The desalination process is aimed at removal of dissolved salt from sea / salt water, typically by using methods like thermal distillation and membrane separation. Conventional thermal methods include: Multi-effect distillation (MED); Multistage flash distillation (MSF); Humidification Dehumidification (HDH); Mechanical-vapor compression (MVC); and Thermal vapor compression (TVC). These processes consume significant thermal energy and in turn, cause carbon emissions (in form of carbon dioxide).
Amongst them, the process of MED & MSF are commercially matured and are operational in various parts of world. As compared to MSF, MED is typically less expensive due to lesser heat transfer surface requirements, better thermal efficiency and low electrical consumption. Membrane separation techniques are also used for water purification, e.g. Reverse Osmosis (RO), Membrane Distillation (MD), and Electro-Dialysis (ED) / Electro-Dialysis Reversal (EDR). Particularly, RO and MD are most commonly used for desalination of sea water, brackish water, groundwater and other salty surface water resources.
RO is the most popular method among the existing commercial membrane technologies for the desalination of sea / brackish water. Rapid developments in membrane technologies have made it cost competitive with the thermal processes. However, the membrane technologies also consume energy, which in turn cause carbon emissions.
Thus, in general, all the conventional desalination process has some disadvantages such as associated energy consumption and resultant emission of CO2. Also these processes do not allow total recovery from incoming sea water or brackish water and generate a reject water stream with concentrated salt content, which is rather difficult to dispose. Some of these shortcomings of traditional desalination techniques could be addressed by the solar desalination processes.
Solar desalination is an integration of a desalination process with a ‘solar energy’ technology. Here, the basic energy necessary for desalination is obtained from the solar energy. Solar desalination systems are broadly classified into: direct method and indirect method. The direct method uses a combined solar energy collection mechanism and the separation (distillation) mechanism.
They are integrated, so as to produce distillate water directly in the solar still / collector. Desalination processes likes MED, MSF & TVC can be operated by direct solar energy. In the Indirect method, the plant has two separate sections – solar energy collection and the traditional desalination plant. The solar energy is collected in a compatible form e.g. in form of electricity (PV solar) or even in form of solar thermal energy, and this energy is provided to the traditional desalination plant. e.g. the electricity produced by the solar Photovoltaic (PV) system can be used to run the traditional desalination processes using techniques like mechanical vapor compression (MVC), reverse osmosis (RO), membrane distillation (MD), electrodialysis (ED) etc.
Direct solar desalination, as compared to the indirect technologies, requires larger land areas due to lower productivity. For direct solar desalination, typical unit size is restricted to a smaller scale. It also suffers from higher cost of construction material. However, it offers a simpler unit than an indirect solar desalination plant. On the other hand, the indirect solar desalination can get benefited from the economy of scale, especially on the desalination side. The indirect method is also suitable for brownfield units where the energy source of the existing traditional desalination units could be switched over to the renewable & sustainable energy source like solar energy. Table 1 provides mapping of various desalination technologies with the options of capturing solar energy.
As compared to MSF, MED is typically less expensive due to lesser heat transfer surface requirements, better thermal efficiency and low electrical consumption. Solar desalination could also be useful for the remote areas that have no electricity / grid connectivity. There are two solar desalination units coming up on commercial scale in the middle-east.
Techno–economic perspective for solar desalination
The cost of a desalination process mainly depends on the source and quality of feed water, site conditions, the plant location, capacity of plant or size, local energy costs and plant lifetime etc. While the traditional desalination technologies like RO and MSF / MED are mature technologies, solar energy capture technologies are in the rapid development stage and their costs (especially that of PV) have decreased rapidly, over last few years.
Typical energy consumption and water product costs are compared in the Table 2. The product water cost of conventional & solar desalination is included direct, indirect & annual operating cost and capex also.
Table 2 information is collected from various source of publications like report of combined solar power and desalination plants; Techno-economic potential in Mediterranean partner countries, June 2009; V.G. Gude et al 2010, renewable energy and sustainable energy reviews, 2641-2654; Ali Al-Karaghouli & L.L.Kazmerski, 2012, WREF, 13-17; Adrian Pugsley et al 2016, renewable energy, 200-219. It may be noted that that the Table 2 data is from the reference articles published in period between 2008-2016 and all of this data may not have factored in the rapid advances / significant cost reduction that has happened in the PV technologies in the last few years.
The present analysis indicates that –
- The energy requirements are largely unchanged for traditional and solar desalination. They vary as per the desalination technique deployed rather than the source of energy.
- While the solar energy is essentially free, there is opex (operating expenses) as well as capex (capital expenditures) associated with the solar plant that impacts the final water product cost. For solar desalination, the plant capacity considered is relatively smaller, resulting in high capex per m3 of product water, which reflects unfavorably in the product cost.
- Per unit cost of thermal energy could vary wildly depending on locations and availability of waste heat etc. That of electricity could also vary based on location, fuel, efficiency of power generation etc.
- The capex of the solar desalination is expected to be higher due to the (high) cost of solar plant. In the recent years, the solar plant costs, especially PV solar costs, have reduced significantly due to rapid technology advancements, which would lower the product cost.
- Is the carbon credits are considered for the renewable energy like solar; it would impact the product costs favorably for solar desalination technologies.
Currently, there are a couple of commercial solar desalination plants of significant capacity, coming up in the Middle-East. It highlights the traction this technology is creating while riding on the solar energy advancements. Solar desalination could also be useful for the remote areas that have no electricity / grid connectivity.
Further advances in solar desalination
With significant pressure from stricter environmental emission norms and leap-frogging of technologies in solar energy capture, especially in PV area, the solar plant costs are decreasing rapidly. This is providing impetus to the attractiveness of the solar desalination. There are also newer technologies like Aqua4 from WaterFX where the solar desalination is coupled with a ‘Zero Liquid Discharge (ZLD)’ concept.
Most of the traditional desalination techniques result in the reject water stream, especially while desalinating sea-water. The Aqua4 technology seems to enhance the water recovery so as to eliminate the reject water stream and instead, aims to recover the salts. Higher water recovery coupled with saleable salt product could help in better product realization. A commercial unit is being planned in California based on this technology. Figure 1 shows a schematic diagram for RO desalination process with Zero Liquid Discharge (ZLD) system, which is powered by PV.
Solar energy is cyclic and hence poses some challenges for continuous / steady operation of desalination. However, these issues are not of “insurmountable” nature, and are being addressed with engineering solutions. Advances in the energy storage technologies are also expected to supplement solar technologies. Hybrid approaches of technologies are also being considered by Ali Al-Karaghouli et al 2010, renewable energy, 323-328 e.g. the PV panels could be cooled with feed water to enhance their performance and the preheated water could also enhance the desalination performance.
Solar desalination deploys renewable solar energy for effecting desalination. It makes the conventional desalination a “greener” option. In the past, higher costs of solar energy capture had an unfavorable impact on the water product cost from solar desalination. However, rapid technology advancements in recent years, especially in the PV solar, are lowering the water product costs from solar desalination.
This is opening up vistas for solar desalination units especially in the areas where there is ample sunlight, shortage of sweet water but have access to sea/brackish water. There are two solar desalination units coming up on commercial scale in the middle-east. There is also one unit coming up in California based on Aqua4 technology that combines solar desalination with Zero Liquid Discharge (ZLD) concept.