(Po,brine-Pi,brine) and (ηbrine,pump) are pressure head and total efficiency of the brine pump which were assumed to be 250 kPa and 0.5 respectively. The power consumption of the brine pump can be reduced by minimising the pressure drop of the secondary loop which subsequently reduces the pressure head. This can be achieved by installing the CO2 refrigeration system near the trigeneration plant.
3.2.1.3 Integration capacity and performance simulations
A simplified schematic diagram of the integrated system displayed in the model is shown in Figure 3.6. A medium temperature vertical multi deck cabinet with refrigeration capacity of 5 kW and a 3 kW low temperature vertical door type cabinet form the loading system of the integrated arrangement. The input parameters of the model include ambient temperature, refrigeration duties and evaporating temperatures of the MT and LT cabinets, circulation ratio, effectiveness of the IHX and delivery brine temperature.
Figure 3.6 A simplified schematic diagram of the integrated arrangement displayed in the model
The main output parameters include COP and power consumption of the individual components and systems, refrigeration capacity of the absorption chiller (Qr,abs), rejected heat from the cascade condenser (Qcond) and heat required to drive the absorption chiller (Qh,abs).
Other parameters such as mass flow rate, degree of sub-cooling and superheating, thermal and flow properties of the CO2 refrigerant at every point in the cycle can also be obtained from the model.
In order to determine the performance characteristics of the individual and the integrated systems at different operating conditions, the following investigations have been carried out using the model:
Variation of the system COP with ambient temperature
Effect of the condensing temperature on the system COP
Effect of the LT evaporating temperature on the system COP
Variation of the COP with load ratio of LT to MT systems
Influence of the use of internal heat exchanger (IHX).
Variation of the COP with circulation ratios
Variation of the system COP with ambient temperature
Figure 3.7 presents the influence of the ambient temperature on the COP of the absorption chiller and the integrated system at different delivery brine temperatures. The minimum condensing temperature of the absorption system and the temperature difference between condensing and ambient temperatures were assumed to be 25 oC and 5 oC respectively. The system was simulated with cascade condenser effectiveness (εcond) 0.88 and LT evaporating temperature (Tevap,LT) -32 oC. The LT and MT loads were maintained constant of 3 and 5.0 kW respectively. Circulation ratio of the MT system was set at CR = 1.2. The condensing temperature of the CO2 refrigeration system varied from -6 oC to -8 oC when the delivery brine temperatures changed from -8
oC to -10 oC.
The COP of the absorption system (COPabs) decreased with increased ambient temperature but increased with brine temperature as can be seen in Figure 3.7. The results show a good agreement with test data Robur (2006) and Suamir et al. (2009).
The COPs of the absorption and integrated systems are stable at ambient temperature below 20 oC but they begin to reduce at ambient temperatures above 20 oC. This is mainly because the condensing temperature of the absorption system was kept constant at 25 oC at ambient temperatures below 20 oC. Figure 3.7 also shows that the COP of the integrated system (COPint) was determined to be 25% lower than COPabs due to the power consumption of the HTF pump as well as the refrigerant pump and compressor of the CO2 refrigeration system. As can be seen in Figure 3.6 the power consumption of HTF pump (oil pump) is nearly 2.5 times that of the CO2 refrigeration system. This indicates that optimisation of the HTF circuit can reduce the electrical energy consumption of the HTF pump which subsequently will improve the COP of the integrated system.
Figure 3.7 Effect of the ambient temperature on the COP of the absorption and integrated systems
Figure 3.8 Seasonal performance of the absorption and the integrated systems 0.0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
-5 0 5 10 15 20 25 30 35
Ambient Temperature (oC)
COP
Tbrine = -10oC Tbrine = -9oC Tbrine = -8oC COPint
Tbrine = -10oC Tbrine = -9oC Tbrine = -8oC COPabs
0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65
0 730 1460 2190 2920 3650 4380 5110 5840 6570 7300 8030 8760 Time (months)
COP
0 1 2 3 4 5 6 7 8 9 10 11 12 COPint
COPabs
The seasonal COPs of the absorption and integrated systems are shown in Figure 3.8.
The simulation results were obtained using London weather data (Met Office, 2009) and MT and LT evaporating temperatures of -8 oC and -32 oC respectively. It can be seen that the COPs are steady in the winter because the minimum condensing temperature is fixed at 25 oC but reduce in the summer due to higher ambient and condensing temperatures.
