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Note: These are some basic methods to of water desalination however technology makes it even batter by a hybrid approach.

Eco sustainable village foundation is studying these articles.

Solar Desalination

Presented at the conference on Desalination Strategies in South Mediterranean Countries, Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean, sponsored by the European Desalination

Society and Ecole Nationale d’Ingenieurs de Tunis, September 11–13, 2000, Jerba, Tunisia. Desalination 137 (2001) 23–29

Solar thermal desalination system with heat recovery

Klemens Schwarzer*, Maria Eugênia Vieira, Christian Faber, Christoph Müller, Fachhochschule Aachen, Solar Institute Jülich, Heinrich-Mussmann-Str. 5, D-52428 Germany

Received 27 July 2000; accepted 17 August 2000

Abstract

This work presents the energy and mass balance equations, the numerical simulation results for the production rate, and the experimental laboratory water tests for a thermal desalination unit with a heat recovery system. The system components are a solar collector and a desalination tower, although the system can be operated with other energy sources. The desalination tower is made of six stages and a water circulation system through the stages to avoid salt concentration. The numerical results calculated using ambient data show that the production rate can reach 25 L/m2/d, which is by a factor of five times greater than the rate of a basin-type solar desalination unit. The water (polluted seawater and well water) laboratory tests results show that the desalination process eliminated the Coliform group bacteria and reduced the salt concentration to very low levels.

Keywords: Desalination; Solar; Heat recovery

1. Introduction

The shortage of drinkable water in many areas of the world is an old problem. In addition, these areas also have a limited supply of conventional energy, although some have a great potential in solar energy. Solar thermal systems that produce potable water from salty water have been studied for quite some years, and the use of solar energy to produce potable water reverts back to ancient Egypt. Various solar thermal systems have been presented in the literature [1]. The most studied model, the still-type distiller, has the advantage of low installation cost, but two important disadvantages: low efficiency and problems associated with the accumulation of salt at the

basin. A new solar thermal desalination system with heat recovery is presented in Fig. 1. Because of K. Schwarzer et al. / Desalination 137 (2001) 23–29 24

(1) Fig. 1. Photo of the desalination system powered by solar collectors. the heat recovery process, this unit can reach a higher thermal performance than the conventional still-type solar distiller. Also, there is a continuous water flow through the stages of the unit avoiding salt accumulation. The disadvantage is the higher installation cost when compared to the still-type unit. The two basic system components are a flat plate collector and a desalination tower. A copper piping system connects these two components.

As solar radiation hits the collector, the working fluid (an oil) is heated up and moves, by natural convection, to the highest point of the system where a heat exchanger is located. The oil flow releases its sensible heat to the salty water on the other side of the equipment. The heat exchanger works as the first stage of the desalination unit. As the oil flow cools, it returns to the solar collector, completing a thermal siphon circuit. The warmed water in the heat exchanger transfers energy by evaporation, radiation, and convection with the second stage of the desalination unit and loses also some energy to the environment. The vapor from the first stage condenses at the bottom wall of the second stage, releasing its latent heat. The condensed water moves through a channel to be collected outside the unit. The energy that is transferred to the second stage warms up the salty water in this stage and the heat recovery process is repeated in the next stages.

The purpose of this work is to present the energy balance equations for the thermal desalination processes, the numerical simulation results for the clean water production, and some experimental results with a unit operating with one stage. The system is powered by solar energy and installed on the northeast coast of Brazil. The water chemical test results for two types of water (polluted seawater and well water) before and after the desalination process indicate the good quality of the water produced. 2. Energy balance equations In the first stage of the solar desalination unit, heated oil from the solar collector enters a heat exchanger and transfers its sensible energy to the salty water mixture on the other side before returning to the solar collector. The salty water increases its temperature and exchanges heat with the second stage by evaporation, convection, and radiation. Some energy is lost to the environment by conduction through the vertical walls and some vapor leakage occurs. Sensible energy is also lost to the environment due to the interior circulation, not only to avoid salt accumulation in the first stage, but also to keep the water level in each stage constant. This latest energy loss can, however, be recovered in other process.

