Las baterías ZEBRA y las de zinc-aire son las más claras alternativas a las de iones de litio, y presentan indudables ventajas en recursos e incluso en densidad energética. El tiempo dirá si llegan a ser una alternativa y se imponen en el mercado, pero merecen más atención, dinero e investigación básica.
Las baterías ZEBRA tienen una densidad de 120 Wh/kg, bastante alta e igual o superior a la de la mayoría de las baterías existentes. Apenas necesitan mantenimiento.
Las baterías de sodio-cloruro de nitrato, conocidas también por baterías ZEBRA, trabajan a altas temperaturas que son apropiadas, por ejemplo, en autobuses que circulen por líneas regulares. En Stabio, en el sur del cantón del Tesino (Suiza) se está construyendo una fábrica par producir pronto estas baterías en serie.
Muchas marcas han probado e introducido estas baterías en sus modelos y en un futuro se espera que se produzcan importantes avances, aunque en los últimos años han pasado a un segundo plano, relegadas por las baterías de iones de litio, en sus múltiples alternativas.
Las baterías Zebra incluso trabajan con cantidades tan grandes como 100 kWh e incluso 10 MWh, pues pueden llegar a almacenar grandes cantidades de electricidad.
La batería Zebra se fabrica a partir de sal común, cerámica y níquel y tiene cuatro veces más energía que una batería de ácido de plomo del mismo peso.
Una batería Zebra se puede cargar alrededor de 1.000 veces. Para mantener el electrolito de sal líquido, la batería requiere una temperatura de entre 270 ° y 350 °C. Debido al aislamiento al vacío, al igual que el utilizado en una botella de termo, la temperatura exterior es sólo de 5 a 10 grados superior a la ambiental y la pérdida de calor asciende a unos 40 W.
La batería se puede cargar en cualquier enchufe ordinario 110 o 220 V de potencia de salida y utilizando una carga rápida es posible cargar el 50% de su capacidad en 30 minutos. El proceso regular de carga lleva 6 a 8 horas y la batería Zebra es 100% reciclable.
Los autobuses eléctricos de la EMT en Madrid van equipados con baterías ZEBRA, al igual que el Renault Twingo Quickshift Electric o los primeros Th!nk City.
El almacenamiento de la electricidad es el nudo gordiano de los vehículos eléctricos, y también de la energía eólica y solar. Todo esfuerzo en investigación y desarrollo es poco, y debería ser uno de los elementos claves de los planes de investigación de los países industrializados, como la Unión Europea, Estados Unidos, Japón y China (que ya no se puede meter en la categoría de en vías de desarrollo).
¿Y España? En España se hace poco o nada y, lo que es peor, apenas hay interés, aunque hay signos alentadores, como el anuncio de Gamesa de dedicar recursos a la investigación del almacenamiento de electricidad.
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ZEBRA battery
The ZEBRA battery operates at 250°C and utilizes molten sodium chloroaluminate (NaAlCl4), which has a melting point of 157°C, as the electrolyte. The negative electrode is molten sodium. The positive electrode is nickel in the discharged state and nickel chloride in the charged state.
Because nickel and nickel chloride are nearly insoluble in neutral and basic melts, intimate contact is allowed, providing little resistance to charge transfer. Since both NaAlCl4 and Na are liquid at the operating temperature, a sodium-conducting β-alumina ceramic is used to separate the liquid sodium from the molten NaAlCl4.
This battery was invented in 1985 by the Zeolite Battery Research Africa Project (ZEBRA) group led by Dr. Johan Coetzer at the Council for Scientific and Industrial Research (CSIR) in Pretoria, South Africa, hence the name ZEBRA battery. In 2009, the battery had been under development for more than 20 years. The technical name for the battery is Na-NiCl2 battery.
The ZEBRA battery has an attractive specific energy and power (90 Wh/kg and 150 W/kg). For comparison, LiFePO4 lithium iron phosphate batteries store 90-110 Wh/kg and the more common LiCoO2 lithium ion batteries store 150-200 Wh/kg. Nano Lithium-Titanate Batteries store energy and power of (116 Wh & 72 Wh/kg) and (1,250 W & 760 W/kg).
The ZEBRA’s liquid electrolyte freezes at 157 °C, and the normal operating temperature range is 270–350 °C. The β-alumina solid electrolyte that has been developed for this system is very stable, both to sodium metal and the sodium chloroaluminate.
The primary elements used in the manufacture of ZEBRA batteries, Na, Cl and Al have much higher worldwide reserves and annual production than the Li used in Li-ion batteries. Lifetimes of over 1500 cycles and five years have been demonstrated with full-sized batteries, and over 3000 cycles and eight years with 10- and 20-cell modules.
Vehicles powered by ZEBRA batteries have covered more than 2 million km. Modec Electric Van uses ZEBRA batteries for the 2007 model. The Th!nk City also uses ZEBRA batteries.
When not in use, ZEBRA batteries are typically left under charge so that they will remain molten and be ready for use when needed. If shut down and allowed to solidify, a reheating process must be initiated that may require up to two days to restore the battery pack to the desired temperature and impart a full charge.
This reheating time varies depending on the state-of-charge of the batteries at the time of their shut down, battery-pack temperature, and power available for reheating.
