Energy Storage Breakthrough?

A cost-effective solution for energy storage, that is not geographically limited, is one key ingredient missing from the vision of a solar and wind dominated electricity generation profile.  Currently, the only cost-effective storage solution for load shifting of renewables are Pumped hydro and Compressed Air Energy Storage (CAES).  Unfortunately, both technologies are limited by geography.  Pumped hydro needs water bodies separated vertically and CAES needs an underground cavern with specific geology.  Other solutions, up to this point, have been batteries that are too expensive and limited by inadequate cycle lives.

Isentropic Energy, a UK-based startup, claims that they now have a solution.  Their technology is the Pumped Heat Electric Storage System (PHES). The storage device works by taking advantage of a heat pump that uses electricity to compress air into a gravel tank on one side and expand air into a second gravel tank on the other side.  The result is one tank with compressed air at 500 degrees celsius and one with expanded air at 150 degrees celsius.  The energy differential can then be used as needed to run a heat engine.

PHES Device Schematic

So key questions are how big, with what duration, and how much does it cost?  As for size, Isentropic claims a land footprint of only 120 square meters for a 16MWh system.  The system would be able to deliver 2MW of power for a duration of 8 hours.  And best of all they claim that they can deliver this solution at a cost of $55/kWh of storage, they hope to be able to get this number down to $10/kWh as they scale up.  This compares favorably to pumped hydro at a fraction of the footprint, would be slightly cheaper that CAES, and would be significantly cheaper than battery technologies like sodium-sulphur and lithium-ion.  (see table below)

A technology like this could be the key to the large-scale integration of wind and solar.  Currently, the intermittency of both technologies is thought to limit the potential for either or both to provide the majority of our power.  However, with a low-cost storage option, like PHES,  that intermittency could be evened out with storage, and wind and solar would then be able to provide consistent power.

As promising as this technology sounds,  Isentropic Energy is still in the prototype development stage.  They are currently developing their fourth prototype and are still probably have another year or two until they are ready for commercial development.

Even if everything goes according to plan and PHES becomes a reality, there is still one more ingredient necessary for a wind and solar dominated grid.  That ingredient is sufficient transmission to access the tremendous wind resources in the Midwest and solar resources in the Southwest.  That’s where we still need policymakers to get things done.  Due to the locally focused structure of electricity transmission and the failure of the national government to step in and get transmission funded and built, a transmission infrastructure necessary to get to most of the wind and solar resource seems very far away.  I mean a 750Kv national transmission infrastructure would only cost $60 billion and what is $60 billion amongst friends or a nation of 300 million people (approx. $200/pp).

Our clean energy future is just two steps away:

1.  Energy storage for wind and solar grid integration

2.  Legislation from the congress and directives from FERC to allow for a nationwide high voltage transmission infrastructure.






1 Comment

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One response to “Energy Storage Breakthrough?

  1. eemc2

    Somehow I am not convinced that Isentropic can deliver on their promise. On their site, they claim roundtrip efficiency of over 72% – 80%. ( )

    Since their device is a heat engine and as such is still subject to laws of Thermodynamics, this efficiency is a pure dream. Maximum heat efficiency of a heat engine is given by the difference of temperatures between the hot and cold heat temperatures at which the device operates. The bigger the difference, the higher efficiency. (That is why in the summer, with high ambient temperatures, coal and nuclear plants but also our A/C units are less efficient.) The formula is simple (any engineering thermodynamics book should have it, I used Thermodynamics by Schmidt, Ezekoye, Howell and Baker): η_max= 1- Tc/Th (Tc and Th are cold and hot reservoir temperatures in Kelvin). Also note how this number will always be smaller than 1, that’s the Thermodynamics telling us we can never get anything “for free”.

    For this device, using the extreme temperature difference (123 and 773K), we get about 84% ideal (or isentropic, if you will) efficiency. However, this figure is quite misleading because considering that as energy is “pumped out” from this contraption, the device will operate with decreasing efficiency. Efficiency at about half the temperature difference (Tc=198K, Th=523K) is only 1-198/523=62%.

    Assuming temperature invariant heat capacity and also invariant heat capacity of gravel, at the point where there is still half the energy stored in the tanks (half temperature difference), the efficiency will drop to 64%. (If I calculate the midpoint to be at 0C for the cold and 335C for the hot, I get about 55% efficiency.) Any more energy will be extracted at best at this efficiency, which is on-par or worse than pumped hydroelectric storage. At 1/3 energy left, we get even lower efficiency.

    It is also important to consider the inherent inefficiency (compressor efficiency maybe 70%, 95% for the electric motor, heat loss, etc – recall that efficiencies multiply). ~Remembering that what we calculated was pure and theoretical maximum, the claims of a high efficiency storage in Isentropic system are exaggerated and will hardly compare to pumped hydro systems (70-85% or plain compressed gas systems ~70-80% we discussed in lecture).

    Sorry, but I am afraid that there seems to be little cool and too much hot air in Isentropic…

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