•Revisit in flow distribution theories in fuel cells.•Analysis of main issues and challenges in concepts and criteria of flow field designs.•Uneven flow distribution as a root cause of low durability ...and reliability after scaling-up.•Characteristic parameters for assessment of uneven flow distribution.•Measures to tackle issues of durability and reliability using flow field designs.
It is a major challenge to transform a laboratory scale production of fuel cells to an industrial scale in terms of throughput, operating lifetime, cost, reliability and efficiency. In spite of a number of efforts, the durability, reliability and cost of fuel cells still remain major barriers to scaling-up and commercialization. Unless these challenges are fully understood there is little chance of overcoming them. In fact, though much fundamental research has been performed, there is still no clear understanding of both the theoretical solution and technical measures needed to solve the durability and performance degradation of fuel cells in the scaling-up process. In this critical review, we will revisit advances in theory of flow field designs. Then, we will analyze main issues and challenges in concepts and criteria of flow field designs and development of theoretical models. We will focus on uneven flow distribution as a root cause of low durability and reliability and performance degradation and why flow field designs are a strategic solution to integrated performance, flow conditions, structure and electrochemical processes. Finally, we will discuss criteria and measures to tackle uneven flow distribution as well as critical durability and performance degradation in the scaling-up of fuel cells.
Since its creation over 170 years ago, and despite major investments and efforts by stakeholders over the last few decades to move this technology to the mainstream, today fuel cells continue to be ...regarded as a fledgling industry. In spite of the commitment by industry leaders, analysis shows that their actions do not address the critical questions facing this technology: Why has scaling-up of fuel cells failed so often when many researchers have stated their successes in the small scale? Why do fuel cell stacks have lower durability, reliability and robustness than their individual cells? Could investments of a hydrogen fueling infrastructure stimulate advancements in the key issues of durability, reliability and robustness and substantially reduce fuel cell costs? In this paper, we will analyze and confront these fundamental questions to improve understanding of the challenges of scaling-up technologies and identify key barriers. Then we will examine options and suggest a procedure for change to substantially improve the durability and reliability of fuel cells and reduce their costs.
•Analysis of chicken-egg issue between hydrogen fueling infrastructure and fuel cell technology.•Identification of technical barriers of durability, reliability and lifetime for fuel cell scaling-up.•Role of flow field design and uneven flow distribution in fuel cell scaling-up.•Measures to substantially improve durability, reliability and robustness.•Opportunities and recommendations for change for widespread adoption of the fuel cell itself.
A general theoretical model based on mass and momentum conservation has been developed to solve the flow distribution and the pressure drop in Z-type configurations of fuel cells. While existing ...models neglected either friction term or inertial term, the present model takes both of them into account. The governing equation of the Z-type arrangement was formulated to an inhomogeneous version of the U-type one. Thus, main existing models have been unified to one theoretical framework. The analytical solutions are fully explicit that they are easily used to predict pressure drop and flow distribution for Z-type layers or stacks and provide easy-to-use design guidance under a wide variety of combination of flow conditions and geometrical parameters to investigate the interactions among structures, operating conditions and manufacturing tolerance and to minimize the impact on stack operability. The results can also be used for the design guidance of flow distribution and pressure drop in other manifold systems, such as plate heat exchanges, plate solar collectors, distributors of fluidised bed and boiler headers.
The effects of climate change on sediment yield and transport dynamics in cold climate regions are not well understood or reported. In this study, the Soil and Water Assessment Tool (SWAT) has been ...built-up, calibrated, and validated against streamflow and sediment load at several monitoring stations in a cold climate region watershed - the Athabasca River Basin (ARB) in Alberta, Canada. The model was then fed with bias-corrected spatial disaggregated high-resolution (~10km) future climate data from three climate models for two emission scenarios (RCP 4.5 and 8.5), and two periods (mid- and end-century). Results show that channel erosion and deposition are the dominant processes over hill slope erosion in the basin. On average, a predicted warmer and wetter future climate has both synergetic and offsetting effects on sediment yield. Changes are sub-region specific and land-use type dependent, thus reflecting a marked spatial and temporal heterogeneity within the basin. Increases on sediment yield in future periods in the agricultural areas are up to 0.94t/ha/yr, and are greater than reported soil formation rates in the region. Similarly, while substantial increases (by more than two fold) in the sediment load transport through the river reaches were obtained, the changes show both temporal and spatial variability, and are closely aligned with the trend of stream flows. We believe that availability of such models and knowledge of the effect of future climatic conditions would help water managers formulate appropriate scenarios to manage such basins in a holistic way. However, significant uncertainties in future sediment yield and transport, as a result of variations in climatic forcing of different climate models, need to be considered in any adaptation measures.
