Isentropic Energy has an unconventional approach to grid-scale energy storage, the holy grail of renewable energy.

When it comes to grid-scale storage, most of the research, funding and hype happens in batteries (Ambri, Aquion, A123, Eos, QuantumScape) and flow batteries (Primus, EnerVault, Deeya), along with some containerized compressed air energy storage (LightSail, General Compression, SustainX).

But there are also some even less conventional approaches, such as that used by Isentropic Energy. (Energy Cache and Gravity Power would also fall in this category.)

Mark Wagner, the Chairman of Isentropic, spoke at a recent energy event and said that the capital cost for Isentropic's energy storage technology was very low, with "a levelized cost of $35 per megawatt-hour." Jonathan Howes, the CTO of Isentropic, has claimed a path to large-scale storage costs that are an order of magnitude lower than lithium-ion batteries or other stored energy technologies.

In 2012, the firm announced a $22 million investment from the U.K.-government-backed Energy Technologies Institute (ETI).

The short-term goal is to deploy a 1.5-megawatt, 6-megawatt-hour storage unit on a U.K. grid-connected primary substation owned by Western Power Distribution, a distribution network operator with 7.7 million customers -- and to get it to demonstration scale.

Wagner, the company's Chairman, shared some conclusions on energy storage based on what he called a "whole-systems approach" in the form of a U.K. grid model developed with a university. The grid model accounts for all generation assets, transmission and distribution, and interconnection.

According to the model, the value of storage decreases after the third hour. Wagner said, "Three hours is enough," adding, "Distributed storage is significantly more valuable than centralized storage."

Wagner takes the stance that "peak shifting is the most valuable thing you can do -- but there is no value after six hours; optimum is about 3 hours." He also said that "The value of storage depends strongly on the nature of the system." With a renewable penetration of greater than 50 percent, storage has a "huge value." Wagner believes that high frequency storage is much less valuable.

Pumped Heat Electricity Storage



Isentropic's Pumped Heat Electricity Storage (PHES) system is based on the First Ericsson cycle and uses a heat pump to store electricity in thermal form. The storage system uses two large containers of gravel, one hot and one cold. Electrical power is input into the machine, which compresses/expands argon gas, which is passed through the two piles of gravel, where it gives up its heat or coldness to the gravel. In order to regenerate the electricity, the cycle is reversed. The temperature difference is used to run the system as a heat engine.

The startup claims that its reversible engine/heat pump boasts three critical features:

Very high round-trip efficiency

High reversibility -- the machine works as both an engine and a heat pump. 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.

Gas-cycle machine -- there is no use of damaging refrigerants, chemicals, or water



Isentropic's technology is compact, has no geographical constraints and claims a round-trip efficiency of 75 percent.

Isentropic's innovations include using aircraft engineering techniques to reduce piston weight and cost, designing new valving to eliminate pressure losses, and using a new sealing technology.

The obstacle to energy storage penetrating the grid is cost. Stephen Berberich of the California ISO has said of energy storage, "It's good stuff, but it's expensive, and we have to find business cases."

Isentropic -- and every other energy storage company out there -- still has to back up its claims and prove the cost piece.