Assessment of blends of CO2 with butane and isobutane as working fluids fo
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26-10-2010, 05:33 PM

Assessment of blends of CO2 with butane and isobutane as working fluids for heat pump applications
arun k
s7 m1
College of Engineering, Trivandrum
2007-11 Batch

.pptx   Assessment of blends of CO2 with butane and.pptx (Size: 596.12 KB / Downloads: 49)

Eco-friendly natural refrigerants as alternative refrigerants

Due to toxicity of NH3, flammability of hydrocarbons, and high pressure and low COP of CO2, blends are more effective.

CO2 and HC mixtures have superior heat transfer properties with reduced cycle pressure and flammability, compared to individual HCs.

In present study, blends of CO2/butane and CO2/isobutane are proposed as working fluids in heat pump for both space heating and industrial process heating.

Mixture properties
The variations of saturation temperature with mixture composition at pressures of 10 bar and 30 bar is shown in Fig.

Difference between dew point temp. and bubble point temp. of R744+R600 blend is higher than that of R744+R600a blend.

Hence R744+R600 blend can yield better performance for higher temperature glide cooling/heating applications

Theoretical modeling and simulation

1 - 2 compression process in compressor.

2 - 3 cooling in condenser/gas cooler to state by rejecting heat to the external fluid (heating effect)

3 - 4 expansion through expansion device and evaporation in evaporator by extracting heat from the external fluid (cooling effect)

The following assumptions have been made for the simplified theoretical analysis:

1. Heat transfer with the ambient is negligible.

2. Evaporation and condensation/gas cooling processes are isobaric.

3. Refrigerant at the evaporator outlet is dry saturated.

4. Refrigerant at the condenser outlet is wet saturated.

5. The temperature approaches for both evaporator and condenser/ gas cooler are a fixed value of 7 ◦C.

6. The compression process is adiabatic but non-isentropic with an isentropic efficiency of 75% for all cases.

Performance of the heat pump has been evaluated based on heating COP given by,

Volumetric heating effect, related to the compressor size for certain heating output, can be evaluated by,

, where,

Hence, the second law efficiency for combined cooling and heating is expressed as:

where, Tes and Tcs are the thermodynamic average temperatures of secondary fluids for cooling and heating, respectively:
2. Simulation code and validation
a simulation code was developed to compute relevant thermodynamic parameters.

For given secondary fluid temp., the code searches for evaporation and condensation temp. and pressures as well as all the state point properties using an effective iteration technique.

subsequently evaluates the first law and second law performance parameters based on the mathematical model presented above.
3.Operating and performance parameters
performance parameters - compressor pressure ratio, heating coefficient of performance and volumetric heating effect

The throttling loss and the system exergetic efficiency are also considered to study the effect of blend on component as well as system irreversibilities.

The reference temperature is taken as 303 K.

the mixture mass fraction is 50/50 for both R744/R600 and R744/R600a.
Results and discussions
Use of blends deteriorate performance, increase compressor pressure ratio
The volumetric capacities for blends are higher compared to R600 and R600a and lower compared to R744
Higher absolute pressure difference across the expansion valve leads to higher throttling loss for the R744 system
Results shows that blends of R744 with R600 and R600a are not suitable for constant temperature (space) cooling and heating applications.
2. Variable temperature cooling and heating

the blend based system is significantly superior in terms of lower compressor pressure ratio and throttling loss, and higher volumetric heating effect, heating COP and second law efficiency;
it is inferior in terms of higher system pressure and compressor discharge temperature compared to R600 and R600a.
the blend based systems are better in terms of lower system pressure, lower throttling loss, higher heating COP and higher second law efficiency, and worse with respect to lower volumetric heating effect, slightly higher compressor pressure ratio and compressor discharge temperature compared to R744.
It is noted that the R744/R600a yields higher performance at a heating outlet temperature of 73 ◦C compared to R744/R600 whereas the trend is reverse at a heating outlet temperature of 100 ◦C due tohigher temperature glide of R744/R600 during condensation.
Results clearly show that the blends can be used very effectively in heat pumps for simultaneous cooling and heating applications and the existing system can be easily retrofitted using these blends as the high side pressure is similar to conventional systems.
3. Effect of mixture composition on performance

4. Effect of using internal heat exchanger on performance
5. Performance comparison with R114 for high temperature heating
6. Heat transfer considerations
Superior in terms of compressor pressure ratio, throttling loss, volumetric heating effect, heating COP and second law efficiency,
Inferior with respect to system pressure and discharge temperature compared to R600 and R600a, whereas, better in terms of system pressure, throttling loss, heating COP and second law efficiency,
worse in terms of volumetric heating effect, compressor pressure ratio and compressor discharge temperature compared to R744 for variable temperature cooling and heating.
The R744/R600a yields higher performance at a heating outlet temperature of 73 ◦C compared to R744/R600 whereas the trend is reversed at a heating outlet temperature of 100◦C due to higher temperature glide of R744/R600.
The blends show superior performance than hydrocarbons in high temperature heating applications in terms of lower compressor pressure ratio, higher volumetric heating effect and higher heating COP.

Although R744 exhibits superior performance than R114 as compared to the blends, the blends do offer a better alternative to R114 in heat pumps for high temperature application due to the excessively high side pressure of R744 systems.
Although heat transfer properties of blends are modestly inferior to its pure refrigerant equivalent, they are significantly higher than R114, which makes the blend a promising alternative to R114.
[1] S.F. Pearson, Natural refrigerants for heat pumps, IEA Heat Pump Center Newsletter 20 (2004) 13–16.

[2] E. Granryd, Hydrocarbons as refrigerants – an overview, Int. J. Refrig. 24 (2001) 15–24.

[3] K.J. Park, T. Seo, D. Jung, Performance of alternative refrigerants for residential air-conditioning applications, Applied Energy 84 (2007) 985–991.

[4] REFPROP, NIST refrigerant properties database, Version 6.01.


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