The Indian subsidiary of SWS (SWS&GB Pvt, Ltd) successfully tested the solar desalination plant installed at Coimbatore (Tamil Nadu). The steam feeding the desalination plant is generated by solar collectors, and the desalination plant is supplied by SWS&GB on turn-key basis. The technology is MED/TVC designed with 6 effects, each equipped with horizontal aluminium tubes over which the sea water is sprayed and forms the evaporating falling film.
The successful test proves the effectiveness of the thermal desalination for the exploitation of the solar energy.
Solar desalination as the name suggests is a technique to desalinate water using solar energy. Reverse osmosis and solar humidification – dehumidification are the two major types of solar desalination.
In the middle of the 20th century, the world had about 4,000 cubic meters of fresh water per person per year, according to DHI Water Group. Now we’re close, globally, to 1,000 cubic meters per person per year — and 1,000 cubic meters per person per year is defined as water scarcity.
As said earlier, if you can get water that is meant for drinking at 22 p per litre, isn’t it a great opportunity. That is the cost of the desalinated water. Detailed costing and references are given below.
In this blog, I have given - What is solar desalination, TERI - Solar desalination unit, Solar water desalination plants in Gujarat, Solar powered desalination plant in Saudi Arabia, Portable solar powered water desalination system by MIT, Solar powered water desalination system by MEMSYS, C water solar powered water desalinator for life tracks, Output of large scale solar desalination unit, World's largest solar desalination unit
This note is meant to list the business opportunity that is there for the Indian entrepreneurs interested in solar energy and in particular solar desalination.
As mentioned below, the CM of Gujarat who is pioneering India's foray into the solar scenario is pretty ambitious.
Gujarats solar policy is aggressive. It makes a lot of sense to set up a solar desalination plants all along the 1600 km of coastline of Gujarat.
It is a business opportunity for all of India. India has a coastline of 6100 km.
It is possible to set up a solar desalination on an average distance of 30 km. That means we can set up about 200 solar desalination plants.
But the costs that I have talked about ie 20 p per litre of potable water is possible only in large plants. These plants may cost about $ 100 m.
I don’t think that should scare anyone so long as you can get saleable water at as low as 20 per litre.
Reverse Osmosis is currently the most popular and favoured technology for desalination. Reverse Osmosis is also the most cost-effective. Reverse osmosis is a pressure-driven process. It sort of forces the separation of fresh water from other constituents through a semi-permeable membrane. Solar energy is collected and converted into electrical or mechanical energy to initiate the process.
The solar humidification-dehumidification (HDH) process is also called the multiple-effect humidification-dehumidification process, solar multistage condensation evaporation cycle (SMCEC) or multiple-effect humidification (MEH). Water is evaporated and then condensed to separate it from other substances. The driving force in this process is thermal solar energy to produce water vapor which is later condensed in a separate chamber as the diagram below explains.
The Reverse Osmosis Solar Installation (known as ROSI) uses membrane filtration to provide a reliable and clean drinking water stream from sources such as brackish groundwater. Solar energy overcomes the usually high-energy operating costs as well as greenhouse emissions of conventional reverse osmosis. ROSI can also remove trace contaminants such as arsenic and uranium that may cause certain health problems, and minerals such as calcium carbonate which causes water hardness.
First solar powered desalination plant in India:
In 2006, Barefoot College set up India’s first solar powered desalination plant at a small voluntary organization called Manthan established in Korti. This desalination plant was based on reverse osmosis. This plant is powered by a 2.5KW power plant which helps it to produce 600 litres of water per hour, for 6 hours every day.
The RO plant reduces the locally available brackish water with a salinity of Total Dissolved Solid (TDS) 4000-6000ppm to 450ppm.
The plant meets the drinking water needs of more than 1,000 men, women and children from Kotri and its surrounding villages. Each family can take 40 litres of water every day for a token amount of Rs.10/ per month.
