Optimisation of mine ventilation fan speeds on demand

In today´s energy crisis, the mining industry has been identified as a promising area for application of energy efficiency techniques. This paper investigates potential for energy/cost saving, by implementation of variable speed drives (VSDs) on ventilation fans underground. In particular, ventilation on demand is considered in the study i.e. in cases when a mine´s ventilation demand may vary throughout the day. By modelling the network with application of Kirchhoff´s Laws and Tellegen´sTheorem, a non-linear constrained minimisation model was developed. The objective was to minimise the cost of energy for the fan which is directly related to the fan´s power consumption. The model also adheres to certain constraints pertaining to the variable demand. Also, fan laws were integrated in the model to find the correct operating point of the fan driven by VSDs to obtain more realistic results. A case study is shown, which is solved using the fmincon function in MatLab. The study suggests if Eskom´s Megaflex time of use tariff (TOU) was followed, an annual energy saving of 459 535 kWh was possible, leading to an annual cost saving of R372 300.