Sodium Nickel Chloride Batteries for Solar PV Systems
A global push for non-polluting energy sources such as solar is currently at its peak. To meet this goal, there’s a need to develop environmentally friendly energy storage systems for solar PV and solar off-grid applications.
Sodium nickel chloride batteries represent one of the most environmentally friendly power storage solutions that can be leveraged for solar PV systems.
Aside from their ecological benefit, these batteries have sparked interest in solar PV systems for other stellar characteristics such as low cost, high energy capacity, and reliability.
But what exactly are sodium nickel chloride batteries and what can they offer solar PV systems?
Let's find out in this comprehensive guide about sodium nickel chloride batteries for solar PV systems.
What is a Sodium Nickel Chloride Battery?
A sodium nickel battery (Na-NiCl2) is a high-temperature energy storage system that uses sodium as the anode and nickel and sodium chloride as the cathode.
The battery works on the basis of electrochemical reactions that involve the transfer of sodium ions between the positive and negative electrodes.
Na-NiCl2 batteries are also known as ‘Zebra batteries’ since they're mainly manufactured by a company under the brand name Zebra.
The sodium nickel chloride battery entered the commercial sphere in the 1990s.
Despite being developed initially for electric vehicles, they have found applications in renewable energy storage systems like solar power plants and wind turbines because of their exceptional characteristics.
Today sodium nickel chloride batteries are an excellent choice for solar PV backup and off-grid energy storage.
Components of Sodium Nickel Chloride Batteries
The Electrodes
Anode
Molten sodium (Na) is used as the anode for sodium nickel chloride batteries.
Cathode
Nickel (Ni) and Sodium Chloride (NaCl) are used in sodium nickel chloride batteries to form a Nickel Chloride (NiCl2) cathode.
Unlike other sodium batteries, the cathode of sodium nickel chloride batteries is located in the middle of the cylindrical-shaped electrochemical cell.
Electrolyte
Sodium nickel chloride batteries use two components as the electrolyte: beta alumina ceramic and a tetrachloroaluminate of sodium such as NaAlCl4 as the secondary electrolyte.
Ceramic electrolyte
The ceramic electrolyte acts as a medium where sodium ions travel between the cathode and anode. It also acts as a physical separation of the positive and negative electrodes.
While the electrolyte is conductive to sodium ions, it acts as an isolator for electrons between the electrodes.
Secondary electrolyte
The secondary electrolyte, molten tetrachloroaluminate, is used to flood the positive electrode to facilitate the electrochemical reactions during charging and discharging.
It also allows rapid transfer of sodium ions between the cathode and ceramic electrolyte.
Independent heaters
The melting point of the secondary electrolyte is 157°C. To maintain it in a liquid (conductive) state, the battery temperature needs to be kept between 270°C and 350°C.
Independent or auxiliary heaters are therefore connected to the battery system for this purpose.
Applications of Sodium Nickel Chloride Batteries
Due to their exceptional characteristics such as operational safety, eco-friendliness, reliability, and flexibility, Na-NiCl2 batteries have found so many applications in the solar renewable energy sector. Here are some example applications:
- Stationary energy storage for solar PV systems
- Emergency power backup for off-grid power applications
- As support energy storage systems for renewable energy distribution grids.
- Residential and commercial buildings solar energy storage solution
- Peak management applications
- In electric vehicles, hybrid electric vehicles, and submarines
Most sodium nickel chloride batteries are currently manufactured in the US and Europe. However, projects involving these energy storage systems have been set up all over the world.
How do Sodium Nickel Chloride Batteries Work?
Sodium nickel chloride batteries leverage electrochemical reactions to charge and discharge.
Charging
During charging, sodium at the cathode is oxidized to sodium ions, and nickel metal is converted to nickel chloride. The sodium ions travel through the secondary electrolyte and secondary electrolyte towards the anode.
