DESALINATION

ELSEVIER Desalination 153 (2002) 25-37 www.elsevier.com/locate/desal

A new seawater desalination process

Efat Chafik

using solar energy

Rtrhr University of Bochum, Department of Mechanical Engineering, Institute for Thermo- and Fluid Dynamics, D-44721 Bochum, Germany

Tel. +49 (234) 32 24423; Fax +49 (234) 32141162; email: chajik@vpt.ruhr-uni-bochum.de

Received 15 April 2002; accepted 30 April 2002

Abstract

This paper presents the development of a new process to desalinate seawater using solar energy. By the proposed process, the solar energy heats airflow up to a temperature between 50 and 80°C. The moderate solar heated air will be humidified by injecting seawater into the air stream. Later on, the water being ti-ee of salt will be extracted from the humid air by cooling it. Using air as a heat carrier and keeping the maximum operating temperature in the process lower than 8O“C enables the use of cost effective polymers as construction material. The main feature of the present process is a successive loading of air with vapor up to a relative high humidity, such as 10 or 15 wt.%. As a result, the air volume flowing through the plant can be substantially reduced. This target will be realized by a new suggested stepwise heating/humidifying-technique. The thermodynamic background of the new process will be described. An optimizing procedure of the desalinating process by selecting of optimum process parameters will be explained. Low cost air heaters for collecting of solar energy are developed. Special designs for air humidification by evaporating of seawater has been constructed and tested. Condensing equipment has been designed to recover desalinated water out ofthe humidified air. This new equipment will be described and the test results of its performance will be delivered. The new collector types and the developed humidifying and dehumidifying equipment are a part of an indoor one-stage-plant consisting of a solar simulator and an air humidifying loop in Bochum, Germany. This plant is running now and serves as a pilot to provide optimum operating conditions and design guidelines for a demonstration plant. Based on these results, an engineering package for the demonstration plant will be completed.

Keywords: Desalination; Solar energy; Renewable energy; Seawater; Brackish water; Collector; Humidifier

Presented at the EuroMed 2002 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 Alexandria University Desalination Studies and Technology Center, Sharm El Sheikh, Egypt, Mq 4-6, 2002.

OOI I -9164/02/$- See front matter 0 2002 Elsevier Science B.V. All rights reserved PII:SOOll-9164(02)01090-l

26 E. Chafik / Desalination I53 (2002) 25-3 7

1. Introduction

Desalination of water, especially of seawater, is effected almost exclusively by evaporating of the salty water and subsequently condensing the arising vapors being free of salt. In most cases, such desalination plants are designed as multi- stage evaporator plants using fuels as energy sources. This process is the basis for a daily production of several millions cubic meters of water [ 11.

For many years, efforts have been made to use solar energy for obtaining potable water from salty water. The solar desalination process offers the advantage of doing practically no ecological damage and creating minimum energy cost. Solar water evaporation plants use collected solar energy for direct heating and evaporating of salty water to gain distilled water [2,3]. In other cases, solar energy has been used to heat seawater and later to inject the warm water into air to humidify it. The subsequently cooling of the humid air delivers the needed water free of salt [4-61. All these processes use water to collect the energy delivered by the sun.

The central issue of the actual investigation is a new process of seawater desalination using solar energy and turning away from merely heating or evaporating ofthe seawater itself as the main idea for water separation. On the contrary, the basic unit operation in the present case is first to use solar energy for heating of an air stream and in the second step to inject seawater into the hot air to evaporate it using the energy initially provided by the sun. Air, as a heat carrier at temperatures less than 8O”C, allows the use of economic structure materials for collecting of solar energy. No corrosion, scaling or plugging can occur in the solar collecting system. The low operating temperature allows the use of low cost polymers for collectors, humidifier, pumps and various equipment.

The second feature of the present solar desali- nation process is the stepwise loading ofair by vapor. The process consists of several steps for air heating, each followed by a humidification stage. This manner of operating makes it possible to

obtain high vapor concentration in the airflow, thus reducing the airflow rate through the plant. A reduction of the power necessary for air blower and the volume of pipes and other equipment can be realized. As a result, low investment and opera- ting costs can be achieved.

2. Thermodynamic background of the process

Heating and humidieing of air can be described using the psychometric chart, also called h-x- diagram shown in Fig. 1 [7,8].

