• Energy Research

Abstract The contact resistance between the gas diffusion layer (GDL) and the bipolar plate has been experimentally estimated as they are assembled in proton exchange membrane (PEM) fuel cells. A number of coated and non-coated GDLs, graphite bipolar plates and a sealing gasket were employed to perform the test. The results show that the contact resistance of the non-coated GDLs (or carbon substrates) is highly influenced by the competing effects of the initial thickness of the GDL and the polytetrafluoroethylene (PTFE) loading. As for the coated GDLs, the PTFE loading present in the microporous layer (MPL) was found to play a positive role in establishing a good contact between the GDL and the bipolar plate.

Flooding of the cathode due to water accumulation is one of the biggest limiting factors in the performance of polymer electrolyte fuel cells (PEFCs). This study therefore attempts to solve this issue by fabricating gas diffusion layers (GDLs) with differently patterned hydrophobic regions. The GDLs in three different patterns (triangular, diamond, and inverted-triangular) were prepared by brushing a Polytetrafluoroethylene (PTFE) solution onto commercial carbon papers through a mask and tested in PEFCs. The patterned GDLs results in superior performance in all cases compared to a uniformly PTFE-treated GDL. Notably, the oxygen transport resistance is significantly reduced, indicating that the water accumulation in the cathode is avoided. This is attributed to the patterned hydrophobicity gradient providing distinct pathways for water and oxygen. The GDL with triangular patterning displays the highest peak power density, due to the fact that the untreated less hydrophobic region is in direct contact with the cathode outlet in this case, facilitating the removal of excess liquid water. Overall, the study confirms that the GDLs with patterned hydrophobicity could be used to enhance the performance of commercial PEFC systems by facilitating water management, potentially leading to improved efficiency and durability.

The Vertical Axis Wind Turbines (VAWTs) have an increasing global market and this emphasis the need for to improve the performance of VAWTs, especially at relatively low wind speed. This paper utilises the Response Surface methodology to optimise the performance of a VAWT. A three bladed VAWT configuration was considered with a NACA0015 profile. Three significant input parameters were selected including the tip speed ratio, the turbine solidity, and the pitch angle. An extended range of each input parameter was chosen in order to gain a good insight into how these input parameters affect the performance of the VAWT. The high-fidelity Computational Fluid Dynamics (CFD) simulations were carried out for the modelling of the turbine. The use of the Response Surface Optimisation based on Multi-Objective Genetic Algorithm (MOGA) along with the CFD simulations is found to be useful in the selection of the optimal design of VAWT. Moreover, the 3D aspects of the VAWT geometry are investigated and these include the turbine aspect ratio and the effect of the blade tip geometry. The implementation of an optimised winglet at the tip of the turbine blades is found to provide a significant enhancement of the cycle averaged power coefficient, especially at low aspect ratios.

In this paper, comprehensive governing differential equations of Stirling engines have been developed by coupling the effect of gas leakage through the displacer gap, gas leakage into the crank case and the shuttle heat loss into the traditional model. Instantaneous pressures and temperatures of the working fluid in the engine were evaluated at same time step. The present model was deployed for the thermal simulation of the GPU-3 Stirling engine and the obtained results were robustly compared to experimental data as well as results from previous numerical models. Then, parametric studies were conducted to assess the impact of geometrical and operating parameters on the performance of Stirling engines working with helium or hydrogen. Results suggest that the modifications made in this model led to better accuracy and consistency in predicting the experimental data of the prototype engine at all speeds, compared with most previous models. It was found that there exists a minimum dimensionless gap number, for every engine pressure below which mass leakage into the compression volume may not impact the brake power and energetic efficiency of the engine. In addition, an optimum mean effective pressure was found for maximum energetic efficiency of the engine. This optimum value is higher for helium gas than for hydrogen gas. Further results indicated that the brake power and energetic efficiency of the prototype Stirling engine can be significantly improved by 30% and 18%, respectively, provided that the heater temperature is raised to 850 °C while the cooler temperature is reduced to 0 °C.

