Future Cleantech Architects – Written evidence (LES0028)


About the authors: Future Cleantech Architects is an independent cleantech think tank working on neglected and hard-to-abate sectors. As such, long-duration energy storage (LDES) is one of our core topics, which is why we are submitting evidence for this call.


1. How much medium- and long-duration energy storage will be needed to reach the Government’s goal of a fully decarbonised power grid by 2035 and net zero by 2050, and by when will it need to be ready?


The grid’s requirements for long-duration storage are directly dependent on the fraction of variable wind and solar in the yearly electricity mix. There is wide variation across studies, but the overall trend is consistently that of an exponential growth in storage needs the higher the fraction of variable renewables (see Figure 1 from Albertus et al.). In other words, going from 90% to 100% renewable is significantly harder than 80% to 90%, and so on.


The bulk of storage (in terms of energy capacity requirements) lies in the seasonal timescale.


Underestimating storage requirements would be a risky strategy in terms of grid resiliency.


2. How sensitive is the amount of storage needed to assumptions about the future balance of supply and demand on the grid?



On the electricity grid, supply and demand must be matched at all times. Variable renewables are an inflexible supply, which means sources of flexibility must be found elsewhere, either on the supply side (dispatchable clean power[1]; grid interconnections; storage) or demand side (demand-side response). These different flexibility tools are not mutually exclusive; on the contrary, combining them is most likely to yield better results (a more resilient energy system thanks to redundancy; lower total system cost; reaching targets faster). The final estimates of storage capacity are very sensitive to the assumptions behind the relative shares of these flexibility tools. In the face of this uncertainty and given the high standards for resiliency of the electricity grid, it’s wiser to plan for the upper bound of storage capacity requirements rather than the median or lower estimates.


Overbuilding and curtailment of renewables is another option to increase flexibility and reduce the amount of storage needed: Cardenas et al. find that for, for a hypothetical 100% renewably powered UK, allowing for 15% curtailment reduces storage requirements by almost two-thirds.


Nuclear power can play an important role in providing clean baseload generation and therefore speeding up the phase-out of fossil fuels and increasing energy security. The caveat is that nuclear generation is usually run as a constant baseload (more due to economics, to get the best return on the high CAPEX invested) i.e. not flexibly, in which case it does not play a significant role helping integrate variable sources such as solar or wind. Nevertheless, the experience of France shows that it is technologically possible for nuclear to perform some degree of load-following, in which case it can help to integrate variable renewables.


Fossil fuels with carbon capture and storage (CCS) could be another source of clean, flexible generation in theory. In practice however, this is a route that should not be promoted, for several reasons:


  1. The long-term safety, effectiveness and cost-effectiveness of large-scale CCS projects remains unproven.
  2. A reliance on speculative future deployments of CCS could delay the development of truly clean energy sources and flexibility tools, such as energy storage, and further delay the phasing out of fossil fuels.



Demand-side management is likely to play a great role, although shifting demand in time can only go so far and is likely to be limited to a few hours/day timescale. It does not constitute a significant lever for seasonal imbalances.


3. Which technologies can scale up to play a major role in storage?


The following graphic overview by Future Cleantech Architects details the storage durations and technology readiness levels of the most prominent long-duration energy storage technologies: https://fcarchitects.org/the-basics-the-gaps-ldes/


As shown in the graph, there are plenty of commercial and pre-commercial storage technologies for medium-duration storage (hours to days), while the challenge lies for technologies suitable for seasonal durations, which have much lower readiness levels.


The most mature and widely deployed storage technology, both globally and in the UK, is pumped hydro (PHS). PHS systems can be designed to store electricity from several hours up to several weeks. While PHS suffers from long development times and geographical constraints, current installed capacities could certainly be significantly expanded, while having a relatively small environmental footprint by using closed-loop systems unlike from major waterways.


