Solar desalination

The solar distillation referred to methods for desalination of sea water using solar energy. This article describes a collection of procedures.

4.1 humid air Gegenstromdestille
4.2 collector system with heat recovery
4.3 Destillationszyclon
4.4 MEH- method - Thermal Desalination using low-temperature heat, for example from solar panels
4.5 Multi -effect distiller
4.6 Aquadestil
4.7 method with direct condensate heat recovery
4.8 method with indirect heat recovery condensate
4.9 Multi-stage seawater desalination (FH Aachen )

5.1 multiple effect evaporation ( MED)
5.2 Multi-stage flash evaporation (MSF )
5.3 Markopulos Patent
5.4 Scheffler seawater desalination plant

Physics of evaporation

By evaporation occurs when the solution temperature is below the boiling point and the steam partial pressure in the surrounding carrier gas ( air) is less than in the liquid., The solvent evaporated then dependent on the temperature and the partial pressure difference. The total pressure of the solution adjacent to the space is greater than the partial pressure of the produced steam. The vapor diffusion distributes the vapor in the carrier gas. At atmospheric pressure, for example, the temperature of water below 100 ° C. is Desalination of sea water by evaporation and subsequent condensation is a naturally occurring process on earth principle. Evaporation and condensation of water in air can take place by utilizing the solar energy at different temperatures and at ambient pressure. Low processing temperatures allow the use of non- concentrating solar collectors, the heat losses can be kept within limits. Lower process temperatures allow the use of inexpensive materials with low requirements on the strength and corrosion resistance. In contrast, the area requirement of evaporation equipment is much larger, since the evaporation performance also depends on the surface. Only low heat and mass flux densities are achieved per unit area. Thus, the same steam output can be ensured as in evaporator systems only have a large area.

Physics of evaporation

Evaporation is a thermal separation process that converts a liquid or a solvent of said solution of a non-volatile solids to the vapor state by changing the temperature and the pressure. The solvent (usually water ) is separated by heating the solution to boiling temperature to the set pressure according partially. The resultant waste steam is in contrast to the only saturated steam distillation of the solvent. The heat content can be used for multi-stage evaporation or Lösungsvorwärmung again. Evaporation can take place under different conditions and thus lead to different manifestations:

Simple solar evaporation plants

Greenhouse Principle

In a flat black (PE, PC) pool with an insulating layer as thermal insulation evaporates (eg sand ) and a tent-shaped, transparent covering of window glass by the absorption of solar radiation the sea or brackish water. The water vapor condenses on the inside of the wind -cooled cover. The condensate is derived by collecting channels for further processing ( blending with salt water). Simple solar stills on this principle have been used for seawater desalination since the end of the 19th century. For installations near the coast after the evaporation process leftover sea water (brine) is pumped back into the sea. The mean production performance of a simple solar desalination plant for the greenhouse principle is in the summer average at up to 6 l / m² • d and approximately 1.2 l / m² • d drinking water in the winter. This applies to annual irradiation services from 1500 to 2000 kWh / m² ( Mediterranean) and a system efficiency of 40%. Therefore, they are very area- consuming if large volumes of water to be obtained. With a service life of 20 years and 8% a drinking water price is of about 2.9 U.S. $ / m³.

Collector and Solardestille

The production of the distillate progressively increases with the water temperature. Therefore, should the still is coupled to a solar thermal collector, and the heat of condensation of the condensing water in the collector to heat the brine in the still to be used. The coupled to a collector distillery provided with good solar radiation compared to the simple Solardestille a production increase of 15 %, based on 1 m² surface ( distillery collector). In contrast is the much higher construction costs and the cost of a flat plate collector over the simple Solardestille. However, experiments have shown that production increases of over 50 % can be achieved by a heat supply in the brine bath ( external heat source with sufficiently high temperature level, possibly waste heat) and the associated higher brine temperatures above 80 ° C.

