Utilising mine-cooling auxiliaries for optimal performance during seasonal changes

Abstract

Electricity is a major concern for energy-intensive consumers. The price of electricity in South Africa has increased by 300% over the last ten years, and the future price increase is forecasted to surpass inflation. This is unfavourable for industries such as the mining industry that consume, on average, 15% of all the electricity generated in South Africa. Due to the growing demand of electricity in South Africa, new ways of reducing the demand of energy-intensive industries such as the mining industry should be explored. The gold mining industry is the largest electricity consumer of all mining industries in South Africa. The industry is under pressure as the increasing electricity tariff, the increasing consumer price index, and the constant need to expand mines to reach deeper deposits contribute to increasing operational costs and decreasing profit margins. The mining industry is thus forced to improve their efficiency and drive any possible energy-saving initiatives by adopting the latest technologies and processes to ensure sustainable operations while maintaining cost-efficiency.. Mine-cooling systems are the largest consumers of energy in the gold mining industry, consuming 22% of a mine’s total electricity demand. As mines become deeper, the cooling requirements also increase, increasing the energy consumption required for operation. With the increased tariffs, these cooling systems need to be as efficient as possible, especially in wintertime when electricity is three times more expensive than in summer. Scope to optimise mine-cooling auxiliaries in terms of cost saving and sufficient cooling during seasonal changes was identified, and a control strategy to utilise mine-cooling auxiliaries for optimal performance during seasonal changes was developed. The strategy consists out of utilising the bulk air coolers (BACs) and low wet bulb (WB) conditions during wintertime. The BAC acts as a precooling tower that cools down the chilled water with the lower WB temperature. The strategy was simulated with simulation software and verified upon implementation, and was developed for combined service delivery and energy efficiency improvement. The strategy was implemented on Mine A, yielding a 4.3°C reduction in precool dam temperatures and an average load reduction of 0.8 MW on the fridge plants. The strategy did not, however, reduce chill dam temperatures, as the fridge plants were already achieving their setpoints prior to implementation. Though the strategy did not improve service delivery, it maintained service delivery with a lower electricity consumption.

In conclusion, the control strategy can be implemented on all mines with suitable fridge plant BAC configurations and appropriate ambient winter conditions. The strategy achieved savings of R40 000 per day; however, these savings could only be achieved on days where the WB temperature was lower than the chilled water temperature for the entire day. The strategy was, therefore, optimised through simulation to achieve savings on days with a higher wet-bulb temperature to ensure that savings are achieved more often throughout the year. The optimised control strategy resulted in lower daily savings but higher annual savings overall due to its ability to be frequently implemented. It was also found that the optimised control strategy was not correctly implemented by control room operators; therefore, two control strategies were also developed to ensure sufficient savings in the future, namely manual implementation or automatic implementation.