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ABSTRACT
The use of hydrofluorocarbons as refrigerants is reduced because of the increasing greenhouse effect. Air refrigeration system is environmentally friendly with air as working medium. In this study, a reverse-bootstrap air refrigeration cycle was proposed for fresh air-conditioning, which eliminated the hot heat exchanger to reduce volume and weight, and a full fresh air refrigerator was developed and tested in a psychrometric test room. Combining the characteristic curves of a motor-driven turboexpander–compressor (MTEC), a numerical model was established to analyze system off-design performance. The relative deviations between the predictions and experimental data were within 10%. The test and calculation were conducted at standard conditions (the outdoor and indoor dry and wet bulb temperature is 35/24°C and 27/19°C, respectively). The MTEC operating at design rotating speed 38 krpm could supply fresh air with a flow rate of 517.4 kg/h and temperature of 12.0°C, and a cooling capacity of 2.93 kW was obtained with a system Coefficient of performance (COP) of 0.315. The optimal COP and corresponding rotating speed will change with the variation of environmental parameters. As the outdoor temperature increases from 32 to 38°C, the optimal COP decreases by 29.6% and the corresponding rotating speed increases from 39.6 to 47.6 krpm. Moreover, when the outdoor relative humidity increases from 35% to 55% and the indoor temperature increases from 23 to 31°C, the COP decreases by 36.8% and increases by 31.9%, respectively. The advanced exergy analysis indicate the system avoidable exergy destructions account for 54.9%. Under the avoidable conditions, the efficiency of compressor, expander, and motor is 0.85, 0.85, and 0.95, respectively, the system COP can reach 0.946. The exergy destruction analysis of components indicates the importance to optimize the compressor.
Nomenclature
| A | = | Heat transfer area (m2) |
| AH | = | Humidity ratio (g·kg−1) |
| cp | = | Specific heat (J·kg−1·K−1) |
| Dh | = | Hydraulic diameter (m) |
| e | = | Specific exergy (J·kg−1) |
| ED | = | Exergy destruction(W) |
| f | = | Friction factor |
| g | = | Gravitational acceleration (m·s−2) |
| h | = | Convective heat transfer coefficient (W·m−2·K−1) |
| H | = | Fin height (m) |
| j | = | Heat transfer factor |
| k | = | Heat transfer coefficient (W·m−2·K−1) |
| l | = | Cutting length (m) |
| L | = | Latent heat of water (J·kg−1) |
| m | = | Mass flow-rate (kg·s−1) |
| Mr | = | Molecular weight (g·mol−1) |
| Nu | = | Nusselt number |
| P | = | Pressure (kPa) |
| Pr | = | Prandtl number |
| Q | = | Cooling capacity (W) |
| Qhe | = | Heat transfer rate (W) |
| Re | = | Reynolds number |
| s | = | Fin spacing (m) |
| t | = | Plate thickness (m) |
| T | = | Temperature (K) |
| w | = | Condensed water mass (g·kg−1) |
| W | = | Work (W) |
| Greek | = | |
| ΔP | = | Pressure loss (kPa) |
| Δtm | = | Logarithmic mean temperature difference(K) |
| η | = | Efficiency |
| ηm | = | Transmission efficiency |
| κ | = | Specific heat ratio |
| λ | = | Thermal conductivity (W·m−1·K−1) |
| μ | = | Dynamic viscosity (Pa·s) |
| ρ | = | Destiny (kg·m−3) |
| Subscript | = | |
| 1 | = | Expander inlet/Indoor |
| 2 | = | Expander outlet |
| 3 | = | Compressor inlet |
| 4 | = | Compressor outlet |
| 5 | = | Heat exchanger hot side inlet/Indoor |
| 6 | = | Heat exchanger hot side outlet |
| Al | = | Aluminum |
| AV | = | Avoidable |
| c | = | Compressor |
| e | = | Expander |
| EN | = | Endogenous |
| EX | = | Exogenous |
| s | = | Saturation state |
| UN | = | Unavoidable |
| Abbreviations | = | |
| ACM | = | Air cycle machine |
| CFD | = | Computational fluid dynamics |
| COP | = | Coefficient of performance |
| DOAS | = | Dedicated outdoor air system |
| HFCs | = | Hydrofluorocarbons |
| MTEC | = | Motor-driven turboexpander–compressor |
| NIST | = | National Institute of Standards and Technology |
| TEC | = | Turboexpander compressor |
Disclosure statement
No potential conflict of interest was reported by the author(s).