The following are the known options for desalinating seawater by distillation. The method described under 4.9 offers the best conditions for the intended application. However, it can certainly be improved by the new hydrodynamic conditions to allow effective desalination further north and south of the subtropical latitudes.
As very large quantities of salt water are processed, it is worth building a terminal customised for this purpose, which takes in water far from the coast and uses screens to prevent marine animals and plants from being sucked in.
As only extremely pure seawater is used, only pure salt should be produced as a by-product during distillation, thus ensuring a process that fulfils the strictest ecological requirements. If other ingredients are produced after distillation, these can be easily separated, as it is easy to completely dry the precipitates. Even before the corresponding container is returned to its original location. The resulting salt can be utilised commercially, e.g. as pure table salt or for energy storage. This eliminates the need to operate salt extraction plants in other locations and could be made available to their operators free of charge.
Table of contents
1 Physics of evaporation
2 Physics of evaporation
3 Simple solar evaporation systems
3.1 Greenhouse principle
3.2 Collector and solar still
3.3 Cascade still
3.4 Watercone
3.5 Methods for emergencies
4 Complex solar evaporation systems
4.1 Humid air countercurrent still
4.2 Collector system with heat recovery
4.3 Distillation cyclone
4.4 MEH process – thermal desalination with low-temperature heat, e.g. from solar collectors
4.5 Multi-effect distillation
4.6 Aquadestil
4.7 Process with direct condensate heat recovery
4.8 Process with indirect condensate heat recovery
4.9 Multi-stage seawater desalination (FH Aachen)
5 Solar evaporation systems
5.1 Multiple-effect evaporation (MED)
5.2 Multi-stage flash evaporation (MSF)
5.3 Markopulos patent
5.4 Simple, autonomous seawater desalination plants
5.5 Scheffler seawater desalination plant
5.6 Laval nozzle – seawater desalination plant
6 Applications
7 Web links
8 Individual references
Physics of evaporation
Evaporation occurs when the temperature of the solution is below the boiling point and the partial vapour pressure in the surrounding carrier gas (air) is lower than in the liquid. The solvent then evaporates depending on the temperature and the partial pressure difference. The total pressure of the space adjacent to the solution is greater than the partial pressure of the vapour produced. Vapour diffusion distributes the vapour in the carrier gas. At normal pressure, the temperature of water, for example, remains below 100°C. Desalination of seawater by evaporation and subsequent condensation is a process that occurs naturally on Earth. Evaporation and condensation of water in air can take place using solar energy at various temperatures and ambient pressures. Low process temperatures allow the use of non-concentrating solar collectors, keeping heat losses to a minimum. Lower process temperatures allow the use of inexpensive materials with low strength and corrosion resistance requirements. In contrast, the surface area required for evaporation systems is much larger, as the evaporation capacity also depends on the surface area. Only low heat and material flow densities are achieved per unit area. This means that the same evaporation performance as with evaporator systems can only be achieved over a large surface area.
Physics of Evaporation
Evaporation is a thermal separation process in which a liquid, or a solvent of the solution of a non-volatile solid, is converted to a vapour state by changing the temperature and pressure. The solvent (usually water) is partially separated by heating the solution to boiling temperature according to the set pressure. Unlike distillation, the resulting vapour is only saturated vapour of the solvent. The heat content can be reused for multi-stage evaporation or to preheat the solution. Evaporation can take place under a variety of conditions, resulting in a variety of manifestations:
Silent evaporation
Boiling only takes place on the surface of the liquid as a result of free convection with a low heating surface load.
Supercooled vaporisation
Boiling takes place before the pressure-dependent boiling temperature of the liquid is reached at high heating surface loads. In this process, bubbles that form locally on the heating surface vaporise and condense simultaneously.
Bubble vaporisation
Vapour bubbles form on the heating surface at medium heating surface loads. The vapour bubbles form on pores and unevenness in the wall surface, which may contain gases or residual vapour. These are nucleation points for the formation of vapour bubbles.