Kaskadendestille

According to results of a project of the Solar Institute Jülich Kaskadendestille is comparatively complicated:

"In the Kaskadendestille the salt water pool is tiered applied in order to keep the distance between the water surface and the inclined cover as low as possible. The Kaskadendestille produced in comparison to a simple Solardestille about 5 % more distillate. However, the higher construction costs and the elaborate cleaning of the cascades can not justify this little extra income. An attempt to preheat the supplied brine in the interspace of a double glass cover by heat recovery of the condensation heat to the cover, brought only unsatisfactory results. The heat losses due to reflection and absorption in the cover are higher than the additional energy input to the heat recovery, so that the effectiveness of the system as a whole is reduced. "

Watercone

The Watercone consists of an absorber bowl and having convex cone. As a material coated polycarbonate is used. The sea or brackish water is manually poured into the absorber pool. Due to the sunlight, the water evaporates and condenses on the cone. The condensed water runs off the cone in a gutter. There the water is stored and can be removed at the end of the process by turning the cone and opening the shutter at the top of the cone. In addition, can be captured with the Watercone soil moisture and used for drinking water. In this application, the cone stands directly on the ground. Moisture condenses on the surface of the cone, is collected in the collecting channel and can be used thereafter.

Advantages: the simplicity of the Watercone is one of its greatest advantages. Even people with a low education level can use it independently without problems. The system is easily explained by pictograms. There is no cost through electricity consumption and maintenance requirements. The material used Makrolon is light, transparent and virtually unbreakable; Watercone multiple devices can be plugged into each other for transport and storage. With a price of less than 50 € per piece with a minimum life of 3 years and a daily production of up to 1.5 liters of water price is less than 3 euro cents per liter, well below the price of bottled drinking water in bottles.

Cons: The total costs are to be paid at the beginning of a project. To this must micro credits or other financing available to make the device available to the poorest. Furthermore, the service life of 3-5 years, is relatively low. Over time, the material used is matt Makrolon.

Collector principle

According to the present project, the collector principle was researched. Following are three typical embodiments of the RSD solar companies, thethermo Bau AG and the SEC in 1000 - represented Solar GmbH.

The RSD Rosendahl system based on a flat-plate collector, run in parallel in the energy, evaporation and condensation in a system. The dosed with a control system according to the sunlight raw water (sea, brackish or polluted fresh water) passes through a raw water chute into the collector and seeps through the wicks on a black absorber fleece. This is a long-term UV-resistant and food safe.

The moistened absorbent fleece heated by the sunlight at 80-90 ° C. About half of the raw water evaporates and condenses on the shortest path to the wind chilled glass cover. The condensate runs into a condensate channel and is led out of the collector. The remaining raw water is collected and discharged under the condensate channel. It carries dirt and salts from the collector. The insulation prevents energy losses through the collector bottom. Depending on the climate can be in the flat-plate collector daily about 6-8 liters of drinking water per m² absorber area win. All metal parts of the collectors who come into contact with water are made of stainless steel or brass - the prerequisite for long life. All external parts are zinc-plated. The glass cover is made of ordinary window glass. The life and safety of the panels is according to the manufacturer about 20 years.

Advantages: The system is decentralized use with solar energy. Thus, a transport of the drinking water to the consumers is not required, since the need is created on site. The water quality is equally good. An extensive pre-filtration or the use of chemicals is not required. The current required can be covered by a PV system. The modules can be combined and can also be at a greater need (up to 100 m³) economic sense to apply. Maintenance is rarely required and can be performed by unskilled persons ( according to the manufacturer ). The production of the collector can be performed directly on site. This means that production costs are kept low and create jobs.

