Renewable Electricity Storage Using Electrolysis
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Renewable electricity storage using electrolysis Zhifei Yana, Jeremy L. Hitta, John A. Turnerb,1, and Thomas E. Mallouka,1 aDepartment of Chemistry, The University of Pennsylvania, Philadelphia, PA 19104; and bPrivate address, Broomfield, CO 80023 Edited by Richard Eisenberg, University of Rochester, Rochester, NY, and approved November 18, 2019 (received for review September 13, 2019) Electrolysis converts electrical energy into chemical energy by function of time for a spring day in California. Starting from storing electrons in the form of stable chemical bonds. The chemical around 4 PM a supply of an additional 13,000 MW of electricity energy can be used as a fuel or converted back to electricity when from nonsolar resources such as natural gas and nuclear within 3 h needed. Water electrolysis to hydrogen and oxygen is a well- is needed to replace the electricity shortfall of solar power (7). established technology, whereas fundamental advances in CO2 elec- The fluctuating power from solar and wind thus requires massive trolysis are still needed to enable short-term and seasonal energy energy storage, both in the short and long terms. There are multiple storage in the form of liquid fuels. This paper discusses the electro- ways that electrical energy can be stored including physical ap- lytic reactions that can potentially enable renewable energy stor- proaches such as pumped hydroelectric and compressed air energy age, including water, CO2 and N2 electrolysis. Recent progress and storage; large-scale batteries such as lead-acid, lithium, sodium major obstacles associated with electrocatalysis and mass transfer sulfur batteries, and flow batteries; and electrolysis, with pumped management at a system level are reviewed. We conclude that hydroelectric being the current leading technology in the energy knowledge and strategies are transferable between these different storage sector (8). Most of these methods suffer from being only electrochemical technologies, although there are also unique com- suitable for short-term storage or are limited by particular geo- plications that arise from the specifics of the reactions involved. graphic requirements. Electrolysis stands out as a means of storing electric energy in the form of stable chemical bonds. The potentially electrolysis | renewable energy | energy storage enormous energy capacity, negligible self-discharge rate, existing infrastructure, and low capital cost of containing renewably gener- uilding sustainable, clean energy systems is one of the most ated carbon-based fuels make them ideal for seasonal storage, a Bcritical problems that the world faces in this century. Pop- feature that is highly desirable given the seasonality of renewable ulation growth and economic development in the coming de- energy sources. The need for such a technology will be increasingly cades will inevitably result in a substantial increase in global acute with the larger anticipated penetration of the low-cost re- energy consumption (1, 2). Traditional fossil sources of energy newable electricity into the power grid, as illustrated in Fig. 1. are carbon-positive and contribute substantially to climate change. While water electrolysis to hydrogen and oxygen is a well- Various alternative sources of carbon-neutral power exist, in- established technology, an efficient and cost-effective means of cluding nuclear fission, biofuels, and electricity generation from storing electrical energy in the form of liquid fuels does not yet renewable resources such as wind and solar energy (1). It is pro- exist. In order for such a technology to become economically jected that renewable electricity can provide a dramatic decrease competitive with abundant fossil fuels, one has to start with very in carbon dioxide intensity, defined as carbon dioxide emissions cheap electricity. A technoeconomic analysis of CO2 electrolysis per unit of energy output, despite the projected increase in overall indicates that a renewable electricity price below $0.04/kWh will energy consumption (3). be needed for electrolysis-derived fuels to be competitive with Electricity from renewable sources such as wind and solar has fossil-derived fuels (9). This is a target that renewable electricity become economically competitive due to years of cost decline is well on its way to reaching based on the International Re- and advances in technology. The globally averaged levelized cost newable Energy Agency renewable cost database (4). of electricity (LCOE) for utility-scale solar photovoltaic (PV) In Fig. 1, we illustrate how a sustainable energy cycle can be power has fallen 73% since 2010, while the LCOE from onshore achieved by coupling electrolysis with electricity generated from wind has reached $0.03kWh in some parts of the world (4). The solar and wind, among other renewable sources. Electrolysis can price of renewable electricity from all currently commercial produce both commodity chemicals and hydrogen, mitigating the sources is projected to be comparable