High temperature batteries: Workhorses for large scale grid storage

Initial conceptualization and work on high temperature batteries was initiated as early as in the 1950s at various national labs in US and was taken up by the Ford Motor company for commercialization in 1967. The initial planned application for these batteries was in electric vehicles owing to its high energy density compared to the available technologies of its time. In the 1980s Japan took up work on the technology to try to adapt it for stationary storage applications under the Moon Light Project by NEDO. In the current scenario, high temperature batteries are mostly considered suitable for stationary storage applications.

General characteristics of High temperature batteries

All types of commercialized high temperature batteries share certain commonalities in terms of design and performance. The operating temperature of the batteries is between 250oC to 450oC depending on the chemistry. This systems are designed so as to allow maintaining this temperature via internal heat generated during operation of the battery. For minimizing thermal losses it is practical to build large systems consisting of 100s of cells tightly packed together and surrounded by a thick insulation. Due to this, the minimum size of these storage systems is 0.5 MWh and above. NAS systems manufactured by NGK are even larger with their minimum battery size being 6 MWh.

These systems are generally characterized by large cell sizes. Individual cells are normally in the size range of 300 – 1500 Wh. The large cell size is practically convenient when assembling large storage systems because of ease of interconnection and assembly. For obtaining optimum cycle life and roundtrip efficiency, a 4-8 hour duration system design is optimum. It is possible for these batteries to deliver short pulses at a much high power but continuous usage under these conditions will impact the longevity of the system.

One of the benefits of this technology is that these ESS systems are insensitive to ambient temperature conditions as the internal temperature is comparatively very high. There are two main reasons why these batteries need to be operated at high temperatures. First one is that the solid electrolyte called Beta Alumina which is used in these systems has good conductivity only at high temperatures. High conductivity of the electrolyte is important for reducing internal resistance losses and improving the efficiency. Another reason is that the sodium (anode) needs to be molten at the operating temperature so as to minimize resistance and obtain a good cycle life from the cell. A lot of ongoing work is focused on reducing the temperature of operation to < 200oC for various reasons discussed in a following section. The variations

There are at least three commercialized high temperature battery chemistries which are being manufactured and others which are currently under late stages of development.

NAS (Sodium-sulphur) NAS batteries are being manufactured by NGK (Japan) who are building 1 MW/7 MWh modular systems. Larger installations can be assembled by putting together these 1 MW systems and the biggest one currently operation in Japan is 34 MW / 245 MWh. It is connected to a 51 MW wind farm which has 35x1.5 MW wind turbines. The ESS is used for stabilizing the temporary fluctuations in wind generation and also for 6-8 hour long duration backup on the following day using wind forecasts. This helps in matching the power generation and grid requirements allowing much higher wind farm utilization.

Each NAS system of 1 MW size consists of 20 stacked modules of 50 kW/300 kWh. Each module consists of 192 cells packed together closely to contain heat effectively. As you may notice from the drawing obtained from an early patent by Ford, the external cell design has not undergone any drastic modifications. It still consists of a tall elongated cell with a tubular solid electrolyte of Beta Alumina. The name NAS is an abbreviation for Sodium (Na) and Sulphur (S) which is the chemistry of this battery. One of the advantages for NAS batteries is that both of its key ingredients are abundantly available on earth. Sodium is present is large quantities in sea water as sodium chloride or table salt. Sulphur is abundantly available on the surface of the earth in deposits near volcanoes and is also obtained as a waste material from desulphurisation of conventional fuels.

The analogue of NAS batteries in the Li-ion world are Lithium-sulphur (LiS) batteries. By contrast, the LiS batteries are very high energy density devices (400+ Wh/kg) designed to operate at room temperatures and this technology is currently in pilot plant production stage. Lithium-sulphur batteries currently use a liquid or gel electrolyte but there are strong ongoing efforts to shift to inorganic solid electrolytes as in NAS batteries.

Na-NiCl2 Sodium-Nickel Chloride batteries share many similarities with NAS batteries. The main differentiator is that the cathode is NiCl2 instead of Sulphur. Two prominent companies working on commercializing this technology are GE (General Electric) and FIAMM. Na-NiCl2 batteries have also been used by some companies such as Think Global for EV applications up to as late as 2012. Currently, Ampower a company based out of China is actively manufacturing this technology. Liquid Metal Batteries (LMBs)

LMBs are a new battery concept which was developed in the lab of Prof. Donald Sadoway at Massachusetts Institute of Technology (MIT) about 15 years ago. The work was initially supported by ARPA-E in US and later led to the formation of a company Ambri which is commercializing the technology. Ambri has built modular 0.5 MW / 1 MWh systems which have been deployed in Hawai, Massachusetts and various other locations in US. This battery chemistry operates at 450oC and consists of 3 liquid layers of anode, electrolyte and cathode. The anode is magnesium metal and the cathode is antimony metal. At the high operating temperature both of these are in the molten state. It is different from the previous two battery types in the sense that here, even the electrolyte is liquid.

When this battery is discharged, the magnesium metal reacts with the antimony to produce an alloy and the energy of this reaction is converted to electrical energy. As a result, when the battery is discharged the magnesium from the top layer is gradually consumed and the thickness of the bottom layer of the cathode increases. Reverse happens during charging and the magnesium layer on the top is restored.

The heat required for maintaining the operating temperature is generated internally while the batteries are being charged or discharged due to the internal resistance. This places a constraint on the system that it must be operated at least once a day to prevent the batteries from cooling down. This may not be a major challenge, however, because grid storage batteries are generally used multiple times a day.

New-chemistries under development

One of the major challenges in all high temperature batteries is in fact the requirement of high temperature. Other than the fact that energy is consumed in maintaining this temperature and extensive thermal insulation is required, it also poses significant challenges in the system design. The problem of corrosion is much higher at these temperatures and sealing of batteries is also a challenge since the common materials used such as plastics, polymers and rubber degrade at these temperatures. As a result, there are multiple approaches being explored for solving this challenge.

One of the approaches being pursued by Ceramatec is to replace the Beta Alumina with another solid electrolyte called NASICON. This electrolyte can operate at temperatures in the range of 120-200oC allowing the cells to operate at

Overall, high temperature batteries have great prospects for stationary storage applications. As the fraction of renewable sources powering our grid increases upwards of 30%, having long duration backup for ensuring continuous power supply will become absolutely necessary. Such applications requiring 100s of MWh of distributed storage could be aptly served by these battery systems.

Published: 24th Sep 2019

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