After a full shut down of the battery pack, three to four days will usually elapse before a fully-charged battery pack loses enough energy to cool and solidify. Beta Research Ltd’s ZEBRA pack, for example, loses heat at the rate of 135W.
Characteristics: High power, high capacity cells suitable for electric traction applications. High Temperature Battery operating at over 270°C. Sodium Nickel Chloride (NaNiCl) chemistry giving a nominal operation cell voltage of 2.58 Volts
Advantages: High energy density (5 times higher than Lead acid). Large cells (up to 500Ah) possible. Cycle life better than 1000 cycles. Tolerant of short circuits. Safer than Sodium Sulfur cells. Typical cell failure is short circuit which does not cause complete failure of the battery. Low cost materials.
Shortcomings: Suitable for large capacity batteries only (> 20KWh). Limited range of available sizes and capacities. (Large multi-cell blocks). High internal resistance. Molten sodium electrode. High operating temperature. Preheating needed to get battery up to the 270°C operating temperature (Up to 24 hours from cold). Uses 14% of its own capacity per day to maintain temperature when not in use. Thermal management needed. Only one factory in the world produces these batteries.
Applications: Traction applications, EVs, HEV, and Railway.
Costs: Expensive
ADVANTAGES OF ZEBRA BATTERIES ZEBRA OTHER SYSTEMS
High energy density >110Wh/kg → Lead acid ~30Wh/kg, NiMH ~50Wh/kg
The above figure is for a complete system → Some other systems require packaging, cooling & management systems
Capacity independent of discharge rate → Lead acid can reduce by >40% at high rates
Performance completely independent of temperature → Most other systems performance affected at high and low temperatures
100% coulombic efficiency, accurate charge capacity → Other systems require charge equalising, capacity not accurately known
No self discharge, high voltage batteries in use ~600V & they require electronic overvoltage protection NiMH, LiIon, Lead acid
Cells fail to short circuit, batteries continue to operate with → Other systems have cells that fail to open circuit
Cells maintenance free, fully sealed, no gassing → Some other systems require maintenance and are not sealed
Indefinite, maintenance free, storage life at ambient temperature → Some other systems require regular maintenance in storage
Demonstrated calendar life >12 years → Some systems have a limited calendar life
Demonstrated cycle life to 100% capacity >2000 nameplate cycles → Some systems can only be cycled to ~80% or cycle life is reduced
Demonstrated >60,000 miles in a road vehicle with no maintenance
Normal charge ~7h, can fast charge ~50% capacity in ~40 mins
The Zebra Battery
In 1998, Mercedes Benz were on the point of launching an all-electric version of the A Class small car. Powered by a 30kWh Zebra battery weighing 370kg (including control system), the vehicle was claimed to have demonstrated a real world range of 120 miles. A fleet of 16 A Class cars tested the Zebra battery in all weather conditions – some of these vehicles are still being used by Mercedes today.
Improvements in Zebra technology since then have reduced the weight of a 30kWh unit to 270kg (120Wh/kg). This specific energy is superior to any automotive LiIon battery available or under development today.
The Zebra battery is suitable for pure BEVs and could be used for PHEVs. It is not suitable for power assist hybrids (HEV0).
The history of Zebra development dates back to the 1970s. The technology was first developed in South Africa. By the 1990s, there were some 8 companies developing sodium beta batteries and 4 pilot production plants were in operation. Today, only MES-DEA in Switzerland manufacture the Zebra battery, although sodium sulphur is still undergoing extensive development by NGK for stationary applications in Japan. A numbre of test installations are in operation. [See Handbook of Batteries, Linden].
As a "hot" battery, ambient temperatures have no effect on battery performance. In sub-zero winter temperatures, a Zebra powered EV will deliver as much power and energy as in mid-summer. This temperature independent capability is a unique feature of "hot" batteries such as the Sodium beta variants (Sodium Sulphur and Sodium Nickel Chloride).
The Zebra battery NaNiCl technology makes far more use of cheap and readily available materials than LiIon and NiMH. The major active materials are nickel, iron and common salt, along with some aluminium. The separator is ceramic beta alumina, a very inexpensive material. The case is made from stainless steel. The only potentially limiting active material is nickel, although MES-DEA state that less than one third as much nickel is required per kWh as NiMH (1.53kg/kWh compared to 6.8kg/kWh for NiMH). Cycle life of over 1400 nameplate cycles has been demonstrated in well over 10 years testing. The battery has a 10 year calendar life.
If nickel availability becomes constrained, Zebra technology has the potential to be developed into an even lower cost variant that would use little or no nickel – the Sodium Iron Chloride battery. This has an open circuit voltage of 2.35V against 2.58V for NaNiCl, but could be manufactured in unlimited quantities from very cheap and ubiquitous active materials (iron and common salt). Specific Energy would only fall by 9% and battery operating temperature could be reduced. We have stated before that development of this NaFeCl battery technology should be prioritised, with the prospect of developing an extremely inexpensive, rugged and high specific energy battery that could approach a cost of $100/kWh and enable widespread adoption of BEVs and PHEVs.
www.cebi.com/content/index_html
www.spinnovation.com/sn/Batteries/Developments_and_Improvements_in_Zebra_Nickel_Sodium_Chloride.pdf
www2.ing.puc.cl/power/paperspdf/dixon/72a.pdf
