A UK company called Isentropic is promoting a pumped heat energy storage system. First some quotes from their website
High reversibility - the machine works both as an engine and a heat pump (an engine turns a temperature difference into work, a heat pump turns work into a temperature difference - every fridge has one). High reversibility means that if it first turns electricity into a temperature difference, it can then regenerate most of the electricity from the temperature difference it has created. No other machine to date can do this efficiently.
Machine layout - the machine re-imagines the first Ericsson cycle of 1833
Isentropic has designed a system that uses the Isentropic heat pump to store electricity in thermal form ("Pumped Heat"). The storage comprises two large containers of gravel, one hot (500C) and one cold (-150C). Electrical power is input to the machine which compresses/expands air to 500C on the hot side and -150C on the cold side. The air is passed through the two piles of gravel where it gives up its heat/cold to the gravel. In order to regenerate the electricity, the cycle is simply reversed. The temperature difference is used to run the Isentropic machine as a heat engine.
The round trip efficiency is over 70%. Because gravel is such a cheap and readily available material the cost per kWh can be kept very low - $80/kWh.
Bath County Pumped Storage, Virginia, USA. Two reservoirs covering 820 surface acres (3.32 sq.km). 30 GWh storage capacity (largest in the world). Pumped Heat Storage Plant of the same capacity would occupy 1/300th of the area.
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.