performance evaluation of co2 heat pump heating system o

PERFORMANCE EVALUATION
OF CO2 HEAT PUMP HEATING SYSTEM
O. KUWABARA(a), M. KOBAYASHI(a), H. MUKAIYAMA(a)
M. ISHIKAWA(b), K. MUKUNOKI(b), N. HONMA(b)
(a)
R&D Headquarters, Sanyo Electric Co., Ltd.
1-1-1, Sakata, Oizumi-Machi, Ora-Gun, Gunma, 370-0596, JAPAN
Fax: 81-276-61-8896, e-mail: o-kuwabara@rd.sanyo.co.jp
(b)
Hokkaido Electric Power Co., Inc.
2-1, Tsuishikari, Ebetsu-City, Hokkaido, 067-0033, JAPAN
Fax: 81-011-385-6558
ABSTRACT
Carbon dioxide as refrigerant is especially suitable for heat pump cycles with high heat rejecting
temperature such as heating boilers and water heaters. In Japan, heating is one of the major parts of
residential energy consumption, so an efficient heating system could reduce much energy
consumption. We evaluated the heating capacity and COP of CO2 heat pump heat generator, which is
based on a heat pump water heater unit. The average COP of CO2 heat pump heating system was
calculated by using the climate conditions and heating load data. The effect of an additional incoming
air heat exchanger was also evaluated, which improved the system COP by lowering the heat-transfer
fluid temperature.
Conclusion: A CO2 heat pump heating system can reduce the CO2 emission in comparison with
an oil boiler.
1. INTRODUCTION
Carbon dioxide is now being watched with interest as an environmental solution for various heat
pump applications because it is non-flammable and non-toxic. CO2 refrigerant is especially suitable
for the heat pump cycle with high heat rejecting temperature because of its low critical temperature.
Therefore, the CO2 heat pump water heater was commercialized and became popular in Japan. In
Japan, heating is a major part (approx. 28%) of residential energy consumption, which is almost as
much as the hot water supply (approx. 27%). The energy consumption of heating is especially high in
cold areas, so an efficient heating system can reduce much energy consumption. CO2 heat pump
heating system may replace conventional oil boilers or electric thermal storage heaters because its
capacity and efficiency is high even at very low ambient temperature, and also because its heat
rejecting temperature is enough high for a radiator panel. We evaluated the heating capacity and the
COP of CO2 heat pump heat generator based on the heat pump water heater unit for cold areas. The
average COP of CO2 heat pump heating system was calculated on a climate condition and a heating
load. The effect of an additional incoming air heat exchanger was also evaluated, which recovered the
heat loss by ventilation and improved the system COP by lowering the heat-transfer fluid
temperature.
7th IIR Gustav Lorentzen Conference on Natural Working Fluids, Trondheim, Norway, May 28-31, 2006
2. EXPERIMENTAL SETUP
Figure 1 shows the schematic diagram of the CO2 heat pump heating system. The heat pump
space heating system consists of two parts: the heat generator unit which is placed outside of the
building and the panel radiator placed inside. The hot heat-transfer fluid (ethylene glycol based water
solution) warms the room by being circulated through the heat generator and panel radiator. The heat
pump cycle reject much heat and require less input with low heat-transfer fluid temperature returned
from the heating device. A fan convector or a large panel radiator can warm the room and lower the
heat-transfer fluid temperature.
Panel Radiator
Pump
Heat Generator Unit
Panel Radiator
Incoming Air Heat Exchanger
Figure 1. Schematic Diagram of the CO2 Heat Pump Heating System
SLHX
T
Evaporator
T P
T
Accumulator
Expansion
Valve
T
T
Thermostatic
Water Supply
Defrost
Valve
T P
T P
2-Stage Compressor
Gas
Cooler
Heat Pump Heat Generator Unit
T
T
Thermo Couple
P
Pressure Sensor
Variable Environment Climate Chamber
Figure 2. Experimental Setup of Heat Generator Unit
Figure 2 shows the experimental setup of the CO2 heat pump cycle. The refrigeration circuit
consists of a 2-stage compressor, gas cooler, expansion valve and an evaporator. The refrigerant
discharged from compressor passes the gas cooler and heat the heat-transfer fluid, reduces pressure at
7th IIR Gustav Lorentzen Conference on Natural Working Fluids, Trondheim, Norway, May 28-31, 2006
expansion valve, evaporates at evaporator, goes back to compressor via the accumulator. The heat
generator unit is placed in a variable climate environmental chamber. The Temperature and the
humidity of evaporator inlet air are changed. Humidity is controlled when the inlet air temperature is
7°C or higher. When the inlet air temperature is 0°C or lower, the humidifier is stopped to prevent the
moisture from freezing on the evaporator coil.
