Theoretical And Experimental Investigation On A Two-Stage Rotary Desiccant Cooling System

Desiccant cooling works in principle of desiccant cooling and evaporative cooling, and can handle the latent load and sensible load separately. The technology is advantageous as it uses only water and moist air in cooling process, and can be driven by low grade thermal energy resources, such as solar energy, geothermic enegy and waste heat. Thus, it attracts more attentions in recent years due to its energy-saving and eco-friendly merits. For solid rotary desiccant cooling units, the effect of adsorption heat is the main reason which results in the dehumidification process deviating from the ideal isothermal process. To improve the process, this thesis proposed an ideal multi-stage desiccant cooling process, the theoretical results indicate that this process has the least irreversible loss, the lowest regeneration temperature and the biggest moisture removal rate. Therefore, how to approach this ideal process is an important way in improving the performance of desiccant cooling.In this work, by using one kind of composite desiccant material developed in Shanghai Jiao Tong University, a novel two-stage rotary desiccant cooling process is proposed. The dehumidification process in this unit is much more close to an isothermal one, regeneration temperature is low (50～90℃), meanwhile the thermal coefficient of the unit is improved to 1.0. The experimental set-up of the two-stage system is built up. Results show that compared with the conventional one-stage system, the novel two-stage system has the merits of low regeneration temperature, high exergy efficiency and high dehumidification capacity. Under the same operation condition, if the regeneration temperature of two-stage system is 70℃, its moisture removal is equal to that of one-stage system which operates in the regeneration temperature of 100℃. Meanwhile, compared with one-stage system, the exergy efficiency of two-stage system increases by about 15%.The gas and solid side-resistance model which can accurately predict the performance of composite desiccant wheel is developed. The model overcomes the shortage of conventional gas side-resistance model which does not take account of the heat conduction and mass diffusion in solid desiccant material. The predicted error of the new model is less than 9%. Also, in order to improve the utilization ratio of desiccant wheel, a new concept of dividing the cross section of one desiccant wheel into four parts is proposed, and one-rotor two-stage rotary desiccant cooling system is developed based on this concept. Experimental results demonstrate the feasibility of this concept and present the effects of wheel thickness, regeneration temperature and rotation speed on system performance.Upon the aforementioned work, an actual two-stage rotary desiccant cooling system is built up inside an office building. It is shown that the system performs well, the thermal COP can reach about 1.0. Besides, according to the thermal comfortable standard, under the condition of the same supply air state, performance of solar driven two-stage rotary desiccant cooling system is compared with that of conventional vapor compression system. The feasibility of two stage rotary desiccant cooling is analyzed under various climate conditions.It is proved that the new two-stage rotary desiccant cooling system has the merits of lower regeneration temperature and good energy efficiency, and will have wide range of application and bright prospect in solar and secondary energy utilization.