Sep 29 2023

Passive Solar Water Desalination

I know we are supposed to be worried about the world supply of fresh water. I have been hearing that at least for the last 40 years, and the statistics are alarming. According to the Global Commission on the Economics of Water:

“We are seeing the consequences not of freak events, nor of population growth and economic development, but of having mismanaged water globally for decades. As the science and evidence show, we now face a systemic crisis that is both local and global.”

Sounds about right. We are not very good at this sort of large-scale management. Everyone just does their thing, oblivious to the big picture, until we have a crisis. Then experts point out the looming crisis which everyone at first ignores. Then we have meetings, summits, and a lot of hand-wringing but next to nothing gets done. Eventually we mostly technology our way out of the problem, but not after significant negative consequences, especially for the world’s poor. The water crisis seems to be following the same playbook.

Now experts are predicting that by 2030 world demand for fresh water will outstrip supply by 40%. This shortage will affect everyone, including people in wealthy developed nations. So now it’s a real crisis. To be clear, I am not trying to minimize this problem at all. The Commission outlines a seven-point plan for properly and fairly managing the world’s fresh water supply, and it all sound very reasonable. It feels like we are in a phase of human history when we are collectively realizing that billions of people have a global effect on the entire planet, and we need to seriously start transitioning from a local focus on securing resources, to globally managing those limited resources.

But I have to admit that my level of worry about the fresh water problem was always mitigated by the sense that we would technology our way out of this one, eventually. There is a lot of water in the world. Water itself is not a limited resource, only fresh water, and we know how to make fresh water out of salt water. It looks like a problem that can be solved by incremental advances in technology. I also admit I know this perspective is a bit naive. I know that 4 billion people already experience water scarcity in the world at least part of the time. This is largely a political problem – a problem of mismanagement and lack of resource equity. Technology can never completely compensate for political malpractice or malfeasance. We need to do both – advance the technology while pushing for proper management.

The primary technology we are talking about is desalination – removing salt from salt water to make fresh water. There are already many desalination plants around the world, but like with everything, it’s ultimately a matter of economics. How much does it cost to desalinate water? It’s also a matter of scale – how much fresh water can we make? (These issues all sound very similar to global warming and energy production.) A recent incremental advance may move us closer to economical fresh water on demand. MIT engineers and their collaborators have improved on the design of passive solar powered water desalination.

The basic technology uses sunlight to evaporate salt water, which will then condense into drinkable fresh water, leaving the salt behind. The problem with this technology is that the salt left behind quickly clogs the system, meaning it has to be cleaned out every few days. This makes the devices labor and cost intensive. What the researchers did was add a circulating current to the water and the salt which is left behind. This removes the salt from the system and keeping it from clogging up. However, the initial design desalinated water at a very low rate. So they tweaked the design into a multi-stage system of evaporation and condensing that removes the salt while maintaining a high water output.

The result is a portable passive system, entirely run by sunlight and physics. Here is how it works:

The heart of the team’s new design is a single stage that resembles a thin box, topped with a dark material that efficiently absorbs the heat of the sun. Inside, the box is separated into a top and bottom section. Water can flow through the top half, where the ceiling is lined with an evaporator layer that uses the sun’s heat to warm up and evaporate any water in direct contact. The water vapor is then funneled to the bottom half of the box, where a condensing layer air-cools the vapor into salt-free, drinkable liquid. The researchers set the entire box at a tilt within a larger, empty vessel, then attached a tube from the top half of the box down through the bottom of the vessel, and floated the vessel in saltwater.

In this configuration, water can naturally push up through the tube and into the box, where the tilt of the box, combined with the thermal energy from the sun, induces the water to swirl as it flows through. The small eddies help to bring water in contact with the upper evaporating layer while keeping salt circulating, rather than settling and clogging.

For a box a square meter in size it can produce 5 liters of drinkable water per hour. The system can also last for several years. Perhaps more importantly, the total price for the water is less than the average price for tap water in the US. The system should also be scalable.

Assuming the best case scenario, that this system works, is scalable, and can be economically mass-produced, it may go a long way to mitigate water insecurity. I doubt something like this will be a solution all by itself. We still need to globally manage our water properly. But at least technology offers us new tools.

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