Investigation of the Impact Factors on the Optimal Intermediate Temperature in a Dual Transcritical CO2 System with a Dedicated Transcritical CO2 Subcooler
Abstract
:1. Introduction
2. System Description
2.1. System Configuration
2.2. Theoretical Analysis
3. Mathematical Model
3.1. The Seeking Process of Optimal Discharge Pressures
3.2. The Simulation Process of the Dual System’s Thermodynamic Performance
4. Results Discussion
4.1. The Verification of the Simulation Model
4.2. Discussion about the Intermediate Temperature
4.3. Predictive Correlation
4.4. Regulation of the Intermediate Temperature
5. Conclusions
- (i)
- After comparison with the measurement data from our previous works, the veracity of the simulation models could be verified, which gives us the convenience to carry on the most discussion based on only the simulation results.
- (ii)
- The existence of optimal intermediate temperature is validated, while optimal values of the intermediate temperature increase with an increase in ambient temperature and water-feed temperature. Subsequently, the system COP also increases with an increase in ambient temperature, but decreases with the water-feed temperature.
- (iii)
- Due to the existence of the optimal intermediate temperature, the system performance rises first and then declines, while the auxiliary cycle’s performance rises gradually with the increase in intermediate temperature. As for the main cycle’s performance, no obvious rule could be observed.
- (iv)
- By using the dual gas cooler (the heat rejection of the auxiliary and main cycles is installed inside a same gas cooler), the negative effects of the pinch point on the heat transfer inside the heat exchanger could be greatly reduced.
- (v)
- A predictive correlation for the optimal intermediate temperature determination, with the ambient and water-feed temperature as the independent variables, is proposed, while the relative prediction errors are no more than 5% across 18 working conditions.
Author Contributions
Funding
Conflicts of Interest
Nomenclature
A | Heat-transfer area (m2) |
Specific heat capacity (kJ∙kg−1∙K−1) | |
d | Diameter (m) |
f | Friction factor |
h | Enthalpy (kJ∙kg−1) |
m | Mass flow rate (kg∙s−1) |
p | Pressure (MPa) |
Pr | Prandtl number |
Q | Heat-transfer rate (kW) |
R | Fouling resistance (K∙m2∙W−1) |
Re | Reynolds number |
T | Temperature (℃) |
Power consumption (kW) | |
Convective heat-transfer coefficient (W∙K-1∙m-2) | |
Thickness of the fin (m) | |
γ | heat leakage coefficient |
Efficiency | |
Conductivity (W∙K-1∙m-2) | |
Density (kg∙m-3) | |
Dehumidification coefficient | |
Subscripts | |
av | Average |
air | air |
comp | Compressor |
eq | Equivalent |
eva | Evaporator |
f | feed |
gc | Gas cooler |
h | Heating |
i | Inner |
in | System inlet |
is | Isentropic |
out | System outlet |
o | Outer |
r | Refrigerant |
single | Single phase |
tube | tube |
v | Volumetric |
w | Water |
wall | Tube wall |
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Components | Detailed Models | |
---|---|---|
CO2 compression model [27] | Each time step was divided into: 1) the isentropic process with constant mass (1-2*); 2) the inner change with constant volume (2*-2). | |
(1) | ||
(2) | ||
(3) | ||
(4) | ||
(5) | ||
CO2 cooling model [29,30] | (6) | |
(7) | ||
(8) | ||
(9) | ||
CO2 evaporating model [31] | (10) | |
(11) | ||
(12) | ||
(13) | ||
(14) | ||
(15) | ||
(16) | ||
Air flow through the fine-tube heat exchanger [32] | (17) | |
(18) | ||
Circulating water through the gas cooler [32] | (19) | |
(20) |
Main Components | Type | Characteristics | |
---|---|---|---|
Main cycle’s compressor | (Panasonic) DC-inventor rotary compressor | Displacement: 8.0 ml/rev | |
Auxiliary cycle’s compressor | (Haili) DC-inverter rotary compressor | Displacement: 3.26 ml/rev | |
Dual gas cooler | Tube-in-tube | Outside tube | Φ22 × 2.45 mm |
Galvanized steel tube | |||
Inside tube | Φ6.35 × 0.7 mm × 2 tubes | ||
Φ7 × 1 mm × 1 tubes | |||
Copper tube | |||
Length | 17.6 m | ||
Main cycle’s evaporator | Wavy-finned tube | Tube | Φ7 × 1 mm |
Number of rows | 2 | ||
Number of tubes per row | 40 | ||
Tube length | 1.5 m | ||
Internal heat exchanger | Plate heat exchanger | Primary channel volume | 0.549 L |
Secondary channel volume | 0.610 L | ||
Material | Channel plate: stainless steel | ||
Brazing: Copper |
Water-Feed Temperature/°C | Water-Supply Temperature/°C | Ambient Temperature/°C | Sought Value of Optimal Intermediate Temperature/°C | Calculated Value of Optimal Intermediate Temperature/°C | Relative Error/% |
---|---|---|---|---|---|
40 | 50 | −20 | 14.74 | 15.30 | 3.65 |
40 | 50 | −10 | 19.55 | 19.65 | 0.51 |
40 | 50 | 0 | 23.53 | 24.00 | 1.97 |
45 | 55 | −20 | 16.99 | 16.70 | −1.72 |
45 | 55 | −10 | 20.84 | 21.05 | 1.01 |
45 | 55 | 0 | 26.41 | 25.40 | −3.97 |
50 | 60 | −20 | 18.65 | 18.10 | −3.07 |
50 | 60 | −10 | 22.79 | 22.45 | −1.51 |
50 | 60 | 0 | 26.78 | 26.80 | 0.07 |
40 | 60 | −20 | 15.87 | 15.30 | −3.71 |
40 | 60 | −10 | 20.07 | 19.65 | −2.12 |
40 | 60 | 0 | 24.31 | 24.00 | −1.29 |
45 | 65 | −20 | 17.09 | 16.70 | −2.33 |
45 | 65 | −10 | 21.02 | 21.05 | 0.16 |
45 | 65 | 0 | 26.36 | 25.40 | −3.80 |
50 | 70 | −20 | 18.72 | 18.10 | −3.42 |
50 | 70 | −10 | 22.73 | 22.45 | −1.24 |
50 | 70 | 0 | 25.56 | 26.80 | 4.65 |
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Song, Y.; Wang, H.; Cao, F. Investigation of the Impact Factors on the Optimal Intermediate Temperature in a Dual Transcritical CO2 System with a Dedicated Transcritical CO2 Subcooler. Energies 2020, 13, 309. https://doi.org/10.3390/en13020309
Song Y, Wang H, Cao F. Investigation of the Impact Factors on the Optimal Intermediate Temperature in a Dual Transcritical CO2 System with a Dedicated Transcritical CO2 Subcooler. Energies. 2020; 13(2):309. https://doi.org/10.3390/en13020309
Chicago/Turabian StyleSong, Yulong, Haidan Wang, and Feng Cao. 2020. "Investigation of the Impact Factors on the Optimal Intermediate Temperature in a Dual Transcritical CO2 System with a Dedicated Transcritical CO2 Subcooler" Energies 13, no. 2: 309. https://doi.org/10.3390/en13020309