Passive Desalination with photovoltaic pumping

[kandrathe]

After hearing so much about the drought in California, and Cape town, I've been musing during my commute recently about optimizing a passive evaporative design, to be more vertical, include active solar for pumping, and a "swamp cooler" technique to chill the evaporation collector pane.

Based roughly from the concepts of horizontal pond,


but again, more vertical will maximize evaporation over the incline. Yielding greater than 8 liters per day per 1m^2.



Example; Bay Area averaged 85.2 gallons (322.5 liters) per person per day while SoCal averaged 119 gallons (450 liters) per person per day. Let's go with 400 for rounding, and population of 3.96 million people, is 1.584 billion liters per day. 1,584,000,000 liters required / 8 liters per m^2 = 198,000,000 m^2 require surface. Aiming for consecutive walls of height of 15m x 100m space to avoid shading, we'd need 132,000 x 1500 m^2 desalination walls to supply the people of Los Angeles. It sound like a lot of work and cost, until you consider that every person in LA is consuming / paying on average 1$ a day for municipal water ($1.45 billion). Spacing walls 20m apart you can get 14 x 80 wall per km^2. That's 117 km^2 or 45 square miles. I guess that's a bit of issue with passive solutions in general.

What issues am I missing?

Towered desalination plant built in Pakistan -- In 1993 a desalination plant was invented in Pakistan, producing 4 liters of water per square meter per day, which is at least ten times more productive than a conventional horizontal solar desalination plant. The structure is a raised tower made of concrete, with a tank at the top. The whole plant is covered with glass of the same shape, but slightly larger, allowing for a gap between the cement tower and the glass. The tank is filled with saline water and water from an outside tank, drop by drop water enters the inner tank. The excessive water from the inner tank drips out onto the cement walls of the tower, from top to bottom. By solar radiation, the water on the wet surface and in the tank evaporate and condense on the inner surface of the glass cylinder and flow down onto the collecting drain channel. Meanwhile, the concentrated saline water drains out through a saline drain. In this process fresh saline water is continuously added to the walls from the top of the tower. After evaporation, the remaining saline water falls down and drains out continuously. The movement of water also increases the energy of molecules and increases the evaporation process. The increase in the tower’s height also increases the production. Whereas in the conventional system water that is filled remains at a standstill for several days, a condenser is provided at the top in an isolated space, allowing cold water to pass through the condenser. The condensed hot vapors and hot water from the condenser are also thrown on the cement wall.

This plant’s base is 3.5 by 1.5 by 10 feet (1.07 m × 0.46 m × 3.05 m) high, and gives about 12 litres (3.2 US gal) of water per day. Built horizontally, a structured plant receives solar radiation at noon only. But Zuberi’s plant is a vertical tower and receives solar energy from sunrise till sunset. From early morning, it receives perpendicular radiation on one side of the plant, while at noon its top gets radiation equivalent to the horizontal plant. From noon till sunset, the other side receives maximum radiation. By increasing the height, the tower plant receives more solar energy and the inner temperature increases as height increases. Ultimately this increases the water yield. Different successive plants were constructed during the 1960s. A number of experiments have been conducted and a much more productive plant has been developed, with further work continuing. This project can be implemented anywhere there is ground water, brine or sea water available with suitable sun. During different experiments a plant 6 feet (1.8 m) high can attain a temperature of 60 °C (140 °F), while a plant of 10 feet (3.0 m) high can reach a temperature of up to 86 °C (187 °F).

TOWERED SOLAR DESALINATION PLANT by Akhter Iqbal Zuberi and Hira Zuberi