Carbon Dioxide Refrigeration System For
Server-room Application
CONTENTS
1. Introduction 2. Free cooling 3. Hybrid cooling
4. Cooling with Propylene Condenser 5. Energy Savings and Conclusions
1. INTRODUCTION
In this study, a CO2 refrigeration system for server room application was modelled using EES.
Refrigeration system operates under three different modes for different temperature ranges. For low ambient temperatures, free cooling is utilized, which is a single stage CO2 cycle. Presently system has been modelled only for subcritical region of the CO2 cycle. For medium ambient temperatures, propylene is used as secondary refrigerant and some of the CO2 from the receiver is let through a three way valve to propylene chiller as can be seen in the process diagram in figure 1.
For higher ambient temperatures in summers, all the CO2 flow is directed through propylene chiller and cooling takes place via propylene chiller alone. All three different modes of operation of cooling system were modelled separately and description of these models along with the results has been elaborated in subsequent sections. In entire study, hot air is assumed to be available at a fixed temperature of 400C and cooling system has been modelled with a cooling capacity of 1MW.
Figure 1. Process diagram for the cooling system
PROPYLENE CHILLER PROPYLENE
CONDENSER
CO2
CONDENSER
CO2 THREE WAY VALVES CO2
EVAPORATOR
PROPYLENE RECEIVER
CO2 RECEIVER
CO2 PUMP
EXPANSION VALVE COMPRESSOR
SERVER ROOM
AMBIENT AIR
CO2 CONTROL VLAVE HOT
AIR
2. FREE COOLING
Free cooling operates entirely on CO2 without need for a secondary refrigerant and significant amount of energy savings are obtained due to obliteration of compressor power input to the system.
The diagram in figure 2 depicts the simple free cooling cycle. System modelled is suitable for ambient temperature range of 1 to 7 0C. Model autometically sets the evaporation and condensation temperatures based on ambient temperature input to the model and results has been plotted in figure 4 for entire range of operation of free cooling cycle. Superheat for evaporator was set to +20C.
Figure 2. Model for Free cooling.
Figure 3 shows p-h diagram for carbon dioxide cycle for an ambient temperature of 70C. The pump work input and refrigerant flow rate are given in figure 5 and figure 6. The hot air flow rate across CO2 evpoator was set by the model for different ambient temperature values for a constant cooling capacity through out the operating range of free cooling cycle. System works for temperaturs till 70C although the hot air flow rate values required to maintain 1MW cooling capacity are not realistic. UA vlaues for CO2 evaporator and condenser were set to be 92 and 338 kW/K respectively whereas ambient air flow was set at 50 kg/s.
Figure 3. Carbon Dioxide Cycle for Ambient air Temperature of 70C
Figure 4. Variation of Evaporation and condensation temperatures for ambient air temperature range for free-cooling
-500 -400 -300 -200 -100 0 100
102 103 104 105
h [kJ/kg]
P [kPa] 15.5°C
1.23°C -11.8°C -25°C
0.2 0.4 0.6 0.8
-1 -0.9
-0.8 -0.7
-0.6 kJ/kg-K
R744
1 2 3 4 5 6 7
20 21 22 23 24 25 26 27 28
Ta[1] [C]
Tc[1]
Tc[1]
Tc[4]
Tc[4]
Evaporation Temperature [C]
Condensation Temperature [C]
Ambient Air Temperature
[C]
Figure 5. Pump Work for free cooling
Figure 6. Refrigerant mass flow rate for a cooling capacity of 1MW
1 2 3 4 5 6 7
0.45 0.5 0.55 0.6 0.65 0.7 0.75
Ta[1] [C]
Wp [kW]
PUMP WORK
1 2 3 4 5 6 7
6.25 6.7 7.15 7.6 8.05 8.5
Ta[1] [C]
mCO2 [kg/s]
Refrigerant mass flow
3. Hybrid Cooling:
The operation of cooling system is switched to hybrid cooling cycle for temperatures more than 70C. In this operation, CO2 cycle is used at its full capacity and rest of the CO2 flow is directed to propylene cycle through a propylene chiller as given in figure 7. The CO2 is cooled to 190C in propylene chiller and this temperature has been fixed for ambient temperature range of operation.
Figure 7. Model for hybrid cooling cycle
Same UA values for CO2 circuit and air flow rate stated in previous section have been used for the present model. UA values for propylene chiller and propylene condenser have been set as 150 and 338 kW/K respectively. Hot air flow rate has been fixed at 67 kg/s. The model was allowed to run till all the capacity of CO2 cycle was exhausted which occurs for an ambient temperature of 240C. The p-h diagrams for individual CO2 and propylene cycles have been plotted on the following page (figure 8 and 9).
