Why cooling?

[Mxyzptlk]

As a non-qualified person interested in science I often come across things I don't understand.
Would one of you brain-boxes explain what I am missing here.

As I understand it, a power station generates electricity from turbines. The turbines are moved by injecting super-heated steam. The 'exhaust' steam is then cooled in those enormous towers and presumably condenses and is then re-heated.
Why does this cooling need to take place? Surely you could feed the heat back to the super-steam boilers or something?

Some or all of this may be completely incorrect!

[Ziggurat]

Surely you could feed the heat back to the super-steam boilers or something?

How? Heat only flows spontaneously from hotter to colder. The temperature of the expanded steam, while hotter than ambient air, is colder than the boilers. So you can't move heat from the expanded steam into the boilers except by expending energy (ie, use a heat pump), which would rather defeat the purpose of a generator.

[fetchbeer]

It's been a while since my thermodynamics classes but as much heat as can be reasonably recycled is.

The basis of the steam cycle is as follows:

1: Saturated steam from the steam generator is expanded in the high pressure (HP) turbine to provide shaft work out put at a constant entropy.
2: The moist steam from the exit of the HP turbine is dried and superheated in the moisture separator reheater(MSR).
3: Super heated steam from the MSR is expanded in the low pressure (LP) turbine to provide shaft work output at a constant entropy.
4: Steam exhaust from the turbine is condensed in the condenser in which heat is transferred to the cooling water under a constant vacuum condition.
5: The feed water is compressed as a liquid by the condensate and feed water pump and the feed water is preheated by the feed water heaters.
6: Heat is added to the working fluid in the steam generator under a constant pressure condition.

The cooling tower is step 4, where the steam is cooled back to water. There are 2 main reasons for this if i remember correctly.

1: It is much more efficient to make high pressure steam from liquid water, than it is to make high pressure steam from low pressure steam. A liquid to gas transition involves a much larger increase in volume than adding the same amount of energy to a gas.
2: Condensation in a turbine damages it. Which is why they don't simply condense the steam in the turbine, and why they have a cooling tower to do it outside of the turbines.

So pretty much, the amount of heat they loose to the environment to recondense the steam makes the entire process more efficient than trying to recycle the steam directly from the turbines.

[Verde]

As I understand it, a power station generates electricity from turbines. The turbines are moved by injecting super-heated steam. The 'exhaust' steam is then cooled in those enormous towers and presumably condenses and is then re-heated.

The exhaust steam has already been cooled by doing the work needed to drive the turbines. At this point there is insufficient heat energy available for any practical use, so it is vented to the atmosphere.

V.

[Salerio]

The exhaust steam has already been cooled by doing the work needed to drive the turbines. At this point there is insufficient heat energy available for any practical use, so it is vented to the atmosphere.

V.

As I understand it the steam from the turbines is low pressure steam and you can extract energy down to about 20 degrees C above ambient (where ambient is the temperature of the coolant). The water used in the boiler is very expensive, very clean water and doesn't get exposed to the atmosphere at all.

The steam is pushed into a heat exchanger where vast amounts of cooling water is used to drop its temperature and condense it back to water (because it's at a low temperature and low pressure there is a vast volume of steam going into the condenser hence the large amount of cooling water needed to condense it). The water is then pumped back into the boiler to be turned back into high pressure steam.

The cooling towers are to cool the coolant in situations where the power station can't extract enough water from a river, or to cool the coolant prior to discharge where environmental concerns dictate this.

[casebro]

Do they actually release the stem to the atmosphere, or do they cool the steam by spraying water on its pipes, the water then turns to steam?

The second system is used on boats operating in water. Double acting compound stem engins- pistons. Cooling the 'used steam' condenses it, actually making a vacuum in that exhaust side of the system. This vacuum means up to umm 12 psi greater pressure differential that helps 'suck' the piston up, while the steam is 'pushing' from the bottom. If that water is only cooled to just under boiling point, that much energy is saved when the water is reheated to repeat the cycle.

I'm going to make one some day, lot's of brass, cast iron cylinders, 10 hp will be enough to power a displacemnt hull of about 16-18 feet, up to hull speed, about 6knts.

[Soapy Sam]

Most turbines are multi-stage, with fan blades iprogressively designed to extract energy from lower and lower temperature and pressure fluids.
There has to be a practical lower limit to that though.

It does seem to me however, that such low temp / pressure steam might be used for something, if only preventing winter frost in greenhouses. I suppose that's where individual situations vary and actual costs dictate the best action.

[Fishstick]

Most turbines are multi-stage, with fan blades iprogressively designed to extract energy from lower and lower temperature and pressure fluids.
There has to be a practical lower limit to that though.

It does seem to me however, that such low temp / pressure steam might be used for something, if only preventing winter frost in greenhouses. I suppose that's where individual situations vary and actual costs dictate the best action.

In many places (for example, an aircraft carrier), the steam is also used for heating and hot water. Eastern europe also has a lot of places where power plants' hot steam is piped to communities for cheap heating and hot water.

