Refrigerants Analysis

NUCLEAR&RENEWABLE ENERGY RESOURCES CONFERENCE with INTERNATIONAL PARTICIPATION

28-29 September 2009 ANKARA – TURKEY

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THEORETICAL INVESTIGATION ON FIRST AND SECOND LAWS OF THERMODYNAMIC IN A TYPICAL POWER CYCLE

Amin Yahyaie , Seyyed Ali Akbar Mirmohammadi, Mohammad Reza Behi,Hamid Reza

Behi, Amin Ebrahimi

The Faculty of Mechanical Engineering, Shahrood University of Technology, Shahrood, Iran Abstract

In this paper, by considering a power cycle working in supercritical and Transcritical regions, the capability of different refrigerants in producing work according to first and second law of thermodynamics was studied. Primarily different power cycles are introduced and classified, then by using EES software, the cycles are simulated. The results show that among R12, R22, R134a, R143a, and R125, R22 and R143a have higher effective and rational efficiencies. R22 shows the maximum potential to produced work; also its effective and rational efficiencies are higher than the other refrigerants. The common factor among refrigerants is the increment of the maximum value of energy added to heat source from Transcritical cycle to supercritical one. Keywords: Efficiency, heat waste recovery, power cycle

1. Introduction Utilizing a low grade heat source for energy production has attracted more and more attention for its potential in reducing fossil fuel consumptions as well as environmental pollution. Studies have shown that some working fluids have higher potential to extract work from a low grade heat source. 2CO could be mentioned as one of this working fluids [1]. Moreover Refrigerant gases have had undetermined behavior in producing work. These gases have been around for many years and are basically condensed under pressure to obtain chilled air. Through the repetitive evaporating and condensing actions of these refrigerant gases, they are able to pull excess heat out of the air and moderate the environment within buildings. Refrigerants also could be used to extract work from low grade heat source. The refrigerants are considered in this study are categorized in table 1.

Table1. Properties of working fluids [1] Substance ASHRAE

No. Molecular Formula

M(kg/kmol) Tcr(oK) Pcr(Bar) ODP

Chlorodifluoromethane R22 CHCLF2 86.47 369.28 49.89 0.055 (CCl3F =1) Dichlorodifluoromethane R12 CC l2 F2 120.9 385.15 41.14 1.0 (CCl3F =1) 1,1,1,2Tetrafluoroethane R134a C2H2F4 102 374.15 40.59 0.02 Pentafluoroethane R125 C2HF5 120 339.17 36.18 0 1,1,1-Trifluoroethane R143a C2H3F3 84.04 345.85 37.61 0

2. Theory and formulation The schematic diagram of the power cycle is shown in figure 1. It is actually a Transcritical and Brayton power cycle in which heat is delivered to the cycle from hot source. The system consists of four main parts, namely: a gas heater, turbine, condenser (gas cooler) and pump. In this system, the refrigerant is first pumped to a supercritical pressure, and then heated in the gas heater. The heated supercritical refrigerant will expand in the turbine. The vapor discharged from the turbine outlet will then be cooled and condensed in a condenser.

E-mail: [email protected]



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Figure1. Power cycle

The performance of a system undergoing a power cycle can be described in terms of the extent to which the

energy added by heat, inQ is converted to a net work output, netw . The extent of the energy conversion from heat to work is expressed by the following ratio, commonly called the thermal efficiency or effective efficiency.

in

neteff Q

wQw

eff (1)

By definition of

nCompressioExpansionnet www ww (2)

And

heaterin QQ Q (3) we have

heater

nCompressioExpansioneff Q

ww )( w(eff

(4)

The Second Law states that it is never possible to convert the full energy content of the energy supplied into work, and the maximum net work that can be achieved is

)( 1201212 ssThhbbwnet sThhbb (5)

Where RR is called the Second Law efficiency or the rational efficiency. If netw is presented by the difference

of compressor and expansion work RR is simplified as

))(.(

)(

12012

.

ssThhm

ww nCompressioExpansionR

sTh

wR

(6)

3. Working conditions Different parts of cycle have shown in schematic of power cycle (figure 1) such as gas heater, gas cooler, pump and expansion machine. The research on the Transcritical and supercritical power cycles for these refrigerants in low grade heat source recovery applications is very limited; therefore, several assumptions are needed to specify the cycle working conditions. The working conditions are presented in table 1 and 2.



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Table 2. Brayton working condition [3] Item Value Unit

Gas heater pressure 200 bar Gas cooler pressure 100 bar Heat source energy 458.1 kj/kg Gas cooler outlet

temperature 413.3 kk

Compression efficiency 85% - Expansion efficiency 85% -

Table 3. Transcritical working condition

4. Results and discussion For both Transcritical and supercritical power cycles, efficiencies of first law and second law and the net work extracted from the cycles are calculated regarding to gas cooler pressure, pressure ratio and the heat added to the heat source. The effects of gas cooler pressure on net produced work have been depicted for Brayton and Transcritical cycles. Basically this comparison is theoretical. The gas cooler pressure has changed in a range of 5 MPa to 10 MPa for Brayton cycle and 500KPa to 3500KPa for Transcritical cycle. As it was evident, for both cycles and all refrigerants, the net work has decreased by increment of gas cooler pressure. The difference in these two graphs is the slope decrement of produced net work. In Transcritical cycle, the rate of net work change was decreasing while this rate is constant for Brayton cycle. In Brayton, we are allowed to decrease the pressure up to critical pressure. In constant pressure ratio, as gas cooler pressure goes towards critical pressure, the net work increases for all refrigerants. In the considered range, R22 has the highest net work. The other point extracted. In former, there have not been seen a unique pattern, but in the latter one, all curves followd the same pattern.