to or lower than that of intermittency of the renewable power. In this scenario, hydrogen- fossil fuel-derived electricity by 2020 (4). One of the major air fuel cells can be used to convert energy that is stored as hy- contributors to the falling LCOE for solar and wind power drogen back to electricity. High-energy-density liquid fuels are the electricity comes from the sharp reduction in installation costs. preferred form for seasonal storage and can form a green energy The global capacity weighted average total installation cost of cycle if CO2 in the air can be concentrated to enable efficient newly commissioned utility-scale PV projects has decreased by 68% to around $1,600/kW since 2010. The construction costs of the new nuclear- and coal-fired power plants are very uncertain This paper results from the Arthur M. Sackler Colloquium of the National Academy of Sciences, “Status and Challenges in Decarbonizing our Energy Landscape,” held October and are subject to various factors; however, total costs between 10–12, 2018, at the Arnold and Mabel Beckman Center of the National Academies of $5,500 and $8,100/kW and $3,500/kW, respectively, have been Sciences and Engineering in Irvine, CA. NAS colloquia began in 1991 and have been suggested and are expected to increase in the future (5, 6). published in PNAS since 1995. From February 2001 through May 2019 colloquia were Despite of the rapid growth of renewable electricity, the cur- supported by a generous gift from The Dame Jillian and Dr. Arthur M. Sackler Foundation for the Arts, Sciences, & Humanities, in memory of Dame Sackler’s husband, Arthur M. rent fraction of power generated from renewable sources in the Sackler. The complete program and video recordings of most presentations are available electric energy mix remains low, with a much greater proportion on the NAS website at http://www.nasonline.org/decarbonizing. of electricity being produced from gas, oil, and coal. However, Author contributions: J.A.T. and T.E.M. designed research; Z.Y. and J.L.H. performed re- based on a reference case prediction, electricity from renewable search; and Z.Y., J.L.H., J.A.T., and T.E.M. wrote the paper. resources is expected to increase from 18% to about 31% be- The authors declare no competing interest. tween 2018 and 2050 (3). One of the issues with solar electricity This article is a PNAS Direct Submission. is the intermittent nature of the diurnal solar cycle, which is not Published under the PNAS license. well matched with demand and thus requires excess generation 1To whom correspondence may be addressed. Email: [email protected] or capacity at the grid level or backup power on the consumer end. [email protected]. For example, Fig. 1 shows the net electrical load fluctuation as a First published December 16, 2019. 12558–12563 | PNAS | June 9, 2020 | vol. 117 | no. 23 www.pnas.org/cgi/doi/10.1073/pnas.1821686116 Downloaded by guest on September 27, 2021 advantage of the faster diffusion coefficient of gases and avoid- ing the low solubility of CO2 in liquid water, as shown in Fig. 2C. Water Electrolysis COLLOQUIUM PAPER Water electrolysis has a long history, the first published dem- onstration dating to 1789, and it is now a well-established com- mercial technology. For example, a large installation of alkaline electrolyzers by Norsk Hydro (1948 to 1990) at a hydroelectric plant was capable of generating about 70,000 kg H2/d (8). The largest wind-hydrogen plant was installed in Norway in 2004 by Norsk Hydro and Enercon, coupling a 600-kW wind turbine with a 48-kW electrolyzer (12). There has been increasing interest in water electrolysis over the past 2 decades because of its potential role in a hydrogen- based energy economy involving electrolytic production, storage, transport, and use of hydrogen as fuel (2). Current research fo- cuses on water electrolyzers that can be categorized according to their electrolytes: alkaline electrolytes, proton-exchange mem- branes (PEM), and solid-state proton or oxide ion conductors. Fig. 1. Sustainable energy utilization. Schematics of energy storage and utiliza- Commercial PEM electrolyzers achieve the best performance tion based on electrolysis. Surplus electrical energy from renewable sources can be (∼70% energy efficiency at current densities of 1.7 A/cm2, Giner stored via electrolysis as chemical fuels. The energy is extracted to levelize demand Inc.) (13) but the large overpotential of the anodic water oxi- on the short time scale and to meet the need for fuel in seasons when the re- dation reaction and the corrosive acidic environment necessitate newable supply is less available. Intermittency plot (Lower Left) data from ref. 7. the use of precious metal catalysts. The membrane electrolyte and manufacture of the bipolar plates also add significantly to cost. Alkaline electrolyzers are the oldest technology with somewhat electrolysis. There is growing research activity on the capture of ∼ carbon from the atmosphere (10), and a few companies are lower system efficiency ( 64% efficiency, at current densities of 1A/cm2, GHW) and lower cost ($800/kW) (13).