A map by Dr Mostafa H Eisharqawy illustrates that regions of global water scarcity are closely ‘in phase’ with areas of terrestial insolation. Solar energy in passive and active modes has great potential to desalt water.
Enery efficiency can be measured in terms of Gain Output Ratio (GOR) – see the image above. High efficiency equates to a high GOR ratio: large freshwater outputs from minimal energy inputs.
Solar still technology has been understood for many years – example cited for Chile in the 1850s. These may be described as passive solar desalination systems. Solar stills are very inefficient in terms of their energy use, but considering the energy is essentially ‘free’ this may not be significant. GOR ratios are typically of the order 1-4 (by comparison to RO at 42).
By comparison to passive solar still technology, Photovoltaic Reverse Osmosis (PV-RO) presently suffers from: high initial cost, sensitivity to water input quality, membrane fouling.
More information on PV-RO to follow...
GOR is usually associated with the MED (multiple effect distillation) process - the amount of produce vs the amount of steam produced producing it.
ReplyDeleteOK, I guess if you use energy as the common denominator in this case then it works equally as a measure of relative energy efficiency. I can see that there is opportunity for mis-interpretation though and I'm always wary of energy values to make sure we are comparing like with like: for example, are cited figures just the energy needed manufacture the water, or do they include pumping/transfer costs etc.
ReplyDeleteOne example that came to mind today was when I was listening to the presentation on mobile desalination barges. I guess if the manufactured water must also be re-pressurised and pumped through an emergency distribution network this could be potentially energy costly.