Simple solar evaporation systems
Greenhouse principle
In a flat, black (PE, PC) basin with an insulating layer as thermal insulation (e.g. sand) and a tent-shaped, transparent cover made of window glass, the seawater or brackish water evaporates by absorbing solar radiation. The water vapour condenses on the inside of the wind-cooled cover. The condensate is drained off via collecting channels for further treatment (blending with salt water). Simple solar stills based on this principle have been used for seawater desalination since the end of the 19th century.[1] In systems near the coast, the seawater (brine) remaining after the evaporation process is pumped back into the sea. The average production capacity of a simple solar seawater desalination plant based on the greenhouse principle is up to 6 litres per square metre per day in summer and approx. 1.2 litres per square metre per day of drinking water in winter. This applies to annual irradiation outputs of 1500 to 2000 kWh/m² (Mediterranean region) and a system efficiency of 40 %. They are therefore very area-intensive if large quantities of water are to be obtained. With a service life of 20 years and an interest rate of 8 %, this results in a drinking water price of approx. 2.9 US$/m³.
Advantages: The construction of the system is simple and can be built and used in a decentralised manner without in-depth specialist knowledge. No electrical supply is required for systems at sea level, as no pumps need to be used. This means that it can be used in regions without infrastructure.
Disadvantages: The output of the system per area is comparatively low, as condensation takes place on the glass surface and the condensation energy cannot be recovered and used to preheat the seawater.
Collector and solar still
The production of distillate increases progressively with the water temperature. The distillery should therefore be coupled with a solar thermal collector and the condensation heat of the water condensing in the collector should be used to heat the brine in the distillery. With good solar irradiation, the distillery coupled with a collector achieved an increase in production of 15 % compared to a simple solar distillery, based on 1 m² of system area (distillery + collector). On the other hand, a flat-plate collector is much more expensive to build than a simple solar still. However, tests have shown that by adding heat to the brine bath (external heat source with a sufficiently high temperature level, possibly waste heat) and the associated higher brine temperatures above 80 °C, production increases of over 50 % can be achieved[1].
Cascade distillation
According to the results of a project by the Solar Institute Jülich[1], cascade distillation is comparatively complex:
„In the cascade still, the salt water basin is arranged in a staircase shape to minimise the distance between the water surface and the inclined cover. Compared to a simple solar still, the cascade still produces around 5 % more distillate. However, the higher construction costs and the more time-consuming cleaning of the cascades cannot justify this small additional yield. An attempt to preheat the brine to be supplied in the space between a double glass cover by recovering the heat of condensation on the cover produced only inadequate results. The heat losses due to reflection and absorption in the cover are higher than the additional energy input of the heat recovery, so that the overall effectiveness of the system is reduced.“
Watercone
Watercones can also be used to recover water from soil moisture
The watercone consists of an absorber bowl and a bulbous cone. The material used is coated polycarbonate. The seawater or brackish water is manually poured into the absorber basin. Solar radiation causes the water to evaporate and condense on the cone. The condensed water runs off the cone into a collecting channel. The water is stored there and can be removed at the end of the process by turning the cone over and opening the cap at the top of the cone. The Watercone can also be used to collect soil moisture and use it for drinking water. In this application, the cone stands directly on the ground. The soil moisture condenses on the surface of the cone, is collected in the collecting channel and can then be used.
Advantages: The simplicity of the Watercone is one of its greatest advantages. Even people with a low level of education can use it independently without any problems. The system is easy to explain using pictograms. There are no costs due to electricity consumption or maintenance requirements. The polycarbonate material used is light, transparent and practically unbreakable; several Watercone devices can be nested for transport and storage. With a price of less than €50 per unit, a service life of at least 3 years and a daily production of up to 1.5 litres, the price of water is less than 3 euro cents per litre (€30/m³) and therefore significantly lower than the price of bottled drinking water.
Disadvantages: The entire costs have to be paid at the beginning of a project. Microcredits or other financing must be available to make the device available to the poorest. Furthermore, the service life of 3-5 years is relatively short. Over time, the polycarbonate material used becomes matt.
Methods for emergencies
The methods of solar seawater desalination for emergencies are based on a larger container that holds the contaminated water, such as a cooking pot or a pit in damp ground. This container is covered with a transparent plastic film that is firmly attached to the edges of the container. A weight that is not too heavy is then placed in the centre of this plastic film so that the plastic film takes on the shape of a cone with an obtuse angle, with its tip pointing downwards. Place a cup under this lowest point of the plastic film to collect the condensation dripping off. It may be necessary to weigh this cup down at the beginning so that it does not float in the sea water, which of course must not be poured too high into the larger container. In principle, this method is a reverse watercone. In addition, the top of this plastic film can also collect rainwater.
