Wind Energy - Water Desalination

Water desalination: an overview

Of all the water present on Earth, 96.5% consists of seawater. The remaining 3.5% is freshwater, but half of that is tied up in ice and consequently not usable. All in all, significantly less than 1% of the world’s water resources are exploitable as drinking water.

Drinking water is characterised by a high degree of purity, which among other things means a low salt content. There are often directives and regulations governing the permissible salt content in drinking water (such as the Drinking Water Ordinance in Germany). According to the limit defined by the World Health Organisation1, a salt content of up to 0.5 g/l is harmless to human health. Oc-casionally, though, the opinion is also heard that water with a salt content of up to 1 g/l is drinkable. The distinction between freshwater, brackish water and seawater is equally inconsistent. In Ger-man usage, the term brackish water usually relates to water with a salt content of 1 – 10 g/l, while in the Anglo-Saxon world water is still referred to as brackish if the salt content is as much as 18 g/l. Seawater has an average salt content of 35 g/l, although salinity can vary between 2 and 45 g/l depending on the locality. Whatever the case, obtaining drinking water or service water from salty seawater or brackish water calls for removal of salt (and other minerals) from the untreated raw water. This is done by separating the raw water into product water, with a low salt content, and brine, with a high salt content.

There are numerous desalination methods, some of which have been in use in large plants for over 100 years. The common, tried-and-tested desalination technologies can be divided into thermal methods (distillation or vaporisation processes) and membrane methods (see Table 1). In addition there are simple (solar) evaporation systems and complex techniques, not yet used commercially, such as freezing (water separation) and ion exchange (salt separation).

Table 1: Thermal and membrane methods of water desalination

Thermal methods	Market share (2005)	Membrane methods	Market share (2005)
Multi-stage flash distillation (MSF)	36%	Reverse osmosis (RO)Reverse osmosis (RO)	46%
Multi-effect distillation (MED)	3%	Electrodialysis (ED)	5%
Vapour compression (VC)	5%

In early 2005 there were more than 10,000 desalination plants with a daily output of at least 100 m³ in service world-wide. The plants are situated in coastal locations where there is a shortage of drinking water, or in semiarid/arid areas where only brackish water is available. The principal consumers are towns, cities and local communities (63%). Desalination plants are also occasionally used on ships. However, the total installed desalination capacity of about 36 million m³/day met only just under 0.3% of global demand for fresh water. The majority of installed desalination plants (53%) are used for seawater desalination; 24% operate with brackish water, 9% treat surface water, and 6% are designed for treating wastewater (Pacific Institute, 2006). Roughly half the installed desalination capacity is located in the Middle East, the Persian Gulf and North Africa (Figure 1); thermal methods are preferred there, mostly in conjunction with combined heat and power plants (cogeneration stations). In countries such as Saudi Arabia, Kuwait and the United Arab Emirates, the main source of water is seawater. In the USA, which accounts for 17% of global desalination capacity, mainly brackish water and surface water is treated (70% on the basis of reverse osmosis).

The most common desalination methods - Thermal methods

The underlying principle behind thermal methods is that the raw water is brought to boiling temperature and the condensed steam is separated from the brine. As the boiling point depends on pressure (water evaporates at 100°C under atmospheric pressure), the principle can be applied in several process stages, gradually reducing both pressure and boiling temperature. The lower the pressure, the less thermal energy is needed for evaporation. All thermal desalination methods make use of the thermodynamic principle that the evaporation process can be optimised by reducing pressure and boiling temperature (cf. Wangnick 2001, Pacific Institute 2006).

Mechanical vapour compression (MVC): Of the various thermal methods, above all vapour compression enters into consideration for wind-powered desalination. As with all the thermal methods, in vapour compression evaporation takes place by reducing the boiling temperature as a result of reducing pressure. However, the process heat required for evaporation is normally generated by a mechanical – electrically operated – compressor and not provided by a steam generator. In addition to mechanical vapour compression plants there are also types known as thermo-compression plants which operate with process steam and use several evaporation stages.

Mechanical vapour compression is the only distillation method operated solely with electrical energy. Usually the mechanical plants consist of a single stage, and reach daily capacities of up to 3,000 m³. They are considered to be reliable and have low maintenance requirements (no biofouling or scaling), are simple to operate, and require only little additional outlay on pretreatment and post-treatment of the raw water and product water. Their specific power demand (compressor, feed pump) is in the region of 7 - 12 kWh/m³.

Membrane methods

This method came onto the market at the end of the 1960s and is based on the idea of reversing the natural tendency to balance the level of concentration between two solutions (osmosis). Reverse osmosis counteracts the osmotic pressure by forcing the salty raw water through membranes in the direction opposite to its “natural” direction of diffusion, leaving the dissolved salt behind. The greater the difference in concentration between the salt solutions in the raw water and the required product water (i.e. the greater the osmotic pressure), the more pressure has to be exerted by reverse osmosis. The amount of electrical energy required for reverse osmosis (usually with high-pressure pumps) therefore rises in direct proportion to the salt water content of the raw water [for a given level of product water quality and product water yield] and to product water yield [for a given salt content in the raw water]. In seawater desalination plants with pressure recovery, specific power consumption (high-pressure pump, feed pump, flushing) ranges between 2.5 kWh/m³ and 7.5 kWh/m³; highly efficient plants are said to consume as little as only 2 kWh/m³. Thanks to modular construction methods, plants vary in size from 100 m³/day to 400,000 m³/day, with specific investment costs of 500 – 1,300 USD/m³/day. There are also micro desalination plants for domestic purposes or mobile use ( 2 litres/minute). Membrane methods such as reverse osmosis are used not only for water desalination, but also for treating wastewater. Depending on the available raw water, the product water yield and the required product water quality, smooth operation of the plants requires more or less elaborate pretreatment of the raw water (filtration and addition of chemicals), regular flushing (in some cases with automatic flushing programmes), frequent replacement of the membranes, avoidance of pressure fluctuations at the membranes (alternating operation), and where applicable post-treatment of the product water (filtration, disinfection).

Important characteristics of the various desalination methods are summarised in Table 2.

Table 2: Characteristics of water desalination methods

	MSF	MED	MVC	RO	ED
Primary energy source	Steam	Steam	Mechanical/ electrical	electrical	electrical
Max. salt concentration of raw water (g/l)	100	100	100	43	3
Product water yield	=<50%	=<50%	30-50%	=<50% (brackish water =<85%)	Brackish water: =< 97%
Product water quality (mg/l)	<10	<10	<10	<500	<500