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A study on the performance of a newly designed heat pump calorimeter
Applied Thermal Engineering 123 (2017) 216–225
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
A study on the performance of a newly designed heat pump calorimeter Kofi Owura Amoabeng a, Jong Min Choi b,⇑ a b
Graduate School of Mechanical Engineering, Hanbat National University, Daejeon 34158, South Korea Department of Mechanical Engineering, Hanbat National University, Daejeon 34158, South Korea
h i g h l i g h t s The heat pump calorimeter is the testing facility to investigate the performance of a heat pump. The heat pump calorimeter consumed much more energy than heat pump. The newly designed calorimeter could save energy greatly rather than the conventional system. It is recommended to adopt small refrigerator for maximizing energy saving rate.
a r t i c l e
i n f o
Article history: Received 7 November 2016 Revised 19 April 2017 Accepted 7 May 2017 Available online 16 May 2017 Keywords: Heat pump Calorimeter Energy consumption Entering water temperature Heat recovery
a b s t r a c t The heat pump calorimeter is the testing facility to investigate the performance of a heat pump using standard test conditions obtained from domestic or international standard organizations. Conventionally, the calorimeter consumes so much energy in order to control and maintain the entering water temperature test conditions of the water-to-water heat pump unit. To minimize this energy, a newly designed calorimeter for the water-to-water heat pump was proposed in this study. Experiments were conducted in both heating and cooling modes with variation in capacity and COP of heat pump unit. In heating mode, entering water temperature test conditions of 40 °C and 5 °C were set for the indoor heat exchanger and outdoor heat exchanger of the heat pump unit respectively. In cooling mode, entering water temperature of 25 °C and 12 °C were set for the outdoor heat exchanger and indoor heat exchanger of the heat pump unit respectively. The analysis of the test results from the experiment showed that the newly designed calorimeter was able to save about 75% or more of the total power consumption comparing with the conventional calorimeter for all heat pump capacity and COP that were investigated. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Energy is a key component in the development of modern society. It promotes economic growth and improves the quality of life. The escalation in worldwide population has contributed to the rising energy consumption, and demand levels are estimated to be 45% higher in 2030 than current levels [1]. In the field of heating, ventilation and air conditioning (HVAC) systems, heat pumps offer the most energy-efficient way to provide heating and cooling in many applications [2]. This is because they have the ability to utilize renewable energy sources and also impact the environment positively than the conventional burning of fossil fuels [3]. Several heat pump types are in existence; some require external mechanical work while others require external thermal energy. Commercial heat pumps based on the vapour compression cycle or the ⇑ Corresponding author. E-mail address: [email protected] (J.M. Choi). http://dx.doi.org/10.1016/j.applthermaleng.2017.05.029 1359-4311/Ó 2017 Elsevier Ltd. All rights reserved.
absorption cycle are operational in several applications in various industries. New heat pump technologies such as the adsorption cycle or the chemical reaction cycle are emerging rapidly, even though they have yet to find major industrial applications [4]. Any heat pump, either for domestic or commercial purpose comes with a set of performance data. This data provides information about the heat pump such as reliability and performance to serve as reference point for end users. However, the heat pump should be tested, rated and certified to estimate the performance before it is distributed. The testing and rating of heat pumps are strictly guided and regulated by standard test conditions provided by international and domestic standard organizations [5–7]. Conventionally, the heat pump testing facility makes use of a calorimeter [8], on the basis of energy balance method in rating and certifying the heat pump [5,9]. The ideal function of the heat pump calorimeter is to control and keep the standard test conditions constant during the testing period while serving as the heat sink and heat source for the heat pump. Heat is transferred to
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217
Nomenclature COP EWT HR HX
coefficient of performance entering water temperature heat recovery heat exchanger
the evaporator of the heat pump by the heating system of the calorimeter and absorbed from the condenser of the same heat pump by the refrigeration system of the calorimeter. Hence the energy consumption of the calorimeter which incorporates the energy used by the heater and refrigeration unit is much higher than that of the heat pump itself. Several research studies have focused on ways to improve the performance of heat pump systems. For instance, the incorporation of a heat-driven ejector to the heat pump has improved system efficiency by more than 20% [10–12]. Also, the development of better compressor technology has the potential to reduce energy consumption of heat pump systems by as much as 80% [13–16]. The evolution of new hybrid systems has also enabled the heat pump to perform efficiently with wider applications [17–20]. Renedo et al. [21] studied on the energy efficiency of reversible water-towater heat pumps and proposed a system that reduces the annual energy consumption for any annual thermal demand of the system. Janghoo et al. [22] optimized the design of HVAC systems to minimize primary energy demand. Neksa et al. [23] studied on the characteristics, system design and experimental results of a CO2heat pump water heating system. The energy consumption of the system reduced by 75% when compared to electrical or gas fired systems. Bahri and Kodal [24] proposed appropriate functions consisting of investment and energy consumption costs for endoreversible refrigerators and heat pumps to find optimal design conditions. Moatassem et al. [25] also developed an optimization model to minimize energy consumption and carbon emission of aging buildings by implementing sustainability measures such as efficient HVAC systems and renewable energy systems. However, due to the high energy consumption of the heat pump calorimeter, it is also of great significance to adopt efficient methods to monitor and control the energy in order to improve system performance. Rodolfo et al. [26] concluded that, the performance of a calorimeter can be improved by using a predictor-based controller known as dead-time compensator after analyzing a model data and a real experimental result. Schroder et al. [27] developed a flow calorimeter which is able to eradicate the guess-work used in determining the specific heat capacity of geothermal water flow. Research work on calorimeter for testing heat pumps is very rare in literature. This study focuses on calorimeter for testing the performance of a water-to-water heat pump unit. The conventional calorimeter is used to test the performance of a water-towater heat pump unit. A newly designed calorimeter is then proposed to execute the same tests. The purpose is to reduce the energy used in the conventional calorimeter. The energy consumption analysis of the newly designed calorimeter is then compared to that of the conventional calorimeter.
2. Experimental setup and test procedure 2.1. Experimental setup The calorimeter set up in this study is a testing facility used to investigate the performance of a water-to-water heat pump unit as shown schematically in Fig. 1. Conventionally, the heat pump calorimeter has two compartments; the indoor side and outdoor
ID HX LWT OD HX
indoor heat exchanger leaving water temperature outdoor heat exchanger
side. Each side of the calorimeter is controlled separately because there is no connection between the indoor and outdoor heat exchanger of the heat pump. Two secondary fluid loops, one for each side, are used to connect the calorimeter to the heat pump unit. One fluid loop connects the indoor side of the calorimeter to the indoor heat exchanger (ID HX) of the heat pump. The other loop connects the outdoor heat exchanger (OD HX) of the heat pump to the outdoor side of the calorimeter. The secondary fluid used in the experiment is mainly water with a 40% concentration of ethylene glycol solution. Each side of the calorimeter is equipped with a constant temperature bath (CTB), a refrigeration unit and electric heater. The refrigeration unit and electric heater are used to set up and maintain the temperature of the secondary fluid located inside the CTB based on standard test conditions [5]. The setting temperature of the secondary fluid in the CTB is used as the entering water temperature (EWT) to the heat exchanger of the heat pump test unit. There is also a volumetric flow meter and circulation pump on each side of the calorimeter as shown in Fig. 1. The flow meter is used to maintain the flow rate of the secondary fluid according to the capacity of the heat pump unit while the circulation pump is used to transport the secondary fluid from the CTB through the heat pump heat exchanger to the refrigerator, electric heater and back to the CTB. In the proposed newly designed calorimeter, a heat recovery heat exchanger (HR HX) unit is used to connect the indoor side flow loop to the outdoor side flow loop as shown in Fig. 1. Hence heat can be transferred from one side of the calorimeter to the other side. As the heat pump capacity change, the COP also change. It is therefore impossible to analyze the calorimeter’s performance on experimental basis by using heat pumps with different capacities and COPs. Hence a heat pump simulator is developed to imitate the variation of heat pump capacity and COP separately. The actual heat pump unit consists of an ID HX, OD HX, a compressor, an expansion valve and a four-way valve as in Fig. 1. However, the heat pump simulator as shown in Fig. 2 consists of two plate type heat exchangers; ID HX and OD HX, two constant temperature water baths and two circulation pumps. The capacities of the ID HX and OD HX are controlled separately by using the temperature and flow rate of each heat pump constant temperature water bath line connected to the ID HX and OD HX. The constant temperature water bath line is used to replace the refrigerant line in an actual heat pump unit. The simulator can realize the change of COP and capacity separately. The experimental rig as shown in Fig. 3 which consist of the heat pump simulator section and the calorimeter section is therefore used for testing the performance of a water-to-water heat pump unit using standard test conditions based on Refs. [28,29] in both conventional and newly designed heat pump calorimeters. The two needle valves located on the indoor side flow line are used to control the amount of heat transfer rate in the HRHX between the indoor side flow and outdoor side flow loops. Temperature measuring devices such as resistance thermal detectors (RTDs) and T-type thermocouple sensors are used to measure temperatures in the experimental set up. The T-type thermocouple and RTD sensors have uncertainty of ±0.1 °C of reading scale. The volumetric flow meters that are used to measure the flow rate of the secondary fluid also have uncertainty of ±0.1% of full scale.
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Calorimeter outdoor loop
Calo rimeter indoor loop
Refrigeraon unit
Refrigeraon unit
Electric heater
Constant Temperature Bath
Electric Heater
Heat recovery HX
Flow meter
Constant Temperature Bath
Flow meter
4 way valve Compressor Outdoor HX
Circulaon pump
Indoor HX
Heat pump unit S
Circulaon pump
Expansion valve
Fig. 1. Schematic representation of calorimeter for water-to-water heat pump unit.
Heat pump Constant Temp. Water Bath
Outdoor HX Pump 2
Refrigerator
Heat pump Constant Temp. Water Bath
Indoor HX Pump 4
Heater
Heater
Refrigerator
Fig. 2. Heat pump simulator.
