Sustainable Air Handling by Evaporation and Adsorption

Abstract

A considerable fraction of today's energy consumption is due to air-conditioning of buildings, involving both heating and cooling. Energy cost and environmental concerns force designers to find sustainable solutions. Desiccant cooling as a sustainable technology is attractive to be investigated for making different design tools. The main subject of this research is cooling and dehumidification by a wheel which contains a matrix of sorption material or desiccants such as silica gel. Desiccants adsorb water vapor due to the difference of water vapor pressure between the surrounding air and their surface. To make the system operate continuously, adsorbed water vapor must be driven out of the desiccant material (regeneration) so that it can be dried enough to adsorb water vapor in the next cycle. In combination with a heat exchanger to recover the regeneration heat and an adiabatic humidifier at the end of this process, cooled air can be attained. The rotary dehumidifier/regenerator wheel appears to be the front runner in the current desiccant system development effort. The main purpose of the research is making the design tools to analyse and to compare any desiccant cooling system with traditional and conventional air handling units. In this thesis, heat and mass transfer are modelled in different components of sustainable air handling systems based on adsorption and evaporation. The sustainable components considered in this thesis range from indirect evaporative coolers to desiccant wheel. Also the traditional air cooler is modelled in order to compare its performance with that of the sustainable components. All models are simulated by the simulation code Simulink (Matlab) with the advantage that any system can be analysed just by linking the inputs and outputs of the various models. Because there is a lack of knowledge about the performance of desiccant wheels, significant attention is paid to validate the heat and mass transfer model. Test facilities were set up in a climate room to investigate the model of desiccant wheel. In general, the accuracy of the models is acceptable within a margin error of practical applications. The validated physical models are used to simulate various air conditioning systems in order to analyze them with respect to capacity, energy consumption, and environmental aspects. In addition, simplified equations are developed for dehumidifier wheels in order to reduce simulation time for year round analyses. With these equations a hybrid evaporative cooling system with desiccant wheel as an auxiliary system is simulated for a reference year and for two different climates (extreme and moderate). The energy costs and carbon dioxide (CO2) emissions are calculated for the different systems in the different climates. Year round simulations show that desiccant air conditioning systems are only advantageous when a free source of thermal energy is available for regeneration. If this is not the case, then it can be made advantageous by a hybrid system with desiccant in operation only during peak hours. The validated simplified models of air cooler and desiccant wheels are implemented in the computer program Enerk. It can be used to analyse and to compare any desiccant system with traditional and conventional air handling units. From a practical point of view it can be considered as the end product of this research.