Fig. 2 shows a schematic drawing of these energy transfers in the first stage. The energy balance equations for the oil and the salty water in the heat exchangers. Directions of the energy transfer in the heat exchanger: first stage.

For the other stages, the energy balance equation has the same form as Eq. (2), except that the energy that enters the system is by condensation, convection, and radiation from the stage below, instead of convection from the oil flow. The mass balance equation represents the steady-state condition in each stage. It states that the amount of water in each tray remains constant, as the amount of water that enters equals the amount that leaves the stage: In order to model the performance of the solar thermal desalination unit, a good knowledge of the rate of heat transfer from the water surface at each stage is necessary. Although solar thermal desalination systems have been published in the iterature, two important limitations are present.

First, there is some confusion about the convection and evaporation heat transfer coefficient for solar desalination applications. Even though the convection heat transfer rate is much smaller than the evaporation rate, this convection coefficient has been used to estimate the evaporation heat transfer coefficient. Some papers have been published in the literature [2–5]. Each of these studies seems to be appropriate for its experimental data at some temperature range, particularly in the interval from 50°C to 70°C where the solar tank distiller operates. Second, the experimental results for the system presented in Fig. 1 show that the water temperature in the first stages can reach values greater than 90°C and the correlation expressions available are not adequate to predict the evaporated mass rate. A one-stage desalination unit was built to study the evaporation and convection heat transfer processes at temperatures ranging from 50°C to boiling. This temperature range represents the practical situation found in the unit of Fig. 1. The evaporation and condensation temperatures are controlled using external electrical heat sources and a digital scale measures the water rate leaving the unit. The upper plate is tilted at 14° with the horizontal plane. The experimental data are used to find the constants to be used in the Colburn analogy to estimate the rate of water produced by the desalination process. The number of measured data points used in the correlation expressions is 379.

3. Experimental results and discussion Using the energy and mass balance equations presented in Eqs. (1) through (3), a numerical simulation code was developed and tested using the global solar energy and ambient temperature data measured at the site where the unit is being used. Fig. 3. Measured global solar radiation and ambient temperature data used in the simulation.

Fig. 4. Clean water production in liters by the desalination unit in 15-min intervals installed. The horizontal dimensions of each stage tray are 0.8m×0.8m, and the distance between stages is 0.1m. Other data used in the simulation are: one solar collector with an area of 2m2 with an average thermal efficiency of 0.5; the lateral and cover walls are insulated with 0.07m mineral wool layers; the amount of water in each stage is 25 L; and six stages are used. The last stage does not produce clean water and is used as the salty water inlet to the unit.

Fig. 5. Accumulated water production rate in one day, using a 2m2 solar collector area.

Table 1

Results of the water laboratory tests before and after the desalination process Item measured Polluted seawater Well water Before After

	Total coliform group bacteria, NMP/100 mL >2419.20 * 3.1 *
	Fecal coliform group bacteria, NMP/100 mL 2419.17 * 2.05 *
	pH, 25°C 7.95 4.72 5.74 5.71
	Color, uH 36 4 2 1
	Turbidity, NTU 7.2 0.48 0.73 0.59
	Alkalinity, mg (CaCO3/L) 114 * 4 4
	Total hardness, mg (CaCO3/L) 4400 4 170 6
	Chloride content, mg (Cl-/L) 16,500 5 235 9
	Conductivity, µS/cm 53,400 56.3 1072 36
	Too small to be measured.

shows the measured global solar radiation and ambient temperature used in the numerical simulation. The integrated value of the global solar radiation for this day is 4.8 kWh/m2. Fig. 4 shows the amount of water produced in liters at the end of each 15-min interval. This water production represents the sum of all stages, that is, the total water amount of water collected at the end of the time interval. From the graph it is seen that the highest amount of clean water produced by the unit per time interval occurs at 2:30 pm for this day.

