In the previous three parts of our series about solar energy storage technologies, we explained the electrochemical properties, pros and contras of various types of rechargeable battery technologies, including Lead-Acid, Lithium-Ion and Nickel-based batteries.
Now in this part 5 we will explore the functioning and characteristics of thermal batteries.
What are Thermal Batteries and how do they work?
Thermal Batteries, as the name implies, are characterized by their operational starting point based on elevated heat.
Different from the ‘conventional’ secondary battery types we discussed before, thermal batteries use liquid electrodes and solid electrolytes.
Thermal batteries use special salts as electrolyte which are solid and thus not active at standard temperature and pressure (STP). Once a thermal battery is required to operate, the solid electrolyte is heated up to its melting point to make it liquid and thus activate the battery. Because of this operational feature, they are also called molten-salt batteries.
As only elevated temperatures activate thermal batteries, they can thus be stored for decades and instantly and without storage time-related performance losses be activated once required.
There are both primary (use-once) and secondary (rechargeable) types available. Primary thermal batteries have been researched and developed since World War II and are nowadays almost exclusively used in military and space applications where long storage capacity and reliability are a priority and short usage periods are acceptable, such as in guided missiles in which the battery becomes active once the solid electrolyte is heated up through a quick pyrotechnic reaction.
Secondary thermal batteries have been researched since the Cold War era. The main types are the German-originating Sodium Sulfur (NaS) batteries and its younger fellow, the Sodium Nickel-Chloride (NaNiCl2) battery.
Sodium Sulfur Batteries
NaS batteries typically come in cylindrical shape, with the outside of the casing made of steel and the inner casing of corrosion-resistant materials such as chromium.
They must be tightly sealed and waterproof as any ingress of water could lead to heavy chemical reactions with the molten sodium anode at the core of the battery cell.
The outer cathode is made of a molten sulfur-soaked carbon structure and separated from the anode by the beta-alumina solid electrolyte (BASE) membrane, a type of Aluminum Oxide (Al2O3). BASE is a fast, efficient conductor of sodium ions and performs poorly in terms of conducting electrons, thus greatly minimizing self-discharge.
NaS batteries are put into operation at a temperature of up to 350°C. Once activated, the charge and discharging of the battery can potentially generate sufficient heat to maintain the required temperature level, however many systems usually are equipped with external heaters – an additional cost driver.
The electrochemical process during discharge basically involves the sodium anode at the core releasing electrons and a dropping sodium level, while the reverse process takes place during charging.
Particular advantages of Sodium Sulfur batteries include their high energy density, roundtrip cycle efficiency of over 90%, potential life-time of many thousands of cycles as well as the relatively flexible way they can be operated.
Moreover, the main components of NaS batteries, including sodium, sulfur and aluminum are cheap and abundant raw materials. However, the aforementioned high activation temperature requirement makes NaS batteries uneconomical for small-scale applications and only with increasing bigger size interesting – for grid energy storage.
Unbeknown to many, NaS batteries are actually not only the major secondary thermal battery type commercially available, but are with around 315MW also the leading battery type in terms of installed electrical energy storage capacity worldwide, right after Pumped Hydro Storage (PHS) and Compressed Air Energy Storage (CAES).
Japan, which has done extensive research on this type, is the dominating user of Sodium Sulfur batteries as grid energy storage, including for wind and solar PV power plants. There are also NaS projects for storage of grid-fed renewable energy underway in China, Germany and the USA.
Sodium Nickel-Chloride Batteries
As Sodium Sulfur batteries were considered uneconomical decades ago, research lead to the invention of the Sodium Nickel-Chloride (NaNiCl2) battery in the 1980s.
Known as ZEBRA battery, named after the research group Zeolite Battery Research Africa Project, the younger brother of the NaS battery is in many aspects comparable to the hyped Lithium-Ion batteries, such as in terms of specific energy of 90-120 Wh/kg (similar to Lithium-phosphate cells) or life-time of 3,000 cycles.
The electrodes of a NaNiCl2 battery are made of molten sodium (anode) and nickel/nickel-chloride (cathode), separated by the BASE electrolyte.
To activate a Sodium Nickel-Chloride battery, an activation temperature of up to 350°C is required. Yet, the heating consumes 1/7 of the battery’s energy per day, thus increasing its self-discharge.
Once activated, Sodium Nickel-Chloride batteries are usually kept in a molten, ready-to-use heated state as it takes enormous time to cool it down (3-4 days) and to re-heat it (up to 2 days). This, along with the high sensitivity to electrical shorts, are the biggest disadvantages of Sodium Nickel-Chloride batteries.
What are the advantages? NaNiCl2 batteries can be charged quickly, are not toxic and, like NaS batteries, feature raw materials that are low-cost and abundant on earth. They are currently mainly employed in vehicles such as submarines, ships, electric cars, and even increasingly as grid energy storage.
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