The silicon age that started in the 60s of the last century has changed the world profoundly, mainly related to the invention and development of microprocessor technology. Meanwhile, the demand for silicon is driven by the photovoltaics industry that consumes about 80% of the high-purity silicon produced worldwide. Independent of the final product, all high-purity silicon has passed through a couple of gas-phase reactions for purification. The most important gaseous species within this production chain are chlorosilanes and monosilane. We will discuss the direct formation of crystalline silicon by homogeneous gas-phase reactions as a direct and highly economical way to produce the required high-purity raw material for silicon solar cells. The direct formation of solid silicon particles from monosilane requires only a fraction of the energy compared to the established Siemens process based on the chemical vapor deposition of silanes. We have developed a method to synthesize nanocrystalline silicon powder using a hot-wall reactor, and the technology was scaled up to the pilot-plant scale. While an economical production strategy is decisive for solar cell production, the structure of the gas-phase product allows for additional, highly promising applications benefiting from the specific properties of the nanoscale particulate material. Both, thermoelectric generators as well as lithiumion batteries benefit from the nanocrystalline structure of the gas-phase product due to high phonon scattering and short diffusion lengths, respectively. First successful examples with regard to these two topics will be discussed. In these fields, silicon finds potential new markets for sustainable energy technology because of its abundant availability and low-cost production.