Integrated approach to reconfigure and optimise the control of a pump in reverse operation

Abstract

Energy is a major expense for industrial consumers such as direct customers of Eskom (South Africa's main energy supplier). The energy tariff has increased drastically over the last few decades despite South Africa being a coal-rich nation. A series of energy reduction methods have also been implemented, namely 'load shedding' for private consumers and 'load-curtailment' for large industries. Furthermore, Eskom has had a resistance towards renewable energy and self-generation. Recently, the government has gradually reduced the restriction of self-generation and renewable energy sources in the private sector. Renewable energy is a growing commodity and is underexploited, especially in South Africa. An underexplored, yet powerful and inexpensive renewable energy source is hydropower, especially on large and integrated systems such as deep-level gold mines. Currently, some forms of hydropower systems exist in South African deep-level gold mines, but these systems are often implemented incorrectly, resulting in inefficient systems plagued with breakdowns. Various forms of hydropower have been implemented in the attempt to reduce energy costs. A reliable method is needed for the consistent and successful implementation of these systems, given the various challenges present in the deep-level mining industry. This study focused on developing a solution method (in a corporate mining environment) to implement a pump as turbine (PAT) with the aid of a thermal-hydraulic simulation suite on an integrated system, following a preset group of key performance indicators (KPIs). A PAT is also known as a pump in reverse operation. With the thermal-hydraulic simulation suite, thebest efficiency point (BEP) of the PAT can be established and the PAT can be controlled accordingly whilst predicting its performance. In this study, a PAT was identified and reclaimed from a sister mine (Mine Y) and was then implemented on Mine X with the aid of the various integrated models. Following the developed solution method, which proved successful within the pre-feasibility and feasibility studies, various simulation models were simulated by means of computational fluid dynamics (CFD) analysis. The feasibility studies yielded favourable results, where approximately 24 234 kWh was possibly recovered whilst maintaining KPI standards. This equates to approximately R10.5 million p.a. based on the non-local authority 2022/2023 Mega Flex tariff, resulting in a 6.2-month payback period (based on a reclamation capital expenditure (CAPEX) scenario). After implementation, the PAT's actual performance was analysed and compared with the simulation results. A significantly low simulation error was observed, namely a 4.54% difference between the actual and simulation-generated energy results. This low difference validates the study as well as the developed solution method, proving its effectiveness. During the actual implementation of the project (after completing the feasibility studies), an additional tariff increase was announced to take effect as of the 1st of April 2023 and therefore should be considered for generation capacity in terms of currency. Approximately 18.65% increase was implemented as from the 1st of April 2023 and adjusted the generated capacity to R 12.45 mil. p.a. Capital expenditure of R 5.4 million is required to realise the project, which will incur a payback of approximately 5.2 months based on the latest tariff increase