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Tuesday, March 25, 2008

Sodium-Sulfur (NaS) battery possesses immense advantages to tackle environmental pollution problems:




Sodium-Sulfur (NaS) battery possesses immense advantages to tackle environmental pollution problems:

Until recently, large amounts of electricity could not be efficiently stored. Thus, to tackle the power cut problems industry used to get the services of Diesel Generating (DG) sets. Diesel generating system runs on diesel and, as we know, pollute environment in many ways. Apart, the quality and economy of electricity produced by DG sets are neither upto the standard nor at par with the electricity obtained from state power grid. Moreover, it helps in improving performance of renewable energy plants, specifically wind farms and solar generation plants, by delivering more reliable power.

A new type of large room-size battery, however, may be poised to store energy for the nation's vast electric grid almost as easily as reservoir stockpiles water, transforming the way power is delivered to homes and businesses. In general, the batteries, use for storage of electricity for many purposes, are plagued by limited life spans or unwieldy bulky.

A Sodium-Sulfur battery is a type of battery constructed from sodium (Na) and sulfur (S). This type of battery exhibits a high energy density, high efficiency of charge/discharge (89—92%), long cycle life, and is fabricated from inexpensive, non-toxic materials. Sodium-Sulfur (NaS) batteries are compact, long-lasting and much efficient to handle large power requirements. Using so-called NaS batteries, utilities could defer for years, and possibly even avoid, construction of new transmission lines, substations and power plants. A suggested application is grid energy storage.

The salient features of NaS batteries are:

(a) High energy density (compact);

(b) High charging-discharging efficiency;

(c) Long calendar lifetime;

(d) Environmentally friendly;

(e) Superior operation and maintenance requirements;

(f) Internal operating temperature is nearly 300 degree Celsius.

The advantages of NaS battery are:

(i) Reduce the investment in power plant and transmission and distribution facilities;

(ii) Improve the efficient use of existing power generation and distribution facilities;

(iii) Reduce power transmission losses;

(iv) Improve the reliability of electricity supply;

(v) They make wind power - wildly popular but frustratingly intermittent - a more reliable resource;

(vi) They provide efficient backup power in case of outages.

Above benefits are critical, as power demand is increasing by leaps and bounds everywhere. The NaS battery is the most advanced of several energy-storage technologies. USA and Japan is the leading user of NaS batteries for their power utilities.

Comparison of Sodium-sulfur (NaS) battery with Lead-Acid battery

Properties

NaS battery

Lead-Acid battery

Energy Density (Volume)

170 kWh/m3

(4.2)

40 kWh/m3

(1)

Energy Density (Weight)

117 kWh/ton

(5.8)

20 kWh/ton

(1)

Charge/Discharge Efficiency

More than 86%

More than 84%

Maintenance

Maintenance free

Regular maintenance

Life

2,500 cycles or more

1,200 cycles

The biggest drawback of NaS bettery, now, is price. The battery costs about $2,500 per kilowatt, about 10% more than a new coal-fired plant. That discourages independent wind farm developers from embracing the battery. Mass production, however, is expected to drive prices down.

Technology: A Sodium-Sulfur (NaS) battery consists of liquid (molten) sulfur at the positive electrode and liquid (molten) sodium at the negative electrode as active materials separated by a solid beta alumina ceramic electrolyte. The electrolyte allows only the positive sodium ions to go through it and combine with the sulfur to form sodium polysulfides.

2Na + 4S = Na2S4

During discharge, as positive Na+ ions flow through the electrolyte and electrons flow in the external circuit of the battery producing about 2 volts. This process is reversible as charging causes sodium polysulfides to release the positive sodium ions back through the electrolyte to recombine as elemental sodium. The battery is kept at about 300 degrees C to allow this process.

Sodium-Sulfur batteries are a possible energy storage application to support renewable energy plants, specifically wind farms and solar generation plants. In the case of a wind farm, the battery would store energy during times of high wind but low power demand. This stored energy can then be discharged from the batteries during peak load periods. In addition to this power shifting, it is likely that sodium sulfur batteries can be used throughout the day to assist in stabilizing the power output of the wind farm during wind fluctuations. These types of batteries present an option for energy storage in locations where other storage options are not feasible due to location or terrain constraints.

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