| Author: | Huang, Junchao |
| Title: | Study on novel solar photovoltaic integrated vacuum glazing for low-energy buildings |
| Advisors: | Yang, Hongxing (BSE) Lu, Lin (BSE) |
| Degree: | Ph.D. |
| Year: | 2021 |
| Subject: | Glazing Heat -- Transmission Building-integrated photovoltaic systems Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Building Services Engineering |
| Pages: | 33, 210 pages : color illustrations |
| Language: | English |
| Abstract: | Building-integrated photovoltaic (BIPV) panels can replace the traditional building envelope materials for simultaneous thermal regulation and on-site power supplies, which becomes an effective approach to energy efficient buildings. Numerous studies have investigated PV integrated windows including single-glazed PV windows, PV insulating glass units, and PV double-skin façades. These types of PV windows are proved with great energy saving potential owing to the generated solar power and enhanced thermal performance, compared with the conventional clear glass or double-pane windows. Attributed to the air gap, hollow PV glazing (PV insulating glass units), and PV double-skin façades have better thermal performances than the single-glazed PV windows, leading to lower air conditioning load. To further improve the thermal performance of PV glazing, vacuum glazing has become an emerging research focus given its remarkable performance in both thermal and sound insulation. The first attempt to laminate glazed PV and vacuum glazing together was made in 2017, followed by very limited studies. Though it is found that the photovoltaic vacuum glazing can offer several benefits, such as generating electricity, thermal insulation, and reducing solar heat gain and noise level. The applicability of such PV envelope systems in diverse meteorological conditions has not been thoroughly discussed and its integration with other architectural design parameters has not been sufficiently addressed. Existing numerical models to predict the thermal behavior of composite photovoltaic vacuum glazing was built based on quasi-steady state and one dimension. There is a lack of a 3D dynamic thermal model to appropriately describe the whole heat transfer processes. Moreover, no research has studied the integration of additional air cavity with photovoltaic vacuum glazing. And the impact from design factors like thermal properties, vacuum pillar dimension and separation, Low-E coating and air cavity width have not been investigated. The overall energy performance of composite photovoltaic vacuum glazing with or without intermediate air cavity also needs further studies. Currently, indoor tests are the main approach to find out the thermal and electrical properties of photovoltaic vacuum glazing, and only limited work has been done for horizontally placed samples. Field tests have not been conducted to measure the real time data for practical performance analysis. To fulfil the research gaps, the first part of this thesis presents a comprehensive investigation of the thermal and power performance of the PV vacuum glazing as well as an integrated design optimization of photovoltaic envelope systems. A prototype office building model with a curtain wall design is first constructed in EnergyPlus to compare the heat gain, heat loss, thermal load, lighting energy and PV generation for different curtain walls. The comparative analysis proves the excellent thermal insulating performance of the PV vacuum glazing, which can reduce up to 81.63% and 75.03% of the heat gain as well as 31.94% and 32.03% of the heat loss in Hong Kong and Harbin, respectively. With the application of PV vacuum glazing in all available facades of the prototype building, net energy savings of 37.79% and 39.82% can be achieved in diverse climatic conditions. Furthermore, screening and variance-based sensitivity analyses are conducted to prioritize building integrated photovoltaic design parameters with respect to specific weather conditions. Selected important design parameters are then optimized with the non-dominated sorting genetic algorithm-II, by which the optimum building design can achieve a net energy consumption reduction of 48.72% and 60.80% compared to benchmarking designs in Hong Kong and Harbin. Secondly, this thesis proposes an integrated photovoltaic vacuum glazing unit with an intermediate air cavity and a calibrated modelling approach to quantify its thermal properties and evaluate the heat transfer performance. Three-dimensional heat transfer models are established and cross-validated against previous publications. The detailed validation demonstrates the reliability of the developed complex models under different circumstances. Furthermore, four PV vacuum glazing configurations are compared in terms of the temperature distribution and overall heat transfer coefficient (i.e., U-value). Simulation results show that the photovoltaic vacuum double glazing can achieve the optimum performance among the four configurations based on simultaneous consideration of the PV module temperature and U-value. Sensitivity analyses of main glazing design factors are also conducted for the U-value, which is greatly reduced by decreasing the density and diameter of vacuum pillars as well as the glass thermal conductivity. The lowest U-value of 0.23 W/(m2·K) is achieved for the hollow PV vacuum glazing and can be further improved with future design optimizations. Thirdly, a comprehensive heat transfer analysis has been carried out to determine the surface heat transfer coefficient and reveal the heat transfer process in the air and vacuum gap. A mathematical heat transfer model is established with MATLAB based on measured physical parameters by indoor tests. A test rig for dynamic experiments is also built to collect surface temperature, ambient temperature, surface heat flux, wind speed, electricity output, and solar irradiance. Under outdoor conditions, the maximum temperature difference between the interior and exterior surface can reach 20.4 °C for hollow PV vacuum glazing, which proves its excellent thermal performance. The heat transfer model is validated against both experimental data and published references. Simulated results are in good agreement with collected data from references and experiments. Based on the calculated U-value and solar heat gain coefficient, the window heat gain and power generation are predicted with the model considering the detailed impact of the Low-E coating. Analysis results show that the integration of the vacuum layer can reduce the U-value and solar heat gain coefficient of hollow photovoltaic glazing by 28% and 15%, respectively. In the hollow photovoltaic vacuum glazing, the Low-E coating is more effective in reducing the window heat gain if applied to the vacuum gap rather than the air gap, contributing to a lower U-value (0.45 W/(m2·K)) and solar heat gain coefficient (0.157) than those of photovoltaic vacuum glazing. The composite glazing is more suitable for a hot climate if the Low-E coating faces outside regardless of the coating's position in the vacuum gap or in the air gap. Compared with the double glazing, the Hollow photovoltaic vacuum glazing can help reduce averagely 75.3% energy consumption for heating and cooling in all the studied orientations and climatic zones. With the energy saving potential proved, a guideline is provided in the thesis for the initial design of the composite PV vacuum glazing to enhance the thermal performance of low-energy buildings for future carbon neutral building development. The validated heat transfer model proposed in this study can be applied to the heat transfer modelling of PV glazing with diverse structures. References are also provided for selecting PV photovoltaic glazing as the building envelope for energy conservation in different climate regions. This comprehensive study on solar PV vacuum glazing provides good reference for future research and industrial development of this new technology. |
| Rights: | All rights reserved |
| Access: | open access |
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