3.1. Calorimeter Control Performance for Heating Operating Mode
Figure 3 and
Figure 4 show the calorimeter performance based on the heating capacity in control (1) in terms of flow loop temperature and energy consumption, respectively. As shown in
Figure 3, for both indoor and outdoor side flow loops, the water temperature at the inlet to the heat recovery unit remained the same as the heating capacity of the heat pump increased because the water flow rate was proportional to the heat pump capacity.
However, the exit water temperature from the outdoor HX decreased, whereas the water exit temperature from the indoor HX increased. As a result, the inlet water temperature to the heat recovery unit from the outdoor flow loop was lower than the IWT setpoint for the outdoor HX, and the inlet water temperature to the HR unit from the indoor flow loop became higher than the IWT setpoint for the indoor heat exchanger.
Therefore, the needle valve at the inlet to the HR unit was regulated to control the quantity of heat transfer from the ID flow loop to OD flow loop such that the inlet water temperature to the indoor heat pump HX was maintained at a setpoint of 40 °C. Moreover, when the heating capacity of the heat pump was increased, the heat exchange rate in the HR unit also increased because the flow rate of water increased. As such, the inlet water temperature to the outdoor refrigerator increased with an increase in heat pump capacity. Also, IWT to the OD refrigerator increased above the water setpoint temperature of 5 °C for the outdoor heat pump HX. Therefore, the calorimeter OD refrigeration unit was operated to extract heat from the secondary fluid in the OD flow loop. The OD loop heater IWT was increased as the heating capacity and flow rate increased. However, the OD heater IWT was below the inlet water setpoint temperature for the outdoor heat pump HX. Therefore the OD heater unit was turned on to maintain the IWT setpoint for the OD heat pump HX. In
Figure 4, the energy used by the ID heater and ID refrigerator was zero because both units were not operated as the heat recovery unit was used to control the inlet water setpoint temperature for the indoor heat pump HX. The energy utilization rate of the OD refrigerator was relatively uniform as the capacity increased because the refrigerator was operated at a constant speed. Also, the energy utilized by the outdoor heater decreased when the heating capacity was increased because the inlet water temperature to the OD flow loop heater also increased. Hence, the total rate of energy utilized by the calorimeter decreased when the capacity of the heat pump unit being tested was increased.
Figure 5 and
Figure 6 represent the calorimeter operating characteristics when the heat pump heating capacity was varied and control (2) was used in heating mode. For
Figure 5, the inlet water temperature to the HR unit from the outdoor flow loop was lower than the IWT setpoint for the OD heat pump HX, and the IWT to the HR unit from the indoor flow loop was higher than the water inlet temperature setpoint for the ID HX.
Therefore, the needle valve was altered to control the heat transfer rate through the HR unit such that IWT to the OD heat pump HX was 5 °C. As a result, IWT values to the OD flow loop refrigerator and heater were similar to the water setpoint temperature for the outdoor HX. In this case, both the refrigerator and electric heater in the OD flow loop were switched off. Since the heat transfer rate in the HR unit from the indoor loop to outdoor loop increased with an increment in capacity, the IWT to ID refrigerator slightly decreased, however small. The ID refrigerator IWT was higher than the IWT setpoint of 40 °C for the ID heat pump HX. Hence, the ID loop refrigerator unit was operated. However, the required amount of heat extraction as a result of the increase in heating capacity was higher than the refrigeration capacity. Therefore, the inlet water temperature to the ID loop heater increased as the heating capacity was increased. Moreover, the indoor flow heater IWT was below the setpoint temperature for the indoor HX. Hence, the ID flow loop electric heater was turned on to control and maintain the IWT for the indoor heat pump HX. For energy consumption analysis, as shown in
Figure 6, the outdoor heater and outdoor refrigerator were turned off due to heat recovery control, so the energy consumption for both units was zero. The indoor refrigerator utilized some energy because the inlet water temperature to the ID refrigerator increased above the IWT setpoint value for the ID heat pump HX.
However, the energy usage rate of the ID refrigerator became relatively the same as the heating capacity increased because the compressor of the refrigeration unit was operated at a constant speed and optimized at a low flow rate. As the inlet water temperature to the ID loop heater increased when the capacity was increased, the energy utilized by the ID heater to maintain the IWT setpoint decreased with an increase in the capacity. The calorimeter total energy utilized then decreased when the heating capacity was increased.
Table 3 indicates the energy consumption analysis of the calorimeter with heat recovery control methods and that of the conventional calorimeter for the water-to-water heat pump unit using the same set of operating test conditions in heating mode. As indicated in
Figure 7, the total energy utilized by the conventional calorimeter showed an upward trend as the heat pump heating capacity increased. This was because, in the conventional calorimeter, there was no heat recovery unit and as such, the electric heater and refrigeration unit on both indoor and outdoor flow loops of the calorimeter were operated simultaneously when testing the heat pump unit.
