Dissertation Defense: Yiyuan Qiao

Title: Personal cooling system with phase change material

Author: Yiyuan Qiao

Committee members:

Dr. Reinhard Radermacher(Chair)

Dr. Ichiro Takeuchi

Dr. Jelena Srebric

Dr. Bao Yang 

Dr. Amir Riaz

 Dr. Yunho Hwang

Date: June 26, 2020, 2:00 pm

Abstract: Personal cooling systems are attracting more attention recently since they can set back building thermostat setpoint to achieve energy savings and provide high-level human comfort by focusing on micro-environment conditions around occupants rather than the entire building space. Thus, a vapor compression cycle (VCC)-based personal cooling system with a condenser integrated with the phase change material (PCM) is proposed. The PCM heat exchanger (PCMHX) works as a condenser to store waste heat from the refrigerant in the cooling cycle, in which the PCM melting process can affect the system performance significantly. Different from most of the previous study, various refrigerant heat transfer characteristics along the condenser flow path can result in the uneven PCM melting, leading to the degradation of the system performance. Therefore, enhancing heat transfer in the PCM, investigating the proposed personal cooling system performance, improving PCMHX latent heat utilization in terms of the distribution of PCM melting, and developing a general-purpose PCM model are the objectives of this dissertation. 

Five PCMHX designs with different heat transfer enhancement including increasing heat-transfer area, embedding conductive structures, and using uniform refrigerant distribution among condenser branches are introduced first. Compared with non-enhanced PCM, the graphite-matrix-enhanced PCMHX performs the best with 5.5 times higher heat transfer coefficient and 49% increased coefficient of performance (COP). To investigate the proposed system performance, a system-level experimental parametric study regarding the thermostat setting, PCM recharge rates, and cooling time was conducted. Results show that the PCS can work properly with a stable cooling capacity of 160 W for 4.5 hours. A transient PCM-coupled system model was also developed for detailed system performance, PCM melting process and heat transfer analysis. From both experiment and simulation work, the uneven PCM melting was presented, which could result in an increase of condenser temperature and a degradation of system COP with time. One significant reason for the uneven PCM melting is the variation of the refrigerant temperature and heat transfer coefficient. Therefore, through experimental analysis, several solutions were proposed to minimize the negative effect of the uneven PCM melting. Results show that the less subcooling is recommended, and the new design with the PCM mass maldistribution can increase the cooling time by 28%. In addition, to extend the PCMHX application, a multi-tube PCMHX model was developed for general-purpose design. A new multi-tube heat transfer algorithm was proposed, and variable tube shape, connection, and topology for tubes and PCM blocks were considered. The comparison with other PCMHX models in the literature shows that the proposed model exhibits much higher flexibility and feasibility for comprehensive multi-tube configurations. The personal cooling system coupled with PCMHX could achieve energy savings for a range of 8-36% depending on the climate and building types in the U.S..