For medium-duration storage, other prominent storage technologies include:


  1. Molten salts, and other forms of high-temperature sensible thermal storage (power-to-heat-to-power). Power-to-heat-to-power systems, such as molten salts with steam power generation, typically have a modest efficiency (around 40% for the traditional systems, and up to 60% for systems employing high-temperature heat pumpscurrently under development). However, molten salt systems have been commercially deployed for over two decades and in several countries (with over 20 GWh of installed capacity world-wide in Concentrated Solar Power systems), and they are also modular, scalable, and rely on abundant materials. Furthermore, power-to-heat-to-power systems can also be used to convert fossil-fired power plants into storage systems using molten salts or other high-temperature thermal storage. This is a particularly promising proposition as it enables to reuse significant parts of infrastructure and to maintain the workforce of the former power plant.


  1. Flow batteries, which have over 1 GWh of storage capacity globally.


  1. Compressed air, which can use either underground caverns or overground artificial pressure vessels.


  1. Liquid air, which has seen rapid development in recent years, with its first demonstration plants installed in the UK.


  1. Batteries based on chemistries other than lithium-ion, such as sodium, potassium, and iron-air.


For seasonal storage, prominent technologies (with low TRL) include:


  1. Green hydrogen and derivatives.
  2. Metal powders, such as iron powder.



(see above answer to overhead question 3)



(see above answer to overhead question 3)



It would be very difficult on multiple fronts:


-          Large volumes would need to be produced, not just because seasonal storage requirements are high, but also to compensate for the efficiency losses in producing, transporting, storing, and using the hydrogen.

-          Building the pipelines to transport hydrogen from production to storage and from storage to peaker power plants.

-          Building a fleet of fuel cell power plants or hydrogen-fired gas turbines (low TRL) just for seasonal storage.


A more achievable alternative would be to establish a national strategic reserve of biogas specifically for seasonal storage: biogas can be easily stored indefinitely and then used in existing peaker plants during dark doldrums (also known as “dunkelflaute”). Biogas is renewable and low-carbon, and easier to secure than hydrogen as long as we don’t waste it in other sectors that have better alternatives (though some competing use cases such as high-temperature industrial processes are important too). Do we have enough biogas to make a difference? The DUKES 2023 report estimates that 3.4 TWh of power generation in 2022 came from biogas from anaerobic digestion alone; this is growing, and is supplemented further by other sources of biogas (landfill and waste). These 3.4 TWh already represent about 4 days’ worth of Britain’s average power demand. Biogas production in the UK is therefore of a scale that is too small to significantly contribute to total yearly power generation, but still large enough to be the right order of magnitude to cover at a minimum several days of dark doldrums in winter, where its added value of providing flexibility to the energy system is actually very high.



Definitely. Power-to-heat-to-power has been mentioned above, so let’s now focus on thermal storage for heat applications. Heat storage is neglected despite being relatively mature, efficient, and cost-effective. For domestic heat, storage is particularly suitable when coupled with district heating schemes, while it has a more limited role when considering single households (although with higher insulation then the home itself can have enough thermal inertia to last for 1-2 days and thus provide flexibility to the grid if the heating is electrified). In industrial settings, heat storage has a very important role, as it couples very well with electrification, both providing flexibility to the electrical grid and allowing the industry to consume green electricity when it is most available (and has a lower cost). Thermal storage however is subject to self-discharge, which means that while it performs very well for hours or days, it is not an efficient choice for seasonal storage, so it is limited at those timescales.


4. What policy support is currently in place to support deployment of storage technologies? Is it sufficient to support deployment at scale?


From discussions on the ground with companies, the consensus is that the current policy framework is insufficient. While there are many innovators springing up to offer various technologies, the customer side of the market is still risk-averse, which slows the development of the crucial first plants that will kick-start the industry and unlock cost reductions. Revenue streams are also too uncertain to give the market enough confidence in the profitability of plants to build them.


11 September 2023

[1] Dispatchable clean power includes a range of sources, such as hydropower, geothermal, biomass, concentrated solar power, and nuclear power.