Thermodynamic study on the air-conditioning system of new DOAS with desiccant wheel

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Thermodynamic study on the air-conditioning system of new DOAS with desiccant wheel

 

Author: Hui, Shui-man
Title: Thermodynamic study on the air-conditioning system of new DOAS with desiccant wheel
Degree: M.Eng.
Year: 2010
Subject: Hong Kong Polytechnic University -- Dissertations
Air conditioning -- Efficiency.
Department: Dept. of Building Services Engineering
Pages: xiii, 88 leaves : ill. ; 31 cm.
InnoPac Record: http://library.polyu.edu.hk/record=b2356372
URI: http://theses.lib.polyu.edu.hk/handle/200/5815
Abstract: The conventional VAV systems, which are the energy efficient and widely used systems in many commercial buildings nowadays. have weaknesses such as over or under-ventilation in multi-zones and improper humidity control in part load conditions. Dedicated outdoor air system (DOAS) is one of the alternative systems for overcoming the restraints of the conventional VAV systems as it can separately deal with the latent and sensible loads. The conventional DOAS is a cooling based system with a problem when the ratio of huge latent to total loads is high. The supply chilled water temperature in the system should lower than the supply air dew point temperature to satisfy the space requirements, which leads to a lower evaporative temperature and higher compressor energy use. The other type of DOAS, which is combined with a desiccant dehumidification recovery (DDC), was developed to eliminate the constraint of the conventional DOAS isolating the latent loads to deal with the DDC and the remaining sensible is handled by the indoor sensible conditioning unit. The system can lower down the evaporative temperature of the chiller. However, the direct evaporative cooler in the system results in unnecessary exergy loss. The direct evaporative cooler converts latent cooling to the sensible potential from the air moisture potential. This study has identified the unnecessary exergy losses in the conventional all air system and the typical DOASs based on the thermodynamic analysis. Then, an innovative DOAS with rotary desiccant system has been developed and evaluated. A case study was pursued for comparison of the thermodynamic performance among the conventional all air system, the conventional DOAS, the typical desiccant wheel based DOAS and the innovative DOAS. The simulation results clearly show that approximately 44% and 58.9% energy savings in chiller by the typical desiccant wheel based DOAS and proposed DOAS with desiccant wheel system respectively as compared to the conventional all air system. It is due to a higher evaporative temperature occurred in the typical and innovative DOAS. Compared with the whole system energy, however, under the design stage conditions, the innovative system has 30.8% energy higher than the conventional all air system. It is because low effectiveness heat recovery unit selected during the simulation. The energy performance can be improved if using a higher effectiveness heat recovery unit and under the part load conditions as heater is needed to overcome the weaknesses of improper humidity control for the conventional all air system. The energy performances of the typical and innovative DOAS were also evaluated and the results show that the total system exergy loss ofthe innovative DOAS is lower than the typical DOAS, with around 78% reduced and 15.1 % increasing on the exergy efficiency for the innovative DOAS compared with the typical DOAS. Both results indicate that the innovative DOAS have potentials for energy saving.

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