Solar concentrating desalination and desal info

one drawback of desalination is the enormous amount of energy it takes to turn saltwater into freshwater. A potential solution has launched in the dry heat of California’s Central Valley, where a pilot project is using solar energy to operate a new kind of desal system.
In San Joaquin Valley’s Panoche water and drainage district, where the experimental solar desalination project is based, the water is brackish—less salty than the ocean but still too salty to be easily used for agriculture. Plants that can handle brackish water, such as pistachios and wheatgrass, dot the landscape, watered by reclaimed runoff. Salts from the soils accumulate every time the water is reused, and eventually the water becomes too salty to be usable.
That’s where the new technology comes in. The salty stuff can now be turned into freshwater by a row of curved mirrors that bend the sun’s rays, focusing it on long tubes containing mineral oil. The heat from the oil generates steam, which separates water from the minerals and salts. Because heat can be held in a thermal storage unit, the system can also run at night or when the sun isn’t shining.


Sound simple? At its core, it is. “Basically, all we’re doing is boiling water,” explains Matt Stuber, cofounder of WaterFX, the company that created the technology. “We’re distilling the water, capturing the heat in the steam so we can reuse it in a very efficient manner.”

With freshwater becoming more and more scarce desalination technologies are popping up everywhere from Israel to Australia. Most use reverse osmosis, which pushes water through a series of membranes to squeeze progressively more stuff out of the liquid. That takes a lot of energy, and only about half the water going in comes out clean. The remaining sludge is a super-salty mixture that often gets discharged back into the ocean, which can have deleterious effects. WaterFX’s technique, on the other hand, makes 93 gallons of clean water for every 100 gallons of brackish water coming in. The remaining material comes out as a solid cake of selenium and salts that can be used as filler in building projects or fertilizers, or purified and sold as sea salt.

Another problem with reverse osmosis is that it only works well near the ocean. “The water chemistry in groundwater is very different from seawater, and the chemistry happens to be predominantly the things that are contribute to technology failures—the worst things you ever want to deal with,” says Stuber. WaterFX’s system only uses about a third of the energy of a similarly sized reverse osmosis operation, which makes the price competitive, Stuber adds.

The folks over at Oak Ridge National Laboratory are hot on the trail of a new graphene desalination membrane, which could free up vast amounts of the world’s water resources for human use. Currently, according to the lab, more than 99% of the world’s water is undrinkable, much of that being locked up in seawater

Somewhat ironically the whole thing is based on methane, the chief component in natural gas. Those of you familiar with natural gas fracking issues might be giving it the stinkeye on that account, but let’s take a look and see what they’re up to.

energy efficient desalination membrane

The Desalination Conundrum

Conventional desalination involves a process called reverse osmosis, in which water is forced through a membrane.

Reverse osmosis is a big step up from distillation in terms of energy consumption, and more efficient systems are in the pipeline (check out this four-in-one desalination process, for example).

Despite recent improvements, though, reverse osmosis still sucks up huge amounts of energy, and part of the problem is the membrane. Conventional membranes are based on polymers (plastics). They tend to get clogged up during the process, and they have to be cleaned regularly in order to keep operating at their personal best.

One emerging solution is solar-powered desalination. Renewable energy helps to reduce dependence on fossil fuels, but it doesn’t address the membrane issue. In an increasingly crowded world, energy efficiency is a critical factor, regardless of whether you’re using fossil fuels or renewables.





An Energy Efficient Graphene Desalination Membrane

That’s where the graphene comes in. And the methane, too.

The new Oak Ridge graphene research is still in the proof of concept stage, but things look promising. The idea is to replace conventional polymer membranes with graphene.

For those of you new to the topic, graphene is a relatively new form of carbon, first discovered in 2004. Since then it has engendered thousands of research papers as scientists dig into its unique properties.

Here’s a schematic look at graphene, showing its unique hexagonal structure (the two blue areas show the chemical bonds of impurities in the graphene sheet):

graphene courtesy of ORNL

Graphene is only one atom thick but it is super-strong. A graphene membrane could be made thinner and more porous than a polymer membrane, so you would need less pressure — and therefore less energy — to push water through it.

The problem is how to make the stuff at commercial scale. Graphene is only one atom thick, so fabricating graphene is a delicate task.

The Oak Ridge team also had to figure out how to punch precisely sized holes in a sheet of graphene, large enough to let water molecules through, but too small for salt ions to pass.

Here’s how the lab describes the methane part of the process for making graphene membranes:

To make graphene for the membrane, the researchers flowed methane through a tube furnace at 1,000 degrees C over a copper foil that catalyzed its decomposition into carbon and hydrogen. The chemical vapor deposited carbon atoms that self-assembled into adjoining hexagons to form a sheet one atom thick.

That was the easy part. The next step involved putting the graphene sheet on a chip of silicon nitride, and exposing it to an oxygen plasma in order to force out selected carbon atoms. That left a hole or pore in the sheet.