Figure 8. Carbon Dioxide cycle for Hybrid cooling at ambient air temperature of 240C
Figure 9. Propylene cycle for ambient air temperature of 240C
-500 -400 -300 -200 -100 0 100
102 103 104 105
h [kJ/kg]
P [kPa] 15.5°C
1.23°C -11.8°C -25°C
0.2 0.4 0.6 0.8
-1 -0.9
-0.8 -0.7
-0.6 kJ/kg-K
R744
-800 -690 -580 -470 -360 -250 -140 -30 80 190 300 10-1
100 101 102 103 104 105
h [kJ/kg]
P [kPa]
30°C
-18.3°C
-55°C
-80°C
0.2 0.4 0.6 0.8
-0.6
-0.2
0.2
0.6
1 kJ/kg-K
Propylene
Figure 10. Energy Balance for Hybrid cooling
Figure 11. Evaporation and condensation temperatures for hybrid cooling
7.5 11 14.5 18 21.5 25
0 200 400 600 800 1000 1200
Qcc[1]
Qcc[1]
Qce[1]
Qce[1]
Qec[1]
Qec[1]
Wp[1]
Wp[1]
Qpc[1]
Qpc[1]
Wc[1]
Wc[1]
Ambient Air Temperature Ta[1] [C]
[kW]
(Cooling Capacity)
(Prolylene Condenser)
(Propylene Chiller)
(CO2 Condenser)
(Compressor Work)
(Pump Work)
7.5 11 14.5 18 21.5 25
10 15 20 25 30 35 40 45 50
Tcc[4]
Tcc[4]
Tc[1]
Tc[1]
Tp[3]
Tp[3]
Tp[4]
Tp[4]
Condensation Temperature (CO2)
Evaporation Temperature (CO2) Condensation Temperature (Propylene)
Evaporation Temperature (Propylene)
[C]
Ambient Air Temperature Ta[1] [C]
Figure 12. COP for Propylene cycle in hybrid cooling
It can be seen from figure 10 that for a constant cooling capacity the capacity of CO2 cycle to condense heat falls with ambient temperature and it is almost insignificant at 240C which has been set as maximum limit of operation for hybrid cycle. Also fraction of refrigerant mass flow of to CO2 condenser falls. Figure 11 depicts evaporation and condensation temperatures for both CO2 and propylene cycles. CO2 evaporation temperature remains constant at 20.07 0C for this operation while condensation temperature varying with the changing ambient temperature. An inconsistency was observed in COP value for propylene cycle for operation at ambient temperature of 80C and has been omitted from figure 12 for variation COP with air temperature which otherwise is reasonable for entire range of temperatures.
7.5 11 14.5 18 21.5 25
0 10 20 30 40 50 60 70 80 90 100
COPp[1]
Ambient Air Temperature Ta[1] [C]
4. Cooling with Propylene Condenser
As seen from last section, it is not feasible to operate cooling system in hybrid cycle mode for ambient temperatures greater than 240C. Following model which is suitable for operations at still higher temperatures is a simple cascade cycle where only propylene condenser operates as shown in figure 13.
Figure 13. Model for cooling with the propylene condenser
The results for this model were plotted for a temperature range of 20 to 300C. Figure 14 shows the p-h diagram for the cycle. Propylene evaporation temperature is fixed by the model at 13.310C for entire temperature range as can be seen in figure 15. Same set of values for fixed flow rates and UA values were used as stated in earlier section for hybrid cycle. Figure 17 shows the variation of COP with ambient temperatures.
Figure 14. Propylene cycle for ambient air temperature at 300C
Figure 15. Evaporation and condensation temperatures for cooling with propylene condenser alone
-800 -690 -580 -470 -360 -250 -140 -30 80 190 300 10-1
100 101 102 103 104 105
h [kJ/kg]
P [kPa]
30°C
-18.3°C
-55°C
-80°C
0.2 0.4 0.6 0.8
-0.6 -0.2
0.2
0.6
1 kJ/kg-K
Propylene
20 22 24 26 28 30
10 15 20 25 30 35 40 45 50 55
Ta[1] [C]
Ta[1] [C]
[C]
Tc[4]
Tc[4]
Tc[1]
Tc[1]
Tp[3]
Tp[3]
Tp[4]
Tp[4]Propylene Evaporation Temperature Propylene Condensation Temperature
Figure 16. Energy Balance for cooling with propylene condenser alone
Figure 17. COP for propylene cycle for cooling with propylene condenser alone
20 22 24 26 28 30
0 200 400 600 800 1000 1200 1400
Ta[1] [C]
Ta[1] [C]
[kW] Qce[1]Qce[1]
Qec[1]
Qec[1]
Wp[1]
Wp[1]
Qpc[1]
Qpc[1]
Wc[1]
Wc[1]
Pump work Compressor work Cooling Capacity Heat Exchange Propylene Condenser
20 22 24 26 28 30
4 4.5 5 5.5 6 6.5 7
Ta[1] [C]
COPp[1]
5. Energy Savings and Conclusions
In these calculations, only energy input considered was from compressor and CO2 pump while energy input from fans for blowing hot and ambient air was ignored. Total energy required to drive the cooling system is plotted in figure 18 for entire range of ambient temperature i.e. 1 to 300C.
There is nearly a smooth transition at temperature of 240C where system switches from hybrid cooling mode to cooling with propylene condenser. Same can be said to be true for COP for propylene which has been plotted in figure 19.
Figure 18. Energy Requirements for Cooling
Figure 19. COP for Propylene Cycle 0
50 100 150 200 250
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29
Total Work Input (kW)
Ambient Air Temperature (C)
Free Cooling Hybrid Cooling Propylene Cooling
0 10 20 30 40 50
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29
COP for Propylene Cycle
Ambient Air Temperature (C)
Hybrid Cooling Propylene Cooling
Figure 20. Energy savings (Percentage) at different temperatures
Figure 20 shows energy savings with variation in ambient temperature. The model with propylene condenser alone was used as reference for making energy saving calculations and it was run was entire range of ambient temperature. It can be seen that there is a significant level of energy savings can be achieved in free cooling region as well as hybrid cooling regions.
0 20 40 60 80 100
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
% Savings
Ambient Temperature (C) FREE
COOLING
HYBRID COOLING