Cons: Compared to large-scale plants, the price of the distillate is too high. A simple collector system ( supply kit F8 -280 ) for 15 liters a day will cost € 1,285.00. With a service life of 20 years ( = depreciation period ) a price (including maintenance costs ), according to Rosendahl of 11.81 € per cubic meter of drinking water is produced. Based on the amount of distillate produced, a high space requirement is needed, however, is available on roofs. Since no heat recovery exists, the radiated solar energy is used only once, which reduces the yield. A complex control and regulation unit can cause failures and additional costs due to the sensitive electronic elements. In addition, these consumed in addition to the pump current, which must be covered by a PV module. Salt water is very aggressive at high temperatures and corrosive to metals. Therefore, a lifetime collector of 20 years must be questioned.

As in Rosendahl - collector system of the company thethermo Bau AG is based on a flat-plate collector. There, the three processes energy, evaporation and condensation take place in parallel.

Advantages: It offers the same advantages as in Rosendahl - collector. The product is heated, the supplied raw water before a built-in heat exchanger. Compared to the Rosendahl collector thus a partial recovery takes place.

Disadvantages: Regarding space requirements, power consumption and life of the pump there are the same disadvantages as the Rosendahl collector. The control and regulation unit is critical to see how the Rosendahl - collector under the given operating conditions. The rate of 3900 € is substantially higher than the Rosendahl system. It will not provide information on performance, whereby the price per cubic meter can not be calculated.

As in Rosendahl - collector, the system of company SEC 1000 is based - Solar GmbH on a flat plate collector. There, the three processes energy, evaporation and condensation take place in parallel. The system is supplied via a pump powered by solar salt, brackish or waste water. The sun shines through the glass on the solar absorber and the water evaporates. The water vapor condenses on the glass and is collected in the gutter below. This is removed as pure drinking water for consumption.

Advantages: It provides pure drinking water of consistent quality. A module produces about 20 liters of potable water per day. If necessary, the system can be extended by another module. The modules operate without mechanical and electronic components. Therefore you are coming from with virtually no maintenance. The system works absolutely emission-free. There are no chemicals or additives needed. The system does not require complicated control. The price per cubic meter ( according to manufacturer's instructions) was reduced to 2.5 € compared to the Rosendahl system. However, this information must be questioned, since no calculations are disclosed.

Cons: See Rosendahl collector, however, some disadvantages of the Rosendahl system could be reduced. For example, the cost could be reduced by avoiding complicated control unit.

Complex solar evaporation plants

The goal of multi-stage solar stills is to use the radiated solar energy more times to achieve maximum distillate yield. Such systems require despite some successes even large research and development efforts. Various concepts are being pursued.

Humid air Gegenstromdestille

This is a closed container. There is no vacuum technology needed, the container should only be airtight. In the larger area of the evaporation module hot water is evaporated oversized towels. The incoming water has a temperature of 80 ° C. On the other side there are capacitors that are traversed by cold seawater. Hot and humid air has a lower density than cold, dry air. Therefore, the hot and humid air rises up. On the other hand it is cooled because of cold seawater flows through this large heat exchanger. The humid air is circulated by a fan itself is not required. Hence the name humid air comes Gegenstromdestille. The plant requires a collector area of 37.5 square meters. This noon heat is cached and evening used to further seawater desalination. However, a 24 -hour operation is not yet possible. The production is 488-536 liters / day. The complex has a specific energy consumption of 106-114 kWh / m³ of water.

Advantages: It is a very simple principle, which offers the opportunity to build low-maintenance systems. Therefore, it is used in distributed configurations. However, the energy of condensation is recovered and used for heating the sea water. Compared to the simple Solardestille the yield can substantially increased and hence the space requirements are reduced.

