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1. INTRODUCTION
One of the greatest problem world is facing nowadays is the scarcity of fresh consumable water. This scarcity is compounding due to increase in population, over exploitation of water resources, industrialisation and many other facts. Whereas the saline water is present in abundance (over 99.3% of the water resource available is saline). As a result of which it has become necessary for us to convert this saline water to consumable water. The United Nation Environmental Programme (UNEP) stated that one third of the world population live in areas where there is scarcity of fresh water. Consequently by 2025 this ratio will be increased to two third (UENP 2012). According to World Health Organisation 20% of the world population has inadequate drinkable water. To reduce the scarcity of water many desalination plants were installed. Desalination is a process in which the saline or sea water for example brackish water is converted to fresh consumable water used for drinking purpose or for agricultural use. According to W.H.O. the concentration of salt in consumable water must not be more than 500 ppm (Achmad Chafidz,Saeed Al-Zahrani et.al 2014). Conventional plants used for desalination purpose us fossil fuels. In oil-rich countries fossil fuel is used as a primary supplement of energy. Due to excessive use of fuel it has become necessary to reduce the use of fossil fuels. Therefore, recently utilisation of renewable energy sources has become a sustainable solution for driving the desalination plants. The semi-arid and arid regions are abundant in solar energy, therefore have a large solar energy utilisation potential. Mainly two options are given in using the solar energy: photovoltaic cell (PV) coupled with the system or solar thermally driven distillation system(M. Shatat et al. 2013). The most widely accepted module for the separation process is the RO module but it has its disadvantages like formation of high polarisation films because of its high pressure operation and by products which generates fouling hence result in high energy consumption with a limited recovery of water. Membrane distillation process is relatively cheap and consumes less energy. As the evaporation and condensation surfaces are tightly packed, it results in compact design i.e. it requires less space. And the combination of the MD system with the solar energy source makes the design more sustainable and reduces the energy cost and consumption of non-renewable energy sources (J. Koschikowiski et al. 2003). This paper gives a complete review of the membranes, types of membranes, membrane distillation process, its combination with the solar energy, design of the equipment in process, exergy analysis, and economic analysis.
2. Membrane Distillation:
Membrane distillation (MD) is a thermally driven separation process that involves phase conversion from liquid to vapour on one side of the membrane and condensation of vapor to liquid on the other side. The exploitation of waste heat energy sources such as solar energy enables MD more promising separation technique for industrial scale. Growing economics and water scarcity are driving desalination as a solution for water supply problems. Membrane distillation in the application of water desalination make this technology a prospective one in the research areas. The membrane facilitates the transport of water vapor through its pores but does not participate in the actual separation process. Low operating temperature and hydrostatic pressure makes MD attractive technology than any other conventional distillation in the field of desalination and food industry .
Membrane distillation can be employed in four different configurations namely:
• Direct contact membrane distillation (DCMD)
• Air gap membrane distillation (AGMD)
• Vacuum membrane distillation (VMD)
• Sweeping gas membrane distillation (SGMD).
Most of the researchers have already focused on the principle, experimentation and application of all the types. But integrated mode of solar power with thermal membrane distillation for potable water utilising low waste heat energy is ongoing research in most of the academic institute. Those of which DCMD and AGMD are best suited for the desalination applications where water is the major permeate component. These two configurations are applied to produce fresh water from a salt solution4. The process thermal efficiency of AGMD is higher than that of DCMD by about 6% due to the presence of the air gap. The permeate flux of DCMD is higher than that of AGMD by about 2.3-fold and 4.8-fold for Thi 80 and 40 °C, respectively. Increase of the thermal conductivity of the membrane material improves the DCMD process by mainly improving the process thermal efficiency and improves the AGMD process by mainly improving the permeate flux (Selvi.S.R., R. Baskaran 2014).
The principle of membrane distillation is briefly described as:
On one side of the membrane, there is brackish water/saline water at a certain temperature, for, example at 80°C. If at the other side of the membrane there is a lower temperature, for example, by cooling the condenser foil to 75°C, then there exists a water vapour partial pressure difference between the two sides of the membrane and thus water evaporates through the membrane. The water vapour condenses on the low-temurature side and distillate is formed. Fig. 1 depicts the principle of membrane distillation.
Therefore there are three different channels in the process: (Joachim Koschikowiski et.al)
• Condensor.
• Evaporator.
• Distillate.
2.2 Membranes Characteristics for Membrane Distillation:
Membrane should have the following characteristics
• The membrane should be porous.
• The membrane should be hydrophobic.
• No capillary condensation should take place inside the pores of the membranes.
• Only vapour should be transported through the pores of the membrane
• The membrane must not alter the vapour equilibrium of the different components in the process liquid and at least one side of the membrane should be in direct contact with the process liquid.(Selvi S.R.et.al 2014)
2.3 Advantages of Membrane distillation (Selvi.S.R. et al 2014):
• Complete rejection of non volatiles (e.g., salts, ions, colloids, cells, and organic non volatiles). Good quality of product water is obtained.
