14.3 Energy recovery devices and process optimization

Energy recovery devices are game-changers in desalination. They slash energy use by up to 60% by reusing pressure from brine. Pressure exchangers and turbines are the MVPs, with pressure exchangers winning the efficiency crown at 98% energy recovery.

Process optimization is all about squeezing every drop of efficiency from desalination. It's a balancing act of tweaking operating conditions, using energy-smart tech, and fine-tuning everything from pumps to control systems. The goal? Minimize energy use per cubic meter of clean water.

Energy Recovery Devices

Pressure Exchangers and Turbines

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• Pressure exchangers transfer pressure from the high-pressure brine stream to the incoming feed water, reducing the energy required for the high-pressure pump
• Work on the principle of positive displacement, using a rotor with multiple ducts that allow direct pressure transfer between two fluids (brine and feed water)
• Turbines convert the potential energy of the high-pressure brine stream into mechanical energy, which can be used to drive the high-pressure pump or generate electricity
• Include Pelton wheel turbines and hydraulic turbochargers (centrifugal turbines coupled with a pump)
• Both pressure exchangers and turbines significantly reduce the specific energy consumption of the desalination process (by up to 60%)

Energy Recovery Ratio

• Energy recovery ratio quantifies the effectiveness of an energy recovery device in recovering energy from the high-pressure brine stream
• Defined as the ratio of the energy recovered by the device to the total energy available in the brine stream
• Higher energy recovery ratios indicate more efficient energy recovery devices and lower specific energy consumption for the desalination process
• Modern pressure exchangers can achieve energy recovery ratios of up to 98%, while turbines typically have lower ratios (around 80-90%)
• Selecting the appropriate energy recovery device based on the energy recovery ratio and other factors (such as capital cost, maintenance requirements, and compatibility with the desalination process) is crucial for optimizing the overall efficiency of the desalination plant

Process Optimization

Energy Efficiency and Specific Energy Consumption

• Energy efficiency is a key factor in the economic viability and environmental sustainability of desalination processes
• Specific energy consumption (SEC) is a measure of the energy required to produce a unit volume of desalinated water (usually expressed in kWh/m³)
• Lower SEC values indicate more energy-efficient processes and reduced operating costs
• Factors affecting SEC include feed water salinity, temperature, recovery ratio, membrane performance, and the efficiency of pumps and energy recovery devices
• Process optimization aims to minimize SEC by selecting appropriate operating conditions, implementing energy-efficient technologies, and optimizing the integration of various process components (pretreatment, membrane modules, post-treatment, and energy recovery)

Pump Efficiency and Variable Frequency Drives

• High-pressure pumps are significant energy consumers in desalination processes, and their efficiency greatly impacts the overall energy efficiency of the plant
• Pump efficiency depends on factors such as pump design, operating conditions (flow rate and pressure), and maintenance practices
• Variable frequency drives (VFDs) can improve pump efficiency by adjusting the pump speed to match the required flow rate and pressure, reducing energy consumption during periods of lower demand
• VFDs also enable soft starting and stopping of pumps, minimizing mechanical stress and extending equipment lifespan
• Proper sizing and selection of pumps, regular maintenance, and the use of high-efficiency motors and VFDs can significantly improve pump efficiency and reduce energy consumption

Process Control Optimization

• Process control optimization involves the implementation of advanced control strategies and automation to maintain optimal operating conditions and minimize energy consumption
• Includes techniques such as feedback control, feed-forward control, and model-based control to maintain key process parameters (pressure, flow rate, temperature, and water quality) within desired ranges
• Real-time monitoring and data analytics can help identify inefficiencies, detect equipment failures, and optimize process performance
• Automated control systems can quickly respond to changes in feed water quality, demand, and other external factors, ensuring consistent product water quality and minimizing energy waste
• Integration of process control with energy management systems can enable real-time optimization of energy use based on factors such as electricity prices, renewable energy availability, and plant load requirements