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Abstract
A novel dedicated outdoor air system consisting of a multi-stage direct expansion coil to generate extra-low temperature outdoor air to avoid moisture-related problems and for desirable air conditions and better energy efficiency is proposed for investigation. In this study, the proposed system's (extra-low temperature-dedicated outdoor air system) performance in achieving the desirable air conditions and better energy efficiency objectives is compared with a conventional direct expansion coil system with variable speed compressor and fan used for air conditioning of a typical office building in Hong Kong. Through hour-by-hour simulations, using equipment performance data of a pilot study and realistic building and system characteristics, it was found that the extra-low temperature-dedicated outdoor air system, as compared to the conventional system, could reduce the annual air-conditioning energy consumption by 22.6%, the indoor discomfort hours by 31% and the condensation risk to zero hours. Root-mean-square error and standard deviation analysis of the resultant relative humidity confirmed that the extra-low temperature-dedicated outdoor air system could better achieve the desired relative humidity. The findings of this study confirm that the extra-low temperature-dedicated outdoor air system is better than the conventional system in terms of both energy consumption and the desirable air conditions when used in office buildings in Hong Kong.
Nomenclature
| ai | = | ith empirical coefficient |
| ConH | = | condensation hour |
| COP | = | coefficient of performance |
| Cpa | = | specific heat capacity (kJ/kg·°C) |
| DB | = | dry bulb |
| DP | = | dew point |
| EIR | = | energy input ratio |
| h | = | specific enthalpy (kJ/kg) |
| Hfg | = | latent heat of evaporation of water (kJ/kg) |
| NCH | = | number of discomfort hours |
| PLR | = | part load ratio |
| PLF | = | part load fraction |
| OA | = | outdoor air |
| Q | = | cooling load (kW) |
| RA | = | return air |
| RH | = | relative humidity |
| RMSE | = | root-mean-square error |
| RTF | = | run time fraction |
| SA | = | supply air |
| SD | = | standard deviation |
| SHR | = | sensible heat ratio |
| t | = | time (hour) |
| T | = | temperature (°C) |
| V | = | volume flow rate (m3/s) |
| w | = | moisture content (kg/kg) |
| W | = | power (kW) |
| WB | = | wet bulb |
Greek symbol
| ϕ | = | space relative humidity |
| ρ | = | air density (kg/m3) |
Subscript
| ci | = | condenser inlet |
| com | = | compressor |
| d | = | design value |
| ei | = | DX coil inlet |
| eo | = | DX coil outlet |
| fan | = | fan |
| lat | = | latent |
| m | = | mean value |
| O | = | reference (environment) state |
| R | = | design condition state |
| sen | = | sensible |
| S | = | supply air |