Complex solar evaporation systems
The aim of multi-stage solar stills is to utilise the irradiated solar energy several times in order to achieve a maximum distillate yield. Despite some successes, such systems still require major research and development efforts. Various concepts are being pursued[2].
Humid air countercurrent distillation
This is a closed container. No vacuum technology is required, the container should only be airtight. Hot water is evaporated via large cloths in the larger area of the evaporation module. The incoming water has a temperature of 80 °C. On the other side are condensers through which cold seawater flows. Hot and humid air has a lower density than cold and dry air. This is why the hot and humid air rises. On the other hand, it is cooled down because cold seawater flows through this large heat exchanger. The humid air circulates by itself. A fan is not required. Hence the name humid air countercurrent stilling. The system requires a collector area of 37.5 square metres. Heat is temporarily stored during the day and used for further desalination in the evening. However, 24-hour operation is not yet possible. Production is between 488 and 536 litres/day. The plant has a specific energy requirement of 106-114 kWh/m³ of water.
Advantages: It is a very simple principle that offers the possibility of setting up low-maintenance systems. It can therefore be used decentrally. Nevertheless, the condensation energy is recovered and used to heat the seawater. Compared to simple solar stills, the yield can be significantly increased and the space requirement reduced.
Disadvantages: Compared to the simple solar still, more equipment is required. Higher investments are therefore to be expected, but these can be reduced by the higher yield and the smaller collector area required. A water price of 10-25 €/m³ is achievable. Although the planned energy storage system ensures uniform distillate recovery, this represents an additional system component and therefore a heat loss zone.
Collector system with heat recovery
Patent DE 100 47 522 A1 is based on an inclined flat-plate collector. Unlike the Rosendahl collector, however, the distillate does not condense on the glass surface, but on condensers provided for this purpose on the back of the absorber. These condensers are shaded by the absorber and thermally insulated from the evaporation chamber. Primary water flows through them and is preheated as a result. The heated primary water then flows over the black absorber fleece, where it is partially evaporated by the solar radiation. The brine flows into the brine tank and is discharged via an overflow. The temperature difference between the evaporation chamber and the condensation chamber creates a circulation of air masses.
Tests with non-optimised prototypes of the system have resulted in distillate yields of up to 20 l/m²d. Due to the similar operating principle, the advantages and disadvantages are summarised in the following article.
Distillation cyclone
This is a system for producing drinking water from sea, brackish or waste water using solar energy. The design can be realised in many different forms. In a favoured design variant, the system is a columnar transparent system consisting of a glass column and an inner hollow column. Solar mirrors are used to focus the sunlight onto the column. The sunlight passes through the transparent area and falls onto the inner hollow column. This is covered on the outside with a black and hydrophilic absorber fleece and heats up strongly due to the effect of the solar radiation. Primary water heated to 95 to 99 °C is channelled over this absorber fleece and evaporates from the absorber surface. The primary water initially serves as cooling water. The humid air rises and cools down inside the hollow column at the condensers provided for this purpose. There, the excess moisture precipitates and condenses as pure water. The condensate is collected in a container at the bottom and drained off. The primary water serves as the cooling medium in the first cooling circuit, which is preheated and from which some of the condensation heat is recovered. A second cooling circuit, which is fed from an external container, is used for further cooling. The cold, humid air (55 °C) sinks and re-enters the area heated by the sun’s rays at the bottom of the hollow column. There the air warms up and can absorb water vapour again, starting a new cycle. Due to the imbalance between hot air masses in the evaporation chamber and cold air masses in the condensation chamber, an independent air mass circulation builds up in the system. The different areas of the system must therefore be thermally insulated against each other. Despite solar radiation, the evaporation process leads to significant cooling of the primary water. This collects in the brine tank. Stratification of the different concentrations takes place there. The brine tank has an overflow that drains the highest concentrations with the help of a culvert. No deposits should form on the absorber fleece as a result of excessive salt concentrations with an associated drop below the solubility limit. The primary water flow should be set high enough accordingly. The circulation of the cooling and brine water and the supply of primary water are ensured by pumps. These can be powered by a solar module. The glass column should have a diameter of 1.4 metres and a height of 7 metres. These dimensions favour the thermodynamic processes inside the column.
Advantages: The system can be used decentrally. The performance values of a functional type are between 17 and 19 l/m²d. This achieves very good heat recovery, as the energy available via solar radiation would have been sufficient for a third to a maximum of half of the distillate quantities achieved. The yield is therefore higher than previously known systems. This makes it possible to save significant collector area or increase the yield with the same collector area.
