# Solar Potential in Saudi Arabia for Southward-Inclined Flat-Plate Surfaces
## 1 Introduction  
Installations with tilted solar collectors for exploiting the renewable energy source of the Sun have long been available in the market as commercial products. Solar flat-plate panels are nowadays widely used for converting solar energy into electricity (PV installations) or hot water (solar thermosyphons, solar heating systems). These stationary systems consist of solar panels that receive solar radiation at fixed tilt angles with a southward orientation in the northern hemisphere. For exploiting the available solar energy at a location or a region, prior knowledge of this potential is necessary. This is gained by solar radiation measurements on both inclined and mostly horizontal surfaces. Unfortunately, solar radiation measurements on horizontal planes are not common worldwide due to the purchase and maintenance costs of the radiometers required [1]. On the other hand, solar radiation measurements on inclined surfaces are really scarce [2]. To fill this gap, solar radiation models have been developed since the second half of the 20 th century. A good account of the most widely used solar radiation models is given in [3]. Internationally, there have been many studies to estimate the performance of static solar systems. The main effort in such studies has been focused on finding the most suitable tilt angle(s) and orientation(s) throughout the year at various locations in the northern hemisphere for maximum retrieval of solar energy by flat-plate collectors in ayear e.g., [4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15], or within seasons [16]. On the other hand, there are works on the technical improvement of solar panels for higher efficiency e.g., [17; 18; 19; 20; 21]. Nevertheless, the scarcity of solar radiation measurements has triggered studies to use solar radiation modelling e.g., [10; 11; 22] in order to derive the optimum tilt angle and orientation for maximum solar energy on solar flat-plate collectors. Other methods use a combination of ground-based solar data and modelling e.g., [2] or utilise solar data from international data bases e.g., [23; 24]. Recently, a new method was presented by [PERSON] and [PERSON] [25] for Greece that estimates the optimum tilt angle of an inclined flat-plate collector with southern orientation for maximum solar energy gain. This method is followed in the present study. As far as Saudi Arabia is concerned, some research has already appeared in the literature related to the present study. [PERSON] et al. [26] estimated the global, direct, and diffuse solar radiation components on horizontal and tilted surfaces at Jeddah. [PERSON] et al. [23] estimated the optimal tilt angle for maximum energy reception by PV installations at 7 locations (cities) of the country. They suggested that the PV panels should be adjusted 6 times in a year for maximum performance, an outcome that burdens installation and maintenance costs because of the moving parts involved. [PERSON] et al. [27] performed solar radiation measurements at 30 stations in Saudi Arabia during the period October 2013-September 2014 (1 year) to assess the solar radiation resource at these locations. The World Bank [28] produced the Global Solar Atlas, which includes Saudi Arabia. This solar map provides the distribution of the 3 solar radiation components over the country and is based on calculations in the period 1999-2018. Finally, [PERSON] et al. [29] provided a study about the economics of solar energy in Saudi Arabia. From the above it is clear that no attempt has been made so far to construct a solar map for Saudi Arabia to show the potential of flat planes inclined southwards for the exploitation of solar energy by appropriate systems. This gap is bridged by the present study, which includes three innovations. (i) For the first time, solar maps for Saudi Arabia for the maximum energy on optimally inclined flat surfaces towards south are derived. (ii) For the first time in Saudi Arabia three energy zones are identified for solar applications.