Cons: Compared to the simple Solardestille a higher expenditure on equipment is necessary. Therefore, higher investments are expected, but these can be reduced by the higher yield and lower required collector area. A water price of 10 € / m³ € -25 / m³ is reached. Although the proposed energy storage ensures a uniform distillate production, however, this represents an additional system component and thus heat loss zone

Collector system with heat recovery

The patent DE 100 47 522 A1 is based on an inclined flat plate collector. Unlike the collector Rosendahl the distillate, however, not fused to the glass surface, but to the designated capacitors on the back side of the absorber. These capacitors are shadowed by the absorber and compared to the evaporation chamber thermally insulated. Through this primary water, which is preheated by flowing. Then the heated primary water flows over the black absorber fleece to evaporate there through the sunlight partially. The brine flowing in the brine bath and is removed via an overflow. Due to the temperature difference between the evaporation chamber and condensation chamber to an air mass circulation develops.

Experiments with non-optimized prototypes of the system have shown distillate yields of up to 20 l / m² d. Because of the similar function principle, the advantages and disadvantages are presented together in the following article.

Destillationszyclon

It is a plant for the production of drinking water from seawater, brackish or waste water using solar energy. The construction can be embodied in many different forms. In a preferred embodiment, the plant is a columnar transparent system that consists of a glass column, and an inner hollow column. By the use of solar mirrors, sunlight is focused on the column. The sunlight passes through the transparent region and is incident on the inner hollow column. This is the outside covered with a black and hydrophilic non-woven absorber and is heated by exposure to the sun's rays strongly. This absorber fleece heated primary water is passed at 95 to 99 ° C which evaporate on the absorber surface. The primary water will initially be used as cooling water. The moist air rises and cools the interior of the hollow column to the appropriate capacitors from. There, the excess moisture condenses and falls out as pure water. The condensate is collected and discharged at the bottom of a vessel. As a cooling medium is used in the first cooling circuit, the primary water, which is thereby pre-heated and is supplied from the recovering part of the heat of condensation. For further cooling a second cooling circuit, which is fed from an external reservoir is used. The cold moist air ( 55 ° C) is lowered and enters the bottom of the hollow column back to the heated portion of the sun's rays. There, the air warms up and can absorb water vapor, so a new cycle begins again. Due to the imbalance between hot and cold air masses in Evaporationsraum air masses in the condensation chamber an independent air mass circulation builds up in the system. The different areas of the equipment must be thermally insulated against each other high. The evaporation process leads despite solar radiation to a significant cooling of the primary water. This accumulates in the brine tank. There, a layering of different concentrations takes place. Brine sump has an overflow which discharges the highest concentrations by means of a siphon. In the absorber fleece, no deposits due to high salt concentrations should form with a concomitant fall below the solubility limit. The primary water flow should be adjusted accordingly high enough. The circulation of the cooling and brine water and the supply of primary water be ensured by pumps. This can be provided 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 in the interior of the column.

Advantages: The system is decentralized applications. The performance values of a function types are 17 to 19 l / m² d Therefore, a very good heat recovery is achieved because the energy available through the solar irradiation would be for a third to half of the maximum reached distillate quantities have been sufficient. The income is therefore higher than previously known systems. So you can save or enhance the yield for the same collector substantial collector.

Cons: With the proposed embodiment, a diameter of 1.4 m and a height of 7 m is no longer easy to transport this system and to handle. It is a complex control and regulation unit is necessary, which are vulnerable under the conditions of southern developing countries. Compared to the simple Solardestille a higher expenditure on equipment is necessary. Therefore, higher investments are expected, but these can be reduced by the higher yield and lower required collector area. The necessary pump and control systems, a power supply is necessary. It shall be verified that the desired circulation flow is formed strong enough. Thus, it is critical to see if no condensate forms on the glass, although the road is the shortest, the glass is wind cooled, and thus a high temperature difference exists toward the glass.

