Solar Energy for Water Desalination in Remote Areas
Introduction
With the increase in the global population and with the ever worsening global warming situation, the world is undergoing a gradual but serious fresh water shortage. The hardest hit areas are the arid regions primarily located in Africa and Asia. Because of this decline of accessible fresh water, there is no other option but to use available technologies in desalination to purify the available saline water from the seas and other sources in order to meet the global demand of fresh water. Because the regions that mostly require additional fresh water are the ones with the highest levels of the sun’s radiation, it is widely accepted that thermal solar desalination is the best technology for using renewable energy sources in the production of this additional fresh water. The main challenge of the utilization of solar thermal energy in the desalination process is that the final output and the entire system’s output are very low. Large scale desalination by this method would also require that a considerable size of land be dedicated to the establishment of the plants. On the other hand because solar energy is freely available, the system is appropriate for small-scale desalination. This is especially appropriate in the arid and remote areas where access to other sources of energy is limited. Solar desalination systems may either comprise separate or integrated constructions for the solar harness and the distillation system. The integrated systems operate through usage of solar radiation to distil saline water to produce fresh water. In separated systems the solar radiation is used to generate electrical energy which is used to power the systems for conventional desalination systems. Both the direct and indirect desalination systems are discussed below.
Direct solar desalination
This system is appropriate for smaller establishments particularly in situations where the daily freshwater requirement is less than 200 m3. The diminished production rate is defined by the reduced functioning steam temperature and pressure. The primary solar still is basically a trough covered with a transparent solid material such as glass, which should be tilted for the whole setup to work. Sea water is placed in the trough and when it is hit by solar radiation, it heats up and evaporates. The water vapour formed mixes with the air that was originally between the water and the glass cover. Once the vapour hits the glass cover, the now humid air cools and condensation happens and the formed water trickles along the glass surface for collection at the edge (Rizzuti, Ettouney and Cipollina 2007, 130; 296). An example of this kind of setup is shown in figure 1 below.
The primary disadvantage for this kind of desalination setup is that it uses thermal energy at a very low efficiency level, which means that the final output of the production is also reduced. Various methods of dealing with this setback have been established and these include the mixing of black dyes into the seawater with an aim of keeping the heat conduction through the walls of the trough and glass cover at a minimum. This strategy also ensures that all the latent heat produced by the condensation process is recycled. Separation of the solar collecting system and the sea water will help in increasing the efficiency of the system through the prevention of damage by corrosion.
Single-effect solar stills
The pre-modern solar desalination constructions had a single coat of glazing covering the still. This made the system very inefficient as most of the latent heat from the condensing water vapour was easily lost by conduction through the still into the atmosphere. With further developments modifications were made particularly in regards to the glazing of the still and better stills with efficiencies of upto forty percent have been developed. These stills can produce over five litres per square metre of the collector surface on a daily basis (Micale, Cipollina and Rizzuti 2009, 21-25). However, this is not the optimum and more research is ongoing to further perfect the system. Some of the developments include the usage of both passive and active solar stills in the construction of the desalination systems. While the passive stills only use the heat generated by the still in the evaporation, active solar stills utilize external heat sources. These external sources include a solar harness or waste heat generated by factories. These development approaches are illustrated in figure 2 below.
Basin stills
Basin stills are studied under two main categories as explained below
Single-slope stills/double slope basin stills
Comparisons of the two categories of the sloping solar stills reveal that because of the constant movement of the sun, the double slope basin still is more efficient as far as the absorption of the sun’s radiation is concerned. The double slope basin stills are more effective in warm climates. The single slope basin still on its part has the added advantage of reduced heat loss to radiation and convection. As a matter of fact, the side under shade can be properly utilised to provide further surface for condensation (Bahadori 2005, 71). The single slope basin stills are more effective in cold climates.
Stills with extra cooling systems
Increasing the difference in temperature between the salt water and the glazing has the compounding effect of raising the rate of evaporation. This effect can be achieved by either providing additional heating to the water or reducing the temperature of the condensational surface through the introduction of a cooling system. One of the most commonly used strategies is to introduce an external condenser into the solar still. The condenser, which is designed as a packed-bed storage tank, is cooled during the night by ensuring that water flows through it and into a radiation-operated cooling surface. This system is illustrated in the figure 3 below.
