11.4.4.2: Heated Volcanic Rocks

In Germany, the Siemens-Gamesa (S-G) Company has built and launched a prototype energy storage installation using an innovative technology. The method is essentially the same as in molten salt technology but volcanic rock blocks are used instead of salt. The rocks are heated up to a temperature as high as 800 degrees Celsius by electric heaters using surplus power generated by wind turbines. The energy is later recovered by passing the heat contained in the rocks to superheated steam which is used next to propel steam turbines and electric generators in the same way as in a conventional coalfired power plant. Undoubtedly, the advantage of this new method is its simplicity compared to the technology of molten salt. The average specific heat and density of the molten “solar salt” in the relevant temperature range can be taken, respectively, as 1.54 kJ/kg K and 1750 kg/m³. The S-G report does not specifically identify what kind of volcanic rocks are being used, but typical values for them are, respectively, 1.1 kJ/kg K (see Page 72 of the report), and 2400 kg/m³, so that the volumetric heat capacities of the salt and the rocks seem to be quite similar. Certainly, however, the construction of a thermally insulated “reservoir” for these rock pieces will be a much easier undertaking than making tanks for molten salts: there is no risk of leaks, there is no risk of corrosion and the price of that solid “storage medium” is certainly much lower than that of “solar salt”. Let’s estimate how much thermal energy U can be stored stored in this S-G prototypical installation. Let’s assume that the temperature range used will be 300 K, from 500 to 800 degrees Celsius:

\[ U = 1.1\;kJ/kg\bullet K \times 1000\;kg/tonne \times 1000\;tonne\times 300\;K= \notag \]

\[=3.3 \times × 10^{8} \;kJ = 330\;GJ = 91.7\;MWh\notag \]

It should be kept in mind, however, that the efficiency of converting thermal energy to electreic power using steam turbines is only 30%, so the “recoverable” energy will be only 30 MWh – or 10 MW of power for three hours. Not a spectacular result, perhaps, but let’s remember that the S-G installation we are talking about is only a prototype. The 1000 tons of rocks occupy less than 500 cubic meters which is about the volume of an American single family house – – but not of a very large family. From a really serious grid-tied installation, we would expect, say, 1 GW of power rather than 10 MW – this means multiplying everything by 100, i.e., 50,000 cubic meters. Which should not frighten us, there are buildings on the OSU campus whose volume is much larger! So building such a thermal reservoir for 1 GW installation will certainly not be a major engineering problem today. In addition, such a reservoir can be hidden partially or even completely underground. The relatively low efficiency of 30% of this type of energy storage remains a problem (this also applies to “molten salt” technology). But if you use such installations to store excess energy from wind turbines and solar farms, the recovery efficiency of 30% is still better than 0%, which we would have had if this energy was wasted. Remember that a general usage system must generate as much power as consumers need. It couldn’t be less! if too little power is generated, some of the users simply have to be disconnected from the grid. But if windmills and solar farms generate too much power then some of them have to be disconnect from the grid! which is just a waste, isn’t it?

There is another important aspect of this new S-G technology. Namely, it can provide a “second youth” to coal-fired power plants that are currently to be decommissioned in accordance with the decarbonisation program. But all you have to do is eliminate their coal boilers and replace them with an energy store. All other machines needed by the new technology turbines, generators are the same as in a traditional power plant. This may be a particularly attractive option, especially for Germany, which are planning to shut down coal power plants in the near future, although they are still in excellent condition. But for many other countries such an option is perhaps no less attractive.