Cheap Desalination

living could use more inventions like this.  

href="">Eye on
Research: Researchers develop low-cost, low-energy desalination process

Sun News Report

Article Launched: 05/27/2007 12:00:00 AM MDT


A low-cost water desalination system developed by New Mexico State
University engineers can convert saltwater to pure drinking water on a
round-the-clock basis ­ and its energy needs are so low it can
be powered by the waste heat of an air conditioning system.

A prototype built on the NMSU campus in Las Cruces can produce enough
pure water continuously to supply a four-person household, said Nirmala
Khandan, an environmental engineering professor in NMSU's Department of
Civil Engineering...

"When you air condition a house, you are pumping the heat outside the
house, and the heat is wasted into the atmosphere," he said. "We want
to capture that heat and use it to power this desalination system."...

It turns out to be very simple.  You could probably build one
yourself.  I suspect it will take some refinement to figure
out the most efficient way to run the system, but that won't take long.

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I looked at the proposed method, and I don't think it will ever be competative. It is a single effect evaporator. The amount of heat needed to evaporate a given amount of water isn't changed by changing the temperature slightly (the heat of evaporation goes up slightly as the temperature decreases). Multiple effect evaporation is commonly used in Saudia Arabia to desalinate sea water. The water is evaporate at one temperature, the condensing water then evaporates another similar quantity of water (the next "effect"), and so on. The number of effects that can be used is limited, usually to 3, but sometimes to 4 or 5. The amount of fresh water generated per unit of heat is multiplied by the number of effects.

What limits how many effects can be used is what temperature limits can be used. At the high end it is affected by corrosion, deposition of scale in the brine, and the pressure exerted by the steam. At the low end, it is affected by the temperature of the cooling water, or other source of cooling, which is necessary to condense the water.

As the heat flows, there are heat transfer resistances that must be overcome, pressure differences for fluids to move. There is a change in the pressure when going from pure water to salt water at a given temperature. This is overcome by evaporating at a hot temperature and condensing at a cooler temperature. I think this is the 15 degrees they were talking about. If your maximum temperature is 220 F, and you have a 15 degree difference between the hot brine and the cool condensate, and have 5 F across each interface, that gives you maybe 30 degrees per effect (very good if you ask me). If your cooling water is ~100 degrees, then you have enough delta T for about 4 effects at most. There is a roughly proportional trade-off between capital for heat exchanger surface and temperature difference for heat transfer.

To go from 4 effects to 5, you would need to drop the delta T from 30 to 24 degrees (of which 15 is still fixed), so your delta T per interface goes from 5 to maybe 2.5 degrees. That takes twice as much surface area (per effect). So to increase your water production from the same fuel from 4 units to 5 units would increase your capital cost from 4 units (1 for each effect) to 10 (2 for each effect). 6 units would require a 20 degree delta T, or maybe 1.25 degrees per interface. That is twice as much again, or 24 units (6 times 4).

The "waste heat" from an air conditioner isn't "free". The temperature of that heat depends on the pressure of the refrigerant at that temperature, and as the temperature goes up, so does the pressure, and the work necessary to compress the refrigerant to that pressure. The cost of the heat shows up as increased electrical usage of the air conditioner.