Exergetic life cycle analysis of hydrogen production and storage systems for automotive applications
Introduction
Hydrogen is generally regarded as a possibly important future energy carrier for automotive and other applications [e.g. [1], [2], [3]]. Hydrogen as an automotive fuel can be used in conventional internal combustion engines, but can also be applied in proton exchange membrane fuel cell vehicles (PEM-FCV) [4]. In order to get a complete view on the various characteristics of fuel chains, it is important to analyse fuel chains with a well-to-wheels approach. Energetic well-to-wheels analyses are mostly based on either the higher or the lower heating value (HHV and LHV, respectively) of fuels under consideration. Examples in literature are numerous [e.g. [5], [6]].
Exergetic well-to-wheels analyses can have considerable additional value compared to these LHV or HHV analyses. Exergy analysis is based on the first and second law of thermodynamics. The exergy content of a certain amount of energy equals the total amount of work obtainable from that energy. Exergy is thus a measure for the quality of energy. It is possible to perform work with energy when this energy is not in equilibrium with the natural environment. This lack of equilibrium can be the result of a temperature or pressure difference with the natural environment (physical exergy), but can also be the result of a difference in the chemical potential (chemical exergy) relative to the natural environment. By defining a reference temperature, pressure and chemical composition of the natural environment, it is possible to define a universal exergy content for every substance (both fuels and non-fuels) and subsequently for all process streams. The most used reference environment is the one defined by Szargut et al. [7]. Extensive information about the theory and the practical applications of the exergy concept is available in literature [8], [9], [10].
According to the second law of thermodynamics, every process has a dissipative, irreversible character and entropy is gained and exergy is lost in every process. Because of this irreversible character, neither exergy nor entropy is balanced for any real occurring process. The unbalance between the incoming and the outgoing exergy of a process is the irreversible exergy loss:Please note that the symbol E in Eq. (1) is used for exergy and not for energy. An exergetic efficiency can be defined asThe incoming exergy resources can be subdivided into a renewable part (consumption rate lower than or equal to the natural regeneration rate) and a non-renewable part (consumption rate higher than regeneration rate) and this subdivision can be used to define a renewability parameter α.The exergy loss is a measure for the loss in energy quality in a process and is a fundamental quantitative and universal measure for the depletion of both fuel and non-fuel resources in a certain process, proposed by a number of authors in recent publications [11], [12], [13], [14], [15], [16]. Mass and elements (except for nuclear reactions) nor energy is lost in a process; it is the quality of the energy (the exergy) that is lost.
Exergy analysis of processes can show the exact location of the process losses and can give information about the cause of these losses. Furthermore, exergy analysis gives insight in the fundamental theoretical thermodynamic limits of processes. Exergy analysis is therefore a useful tool in process comparison and process improvement. The exergy content of a process emission shows how far the emission is chemically and or physically out of equilibrium with the natural environment. The relation between the environmental damage of an emission and the exergy value of the emission is by no means linear or straightforward [7]. Exergy analysis can, however, be used to quantify the effort it would theoretically and/or practically take to abate emissions or to recycle emissions. Attempts to do so can be found in recent work by Dewulf et al. [14], Gong et al. [15] and Wall [16].
The aim of this paper is to give the results of an exergy analysis of eight different hydrogen fuel chains, differing in the way hydrogen is produced, transported and stored inside the vehicle and to use the results as an example to show the value of exergy analysis with respect to the aspects described above.
Section snippets
Description of the investigated production routes and hydrogen storage systems
Route 1–4 start with the conventional large-scale steam reforming of natural gas (NG), followed by pipeline transportation of the produced hydrogen to a hydrogen retail station with the capacity to serve an equivalent number of vehicle kilometres as a present day petrol retail station in the Netherlands. Route 5–8 start with the electrolysis of water at a hydrogen retail station. For each of these two main groups, four hydrogen storage systems inside the automobile are analysed: storage as
Hydrogen production processes: large-scale steam reforming and retail-scale electrolysis
The exergy analysis of the large-scale steam reforming plant is based on design information of a hydrogen producing plant at the Rayong Refinery in Thailand [17]. The reformer consists of a desulphurisation, reformer and water gas shift reaction section and a hydrogen purifying pressure swing adsorption (PSA) unit. NG enters the plant at a pressure of , the hydrogen product leaves the PSA unit at a pressure of . For the purpose of this study, the hydrogen product is
Exergy, the recyclebility of equipment and the abatement of emissions
In the calculation of the exergy requirements to produce equipment, two assumptions are made. The materials are assumed to be produced out of raw natural resources (e.g. metal ores) and the exergy of the materials is assumed to be lost exergy. The calculations thus represent a worst case. In practice, the equipment can be made out of recycled materials (e.g. metal scrap) and the materials still have an exergy content after usage and can be used for the production of new equipment. A distinction
Conclusions
Exergetic well-to-wheels analyses of fuel chains are in this paper shown to have considerable additional value compared to conventional analyses based on the LHV of fuels under consideration. The exergy loss is a fundamental universal measure for the depletion of both fuel and non-fuel resources and can thus be a useful tool in the quantification of resource depletion in these kind of studies. Furthermore, exergy can give insight in the theoretical limits of processes, in the locations and
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