Fig. 5 shows the accumulated clean water production vs. the hour of the day. Each value represents the amount of clean water that has been produced by all stages from the start of the day up to the end of the specific time interval. It is seen that 50.2 L are collected at the end of the day. This represents a rate of 25L/m2 for a value of 4.8 kWh/m2/d of solar radiation, according to a rate of 5.2 L/kWh m2. To study the quality of the water produced by the desalination unit, the first stage is tested and the two different types of water are analyzed before and after the process. Table 1 presents the conditions of the water before and after the desalination process.

4. Conclusions

The solar desalination unit can reach very good thermal performance, as shown by the numerical simulation, when well built. The results show that it can reach a water production rate of 25L/m2/d for a value of 4.8kWh/m2/d of solar radiation. This represents a rate of 5.2 L/kWh m2 and a factor of five to six times greater than the tank-type distiller. The problem of salt concentration has been taken into consideration through the continuous water flow through the unit. The laboratory tests presented in Table 1 for very polluted seawater and for well water show that the desalination process eliminated the total coliform group bacteria and the coliform group fecal bacteria to such low levels that they could not be detected by the instrument. The physical– chemical analysis also shows a significant reduction in all items tested.

5. Symbols

	A — Area, m2
	cp — Specific heat at constant pressure,
	J/kg K
	h — Heat transfer coefficient, W/m2 K
	k — Overall heat transfer coefficient,
	W/m2 K
	M — Mass, kg
	m — Mass rate, kg/s
	Q — Rate of energy transfer, W
	T — Average temperature, K
	T — Temperature, K
	t — Time, s
	Greek
	— Emissivity
	— Thermal conductivity
	— Stefan-Boltzmann constant, 5.68×
	1008 W/m4 K
	Subscripts
	c — Convection
	circ — Circulation
	e — Evaporation
	hx — Heat exchanger
	in — Inlet
	k — Conduction
	l — Loss
	leak — Leakage
	o — Oil
	out — Outlet
	r — Radiation
	w — Water

References

[1] G.O.G. Lof, J.A. Elbling and J.W. Bloemer, AIChe J.,

7 (1961) 641.

[2] J.A. Clark, Solar Energy, 44 (1960) 43.

[3] R.V. Dunkle, Intl. Heat Transfer Conf., 5 (1961) 895.

[4] R.S. Adhikari, A. Kumar and A. Kumar, Intl. Solar

Energy Res., 14 (1990) 737.

[5] A.T. Shawaqfeh and M.M. Farid, Solar Energy, 55

(1995) 527.

REVERSE OSMOSIS AND ELECTRO-DEIONIZATION (EDI)

THE ULTRAPURE SOLUTION

Introduction

One the first and most popular applications of Reverse Osmosis (R.O.) was to use R.O. as pre-treatment for deionization (D.I.). R.O. proved to be a cost-effective method of producing ultrapure water by reducing the frequency of regeneration of the ion exchange resin in the D.I. system. With the introduction of Electro-Deionization (EDI) as an alternative to conventional D.I., the system configurations available to the ultrapure water system provider have expanded, with many possibilities for combining R.O. and EDI to provide the most reliable, efficient and cost-effective design. Applications for RO/EDI EDI can be used anywhere in general industry where deionized water is advantageous. Ultrapure water is used for microelectronic and semiconductor production, for biomedical and laboratory use, by pharmaceutical compounders, as pretreatment for stills, for boiler water during power generation, and in the food and beverage industry. A typical EDI module will produce permeate water of approximately 15-17 mega-Ohm (MW) resistivity when installed and operated per the manufacturers recommendations. Resistivity in the this range is far better than most pharmaceutical applications, generally adequate for electronics and other traditional D.I. applications, and slightly below the requirements for the 18.2 resistivity required for some semiconductor applications.

