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

[Jester]

I'm no engineer, but I've always wondered why large-scale desalination isn't the obvious solution to fresh water shortages, especially on the coasts.

From an economic perspective, I'd want to know: How much does it cost to install and maintain? How difficult is it to get fresh water from the towers to where it's needed? How hard is it to find a viable space 11x11 km? (That is a really huge area to cover with anything...) And, of course, how does all this square against alternative desalination options?

-Jester

Afterthought: If they're all concentrated in one area, how do you stop the area around from becoming a barren salt waste? Not only will that be an ecological issue, but it will reduce the efficiency of the plants as the intake becomes increasingly saline. Or is the ocean so efficient at circulating water that this is not an issue?

[kandrathe]

I'm no engineer, but I've always wondered why large-scale desalination isn't the obvious solution to fresh water shortages, especially on the coasts.

Yes, me as well. I wonder why North Africa couldn't use it's solar power to reclaim parts of the Sahara from west to east, increasing the relative humidity of the entire region.

From an economic perspective, I'd want to know: How much does it cost to install and maintain?

I wouldn't start with 11x11 Km, but work on one prototypical implementation minimizing materials, and cost.

How difficult is it to get fresh water from the towers to where it's needed?

Probably just a little more difficult than it is currently, where water is stored in reservoirs up in the hills, or uphill drawn from the velocity pressure of Colorado river.

How hard is it to find a viable space 11x11 km? (That is a really huge area to cover with anything...)

"On April 20, 2012, President Barack Obama signed a proclamation designating a 14,651-acre (5,929 ha) portion of the former post as the Fort Ord National Monument. In his proclamation, the President stated that, "The protection of the Fort Ord area will maintain its historical and cultural significance, attract tourists and recreationalists from near and far, and enhance its unique natural resources, for the enjoyment of all Americans."

And, of course, how does all this square against alternative desalination options?

-Jester

http://www.poseidonwater.com/carlsbad-desal-plant.html

"The plant is expected to produce 50 million US gallons (190,000 m3) of water per day (0.069 cubic kilometres per annum) with energy use of ~3.6[33] kWh for 1 m3 fresh water, or ~38 MW of average continuous power. Another estimate has the plant requiring 40 MW to operate, and a cost of $49 million to $59 million a year. It will provide about 7% of the potable water needs for the San Diego region. The San Diego County Water Authority signed a contract with the plant operator to purchase a minimum 48,000 acre-feet per year of water, but it can also demand up to a maximum of 56,000 acre-feet per year. This is equivalent to 43 million gallons per day (mgd), or about 86% of the plant's output. The cost of water from the plant will be $100 to $200 more per acre-foot than recycled water, $1,000 to $1,100 more than reservoir water, but $100 to $200 less than importing water from outside the county. As of April 2015, San Diego County imports 90% of its water. A conglomerate of California-based environmentalist groups, the Desal Response Group, claims that the plant will cost San Diego County $108 million a year."

Afterthought: If they're all concentrated in one area, how do you stop the area around from becoming a barren salt waste? Not only will that be an ecological issue, but it will reduce the efficiency of the plants as the intake becomes increasingly saline. Or is the ocean so efficient at circulating water that this is not an issue?

In the passive plans, the slightly increased saline is returned to the ocean before its brine level is ever close to being toxic to sea life.

[Lissa]

I'm no engineer, but I've always wondered why large-scale desalination isn't the obvious solution to fresh water shortages, especially on the coasts.

Yes, me as well. I wonder why North Africa couldn't use it's solar power to reclaim parts of the Sahara from west to east, increasing the relative humidity of the entire region.

Well, the issue with the Sahara is due to the change in tilt of the Earth that occurred 25k years ago. Prior to that the Sahara was very lush, but after the change in tilt, the desert grew.

Another aspect is that ancient man unknowingly caused some deserts to grow due to irrigation with sea water. A prime example of this is the growth of the Arabian desert. At one time, probably close to 6k AD, people started irrigating parts of Arabia using waters from the Persian Gulf, Red Sea, and Arabian Sea and this salted the earth there and destroyed the arable lands there thus causing the desert to grow to its present state.

