Energy storage - all in flux

Energy storage is all the rage, as costs fall and the use of variable renewables expands. The boom started back in 2016, when grid linked battery storage grew by 50% to 1.7GW globally, according to IRENA. By then there was 1.6 GW of electro-mechanical storage, 3.1GW of heat storage, but all of this dwarfed by the 150GW of pumped hydro. The boom has continued since, for Lithium Ion batteries especially, with Tesla making much of the running, and some see that as continuing, with 55% growth per annum expected up to 2022:

http://www.greentechmedia.com/articles/read/lithium-ion-storage-installations-could-grow-by-55-percent-annually - gs.AKYlRrU That’s led to lots of guides to the market and how to get in on the act: http://www.adlittle.com/sites/default/files/viewpoints/adl_future_of_batteries-min.pdf and https://energystorageforum.com/files/whitepaper

Although the media is also awash with stories other clever, or sometimes odd, new ideas, including mass-based gravity stores http://www.forbes.com/sites/mikescott/2018/02/09/dropping-a-weight-down-a-disused-coal-mine-is-the-new-way-to-store-energy-and-it-might-just-work/ - 2118f5fd1497, liquid air stores http://www.powermag.com/grid-scale-liquid-air-energy-storage-plant-is-launched/ and giant underground caverns storing pressurised air or hydrogen. http://www.greentechmedia.com/articles/read/enlisting-abandoned-oil-and-gas-wells-as-electron-reserves As well as batteries for everything!

However, while the field is full of innovation at present, pumped hydro storage continues to dominate, although that has limits and there are few other major options as yet for large scale grid balancing, especially for longer-term energy storage, as this EASAC report makes clear: http://www.easac.eu/fileadmin/PDF_s/reports_statements/Electricity_Storage/EASAC_Electricity_Storage_Full_Report.pdf

It is also worth noting that storage only one option for grid balancing: ‘There is nothing that storage can do that something else can’t do - flexibility can be provided by a lot of competitors’. http://www.euractiv.com/section/electricity/news/storage-not-fundamentally-needed-for-future-power-grid-scientists-say/  Storage offers one way to respond to the variability of some renewables, but there are other options, including smart grid demand management (to time-shift demand peaks) and supergrid imports and exports (to balance local supply and demand variations across wide areas).

What is needed varies depends crucially on whether the requirement is for short term balancing (for a few seconds, minutes or perhaps a few hours) or longer term storage (for days, or even weeks). Batteries, capacitors, and flywheels, along with smart-grid demand adjustments, may all be fine for brief periods, dealing with short-term variations in renewable inputs, but are not much use for longer-term lulls in renewable availability. Pumped hydro projects may be able to deliver power for perhaps a day or so, depending on their scale, but for longer term storage that’s when big hydrogen gas or compressed air underground storage facilities may come into their own- linked to back-up generators. The stores can be charged using green energy already produced, when there was surplus, locally or on a wider basis, with supergrid links for transfers. That’s some way off, although a system is being developed with a 2.1 GW wind farm in Wyoming sending electricity by a HVDC power grid 525 miles to an underground salt cavern compressed air storage facility in Utah and then, after conversion back to electricity, 490 miles on to Los Angeles.

So there are really two different markets- one for large systems and longer term storage, the other, usually for small scale systems for short term grid balancing. However, there is some overlap. Indeed, confusingly, while domestic scale battery storage, to back-up home PV arrays, is quite popular these days, some say that storage is more efficient, in energy and cost terms, at the utility scale, though there are conflicting views and interpretations: http://www.nature.com/articles/nenergy20171

Also https://eandt.theiet.org/content/articles/2017/01/household-solar-storage-increases-emissions-study-concludes/ and https://news.utexas.edu/2017/01/30/storing-solar-power-increases-energy-use-and-emissions

