Solar seawater desalination

The solar distillation describes methods for the desalination of sea water with the help of solar energy . This article describes a compilation of the procedures.

Physics of evaporation

From evaporation occurs when the temperature of the solution below the boiling point is located and the steam partial pressure in the surrounding carrier gas (air) is less 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 resulting vapor. The vapor diffusion distributes the vapor in the carrier gas. At normal pressure z. B. the temperature of water below 100 ° C. The desalination of sea water through evaporation and subsequent condensation is a process principle that occurs naturally on earth. Evaporation and condensation of water in air can take place using solar energy at different temperatures and at ambient pressure. Low process temperatures allow the use of non- concentrating solar collectors , whereby the heat losses can be kept within limits. Lower process temperatures enable the use of inexpensive materials with low demands on strength and corrosion resistance. On the other hand, the area required by evaporation systems is much greater, since the evaporation capacity also depends on the surface. Only low heat and material flow densities are achieved per unit area. This means that the same steam output as with evaporator systems can only be guaranteed over a large area.

Physics of evaporation

Evaporation is a thermal separation process that converts a liquid or a solvent of the solution of a non-volatile solid into the vapor state by changing the temperature and pressure. The solvent (usually water) is partially separated off by heating the solution to the boiling point according to the pressure set. In contrast to distillation, the resulting vapor consists only of saturated steam from the solvent. The heat content can be reused for multi-stage evaporation or for solution preheating. Evaporation can take place under different conditions and thus lead to different manifestations:

Silent evaporation: Boiling takes place only on the surface of the liquid as a result of free convection with a low heating surface load.
Supercooled evaporation: Boiling takes place before the pressure-dependent boiling point of the liquid is reached with high heating surface loads. In the process, bubbles that arise locally on the heating surface evaporate and condense simultaneously.
Bubble evaporation: Steam bubbles form on the heating surface when the heating surface is exposed to medium loads. The vapor bubbles arise on pores and unevenness in the wall surface, which can contain gases or residual steam. These represent the nucleus for the formation of vapor bubbles.

Simple solar evaporation systems

Greenhouse principle

In a flat, black (PE, PC) basin with an insulation layer as thermal insulation (e.g. sand) and a tent-shaped, transparent cover made of window glass, the sea or brackish water evaporates through the absorption of solar radiation . The water vapor condenses on the inside of the wind-cooled cover. The condensate is drained by means of collecting channels for further processing (blending with salt water). Simple solar stills based on this principle have been used for seawater desalination since the end of the 19th century. In systems close to the coast, the sea water ( brine ) left over from the evaporation process is pumped back into the sea. The average production output of a simple solar seawater desalination system based on the greenhouse principle is on average up to 6 liters per square meter per day in summer and around 1.2 liters per square meter per day of drinking water in winter. This applies to annual irradiation capacities of 1500 to 2000 kWh / m² ( Mediterranean area ) and a system efficiency of 40%. Therefore, they are very space-consuming when large amounts of water are to be extracted. A service life of 20 years and an interest rate of 8% results in a drinking water price of approx. US $ 2.9 / m³.

Advantages: The construction of the system is simple and can be built and used locally without in-depth specialist knowledge. No electrical supply is required for systems at sea level, as no pumps have to be used. This means that it can be used in regions without infrastructure.

Disadvantages: The performance of the system per area is comparatively low, since the 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. Therefore, the still should 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 still. The still coupled with a collector produced a production increase of 15% with good solar irradiation compared to the simple solar still, based on 1 m² of system area (still + collector). On the other hand, there is the much higher construction effort and the higher costs of a flat- plate collector compared to the simple solar still. However, tests have shown that by supplying 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.

Cascade still

According to the results of a project by the Jülich Solar Institute , the cascade still is comparatively complex:

“In the cascade still, the salt water basin is laid out in the shape of a staircase in order to keep the distance between the water surface and the sloping cover as small as possible. The cascade still produces around 5% more distillate compared to a simple solar still. However, the higher construction costs and the more complex 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 only produced unsatisfactory results. The heat losses through 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 extract water from soil moisture

The watercone consists of an absorber bowl and a bulbous cone. Coated polycarbonate is used as the material . The sea or brackish water is poured manually into the absorber basin. As a result of the sun's rays, the water evaporates and condenses on the cone. The condensed water runs off the cone into a collecting gutter. The water is stored there and can be removed at the end of the process by turning the cone and opening the closure at the tip of the cone. In addition, the Watercone can 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.