Disadvantages: With the proposed design, a diameter of 1.4 metres and a height of 7 metres, this system is no longer easy to transport and handle. A complex control and regulation unit is required, which is vulnerable in the conditions of southern developing countries. Compared to the simple solar still, more equipment is required. Higher investments are therefore to be expected, but these can be reduced by the higher yield and the smaller collector area required. A power supply is required for the necessary pumps and control systems. It must be proven that the desired circulation flow is strong enough. It is therefore critical to ensure that no condensate forms on the glass, even though the path is the shortest, the glass is wind-cooled and there is therefore a high temperature difference towards the glass.
MEH process – thermal desalination with low-temperature heat, e.g. from solar collectors
The multi-effect humidification/dehumidification (MEH) process is another thermal process for decentralised seawater desalination in the small and medium-scale production range up to approx. 50,000 litres per day. Systems using the MEH process are based on the supply of thermal energy from low-temperature sources (e.g. solar collectors). The heat is fed into a closed desalination module in which the natural water cycle with evaporation and condensation is efficiently simulated. Sufficiently large evaporation and condensation surfaces, in relation to the energy turnover, enable the greatest possible recovery of the evaporation heat in the condenser. In this way, production rates of over 25 litres/m² per day can be achieved with a solar-powered system. Waste heat from other processes or diesel generators can also be fed into the process. This process was developed to application maturity at the Bavarian Centre for Applied Energy Research (ZAE Bayern).
Mechanical engineers at the Ruhr University Bochum (RUB) have developed a particularly space-saving, transportable prototype based on this principle. By using air as a heat transport medium, the system can be operated at particularly low temperatures. In the system, heated seawater trickles through an evaporative humidifier, which heats the incoming air and additionally enriches it with water vapour from the seawater. This results in a production rate of around 20 litres per m² of collector surface per day (based on ten hours of sunshine per day)[3][4] Research into this was funded by the EU as part of the Soldes project. [5][6] In a multi-stage system, also funded by the EU as part of the Soldes project, with air collectors and evaporative humidifiers connected alternately in series, only the circulating air, but not the brine, is heated by solar collectors.[7] The air is heated and humidified in stages.
ZAE-Bayern planned and built a solar seawater desalination plant in Oman in 2000. The system consists of an array of 40 m² vacuum flat-plate collectors, an insulated steel tank (3.2 m³) and a thermally operated desalination tower. The daily output is approx. 800 litres. The distillation process works at ambient pressure. Heated seawater is distributed over a large evaporator. A convection roller, which is driven by density and humidity differences, transports moist air to double polypropylene plates arranged in the module. These serve as condensation surfaces and cold seawater flows through them. The condensation of the humid air on the surface of the panels heats the seawater to 75 °C.
Advantages: The geometric arrangement of evaporation and condensation surfaces enables a material and heat flow that can otherwise only be realised with a complex multi-chamber system. This achieves a heat recovery that reduces the thermal energy requirement of the desalination plant to around 100 kWh/m³ of distillate compared to the evaporation enthalpy of 690 kWh/m³ for water. The heat recovery is therefore only slightly below the maintenance and technology-intensive vacuum evaporation plants. The system is therefore ideal for decentralised use in structurally weak areas.
Disadvantages: Compared to simple solar stills, more equipment is required. Higher investments are therefore to be expected, but these can be reduced by the higher yield and the smaller collector area required. However, the condensation heat is only partially recovered. Pumps are also required for water circulation.
The Fraunhofer Institute for Solar Energy Systems has utilised this principle in the „SODESA“ project. This test system has a 56 m² collector field. In the project, collectors were developed in which the hot seawater could flow directly through the absorber. For this reason, it was not allowed to use a copper absorber, as this material corrodes immediately. Collectors were developed in which the absorber was made of glass.
