Effect of Vibration on the Liquid Water Transport of PEM Fuel Cells

The objective of this research is to analyze the effects of vibration on the performance of hydrogen PEM fuel cells. It has been reported that if the liquid water transport across the gas diffusion layer (GDL) changes, so does the overall cell performance. Since many fuel cells operate under a vibrating environment –as in the case of automotive applications, this may influence the liquid water concentration across the GDL at different current densities, affecting the overall fuel cell performance. The problem was developed in two main steps. First, the basis for an analytical model was established using current models for water transport in porous media. Then, a series of experiments were carried, monitoring the performance of the fuel cell for different parameters of oscillation. For sinusoidal vibration at 10, 20 and 50Hz (2 g of magnitude), a decrease in the fuel cell performance by 2.2%, 1.1% and 1.3% was recorded when compared to operation at no vibration respectively. For 5 g of magnitude, the fuel cell reported a drop of 5.8% at 50 Hz, whereas at 20 Hz the performance increased by 1.3%. Although more extensive experimentation is needed to identify a relationship between magnitude and frequency of vibration affecting the performance of the fuel cell as well as a throughout examination of the liquid water formation in the cathode, this study shows that sinusoidal vibration, overall, affects the performance of PEM fuel cells.

Steam Turbine Driving Compressor for Gas-Steam Combined Cycle Power Plant

Gas turbine is widely applied in power-generation field, especially combined gas-steam cycle. In this paper, the new scheme of steam turbine driving compressor is investigated aiming at the gas-steam combined cycle power plant. Under calculating the thermodynamic process, the new scheme is compared with the scheme of conventional gas-steam combined cycle, pointing its main merits and shortcomings. At the same time, two improved schemes of steam turbine driving compressor are discussed.

Thermo-Economic Optimization in the Design of Small-Scale and Residential Cogeneration Systems

In recent years, gas-turbine engines have undergone major improvements both in efficiency and cost reductions. Several inexpensive models are available in the range of 30 to 250 kWe, with electrical efficiencies already approaching 30%, due to the use of a basic air-compressor associated to an internal air pre-heater. Gas-turbine engines offer significant advantages over Diesel or IC engines, particularly when Natural Gas (NG) is used as fuel. With the current market trends toward Distributed Generation (DG) and the increased substitution of boilers by NG-fuelled cogeneration installations for CO2 emissions reduction, small-scale gas turbine units can be the ideal solution for energy systems located in urban areas. A numerical optimization method was applied to a small-scale unit delivering 100 kW of power and 0.86 kg/s of water, heated from 318 to 353K. In this academic study, the unit is based on a micro gas-turbine and includes an internal pre-heater, typical of these low pressure-ratio turbines, and an external heat recovery system. The problem was formulated as a non-linear optimisation model with the minimisation of costs subject to the physical and thermodynamic constraints. Despite difficulties in obtaining data for some of the components cost-equations, the preliminary results indicate that the optimal compressor pressure ratio is about half of the usual values found in large installations, but higher than those of the currently available micro-turbine models, while the turbine inlet temperature remains virtually unchanged.

Numerical Modeling of Velocity and Temperature Distributions in a Bipolar Plate of PEM Electrolysis Cell With Greatly Improved Flow Uniformity

In this present work, finite volume method was used to simulate the three-dimensional water flow and heat transfer in a flow field plate of the proton exchange membrane (PEM) electrolysis cell. The standard k-ε model together with standard wall function method was used to model three-dimensional fluid flow and heat transfer. First, numerical simulations were performed for a basic bipolar plate and it was found that the flow distribution inside the channels in not uniform. The design of the basic bipolar plate has been changed to a new model, which is featured with multiple inlets and multiple outlets. Numerical results show that the flow and temperature distributions for the new design become much homogeneous.

Energy Saving by ESCO (Energy Service Company) Project in Japanese Hospital

This paper describes a study starting from an analysis of typical energy demand profiles in a hospital setting followed by the case study of a cogeneration system (CGS) by an ESCO (Energy Service Company) project. The concept is a future autonomous system for the combined generation of electrical, heating and cooling energy in the hospital. The driving cogeneration units are two high-efficiency gas engines; this is used to produce the electrical and heat energy. Gas engine is used as a driving unit because of high needs for electrical and heating energy. The natural gas-fuelled reciprocating engine is used to generate 735kW of power. In our case electrical energy will be used only in the Hospital. A deficit in electricity can be also purchased from the public network. The generated steam will be used to drive three steam-fired absorption chillers and delivered to individual consumers of heat. This system is capable of doing simultaneous heating and cooling. No obstacles were recognized for the technical feasibility of CGS. The average ratio between electric and thermal load in the Hospital is suitable to make CGS system operate. An analysis performed for a non-optimized CGS system predicted a large potential for energy savings.

Experimental Evaluation of a Control Strategy for Real-Time Optimization of Low Temperature PEM Fuel Cell Stack

A robust control strategy which ensures optimum performance is crucial to proton exchange membrane (PEM) fuel cell development. In a PEM fuel cell stack, the primary control variables are the reactant’s stochiometric ratio, membrane’s relative humidity and operating pressure of the anode and cathode. In this study, a 5 kW (25-cell) PEM fuel cell stack is experimentally evaluated under various operating conditions. Using the extensive experimental data of voltage-current characteristics, a feed forward control strategy based on a 3D surface map of cathode pressure, current density and membrane humidity at different operating voltages is developed. The effectiveness of the feed forward control strategy is tested on the Green-light testing facility. To reduce the dependence on predetermined system parameters, real-time optimization based on extremum seeking algorithm is proposed to control the air flow rate into the cathode of the PEM fuel cell stack. The quantitative results obtained from the experiments show good potential towards achieving effective control of PEM fuel cell stack.

