Indoor and outdoor radiant cooling systems for buildings




Moftakhari, Ardeshir

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Heating, Ventilation and Air Conditioning (HVAC) systems account for a major portion of overall energy consumption in buildings. Most HVAC systems use conventional and well established all-air thermal distribution systems that rely on convection as the major heat transfer mechanism for heat extraction. However, energy conservation efforts encouraged research related to radiant systems that can improve efficiency of HVAC systems. Radiant systems can be integrated into the building envelope by means of radiant cooling panels or slabs. These systems can increase the efficiency of the HVAC system by allowing chillers to operate more efficiently at a higher chilled water temperature compared to all-air systems. Another example of using radiative heat transfer for energy efficiency is in systems that use exterior spectrally-selective surfaces to intensively radiate heat to the outer space under suitable atmospheric conditions or passive radiative cooling (PRC). These systems utilize materials with high albedo coating and emit thermal infrared radiation that exploits the atmospheric window, resulting in sub-ambient surface temperatures, even during sunny days. Both indoor and outdoor radiant cooling systems need further development before they show full potential for increasing building energy efficiency. However, there are many questions about these systems that need to be addressed. For indoor radiant cooling systems, there is no clearly defined design process. Currently, design relies on procedures developed for all air systems, which in some situations are systems that cannot deliver the load when considering effective control of indoor environmental conditions. Also, there is a need for better understanding of the dynamics of heat transfer in rooms with radiant cooling systems and inclusion of this into the design procedure. When considering novel outdoor radiant cooling systems, all engineering aspects of these systems are very much unknown as these systems just emerged and there are few full-scale prototypes. From the theoretical studies (Zhu et al., 2013), we know that these PRC systems that are based on selective surfaces can produce cooling power of up to 100 (W/m²); however, there is little to no information how ambient condition, application specifics, and material affects performance of these systems. Additionally, there are no models associated with selective PRC films that can be implemented in the design or energy performance modeling tools. My PhD thesis work utilizes the state-of-the-art facilities, such as Thermal Façade lab and Roof Testing lab at the University of Texas at Austin, to conduct a set of full-scale experiments and combine experimental results with analytical and numerical modeling to address several research and engineering challenges related to the system design process, operation and energy performance. Specifically, my dissertation: (a) investigates the complex heat transfer dynamics of spaces conditioned with radiant cooling panels, (b) develops a system design that integrates building thermal response to these radiant cooling systems, (c) advances the system control strategy by integrating all influencing parameters in the model that provides optimal thermal comfort and energy saving, (d) evaluates thermal performance of PRC-based roofs in a full-scale prototype, and (e) develops sky radiation models for energy analysis of the PRC-based roofing technology. The research builds upon fundamental science and principles while the findings are incorporated in the ASHRAE and other relevant building design guidelines used worldwide by HVAC designers and practitioners.


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