Multi-effect still
The multi-effect still works according to the multi-stage principle, in which the condensation heat is used as an energy source for the next stage. The incident solar radiation heats the absorber plate located under a glass pane. A viscose cloth is applied to the back of the plate, which is filled with salt water. A proportion of the salt water evaporates, condenses on the cooler sheet underneath and transfers the condensation heat to the next stage. Analyses of the test results for a four-stage prototype revealed high heat recovery factors in the individual stages (approx. 70 %). However, the maximum distillate yields achieved are only around 50 % higher than the results of the simple solar still. This is due to the high heat losses in the first absorber stage, in which only approx. 20 % of the incident radiation is converted into useful energy. Improvements to the system through a double glass cover or transparent thermal insulation and/or the use of a selectively coated absorber can therefore be expected to increase yields further. Humidification of the system with the aid of viscose cloths has proved to be very effective. No dripping of the brine or mixing of the salt water with the distillate was observed. Salinity tests revealed very good distillate quality. However, the high cost of operating and maintaining the system must be taken into account.
Aquadestil
The seawater or brackish water (15-25 °C) flows through the condensers from bottom to top and heats up in the process. At the top end, the cooling water flows out of the condensers onto the evaporation surface (perforated dimpled surfaces). Above the evaporation surface is a radiator through which solar-heated thermal oil flows. The collectors required for this are located outside the system. The water flows over the surface, heats up and evaporates in the process. The moist air rises and distillate condenses on the condensers. The condensate flows off, is collected in a collecting channel and discharged from the system. The excess brine flows back into the sea. According to the manufacturer, a 1.5 kW solar collector produces a distillate output of 12-18 litres per hour. This puts the distillate price at €3.9-5.7/m³.
A further development uses the resulting vapour for preheating. Vaporisation then takes place in step evaporators.
Advantages: The system has a simple and compact design and is therefore suitable for decentralised use. No control is required for this system. To produce large quantities of drinking water, the modules can be stacked to reduce the space required.
Disadvantages: Compared to the simple solar still, more equipment is required. Higher investment costs are therefore to be expected, but these can be reduced by the higher yield and the smaller collector area required. However, the condensation heat is only partially recovered. Pumps are also required to circulate the water.
Process with direct condensate heat recovery
In this case, evaporation and condensation take place in several stages. Air circulates in the chambers of the individual stages due to natural convection. There is no air exchange between the individual stages. The process is suitable for very small systems as no fan is required. A pump can also be dispensed with if the raw water is taken from a higher tank and a thermosyphon collector is used. For a system with a collector area of 2 m², a theoretical production capacity of 25 l/m²d with an annual irradiation of 1750 kWh/m² was calculated. This has not yet been confirmed experimentally. (Application at FH Aachen, section 1.3.9)
Process with indirect condensate heat recovery
In order to be able to transfer a large proportion of the condensation heat into sensible heat of the heat transfer medium, relatively large mass flows must be generated, which require corresponding pump capacities. Despite this fact and the energy disadvantages discussed in the previous section compared to a direct transfer of … As the evaporative humidifier and the condenser do not represent a unit with direct thermal contact, there are many possible designs for the two system elements. In order to achieve the largest possible surface area in the humidifier, a wide variety of materials can be used, such as wooden louvres (Nawayseh et al. 1997), thorn bushes (Gräf 1998) or polypropylene mats (Fuerteventura). With a collector area of 47.2 m², Müller-Holst and Engelhardt (1999) state daily outputs of 11.7 to 18 l/(m² day) for this system. In order to achieve these outputs, flat-plate collectors with brine flowing through them and a thermal energy storage tank were specially developed for the process. The storage tank enables the system to operate for 24 hours. The production costs are estimated at around €11/m³ distillate.
Multi-stage seawater desalination (FH Aachen)
An optimised desalination plant has been developed at the Solar Institute Jülich, which is designed to deliver many times the output of conventional solar stills with the same energy input. With the development of an optimised prototype plant and a dynamic calculation model as a dimensioning aid for thermal seawater and brackish water desalination plants, the prerequisites for commercialisation have been created. External heat is used to heat salt water in the lower stage to approx. 95 °C and then evaporate it. The water vapour in the rising humid air condenses on the underside of the evaporator stage above. The condensate runs along the slopes into a collecting channel and from there into a collecting tank. The enthalpy of vaporisation released by condensation (i.e. = 2250 kJ/kg) is transferred to the stage above and in this way heats the salt water located there. This process in turn leads to evaporation and condensation in the next higher stage. As the condensation heat is used several times for evaporation in the next stages, the desalination rate of this type of system is many times higher than that of simple distilleries. For example, with this method of heat recovery from the condensation heat in the next higher stage, approximately three times the amount of distillate can be obtained with the same energy input in a four-stage system. Optimisations that have already been carried out predict an energy requirement of 180 kWh/m³ distillate for a five-stage distillery. This corresponds to less than a quarter of the energy requirement of a simple distillery. Many factors influence evaporation and condensation, as these are coupled diffusion and convection transport processes. Evaporation and condensation temperature and geometric factors (distance between the surfaces, angle of inclination of the condensation surface) have a particular impact.
