El grupo Xcel Energy trabaja actualmente en una proyecto para almacenar energía eólica, proyecto que se está probando en un parque eólico en Minnesota y que se trata de una batería compuesta por 20 módulos de 50 kilovatios cada uno.
Esta gigantesca batería puede almacenar hasta 7,2 megavatios/hora de energía, lo suficiente para abastecer de electricidad a 500 hogares durante siete horas.
"El almacenamiento de energía es la clave para ampliar el uso de las energías renovables", sostiene Dick Kelly , representante de la compañía Xcel y agrega que “Esta tecnología tiene el potencial para reducir los efectos causados por la variabilidad de energía eólica”.
Este tipo de baterías, basadas en sulfuro de sodio, ya se han utilizado en varias aplicaciones en Estados Unidos, pero es la primera vez que se aplica a la energía eólica, sostienen las autoridades de Xcel. “Como líderes nacionales en la distribución de la energía eólica, esto será muy importante tanto para nosotros como para nuestros clientes”, asegura Kelly
Las baterías hechas de sodio y azufre como electrodos, son conocidas desde que se estudió la estructura atómica de los elementos con el objetivo de encontrar pares capaces de ceder-absorber electrones, y formar así baterías. Se creé que fue Ford Motor Co, la que al investigar una posible batería para automóviles eléctricos, descubrió una forma práctica de hacer una batería de estos elementos. En la actualidad otras empresas han continuado con la investigación y ya se tienen bancos de baterías de este tipo en el comercio.
Como todas las baterías, la de sodio-azufre (Na y S, del latín natrium y sulfurium), tiene dos electrodos, uno de sodio, (Na), y otro de azufre, (S), separados en este caso por un electrolito en cerámica (alúmina), capaz de conducir iones. El electrodo negativo es el sodio, que en presencia del electrolito se combina químicamente con el azufre para formar polisulfuros de sodio, produciendo además, una corriente de iones, que dan lugar a una corriente eléctrica. El electrodo positivo es el azufre.
La reacción que se presenta es la siguiente: 2 Na + xS = Na2 + Sx, que es reversible.
Esta reacción, tal como se presenta, serviría para descargar una batería. Pero al hacerle pasar una corriente eléctrica en sentido contrario, esta reacción es reversible, produciendo el sodio y el azufre en su estado original.
Cabe aclarar que para que se efectue esta reacción, es necesario que los dos componentes estén líquidos, lo cual sucede, para el sodio, a partir de 98 grados, y para el azufre, a partir de 113 grados C. Sin embargo, debido a la estructura propia de las moléculas del azufre, que forman grandes cadenas, éste tiene calentarse hasta una temperatura próxima a los 300 grados. Esta elevación de temperatura tiene que hacerse con la celda totalmente cerrada, pues ambos elementos tienden a inflamarse cuando se calientan expuestos al aire.
Las baterías de Na-S tienen una muy alta densidad de carga, pues alcanzan de 100 a 120 Wh/kg, si las comparamos con las comunes de plomo ácido, que solamente llegan a tener del orden de 35 Wh/kg.
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Xcel Energy launches groundbreaking wind-to-battery project
MINNEAPOLIS – Xcel Energy soon will begin testing a cutting-edge technology to store wind energy in batteries. It will be the first use of the technology in the United States for direct wind energy storage.
Integrating variable wind and solar power production with the needs of the power grid is an ongoing issue for the utility industry. Xcel Energy will begin testing a one-megawatt battery-storage technology to demonstrate its ability to store wind energy and move it to the electricity grid when needed. Fully charged, the battery could power 500 homes for over 7 hours.
“Energy storage is key to expanding the use of renewable energy,” said Dick Kelly, Xcel Energy Chairman, President and CEO. “This technology has the potential to reduce the impact caused by the variability and limited predictability of wind energy generation. As the nation’s leader in distributing wind energy, this will be very important to both us and our customers.”
Xcel Energy has signed a contract to purchase a battery from NGK Insulators Ltd. that will be an integral part of a project. The sodium-sulfur battery is commercially available and versions of this technology are already being used in Japan and in a few US applications, but this is the first U.S. application of the battery as a direct wind energy storage device.
The 20 50-kilowatt battery modules will be roughly the size of two semi trailers and weigh approximately 80 tons. They will be able to store about 7.2 megawatt-hours of electricity, with a charge/discharge capacity of one megawatt. When the wind blows, the batteries are charged. When the wind calms down, the batteries supplement the power flow.
The project will take place in Luverne, Minn., about 30 miles east of Sioux Falls, S.D., with the battery installation beginning this spring adjacent and connected to a nearby 11-megawatt wind farm owned by Minwind Energy, LLC. S&C Electric Company will install the battery and all associated interconnection components. The battery is expected to go on-line in October 2008.
