Design Optimization and Performance Analysis of Integrated Solar Thermal Air-Conditioning System Configurations

Abstract

Air conditioning has become an integral part of buildings due to climate change and global warming. About 55% of the world electricity is being consumed by the building sector with around 20% share of cooling only. Currently, the air-conditioning demand is mostly fulfilled by the vapor compression systems based on CFCs and HFCs as refrigerants causing ozone layer depletion. Therefore, the evident solution is the renewable energy based eco-friendly airconditioning systems such as efficient evaporative cooling techniques, solar thermal energy driven desiccant cooling systems. However, desiccant air conditioning systems with conventional evaporative coolers are characterized by low COP and also have certain performance limitations in humid climates. Therefore, an integration of efficient systems and components is required to develop energy efficient configurations capable to separately handle sensible and latent loads. It would result in better system performance to achieve the desired comfort conditions in subtropical and humid climates. In the current work, two integrated solar assisted desiccant cooling system configurations; (1) solar desiccant integrated Maisotsenko cycle (SDI-MC), and (2) solar desiccant integrated vapor absorption system (SDI-VAS) are experimentally evaluated for selecting an efficient system configuration compared to conventional standalone desiccant air-conditioning (DAC) system. Initially, an array of solar thermal source is evaluated by integrating flat plate collectors with evacuated tubes and parabolic trough collectors to overcome its heat losses. The main performance parameters include solar energy gain, efficiency, and solar fraction of the system. Secondly, a desiccant wheel that is a key component of the desiccant cooling system is theoretically and experimentally analysed to determine the effect of key operating parameters such as regeneration temperature, wheel rotational speed along with air inlet temperature and humidity. Afterwards, a cross flow Maisotsenko cycle (MC) indirect evaporative cooler is designed and developed for integration with the solid desiccant cooling system instead of direct evaporative cooler. For the system configurations analysis, SDI-MC configuration is developed first by integrating MC cooler on supply side of DAC. Similarly, SDI-VAS configuration is developed by integrating gas fired absorption system with DAC. Both configurations are experimentally evaluated under a wide range of operating conditions including air inlet temperature, air inlet humidity, and regeneration temperature. The performance evaluation parameters include thermal COP, dehumidification effectiveness, and cooling capacity. After comparison it was determined that SDI-MC system resulted in better performance than other configurations. Therefore, the SDI-MC configuration is further analysed under actual transient subtropical climate conditions of Taxila, Pakistan. The analysis consists of solar fraction, auxiliary energy share, efficiency of solar source, thermal COP, and cooling capacity for selected days of three different months i.e. May, June, and July. Subsequently, an office building model is developed in TRNBuild to generate hourly dynamic air-conditioning loads. Then simulation models of the SDI-MC and traditional DAC are developed in TRNSYS and validated with experimental data. The building model is incorporated in the simulation models of SDI-MC, and DAC. Finally, the overall SDI-MC model is optimized by coupling TRNSYS with GenOpt. The simulation based optimization is performed by considering COP, and solar energy gain of the system as cost functions using Hook Jeeves algorithm. Afterwards, the optimized SDI-MC model is subjected to seasonal transient energy analysis in subtropical climate condition of Taxila Pakistan. Finally, the life cycle cost analysis (LCC) is performed. The experimental results revealed that both SDI-MC and SDI-VAS can provide comfort conditions even at low regeneration temperature of 70°C. However, SDI-MC results in higher thermal COP ranging from 0.45 to 0.81. Similarly, the efficiency of solar thermal source ranges from 25% to 70% for hybrid array with evacuated tube collector and 20% to 60% for hybrid array with parabolic trough collector. Similarly, transient seasonal results revealed that monthly average thermal COP of the SDI-MC ranges from 0.57 to 1. Whereas, the COP of the traditional system ranges from 0.37 to 0.71. Therefore, efficiency enhancement of the SDI-MC is 43% to 47%