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Nanofluids of graphene nanoplatelets (GNP) have superior thermal performance characteristics and good stability, are relatively affordable, and can easily be prepared by the two-step method. This review performs an in-depth analysis of the preparation, stability, and thermophysical properties of GNP nanofluids, and their applications in heat exchangers, solar thermal collectors, and heat pipes. This study analyses in detail the performance improvements achieved with pristine, covalent functionalised, and non-covalent functionalised GNP nanofluids compared to water. Covalent functionalisation was found to be superior to non-covalent functionalisation in terms of stability and heat transfer coefficients. Functionalisation by electrophilic addition and free-radical grafting were found to be more environmentally friendly compared to acid treatment. In terms of convective heat transfer coefficient, pristine GNP outperformed functionalised GNP, but both types showed large improvements compared to water. It was found that stability and heat transfer performance improved as particle size was decreased, while thermal conduction and convection coefficients increased with nanofluid concentration and temperature. Thermal conductivity improvements of over 30% were found for both pristine and covalently functionalised GNP nanofluids at 0.1 wt% concentration. A maximum convection heat transfer coefficient increase of 200% was achieved using 0.1 wt% pristine GNP nanofluid. By comparison, a maximum improvement of 119% was achieved using covalently functionalised GNP. The convective heat transfer enhancement seemed to increase with decreasing tube diameter. In flat plate solar collector applications, efficiency improvements over 20% were obtained for covalently functionalised GNP nanofluids at 0.1 wt%, while an efficiency improvement of over 65% was obtained using pristine GNP in an evacuated tube solar collector. Applications in cooling, heat pipes, and direct absorption solar collectors were also reviewed. From this study, it could be inferred that GNP nanofluids are a viable alternative working fluid. Further research is needed to optimise their performance.

An efficient phase change material (PCM) should not only exhibit high latent heat, but also high thermal conductivity. Combination of these favourable properties leads to an efficient latent heat thermal energy storage. Paraffin wax is one of the popular options for organic PCMs. However, paraffin wax has a low thermal conductivity, hindering its heat charging and discharging process. This study explores the effect of multi-walled carbon nanotube (MWCNT) weight percentage (0–0.08 wt%) and various surfactants (gum arabic (GA), polyvinylpyrrolidone (PVP), sodium docecylbenzene sulfonate (SDBS), and sodium dodecyl sulfate (SDS)) on the thermal conductivity, melting temperature, melting latent heat and thermal stability of paraffin wax. It is found that the maximum thermal conductivity enhancement (48%) is exhibited by paraffin wax/0.06 wt% MWCNT without surfactant compared to paraffin wax. The thermal conductivity of paraffin wax/0.08 wt% MWCNT added with PVP, SDBS and SDS are higher than samples added with GA. Overall, all samples exhibit single-step decomposition characteristics, as shown in thermogravimetric analysis (TGA).

© 2019 Spent Coffee Ground (SCG) was studied as a potential source of oil feedstock for biodiesel production as an alternative waste utilisation instead of being disposed as municipal waste. This study evaluated the ultrasonic assisted oil extraction from SCG, which was followed by biodiesel conversion via transesterification. Hexane was found to be the most effective extractant for SCG oil. Soxhlet extraction could obtain maximum SCG oil yield of 12.5% within 3 h whereas the highest SCG oil yield (14.52%) using ultrasonic extraction was obtained at hexane to SCG ratio of 4 mL g−1 and at 30% ultrasonic amplitude for 30 min. The improved oil yield with shorter extraction time was due to the ultrasonic fragmentation on SCG cells, which enhanced the interactions between oil and solvents. Fourier transform infrared analysis showed that the SCG oil possessed suitable functional groups for biodiesel conversion. Then, the SCG oil was successfully converted to biodiesel via ultrasonic assisted transesterification. The optimal FAME yield (97.11%) was achieved with molar ratio of methanol to SCG oil of 30:1, 4 wt% of catalyst concentration, at 30% ultrasonic amplitude and for 3 h. The produced SCG biodiesel has promising properties which adhere to the biodiesel standards but acid value was beyond the permissible limit which could be overcome by utilising as blend feedstock with other commercially available biodiesel. The high calorific value along with low viscosity, density and corrosion properties suggested SCG biodiesel as an interesting and viable option for biodiesel blending.