3.1. Performance Characteristics of New Cascade Heat Pump Cycles
Figure 5 shows heating capacity of the cascade heat pump cycle with AHX in desuperheater, heater and parallel positions at the LS. Capacity of the cycle with AHX in desuperheater position was higher than that with AHX in heater and parallel positions at all OD
EWT. When AHX was adopted in the LS, heat transfer to secondary fluid of the HS cycle occurred in two stages. Firstly, there was heat transfer to secondary fluid of the HS cycle in the AHX, followed by heat transfer to HS secondary fluid from the AHX in the ID HX. Heat transfer to the HS secondary fluid in the AHX was such that, compressed superheated LS refrigerant exchanged heat with HS secondary fluid in the AHX, resulting in significant heat transfer to the HS secondary fluid when AHX was in desuperheater position. In heater position, subcooled LS refrigerant exchanged heat with HS secondary fluid in the AHX, resulting in heat transfer to the LS refrigerant because AHX
EWT was much higher than temperature of LS refrigerant entering the AHX. With AHX in parallel position, high temperature refrigerant from the LS compressor divided into two flow paths. The first part went into the AHX for heat exchange with the HS secondary fluid, while the other part went into the cascade heat exchanger for heat exchange with the HS refrigerant. Low stage refrigerant mass flow rate in the AHX was therefore lower than that of the cycle with AHX in desuperheater and heater positions, as shown in
Figure 6. This resulted in lower heat transfer to secondary fluid of the HS cycle in the AHX compared to that of the cascade heat pump cycle with AHX in desuperheater position, even though LS refrigerant temperature entering the AHX was almost similar in both cycles. Consequently, AHX leaving water temperature (
LWT) was highest in the cycle with AHX in the desuperheater position, followed by that of the AHX in parallel position in all OD
EWT conditions, as shown in
Figure 7. At 0
OD
EWT, AHX
LWT was 56.2
, 55.7
and 53.9
for the cascade heat pump cycles with AHX in desuperheater, parallel and heater positions respectively.
From the AHX, secondary fluid of the HS cycle proceeded to the ID HX for heat exchange with the HS refrigerant. Thus,
LWT of the AHX became the ID
EWT. Due to heat transfer between LS refrigerant and HS secondary fluid in the AHX, LS condensing temperature of the cascade heat pump cycles with AHX in desuperheater and parallel positions were lower than that of the cycle with AHX in heater position shown in
Figure 8.
Evaporating temperature of the HS cycle was also highest in the cycle with AHX as heater due to its highest LS condensing temperature trend, while HS evaporating temperature of the cycle with AHX in desuperheater position was higher than that of the cycle with AHX in parallel position as shown in
Figure 9. This was due to higher heat transfer from the LS to HS in the cascade cycle with AHX in desuperheater position, as compared to that of the cascade cycle with AHX in parallel position, as a result of higher LS refrigerant mass flow rate in the cascade cycle with AHX in the desuperheater position. In the HS cycle, condensing temperature was almost similar in the cascade cycles with AHX in desuperheater and heater positions, while that of the cycle with AHX in parallel position was slightly lower, as shown in
Figure 10. Furthermore, HS refrigerant mass flow rate was highest in the cycle with AHX as heater followed by that with AHX in desuperheater position as shown in
Figure 11. This was due to the trend of their HS pressure difference as shown in
Figure 12. Therefore, heat transfer to secondary fluid in the ID HX was highest in the cascade cycle with AHX as heater followed by that with AHX in desuperheater position. However, because ID
EWT was highest in the cascade cycle with AHX in desuperheater position,
LWT of the ID HX was slightly higher in the cascade heat pump with AHX as desuperheater compared to the cascade cycles in heater and parallel positions as shown in
Figure 13. This resulted in higher heating capacity in the cycle with AHX as desuperheater compared to the cascade heat pump cycles with AHX in heater and parallel positions. Thus, heat transfer to HS secondary fluid in the AHX had greater effect on the heating capacity than that in the ID HX. Heating capacity of the cascade cycle with AHX in desuperheater position was higher than that of the cascade cycle with AHX in heater position by 25.1% and 26.0% at OD
EWT of −5
and 0
respectively. Additionally, heating capacity of the cascade cycle with AHX in desuperheater position was higher than that of the cascade cycle with AHX in parallel position by 40.9% and 38.0% at OD
EWT of −5
and 0
respectively.
