Citation: Jafari, S.; Sohani, A.; Hoseinzadeh, S.; Pourfayaz, F. The 3E Optimal Location Assessment of Flat-Plate Solar Collectors for Domestic Applications in Iran. Energies 2022, 15, 3589. https:// doi.org/10.3390/en15103589 Academic Editor: George Kosmadakis Received: 17 February 2022 Accepted: 7 May 2022 Published: 13 May 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). energies Article The 3E Optimal Location Assessment of Flat-Plate Solar Collectors for Domestic Applications in Iran Sina Jafari 1 , Ali Sohani 1 , Siamak Hoseinzadeh 2, * and Fathollah Pourfayaz 2 1 Laboratory of Optimization of Thermal Systems’ Installations, Faculty of Mechanical Engineering-Energy Division, K.N. Toosi University of Technology, No. 15-19, Pardis Street, Mollasadra Avenue, Vanak Square, Tehran 1999 143344, Iran; sinajafari96@gmail.com (S.J.); asohani@mail.kntu.ac.ir (A.S.) 2 Department of Renewable Energies and Environment, Faculty of New Sciences and Technologies, University of Tehran, Tehran 1439 957131, Iran; pourfayaz@ut.ac.ir * Correspondence: s.hoseinzadeh@ut.ac.ir Abstract: The analytical hierarchy process (AHP) was utilized to determine the optimal location on which to install flat-plate solar thermal collectors for residential buildings in a number of cities in Iran under diverse climatic conditions. The payback period of investment (IPBP) was chosen as one of the decision criteria, while payback periods of energy and greenhouse gas emissions (EPBP and GGEPBP), being two recently introduced concepts, were also taken into account to provide a broader insight from the energy, economic, and environmental (3E) benefits of the system. The novelty of this work is proposing a method to find places with the greatest potential to install flat- plate solar collectors. It was performed using AHP as a systematic decision-making tool, and based on energy, environmental, and economic criteria, which are the key aspects of an energy system. Codes developed in the MATLAB software were employed to determine the values for different investigated cities. According to the results, Yazd, located in the center of the country, was found to be the best place to install the system. This city enjoys EPBP, IPBP, and GGEPBP scores of 2.47, 3.37, and 0.71 years, respectively. The collector area for this city was also found to be 109.8 m 2 . Yazd gained a score of 26.5 out of 100. With scores of 24.4, 18.6, 15.9, and 14.6 out of 100, Tehran, Bandar Abbas, Rasht, and Tabriz were found to be the second, third, fourth, and fifth priorities for utilizing the system, respectively. Keywords: analytical hierarchy process (AHP); domestic application; solar thermal collectors; payback time of energy; payback period of greenhouse emission 1. Introduction The depletion of natural oil resources and their impact on climate change are an undeniable facts [1]. Hot–arid climate countries, such as Iran, are consistently improving their utilization of renewable energy technologies in residential buildings [2]. In this post- pandemic world, much of the energy demand is based on building air-circulation and ventilation [3]. As such, the use of solar thermal collectors for hot water and cleaning could be realized by understanding the optimal performance and determining a suitable location for higher heat transfer [4]. The harnessing of solar energy for residential buildings has gained great importance in the supply of heating and cooling energy, as well as electricity, in both small and large scales [5–7]. One of the most common technologies used to harness solar irradiation and its con- version into thermal energy is solar thermal collectors for solar heating applications [8]. Due to their low cost, simple design, and the easy installation of flat-panel solar collectors compared to other forms of solar hot water technologies, their use in buildings (residential and commercial) is being explored in hot–arid countries such as Iran [9]. It is pertinent to discuss the various research literature on the simulation and evaluation of the performance Energies 2022, 15, 3589. https://doi.org/10.3390/en15103589 https://www.mdpi.com/journal/energies