On 14/02/2021 12:58, The Natural Philosopher wrote:
On 14/02/2021 05:36, jon wrote:
On Sun, 14 Feb 2021 03:22:29 +0000, The Natural Philosopher wrote:

On 13/02/2021 21:36, newshound wrote:
On 13/02/2021 12:24, Fredxx wrote:
On 13/02/2021 10:43, newshound wrote:

Sorry, just can't see it. Not sure I can even see a large scale solar
still with photovoltaic pumps working for coastal deserts. Otherwise
someone would be doing it.

You need a source of salt water, not normal in a desert.

Which is why I said coastal. Although of course brackish water is
available in many deserts, you just have to dig deep enough and then
pump it up.



Your low grade heat from a power station operated in the usual way
isn't going to evaporate water quickly. Then you need massive
condensers to collect it, pumping seawater through them to keep them
cool with the associated pumping and pipe friction losses.

Low grade heat could be 100+C, enough to boil water.

The waste heat from a power station is at about 20 deg C. Because all
the practicable energy has been removed from it.

well the exit from the final steam turbine is at 60C give or take.

after that its all lost heat even if the condenser outflow is at 20.



And you don't need to boil the water, merely to evaporate it. This is
easier if you create some vacuum (although that requires energy). But
you still need to provide some latent heat to get it into the vapour
phase (to separate the salt) and then remove it again.


Given you can heat incoming water by the condensate much of the energy
can be reclaimed. That would make condensers more manageable and a
power station would normally need a condenser of some size.

FFS reclaiming this energy is just not useful. The point is that to
keep the process going you have to remove the latent heat from the
vapour continuously, and them MOVE IT SOMEWHERE ELSE. This requires
capital hardware and some energy.


In a combined electricity/desalination plant it *might* make sense to
take bled steam from the HP turbine outlet to do the evaporating,
this still has a decent energy density so your plant volume (and
hence capital cost) is lower for a given throughput.

You only need a little higher than 100C to boil water. HP steam can be
up to 600C, and there are many stages to a generating turbine to bleed
steam off, I might suggest nearer the low pressure exhaust side.

Trust me, engineers who design such systems do the sums properly. In
most cases where process steam is taken out of a power station a
smaller "volume" at higher temperature is usually removed, because this
provides more flexibility and lower cost.

Greenhouses are a bit different because all they usually need is
condenser outlet temperature.


Of course if you want a higher temperature exhaust it means the
turbine could be a few stages shorter and correspondingly cheaper.

I suspect that reverse osmosis would still make better commercial
sense.

Of course it does. That is exactly my point, this is the way that
everyone does it. Not by faffing around with thermal cycles.


Really it all depends on the cost. It may be that if you have pretty
much 'free' heat that distillation trumps osmosis

Is that with an entry temperature of 600C ?

no. That's where you get your power from. Most multistage turbines exit
IIRC about 50°C.

They could as easily exit at 100°C with a significant loss in efficiency
of you wanted more heat for desalination.

But I am too rusty to do the sums. Its all about capital cost versus
loss of income from electrical power.



Also, condensing the vapour is just as necessary as evaporating it. That
has a capital cost too.