The temperatures are monitored at the selected locations according to ASHRAE standard 41.1 [30]. The temperatures and flow rates of the secondary fluid at relevant locations in the experimental set up are the measured parameters used to estimate the heating and cooling capacities of heat pump unit with the equation;
Q¼
V q Cp DT 60; 000
ð1Þ
where Q represent the cooling or heating capacity of the heat pump in kW, V is the secondary fluid flow rate in liters per minute (LPM), q is the density of the secondary fluid in kg/m3, Cp is the heat capacity of the secondary fluid in kJ/kg°C and DT is the secondary fluid temperature difference across the heat pump heat exchanger in °C. The power consumption of refrigerator and electric heater are measured by power meter with an uncertainty of ±0.01% of reading scale. The specifications of the various components in the experimental set up are shown in Table 1. 2.2. Test procedure The focus of the study was to investigate the performance of the conventional calorimeter and the proposed newly designed calorimeter for a water-to-water heat pump unit using the same set of standard operating test conditions. Experiments were conducted in both heating mode and cooling mode for the heat pump unit. Heat pump units were tested in both the conventional and
newly designed calorimeter using the same rated capacity for the refrigeration unit. Afterwards, the same tests were performed with the newly designed calorimeter but with a small refrigerator capacity as shown in Table 1. Table 2 shows the test conditions used for the investigation. To execute experiment in heating mode, the EWT to the ID HX and OD HX were set to the standard conditions of 40 °C and 5 °C in the indoor and outdoor CTB respectively. In using the conventional calorimeter, the needle valve 1 at the indoor flow line close to the inlet to the HR HX was fully closed while needle valve 2 was fully opened. Hence, the secondary fluid at the indoor flow loop does not flow through the HR HX. Therefore there was no flow connection between flow lines on either side of the calorimeter. In the outdoor flow loop, the secondary fluid from CTB at the outdoor side of the calorimeter flowed through the OD HX. The OD HX absorbed heat from the secondary fluid. As a result, secondary fluid leaving water temperature (LWT) at exit of the OD HX decreased. The low temperature secondary fluid from exit of OD HX then proceeded continuously through the HR HX to the outdoor refrigerator (OD refrigerator) and to the outdoor electric heater (OD heater) back to the CTB at the outdoor side of the calorimeter. The OD refrigerator was turned off in the process because the temperature of the secondary fluid at inlet to OD refrigerator was lower than the setting temperature in the outdoor CTB. The OD heater was then adjusted to control and maintain the secondary fluid EWT condition of 5 °C to the OD HX in the CTB. For
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Calorimeter
Condenser 1
Condenser 2
Compressor 2
Exp. Valve 1
S
S
Compressor 1
OD Refrigerator
ID Refrigerator
Evaporator 1
Evaporator 2
Exp. Valve 2
Ball Valve
OD Heater
ID Heater Heat recovery HX Needle valve 2
Calorimeter Constant Temp. Bath
Needle valve 1
Pump 1
Flow meter 1
Heat pump Constant Temp. Water Bath
Outdoor HX
Flow meter 2
Heat pump Constant Temp. Water Bath
Indoor HX
Pump 2
Refrigerator
Pump 3
Calorimeter Constant Temp. Bath
Pump 4
Heater
Heater
Refrigerator
Heat pump simulator Fig. 3. Schematic representation of the experimental test rig.
the indoor flow loop, the secondary fluid from CTB at the indoor side of the calorimeter flowed through the ID HX. The secondary fluid absorbed heat in the ID HX. As a result, secondary fluid leaving water temperature (LWT) at exit of the ID HX increased. The high temperature secondary fluid from exit of ID HX then proceeded continuously through the HR HX bypass flow line to the indoor refrigerator (ID refrigerator) and to the indoor electric heater (ID heater) back to the CTB at the indoor side of the calorimeter. The ID refrigerator was turned on in the process to extract heat from the secondary fluid because the temperature of the secondary fluid at inlet to ID refrigerator was higher than the setting temperature in the indoor CTB. The ID heater was further adjusted to control and maintain the secondary fluid EWT condition of 40 °C to the ID HX in the CTB. In using the proposed newly designed calorimeter, the needle valve 1 was adjusted to allow secondary fluid from exit of ID HX to flow through HR HX unit. Heat was then transferred from the high temperature secondary fluid at exit of ID HX to the low temperature secondary fluid at exit of OD HX. This increased the secondary fluid temperature at the outdoor flow line as it flowed from the HR HX through the OD refrigerator to the OD heater and back to the CTB. The amount of OD heater power needed to adjust and maintain the secondary fluid EWT condition of 5 °C in the outdoor CTB then reduced greatly. Also, the amount of heat extracted in the ID refrigerator reduced since secondary fluid temperature at inlet to the ID refrigerator decreased in the process of heat recovery in the HR HX. The amount of heat recovery in the HR HX varied as the heat pump capacity and COP varied.
Similarly, to execute experiment in cooling mode, the EWT to the ID HX and OD HX were set to the standard conditions of 12 °C and 25 °C in the indoor and outdoor CTB respectively. In the conventional mode, the needle valve 1 at the indoor flow line close to the inlet to the HR HX was fully closed while needle valve 2 was fully opened. Hence, the secondary fluid at the indoor flow loop fully bypassed the HR HX. Hence no flow connection between indoor and outdoor line on either side of the calorimeter. The secondary fluid from CTB at the indoor side of the calorimeter flowed through the ID HX. The ID HX absorbed heat from the secondary fluid. As a result, secondary fluid LWT at exit of the ID HX decreased. The secondary fluid at low temperature from exit of ID HX then proceeded continuously through the HR HX bypass flow line to the ID refrigerator and then to the ID heater back to the CTB at the indoor side of the calorimeter. The ID refrigerator was turned off in the process because the temperature of the secondary fluid at inlet to ID refrigerator was lower than the setting temperature in the indoor CTB. The ID heater was therefore adjusted to control and maintain the secondary fluid EWT condition of 12 °C to the ID HX in the CTB. However, in the outdoor flow loop, the secondary fluid from CTB at outdoor side of the calorimeter absorbed heat as it flowed through the OD HX. As a result, leaving water temperature (LWT) of the secondary fluid at exit of the OD HX increased. The secondary fluid at high temperature from exit of OD HX then proceeded continuously through the HR HX to the OD refrigerator and then to the OD heater back to the CTB at the outdoor side of the calorimeter. The OD refrigerator was turned on in the process to extract heat from the secondary fluid
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Table 1 Specification of components in test rig. Component
Specification
stable conditions, 30 mins performance data was recorded every second with a data acquisition system and average values from the data were used to investigate heat pump performance. The power consumption of the refrigerator and electric heaters were recorded by using a power meter.
Circulation pump
3-phase motor
Rated frequency: 60 Hz
Volumetric flow meter
Magnetic type
Flow range: 0–40 LPM Uncertainty: ±0.1% of full scale
Electric heater
Flange heater
Rated power: 12 kW
3. Results and discussion
Thermocouple
T-type
Range: 200 °C to 40 °C Uncertainty: ±0.1 °C of reading scale
3.1. Performance in heating mode
RTD sensor
A-class-type
Range: 200 °C to 600 °C Uncertainty: ±0.1 °C of reading scale
Power meter
3 phase/3 wire
Voltage range: 0–600 V Current range: 0–20 A Uncertainty: ±0.01% of reading scale
ID/OD heat exchanger
Plate type
Rated capacity: 10.5 kW
Refrigerator
Scroll compressor (constant speed)
Condenser
Refrigerant: R410 A Rated voltage: 400 V Rated frequency: 50 Hz Rated capacity: 14.4 kW (conventional/newly designed) Rated capacity: 3.6 kW (newly designed) Electronic Expansion Valve (EEV) Input voltage: 12 VDC ± 10% Rated capacity: 20 kW
Plate type
Rated capacity: 10.5 kW
Heat recovery HX
Table 2 Test conditions. Parameter
Heating mode
EWT for ID HX EWT for OD HX Heat pump capacity (secondary fluid flow rate) Heat pump COP
40 °C 12 °C 5 °C 25 °C 3 kW (7.29 LPM), 5 kW (12.14 LPM) 7 kW (17.00 LPM), 9 kW (21.86 LPM) 3, 4, 5, 6
Cooling mode
because the temperature of the secondary fluid at inlet to OD refrigerator was higher than the setting temperature in the outdoor CTB. The OD heater was further adjusted to control and maintain the secondary fluid EWT condition of 25 °C to the OD HX in the CTB. In the newly designed, there was a similar test procedure as in heating mode. The needle valve 1 was adjusted to allow the secondary fluid from exit of ID HX to flow through HR HX unit. Heat was then transferred from the high temperature secondary fluid at exit of OD HX to the low temperature secondary fluid at exit of ID HX. This increased the secondary fluid temperature at the indoor flow line as it flowed from the HR HX through the ID refrigerator to the ID heater and back to the CTB. The amount of ID heater power required to adjust and maintain the secondary fluid EWT condition of 12 °C in the indoor then reduced considerably. Again, the amount of heat extracted in the OD refrigerator also reduced since secondary fluid temperature at inlet to the OD refrigerator decreased as a result of the HR HX. The same secondary fluid flow rates were set for the two flow lines that connect the calorimeter to the ID HX and OD HX of the heat pump unit according to the capacity of the heat pump unit. The refrigerator of each flow line had higher capacity by 20% than the maximum heat extraction rate of the heat pump or the maximum difference of heat amount between indoor and outdoor heat exchanger in the heat pump. Steady state conditions were attained when temperatures and flow rates were within ±0.1 °C and ±0.1 LPM respectively. After
60 Test unit conditions Operating mode: Heating COP: 4.0 o o OD HX EWT: 5 C ID HX EWT: 40 C
55 50 o
Expansion valve
Entering water temperature ( C)
Evaporator
Performance comparison of the conventional calorimeter and newly designed calorimeter were analyzed according to heat pump capacity in heating mode. The newly designed calorimeter had a small capacity refrigerator comparing with that of the conventional calorimeter. As shown in Fig. 4, for the conventional calorimeter, the EWT to the OD heater was lower than the setting EWT to OD HX, because OD HX absorbed heat from the secondary fluid. However, the EWT to the OD heater remained the same according to an increment of heat pump capacity, because heat pump capacity and flow rate increased. The EWT to the ID refrigerator was higher than the setting EWT to ID HX, because heat was absorbed by the secondary fluid from ID HX. The EWT to the ID heater decreased because there was heat extraction in the ID refrigerator. As the heat pump heating capacity increased, the EWT to the ID heater also increased. In the case of the newly designed calorimeter, the EWT value to the OD heater was higher than that of the conventional calorimeter as a result of heat transfer from indoor flow line to outdoor flow line. This reduced the EWT to the ID refrigerator. The EWT to the ID heater also decreased due to heat extraction in the refrigeration unit located at the indoor flow loop of calorimeter. However, the EWT to ID heater was higher in newly designed calorimeter than in conventional system, because the newly designed calorimeter had a small capacity refrigerator. As the heat pump capacity changes in heating mode, the power consumption of the OD heater, ID refrigerator and ID heater for both conventional and newly designed calorimeter are shown in Fig. 5. The OD heater power increased as heat pump capacity increased. This is because more heater power was required as heat pump capacity and flow rate were increased in order to maintain EWT of 5 °C to OD HX. But the increment was 63–76% higher in conventional calorimeter than in newly designed calorimeter as capacity increased from 3 kW to 9 kW respectively because EWT to OD heater of the former was lower than the latter. The ID refrig-
45 40 35 30 25 20
Newly designed
Conventional
15 10
ID Refrigerator
ID Refrigerator
ID Heater
ID Heater
OD Heater
OD Heater
5 0 -5 1
3
5
7
9
11
Test unit capacity (kW) Fig. 4. EWT of calorimeter with the variation of heat pump capacity in heating mode.