Figure 14 shows
COP of the cascade heat pump cycles adopting AHX.
COP was highest in the cascade cycle with AHX in desuperheater position followed by that with AHX in heater position. The cascade cycle with AHX in parallel position had the lowest
COP at all OD
EWT conditions. In the cascade cycles with AHX in desuperheater and parallel positions, heat transfer occurred between compressed high temperature LS refrigerant and HS secondary fluid in the AHX. However, heat transfer to secondary fluid in the AHX was higher in the cascade cycle with AHX in desuperheater position because LS refrigerant mass flow rate in the AHX was higher in system with AHX in desuperheater position than that of the cascade cycle with AHX in parallel position. Thus, the LS EEV opening was decreased in the system with AHX in parallel position in order to maintain the same superheat at the LS cycle in both systems. This caused lower LS pressure difference in the cascade cycle with AHX in desuperheater position, as compared to that of the cascade cycle with AHX in parallel position as shown in
Figure 15. Moreover, LS pressure difference of the cascade cycle with AHX in heater position was highest due to heat gain by LS refrigerant in the AHX. This resulted in the cascade cycle with AHX in heater position having the highest LS power consumption, followed by that of the cycle with AHX in parallel position. Power consumption of the LS cycle was lowest in the cascade cycle with AHX in desuperheater position.
In the HS cycle, power consumption of the cascade cycle with AHX in heater position was lower than that of the cascade cycle with AHX in desuperheater and parallel positions. This is because HS evaporating pressure was higher in the cascade cycle with AHX in heater position, resulting in lower pressure difference in the HS cycle. Additionally, HS power consumption of the cycle with AHX in desuperheater position was slightly lower than that of the cycle with AHX in parallel position because pressure difference of the HS cycle was lower in the cycle with AHX in desuperheater position. Consequently, total power consumption of the cascade cycle with AHX as heater was highest, due to its high LS power consumption, followed by that of the cascade cycle with AHX in parallel position as shown in
Figure 16. Total power consumption of the cascade cycle with AHX as desuperheater was lowest because of its low LS power consumption. Thus, power consumption of the LS cycle had dominant effect on total power consumption of the cascade cycles with AHX.
The high heating capacity of the cascade cycle with AHX in desuperheater position, coupled with its low total power consumption resulted in it having the highest COP among the three cascade heat pump cycles with AHX. At 0 OD EWT, COP of the cascade cycle with AHX in desuperheater was higher than that of the cascade cycles with AHX in heater and parallel positions by 39.0% and 42.5% respectively. Also, at −5 OD EWT, COP of the cascade cycle with AHX in desuperheater position was higher than that of the cascade cycles with AHX in heater and parallel positions by 39.2% and 44.3% respectively. Therefore, OD EWT had no significant effect on the difference in COP between the cascade heat pump cycles with AHX.
