Winds of Change for Reverse Osmosis Seawater Desalination?
Joachim Kaufler, Robert Pohl, and Hadi Sader
Water World
12/07/2011
Energy consumption of industrial scale seawater desalination has decreased significantly over the past few decades. Advantages in membrane technology and energy recovery devices have led to a relatively low specific energy consumption (SEC) of reverse osmosis (RO) processes to around 2 – 4 kWh/m³.
Lower energy consumption per unit, lower investment costs, a simple process control and a very flexible plant design have all helped RO to gain a significant market share within the field of small and large-scale desalination plants (< 100,000 m³ daily).
However, due to the ever expanding desalination market, total electric energy consumption of desalination, especially RO, will increase. With nearly all electric energy for commercial seawater reverse osmosis (SWRO) generated by fossil fuels, this presents challenges and concerns. This includes costs (energy can represent 30% to 60% of total water production (RO) costs), through to reliability. For example, if fossil fuel energy sources are supplied internationally, then reliability of supply could be affected negatively by political conflicts.
These concerns can be avoided or minimised by using locally available renewable energies, such as wind or solar power. And feasibility studies carried out could see wind powered RO desalination proven in Bahrain, in the Middle East.
In February 2011 a Memorandum of Understanding was signed between SYNLIFT Systems and Jade Consultancy to help develop the concept of wind powered seawater desalination. Extensive laboratory studies simulating Gulf region conditions suggest the concept is both technically and commercially available. The basis of which will be presented in the following article.
Why wind power?
As a freely accessible source, wind energy offers low and long-term stable power generation costs, as demonstrated in Table 1 below. Considering the steadily increasing grid tariffs, the direct consumption of wind power within a local sub-grid constellation is becoming the most beneficial option for an increasing number of sites.
Principles of seawater desalination
At an industrial scale, thermal as well as membrane processes are used for seawater desalination. The thermal desalination processes are characterised by the phase change of the seawater in which the product is separated by condensation of the vapour. Well-known applications of thermal desalination at industrial scale are the Multistage Flash Evaporation (MSF), the Multi Effect Distillation (MED) and the Mechanical or Thermal Vapour Compression (M/T VC).
Membrane processes operate without phase change. By regulation of the chemical potential across a semi-permeable membrane a partial flow of seawater is separated as permeate. Within industrial seawater desalination, RO is increasingly important. For desalination of brackish water with lower salinities the Electro Dialysis is as well applied.
Compared to other desalination principles, certain characteristics make RO more suitable for a wind powered desalination process compared to other desalination principles [1-4]. These include a high process dynamic – the ability to adjust the power of the desalination plant quickly in the event of fluctuating wind power.
Principle of RO-process for seawater desalination
RO is applied as cross-flow filtration and pressurised seawater with feed flow flowing along the semi-permeable membrane. If feed pressure exceeds the osmotic pressure of the seawater a fraction of water diffuses through the membrane and is collected as permeate flow.
The retentate flow is depressurised by the downstream energy recovery device. Recovered energy is transferred as hydrostatic pressure directly to an equal fraction of the feed flow or indirectly by boosting the high-pressure pump flow.
The power of the high-pressure pump could be adjusted continuously within a broad range of feed pressure and flow in order to adjust the permeate production to varying water demand and feed conditions or – relevant for WIP – to the fluctuating wind power generation.