Advantages: Various heat sources can be used to drive the system, such as solar energy coupled in via collectors or waste heat from diesel generators or other mechanical machines. Due to the comparatively low investment costs, decentralised application is possible. Compared to simple solar stills, the yield can be significantly increased and the space requirement reduced. With an appropriate arrangement, a circulation pump is not necessary.
Disadvantages: Compared to the simple solar still, more equipment is required. Higher investment costs are therefore to be expected, but these can be reduced by the higher yield and the smaller collector area required.
Solar evaporation systems
Multiple-effect evaporation (MED)
El-Nashar et al. (1987) provided results from the one-year test phase of a MED system operated with evacuated tube collectors in Abu Dhabi in the United Arab Emirates. The production rate was 100 m³/day with a collector area of 1860 m². This plant thus supplied an average of 54 litres of distillate per day and m² of collector area. Milow and Zarza (1997) report on several years of operating experience with a 14-stage MED pilot plant in Almería, Spain. This system is operated by parabolic trough collectors in combination with an absorption heat pump and produces around 72 m³/day. The water production costs are stated at 2.5 €/m³ for the location in southern Spain if 45 per cent of the required process heat is provided by conventional energy sources. For medium-sized solar seawater desalination plants, a combination of thermal solar collectors with a thermal storage tank is considered an economical solution. For such plants with a daily output of 270 m³ of distillate, a water production price of 2-2.5 €/m³ is given. The production output is 7.8 litres/m² of distillate.
Multi-stage flash vaporisation (MSF)
There were trials in Kuwait in the 1980s with a twelve-stage MSF plant operated by parabolic trough collectors for solar seawater desalination. With a collector area of 220 m², the system produced around 300 l/h at maximum irradiation. A 7 m³ tank serves as thermal storage and enables 24-hour operation. AQUASOL project: The AQUASOL project was realised by ZAE-Bayern in cooperation with the Moik company and the Technical University of Munich. The functional principle of the AQUASOL process is based on single-stage flash evaporation with subsequent air humidification. Water is heated in a pressurised circuit to just below the respective boiling point and then expanded to ambient pressure. Heating to 120 °C at an absolute pressure of 2 bar was determined as a suitable operating parameter. For a solar-powered system, 6 m² of STIEBEL ELTRON SOL 200 A vacuum tube collectors are required. The standard module heads of the collectors were rebuilt from seawater-resistant steel 1.4539 and replaced. The solar modules were equipped with a single-axis tracking device.
Advantages: Due to its size, the system can be used decentrally and can be operated using solar energy.
Disadvantages: As the system is only operated in a single stage, the system efficiency is too low. The energy requirement is very high due to evaporation and the high temperatures. In addition, the technical complexity of the system components required, such as the expansion chamber with pressurised circuit and pressure vessel as well as a seawater-resistant circulation pump, is very high. Therefore, the goal of building a particularly maintenance-friendly system was not achieved. The system cannot be maintained independently by the local population. The system is therefore very expensive. Due to the many disadvantages, ZAE-Bayern decided not to pursue this technology any further and instead opted for an evaporation column with packing to increase the evaporation surface.
Markopulos patent
This is an EU-funded project based on the Markopulos patent. Its aim is to produce drinking water by evaporating seawater with the aid of thermal solar collectors and PV cells: It consists of a vacuum evaporation vessel and a condensation vessel inside it, which is under normal pressure and immersed in the liquid phase of the evaporation vessel. A vacuum pump conveys vapour from the evaporation vessel into the condensation vessel. There, the vapour condenses on the heat exchanger through which seawater flows and transfers the energy to the seawater to be evaporated. Operating the system at negative pressure (50 mbar) enables the use of low-temperature heat (33 °C), which reduces heat losses to the environment. According to the patent, the energy balance of this system should be more favourable than that of previous systems.