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Test unit conditions Operating mode: Heating o OD HX EWT: 5 C o ID HX EWT: 40 C COP: 4.0
Power consumption (kW)
14 12 10
Conventional ID Refrigerator ID Heater OD Heater
Newly designed ID Refrigerator ID Heater OD Heater
8 6 4 2 0 -2 1
3
5
7
9
11
Test unit capacity (kW) Fig. 5. Power consumption of calorimeter the variation of heat pump capacity in heating mode.
erator power consumption changed slightly as heat pump capacity increased because the refrigeration unit adopted a constant speed compressor. But the value was about 82% higher in conventional than in newly designed because of large refrigerator capacity in conventional calorimeter. Therefore to control and maintain EWT of 40 °C to the heat pump ID HX, the ID heater was adjusted. The ID heater power consumption decreased as capacity increased because EWT to ID heater increased as heat pump capacity increased. However, the ID heater power consumption in conventional was 76–95% higher than in newly designed as capacity increased from 3 kW to 9 kW respectively because EWT to ID heater was lower in conventional calorimeter than newly designed. As COP of heat pump increased at fixed heat pump heating capacity, the OD HX capacity also increased but the ID HX capacity remained constant. Therefore the LWT at OD HX decreased because heat absorbed from the secondary fluid at OD HX increased but the LWT at ID HX remains constant. As can be seen in Fig. 6, the EWT to the OD heater decreased in conventional calorimeter as heat pump COP increased. The EWT to the ID refrigerator remained the same as heat pump COP increased since LWT at ID HX remained constant. In the case of newly designed calorimeter, the EWT to the OD heater represented higher values than the conventional calorimeter at all heat pump COP conditions. This was because the needle
valve was adjusted to control the amount of heat transfer in HR HX to maintain the same EWT to the OD heater. The EWT to the ID refrigerator was also decreased. After heat extraction in refrigeration unit, the EWT to the ID heater also decreased but the value was higher in newly designed calorimeter as compared to conventional calorimeter because of small refrigerator capacity, in newly designed calorimeter. Fig. 7 shows the power consumption in both conventional and newly designed calorimeter. The power consumption of OD heater increased as COP increased in order to maintain the setting EWT of 5 °C to the OD HX. The OD heater power consumption was however 54–59% higher in conventional calorimeter than newly designed system as COP decreased from 6 to 3 respectively because EWT to OD heater was lower in conventional calorimeter. The ID refrigerator power was higher in conventional than in newly designed by 80% because of large capacity refrigerator in conventional calorimeter. Also, to maintain the setting EWT of 40 °C to ID HX, ID heater power consumption in newly designed system was lower than conventional because EWT to ID heater increase in newly designed system. Figs. 8 and 9 show performance comparison of newly designed and conventional calorimeter when the refrigerators for systems
16 Test unit conditions Operating mode: Heating o OD HX EWT: 5 C o ID HX EWT: 40 C Capacity: 5 kW
14
Power consumption (kW)
16
12 10
Conventional ID Refrigerator ID Heater OD Heater
Newly designed ID Refrigerator ID Heater OD Heater
8 6 4 2 0 -2 2
3
4
5
6
7
Test unit COP Fig. 7. Power consumption of calorimeter with the variation of heat pump COP in heating mode.
60 60
o
Entering water temperature ( C)
o
Entering water temperature ( C)
50 45 40 35 30 25
Newly designed
Conventional
20 15 10
Test unit conditions Operating mode: Heating Capacity: 5 kW o o OD HX EWT: 5 C ID HX EWT: 40 C
55
Test unit conditions Operating mode: Heating Capacity: 5 kW o o OD HX EWT: 5 C ID HX EWT: 40 C
55
ID Refrigerator
ID Refrigerator
ID Heater
ID Heater
OD Heater
OD Heater
5
50 45 40 35 30 25
Newly designed
Conventional
20
ID Refrigerator
ID Refrigerator
15
ID Heater
ID Heater
10
OD Heater
OD Heater
5 0
0
-5
-5
2 2
3
4
5
6
7
3
4
5
6
7
Test unit COP
Test unit COP Fig. 6. EWT of calorimeter with the variation of heat pump COP in heating mode.
Fig. 8. EWT of calorimeter with the variation of heat pump COP in heating mode (the same refrigerator for the conventional and the newly designed calorimeters).
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16
o
12
Entering water temperature ( C)
Power consumption (kW)
14
10 8
Conventional ID Refrigerator ID Heater OD Heater
6
Test unit conditions Operating mode: Cooling o OD HX EWT: 25 C o ID HX EWT: 12 C COP: 4.0
55
Test unit conditions Operating mode: Heating Capacity: 5 kW o o OD HX EWT: 5 C ID HX EWT: 40 C
Newly designed ID Refrigerator ID Heater OD Heater
4 2 0
50 45 40
Newly designed
Conventional OD Refrigerator
OD Refrigerator
ID Heater
ID Heater
OD Heater
OD Heater
35 30 25 20 15 10 5 0 -5
-2 2
3
4
5
6
1
7
3
Fig. 9. Power consumption of calorimeter with the variation of heat pump COP in heating mode (the same refrigerator for the conventional and the newly designed calorimeters).
were of the same capacity. The OD heater EWT was lower in conventional calorimeter than newly designed system because of heat transfer in HR HX in the case of newly designed system. The EWT to the ID refrigerator was higher in conventional system than newly designed. However since the same rated capacity refrigerator was used, the heat extraction rate in conventional system was similar to it in newly designed calorimeter at almost the same EWT to the refrigerator. Based on that, the EWT to the ID heater in newly designed system was lower than it in conventional system. It can be seen that there was increase in power consumption of OD heater as heat pump COP increased in conventional and newly designed calorimeter. To compare conventional and newly designed, the OD heater power consumption was about 56–63% higher in conventional than in newly designed system because EWT to OD heater was lower in conventional than it in newly designed system at all heat pump COPs. The power consumption of ID refrigerator in conventional system was similar to that in newly designed system, because the temperature difference between refrigerator inlet in conventional and newly designed system was relatively small. The power consumption of ID heater in this case became 18% higher in newly designed system than in conventional system because EWT to the ID heater represented lower values in newly designed system than conventional system.
7
9
11
Fig. 10. EWT of calorimeter with the variation of heat pump capacity in cooling mode.
As heat pump cooling capacity increased, the EWT to OD heater increased. However, it was higher in newly designed than in conventional as heat pump capacity changed from 9 to 3 kW respectively because the newly designed used a small capacity refrigerator as compared with that in conventional system. The power consumption analysis is shown in Fig. 11. The ID heater power increased greatly as capacity increased in the conventional system. This is because more heat input was required as capacity and flow rate were increased in order to maintain the setting EWT of 12 °C to ID HX. The ID heater power was almost the same in newly designed system as capacity increased because EWT to ID heater was slightly lower than the EWT set condition to ID HX as capacity varied from 3 to 9 kW. The ID heater power was 45–90% higher in conventional calorimeter than in newly designed system as capacity varied from 3 to 9 kW respectively. The OD refrigerator power consumption was about 82% higher in conventional system than in newly designed system, because the conventional calorimeter used large capacity refrigerator for heat extraction. The OD heater power decreased as heat pump capacity increased because EWT to OD heater increased as the heat pump capacity increased. As heat pump cooling capacity changed from 3 to 9 kW, the OD heater power consumption was up to 89% higher
16 Test unit conditions Operating mode: Cooling o OD HX EWT: 25 C o ID HX EWT: 12 C COP: 4.0
14
Power consumption (kW)
3.2. Performance in cooling mode In cooling mode, the ID HX absorbed heat from the secondary fluid while the OD HX rejected heat to the secondary fluid. As a result, the LWT increased at OD HX and decreased at ID HX. Performance comparison of the conventional calorimeter and newly designed calorimeter were analyzed according to heat pump capacity in cooling mode. The newly designed calorimeter had a small capacity refrigerator comparing with that of the conventional calorimeter. As shown in Fig. 10, the EWT to the ID heater was almost the same in both conventional and newly designed as heat pump capacity was increased. But it was higher in newly designed calorimeter than it was in conventional system because of heat transfer in HR HX unit from outdoor flow line to indoor flow line to increase the LWT at ID HX exit. The OD refrigerator EWT was higher in conventional calorimeter than newly designed because there was no usage of HR HX in the conventional calorimeter.
