|Fig. 1: An Avista utilities containerized vanadium flow battery. Maintained by UniEnergy Technologies. (Source: Wikimedia Commons)|
The push for completely renewable, clean energy is ever-present in today's world. Unfortunately, this heavily sought after goal is incredibly difficult to attain due to a simple problem, renewable energy sources such as wind and solar that rely on the environment often times are not able to complete the energy demand at peak hours. This is due to the fact that energy sources that rely on the sun and wind, are dependent on those environmental factors to be present when energy is needed and often times peak energy usage hours (i.e. early mornings, or afternoon evenings) do not fall when those environmental factors are present. This means that in order provide more widespread use of renewable energy sources, there must be an efficient and cheap way to store the electricity they generate until it is needed. The most common solution to this problem has been the use of batteries, but many times these can be very expensive or inefficient. This dichotomy of cost versus performance is ongoing, but the three most promising contenders in this field, NaS batteries, Li-ion batteries, and Flow batteries seek to tackle the cost/performance issue.
NaS batteries, or sodium sulfur batteries are a type of finite state battery that uses sulfur as the positive electrode and sodium as the negative.  These electrodes are separated by a solid ceramic which serves as the electrolyte.  The process by which discharge and recharge occurs happens at about 350 degrees Celsius which means these batteries get fairly hot, however they are efficient at about an 80% efficiency. 
Li-ion batteries, or lithium ion batteries are also a type of finite state battery, however li-ion batteries use an intercalated lithium compound as one electrode material, while lithium ions move from the negative electrode to the positive electrode during discharge.  These batteries stay relatively cool and have an efficiency of about 92%. 
The last battery is a flow battery, which differs from a finite state battery in that it contains two chemical components dissolved in liquid that are separated by a membrane.  The flow battery converts chemical energy directly into electricity due to the movement of ions across the membrane.  The vanadium redox battery is a popular flow battery, and it has an efficiency of about 73%.  A picture of a containerized vanadium flow battery can be seen in Fig. 1.
Sodium sulfur batteries have a fairly low cost, about 500 kwh (kilowatts per hour) making them an economically viable solution.  Unfortunately, the fact that they run at such a high temperature makes them slightly unsafe, and they use toxic materials which mean that once their use is up they are difficult to discard.  Li-ion batteries have a high energy density (they can pack a lot of energy into a small area) and they are very thermally stable which makes them ideal for room temperature conditions.  Unfortunately, li-ion batteries efficiency drops severely after about 500 deep cycles (where the battery goes from full charge to 20%) which makes them difficult to be a viable source for long term grid electricity storage.  Also they have a fairly high cost of about 1750 kwh which is being reduced by emerging technologies, but is still keeping the li-ion battery from being a large scale solution.  Lastly, flow batteries have a low cost comparable to sodium sulfur batteries at 500 kwh, and they are easy to scale.  This is because if you want to double the size of a traditional battery, you have to double the price (because you essentially are just getting two units), but with a flow battery you simply increase the tank size which means that the price per kwh goes down much more then the price increases.  Unfortunately flow batteries are not very energy dense, which means they require more land to employ. 
In conclusion, the three battery technologies on the forefront of grid energy storage still have their own unique downsides, but with advancement in technology happening daily, the future for grid battery storage looks promising.
© Cody Smith. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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