Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener Development of PCM-to-air heat exchanger for integration in building envelope–modeling and validation Mohamed Dardir a,b , Mohamed El Mankibi c , Fariborz Haghighat a, ⁎ , Lubomir Klimes d a Energy and Environment Group, Department of Building, Civil and Environmental Engineering, Concordia University, Montreal, Canada b Department of Architectural Engineering, Ain Shams University, Cairo, Egypt c ENTPE-Université de Lyon, 3 Rue Maurice Audin, 69120 Vaulx en velin, France d Sustainable Process Integration Laboratory–SPIL, Brno University of Technology, Brno, Czech Republic ARTICLE INFO Keywords: Energy storage Radiative cooling PCM-to-air heat exchangers Numerical modeling Experimental validation ABSTRACT Earlier applications of phase change material (PCM)-to-air heat exchangers (PAHXs) reported the insufficient cooling charging energy needed for complete solidification of the PCM in free cooling systems. Also, the pre- diction of PAHX performance under low airflow regimes is a system limitation for free cooling applications. Besides, the implementation of the long wave thermal radiation cooling concept has not gained much attention in the free cooling design of PAHX units. This paper reports the development of PAHX system for building envelope applications that promotes the thermal radiation loss to the sky during night-time to maximize the cooling potential. A 2D numerical model has been developed considering the PCM thermal behavior, short and long wave radiation, and convection phenomena. New thermal boundaries of long-wave radiation have been proposed between system elements and the sky temperature. In addition, the model considered various forms of convective heat transfer phenomena. The apparent heat capacity method was used to simulate the thermal storage process. Experimentally obtained data and inter-model comparison were used to validate the proposed model. Two full-scale prototypes of the developed PAHX system were designed and tested under real conditions using two different types of PCMs. A parametric analysis was conducted to investigate the system thermal behavior under various air velocity profiles in the air channel and various inlet air temperature conditions. The results indicate that the building envelope integrated PAHX can use the sky radiation as a cooling source. 1. Introduction Phase Change Materials (PCMs) are being used in building appli- cations due to their thermal storage abilities. Generally, the principle of thermal storage using PCMs is the release of a large amount of energy, the latent heat of fusion, during the phase change process at a relatively narrow range of temperatures called the Phase Change Temperature (PCT) range. In free cooling systems, thermal storage using PCMs is a reversible process through the cooling charging process (solidification of PCM/heat release from PCM) and the cooling discharging process (melting of PCM/heat storage to PCM) (Dardir et al., 2019). In some latent heat thermal energy storage systems, air or water, as a heat transfer fluid, is used to charge and discharge the PCM (Nkwetta et al., 2014; El-Sawi et al., 2013). Applications of PCM-to-air heat exchangers (PAHXs) have been widely discussed in the literature. Those applica- tions can be classified into two main categories: units integrated into ventilation systems, and ventilated double skin facades. Both types involve the convective heat transfer process between air and PCM within the heat exchanger. They differ in the system configuration, the amount and shape of PCM, system dimensions, installation challenges, and heat gains and losses (de Gracia et al., 2015; Mosaffa et al., 2013). They also have different boundary conditions, as the ventilated facade type can involve significant radiative component and asymmetric conditions along plate thickness. Borderon et al. (2015) investigated multiple units of a flat micro- encapsulated plated PAHX with different ventilation modes under cli- mates of four different cities in France for summer cooling applications. The system performance was numerically investigated using a 2D heat transfer model utilizing the apparent heat capacity method considering forced convection. Waqas and Kumar (2011) investigated the perfor- mance of flat encapsulated configuration using a 2D numerical model utilizing the enthalpy method for both laminar and turbulent flows. Darzi et al. (2013) studied the performance of a flat plated PAHX during the melting process under various airflow rates, and a 2D numerical https://doi.org/10.1016/j.solener.2019.08.003 Received 31 May 2019; Received in revised form 29 July 2019; Accepted 2 August 2019 ⁎ Corresponding author. E-mail address: Fariborz.Haghighat@concordia.ca (F. Haghighat). Solar Energy 190 (2019) 367–385 Available online 20 August 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved. T