5
Test unit capacity (kW)
Test unit COP
12 10
Conventional OD Refrigerator ID Heater OD Heater
Newly designed OD Refrigerator ID Heater OD Heater
8 6 4 2 0 -2 1
3
5
7
9
11
Test unit capacity (kW) Fig. 11. Power consumption of calorimeter with the variation of heat pump capacity in cooling mode.
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in conventional calorimeter than in newly designed system because the EWT of OD heater was lower in conventional than in newly designed calorimeter. Figs. 12 and 13 analyzed the performance of newly designed calorimeter using the same capacity refrigerator in conventional calorimeter and small refrigerator capacity in terms of EWT and power consumption respectively. The EWTs of ID heater in both cases were almost the same because flow control in HR HX unit was adjusted in the same manner by using the needle valves. The EWT to the OD refrigerator was also the same since heat pump capacity and flow rate were the same. However, for the same EWT to OD refrigerator, more heat extraction occurred when the refrigerator capacity was large. Hence, the EWT to the OD heater was lower when large refrigerator capacity was used as compared to small capacity refrigerator. However in both cases, as heat pump capacity was increased, the EWT to the OD heater also increased. The ID heater power consumptions were also almost the same because EWT to ID heater were controlled to be same. OD refrigerator power consumption was about 79% higher for large capacity refrigerator compared to small capacity refrigerator. This was because heat extraction increased when large capacity refrigerator was used. Also, for the OD heater power consumption, it was
60
o
Entering water temperature ( C)
55 Newly designed (Qref = 14.4 kW)
50 45 40
Newly designed (Qref = 3.6 kW)
OD Refrigerator
OD Refrigerator
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ID Heater
OD Heater
OD Heater
35 30 25 20
greatly higher with large capacity refrigerator than with small capacity refrigerator because EWT to the OD heater was lower in large capacity refrigerator. In both cases, as heat pump capacity increased, the power consumption of OD heater also increased because flow rate was also increased. As COP was increased in cooling mode, the OD HX capacity decreased but ID HX capacity remained the same. The heat rejected by the OD HX to the secondary fluid then decreased and that also decreased the LWT at OD HX exit. Performance comparison of the conventional calorimeter and newly designed calorimeter were analyzed according to heat pump capacity in cooling mode. The newly designed calorimeter had a small capacity refrigerator comparing with it of the conventional calorimeter. As can be seen in Fig. 14, the EWT to the ID heater was constant as COP increased since the LWT at ID HX exit remains constant in both conventional and newly designed. The ID heater EWT was higher in newly designed calorimeter because of flow control in HR HX to transfer heat from LWT of OD HX to LWT of ID HX. The EWT of OD refrigerator decreased slightly as COP increased because LWT of OD HX decreased. But the OD refrigerator EWT was higher in conventional calorimeter than newly designed system because of heat transfer in HR HX. The large capacity refrigerator in conventional system also caused an increase in the heat extraction rate and that decreased the EWT to the OD heater in conventional than in newly designed which used a small capacity refrigerator as shown in Fig. 14. As shown in Fig. 15, the power consumption of ID heater remained the same as heat pump COP increased because the LWT of ID HX was the same. Due to higher ID heater EWT in newly designed calorimeter, the ID heater power was higher in conventional calorimeter than in newly designed system by 76%. The OD heater power was much higher in conventional calorimeter than in newly designed system, because EWT was higher in newly designed calorimeter as compared to conventional calorimeter.
15
3.3. Total power consumption analysis
10 5
Test unit conditions
0
Operating mode: Cooling
COP: 4.0
o OD HX EWT: 25 C
o ID HX EWT: 12 C
-5 1
3
5
7
9
11
Test unit capacity (kW) Fig. 12. EWT with the variation of heat pump capacity for newly designed calorimeter.
Total power consumption for the conventional and the newly designed calorimeter were investigated according to heat pump capacity and COP. The newly designed calorimeter had a small capacity refrigerator comparing with that of the conventional calorimeter. As shown in Figs. 16 and 17, the total power consumption in heating mode was more than in cooling mode as capacity and COP of heat pump unit were varied. This was due to high
16 60 Newly designed (Qref = 14.4 kW) OD Refrigerator ID Heater OD Heater
50
10 8
Test unit conditions Operating mode: Cooling COP: 4.0 o o OD HX EWT: 25 C ID HX EWT: 12 C
6
Test unit conditions Operating mode: Cooling o OD HX EWT: 25 C o ID HX EWT: 12 C Capacity: 5 kW
55
o
12
Newly designed (Qref = 3.6 kW) OD Refrigerator ID Heater OD Heater
Entering water temperature ( C)
Power consumption (kW)
14
4 2 0
45 40
Newly designed
Conventional OD Refrigerator
OD Refrigerator
ID Heater
ID Heater
OD Heater
OD Heater
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3
5
7
9
11
Test unit capacity (kW)
-5 2
3
4
5
6
7
Test unit COP Fig. 13. Power consumption with the variation of heat pump capacity for newly designed calorimeter.
Fig. 14. EWT of calorimeter with the variation of heat pump COP in cooling mode.
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14 12 10
Conventional OD Refrigerator ID Heater OD Heater
Newly designed OD Refrigerator ID Heater OD Heater
8 6 4 2 0 -2 2
3
4
5
6
7
Test unit COP Fig. 15. Power consumption of calorimeter with the variation of heat pump COP in cooling mode.
25
Total power consumption (kW)
Test unit conditions o o Heating : OD HX EWT: 5 C ID HX EWT: 40 C o o Cooling : OD HX EWT: 25 C ID HX EWT: 12 C COP: 4.0
20
15
10 Newly designed Heating mode Cooling mode
Conventional Heating mode Cooling mode
5
0
4. Conclusion
-5 1
3
5
7
9
11
Test unit capacity (kW) Fig. 16. Total power consumption of calorimeter according to heat pump capacity.
25 Test unit conditions o o Heating : OD HX EWT: 5 C ID HX EWT: 40 C o o Cooling : OD HX EWT: 25 C ID HX EWT: 12 C Capacity: 5 kW
20
Total power consumption (kW)
EWT test condition to the ID HX in heating mode than in cooling mode. In cooling mode, the refrigerator capacity was reduced because EWT to the refrigeration unit decreased. Therefore, the power consumption of the electric heater at the exit of the refrigerator in cooling mode was lower than in heating mode. The total power consumption of conventional calorimeter showed an increasing trend in both heating and cooling mode as heat pump capacity was increased. However, the total power consumption showed a decreasing trend as heat pump capacity increased in newly designed calorimeter. The total power consumption in conventional calorimeter was about 15–24% higher in heating mode than in cooling mode as heat pump capacity varied from 3 to 9 kW respectively. In newly designed calorimeter, total power consumption in heating mode was about 32% higher than in cooling mode. In heating mode, it was seen that as heat pump capacity increased from 3 to 9 kW, the total power consumption was about 74–87% higher in conventional than in newly designed respectively. Also, in cooling mode, the total power consumption was about 77–89% higher in conventional than in newly designed. Total power consumption of the conventional and newly designed calorimeter showed nearly constant trend in both heating and cooling mode as COP was increased. The total power consumption in conventional calorimeter was about 18% higher in heating mode than in cooling mode as COP varied from 3 to 6. In newly designed calorimeter, total power consumption in heating mode was about 21% higher than in cooling mode. It was seen that the total power consumption was about 78% higher in conventional than in newly designed in heating mode. Also, in cooling mode, the total power consumption was about 80% higher in conventional than in newly designed as COP varied from 3 to 6. The newly designed calorimeter could save much more energy comparing with the conventional system by heat recovery between indoor flow and outdoor flow lines, and the former system could use smaller refrigerator than the latter.
15
10
Newly designed Heating mode Cooling mode
Conventional Heating mode Cooling mode
5
0
-5 2
3
4
5
6
7
Test unit COP Fig. 17. Total power consumption of calorimeter according to heat pump COP.
The heat pump calorimeter is a testing facility used to investigate the performance of a heat pump. To minimize a lot of energy consumption of the calorimeter, a newly designed calorimeter was proposed in this study. In the proposed newly designed calorimeter, a heat recovery heat exchanger is used to connect the indoor side flow loop to the outdoor side flow loop. Experiments were conducted according to the variation of heat pump capacity and COP in both heating and cooling modes for the conventional calorimeter and the proposed newly designed calorimeter. When the refrigerators for the conventional and the newly designed calorimeter were of the same capacity in heating mode, the power consumption of OD heater was higher in conventional system than in newly designed system. However, the power consumption of ID heater represented higher values in newly designed system than in conventional system. Finally, the performance enhancement of the newly designed system was small or negligible. For the cooling mode of heat pump, the power consumption of OD heater was greatly higher in the newly designed calorimeter with large capacity refrigerator than with small capacity refrigerator because EWT to the OD heater was lower in the system with large capacity refrigerator. When the newly designed calorimeter had a small capacity refrigerator comparing with that of the conventional calorimeter, total power consumption increased in conventional calorimeter but there was a decreasing trend in total power consumption of newly designed calorimeter as heat pump capacity increased. Both conventional and newly designed calorimeter showed increasing
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trend in total power consumption as COP increased. The analysis of the test results from the experiment showed that the newly designed calorimeter was able to save about 75% or more of the total power consumption in conventional calorimeter for all heat pump units that were investigated. The newly designed system utilized a heat recovery heat exchanger and as such required a smaller refrigeration unit than the large refrigeration unit in the conventional system. Normally, the refrigeration unit represents higher price than the heat exchanger. As a result, the newly designed system has lower capital cost and operating cost than the conventional system. However, the economic benefits may change according to the system size and operating time. Therefore, future research work would consider a detailed economic analysis of the newly designed system. Acknowledgement This work was supported by the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry& Energy, Republic of Korea (No. 20132020101980). References [1] [2] [3] [4]
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Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
A study on the performance of a newly designed heat pump calorimeter Kofi Owura Amoabeng a, Jong Min Choi b,⇑ a b
Graduate School of Mechanical Engineering, Hanbat National University, Daejeon 34158, South Korea Department of Mechanical Engineering, Hanbat National University, Daejeon 34158, South Korea
h i g h l i g h t s The heat pump calorimeter is the testing facility to investigate the performance of a heat pump. The heat pump calorimeter consumed much more energy than heat pump. The newly designed calorimeter could save energy greatly rather than the conventional system. It is recommended to adopt small refrigerator for maximizing energy saving rate.
