Editing IPCC:AR6/SR15/Chapter-4 (section)

== 4.3.1.4 Energy storage ==

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The growth in electricity storage for renewables has been around grid flexibility resources (GFR) that would enable several places to source more than half their power from non-hydro renewables (Komarnicki, 2016) <sup>[[#fn:r138|138]]</sup> . Ten types of GFRs within smart grids have been developed (largely since AR5)(Blaabjerg et al., 2004; IRENA, 2013; IEA, 2017d; Majzoobi and Khodaei, 2017) <sup>[[#fn:r139|139]]</sup> , though how variable renewables can be balanced  without hydro or natural gas-based power back-up at a larger scale would still need demonstration. Pumped hydro comprised 150 GW of storage capacity in 2016, and grid-connected battery storage just 1.7 GW, but the latter grew between 2015 to 2016 by 50% (REN21, 2017) <sup>[[#fn:r140|140]]</sup> . Battery storage has been the main growth feature in energy storage since AR5 (Breyer et al., 2017) <sup>[[#fn:r141|141]]</sup> . This appears to the result of significant cost reductions due to mass production for electric vehicles (EVs) (Nykvist and Nilsson, 2015; Dhar et al., 2017) <sup>[[#fn:r142|142]]</sup> . Although costs and technical maturity look increasingly positive, the feasibility of battery storage is challenged by concerns over the availability of resources and the environmental impacts of its production (Peters et al., 2017) <sup>[[#fn:r143|143]]</sup> . Lithium, a common element in the earth’s crust, does not appear to be restricted and large increases in production have happened in recent years with eight new mines in Western Australia where most lithium is produced (GWA, 2016) <sup>[[#fn:r144|144]]</sup> . Emerging battery technologies may provide greater efficiency and recharge rates (Belmonte et al., 2016) <sup>[[#fn:r145|145]]</sup> but remain significantly more expensive due to speed and scale issues compared to lithium ion batteries (Dhar et al., 2017; IRENA, 2017a) <sup>[[#fn:r146|146]]</sup> .

Research and demonstration of energy storage in the form of thermal and chemical systems continues, but large-scale commercial systems are rare (Pardo et al., 2014) <sup>[[#fn:r147|147]]</sup> . Renewably derived synthetic liquid (like methanol and ammonia) and gas (like methane and hydrogen) are increasingly being seen as a feasible storage options for renewable energy (producing fuel for use in industry during times when solar and wind are abundant) (Bruce et al., 2010; Jiang et al., 2010; Ezeji, 2017) <sup>[[#fn:r148|148]]</sup> but, in the case of carbonaceous storage media, would need a renewable source of carbon to make a positive contribution to GHG reduction (von der Assen et al., 2013; Abanades et al., 2017) <sup>[[#fn:r149|149]]</sup> (see also Section 4.3.4.5). The use of electric vehicles as a form of storage has been modelled and evaluated as an opportunity, and demonstrations are emerging (Dhar et al., 2017; Green and Newman, 2017a) <sup>[[#fn:r150|150]]</sup> , but challenges to upscaling remain.

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