Air fuelled zero emission road transportation: A comparative study Haisheng Chen a,b , Yulong Ding a,c, * , Yongliang Li a , Xinjing Zhang a,b , Chunqing Tan b a Institute of Particle Science & Engineering, University of Leeds, Leeds, UK b Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing, China c Institute of Process Engineering, Chinese Academy of Sciences, Beijing, China article info Article history: Received 17 April 2010 Received in revised form 24 June 2010 Accepted 2 July 2010 Available online 13 August 2010 Keywords: Zero emission Road transportation Compressed air Liquid air Engine abstract Road transportation using air as a fuel has attracted much attention over the past decade. The fuel (air) can be in two forms, compressed gas form and cryogenic liquid form and engines based on both forms of air have been investigated. Prototypes of air powered road vehicles are expected to emerge over the next few years. However, there have been debates over the advantages and disadvantages of the two technol- ogies. This paper aims to compare the two technologies from the technological point of view. Engines for a typical small scale passenger car are used for the analyses and the comparison is based on the shaft work, coolth, efficiency and energy density. It is shown that the shaft work outputs and the coolth avail- able to engines powered by both fuels increase with increasing working pressure and temperature. Given the working pressure and temperature, liquid air powered engines have a slightly lower specific work outputs than compressed air powered engines. The volumetric energy density of liquid air, however, is much higher than that of compressed air, and liquid air has much higher coolth than compressed air. On the other hand, the efficiency of the compressed air powered engines is higher than that of liquid air powered engines mainly because of the higher energy consumption of liquefaction plants. The anal- yses also suggest that an effective use of coolth be a key to improve the overall efficiency of liquid air powered engines. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Pressure has been growing over decades to curtail global green- house gas emission. A major source of the greenhouse gases is the burning of fossil fuels in internal combustion engines (ICE) for road transportation [13–15,1,16]. This is particularly true in the urban areas. Three types of zero emission road transportation technolo- gies have been proposed and investigated extensively over the past two decades, vehicles based on hydrogen energy (e.g. fuel cell vehi- cles and hydrogen burning ICE), battery electric vehicles (e.g. nickel metal hydride, Lithium ion batteries) and air vehicles [8,7,13–15], Kreeith et al. [6], [9,12,17,16,10,2]. A review has been published re- cently [11] on the three technologies. It was found that, among the three technologies, the battery electric technologies have the high- est energy efficiency but with toxic remains; the hydrogen energy technologies have the highest energy density but with the lowest efficiency, the lowest maturity and toxic remains; the compressed air technology promises an efficiency similar to that of battery electric technology, a high maturity and complete zero emission [11,5]. This paper is concerned with road transportation using air as fuel. Physically, air can be in three forms, compressed gas form, cryogenic liquid form and slurry form (mixture of liquid and solid air). Engines based on compressed and liquid air have been inves- tigated and prototypes of the two types of air powered road vehi- cles are expected to emerge in the next few years [2]. However, there have been debates over the advantages and disadvantages of the two technologies [8,7,18,4,17,10]. This paper aims to com- pare the two technologies from a technological point of view. En- gines for a typical small scale passenger car will be used for the analyses and the comparison will be based on the shaft work, cool- th, efficiency and energy density. Note that only theoretical analy- ses are carried out in this work, the engines considered are virtual power systems. 2. Description of the technologies Typical compressed air and liquid air power systems are shown schematically Figs. 1a and b, respectively. The principle of the com- pressed air power system is straightforward – compressed air ex- pands through an expander to release the pressure potential producing work to drive the car. For liquid air engines, many ther- modynamic cycles have been reported in the literature. The most extensively investigated one is the Rankine cycle as shown in Fig. 1b [8]. Liquid air stored in a tank is pumped in the liquid state 0306-2619/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2010.07.002 * Corresponding author at: Institute of Particle Science & Engineering, University of Leeds, Leeds, UK. Tel.: +44 113 343 2747; fax: +44 113 343 2405. E-mail address: y.ding@leeds.ac.uk (Y. Ding). Applied Energy 88 (2011) 337–342 Contents lists available at ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy