View metadata, citation and similar papers at core.ac.uk brought to you by CORE EnErgy StoragE rESEarch in SwitzErland – thE SccEr hEat & ElEctricity StoragE provided by Infoscience CHIMIA- École polytechnique2015, 69, No. 12fédérale753 de Lausanne doi:10.2533/chimia.2015.753 Chimia 69 (2015) 753–758 © Schweizerische Chemische Gesellschaft Redox Flow Batteries, Hydrogen and Distributed Storage C. R. Dennisona, Heron Vrubela, Véronique Amstutza, Pekka Peljoa, Kathryn E. Toghillb, and Hubert H. Girault*a Abstract: Social, economic, and political pressures are causing a shift in the global energy mix, with a prefer- ence toward renewable energy sources. In order to realize widespread implementation of these resources, large-scale storage of renewable energy is needed. Among the proposed energy storage technologies, redox flow batteries offer many unique advantages. The primary limitation of these systems, however, is their limited energy density which necessitates very large installations. In order to enhance the energy storage capacity of these systems, we have developed a unique dual-circuit architecture which enables two levels of energy stor- age; first in the conventional electrolyte, and then through the formation of hydrogen. Moreover, we have be- gun a pilot-scale demonstration project to investigate the scalability and technical readiness of this approach. This combination of conventional energy storage and hydrogen production is well aligned with the current tra- jectory of modern energy and mobility infrastructure. The combination of these two means of energy storage enables the possibility of an energy economy dominated by renewable resources. Keywords: Electrical energy storage · Hydrogen · Redox flow batteries 1. Introduction resources.[2] Moreover, these values are set such as wind and solar, this assumption is to grow significantly in the coming years. rapidly losing its validity.[4] 1.1 Growing Challenges for the By 2020, the European Union is targeting For grids with a large penetration of re- Electrical Grid 20% reliance on renewables for its overall newable energy, both supply and demand Around the world, concerns about en- energy mix.[3] These targets are primarily become unpredictable. Solar irradiation ergy security, sustainability, and the en- being met through the installation of wind, can vary significantly from minute to min- vironment have prompted a re-evaluation photovoltaic, and concentrated solar gen- ute depending on cloud formations and of the ways in which we produce and eration facilities. However, the growing other atmospheric conditions. A passing consume (or more precisely, convert) en- penetration of electric vehicles also plays cloud can cause megawatts of solar genera- ergy. As a result, there is a growing effort a key role in shifting the energy mix by tion to suddenly disappear from the grid, to transition the global energy mix from reducing the need for petroleum. necessitating other generating stations conventional sources, such as fossil fuels While these changes to the energy mix to rapidly ramp up their output to main- and nuclear energy, to more sustainable represent significant progress toward so- tain the stability of the grid. As the cloud sources such as hydro, wind and solar. In- cial, environmental, and political goals, passes, the solar generation becomes avail- deed, from 2002 to 2012, the net renewable they also represent a growing challenge able again, requiring the other generating electricity generation worldwide increased for the world’s electrical grids. Currently, stations to suddenly curtail their output by 62.5%.[1] As of 2013, 25.4% of Europe- most electrical grids are designed to pro- to compensate. The availability of wind an electricity, and 13.6% of overall energy duce electricity ‘just in time’ – as addi- is equally unpredictable. If these fluctua- (including transport, electricity, and heat- tional load is added to the grid, generating tions are not compensated for, the power ing/cooling) was derived from renewable stations must simultaneously ramp up to quality (i.e. line voltage and frequency) on meet the demand and keep the grid volt- the grid can deteriorate, eventually causing age and frequency stable. As consumer localized or even cascading power outages. demand is inherently unpredictable, some As the energy mix continues to evolve, fa- ‘reserve’ generation capacity needs to be vouring renewable resources, these fluctu- available at all times to cope with large ations will become increasingly disruptive, increases in demand. This reserve capac- pushing beyond the ramping limitations of ity has traditionally taken the form of re- conventional power stations. A larger share dundant generating stations which are idle, of ‘spinning