Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser Exergy and sustainability index of photovoltaic thermal (PVT) air collector: A theoretical and experimental study Ahmad Fudholi a, ⁎ , Muhammad Zohri a,b , Nurul Shahirah Binti Rukman a , Nurul Syakirah Nazri a , Muslizainun Mustapha a , Chan Hoy Yen a , Masita Mohammad a , Kamaruzzaman Sopian a a Solar Energy Research Institute, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia b College Computer Information Management (STMIK), Mataram, Indonesia ARTICLE INFO Keywords: Solar energy Exergy analysis Solar collector Thermal Electrical Improvement potential ABSTRACT This paper presents a review of the exergy and sustainability index of solar thermal systems. The review includes exergy analyses of solar collectors, solar drying systems and photovoltaic thermal (PVT) systems. Solar collec- tors, the most essential components of solar thermal systems, receive solar energy and convert it into thermal energy. The PVT collector is a popular means of harvesting solar energy. A PVT collector can generate electrical and thermal energies simultaneously. Experimental and theoretical approaches for a PVT air collector with a ∇-corrugated absorber are investigated in this study. A steady-state energy analysis of the PVT air collector is performed to predict photovoltaic (PV) and air outlet temperatures. Experimental results are in close agreement with the results of the theoretical study. The percentage errors of PV and air outlet temperatures between experimental and theoretical values are 5.49% and 3.75%, respectively. The PVT exergy efficiency of the PVT air collector with a ∇-corrugated absorber is 13.36% and 12.89% for the theoretical and experimental study, re- spectively. Furthermore, a sustainability index is proposed. The sustainability index of the PVT air collector is 1.168 and 1.148 for the theoretical and experimental study, respectively. 1. Introduction As the human population continues to grow, the economic and energy demands of countries sharply increase. Thus, energy consump- tion increases tremendously annually. Most of this energy is generated by a carrier from the centre of a conventional type of energy, such as coal and natural gas. Considering that renewable energy sources are economical and practical, researchers continue to focus on them be- cause of the increasing energy demand and oil prices. The amount of greenhouse gas simultaneously increases because of the increasing usage of electricity for heating, refrigeration and air conditioning. Green technologies, such as solar photovoltaic (PV) systems, wind turbines, hydrogenation plants and biomass and photovoltaic thermal (PVT) systems, can overcome the decreasing conventional energy. PVT systems have been developed for various applications over the past few years. However, new studies are still being conducted to im- prove the electrical and thermal efficiencies of PVT. Modules and building integration photovoltaic (BIPV) systems have become in- creasingly popular in many areas, particularly in industrialised coun- tries where government support has accelerated the installation of PV systems to grid connections. Connection to PVT increases the efficiency of PV. Various studies have been conducted in recent years, and PVT systems with water and air as the heat carrier have been developed and reviewed [1–5]. Various experimental and theoretical studies on PVT collectors are available in the literature. Kern and Russell [6] proposed PVT collectors that use air or water as a heat removal fluid. Ra- ghuraman [7] introduced methods for predicting the efficiency achieved by the flat plates of air and water PVT collectors. Prakash [8] conducted a detailed study on the effect of air duct length, depth, mass flow rate and area fraction of an absorber plate surrounded by solar cells. Caliscan [9] analysed the performance of solar collectors. The per- formance analysis included exergoenviroeconomic, exergoenviron- mental, enviroeconomic, environmental, exergy and energy analyses. He reported that the sustainability index and exergy efficiency of solar collectors are 1.0073% and 0.732%, respectively. The sustainability index was initially implemented for this type of systems in open lit- erature. However, to the best of the authors’ knowledge, only one da- taset regarding the sustainability index of solar energy systems cur- rently exists, and no information is available with regard to the https://doi.org/10.1016/j.rser.2018.10.019 Received 3 December 2017; Received in revised form 14 July 2018; Accepted 15 October 2018 ⁎ Corresponding author. E-mail address: a.fudholi@ukm.edu.my (A. Fudholi). Renewable and Sustainable Energy Reviews 100 (2019) 44–51 Available online 30 October 2018 1364-0321/ © 2018 Elsevier Ltd. All rights reserved. T