Air conditioning accounts for more than 20% of the total energy consumption in Hong Kong buildings. An adsorption cooling system is a good alternative to replace the traditional vapor compression (VC) system due to its environmental friendliness, energy saving potential, featuring no moving parts and lower noise level and vibration.

However, VC systems still dominate in most applications, since adsorption cooling systems have lower cooling efficiency, and the size is relatively bukly. In order to address this issue, a simulation was conducted first to investigate the effect of different operating parameters on the adsorption cooling system’s performance. Pressures variations of different system’s components were considered in the simulation. The adsorbent-adsorbate working pai...[ Read more ]

Air conditioning accounts for more than 20% of the total energy consumption in Hong Kong buildings. An adsorption cooling system is a good alternative to replace the traditional vapor compression (VC) system due to its environmental friendliness, energy saving potential, featuring no moving parts and lower noise level and vibration.

However, VC systems still dominate in most applications, since adsorption cooling systems have lower cooling efficiency, and the size is relatively bukly. In order to address this issue, a simulation was conducted first to investigate the effect of different operating parameters on the adsorption cooling system’s performance. Pressures variations of different system’s components were considered in the simulation. The adsorbent-adsorbate working pair used was silica-gel and water. On the other hand, an adsorption cooling system prototype was built and studied experimentally for verifying the model. The effect of desorption temperature, adsorption – desorption phase time, cooling water inlet temperature and chilled water inlet temperature on the specific cooling power (SCP) were investigated. At a typical working condition, the SCP of the adsorption cooling system can be achieved at about 80 W/kg. Besides, it was found that the experimental results were matched with the simulation.

Some nanofluids (i.e. Al₂O₃/Water and TiO₂/water) were tested as an adsorbate in the adsorption cooling system due to their enhanced effective thermal conductivity, convective heat transfer coefficient and critical heat flux. In order to study the enhancement of the effective thermal conductivity of nanofluids, a semi-analytical model was derived from the steady heat condition equation in spherical coordinates. The effects of nanolayer thickness, nanoparticle size, volume fraction, thermal conductivity of nanoparticles and base fluid are discussed. A linear thermal conductivity profile inside the nanolayer is considered in the mode. The model, while investigating the impact of the interfacial nanolayer on the effective thermal conductivity of nanofluids, provides an equation to determine its nanolayer thickness for different types of nanofluids. Hence, different relationships between the nanolayer thickness and the nanoparticle size are found for each type of nanofluid. The results also showed that the effective thermal conductivities of different types of nanofluids calculated by the model were in good agreement with the experiments and the prediction was better than some conventional models.

Saturated vapor pressure, enthalpy of evaporation and the evaporation rate of nanofluids were also studied, and it was found that the values can be increased or decreased, depending on their volume concentration and the types of nanofluids. For an engineering purpose, a semi-analytical model for estimating the evaporation rate of water as a function of temperature, humidity and air velocity was developed. The model can also be used to estimate the evaporation rate of nanofluids when the saturated vapor pressure of water is replaced by that of nanofluids. After comparing the modeling results with the experiment, the agreement between them was satisfactory. Lastly, this thesis indicates that using 0.01% TiO₂ nanofluid as the adsorbate in the adsorption cooling systems, the SCP was enhanced by about 9%, meaning that 0.01% TiO₂ nanofluid is another potential adsorbate to be used in adsorption cooling systems.