In sub-critical thermal power cycle, the thermodynamic fluid absorbs certain amount of latent heat during phase change from water to steam but in a super critical unit, this absorption is zero. Now question is, how much this factor helps in improving the cycle efficiency of a super critical thermal power cycle?
Practising Efficiency engineers in large thermal power plants can answer this question.
I don't think that absorbing heat in a region where there is no latent heating is important or necessary. However, in order for a cycle to be nicely recuperated, so that as much as possible heat is added at high temperature and rejected at low temperature, it is useful if the gradient of the hot-side and cold-side T-H curves can be made as close to each other as possible. This becomes difficult in you have one side which is latent (flat) and another side which is sloped.
In the case of a subcritical Rankine cycle, everything is fine, since recuperation from saturated high-pressure vapour to saturated low-pressure liquid is possible: two flat (but parallel) lines; Latent-latent transfer. In the case of a supercritical Brayton cycle, it's also fine, sensible-sensible (two sloped lines). But cases where you are forced to recuperate sensible-latent or vice versa are going to be a problem.
I'll be interested to see any other answers to this question -- those are just my thoughts on this.
Technologically, we have prepared and adopted this T-H/T-Phi diagrams for a understanding of the matter, but scientifically how much it is clarifying and justifying the concept. We should collect data from practical cycles and display for dissemination and we can know how far it is helpful for efficiency improvement. As pressure rises latent heat absorption decreases and at super critical pressure say 225kg/sq.cm for water-steam , the latent heat absorption reduces to zero. This zero absorption of latent heat : whether it has any role in improving the thermodynamic cycle efficiency. As such systems working on super critical pressure cycle are 4/5 % more efficient than sub-critical operating units.
dear Radhashyam Muduli,
I think, basically, the reason of a better efficiency are correlated (1) with a higher mean temperature of introduction of the heat; (2) a better coupling between the isobar of the steam heating and the line of the (cooling) hot combustion gases; (3) with better performances of turbines.
However, the large super-critical steam cycles are always accompanied by (at least two) re-heating.
you can see also:
http://www.elp.com/articles/print/volume-81/issue-1/power-pointers/primer-on-supercritical-steam.html
Dear Constante Mario Invernizzi
You are right to state the reasons for higher efficiency of super critical units for less heat rate and less fuel consumption. I was interested to know this aspects zero latent heat absorption in Boiler leading to low heat absorption in total cycle leading to improved efficiency in such system from practising engineer s in thermal power plants.
There is little pressure margin to reheat twice.once reheat is adequate at entrance of intermediate pressure turbine. After all supercritical unit are more efficient.
It improves it by the percentage of Total energy inputs absorbed in the heating of already Supercritical Steam/ divided by the percentage of the Total energy inputs absorbed by heating Subcritical steam multiplied by the Supercritical Carnot efficiency divided by the average Carnot efficiency of the Subcritical section. A weighted percentage in effect.
Mr.Derek Auret,Thank you for your answer. Can you please calculate these figures/Data derived from a 660 MW or 800 MW coal fired unit known to you and display the same public vision or send it to me at [email protected]? My querry was: in a super critical thermal power plant, thermodynamic fluid absorbs no(zero) latent heat while converting from water to steam in Boiler while in sub-critical unit this value is something positive say 150 or 200 kcal/kg. How this feature helps in improving the efficiency of a super critical unit that is accepted as more efficient unit?
Dear Mr Muduli Sir.
I spent 3 years building a collapsing steam water pump some 30 years ago, my introduction to steam. Turns out that efficiency of such a device is limited to 3.2%, by virtue of the huge cost of energy involved in phase change as you rightly put, and as I put, the weighted average of that cost. Then I learned that a steam engine on a train, running at perhaps 30 bar, has an efficiency limit of perhaps 15%. This low figure is also due correctly to the huge weight of the phase change cost in any average. So when you state that a trans-critical operation is free of that cost, I completely understand what you mean.
