Cost and energy savings on mine surface cooling systems

Abstract

The South African national power utility, Eskom, is struggling to meet the high electrical energy demand. The problem is due to the shortage of generating capacities as a result of delays in the building of new power stations. Unplanned maintenance on the existing power stations also adversely affects the power supply. Integrated Demand Management (IDM), a division of Eskom, initiated a Demand Side Management (DSM) programme to achieve energy efficiency and to apply load management to energy-intensive industries. The South African mining industry consumes approximately 14% of the country’s total generated electricity capacity. Gold mines in South Africa are the deepest in the world, reaching depths of up to 4 km. Such depths require large cooling systems to maintain acceptable and productive working conditions. Up to 25% of a mine’s total electricity usage is consumed by these large cooling systems. Electricity costs are increasing at a higher rate than inflation. For this reason, mines require more electrical energy saving measures to reduce their operating costs. The increase in operating costs reduces the competitiveness of the mines and thus affects their economic viability. Deep-level mine surface cooling systems were investigated to identify potential cost and energy savings. Typical surface cooling systems must supply air at approximately 8°C Wet-bulb (WB) to the underground working areas to maintain acceptable working conditions. Lower ambient air temperatures during the evenings reduce the cooling demand, hence the mine surface cooling systems power load can be reduced during Eskom evening peak period. The reduced power load on the cooling systems during the Eskom expensive evening peak period will result in electricity costs savings for the mine. A versatile load management strategy was developed, which can be implemented on closed or semi-closed loop mine surface cooling systems. Load reduction tests were conducted on the surface cooling systems of Mine A (closed loop) and Mine B (semi-closed loop) during the Eskom evening peak period. The underground temperatures and relative humidity were monitored during the evening peak period. These tests verified that the strategy can be implemented without adversely affecting the underground working conditions. An Energy Management System (EnMS) was utilised to execute the developed strategy. The underground temperatures of all three levels (95L, 100L and 110L) at Mine A vary between 23°C WB and 27°C WB after implementation. The underground Bulk Air Coolers (BACs) inlet air temperature on 84 level is monitored at Mine B, and the WB temperatures vary between 17°C WB and 21°C. These temperature ranges are within the operational safety boundaries of the mines. Therefore, the mine surface cooling systems can continuously be offloaded during the Eskom expensive evening peak period. An average evening load reduction of 1.8 MW was achieved at Mine A, with annual cost savings of R900 000. The load reduction for Mine B was an average of 2.7 MW and was achieved with the annual cost savings of R1.4 million. The developed load management strategy is versatile and can be implemented to any closed or semi-closed loop mine surface cooling system.