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Solar energy is deemed to be one the most efficient and clean energy resources to generate electricity. Photovoltaic technologies have a promising future in space and terrestrial applications. Photovoltaic concentrating is a technique to increase the conversion efficiency of high-efficiency solar cells. Multi-junction solar cells are designed to exploit a larger range of solar spectrum photons and convert to electricity. In this study, triple-junction III-V solar cells compound consisting of GaInP/GaInAs/Ge semiconductor materials is considered. This work investigates terrestrial multi-junction solar cells performance characterisation, which is important for the design of high concentration photovoltaic systems. The research has developed a model of a III-V solar cell operating at high flux conditions induced by light concentration. The thermal management on such an assembly is a focus of this work. This research also presents the effects of Air Mass (AM) on solar cell performance. This atmospheric parameter has a strong influence on the behaviour of high concentrating photovoltaic solar cells. As air mass increases, the corresponding Direct Normal Irradiance (DNI) and Cell Temperature (Tc) decrease. The effects of air mass (AM =1-10D) atmospheric changes on triple-junction solar cells have been assessed. For High Concentration Photovoltaic (HCPV) the light concentration on to a relatively small solar cell area leads to high power densities. Effective thermal management is essential to avoid damaging high temperatures. A thermal model by using a convergent iterative technique has been developed; the predicted convergent cell temperature limit is ≤ 80oC. The proportion of the incident radiation not converted to electricity leads to the generation of heat; this is a function of material temperature coefficients and current mismatch in variable atmospheric conditions and results in an increase in cell temperature. The rate of heat loss by convective transfer is also considered for air mass values AM =1.5, 4 and 8D. In addition, a Finite Element Method (FEM) model is developed in COMSOL Multiphysics® in order to predict the temperature distribution of the PV cells and thermal behaviour of the receiver assembly. Furthermore, in this study, a transient model of the HCPV cell has been developed using MATLAB® Live-Link with COMSOL Multiphysics. In order to characterise the behaviour of a triple-junction solar cell, it is essential to find the transient cell operating temperature. The behaviour of electrical parameters of the Jsc, Voc, FF and conversion efficiency are considered. However, in the proposed model, a dynamical efficiency is compared with constant efficiency and the error is about 12%. The research has given a better understanding of the overall daily/annual performance prediction of CPVs and is important for future system design in variable environment conditions. At higher values of DNI, Tamb and lower AM the thermal response needs enhanced/forced convection to maintain cell operation within/below safe operating temperature and to optimise energy yield. For long-term performance evaluation, the average of monthly variations of atmospheric parameters throughout the year is considered. Thus, during the summer months, a higher record of the atmospheric parameters values in which need more consideration. The annual cell operating temperature of ˃ 80oC represents about 13% of the time, which happened during the Summer season. As is noted, the cell temperature between 65-70°C is predominate in the Spring and Autumn seasons and represent about 24%, (the highest frequency).

The Fischer-Tropsch synthesis is a highly exothermic, indirect, catalytic, gas (syngas) liquefaction chemical process. Temperature control is particularly critical to the process in order to ensure longevity of the catalyst, optimise the product distribution, and to ensure thermo-mechanical reliability of the entire process. This thesis proposes and models the use of encapsulated, phase change material, in conjunction with a supervisory temperature control mechanism, as diluents for the catalytic, multi-tubular fixed bed reactor in order to help mitigate the heat rejection challenges experienced in the process. The modelling was done using the Finite Element Analysis (FEA) software, COMSOL Multiphysics. In the main, three studies were considered in this thesis. In the first study, a two dimensional quasi-homogeneous, reactor model, without and with the dissipation of the enthalpy of reaction into a near isothermal phase change material (silica encapsulated tin metal) heat sink, in a wall-cooled, single-tube fixed bed reactor was implemented and the results were presented. The encapsulated phase change material was homogeneously mixed with the active catalyst pellets. The thermal buffering provided by the phase change material were found to induce up to 7% increase in selectivity towards the C5+ and a 2.5% reduction in selectivity towards CH4. Although there was a reduction in the conversion per pass of the limiting reactant and hydrocarbon productivity due to a reduction in reactor temperature, it was observed that for a unit molar reduction in the productivity of C5+, there was a corresponding 1.5 moles reduction in methane production. In the second study, a modified, one dimensional, α-model was derived which accounted for the heat sink effect of the phase change material diluent. The resulting, less computationally cumbersome, yet sufficiently accurate model was benchmarked against the more rigorous two-dimensional quasi-homogeneous model in order to check its fidelity in predicting the reactor performance. As in the first case study, a homogeneous distribution of the phase change material and active catalyst pellets was assumed. The α-model was able to approximate the reactor temperature profile of the 2D-quasi-homogeneous reactor model to within 4% error, and consistently, slightly over-predicted the limiting reactant conversion by about 3%. Based on these comparisons, the α-model was deemed sufficiently accurate to predict the reactor performance in place of the 2D model for the optimisation simulation in the third study. The third case study entailed simultaneously maximising the production of long chain hydrocarbon molecules and ensuring proper heat rejection from the reacting system, two desirable yet often conflicting operational requirements. The homogeneous distribution of the active catalyst pellets and the phase change material diluents was abandoned for a multi-zonal axial distribution in which, individual zones of the catalyst bed were diluted to varying extents. The best dilution and distribution "recipe" was determined using optimisation techniques and the previously derived modified α-model. The multi-zonal axial dilution of the catalyst bed brought about a marked increase (up to 19%) in the productivity of the long chain hydrocarbons, while ensuring a more judicious use of the catalyst bed in contrast to the homogeneous catalyst/phase change material arrangement in the previous two studies. The latent enthalpy of the metallic phase change material combined with its good thermal conductivity helped push the limits of the catalyst bed by increasing the conversion per pass beyond the typical 20-30% reported in literature, with less likelihood of either early catalyst deactivation or thermal unreliability of the reacting system. In the main, it was observed that the overall productivity of the desired C5+ could be enhanced by reducing the quantity of the catalyst pellets by a pre-defined reactor volume. In addition, the reactor productivity benefits from a highly active zone situated at the reactor entrance, immediately followed by a less reactive zone. This arrangement has the effect of ramping the reaction rate (and in effect the reactor temperature) early on, and this is kept in check by the less reactive zone immediately adjacent to the reactive one at the reactor entrance.