An air flow with initial temperature and initial humidity as indicated by Item 1, i.e. 25OC and 10 g water/kg dry air, can then be heated up to 80°C (Item 2) and humidified by adiabatic injection of water to increase its humidity up to 30 g/kg (Item 3) accompanied by a temperature decreasing to approximately 30°C.

In this case, the amount of water that can be gained is 20 g/kg air. The high air to water ratio (5O:l) and the necessary low temperature for following condensation of the moistness are the main economic disadvantages of this procedure.

80

30 25

Humidity x in g water/ kg dry air

Fig. 1. Heating and humidi@ing process in h-x-diagram.

E. Chajk / Desalination 153 (2002) 25-37 27

Therefore this method can only be used for the desalination of very low rates of water.

Nevertheless, it is theoretically possible to load air with a substantial amount of vapor humidity, for example I5 wt. %, by extensive heating it up to very high temperatures followed by adiabatic injecting seawater into the hot air. This process cannot be economically realized by means of solar energy because of the necessary heating of air up to very high temperatures about 500°C.

The following procedure shows another possibility how to obtain such high humidity con- tents by means of solar energy at moderate temp- eratures and then to gain this humidity as water, which is free of salt. This task is solved by a gradual heating and subsequent humidification of air [9]. Here, the air is heated stepwise by means of solar energy in a simple collector up to temperatures of about 5080°C. The preheated air is humidified by injecting salty water (e.g. seawater) into the air stream. The course of this procedure can be followed in an example shown in the following h-x-diagram (Fig. 2).

Based on initial air with a temperature of 25°C and a humidity of e.g. 10 g water/kg of dry air, the first heating step up to 50°C is made in a solar air-heating collector. In the following humidification stage, seawater is blasted into the preheated air

until the air becomes approximately saturated. Assuming adiabatic humidification, the humidity content in the air increases to 21 g water/kg of dry air and the air-cools down to the wet bulb temp- erature of 23°C.

In the following, the air flows to a second heating step and is now heated up to 50°C then being loaded up to 28 g water/kg of dry air in the second humidification stage. Here, its temperature goes down to 3 1°C. During the third step, the air is heated up to 56°C and humidified to 36 g/kg at 35°C etc. After the 15th humidification stage (in the example shown here) the air is loaded with approximately 148 g water/kg dry air at 60°C as shown in Fig. 2.

This high water content is identical to the humidity that can be achieved by injecting of water into air preheated up to about 450°C.

The humid air can now be cooled to obtain water free of salt by condensing the moisture in a condenser, using fresh seawater as a coolant. In the condenser, the air cools down to 25°C and is dehumanized to approximately 20 g water/kg of dry air. That means that an extracted quantity of approximately 128 g water free of salt can be ob- tained per 1 kg of dry air (ratio air/water appro- ximately 8: 1).

Fig. 3 shows a simplified example of a solar

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

Humidity x in g water / kg dry air

Fig. 2. h-x-diagram with stepwise heatinglhumidifling process.

28 E. Chajk / Desalination 153 (2002) 25-37

Recycling fan

humidifying stages)

Fig. 3. Scheme ofthe proposed process.

desalination plant operating according to the procedure explained above.

The main equipment needed for the new desalination process is: l Collectors for heating up of air l Humidifiers l One dehumidifier

3. Development pracedure of the new process and design criteria of a desaliaration plant

The following four issues have been investi- gated to deliver most effective equipment and to establish design criteria for a plant using the new process: l Cost-effective collector designs for solar heating

of air have been developed and tested. l Humidifier designs and their optimum operating

parameters have been provided. l A design of an optimized dehumidifier could

be delivered. l The optimum operating conditions ofthe process

and subsequently the completion of an optimized process flow sheet of a demonstration plant

are now under investigation using an indoor pilot plant.

3.1. Development and selection of collectors

The main investment costs of a solar desalination plant are caused by the required large area of solar collectors. Therefore it is essential for the economy of a desalination process to minimize the costs of the collectors used. An important criterion in this issue is the effkiency of the collectors and its dependence on the average operating temperature. The material of the collectors will also play an important role in relation to the optimization of the plant economy.

In general, the various callector designs and their efficiencies can be divided into three types [IO] as shown in Fig. 4. l The simplest type is the collector with an over-

flow absorber tid one cover (collector 1, Fig. 4). The air flows between an absorbing surface, which is heated by absorbing of solar energy, and a light transparent coveting, which norm- ally consists of glass. The air stream between absorber and cover is acting as a heat carrier.

E. Chafk / Desalination I53 (2002) 25-37 29

Collector Type 1 Collector Type 2 Collector Type 3

0.8

2 c 0.6 .a, 2

L" 0.4

0.0

0.00 0.02 0.04 0.06 0.08 0.10 (Tm-TaYG [K’qmiWl

41.5 52 63.5 69 74.5 Tm'C I I I I I

Fig. 4. EBiciency of different collector types. Tm, mean temperature of air in the collector, ‘C; Ta, ambient temperature, “C; G solar irradiation, W/m2; Aq, collector area, m2; C: air velocity, m/s.

To decrease heat losses, the absorbing surface is being insulated to the bottom. This type of collector is characterized by its simple con- struction. However, its efficiency is low as there is no insulating layer between glass covering and flowing heated air. The just mentioned disadvantage is largely being removed by the installation of a second glass covering above the air channel (collector 2, Fig. 4). A layer of stagnating air between the two covers acts as an insulating space. A further advantage can be achieved by using air collectors with double stagnating air layers (collector 3, Fig. 4). This design provides the best efficiency.

The goal therefore must be to design simple solar collectors, which combines the advan-

tageous characteristics ofthe collectors 2 or 3 shown in Fig, 4, but this at favorable manufacturing costs.

When developing the new collector, great importance is attached to the attempt that nearly the whole collector components can be assembled by parts already available in the market and usually manufactured in mass production, which results in low manufacturing and assembling costs. This target could be reached, as will be de- scribed below.

3. I. I. Description of the new collector designs

The new collector consists of a multiple web plate of polycarbonate. Large amounts of such plates are used, for instance, as roof covering for greenhouses or as light transmitting walls for halls, factory buildings, etc. The plates operate

30 E. Chafik /Desalination 153 (2002) 25-37

also as a heat insulator. The used polycarbonate has a high light transmittance and an excellent water resistance - also against hail [ 111. A lo-year lifetime is granted by the providers of these plates.

A multiple web plate consists of two, three or four layers of approximately 1 mm thick sheets with a distance of 4 to 36 mm between each other. By means of vertical or inclined webs, these layers are being connected with each other already at extrusion. This results, for example, in a frame- work double plate (FWDP) or the four-fold web plate (FFWP), as shown in Fig. 5. By last mentioned design, four layers are being connected with each other by means of webs and thus resulting three channels being arranged over each other. The cross-section of one channel has a dimension of approximately 24x7 mm.

These polycarbonate plates can be easily con- verted into collectors for solar heating of air flow by inserting of absorbing strips into the plate in such a way that at least one layer of stagnating air lies between air flow channel and surroundings. Two transparent covers are then covering the flow of heated air [12].

Fig. 6 shows how a prefabricated framework double web plate (FWDP) is converted into a collector (No. 1, type FWDC) by installing black- ened aluminum strips. This provides a collector with underflow absorber and two-fold covering similar to collector type 3 shown in Fig. 4. The air to be heated flows through the lower channel under the absorber. This channel is separated from the transparent upper polycarbonate layer turned towards the sun by a chamber filled with stag- nating air. This considerably reduces the heat losses by upward convection. In this case an additional insulation of the bottom plate is necessary.

A test collector, FWDC-type, is presented in Fig. 7. The lower side of the collector is insulated by means of a polystyrene plate. The collector ends are fitted into two insulated tubes as distributor and gatherer of the air streams.

Another collector design is the four-fold web collector (No. 5, type FFWC) according to Fig. 8.

Frame-Work Double-Plate (FWDP)

Four-Fold Web Plate (FFWP)

Fig. 5. Cross-sections of polycarbonate plates FWDP and FFWI?

17mm

Fig. 6. Framework double-web collector (collector 1) with absorber strips.

Fig. 7. Test collector FWDC-type.

E. Chajik / Desalination 153 (2002) 25-3 7

Absorber Strips

31

Closed channel /

Soft rubber Y

Fig. 8. Four-fold web-plate collector (collector 5).

The middle chamber of a four-fold web plate is formed as a flow channel of the air to be heated. With the help of unilaterally blackened strips of aluminum or some other good heat-conducting material, which is put loosely in this middle channel, the solar energy can be absorbed. The upper row of chambers serves as an insulating layer in which the air stagnates. Effective heat insulation downwards can be achieved by affixing a plastic film below the lower channels.

3.1.2. Experimental investigation on the new collector designs

In the course of the actual R&D work, the efficiencies of several collector designs have been experimentally investigated using an existing set- up for indoor testing of collectors (Fig. 9).

3.1.3. Experiments on collectors and results

The main objective of these experiments was to find out the efficiency of the different types of the polycarbonate collectors. Graphs ofefftciency as a function of the collector mean temperature were established for variable collector operating conditions. Solar irradiation intensity, airflow rate, wind velocity and temperature and air inlet temp- erature were variable and adjusted to get the

Fig. 9. Set-up for indoor collector tests.

collector efftciency at every possible location in the process. The indoor collector experiments were performed at three different solar irradiations G (590, 800 and 1000 W/m”). Inlet temperatures were 25, 35, 45, 55, 65°C at different air flow rates for each irradiation.

32 E. Chafk / Desalination I.53 (2002) 25-3 7

Fig. IO. Effvziency of the different collectors.

Fig. 10 shows the efficiency of both above mentioned collector designs as found by the indoor tests as a function of the value (T, - r,,,) whereas T, is the mean air temperature in the collector and TOMh is the ambient temperature. Furthermore, the efficiency of a commercial collector is drawn.

It should be mentioned that the efficiency of some commercial collectors is about three times of those found in the new developed collectors. Nevertheless, as the price of the new collectors is less than 20 % of that of a commercial one (price differs between 40 and 250 e), it is still more economical to use new collectors despite their low efficiency. An additional advantage of the new collectors is their low weight. Substantial savings will be possible by using of light bearing con- structions.

3.2. The humidljkation of solar heated air

Two humidifier types has been developed: a) The pad humidifier: The pad humidifier unit

(Fig. 11) contains cassette made of corrugated cellulosic material, which consists the wetted surface. The cross sectional area of the pad is 0.45 mx0.55 m, while the thickness of the pad is 0.30 m. At the top, there is a liquid distributor, which can feed the cassette with water, while at the bottom there is a liquid collector, where the water is collected as it drains down the cassette. Thus, the water flows downward, while the air passes in a cross flow direction through the openings

1. Humidifying pad GLASdek 4. Bleed-off or CELdek 5. Water distributor

2. Water inlet 3. Drain

Fig. 11. A scheme of a pad humidifier unit.

of the cassette. The air is humidified as it comes in contact with the wetted surface of the pad.

b) The U-tube spray chamber humidifier: The U-tube humidifier consists of a tube 0.30 m in diameter and 4 m in length made of polypropylene. The tube has a U shape. Four nozzles made of polypropylene are placed in one leg of the U tube to spray a flow rate of 1.5 Vmin water per nozzle at 3 bar water pressure. On the outside, the humidifier is insulated with a layer of glass wool covered with a polypropylene sheet. Fig. 12 shows photos of both tested humidifiers.

3.2.1. Test procedure on humidiJiers

Both humidifying equipment were a subject matter of a test program to get the optimum humidification efficiency whereas can be defined, using the h-x-diagram (Fig. 13), as

?J,,,= 100{(X2-X1)/(Xs-x1)} (1)

whereas xl - inlet humidity; x2 - outlet humidity; xs - maximum achievable saturation humidity.

E. Chafik / Desalination I53 (2002) 25-37 33

Fig. 12. Photos of the pad and the U-tube humidifiers.

“= 80 .-

f tii t

?50 r”

20 30 40 50 60 70 80 Humidity x g water/ kg dry air

Fig. 13. Definition of the humidification efficiency q,,.

The heated air at outlet of solar collectors has to be loaded with vapor by injecting of seawater. In order to avoid condensation in the humidifying chamber, the humidifying operation has to be non- adiabatic, i.e. the temperature of the seawater injected into the solar heated air has to be higher than the wet bulb temperature corresponding to the condition of air at humidifier inlet.

The temperature of water to be injected in the humidifier will determine the achievable humidity of air. This subject is comprehensively investi- gated in [ 131 and [ 141. On the assumption that there is a balanced heat and mass transfer process between airflow and injected water flow, the water enthalpy reduction will be identical to the enthalpy increase of air. Therefore the exchange will be governed by the heat balance equation:

m,.Ah,=~p,.riz,.(f,~-t,) or

(2)

where ti, is air flow rate, kg/h; tiiz,- flow rate of injected water, kg/h; t, - water inlet temperature, “C; tWA - water outlet temperature, OC; cpW - specific heat of water, kJ/kg”K; Ah, - enthalpy change of air flow, kJ/kg.

