Energy and Exergy Analysis of Transcritical CO2 Cycles for Heat Pump Applications
Abstract
:1. Introduction
- The reference cycle and also the most important cycle modifications are analyzed;
- The optimal operating conditions are more exhaustively explained by means of a comprehensive exergy analysis including the entire cycle, not just the irreversibility in the gas cooler;
- The performance is assessed with reference to a constant heat demand, while in the literature, the compressor displacement is often taken as a constant, and taking into account a wide range of possible rated operating conditions (with regard to the temperature profile of the heat sink and the ambient temperature).
2. Methods
2.1. General Assumptions
2.2. Reference Cycle
2.2.1. Thermodynamic States
- Evaporation temperature , with the corresponding pressure ;
- Superheating at the compressor inlet ;
- Gas cooler pressure ;
- Temperature at the gas cooler exit .
2.2.2. Cycle Performance
2.2.3. Design Parameters
- 1.
- : rate of heat transferred in the gas cooler;
- 2.
- : water inlet temperature at the gas cooler;
- 3.
- : water temperature glide in the gas cooler;
- 4.
- : minimum temperature difference between CO2 and water in the gas cooler;
- 5.
- : air inlet temperature in the evaporator;
- 6.
- : air temperature decrease in the evaporator;
- 7.
- : minimum temperature difference between air and CO2 in the evaporator;
- 8.
- : superheating at the compressor inlet;
- 9.
- : compressor overall efficiency;
- 10.
- : compressor mechanical efficiency;
- 11.
- : gas cooler pressure.
2.2.4. Constrained Optimization
2.3. Cycle with Internal Heat Exchanger
2.4. Parallel Compression
2.5. Cycle with Ejector
3. Results and Discussion
3.1. Influence of Gas Cooler Pressure (Cycle Optimization)
3.2. Optimized Cycles: Influence of End-User Temperature Profile and Ambient Temperature
3.2.1. Reference Cycle
3.2.2. Cycle Modifications
4. Conclusions
- 1.
- The temperature profile of the end user determines the number and locations of pinch points in the gas cooler for the maximum COP configuration: for relatively low inlet temperature and high temperature glides, the optimal pressure is the one that gives rise to two pinch points, one at the cold end and another inside the gas cooler (2PP configurations), while for relatively high inlet temperatures and low temperature glides, the optimal pressure is higher than the one that results in two pinch points, and in the optimal configuration, there is only one pinch point in the gas cooler, located at the cold end (1PP configurations).
- 2.
- The transition from the 2PP to the 1PP region mainly depends on the average water temperature and the temperature glide, and is explained by the exergy destruction that takes place not only in the gas cooler but also in the valve.
- 3.
- The optimal gas cooler pressure increases with the average water temperature but at different rates in the 2PP and 1PP regions. In particular, the rate of increase is much higher for 1PP configurations.
- 4.
- The flow rate of CO2 changes with the average water temperature in different ways in the 2PP and 1PP regions: it increases in the former and decreases in the latter. Furthermore, it decreases with the ambient air temperature.
- 5.
- The cycle with IHX and ejector is the most efficient among those analyzed: with respect to the reference cycle, the COP increases by 13.3–26.1% and the second-law efficiency by 21.0–37.6%, depending on the operating conditions.
Author Contributions
Funding
Data Availability Statement
Conflicts of Interest
Abbreviations
1PP | Configurations with optimal pressure resulting in one gas cooler pinch point |
2PP | Configurations with optimal pressure resulting in two gas cooler pinch point |
ASHP | Air Source Heat Pump |
COP | Coefficient Of Performance |
HTF | Heat Transfer Fluid |
HP | Heat Pump |
IHX | Internal Heat Exchanger |
PC | Parallel Compression |
Appendix A. Optimized Thermodynamic Cycles
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Parameter | Range | Parameter | Value | Parameter | Value |
---|---|---|---|---|---|
20–50 °C | 100 | 0.85 | |||
20–60 K | 3 | 0.85 | |||
−20–0 °C | 3 | 0.80 | |||
3 | 0.95 | ||||
5 | |||||
0.90 | |||||
Equation (4) |
Case | |||
---|---|---|---|
A | 20 | 50 | 0 |
B | 40 | 30 | 0 |
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Gambini, M.; Manno, M.; Vellini, M. Energy and Exergy Analysis of Transcritical CO2 Cycles for Heat Pump Applications. Sustainability 2024, 16, 7511. https://doi.org/10.3390/su16177511
Gambini M, Manno M, Vellini M. Energy and Exergy Analysis of Transcritical CO2 Cycles for Heat Pump Applications. Sustainability. 2024; 16(17):7511. https://doi.org/10.3390/su16177511
Chicago/Turabian StyleGambini, Marco, Michele Manno, and Michela Vellini. 2024. "Energy and Exergy Analysis of Transcritical CO2 Cycles for Heat Pump Applications" Sustainability 16, no. 17: 7511. https://doi.org/10.3390/su16177511