environmentalresearchweb blog

« The oldest ideas about glaciers | Main | Green energy for China »

Energy for global water cycle - we already use a lot of sun…what more do we want?

According to Japanese researcher Taikan Oki and Shinjiro Kanae, approximately 500,000 cubic km of water per yr are evaporated over the ocean (437,000 cubic km) and land (66,000 cubic km).¹ With water at a density of 1000 kg per cubic meter, this is 5x10^17 kg of water evaporated per year. Using a latent heat of vaporization for water of 2,270 kJ/kg, this means that a minimum approximately 1,135,000 exajoules per year (1 exajoule, or EJ, = 10^18 J) of solar energy are used to evaporate the world’s water and drive the much of the hydrologic cycle of the planet.

Given that humans consume approximately 500 EJ/yr in primary energy, this means that the Earth’s water consumes at least 2,000 times more solar energy each year when evaporating water that we consume in primary energy resources. Eighty-seven percent of this water is desalinated by evaporating from the oceans. So when we talk about desalinating water, or recycling water, just remember that it means that we are inherently deciding that 2,000 times our direct consumption of primary energy resources for the creation of fresh water is not enough!

So when we think if using desalination, but matching it up with carbon-free sources or technologies that are more efficient than a couple of decades ago, what we are really saying is that our original use of solar energy for water desalination is no longer sufficient for our purposes. With that mindset, should we rearrange our priorities in terms of the uses of water and locating people to where the solar resource combines with precipitation patterns and the Earth’s contours to deliver water to us renewably? Or should we continue to bet that energy will be cheap such that we become more dependent upon it for delivering fresh water? These kinds of questions are mainly for rich countries, as we can only be so lucky to have these options.

¹Oki, Taikan and Kanae, Shinjiro (2006). Global Hydrological Cycles and World Water Resources. Science, 25 August 2006, Vol. 313. no. 5790, pp1068–1072, DOI: 10.1126/science.1128845.

Posted by Carey King on February 18, 2010 2:32 AM | Permalink