The R.O. and D.I. in the Conventional Ultrapure Water Process

As indicated in Figure 1 below, the conventional ultrapure water processes have included reverse osmosis as the primary method to reduce total dissolved solids (TDS) and deionization as the secondary method to remove TDS to “ultrapure” levels. In cases where the feed water TDS is high or a reduction in the frequency of the D.I. regeneration is desired, a double-pass R.O. system can be applied as the primary TDS-reducing component of the overall ultrapure water system. The D.I. technology is often applied in two steps as well, with a primary D.I. system satisfactory for most applications and a polishing system when 18.2 MW water is the requirement. R.O. / EDI- The Ultrapure Solution Page 2

Figure 1: Conventional Ultrapure Water Process

A Brief Introduction to EDI

EDI is a water treatment process that combines two well-established water purification technologies electro-dialysis and ion-exchange resin deionization, to remove ions from an aqueous stream. EDI equipment consists of alternating cation- and anion-permeable membranes and spacers that form compartments. Alternate compartments are filled with mixed ion-exchange resins. The result is a repeating element called a cell pair, as shown

in Figure 2.

EDI Product Concentrate Inlet Concentrate Outlet Concentrate Outlet Resin

	A = Anion-Permeable Membrane
	C = Cation-Permeable Membrane
	R.O. / EDI- The Ultrapure Solution

Feed water enters all the compartments in parallel. The imposition of a direct electrical current (DC) serves to motivate ions to move from the compartments containing the resins into the adjoining sections. The charge selective displacement of ions by the transverse DC electric field depletes them from the ion-exchange-containing compartments, which become diluting regions. The transfer of ions across the appropriately charged boundary membranes into neighboring compartments transforms these compartments into concentrating sections. Water does not flow through the membranes. It is the ions that make this passage, directionally motivated by the direct current.

The typical EDI module is constructed using three separate streams of water, each for a specific purpose. The feed/product stream provides the entry of feed water to the EDI module and the exit of EDI permeate from the EDI module. Recovery of 90-95% is typical, leaving 5-10% of the feed water to the other two streams. The concentrate stream provides the means to collect the ions which have exited the feed stream, and remove them from the EDI module. The electrolyte stream provides a continuous flow of water across the EDI electrodes to prevent accumulation of ions at the electrodes. In most applications, the feed water will be split into the three streams. However, in some applications and with some EDI modules, the concentrate stream will be fed with a higher concentration of water than the EDI feed water to improve ion transport efficiency. In these applications, the concentrate solution is derived from a separate brine dosing system and/or by recycling the EDI concentrate back into the EDI concentrate feed.

EDI systems are configured by skid-mounting one or more EDI modules with interconnecting piping, valves, instrumentation, and a DC power supply. The capacity of EDI systems range from less than 1 GPM of product to hundreds of gallons-per-minute, completely scalable as a function of the number of EDI modules installed on the skid.

The R.O. / EDI Ultrapure Water Process

Electro-deionization can be substituted for the primary D.I. system step of the conventional process. EDI is not a viable alternative to reverse osmosis for the primary TDS-reduction step unless the feed water conditions provide feed conductivity of approximately 4 – 30 µs/cm. As a replacement for DI, an EDI system offers a number of advantages, the most significant being the elimination of chemical regeneration.

No Chemical Regeneration: EDI does not require chemical regeneration of the resin due to a special feature of this process that causes the water molecules to “split” into hydroxyl(OH-) or hydrogen (H+) atoms. The local production if OH- and H+ within the mixed ion-exchange (IX) resins results in the constant regeneration of the resins without the addition of chemicals. Eliminating the need for chemical regeneration R.O. / EDI- The Ultrapure Solution reduces operating costs, simplifies the control process, and makes the system more easily expandable.

Continous Operation: In addition to the elimination of the need for hazardous chemicals, and RO/EDI system may be operated continuously because there is no down-time needed for regeneration.

No Resin Fines: Mixed beds using large quantities of ion-exchange (IX) resins tend to deteriorate over time and generate resin fines. Resin traps are not required with EDI.

