When it comes to green energy, the intermittent nature of renewable sources like wind, solar, and tidal power presents a difficult problem for the electrical grid management. Peak energy production often doesn’t correlate well with peak energy demand, necessitating a means of storing excess energy when consumption is low. As renewable energy sources become more prevalent, and the need to curb fossil fuel emissions continues to increase, finding a new grid energy storage solution has never been more important. It is the final piece of technology required to bring about wide scale adoption of renewable energy sources like solar panels and wind turbines.
- What is a Molten Salt Battery?
Molten salt batteries, especially liquid metal batteries, are increasingly gaining interest from the energy community as a grid energy storage solution for renewable energy sources. Combining high energy and power densities, long life times, and low cost materials, they have the potential to meet the unique demands of grid scale energy storage. A molten salt battery is a class of battery that uses a molten salts electrolyte. The components of molten salt batteries are solid at room temperature, allowing them to be stored inactive for long periods time. During activation, the cathode, anode and electrolyte layers separate due to their relative densities and immiscibility. The molten salt layer in the middle serves as an electrolyte with a high ionic conductivity, and is the medium through which the ionic species travel as the battery charges and discharges.
- Advantages of Molten Salt Batteries
Molten Salt Batteries carry several inherent advantages over their solid state contemporaries. Since some (or all in the case of liquid metal batteries) of the components are liquid, the batteries possess a higher current density, longer cycle life, and simplified manufacturing scheme in large scale applications. Since no membranes or separator systems are involved, cycle life is higher and energy efficiency can be retained over a longer period of time. The grid scale energy storage company Ambri has previously shown that a lead-antimony and lithium liquid metal battery should retain 85 percent of its initial efficiency over a decade of daily charge/discharge cycles. Since the battery is essentially a container containing 3 liquid phases, construction is as simple as pouring the heavier metal into the bottom, the electrolyte in the middle, and the lighter electrode on top. The major drawback of this design is the high operating temperature required to keep the components in the liquid state. However in a grid scale application these elevated temperatures can easily be maintained using the heat generated during the charge and discharge cycles.
- History of the Molten Salt Batteries
The first molten salt batteries actually weren’t intended to operate for very long periods of time at all, but were instead used as single activation primary batteries for bombs and rockets. Invented by German WWII era scientist Georg Otto Erb, the first practical cells were called thermal batteries and while they were never used during the war, the United States Ordnance Development Division would eventually acquire the technology and use it to power rockets, bombs, and even nuclear weapons. These early batteries could last indefinitely (over 50 years) in the solid state while supplying a huge burst of power. Today thermal batteries are still used as the primary source of power for missiles like the AIM-9 Sidewinder, BGM-109 Tomahawk and the MIM-104 Patriot.
In 1966 Ford Motor Company invented the Sodium-Sulfur (NaS) liquid metal battery for electric vehicle application. The high power density and high energy capacity looked promising but the high operating temperature of 290-390 °C caused Ford to drop research and development. In 1983, Tokyo Electric Power Company (TEPCO) and Nippon Gaishi Kaisha (NGK) realized the potential for NaS battery system as a solution for grid storage and began research and development of the technology. In 1993 the first large-scale prototype of such a system was field tested at TEPCO’s Tsunashima substation. The system consisted of three 2 MW, 6.6 KV battery banks. This laid the groundwork for NGK/TEPCO consortium’s current line of grid storage NaS batteries, which produce 90 MW of storage capacity every year.
Meanwhile in Pretoria, South Africa, 1985, the Zeolite Battery Research Africa Project (ZEBRA) led by Dr. Johan Coetzer at the Council for Scientific and Industrial Research, invented the first sodium nickel chloride battery. It had a specific energy of 90 Wh/kg, a notably stable beta alumina solid electrolyte, and enhanced corrosion resistance over NaS. This design, while novel, has yet to see large scale commercial grid storage applications and remains a hot topic in battery research and development. They have however been deployed by FIAMM Sonick and used in the Modec Electric Van.
- Sodium-Nickel Chloride Batteries
Sodium Nickel Chloride (Na-NiCl2) Batteries also use a molten sodium core, but instead uses the nickel as the positive electrode in the discharged state and nickel chloride in the charged state. Both forms of nickel electrode are insoluble in their liquid states and a sodium conducting beta alumina ceramic is used as the separator. In place of the pure elemental sodium found in NaS batteries, a tetrachloroaluminate (NaAlCl4) core is preferred. Na-NiCl2 batteries are sometimes called sodium metal halide batteries and in addition to boasting long operating life spans, the ability to be assembled in the discharged state, and a safer chemistry than NaS. Normal operating temperature range of Na-NiCl2 batteries are in the 270-350 °C range, but one company, Sumitomo was able to develop a similar chemistry using a salt that melts at 61 °C and operates at 90 °C; they had initially slated commercial trials for late 2015 so only time will tell how they function in the market.
The Future of Grid Storage and Molten Salt Batteries
It has become clear that energy storage is the last piece of the puzzle for the world to fully reap the benefits of renewable energy sources. Money spent on investment into grid scale energy storage will carry more weight than money spent in better renewable energy technologies.
If you would like to know more about getting safe, reliable and recyclable molten salt battery storage, for your own home, business or micro-grid visit us at http://quantum.GridEdge.com.au