Therefore, as the heat pump capacity was increased, the total power consumption also increased based on standard test conditions. However, the total energy utilization rate of the calorimeter with a heat recovery unit showed a downward trend as the heat pump heating capacity was increased. This was because the heat transfer rate in the heat recovery unit increased as the heating capacity increased, thereby decreasing the total power consumption of the calorimeter. Also, in
Figure 7, as the heat pump heating capacity increased from 3 to 9 kW, the total energy utilization rate of the conventional calorimeter was about 73 to 89% higher than the calorimeter with heat recovery control methods. This was because the conventional calorimeter made use of two electric heaters and refrigeration units concurrently, whereas the calorimeter with a heat recovery unit only made use of one electric heater and refrigeration unit at the same standard test conditions for the heat pump unit.
Also, the refrigeration unit for the conventional calorimeter used a refrigerator capacity which was much larger compared to the refrigeration unit for the calorimeter with a heat recovery unit. Moreover, for the calorimeter with heat recovery control methods, when the control (1) method was adopted in the HR unit, the total energy increased between 8% and 13% compared to control (2). This showed that, for testing heat pumps in heating mode, it was better to use the heat recovery unit to control and maintain the inlet water temperature setpoint for the outdoor HX. Thus, control (2) was an optimum control option for the calorimeter with heat recovery compared to control (1) in heating mode.
The heat recovery control (2) resulted in a lower energy consumption for the calorimeter than control (1) in heating mode. Hence, the calorimeter performance analysis in relation to heat pump COP variation was executed for the control (2) method.
Figure 8 shows the measured flow loop temperature analysis. The inlet water temperature to the HR unit from the outdoor flow loop HX decreased as the COP increased, whereas the IWT from the indoor flow loop HX to the heat recovery unit was unchanged because the ID heat pump HX capacity remained unchanged. The inlet water temperature to the OD refrigerator was nearly the same as the inlet water temperature to the OD loop heater as a result of the control technique in the HR unit.
The inlet water temperature to the ID loop refrigerator decreased as the COP increased because the heat transfer rate in the HR unit was constant and therefore, IWT to the ID loop heater also decreased with an increment in COP.
Figure 9 also represents the calorimeter energy analysis due to heat pump variation in COP. The OD heater and OD refrigerator energy consumption were zero because the OD flow loop was controlled with the HR unit. The ID loop refrigerator utilized energy as a result of heat extraction. However, ID refrigerator energy consumption was relatively uniform as the COP increased as a result of the constant operating speed of the compressor. Since the inlet water temperature to the indoor loop heater decreased because COP increased, the energy used by the ID heater to maintain the IWT setpoint also increased. The calorimeter total energy utilization rate increased by about 8% with an increase in the COP of the heat pump unit being tested.
3.2. Calorimeter Control Performance in Cooling Operating Mode
For test measurement in cooling operating mode, the water exit temperature from the OD heat pump HX increased, whereas the water exit temperature from the ID heat pump HX decreased.
Figure 10 and
Figure 11 show the calorimeter performance analysis according to the cooling capacity in control (1). For the flow loop temperature analysis as shown in
Figure 10, the water entering temperature to the heat recovery unit was relatively the same as the capacity increased for both indoor and outdoor flow loops because the flow rate was proportional to the heat pump cooling capacity. However, the inlet water temperature to the HR unit from the OD flow loop was higher than the IWT setpoint for the outdoor heat pump HX, whereas the inlet water temperature to the HR unit from the ID flow loop was lower than the IWT setpoint for the heat pump outdoor HX. Hence, the needle valve at the inlet of the HR unit was regulated to control the heat quantity through the HR unit so that the inlet water temperatures to the indoor loop refrigerator and heater were relatively equal to the inlet water temperature setpoint of 12 °C for the ID heat exchanger. Therefore, the ID refrigerator and ID heater were not activated. Also, due to heat transfer in the HR unit, IWT to the OD loop refrigerator slightly decreased when the cooling capacity was increased. However, the OD refrigerator IWT was higher than the standard temperature condition of 25 °C for the OD heat pump HX. As a result of heat discharged from the OD refrigerator, the inlet water temperature to the heater at the OD flow loop also decreased. But the IWT to the OD heater increased as the cooling capacity of the heat pump increased because the flow rate through the OD flow loop increased. The IWT to the OD loop heater was lower than the standard IWT setpoint for outdoor HX and so the OD heater was turned on.
For energy consumption analysis using HR control (1) as shown in
Figure 11, both the indoor heater and refrigerator were turned off since the ID flow loop control was conducted with the heat recovery unit. The OD refrigerator utilized some energy as a result of heat extraction. However, the rate of energy utilized by the OD loop refrigerator was somewhat unchanged as the cooling capacity increased due to the constant speed operation. The rate of energy metered by the OD loop heater decreased when the cooling capacity was increased because IWT to the OD loop heater also increased. Therefore, the total rate of energy utilized by the calorimeter also decreased when the cooling capacity was increased.