In Figure 3, a single-pass R.O. system is identified as the primary method of TDS removal with EDI as the secondary method. In this configuration, feed waters with TDS of up to 500 mg/L can effectively purified to yield EDI permeate of approximately 15-17 MW resistivity. If higher resistivity is required, a polishing system consisting of an additional EDI system or a mixed-bed D.I. system would be included. In most applications, mixed-bed D.I. is the preferred approach for the polishing system, for the following reasons:

With EDI permeate as the feed water to the polishing D.I. system, the need for regeneration of the polishing D.I. resin is practically eliminated.

The efficiency of rejection of the EDI module declines when the feed conductivity is outside the specified range of 4 –30 µs/cm. In the polishing application with feed

water resistivity of 15 –17 MW, there is insufficient conductivity in the EDI feed and concentrate streams to enable efficient ion transport.

Figure 3: RO/EDI Ultrapure Water Process

Overall system recovery can be increased by recycle of the EDI concentrate to the R.O. feed. Although with typical EDI recovery of 90-95% the TDS of the EDI concentrate can be 10-20 times the EDI feed TDS, the EDI concentrate is still likely to have a diluting effect on the R.O. system feed water. R.O. / EDI- The Ultrapure Solution  Use of Double-Pass R.O. with EDI

The decision to use single-pass or double-pass R.O. as pre-treatment for EDI is driven by the need to keep the EDI feed conductivity between 4 – 30 µs/cm. In most applications, double-pass R.O. will yield permeate of less than 4 µs/cm (typically about 1 mg/L TDS), therefore double-pass R.O. may not necessarily be an improvement over single-pass R.O. as a pre-treatment for EDI. There are situations when double-pass R.O. will be the preferred method for primary TDS reduction when EDI is the secondary TDS reducing method:

High Feed TDS: If the feed TDS to the R.O. system is in excess of 1,000 mg/L, most single-pass R.O systems will yield permeate with TDS towards the high end or outside the 4 –30 µs/cm range for EDI feed. Although the 4 –30 µs/cm EDI feed range is not an absolute requirement, but a guideline, with feed TDS over 1,000 mg/L, double-pass R.O. should be considered.

High Recovery: The overall system recovery can often be increased by blending some 1-pass permeate water with some 2-pass R.O. permeate water to yield a blended permeate with TDS higher than the pass-2 permeate, but lower than the pass 1 permeate. As is the case with the single pass R.O., the EDI concentrate can be recycled to the feed stream of the 1^st pass R.O. system to increase overall system recovery.

Summery

An EDI/RO system, when properly designed and maintained, offers many advantages over conventional DI-based ultra pure water treatment systems. Water treatment solution providers who offer EDI/RO systems can provide their clients with the best solution possible for their situation. Table 1 summarizes the water treatment technologies recommended depending on the feed water conditions and the product water requirements.

Table 1: Recommended Water Treatment Technologies

PRODUCT WATER REQUIREMENTS

	INDUSTRIAL HIGH-PURITY
	FEED WATER PROCESS PROCESS ULTRAPURE DEIONIZED
	CONDITIONS (< 10 ppm TDS) (< 1 ppm TDS) (1-17 MW) (18.2 MW)
	< 30 µs/cm ------ 1-Pass R.O. EDI 1-Pass RO --> EDI --> DI
	15 - 1000 ppm TDS 1-Pass R.O. 2-Pass R.O. 1-Pass R.O. --> EDI 1-Pass RO --> EDI --> DI
	over 1000 ppm TDS 2-Pass R.O. 2-Pass R.O. 2-Pass R.O. --> EDI 2-Pass RO --> EDI --> DI
	R.O. / EDI- The Ultrapure Solution

The key points to remember when considering R.O. and EDI are:

R.O and EDI are complementary: EDI performs best when fed with R.O. permeate and in most cases requires R.O. or a similar technology to pre-treat the EDI feed water. EDI is not a replacement for double-pass R.O.: Single-pass and double-pass R.O. are two alternative methods of pre-treatment for EDI, but it is likely that some type of R.O. system will be needed.