[kandrathe]

I'm no engineer, but I've always wondered why large-scale desalination isn't the obvious solution to fresh water shortages, especially on the coasts.

Yes, me as well. I wonder why North Africa couldn't use it's solar power to reclaim parts of the Sahara from west to east, increasing the relative humidity of the entire region.

Well, the issue with the Sahara is due to the change in tilt of the Earth that occurred 25k years ago. Prior to that the Sahara was very lush, but after the change in tilt, the desert grew.

Another aspect is that ancient man unknowingly caused some deserts to grow due to irrigation with sea water. A prime example of this is the growth of the Arabian desert. At one time, probably close to 6k AD, people started irrigating parts of Arabia using waters from the Persian Gulf, Red Sea, and Arabian Sea and this salted the earth there and destroyed the arable lands there thus causing the desert to grow to its present state.

Interesting. I've heard that even the Sphinx in Egypt was originally eroded by water, not sand.

Humans are also not helping ... explanation by a Namibian... "There are many factors at play, but it appears that the most important reason is the desertification of the Sahel, South of the Sahara. This semi-arid region used to grazed by livestock only during the rainy season, because there was no drinking water during the dry season (the nomadic livestock farmers would trek to the Southern, high-rainfall agricultural areas in the dry season where they would graze on the crop left-over material after the harvest season, simultaneously fertilising the fields). The crop farmers in the South changed their crops to cash crops with later and longer harvest seasons (and little to eat for the livestock) while in the Sahel, more and more boreholes were drilled in order to provide the livestock with permanent drinking water throughout the year. Due to the piosphere effect around the water points where the veld no longer had any rest period, but were being continually grazed and trampled throughout the year, the top soil, without any vegetative cover, is being removed by wind… the effect is first circles of desertification around the permanent water points that increasingly become larger (as the livestock graze further away in order to get enough food) until it forms continuous areas of desert, joining the Sahara to the North. This effect is exacerbated by periodic droughts."

[kandrathe]

I'm gonna just hang this out here... us woke people are into desalination.

[Taem]

I often pondered of a quick evaporation system, such as with heating rods, where the coastal cities could just sell the water inland to recoup the costs of their plant. However, I found out that these systems still require pumps and filters, as the evaporated water still has salinated particles in it. Santa Barbara authorities found it would be cheaper to just build a complex filtering system and replace the filters every month and pawn the additional costs to the tax payers.

[LavCat]

The current issue of The Economist has an article on desalination.

[kandrathe]

I often pondered of a quick evaporation system, such as with heating rods, where the coastal cities could just sell the water inland to recoup the costs of their plant. However, I found out that these systems still require pumps and filters, as the evaporated water still has salinated particles in it. Santa Barbara authorities found it would be cheaper to just build a complex filtering system and replace the filters every month and pawn the additional costs to the tax payers.

Possibly dual use... Solar reflected to a tower to melt salt, used to boil salt water to steam. Power turbines with desalinated steam, and collect the condensed, now purified water for drinking. I like the prospect of integrating two systems to provide renewable energy and purified water. Need to consider birds flying into the solar beams... possibly focus in two steps, parabolic mirror to deliver low concentration to the tower, the second lens at the tower wall to focus to a pinpoint.

[kandrathe]

The current issue of The Economist has an article on desalination.

This one? A membrane that can remove salts from water more efficiently.

Based upon this paper in Science...

[LavCat]

The current issue of The Economist has an article on desalination.

This one? A membrane that can remove salts from water more efficiently.

Based upon this paper in Science...

Yes, that's the one.

[eppie]

How do you prevent the slat from crystalizing out everywhere? I understand these systems work by just evaporating a little bit of water from sea water so that what they send back is just slightly more saline, but in every angle or with every splash you get salt forming in these devices. That can be very difficult to clean up.

[kandrathe]

How do you prevent the slat from crystalizing out everywhere? I understand these systems work by just evaporating a little bit of water from sea water so that what they send back is just slightly more saline, but in every angle or with every splash you get salt forming in these devices. That can be very difficult to clean up.

I guess I would explore a marine tolerant concrete, and use sealer coating like Siloxa-Tek 8510. I would expect part of the engineering would be to find a design needing minimal manual maintenance.

Thanks to ancient Romans... saltwater resistant concrete