Isentropic Energy – A Breakthough in Energy Storage?

One of the primary problems that the electric power industry has faced since its inception is the fact that electricity cannot be economically stored throughout the grid. Electricity is generated and transported via transmission lines, but it must be either used or disposed of: either too much or too little power on the grid causes instability problems that can lead to system-wide failure. Unlike oil or gas, there are no storage tanks to help buffer against even hourly supply and demand changes.

This is why the price of power varies as much as 3X from off-peak hours in the middle of the night to peak demand in the afternoon.  Although demand varies, there is no way to smooth out the supply. Further, the entire grid is overbuilt to ensure that the worst case local demand on the worst case days is met. The so-called “peaker plants” that are built to handle these demand spikes are intentionally run at reduced capacity the majority of the time just so they have enough available extra capacity to “spin-up” and meet the peak demand. This approach is very costly.

Today, there are really only two economically viable electricity storage options for bulk power: pumped hydro, and compressed air energy storage (CAES).  But unfortunately, even these two options have significant geographical limitations and their net environmental impact is still being debated. While advances in batteries (Li-ion, NaS, Flow, advanced lead acid) are taking place, they are still currently far more expensive than pumped hydro and CAES. Considering that the average price of generation is around $0.04/kWh and the average retail electricity price to end customers is ~$0.10/kWh, you can quickly see how a utility would only be interested in storage technologies that can get to pennies/kWh or less.

The chart below, from the Electricity Storage Association (ESA), shows the overall cost of the various storage options.

Apart from cost differences, not all of the storage technologies are necessarily a good fit for bulk time-shifting applications. The plot below, also from ESA, shows the various storage options plotted against two of the most important variables: discharge time (how long power can be delivered) and rated power (how much power can be delivered). For time-shifting applications, what we want is the ability to deliver lots of power for long periods of time.

Further  complicating things is that as more renewables come online and become a greater percentage of overall generation capacity, their weather-dependent intermittency will only pose further problems for utilities and grid operators.

What is desperately needed is cheap, reliable, ubiquitous, bulk electricity storage.

That’s what this new company Isentropic Energy could provide. Their system uses the First Ericsson Cycle and improves on a thermodynamic heat engine from the 1800’s to convert electricity into a temperature difference, store that temperature, and then use the temperature difference in reverse to create electricity.

From the company’s website:

“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.”

Here’s a schematic of their system:

Apparently, they have used advancements in aircraft technology to make significant improvements in the heat engine mechanics, resulting in roundtrip efficiencies in the range of 72-80%. Also, because the system uses gravel, which is a cheap and readily available material, they project installed costs at around $55/kWh, and as low as $10/kWh at scale. This translates into very low $/kWh costs over the lifetime of the system as shown in their table below:

The overall benefits of this system are that it is very low cost, is not geographically constrained, has high overall roundtrip efficiency, is environmentally inert, and is both modular and scalable. While the technology is still in early development (according to this GreenTechMedia article, they have completed prototype number 3), it definitely holds promise for solving one of the power industries biggest challenges.

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