[lauwersw]

Do they actually release the stem to the atmosphere, or do they cool the steam by spraying water on its pipes, the water then turns to steam?

Generally the steam stays in a closed circuit (fetchbeer wrote "under constant vacuum conditions", which indicates there's some good reasons for that), so they spray water on the pipes where the steam is running through.

[Mxyzptlk]

Thanks for these great replies, people. Fetchbeer was especially helpful. I think I get it now and I can see that with secondary turbines and pre-heating that any residual heat is used as much as possible, and I see why the steam has to be condensed.
So that's all good, and I would find it really helpful if you could expand on this:

Quote:
1: Saturated steam from the steam generator is expanded in the high pressure (HP) turbine to provide shaft work out put at a constant entropy.

Entropy is another idea that I have trouble with! I think it is basically the energy 'lost' from a system because of friction and other inefficiencies, though it's not lost, just dissipated into the surrounding environment, ie out side the system.
So what is constant entropy and is it important that it's constant?

[Dilb]

Entropy is another idea that I have trouble with! I think it is basically the energy 'lost' from a system because of friction and other inefficiencies, though it's not lost, just dissipated into the surrounding environment, ie out side the system.
So what is constant entropy and is it important that it's constant?

Entropy is a physical property of things, like pressure, or temperature, or internal energy. Entropy can be produced by "irreversible" things, like
1. Friction
2. Electrical resistance
3. Transferring heat from something hotter to something colder

However, if we don't produce any entropy, it's still conserved, the same way that energy is always conserved.

Suppose we have a cylinder of a gas, with a piston that can move up or down, allowing us to compress or expand the gas. If we have a perfectly greased, smooth cylinder, we won't have any friction to generate entropy. If we insulate the cylinder, then we won't transfer any heat, so we can't generate any entropy that way, either. So if we compress the gas in this frictionless, perfectly-insulated cylinder, we have a process where the entropy has to stay the same.

Obviously this is only an approximation, but it's a useful approximation. Since the entropy doesn't change, it's called an isentropic process, or constant entropy process, or a reversible process.

[bobdroege7]

Most turbines are multi-stage, with fan blades iprogressively designed to extract energy from lower and lower temperature and pressure fluids.
There has to be a practical lower limit to that though.

It does seem to me however, that such low temp / pressure steam might be used for something, if only preventing winter frost in greenhouses. I suppose that's where individual situations vary and actual costs dictate the best action.

In a modern power plant it is used to preheat the feed water, recovering some of the energy that way.

[bobdroege7]

The exhaust steam has already been cooled by doing the work needed to drive the turbines. At this point there is insufficient heat energy available for any practical use, so it is vented to the atmosphere.

V.

No it's not.

It is condensed for reuse.

[bobdroege7]

How? Heat only flows spontaneously from hotter to colder. The temperature of the expanded steam, while hotter than ambient air, is colder than the boilers. So you can't move heat from the expanded steam into the boilers except by expending energy (ie, use a heat pump), which would rather defeat the purpose of a generator.


But it is not colder than the feedwater and it can be used to preheat the feedwater.

In a GE BWR-6 one of the feedwater preheaters was located inside the main condenser and it used the waste heat from the turbine to preheat the feedwater to achieve improvements in efficiency.

Do some googleing on the steam cycle or Carnot engine for enlightenment

[Mxyzptlk]

Entropy is a physical property of things, like pressure, or temperature...
So if we compress the gas in this frictionless, perfectly-insulated cylinder, we have a process where the entropy has to stay the same.

Obviously this is only an approximation, but it's a useful approximation. Since the entropy doesn't change, it's called an isentropic process, or constant entropy process, or a reversible process.

Thanks Dilb, just to make it clear, why isn't this called zero entropy?

[Ziggurat]

But it is not colder than the feedwater and it can be used to preheat the feedwater.

And it often is. But that's not really what the OP was asking about.

Do some googleing on the steam cycle or Carnot engine for enlightenment

I'm more than sufficiently familiar with the Carnot cycle, but it also has little relevance here. There are no engines which operate on the Carnot cycle, and there would be no point in trying to make one which did.

[Ziggurat]

Thanks Dilb, just to make it clear, why isn't this called zero entropy?

Because it's generally not zero. Entropy is basically the logorithm of the number of possible microstates of a system, for example, the number of different positions and velocities that each of the gas molecules in a cylinder has. It's only zero when your system only has one available microstate, which only happens at zero temperature.

[Dilb]

Thanks Dilb, just to make it clear, why isn't this called zero entropy?

Because it's the "change in entropy" that's zero. If you have an expansion at a constant temperature, you don't call it a zero-temperature expansion, because that sounds like you're expanding something when the temperature is zero, which a scientist or engineer would probably think of as absolute zero. Entropy also goes to an absolute value of zero (at a temperature of absolute zero).