At the complete charging, all the sodium ions will have moved to the anode and are in an unstable state, waiting to revert to their original state.
Discharging
The ions and electrons flow is reversed during discharge. The sodium ions migrate back to the cathode, leaving sodium metal at the anode.
The sodium ions recombine with chloride ions at the cathode to form sodium chloride. The nickel chloride is also converted to original nickel metal. Available electrons travel through an external circuit through a load to power it.
Reaction at anode:
2Na+ + 2e- 2Na
Reaction at cathode:
Ni + 2NaCl NiCl2 + 2Na+ + 2 e-
Overall reaction:
2NaCl + Ni NiCl2 + 2Na
The forward reactions represent the charging process while the backward reactions represent the discharging process.
Please note: There're no secondary reactions in sodium nickel chloride batteries. In case of an overcharge, the nickel in the positive electrode reacts with sodium chloroaluminate stopping the current flow through the cells.
This acts as a protection mechanism to prevent damage to the battery components.
Characteristics of Sodium Nickel Chloride Batteries
Price
The cost at the cellular level for sodium nickel chloride batteries is $110 per kWh. This is a good range compared to the cost of other solar rechargeable batteries.
However, the capital cost for these batteries is in the $500-$600 per kWh range.
This makes Na-NiCl2 generally more expensive than other conventional PV batteries such as lithium-ion.
Overcharge capabilities
Sodium nickel chloride batteries have inherent overcharge capabilities since they can withstand high temperatures that are likely to arise in such scenarios.
The battery components can also withstand high current passes and hence are more likely to survive overcharge conditions.
Size range
The size of a single sodium nickel chloride can vary from 4 to 25 kWh.
However, installations can be in the megawatt-hour range depending on the number of single sodium nickel chloride batteries used.
Depth of discharge
The DOD for Na-NiCl2 batteries is 80% at around 3000 cycles.
While this means that 20% of the energy stored in these batteries can't be used, you still get a reliable energy storage solution for your solar PV for an extended period.
Discharge time
A typical Na-NiCl2 battery discharges within 2 hours. This time may, however, increase to several hours depending on the capacity of the battery.
Self discharge rate
Thanks to the ceramic electrolyte, Na-NiCl2 batteries don’t experience the typical self-discharge that occurs in conventional rechargeable batteries.
On the flip side, though, sodium nickel chloride batteries lose most of their stored energy to meet the thermal requirements of the electrolyte.
These batteries consume about 15% of the stored energy per day to reach and maintain the electrolyte activation temperature.
Cycle life
Sodium nickel chloride batteries can do 2000-4500 cycles at 80% depth of discharge. This is a decent cycle life that pretty much covers the ROI and extra benefits.
However, it's still low compared to other batteries such as lithium-ion, which can do as high as 10,000 cycles.
Round trip efficiency
Sodium nickel chloride batteries show a round trip efficiency of about 85 to 95% for solar power output. In these high-temperature batteries, any electrical loss is converted to heat.
Charge resistance
As nickel and nickel chloride is insoluble in molten salts, contact is allowed making these batteries have very low charge resistance.
Expected lifespan
The expected lifespan or useful life of sodium nickel chloride batteries is around 15 years. This, combined with the high cycle life means that these batteries are reliable to use for a solar PV system.
Operating temperature
Na-NiCl2 batteries have a wide operating temperature of -4°F to 140°F (-20°C to 60°C). These batteries can maintain optimum performance even when temperatures fluctuate.
Their charge and discharge processes remain constant even if ambient temperatures keep changing.
Power to energy ratio
One very interesting characteristic of sodium chloride batteries is that they have a flexible power to energy ratio.
This means that these batteries can be cooled to ambient temperatures without damaging the essential components.
Energy density
Sodium nickel chloride batteries have a high energy density of around 100-170 Wh/kg.
Thanks to this characteristic, Na-NiCl2 batteries have found application in large-scale, stationary energy storage applications.
