Experimental and numerical study on a novel low temperature façade solar thermal collector to decrease the heating demands: A south-north pipe-embedded closed-water-loop system Mohamad Ibrahim a,b,⇑ , Etienne Wurtz a,b , Jocelyn Anger a,b , Oussama Ibrahim c a Univ. Grenoble Alpes, INES, F-73375 Le Bourget du Lac, France b CEA, LITEN, Department of Solar Technologies, F-73375 Le Bourget du Lac, France c Faculty of Engineering, Lebanese University, Beirut, Lebanon article info Article history: Received 20 October 2016 Received in revised form 14 February 2017 Accepted 21 February 2017 Keywords: Active pipe-embedded wall Façade solar thermal collector Energy efficiency Low-grade energy source Thermally-active envelope abstract Recently, more and more research is being conducted on thermo-activated building walls with the use of circulating water or other fluids for the aim of decreasing and shifting the heating and/or cooling loads. In this context, we present a novel concept of the active embedded-pipe envelope systems. The system con- sists of an active closed-loop-water-pipes embedded in the building exterior walls to harvest and utilize the solar energy gain on the south wall exterior surface to decrease or offset the heat loss through the north wall and enhance thermal comfort. During non-cloudy winter days, a significant amount of solar energy hitting the south (insulated) facade is not transferred to the inside environment. In this study, a comparative experimental set-up is carried out to test the system’s efficiency and com- pare its performance to that of a static insulated envelope without the system. Also, a numerical model is developed and validated against experimental measurements. Numerical simulations with a parametric study are carried out to examine the active wall loop system’s efficiency for different design and operat- ing parameters. The main conclusion derived from this study is that the system performs very well in the Mediterranean climates (or similar climates) and to a lower extent in the cooler ones. However, its per- formance is highly dependent on several design, climatic, and operating variables which should be opti- mized to have the best performance. Ó 2017 Elsevier Ltd. All rights reserved. 1. Introduction The building fabric plays a major role in regulating the indoor environment. Through influencing the energy flows between the indoors and the outdoors, the building walls are key components for energy saving in the building enclosure structure. Traditionally, the building envelope has been treated as a static or passive system in the building thermal energy system allowing or resisting heat energy transfer from the inside to the outside or vice versa. However, as our needs have evolved and technologies have advanced, the demand placed on designers to integrate a wide range of increasingly complex materials, components, and systems into the building enclosure has grown. One of the methods is to use low-grade energy sources such as solar energy, underground water, cool air, and geothermal energy in an active embedded-pipe envelope system that utilizes the cir- culating water to transfer heat to or from the inside space. This is quite popular for the ceiling and floor, such as chilled ceiling sys- tems and under-floor heating systems. A comprehensive review of the research and application of active hollow core slabs in building systems for utilizing low energy sources is presented in Xu et al. (2014) and Xu et al. (2010). Fewer studies have dealt with embedded-pipe systems in the building exterior fabric, particularly the exterior walls, with water or fluid circulating inside for heating and cooling. As reported by Yu et al. (2016), this approach can work with lower temperature gradients, leading to relatively easier utilization of low-grade energy sources and consuming less energy compared to air-based systems. Recently, more and more research is being conducted on thermo-activated building walls with the use of circulating water or other fluids for the aim of decreasing and shifting the heating and/or cooling loads. In a very recent study, Yu et al. (2016) http://dx.doi.org/10.1016/j.solener.2017.02.036 0038-092X/Ó 2017 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: CEA, LITEN, Department of Solar Technologies, INES, F-73375 Le Bourget du Lac, France. E-mail address: mohamad.ibrahim@cea.fr (M. Ibrahim). Solar Energy 147 (2017) 22–36 Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener