Abstract of Water desalination

            Water desalination is actually a competent application of water desalination, particularly in the areas where fresh water is not that easily available and sunlight is present. In this research, a MED or Multi Effect Distillation which is solar-powered is evaluated that includes a TSU or Thermal Storage Unit and a MED unit. A duo of tanks for thermal storage are involved in the TSU along with a collector array which operate like a brine heater. The system of dual-tank seems to introduce a delay among the MED’s application and accumulation of solar energy, which enables a better manipulation of temperatures and mass flows in the MED. At a high temperature, the medium fluid flows to the charging tank from the discharging tank in the MED through heat exchange while providing suitable heat for the production of distillate. The medium fluid in the charging tank is heated again by circulation through solar array. For alternating the roles at almost sundown, the system is formulated for the tanks. At a real environment, the new design was evaluated experimentally and a digital pyranometer was utilized for measuring the solar insolation. Days were selected randomly and experiments were conducted.

introduction of Water desalination

            The presence of water in the time to come will be quite worrisome for various nations over the globe [1]. The demand for water is increasing two times the rate of population. That is why, the importance of desalination will rise, especially for the nations with minimal rainfall.

            Even though the desalination of sea water needs intensive use of fossil fuel, it is used broadly. Almost nineteen thousand plants of desalination with a capacity of sixty million cubic meters daily have been implemented as of 2014 in different countries [1]. When it comes to the expansion of potable water, the demand of energy is quite a significant barrier [2]. Almost 356 thousand tons of petroleum are used every other day for producing thirteen million cubic meters of drinkable water [3]. An undesirable link is created by this between water costs and energy. A recent study for instance has explained that for satisfying a three percent annual increment in the demand of water using desalination, an investment of 500 billion dollar would be needed annually [4].

            For the technology of desalination, there are 2 primary categories. First one is the change of phase which means thermal processes like MSF or multi-stage-flash and MED systems. The second one is the single-phase which means processes of membrane like RO or reverse-osmosis systems. Altogether, RO, MSF, and MED systems are responsible for 94 percent of all desalination capacity in the world [5].

            MED technology in large thermal plants is used due to its low TBT or top brine temperature, normally between 60-90 °C along with its low specific requirements in terms of energy consumption [5]. Furthermore, MED seems to have only half of the MSF’s pumping requirements [6][7].

            Desalination is quite a reliable application of solar energy, especially in the regions where water is not available and sunlight is available. In this application, energy can be sent in as thermal energy which is used in technologies of thermal storage or electrical energy which is used in PV or photovoltaic technologies. MSF and MED technologies can easily be joined with thermal solar systems for meeting their large requirements of thermal energy.

            A unique design of thermal storage is presented by this work which is capable of driving continuously an MED over twelve months. According to seasons, the rates of production change for optimizing the solar energy transfer to the seawater. A duo of tanks for solar storage are used by the design, one is used for offering the MED while the other one for holding the seawater that circulates through the solar array. It can be said that this design acts to isolate the Multi Effect Distillation from solar variations, enabling more efficient operations.

Multi-Effect Desalination (MED)

            The function of MED depends on the transfer of steam and hot brine through various effects or units. Each and every effect has a system of spaces for the collection of brine and vapor, nozzles for spraying the brine, and tubes which are heat exchanger. Just as show in the Figure 1, the conventional systems of MED pass the source steam through exchanger tubes in the very first effect. Partially, the sprayed brine evaporates and provides both brine and steam for driving the second effect. From the exchanger tubes, the distillate is collected from the second effect till the last effect. In each and every effect, the temperature falls eventually as stages pass. For promoting the evaporation of brine, the pressure in each and every effect is manipulated using a vacuum project so that each and every effect is at lower pressures compared to the previous one. Just as illustrated in Figure 1, the vapor which is produced by the last effect is transferred through a condenser due to which distillate is produced and income seawater is heated for the system. Moreover, MED can be utilized for plants at a large-scale just like the one which began in Saudi Arabia in 2009 and produced more than 800,000 cubic meters daily [8].

Figure 1. MED layout

        For MED desalination, the market receded after the production and development of competing technology which include distillation of multi-stage flash in late 60s and RO or Reverse Osmosis in mid 70s.

Solar Desalination Approaches

            For sure, solar energy has the capability of reducing the fossil fuel that is used for the process of desalination, even though it is difficult at present to make the desalination competitive with different conventional techniques. At present, desalination capacity of renewable energy is measures to lower than one percent of the capacity for desalination plans which are conventionally fueled [4]. Furthermore, such systems have a high capital cost of up-front and need additional maintenance or care.

            At present, there are 2 important approaches to the solar desalination: indirect and direct. Brine is heated by direct systems and medium fluid is heated by indirect systems which is transfers heat to brine in the unit of exchange if solar energy is utilized for driving MSF or MED units as this is known as indirect desalination because the brine is given temperature at the exchange unit. RO is yet another indirect method that utilized membranes and a solar PB can be utilized for producing electricity which the membrane needs.

SYSTEM DESCRIPTION of Water desalination

        Three components are included in the system of solar desalination: solar collectors forming an array, two tanks for thermal storage, and an MED unit. The overall scheme is illustrated in the Figure 2. In the figure, the process for the tanks is illustrated as one tank is charged while the other one is discharged. Daily, the roles of tanks are switched as:

In the position A, switches indicate that the first tank is discharging while the second tank is charging.

In the position B, switches indicate that the first tank is charging while the second one is discharging.

Modified MED

            In the first effect, a heat exchanger is included in the conventional MED which takes steam from an external source of heat like boiler for the facility of power. In the heat exchanger, the steam is condensed in the first effect for evaporating and heating the brine. This task is performed by the modified MED with sufficient heat from the medium fluid which is stored in the discharging tank. A different area is required by it for the unit of heat exchange in the first effect. The remaining operations of MED are not altered as the vapors generated are used like a heating source for the 2nd effect till the last effect.

            In the charging tank, the medium fluid is heated every day by circulating it through the array which raises its temperature to the highest value for determining the output of MED for the following day. That is why, the MED should be capable of accommodating the daily changes in the supply temperature of medium fluid. It is obtained by upgrading the mass flow of incoming seawater every other day when the roles of tanks are switched.

Two-Tank System

        Two insulated tanks which are identical are included in the system of thermal storage with enough volume for supplying the MED with the medium fluid for a whole day. During operation, the design of charging tank is made is such a way to be full of warm fluid at sunset every other day at the time when roles are switched by the tanks. Using this approach, the charging tank is allowed to increase the temperature of medium fluid throughout the day while minimizing the losses of heat.

        Just as sufficient heat is passed to the first effect from the discharging tank, the temperature is changed. This change together with the mass flow of medium fluid relies on the kind of medium fluid, the efficiency of heat exchanger, the required level of MED production, and the top temperature of brine in the MED’s first effect. Water is selected to be the medium fluid with similar specific heat as the water from sea.

Solar Heating

        Circulation heats the medium fluid. The circulation of mass flow through the arrays is selected to increase the transfer of heat to the medium fluid, and this rate seems to change as an operation of the solar energy. Medium fluid’s maximum temperature and the distillate production for each day is changed in accordance with the average values of daily insolation.

Figure 2. Overall schematic of proposed design

 Experimental setup of Water desalination

Structure and operation of  modified MED

A picture of the experimental apparatus is shown in Figure 3 which is used in the research. Following components are included in the system of solar desalination:

1. Solar collectors which are flat plate.

2.  A duo of storage tanks.

3.   A desalination unit with single-effect.

Figure 3. Experimental apparatus of MED

The single effectunit

            Condenser or evaporator tubes of heat exchange, demister, a system of water distribution, brine pool, and a vapor space are included in the effect. Furthermore, the height, breadth, and length are 35, 35, and 70 centimeters respectively. The design of effect is like a rectangular chamber of coated iron for reducing thermal losses and preventing corrosion.

The condenser/preheater unit

            Additionally, the condenser is a galvanized heat exchanger with a shell-and-tube where condensed vapor’s latent heat is transferred for taking the seawater in. The height, breadth, and length of the effect are 35, 35, 100 centimeters respectively. Across the condenser unit, the tubes are made up of iron with 1.9 cm diameter. The condenser’s outer surface is covered totally with an insulating material.

This scheme is shown in the Figure 4 and there are no headers in the figure.

Figure 4. inside tubes of the effect and the condenser

Distillate storage tank

        In a tank, the desalinated water is accumulated and located right beside the condenser. At the end of the side of shell, a discharge tube is adjusted for directing the desalinated water to the tank of storage.