The brackish water, coming to the village through the regular government pipelines, is pumped through the RO plant and is stored in a 5,000 litre tank. The plant consists of a booster pump that costs INR 4,000, a sand filter, a cartridge and a carbon filter that prevents waste and impurities in the water from mixing with the desalinated water. It costs INR.15.5 lakhs to install a mini-RO plant specially designed for operating in a village to bring drinking water to its rural community.
TERI designed a solar desalination unit. It is done in collaboration with the Solar Energy Centre of the Ministry of Non-conventional Energy Sources. The prototype has been tried and tested at the Solar Energy Centre.
TERI’s current solar desalination unit consists of 10 flat-plate solar collectors. It has 4 trays that hold brackish water and a neat and efficient compact heat exchanger in the bottom tray.
Schematic assembly arrangement of TERI’s solar desalination unit
The unit is pretty capable to deliver 100 litres of water. No-fuss, closed-loop design and the total absence of conventional fuels of this unit makes it totally sustainable, affordable, and totally eco-friendly. The unit is easy to maintain since the system does not depend on pumps or any other active component that requires daily maintenance.
Economic analysis of the process establishes that the annual cost of desalinated water from the TERI-SEC prototype desalination system is less than that of water from a solar still.
Solar water desalination plants in Gujarat:
Gujarat government is planning to develop solar powered desalination water treatment plants on the 1600km long coastline of Gujarat. The desalination plants will be set up in 50 cities across the state. This is a terrific idea as most villages and towns along the coast can get adequate water. Solar power can light up the economies of the coastal villages and towns and what with good drinking water making them highly inhabitable. Gujarat has been in the forefront of solar and is already well known for its forward looking solar policy and its plan for solar city.
A mini solar desalination plant has been in operation in Ahmedabad, Gujarat since May 2008. The plant produces pure distilled water from saline ground water. It is also capturing and recycling the solar heat that is stored in the water. The stored energy can be converted to electricity using the ocean thermal energy conversion system.
World’s largest solar desalination plant - Solar powered desalination plant in Saudi Arabia:
Saudi Arabia expects to start its first solar-powered sea water desalination plant by 2013. This will be the world’s largest solar powered desalination plant which will produce 30,000 cubic meters of water for 100,000 people living in al-Khafji, Saudi Arabia.
The plant will be powered by ultra-high concentrator photovoltaic technology – a system with a concentration greater than 1,500 suns. IBM has joined with KACST (King Abdulaziz City for Science and Technology) to build the plant.
According to KACST scientists, the two most commonly used methods for seawater desalination are thermal technology and reverse osmosis. Both methods are high energy users with costs ranging from 2.5 to 5.5 Saudi Riyals per cubic meter (around US$1.50).
So the IBM-KACST team is also working to improve nano membrane technology that filters out salts as well as potentially harmful toxins in water while using less energy than other forms of water purification. The organizations say that by combining solar power with the new nano membrane, they will be able to significantly reduce the cost of desalinating seawater at these plants.
Researchers from IBM and KACST developed chlorine-resistant and fouling-resistant polymers that increase the permeability of the membranes without sacrificing selectivity.
Portable Solar-Powered Water Desalination System by MIT:
A team from the Field and Space Robotic Laboratory of Massachusetts Institute of Technology (MIT) has designed a solar-powered water desalination system which can be easily packed up for delivery to emergency areas.
The system’s photovoltaic panel is able to generate power for the pump, which in turn pushes undrinkable seawater through a permeable membrane. Once the salt and other minerals are removed, the water can then be drunk. The system even has sensors that enable water purification even without high levels of sunlight.
The water desalination system produces 80 gallons of drinking water per day, depending on weather conditions. A larger version is also being designed, which will cost $8,000 and will be able to provide 1,000 gallons of water daily.
Solar Powered Water Desalination System by Memsys:
Memsys, Singapore developed a new technology called vaccum multi-effect membrane distillation which combines the two most popular forms of desal tech, thermal distillation and membrane distillation. The desalination unit is completely mobile, does not require any external parts, and fits inside a 20-foot shipping container. It is powered using both solar thermal and solar PV, and requires little maintenance. Unlike existing desalination technology, it does not need chemicals, diesel generators, or any other external power. Depending on the amount of sunlight, it is expected to produce, on average, 1000 litres of drinkable water per day.
