Liquid-based photovoltaic/thermal (PV/T) co-generation system in real building application

Abstract

In the 21st century, all countries are competing energy recourses to sustain their rapid economic growth and development. At the same time, the global greenhouse effect is becoming serious due to the excessive energy consumption from combustion of fossil fuels. Consequently, there are strong urges for the advanced development of renewable energy technologies. Solar energy has been identified for long time as one of the promising renewable energy sources. Hong Kong is located at the sub-tropical climate region of East Asia where abundant of sunshine is available throughout a year. There is great potential for the wider-application of solar energy systems to reduce the dominant reliance on energy supply from the power utilities. Applications on the building sector can be most effective as buildings bare the largest energy uses among all the sectors in Hong Kong. One of the major constraints for installing solar energy systems in buildings is the limitation of spaces for the system accommodation. Either photovoltaic or solar thermal system has to be designed with large panel area to maximize the collection of solar irradiance. Hybrid photovoltaic/thermal (PV/T) system is a desirable option to overcome the installation barrier. Hybrid (PV/T) system is formed by an integration of photovoltaic and solar thermal systems together to a single unit system with dual generations of electrical and thermal energy. The total energy output per unit collector area of a (PV/T) system is significantly increased as compared with either an individual photovoltaic or solar thermal system. So far, (PV/T) system research emphasizing on real building operation has been found to be inadequate. To meet the current needs, the main scope of this PhD research is to investigate the application potential of water-based photovoltaic/thermal (PV/T) co-generation system in real office building in the warm climate region. Initially, an individual electrical photovoltaic (PV) system and an individual indirect thermosyphon solar water heating (ST) system were erected on the roof top of an office building located at the university campus. The (PV) system is a stand-alone type system with battery storage. The electricity output from the (PV) system is to support the power demand of a group of LED exit-sign lighting inside the building. The (ST) system provides domestic hot water service to the occupants at the selected pantries. Water is heated in an in-direct way via a heat exchange coil set which is specially designed to reduce the overall friction loss of the thermospyhon flow route and maintain the thermal stratification along the vertical height of the storage tank. In the numerical analysis, the electrical output of the (PV) system is modeled by an equivalent single diode, "five parameters electrical circuit. The indirect heating process in the (ST) system is modeled based on the principle of buoyant pipe flow and the logarithmic mean temperature difference relationship. Both the (PV) and (ST) numerical models are successfully developed with the use of the Euler Explicit finite difference computation techniques and well validated with experimental data. The above works facilitate a comprehensive computer analysis on the year-round energy performance of a hypothetical (PV/T) system as applied in a real office. Based on the two separate (PV) and (ST) system models, a new photovoltaic/thermal system model is constructed by a proper integration of the former two models. With the readiness of the three system models (PV), (ST) and (PV/T), the numerical models of a full scale hybrid (PV/T) system and a side-by-side (PV) and (ST) system aiming to provide electricity support of all the LED exit-sign lighting and pre-heating process of the hot water supply at all pantries of the real office building are derived. Both the (PV/T) and the side-by-side system are designed under the circumstance that they are offering equivalent amount of electricity savings for the real building in a typical year. Euler Explicit finite difference computation techniques are adopted in both systems' computational model programs. The climate hourly data input to the simulation process is the Typical Meteorological Year (TMY) dataset of Hong Kong. The incident solar irradiation levels on the tiled surface panels of the solar energy systems are computed by anisotropic Perez's model. Finally, economic and environmental life cycle analyses (LCA) for both hybrid (PV/T) and side-by-side (PV) and (ST) systems are performed to evaluate their application potentials for a real office building in the sub-tropical region with Hong Kong as an example. The evaluations are in terms of investment cost, energy use and greenhouse emissions. The energy payback time, greenhouse gas payback time and the cost payback time of both systems are relatively the same. They are 6.8 years, 2 years and 14.5 years respectively. The results imply that the applications of either the (PV/T) system or the side-by-side system are encouraging. Incorporations of solar energy systems in such a real office building are generally recommended. The (LCA) results also help consolidate a conclusion that at the product mature stage, hybrid (PV/T) has a definite advantage over the conventional side-by-side system for real office building applications as the energy output per unit collector coverage area of a (PV/T) system is certainly higher than a side-by-side (PV) and (ST) system. The contemporary building design trend is towards the goal of Low-carbon/Zero-carbon buildings (LCBs/NCBs). Application of (PV/T) system maximizes the renewable energy yields of a building and makes possible to achieve the (LCBs/ZCBs) targets.

Date of Award	15 Jul 2014
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Tin Tai CHOW (Supervisor) & Chung Leung Johnny CHAN (Co-supervisor)