applied sciences Article Thermal Characteristics and Parametric Analysis of an Improved Solar Wall Xi Zhao 1 , Jiayin Zhu 2,*, Ruixin Li 2 , Weilin Li 2 and Bin Chen 3 1 College of Architecture, Texas A&M University, College Station, TX 77843, USA; [email protected] 2 Department of Civil Engineering, Zhengzhou University, Zhengzhou 450001, China; [email protected] (R.L.); [email protected] (W.L.) 3 Department of Construction Engineering, Dalian University of Technology, Dalian 160003, China; [email protected] * Correspondence: [email protected]; Tel.: +86-135-2658-2041 Abstract: Solar air collectors installed on buildings can significantly reduce conventional energy consumption in winter and summer. However, some problems arise in the utilization process, such as overheating, inconvenient operation control and low energy efficiency, etc. This work is a parametric analysis focusing on the automatic control and thermal efficiency improvement of the solar wall. An improved color-changing solar wall integrated with automatic control components, such as a photoelectric fan and temperature-controlled damper, was proposed in this paper. Based on the exper- imental data, the average daily heat output of the color-changing solar wall is 1.08 MJ per unit floor area on clear days in winter and the average thermal efficiency is 56.8%. Meanwhile, a quantitative analysis was carried out based on monitoring experiments for evaluating the thermal characteristic of automatic control components. Furthermore, in order to improve the thermal performance of the solar wall, parametric analysis was performed by numerical simulation. Results from this paper can provide a theoretical basis for the application of solar air collectors in modern buildings. Citation: Zhao, X.; Zhu, J.; Li, R.; Li, W.; Keywords: solar energy; an improved color-changing solar wall; automatic control components; Chen, B. Thermal Characteristics and field measurement; parametric analysis Parametric Analysis of an Improved Solar Wall. Appl. Sci. 2021, 11, 6325. https://doi.org/10.3390/app11146325 1. Introduction Academic Editor: Sergio Montelpare As a type of renewable energy, solar energy is widely used as an alternative to fossil energy, and has the highest potential to meet the ever-increasing energy demands. Since Received: 31 May 2021 the middle of the last century, solar air heating has aroused great interest in the community Accepted: 7 July 2021 of solar researchers because it is a relatively simple, inexpensive and low maintenance Published: 8 July 2021 application [1]. As a typical component of passive solar air heating, solar air collectors are mainly used to convert incident solar radiation into useful heat in different types of Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in buildings. These heat collectors have been adopted around the world to reduce fossil fuel published maps and institutional affil- consumption for space heating and domestic hot water production [2]. Along these years, iations. several types of solar air collectors, differentiated by the type of solar absorber element (flat plate, V-corrugated plate, cylindrical tubes, plates with fins, etc.) were designed, mathematically modelled and experimentally tested [3,4]. The thermal performance of different types of solar air collectors is studied and compared in Table1. Among them, the most common system used for absorbing solar irradiation is the flat-plate solar air Copyright: © 2021 by the authors. collectors, which are always integrated on building façades as passive heating components. 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/). Appl. Sci. 2021, 11, 6325. https://doi.org/10.3390/app11146325 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, x FOR PEER REVIEW 2 of 21 Appl. Sci. 2021, 11, x FOR PEER REVIEW 2 of 21 Appl. Sci. 2021, 11, x FORTable PEER REVIEW1. Investigations on thermal performance of different types of solar air collectors. 2 of 21 Appl. Sci. 2021, 11, x FOR PEER REVIEW 2 of 21 Appl. AuthorSci. 2021, 11, x FOR PEER TypesREVIEW Diagram Inference 2 of 21 Table 1. Investigations on thermal performance of different types of solar air collectors. Appl. Sci. 2021, 11, x FOR PEER REVIEW 2 of 21 Author solarTable airTypes 1. heater Investigations duct on thermal performanceDiagram of different types of solar air collectors.Inference Appl. Sci. 2021, 11, 6325 Table 1. 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