Energy efficiency through variable speed drive control on a cascading mine cooling system
Van Greunen, Declan
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An ever-expanding global industry focuses attention on energy supply and use. Cost-effective electrical energy production and reduced consumption pave the way for this expansion. Eskom’s demand-side management (DSM) initiative provides the opportunity for reduced electricity consumption with cost-effective implementation for their respective clients. South African gold mines have to extend their operations to up to 4000 m below the surface to maintain profitable operations. Deep-level mining therefore requires large and energy-intensive cooling installations to provide safe working conditions. These installations generally consist of industrial chillers, cooling towers, bulk air coolers and water transport systems. All of these components operate in unison to provide chilled service water and cooled ventilation air underground. In this study the improved energy efficiency and control of a South African gold mine’s cooling plant is investigated. The plant is separated into a primary and secondary cooling load, resulting in a cascading cooling system. Necessary research was conducted to determine the optimal solution to improve the plant’s performance and electrical energy usage. Variable speed drives (VSD) were installed on the chiller evaporator and condenser water pumps to provide variable flow control of the water through the chillers, resulting in reduced motor electricity usage. Potential electricity savings were simulated. Proposed savings were estimated at 600 kW (13.6%) daily, with an expected saving of R 2 275 000 yearly, resulting in a payback period of less than 9 months. Results indicated are based on total savings, as VSD savings and control savings were combined. The VSDs that were installed, were controlled according to an optimum simulation model’s philosophy. A real-time energy management program was used to control the VSDs and monitor the respective systems. The program’s remote capabilities allow for off-site monitoring and control adjustments. A control strategy, which was implemented using the management program, is discussed. Energy efficiency was achieved through the respective installations and control improvements. The results were analysed over an assessment period of three months to determine the viability of the intervention. A newly installed Bulk Air Cooler (BAC) added to the service delivery of the cooling plant post installation of the VSDs. Focusing on service delivery to underground showed a savings of 1.7 MW (33.6%) daily and a payback period of 3.6 months (0.3 years). The overall implementation showed an average energy saving of 2.3 MW (47.1%) daily, with the result that a daily saving of R 23 988.20 was experienced, reducing the payback period to 2.3 months (0.2 years). Through the installation of energy-efficiency technology and a suitable control philosophy, a cost-effective, energy-efficiency improvement was created on the case-study cooling plant.
- Engineering