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evaporative solar desalination (one more time with love)

There are many high-mineral content aquifers inland so this does not just work in coastal areas.
  [vote for,

Evaporative solar desalination sounds great in principle. The problem to date has been system efficiency. Firstly this is governed by the difference in vapour temperature and condensor temperature. The greater the temperature difference, the more water is collected from each cubic metre of air. Many previous designs have played around with condensors operating at ambient temperatures, and have observed low efficiencies. The fact is if the condensor surface is not able to cool the airstream back to ambient temperatures, then you are ejecting valuable energy from the system. Secondly, the heat of condensation is a huge source of energy. If you are not re-using this energy (deposited by the steam onto the condensation surface) then you have a poor design. In addition to this the condensor extracts energy from the vast amounts of air you have just spent your precious solar energy heating, and any successful design must be able to reuse this.

In summary 1) any design which ejects vapour at above ambient temperature is going to result in low efficiency. 2) any design which ejects the heat of condensation from the system is going to result in low efficiency.

This design overcomes both constraints.

Here’s my dream project and it can be adapted to many other countries. The technology, once built, can be maintained by the average modern farmer. We all know they’re a pretty handy lot.

OK find yourself a salt pan. See whether there is a mineral rich aquifer beneath it. In Australia alone, thousands of salt pans all over the country fit the profile. The water would contain salt but also more valuable magnesium salts.

On one of these salt pans, construct a thin film or polycarbonate greenhouse similar in design Jörg Schlaich’s solar tower, only smaller. I am thinking a diameter of under 1000m. Put a low earth-wall around it, ready for flooding to a shallow depth of say 30cm.

At the centre of the greenhouse build two chimneys side by side. One chimney conveys the vapour from the greenhouse upward by normal convection. At the top is a U-bend connecting this chimney to the other chimney next to it. Near the top of the other chimney is the condensor harvesting our precious water.

The condensor is fed with chilled brine at such a temperature that it cools the airstream to just below ambient. This creates a strong downdraft of dehumidified air in the second chimney. What do we do with this downdraft? Well we drive the turbine compressor to chill our brine! What happens to the heat from the compressor? Well we inject it into the greenhouse! If you can come up with a better design to reuse the heat of condensation let’s hear it!

The chimneys don’t have to be 1000m or even 400m. A modest 100m chimney would suffice, delivered onsite in prefab sections. Marine-grade concrete, perhaps HDPE lined.

And I don’t want to hear about solar desalination only being useful in coastal areas. Many inland farming communities have ample water, which is increasingly getting saltier. These communities are probably able to quantify cost of every ppm rise in salinity to the nearest dollar. An installation such as this might cost USD $5 million, and deliver 3 gigalitres per annum. Alex Fiedler hot and arid South Australia

fiedag, Mar 06 2006


       It's interesting to think about whether this would work or not. My first stab at it is that the power from the potential Energy of the cold column would need to be about 1/3 of the power from the condensation, because a typical heat pump can pump about 3 times the power input.
Using some typical numbers, I think it ought to be possible to work out how high the column should be.
Offsetting that: the pump is lifting heavier, cold brine to the top of the column, whereas lighter, hot brine is pumped down.
Tis a tricky one indeed.
Ling, Mar 06 2006

       More Agrarian Socialist nonsense.   

       Current price of 3 gigalitres in Aust is around USD90K. Discount rate on a USD5M structure say 8% gives base cost of capital USD400K. Add maintenence and operation costs etc and you want to construct a business that more than quintuples the price of water, and therfore greatly increases the price of every product in the chain?   

       None of the solar tower / solar desalination/ utility scale solar power ideas I have ever seen has adressed the problem of decay in insolation over time through material degredation and surface fouling of the panels** - your idea would also require continual cleaning of your 1km wide structure or it would quickly degrade in efficiency. (**There is one new idea using giant plastic bubbles that can rotate through a cleaning solution - still early days)   

       Your capicity of 3 Gl would represent approx 0.01% of national consumption, whereas the infrastructure cost you are proposing would, if applied to produce that national consumption, chew up USD50B or about 10% of GDP. That's out of all reasonable proportion.   

       When will we learn that our agricultural industry is completely overexpanded and should be withdrawn from the marginal land it now occupies.   

       PS, how will you replenish the aquifers? or is that also some problem that can be left to future taxpayers?
ConsulFlaminicus, Mar 06 2006

       /how will you replenish the aquifers?/ - Are Central Australian aquifers replenished, or is it "fossil water" - remnants of ancient deposits which will be gone after depletion?   

       I am a great fan of nuclear desalination.
bungston, Mar 06 2006

       [bungston] Most of our aquifers are contained in one giant system that straddles much of the continent - this source has been severely degraded, but it is so vast that protecting it always seems to be 'not so urgent'. We have a program, proceeding slowly, to cap thousands of bores that currently flow freely, but in much of Australia these bores and their associated drainage channels are the only source of water and whole ecologies have grown up around them.   

       We are also investigating the diversion of flood waters into aquifer storage, but need more surveys to establish that the target aquifers are suitable (not fractured etc.). Replenishment of the vast aquifer system mentioned earlier, known as 'The Great Artesian Basin', has been assessed as a national priority.   

