Performance modelling and comparative assessment of desiccant evaporative cooling system for hot and humid climate
thesisposted on 18.06.2020 by Ismanizam Abd-Manaf
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In hot and humid climates, air conditioning (AC) systems are essential for providing indoor thermal comfort. Energy consumption of the conventional AC system in building constitutes up to 60% of building energy usage. Conventional vapour compression (VC) refrigeration is the common type in air-conditioning systems used in buildings as these are simple and commercially available. However, these systems have a great impact on the environment to the extent that refrigeration technologies can also be part of solutions for mitigating global warming. A desiccant based air dehumidification air conditioning system is an alternative solution to VC systems and is used mainly in hot and humid climate for reduction of air humidity and temperature. The system is attractive as it can be used with renewable energy sources such as solar and waste heat. The relative operational simplicity of DEC systems, when compared to conventional HVAC systems, fosters a worldwide applicability potential, notably in low-income developing countries.
Recent studies suggest that one of the main barriers to wider use of desiccant-assisted cooling systems is the lack of knowledge on the design of such systems. Hence, this research investigates various DEC system configurations performance, and identifies one that is suitable for hot and humid climate by using commercial TRNSYS simulation software.
In establishing a credible simulation result, modelling and validation techniques for a wide range of individual components, a complete system has been developed. The critical parameters in desiccant wheel model for fixed isopotential effectiveness (εF1 and εF2) parameter values of 0.165 and 0.671 respectively have been validated using published data and a manufacturer software was recommended. Optimum parameters have been identified that can be applied in modelling other DEC configurations at specified ambient conditions.
Based on literature review, there are six DEC system configurations identified and investigated such as parallel direct/direct (PDD), parallel indirect/direct (PIDD), series direct/direct (SDD), series direct/indirect (SDID), series indirect/direct (SIDD) and series indirect/indirect (SIDID).The performance of these six DEC configurations was analysed under four main criteria; i) individual components performance, ii) system performance, iii) comfort condition, and iv) energy.
The six configurations are simulated under two operating condition parameters; mass flow rate of 1.0 to 2.5 kg/s and regeneration temperature of 80 to 120°C. The ambient environmental range is set between 28-45°C temperature and 13-30 g/kg humidity ratio mimic hot and humid weather condition. The results identified that the SIDID configuration gave the highest thermal coefficient of performance of 0.44 at regeneration temperature of 80°C and mass flow rate 1 kg/s while the air handling coefficient of performance is 1.15 for regeneration temperature of 80°C and mass flow rate 2.5 kg/s. However, the highest evaporative cooler effectiveness of 1.14 is found in SIDD when operated at 100°C regeneration temperature and 1 kg/s mass flow rate. In the observation of zone comfort, the SIDD configuration has provided better temperature compared to the other DEC system model with lowest indoor zone temperature between 18-22°C while operating at 120°C regeneration temperature and 2 kg/s mass flow rate.
Nevertheless, the lowest humidity ratio is found in SIDID with 0.008-0.012 kg/kg when operating at 1 kg/s mass flow rate and 100°C regeneration temperature. Therefore, the zone comfort is best found in SIDD and SIDID when operated at 120°C regeneration temperature and 2 kg/s mass flow rate while other four configurations operated at a higher mass flow rate of 2.5 kg/s. In the 8760 hours operation for Kuala Lumpur, Malaysia condition, the annual regeneration energy calculated is 977,364 kWh/yr and 948,446 kWh/yr for SIDD and SIDID respectively. While the other four configurations produce more energy of above 1,210,000 kWh/yr. It worth mentioning, prioritising low mass flow rate in the DEC system reduces not only energy consumption and noise, but also the equipment size and the ductwork would be much smaller together with lower operational and maintenance cost. However, the source of regeneration heater should also be considered as a low mass flow rate requires higher regeneration temperature, which in turns would end up with big heater equipment. The best optimum configuration identified is SIDD configuration then modelled and simulated for two-stage dehumidification for more in-depth investigation. The results found that the two-stage DEC system provides better zone indoor comfort with less regeneration temperature and mass flow rate to run the system. For example, the zone temperature in two-stage dehumidification (2SD) is 22.4°C when operated at 70°C regeneration temperature compared to one-stage dehumidification (1SD) that needs to be operate at 110°C regeneration temperature (at 2.5 kg/s mass flow rate). Also, the two-stage zone humidity ratio is 0.013 kg/kg when operated at 80°C regeneration temperature compared to 1SD operated at 110°C regeneration temperature with similar 2.5 kg/s mass flow rate. With all findings, a design outline on the operating parameters and system performance of various DEC systems is presented at the end of the thesis.
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