The battery, which promises to provide a viable solution for stationary grid storage of energy supplied by renewables, uses three inexpensive liquid layers as electroactive components, including a liquid metal positive electrode, a fused salt electrolyte, and a liquid metal negative electrode. The system operates at elevated temperature maintained by self-heating during charging and discharging.
Professor Donald R Sadoway of the Department of Materials Science and Engineering at MIT, explains more: “The battery consists of three liquid layers. On top is a low-density metal. On bottom is a high-density metal. In between there is molten salt, not salt dissolved in water but rather salt that has itself melted and turned to liquid. The contiguous layers are insoluble in one another and so they stratify according to density. There is no need for membranes or separators. The battery produces current when the metal on top alloys with the metal on bottom. Recharging the battery causes the metal on bottom to be purified, and sends the metal from the top back to its original location.”
Ambri has a liquid metal battery factory which can produce 20 MWh a year. It only required a few million dollars of capital, one of the advantages Ambri has. A full-size, 500 MWh per year battery factory would only require about $50 million of capital investment.
Ambri’s factories will require one fourth to one tenth of the capital investment to produce an equivalent amount of electricity storage per year as other technologies. The active components of Ambri’s cells are housed in steel containers and cell tolerances are in millimeters not microns. Cells are put together in systems using steel racking and other basic components. Consequently, Ambri will be able to leverage workers that have experience building and assembling steel parts, a ubiquitous skill set. This gives rise to our manufacturing strategy of building Ambri’s Liquid Metal Batteries around the world through a network of manufacturers that will serve local and regional markets on a global basis.
Cells operate at elevated temperature and, upon melting, these three layers self-segregate and float on top one another due to their different densities and levels of immiscibility. For our initial chemistry, we worked with magnesium (Mg) and antimony (Sb) electrodes and have since moved to a chemistry with a higher voltage and lower cost. In a charged state, there is potential energy between the top metal layer and the bottom metal layer which creates a cell voltage. To discharge the battery, the cell voltage drives electrons from the Mg electrode, delivering power to the external load (e.g., light bulb), and the electrons return back into the Sb electrode. Internally, this causes Mg ions to pass through the salt and alloy with Sb, forming a Mg-Sb alloy. To recharge, power from an external source (e.g., wind turbine) pushes electrons in the opposite direction, pulling Mg from the Mg-Sb alloy and re-depositing Mg back onto the top layer, returning the system to three distinct liquid layers. The cell design is simple, uses low-cost materials, and the all liquid design avoids the main failure mechanisms experienced by solid components in other battery technologies.
Given that the battery (chemistry) depends on high temperatures to operate, is there a trade-off between energy required and generated? “We have many variants of the technology. One has it operating at 450°C. Some of the energy is lost in generating enough heat to keep the battery at that temperature. With proper insulation we have shown round-trip energy efficiency exceeding 75%, which compares favorably with such methods as pumped hydro storage and compressed air storage. So the trade-off is acceptable.”
Trials of the new storage, installed alongside renewables, are planned to take place at the Pearl Harbor naval base, Hawaii, in late 2015, and the Cape Cod military base in the same timeframe.
“We look at the world from a cost-of-electricity standpoint.” Places with high electricity costs will place a very high value on energy storage.
Ambri isn’t just looking at the frequency regulation market, but also storage for up to 24 hours where that can be helpful and cost competitive.
“2 MWh / 1 MW units strung together will give you the best economics.”
“Next year we [Ambri] will deploying five 35kWh prototype systems to customers in Massachusetts, Hawaii, New York and Alaska. Following the successful demonstration of these prototypes, Ambri will ramp up manufacturing, marketing and sales.”
And what about future developments? “There are more discoveries occurring on campus at MIT,” reveals Sadoway, “New chemistries that will be even more efficient than what we have discovered so far.”
SOURCES – Ambri, Battery Show, Cleantechnica
Brian Wang is a Futurist Thought Leader and a popular Science blogger with 1 million readers per month. His blog Nextbigfuture.com is ranked #1 Science News Blog. It covers many disruptive technology and trends including Space, Robotics, Artificial Intelligence, Medicine, Anti-aging Biotechnology, and Nanotechnology.
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