Finding the coefficient of performance (COP) of transcritical CO2 refrigeration cycle
TOPIC - Finding the coefficient of performance (COP) of transcritical CO2 refrigeration cycle with double tube heat exchanger.
PROBLEM STATEMENT -
To improve the performance of transcritical CO2 refrigeration system by using internal double tube heat exchanger. Double tube heat exchanger will increase heat transfer of the system. The performance comparison will be made between with IHX and without IHX tested in the transcritical refrigeration system. Finally the recommendations are made and conclusions are drawn based on the improved performance of the cycle using the IHX.
Here above we can see the graph of various equipment. The equipment states that the IHX (Internal heat exchanger) has low performance with respect to COP than other equipment. So we have to choose such a designand type of heat exchanger that it will increase the COP.
OBJECTIVES -
To improve the heat transfer rate, improve the COP of the system. The project scope is to run a python code to determine the pressure enthaply (ph) diagram and how it will help to increase COP of a small CO2 refrigeration system that can operate in transcritical regions.
SCOPE -
There are several benefits associated with the successful completion of a transcritical co2 refrigeration system with CO2 as the working fluid. To reduce the emission of greenhouse gases. This refrigeration system provides a benefit that traditional refrigerators do not, a nonhazardous working fluid. The system is very reliable and maintenance cost is less.
INTRODUCTION -
Due to global environmental concerns, the usage of natural working fluid is becoming more interesting theme to be discussed. Transcritical CO2 cycle is recently considered as one of the most influential refrigerant for its characteristics such as non-ODP, non-flammable and non-toxicity, despite the drawback of high working pressure.CO2 has rather long old history as refrigerant, widely used during (1930). However it was abandoned later due to invention of synthetic refrigerant which were more effective. But refrigeration system with CO2 as a working fluid needs to operate above critical point due to its low critical temperature (31.1℃). So it requires replacement of conventional condenser with a gas cooler to the existence of single phase above critical point. The coefficient of performance for CO2 based transcritical system is low, it is even lesser when operated at higher ambient temperature regions due to requirement of high pressures and temperature to which the gas needed to be compressed for effective heat rejection and also for increasing COP of the transcritical refrigeration system . In this heat exchanger the carbon dioxide at the outlet of gas cooler exchanges heat with the carbon dioxide flowing out of the evaporator before entering into compressor. Employing the internal heat exchanger increases the specific refrigerating effect, on other hand the specific volume of the refrigerant vapour at the beginning of the compression rises and also the specific compression work increases too. Hence the COP of the system will be higher than the refrigerating system without internal heat exchanger.
CASE STUDY -
Basic transcritical CO2 refrigeration cycle -
The above figure shows the basic transcritical refrigeration system. A basic CO₂ transcritical cycle needs to operate above the critical point because of very low critical point of CO₂. Hence in this system the condenser is replaced by the gas cooler. The cycle of the transcritical system is in both subcritical and supercritical pressure.In the transcritical cycle the evaporation pressure is subcritical i.e. it is lower side pressure and gas cooler pressure is supercritical due to low critical point of CO₂. Hence the pressure in the gas cooler has the great influence on the COP of the system. Hence it is necessary to control this pressure.
The figure consists plot of specific enthaply and pressure.As shown in the figure the transcritical system begins with compression of CO2 vapour from state 1 to 2 then it is condensed in the gas cooler at state 2 to 3 above the critical point. Then the condensed CO2 is expanded upto the evaporator pressure at state 3 to 4 in the expansion device and then refrigerating effect is obtain in the evaporator at state 4 to 1. But the COP of the transcritical refrigeration system is very low because it is very difficult to maintain the gas cooler pressure. Hence the following modifications are done in the transcritical cycle to increase the COP.
Transcritical Refrigeration CO2 cycle with Internal heat exchanger -
The Montreal and Kyoto Protocol are two frameworks to works single goal of environment safety. This protocol suggest prohibiting the usage of synthetic refrigerants to prevent ozone layer depletion and control global warming well. Such conditions encourage us to consider CO2 as a working fluid for refrigeration and air conditioning system. In the above figure thermodynamic analysis of CO2 based transcritical cycle is presented. The operating parameters consider, including heat rejection pressure in gas cooler, evaporator temperature and gas cooler exit temperature.COP of the system decrease with increase in gas cooler inlet temperature of external fluid. Hence gas cooler inlet temperature of external fluid should be low which depends on ambient conditions. An increasing evaporator temperature resulted in increasing COP of the system. Effectiveness of internal heat exchanger has no effect on COP of the transcritical system.
p-h Diagram of the trans-critical cycle with the internal heat exchanger
Double Tube Heat Exchanger -
Heat exchangers are a device that exchange the heat between two fluids of different temperatures that are separated by a solid wall. The temperature gradient, or the differences in temperature facilitate this transfer of heat. Transfer of heat happens by three principle means: radiation, conduction and convection. In the use of heat exchangers radiation does not take place. However, in comparison to conduction and convection, radiation does not play a major role. Conduction occurs as the heat from the higher temperature fluid passes through the solid wall. To maximize the heat transfer, the wall should be thin and made of a very conductive material. The biggest contribution to heat transfer in a heat exchanger is made through convection.