Post-treatment may be required after an R.O./EDI system: EDI will not provide suitable

final microbiological protection required for pharmaceutical applications or 18.2 MW resistivity needed for some semiconductor applications. In these cases, additional equipment such as a polishing D.I., sub-micron filter, ultraviolet sterilizer, or other similar devices may be required.

The plants shown introduce the Rosendahl-type flat collectors. RSC Rosendahl System S.L. Canarias, Gran Canaria is the only and exclusive licensor world wide.
 

yellow = sun rays, dark blue = crude water, light blue = condensate, olive = brine

The picture at the right side shows a flat collector which spends drinking water at a farm located on the island Gran Canaria. The crude water is taken out of a small lake. The unit works for several years very reliable and economically. Prior to the installation the farmers bought the drinking water in plastic canisters. The price of the canister water was approximately 100 € per cubic metre. A calculation after some years of operation led to a price of 4.06 € per cubic metre using Rosendahl’s solar water treatment with flat collectors.

Once large plants are installed the installation costs decrease at an enormous rate. As a major result the water price drops as well. Most of it are the costs of financing due to the fact that operation costs are practically not existing.

Examine the picture at the left side to see two flat collectors at the island of Puerto Rico. Almost all families distill their water with electric distillation apparatus consumpting approximately 20 kWh of electrical energy per day. The crude water is polluted water which is stored in the stainless steel tanks visible at the rear. The tank size should be calculated and planned in advance to avoid lacks of water if the supply is interrupted.

Once there is no fresh water available for a longer period one can operate the distillation plant with ocean water or brackish water without problems. The collectors are constructed to work with every type of crude water and resist long stops without damages. Operation temperatures of 80°C and 140°C during stops are normal.

Sometimes it is not possible to mount the collectors on roof tops as it can be seen on the photograph at the right side which is taken at the Philippines. In such cases they can be mounted in open areas and should be protected of children or animals. The picture shows the installation of the collector. The crude water comes from an ocean water well at the beach underneath. This ensures that no fish nor animals or plants of the ocean can reach the pre-filtering unit.

If there is lack of space, the collectors can be mounted as well on scaffoldings to form roofs, shacks or garages and have a double use, for example to spend shadow, protect from rain, collect rain, etc.

Bigger collector fields can be installed parallel as shown on the left picture. In collector groups of up to 64 pieces they use a common control unit and crude water intake to save costs. Collector fields deliver water of highest quality just like single plants. The water can not only be used as drinking water but also for technical purposes (battery water, cosmetic or medical use) or to water plants.

The outstanding water quality was honoured 1999 by the American Water Quality Association WQA with the gold medal. No other known system disintegrates salts and minerals better than the Rosendahl flat collectors. A copy of the certificate can be mailed to you upon request.
 

Our water purification unit F6-mini (80 x 60 cms = 0,48m²) is quite simple but very reliable though. Depending on climate and radiation it produces 3.14 litres of pure water per day. The supply of salt, brackish or dirt water is regulated by a droplet control which can be obtained at every pharmacy. The other option is the operation fully automated by electronic controls. The crude water should be filtered to ensure proper operation.
Once you know your insolation you know the production of the collector. In case you install 10 collectors aside in an array they produce 10 times more than one. Easy to figure out.

Depending on the crude water quality it is from time to time useful to put an activated carbon filter between the collector do avoid odours. In case you decided to buy the collectors with droplet control you can figure out the neccessary number of activated carbon filters at the [Calculation] by checking the radio button “electronic controls”.
Please click on either of the following “image” buttons to receive a pop up window with a picture of a droplet and an electronically controlled crude water supply:   

 Introduction

Brief introduction of our company is that We have executed a work of Rs. 56 million in Balochistan on renewable. We have acquired a lot of experience in Renewable (Wind & Solar). We are in contact with a number of international companies, through which we are introducing gradually, the solar/ wind technology in Pakistan.