a r t i c l e
i n f o
Article history: Received 7 November 2016 Revised 19 April 2017 Accepted 7 May 2017 Available online 16 May 2017 Keywords: Heat pump Calorimeter Energy consumption Entering water temperature Heat recovery
a b s t r a c t The heat pump calorimeter is the testing facility to investigate the performance of a heat pump using standard test conditions obtained from domestic or international standard organizations. Conventionally, the calorimeter consumes so much energy in order to control and maintain the entering water temperature test conditions of the water-to-water heat pump unit. To minimize this energy, a newly designed calorimeter for the water-to-water heat pump was proposed in this study. Experiments were conducted in both heating and cooling modes with variation in capacity and COP of heat pump unit. In heating mode, entering water temperature test conditions of 40 °C and 5 °C were set for the indoor heat exchanger and outdoor heat exchanger of the heat pump unit respectively. In cooling mode, entering water temperature of 25 °C and 12 °C were set for the outdoor heat exchanger and indoor heat exchanger of the heat pump unit respectively. The analysis of the test results from the experiment showed that the newly designed calorimeter was able to save about 75% or more of the total power consumption comparing with the conventional calorimeter for all heat pump capacity and COP that were investigated. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Energy is a key component in the development of modern society. It promotes economic growth and improves the quality of life. The escalation in worldwide population has contributed to the rising energy consumption, and demand levels are estimated to be 45% higher in 2030 than current levels [1]. In the field of heating, ventilation and air conditioning (HVAC) systems, heat pumps offer the most energy-efficient way to provide heating and cooling in many applications [2]. This is because they have the ability to utilize renewable energy sources and also impact the environment positively than the conventional burning of fossil fuels [3]. Several heat pump types are in existence; some require external mechanical work while others require external thermal energy. Commercial heat pumps based on the vapour compression cycle or the ⇑ Corresponding author. E-mail address: [email protected] (J.M. Choi). http://dx.doi.org/10.1016/j.applthermaleng.2017.05.029 1359-4311/Ó 2017 Elsevier Ltd. All rights reserved.
absorption cycle are operational in several applications in various industries. New heat pump technologies such as the adsorption cycle or the chemical reaction cycle are emerging rapidly, even though they have yet to find major industrial applications [4]. Any heat pump, either for domestic or commercial purpose comes with a set of performance data. This data provides information about the heat pump such as reliability and performance to serve as reference point for end users. However, the heat pump should be tested, rated and certified to estimate the performance before it is distributed. The testing and rating of heat pumps are strictly guided and regulated by standard test conditions provided by international and domestic standard organizations [5–7]. Conventionally, the heat pump testing facility makes use of a calorimeter [8], on the basis of energy balance method in rating and certifying the heat pump [5,9]. The ideal function of the heat pump calorimeter is to control and keep the standard test conditions constant during the testing period while serving as the heat sink and heat source for the heat pump. Heat is transferred to
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217
Nomenclature COP EWT HR HX
coefficient of performance entering water temperature heat recovery heat exchanger
the evaporator of the heat pump by the heating system of the calorimeter and absorbed from the condenser of the same heat pump by the refrigeration system of the calorimeter. Hence the energy consumption of the calorimeter which incorporates the energy used by the heater and refrigeration unit is much higher than that of the heat pump itself. Several research studies have focused on ways to improve the performance of heat pump systems. For instance, the incorporation of a heat-driven ejector to the heat pump has improved system efficiency by more than 20% [10–12]. Also, the development of better compressor technology has the potential to reduce energy consumption of heat pump systems by as much as 80% [13–16]. The evolution of new hybrid systems has also enabled the heat pump to perform efficiently with wider applications [17–20]. Renedo et al. [21] studied on the energy efficiency of reversible water-towater heat pumps and proposed a system that reduces the annual energy consumption for any annual thermal demand of the system. Janghoo et al. [22] optimized the design of HVAC systems to minimize primary energy demand. Neksa et al. [23] studied on the characteristics, system design and experimental results of a CO2heat pump water heating system. The energy consumption of the system reduced by 75% when compared to electrical or gas fired systems. Bahri and Kodal [24] proposed appropriate functions consisting of investment and energy consumption costs for endoreversible refrigerators and heat pumps to find optimal design conditions. Moatassem et al. [25] also developed an optimization model to minimize energy consumption and carbon emission of aging buildings by implementing sustainability measures such as efficient HVAC systems and renewable energy systems. However, due to the high energy consumption of the heat pump calorimeter, it is also of great significance to adopt efficient methods to monitor and control the energy in order to improve system performance. Rodolfo et al. [26] concluded that, the performance of a calorimeter can be improved by using a predictor-based controller known as dead-time compensator after analyzing a model data and a real experimental result. Schroder et al. [27] developed a flow calorimeter which is able to eradicate the guess-work used in determining the specific heat capacity of geothermal water flow. Research work on calorimeter for testing heat pumps is very rare in literature. This study focuses on calorimeter for testing the performance of a water-to-water heat pump unit. The conventional calorimeter is used to test the performance of a water-towater heat pump unit. A newly designed calorimeter is then proposed to execute the same tests. The purpose is to reduce the energy used in the conventional calorimeter. The energy consumption analysis of the newly designed calorimeter is then compared to that of the conventional calorimeter.
2. Experimental setup and test procedure 2.1. Experimental setup The calorimeter set up in this study is a testing facility used to investigate the performance of a water-to-water heat pump unit as shown schematically in Fig. 1. Conventionally, the heat pump calorimeter has two compartments; the indoor side and outdoor
ID HX LWT OD HX
indoor heat exchanger leaving water temperature outdoor heat exchanger
side. Each side of the calorimeter is controlled separately because there is no connection between the indoor and outdoor heat exchanger of the heat pump. Two secondary fluid loops, one for each side, are used to connect the calorimeter to the heat pump unit. One fluid loop connects the indoor side of the calorimeter to the indoor heat exchanger (ID HX) of the heat pump. The other loop connects the outdoor heat exchanger (OD HX) of the heat pump to the outdoor side of the calorimeter. The secondary fluid used in the experiment is mainly water with a 40% concentration of ethylene glycol solution. Each side of the calorimeter is equipped with a constant temperature bath (CTB), a refrigeration unit and electric heater. The refrigeration unit and electric heater are used to set up and maintain the temperature of the secondary fluid located inside the CTB based on standard test conditions [5]. The setting temperature of the secondary fluid in the CTB is used as the entering water temperature (EWT) to the heat exchanger of the heat pump test unit. There is also a volumetric flow meter and circulation pump on each side of the calorimeter as shown in Fig. 1. The flow meter is used to maintain the flow rate of the secondary fluid according to the capacity of the heat pump unit while the circulation pump is used to transport the secondary fluid from the CTB through the heat pump heat exchanger to the refrigerator, electric heater and back to the CTB. In the proposed newly designed calorimeter, a heat recovery heat exchanger (HR HX) unit is used to connect the indoor side flow loop to the outdoor side flow loop as shown in Fig. 1. Hence heat can be transferred from one side of the calorimeter to the other side. As the heat pump capacity change, the COP also change. It is therefore impossible to analyze the calorimeter’s performance on experimental basis by using heat pumps with different capacities and COPs. Hence a heat pump simulator is developed to imitate the variation of heat pump capacity and COP separately. The actual heat pump unit consists of an ID HX, OD HX, a compressor, an expansion valve and a four-way valve as in Fig. 1. However, the heat pump simulator as shown in Fig. 2 consists of two plate type heat exchangers; ID HX and OD HX, two constant temperature water baths and two circulation pumps. The capacities of the ID HX and OD HX are controlled separately by using the temperature and flow rate of each heat pump constant temperature water bath line connected to the ID HX and OD HX. The constant temperature water bath line is used to replace the refrigerant line in an actual heat pump unit. The simulator can realize the change of COP and capacity separately. The experimental rig as shown in Fig. 3 which consist of the heat pump simulator section and the calorimeter section is therefore used for testing the performance of a water-to-water heat pump unit using standard test conditions based on Refs. [28,29] in both conventional and newly designed heat pump calorimeters. The two needle valves located on the indoor side flow line are used to control the amount of heat transfer rate in the HRHX between the indoor side flow and outdoor side flow loops. Temperature measuring devices such as resistance thermal detectors (RTDs) and T-type thermocouple sensors are used to measure temperatures in the experimental set up. The T-type thermocouple and RTD sensors have uncertainty of ±0.1 °C of reading scale. The volumetric flow meters that are used to measure the flow rate of the secondary fluid also have uncertainty of ±0.1% of full scale.
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Calorimeter outdoor loop
Calo rimeter indoor loop
Refrigeraon unit
Refrigeraon unit
Electric heater
Constant Temperature Bath
Electric Heater
Heat recovery HX
Flow meter
Constant Temperature Bath
Flow meter
4 way valve Compressor Outdoor HX
Circulaon pump
Indoor HX
Heat pump unit S
Circulaon pump
Expansion valve
Fig. 1. Schematic representation of calorimeter for water-to-water heat pump unit.
Heat pump Constant Temp. Water Bath
Outdoor HX Pump 2
Refrigerator
Heat pump Constant Temp. Water Bath
Indoor HX Pump 4
Heater
Heater
Refrigerator
Fig. 2. Heat pump simulator.
