Air fuelled zero emission road transportation: A comparative study
Introduction
Pressure has been growing over decades to curtail global greenhouse gas emission. A major source of the greenhouse gases is the burning of fossil fuels in internal combustion engines (ICE) for road transportation [13], [14], [15], [1], [16]. This is particularly true in the urban areas. Three types of zero emission road transportation technologies have been proposed and investigated extensively over the past two decades, vehicles based on hydrogen energy (e.g. fuel cell vehicles and hydrogen burning ICE), battery electric vehicles (e.g. nickel metal hydride, Lithium ion batteries) and air vehicles [8], [7], [13], [14], [15], Kreeith et al. [6], [9], [12], [17], [16], [10], [2]. A review has been published recently [11] on the three technologies. It was found that, among the three technologies, the battery electric technologies have the highest 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 investigated and prototypes of the two types of air powered road vehicles 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 compare the two technologies from a technological point of view. Engines for a typical small scale passenger car will be used for the analyses and the comparison will be based on the shaft work, coolth, efficiency and energy density. Note that only theoretical analyses are carried out in this work, the engines considered are virtual power systems.
Section snippets
Description of the technologies
Typical compressed air and liquid air power systems are shown schematically Figs. 1a and b, respectively. The principle of the compressed air power system is straightforward – compressed air expands through an expander to release the pressure potential producing work to drive the car. For liquid air engines, many thermodynamic 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
Methodology of the analyses
Fig. 2 shows the theoretical working cycles of the two engines in the temperature–entropy plane (T–S diagram). For the compressed air engine, the working process is simple as shown in Fig. 2a. It consists of only one expansion Process 1–0, in which the compressed air expands isothermally to produce work. The working process of the liquid air engine consists of three processes (Fig. 2b): pumping Process 1–2 in which liquid air from the cryogen tank (State 1) is pumped isentropically to a certain
Theoretical shaft work
Fig. 3, Fig. 4 show respectively the theoretical total (WT) and net work output (Wnet) for the two engines as functions of the working pressure and temperature.
In Fig. 3, the working temperature is assumed as 300 K and the final pressure is 1.013 bar (discharge to the ambience). For the range of working pressure shown (50–400 bar), both the theoretical total and net work output increase monotonically with increasing pressure and the increase tends to level off at high pressures. The theoretical
Concluding remarks
Two types of air fuelled engines for zero emission road transportation are compared in terms of their shaft work, coolth, efficiency and energy density. It is found that the shaft work output and the coolth of both the fuels increase with increasing working pressure or temperature. Given the working pressure and temperature, liquid air powered engines have a slightly lower specific work outputs than compressed air powered engines. At P = 300 bar and T = 300 K, the practical net work outputs of the
Acknowledgements
The authors would like to thank the following organizations for financial support of the work: UK EPSRC under Grant EP/F060955/1 (Y. Ding and C. Tan), China NSFC under Grant No. 50906079 (C. Tan and H. Chen), China Scholarship Council (X. Zhang) and a grant provided by the State Key Laboratories of Multiphase Flow and Complex Systems of Institute of Process Engineering of Chinese Academy of Sciences (Y. Ding).
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