Effect of the condensing temperature on the system COP
The condensing temperature of the DX-volatile CO2 system is the same as the evaporating temperature of the MT system which can be varied by modulating the delivery brine temperature. The effect of the condensing temperature on the performance of the CO2 refrigeration and integrated systems is shown in Figure 3.9. The simulations were carried out at constant refrigerant mass flow rate for both LT and MT systems; εcond = 0.88; Tevap,LT = -32 oC; CR = 1.2 and Tamb = 25 oC.
Figure 3.9 Variation of system COP with condensing temperature
Figure 3.9a shows that increasing the condensing temperature from -10 oC to -1 oC reduced the COP of the MT refrigeration system (COPMT) by 14%. This is because the MT refrigeration capacity reduced from 5.2 to 4.7 kW (about 9.6% reduction) and increased the pump power by 5%. The increase of the condensing temperature also
Tcond (oC)
system (b) COP of the absorption and integrated systems
significantly reduced the COP of the LT system (COPLT) and the overall COP (COPCO2,overall) by 47% and 46% respectively. This is mainly due to the increase in the compressor power consumption due to the higher discharge pressure and temperature.
Figure 3.9b shows that increasing the condensing temperature, however, enables the absorption system to operate at higher delivery brine temperature which improves the COPabs and COPint by approximately 35% and 21% respectively. This shows that the integrated arrangement will be more efficient if a higher MT evaporating temperature is used to satisfy the refrigeration requirements.
Effect of the LT evaporating temperature on the system COP
Figure 3.10 shows the effect of the LT evaporating temperature on the COP of the individual and the integrated systems. The simulation was carried out at Tevap,MT = -8 oC, CR = 1.2, Tamb = 25 oC and at constant refrigerant mass flow rate. Two interesting simulation results are noted. Firstly, increasing the LT evaporating temperature can considerably improve the COPLT and COPCO2-overall but it does not influence the COPMT. This indicates that operating at higher LT evaporating temperatures will improve the overall system efficiency.
Figure 3.10 Variation of system COP with LT evaporating temperature (a) COP of the CO2 refrigeration
system
(b) COP of the absorption and integrated systems
Secondly, the LT evaporating temperature has insignificant effect on the COPabs. It just slightly increases the COPint (Figure 3.10b) even though the increase of the COPCO2,overall is significant (Figure 3.10a).
Variation of the COP with load ratio of LT to MT systems
The load ratio of LT and MT refrigeration systems (LRLM) may vary in each supermarket. For medium size supermarkets the LRLM can range from 14% to 52%
with the average for Tesco (Tesco, 2009) being 32% representing average electrical energy consumption ratio for the LT and MT systems to the overall refrigeration system including display cabinets of 20% and 42% respectively as described in Section 1.1 (Chapter 1).
Figure 3.11 Variation of COP with load ratio of LT and MT systems (LRLM) (Investigated at: Tevap,MT = -8 oC, Tevap,LT = -32 oC, CR = 1.2, Tamb = 25 oC)
Figure 3.11 illustrates the influence of LRLM on the COP of the CO2 system. The overall COP (COPCO2,overall) reduces when the LRLM increases even though the COPMT
and COPLT are maintained constant. The overall COP of the CO2 system was found to be more sensitive to load ratio below LRLM of 80%.
Influence of the use of internal heat exchanger (IHX)
The CO2 system was designed with a liquid-suction heat exchanger (IHX) to enable the system to be evaluated at different suction superheating and liquid line subcooling. The
0 10 20 30 40 50 60
0 20 40 60 80 100 120 140 160 180 200 220 240 Ratio LT and MT loads (%)
COP COP_MTLT
COP_MT COP_LT COCO2,overall
COPMT
COPLT
LRLM (%)
The LRLM range applied in typical UK super-stores (Tesco, 2009)
model was used to simulate the effect of the suction superheating and liquid line subcooling on the COP of the LT DX system. The model was also used to determine a suitable effectiveness for the IHX to provide suction superheating below 20 K as recommended by Bock (2009). For design purposes, the suction superheating and liquid line subcooling were assumed to be in the range 8 to 12 K and 2 to 3 K respectively.