The temperatures are monitored at the selected locations according to ASHRAE standard 41.1 [30]. The temperatures and flow rates of the secondary fluid at relevant locations in the experimental set up are the measured parameters used to estimate the heating and cooling capacities of heat pump unit with the equation;
Q¼
V q Cp DT 60; 000
ð1Þ
where Q represent the cooling or heating capacity of the heat pump in kW, V is the secondary fluid flow rate in liters per minute (LPM), q is the density of the secondary fluid in kg/m3, Cp is the heat capacity of the secondary fluid in kJ/kg°C and DT is the secondary fluid temperature difference across the heat pump heat exchanger in °C. The power consumption of refrigerator and electric heater are measured by power meter with an uncertainty of ±0.01% of reading scale. The specifications of the various components in the experimental set up are shown in Table 1. 2.2. Test procedure The focus of the study was to investigate the performance of the conventional calorimeter and the proposed newly designed calorimeter for a water-to-water heat pump unit using the same set of standard operating test conditions. Experiments were conducted in both heating mode and cooling mode for the heat pump unit. Heat pump units were tested in both the conventional and
newly designed calorimeter using the same rated capacity for the refrigeration unit. Afterwards, the same tests were performed with the newly designed calorimeter but with a small refrigerator capacity as shown in Table 1. Table 2 shows the test conditions used for the investigation. To execute experiment in heating mode, the EWT to the ID HX and OD HX were set to the standard conditions of 40 °C and 5 °C in the indoor and outdoor CTB respectively. In using the conventional calorimeter, the needle valve 1 at the indoor flow line close to the inlet to the HR HX was fully closed while needle valve 2 was fully opened. Hence, the secondary fluid at the indoor flow loop does not flow through the HR HX. Therefore there was no flow connection between flow lines on either side of the calorimeter. In the outdoor flow loop, the secondary fluid from CTB at the outdoor side of the calorimeter flowed through the OD HX. The OD HX absorbed heat from the secondary fluid. As a result, secondary fluid leaving water temperature (LWT) at exit of the OD HX decreased. The low temperature secondary fluid from exit of OD HX then proceeded continuously through the HR HX to the outdoor refrigerator (OD refrigerator) and to the outdoor electric heater (OD heater) back to the CTB at the outdoor side of the calorimeter. The OD refrigerator was turned off in the process because the temperature of the secondary fluid at inlet to OD refrigerator was lower than the setting temperature in the outdoor CTB. The OD heater was then adjusted to control and maintain the secondary fluid EWT condition of 5 °C to the OD HX in the CTB. For
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Calorimeter
Condenser 1
Condenser 2
Compressor 2
Exp. Valve 1
S
S
Compressor 1
OD Refrigerator
ID Refrigerator
Evaporator 1
Evaporator 2
Exp. Valve 2
Ball Valve
OD Heater
ID Heater Heat recovery HX Needle valve 2
Calorimeter Constant Temp. Bath
Needle valve 1
Pump 1
Flow meter 1
Heat pump Constant Temp. Water Bath
Outdoor HX
Flow meter 2
Heat pump Constant Temp. Water Bath
Indoor HX
Pump 2
Refrigerator
Pump 3
Calorimeter Constant Temp. Bath
Pump 4
Heater
Heater
Refrigerator
Heat pump simulator Fig. 3. Schematic representation of the experimental test rig.
the indoor flow loop, the secondary fluid from CTB at the indoor side of the calorimeter flowed through the ID HX. The secondary fluid absorbed heat in the ID HX. As a result, secondary fluid leaving water temperature (LWT) at exit of the ID HX increased. The high temperature secondary fluid from exit of ID HX then proceeded continuously through the HR HX bypass flow line to the indoor refrigerator (ID refrigerator) and to the indoor electric heater (ID heater) back to the CTB at the indoor side of the calorimeter. The ID refrigerator was turned on in the process to extract heat from the secondary fluid because the temperature of the secondary fluid at inlet to ID refrigerator was higher than the setting temperature in the indoor CTB. The ID heater was further adjusted to control and maintain the secondary fluid EWT condition of 40 °C to the ID HX in the CTB. In using the proposed newly designed calorimeter, the needle valve 1 was adjusted to allow secondary fluid from exit of ID HX to flow through HR HX unit. Heat was then transferred from the high temperature secondary fluid at exit of ID HX to the low temperature secondary fluid at exit of OD HX. This increased the secondary fluid temperature at the outdoor flow line as it flowed from the HR HX through the OD refrigerator to the OD heater and back to the CTB. The amount of OD heater power needed to adjust and maintain the secondary fluid EWT condition of 5 °C in the outdoor CTB then reduced greatly. Also, the amount of heat extracted in the ID refrigerator reduced since secondary fluid temperature at inlet to the ID refrigerator decreased in the process of heat recovery in the HR HX. The amount of heat recovery in the HR HX varied as the heat pump capacity and COP varied.
Similarly, to execute experiment in cooling mode, the EWT to the ID HX and OD HX were set to the standard conditions of 12 °C and 25 °C in the indoor and outdoor CTB respectively. In the conventional mode, the needle valve 1 at the indoor flow line close to the inlet to the HR HX was fully closed while needle valve 2 was fully opened. Hence, the secondary fluid at the indoor flow loop fully bypassed the HR HX. Hence no flow connection between indoor and outdoor line on either side of the calorimeter. The secondary fluid from CTB at the indoor side of the calorimeter flowed through the ID HX. The ID HX absorbed heat from the secondary fluid. As a result, secondary fluid LWT at exit of the ID HX decreased. The secondary fluid at low temperature from exit of ID HX then proceeded continuously through the HR HX bypass flow line to the ID refrigerator and then to the ID heater back to the CTB at the indoor side of the calorimeter. The ID refrigerator was turned off in the process because the temperature of the secondary fluid at inlet to ID refrigerator was lower than the setting temperature in the indoor CTB. The ID heater was therefore adjusted to control and maintain the secondary fluid EWT condition of 12 °C to the ID HX in the CTB. However, in the outdoor flow loop, the secondary fluid from CTB at outdoor side of the calorimeter absorbed heat as it flowed through the OD HX. As a result, leaving water temperature (LWT) of the secondary fluid at exit of the OD HX increased. The secondary fluid at high temperature from exit of OD HX then proceeded continuously through the HR HX to the OD refrigerator and then to the OD heater back to the CTB at the outdoor side of the calorimeter. The OD refrigerator was turned on in the process to extract heat from the secondary fluid
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Table 1 Specification of components in test rig. Component
Specification
stable conditions, 30 mins performance data was recorded every second with a data acquisition system and average values from the data were used to investigate heat pump performance. The power consumption of the refrigerator and electric heaters were recorded by using a power meter.
Circulation pump
3-phase motor
Rated frequency: 60 Hz
Volumetric flow meter
Magnetic type
Flow range: 0–40 LPM Uncertainty: ±0.1% of full scale
Electric heater
Flange heater
Rated power: 12 kW
3. Results and discussion
Thermocouple
T-type
Range: 200 °C to 40 °C Uncertainty: ±0.1 °C of reading scale
3.1. Performance in heating mode
RTD sensor
A-class-type
Range: 200 °C to 600 °C Uncertainty: ±0.1 °C of reading scale
Power meter
3 phase/3 wire
Voltage range: 0–600 V Current range: 0–20 A Uncertainty: ±0.01% of reading scale
ID/OD heat exchanger
Plate type
Rated capacity: 10.5 kW
Refrigerator
Scroll compressor (constant speed)
Condenser
Refrigerant: R410 A Rated voltage: 400 V Rated frequency: 50 Hz Rated capacity: 14.4 kW (conventional/newly designed) Rated capacity: 3.6 kW (newly designed) Electronic Expansion Valve (EEV) Input voltage: 12 VDC ± 10% Rated capacity: 20 kW
Plate type
Rated capacity: 10.5 kW
Heat recovery HX
Table 2 Test conditions. Parameter
Heating mode
EWT for ID HX EWT for OD HX Heat pump capacity (secondary fluid flow rate) Heat pump COP
40 °C 12 °C 5 °C 25 °C 3 kW (7.29 LPM), 5 kW (12.14 LPM) 7 kW (17.00 LPM), 9 kW (21.86 LPM) 3, 4, 5, 6
Cooling mode
because the temperature of the secondary fluid at inlet to OD refrigerator was higher than the setting temperature in the outdoor CTB. The OD heater was further adjusted to control and maintain the secondary fluid EWT condition of 25 °C to the OD HX in the CTB. In the newly designed, there was a similar test procedure as in heating mode. The needle valve 1 was adjusted to allow the secondary fluid from exit of ID HX to flow through HR HX unit. Heat was then transferred from the high temperature secondary fluid at exit of OD HX to the low temperature secondary fluid at exit of ID HX. This increased the secondary fluid temperature at the indoor flow line as it flowed from the HR HX through the ID refrigerator to the ID heater and back to the CTB. The amount of ID heater power required to adjust and maintain the secondary fluid EWT condition of 12 °C in the indoor then reduced considerably. Again, the amount of heat extracted in the OD refrigerator also reduced since secondary fluid temperature at inlet to the OD refrigerator decreased as a result of the HR HX. The same secondary fluid flow rates were set for the two flow lines that connect the calorimeter to the ID HX and OD HX of the heat pump unit according to the capacity of the heat pump unit. The refrigerator of each flow line had higher capacity by 20% than the maximum heat extraction rate of the heat pump or the maximum difference of heat amount between indoor and outdoor heat exchanger in the heat pump. Steady state conditions were attained when temperatures and flow rates were within ±0.1 °C and ±0.1 LPM respectively. After
60 Test unit conditions Operating mode: Heating COP: 4.0 o o OD HX EWT: 5 C ID HX EWT: 40 C
55 50 o
Expansion valve
Entering water temperature ( C)
Evaporator
Performance comparison of the conventional calorimeter and newly designed calorimeter were analyzed according to heat pump capacity in heating mode. The newly designed calorimeter had a small capacity refrigerator comparing with that of the conventional calorimeter. As shown in Fig. 4, for the conventional calorimeter, the EWT to the OD heater was lower than the setting EWT to OD HX, because OD HX absorbed heat from the secondary fluid. However, the EWT to the OD heater remained the same according to an increment of heat pump capacity, because heat pump capacity and flow rate increased. The EWT to the ID refrigerator was higher than the setting EWT to ID HX, because heat was absorbed by the secondary fluid from ID HX. The EWT to the ID heater decreased because there was heat extraction in the ID refrigerator. As the heat pump heating capacity increased, the EWT to the ID heater also increased. In the case of the newly designed calorimeter, the EWT value to the OD heater was higher than that of the conventional calorimeter as a result of heat transfer from indoor flow line to outdoor flow line. This reduced the EWT to the ID refrigerator. The EWT to the ID heater also decreased due to heat extraction in the refrigeration unit located at the indoor flow loop of calorimeter. However, the EWT to ID heater was higher in newly designed calorimeter than in conventional system, because the newly designed calorimeter had a small capacity refrigerator. As the heat pump capacity changes in heating mode, the power consumption of the OD heater, ID refrigerator and ID heater for both conventional and newly designed calorimeter are shown in Fig. 5. The OD heater power increased as heat pump capacity increased. This is because more heater power was required as heat pump capacity and flow rate were increased in order to maintain EWT of 5 °C to OD HX. But the increment was 63–76% higher in conventional calorimeter than in newly designed calorimeter as capacity increased from 3 kW to 9 kW respectively because EWT to OD heater of the former was lower than the latter. The ID refrig-
45 40 35 30 25 20
Newly designed
Conventional
15 10
ID Refrigerator
ID Refrigerator
ID Heater
ID Heater
OD Heater
OD Heater
5 0 -5 1
3
5
7
9
11
Test unit capacity (kW) Fig. 4. EWT of calorimeter with the variation of heat pump capacity in heating mode.