In this setup, water is used as heat-transfer fluid. Water is provided to the gas cooler by a
thermostatic water supply, and a valve is controlled to keep the flow rate constant. The gas cooler has
counter-flow channels for the water and the refrigerant. The compressor speed and the expansion
valve are controlled to keep the heating capacity constant. The compressor speed is controlled by a
variable frequency inverter drive. The expansion valve is driven by a stepping motor, and is set
manually. The upper limits of the Compressor speed, discharge pressure, discharge temperature and
the inverter current set limit to the operating condition.
The heating capacity is calculated from the temperature difference between the inlet water and
the outlet water of the gas cooler, and its flow rate. The input power, which is the sum of the
compressor, evaporator fan and the controller board, is measured for the heat generator unit. The COP
is calculated using the input power of the heat generator unit. The suction, intermediate and discharge
pressures of the refrigerant are measured with pressure sensors. The air, water and refrigerant
temperatures are measured with thermocouples.
The additional incoming air heat exchanger, which recovered the heat loss by ventilation, is
evaluated. The heat-transfer fluid returned from panel radiator heats the incoming air from outdoors
at the heat exchanger. The heating capacity of the heat exchanger is calculated from the temperature
difference between the water inlet and outlet temperatures, and the water flow rate. The water flow
rate and the air flow rate are set to certain points.
3. COMPONENTS AND TEST CONDITIONS
The specifications of the heat generator unit and its components are shown in Table 1. All of the
components are the same as the heat generator unit of water heater system, the heating capacity of
which is 6.0kW.
The air inlet temperature was varied from -20 to 7°C. The heating capacity was set to the
maximum, and set to 4.5kW when the maximum capacity was higher than 4.5kW. The evaporator fan
speed was fixed at 650 RPM (1600m3/h). The water inlet and outlet were set at various temperatures
between 30 and 50°C. The amount of the refrigerant charge was 860g.
The air flow rate of the incoming air heat exchanger was set to 150 and 200m3/h, and water flow
rate was varied between 2 and 4L/min. The temperature difference between inlet water and inlet air
was varied between 40 and 70K. These test conditions are summarized in Table 2.
Table l. Specifications of the Heat Generator Unit
Cylinder Volume (1st/2nd)
Rated Output
Fin Tube Type Evaporator
Face area (W*H)
Dimensions (W*H*D)
Weight
2-Stage Rotary Compressor
3.9/2.5cm3
1300W
810*630mm
840*690*290mm
65kg
7th IIR Gustav Lorentzen Conference on Natural Working Fluids, Trondheim, Norway, May 28-31, 2006
Table 2. Test Conditions
Heat Generator Unit
Temperature
Evaporator Inlet Air
Flow Rate
Gas Cooler Inlet Water Temperature
Gas Cooler Outlet Water Temperature
Heating Capacity
Incoming Air Heat Exchanger
Temperature
Inlet Air
Flow Rate
Temperature
Inlet Water
Flow Rate
-20, -10, 0, 7°C
1600m3/h
30-50°C
45-75°C
4.5kW and the maximum
-10, 0°C
150, 200m3/h
40, 50, 60°C
2, 3, 4L/min
4. SIMULATION CONDITIONS
The annual electric power consumption was calculated as follows.
