How to reverse global warming - IRRIGATE deserts

[CaptainPanic]

Evaporating / multi-effect evaporators

While the heat of vaporization of water is 2260 kJ/kg, with proper heat integration and using heat pumps, the heat requirements per kg of water are much lower, with savings of up to 80%, according to this article (warning: .pdf!).

Let's assuming that energy costs around 10 euro/GJ (I can't be bothered to back that up now, please ask if you don't believe this), if you use a common large scale gas heated boiler. So, even with 80% energy savings, a ton (or m³) of water would cost around 5 euro/m³. It will only go up if you use the sun to make water, because the investment costs are higher.

 

Reverse osmosis

As was stated before by Enthalpy, reverse osmosis is the modern method, and costs only around 0.5 euro/m³. This wikipedia article states a list of prices of water in $/m³, depending on the location. I conclude that reverse osmosis is around 10x cheaper than evaporation.

 

Fill empty tankers with European river water

Finally, how about this idea: The average cost of oil transport by tanker amounts to only two or three United States cents per 1 US gallon (3.8 L) (source: wikipedia). That amounts to about 5 euro/m³. However, tankers are full when going from arid places (e.g. Arab countries) to wet places (e.g. Rotterdam, Netherlands). If you can make them carry water on the way back (they typically go back empty), then the costs of this water should be a fraction of the costs. If it would be only 10%, the costs of water are similar to the costs of the desalination through reverse osmosis: around 0.5 euro/m³. Obviously, there is that issue that some residual oil in the tanker would contaminate the water. I haven't solved that yet.

 

Costs of the forest

Interestingly, with a price for water of 0.5 euro/m³ (reverse osmosis or cheap tankers), we can also calculate the costs of the forest. Assuming that the forest needs 1 m of 'rainfall' (or irrigation) per year, or about 1 million m3 per square kilometer, the costs for the water for 1 km² of forest is 500,000 euro/year (in which I totally ignore the costs of the actual irrigation system, or maintenance).

The production of for example eucapyptus can be as much as 53 ton/ha/yr (pdf warning), or 5300 ton/km²/yr.

That simply means that the estimated costs of the water alone for that wood would be very close to 100 euro/ton of wood... which is rather expensive. (Normally, wood in bulk sells for prices up to 100 euro/ton, but that includes everything... from the profit for the land owner, to the harvesting and transport, and not just the water for growing it).

 

I don't believe in planting trees with the sole purpose to save the planet. In this modern capitalist world, it is more likely to work if someone makes money while saving the planet.

p.s. I hope I didn't make any mistakes in this back-of-the-envelope calculation.