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Test unit conditions Operating mode: Heating o OD HX EWT: 5 C o ID HX EWT: 40 C COP: 4.0
Power consumption (kW)
14 12 10
Conventional ID Refrigerator ID Heater OD Heater
Newly designed ID Refrigerator ID Heater OD Heater
8 6 4 2 0 -2 1
3
5
7
9
11
Test unit capacity (kW) Fig. 5. Power consumption of calorimeter the variation of heat pump capacity in heating mode.
erator power consumption changed slightly as heat pump capacity increased because the refrigeration unit adopted a constant speed compressor. But the value was about 82% higher in conventional than in newly designed because of large refrigerator capacity in conventional calorimeter. Therefore to control and maintain EWT of 40 °C to the heat pump ID HX, the ID heater was adjusted. The ID heater power consumption decreased as capacity increased because EWT to ID heater increased as heat pump capacity increased. However, the ID heater power consumption in conventional was 76–95% higher than in newly designed as capacity increased from 3 kW to 9 kW respectively because EWT to ID heater was lower in conventional calorimeter than newly designed. As COP of heat pump increased at fixed heat pump heating capacity, the OD HX capacity also increased but the ID HX capacity remained constant. Therefore the LWT at OD HX decreased because heat absorbed from the secondary fluid at OD HX increased but the LWT at ID HX remains constant. As can be seen in Fig. 6, the EWT to the OD heater decreased in conventional calorimeter as heat pump COP increased. The EWT to the ID refrigerator remained the same as heat pump COP increased since LWT at ID HX remained constant. In the case of newly designed calorimeter, the EWT to the OD heater represented higher values than the conventional calorimeter at all heat pump COP conditions. This was because the needle
valve was adjusted to control the amount of heat transfer in HR HX to maintain the same EWT to the OD heater. The EWT to the ID refrigerator was also decreased. After heat extraction in refrigeration unit, the EWT to the ID heater also decreased but the value was higher in newly designed calorimeter as compared to conventional calorimeter because of small refrigerator capacity, in newly designed calorimeter. Fig. 7 shows the power consumption in both conventional and newly designed calorimeter. The power consumption of OD heater increased as COP increased in order to maintain the setting EWT of 5 °C to the OD HX. The OD heater power consumption was however 54–59% higher in conventional calorimeter than newly designed system as COP decreased from 6 to 3 respectively because EWT to OD heater was lower in conventional calorimeter. The ID refrigerator power was higher in conventional than in newly designed by 80% because of large capacity refrigerator in conventional calorimeter. Also, to maintain the setting EWT of 40 °C to ID HX, ID heater power consumption in newly designed system was lower than conventional because EWT to ID heater increase in newly designed system. Figs. 8 and 9 show performance comparison of newly designed and conventional calorimeter when the refrigerators for systems
16 Test unit conditions Operating mode: Heating o OD HX EWT: 5 C o ID HX EWT: 40 C Capacity: 5 kW
14
Power consumption (kW)
16
12 10
Conventional ID Refrigerator ID Heater OD Heater
Newly designed ID Refrigerator ID Heater OD Heater
8 6 4 2 0 -2 2
3
4
5
6
7
Test unit COP Fig. 7. Power consumption of calorimeter with the variation of heat pump COP in heating mode.
60 60
o
Entering water temperature ( C)
o
Entering water temperature ( C)
50 45 40 35 30 25
Newly designed
Conventional
20 15 10
Test unit conditions Operating mode: Heating Capacity: 5 kW o o OD HX EWT: 5 C ID HX EWT: 40 C
55
Test unit conditions Operating mode: Heating Capacity: 5 kW o o OD HX EWT: 5 C ID HX EWT: 40 C
55
ID Refrigerator
ID Refrigerator
ID Heater
ID Heater
OD Heater
OD Heater
5
50 45 40 35 30 25
Newly designed
Conventional
20
ID Refrigerator
ID Refrigerator
15
ID Heater
ID Heater
10
OD Heater
OD Heater
5 0
0
-5
-5
2 2
3
4
5
6
7
3
4
5
6
7
Test unit COP
Test unit COP Fig. 6. EWT of calorimeter with the variation of heat pump COP in heating mode.
Fig. 8. EWT of calorimeter with the variation of heat pump COP in heating mode (the same refrigerator for the conventional and the newly designed calorimeters).
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16
o
12
Entering water temperature ( C)
Power consumption (kW)
14
10 8
Conventional ID Refrigerator ID Heater OD Heater
6
Test unit conditions Operating mode: Cooling o OD HX EWT: 25 C o ID HX EWT: 12 C COP: 4.0
55
Test unit conditions Operating mode: Heating Capacity: 5 kW o o OD HX EWT: 5 C ID HX EWT: 40 C
Newly designed ID Refrigerator ID Heater OD Heater
4 2 0
50 45 40
Newly designed
Conventional OD Refrigerator
OD Refrigerator
ID Heater
ID Heater
OD Heater
OD Heater
35 30 25 20 15 10 5 0 -5
-2 2
3
4
5
6
1
7
3
Fig. 9. Power consumption of calorimeter with the variation of heat pump COP in heating mode (the same refrigerator for the conventional and the newly designed calorimeters).
were of the same capacity. The OD heater EWT was lower in conventional calorimeter than newly designed system because of heat transfer in HR HX in the case of newly designed system. The EWT to the ID refrigerator was higher in conventional system than newly designed. However since the same rated capacity refrigerator was used, the heat extraction rate in conventional system was similar to it in newly designed calorimeter at almost the same EWT to the refrigerator. Based on that, the EWT to the ID heater in newly designed system was lower than it in conventional system. It can be seen that there was increase in power consumption of OD heater as heat pump COP increased in conventional and newly designed calorimeter. To compare conventional and newly designed, the OD heater power consumption was about 56–63% higher in conventional than in newly designed system because EWT to OD heater was lower in conventional than it in newly designed system at all heat pump COPs. The power consumption of ID refrigerator in conventional system was similar to that in newly designed system, because the temperature difference between refrigerator inlet in conventional and newly designed system was relatively small. The power consumption of ID heater in this case became 18% higher in newly designed system than in conventional system because EWT to the ID heater represented lower values in newly designed system than conventional system.
7
9
11
Fig. 10. EWT of calorimeter with the variation of heat pump capacity in cooling mode.
As heat pump cooling capacity increased, the EWT to OD heater increased. However, it was higher in newly designed than in conventional as heat pump capacity changed from 9 to 3 kW respectively because the newly designed used a small capacity refrigerator as compared with that in conventional system. The power consumption analysis is shown in Fig. 11. The ID heater power increased greatly as capacity increased in the conventional system. This is because more heat input was required as capacity and flow rate were increased in order to maintain the setting EWT of 12 °C to ID HX. The ID heater power was almost the same in newly designed system as capacity increased because EWT to ID heater was slightly lower than the EWT set condition to ID HX as capacity varied from 3 to 9 kW. The ID heater power was 45–90% higher in conventional calorimeter than in newly designed system as capacity varied from 3 to 9 kW respectively. The OD refrigerator power consumption was about 82% higher in conventional system than in newly designed system, because the conventional calorimeter used large capacity refrigerator for heat extraction. The OD heater power decreased as heat pump capacity increased because EWT to OD heater increased as the heat pump capacity increased. As heat pump cooling capacity changed from 3 to 9 kW, the OD heater power consumption was up to 89% higher
16 Test unit conditions Operating mode: Cooling o OD HX EWT: 25 C o ID HX EWT: 12 C COP: 4.0
14
Power consumption (kW)
3.2. Performance in cooling mode In cooling mode, the ID HX absorbed heat from the secondary fluid while the OD HX rejected heat to the secondary fluid. As a result, the LWT increased at OD HX and decreased at ID HX. Performance comparison of the conventional calorimeter and newly designed calorimeter were analyzed according to heat pump capacity in cooling mode. The newly designed calorimeter had a small capacity refrigerator comparing with that of the conventional calorimeter. As shown in Fig. 10, the EWT to the ID heater was almost the same in both conventional and newly designed as heat pump capacity was increased. But it was higher in newly designed calorimeter than it was in conventional system because of heat transfer in HR HX unit from outdoor flow line to indoor flow line to increase the LWT at ID HX exit. The OD refrigerator EWT was higher in conventional calorimeter than newly designed because there was no usage of HR HX in the conventional calorimeter.
5
Test unit capacity (kW)
Test unit COP
12 10
Conventional OD Refrigerator ID Heater OD Heater
Newly designed OD Refrigerator ID Heater OD Heater
8 6 4 2 0 -2 1
3
5
7
9
11
Test unit capacity (kW) Fig. 11. Power consumption of calorimeter with the variation of heat pump capacity in cooling mode.
