Development of some exergetic parameters for PEM fuel cells for measuring environmental impact and sustainability

Abstract

This paper presents some new exergy-based parameters for PEM fuel cells to study how some of their operating aspects and system characteristics affect the environment and sustainability, based on some actual and literature data. The exergetic parameters of a PEM fuel cell developed here, in conjunction with environmental impact and sustainable development, are exergy efficiency, exergetic stability factor, environmental benign index and exergetic sustainability index. Any increase in efficiency improves exergetic sustainability. However, any increase in waste exergy ratio, exergy destruction factor, environmental destruction coefficient and environmental destruction index results in an increasing environmental impact of the PEM fuel cell and hence, a decreasing sustainability. Such parameters are expected to quantify how PEM fuel cells become more environmentally benign and sustainable.

Introduction

It is well-known that micro-level system parameters may have some great impact on macro-level energy aspects, the environment, and sustainability. Of course, if one wants to approach these thermodynamically, there are two ways: energy analysis through the first-law of thermodynamics and exergy analysis through the second-law of thermodynamics. Exergy analysis is an essential tool to expose the impacts of a power generating device on exergy-based sustainability, sustainability is necessary to overcome current ecological, economic, and developmental problems [1]. Sustainability concerns often lead local and national authorities to incorporate environmental considerations into energy planning. In this regard, sustainable development demands a sustainable supply of a fuel such as hydrogen that, in the long term, is readily and sustainably available at reasonable cost and can be utilized for all required tasks without causing negative societal impacts. If so, the promotion of the power generating systems consuming hydrogen such as fuel cells for exergy-based sustainability should become one of the primary goals of energy policy makers all over the world. In this regard, it should be emphasized that, in terms of exergy-based sustainability, PEM fuel cells consuming hydrogen will play an important role to generate clean power in the near future. Actually, in order to produce electricity more efficiently, and reduce pollution and greenhouse gas emissions, several new technologies are under investigation [2]. Of these, PEM fuel cell, which is the focus of this manuscript, is a hydrogen fuel cell that is a device that converts hydrogen and oxygen directly into electricity, water, and waste heat while producing none of the noxious byproducts typical of combustion processes [3]. This fuel cell is particularly attractive for low (less than 1 kW) to intermediate power levels (up to 50 kW), and for applications that required rapid start up and quick response to load changes [4]. The electrolyte in this fuel cell is an ion-exchange membrane (fluorinated sulfonic acid polymer or other similar polymer) that is an excellent proton conductor. The only liquid in this cell is water; thus corrosion problems are minimal. Water management in the membrane is critical for efficient performance; the fuel cell must operate under conditions where the byproduct water does not evaporate faster than is produced because the membrane must be hydrated. Because of limitation on the operating temperature imposed by the polymer, usually less than 120 °C, and because of problems with water balance, a H₂-rich gas with minimal or no CO (a poison at low temperature) is used [5]. It may then be said that PEM fuel cell is arguably the most appropriate type for mobile applications due to its low operating temperature, its solid phase electrolyte, and its ability to operate with CO₂ in the air stream [4], [6]. All these can be emphasized in terms of the first-law of thermodynamics. However, in terms of the second-law of thermodynamics, minimizing irreversibilities in PEM fuel cells is a significant challenge for better efficiency, environment, and sustainability. This should be investigated and exergetically discussed for better understanding the environmental and sustainability aspects and benefits of PEM fuel cells. Considering the above explanations, a detailed literature review has been performed on exergy-based sustainability parameters of a PEM fuel cell.

Through a comprehensive literature review, it is noticed that various aspects of a PEM fuel cell have been studied [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. Regarding exergy-based sustainability, it is also possible to find some works on exergy-based sustainability parameters for different applications in the literature [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46]. However, no studies exist on exergy-based sustainability parameters of a PEM fuel cell in the open literature. Lack of such parameters makes the paper original and becomes the main motivation behind this work. In addition, there are some key reasons to conduct the present study: (i) PEM fuel cells have received considerable attention recently, notably as a potential replacement for the conventional internal combustion engine in transportation applications [47]. (ii) Hydrogen is becoming a much more cost effective energy carrier than coal and synthetic fossil fuels, as well as being the environmentally most compatible fuel [48]. (iii) Hydrogen will play a key role in replacing fossil fuels [49], [50], [51], [52], (iv) hydrogen as a fuel is more exergetically efficient in power generation systems (including fuel cells) [53], (v) environmentally benign and sustainable hydrogen production and utilization will be one of the potential solutions in combating global warming [54], [55], (vi) PEM fuel cell has been widely recognized as the most promising candidates for future power generating devices in the automotive, distributed power generation and portable electronic applications [56], (vii) there has recently been an increasing interest in the use of PEM fuel cell for both mobile and stationary applications as environmentally friendly power source. Emphasis is placed on high power density with adequate energy conversion efficiency for mobile applications, and on high-energy efficiency with adequate power density for stationary applications [9]. Considering such important facts, as the scientific and industrial benefits, this study, which includes all details on exergy-based sustainability aspects of a PEM fuel cell, aims to contribute to:

• develop energetic, environmental and sustainability parameters in conjunction with PEM fuel cells and their integrated systems,

• understand new exergetic dimensions of a PEM fuel cell consuming hydrogen produced from non-fossil fuel sources such as water, clean biomass,

• introduce some main concepts and methods about the exergy-based sustainability parameters of a PEM fuel cell,

• develop some new parameters such as exergetic efficiency, waste exergy ratio, exergy destruction factor, environmental destruction coefficient, environmental destruction index, environmental benign index, exergetic stability factor, and exergetic sustainability index, and

• encourage the strategic use of PEM fuel cell systems for ensuring exergy-based sustainability.

Under these important considerations, as understood from the above literature works, nowadays there is a great interest in studying the linkages between hydrogen and exergy-based sustainability. If so, it can be said that one of the best ways to link hydrogen and exergy-based sustainability is to use PEM fuel cell device consuming hydrogen from non-fossil fuel sources such as water in order to contribute to ensure the exergy-based sustainability. As differing from the above studies, this paper presents some exergy-based sustainability parameters of a PEM fuel cell in terms of the second-law of thermodynamics. Actually, in this paper, we bring completely a new dimension about hydrogen consuming devices such as PEM fuel cells (so-called: new sustainability dimensions of a PEM fuel cell) in order to contribute to sustainability. For this purpose, some exergetic parameters of a PEM fuel cell for exergy-based sustainability are developed and proposed by taking into account the general characteristics of a PEM fuel cell.