The team was able to tune the number and size of the pores by varying the length of time that the carbon sheet was exposed to the plasma.

That’s a whole story in itself. To calculate the most effective pore size, the team went over to a shared science user facility at Oak Ridge called the Center for Nanophase Materials Sciences, and asked to borrow their scanning transmission electron microscopy (STEM) gear.

STEM provided the team with an atom-scale image of their graphene sheet, which they used to correlate porosity with its transport properties. That enabled them to calculate the optimal pore size, and distribution level, for desalination.

In case you’d like to try this at home, that would be pores in the range of 0.5 – 1 nanometers across, distributed at a rate of one per 100 square nanometers.

The topmost image in this article shows the red graphene membrane stabilized with yellow silicon atoms. The circular figure is an enlargement to show off the honeycomb structure. Ignore the orange areas — those are residual blotches of a polymer.

Just What The World Needs: A Methane Based Graphene Desalination Membrane — No, Really

So far the graphene desalination membrane has passed its tests with flying colors, achieving almost 100 percent rejection of salt ions while allowing water to flow through at a rapid pace.

To ice the cake, according to Oak Ridge the methane-based fabrication method could be scaled up to a commercial level.

That’s not such great news when you factor in the rapid increase in environmental, public health and quality-of-life baggage carried by oil and gas fracking operations. In the US, for example, fracking (short for hydrofracturing) was practically a non-issue when it was confined to thinly populated areas in western regions, but in recent years it has exploded into more heavily populated areas as the result of new shale discoveries.

The use of methane in water purification particularly ironic, given that one of the major issues in natural gas fracking is water contamination from both fracking fluid and fracking wastewater disposal.

On the other hand, when you consider the growth of methane-rich, renewable biogas sources, perhaps some day in the sparkling green future that super-efficient graphene desalination membrane can trace its roots to your friendly neighborhood hog farm.

The Baja California peninsula is one of the most arid areas in Mexico and water shortages are becoming critical, especially along the southern coastline which has matured into one of the most desirable jet-set locations in the world.

Desalination, which involves removing the salts from seawater or brackish water to provide drinking water, is one viable option to ensure future water security for the region. There are already about 70 desalination plants on the Peninsula, though most are very small (25 liters/second or less) and are powered by conventional electricity. Several larger desalination projects on the Baja California Peninsula, some of which will rely mainly on solar power, are currently in the planning stages.

Map of Baja California PeninsulaLa Paz, the capital of the state of Baja California Sur, faces a particularly serious water supply problem. The local aquifer is reported to be already overexploited and suffering from salt water intrusions. Because of its greater density, seawater normally underlies freshwater in coastal areas. Salt water intrusions occur when so much fresh water is pumped out of coastal aquifers that it is replaced by the underlying salt water. The water supply issues have led to water rationing, in which almost half of La Paz’s 250,000 residents receive water only 12 hours or less each day.

Obtaining water from the desalinization of sea water is more expensive than abstracting water from aquifers via wells, but avoids the possibility of salt water intrusions.

A recent Ooska news article provides details of the desalination plants already built or being planned:

Baja California Sur:

Cabo San Lucas, opened in 2007, treats approximately 230 liters a second (60 gallons/s), equivalent to 20 million liters (5 million gallons) a day.

La Paz. Still at the proposal stage is a desalination plant capable of treating 200 liters a second.

Sierra de la Laguna. A Canadian mining company (Vista Gold Corp.) planned a desalination plant to provide water for its proposed Concordia open-pit mine. However, the mining plan was refused an essential permit by the Mexican government.

Baja California:

Ensenada. The 28-million-dollar El Salitral desalination plant is a “highly innovative project that would put the region on the map globally for desalination”. Construction is due to start later this year, and the plant should be operational by the end of 2012, when it will treat 250 liters of seawater a second. The plant would supply 96,000 people with potable water.

Rosarito. Preliminary geological and environmental impact studies are underway for a desalination facility in La Misión large enough to supply the needs of 96,000 people. Still in the concept stage is a second desalination plant  which would supply water across the border to San Diego in California.

San Quintín. Plans exist for a desalination plant with a capacity of 150 liters a second.

Main source: The OOSKA News Weekly Water Report for Latin America and the Caribbean (16 February 2011)

What a difference a month makes. Back in June, the California Farm Bureau Federation reported that local officials were still a bit iffy over the prospects for scaling up a relatively small, demonstration-scale solar desalination plant for the water-starved San Joaquin Valley. We have no idea what changed their minds, but just last week the desalination plant’s developer, WaterFX, announced plans for bouncing the project up to a commercial-scale facility capable of producing 1.6 billion gallons of fresh water per year.