The special process combines thermal distillation and membrane distillation, the most common process used by desalination systems. Water is boiled in a small vacuum at lower temperatures, 50-80 degrees Celsius (or 122-176 Fahrenheit), and the steam is passed through several membrane distillation stages, also at lower temperatures and pressures. Energy is recovered after each step in order to power the next, making it a far more efficient system than current technologies. The system is also able to handle very high salt concentrations, like those found in saline groundwater.
C-Water: Solar Powered Water Desalinator for Life Rafts:
Chinese Engineer Chao Gao has designed an award-winning experimental desalination product called C-water that uses solar power to make drinking water. Its distillation technique is very simple.
The C-Water can be placed on a damp surface or on the water in direct sunlight and, like a greenhouse; it uses solar rays to heat the water, separating out any impurifications. The evaporated water vapor then condenses on the C-water’s roof, where it is collected in a separate area. Within 47 hours the unit can produce enough fresh water.
The device could also be deployed a larger scale to produce great amounts of water, but for now the idea is being discussed as a life-saving solution for life rafts.
Output of large scale solar desalination plant:
A theoretical proposal is outlined for large scale solar desalination using multi effect humidification. It involves the use of a large area solar collector, multi effect distillation and boiling at reduced pressure. The configuration devised is a circular tank of one kilometer diameter containing water to a depth of 10 meters with a sealed double glazed dome, operating at 0.1 atmosphere pressure with a working temperature below 50° C. A solar absorber placed just above the water level, abundantly perforated but covering the entire area, sets up convection currents that evaporate the sea water and condense the vapour. Incoming seawater recovers energy from outgoing clean water and brine in a counter current heat exchanger. Water flow is driven by solar distillation and hydrostatic pressure. It is estimated that the structure would have 95% energy efficiency and a gained output ratio of 20. In sunbelt countries with average isolation of 6kwh/m2/day the desalination plant would produce 100,000 m3/d distilled water at a speculative cost of $0.28/m3.
Consider the desalination plant described in Figure of diameter 1 kilometer.
Area solar absorber πr2 = 3.14 x 0.5 x 0.5 = = 0.785 km2
In sunbelt countries average annualised insolation is 6kwh/m2/day
Assume 80% transmittance through glass
90% efficiency solar absorber
Total solar energy absorbed = 0.8 x 0.9 x 0.785 x 106 x 6000 x 60 x 60 = = 12 x 1012 joules/day
Energy required to heat 1 gram water from ambient temperature to 50° C and to evaporate = 30 + 540 calories
= 570 x 4.2 joules
= 2,400 joules
But if there is a gained output ratio of 20
Actual amount of energy consumed per gram water desalinated is 120 joules
Thus mass of water desalinated is 12 x 1012/120 grams/day = 100 million litres/day
Such a capacity would be comparable to some of the world’s largest desalination plants. Note that water demand in the UK is 150 litres/day and in developing countries 10 litre/day per person. The desalination plant above would meet the daily demand of 600,000 people in an advanced country or 10 million people in the developing world.
Cost of desalinated water:
One can only hazard a guesstimate cost for the plant described above to give an order of magnitude figure. The installation is very large but technically very simple with nothing that is high tech. Let us say that construction would cost $100 per square metre giving a total cost of $100 million.
If the capital cost is discounted over 10 years with average production of 100,000 m3/d, then the capital cost is equivalent to
$ 100million = $0.28/m3
100,000 x 365 x 10
The cost of desalinated water from the world’s most recent large plants is about $0.5/m3
The running costs of the solar desalination plant would be very low with its fuel free. This order of magnitude calculation indicates that large scale solar desalination could compete with present desalination technologies.
All the above information about the costs of solar desalination helps, entrepreneurs in gujarat, maharastra, Rajasthan, Tamilnadu, Karnataka, Kerala, Andhra pradesh, Orissa, West Bengal - the coastal states for setting up coastal solar desalination plants.
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