Pros: The simplicity of the Watercone is one of its greatest advantages. Even the population 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 for electricity consumption or maintenance. The polycarbonate material used is light, transparent and practically unbreakable; several Watercone devices can be plugged into each other for transport and storage. With a price of less than € 50 per piece with a lifespan of at least 3 years and a daily production of up to 1.5 liters, the water price is less than 3 euro cents per liter and thus significantly below 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 is relatively short at 3-5 years. Over time, the polycarbonate material used becomes matt.

Methods in case of emergency

Emergency solar seawater desalination methods rely on a larger container to hold the contaminated water, such as a saucepan or a pit in the damp ground. This container is covered with a transparent plastic film , which is well attached to the edges of the container. In the middle of this plastic film you then put some not too heavy weight so that the plastic film assumes the shape of a cone with an obtuse angle , the tip of which points downwards. A cup is placed under this lowest point of the plastic film to catch the condensation water that drips off here. You may have to weigh down this cup at the beginning so that it does not float in the sea water, which of course must not be filled too high into the larger container. In principle, this method is an inverted watercone. In addition, the top of this plastic film can then also catch rainwater .

Complex solar evaporation systems

The goal of multi-stage solar stills is to use the radiated solar energy several times in order to achieve maximum distillate yield. Despite some successes, such systems still require a great deal of research and development. Various concepts are being pursued.

Moist air countercurrent still

This is a closed container. No vacuum technology is required, the container should just be airtight. In the larger area of the evaporation module, hot water is evaporated using large cloths. The incoming water has a temperature of 80 ° C. On the other side there are condensers through which cold sea water flows. Hot and humid air has a lower density than cold and dry air. That's why the hot and humid air rises. On the other hand, it is cooled down because cold sea water flows through this large heat exchanger. The humid air circulates by itself. A fan is not required. Hence the name moist air countercurrent still comes from. The system requires a collector area of 37.5 square meters. Heat is temporarily stored at lunchtime and used for further seawater desalination in the evening. However, 24-hour operation is not yet possible. The production is between 488 and 536 liters / day. The system has a specific energy requirement of 106–114 kWh / m³ water.

Advantages: It is a very simple principle that offers the possibility of building low-maintenance systems. It can therefore be used decentrally. Nevertheless, the condensation energy is recovered and used to heat the seawater. Compared to the simple solar still, the yield can be significantly increased and thus the space requirement can be reduced.

Disadvantages: Compared to the simple solar still, more equipment is required. Therefore, higher investments are 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. The envisaged energy storage ensures that the distillate is evenly extracted, but it is an additional system component and thus a heat loss zone.

Collector system with heat recovery

The patent DE 100 47 522 A1 is based on an inclined flat collector. In contrast to the Rosendahl collector, the distillate does not condense on the glass surface, but on the condensers provided on the back of the absorber. These capacitors are shaded by the absorber and thermally insulated from the evaporation space. Primary water flows through this and is thereby preheated. The heated primary water then flows over the black absorber fleece, where it partially evaporates from the sun's rays. The brine flows into the brine tub and is discharged via an overflow. The temperature difference between the evaporation space and the condensation space creates an air mass circulation .

Tests with non-optimized prototypes of the plant have shown distillate yields of up to 20 l / m²d. Because of the similar principle of operation, the advantages and disadvantages are presented together in the following article.

Distillation cyclone

This is a system for the production of drinking water from sea, brackish or waste water using solar energy. Construction can take many different forms. In a preferred variant, the system is a column-like transparent system that consists of a glass column and an inner hollow column. The use of solar mirrors focuses the sunlight on the column. The sunlight passes through the transparent area and falls on the inner hollow column. This is covered on the outside with a black and hydrophilic absorber fleece and heats up strongly due to the action of solar radiation . Primary water heated to 95 to 99 ° C is passed over this absorber fleece and evaporates from the absorber surface. The primary water is initially used as cooling water . The humid air rises and cools down on the condensers provided inside the hollow column. There the excess moisture precipitates and condenses as pure water. The condensate is collected in a vessel 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 part of the condensation heat is recovered. A second cooling circuit, which is fed from an external container, is used for further cooling . The cold, moist 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 vapor again, which starts a new cycle. Due to the imbalance between hot air masses in the evaporation room and cold air masses in the condensation room, an independent air mass circulation builds up in the system. The different areas of the system must therefore be highly insulated against each other. Despite solar radiation, the evaporation process leads to a significant cooling of the primary water. This collects in the brine tub. The different concentrations are stratified there. The brine tub has an overflow which, with the help of a culvert, discharges the highest concentrations. No deposits should form on the absorber fleece as a result of excessively high salt concentrations with the associated dropping below the solubility limit. The primary water flow should be set high enough accordingly. The circulation of the cooling and brine water as well as the supply of primary water are ensured by pumps. These can be supplied with a photovoltaic module. The glass column should have a diameter of 1.4 m and a height of 7 m. These dimensions favor 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, because the energy available through the solar irradiation would have been sufficient for a third to a maximum of half of the distillate quantities achieved. The yield is thus higher than previously known systems. In this way, significant collector area can be saved or the yield can be increased with the same collector area.