With the two systems being completely different, for example in one the cost of promoting the liquid to gas is cyclic, and in the other non existent, perhaps the theory should just be understood, and the facts of results left to speak for themselves.
On re-reading the other answers, I am completely taken aback that you are only talking about super-critical operation as a hobby. I quote .... 2 reheatings, and note your reference to not much of a pressure range. From that you might denote my naivety as I generally work from first principles.
Naivety aside, I propose that you do away with water-tube technology . Towards that, I show you the mechanism to bypass conductive restrictions in heat transfer. https://youtu.be/phGaFavF5GQ .
Mr.Derek Auret: Still then you have not understood the point. The super critical thermal power units have efficiency 42% while sub critical are only 37% efficient with 5% more efficient. The analysis is out of 5% increase in efficiency ,how much latent heat zero absorption factor contributes in enhancing this efficiency in super critical units? Hobby will not help in any way for a detail study.
The starting point of thermodynamic logic is: 1) opposing perpetual motion. 2) The irreversibility of dynamics. Let's look at how to get Carnot efficiency = 1-T1/T2.
1.1 There are many kinds of type 2 perpetual motors, each of which is a natural phenomenon.
A) Machine A: against the irreversibility of thermodynamics (diffusion, heat conduction, friction, etc) - dynamics;
B) Machine B: Utilizing the Difference of Carnot Efficiency (Reversible Thermodynamics) - Thermodynamics
1.2 A and B belong to different disciplines. There is a parallel relationship between them and there is no logical mutual inevitability.
1.3. Logic of the Second Law of Thermodynamics:
Experience induction, deny that A machine==> B machine can not be manufactured==> All material Kano efficiency: 1-T1/T2.
1.4 The logic of the second law of thermodynamics violates the physical logic that A and B cannot be inferred from each other.
1.5 The second law of thermodynamics elevates the "irreversibility", which is in fact only a kinetic experience.
📷 Detailed discussion can be found in the following figure.
Dear Bo Miao, Thank you for your attempt and answer.
Your answer is theoretical and it does not reflect any efficiency and how can it be enhanced in a traditional thermal power plant using coal as principal fuel.
The increase in efficiency by and around 5% in super critical thermal power units is effected by following factors,
1. Larger unit capacity say 660 MW in normal 500 MW units
2. Higher steam parameters say pressure 260 kg/sq.cm in stead of 171 kg/sq.cm and 580 degree Centigrade in stead of 540 degree Centigrade.
3. During process of evaporation say phase change from water(liquid) to steam(vapor or gas), since system operates above critical pressure of water/steam (225 kg/sq.cm) it absorbs nil/no latent heat.
My question is how that 5% increase in efficiency can be shared or distributed in above three heads.
You can consult your professors on Thermodynamics and Heat Engines and suggest a plausible answer.
Comparison of New and Old Thermodynamics
1. Logic of the Second Law of Thermodynamics: Subjectivism, Logical Jump, Interdisciplinary Argumentation.
2. New thermodynamics pursues universality, two theoretical cornerstones:
2.1 Boltzmann formula: ro=A*exp(-Mgh/RT) - Isotope centrifugal separation experiments show that it is suitable for gases and liquids.
2.2. Hydrostatic equilibrium: applicable to gases and liquids.
3. The second and third sonic virial coefficients of R143a derived from the new thermodynamics are in agreement with the experimental results.
3.1. The third velocity Virial coefficient derived is in agreement with the experimental data, which shows that the theory is still correct when the critical density is reached.
4. See Appendix Pictures and Documents for details.
Dear Bo Miao, Thank you for your presentation and it is a nice one based on mathematics and thermodynamics. I am not that much qualified to understand your theory. If you can, please provide a break up of 5% improvement in thermal cycle efficiency as it is experienced in super critical thermal power units in three heads as mentioned in my last question.
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