It is assumed that the temperature of the panel radiator inlet water is set according to the ambient
temperature. The hourly heating load of a house was calculated based on the ambient temperature
data with a simulation program, taking the heat gain from the sun and the heat loss of ventilation. The
temperature of the panel radiator outlet water or the incoming air heat exchanger outlet water was
calculated from the temperature of the panel radiator inlet water and the heating load. The COP was
evaluated approximately from the ambient temperature and the gas cooler outlet water temperature.
The electric power consumption was then calculated from the COP and heating load. The simulation
was carried out based on the climate data of Sapporo, a city located northern in Japan.
The house is detached and average area. The room temperature was set to 20°C. The total
heating capacity of the panel radiators was 8.6kW at temperature difference 35K. The panel size is
approximately 60% larger than that arranged for temperature difference 50K, so that the heat-transfer
fluid temperature returned from the panel radiator can be lower than conventional central heating
system. The COP and the heating capacity fell at low ambient temperature. If the heating capacity
was lower than the heating load, the electric heater (COP=1) compensated. The incoming heat
exchanger was placed at the intake of the ventilation, and operated at the heating load higher than
3.0kW. Simulation conditions are summarized in Table 3.
Table 3. Simulation Conditions
Climate in Sapporo
(Hourly Climate Data by
SHASE)
House Data
Latitude
Heating Period Average
Temperature
Minimum Temperature
Floor area
Isolation(Q-value)
Ventilation
Heating and Cooling Load Simulation Program
43°N
2.3°C
-13.2°C
140m2
1.5W/m2/K
150m3/h
SMASH
(Distributed by IBEC)
7th IIR Gustav Lorentzen Conference on Natural Working Fluids, Trondheim, Norway, May 28-31, 2006
5. RESULTS AND DISCUSSION
3.1. Test Results and Discussion
The higher gas cooler inlet water temperature or the lower inlet air temperature decreased the
COP and the maximum heating capacity while the gas cooler outlet water temperature had minimal
effects. The maximum heating capacities were 3.43kW (inlet air: -20°C, inlet/outlet water: 40/55°C),
and 6.57kW (inlet air: 7°C, inlet/outlet water: 40/55°C). The heating capacity and the COP are shown
in Figure 3 and 4.
7.0
Maximum add Fixed Capacities (4.5kW)
Heating Capacity [ kW]
6.0
for Each Gas Cooler Inlet / Outlet Temperature
5.0
4.0
3.0
30/ 45°C
40/ 55°C
50/ 65°C
40/ 55°C max. capacity
2.0
1.0
30/ 55°C
40/ 65°C
50/ 75°C
0.0
- 25
- 20
- 15
- 10
-5
0
5
10
5
10
Evaporator Inlet Air Temp. [ °C]
3.5
COPs for Each Gas Cooler Inlet / Outlet Temperature
3.0
COP[ - ]
2.5
2.0
1.5
1.0
30/ 45°C
40/ 55°C
50/ 65°C
40/ 55°C max. capacity
0.5
30/ 55°C
40/ 65°C
50/ 75°C
0.0
- 25
- 20
- 15
- 10
-5
0
Evaporator Inlet Air Temp. [ °C]
Figure 3 and 4. Heating Capacity and COP of Heat Generator Unit
The temperature differences between the inlet air and the evaporating refrigerant varied between
4 and 8K. There was a suction line heat exchanger, but the superheat varied from -2 to 10K. The upper
limit of discharge temperature set the limit to opening of expansion valve at inlet air temperature
lower than -10°C. The discharge pressure varied between 8.5 and 13.5MPa. The higher the gas cooler
inlet water, the higher the discharge pressure. The temperature differences between the gas cooler
outlet water and the gas cooler inlet refrigerant varied between 42 and 70K. The gas cooler outlet
7th IIR Gustav Lorentzen Conference on Natural Working Fluids, Trondheim, Norway, May 28-31, 2006
water temperature can be higher with even COP, because the temperature difference is large enough.
The diagram of the CO2 heat pump cycle is shown in Figure 5.
The capacity of the incoming air heat exchanger was increased with high water flow rate or high
air flow rate. The temperature differences between the outlet air and the inlet water varied between
0.8 and 6.2K. The temperature difference was minimal, because the water-side heat capacity was
much larger than that of the air-side, and because the heat-transfer performance of the heat exchanger
was high enough.