Raw materials availability
Nickel, alumina, and sodium chloride, the materials used in these batteries, are abundant materials that are easily obtainable and cheap to manufacture.
The same can't be said for other batteries such as lithium-ion that require scarce and pretty expensive materials to design.
Recyclability
Sodium nickel chloride batteries use non-toxic raw materials that are fully recyclable.
Currently, dead and old sodium nickel chloride batteries are used in the production of stainless steel and materials for road paving.
Advantages of Sodium Nickel Chloride Batteries
Safe to use
Sodium nickel chloride batteries have zero emissions hence are safe to use. They also have no fire risks due to their relatively inert chemistry.
Eco-friendly
The materials used to build these batteries are highly recyclable, so they don't pollute the environment. Even better, recycling technologies for these batteries already exist in the steel and road paving industries.
Maintenance-free
Sodium nickel chloride batteries don't require any kind of maintenance, such as air conditioning needed for other batteries. Solar PV applications are, therefore, a breeze with these batteries.
Great scalability and flexibility
Several battery cases or containers can be wired together to form a huge energy storage system.
Therefore, sodium nickel chloride batteries can be used for both small-scale and large-scale PV energy storage.
Disadvantages of Sodium Nickel Chloride Batteries
Fragility of the ceramic electrolyte
Although the ceramic electrolyte is a good conductor of sodium ions, it easily degrades, reducing the battery’s lifespan.
Batteries with a 15-year lifespan are not bad, but the properties of the ceramic electrolyte could be improved to make them last longer.
Efficiency limitations due to design
Sodium nickel chloride batteries have a cylindrical design that doesn't allow for efficient energy storage and discharge.
Even though other batteries such as lithium-ion have material, costs, and safety concerns, they're considered better than Na-Nicl2 batteries due to their enhanced performance.
Therefore, the design for Na-NiCl2 batteries needs to be improved in order to compete with the leading solar rechargeable batteries.
Extreme thermal needs
The electrolyte used on nickel chloride batteries needs to be maintained in a molten state to be conductive to sodium ions.
Cooling the electrolyte is a big hassle, and it could take up to 4 days, not to mention that reheating it can take another two days.
These thermal constraints and the infrastructure that goes into it are a big drawback for Na-NiCl2 batteries.
A less suitable choice for homes
Although sodium nickel chloride batteries can be scaled for use in homes, their size, high sensitivity to electrical shorts, and high operating temperatures make them less suitable for home use.
It can be quite costly to install these batteries along with the thermal infrastructure for home use.
However, with design improvements, sodium nickel chloride batteries can still be made appropriate for home use.
Are Sodium Nickel Chloride Batteries the Best Batteries for Solar PV?
In terms of reliability, safety, and environmental friendliness, sodium nickel chloride batteries could be considered a good choice for solar PV.
However, in terms of performance and cycle life, other batteries like lithium-ion have a competitive advantage.
The Future of Sodium Nickel Chloride Batteries
The following improvements have great potential to solve most of the problems associated with solar rechargeable batteries for PV systems:
- Development of new glass materials that can be used for these batteries to enhance their life cycle.
- Automation of the production of sodium nickel chloride batteries to reduce overall cost and increase production volume.
- Swapping the cylindrical battery design with a flat disk design can increase the battery’s performance by 30% at low temperatures.
- Improvement of the positive electrode used in these batteries to better the charging and discharging capabilities.
Conclusion
To sum it up, sodium nickel chloride batteries are well suitable for solar PV and off-grid applications despite the current limitations.
If you're looking for a reliable and flexible backup and energy storage system for your solar PV system, sodium nickel chloride batteries are a good pick.
If the suggested design improvements are made, you can expect the performance of sodium nickel chloride batteries to be at par with that of leading solar rechargeable batteries.
NaNiCl2 batteries have great potential for solar PV, and it’s very likely that these batteries will dominate the market for renewable energy storage systems in the near future.