MEH- method - Thermal Desalination using low-temperature heat, for example from solar panels

Another thermal process for decentralized desalination in the small and mittelskaligen production range up to 50,000 liters a day is the Multi-Effect-Humidification/Dehumidification-Verfahren ( MEH ) dar. systems based on the MEH- process based on thermal energy supply from low-temperature sources ( z. example, solar panels ). The heat is supplied to a completed module desalination in which the natural water cycle is modeled with evaporation and condensation in an efficient manner. Sufficiently high evaporation and condensation surfaces, with respect to the energy conversion, permit a widest possible recovery of the heat of evaporation in the condenser. In this manner, production rates of more than 25 l / m² per day can be obtained with a solar powered system. Equally, however, can also be the heat generated by other processes or by diesel generators are fed into the process. This procedure was performed at the Bavarian Center for Applied Energy Research for practical application.

A developed according to this principle functioning, particularly space -saving, portable prototype machine builders of the Ruhr- University Bochum (RUB ). By using air as the heat transport medium, the plant can be operated with very low temperatures. In the complex heated sea water trickles through an evaporative humidifier that heats incoming air and additionally enriched with water vapor from the sea water. Here, a production rate of about 20 liters per m² collector area per day results (based on ten hours of sun a day). This research were funded by the project Soldes by the EU. In a likewise supported by the EU within the project Soldes multistage system with alternately connected in series air collectors and Verdunstungsbefeuchtern is only the circulating air, but not the sole, heated by solar panels. The air is gradually heated and humidified.

The ZAE Bayern planned and built in 2000 a plant for solar desalination in Oman. The system consists of a field of 40 m² vacuum flat solar panels, an insulated steel tank (3.2 m³ ) and a thermally driven desalination storm. The daily output is about 800 liters. The distillation process is carried out at ambient pressure. This heated seawater is distributed over a large-scale evaporator. A convection which is driven by density differences and moisture, moist air to be transported is arranged in the module polycarbonate sheets of polypropylene. These serve as condensation surfaces and are traversed by cold seawater. By the condensation of the moist air at the plate surface, the sea water is heated to 75 ° C.

Advantages: The geometric arrangement of evaporation and condensation surfaces allow mass and heat flow, which may otherwise be realized only through a complex multi-chamber system. Therefore, heat recovery is achieved, the lower the thermal energy requirement m³ of desalination to about 100 kWh / m³ of distillate over the enthalpy of vaporization of water at 690 kW /. The heat recovery is thus only slightly below the maintenance and technology complex vacuum evaporation systems. Consequently, investment for decentralized applications offers in structurally weak areas.

Cons: Compared to the simple Solardestille a higher expenditure on equipment is necessary. Therefore, higher investments are expected, but these can be reduced by the higher yield and lower required collector area. Wherein the heat of condensation is recovered only partially. In addition, pumps are required for water circulation.

The Fraunhofer Institute for Solar Energy Systems has used this principle in the project " SODESA ". This experimental system has a 56 m² collector field. In the project, collectors have been developed in which the hot sea water could flow through directly through the absorber. There could therefore be no copper absorber, since this material corrodes immediately. It panels have been developed in which the absorber consists of glass.

Multi-effect distiller

The multi-effect distiller operates according to the principle of multi-stage, in which the condensation heat is used as an energy source for the next following stage. The incident solar radiation heats the absorber plate lying under a sheet of glass. On the back of the sheet a Viskosetuch is applied, which is loaded with salt water. A portion of the salt water evaporates, condenses on the cooler underlying plate and returns the heat of condensation to the following ex. The evaluations of the experimental results for a four-stage prototype revealed high recovery factors in the individual stages (about 70 %). The distillate yields maximum achieved, however, are only about 50 % higher than the results of the simple Solardestille. Attributable to the high heat losses in the first absorber stage, are implemented in only about 20% of the incident radiation into useful energy. Improvements to the system by a double glass cover or a transparent thermal insulation and / or by the use of a selectively coated absorber can therefore expect further increases in yield. Has proven good humidification system using the viscose. Dripping the brine and mixing of the salt water to the distillate was not observed. Salinity tests showed a very good distillate quality. However, must be taken of the high cost of operation and maintenance of the plant.

