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Hi all, lurker for a while. I'm a chemical engineer for the pulp and paper industry in the U.S.
I don't no why you'd ever use photovaltaic for this application. Where you only have a 14-30% electrical generating efficiency.
I rather use solar thermal multi effect evaporation. A 7 effect multi effect evaporator gets about 5.6 steam economy meaning, you can evaporate 5.6 lbs of water for every lb steam you generate. Further you'd only need low pressure steam generated. Low temperature steam say 5-6 bar is sufficient for multi effect evaporation. Since we are not trying to generate electricity high temperatures 400-600 C are not needed for high generating efficiency and thus this simplifies the solar thermal plant.
Water is solely a capital cost problem in Saudi Arabia. They have plenty of sun and access to sea water. And with current oil prices they have plenty of money. Further the intermittance of the solar is not a problem. This is not an electrical storing problem as it is for other solar thermal generating schemes. This is a water storing problem. Just desalinate large amounts of water during daylight hours and store during nighttime. Or if you want better return on investment on you ME evaporation plant use solar thermal steam generation during the daytime and then switch fossil at night.
I am NOT an engineer, but I'd like to see why this wouldn't be superior to photovoltaic. Each conversion of energy loses a lot. And couldn't the materials cost of this be a lot lower too?
Just price out a sq. meter or solar cells ($1000) and a sq. meter of mirror ($5), then factor in the 4 to 1 efficiency (20% vs 90%) advantage of mirrors, add the simplicity advantage of evaporation over reverse osmosis, it is hard to see what could overcome this 1000 to 1 cost advantage.
Hi Veramcor,
Thanks for your comment. Please let me rephrase the question. Anyone have any sense of how many acres of solar thermal would be needed to desalinate water to irrigate an acre of farm land?
Thanks,
Dave
Yes, of course. The calculation for PV that I made is only in order to get a feeling of the order of magnitude involved in the worst case hypothesis. Systems based on solar heat would be more efficient and less expensive. A very promising area.
Indeed solar thermal desalination has already been used in Saudi Arabia to desalinate water;
http://www.un.int/saudiarabia/solar3
And here is one system in use in Jeddah, Saudi Arabia.
http://www.dwc-water.com/technologien/thermische-entsalzung/index.html
Intuitively, I tend to agree with you re solar evaporators versus PV-powered reverse osmosis. However, I think one really needs to compare the overall energy consumption of the entire desalination system. While a PV collector collects less energy per unit area of colletor than a mirror-type thermal system, one has to look at the total amount of energy consumed per unit volume of desalinated water produced.
Part of that evaluation would be comparing the energy efficiency of reverse osmosis versus multi-effect evaporation. I think RO generally comes out ahead in such a one-to-one comparision, particularly if you're not dealing with full-strength sea water. But that must be weighed against the lower collector efficiency of the PV collector in comparison to the mirror thermal system. Then we need to consider the heat losses of the evaporator system.
Taking all these factors into consideration, I'm not so sure it's as clear-cut as you might think.