coatings Article Indoor and Outdoor Performance Study of Metallic Zinc Particles in Black Paint to Improve Solar Absorption for Solar Still Application Muhammad Shakeel Ahmad 1 , Shwe Sin Han 1 , Amad Zafar 2 , Usman Ghafoor 3 , Nasrudin Abd Rahim 1 , Muhammad Umair Ali 4, * and You Seung Rim 5, *   Citation: Ahmad, M.S.; Han, S.S.; Zafar, A.; Ghafoor, U.; Rahim, N.A.; Ali, M.U.; Rim, Y.S. Indoor and Outdoor Performance Study of Metallic Zinc Particles in Black Paint to Improve Solar Absorption for Solar Still Application. Coatings 2021, 11, 536. https://doi.org/10.3390/ coatings11050536 Academic Editor: Luigi Calabrese Received: 6 April 2021 Accepted: 28 April 2021 Published: 30 April 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 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/). 1 Higher Institution Centre of Excellence (HICoE), UM Power Energy Dedicated Advanced Centre (UMPEDAC), Level 4, Wisma R&D, University of Malaya, JalanPantai Baharu, Kuala Lumpur 59990, Malaysia; shakeelalpha@gmail.com (M.S.A.); shwesinhan2018@gmail.com (S.S.H.); nasrudin@um.edu.my (N.A.R.) 2 Department of Electrical Engineering, University of Lahore, Islamabad 54590, Pakistan; amad.zafar@ee.uol.edu.pk 3 Department of Mechanical Engineering, Institute of Space Technology, Islamabad 44000, Pakistan; usmanghafoor99@gmail.com 4 Department of Unmanned Vehicle Engineering, Sejong University, Seoul 05006, Korea 5 Department of Intelligent Mechatronics Engineering and Convergence Engineering for Intelligent Drone, Sejong University, Seoul 05006, Korea * Correspondence: umair@sejong.ac.kr (M.U.A.); youseung@sejong.ac.kr (Y.S.R.) Abstract: In this study, the effects of metallic zinc (Zn) particles were studied to increase surface temperature on a solar-still absorber, which is a major component of increased production. Various concentrations of Zn particles were mixed in black paint and applied to the absorber plate. SEM and XRD were used to examine and confirm the surface morphology and phase identification of as-received powder. UV-Vis spectroscopy was used to examine light-absorption properties. Finally, extensive indoor testing (using an improvised solar emulator) and outdoor testing were conducted to optimize the concentration. The specimens containing 10 wt% Zn in black paint showed the highest increase in temperature, i.e., 103.53 ◦ C in indoor conditions at 1000 W/m 2 irradiation, which is 59.17% higher than a bare aluminum plate and 17.57% higher than an only black-paint-coated aluminum plate. On the other hand, specimens containing 10 wt% Zn reached just 87.53 ◦ C, compared to 80.00 ◦ C for an only black-paint-coated aluminum plate and 60.62 ◦ C for bare aluminum. Keywords: absorber; desalination; zinc; solar still; temperature; thermal conduction 1. Introduction Freshwater is essential for human beings’ survival, and it plays a crucial role in a sustainable environment. Therefore, the demand for potable water is increasing gradually every year due to population enlargement. Although 70% of the earth is covered with water, most of the accessible water cannot be utilized for drinking due to salinity and water contamination [1]. Various techniques, such as membrane desalination [2–4] and dialysis [5,6] are used in the purification of water. One of the potential alternatives to produce fresh water and decrease the cost of distillation plants is the utilization of solar energy directly using flat collections of concentrated collectors [7]. Solar distillation is one technique to produce fresh water at a lower cost than the other possible seawater desalination processes [8,9]. Solar distillation using solar stills is the most feasible and environment-friendly way to supply fresh water in arid and remote regions [10]. Solar stills work on evaporation and condensation processes and generally consist of a water basin and a transparent cover that allows sunlight penetration and stimulates the evaporation process. The evaporated water condenses on the backside of the top cover and Coatings 2021, 11, 536. https://doi.org/10.3390/coatings11050536 https://www.mdpi.com/journal/coatings