• Operation at near-atmospheric pressure compared to the high operating pressures of membrane processes like RO, etc., and lower operating temperatures (40–100 °C) than conventional multiple effect distillation.
• Much reduced need for vapour space compared to conventional distillation processes.
• Much reduced mechanical strengths needed for the membrane and the module.
• Membranes used in MD are tested against fouling and scaling.
• Chemical feed water pretreatment is not necessary.
3. Solar powered membrane distillation:
Solar energy can be used to convert saline water into fresh water with simple, low cost and economical technology and thus it is suitable for small communities, rural areas and areas where the income level is very low. Recent developments have demonstrated that solar powered desalination processes are better than the alternatives membrane desalination technology like RO. Two types of solar power MD is classified as direct and indirect systems. In direct systems are those where the heat gaining and desalination processes take place naturally in the same device,(Solar still) .In indirect method, the plant is separated into two subsystems, a solar collector and a desalination unit. The solar collector can be a flat plate, evacuated tube, solar pv cell or solar concentrator and it can be coupled with any of the distillation unit types which use the evaporation and condensation principle, such as MSF, MED and MD for possible combinations of thermal desalination with solar energy. Systems that use PV devices tend to generate electricity to operate thermal membrane distillation in solar thermal MD8. -100 MD techniques hold important advantages with regard to implementation of stand-alone operating desalination systems. The most important advantages are the operating temperature of the MD process is in the range of 40. This is a temperature level at which thermal solar collectors perform well. Intermittent operation of the module is possible9. According to simulated calculations, Spiral wound membrane module in SPMD pilot plant can distill 150 lit/day of water in the summer in a southern country .Commercialised solar power membrane distillation like SCARAB plant consumes 5-12 kwh/m3 thermal energy consumption. The types of solar desalination technologies are as follows:
3.1 Direct solar desalination:
The direct systems are those where the thermal desalination processes take place in the same device and it is mainly suited to small production systems, such as solar stills, in regions where the freshwater demand is less than 200 m3 /day (Ma & Lu, 2011 ). Solar still distillation represents a natural hydrologic cycle on a small scale. The solar still is working as a trap for solar radiation that passes through a transparent cover it consists of a basin containing salt water, a pair of glass or plastic panels sloping at an angle above the basin and meeting at the apex, creating a structure much like a greenhouse. The basin is generally painted black to maximize the absorption of long wave radiation falling on the surface. Solar radiation falls on the sloping panels and the greenhouse effect that is produced in the inside raises the temperature of the salt water held in a basin. Water at the surface is evaporated, the water vapour rises in the still and reaches the sloping panels, where it condenses to liquid water and runs down the sides of the panels. The water is collected and drawn off to provide fresh water. Solar stills can produce 3–4 L of fresh water per day per square metre. Because of low production rates, it is important to minimize capital costs by using very inexpensive construction materials. Efforts have been made by various researchers to increase the efficiency ofsolar stills by changing the design, by using additional effects such as multi-stage evacuated stills and by adding wicking material, and these modifications have increased production per unit area (Buros, 2000). In the simple solar still shown in Fig. 11 , the latent heat of condensation is dissipated to the environment. However, the latent heat of condensation can be used to pre-heat the feed-water, and this leads to an improvement in the efficiency. Solar still technology requires a large area for solar collection so it is not viable for large-scale production, especially near cities where land is scarce and expensive. The comparative installation costs tend to be considerably higher than those of other systems. Solar stills are also vulnerable to damage by the weather. Labour costs are likely to be high due to the need for routine maintenance to prevent scale formation and to repair vapour leaks and damage to the glazing panels (Buros, 2000; Miller, 2003). However, they can be economically viable for small-scale production for households and small communities, especially where solar energy and low cost labour are abundant.
