# Thermal Characteristics and Parametric Analysis of an Improved Solar Wall
## 1 Introduction  
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 the middle of the last century, solar air heating has aroused great interest in the community of solar researchers because it is a relatively simple, inexpensive and low maintenance 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 buildings. These heat collectors have been adopted around the world to reduce fossil fuel consumption for space heating and domestic hot water production [2]. Along these years, 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 Table 1. Among them, the most common system used for absorbing solar irradiation is the flat-plate solar air collectors, which are always integrated on building facades as passive heating components. To improve the efficiency of solar energy, there are extensive studies and efforts have focused on improving the heat efficiency and optimizing the structure of solar air collectors [12; 13; 14; 15]. A more effective approach is to maximize the contact between the circulating air and the absorber plate through forced turbulence in double channel, including V-corrugated or sine-wave absorber plates, with fins, baffles or ribs [16; 17; 18]. Taking the instantaneous efficiency of the collector as an evaluation index, [PERSON] et al. [19] proposed optimal structure design parameters of the flat-plate solar collector by numerical simulations. To simulate the thermal behavior of single/two-glass-cover solar air collector systems, [PERSON] et al. [20] built a mathematical model. [PERSON] et al. [21] investigated the performance of the solar air collector in North Eastern India, and various parameters considered for the present investigation were collector tilt angles, single and double glazing, mass flow rate and two different absorber plates. In reference [22], an innovative solar water heater integrated with a linear parabolic reflector was proposed and experimentally tested, and the experiments showed that the device was effective during winter. Its mean daily efficiency varied between 36.4% and 51.6%. [PERSON] et al. [23] investigated the performance of two types of solar air collectors with aluminum absorbers at three different air mass flow rates. Results showed that an enhancement of 15.9-41.2% in thermal efficiency was achieved in comparison to the flat plate solar air collector. [PERSON] [24] compared the performance of three different types of designed flat-plate solar air heaters by the energy and exergy analyses. He found that the heater with double glass covers and fins is more effective. There are different factors affecting the solar air heater efficiency, e.g., aspect ratio of collector, absorber materials, vent size, wind speed, etc. [25; 26; 27; 28]. To improve the dis