Aquadestil

The sea water or brackish water ( 15-25 ° C) flows through the capacitors from the bottom up and heats up. At the upper end of the cooling water from the condensers flows to the evaporation surface ( perforated studs areas). There is a radiator, which is traversed by solar heated thermal oil above the evaporation surface. The necessary panels are located outside the plant. The water flows over the surface, heats up and evaporates there. The moist air rises, and the distillate condensed in the condensers. The condensate is drained off, is collected in a gutter, and out from the plant. The excess brine flows back into the sea. According to the manufacturer resulting in a 1.5 kW solar collector a distillate output of 12-18 l / h Thus, the distillate price is 3.9 to 5.7 € / m³.

A further uses the resulting steam to preheat. Subsequently, the evaporation takes place in stages evaporators.

Advantages: The system is simple and compact and is therefore suitable for decentralized use. No provision is required for this property. To produce large quantities of drinking water, the modules can be stacked to reduce the space requirements.

Cons: Compared to the simple Solardestille a higher expenditure on equipment is necessary. Therefore, higher investment costs are expected, but these can be reduced by the higher yield and lower required collector area. Wherein the heat of condensation is recovered only partially. In addition, pumps are required for water circulation.

Method with direct condensate heat recovery

In this case, the evaporation and condensation takes place in multiple stages. In the chambers of the single stage air circulates by natural convection. Between the individual steps, no air exchange takes place. The method is suitable for small systems, because there is no fan needed. On a pump can also be waived if the raw water taken from an elevated tank and a Thermosyphonkollektor used. For a system with 2 m² collector a theoretical production capacity of 25 l / m² d at an annual insolation of 1750 kWh / m² was calculated. Previously, this could not be confirmed experimentally. (Application at FH Aachen, Section 1.3.9 )

Method with indirect heat recovery condensate

In order to transfer a large part of the heat of condensation in sensible heat of the heat transfer medium, relatively large mass flows must be generated which require corresponding pump performance. Despite these facts and the issues discussed in the previous section energetic disadvantages compared to a direct transfer of the ... Since the evaporative and condenser are not unit with direct thermal contact, many design options for the design of the two contact elements are possible. To achieve the largest possible surface in the humidifier, a variety of materials such as wood slats ( Nawayseh et al. 1997), thorn bushes ( Graef 1998) or polypropylene mats ( Fuerteventura) can be used. With a collector area of 47.2 m² give Müller- Holst and Engelhardt ( 1999) daily output of 11.7 to up to 18 l / (m day) for this system. In order to achieve these benefits through by Sole flat plate collectors and a thermal energy storage have been developed specifically for the method used. The memory provides a system operating 24 hours. Production costs are estimated at about 11 € / m³ distillate.

Multistage Desalination (FH Aachen )

At Solar Institute Jülich optimized desalination plant has been developed which will provide a multiple conventional solar stills with the same energy input. With the development of an optimized prototype system and a dynamic computing model as a dimensioning aid for thermal seawater and brackish water desalination plants created the precondition for marketing. By external heat supply salt water is heated in the lower stage to about 95 ° C and then evaporated. The water vapor in the ascending moist air condenses on the underside of the overlying evaporator stage. The condensate passes along the slope in a collecting channel and from there into a collection container. The condensation released enthalpy of vaporization (ie = 2250 kJ / kg) is released to the overlying each stage and thus heated 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 in the next steps for evaporation, desalination rate of this type of system compared to simple stills is higher by a multiple. Thus, with this method, the recovery of the heat of condensation in the next higher level, for example, in a four-stage system about three times the amount of distillate are obtained at the same energy input. Already optimizations performed by predicting an energy consumption 180 kWh / m³ distillate at a five- distillery. This corresponds to less than a quarter of the energy requirement of a simple still. Many factors affect the evaporation and condensation, since it is coupled diffusion and convective. Notably is evaporating and condensing temperature and geometrical factors (distance between the surfaces, angle of inclination of the condensing surface ) affect.