Processing Analysis of the Ternary LiNH2-MgH2-LiBH4 System for Hydrogen Storage

The ternary LiNH2-MgH2-LiBH4 hydrogen storage system has been extensively studied by adopting various processing reaction pathways. The stoichiometric ratio of LiNH2:MgH2:LiBH4 is kept constant with a 2:1:1 molar ratio. All samples are prepared using solid-state mechano-chemical synthesis with a constant rotational speed, but with varying milling duration. All samples are intimate mixtures of Li-B-N-H and MgH2, with varying particle sizes. It is found that the samples with MgH2 particle sizes of approximately 10nm exhibit lower initial hydrogen release at a temperature of 150°C. The as-synthesized hydrides exhibit two main hydrogen release temperatures, one around 160°C and the other around 300°C. The main hydrogen release temperature is reduced from 310°C to 270°C, while hydrogen is first reversibly released at temperatures as low as 150°C with a total hydrogen capacity of 6 wt.%.

Short Term Wind Speed Forecasting Based on Bayesian Model Averaging Method

Wind energy has been the world’s fastest growing source of clean and renewable energy in the past decade. One of the fundamental difficulties faced by power system operators, however, is the unpredictability and variability of wind power generation, which is closely connected with the continuous fluctuations of the wind resource. Good short-term wind speed forecasting methods and techniques are urgently needed since it is important for wind energy conversion systems in terms of the relevant issues associated with the dynamic control of the wind turbine and the integration of wind energy into the power system. This paper proposes the application of Bayesian Model Averaging (BMA) method in combining the one-hour-ahead short-term wind speed forecasts from different statistical models. Based on the hourly wind speed observations from one representative site within North Dakota, four statistical models are built and the corresponding forecast time series are obtained. These data are then analyzed by using BMA method. The goodness-of-fit test results show that the BMA method is superior to its component models by providing a more reliable and accurate description of the total predictive uncertainty than the original elements, leading to a sharper probability density function for the probabilistic wind speed predictions.

Performance Based Cost Modeling of Phase Change Thermal Energy Storage for High Temperature Concentrating Solar Power Systems

Sizing and cost models were developed for thermal energy storage (TES) systems utilizing cascaded phase change materials (PCM) as the storage media in a variety of configurations. The sizing model is based on an energy balance around a characteristic fundamental element of the system, consisting of a steel pipe embedded in a matrix of phase change material. Due to the transient behavior PCM system, the sizing model requires time and space integrations. The model accounts for decreases in thermal performance caused by precipitate formation on the surface of the pipe and predicts the resulting transient power output. The model calculates the required tank and pipe sizes, the amounts of heat transfer fluid and PCM, as well as the land area for the configuration. Using a cost metric approach, the cost of each system component is estimated. Furthermore, the effect of several technological pitfalls, including: pinch point heat transfer, precipitate buildup, and transient energy output have been investigated. Prices are shown to depend heavily on system configuration. Specifically, prices are shown to be most dependent on precipitate formation during discharge and consequently the size of the necessary heat transfer area of heat exchangers. The cost of different configurations vary from $40/kWh to $100/kWh.

Analysis of Energetic, Design and Operational Criteria When Choosing an Adequate Working Fluid for Small ORC Systems

Small cogeneration (CHP) systems may lead to a significant reduction of primary energy consumption and harmful emissions. Low temperature Rankine cycles, that can be assisted by solar energy, are a possible solution for producing combined electricity and useful heat. These cycles usually use an organic working fluid. This study presents an analysis of the energetic, design and operational features, that have to be taken into account when choosing an adequate working fluid for these Organic Rankine Cycles (ORC). When using renewable energies as a heat source, like solar or geothermal, the cycles may operate at temperatures between 120°C and 230°C. A system producing 5 kW of electricity was considered as a basis of comparison. Several fluids were analysed: n-dodecane, water, toluene, cyclohexane, n-pentane, HFE7100, R123, isobutane and R245fa. The organic dry fluids, with a positive slope of the saturated vapor curve in a T-s diagram, are in principle desirable for low temperature applications, simplifying turbine design. The degree to which the fluids are drying, is generally related to their molecular weight or molecular complexity. Practical issues, like thermal stability, toxicity, flammability and cost are considered. The thermodynamic cycle efficiency is also important. The saturated vapor specific volume gives an indication of condenser size, which is related to system initial cost. A super-atmospheric (>100 kPa) saturation pressure eliminates infiltration gases, which is important for operational reasons, because infiltration reduces system efficiency. The degree of superheating was optimized for maximum cycle efficiency, with a quadratic approximation method. This optimization makes it possible to decide if it is better to have saturated vapor or superheated vapor at turbine inlet, for a fixed turbine inlet temperature. For a heat source temperature of 120°C, only toluene and isobutane present a small advantage in superheating. It is difficult to find the best fluid, which has simultaneously: high cycle efficiency, low vapor specific volume at turbine outlet, super-atmospheric saturation pressure, good thermal stability, small environmental impact, small toxicity and no flame propagation. From the point of view of cycle efficiency, n-dodecane presents the best performance. However, this fluid presents the highest saturated vapor specific volume (resulting in a larger condenser) and the smallest condenser saturation pressure (resulting in infiltration of gases). The best candidates for the cycle regarding all the aspects are: toluene, cyclohexane and n-pentane. Comparing the three fluids, toluene presents the highest efficiency, the highest impact in environment and the biggest vapor specific volume. N-pentane presents the smallest cycle efficiency and smallest vapor specific volume, but is the unique fluid with super-atmospheric saturation pressure. Cyclohexane is the fluid with lowest impact in environment.