In a bid to help bring greater access to clean drinking water to the developing world, WaterStillar has created a solar-distillation system designed to produce clean drinking water from almost any source. Conceived as a cheap, efficient, modular system that can be scaled up to produce thousands of liters per day, Water Works is installed with no upfront costs and requires minimal maintenance or training to operate.
The WaterStillar Water Works was first conceived in 2004. Like nature's water cycle, it works by heating water until it evaporates and condenses to rid it of any contaminants. 
Water is gravity fed into the Works unit from a tank above, so that no pump is required. The unit is split vertically into a number of sections, with the water routed evenly into each.
The water routed to the lowest section of the unit is heated by vacuum tube solar collectors (thermal solar panels). As it heats up, it begins to evaporate and the resulting vapor rises to the top of the section, leaving any contaminants as run-off. The run-off is recirculated and diluted with fresh source water.
When the vapor hits the distillation panel at the top of its section, it condenses to form clean water droplets. Held on by surface tension, the droplets then run down the angled panel to an outlet ready for consumption. The residual heat from each lower layer used to heat the layer above. In this way, the system is able to be highly efficient.
A standard installation produces 2-300 liters of clean water per day, but can be scaled up to 10,000 liters.
The WaterStillar Works system uses no electricity or chemicals and produces no emissions. The clean water produced is said to have a neutral taste and be free of bacteria or viruses. Minerals can be added to the water after the process, but this is not built in.
On cloudy days, evaporation can still take place, but the system may work at a slower rate. It is also possible to use photovoltaic or mains power as an alternative way of to providing heat to the system.
WaterStillar says the Works system is manufactured using materials and parts with the maximum possible lifespan against the environmental impact of producing them. Correctly installed, it should last for 20 years and require very little maintenance. The system is remotely monitored to ensure the water is safe and local WaterStillar partners periodically check in and provide any required maintenance.
WaterStillar's plan is to offer the system at no up-front capital cost, with the customer paying a quarterly invoice based on a metered supply. The company says it has 80 Works systems awaiting dispatch from warehouses in Copenhagen and Mexico City and that it is currently seeking an NGO partner to help with distribution.

Researchers have developed a new solar-powered desalination system that produces high amounts of drinkable water and uses a technique inspired by the ocean to avoid the problem of salt clogging. Scaled up, the system could provide enough drinking water to fulfil the daily needs of a small family.


There have been a number of solar-powered desalination systems developed in recent years to address the issue of maintaining a sustainable supply of freshwater. However, a frequent challenge is salt accumulation, which clogs the system and affects the water production rate.

To overcome this issue, researchers from MIT and Shanghai Jiao Tong University took inspiration from a natural phenomenon, namely, how deep-ocean currents are driven by differences in the water’s density, a process known as thermohaline circulation.

The researchers’ new system improves upon their previous design, a similar concept comprising multiple layers called ‘stages’. Each stage contained an evaporator and a condenser that used sunlight to passively separate salt from incoming water. While it efficiently used the sun’s energy to evaporate water, it became clogged after a few days due to salt accumulation. So, the researchers tried a version of thermohaline circulation as a way of mitigating salt build-up.



The new design features a single stage that looks like a thin box, topped with a dark material that absorbs heat from the Sun. Inside, the box is separated into an upper and lower section. Water flows through the upper half, where the ceiling is lined with an evaporator layer that uses solar heat to warm up and evaporate any water that comes into direct contact. The water vapor is funneled to the lower section, where a condenser layer air-cools the vapor into salt-free, drinkable water.

The entire box is tilted, which, in combination with the thermal energy from sunlight, induces the water to swirl as it flows through. This movement helps bring the water in contact with the upper evaporating layer while keeping the salt circulating and preventing it from settling and clogging.

A graphical representation of how the solar-powered desalination system works
A graphical representation of how the solar-powered desalination system worksGao et al.

“We introduce now an even more powerful convection that is similar to what we typically see in the ocean, at kilometer-long scales,” said Zhenyuan Xu, one of the study’s corresponding authors. “When seawater is exposed to air, sunlight drives the water to evaporate. Once water leaves the surface, salt remains. And the higher the salt concentration, the denser the liquid, and this heavier water wants to flow downward. By mimicking this kilometer-wide phenomena in [a] small box, we can take advantage of this feature to reject salt.”

The researchers found that their system produced fresh water at a range of salt concentrations, from natural seawater to water that was seven times saltier. They say that if the system was scaled up to the size of a small suitcase, it could produce 4 to 6 L (1.1 to 1.6 gal) of water per hour and last several years before requiring replacement parts.

Because of the system’s high water-production rate, high salt rejection, extended lifetime and the fact that it’s solar-powered and doesn’t require electricity, the researchers say that the overall cost of running the system would be cheaper than the cost of producing tap water in the US.

“We show that this device is capable of achieving a long lifetime,” said Yang Zhong, a co-author of the study. “That means that, for the first time, it is possible for drinking water produced by sunlight to be cheaper than tap water. This opens up the possibility for solar desalination to address real-world problems.”

The study was published in the journal Joule.g