Partners in the project with Xcel Energy include the University of Minnesota, the National Renewable Energy Laboratory, the Great Plains Institute and Minwind Energy, LLC. Xcel Energy is testing emerging technology and energy storage devices as part of its overall Smart Grid strategy, which modernizes and upgrades the grid to allow for easier integration of renewable energy sources.
The project has been selected to receive a $1 million grant from Minnesota’s Renewable Development Fund, pending Minnesota Public Utilities Commission approval this spring.
Xcel Energy (NYSE: XEL) is a major U.S. electricity and natural gas company with regulated operations in eight Western and Midwestern states. Xcel Energy provides a comprehensive portfolio of energy-related products and services to 3.3 million electricity customers and 1.8 million natural gas customers through its regulated operating companies. Company headquarters are located in Minneapolis.
NAS Batteries
Due to the development of a high-technology-society, power demand has been increasing year after year. A big problem under such an environment is the big difference of the electric demand between day and night, and also throughout the seasons.
It is getting difficult to construct power stations and transmission facilities which cover peak power demands not only from the point of construction cost but also conservation of resources. If we succeed to reduce these fluctuations, we can contribute to efficient operation at power stations, decrease of operation cost, and conservation of resources. Our focus has been on a power storage system which applies high energy density and high efficient sodium-sulfur(NAS) batteries.
This system reduces fluctuation by load leveling and peak shaving. We have jointly developed this "NAS battery" with the Tokyo Electrical Power Company. After extensive testing and demonstration, the prospect of commercial utilization has now been realized. Our NAS battery is expected to play an important role in reducing power demand fluctuation. NAS batteries can function as a power station to charge electric power in the base power source at low demand and discharge it at peak demand. By applying it to a consumer, reduction of an electricity bill and the improvement of electric power quality will be possible.
Nowadays, another topic for power generation is how to realize maximum utilization of green energy which does not produce CO2 emission in order to help restrain global warming for our future generations. Use of intermittent renewable energy such as wind turbines and solar PV generation, currently being restrained for system reliability , are now encouraged by proper solutions for not only renewable energy developers but also for grid operators worrying about fluctuation of grid system stability by inflowing of unlimited intermittent power.
Our NAS battery has made an answer for this problem, that is, hybrid system of NAS battery combined with wind turbine generations and with solar PV generations has made possible maximum use of these unlimited power generation resources without producing CO2 by offsetting intermittency by use of sophisticated control of charge-discharge program with our NAS battery.
By applying Beta alumina for its solid electrolyte, the NAS battery is free from secondary reaction due to charge/discharge. In addition, the thermal energy loss caused by internal resistance is used to keep the battery warm. As a result, the NAS battery can obtain high efficiency.
The NAS battery’s key technology, Beta alumina solid electrolyte, which NGK developed by making the best use of its ceramic technology, makes the high reliable battery available.
Features of the NAS Battery System
Large capacity by connecting many batteries, several MW system can be easily constructed
Compactness required area for installation is approx. one third of that for a lead acid battery
Long-term durability Long-term durability : 15years
Preservation of the environment no discharging of any pollution gases, no vibrating, low noise
Stabilizing Intermittent Renewable Energy
Although renewable energy resources are environmentally beneficial; the variable output of wind, solar and wave generation causes voltage and frequency fluctuations on the power network. NAS system smoothes the output from these resources, bringing increased benefits to society.
For small islands or isolated grids, NAS system optimizes the use of renewable energy.
Substation Applications
NAS system stores energy during off-peak periods and discharges at peak periods. The NAS system can replace other types of peaking generation or standby plant to meet growth in peak demand or replace retiring plants.
Principle of the NAS Battery
NAS battery consists of sulfur at positive electrode, sodium at negative electrode as active materials, and Beta alumina of sodium ion conductive ceramic which separates both electrodes. This hermetically sealed battery is operated under the condition that the active materials at both electrodes are liquid and its electrolyte is solid.
If a load is connected to terminals, electric power is discharged through the load. During the discharge, sodium ions converted from sodium in a negative electrode pass through solid electrolyte then reach to sulfur in positive electrode. The electrons finally flow to outside circuits. The electric power is generated by such current flow. With the progress of the discharge, sodium polysulfide is formed in positive electrode; on the contrary, sodium in negative electrode will decrease by consumption. During the charge, the electric power supplied from outside form sodium in negative electrode and sulfur in positive electrode by following the reverse process of the discharge. Because of this, the energy is stored in the battery.
www.ngk.co.jp/english/products/power/nas/index.html
www.uniovi.es/QFAnalitica/quimica_fisica/ElectroPilas/BateriaSodioAzufre.pdf