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•Climate change impact analysis on erosion and sediment transport of the Athabasca River Basin (ARB) using the SWAT model•Future (mid- and late-century) climate data generated by three climate models for RCP 4.5 and 8.5 emission scenarios•Impacts due to climate change are found to be region specific and land-use type dependent.•Channel erosion and deposition were the dominant processes over hill slope erosion.•Increment on soil erosion rate due to climate change is greater than reported soil formation rates.
As resource scarcity, extreme climate change, and pollution levels increase, economic growth must rely on more environmentally friendly and efficient production processes. Fuel cells are an ideal ...alternative to internal combustion (IC) engines and boilers on the path to greener industries because of their high efficiency and environmentally friendly operation. However, as a new energy technology, significant market penetration of fuel cells has not yet been achieved. In this paper, we perform a techno-economic and environmental analysis of fuel cell systems using life cycle and value chain activities. First, we investigate the procedure of fuel cell development and identify what activities should be undertaken according to fuel cell life cycle activities, value chain activities, and end-user acceptance criteria. Next, we present a unified learning of the institutional barriers in fuel cell commercialization. The primary end-user acceptance criteria are function, cost, and reliability; a fuel cell should outperform these criteria compared with its competitors, such as IC engines and batteries, to achieve a competitive advantage. The repair and maintenance costs of fuel cells (due to low reliability) can lead to a substantial cost increase and decrease in availability, which are the major factors for end-user acceptance. The fuel cell industry must face the challenge of how to overcome this reliability barrier. This paper provides a deeper insight into our work over the years on the main barriers to fuel cell commercialization, and discusses the potential pivotal role of fuel cells in a future low-carbon green economy. It also identifies the needs and points out some directions for this future low-carbon economy. Green energy, supplied with fuel cells, is truly the business mode of the future. Striving for a more sustainable development of economic growth by adopting green public investments and implementing policy initiatives encourages environmentally responsible industrial investments.
Stream temperatures, which influence dynamics and distributions of the aquatic species and kinetics of biochemical reactions, are expected to be altered by the climate change. Therefore, predicting ...the impacts of climate change on stream temperature is helpful for integrated water resources management. In this study, our previously developed Soil and Water Assessment Tool (SWAT) equilibrium temperature model, which considers both the impacts of meteorological condition and hydrological processes, was used to assess the climate change impact on the stream temperature regimes in the Athabasca River Basin (ARB), a cold climate region watershed of western Canada. The streamflow and stream temperatures were calibrated and validated first in the baseline period, using multi-site observed data in the ARB. Then, climate change impact assessments were conducted based on three climate models under the Representative Concentration Pathways 4.6 and 8.5 scenarios. Results showed that warmer and wetter future condition would prevail in the ARB. As a result, streamflow in the basin would increase despite the projected increases in evapotranspiration due to warmer condition. On the basin scale, annual stream temperatures are expected to increase by 0.8 to 1.1 °C in mid-century and by 1.6 to 3.1 °C in late century. Moreover, the stream temperature changes showed a marked temporal pattern with the highest increases (2.0 to 7.4 °C) in summer. The increasing stream temperatures would affect water quality dynamics in the ARB by decreasing dissolved oxygen concentrations and increasing biochemical reaction rates in the streams. Such spatial-temporal changes in stream temperature regimes in future period would also affect aquatic species, thus require appropriate management measures to attenuate the impacts.
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•Climate change impact on the stream temperature regimes in the Athabasca River Basin was assessed.•Stream temperatures are expected to increase in the basin due to the warmer climate.•Stream temperature changes showed marked temporal pattern with highest increases in summer.•Future warmer stream temperatures would affect the fish species and water quality dynamics.
•Regional-scale modelling of ecosystem (Reco) and autotrophic respiration (Ra).•Temperature and dissolved oxygen (DO) levels mostly influence Reco and Ra.•Water-filled pore space has lowest influence ...on Reco, Ra and soil respiration (Rs).•Soil and air temperature and DO contributes >65% for Reco, Ra and Rs simulation.•First time Reco, Ra modelling using the Soil and Water Assessment Tool (SWAT).
