| Author: | Jia, Linrui |
| Title: | Development of a novel ground heat exchanger-assisted active radiative sky cooling system for efficient heat rejection |
| Advisors: | Lu, Lin (BEEE) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Buildings -- Energy conservation Geothermal resources Heating Air conditioning Ventilation Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Building Environment and Energy Engineering |
| Pages: | xxiv, 281 pages : color illustrations |
| Language: | English |
| Abstract: | Geothermal energy, characterized by its low greenhouse gas emissions, is a kind of renewable energy source that harnesses cooling (heating) energy from the Earth’s interior. However, the geographical distribution of high ground temperatures in tropical regions poses challenges for developing geothermal energy-based cooling systems, given the predictable issues such as degradation in energy efficiency and thermal imbalance due to the annually dominated cooling requirements. To promote the widespread adoption of this cooling technology across different climate regions, a supplementary cooling technology with low electricity consumption, cost-effectiveness, and significant cooling efficiency is imperative. Fortunately, radiative sky cooling (RSC) emerges as a passive cooling technology that harnesses the cold energy from deep space through the atmospheric window, offering an ideal solution. RSC provides several advantages over traditional cooling methods, including energy efficiency, environmental friendliness, and cost-effectiveness. By radiating a portion of heat into deep space, RSC aids in reducing the amount of heat rejected into the ground. This collaborative working mode not only addresses the challenges faced by geothermal energy-based cooling systems but also maximizes the cooling potential of RSC, surpassing its previous cooling limitation of 100 W/m2. In this thesis, our objective is to develop a novel geothermal energy-coupled RSC cooling system that realizes effective heat rejection for buildings. This endeavour aims to provide a new alternative to traditional cooling technologies and contribute to the global decarbonization efforts. Firstly, in order to disclose the cooling potential of the RSC radiator (RSCR, to produce cold water) with different interior configurations, an analytical model has been developed. The Laplace transformation and Convolution theory are applied to solve the problem of nonuniform temperature distribution on the RSCR. Then, an outdoor experiment is carried out to verify the proposed model. Subsequently, the influence of different channel geometries, flow rates, tilt angles and wind velocities on the cooling performances of RSCRs are investigated. The results indicate that increasing the RSCR’s length, enlarging the pipe spacing, and decreasing the flow rate and thickness of RSCR can definitely intensify the cooling performances. The application suggestions for S-channel RSCR (SRSCR) and I-channel RSCR (IRSCR) are also given. This research provides an effective analytical tool in the evaluation of cooling power generation, contributing valuable insights for the optimization of cooling efficiencies of RSCRs under varied configuration scenarios. For long-term prediction, a numerical RSCR model with high calculation efficiency and simulation accuracy has been developed, considering the spectral selectivity of cooling coatings and atmospheric windows, the influence of tilt angles and cloud cover fractions. Based on this numerical model, a novel deep space-source heat pump (DSSHP) system has been further developed to investigate its feasibility for stand-alone building cooling. Subsequently, the long-term operation performances of the DSSHP are investigated under various working conditions. The results indicate that the optimal DSSHP operation conditions include a lower flow rate, horizontal installation of environment-side water-to-air radiators, and clear sky. Moreover, this cooling system also shows high geographical adaptability for actual applications. This technology can provide a novel application way of cooling coatings and a new cooling source for HP systems, with the advantages of water saving, energy saving, and high energy efficiency. For city-scale applications with abundant cooling demands, a novel radiative sky cooling radiator (RSCR)-assisted GSHP (RGSHP) system to maximize the cooling potential of using RSCR and GSHP has been developed. The RGSHP can take full advantage of the ground’s thermostability to improve the cooling performance of the RSCR. Based on this developed hybrid system, the effects of designing parameters, weather conditions, and spectral properties of coatings on the performances of radiative cooling have been comprehensively investigated. Due to less heat rejected into the ground by RGSHP, the borehole wall temperature shows smaller values than stand-alone GSHP. For a ten-year simulation, the RGSHP system can reduce borehole wall temperature by 10.2%. Moreover, using the RGSHP can also achieve cooling storage in the ground during the non-cooling seasons, and thus the RGSHP outperforms the GSHP by 13.2% regarding the cooling season average COP. Then, this thesis systematically evaluates the advantages in the energy efficiency of the hybrid system, the temporal cooling potential variability of RSCR in seven geographic regions of China, and the effects of varied radiative coatings with different spectral properties on the cooling capacity. Compared with the stand-alone GSHP system, the heat rejected into the ground can be reduced by 20.6% on the nationwide average, and the nationwide average coefficient of performance (COP) of the proposed system can be increased by 18.6% due to the additional cooling contributions from RSCRs. Finally, for better tailoring the building-scale demands, this thesis has newly presented a vertical fin-intensified radiative sky cool radiator (VRSCR), which is able to synergistically couple the radiative sky cooling and convective cooling for overcoming the drawback of low radiative-cooling energy density. This chapter then comprehensively compares the application potentials of five cooling technologies to give promotion suggestions for these cooling technologies, including the passive radiative cooling building, HRSCR, VRSCR, stand-alone GSHP, and the commonest air-source heat pump system (ASHP). The results indicate that the difference in inlet water temperature relative to ambient air temperature dominates the adaptability of VRSCR and HRSCR. If the RSCR cool the circulating water at the ambient level, the back-insulated HRSCR is recommended. While with the inlet water temperature over ambient air temperature, the VRSCR without any thermal insulation layer is practically prioritized. The study sheds light on the promising benefits of renewable and sustainable cooling sources for achieving energy-efficient buildings in China. Overall, this thesis has developed spectral selective analytical and numerical models of water-to-air radiators, which allow for efficient radiative cooling behaviour predictions and relevant cooling potential evaluations. Then, this thesis further combined two cooling sources: geothermal energy and radiative sky cooling energy, which have theoretically distinctive mechanisms, aiming to intensify the systematical cooling efficiency towards city-level heat rejection. This work provides a new paradigm combination and inspires a new suggestion for effectively harvesting a variety of cooling sources towards a building’s high-efficiency heat rejection. |
| Rights: | All rights reserved |
| Access: | open access |
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