3.2 Indirect solar desalination
In these systems, the plant is separated into two subsystems, a solar collector and a desalination unit. The solar collector can be a flat plate, evacuated tube or solar concentrator and it can be coupled with any of the thermal desalination processes types which use the evaporation and condensation principle, such as multistage flash distillation (MSF), vapour compression (VOC),multiple effect evaporation (MED), and membrane distillation (MD) for possible combinations of thermal desalination with solar energy. Systems that use photovoltaic (PV) devices tend to generate electricity to operate reverse osmoses (RO) and electro dialysis (ED) desalination. (Mahmoud Shatat ∗, Mark Worall, Saffa Riffat)
4. Design of solar powered membrane distillation:
Membrane distillation is very suitable for compact, solar powered desalination units providing small and medium range output <10000 l/day. The minimum sized compact membrane distillation system designed to produce a drinking water output of 150 l/day from sea-or brackish water. The main aim of the system design is a simple, self-sufficient, low maintenance and robust plant for target markets in arid and semi-arid areas of low infra structure. The required thermal energy is supplied by a 6.5 m3 solar thermal collector field. Electrical energy is supplied by a 75 W PV-module. This must be equipped with sea water resistant pipes. The required components to design a compact solar powered membrane distillation system of capacity 150 lit/day at 6.5 KWhr. /dm2 are represented in the below Table. (Baskaran.R 2014)
5. Exergetic analysis:
The property exergy is the potential of a system in a specified environment and it is defined as the maximum amount of useful work that can be obtained as the system is brought to equilibrium with the environment. Unlike energy, exergy is not conserved (except for ideal or reversible processes). Rather exergy is consumed or destroyed, due to irreversibilities in any real processes. The following equation represents a balance of exergy for any system and process.
(Total exergy entering)-(Total exergy leaving)-(Total exergy destroyed)=(Change in total exergy)
The use of exergetic analysis can shed light on the plant components which can be improved further. On the basis of the experimental results obtained, this section presents an exergy analysis of the compact and large desalination systems. Such analysis helps to determine the sites of the highest entropy generation and thus exergy destruction, and to identify the components responsible for the greatest losses in the systems. In general, when the exergy losses are high in one part, we should consider improving this part first.
The exergetic analysis of these MD plants was performed by implying the following assumptions: (Fawzi Banat,Nesreen Jwaied 2008)
1. The salinity of the feed water is constant.
2. The kinetic and potential energies of fluid streams are negligible.
3. Adiabatic heat exchanger.
6. Estimation of energy requirements
6.1. Thermal energy
The objective of the study was to produce 100 l/h of distillate water by a VMD process to be on average 600–800 l/day for 6–8 h of operation. The collector area depended on the level of inlet collector temperature that must be maximized. We proposed to recover the condensation energy. The optimum operating temperature was 75 ◦C; this temperature ensures high productivity without compromising the mechanical strength of the membranes which cannot withstand high temperatures. The heat quantity to be supplied by the collector was defined by the specific energy process consumption Csp which was the number of kWh required to produce an m3 of distillate. (Samira Ben Abdallah∗, Nader Frikha, Slimane Gabsi)
This consumption was based on:
- Energy requirements to evaporate water at the membrane;
- Heat recovery method and efficiency (the condenser position)
- Thermal losses at the various elements of the pilot plant.
Csp = Lv + losses − Recovery
Lv was vaporization latent heat
Lv = 21106 J kg−1 ≈ 580 kWh m−3
The heat recovered quantity by condensation depended on the vapor temperature and thus on the membrane module configuration (number of compartments per module, a passes number). This quantity represents in reality the fraction of heat the recovered quantity that can be expressed with the following equation:
Recovery = FeffLv
With Feff was the condensation efficiency factor. An effective thermal insulation of the various plant organs carried to minimize heat losses. Indeed the specific system consumption can be expressed as follows:
Csp = (1 − Feff)Lv
Operating without heat recovery, the specific consumption is equal to the evaporation latent heat. Thus evaporating 1 kg of water requires 0.58 kWh (Csp = Lv = 2.1 × 106 J kg−1 ≈ 580 kWhm−3). Recovering 50% of the condensation heat, the specific consumption is equal to 0.29 kWh per kg of distilled water. The collector’s choice will be done after an economic study to determine the minimum collector cost.
6.2. Electric energy
The plant operation required the electricity. The electrical energy used to operate the circulating, feed and peristaltic pumps. We tried to assess its needs in addition to the need for local lighting and operation of the computer and the data acquisition chain. We estimated the total electrical energy required to 2 kW. This utility is not available in arid regions. We proposed to use PV cells to ensure the installation autonomy in electrical energy. The cells mounting were easy and fast. The panels were relatively light. The PV module lifetime was over 20 years. While the battery lifetime reaches 8–10 years. In addition, operating cost was low and their maintenance limited to one or two visits of a professional each year. We also note the positive environmental aspects that are not negligible. Indeed, no noise and smell, no additional electric line, no pollution and few waste.
