Electrochemical

Energy Conversion and Management 48 (2007) 1255–1265 www.elsevier.com/locate/enconman

Electrochemical reduction of CO2 to hydrocarbons to store renewable electrical energy and upgrade biogas
M. Gattrell *, N. Gupta, A. Co
Institute of Chemical Process and Environmental Technology, National Research Council, 1200 Montreal Road, Ottawa, Ont., Canada K1A 0R6 Received 25 March 2006; accepted 27September 2006 Available online 14 November 2006

Abstract An approach for synthesis of a hydrocarbon fuel from CO2 has been evaluated. The use of electrochemical reduction of CO2 at a copper electrode to a hydrocarbon mixture looks like a promising technology, especially when applied to upgrading biogases. The resulting gas mixture, containing methane and hydrogen with small amounts of ethyleneand carbon monoxide, has a similar energy density and composition to Hythane. Therefore, it is hoped that the gas mixture could be directly used in existing gasoline engines with minimal modifications. Estimating from existing literature data, the energy storage efficiency is poor (around 38% based on the fuel product HHV). However, the process equipment requirements are not complex, and the fuelproduct is also relatively readily used within standard engines. Finally, the paper discusses some of the key areas for research to verify and improve this technology. Ó 2006 Elsevier Ltd. All rights reserved.
Keywords: Renewable energy; CO2 conversion; Electricity storage; Electrochemistry; Hythane; Biogas upgrading

1. Introduction 1.1. Background A key to reducing emissions of CO2 is toutilize energy sources that do not emit CO2 during their operation. These can include renewable energy sources such as wind, hydro and solar, as well as nuclear energy. Issues arise in integrating such energy sources if they are not matched with energy demands in time and space. These can be due to the energy sources not being able to respond to demand requirements, the energy source beinggeographically distant from the demand and the demand being mobile (transportation needs). Many non-CO2 emitting power sources are poorly able to respond to energy demands. Wind, solar and tidal power produce electricity only when their respective energy source is naturally available. Nuclear, run-of-river hydro and bio*

Corresponding author. Tel.: +1 613 990 3819; fax: +1 613 991 2384. E-mail address:michael.gattrell@nrc-cnrc.gc.ca (M. Gattrell).

mass based electricity generation facilities that rely on complex processing (e.g., gasification) are best suited to providing base load, constant power. Many technologies have been developed to deal with such mismatches between power availability and demand through power storage. For electrical power in particular, these can include pumped hydro,compressed air/gas turbine, batteries and mechanical storage (flywheels) [1,2]. Many large sources of renewable energy are geographically located far from areas of high energy demand. Some authors have looked at trying to utilize solar energy better in desert regions [3] and hydroelectric resources in northern Canada [4]. Also, in Canada, a significant amount of wind energy is located along the NorthAtlantic coast (especially Newfoundland and Labrador), while most power demand is in southern parts of the country [5]. Finally, transportation requires a portable form of energy. The most flexible method to address these issues would be to store the available energy in a portable chemical form (i.e., as a fuel) for use at a later time or different location. It should be noted, however, that such fuelswould only

0196-8904/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2006.09.019

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M. Gattrell et al. / Energy Conversion and Management 48 (2007) 1255–1265

sensibly be produced if ‘‘true’’ (as opposed to time-of-day) local excesses of renewable energy existed. Time-of-day excesses of renewable energy could, through the storage technologies...