       [bigsleep] the bubble cleaning happens through another method, not through sun bleaching.
ConsulFlaminicus, Mar 07 2006

       Hi ConsulFlaminicus, I am never comfortable rejecting an engineering design because of an unfavourable cost/benefit analysis based on current information. There is often wide variability in the income and cost assumptions over time and in different markets. You would agree that the market often reaches very divergent conclusions about the value of a particular product. Your critique, while very welcome (actually I am overjoyed that anyone gives a shit), does not address the idea's engineering merits. I want to reply to the points you raised: 1) if the cost of salinity exceeds $400K p.a. for a particular customer, and this product is the ONLY PRODUCT that can remove that cost, then the purchase will be economical for that customer. True? 2) Continual cleaning of greenhouse structures is not considered a "showstopper" for most commercial produce growers. The efficiency loss can be accepted. An automated radial cleaning assembly is an option in any case, with a 100K capital cost, and perhaps 10K annual running costs. Moreover any dust on the canopy would still heat the polycarbonate, and much of this heat (because of the rapid air-draft) in turn would still end up in the air stream. 3) I am not proposing to compete with conventional forms of water production. The market here would be remote areas only, with no other source of water. Think of it as a niche product.   

       4) The aquifers I am targeting here would all be heavily mineralised, with no other value to man or beast. They would not need replenishing.   

       In summary, no-one in Europe would see the economic benefit of a RO desalination plant. But people in other parts of the world, are building them like they're going out of style. Don't make assumptions about what the market does and does not want. Just concentrate on what is possible from an engineering point of view.
fiedag, Mar 09 2006

       someone should find some better numbers on this-- consider the cost and plastic longevity of existing greenhouse designs, rather than grandiose $5mil mega-greenhouses.
sninctown, Mar 09 2006

       [fiedag] Points taken on board. Allow me to address them! If by //cost of salinity exceeds $400k pa'// you mean that the land is unusable (by man), how is covering it with a structure going to make it usable for agriculture? Perhaps you mean that the fresh water produced could make large tracts of surrounding salinity affected land usable, but putting fresh water on saline land doesn't rectify it - the water evaporates and the surface salt remains or is carried away in runoff to be someone else's problem.   

       re the comments about 'market/s' : well informed markets will reach closely similar conclusions about *price* of a product or service, *value* as you say can be perceived differently. Mature economies work on efficient allocation of resources including capital, so price should win out over value, except where there are votes about that can be gained through the inefficient allocation of capital that is not your own.   

       Some engineering issues with the design - a couple of hundred meter towers are not going to be simple, particularly if there are some serious condensers and turbine compressors mounted near the top?   

       //a strong downdraft of dehumidified air in the second chimney// dehumidified air is lighter than humid air, though chilled air is heavier than heated air. if these two towers are close together, than the positive pressure pushing air up tower one and across the top to tower two is also going to be pushing air up tower two, non? and much of the //downdraft pressure// is being used to drive your turbine? I think you'll need plenty more energy input to make it circulate.   

       Also suspect that much of the systems pressure of heated air inside will simply leak out the side, seeking path of least resistance (you have added enormous resistance to the chimney route, which is open in 'normal' solar chimney models)   

       How strongly would the structure and panels need to be to withstand even light breezes acting over such a huge surface area? Very extensive factory buildings have protocols regarding when they can open their doors because of this problem.   

       Lots more. Sorry. Bored now.
ConsulFlaminicus, Mar 09 2006

       You should be able to build a much smaller scale one (desktop size) to see if it really works.   

       Injecting waste heat from the compressor into the greenhouse would likely be small and also would hurt the compressor efficiency & reliability.   

       If the engineering works (which I'm suspicious of but can't see any 2nd law violation yet...), you still have to worry about biology. All that humid air & sunlight will grow you some nice bacteria & algae. Cleaning the plastic is just one part. You also need to clean the water.   

       Overall, interesting, new, and on the right track. We'll get solar desalinization right sometime in my lifetime (I expect/hope).
sophocles, Mar 09 2006

       I take your point that if there is impedance at the top of the chimneys you will get leakage. However if the condenser cools the airstream sufficiently, then this "suck the air" out of the first chimney. Rapid air flow is key to reducing wasted energy. Leakage due to a strong breeze is probably more of a worry. I don't accept that the structure and panels will have to be that strong. The whole canopy is open and pressure can equalise. The compressor heat exchanger at the canopy intake does create some input impedance, which reduces system air flow. I am hoping this is still acceptable given the benefits of reusing all that waste heat.   

       The bacteria should not get a chance. Air and water flow are too rapid. Importantly the canopy will be UV transmissive creating a natural sterilisation process for the brine.
fiedag, Mar 21 2006

       actually the most cost-effective way to desalinate inland saltwater supply or any inland hard water supply is AC pulsed electrolysis with a hydrogen burning recuperator which produces your water and part of the power you consumed in the electrolysis process. Researchers have gotten the electrolysis to be 91.7%-96.2% efficient and a hydrogen burning turbine can easily be 95-99% efficient, with a high-speed alternator 95-99% efficient, for a total minimum round-trip efficiency of about 82%. Which is about a 50% increase over current state of the art systems.
costellogroup, Jul 14 2007


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