Siemens Proposes Energy Storage in Heated Rock

The German company Siemens is developing an energy storage system that will convert electrical energy to sensible heat storage in hot rocks at a temperature greater than 600°C, and then reconvert the stored energy to electricity using a steam turbine. I stumbled on this energy storage concept in an article published by energydigital.com. The advantage of such an energy storage system is low capital costs and very long cycle life. The major disadvantage is low round trip efficiency. Siemens claims that its preliminary prototype system is running at 25% efficiency (i.e. 25% percent of electrical energy input to the system is retrieved as electrical energy at the of the storage/steam generator system) but that it believes an efficiency as high as 50% can eventually be achieved.

One can question whether 50% efficiency is really obtainable. In this Mitsubishi Heavy Industries, LTD. presentation on advanced turbine technologies for high efficiency coal-fired power plants Mitsubishi claims that advanced steam turbines can obtain a net LHV efficiency (i.e. percentage of chemical energy converted to electrical energy) in the range of 43% to 45%. The upper end of the range requires operation at 625°C whereas the lower end correspond to the more usual operating temperature of 565°c. This reference claims that typical steam boiler efficiency (i.e. the percentage of chemical potential energy converted to energy in super-heated steam is 85%. One can use this number to calculated the efficiency of the rest of the steam generator system (=ηS) according to the following equation:

0.85×ηS=0.43/0.45

For for 43% total efficiency we find ηS=50.5%, and for 45% total efficiency we find ηS=52.9%. Since electrical energy can be converted to heat at close to 100% efficiency it is technically possible to obtain 50% total efficiency if heat can be transferred in and out of the storage system with very low parasitic losses. The required efficiencies of heat transfer (=ηS) can be calculated using the equation:

ηS×.505/.529 = 0.50

For the low efficiency case we find ηH = 99.0% and for the high efficiency case we find ηH = 94.5%. Keeping the parasitic losses in the heat transfer process under 1% seems unlikely to me, so that the most advanced steam turbine technology will be required to obtain 50%. Even then there is no guarantee that the heat transfer efficiency obtain in a practical implementation will meet the required limit.

Of course even 50% is not very high efficiency. Any electrical energy coming out of this system will cost twice as much per kwh as the electrical energy going in based on efficiency consideration alone. And on top of this efficiency cost must be added the cost of the energy storage and reconversion system. However, this extra cost may be fairly low. Rocks are cheap and the cycle life of this system could be extremely long. Steam turbines are a mature technology with a life expectancy of thirty years. Furthermore there is good reason to believe that the low costs of this energy storage system would survive a major volume scale up since it uses earth abundant materials.

On question of interest is whether the cost of this storage system is low enough to allow storage times of more than a few hours which seems to be the limits for currently popular but still expensive lithium ion battery storage systems. In their press release Siemens talks about building a full system capable of delivering electrical power continuously for 24 hour from a fully charged rock reservoir, which is a time period far beyond what anyone is proposing for lithium ion batteries.

Siemens is a major manufacturer of wind turbines and they are specifically proposing using this storage system for storing wind energy.

Energy Storage in Molten Silicon

Science Daily recently posted an article about some Spanish scientists who have modeled a thermal energy storage system based on molten silicon at a temperature of 1410C. They refer to this system as a phase change energy storage system, implying that in the discharged state the silicon will a solid at close to the melting temperature and most of the stored energy will be used to convert the silicon the the liquid phase rather than to raise the temperature of the storage mass. Presumably the best use of such an energy storage system would be in a high concentration dual axis solar field. The thermal energy storage density would be 5 to 10 times higher than the molten salt systems currently used for energy storage in concentrated solar power (CSP) applications.

The scientists specifically model the use of thermo photovoltaic cells (TPV) as the thermal to electric power conversion technology. It is not clear to me that TPV would outperform a steam engine or a closed cycle gas turbine running off the same heat source in cost and/or conversion efficiency. It is far from clear that his idea is anything other a calculational curiosity, but at least it is a new idea in energy storage. One good thing about using silicon as an energy storage medium is that it is extremely cheap and abundant in the earth’s crust.