Electrical power out of R134a phase changes

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Discussion Overview

The discussion revolves around the application of the Rankine cycle using R134a as a working fluid to extract 5 kW of electrical energy. Participants explore various aspects of the system design, including turbine size, heat exchange parameters, and the necessary properties of R134a, while addressing the challenges faced by a second-year engineering student.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant seeks guidance on determining the turbine size needed to extract 5 kW of power from the Rankine cycle using R134a.
  • Another participant questions the specific requirements, such as the amount of R134a needed, the frequency of the cycle, and the heat extracted from the heat reservoirs.
  • Clarification is requested regarding what is meant by "size" of the turbine, with emphasis on the need to deduce parameters like the amount of R134a and the cycle frequency.
  • A participant suggests that knowing either the amount of R134a or the cycle frequency could allow for calculations of the other based on the inlet and exhaust temperatures and power requirements.
  • Discussion includes the importance of steam tables for R134a to compute cycle efficiency and the impact of non-reversibility on actual efficiency.
  • One participant expresses doubt about the availability of advanced and detailed information on the forum for the specific queries raised.

Areas of Agreement / Disagreement

Participants express varying levels of clarity regarding the specific goals of the original poster, with some seeking more detailed information while others provide general theoretical insights. No consensus is reached on the specific calculations or design parameters needed.

Contextual Notes

Limitations include the lack of specified assumptions about the cycle's idealization, the need for specific flow rates, and the dependency on the properties of R134a as outlined in steam tables. The discussion does not resolve the uncertainties surrounding the design parameters.

Who May Find This Useful

Engineering students, particularly those studying thermodynamics and energy systems, may find the discussion relevant for understanding the complexities of designing a Rankine cycle system.

jonks
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Hi

My 2'nd year of engineering doesn't quite let me to find an answer to the following problem:

In a nutshell:
I want to use rankine cycle to extract 5kW worth of electrical energy from a system utilizing R134a as working fluid.

The properies of R134a are as follows:
Tag R134a
Compound CH2FCF3
MolMass 102.03
Critical T 101.08
Critical P 4060.3
Critical D 515.3
Critical V 0.00194
Gas Cosnt 0.0815

http://www.pandikas.ee/kool/r134a/propertiesR134a.jpg

Available heat source is 70C and cooling 20C
The evaporator needs to be of X size, the Condenser needs to be of X size and I am sorry to say the turbine size isn't preset either - they just have to be big enough. No real idea on how to go a bout it.
Now, working with handy materials I figured the standard 2.54cm coiled copper tube will do nicely as evaporator emerged in the 70C water.
This assumption gives me the force value of 1072.91 kg*m/s - the other values I dare not to put down in here as they seem way too unrealistic...

Could someone just guide me to right direction on how to go about those multiple problems I've encountered here?

Br,
J

My previous work can be found in attachment area.
 

Attachments

Last edited by a moderator:
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What exactly are you trying to determine? No. of kg of required R134A? Speed (frequency) of the cycle? Amounts of heat extracted from the 70C reservoir and dumped into the 20C reservoir per cycle?
 
Yea it's not really clear what exactly you are looking for...

You already know the operating temperatures and specified power output but beyond knowing what you are looking for we don't know if you are idealizing the cycle either or if you are counting in isentropic efficiencies
 
Thank you for your answer and a happy new year to you all.

I am not trying to idealize at all.

To put it simply, I am trying to figure out the turbine size needed to extract the 5kW.

Br,
J
 
jonks said:
Thank you for your answer and a happy new year to you all.

I am not trying to idealize at all.

To put it simply, I am trying to figure out the turbine size needed to extract the 5kW.

Br,
J

Question is, what you mean by "size".

The important parameters to be deduced are the amount of R134 and the frequency of the Rankine cycle. If you were to specify one, the other could be computed based on your inlet and exhaust temperatures, and electrical power requirements. We could compute an answer based on a reversible (ideal) cycle and also take a shot at a realistic efficiency for your power plant.
 
The size of the turbine: I would like to calculate the approximate height, depth and width of the blades. To visualize, here is the turbine project.
http://www.pandikas.ee/kool/r134a/turbine191212.jpg

This is what I have so far - but aside from the general idea - for the moment at least, I don't have much else to work with.

As for the amount of R134a - this is interesting: This is something that should be possible to calculate from the evaporator outlet diameter and pressure. However, this would again lead me to the point of where I'd need to know the rate of flow - which I don't. =/


rude man said:
Question is, what you mean by "size".

The important parameters to be deduced are the amount of R134 and the frequency of the Rankine cycle. If you were to specify one, the other could be computed based on your inlet and exhaust temperatures, and electrical power requirements. We could compute an answer based on a reversible (ideal) cycle and also take a shot at a realistic efficiency for your power plant.
 
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Jonks, I finally looked at your work and I think you're looking for some pretty advanced and detailed info which I doubt you'll get on this forum, but maybe you will.

I looked at this from a pretty basic, theoretical viewpoint. In this view the salient features of the Rankine cycle are the two temperatures and the type of working fluid (specifically its 'steam tables' from which enthalpies and entropies are either looked up or calculated. A p-v diagram for R134 would be a big help also.) This enables one to compute the efficiency of the cycle (assuming reversibility) = work output/heat input at the higher temperature. The latter two are computed from the aforesaid steam table entries on a per-unit-weight of working fluid basis ("specific" entropies and enthalpies). Then, you'd have to make allowances for the fact that no part of the cycle is really reversible. That drops the efficiency by at least half.

So to get output power you'd need to know the amount of fluid which would give you the work output per cycle. Then the work output per unit time, which is your power, is just that work times the frequency of running the cycle (set by the pump adiabatically pushing the pressure up from p2 to p1 and corresponding temperature up from T2 to T1.

You'd need this info in any case, on top of your other design details you're looking for, seems to me. Anyway, I hope you find what you're looking for, here or elsewhere.