Most efficient temperature to evaporate water?

[mcharbs55]

Hello,

I have a question for you guys and gals. I am working on a Desalination project and am ironing out questions about efficiency. What would be the most efficient temperature to evaporate water? 50, 100, 110 degrees? Does it make a difference, or is the energy required to evaporate a fixed amount of water a constant? Also, does it matter which method is used in terms of efficiency (induction, resistance, etc.). If possible could you please point me in the direction of any good sources on this subject.

Thanks for any help.

 

[rumborak]

Isn't the rather obvious answer here, the hotter the better? I mean, people boil water to create steam after all.

 

[mcharbs55]

Sorry, I should have been more specific. I meant in terms of energy efficiency.

 

[rumborak]

It's really a question of how exactly you pose the energy question. I mean, if you have water and just leave it at whatever temperature it already is, you will expend no energy, but there will always be a certain amount of evaporation. So, when posed that way, you already have perfect efficiency and there's no way to improve.

 

[russ_watters]

In terms of energy efficiency, ambient temperature is the most efficient because it requires no artificial heat input.

 

[mcharbs55]

Okay thank you, I know it sounds like a stupid question. I guess what I'm really asking is; as temperature of water increases as well as the energy consumption, does the evaporation rate increase linearly? If it is not linear then I want to find "the sweet spot" (not taking all day to evaporate but not using crazy amounts of energy) to evaporate the water.

Thanks again for any help.

 

[russ_watters]

It's probably best to insulate the container and boil the water, that way you can be sure you aren't losing any heat in any other way but to the boiling. When you evaporate, you rely on cool, dry air pulling away the moisture, which also pulls away heat.

 

[rumborak]

There will essentially be no other way than to do testing, since it depends on so many factors. For example, while physical formula do exist for vapor pressure in equilibrium, your scenario is decisively not in equilibrium, since the point of desalination is to transport away the vapor to a different place where it can then recondensate. So, you actively ty to stay away from equilibrium (since that would mean no more vapor production) and thus it now heavily depends on how fast you are transporting the vapor away.
Not only that, the remaining water will slowly increase in salinity, which will influence things. How fast it increases then becomes a question how much total water there is. Et cetera, et cetera.

 

[NascentOxygen]

I am working on a Desalination project and am ironing out questions about efficiency.

Are you reducing the pressure so that it boils (rapidly vaporizes) at a lower temperature?

 

[John Park]

Okay thank you, I know it sounds like a stupid question. I guess what I'm really asking is; as temperature of water increases as well as the energy consumption, does the evaporation rate increase linearly?

The vapour-pressure of water increases much more than linearly with in creasing temperature. But vapour pressure isn't the only factor that determines the evaporation rate. In general air flowing over a water surface (or bubbled through water) will cause faster evaporation than still air (the basis of how clothes driers work). There may be a "sweet spot" in terms of dividing your input energy between heaters and fans or air pumps--but you'd probably have to find it for your own set-up.

 

[DrStupid]

Does it make a difference, or is the energy required to evaporate a fixed amount of water a constant?

It is almost constant. Maybe there is a potential for minor optimizations but it is negligible compared to other factors like energy recycling with a heat exchanger.

 

[OmCheeto]

Okay thank you, I know it sounds like a stupid question. I guess what I'm really asking is; as temperature of water increases as well as the energy consumption, does the evaporation rate increase linearly? If it is not linear then I want to find "the sweet spot" (not taking all day to evaporate but not using crazy amounts of energy) to evaporate the water.

Thanks again for any help.

It is not a stupid question.
One of our mentor-emeriti asked a very similar question a while back, and I don't think he's stupid.

Here is one thing I found out:

No matter what I do, I always end up having to add 2265 kJ, for every kg of evaporated water.

Of course, your problem is a bit different, so there may be different techniques to consider.

Off the top of my head, I would look into aeration. as it would lead to a larger surface area.

Just a thought. I have no idea if it's stupid or not.

[edit: but as @stevendaryl said, if this idea makes you a millionaire, you'd better share... ]

 

[John Park]

It might help if you told us a bit more about your set-up. What sort of quantities of water, in what sort of vessels? (Other things being equal, water evaporates faster from a shallow open pool than from a deep tank.) What are your heat sources?

 

[rumborak]

Based on this thread I read the Wikipedia article on the subject. Very cool, I had no idea that desalination by freezing is actually a thing.

 

[mcharbs55]

There will essentially be no other way than to do testing, since it depends on so many factors. For example, while physical formula do exist for vapor pressure in equilibrium, your scenario is decisively not in equilibrium, since the point of desalination is to transport away the vapor to a different place where it can then recondensate. So, you actively ty to stay away from equilibrium (since that would mean no more vapor production) and thus it now heavily depends on how fast you are transporting the vapor away.
Not only that, the remaining water will slowly increase in salinity, which will influence things. How fast it increases then becomes a question how much total water there is. Et cetera, et cetera.

I agree, there are just too many variables as you have stated. Thank you for your input and assistance!

 

[mcharbs55]

Are you reducing the pressure so that it boils (rapidly vaporizes) at a lower temperature?

No, I had not even thought of that but it is definitely something I need to look into.

 

[mcharbs55]

It is not a stupid question.
One of our mentor-emeriti asked a very similar question a while back, and I don't think he's stupid.

Here is one thing I found out:
Of course, your problem is a bit different, so there may be different techniques to consider.

Off the top of my head, I would look into aeration. as it would lead to a larger surface area.

Just a thought. I have no idea if it's stupid or not.

[edit: but as @stevendaryl said, if this idea makes you a millionaire, you'd better share... ]

Thank you for your reply, I appreciate it! I am going to look into aeration!

 

[mcharbs55]

It might help if you told us a bit more about your set-up. What sort of quantities of water, in what sort of vessels? (Other things being equal, water evaporates faster from a shallow open pool than from a deep tank.) What are your heat sources?

I am still in the design phase so I have not decided things like basin size! I am currently looking at using something around a 1000W electric element powered by a solar array. Still waiting on details of solar specifications before I chose the element I wan to use.

 

[mcharbs55]

It is almost constant. Maybe there is a potential for minor optimizations but it is negligible compared to other factors like energy recycling with a heat exchanger.

Okay thank you, multiple sources have learned toward this.

 

[NascentOxygen]

Modern commercial desal plants use reverse osmosis with membranes, they don't evaporate and condense. I don't know how well they scale down.

 

[Andy SV]

Solar to electric to heat is not efficient
1000W of solar panels are a lot bigger than the equivalent solar water heater
And that is not even accounting for transmission losses

My brother is an EPA wast water treatment engineer but he started at a truck wash treating the wash water lagoons
When he got a algae bloom of a bad species he had to kill it but could not poison the water for obvious reasons. What he did was to add a black dye to the water
It got almost hot enough to not want to touch.