In Australia, a startup company CCT Energy Storage has created an unusual “thermal battery” using not molten salt, but ... molten silicon. This is a remarkable achievement, given that the melting point of silicon is as high as 1410 degrees Celsius (or 1683 K), almost twice as high as the highest temperature used in the molten salt technology. In addition, silicon’s unique feature is its unusually high latent heat of fusion (denoted as Lf , it’s the energy needed to melt a material after heating it up to its melting point; its value is usually given in Joules per mole, or Joules per kg). For silicon, Lf = 1787 kJ/kg, or 0.496 kWh/kg. The density of silicon at its melting temperature is about 2300 kg/m3 – taken together, it means that for melting one cubic meter of silicon the energy of about 1.2 MWh is needed – and, of course, the same amount of energy can be recovered on the transition from the molten phase back to the solid phase. And it should be stressed that during the entire process of “extracting” heat from the “battery” the temperature will remain at the constant level of 1683 K.
If this silicon “thermal battery” is to be used to store electricity, then “charging” it will be a simple operation: resistance heaters convert electricity into thermal energy with 100% efficiency. But recovering electricity will be more complicated because a heat engine driving an electric generator must be used for this. A heat engine, as discussed in Chapter 3 (see Section 3.4.6 and the Equation. 3.15), will never convert all heat into mechanical energy. But the high temperature of silicon is a favorable factor here. It can be expected that at Th as high as 1787 K, the efficiency of the heat engine can be at least 50%. Thus, one can realistically expect that from such “thermal battery” about 0.6 MWh could be recovered from every cubic meter of silicon.
Let’s try to compare this with the performance of a molten salt installation. In contrast to the silicon thermal battery, which uses the phase transformation process, i.e. the transition from solid to liquid state taking place at a constant temperature, in the molten salt technology only the liquid phase is involved and the temperature changes it increases when heat is added and decreases when it is extraced. On the Stanford University website the volumetric heat capacity of the salt mixture used in this method is given as 3770 kJ/m3 K. Assuming that the temperature decreases by 200 K, this gives the total amount of stored heat energy as 754 MJ/m3, or 0.209 MWh/m3. At the same time, the efficiency of converting thermal energy into electricity in this temperature range, which is used in the molten salt method, can be at most 30%, which effectively gives about 0.06 MWh/m3 so using molten silicon you can get the same storage capacity with a ten times smaller volume of the storage medium! Let us add that the needed silicon does not have to be highly purified, i.e. it is easily available and inexpensive. So one can expect that these silicon “thermal batteries” will find a number of interesting practical applications.