Wednesday, February 6, 2008

Seawater and sunlight

Here's a funny thing about the water shortage problem: water is the most abundant substance on Earth. More than 70% of the planet is covered by water thousands of feet deep.

Of course most of this is salt water.

Question: is there an economically feasible and environmentally friendly way to remove the salt from seawater and make it safe for human use?

Answer: Yes. Solar desalination appears to be the answer. Research over the last couple of decades has shown that it works.

One project found that a $500,000 plant could supply more than 5,000 liters of fresh water per day for an estimated cost of $0.02/liter:

This price was considered prohibitively expensive in comparison with existing US municipal water rates, but design enhancements may improve efficiency, and a higher price for water still trumps dying of thirst.

The idea is a relatively simple one. Parabolic reflectors capture and concentrate sunlight that is used to heat seawater. The hot water is sprayed into the top of an "evaporator" or "cooling tower." Dry air is pumped up the tower as the water droplets fall down, evaporating the water. The water vapor flows into a condenser and the salt and other particulates fall to the bottom of the evaporator. See a basic schematic here:

Another type of solar powered desalination is the "membrane distillation" model. This plant may actually return surplus energy to the grid. See this pdf document for details:

The US is spending an estimated $250,000,000 per day on the war in Iraq. If we could divert just one day's outlay of Iraq war funds, we could finance the construction of 500 solar seawater desalination projects. What if we appropriate a week's Iraq funds? We could build desalination plants in coastal areas all over the planet and connect them to population centers with pipelines.

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