Desalination Plants

What would you call it if you said that the process can produce more energy than is consumed by the process?

I guess the answer is, yes it CAN be converted back into electricity, but not for a profit.

can you make it more clear please? How can it be that a surplus energy is produced ? Which technology?

Sorry,

You can't produce surplus energy. That's the (poorly communicated) point.

That would be perpetual motion, or over unity.

as far as I know, energy is needed to produce fresh water via desalination process, I cant see how we can produce energy with this process:)

You can't produce more energy than the process consumes.

Never mind.

You lit both ends of the candle on that Rule 75...

See my response #8 above.

I think what your opinion is not clear much , I am asking just if we can produce electricity from the desalination process to sell it, or this is just a misinformation from the book author? For example siemens constructed a plant which uses its remnant energy out of gas combustion so as to produce fresh water , here the aim is to produce electricity, what I am asking is one in which the aim is desalination and by-product is electricity... thanks in advance...

I give up.

There are two common processes for desalinazation. One is evaporation- essentially, you boild the water off, then condense it again. Most of the energy is consumed in the phase change of the water from liquid to gas. Theortectically, one could possibly recover some of the heat from the condensation phase, but it would be very limited, and very difficult to use the recovered energy for much of anything. If you use, say, a natural gas burner to evaporate the water, there will be waste heat going up the exhaust stack, but if that were easy to capture, you are spending WAY too much money evaporating your water.

The second process is Revers Osmosis, a process in which most of the energy is used to raise the pressure of the raw water. It is very unlikely that there is enough energy remaining in the brine discharge stream or in the product water stream to do much of anything with.

It sounds to me as though the author of the bood does not have a really good handle on reality...

I believe the Siemens plant uses gas to produce electricity on a large scale (MWs). This is done with steam turbines, and the left-over steam can be used for a variety of purposes - operating a desalination process being one of them. This gives a use for some of the 30% or so of original energy input which cannot be used in the turbines. When you try to reverse the set-up, energy used in the desalination process is a fraction of what would be required to produce a reasonable amount of electricity, so it would be more efficient in a desalination plant to cut down production rather than try to make electricity from the left-over power. That said, if your expectations were reasonable - only want power for the office/canteen - it may be useful in reducing your bills, rather than exporting the electricity for money.

I think it must be so regarding the approach to the question:)

A desalination plant is already a boiler....

Then you can choose a condensing scheme that might do more than just one thing.

I have not heard of any energy 'released' in the desalination process. Plenty is consumed, and the only byproduct I know of is very salty water. If this extra salinity possesses any electrical properties, that would be where to look. When there is a difference in potentials between two bodies, sometimes electrical energy can be produced. I just don't know personally of any process from desalination that could have a byproduct of electricity.

Sometimes waste energy, like the discharge flow and pressure in reverse osmosis, can be recovered to improve the efficiency of the process, but turning that waste energy into electricity would only be practical if the primary energy source was not electric.

"For example siemens constructed a plant which uses its remnant energy out of gas combustion so as to produce fresh water"

I think the OP is trying to say that a byproduct of gas combustion was fresh water. It's true that many fuels, when combusted, release water (and CO2, CO, NO2, etc.) as a result of combustion. The system he's talking about may capture the water from the exhaust.

I guess it could be done. If you could condense the water, I'd imagine it would be heavily contaminated with the other combustion products. It would probably need a lot of processing anyway. I did a quick google search on "capturing water from exhaust". It didn't net much.

I guess it could be done.

My own choice would be to scavenge the waste heat from the combustion and use it to boil water to make steam to generate electricity

My own choice would be to scavenge the waste heat from the combustion and use it to boil water to make steam to generate electricity

And if you condense the spent steam at a high enough elevation above the boiler, you can use the energy released by the falling water to generate more electricity.

hi cr4 freinds,

in general , there are three types of desalination plants inthe world, multy stage flash, reversosmosis and multy effect .

mainly desalination plant is consisting of 20 to 50 rooms and the size of each is between 4x 11 meter and it depends on the design.

the tempreture will be changed in each room due to changing of presure . the chamber will be vacumed by ejectors so the last chumber ( room ) presure will be negative , less than zero . so sea water with high solts will be evaporated then it will be condensed . so it can,t be drinked as it does not contain any chimical properties like managnez, calcium,sulphat .....ext.

in r.o .process the rate of electricity is very high at the start-up the plant and been explained by others . for more details of desalination plant , advantages and dis-advantage , pls check in google search.

the only way you can get electricity from desalination plan is indirect by utilizing extra steam to run steam turbine . you can do it by you are going to spend 5$ to buy coca-cola while you can buy it for less than onse $.

jinxnao: quoting from one of your posts; "...constructed a plant which uses its remnant energy out of gas combustion...", I appropriately conclude that you are referring to "evaporation-essentially, you boild the water off, then condense it again".

Yes it is possible in that particular case to produce electricity by use of ramnant energy out of gas combustion. Heat Recovery Steam Generators (HRSG) is the commercial name of the equipment.

HRSG are extensively being used in Combined Cycle Power Plants; The hot gases from a Gas Turbine are fed into a boiler (HRSG) and the steam thus produced operates a steam turbine generator.

In case of Reverse Osmosis If you are using a gas turbine to build water pressure, HRSG can be applied to turn gas turbine Power unit.

Sea water desalination plants evaporate sea water, raise steam,use for power generation and the condensate goes for chilling to get demineralized water. Does your book mean this?

It costs upwards of 3kWh per tonne of water produced, using an RO plant with energy recovery. An evaporation plant costs perhaps 10-50 times more.

So where is this electricity going to be produced - by what process?

I'm not sure I follow your question. However, If your evaporative desalinization set up doesn't use electricity to evaporate and condense the H20, you might be able to use the highly concentrated saline solution to oxidize scrap steel and create a current of electricity that could be considered profitable provided your labor is cheap enough to re-load and re-wire your "salt battery" and you have access to waste iron at no cost. Gary

I think you are confusing the issue: You read somewhere about using something like a desalination process to create a differential pressure that can be used to drive a turbine (water) and generate electricity.

That is opposite of the desalination: Osmosis against reverse Osmosis. Use water from a river (for example) and water from the sea (Salty) on each side of a membrane... Details where you started reading...?

Re-read carefully.

The electrodialysis process operating in reverse can produce DC electrical power. But now you're receiving two streams of water with different salinities and mixing them, which kinda defeats the purpose if your goal is power + fresh water.

I imagine that you could use a conventional distillation-type process and tack on reverse electrodialysis.

The thermal (possibly solar) plant would take saltwater in and produce a stream of brine and another of fresh water. The fresh water would go to the consumer, of course.

Now you have a stream of highly saline water - much saltier than seawater - going back into the ocean as waste. But what if, instead of just allowing it to mix freely with the ocean, you had it mix with seawater in an electrodialyzer operating in reverse. You could generate power from that. I don't know how much.

One limitation on such a process is that there is a limit to how much you can concentrate that waste brine and still operate the plant - above a certain salt concentration, scale begins to form and plant maintenance becomes a big problem.

That will depend on which method you use to perform desalination. If it's reverse osmosis then no, it's impossible. However, if it's distillation, then yes, the steam can be used to drive a turbine. However, the amount of electricity generated will be far less than is consumed by the distillation process. However, it may still be sufficient to power a couple of pumps to supply seawater to your raw water storage tanks.