Thermal characteristics of a building-integrated dual-function solar collector in water heating mode with natural circulation Jie Ji a, * , Chenglong Luo a, b , Tin-Tai Chow c , Wei Sun a , Wei He a a Department of Thermal Science and Energy Engineering, University of Science and Technology of China, 96# Jinzhai Road, Hefei, Anhui 230026, China b China Nuclear Power Operation Technology Corporation, LTD, Wuhan 430223, China c School of Energy and Environment, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China article info Article history: Received 31 December 2009 Received in revised form 29 July 2010 Accepted 2 October 2010 Available online 20 November 2010 Keywords: Dual-function solar collector Passive space heating Water heating Natural circulation Summer overheating abstract As a modified building-integrated solar thermal system, building-integrated dual-function solar collector here proposed is able to provide passive space heating in cold winter, and water heating in warm seasons. In this study, evaluations were made on this modified collector system for the warm period operation under the water heating mode with natural circulation of flow. A dynamic numerical model has been developed and validated by experimental data. Based on practical air-conditioned room design conditions, numerical analysis was performed to study the water heating performance, as well as to compare the solar transmission through building facade in different seasons with or without its pres- ence. The results show that when working in the water heating mode, the system performs well in providing services hot water in the warm seasons without bringing in summer overheating problem. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Energy consumed in buildings accounts for approximately 30e40% of the global energy consumption. Some major consump- tions are in lighting, ventilating, space heating/cooling, and water heating. To fight against the worldwide deteriorating energy wastage and pollution problems, much effort in the building sector has been on promoting solar energy utilization. To this, the appli- cations of solar thermal technology e with passive space heating and water heating as the two major approaches e have been highly successful. For passive space heating in buildings, many distinct design features have been put forward, such as the Trombe wall system [1,2], composite TrombeeMichel wall [3], and PV-Trombe wall [4]. Also introduced in the BarraeCostantini system is the thin metal- plate absorber [5]. While these named systems have been well developed, hurdles remain such as the low annual utilization rate in places with hot and lengthy summer. Summer overheating is also found to be problematic [6,7]. Although in both the Trombe wall system and BarraeCostantini system the glazing is provided with two top-and-bottom vents for summer heat dissipation, this provision is often not adequate to remove the absorbed solar heat at the building façade. Besides, the design also introduces other operational inconvenience, such as the difficulty in getting access to the two vents at the outside. In order to overcome the above barriers, a hybrid solar collector system e here named as the “building-integrated dual-function solar collector” e is introduced. The system involves a modified solar collector, which is modified from the conventional flat-plate collector by broadening the air gap between the absorber plate and the backboard, on which two vents are added. This system not only performs passive space heating in cold winter, but also as a façade water heating collector when space heating is no longer required. Such integrated design makes it more cost-effective than those conventional systems solely for solar water heating or passive space heating. On the other hand, the elimination of the vent openings at the front surface makes the application more conve- nient and practical. The concept of facade water heating collector has been proposed for decades, but very few studies were reported in literature. Rockendorf and Janssen [8] carried out experimental study on a glazed solar water heater without thermal insulation layer between absorber and wall; the result showed that more than 75% of the absorbed solar heat could be removed by the circulating water stream. Sourek [9] analyzed the active water heating system with facade solar collector and compared it with the conventional roof-mounted water heating collectors; the study showed that * Corresponding author. Tel.: þ86 551 3607346; fax: þ86 551 3607346. E-mail address: jijie@ustc.edu.cn (J. Ji). Contents lists available at ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy 0360-5442/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2010.10.004 Energy 36 (2011) 566e574