The evaporation vessel is heated by a solar collector, which compensates for the heat losses of the system. This is achieved by a separate solar circuit with a fluid heat transfer medium that flows through the solar collector, the heating device inside the evaporator and through the circulation pump. The electrical components of the system, such as pumps, valves and controls, should be powered by a PV module. The entire system is housed in a container, making it very easy to transport and set up at the application site. According to the Markopulus patent, an exemplary design of the evaporation vessel with a base area of 1.2 × 2 m enables a drinking water production capacity of 50 m³/h. A negative pressure of 50 m³/h should be maintained. A negative pressure of 50 mbar should prevail. This enables an evaporation temperature of 33 °C. A temperature of 70 °C is reached in the condenser.
Advantages: The system has a compact design, is easy to transport and can therefore be used decentrally. A sustainable and self-sufficient energy supply is guaranteed when using renewable energies (solar, wind). Waste heat can also be utilised.
Disadvantages: The stated drinking water production of 50 m³/h appears dubious. This would require 1.25 million m³ of vapour per hour to be extracted using a vacuum pump. This does not appear to be feasible. The energy required to generate the vacuum is enormous and represents the main energy requirement of the plant. Vaporising this amount of water would require 30 MW of power, comparable to a small power station. However, the plant is too small for this with too little heat exchanger surface. In contrast, a steam production of 50 m³/h seems feasible with such an apparatus. However, the heat exchanger surface also appears to be too small for this, as heat transfer can only take place on the outer surfaces of the condenser and at the openings. Generating a negative pressure of 50 mbar is energy-intensive and therefore also questionable.
Simple, autonomous seawater desalination plants
The aim of the development is an inexpensive, easy-to-operate system for the water requirements of a family or a small village. It should be possible to feed salt water directly into the system without any complex pre-treatment. Any deposits and incrustations that form over time should be easy to remove. The system should only consist of simple components (no pressure vessels etc.).
Scheffler seawater desalination system
The proven 2 m² (8 m² for larger systems) Scheffler reflectors, which bring the salt water to the boil, are planned as the energy source. A multi-stage prototype was built between August and November 2000. Salt water boils in the centre. The resulting pure water vapour condenses on a cylinder. The condensation heat released in turn heats salt water, which seeps down through a mesh on the other side of the cylinder. When heated, it also releases pure water vapour, which then condenses on the next cylinder. The use of four condensation stages increases the yield of pure drinking water by a factor of three compared to just one stage. The principle is not new, but has been implemented here in a very compact and material-saving way by using a Scheffler mirror, which can provide heat at over 100 °C with very good efficiency.
Advantages: Decentralised application is possible. The proven Scheffler reflectors are used. However, the concentrated energy is used for seawater desalination instead of cooking.
Disadvantages: The prototype encountered problems with the operability of the system. In addition, some materials were unsuitable. Other materials need to be found. The system should still be tested in practice. Instead of the cylindrical surfaces, a tent-like structure made of resistant foils should be used.
Laval nozzle – seawater desalination plant
The system consists of an evaporation chamber and a condensation chamber. The evaporation chamber is heated by direct solar radiation and also by collectors. The chamber is preferably filled with „thorn bush branches“ in order to optimise the radiation absorption, the evaporation surface and the target points for the precipitation of salts. The resulting vapour is fed through Laval nozzles into the condensation chamber, which is cooled with primary water. As it passes through the Laval nozzles, the vapour accelerates, expands and cools simultaneously (adiabatically). As a result, it constantly „rains“ in the condensation chamber.
The system (DE 20 2012 009 318.5) should still be tested in practice.
Applications
In many areas where drinking water is obtained from seawater or where seawater desalination is considered to have great potential (developing countries), a combination of desalination plants with renewable energies such as wind and solar energy is an obvious choice. In Tenerife, for example, an Enercon seawater desalination plant has been in operation since 1997, which is powered by wind energy.
There are plans to utilise the pressure at the base of downdraught wind turbines to produce drinking water using reverse osmosis. The necessary pressure of approx. 70 bar would be achieved with economic (and technically feasible) dimensions of the drop tower of 1200 m height and 400 m diameter. The coastal areas of North Africa and the Gulf region would be particularly suitable for such projects[8].
Solar and freely scalable drinking water treatment with decentralised systems that can produce drinking water from almost any raw water are ideal for use not only in developing countries, but in almost any country where there is sufficient sunshine and sufficient „raw water“. Such systems have been running maintenance-free for many years using the „RSD Rosendahl System“ in Puerto Rico and many other countries.
A pioneer in the field of seawater desalination was the British physician James Lind, who discovered in 1758 that drinkable water that tasted like rainwater could be obtained from the vapour of heated seawater.