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in conventional calorimeter than in newly designed system because the EWT of OD heater was lower in conventional than in newly designed calorimeter. Figs. 12 and 13 analyzed the performance of newly designed calorimeter using the same capacity refrigerator in conventional calorimeter and small refrigerator capacity in terms of EWT and power consumption respectively. The EWTs of ID heater in both cases were almost the same because flow control in HR HX unit was adjusted in the same manner by using the needle valves. The EWT to the OD refrigerator was also the same since heat pump capacity and flow rate were the same. However, for the same EWT to OD refrigerator, more heat extraction occurred when the refrigerator capacity was large. Hence, the EWT to the OD heater was lower when large refrigerator capacity was used as compared to small capacity refrigerator. However in both cases, as heat pump capacity was increased, the EWT to the OD heater also increased. The ID heater power consumptions were also almost the same because EWT to ID heater were controlled to be same. OD refrigerator power consumption was about 79% higher for large capacity refrigerator compared to small capacity refrigerator. This was because heat extraction increased when large capacity refrigerator was used. Also, for the OD heater power consumption, it was
60
o
Entering water temperature ( C)
55 Newly designed (Qref = 14.4 kW)
50 45 40
Newly designed (Qref = 3.6 kW)
OD Refrigerator
OD Refrigerator
ID Heater
ID Heater
OD Heater
OD Heater
35 30 25 20
greatly higher with large capacity refrigerator than with small capacity refrigerator because EWT to the OD heater was lower in large capacity refrigerator. In both cases, as heat pump capacity increased, the power consumption of OD heater also increased because flow rate was also increased. As COP was increased in cooling mode, the OD HX capacity decreased but ID HX capacity remained the same. The heat rejected by the OD HX to the secondary fluid then decreased and that also decreased the LWT at OD HX exit. Performance comparison of the conventional calorimeter and newly designed calorimeter were analyzed according to heat pump capacity in cooling mode. The newly designed calorimeter had a small capacity refrigerator comparing with it of the conventional calorimeter. As can be seen in Fig. 14, the EWT to the ID heater was constant as COP increased since the LWT at ID HX exit remains constant in both conventional and newly designed. The ID heater EWT was higher in newly designed calorimeter because of flow control in HR HX to transfer heat from LWT of OD HX to LWT of ID HX. The EWT of OD refrigerator decreased slightly as COP increased because LWT of OD HX decreased. But the OD refrigerator EWT was higher in conventional calorimeter than newly designed system because of heat transfer in HR HX. The large capacity refrigerator in conventional system also caused an increase in the heat extraction rate and that decreased the EWT to the OD heater in conventional than in newly designed which used a small capacity refrigerator as shown in Fig. 14. As shown in Fig. 15, the power consumption of ID heater remained the same as heat pump COP increased because the LWT of ID HX was the same. Due to higher ID heater EWT in newly designed calorimeter, the ID heater power was higher in conventional calorimeter than in newly designed system by 76%. The OD heater power was much higher in conventional calorimeter than in newly designed system, because EWT was higher in newly designed calorimeter as compared to conventional calorimeter.
15
3.3. Total power consumption analysis
10 5
Test unit conditions
0
Operating mode: Cooling
COP: 4.0
o OD HX EWT: 25 C
o ID HX EWT: 12 C
-5 1
3
5
7
9
11
Test unit capacity (kW) Fig. 12. EWT with the variation of heat pump capacity for newly designed calorimeter.
Total power consumption for the conventional and the newly designed calorimeter were investigated according to heat pump capacity and COP. The newly designed calorimeter had a small capacity refrigerator comparing with that of the conventional calorimeter. As shown in Figs. 16 and 17, the total power consumption in heating mode was more than in cooling mode as capacity and COP of heat pump unit were varied. This was due to high
16 60 Newly designed (Qref = 14.4 kW) OD Refrigerator ID Heater OD Heater
50
10 8
Test unit conditions Operating mode: Cooling COP: 4.0 o o OD HX EWT: 25 C ID HX EWT: 12 C
6
Test unit conditions Operating mode: Cooling o OD HX EWT: 25 C o ID HX EWT: 12 C Capacity: 5 kW
55
o
12
Newly designed (Qref = 3.6 kW) OD Refrigerator ID Heater OD Heater
Entering water temperature ( C)
Power consumption (kW)
14
4 2 0
45 40
Newly designed
Conventional OD Refrigerator
OD Refrigerator
ID Heater
ID Heater
OD Heater
OD Heater
35 30 25 20 15 10 5 0
-2 1
3
5
7
9
11
Test unit capacity (kW)
-5 2
3
4
5
6
7
Test unit COP Fig. 13. Power consumption with the variation of heat pump capacity for newly designed calorimeter.
Fig. 14. EWT of calorimeter with the variation of heat pump COP in cooling mode.
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K.O. Amoabeng, J.M. Choi / Applied Thermal Engineering 123 (2017) 216–225 16 Test unit conditions Operating mode: Cooling o OD HX EWT: 25 C o ID HX EWT: 12 C Capacity: 5 kW
Power consumption (kW)
14 12 10
Conventional OD Refrigerator ID Heater OD Heater
Newly designed OD Refrigerator ID Heater OD Heater
8 6 4 2 0 -2 2
3
4
5
6
7
Test unit COP Fig. 15. Power consumption of calorimeter with the variation of heat pump COP in cooling mode.
25
Total power consumption (kW)
Test unit conditions o o Heating : OD HX EWT: 5 C ID HX EWT: 40 C o o Cooling : OD HX EWT: 25 C ID HX EWT: 12 C COP: 4.0
20
15
10 Newly designed Heating mode Cooling mode
Conventional Heating mode Cooling mode
5
0
4. Conclusion
-5 1
3
5
7
9
11
Test unit capacity (kW) Fig. 16. Total power consumption of calorimeter according to heat pump capacity.
25 Test unit conditions o o Heating : OD HX EWT: 5 C ID HX EWT: 40 C o o Cooling : OD HX EWT: 25 C ID HX EWT: 12 C Capacity: 5 kW
20
Total power consumption (kW)
EWT test condition to the ID HX in heating mode than in cooling mode. In cooling mode, the refrigerator capacity was reduced because EWT to the refrigeration unit decreased. Therefore, the power consumption of the electric heater at the exit of the refrigerator in cooling mode was lower than in heating mode. The total power consumption of conventional calorimeter showed an increasing trend in both heating and cooling mode as heat pump capacity was increased. However, the total power consumption showed a decreasing trend as heat pump capacity increased in newly designed calorimeter. The total power consumption in conventional calorimeter was about 15–24% higher in heating mode than in cooling mode as heat pump capacity varied from 3 to 9 kW respectively. In newly designed calorimeter, total power consumption in heating mode was about 32% higher than in cooling mode. In heating mode, it was seen that as heat pump capacity increased from 3 to 9 kW, the total power consumption was about 74–87% higher in conventional than in newly designed respectively. Also, in cooling mode, the total power consumption was about 77–89% higher in conventional than in newly designed. Total power consumption of the conventional and newly designed calorimeter showed nearly constant trend in both heating and cooling mode as COP was increased. The total power consumption in conventional calorimeter was about 18% higher in heating mode than in cooling mode as COP varied from 3 to 6. In newly designed calorimeter, total power consumption in heating mode was about 21% higher than in cooling mode. It was seen that the total power consumption was about 78% higher in conventional than in newly designed in heating mode. Also, in cooling mode, the total power consumption was about 80% higher in conventional than in newly designed as COP varied from 3 to 6. The newly designed calorimeter could save much more energy comparing with the conventional system by heat recovery between indoor flow and outdoor flow lines, and the former system could use smaller refrigerator than the latter.
15
10
Newly designed Heating mode Cooling mode
Conventional Heating mode Cooling mode
5
0
-5 2
3
4
5
6
7
Test unit COP Fig. 17. Total power consumption of calorimeter according to heat pump COP.
The heat pump calorimeter is a testing facility used to investigate the performance of a heat pump. To minimize a lot of energy consumption of the calorimeter, a newly designed calorimeter was proposed in this study. In the proposed newly designed calorimeter, a heat recovery heat exchanger is used to connect the indoor side flow loop to the outdoor side flow loop. Experiments were conducted according to the variation of heat pump capacity and COP in both heating and cooling modes for the conventional calorimeter and the proposed newly designed calorimeter. When the refrigerators for the conventional and the newly designed calorimeter were of the same capacity in heating mode, the power consumption of OD heater was higher in conventional system than in newly designed system. However, the power consumption of ID heater represented higher values in newly designed system than in conventional system. Finally, the performance enhancement of the newly designed system was small or negligible. For the cooling mode of heat pump, the power consumption of OD heater was greatly higher in the newly designed calorimeter with large capacity refrigerator than with small capacity refrigerator because EWT to the OD heater was lower in the system with large capacity refrigerator. When the newly designed calorimeter had a small capacity refrigerator comparing with that of the conventional calorimeter, total power consumption increased in conventional calorimeter but there was a decreasing trend in total power consumption of newly designed calorimeter as heat pump capacity increased. Both conventional and newly designed calorimeter showed increasing
K.O. Amoabeng, J.M. Choi / Applied Thermal Engineering 123 (2017) 216–225
trend in total power consumption as COP increased. The analysis of the test results from the experiment showed that the newly designed calorimeter was able to save about 75% or more of the total power consumption in conventional calorimeter for all heat pump units that were investigated. The newly designed system utilized a heat recovery heat exchanger and as such required a smaller refrigeration unit than the large refrigeration unit in the conventional system. Normally, the refrigeration unit represents higher price than the heat exchanger. As a result, the newly designed system has lower capital cost and operating cost than the conventional system. However, the economic benefits may change according to the system size and operating time. Therefore, future research work would consider a detailed economic analysis of the newly designed system. Acknowledgement This work was supported by the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry& Energy, Republic of Korea (No. 20132020101980). References [1] [2] [3] [4]
[5]
[6] [7] [8]
[9]
[10]
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