The double tube heat exchanger is one of the simplest types of heat exchangers. It is called a double tube exchanger because one fluid flows inside a pipe and the other fluid flows between that pipe and another pipe that surrounds the first. This is a concentric tube construction. Flow in a double-pipe heat exchanger can be co-current or counter-current. There are two flow configurations: co-current is when the flow of the two streams is in the same direction, counter current is when the flow of the streams is in opposite directions.
In this double tube heat exchanger a hot process fluid flowing through the inner pipe transfers its heat to cooling water flowing in the outer pipe. The system is in steady state until conditions change, such as flow rate or inlet temperature. These changes in conditions cause the temperature distribution to change with time until a new steady state is reached. The new steady state will be observed once the inlet and outlet temperatures for the process and coolant fluid become stable. In reality, the temperatures will never be completely stable, but with large enough changes in inlet temperatures or flow rates a relative steady state can be experimentally observed.
CAD model of the double tube heat exchanger for transcritical refrigeration CO2 cycle
PYTHON CODE -
Here we have divided this project in two case .
Case 1 - To plot the pressure enthaply (ph) diagram without the use of internal heat exchanger and calculating the coefficient of performance.
Case 2 - To plot the pressure enthaply (ph) diagram with the use of internal heat exchanger and calculating the coefficient of performance.
CASE 1 - SAMPLE CODE
# Finding the COP and plot of pressure and enthaply with and without internal heat exchanger
import math
import matplotlib.pyplot as plt
import matplotlib.patches as mpatches
# Input data
evaporator_pressure = 2.5 # Evaporator pressure at state 1 i.e., evaporation
condenser_pressure = 10 # Condenser pressure at state 3 i.e., condensation
# Finding the pressure at state 3 i.e., compression and state 4 i.e., expansion
evaporator_pressure = p1 = 2.5
condenser_pressure = p3 = 10
# During compression process the pressure which is raised from evaporator is equal to condenser pressure i.e., p2=p3
p2 = p3 = 10 # pressure at state 2 i.e., compression process
# During expansion process the pressure which is lowered from condenser is equal to evaporator pressure i.e., p1=p4
p4 = p1 = 2.5 # pressure at state 1 i.e., expansion process
# Calculating enthaply without the use of internal heat exchanger
# The enthaplies were calculated through the ph graph for R744 refrigerant
h1 = 490
h2 = 570
h3 = 310
h4 = 330
plt.plot([h1,h2,h3,h4,h1] , [p1,p2,p3,p4,p1])
plt.xlabel('enthapy KJ/KG')
plt.ylabel('pressure MPa')
phplot_without_heatexchanger = mpatches.Patch(color='red' , label = 'Without_heatexchanger')
plt.legend(handles = [phplot_without_heatexchanger])
plt.show()
# Calculating the Coefficient of performance , refrigeration_effect , work_done
refrigeration_effect=(h1-h4)
work_done=(h2-h1)
COP = ((h1-h4)/(h2-h1))
print(refrigeration_effect)
print(work_done)
print(COP)Here above we have performed a programming to calculate the coefficient of performance (COP) and plot of pressure enthaply (ph) diagram.
STEPS -
First step - Here we have first imported the modules that are math , matplotlib , mpatches this are the required modules from the programming point of view.
Second step - Since the refrigerant used to perform the transcritical cycle is CO2 the standard evaporator and gas cooler pressure are 2.3MPa and 10MPa. So, we have given them the citation as p1 and p3.
Third step - To perform the complete cycle we required pressure at compressor and expansion valve. Where in for transcritical cycle the evaporator pressure and expansion valve pressure are same i.e., 2.5MPa . Therefore, p1=p4 and gas cooler pressure and compressor pressure are same i.e, p2=p3=10MPa.
Fourth step - Next we have calculated the entaply through the ph graph of R744 i.e., CO2 refrigerant . The plot images is given below there the enthaplies are marked.
If we look in above diagram we have marked the values for enthapies that are h1`,h2`,h3`,h4`. These values are marked at 2.5MPa and 10MPa pressure. So after marking the values for enthaplies we have total four enthaplies. h1` = 490, h2` = 570, h3` = 310, h4` = 330.
Fifth step - Since now we have values of pressure and entaplies, the plot is generated.
Above is the plot which is exactly the same as mentioned in above case study without the double tube heat exchanger.
Sixth step - So, now we have to calculate the coefficient of performance (COP) .The formula for finding the COP is , COP = (refrigeration effect/work done). To calculate the refrigeration effect we have differences of (h1-h4) and to calculate work done we have differences of (h2-h1). So, the COP is ((h1-h4)/(h2-h1)). After substituting the values we have results as -
COP = 2.