During the night, the cooling panel uses the cold sky temperature to reduce the heat in the condenser (Haddad et al. 2000). In this way, the temperature within the holding tank is reduced to almost the same level as the effective sky temperature. Once daytime begins water is cleared from the storage tank just as vapour begins to collect on the solar still collecting panel. The buoyancy forces within the condenser make it easy to drive the vapour through the channel between the still and the condensation system. As such, the system does not require more external force to move the collected water through the system. The above construction has two primary advantages. First, is that the diminished heat loss within occasioned by lowering of the temperature inside the still coupled with the reduced partial pressure in the still make the rate of evaporation go up. Secondly, the reduction of temperature inside the condenser increases the condensation rate (Haddad et al. 2000, 464).
Stills combined with green houses
A combination of solar stills with green houses has been used in the establishment of small scale cultivation in regions where the only water available is salty. One of such constructions is illustrated in figure 4 below and its design includes using as part of one of slopes of the roof of the greenhouse.
For this system, water is pumped from a storage tank to the top of the roof of the greenhouse in daytime. From here it is well circulated across evaporation surface of the still. The cover of the still is made of a basic sheet of glass while the bottom is made of a translucent material which absorbs some of the solar radiations but lets through wavelengths that are suitable for the photosynthesis process of the crops below (Haddad et al. 2000, 459-469).
Because a big percentage of the heat radiation is taken up in the still, there is a substantial reduction in the temperature of the air in the greenhouse. This ends up creating a favourable climate for the crops coupled with a reduced requirement for ventilation. Consequently, the crops in the greenhouse consume less water hence making the greenhouse very efficient.
The vapour that arises from the condensation process on the top glazing flows across the inner wall of the cove and is tapped elsewhere as freshwater (Haddad et al. 2000). The returned feed water is collected elsewhere and is fed into the cycle once more. This system can operate during both daytime and at night mainly because the excess heat collected during the day is stored with the saline water reservoir and can be reused when the sun goes down.
As far as the benefits of the greenhouse are concerned, the combination of the desalination system and the greenhouse results in a substantial reduction in the harshness of the climate inside the greenhouse.
Also, because half of the greenhouse roof is made of the solar still, there is more freshwater available for irrigating the low canopy crop. Gains in the desalination system indicated that the integrated system is more economical than the basin stills and single effect solar desalination systems by 34.5% and 49% respectively (Haddad et al. 2000, 459-469).
Multi-effect solar stills
These types of solar still reuse a certain amount of the latent heat generated by the condensing vapour on the cooling panel. The latent heat is direct to either preheat the salty feed water or to heat the water inside the still (Wang, Chen and Shammas 2010, 536). Preheating the feed water is achieved by combining the feed water channels and the condensation panel. In this way the seawater is continuously heated by the heat generated by the condensing vapour while at the same time ensuring that the condensation area is well cooled. Such a construction can produce more that twenty litres per square metre on a daily basis.
Solar still with preheated feed water
This type of solar still is split into upper and lower chambers. The upper chamber is used for the evaporation process while the lower is used for the condensation. The two chambers are separated by a sheet of metal leaving only a small opening in the top area of the still.
The freshwater is collected in a separate reservoir placed below the condensation chamber. Figure 5 is an illustration of a typical desalination system with preheated feed water.
Water desalination with humidification-dehumidification
One of the major setbacks of the desalination systems and which greatly affects the performance of the systems is the direct contact between the collector and the salty water. This is because, such a direct contact may end up causing corrosion within the still resulting in a reduction in the thermal efficacy of the system. In humidification-dehumidification desalination air takes up the role of a working fluid, thereby decreasing the corrosive/scaling effect of the salt in the stills (Borouni et al. 2001, 167-176). These systems comprise a well sealed unit made up of two heat exchangers-one of evaporation and the other for condensation (Fath and Ghazy 2002, 119-133). The construction of such a system is cost effective mainly because the material used is lightweight and cheap. The system operates at atmospheric pressure and this makes it affordable to operate.
Desalination with humidification.
This system is also described as dehumidification using an open system and it operates by first heating saline water using solar radiation in a collector and then spraying it onto the wall of the humidifier (Fig. 6). The humidifier’s wall’s surface assumes the structure of a honeycomb and once air is blown through it, it becomes hot and humid.
