will require any business strategy for flywheel energy storage to adapt as these technologies mature. References: Dell, Ronald M. and David A. J. Rand. 2004. Clean energy. Royal Society of Chemistry, Cambridge. Hall, Peter J. and Euan J. Bain. 2008. Energy-storage technologies and electricity generation Energy Policy 36 2008 4352-4355. 5
Howell, David. Progress Report for Energy Storage Research and Development. US Department of Energy, 2009. Ibrahim, H., A. Ilinca, and J. Perron. 2006. Energy storage systems-characteristics and Comparisons Renewable and Sustainable Energy Reviews 12 2008 1221-1250. Lazarewicz, Matthew, and Alex Rojas. Grid Frequency Regulation by Recycling Electrical Energy in Flywheels. http://www.beaconpower.com/products/energystoragesystems/docs/grid%20freq%20reg%20white%2 0Paper.pdf Accessed February 22, 2009 Liu, Haichang and Jihai Jiang. 2006. Flywheel energy storage-an upswing technology for energy sustainability Energy and Buildings 39 2007 599-604. US Department of Energy. 2003.. DOE/EE-0286. 6