Fossil and Nuclear power plant heat rejection...

[Koros]

Hello,

For a fossil power plant and a nuclear power plant with the same capacity of power generation(i.e 700 MW)which plant would reject more heat to atmosphere and could you please say why?

 

[BrianPetersen]

It depends on the overall thermal efficiency ... and nothing else. Divide the electrical power output by the overall thermal efficiency of the entire plant and you have the input heat rate. Subtract the electrical power rate from the heat rate and you have the total amount of heat rejection. All very simple first-law thermodynamics stuff.

I don't think the nuclear plants are using as high superheat and reheat temperatures as the best combustion boilers can, but on the other hand, the nuclear plants aren't throwing heat out a chimney. It's likely a wash, but I'm open to seeing better information from others.

 

[IRstuff]

It depends on the design of the nuclear reactor. San Onofre in California is designed specifically to be cooled by seawater and therefore has very little rejection to the atmosphere:

http://en.wikipedia.org/wiki/San_Onofre_Nuclear_Generating_Station

Diablo Canyon appears to be similar:

http://en.wikipedia.org/wiki/Diablo_Canyon_Power_Plant

There are presumably others that are designed to dump their heat load into the atmosphere, like:

http://www.nrc.gov/info-finder/reactor/susq1.html

TTFN

FAQ731-376

 

[IRstuff]

You do realize that there are power generation as well as nuclear engineering forums where this might be a more appropriate topic?

TTFN

FAQ731-376

 

[BrianPetersen]

Obviously there is an interpretation that I hadn't considered. Is the original poster concerned with the total amount of heat being let out into the environment (the word "atmosphere"??) or is the original poster concerned only with heat emitted to atmosphere while pretending that heat emitted in other directions (e.g. seawater or lakewater), but which is nevertheless "there", doesn't exist?

If it's the latter, the question can't be answered, because there are examples of power plants regardless of power source that are cooled *directly* by varying combinations of seawater, lakewater, or atmosphere ... All that heat is going *somewhere*, though. One thing that can be stated is that the fossil fuel plants always have a chimney, and the nuclear plants don't (at least, not for the main power source).

 

[zdas04]

I took the word "atmosphere" to mean "environment". If he really meant "atmosphere" then I would expect to see a fossil fuel plant stack heat plus evaporated cooling water to be a higher heat load than just the evaporated cooling water in a nuc plant (even with sea water cooling, the heat from the plant will find its way into the atmosphere eventually).

The question of heat to the environment is indeterminent. You would have to do a heat balance on each particulare plant to compare the benefits of superheat vs. not having combustion ineffeciencies vs. steam generator fouling. With two plants both operating within design, I would expect that the pluses and minuses would cancel and two 700 MW plants would each dump about the same amount of waste heat to the environment.

David

 

[IRstuff]

That parity exists is a bit unclear. I suspect that a nuclear plant has additional heating that isn't strictly comparable to a fossil fuel plant. The typical pressurized water reactor has an additional loop that a conventional generation plant would not have, i.e., a conventional plant would be able to heat the steam generator directly, so all its available heat is directed to steam generation.

In a PWR, the steam generation is done by a pressurized water system that is heated by the reactor. Thus, a potentially sizable portion of the reactor heat might be lost in the reactor, as well as in the primary pressurized water loop.

TTFN

FAQ731-376

 

[Koros]

Thank you allfor your comments,

Let me clarify my means: I would like compare the heat rejection of a CANDU nuclear plant(700 MW) with a coal-fired fossil plant(700MW) both via the main condenser to a cooling tower, just learn me which one has overally more heat rejection than the otherone.

 

[BrianPetersen]

So, what's the overall thermal efficiency of both plants? The answer to that question will tell you the answer to your question. I suspect that the coal plant will have higher superheat and reheat temperatures in the Rankine portion of the cycle, and thus higher efficiency in that part of the cycle, but the coal plant will be throwing some heat out a chimney that the nuclear plant doesn't have.

 

[IRstuff]

For what purpose are you asking? You've picked the portion of the systems that probably have the least amount of differences.

TTFN

FAQ731-376

 

[byrdj]

This seems to be a very simple thermo question, unfortunately I had problems with even simplier thermo questions

but using a SWAG I would "think" the conditions at the condensor indeed would be the same. However given the intial conditions are so much less for the Nuke, the steam flow to produce the same MW as the fossil would be more those more more BTUs/hour into and out of the condesor.

I should have heat balance charts, but I can't find them at this time. (since I never really cared, I don't recall where they might be packed away)

 

[IRstuff]

Again, I think that's unclear. Assuming a generator sized to deliver 700 MW, the drive requirement for that generator is the same in both cases, so the amount of energy required to drive the generator is the same. From the perspective of the steam/condenser loop, a plausibly optimum steam loop to drive the generator would be the same in both cases. Given that assumption, the efficiency from the steam loop to MW out would be very similar.

What the nuclear reactor does with its excess capacity would be a different question. San Onofre and Diablo Canyon are both designed around 1100 MW generators, so there's hypothetically a design that would suit a 700 MW generator, although the overall cost/benefit might be too low for 700 MW, which is yet another question.

TTFN

FAQ731-376

 

[byrdj]

the CANDUs are just under 700 MWs

 

[electricpete]

Nuclear plants typically have lower steam temperatures... a result of material temperature limitations on the primary side. Lower source temperature with the same sink temperature means lower Carnot efficiency, generally lower efficiency, and more heat rejected per energy output (for nuclear)

=====================================
(2B)+(2B)' ?

 

[ScottyUK]

I agree with ePete. For a given generator power output and both driven by condensing turbines, the turbine with the higher inlet temperature will have higher thermal efficiency. This will potentially weigh in favour of the fossil fueled plant, assuming that it is a relatively modern plant capable of operating at temperatures and pressures greater than those at which the nuke can achieve.

byrdj,

Yes, thermodynamics is a creation of the devil.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[zdas04]

Thermodynamics is a thing of beauty. It was that electrical stuff that the devil spawned.

David

 

[byrdj]

Nuclear intial conditions are approx 1200 - 1500 psig saturated (approx 550 - 600F) with no (or very little)intermedate reheat

versus a drum at 2400 psig 1050F or
a once through at 3500 psig 1050F
and with 1000F reheat

 

[Koros]

Thank you all again for participating on this topic,

I agree with "electricpete", he got answer of my question.

 

[rmw]

The lower operating pressure of the Nuclear cycle is part of the equation too. ElectricPete mentioned the lower temperatures of the Nuclear cycle, and those same limitations dictate lower pressures too.

Hence a X hundred MW nuclear plant due to the fact that it operates at lower temperatures AND pressures requires MUCH more steam flow for the same MW output than a fossil plant (assume the highest efficiencies obtainable - ultra supercritical double reheat machines) would.

The steam flow that ultimately gets to the condenser(s) dictate what the heat rejection to the environment is.

I think if you can find a copy of the book STEAM by B&W, you will find some heat balances for similalr sized fossil and nuclear plants, if memory serves. I know they have heat balances for some fossil sized units about the size you mentioned.

rmw