The hot and humid air is then released into the condensing surface between the tubes carrying feed water. Here condensation happens and fresh water flows into a separate collection reservoir. As a means of raisin the thermal efficiency, the preheated seawater which is not collected by the air in the honey comb wall of the humidifier is tapped and channelled back to the tank containing saline water from where it is passed through the system for another cycle. The efficiency of the system is increased by reduced pressure loss owing to the fact that air in the chambers travels in straight lines. The thermal efficiency of such desalination constructions can be as high as 85%.
Solar air-heating with stepwise humidification
The reduced amount of water produced by typical skills is to a large extent a factor of the small difference between the moisture content of the air prior to the condenser and after leaving it (Chafik 2002, 26). This problem can be solved by heating and humidifying the air through a series of steps within a basic solar collector. This has the double effect of increasing the vapour content in the air and reducing the needed air flow rate. In this type of setup, air with a particular temperature and humidity (such as 25ºC and 10 g vapour/kg dry air) is passed through the solar collection area (Chafik 2002, 25-37). Here the temperature of the air is increased to 50ºC and the released product is sprinkled with sea water to increase its humidity levels. The temperature of the air after this process drops to 23ºC. Next the air is subjected to the heating and humidification process again until its temperature rises to 50ºC and moisture content increases to 28g vapour per kilogram of dry air. After humidification, the air temperature decreases to approximately 31ºC. After this, the air is heated to 56ºC before it is sprinkled with water to raise the humidity levels to 36g of vapour per kilogram of dry air. After humidification in this step, the temperature of the air drops to 35ºC (Chafik 2002, 25-37).
The heating and successive humidification happens 15 times until the air reaches a temperature of 60 degrees Celsius and a humidity level of 148g of vapour per kilogram of dry air (Chafik 2002, 27). Cooling the warm and humid air released after the final step to 25ºC results in a release of 127g of freshwater per kilogram of dry air (Chafik 2002, 25-37).
Indirect solar desalination
Across the globe desalination industries are responsible for the generation of approximately 236 m3 of fresh water (Garcia-Rodriguez, & Gomez-Camacho 2000, 213-218). It is for this purpose that large scale desalination systems have been developed. Most of the industrial desalination establishments are either multi-effect, multistage flash or reverse osmosis (Garcia-Rodriguez, & Gomez-Camacho 2000, 213-218). The three types of systems detailed above are more often than not powered using fossil fuels to generate the type of energy required for heating or generation of electric power. It is because of this usage of fossil fuels that makes the systems characteristically expensive to run. In recent days, however, renewable sources of energy are gradually taking over the role of fossil fuels in the desalination plants. Solar energy has particularly found usage because it is readily available, cheap and comparably easy to maintain.
The energy can be integrated into the three primary desalination systems resulting in the production of clean water at a reduced cost. Below is a brief discussion of how this integration can be set up (Fiorenza, Sharma and Braccio 2003, 2217-2240).
Multi effect
The multi effect desalination system is currently the most reliable of all other techniques. In the past this system was only used for production of over 100m3 per day primarily because it requires extensive technical maintenance and a large supply of electricity. However, the usage of solar systems to generate the required electrical energy to power the plants has made the systems more economically viable.
The multi-effect desalination system principally operates by stepwise evaporation of salty water. This is achieved using heat transferred from condensing steam while being transported through a number of tubes. The tubes in this instance are arranged into a number of bundles and the evaporation takes place in an average of 12 steps (Seimat 2000, 54-65).
In the initial steps, the condensing steam is produced outside the system using fossil fuels or through solar systems. After this, brine is evaporated to generate steam that is available at a lower pressure and temperature and this is used for more evaporation of the salty water. Research is till being carried out for the development of multi-effect desalination systems which utilize solar heating (Manwell and McGowan 1994, 229-241).
Multi-Stage Flash
This is the most commonly utilised desalination procedure for the production of larger amounts of fresh water. The system is responsible for the daily production of over ten million tonnes of fresh water across the globe. The multi-stage flash system functions using pressurised sea water which is forced through sealed tubes across which an exchange of heat is made with condensing vapour (Fath 1998, 45-56). Once the saline water is heated to a certain temperature it is directed into a low pressure chamber. Inside this chamber, flash evaporation is done yielding vapour which is then condensed and tapped as fresh water.