7. DEVELOPMENT OF THE SYSTEM:
For reliable systems all components are important. The operating conditions concerning the handling of hot seawater and strong ambient conditions as expected in many potential installation locations are quite difficult. Therefore, special stress tests must be carried out on each component of the system in the laboratory before field tests of systems can be conducted successfully. Three different test stands were set up:
7.1 Seawater test facility:
Resistance against hot seawater is not given for most metallic materials. The special conditions caused by the intermittent operation of solar-powered systems thus complicate the choices of materials. For example, CuNi10Fe is used in many desalination plants and withstands hot seawater but needs a steady flow rate, or else pitting corrosion can occur and destroy components in a short time. Polymer materials also have to be tested. To give an example here, the maximum temperature to which standard polymer materials (PP, PE, PVC) can be used is in the same range in which the MD module is operated, but the stagnation temperature of the solar collector field is much higher. . The test facility consists of a fluid cycle with test lines for components made from different materials. MD modules can also be integrated and operated in the loop. Thus, long-term desalination performance can be tested by measuring the salinity of the distillate. (Joachim Koschikowski*, 2003)
7.2 Pump test facility:
The fact that the desalination systems will be operated as stand-alone systems requires a PV system as the supply for electrical auxiliary equipment. Most of the electrical power is consumed by the pump. Thus, the efficiency of the pump is an important influence on the design layout of the PV area and therefore on the investment costs of the total system. The pressure drop of the MD module and all other components in the loop should be as low as possible to minimize the pump energy. A test facility was set up to carry out investigations on different pump types concerning their specific energy consumption, their starting characteristics and their coupling to PV. Different pumps, auxiliary parts or the MD module can be integrated into the circuit. The pressure drop, temperature and volume flow can be measured as well as the electrical power consumption of the pump. (Joachim Koschikowski*, 2003)
7.3 Performance test facility:
Two important tasks of the work to be carried out are the system design for complete test systems by simulation calculations and the development of new spiral-wound MD modules. To carry out system simulation calculations, an empirical simulation model of the MD module must be developed, which is based on measured performance data. Exact performance measure-ments must be feasible to determine improvements concerning new module constructions. The dynamical behaviour of the MD module is a very important question for the system design with respect to intermittent operation of a solar collectors as a heat source. A test facility was set up that allows the determination of the module’s GOR and ἠ thermal value for different inlet temperatures and different feed volume flows. Also the dynamic start-up and cool-down behaviour can be monitored. (Joachim Koschikowski*, 2003)
8. Cost estimation:
The cost of desalination is usually a function of: plant capacity, feed water quality, pretreatment, process technology, energy cost, plant life and investments amortization. The major cost elements for desalination plants are capital cost and annual operating costs. Capital cost covers purchasing cost of equipment, auxiliary equipment, land, installation charges and pretreatment of water. Annual operating costs are the total yearly costs of owing and operating a desalting plant. These include Amortization or fixed charges, operating and maintenance (O&M) costs and membrane replacement costs (Fawzi Banat*, Nesreen Jwaied 2008).The parameters that affect the cost of the system are shown in the figure below.(Lilian Malaeb et al 2014).
8.1 Capital cost:
Calculation of the capital cost depends on the process capacity and design features. Table 1 lists the technical values used to estimate the water production cost for the desalination units. The following cost basis was used in the estimation of the capital investment of the desalination units: (Fawzi Banat*, Nesreen Jwaied 2008)
•Membrane price is $36/m2 .
•PV-module price is $5/Wp .
•The installation cost is 25% of the purchased equipments costs.
8.2 Annual operating costs:
Annual operating cost covers all expenditure incurred after plant commissioning and during actual operation. These include:
8.2.1 Amortization or fixed charges:
When the required funds for the project have to be borrowed, there will be an interest charges for the use of the required funds. This item accounts for annual interest payments for capital cost. It is obtained by multiplying the capital cost by an amortization factor a, which is given by where i is the annual interest rate (%) and n is the life time of the facility. (Fawzi Banat*, Nesreen Jwaied 2008)
8.2.2 Membrane replacement cost:
Replacement rate may vary between 5% and 20% per year. The lower bound applies to low salinity brackish water and the upper would reflect the high salinity seawater. Membrane replacement cost is estimated for a 1-year period and divided by the quantity of water to be produced during the year to determine the overall water cost.
The average annual cost of a desalination system depends on the expected lifetime, the interest rate, and the total initial investment. (Fawzi Banat*, Nesreen Jwaied 2008)
The values of the common economic parameters are listed below.
•Plant life expectancy is 20years
•Operating and maintenance (O&M) costs are 20% of plant annual payment
•Zero pretreatment costs
•Interest rate is 5%.
•Annual rate of membrane replacement is 20%.
9. CONCLUSION:
The development of solar powered membrane distillation system is an important task to provide people on rural remote areas with clean potable water. The fact that the lack of drinking water often corresponds with a high solar insolation speaks for the use of solar energy as a driving force for a water treatment system. MD system is a process with several advantages regarding the integration into a solar thermally driven desalination system. Recent development and technical improvements in desalination technologies have significantly reduced the cost of desalination in recent years. Also it has reduced the energy requirement by using Solar as well as the waste heat driven MD process. Due to increase in use of solar technology the over consumption of fuel or other conventional sources has reduced. Though the solar powered systems are less efficient in desalination of water than the fuel powered system, they have an advantage of reduction in the operating cost or energy cost. As a result the solar powered MD system has proved to be a reliable technology.