Advantages: To drive the system different heat sources can be used, such as via collectors coupled solar energy or waste heat from diesel generators and other mechanical equipment. Due to the comparatively low investment costs a decentralized application is possible. Compared to the simple Solardestille the yield can substantially increased and hence the space requirements are reduced. At an appropriate arrangement, a circulating pump is not required.

Solar evaporation systems

Multiple effect evaporation ( MED)

El- Nashar et al. (1987 ) provided results from the one-year test phase of an evacuated tube collectors operated with MED plant in Abu Dhabi in the United Arab Emirates. The production rate was 100 m³ / day with a collector area of 1860 m². In order for this system delivered an average of 54 liters of distillate per day and m² of collector area. Milow and Zarza (1997 ) report on the multi-year operating experience with a 14 -stage MED pilot plant in Almeria, Spain. This plant is operated by parabolic trough collectors in combination with an absorption heat pump and generates about 72 m³ / day. The water production costs come to 2.5 € / m³ for the location of Southern Spain, when 45 percent of the required process heat is provided by conventional energy sources. For medium-sized solar sea water desalination plants, a combination of solar thermal collectors is viewed with a thermal memory as an economic solution. For such plants with a daily capacity of 270 m³ a distillate water production rate of 2-2.5 € / m³ is specified. The output results 7.8 l / m² distillate.

Multi-stage flash evaporation (MSF )

There were in Kuwait in the 80s experiments with a powered by parabolic trough collectors twelve -stage MSF plant for solar desalination of seawater. With a collector area of 220 m², the plant produced at maximum irradiance about 300 l / h A 7 m³ large tank acts as a thermal reservoir and allowed a 24 -hour operation. AQUASOL Project: The project was implemented by AQUASOL ZAE Bayern in cooperation with the firm Moik and the Technical University of Munich. The operating principle of AQUASOL method relies on only a single-stage flash evaporation with subsequent humidification. Water is to just heated in a pressure circuit below the respective boiling point and then decompressed to ambient pressure. As suitable operating parameters, the heating at 120 ° C was determined at an absolute pressure of 2 bar. In a solar powered system 6 m² vacuum tube collectors STIEBEL ELTRON SOL 200 A are required. The standard module heads of collectors were rebuilt and replaced in seawater resistant steel 1.4539. The solar panels were fitted with a single-axis tracker.

Advantages: The system is decentralized in use due to their size and can be operated by solar energy.

Cons: Since the plant is only operated in one stage, the system efficiency is too low. Here, the energy consumption due to the evaporation and the high temperatures is very large. In addition, the technical effort required by the system components, such as expansion chamber with pressure cycle and pressure vessels and a sea water resistant circulation pump very large. Therefore, the goal is to build a very low-maintenance plant, not reached. The system can not be maintained independently of the local population. Therefore, the machine is very expensive. Because of the many disadvantages of the ZAE Bayern has decided not to pursue this technology and instead rely on a vaporizer column with packing to increase the evaporation surface.

Markopulos Patent

It is an EU-funded project on the basis of Markopulus patent. This, with the help of solar thermal collectors and PV cells to gain the goal by evaporation of sea water into drinking water: It consists of a vacuum evaporation vessel and a lying therein condensing vessel, which is under normal pressure and is immersed in the liquid phase of the evaporation vessel. A vacuum pump conveys the steam from the evaporation vessel to the condensation vessel. There, the vapor condenses on the flow-through of seawater heat exchanger and returns the energy so as to to be evaporated seawater. The operation of the system under reduced pressure ( 50 mbar) allows the use of low-temperature heat ( 33 ° C), which reduces heat losses to the environment. The energy balance of this will be the patent accordingly more favorable than in previous systems.

