They show a diagram of two gravel containing cylinders 7 meters high and 8 meters in diameter, one at 500ºC and the other at -150ºC, and claim that the recoverable electrical energy is 30MWh. The electrical energy density is therefore:

Energy Density = 30,000,000Wh/[(2×7×pi×4^2 m^3)×1000 liters/m^3] = 42.6Wh/liter

This energy density is similar to the energy density of lead acid batteries. As a sanity check I will assume that the heat capacity per unit volume of the gravel containers is 0.55 times the heat capacity of water (The specific heat of rock is about 0.20 and the density of rock is 3 times that of water. I assume 10% airspace in the container.) Since water has a heat capacity of 1.16Wh/°C liter, gravel should have a heat capacity of 0.64Wh/°C liter. Therefore the thermal energy density of the storage system is given by:

Energy Density = 0.5*0.64Wh/°C liter × 650°C = 208Wh/liter

In order to achieve the advertised electrical energy production 100×42.6/208 = 20.6% of the stored thermal energy must be converted into electricity.

The problem I see with this scheme is not the thermal storage per se (although good insulation will be required for extended storage times) but the heat pump itself. Clearly Isentropic is talking about an external combustion engine such as a Stirling engine or an Ericsson cycle engine. Both of these engine types were invented in the nineteenth century, and they have remained stubbornly uneconomical because the power to weight ratio and the efficiency of real implementations have remained low. If Isentropic has created an economical and efficient version of one of these engines then they have accomplished a great engineering feat. Personally, however, I have filed their claims under the heading "If it sounds too good to be true then it probably is". I hope that they prove me wrong.

September 13, 2009 Energy Storage News

rogerkb [at] energystoragenews [dot] com