Reverse osmosis
A substantial number of regions around the globe are faced with extreme water shortages and as such are forced to depend on desalination to purify the available water for domestic use. Reverse osmosis is one of the desalination methods that has found great usage in the Mediterranean, Middle East and North African areas. Reverse osmosis is a desalination process that works by using semi-permeable membranes to separate water from concentrate salt (Glater 1998, 297). The membranes for use in this process are commercially available with some of them retaining upwards of 90% of salt from sea water while using pressures of about 60 bar. Unfortunately, these systems online yield about 46% of fresh water from collected sea water. Reverse osmosis can be combined with solar radiations to yield even more fresh water. This is a technology that was first tried and tested in the late 1970s to create state of art solar-powered reverse osmosis desalination systems. Over the years research has been carried out in countries with conditions that favour the operation of solar driven desalination. These countries are distinguished by remoteness, extreme water scarcity and intense solar radiation (Garcia-Rodriguez et al. 2002, 135-142).
There are three main categories under which solar powered reverse osmosis systems can be classified, all depending on the kind of technology employed in the solar sub-unit (Garcia-Rodriguez et al. 2002). These are briefly detailed below:
The Photovoltaic powered reverse osmosis systems
For this system, reverse osmosis membranes are coupled with a number of different photovoltaic modules. Photovoltaic cells found prominent usage in solar-powered desalination systems primarily because they were the first form of technology that was commercially developed to tap energy from the sun. Photovoltaic is still considered the favorite technology for the purpose. In the photovoltaic reverse osmosis desalination systems, the direct current created by the solar cells is channeled to drive the pumps which create the pressure needed to push the water through the filtration membranes. This system has undergone considerable improvements over the years. Unfortunately it is yet to find widespread commercial acceptance owing to the low conversion efficiencies (approx. 15-16%) as well as the prohibitive retail cost of the photovoltaic modules.
Components of the photovoltaic-powered reverse osmosis desalination systems
Solar sub-units
These are basically the photovoltaic modules and they are classified into two categories, the mono-crystalline ad the multi-crystalline modules. The orientation of the modules is a critical determinant of the system’s efficacy especially because it determines the total output of generated power. Modules that have fixed axes retain a particular angle and position while those with flexible axes can easily be re-adjusted depending on the season for maximum efficiency. The latter can be modified to include a tracking system such that the modules automatically follow the path of the sun. Such tracking systems have not been widely used because of their high costs. As an alternative, the cheaper Maximum Power Point Trackers (MPPTs) are used to assess the usage of power and ensure that the system attains maximum efficiency while utilizing the least amount of power (Harrison, Ho and Mathew 1996, 509-513).
Water extraction unit
The pumps used to direct the collected feed water to the reverse osmosis pretreatment system is powered using electricity generated by the photovoltaic modules. Such solar-powered pumps are very reliable particularly in remote regions besides having the added advantage of being cheaper to maintain (Alawaji et al. 1995, 283-289).
Pre-treatment unit
This is basic reverse osmosis pre-treatment system comprising a filtration grid with a 5 μm pore size which is preceded by a 25 μm filter (Cheah 2004). This grid is followed by an active carbon dependent fine filtration system. This active carbon system is mainly used to trap the free chlorine particles which can easily damage the reverse osmosis membranes. If the feed water contains a very high level of bacteria, a chlorination/ozonation system can be included to prevent biofouling of the membranes (Cheah 2004, 22).
High pressure pump and motor
In this section, only positive displacement pumps can be utilized since this is a part of the desalination system that requires extreme energy efficiencies at low flow rates. The pump motors can be powered using direct current generated by the photovoltaic modules. In case the only pumps available require alternating current, an inverter can be introduced between the photovoltaic modules and the motors.
Reverse osmosis membranes
These membranes are wound in a spiral manner in order to increase the contact surface. The most commonly used reverse osmosis design is the single pass system whereby all the membranes are arranged in series with the pressure systems (Richards and Schäfer 2003, 2013-2022). In order to increase the total recovery, the emerging concentrate can be re-circulated through the pressure vessels and across the membranes.
Aside from these primary components of the photovoltaic reverse osmosis desalination plants, a number of other elements can also be included into the system. These are briefly discussed below:
AC/DC inverter
Desalination systems whose functioning is based on pressure pumps driven by alternating current powered motors need to have inverters to convert the direct current generated from the photovoltaic cells into the required alternating current. The usage of motors driven by direct current gets rid of the inversion systems. However, these motors are more expensive than those driven by alternating current.