Assuming the same outlet temperature of air and water, the above equation must be solved by try and error, using the h-x-diagram in order to get M, and water outlet temperature t,,.

The influence of the mass flow ratio water to air +z, / ti, on the efficiency of the humidification as well as the dependency of the efficiency q,, on the air velocity and the length of the humidification chamber have been investigated and taken into consideration. It could be found that n,, for both tested humidifying equipment to be about 95% [ 1.51.

3.3. DehumidiJication and heat recovery

The extraction of water from humid air can be effected in a condenser cooled by seawater. On the other hand, it is required to make a first concept for an energy efficient process improving the plant

34 E. Chafik / Desalination 153 (2002) 25-37

3000 kg/h from sea 20”C

to sea 4 ~

1800 kg/h (from seawater tank)

40°C

t-l t-i ‘I’ 1 .O kg/h 17-1 1.5 kg/h

Destillat (Product) Destillat (Product)

HREC I HREC II

Fig. 14. Heat recovery and dehumidification.

efficiency by heat recovery. An energy optimizing study of the new process is presented in [16]. A possible measure to recover heat is to use the heat content of the humid air flowing from the last humidifier to preheat the seawater before its injec- tion into the humidifiers. Three heat exchangers HRJZC I, HRECII and HRECIII (Fig. 14) will be installed for heat recovery by successive heating of the circulating seawater using the enthalpy of air after the last stage. This measure is substantial by higher insolation. A part of desalinated water will precipitate in these heat recoveries. Addition- ally, a cooler DEHEX is supposed to complete the dehumidification of back flowing air by condens- ation of the rest humidity. The shown scheme is made for a very low temperature level. At high insolation, a substantial increase of water temp- erature can be achieved.

3.4. Optimization of the number of heating/ humidijcjng stages

The further development of the desalination process is to strive for an optimized number of required heating/humidiljGng stages. The base of this investigation is the cost of a complete stage

r’l 1.5 kg/h

Dehumidified air

-I- 36.4 kglh

Destillat (Product) Destillat (Product)

HREC Ill DEHEX

including collectors and humidifier. In a subsequent economical optimization, an optimum of the number of required stages will be provided by variation of the hourly mass flow through the plant. As an example is a four-heating/humidifying stages by a circulating airflow of 1100 kg dry air/h and an insolation of about 600 W/m2. The h-x-diagram for that process could be drawn as shown in Fig. 15. This h-x-diagram is based on the injection of preheated seawater into the air. The temperature

water inlet in the humidifiers t,,,, was in that

Fig. 15. h-x-dim for a four stage desalination process.

E. Chafik / Desalination 153 (2002) 25-37 35

case 43°C. A slight superheating of the humid air after the fourth stage is required to prevent water condensation in the duct to the dehumidifier.

4. Final process flow sheet

All results gained by the above-mentioned investigation are included in a process flow sheet

of a demonstration plant as shown in Fig 16. The solar heating of circulating air takes place

in the collecting areas consisting each of 15 parallel mounted multi-web polycarbonate plates. Each plate is 980 mm wide and 3 m long. The dry airflow rate through each polycarbonate plate is about 75 m3/h in the shown plant. The air velocity through each channel is about 4 m/s.

To Sea 1 43 B 1800

0 Temperature in ‘C Humidity in g/kg

Fig. 16. Process flow sheet of a Cstage demonstration plant.

1

Flow rate in kg/h Pressure in bar

36 E. Chafik / Desalination I53 (2002) 25-3 7

t

Fig. 17. Isometric view of a 4-stage demonstration plant.

Fig. 17 shows an isometric view of a possible plant to deliver daily 400 1 potable water for a mean insulation value of 550 W/m*.

5. Conclusion

A new seawater solar desalination process is described. The main equipment have been de- veloped, designed and tested. The running data of the process are presented for a small demon- stration plant. The process flow sheet of an example plant is provided.

The main plant components are selected. A new design of a low-cost collector for solar air heating has been developed. Two different geometries of the collector have been presented and will be further investigated to increase their efficiencies. An efficient humidifier design is

developed and tested. The engineering of the

humidification and dehumidification units is

completed.

This research work is a subject matter of an international R&D project with the participation of four countries. The project is supported partly by the European Commission and has a duration of 4 years ending in November 2002.

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