The steadily rising world population in connection with changing
climatic conditions is not only leading to nutrition problems, but also above
all, to those regarding the potable water supply. You can study it in the
Middle-East region dramatically. Between 1940 and 2000, the world population
has more than doubled. Simultaneously water consumption per capita doubled over
the same period. Following this development total water consumption has more
than quadrupled, making potable water an increasingly scarce commodity in many
parts of the world also in colder latitudes despite ample total reserves.
Apart from an economical use of already existing resource, increasing
importance is being attached to the development and expansion of treatment
facilities for potable water from sea and brackish water. In particular,
decentralized units offer good solutions because they can be geared to local
water needs and to existing sources of energy.
Over the past 30 years, the production of potable water by seawater
plants has increased from zero to almost 20,000,000 m³/day. In many countries
they contribute significantly to the supply of potable water.
Of the various processes available today, vapour compression has emerged
to be most sophisticated for capacities of up to
2,000 m³/day. This simple but effective method ensures max. reliability with an
almost maintenance-free operation. Both the reliability and economic efficiency
of such plants have been demonstrated in numerous installations worldwide.
Growing interest in decentralized systems has also prompted efforts to
design new plants that operate off the public electric grid and do not,
necessarily, require any infrastructure for the supply of fossil sources of
energy and also chemicals. Especially remote areas need to run their own
desalination plants because water transport is expensive. Through poor
infrastructure in these areas a desalination plant that is dependent on the
existence of an electric grid in many cases is no solution to the water
problem. The use of wind energy allows the grid-independent operation of a new
desalination system and besides, having considerable ecologic advantages, is a
very economic solution.
The stand-alone windpowered mechanical vapour compression (MVC) seawater
desalination would be of special interest to the tourism industry as it is
ideally suited for remote areas and islands in particular. Large numbers of
tourists lead to an substantial increase in water consumption per capita,
mostly in areas in which water is already scarce.
The idea to use wind power as an alone or main energy source for
desalination is not new. Wind conditions for example in coastal areas are often
in favour of this kind of desalination system. The new generation of small and
medium sized wind turbines that has been developed in the past years offers a
high amount of reliability in service combined with low investments costs.
The variable nature of wind power is not a problem, because water can be
stored inexpensively even for long periods of time without deterioration. With
a plant that is dimensioned according to the local wind conditions water
becomes available any time.
For the operation of a windpowered desalination plant it is most
important to have a plant that has to be insensitive against repeated start up
and shut down cycles caused by sometimes rapidly changing wind conditions.
MVC plants are widely used all around the world and have proven to be
efficient and economic for many applications. Both, wind turbine and high
pressure blower of the MVC-plant, are fluid flow machines with similar
characteristics. There is therefore a natural affinity of both machines. By
variation of the compressor speed and the evaporation temperature the power
consumption can be adapted also to rapid changes in wind conditions.
A combination of a wind turbine and a MVC plant is therefore able to
utilize wind energy to a very high degree at all times and conditions (Fig. 1).
The plant described in the following is therefore a MVC-plant with a vertical
tube evaporator-condensor-unit.
I. DESCRIPTION OF THE DESALINATION PLANT
The filtered seawater is pumped into an intermediate tank of about 4 m³.
From here the water is pumped into the desalination plant (Fig. 2).
For preheating incoming seawater flows in two parallel lines through
plate heat exchangers. In these exchangers heat is transferred from the
outgoing distillate and brine to the incoming seawater.
The pressure in the following degasifyer is reduced to almost the level of
the evaporation pressure of the preheated seawater using a vacuum pump. The
gaseous components (air and CO2) dissolved in the seawater escape
and are removed.
Figure. 1. MVC-plant with a
vertical tube evaporator-condensor-unit.
The more water evaporates the higher is the salt concentration in the
remaining seawater. Usually the salt concentration should not become higher
than 7 - 9 %, therefore continuously concentrated seawater is led back into the
sea and fresh seawater is supplied.
II. DESIGN OF THE MAIN COMPONENT - THE
EVAPORATOR-CONDENSOR UNIT (ECU)
The evaporator-condensor unit (ecu) is the heart of the plant.
It consists of a bundle of tubes placed between the upper and the lower
tube sheets. The tube sheets divide the ecu in 2 spaces: the seawater contacted
room and the distillat contacted.
The distillate contacted walls consist of conventional stainless steel
1.4571, but the seawater contacted walls of steel of the quality 1.4565 S.