Solar collector (feed water heater)

        In this study, the heater of feed water is a solar collector with a flat plate. At the 45 degree slope angle, the collector is operated. The aspect degree of collector is 180south. Using a galvanized iron sheet (120 cm ×70 cm), the solar collector is designed. A heat absorbing plate is included in the flat plate collector which is painted black for increasing the absorption of solar radiation. At the end of shell, a discharge tube is adjusted for directing the desalinated water to the tank of storage. Absorber plate and the pipes are enclosed in a metal box which is insulated with a glass sheet.

Tubes:

        Between the unit and the collector, the connection tubes are made of plastic material which is insulated. Plastic is used for making tubes for reducing the losses of heat. Plastic tubes’ diameter is 1.9 centimeters. Tank switching’s control valves are shown in the Figure 5. 

Figure 5. The control valves of tanks switching

Storage tanks,

The tanks for storage are atmospheric vessels with an 80 gallon operational volume. Fiber glass fabricates the tank. Warm medium fluid supplied by tanks to the effect.

Measurements of experimental data

Temperature

System’s different temperatures are determined by using the Lascar Thermocouple EL-USB-TC-LCD.

Solar radiation

A solar radiator pyranometer is used for measuring solar radiation which is placed on the surface of collector.

Wind velocity

Digital Anemometer MagiDeal MS6252B is utilized for measuring the speed of wind in m/s.

Flow rates

DIGITEN G1/2" Sensor Meters of Flow Water are utilized for measuring outlet brine, inlet feedwater, and condensate.

Results of Water desalination

            A little desalination unit of solar multi-effect is researched and the experiment was proceeded at the western region of Saudi Arabia. The experiment was conducted on days which were selected randomly during the month of June. A single effect, collector, and two storage tanks are included in the experiment. During the month, the total solar irradiance and its average on the surface collect is approximately 8.23 kW h/ m2. The highest value at 618kg/day was produced by the system with 1.68 m2 collector area. Solar radiation’s hourly variations are shown in the Figure 6 during the month of June. Wind speed’s hourly variations are illustrated in the Figure 7.

Figure 6.  The hourly variation in solar radiation during June 2017

Figure 7. Wind speed

        For visualizing the daily changes better in temperature and mass flows, bar graphs are illustrated in the Figure 8 for daily production, supply temperatures, and solar intensity during the chosen days. The daily solar intensity and the corresponding change in temperature are shown in the upper bar graph for the medium fluid in the graph at middle.

        Distillate production is shown in the bottom graph. Daily solar temperature and intensity drops seem to determine the daily development. The supply temperature of medium fluid closely tracks the useful heat. Daily production and temperature of medium supply are predicted by the previous day’s solar intensity.

        For this specific feed water and supply temperature, produced distillates’ hourly variations in June 21 is analyzed in the Figure 9. The ratio of recovery simplified as the discharging brine’s percentage converted to distillate is actually illustrated in the Figure 9. The ratio of recovery ranges to sixty from forty percent.

Figure 8. The hourly variation in solar radiation during some day of June (top). Medium fluid supply temperature(middle). Daily production (bottom).

Figure 10 illustrates the hourly productivity of system on the same day. This day’s accumulative productivity is almost 618 kilograms.

 Figure 9. Total distillate production and recovery ratio

THE SEC or specific consumption of thermal energy is simplified as the input of thermal energy to the solar collector which is divided by the production of distillate. Experimental data was used for calculating the SEC and the analyzed to be 1073 for June 21.

 Figure 10. Accumulative hourly productivity

Conclusions of Water desalination

        In this study, a new desalination design of solar-powered MED is evaluated, used, and built. Two individual tanks for thermal storage are included in the system and MED feeding so that tanks change their roles every other day. Sufficient working fluid is held by the discharging tank at the highest supply temperature for driving the MED. As it is discharging through the MED, working fluid is accepted by the charging tank from the MED’s first effect. Circulation heats it through a solar array.

        The design of dual-tank with medium fluid at low-temperature is right for the MED which needs a lower TBT compared to the MSF technology. The medium fluid is allowed by this system for gradually rising to the optimal temperature every day for minimizing the losses and available solar energy is used by it. This design also seems to have an advantage of permitting the system of MED to work at an output’s constant level every other day so that it is not efficient for tracking the solar activity on an hourly basis.

            For a certain location, the experimental work was carried out for the western area of KSA, using only a single effect. The rates of mass flow are adjusted for tracking seasonal changes in the solar power for maintaining effective TBT. A case study’s outcomes indicate that this system is capable of producing distillate’s 560 kg daily at an average rate using 2 collectors of flat plate with 1.68 m2 area. 1073 kJ/kg is the specific consumption of thermal energy.