The heating of the evaporation vessel is provided by a solar collector, which thus compensates for the heat loss of the system. This is done by a separate solar circuit with a fluid heat carrier medium which flows through the solar collector, the heating device in the interior of the evaporator and the circulating pump. The electrical components of the system, such as pumps, valves and control system should be powered by a PV module. The entire system is housed in a container and is very easy to carry and at the application site put up. After Markopulus patent enables an exemplary embodiment of the evaporation vessel with an area of 1.2 × 2 m a drinking water production capacity of 50 m³ / h It is to prevail, a negative pressure of 50 mbar. This enables an evaporation temperature of 33 ° C. In the condenser to a temperature of 70 ° C is reached.

Benefits: The plant is compact, easy to transport and can therefore be used locally. A sustainable and self-sufficient energy supply is ensured by a controlled use of renewable energies (solar, wind ). And waste heat can be used.

Cons: The specified drinking water production of 50 m³ / h appears doubtful. This would require 1.25 million m³ per hour steam are aspirated with a vacuum pump. This does not appear feasible. The energy required to generate the negative pressure is enormous and represents the main energy demand of the plant dar. To evaporate this amount of water for 30 MW of power would be required, similar to a small power plant. However, given the plant is too small with too small a heat transfer surface. On the other hand, a steam production of 50 m³ / h with such an apparatus seems feasible. However, the heat exchanger surface is also available for this purpose is too small, since a heat transfer can take place only on the outer surfaces of the capacitor and at the apertures. The generation of a reduced pressure of 50 mbar is energy consuming and therefore to draw also in doubt.

Scheffler seawater desalination plant

The aim of the development is a low producible, easy -to-use system for the water needs of a family to which a small village. Salt water should be supplied without complex pretreatment directly the plant. By the time resulting deposits and encrustations are located easily remove. The plant will consist only of simple components ( no pressure vessels, etc.). The energy source is the proven 2 -m ² (8 m² for larger systems ) are provided Scheffler reflectors that bring the salted water to a boil. Between August and November 2000, a multi-stage prototype has already been built. In the middle of cooking salt water. The resulting pure water vapor condenses on a cylinder. The released heat of condensation again heated salt water that seeps down through a tissue on the other side of the cylinder. When heated it emits also pure water vapor, which then condenses on the next cylinder. The use of four levels of condensation increases the yield of pure water by a factor of 3 with respect to only one step. The principle is not new, but has been here through the use of a Scheffler mirror, which can provide heat at about 100 ° C with very good efficiency, implemented in a very compact and material-saving.

Advantages: A distributed application is possible. It uses the proven Scheffler reflectors. Instead of the concentrated energy for cooking but is used for seawater desalination.

Cons: In the prototype, problems occurred in the operation of the plant. In addition, some materials were unsuitable. It is necessary to seek other materials. The system should be tested in practice. Instead of the cylindrical surfaces of a tent- like structure should be used for resistant films.

Applications

In many areas where drinking water is obtained from sea water or sea water desalination is attributed to a large potential (developing countries ), a combination of desalination plants with renewable energy sources such as wind and solar energy offers. So runs on Tenerife since 1997, a desalination plant Enercon, which is powered by wind energy.

There are considerations to take advantage of the pressure at the bottom of case, wind power plants to produce with the help of reverse osmosis drinking water. The required pressure of about 70 bar would be achieved if economic ( and technically feasible ) dimensions of the drop tower of 1200 m height and 400 m in diameter. Particularly the coastal areas of North Africa and the Gulf region would be suitable for such projects.

Solar and freely scalable decentralized drinking water treatment systems that can produce drinking water from almost any raw water, are ideal for use not only in developing countries but in almost every country where there is enough sun sufficient " raw water " is present. Such plants operate according to the " RSD Rosendahl system " for many years maintenance free, inter alia, in Puerto Rico and in many other countries.

A pioneer in the field of seawater desalination was the British physician James Lind, who discovered in 1758 that could win potable water from the vapor of heated sea water that tasted like rainwater.

Solar thermal collector#Flat plate collectors Energy development Energy tower (downdraft)

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