Real Time Monitoring and Control of Scale Formation in the Geothermal Energy Generation Systems: A case study of Olkaria II, Kenya

Abstract

Exploration of Geothermal power has gained momentum in the recent past as it has proved to be dependable, has reduced green gas emissions, meet diversification needs, provides least cost base load mode of generation and is inexhaustible for billions of years to come. However, the development and exploitation of geothermal resource face a notable challenge of scale formation on the steam lines and most surface equipment, leading to reduced and expensive production. Scaling occur mainly due to deposition of solids carried by steam. One of the important scale solubility factor is the pH of the brine. Low pH levels are for example associated with Non Condensable Gases (NCGs) carryover which enhances scaling. Currently, use of Organic Rankine Cycle (ORC) and a secondary working fluid to drive turbines for initially differently designed geothermal systems, use of combined cycle plants or use of pH mod has led to reduced scaling and additional production. Use of ORC and combined cycle plant is not cost effective especially for modular wellhead plant while pH mod considers only scaling in reinjection pipelines. Scaling challenge therefore is still at large especially for initially installed single flush geothermal stations like Olkaria II leading to losses of large amount of energy. This study explored the design and implementation of a virtual, real time scale level monitoring and control for geothermal energy generation system based on a physical T5554 analytic process control system. A virtual system was designed using Siemens NX and programmed using Totally Integrated Automation (TIA) software, PORTAL V14 via Programmable Logic Controller -1200 PLC, CPU 1214C DC/DC/DC 6ES7 214-1AG40- OXBO hardware. A set point pH of 6.5, an appropriate point to prevent silica scaling, as guided by the practice in Olkaria II was used with the PID of the PLC to control the pH of the analytic process control system. The virtual and the physical model were then linked to achieve communication through a channel called Open Platform Communication (OPC) via KEP server, which is an interoperability standard for secure and reliable exchange of data in industrial automation. Siemens NX design was configured to communicate with KEP server via External Signal Configuration feature. This facilitated the merger of control signals between Siemens NX design and TIA design. The TIA portal used links that specified the sensor and actuator control signals. The system was verified by taking pH readings of the two systems concurrently. When the pH Data of of the two systems were compared, they indicated a standard deviation of 8.25578E-4 in an acidic condition and a standard deviation of 0.01325 upon acidic condition correction by a metering pump of the physical T5554 analytic process control system, to a value around the set point. The deviation was so small that it did not affect the working of the system. This confirmed that the digital model could be used to accurately represent the physical system to achieve scale monitoring and real time control through managing the always changing brine pH in geothermal fields.