Figure2. Brayton Figure3. Transcritical

Figures 2and 3 show the effects of pressure ratio on effective efficiencies. In graphs, R22 presents the best result. R22 in comparison with five other refrigerants has the highest efficiency in the pressure ratio range of 3 to 8. In Transcritical cycle, after R22, R12 and R143a have higher efficiency but in Brayton R12 and R143a are situated.

Item Value Unit Gas heater pressure 120 bar Gas cooler pressure 35 bar Heat source energy 458.1 kj/kg Gas cooler outlet

temperature 300.3 kk

Compression efficiency 85% - Expansion efficiency 85% -



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These two refrigerants are analogous. The curves cross each other at 4.7 for Brayton and 4.4 for Transcritical cycle. On the left of meeting point R134a has higher efficiency than R12 and on the right is vice versa. In both cycles, R134a and R125 have the lowest efficiency, excluding in Transcritical cycle at low pressure ratios, two curves overlap. Obviously, in a special pressure ratio, Transcritical efficiency is higher than Brayton one.


For rational efficiency versus pressure ratio some replacements among working fluids are figured, though the behavior of graphs is alike. R22 shows the highest efficiency again but R12 is last working fluid which it means lowest rational efficiency, in spite of former graphs (figures 2, 3).

Figure6. Brayton Figure 7. Transcritical

According to figure 6 and 7, the net work obtained from cycles regarding to pressure ratio, have been depicted. As it is obvious, increment of pressure ratio leads to growth in net work. These 2 graphs have similar behavior to figure 4 and 5. In figures 8 and 9, the effects of heat applied in heat source on cycle efficiency have been presented. Based on the figures 8 and 9, between 200 to around 500 (kj/kg) the efficiency has been increasing with a steep slope and achieving the maximum value, after that efficiency decreases with a smooth slope. This scenario is correct for both cycles. According to the figures, the behavior of these refrigerants is different. In both cycles, R22 has the highest efficiency and the highest increment rate of efficiency. It is worth mentioning that curve behavior is the same as Transcritical cycle for high value of energy added to heat source.



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Table 4. Transcritical

Refrigerant Maximum efficiency Qboiler(kj/kg) Max. Sec.

efficiency Qboiler(kj/kg)

R22 0.1087 531 0.3721 200 R12 0.0922 531 0.2512 282.8

R143a 0.08825 503.4 0.3365 282.8 R125 0.07898 365.5 0.3233 282.8 R134a 0.07682 531 0.2665 337.9

Table 5. Brayton

Refrigerant Maximum efficiency Qboiler(kj/kg) Max. Sec.

efficiency Qboiler(kj/kg)

R22 0.1 669 0.2811 255.2 R12 0.07818 724.1 0.161 365.5

R143a 0.07788 696.6 0.2384 448.3 R125 0.06922 531 0.2229 337.9 R134a 0.06824 669 0.1984 393.1

Also figures 10 and 11 have shown the effects of heat applied in heat source on rational efficiency. In accordance with these figures among 200 to around 300 (KJ/Kg), the efficiency has been increasing with a steep slope and achieving the maximum value (except R125 in Transcritical cycle which is continually descending), after that efficiency decreases with a smooth slope. This story is correct for both cycles. According to the graphs, the



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behavior of these refrigerants is different. Similar to last part in both cycles, R22 again has the highest efficiency and the highest increment rate of efficiency. It is worth mentioning that curve behavior is the same as Transcritical cycle for high value of energy added to heat source. In following tables the maximum efficiency are listed. It is worth mentioning that, the comparison between these tables illustrates that maximum value of energy added to heat source is increasing from Transcritical cycle to supercritical one. 5. Conclusions In present study, supercritical and trancritical power cycles of 5 refrigerants were considered and their efficiencies and capability of producing works were illustrated by the aim of some graphs. According to the figures, the behavior of the refrigerants is considerably different. Among them R22 shows the maximum potential to produced work, also its efficiency is higher than the other refrigerants. The common factor among refrigerants is the increment of the maximum value of energy added to heat source from Transcritical cycle to supercritical one. Acknowledgment This work was supported by research grant from Shahrood University of Technology. References [1] Novel Cycles Using Carbon Dioxide as Working Fluid New Ways to Utilize Energy from Low Grade Heat

Sources “Licentiate Thesis by Yang Chen Stockholm, May 2006 [2] M. Farzaneh, M. Sadi,”Theoretical investigation of 2CO power cycle application in internal combustion

engine in order to use exhausting gas”, international conference of modelling and simulation, Sharjeh, UAE 2009

[3] Y. Chen, P. Lundqvist, P. Platell b "Theoretical research of carbon dioxide power cycle application in automobile industry to reduce vehicles fuel consumption"

[4] Yang Chen. "Novel Cycles Using Carbon Dioxide as Working Fluid New Ways to Utilize Energy from Low Grade Heat Sources" Licentiate Thesis

[5] Sonntag Van Wilen "Fundamental of Thermodynamics (Fifth Edition)" [6] Desmond E Winterbone "Advanced Thermodynamics for Engineers" John Wiley & Sons, Inc [7] Refrigeration & Air Conditioning Technology, By William C. Whitman, William M. Johnson, John

Tomczyk, Published by Cengage Learning, 2004, ISBN 1401837654, 9781401837655

Refrigerants Analysis

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