Disadvantages: With the proposed embodiment, a diameter of 1.4 m and a height of 7 m, this system is no longer easy to transport and handle. A complex control and regulation unit is necessary, which are susceptible under the conditions of southern developing countries. Compared to the simple solar still, a higher expenditure on equipment is necessary. Therefore, higher investments are 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 demonstrated that the desired circulation flow is strong enough. It is therefore critical to see whether no condensation forms on the glass, although the path is the shortest, the glass is wind-cooled and there is therefore a high temperature difference to the glass.

MEH process - thermal desalination with low temperature heat e.g. B. from solar panels

The multi-effect humidification / dehumidification process (MEH) is another thermal process for decentralized seawater desalination in the small and medium-scale production range up to approx. 50,000 liters per day. Systems based on the MEH process are based on thermal energy input from low-temperature sources (e B. Solar panels ). The heat is fed into a closed desalination module, in which the natural water cycle with evaporation and condensation is reproduced in an efficient manner. Sufficiently large evaporation and condensation surfaces, based on the energy consumption, enable the greatest possible recovery of the evaporation heat in the condenser. In this way, production rates of over 25 l / m² per day can be achieved with a solar-powered system. The waste heat from other processes or from diesel generators can also be fed into the process. This process was made ready for use at the Bavarian Center for Applied Energy Research (ZAE Bayern).

A machine builder at the Ruhr University Bochum (RUB) developed a particularly space-saving, transportable prototype that works according to this principle . By using air as a heat transport medium, the system can be operated at particularly low temperatures. In the system, heated sea water trickles through an evaporative humidifier, which heats the incoming air and additionally enriches it with water vapor from the sea water. This results in a production rate of around 20 liters per m² of collector surface per day (based on ten hours of sunshine a day). Research on this was funded by the EU as part of the Soldes project. In a multi-stage system, also funded by the EU as part of the Soldes project, with air collectors and evaporative humidifiers alternately connected in series, only the circulating air, but not the brine, is heated by solar collectors. The air is gradually heated and humidified.

The ZAE-Bayern planned and built a plant for solar seawater desalination in Oman in 2000 . The plant consists of a field 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 liters. The distillation process works at ambient pressure. Heated sea water is distributed over a large evaporator. A convection roller, which is driven by differences in density and humidity, transports moist air to double-walled polypropylene sheets in the module . These serve as condensation surfaces and are traversed by cold sea water. The seawater heats up to 75 ° C due to the condensation of the moist air on the surface of the plate.

Advantages: The geometric arrangement of evaporation and condensation surfaces enable a flow of material and heat that can otherwise only be achieved with a complex multi-chamber system. This achieves heat recovery that reduces the thermal energy requirement of the desalination plant to around 100 kWh / m³ of distillate compared to the enthalpy of evaporation for water of 690 kWh / m³. The heat recovery is therefore only slightly below the maintenance and technology-intensive vacuum evaporation systems. The system is therefore suitable for decentralized use in structurally weak areas.

Disadvantages: Compared to the simple solar still, more equipment is required. Therefore, higher investments are to be expected, but these can be reduced by the higher yield and the smaller collector area required. The condensation heat is only partially recovered. Pumps are also required for water circulation.

The Fraunhofer Institute for Solar Energy Systems used this principle in the “SODESA” project. This test facility has a 56 m² collector field. In the project, collectors were developed in which the hot sea water could flow directly through the absorber. It was therefore not allowed to be a copper absorber, as this material corrodes immediately. Collectors were developed in which the absorber is 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 sheet located under a pane of glass. A viscose cloth is attached to the back of the sheet and is filled with salt water. Some of the salt water evaporates, condenses on the cooler sheet below and transfers the heat of condensation to the next stage. The evaluation of the test results for a four-stage prototype showed high heat recovery factors in the individual stages (approx. 70%). However, the maximum distillate yields achieved are only about 50% higher than the results of the simple solar still. The cause is 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 through the use of a selectively coated absorber therefore lead to the expectation of further increases in yield. Moistening the system with the help of viscose cloths has proven to be effective. No dripping of the brine or mixing of the salt water with the distillate was observed. Salt content tests showed a very good quality of the distillate. However, the high expenditure for operation and maintenance of the system must be taken into account.