Nickel foams feature superior structural and transport characteristics and are therefore strong candidates to be used as gas diffusion layers (GDLs) in polymer electrolyte fuel cells (PEFCs). In this work, the impact of compression on the key structural and transport properties has been investigated, including employing a specially designed compression apparatus and X-ray computed tomography. Namely, 20 equally spaced two-dimensional CT based images and numerical models have been used/developed to investigate the sensitivity of the key properties of nickel foams (porosity, tortuosity, pore size, ligament thickness, specific surface area, gas permeability and effective diffusivity) to realistic compressions normally experienced in PEFCs. Wherever applicable, the anisotropy in the property has been investigated. One of the notable findings is that, unlike porosity and ligament thickness, the mean pore size was found to decrease significantly with compression. The mean pore size is around 175 μm for uncompressed nickel foam and it decreased to around 110 μm for a 20% compression ratio and to around 70 μm for a 40% compression ratio. Further, unlike the effective diffusivity, the gas permeability was shown to be highly anisotropic with compression; this fact is of particular importance for PEFC modelling where the properties of GDLs are often assumed isotropic. All the computationally estimated properties have been presented, validated and discussed.

This paper numerically investigates the effect of the trailing edge profile on the performance of the straight-blade vertical axis wind turbine (SB-VAWT). In a 2-D cross-section of the SB-VAWT model, four trailing edge profiles are investigated, namely sharp, rounded, S-blunt, and R-blunt. The numerical investigation is based on the unsteady Reynolds averaged Navier-Stokes (URANS) equations combined with the transition shear stress transport (SST) model in order to account for the transition in the boundary layer in the vicinity of the airfoils. It has been found that the trailing edge profile may play a significant role in improving the turbine performance and should be accurately accounted for in the design process of the SB-VAWT.

There has been an increase in the use of biomass for power generation by means of co-firing with coal as well as by the combustion of 100% biomass. Despite the advantages of biomass in reducing carbon emissions from the electricity sector, the co-firing of high percentages of biomass can potentially aggravate ash related problems in the boiler. In order to develop mitigation strategies for the formation of deposits, an understanding of the ash behaviour during the combustion of high percentages of biomass is required. In this work, ash samples from El Cerrejon coal and pine biomass were characterised for their inorganic composition by X-ray fluorescence and wet chemical methods. Relationships between these two methods were found. Furthermore, the melting behaviour of ashes from pure coal, pine, and their blends were studied through ash fusion tests (AFT) and via a method using a simultaneous thermal analyser coupled to mass spectrometer (STA-MS) for the evolved gas analysis. Pine ash has lower slagging potential than El Cerrejon coal ash and results show that for 20:80 and 80:20 pine:coal ash belends the characteristic ash fusion temperatures increase with increasing pine ash content. There is unusually higher slagging potential (lower ash fusion temperatures) at a 50:50 blend ratio. Viscosity models produced sensible results for coal and coal/pine blends, but further refinement is required for modelling the viscosity of pure biomass ash. Thermodynamic modelling of slag formation was undertaken using the FactSage model. This model was successful in predicting the changes of gas, solid and liquid phases during pure pine, coal and co-combustion.

AbstractThe OxyCAP-UK (Oxyfuel Combustion - Academic Programme for the UK) programme was a £2M collaboration involving researchers from seven UK universities, supported by E.On and the Engineering and Physical Sciences Research Council. The programme, which ran from November 2009 to July 2014, has successfully completed a broad range of activities related to development of oxyfuel power plants. This paper provides an overview of key findings arising from the programme. It covers development of UK research pilot test facilities for oxyfuel applications; 2-D and 3-D flame imaging systems for monitoring, analysis and diagnostics; fuel characterisation of biomass and coal for oxyfuel combustion applications; ash transformation/deposition in oxyfuel combustion systems; materials and corrosion in oxyfuel combustion systems; and development of advanced simulation based on CFD modelling.