Performance optimization of low-to-moderate temperature thermal systems: a multi-pronged investigation

Abstract

Solar energy plays a vital role for the future of clean energy production due to its availability all year round. To harvest solar energy, solar collectors are used, where they can be classified into categories of concentrating and non-concentrating solar collectors. In this work, both concentrating and non-concentrating solar collectors are studied through modeling and experimental methods for different application purposes. Moreover, the use of thermal energy storage through incorporation of phase change material (PCM) to improve the functionality and heat transfer performance of different systems was studied.

In chapter 1, the performance of a heat pipe evacuated tube collector (HPETC) is investigated through the incorporation of a primary compound parabolic concentrator (CPC)-involute reflector which can be retrofitted to an existing single HPETC. A hybrid numerical model is developed through optical ray tracing methods from SolTrace and ANSYS Fluent. The heat flux intensity around the absorber tube is compared for different times of the day. The results show that the use of a primary CPC-involute reflector increases the overall average heat flux received by the absorber tube by a maximum of 108% and yields to uniform heat flux distribution. A transient computational fluid dynamics (CFD) modeling is also developed, using SolTrace results as a boundary condition in ANSYS Fluent. The novel approach pursued herein is validated with ongoing experimental work, with a maximum deviation of 7.2%. Then, paraffin wax based phase change material (PCM) is incorporated into the collector tube. With the CPC-involute reflector, the paraffin wax PCM was able to achieve 100% melting, whereas conventional system without reflector only reached a maximum of 75% melting fraction.

In chapter 2, a self-contained PCM-based solar ice control ground radiant heating system (RHS) is investigated to be retrofitted as a layer onto the surface of an existing concrete pavement to prevent snow/ice accumulation during winter season. This work is funded by the United States Department of Transportation (DOT). First and foremost, material characterization and thermal analysis of mortar samples are conducted to determine the samples' thermophysical properties. Then, CFD study of different RHS model configuration is conducted under winter and summer ambient conditions to select one with the most optimal performance. Lastly, a PCM optimization study is conducted to investigate different types of PCM and PCM slurry composition that performs in both winter and summer conditions. Results from this study shows that the PCM blocks located between pipe loops configuration yields superior performance when compared to the other configurations, while a low temperature PCM slurry mixture composed of liquid (at room temperature) PureTemp4 and micro-encapsulated PCM28 is selected as the PCM type to be incorporated into the PCM blocks of the RHS. Results from this work leads to several confirmed permutations for the following experimental phase of this project.

Chapter 3 of this work involves thermal performance enhancement of solar photovoltaic/thermal (PV/T) panels. Solar photovoltaic (PV) panels are popular solar energy devices that converts sunlight into electricity. However, such systems experiences low efficiencies conversion as PV cells tend to overheat. To overcome this problem, a hybrid PV/T system are often used instead. PV/T systems have the ability to provide both electricity and useful heat. This study aims to improve the thermal energy output as well as PV efficiency through the use of highly absorptive multi-walled carbon nanotube (MWCNT) selective absorber coating using CFD modeling method. Results from this study show that by using MWCNT selective absorber coating, a maximum increase in thermal energy output of ~7% can be achieved while PV cell efficiency enhancements up to ~0.8% can be obtained. Future work of this study includes investigating nanofluid as a heat transfer fluid as well as incorporating nano-enhanced thermal energy storage to further amplify the thermal energy output and PV cell efficiency.