Now we will introduce the solar Desalination system which is proposed for the Rangers Posts in the deserts areas. The basic principles of solar water distillation are simple yet effective, as distillation replicates the way nature makes rain. The sun’s energy heats water to the point of evaporation. As the water evaporates, water vapor rises, condensing on the glass surface for collection. This process removes impurities such as salts and heavy metals as well as eliminates microbiological organisms. The end result is water cleaner than the purest rainwater. The still is a passive solar distiller that only needs sunshine to operate. There are no moving parts to wear out.

A similar 6000 gallon Solar stills have already been constructed at Gwader , which are functional . Since that time, technologies have improved a lot and all those new things will be incorporated in our new proposed Solar stills near Yazman Mandi.

Outline Plan 

Construction of Solar units is planned in 3 phases.

Phase1

In phase 1, we have planed that a small proto type unit will be prepared at Punjab Rangers Headquarter Lahore , this is planned in view of training of staff on new technology , procurement of materials and for optimizing the performance of the unit. This is designed for approx 30-40 gallons per day.

Phase 2

In phase 2 we will go for a pilot project of 100 Gallons. After observing the site difficulties and output of plant Proper monitoring and evaluation will be carried out after its completion. This is designed for approx 400-500 gallons per day.

Phase 3

After thorough studies, later on the main project for 2000 gallon will be undertaken. 

Cost Estimates

Cost estimates For Phase 1

Cost estimates For Phase 2

Cost estimates For Phase 3

Will be determined , after the completion of phase 2.

Conclusion

Solar still desalination of water is the most economical method of desalination. This technology is being widely used all over the world. Being a simple and no maintenance cost , it is recommended that this type of tech may please be adopted in comparison of RO ( Reverse Osmosis System) which are quite expensive to maintain.

A novel idea for a salt water desalination

I have tried several methods to desalinate water many which appear on the energy21 collection of web sites.

The methods I have tried so far tend to work but are not very efficient unfortunately. Mostly I had to complete against pressure as the closed container heated up the water vapor ceased to be produced at a point where the pressure inside the vessel could not be increased and the vaporizing and condensation reaches a standstill.

It seems water under pressure takes a lot more heat to allow it to boil,(such as case in super heated steam in most power generators.
However the opposite is true when the pressure on the water is reduced such as in a partial vacuum.
This method below although not yet tried by me may provide a better result.

The idea for this design came to me after seeing a science demonstration on Australian channel seven afternoon television childrens program called The Big Arvo.

If you wish to reproduce the experiment for yourself ,here are some brief details.
The science demonstration explained of how to produce a cloud.
It consist of a coke cola plastic 2 litre bottle.
The bottle had some hot water added to it ,probably about ½ litre or less in volume.
A match was lit and some ash was added from the burning match to the plastic bottle.
The bottle was again resealed and then plastic bottle was squeezed to compressed the contents (air) water can not be compressed.
When the coke bottle was re opened water vapor came out the bottle in the form of a small water cloud.

The Suggested method of desalination

I have tried to think of a method where this means could be used to produce clean water using solar energy and have come up with idea that may work although I have not tried it yet myself.‘

If you look at the drawing above you will see three large plastic ( or metal cylinders) ,I see them as being 6 inch plastic PVC tube of a suitable length and fitted with end caps
One Tube has one end cap with a brass fitting at bottom and this then is used as a water replacement container and allows feed water (saline) into the second tube and into a spiral enclosed within the second cylinder, the pathway is shown in red.

All cylinders are made airtight apart from the water storage section and the spiral coil must be wound into PVC tube to enter at the bottom and exit at the top and the reverse is true for the third cylinder as per diagram above.

I expect the water level to be maintained so that water in second tubing to be at a level somewhere near the top area as shown in red spiral path in diagram.
The saline water feed tube starts at the bottom and works it way to the exit at top of cylinder and pushes vapor into the next third cylinder.

