Thermal batteries are thermal storage and conversion systems that serve the function of a grid scale rechargeable batteries. A thermal battery takes in electricity from the grid, and supplies electricity back to the grid whenever needed (i.e., with a roundtrip efficiency targeted at 50%).
Variable Depth of Discharge
How the technology works
The thermal battery works by converting excess electricity from the grid to extremely high temperature heat at 2400 Celsius (4350 F), using resistance heaters that are similar to an incandescent light bulb filament. The heat is transferred to an inexpensive (nominally $0.5/kg) solid graphite medium, which then stores the energy as heat. When electricity is desired the heat is transferred to a power conversion unit that uses photovoltaic cells to convert the light emitted by the glowing white-hot infrastructure back to electricity. To prevent oxidation of the materials of construction, the entire system is held within an inert environment. In this way, the thermal battery is able to serve as an inexpensive rechargeable battery for the grid that can eventually enable full penetration of renewables onto the grid.
To charge the thermal battery, excess electricity from the grid is used to power resistance heaters at approximately 2500 C. The heaters convert electricity to heat via joule heating and then transfer the heat to dense graphite pipes via thermal radiation. The graphite pipes then transfer heat via convection to liquid metal tin, which is used as a heat transfer fluid. The liquid tin is mechanically pumped, and is nominally heated from 1900 C to 2400 C, after which it is routed to a large energy storage unit made of inexpensive graphite blocks. The liquid tin then heats the blocks by thermal radiation from the outer pipe walls and the tin correspondingly cools, after which it is routed back to the heaters to be reheated. In this way, the storage unit of graphite blocks is continually heated until it reaches the peak temperature, at which point the thermal battery is fully charged.
When electricity is desired the liquid tin is pumped through the graphite storage blocks until it reaches the peak temperature, to remove their heat. The tin is then routed to a power conversion unit made of a large array of parallel graphite pipes carrying the tin. The pipes form repeat unit cells that are lined with tungsten foil. The tungsten foil serves as a diffusion barrier to prevent deposition of the graphite onto the converter. Water cooled heat sinks, covered in photovoltaic cells are then mechanically inserted into the cavity, which contains inert Argon gas. The water cooling keeps the cells near room temperature, as the water absorbs the wasted heat, and transfers it to the environment via a dry cooling unit. The light emitted by the tungsten foil is converted to electricity by the cells, which is then supplied back to the grid. The photovoltaic cells (i.e., also termed thermophotovoltaic (TPV) cells) are backed by a highly reflective mirror that returns any unused light back to the hot side of the infrastructure. This preserves the energy that is not converted, since it is reabsorbed by the hot side, and this light recycling process is critical to the system’s efficiency. As the liquid tin passes through the power conversion unit it cools back to 1900C at which point it is routed back to the graphite storage blocks to be reheated back to the peak temperature. In this way, the system can continually supply electricity back to the grid, which is how the thermal battery discharges.
When it comes to cost, we believe in being open and transparent, so that potential investors can evaluate our value proposition by comparison to competing technologies. The details associated with how we developed our cost estimates are provided in two references [1, 2]. The estimated cost of our thermal battery technology is low, because almost the entire system is made out of carbon, which is one of the most abundant and inexpensive materials on earth. The total capital cost for a thermal battery depends on the configuration, and most critically the amount of energy stored and the discharge rate. The bar charts shown here, provide our estimated capital costs for a 1 GWh thermal battery with a 100 MW discharge rate. The total estimated capital cost for a thermal battery is the cost per unit power (CPP) multiplied by the discharge rate, plus the cost per unit energy (CPE) multiplied by the number of hours of storage and the discharge rate. To the best of our knowledge, no other competing technology has full system costs this low.