PLANT WASTE SOLAR PANELS

 
 High purity silicon is essential for manufacturing solar panels. Unfortunately this prerequisite conversion of silica to elemental silicon requires a lot of energy, and the associated greenhouse gas emissions are significant. It has now been demonstrated that the ashes from burning biomass (rice hulls in this case) can provide a rich source of silica than can be reduced to give solar grade silicon.
For the preparation of the silica from rice hull ash only dilute acid and hot water are required. The energy requirement to then produce 99.9999% pure silicon is an order of magnitude less than the conventional
process and is actually lower than the energy created by burning the rice hulls in the first place. As the carbon dioxide generated by burning biomass is originally fixed from the atmosphere by plants, the carbon footprint for the production of this sustainable silicon is very low.
Polycrystalline silicon, with impurity levels lower than those of the SEMI III standard for solar grade silicon feedstock (≈99.9999% pure), was produced using rice hull ash (RHA) as a biogenic silica source. The RHA is first purified using very simple, low cost, low energy, acid milling/boiling water wash purification steps and pelletization followed by carbothermal reduction using an experimental 50 kW electric arc furnace (EAF) operated at 1700–2100 °C in batch mode. Typical processing involves adding 3.6 kg of pellets to the EAF followed by introduction of an additional 3.6 kg charge every 6 h after the start of carbothermal reduction. This approach produces up to 1.6 kg of silicon per batch. Purities, determined by inductively coupled plasma optical emission spectrometry (ICP-OES), were reproducibly found to be 99.9999 wt% (6 Ns) with B contents of ≈0.1 part per million by weight. This process escapes multiple process steps including the intermediacy of metallurgical grade silicon and the production and reduction of chlorosilanes as currently used in the Siemen's process. Furthermore, burning rice hulls to produce electricity and RHA, generates more energy than required for the overall process. Finally, the carbon footprint for the process discussed here is very low. The rice plant “fixes” CO₂ as it grows. The recovered hull contains sufficient amounts of this carbon that it can be burned to generate electricity returning part of this carbon to the atmosphere as CO₂. The carbon retained in the RHA is still from fixed CO₂ and provides the carbon source (especially in the Path 2 process) for carbothermal reduction returning the remaining carbon to the atmosphere as CO₂. A further point is that the alternative of landfilling with RHA or especially rice hulls would lead to generation of methane, a known green house gas. Thus, one might even argue that the carbon footprint for the process described here is actually negative.