The refrigeration effect = 160(KW) .
The work done = 80(Joule) .
CASE 2 - SAMPLE CODE - To plot the pressure enthaply (ph) diagram with the use of internal heat exchanger and calculating the coefficient of performance.
# Finding the COP and plot of pressure and enthaply with and without internal heat exchanger
import math
import matplotlib.pyplot as plt
import matplotlib.patches as mpatches
# Input data
evaporator_pressure = 2.5 # Evaporator pressure at state 1 i.e., evaporation
condenser_pressure = 10 # Condenser pressure at state 3 i.e., condensation
# Finding the pressure at state 3 i.e., compression and state 4 i.e., expansion
evaporator_pressure = p1 = 2.5
condenser_pressure = p3 = 10
# During compression process the pressure which is raised from evaporator is equal to condenser pressure i.e., p2=p3
p2 = p3 = 10 # pressure at state 2 i.e., compression process
# During expansion process the pressure which is lowered from condenser is equal to evaporator pressure i.e., p1=p4
p4 = p1 = 2.5 # pressure at state 1 i.e., expansion process
# Calculating enthaply with the use of internal heat exchanger
# The enthaplies were calculated through the ph graph for R744 refrigerant
h_1 = 460
h_2 = 520
h_3 = 350
h_4 = 320
plt.plot([h_1,h_2,h_3,h_4,h_1] , [p1,p2,p3,p4,p1])
plt.xlabel('enthapy KJ/KG')
plt.ylabel('pressure MPa')
phplot_with_heatexchanger = mpatches.Patch(color='red' , label = 'With_heatexchanger')
plt.legend(handles = [phplot_with_heatexchanger])
plt.show()
# Calculating the Coefficient of performance1 , refrigeration_effect1 , work_done1
refrigeration_effect1=(h_1-h_4)
work_done1=(h_2-h_1)
COP1 = ((h_1-h_4)/(h_2-h_1))
print(refrigeration_effect1)
print(work_done1)
print(COP1)Here above we have performed a programming to calculate the coefficient of performance (COP) and plot of pressure enthaply (ph) diagram.
STEPS -
First step - Here we have first imported the modules that are math , matplotlib , mpatches this are the required modules from the programming point of view.
Second step - Since the refrigerant used to perform the transcritical cycle is CO2 the standard evaporator and gas cooler pressure are 2.3MPa and 10MPa. So, we have given them the citation as p1 and p3.
Third step - To perform the complete cycle we required pressure at compressor and expansion valve. Where in for transcritical cycle the evaporator pressure and expansion valve pressure are same i.e., 2.5MPa . Therefore, p1=p4 and gas cooler pressure and compressor pressure are same i.e, p2=p3=10MPa.
Fourth step - Next we have calculated the entaply through the ph graph of R744 i.e., CO2 refrigerant . The plot images is given below there the enthaplies are marked.
If we look in above diagram we have marked the values for enthapies that are h1,h2,h3,h4. These values are marked at 2.5MPa and 10MPa pressure. So after marking the values for enthaplies we have total four enthaplies. h1 = 460, h2 = 520, h3 = 350, h4 = 320.
Fifth step - Since now we have values of pressure and entaplies the plot is generated.
Above is the plot which is exactly the same as mentioned in above case study with the double tube heat exchanger.
Sixth step - So, now we have to calculate the coefficient of performance (COP) .The formula for finding the COP is , COP = (refrigeration effect/work done). To calculate the refrigeration effect we have differences of (h_1-h_4) and to calculate work done we have differences of (h_2-h_1). So, the COP is ((h_1-h_4)/(h_2-h_1)). After substituting the values we have results as -
COP = 2.33
The refrigeration effect = 140(KW) .
The work done = 60(Joule) .
% Increased in COP -
% (COP)increased = ((2.33 – 2)/(2))
= 16.5%
Final Result -
| CASES | Coefficient of performance | Refrigeration effect(KW) | Work done(Joule) |
| Without double tube heat exchanger (Case 1) | 2 | 160 | 80 |
| With double tube heat exchanger (Case 2) | 2.33 | 140 | 60 |
CONCLUSION -
In this report an experimental transcritical system with carbon-dioxide working as refrigerant in system when double tube internal heat exchanger is installed in system. Various working condition are considered when using CO2 as refrigerant varying the pressure at the evaporator and gas cooler . Using the double tube internal heat exchanger the COP is increased by 16.5%. Some considerations about the COP, the refrigeration power and carbon dioxide mass flow are not reported. The experimentation suggests the use of internal heat exchanger working with carbon dioxide in a transcritical cycle to cool in residential application.
FUTURE SCOPE -
The performance of CO2 transcritical refrigeration system is increased by using internal double tube heat exchanger. Day by day there is increased in global warming and ozone depletion due to simple VCC refrigeration system, but from CO2 transcritical system there is no pollution and emission of harmful gases in environment. Hence CO2 transcritical system can be used in future on a very large scale.