Electrical storage
The excess electrical energy generated by the photovoltaic cells can be stored in batteries and then used to run the system during night time. Unfortunately, even though batteries make the desalination system produce steadily they come with a number of disadvantages. First, the installation of the batteries and subsequent replacement substantially increases the expenditures associated with the project. Secondly, after installing other components such as charge controllers which increase the effectiveness of the batteries, the entire system becomes more complicated demanding the need of a trained professional to run the system (Mohamed et al. 2008, 17-22). If proper maintenance is not provided, the life of the batteries is reduced hence translating to more operating costs for the systems.
Solar-thermal powered reverse osmosis desalination
This system operated by harnessing heat from solar radiation and transfers it through an appropriated fluid to a thermodynamic steam-powered system for the generation of the power needed for the filtration process (Delyanis 1987, 3). These systems, even though effective, have been less utilized in the desalination process as compared to the photovoltaic powered systems. In early experiments, salinity gradient solar ponds and flat plate collectors were utilized in the systems owing to their non-concentration traits (Delyanis 1987, 3-19).
Salinity-gradient solar ponds
Solar ponds are shallow collections of water that have the ability to store thermal energy in lower layers. When the sun shines across a still mass of water, some of the radiation is absorbed by the water resulting in a rise in temperature. The water on the surface releases this additional heat into the atmosphere as a result of convection in the air around. This loss of heat makes the water at the bottom warmer and lighter than the water at the top thereby creating a convective circulation with the water below rising and the denser water at the top sinking to the bottom (García-Rodríguez and Delgado-Torres 2007, 319-327). In order to prevent the top and low layers from mixing, a salinity barrier is created between them. A basic solar pond comprises three layers with varying temperatures and salinity. The layer at the top which is approximately a meter deep is generally at the same temperature as the atmosphere above and has a low amount of salt. The middle layer is referred to as the gradient zone and it is usually about two meters deep. The temperature and salinity of the water in this region increase with depth. The third and bottom layer of the salinity gradient solar pond also known as the storage zone is usually extremely dense and can contain temperatures of upto 100 degrees Celsius (Hussain 2003). The middle layer basically acts as insulation for the bottom layer mainly because the water in this region cannot rise to displace the lighter water above it neither cannot go down to take the space of the denser water below. This prevents convection resulting in heat being retained in the bottom layer and it is utilized by circulating the brine from this layer through a separate heat exchanger. These types of solar ponds usually need a lot of land and this is the primary reason why it makes sense to position them in deserts or other remote areas. The salinity gradient solar ponds have the advantage of cost effectiveness, owing to the fact that they do not use fossil fuels for heating the desalination plants.
Flat plate collectors
Flat plate collectors are made of a clear flat front plate, followed by an insulating region and an absorbing back plate. Channels are created between the plates to provide space through which the heating fluid flows. These collectors are very effective in tapping thermal energy and they can reach temperatures as high as 90°C.
Hybrid solar reverse osmosis desalination systems
This technology combines the power generated by the solar systems and electrical power generated by other systems such as grid electricity and wind power. In this instance, solar is the primary source of power for operating the desalination system, while the auxiliary sources are used to support the solar system thereby increasing the amount of time that the system can be in operation per day.
Conclusion
Solar heat energy in the desalination of salty water has mainly found usage in small scale establishments in the remote and arid regions. As has been explained in this essay this is primarily because the solar systems are very inefficient aside from having a diminished production rate. It is because of this fact that less than 0.025 percent of all the fresh water produced by desalination is produced using solar energy. However, these trends are bound to change primarily because of a reduction in the global supplies of fossil fuel. The world’s fresh water requirement is increasing gradually especially with farmers turning away from rain-fed agriculture to irrigation. The combination of renewable energy with conventional desalination systems has the potential of offering the best solution for the challenges of fresh water supply. This report has provided an extensive study of the available solar desalination systems outlining both the positives and negatives of each system. The research has been heavily dependent on published literature pertaining to the topic. This is because in Technology and Engineering, like with any other academic field, chances are that extensive research has been carried out by professionals in the field before. Consequently, in order to establish the backbone of a given research project, it is only necessary that extensive review of literature be carried before identifying seeking first hand information from the field. The latter, i.e. information collected from the field is also necessary since it helps give professional credibility to the project. Combining results from both sources would serve to foster their symbiotic relationship with one offering background information and the other presenting up-to-date information on the topic. Empirical data has been used and backed with numbers and figures to give the discussion the kind of credibility that this type of scientific research demands. It is however worth noting that the study is not conclusive especially because more research on solar desalination is still ongoing.
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