Journal of Power Sources 196 (2011) 1163–1170 Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour A comparison of high-speed flywheels, batteries, and ultracapacitors on the bases of cost and fuel economy as the energy storage system in a fuel cell based hybrid electric vehicle Reed T. Doucette ∗ , Malcolm D. McCulloch Department of Engineering Science, University of Oxford, Thom Building, Parks Road, Oxford, OX1 3PJ, United Kingdom article info Article history: Received 9 June 2010 Received in revised form 19 July 2010 Accepted 29 August 2010 Available online 6 September 2010 Keywords: Flywheel Ultracapacitor Battery Hybrid Electric Vehicle abstract Fuel cells aboard hybrid electric vehicles (HEVs) are often hybridized with an energy storage system (ESS). Batteries and ultracapacitors are the most common technologies used in ESSs aboard HEVs. High-speed flywheels are an emerging technology with traits that have the potential to make them competitive with more established battery and ultracapacitor technologies in certain vehicular applications. This study compares high-speed flywheels, ultracapacitors, and batteries functioning as the ESS in a fuel cell based HEV on the bases of cost and fuel economy. In this study, computer models were built to simulate the powertrain of a fuel cell based HEV where high-speed flywheels, batteries, and ultracapacitors of a range of sizes were used as the ESS. A simulated vehicle with a powertrain using each of these technologies was run over two different drive cycles in order to see how the different ESSs performed under different driving patterns. The results showed that when cost and fuel economy were both considered, high-speed flywheels were competitive with batteries and ultracapacitors. © 2010 Elsevier B.V. All rights reserved. 1. Introduction High-speed flywheels are an emerging technology with charac- teristics that have the potential to make them viable energy storage systems (ESSs) aboard vehicles. This paper investigates the com- petitiveness of high-speed flywheels on the bases of cost and fuel economy when compared to the more well established energy stor- age technologies of batteries and ultracapacitors in a fuel cell based series hybrid electric vehicle (HEV). Amidst growing concerns over energy security, climate change, air pollution, and fossil fuel reserves, alternatives to conventional automobile powertrains based on internal combustion engines (ICEs) are being investigated [1,2]. Powertrains based on fuel cells are one such alternative that have the potential to overcome many of the problems endemic to ICEs [3,4]. Fuel cells typically have a higher “tank to wheel” efficiency than ICEs, and depending on how the hydrogen fuel is generated they have the potential to emit significantly fewer pollutants [5]. Hybridizing a fuel cell with an ESS can have several positive impacts [6]. The ESS can be designed to meet the transient power demands that characterize normal driving conditions. With the ESS ∗ Corresponding author. Tel.: +44 (0) 7769715340; fax: +44 (0) 1865273010. E-mail addresses: reed.doucette@gmail.com (R.T. Doucette), malcolm.mcculloch@eng.ox.ac.uk (M.D. McCulloch). handling the transient loads, the fuel cell only has to provide the average power [2]. This enables the fuel cell to be downsized which reduces costs and typically improves efficiency [7]. The ESS pro- vides the additional benefit of being able to store energy captured through regenerative braking [2]. Most of the work done in designing and optimizing series HEVs has only considered batteries and/or ultracapacitors as the ESS [2,8–11]. While previous research has produced a great deal of information about optimal ESS technologies and configurations, it has largely neglected high-speed flywheels as an ESS technology that could compete with ultracapacitors and batteries. Flywheels are a mature energy storage technology, but in the past, weight and volume considerations have limited their appli- cation as vehicular ESSs [12]. The energy, E, stored in a flywheel is expressed by E = 1 2 Jω 2 (1) where J is the inertia and ω is the angular velocity. From Eq. (1), it can be seen that greater energy gains come from increasing the speed of a flywheel than from increasing the inertia. Improvements in low friction bearings and high tensile strength and low density materials have now made high speeds attainable hence making lightweight flywheels a reality [13]. For instance, the flywheel used in this study weighs 15 kg (including packaging), has a maximum speed of 60,000 rpm, and is capable of storing 540 kJ [14–16]. The Ragone plot in Fig. 1 shows that flywheels achieve specific energy 0378-7753/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2010.08.100