Figure 12 and
Figure 13 represent the calorimeter operating characteristics with the variation in capacity when control (2) was used for testing in cooling mode. In
Figure 12, the inlet water temperature to the HR unit from the OD flow loop was higher than the inlet water setting temperature for the outdoor HX, whereas the IWT to the HR unit from the ID flow loop was lower than the IWT setpoint for the indoor HX. As such, the needle valve at the HR unit inlet was used to control the quantity of heat through the HR unit such that the IWT at the inlet to the OD heat pump HX was maintained at a setpoint of 25 °C. As the cooling capacity increased, the heat transfer rate in the HR unit increased, which then increased the inlet water temperature to the indoor refrigerator. Also, the inlet water temperature to the ID refrigerator was above the setpoint temperature of 12 °C for the ID heat pump HX. Therefore, the ID refrigeration unit was operated for heat extraction. However, the refrigerator capacity in the ID flow loop did not change so much. As a result, the ID heater IWT also increased as the cooling capacity was increased. However, the ID heater IWT was lower than the inlet water setpoint temperature for the ID heat pump HX. Therefore, the indoor loop heater was turned on to maintain the IWT setpoint for the ID heat pump HX.
As shown in
Figure 13, the OD heater and OD refrigerator were turned off to save energy because the IWT setpoint to the outdoor heat pump HX was controlled due to heat transfer between the indoor and outdoor flow loops through the HR unit. However, the ID refrigerator utilized energy as a result of heat extraction. The energy consumed by the ID refrigerator recorded almost the same value as the cooling capacity was increased because the refrigerator was operated at a constant speed. For the ID heater, since the IWT to ID heater was increased with an increment in cooling capacity, the energy utilized by the indoor heater to maintain the IWT setpoint decreased when the cooling capacity was increased. Therefore, the calorimeter total energy consumption decreased when the cooling capacity was increased.
Table 4 indicates the energy consumption analysis of the calorimeter with heat recovery control methods and that of the conventional calorimeter for the water-to-water heat pump unit with the same set of operating test conditions in cooling mode.
Figure 14 compares the total energy consumption of the calorimeter with heat recovery control methods to that of a conventional calorimeter. Similar to the heating operating mode, the total energy utilized by the conventional calorimeter showed an upward trend as the heat pump cooling capacity increased. This was because the conventional calorimeter without a heat recovery unit utilized both the electric heater and refrigeration unit in each flow loop of the calorimeter simultaneously.
Therefore, as the heat pump cooling capacity increased, the total power consumption also increased based on standard test conditions. However, the total energy utilization rate of the calorimeter with a heat recovery unit showed a downward trend as the heat pump cooling capacity was increased. This was because the heat transfer rate in the heat recovery unit increased as the cooling capacity increased. Hence, the total power consumption decreased with an increase in the cooling capacity.
Also, in
Figure 14, as the heat pump cooling capacity increased from 3 to 9 kW, the total energy utilization rate of the conventional calorimeter was about 71 to 92% higher than that of the calorimeter with heat recovery control methods. The reason for this is that the conventional calorimeter utilized two electric heaters and refrigeration units at the same time, whereas the calorimeter with a heat recovery unit only made use of one electric heater and refrigeration unit under same heat pump test conditions.
Again, the refrigeration unit of the conventional calorimeter utilized a much larger refrigerator capacity compared to the case of the calorimeter with a heat recovery unit. Also, when the control (1) method was adopted in the HR unit, the total energy decreased between 6.4% and 21% as compared to control (2). This implied that, for the experiment in heat pump cooling mode, it was better to use the heat recovery unit to control and maintain the IWT setpoint for the ID heat pump HX. Thus, control (1) was an optimum control option when compared to control (2) in cooling mode.
Figure 15 and
Figure 16 illustrate the calorimeter performance in terms of heat pump variation in COP using the heat recovery control (1) method in cooling mode. As indicated in
Figure 15, the inlet water temperature to the HR unit from the indoor heat pump HX was relatively the same as the COP increased because the heat pump indoor HX capacity remained constant. The exit water temperature from the outdoor heat exchanger to the HR unit decreased because the OD heat pump HX capacity also decreased.
Therefore, needle valves were altered to control the heat exchange through the HR unit such that inlet water temperatures to the ID loop heater and refrigerator were equivalent to the IWT setpoint of 12 °C. The IWT to the OD loop refrigerator decreased as the COP was increased since the heat exchange rate in the HR unit was constant. Based on OD refrigerator heat extraction at a constant speed, IWT to the OD heater also decreased as the COP increased.
Figure 16 represents the energy consumption analysis as a result of COP variation. The ID loop heater and refrigerator energy consumption were zero because the ID flow loop was controlled with the HR unit. The OD loop refrigerator utilized energy because of heat extraction. However, the energy used by the OD refrigerator was relatively unchanged as the COP was increased. Since the IWT to the OD loop heater decreased as a result of the increase in the COP, the energy used by the OD heater to maintain the IWT setpoint increased when the COP was increased. The calorimeter total energy utilization rate increased by about 6.5% with an increase in the COP of the heat pump system being tested in cooling operating mode.