In the early 1990s the discrete element method (DEM) was used for the first time to simulate media motion in tumbling mills. Although it has been over a quarter of a century since this tool was first used to predict media motion it has not yet reached maturity to be used in predicting product size distribution and throughput of tumbling mills. However, there has not been shortage of attempts to do so. The literature is relatively vast in the topic and researchers who embark in this area of research will find it difficult to understand the current status of development and also the similarities and fundamental differences that exist amongst the various approaches that have been proposed and pursued over the years. The paper reviews the literature on the application of models based on distributed collision energy information to predict size reduction in tumbling mills, in particular ball mills, analyzing critically various approaches proposed, their limitations and achievements, identifying areas that still require development until the technology becomes ready for being used for optimizing and designing ball mills. Finally, the advances recently accomplished on the approach proposed by the author and his co-workers are then reviewed in greater detail.

Biomass grinds are typically non-spherical and are composed of particles with wide range of sizes that may vary up to 10× between the smallest and largest particle. Since fluidized bed system is often used to convert biomass into fuels, chemicals and products, the viscous and kinetic energy losses’ coefficients in the Ergun equation were determined to incorporate these unique characteristics of biomass grinds. The revised Ergun’s equation, validated using loblolly pine wood grinds, and data from other published work resulted in estimated Ergun’s K₁ and K₂ coefficients of 201 and 2.7 respectively. In addition, the relative mean deviation between experimental and predicted pressure drop was in general better with the modified Ergun’s equation when compared to the original Ergun’s equation.

Nanostructures in solar cells are used both for the active layers and for light management techniques. Particularly thin-film solar cells will benefit strongly from such nanoscale approaches as the light absorption needs to be improved. Nanoparticles produced by wet chemical techniques, sometimes in the form of quantum dots, are currently used to fabricate thin-film solar cells for research purposes. Light management studies use nanostructures that are often created by lithographic methods but which are too expensive for an industrial realisation. In this review paper, the opportunities for using nanoparticles as a bottom-up approach for both the active layer and light management nanostructures is discussed. Since both the wet chemical method and lithographic techniques have considerable limitations, the use of gas aggregation cluster sources is proposed as a promising method to advance the use of bottom-up nanoparticles for solar cells. Plasmonics, Mie scattering, quantum dots and new materials are reviewed with respect to the nanoparticle potential. The increase of solar cell efficiency by using ultra-clean and crystalline nanoparticles which are produced with a vacuum-compatible technique at low temperatures should be very interesting for science and technology, ultimately leading to industrial products.