## Two recent papers on Adiabatic Compressed Air Energy Storage (ACAES)

August 4, 2021

Adiabatic Compressed Air Energy Storage (ACAES) and other next generation CAES concepts have long been touted as potential large-scale energy storage options for the future, however thus far this potential has never been realised. In our two latest papers, Why is adiabatic compressed air energy storage yet to become a viable energy storage option? in iScience and Adiabatic compressed air energy storage in Joule, we explore the lack of success in ACAES, presenting a novel take on the challenges and opportunities for this potentially game-changing energy storage option.

Most people are familiar with the act of blowing up a balloon, and then releasing the end and watching it fly around the room. This is an everyday example of energy stored in compressed air. Of course, while entertaining, this isn’t a particularly useful exercise. However, the concept of compressed air energy storage can also be deployed for utility scale storage of electricity. On the face of it, it sounds simple: To store energy, run a compressor and store the high pressure compressed air. To release energy, just release the air from its container, using the pressure difference between the stored air and the ambient atmosphere to drive a turbine and generate work.

An entertaining but useless compressed air energy storage system.

However, there are several challenges to be able to do this on a utility scale. When air is compressed to a high pressure (required to get a reasonable energy density), it gets very hot. This hot air is more difficult to store than cool air and it is challenging for the compressors’ operation. Therefore, industrial compressors which reach high pressures normally cool the air during the compression process, removing heat during and after the compression stages. This stops the air reaching very high temperatures.

The problem with this is that when cool air is expanded, it rapidly gets very cold. For example, using $T_{out}=T_{in}(\frac{p_{out}}{p_{in}})^{\frac{\gamma-1}{\gamma}}$, we find that when the inlet air is $300$K ($\sim27$C) and the expansion pressure ratio is $10$, the outlet temperature is $\sim155.4$ K ($-117.8$C), introducing condensation and possibly ice crystals into the turbines which are both highly damaging to the blades. The expansion of this cool air also generates much less work than was required to compress it, so the system isn’t very efficient. Therefore, the process in a utility scale compressed air energy storage heats the cool compressed air prior to the expansion through the turbines. This heating is achieved using combustion of natural gas, allowing the air to enter the turbine inlets at temperatures much higher than those achieved in the compression. This process is known as Diabiatic Compressed Air Energy Storage or DCAES. There are two DCAES plants that operate today, which together have more than 80 years of operational experience. Huntorf, Germany and McIntosh Alabama, USA.

The compression train at the Huntorf CAES plant

You might be thinking that the DCAES process sounds an awful lot like a gas turbine now… and you’d be right. The DCAES process is similar to an Open Cycle Gas Turbine (OCGT), with the major difference being that the compressors and turbines operate at different times. This temporal difference and the much higher air pressures (up to 72 bar in storage) give DCAES a significant storage element. For example, of the two existing DACES plants, the more modern McIntosh plant requires 0.69 kWh of off-peak electricity and 1.17 kWh natural gas to produce 1 kWh of peak electricity. This can be compared to an OCGT by itself, which may achieve an energy efficiency of around 40%, thus requiring approximately 2.5 kWh of natural gas to produce 1 kWh of peak electricity. But these DCAES systems still require fossil fuels, and therefore, are not the sustainable energy storage we need for the future.

Enter Adiabatic CAES or ACAES. ACAES removes the need for fossil fuels by storing the heat from the compression, using it to later heat the air entering the expanders. Again, sounds simple, right? Just use a Heat Exchanger (HEX) and a Thermal Energy Store (TES) instead of the combustion chamber. Unfortunately, as you may have realised if you have been reading carefully above (and as you may anyway suspect!), it isn’t that simple. To reach the high pressures, the compression has to be staged to avoid high temperatures, which as noted above are lower than the turbine inlet temperatures. Hence, the compression heat cannot be used to fully reheat the air entering the turbine or the temperatures would be too low. This means that the compression and expansion processes for ACAES must be re-designed, and, this re-design goes against current conventional wisdom.

What do I mean by that? Well, a sensible, practical measure of the performance of a compressor is cost per unit of compressed air (at the target pressure) produced. Since cost is generally proportional to work input, minimising work input is desirable. This involves having lots of intercooled compression stages with lower outlet temperatures. Conversely for a turbine, maximising work output involves higher inlet temperatures, typically with fewer stages. On top of this, another important reason for more compression stages and fewer expansion stages relates to the air flow. In a compressor, the air mass flows against the pressure gradient; for example, in an axial compressor the flow is accelerated by the rotor and decelerated by the stator. The deceleration produces the pressure change and hence the pressure increase available per stage is limited by the relative velocity between the rotor and the airflow. Conversely, in the turbine the flow is accelerated along the pressure gradient. The combination of these reasons means that when considered independently, the best-practice is to do the compression in lots of stages but the expansion in just a couple (two in both current DCAES plants).

However, our knowledge of thermodynamics says that we only get a reasonable work output if the expansion path follows the reverse of the compression path. Hence, the compressors and turbines in an ACAES system must be specially designed with the coupling between the compression and expansion at the forefront of the design considerations.

PV-Diagram for a 2-stage ACAES system. The efficiency (work-in/work-out) of an ACAES system is maximised by using a reversible compression path and following this during the expansion.

There are also challenges with the heat exchangers, since the typical approach to designing a high-effectiveness HEX for cooling is to use a large temperature difference between the fluid-to-be-cooled and the coolant. However, if this coolant must be used to reheat the air during the expansion, it needs to be as hot as possible, which in turn means the temperature difference between the air-to-be-cooled and the coolant must be minimised! This compromise between reversibility (the minimisation of temperature difference) and effectiveness is a major design challenge.

Overall, there have been lots of studies which predict that ACAES can have a reasonable performance (up to 75% RTE), however these studies typically use black-box component models (models which take a specified number of inputs and produce outputs without considering the internal component design details). While conceptually useful, these studies cannot accurately predict operation of real systems and hence the few pilot experimental systems that exist have not produced the anticipated performance level.

In our two recent papers (in iScience and Joule), we discuss these issues in more detail, along with developing a novel, robust analytical exergy analysis for an idealised system with variable pressure storage. We look at the historical development of ACAES and examine the multitude of failures (Lightsail, SustainX, project ADELE) to produce a viable compressed air energy storage system without fossil fuels. We highlight that ACAES research needs to consider a whole-system design philosophy, where components are designed with the charge-discharge coupling in mind, if simulations are to reliably predict the operation of pilot plants. We also highlight the need for further large-scale experimental work, accompanied by transparent documentation, to counterbalance a number of opaque and unverifiable performance claims which are currently hindering the development of ACAES and other next-generation CAES systems.