reserves’, primarily gas-fired but synchronized with the grid so that they generators, will need to be allocated to sta- *Correspondence: Prof. Dr. H. H. Giraulta can react immediately (so called ‘spinning bilize the system. In effect, this means that E-mail: [email protected] reserve’). These resources are both ineffi- it becomes increasingly necessary to burn aLaboratoire d’Electrochimie Physique et Analytique (LEPA) cient and costly, as they are spinning (i.e. fossil fuels simply to utilize renewable en- École Polytechnique Fédérale de Lausanne (EPFL) – consuming fuel), but primarily operating at ergy sources, and of course this largely un- Valais Wallis zero-load. Nonetheless, this model for the dermines the goal of using renewable en- Rue de l’Industrie 17 Case Postale 440 electrical grid is quite satisfactory for cop- ergy in the first place (e.g. reduced depen- CH-1951 Sion ing with highly variable demand for elec- dence on fossil fuels, reduced atmospheric bDepartment of Chemistry tricity, provided that the supply of electric- emissions, etc.). Lancaster University ity is reliable. However, with the growing The growing use of electric vehicles Lancaster LA1 4YB, United Kingdom implementation of renewable resources can create similar problems of grid insta- 754 CHIMIA 2015, 69, No. 12 EnErgy StoragE rESEarch in SwitzErland – thE SccEr hEat & ElEctricity StoragE bility. Owners of these vehicles expect to production by essentially decoupling ener- value of these systems lies in their energy be able to recharge in a timeframe which gy production from consumption. Instead, capacity. is reasonably similar to conventional liq- such a grid operates on an ‘as available’ or Several technologies are being con- uid-fuelled vehicles. These quick charging ‘on demand’ basis – energy is stored when- sidered to address the growing technical stations can easily consume over 100 kW ever it is available from the generation in- needs and market opportunities. Indeed, each, and charging may take up to an hour frastructure, and supplied to consumers from a technological standpoint large- or more. Moreover, drivers will expect according to demand. In effect, the energy scale electrical energy storage is not a new to be able to charge whenever necessary, storage infrastructure acts as a buffer be- concept. Large pumped hydroelectric sta- creating a large magnitude, unpredictable tween generation and consumption.[5] This tions have long been used to store energy load on the grid. Finally, each driver who buffering is critical for the next evolution by pumping water from a low elevation to replaces a petroleum-fuelled vehicle with of the electrical grid for two reasons; it pro- a higher elevation. To recover this energy, an electric vehicle is essentially shifting tects consumers from variability in genera- the water is allowed to flow downhill under load from the petroleum infrastructure to tion (e.g. clouds passing over a solar sta- the influence of gravity, passing through a the electrical grid. This will increase the tion), and it protects generators from large turbine along the way. Unfortunately, this total load on the electrical grid, which will variability in demand (e.g. electric vehicle approach is highly geographically con- not only require additional generators, but charging). Thus, grid-scale energy storage strained, as it typically requires existing will also tax the electrical transmission and is necessary to enable the widespread im- natural features such as mountain lakes to distribution infrastructure. As the infra- plementation of renewable energy sources be practical.[5,8] In a similar approach, air structure reaches its limits, blackouts will and electric vehicles. is compressed into a reservoir to store en- become increasingly common.[4] However, there are numerous additional ergy, and allowed to expand through a tur- Hydrogen-fuelled vehicles have been benefits to electrical energy storage. Such bine to recover the energy. These systems considered as an alternative to fossil- systems can be used to offset the ‘peaks’ tend to be quite large, and have a relatively fuelled vehicles for years. Unfortunately, in electrical demand. In such a regime, low efficiency due to heat rejection during however, hydrogen is commonly obtained energy is stored when demand is low, and the compression step.[5,8] Instead, it is de- by reforming fossil fuels, resulting in hy- then time-shifted to periods when demand sirable to have an efficient, scalable, flex- drogen which is not truly ‘clean’ (in the is high. Large electrical consumers (e.g. ible technology that can be deployed to a sense of being carbon neutral). Electroly- steel mills) may engage in this practice to variety of locations. sers may be used to obtain hydrogen by reduce