[MervinFerd]

I'm more than sufficiently familiar with the Carnot cycle, but it also has little relevance here. There are no engines which operate on the Carnot cycle, and there would be no point in trying to make one which did.

But, the Carnot cycle sets the absolute minimum of heat that must be disposed of, given the temperatures of the Source and Sink. IOW, the Carnot cycle is the most efficient cycle possible. All real cycles will be less efficient.

Efficiency < T1/(T2-T1); T in Kelvins.

Right?

[Ziggurat]

But, the Carnot cycle sets the absolute minimum of heat that must be disposed of, given the temperatures of the Source and Sink. IOW, the Carnot cycle is the most efficient cycle possible. All real cycles will be less efficient.

Efficiency < T1/(T2-T1); T in Kelvins.

Right?

Yes, though any absolute temperature scale (such as Rankine) will work, not just Kelvin. I'm not sure what point you're trying to make, since we weren't discussing maximum theoretical efficiencies.

[ben m]

In many places (for example, an aircraft carrier), the steam is also used for heating and hot water. Eastern europe also has a lot of places where power plants' hot steam is piped to communities for cheap heating and hot water.

Not just Eastern Europe---New York, San Francisco, Pittsburgh, Harrisburg, San Diego, Minneapolis, Seattle, and Detroit all have district heating---I presume it's all waste heat from power plants.

[MervinFerd]

I'm not sure what point you're trying to make, since we weren't discussing maximum theoretical efficiencies.

Only that waste heat is completely unavoidable. From first principles.

I didn't think that point was made cleanly in the previous posts.

If anybody has the data, it would be interesting to compare the actual efficiencies with the theoretical efficiencies.

[rjh01]

But, the Carnot cycle sets the absolute minimum of heat that must be disposed of, given the temperatures of the Source and Sink. IOW, the Carnot cycle is the most efficient cycle possible. All real cycles will be less efficient.

Efficiency < T1/(T2-T1); T in Kelvins.

Right?

The above formula says
1. If the difference between T1 and T2 is large then Efficiency is small
2. If T1 is small then Efficiency is small.
3. If the difference between T1 and T2 is small and T1 is small then efficiency is unstable.

Is that right?

[ben m]

The above formula says
1. If the difference between T1 and T2 is large then Efficiency is small
2. If T1 is small then Efficiency is small.
3. If the difference between T1 and T2 is small and T1 is small then efficiency is unstable.

Is that right?

Actually, that equation was upside-down; it's e < (T2 - T1)/T2, with T2 being the higher temperature. Nothing pathological happens when T2 goes to zero.

[rjh01]

Actually, that equation was upside-down; it's e < (T2 - T1)/T2, with T2 being the higher temperature. Nothing pathological happens when T2 goes to zero.


That looks better.

[MervinFerd]

Actually, that equation was upside-down; it's e < (T2 - T1)/T2, with T2 being the higher temperature. Nothing pathological happens when T2 goes to zero.

Right. Dumb mistake.

[Mxyzptlk]

Because it's generally not zero. Entropy is basically the logorithm of the number of possible microstates of a system, for example, the number of different positions and velocities that each of the gas molecules in a cylinder has. It's only zero when your system only has one available microstate, which only happens at zero temperature.

I see. Would this be what happens in a superconductor?

[Mxyzptlk]

Actually, that equation was upside-down; it's e < (T2 - T1)/T2, with T2 being the higher temperature. Nothing pathological happens when T2 goes to zero.

This is interesting and relevant to me. With my very limited math, your equation seems to say that the greater the difference between the input and output temperature, the greater the efficiency?
Therefore cooling makes it more efficient?

Is that anywhere near correct or have I gone way off?

[Ziggurat]

I see. Would this be what happens in a superconductor?

Not quite. They do indeed enter a state where many of the conduction electrons enter a single ground state, but the nature of that state, and the energy gap between it and any excited states, is critical to superconductivity and cannot be understood purely in terms of entropy.

[MervinFerd]

the greater the difference between the input and output temperature, the greater the efficiency?
Therefore cooling makes it more efficient?

The maximum possible efficiency is increased by increasing the difference between the source and sink. Ordinarily, it's easier to make the source hotter than to make the sink colder. Some kind of fire for the source and a local river or the atmosphere for a sink.

I'm not sure how relevant this is to steam engines. The fire can be extremely hot, but what matters is the temperature of the working fluid. I imagine that is limited by the properties of available steel alloys. But the Carnot (maximum) efficiency may be much higher than the practical efficiency. ETA: If the source pushes 1000 C (not sure if this is possible) and the sink is room temperature, the Carnot efficiency would be around 75%. Other losses may dominate. To be very clear--I'm just speculating here. Never tried to design a steam engine.

I think it is relevant to alternative energy sources, where you would like to run some kind of engine or pump off a cheap solar collector or other low-intensity source. The heat source may be very cheap, but the possible efficiency is low. ETA: If the source is 100 C and the sink is RT, I calculate the Carnot efficiency to be 25%. That's before any of the losses that would happen in a real engine.