Aqua style

The sea or brackish water (15-25 ° C) flows through the condensers from bottom to top and heats up in the process. At the upper end, the cooling water flows out of the condensers onto the evaporation surface (perforated knob surfaces). A heating element is located above the evaporation surface, 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 humid air rises and the distillate condenses on the condensers. The condensate flows off, is collected in a collecting channel and led out of the system. The excess brine flows back into the sea. According to the manufacturer, a 1.5 kW solar collector has a distillate output of 12-18 l / h. The distillate price is thus € 3.9–5.7 / m³.

A further development uses the resulting steam for preheating. Then the evaporation takes place in stage evaporators.

Advantages: The system is simple and compact and is therefore suitable for decentralized use. No regulation is required for this system. In order 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. Therefore, higher investment costs are to be expected, but these can be reduced by the higher yield and the smaller collector area required. The condensation heat is only partially recovered. Pumps are also required for water circulation.

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 exchange of air between the individual stages. The process is suitable for very small systems because no fan is required. A pump can also be dispensed with if the raw water is taken from a tank at a higher level and a thermosyphon collector is used. A theoretical production output of 25 l / m²d with an annual irradiation of 1750 kWh / m² was calculated for a system with 2 m² collector surface. 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 part of the condensation heat into sensible heat of the heat transfer medium, relatively large mass flows must be generated, which require corresponding pump outputs. Despite this fact and the energetic disadvantages compared to a direct transfer of the ... As the evaporative humidifier and the condenser do not represent a unit with direct thermal contact, there are many design options for the design of the two system elements. In order to achieve the largest possible surface in the humidifier, a wide variety of materials such as wooden lamellas (Nawayseh et al. 1997), thorn bushes (Gräf 1998) or polypropylene mats ( Fuerteventura ) can be used. With a collector area of 47.2 m², Müller-Holst and Engelhardt (1999) give daily outputs of 11.7 to 18 l / (m² day) for this system. In order to be able to achieve this level of performance, flat-plate collectors through which brine flows, specially developed for the process, and a thermal energy store were used. The memory enables the system to be operated for 24 hours. The production costs are estimated at around 11 € / m³ of distillate.

Multi-stage seawater desalination (FH Aachen)

At the Solar Institute Jülich optimized desalination system was developed with the same conventional energy supply is a multiple solar stills is intended to provide. With the development of an optimized prototype system and a dynamic calculation model as a dimensioning aid for thermal sea and brackish water desalination systems, the prerequisites for marketing were created. By external heat supply, salt water is heated in the lower stage to approx. 95 ° C and then evaporates. The water vapor in the rising moist 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 container. The enthalpy of evaporation released by condensation (i.e. = 2250 kJ / kg) is transferred to the respective level above and in this way heats the salt water located there. This process in turn leads to evaporation and condensation in the next higher level. Since the heat of condensation 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 stills. With this method of heat recovery of the heat of condensation in the next higher level, for example in a four-stage system, approximately three times the amount of distillate can be obtained with the same amount of energy. Optimizations that have already been carried out predict an energy requirement of 180 kWh / m³ of distillate for a five-stage still. That corresponds to less than a quarter of the energy requirements of a simple still. Many factors influence evaporation and condensation, as it is a question of coupled diffusion and convection transport. The evaporation and condensation temperature as well as geometric factors (distance between surfaces, angle of inclination of the condensation surface) have a particular effect .

Advantages: Various heat sources can be used to drive the system, such as B. solar energy coupled in via collectors or waste heat from diesel generators or other mechanical machines. A decentralized application is possible due to the comparatively low investment costs. Compared to the simple solar still, the yield can be significantly increased and thus the space requirement can be reduced. With a corresponding arrangement, a circulation pump is not necessary.

Disadvantages: Compared to the simple solar still, more equipment is required. Therefore, higher investment costs are 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 an 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 means that this system delivered an average of 54 liters of distillate per day and m² of collector surface. Milow and Zarza (1997) report on several years of operational experience with a 14-stage MED test facility in Almería , Spain. This system is operated by parabolic trough collectors in combination with an absorption heat pump and generates around 72 m³ / day. The water production costs are given as 2.5 € / m³ for the southern Spain location if 45 percent 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 store is seen as an economical solution. For such systems 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 l / m² of distillate.

