Performance evaluations of façade integrated heat pipe systems

Abstract

The burning of fossil fuels brings about energy and environment challenges, such as air pollution, urban heat island effect, and global warming. With the increasing energy cost and carbon emission penalty, people desire low-carbon or zero-carbon life style nowadays. The two major solution paths are renewable energy utilization and energy efficiency enhancement. For the metropolitan cities like Hong Kong, the building integrated systems have great application potential. As heat pipe is a high-efficiency heat transfer device, evaluated in this study were the performances of two innovative facade integrated heat pipe systems. One is the building-integrated heat pipe photovoltaic/ thermal (BiHP-PVT) system, which has heat pipe photovoltaic/thermal (HP-PVT) collectors constructed outside the building envelope, and the other is the building integrated heat pipe embedded (BiHPe) system, which has heat pipes embedded within the envelope. The BiHP-PVT panel is for placement at the exposed façade surface and so contributes to the tone of the architecture. It carries three distinct functions: electricity generation, water heating and air-conditioning energy saving. Accordingly, the heat pipes are welded to the back of an absorber plate which has photovoltaic (PV) plate attached to the front. The heat pipe operation provides a cooling effect to the solar cells and therefore improving the photovoltaic efficiency. The heat absorbed by heat pipes goes to a water storage tank for domestic water heating purpose. At the same time, the solar heat transmission to the air-conditioned space is reduced, and hence results in space cooling load reduction. For the BiHPe system, the heat pipes are buried inside the vertical wall and hence leave the architectural features intact. The embedded heat pipes absorb a portion of the solar heat that passes through the solid wall and convey this to the water storage tank, meanwhile, the heat transmission to the air-conditioned space can be reduced. To facilitate the development of numerical models for the façade integrated heat pipe systems, two traditional heat pipe applications in buildings were first investigated: the horizontal air-to-air heat pipe heat exchanger (HPHX) for use in a ventilating duct system, and the evacuated tube heat pipe solar water heating system. The numerical models of these two conventional applications were first developed and validated by published data in literatures. The control-volume semi-implicit finite difference method (FDM) was adopted in this study. The energy balance equation set of the HPHX was first formulated and then solved using the semi-implicit FDM. The same approach was applied to the heat pipe solar water heater. Afterwards, the HP-PVT system model was also developed using the very similar approach. This model was validated by the measured data obtained from other researchers based on an experimental plant in the Guangdong Province of China. From the overall building sustainability point of view, the HP-PVT collectors can be installed at the outside surface of a vertical wall and so the BiHP-PVT system was proposed. The energy performance of the BiHP-PVT system was then evaluated and compared with an alternative building-integrated design option, the building integrated photovoltaic thermal/water (BiPVT/w) system. Based on a hypothetical case study in Hong Kong, it was found that the BiHP-PVT system performed better than the BiPVT/w system in terms of reduction of overall heat transmission through the wall, and its thermal and electrical efficiencies were similar to those of the BiPVT/w system. Then, another novel so-called BiHPe system with heat pipes embedded in the wall was proposed. An experiment rig for the BiHPe system with an optimum design was constructed in the Jiangsu Province of China. The computer model of this solar façade was validated by measured data acquired during winter period. Finally, the year round energy performance of this innovative façade-integrated system was predicted for Hong Kong and Shanghai applications. Through applying the BiHPe system to a hypothetical gymnasium, the overall heat transmission through the wall was reduced significantly in Hong Kong application, but this was changed slightly in the case of Shanghai as the reference wall was well insulated. All the dynamic simulation models for the above heat pipe applications have been successfully developed based on the energy balance equations and solved by the semi-implicit FDM. The semi-implicit FDM, which treats the linear parts of the equations implicit and the nonlinear parts explicit, was used to discretize the energy balance equation set. It was found that the semi-implicit FDM was a better choice than the fully implicit or fully explicit FDM for solving the mathematical models of the heat pipe applications. The semi-implicit FDM has a range of advantages, i.e. no iteration demand, unconditionally stable, good computational efficiency and accuracy. It is expected that the developed models for the innovative heat pipe applications can be used for developing the component modules in building energy performance simulation tools to study the energy saving potential in real buildings.

Date of Award	15 Jul 2015
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Tin Tai CHOW (Supervisor) & Chung Leung Johnny CHAN (Co-supervisor)