This second 6 inch tube must be made air tight painted black to allow maximum heating of outer tube by solar energy and has some water or some other fluid also added in it.
A value is also fitted to allow a vacuum pump to reduce the pressure in the second six inch PVC tubing and then when exhausted closed ,so that a low pressure area is maintained within the tube.

If you don’t have a vacuum pump,
Another method may be to placed some water in this outer tube and then heat it by solar means, so that as the water warms, the air will be driven out though this valve.
This then closed off.
Putting very Hot water in this PVC tube may also work. But as a whole the greater the vacuum you can get in this area the better I think it will work.

The third cylinder

The third cylinder is constructed as previously described as the second cylinder.

But the difference is that this tube is pressurized and wrapped in aluminium foil to reflect any heat from the sun.

I think that by inserting a bike value in the end cap at top position and by using a bicycle pump to pressurized this third cylinder.

An idea, also occurs to me that this third cylinder could also be submerged in a water bath( a 44 gallon drum perhaps ) as well,this bath would completely cover this cooling vessel and normal evaporation from this bath would also help to keep this cooling tube area cool as well.

Both cylinders would need a small amount water placed in them, but separate from that which is flowing through the two spiral tubes from the water storage area and into the distilled water collection point as show in diagram above.

This is how I feel the unit would work,

Saline Water is fed by means of gravity and through the plastic spiral where it is heated from the heat gathered from the environment and because of the vacuum means a lower boiling point heat will be transferred to flowing saline water and allows it to vaporize at top of the tube.
This vapor is then pushed through the spiral plastic tube and then enters into the pressurized region of the third tube where it condenses and flows in to a collection area below the tube.

Making a good seal in end caps.

With previous experiments when inserting brass fittings I found it impossible to get a good air tight seal ,then I tried adding some fibre glass resin in the end caps and then letting it set around the brass fittings, I found the fibre glass has a tendency to close around metal objects and this makes it an excellent means for making air tight seals especially in slight pressurized applications as described here.

Then Plastic PVC cement is applied to the end caps and the 6 inch PVC cylinders walls and pressed fitted together and let dry.

I am unsure how it would work in a slightly low pressure area as is the case for the third cylinder as describe here within.

I have suggested using plastic tube (1/4”) for the two spiral coils in the cylinder ,but however I feel copper piping would allow better heating and cooling transfers to the liquid within.

I see the unit as a stand up affair with the each PVC pipe standing or angled to make the maximum use of the solar heating in the area especially the second tube.

The third section cooling section could be placed in the shade or in a cooler location.

Like some comment

As I have mentioned before this is only an idea and the idea may have some flaws ,and if you have some additional comments or partial experience in this type of thing I would like to hear from you.

I see some flaws on your design. One. The vacuum on the middle cilinder will not get through the spiral tube, and if it goes (making the tube from some kind of plastic) it will be there as long as there are not water vapor presure inside it. As a matter of fact, the cooling on the last cilinder could be more effective on lowering the presure on the Tube as the vapour condenses. Two. As the water evaporates from the coil it will leave salt on the tube and increase the concentration on the remaining water which also increases the boiling point making it difficult to evaporate more. You need to change this water and loose the heat it gained or put so much of it that the increase in salt will not be a problem.

I still prefere the oldest method. A pan not to deep (12 inch diam. 4 inch deep) painted in black with a tube in the middle going through the bottom of the pan. Put a funnel on the tube and cover the pan with some clear plastic like the one of fruit bags and tie it to the sides of the pan. Put a little stone on the plastic just in top of the funnel and put it all to the sun and fill with salt water.

As the water evaporates it will condensate on the plastic and flow to the lower part (the stone) and drip on the funnel out of the pan.You can change the water in the morning as it will be the coldest to keep the salt level down. You can put leaves insted of salt water or chppoed cactus in the desert and they all will evaporate and give clean water.

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