3.2. Performance Comparison between Conventional Cascade Heat Pump Cycle and New Cascade Heat Pump Cycle with AHX in Desuperheater Position
Figure 17 shows capacity of the conventional cascade heat pump and that of the cascade heat pump adopting AHX in desuperheater position. The capacity of the cascade heat pump adopting AHX in desuperheater position was higher than that of the conventional cascade heat pump at all OD
EWT conditions. Capacity of cascade heat pumps for hot water generation greatly depends on heat transferred to secondary fluid of the HS cycle. In the cascade heat pump with AHX in desuperheater position, heat transfer to HS secondary fluid occurred in the AHX, between LS refrigerant and HS secondary fluid, and in the ID HX, between HS refrigerant and HS secondary fluid. However, in the conventional cycle, heat transfer to secondary fluid of the HS cycle occurred only in the ID HX. The indoor
EWT of cascade heat pump with AHX in desuperheater position was higher than that of the conventional cascade heat pump as shown in
Figure 18, due to heat transfer from the LS refrigerant to HS secondary fluid in the AHX. Heat transfer in the ID HX of the cycle with AHX in desuperheater position, therefore occurred between HS refrigerant and HS secondary fluid with
EWT higher than that of the conventional cycle. For the conventional cycle, LS condensing temperature was higher than that of the cycle with AHX in desuperheater position, as shown in
Figure 19. This resulted in higher heat transfer to the HS cycle in the cascade heat exchanger, and higher HS evaporating temperature compared to the cycle with AHX as desuperheater as shown in
Figure 20. High stage refrigerant mass flow rate in the conventional cycle was therefore higher than that of the cycle with AHX in desuperheater position as shown in
Figure 21, which consequently resulted in higher heat transfer between HS refrigerant and secondary fluid in the ID HX. Even though heat transfer to HS secondary fluid in the ID HX was lower for the cycle with AHX as desuperheater, total heat transfer to the HS secondary fluid was higher than that in the conventional cascade heat pump at all OD
EWT conditions. Therefore,
LWT of the ID HX was higher in the cycle with AHX as desuperheater as shown in
Figure 22, resulting in higher heating capacity for the cycle with AHX in desuperheater position at all OD
EWT conditions.
Furthermore, difference in heating capacity between the cycle with AHX as desuperheater and that of the conventional cycle increased as OD EWT increased, due to the increase in heat transfer to secondary fluid of the HS cycle in the AHX. Heating capacity of the cycle with AHX as desuperheater was higher than that of the conventional cycle by 3.7%, 6.1%, 6.6% and 7.4% at OD EWT of −5 , 0 , 5 and 10 respectively.
Figure 23 shows
COP of the conventional cascade heat pump and that of the cycle with AHX as desuperheater according to OD
EWT variation.
COP of the cascade heat pump cycles increased according to increase in OD
EWT due mainly to increase in heating capacity. Even though total power consumption of the cascade heat pump cycles increased as OD
EWT increased, as shown in
Figure 24, the slope of increase of the heating capacity was higher than that of the total power consumption, resulting in the increasing
COP trend as OD
EWT increased.
Furthermore,
COP of the cascade heat pump with AHX as desuperheater was higher than that of the conventional cycle at all OD
EWT conditions. Low stage power consumption of the cascade heat pump with AHX as desuperheater was significantly lower than that of the conventional cycle at all OD
EWT conditions as shown in
Figure 25. This was due to lower LS pressure difference of the cycle with AHX in desuperheater position as shown in
Figure 26, caused by lower LS condensing pressure as a result of heat transfer to the HS secondary fluid in the AHX. Nonetheless, HS power consumption of the cascade cycle with AHX in desuperheater position was slightly higher than that of the conventional cycle at all OD
EWT conditions. Therefore, total power consumption of the conventional cycle was higher than that of the cycle with AHX as desuperheater. Combined effect of higher heating capacity and lower total power consumption in the cycle with AHX as desuperheater resulted in it having higher
COP as compared to that of the conventional cascade heat pump at all OD
EWT conditions.
COP of the cascade heat pump with AHX as desuperheater was higher than that of the conventional cascade heat pump by 12.2%, 14.9%, 13.2% and 12.4% at OD
EWT of −5
, 0
, 5
and 10
respectively.
Figure 27 shows P-h (Pressure-enthalpy) diagram of the conventional cascade heat pump cycle and the new cascade cycle with AHX as desuperheater at standard
EWT conditions. Compared to the conventional cycle, the new cascade cycle had lower LS condensing pressure and lower enthalpy difference across the LS compressor. This resulted in lower LS power consumption of the new cascade cycle. Furthermore, enthalpy difference across the ID HX of the new cycle was slightly higher than that of the conventional cycle, representing slightly higher heat transfer in the ID HX of the new cascade cycle; while enthalpy difference across HS compressor of the new cascade cycle was higher than that of the conventional cycle, showing higher HS power consumption of the new cascade cycle. However, enthalpy difference across the LS and HS compressors shows lower total power consumption of the new cascade cycle than that of the conventional cycle.