Certainly though, at whatever sale, batteries are doing well, with falling prices for Lithium Ion batteries being the main driver- making use of technology initially developed for electric vehicles. Bloomberg New Energy Finance says that they expect the lithium-ion battery market for energy storage to be worth at least $239 bn between now and 2040: ‘Utility-scale batteries increasingly compete with natural gas to provide system flexibility at times of peak demand. Small-scale batteries installed by house holds and businesses alongside PV systems will account for 57% of storage worldwide by 2040’. https://about.bnef.com/new-energy-outlook/

However, while Lithium Ion batteries seem to be a winner for now in the battery field, the storage field generally is expanding rapidly, with a dizzying array of new battery technologies and ambitious claims as to their performance potentials. Here are just two battery examples: http://www.renewableenergyworld.com/articles/2017/02/new-hydronium-ion-battery-presents-opportunity-for-sustainable-grid-scale-storage.html and http://news.stanford.edu/2017/02/07/stanford-engineers-create-low-cost-battery-storing-renewable-energy/

Flow batteries are often seen as the big new thing. They involve the use of two separate fluids, but so far these have been toxic and can degrade, so some efforts have been centred on looking for alternatives e.g. organic materials: http://aziz.seas.harvard.edu/electrochemistry and long lasting materials: http://www.seas.harvard.edu/news/2017/02/long-lasting-flow-battery-could-run-for-more-than-decade-with-minimum-upkeep. For a market overview see: http://www.idtechex.com/redox Some Flow Battery ideas are quite dramatic, for example, underground cavern storage of the binary fluids for a huge redox flow battery system: http://www.renewableenergyworld.com/articles/2017/06/german-utility-planning-storage-project-in-salt-caverns.html

So that’s an option for large scale or longer term storage. But, for that, others also look to other electro-chemical conversion routes, such as electrolysis- to produce hydrogen gas, which can be stored (if necessary for long periods) ready for use in a fuel cells or gas turbine to generate power when needed. The Power to Gas (P2G) idea is being followed up in Germany, using surplus electricity from wind projects, with, in some cases, the resultant hydrogen being converted to methane gas, which has multiple uses.  Since P2G is a multi-stage process, the overall energy conversion efficiency can be low, and costs high, but efficiencies are improving and a study has suggested that, longer term, P2G could still win against pumped hydro on environmental sustainability terms: http://link.springer.com/article/10.1007%2Fs10098-016-1250-8

As can be seen, the storage field is still wide open, with many rival routes being promoted at a range of scales: there are many more than looked at above: http://iopscience.iop.org/book/978-0-7503-1531-9. It can be hard to assess all the claims. Some do not stay the course, with some projects collapsing e.g.: http://www.greentechmedia.com/articles/read/Redflow-halts-delivery-of-residential-flow-batteries Some also back out strategically: http://www.greentechmedia.com/articles/read/bosch-abandons-ev-battery-manufacturing Given this fluid situation, there can certainly been problems for those trying to decide which options to back.

Moreover, all this, so far, has been about storing electricity, directly or indirectly. Arguably, heat is a much better storage medium- higher energy density and low cost, with some renewables able able to produce it directly (solar, biomass, geothermal) other indirectly (e.g. wind powered mechanical heat churns, or just by resistance heating). But that’s another story, with a different set of issues, including debates over whether heat should be stored locally near the point of use, or whether it can be sent long distances efficiently. Some say so (20-30 miles), with the trade offs being between temperature, pipe diameter and heat flow rates. Big biogas-fired CHP is then an option, backed up by big heat stores, offering flexible heat and power outputs. That may turn out to be the best energy balancing option in some locations.

However, for the moment, given its low cost, we will probably just continue to use (fossil) gas fired turbines for most grid balancing. Certainly storing gas before combustion is still a much easier option than storing electricity once it has been generated. But hopefully some of these conventional gas plants will be switched over to using biogas or green syngas from P2G, so as to reduce the carbon emissions, with waste heat also being recycled wherever possible. That would be part of the change that is underway in the overall power system. The new post-generation storage options looked at above can also be part of it, including electricity and heat storage options, but the whole thing needs to be reconfigured into an optimal flexible balancing package. See: http://iopscience.iop.org/book/978-0-7503-1230-1