Optimising deep-level mine refrigeration control for sustainable cost savings
Peach, Pieter Frederik Hames
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Declined productivity and increased operational costs have seen South African deep-level gold mines become marginal operations during the past few years. The price of electricity is one of the major contributors to an increase in the operational costs. This is mainly due to gold mining depending heavily on electricity for its operations and the price of electricity increasing at higher-than-inflation rates. The past few years also saw South Africa’s power utility, Eskom, struggling to keep up with its economy and to adequately supply the electricity demand. Ageing infrastructure and mismanagement led to power outages being experienced throughout the country. Since gold mining is such an electricity-intensive operation, it is very sensitive to Eskom’s ability to meet the demand. During peak periods, Eskom regularly operates without adequate reserve margins, subsequently reverting to load curtailment of large users in order to ensure grid stability. This directly influences the productivity of industries such as gold mines. The problems experienced by Eskom led to the establishment of Demand Side Management (DSM). Energy Services Companies (ESCos) are employed by Eskom to implement DSM initiatives on large electricity consumers such as gold mines. However, research proves that implemented load management initiatives (part of DSM) were not sustainable in the past. This is mainly due to the structure of the old DSM model, which only required ESCos to maintain targeted savings for a period of three months after implementation. The DSM model was revised in recent years to ensure that the performances of implemented load management initiatives are more sustainable. The revised DSM model, however, brought new challenges for ESCos as they are forced to sustain the performances of implemented projects for a period of 36 months. Project funding for infrastructure and implementation time were also drastically reduced with the new model. This presented the need for ESCos to develop creative and sustainable load management techniques that will be cost effective and easy to implement. One of these techniques is developing optimised control strategies for the electricity consuming systems of deep-level mines. Due to their large electricity consumption patterns, two deep-level gold mine refrigeration systems were identified for optimised control. The development commenced by determining the theoretical impact of the developed optimised control strategies through verified simulation models. The simulation results were then validated in the form of practical tests on the respective refrigeration systems. Validation proved the accuracy of the optimised strategies with a correlation error of 2% for Mine D and 3% for Mine R, measured against the average summer evening peak period load reduction. The optimised control strategies were implemented on the refrigeration systems of Mine D and Mine R. By the time of this study, load management proved to be sustainable for a consecutive period of eight months at Mine D and nine months at Mine R. An average evening peak period load reduction of 7.28 MW (Mine D) and 2.00 MW (Mine R) was measured at the respective mines. This accumulates to financial cost savings of R 1.17 million (Mine D) and R 143 000 (Mine R) over the respective measuring periods. Assuming sustainability over a period of 36 months, a total financial saving of approximately R 6 million is expected. The study proved that sustainable cost savings on deep-level gold mine refrigeration systems are possible through the implementation of optimised control strategies. It is, however, important to note that the sustainability is not solely measured on cost savings, but also determined by the effect of load management on system operational parameters such as temperatures, dam levels and safety regulations.
- Engineering