Ecosystem respiration (Reco) and its components, the autotrophic respiration (Ra) and soil respiration (Rs) are the essential indicators of the global carbon cycle. They are represented as functions of either temperature or soil moisture, or a combination of both in the widely-used Earth System Models (ESMs). Thus, it is difficult to evaluate the influence of other environmental factors (such as, precipitation, soil temperature, dissolved oxygen level and oxidation reduction potential (ORP)) on Ra, Rs and Reco. Here we introduced microbially mediated, detailed carbon cycle processes within our mechanistic model to address this issue. Dominance analysis using a multivariate approach was performed to find out the influence of individual environmental factors on Ra, Rs and Reco in the cold climate regions of Athabasca River Basin (ARB), Canada. Contribution of the 6 predictor variables, including air temperature, precipitation, soil temperature, water-filled pore space (WFPS) used as a proxy of soil moisture, dissolved oxygen level, and ORP, on Ra, Rs and Reco were estimated based on the R2 values originated from multiple regression analyses. Our results showed that the prevailing temperature (both air and soil) and dissolved oxygen levels are the major influencing factors on Ra, Rs and Reco. WFPS is found to be the least influential factor on respiration estimation. Output of this study can be used to consider the crucial roles of environmental drivers in Ra, Rs and Reco estimation in the development of future ESMs.
•Technical challenges and obstacles broken down during the scaling-up of fuel cells.•Analysis of durability and reliability of fuel cell scaling-up using life cycle procedure.•Three operating windows ...for different stages: components, individual cells, and stack.•Operating window narrowing due to uneven flow distribution and dynamic load.•Connection points among components, flow fields, cells, stack and system control.
The technical challenges and obstacles to scaling-up of fuel cells are diverse, including such issues as water, heat, materials, catalyst, and flow fields because of multiple chemical and physical interactions at the atomic level and stack system level. The current results and data, even assumptions and guidelines are separated, inconsistent or unconnected. The unconnected data is partly the result of different disciplines. This paper is a first attempt toward understanding and analyzing the massive but spread-out work, which has been done and reported in the literature on fuel cell performance, reliability and durability. In this, we analyze the procedure of fuel cell research and development, and break down the barriers of scaling-up into four different stages: component, individual cell, stack and system control. We find that there are three different operating windows at each stage of the components, individual cells, and stack. While the operating window of components (e.g., membrane) are defined as ranges of temperature and relative humidity (RH), the operating window of a cell must include channel velocity and pressure drop within the cell. The operating window of a stack becomes narrower than that of its individual cells due to uneven flow distribution and load change. We have also found that there are knowledge gaps in the different stages of development. A solution for fuel cell scaling-up and a connection can be built among the components, cells, stack, process and system control through the operating windows and flow fields. The concepts of the three operating windows and flow field designs can build a connection among properties of the material and structures of components (e.g., wettability, porosity, and hydrophobicity), flow field, cells and performance of a stack and macro operation conditions (e.g., pressure, humidity and flow rates). This clarifies key ambiguities and converges our future directions on how to bridge different stages or disciplines of research and development. These can provide a new insight for future research to address the key issues of durability and reliability that remain unsolved.
A bent copper–water heat pipe with grooved inner surface has been investigated experimentally. A comparison between the bent and the straight heat pipes was performed at different inclination angle. ...Experimental results show that there is a small temperature difference between the condenser of the straight and that of the bent at the vertical orientation. The temperature difference increases as an inclination angle increases. Furthermore, the response time increases as the inclination angle increases. The thermal response of the straight to a sudden heat load is slightly faster than that of the bent. However, as the inclination angle increases to after the horizontal, the heat flux at the condensers decreases nonlinearly and the response time increases nonlinearly. A two-phase flow map has been proposed to explain the nonlinear performance of the thermal response and the heat flux, based on force balance among gravity, capillary, friction and buoyancy force acting on the working fluids. The nonlinear performance of the thermal response and the heat flux results from the capillary blocking due to formation of liquid bridge of two-phase flow. It was also found that the bent heat pipe is more sensitive to the change of the inclination angle than the straight in terms of the thermal response time and the heat flux of the condenser. The heat flux of the bent decreases faster than that of the straight after the horizontal orientation.
An analytical model based on mass and momentum conservation has been developed to solve the flow and pressure distribution in fuel cell stacks. While existing models neglected either friction effect ...or inertial effect, the present model takes both of them into account. The analytical solutions are fully explicit so that the velocity and pressure distribution in fuel cell stacks are directly correlated with the geometrical parameters of fuel cell stacks. Parameter Sensitivity is also analysed to determine the influence of geometrical structures and parameters on flow performance of fuel cell stacks. It is found that friction and momentum effects work in opposite directions, the former tending to produce a pressure drop and the latter a pressure rise. The proper balance of the two effects can result in less non-uniformity and an optimal design. Furthermore, the existing solution by Bassiouny and Martin Flow distribution and pressure drop in plate heat exchanges. Part I. U-type arrangement. Chem Eng Sci 1984;39(4):693–700 is a special case of the present solutions without the friction effect and those by Kee et al. A generalized model of the flow distribution in channel networks of planar fuel cells. J Power Sources 2002;109:148–59 and Maharudrayya et al. Flow distribution and pressure drop in parallel-channel configurations of planar fuel cells. J Power Sources 2005;144:94–106 are another special case without inertial effect.
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