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the energy storage riddle - how do you squeeze a balloon?

Updated: Jun 1, 2022

written by Huw Thomas & Jim Isherwood

Following our recent article, “insights from recent hydrogen projects” and the considerable interest generated from it, we are happy to share our insights from recent stored energy projects, a critical piece in the energy transition jigsaw.

The roll out of variable renewable energy (VRE) sources continues around the world at pace and the need for energy storage to back up the supply with dispatchable electricity is becoming increasingly important. Energy storage can be provided via various technologies, including batteries, compressed or liquefied air and, pumped hydro, amongst others - each with an application niche. io consulting’s stored energy experts explain some of the considerations to be made when evaluating and designing large capacity, long duration energy storage facilities.

round trip efficiency

For stored energy technology, the round trip efficiency (RTE), the ratio of the energy out over the energy in, is a common measure of performance. One might assume that the higher the RTE the better, but maximising RTE is not always a cost effective approach. Based on our experience of evaluating and optimising energy storage systems, there will be a sweet spot of RTE that translates into the lowest lifecycle cost of energy stored. This sweet spot can be driven by the thermodynamics of the system but can also be due to the practicalities of scaling up to the large industrial applications necessary for grid level of support for hundreds, if not thousands, of MWh of energy stored. What is optimum on a pilot plant can lead to solutions that rapidly become absurd at industrial scale. A pragmatic understanding of equipment performance and sizing is key to establishing the RTE sweet spot and providing the lowest lifecycle cost of energy stored.

systems approach

Stored energy projects are complex in nature and balancing the cost vs efficiency (as described above) demands a systems approach. Further, when executing new solutions in different surroundings, the need to think holistically and applying true system engineering methods is fundamental to success, particularly when results are not always as expected. In compressed air energy storage (CAES), the results were sometimes counter intuitive. A simple example of this is in a water compensated CAES facility of fixed storage capacity, where it is cheaper to construct a 600m deep underground cavern than a shallower 300m deep cavern, as the volume required for the deeper cavern is reduced due to the higher storage pressure – perhaps obvious to those familiar to the mass, density and volume relationship, but it does get the grey cells moving nevertheless.

The majority of mechanical and thermal storage technologies i.e., CAES, liquid air energy storage (LAES) et al, are closed circuit, that is: power in – energy stored – power out. The relationships and interactions between the systems need to be understood to ensure no unintended consequences are caused. A good analogy, articulated by our friends at Hydrostor, is

“the energy storage system is much like a balloon, as you squeeze one end, the other expands.”

Striking the balance between competing factors is key to developing cost effective solutions.

To fully understand and identify the nuances of such a system, io’s system architects developed physics models to understand the fundamental relationships and interactions that exist within an CAES facility. To do so requires engineers with a solid foundation in thermodynamics as they do not shy away when enthalpy, entropy and exergy are discussed. The benefit of such a model was the ability to apply a wide range of operating scenarios and conditions to test and refine the complete design whilst maintaining a clear view of cost implications – a good example of this is provided in our previous Insights article “how systems modelling optimised A-CAES”.

benefits of scale

The ability to provide large scale, long duration energy storage facilities is what differentiates the mechanical, thermal and chemical technologies from their electrochemical alternative – the battery. How each technology compares and how their unique characteristics best serve the increasing energy storage demand is well understood, as shown in this chart.

However, experience from recent large scale capital intensive projects has demonstrated that the range of capacities for which the technologies are best suited is likely smaller, with a handful of sweet spots where the storage technology can achieve its best CAPEX/kW or CAPEX/kWh without incurring intolerable risk. The reason for this is due to the equipment and infrastructure applicable to these types of technology solutions.

In CAES, for example, the rotating equipment items represent a large proportion of the surface facilities costs and increases in capacity are typically accompanied with step changes in cost as capacity moves through frames sizes. Another consideration is that reduced scope, or fewer items installed, is generally less costly. Three large process trains compared to five smaller trains are almost always more cost effective but going big can bring risk, especially when technology limits are stretched.

In addition to selecting the equipment that best fits the solution, making the most of the design opportunities and overcoming the challenges that inevitably present themselves when working at large scale is key to identifying the sweet spot. One example concerns optimisation of the CAES plant layout - such facilities are mostly non-hazardous creating opportunities to challenge the convention of hydrocarbon process industries. The usual trade offs between bulk costs, such as piping and electrical cabling, are disrupted, and new best practices are identified that, when applied, bring considerable cost savings.

location considerations

The selection of the right energy storage solution for a particular application depends on the desired performance characteristics, including the dispatch capacity, the total energy stored and the need for grid support services. However, location is an important aspect that may drive the selection, for instance there may be existing natural or manmade infrastructure that has a synergy for a particular storage technology. As an example, a salt cavern or deep mines could be used for compressed air or hydrogen storage, providing advantage in cost and scale that is perhaps not available to LAES.

Such synergy is not just theoretical. For example, there are plans for the existing deep mines in the Broken Hill area of Australia to be converted into an air storage cavern as part of a CAES facility. This conversion dramatically reduces the lifecycle cost of the CAES facility, providing economic advantage for a necessary energy storage requirement and, by making use of existing infrastructure, there are wider benefits for the residents and workers of Broken Hill.

io’s approach

The race to develop and install “build anywhere” long duration energy storage technologies at scale, that can rival pumped hydro for cost effectiveness, is speeding up. The world's largest non-hydro long duration energy storage project, which is currently under construction and due to be fully commissioned in 2022, is Highview Power’s 50MW/250MWh liquid air energy storage (LAES) facility in Greater Manchester, England with more similar size facilities planned for Spain, UK, US, and Chile. These will however be eclipsed by the recent announcement from Hydrostor of two 500MW/5GWh ‘advanced’ compressed air long duration energy storage (A-CAES) projects to be built in California, USA. One reason these two technologies are first past the post is likely due to the fact that the technologies employed are not new. However, the combination is novel.

Interfaces within any design are always challenging to get right and, when developing a novel design or concept which includes systems and technologies that have not previously been deployed in combination, the need for a robust system integrator becomes paramount.

io is a project architect and system integrator created specifically with this in mind, integrating the experience and technologies of our parents Baker Hughes and McDermott. The application of a systems approach in the early stages of project development enables quality project decisions that lead to cost effective solutions that meet the business opportunity.

To find out more about our energy transition experience and capabilities, please contact:

Phil Penfold at io:

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