16
Gascooler
Outlet
14
49°C
Pressure [ MPa]
12
<- - - - - -
Compressor
Discharge
Rejected Heat
----->
39°C
125°C
110°C
10
8
6
4
2
0
500
40/ 65°C
50/ 65°C
Water Temperature Inlet: 40°C, 50°C Outlet: 65°C
550
600
650
700
Enthalpy [ kJ/ kg]
750
800
850
Figure 5. P-h Diagram of the CO2 Heat Pump Cycle
3.2. Simulation Results and Discussion
The sum of heating load for heating period (from Oct. 1st to Apr. 30th) was 10,918kWh, and the
peak heating load was 5.68kW. The sum of input power for heating period was 6,513kWh and the
average COP was 1.68, without the incoming air heat exchanger. The sum of the electric heater input
was 793kWh (12% of input). The incoming air heat exchanger lowered the heat-transfer fluid
temperature returned from the panel radiator between 13 and 22K. The sum of input power for
heating period was 5,996kWh and the average COP was 1.82, with incoming air heat exchanger. The
sum of the electric heater input was 558kWh (9% of input).
The heating capacity of the heat generator unit was lower than the heating load at inlet air
temperature lower than approx. -2°C, however, the minimum heating capacity of the electric heater to
compensate the shortage of the heating capacity was approx. 2.8kW. The incoming air heat exchanger
made the gas cooler inlet water temperature low, and improved the average COP of the heat generator
unit from 1.77 to 1.91. It is important for a high average COP to make the gas cooler inlet water
temperature low. Radiator devices of high heating capacity with even low temperature heat-transfer
fluid are necessary - large panel radiators, fan convector, for example. It is also important to suppress
the noise and draft felling from the radiator devices.
The sum of CO2 emissions from the energy consumption of the CO2 heat pump heating system
and the emissions from other heat generators were calculated by using the average CO2 emission
intensity in Japan. The comparison is shown in Table 4. A CO2 heat pump heating system can reduce
the CO2 emission in comparison with an oil boiler. The reduction of the CO2 emission is 366kgCO2
/year (12%) without incoming air heat exchanger, and is 584kgCO2 /year (19%) with it.
7th IIR Gustav Lorentzen Conference on Natural Working Fluids, Trondheim, Norway, May 28-31, 2006
Table 4. CO2 Emissions
Heat Generator
or Heating System
Fuel / Input
Intensity
[kgCO2/kWh]
Boiler
Efficiency
CO2 Emission
[kg/Year]
Ratio to
Oil Boiler
CO2 HP with
Incoming Air
Heat Exchanger
Electricity
CO2 HP without
Incoming Air
Heat Exchanger
←
Kerosene
LPG
0.421
←
0.242
0.217
-
-
85%
←
2,524
2,742
3,108
2,787
81%
88%
100%
90%
Boiler
6. CONCLUSIONS
The heating capacity of a CO2 heat pump heating system is enough high for a house in cold areas
in Japan. An electric heater of 3kW can compensate the shortage and annual average COP drops
minimally. The heat generator unit for a CO2 heat pump water heater system of 6.0kW can be used for
a heating system. A panel radiator of the same type which is used for conventional central heating
system can be used, however, high heating capacity with even low heat-transfer fluid temperature is
necessary for high COP. A CO2 heat pump heating system can reduce the CO2 emission in
comparison with an oil boiler. The incoming air heat exchanger let the gas cooler inlet water
temperature low, and improves the average COP of the heating system.
The performance data of the heat generator unit with split cycle is going to be tested. The split
cycle is consists of a 2-stage compressor, additional internal heat exchanger and an additional
expansion valve. The heating capacity and the COP at low ambient temperature will improve with
this cycle.
7th IIR Gustav Lorentzen Conference on Natural Working Fluids, Trondheim, Norway, May 28-31, 2006
7th IIR Gustav Lorentzen Conference on Natural Working Fluids, Trondheim, Norway, May 28-31, 2006
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