Figure 3.12 Variation of superheating, subcooling and COPLT with different IHX effectiveness (Investigated at: LT refrigeration duty 3 kW, Tcond = -8 oC, Tevap,LT = -32 oC, Tamb = 25 oC)
The simulation results are presented in Figure 3.12. It can be seen that the suction superheating and liquid line subcooling increase with the effectiveness of the IHX. The effectiveness of the IHX which could provide the designed superheating and subcooling was found to be in the range between 0.2 and 0.35. The figure also shows that increasing the suction superheating and liquid line subcooling slightly reduces the COPLT. The COPLT will drop by less than 5% when the suction superheating and liquid line subcooling increase to 20 K and 8 K respectively.
Variation of the COP with circulation ratio
The circulation ratio (CR) indicates the amount of refrigerant flowing through a flooded evaporator coil. Stoecker (1998) defines the circulation ratio as the ratio of refrigerant flow rate circulated through the evaporator to the flow rate of refrigerant vaporised.
CR = 1 means that just enough refrigerant circulates to provide complete evaporation without superheating. CR = 2 means that the mass flow rate of refrigerant that flows through the evaporator is twice that required for complete evaporation. To investigate
COPLT Subcooling Suction superheat
Subcooling and superheat (K)
0 1 2 3 4 5 6
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Effectiveness of the IHX
COP of the LT System
0 5 10 15 20 25 30
Subcooled/Superheat (K)
COP_LT SubcooledSubcooling Suction_superheatSuperheating COPLT
the effects of the circulation ratio on the system COP, a correlation between CR and refrigeration duty of the MT evaporator coil (Qevap,MT) was applied. The correlation, which is presented in Figure 4.7 (Chapter 4), was established using the CO2 evaporator model.
Figure 3.13 Effect of circulation ratio (CR) on the COP, refrigeration duty and power consumption of the MT refrigeration system
Figure 3.13 shows the influence of the circulation ratio on the COP, refrigeration duty and pump power of the MT CO2 system at Tevap,MT = -8 oC, Tevap,LT = -32 oC and Tamb = 25 oC. The load ratio of LT to MT systems (LRLM) was in the range between 58% and 61%. It can be seen that the CO2 pump power (WCO2,pump) increases linearly with the circulation ratio. The MT refrigeration duty (Qevap,MT) increases slightly up to CR = 2.5 but then remains relatively constant. The COPMT reduces sharply as the CR increases above 1.0.
A circulation ratio was also found to influence the COPCO2,overall as shown in Figure 3.14. Increasing the circulation ratio from 1.0 to 4.0 reduced the COPCO2,overall from 11.0 to 8.0. The CR has only a very small effect on the COP of the integrated system.
To enable investigation of the effect of the circulation ratio on the system performance, the test rig was designed to provide a circulation ratio of up to 2. Other design conditions for the system and estimated performance parameter are listed in Table 3.3.
Qevap,MT
COPMT
WCO2,pump
A display of the EES model developed for the design is shown in Figure B-1 (Appendix B).
Figure 3.14 Effect of the circulation ratio (CR) on the overall COPs of the CO2 system and the integrated arrangement
Table 3.3 Design conditions and estimated performance parameters
Design conditions Estimated performance parameters
Parameters Value Parameters Value
Ambient temperature (oC) 25 MT refrigeration capacity (kW) 5 Delivery brine temperature (oC) -10 LT refrigeration capacity (kW) 3 Condensing temperature (oC) -8 COP of MT CO2 system 28.6 MT evaporating temperature (oC) -8 COP of LT CO2 system 4.3 LT evaporating temperature (oC) -32 COP overall CO2 system 9.2 Circulation ratio 2 COP of the absorption system 0.55 Load ratio LT to MT (LRLM) (%) 60 COP of the integrated system 0.41 LT evaporator superheat (K) 5 LT compressor power (kW) 0.70 LT compressor suction superheat (K) 9.8 CO2 pump power (kW) 0.18 LT liquid line subcooling (K) 2.4 Brine pump power (kW) 0.69 Condenser effectiveness (%) 88 HTF pump power (kW) 1.93 Max. capacity of the absorption chiller (kW) 10.95 Heat rejected in the condenser (kW) 8.56 Mass flow rates
Brine mass flow rate (kg/s)* 0.725 MT CO2 mass flow rate (kg/s) 0.040 HTF mass flow rate (kg/s) 1.20 LT CO2 mass flow rate (kg/s) 0.011 CHP exhaust gas mass flow rate (kg/s) 0.81
* Water-glycol mixture: propylene glycol with 40% mass fraction in water solution was considered.
COPCO2,overall
COPint
COP of the integrated system
COP of the CO2 system