Dr. Chuyang Y. Tang, Associate Professor of Civil Engineering, shared his recent development on harvesting energy by the dilution of waste water during desalination of sea water.
Desalination is making its way back to Hong Kong. The first desalination plant in Hong Kong was built in the 1970s during which water shortage prevailed. The thermal-based plant was known to be too costly to run, causing it to be decommissioned. A new desalination plant is now planned. Upon its completion in 2020, the initial phase of the plant can provide about 5% of the total water supply in Hong Kong.
The new desalination plant adopts the state of the art reverse osmosis (RO) technology, where a high pressure is applied to "squeeze" water through a dense membrane. Despite its much higher energy efficiency compared to thermal distillation (see Table 1), RO desalination remains relatively energy intensive due to its requirement of high pressure pumping – 3-4 kWh of electricity is typically required to produce 1 m3 of clean water. Furthermore, desalination produces an undesirable byproduct known as brine that contains the salts removed from the seawater. Discharge of the highly saline brine (containing ~ 70 g salts/L) in an uncontrolled manner can cause severe ecological damage.
| |
Principle |
Electricity requirement
(kWh/m3) |
Thermal energy requirement
(kWh/m3) |
Current installation |
| Thermal distillation |
Evaporation of water at high temperature |
1.5 – 5.5 |
25-120 |
Mainly in the Middle East |
| Reverse osmosis |
High pressure pumping of water through a salt-rejecting membrane |
3-4 |
Nil |
The majority of new desalination plants (U.S.A., Australia, Singapore, and some Middle East countries) |
Table 1: Comparison of thermal distillation and reverse osmosis technologies
A research team at the University of Hong Kong has been studying the beneficial use of brine for electricity generation. The team, led by Associate Professor C.Y. Tang from the Department of Civil Engineering, draws experts from desalination, chemistry, and power generation. The core technology is a process known as reverse electrodialysis or simply RED. By coupling the saline brine to a low-salinity solution (e.g., treated wastewater), the RED process is able to extract the salinity gradient power (also known as osmotic power) by converting the chemical potential energy arising from the salinity difference into electricity. A typical RED stack (Figure 1) consists of several cation and anion exchange membranes in an alternative sequence. Under the concentration difference between the brine and dilute solution, positively charged ions (e.g., sodium) transport through cation exchange membranes, while negatively charged (e.g., chloride) through anion exchange membranes. The ionic current will then be converted to electricity at the electrodes. According to the team’s estimation, the hybridization of RO-RED technology can reduce energy consumption of desalination by approximately 30%. Furthermore, the RED treated brine has reduced salinity for its subsequent environmentally-friendly disposal. The team, supported by the Strategic Research Theme in Clean Energy of the University, is looking for ways of improving the power density and conversion efficiency of the process.
Figure 1: Schematics of a reverse electrodialysis (RED) setup. The RED stack consists of several cation exchange membranes (golden color) and anion exchange membranes (silver color) in an alternative manner. An ionic current is formed by the directional movement of cations (towards the right) and anions (towards the left) under their respective concentration gradient. The ionic current is converted to electric current at the electrodes (grey color).
Dr. Tang has more than a dozen years' experience in the area of desalination and wastewater reclamation. He has been recently named as one of the "Desalters to Watch" by the Water Desalination Report (Vol. 51(6), 2015) for his work on membrane based desalination and salinity power harvesting. He is also serving as a guest editor for Energies for a special issue on Osmotic Power that covers a wide range of technologies for salinity gradient power harvesting. Dr. Tang is a recipient of the Finland Distinguished Professor Program Fellow, the International Desalination Association Fellow, and the Singapore Minister for National Development (MND) R&D Merit Award for his innovative work on membrane technology and desalination.
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