Multi-stage flash evaporation (MSF)

In Kuwait there were attempts in the 1980s with a twelve-stage MSF system 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 implemented by ZAE-Bayern in cooperation with Moik and the Technical University of Munich. The functional principle of the AQUASOL process relies on only one-stage expansion evaporation with subsequent air humidification. Water is heated in a pressure circuit to just below the respective boiling point and then expanded to ambient pressure. The heating to 120 ° C. at an absolute pressure of 2 bar was determined as a suitable operating parameter. A solar powered system requires 6 m² of STIEBEL ELTRON SOL 200 A vacuum tube collectors. The standard module heads of the collectors were made from seawater-proof 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 by solar energy.

Disadvantages: Since the system is only operated in one stage, the system efficiency is too low. The energy requirement is very high due to the evaporation and the high temperatures. In addition, the technical effort due to the required system components, such as a relaxation chamber with pressure circuit and pressure vessel and a seawater-proof circulation pump, is very high. Therefore, the goal of building a particularly maintenance-friendly system was not achieved. The system cannot be serviced independently by the local population. The system is therefore very expensive. Due to the many disadvantages, the ZAE-Bayern decided not to pursue this technology any further and instead to rely on an evaporation column with packing to increase the evaporation surface.

Markopulos patent

This is an EU funded project based on the Markopulus patent. The aim of this is to obtain drinking water by evaporating seawater with the help of thermal solar collectors and PV cells: It consists of a negative pressure evaporation vessel and a condensation vessel located in it, which is under normal pressure and immersed in the liquid phase of the evaporation vessel. A vacuum pump conveys steam from the evaporation vessel into the condensation vessel. There, the steam condenses on the heat exchanger through which the sea water flows and transfers the energy to the sea water 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 is said to be more favorable than that of previous systems.

The evaporation vessel is heated by a solar collector, which compensates for the heat losses in the system. This takes place through 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 fed by a PV module. The entire system is located in a container and is therefore very easy to transport and set up at the place of use. According to the Markopulus patent, an exemplary embodiment of the evaporation vessel with a base area of 1.2 × 2 m enables a drinking water production capacity of 50 m³ / h. There should be a negative pressure of 50 mbar. This enables an evaporation temperature of 33 ° C. A temperature of 70 ° C is reached in the condenser.

Advantages: The system is compact, easy to transport and can therefore be used decentrally. A sustainable and self-sufficient energy supply is guaranteed with the use of renewable energies (sun, wind). Waste heat can also be used.

Disadvantages: The specified drinking water production of 50 m³ / h appears doubtful. To do this, 1.25 million m³ of steam per hour would have to be extracted with a vacuum pump. This does not seem feasible. The energy expenditure to generate the negative pressure is enormous and represents the main energy requirement of the system. To evaporate this amount of water, 30 MW of power would be required, comparable to a small power plant . For this, however, the system is too small with a heat exchanger surface that is too small. In contrast, a steam production of 50 m³ / h appears to be realizable with such an apparatus. However, the heat exchanger surface also appears to be too small for this, since heat can only be transferred to the outer surfaces of the condenser and to the openings. The generation of a negative pressure of 50 mbar is energy-consuming and therefore also to be questioned.

Simple, autonomous seawater desalination plants

The aim of the development is an inexpensive, easy-to-use system for the water needs of a family up to that of a small village. Salt water should be able to be fed directly into the system without complex pretreatment. Deposits and encrustations that develop over time should be easy to remove. The system should only consist of simple components (no pressure vessels, etc.).

Scheffler seawater desalination plant

The tried and tested 2 m² (8 m² for larger systems) Scheffler reflectors, which bring the salt water to a boil, are provided as an energy source. A multi-stage prototype was built between August and November 2000. Salt water is boiling in the middle. The resulting pure water vapor condenses on a cylinder. The released heat of condensation in turn heats salt water, which seeps down through a fabric on the other side of the cylinder. When heated, it also gives off pure water vapor, which then condenses on the next cylinder. The use of four condensation stages increases the yield of pure drinking water by a factor of 3 compared to just one stage. The principle is not new, but has been implemented in a very compact and material-saving manner by using a Scheffler mirror, which can provide heat at over 100 ° C with very good efficiency.

