Real Time Monitoring and Control of Scale Formation in the Geothermal Energy Generation Systems

Abstract

Exploration of Geothermal power has gained momentum in the recent past as it has proved dependable, has reduced green gas emissions, met diversification needs, provides least cost base load mode of power 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 associated with Non Condensable Gases (NCGs) carryover which enhances scaling. Reduced production due to scaling is demonstrated. For example, it was established from Olkaria production logs that from Nov 2018-Feb 2019, when the unit was experiencing scaling, KenGen lost a total of 7,825,685KWh translating to US$ 688,660.28, considering electricity sale price as 0.008 US$/ Kwh due to scaling. The recent used strategies like use of Organic Rankine Cycle (ORC) bottoming plant to the existing facility for initially designed geothermal systems, use of combined cycle plants and pH mod has led to reduced scaling and additional production. ORC involves use of heat exchanger and a secondary working fluid to drive turbines. pH mod involves online monitoring and control of silica in the reinjection pipelines using a chemical process treatment equipment. Combined cycle technology involves use of a steam turbine and ORC technology in one plant. ORC is not typically cost effective especially for modular wellhead plant while pH mod considers only scaling in reinjection pipelines. This means that scaling challenge 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 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 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 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 B O O K O F A B S T R A C T S , 2 0 2 1

4th Annual 1nternational Conference Innovative Technology for a Dynamic World 61

verified by pH readings of the two systems. The pH Data of the of the two systems indicated a standard deviation of 8.25578E-4 before acidic condition correction by the metering pump and a standard deviation of 0.01325 upon correction. The time lag 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 and achieve scale monitoring and real time control through managing the always changing brine pH in geothermal fields.