4 Experimental Study
4.7 Data analysing methods
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The calculated air flows including the outdoor airflow, recirculation airflow and other airflows of the CAHP operated in different seasons and different cities are listed in Table 4.15 and Table 4.16.
According to the EU standard for ventilation [6], the fresh air in the CAHP system was designed to be 4L/s per person.
Table 4.15 Airflow rates of CAHP for different cities in summer
Cities Recirculation air (L/s)
Outdoor air to test room (L/s)
Regeneration air (L/s)
Air for excess heat (L/s)
Exhaust air from room (L/s)
Copenhagen 190 60 125 120 60
Milano 190 60 125 120 60
Colombo 190 40 115 130 40
Table 4.16 Airflow rates of CAHP for different cities in winter
Cities Recirculation air (L/s)
Outdoor air to test room (L/s)
Regeneration air (L/s)
Exhaust air from room (L/s)
Copenhagen 190 60 95 60
Milan 190 60 95 60
After the calculation, the experiments were conducted using the designed conditions. The measured data were then analyzed comparing with conventional heating or air-conditioning systems.
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Figure 4.8 Schematic diagram of the reference air source heat pump system operated in summer mode
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Figure 4.9 Schematic diagram of the reference air source heat pump system operated in winter mode
The indoor thermal loads of the test room are same for both the CAHP and the conventional air source heat pump systems. The COPs of the conventional air source heat pump were calculated with different condensing and evaporating temperatures in different classes of outdoor climates.
During the COP calculations, the condensing and evaporating temperatures of the conventional air source heat pump were calculated referring to the measured air and refrigerant temperature
difference in the CAHP. The calculating methods are shown in the following equations.
) ( con CAHP con outletCAHP
ref outlet con ref
con T T T
T (31) )
( eva CAHP eva outlet CAHP
ref outlet eva ref
eva T T T
T (32) Where:
ref
Tcon ,Tevaref are the calculated condensing and evaporating temperature of the reference air source heat pump, °C;
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ref outlet
Tcon ,Tevaoutletref are the designed air temperature at the outlet point of the condenser and evaporator of the reference air source heat pump, °C;
CAHP
Tcon ,TevaCAHP are the measured condensing temperature and evaporating temperature of the heat pump in the CAHP system, °C;
CAHP outlet
Tcon ,TevaoutletCAHP are the measure air temperature at the outlet point of the condenser and evaporator of the heat pump in the CAHP system, °C;
The entropy efficiency of the compressor in the reference air source heat pump system was assigned referring to the measured entropy efficiency of the compressor in the CAHP.
Besides the energy use comparisons with reference air source heat pump, the CAHP is futher compared with another reference system which use air source heat pump in summer but use gas boiler provide heating in winter. Due to that the energy sources of the CAHP and gas boiler are different, the comparions of CAHP with gas boiler in winter mode is based on the primary energy use. During the calculation, the overall efficiency of boiler and heat exchangers was assumed to be 81%. The primary energy factors of natural gas and power in Copenhagen and Milan was
investigated and listed in Table 4.17 [114].
Table 4.17 Primary energy factors of power and natural gas in Copenhagen and Milan Copenhagen (Denmark) Milan (Italy)
Natural Gas Power Natural Gas Power
1 2.5 1 2.18
To ensure the comparisons between the CAHP and the reference systems impartial enough, the comparisons were made based on both systems providing same indoor air quality and same indoor thermal environment in the test room. Since the CAHP has a very strong ability on indoor air
cleaning, the airflow rates of outdoor air and indoor recirculation air are quite different in the CAHP and the reference systems. Based on the previous study [88], 80% of indoor recirculated air is cleaned by the silica gel rotor and can be used to substitute for outdoor air.
The flow rate of outdoor air in the reference system should be equivalent to the flow rate of clean air delivered into the test room by the CAHP system and was calculated with the following equation.
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CAHP r CAHP
f ref
f V V
V (33) Where:
ref
Vfre is the outdoor fresh airflow rate in the reference system, l/ ; s
CAHP
Vfre is the fresh airflow rate in the CAHP system, and here it was taken to be 4L/s per person;
is the air cleaning efficiency of the silica gel rotor. Referring to the study of Fang et al. [88], the value was taken to be 0.8;
CAHP
Vrec is the recirculation airflow rate in the CAHP system, l/ . s
Thus, the outdoor airflow rates and the recirculation airflow rates of CAHP and the reference system were calculated as shown in Table 4.18.
Table 4.18 Outdoor and recirculation airflow rates of CAHP and reference system
Cities Outdoor air (L/s) Recirculation air (L/s) CAHP reference system CAHP reference system
summer
Copenhagen 60 212 190 38
Milan 60 212 190 38
Colombo 40 192 190 38
winter
Copenhagen 60 212 190 38
Milan 60 212 190 38
With the airflow rates in the reference system (Table 4.18) and the outdoor climate conditions listed in Table 4.8-Table 4.12, the hygrothermal load and the energy use of reference system can be calculated.
With the calculated energy use of reference systems and the measured energy use of the CAHP, the energy saving potential of the CAHP was thus estimated. Overall, the comparisons were made based on both systems providing the same indoor air quality and indoor thermal environment in the same test room. The hygrothermal conditions of the supply air delivered to the ventilated room in the reference system were equal to that in the CAHP system. The energy uses of the proposed
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CAHP were measured with experimental studies. The energy uses of the reference conventional systems were calculated. During the calculation, the indoor air, outdoor air and supply air thermal conditions in the reference systems were designed referring to the measured values from the experimental study of the CAHP. In other words, the thermal parameters of indoor air, outdoor air and supply air in the two systems were equal.
After the measurements and calculations of energy use of the CAHP and the reference system, the energy saving ratio of the CAHP was calculated with the following equation.
referece CAHP
referece S
E (E E )/E
(34)
Where:
ES
is the energy saving potential of the CAHP;
ECAHP is the energy use of CAHP, kW;
referece
E is the energy use of the reference system, kW.