Advantages: A decentralized application is possible. The tried and tested Scheffler reflectors are used. Instead of cooking, however, the concentrated energy is used for seawater desalination.

Disadvantages: The prototype encountered problems with the operability of the system. In addition, some materials were unsuitable. Other materials must be sought. The system should still be tested in practice. Instead of the cylindrical surfaces, a tent-like structure made of durable foils should be used.

Laval nozzle - seawater desalination plant

The system consists of an evaporation and a condensation chamber. The evaporation chamber is heated by direct sunlight and also by collectors. The chamber is preferably filled with "thorn bush branches" in order to optimize the radiation absorption, the evaporation area and the desired locations for the precipitation of salts. The resulting steam is fed through Laval nozzles into the condensation chamber, which is cooled with primary water. When passing through the Laval nozzles, the steam accelerates, relaxes and cools at the same time (adiabatically). As a result, it "rains" constantly in the condensation chamber.

The system (DE 20 2012 009 318.5) should still be tested in practice.

Applications

In many areas in which drinking water is obtained from seawater or in which seawater desalination is ascribed great potential (developing countries), a combination of desalination plants with renewable energies such as wind and solar energy is an option . A seawater desalination plant from Enercon , which is operated with wind energy , has been running on Tenerife since 1997 .

There are considerations to use the pressure at the foot of down wind power plants to produce drinking water with the help of reverse osmosis . The pressure of approx. 70 bar required for this 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.

Solar and freely scalable drinking water treatment with decentralized systems, which can obtain drinking water from almost any raw water, are ideally applicable not only in developing countries, but in almost every country where there is enough sun and sufficient "raw water". Such systems have been running maintenance-free and according to the "RSD Rosendahl System" for many years. a. in Puerto Rico and in many other countries.

A pioneer in the field of seawater desalination was the British doctor James Lind who discovered in 1758 that potable water that tasted like rainwater could be obtained from the steam of heated seawater.

Web links

Hendrik Müller-Holst: Solar desalination with the MEH process , dissertation at the Technical University of Munich 2002 (PDF, 155 p., 8.4 MB).
Thomas Brendel: Solar seawater desalination plants with multi-stage evaporation Dissertation, Faculty of Mechanical Engineering of the Ruhr University Bochum , 2003 (PDF, 175 p., 3.68 MB).
Book of Synergy: Simple water treatment systems (further information on RSD and MEH processes, Watercone and others).

Individual evidence

↑ ^a ^b ^c project 252 001 91; Subproject: Solar thermal applications (solar thermal seawater desalination / water treatment). (PDF) Solar-Institut Jülich, accessed on July 19, 2008 .
^ Tapping A Market . In: CNBC European Business. Retrieved October 1, 2008 .
↑ With sunlight to drinking water. Bochum seawater desalination uses solar energy. RUB machine builders develop new process and prototype. November 5, 2003, accessed July 19, 2008 .
↑ RUB machine builders develop a new process for seawater desalination. November 6, 2003, accessed July 19, 2008 .
↑ Development and optimization of a new process for desalination of sea water by means of solar energy. Retrieved July 19, 2008 .
↑ Solar drinking water desalination . In: Archive, finetech.net - information portal and museum for renewable energy. (No longer available online.) Archived from the original on August 3, 2008 ; Retrieved July 19, 2008 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2
↑ Thomas Brendel, dissertation. P. 28
↑ Issue No. 7 Energy Towers Dan Zaslavsky ( Memento from August 14, 2006 in the Internet Archive )

[sij-1] project 252 001 91; Subproject: Solar thermal applications (solar thermal seawater desalination / water treatment). (PDF) Solar-Institut Jülich, accessed on July 19, 2008 .

[2] Tapping A Market . In: CNBC European Business. Retrieved October 1, 2008 .

[3] With sunlight to drinking water. Bochum seawater desalination uses solar energy. RUB machine builders develop new process and prototype. November 5, 2003, accessed July 19, 2008 .

[4] RUB machine builders develop a new process for seawater desalination. November 6, 2003, accessed July 19, 2008 .

[5] Development and optimization of a new process for desalination of sea water by means of solar energy. Retrieved July 19, 2008 .

[6] Solar drinking water desalination . In: Archive, finetech.net - information portal and museum for renewable energy. (No longer available online.) Archived from the original on August 3, 2008 ; Retrieved July 19, 2008 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2

[7] Thomas Brendel, dissertation. P. 28

[8] Issue No. 7 Energy Towers Dan Zaslavsky ( Memento from August 14, 2006 in the Internet Archive )