## 1 Introduction  
Solar flat-plate panels are widely used for converting solar energy into electricity. They are classified into two big groups: stationary (fixed) and dynamic (moving along one axis or dual axes) modules [1]. The stationary systems consist of solar panels that operate (receive solar energy) at fixed tilt angles with a southern orientation in the northern hemisphere. The dynamic group includes solar installations comprised of moving solar panels, which follow (track) the Sun. They are divided into one-axis and dual-axis systems. It is self-evident that the best performance of a solar system is from a dynamic one with dual-axis operation. This way, the solar panels are always normal to the solar rays throughout the day, receiving the maximum possible solar energy. Nevertheless, dynamic solar systems have higher costs than stationary ones due to their moving parts. In practice, solar radiation measurements on a horizontal plane are infrequent worldwide due to the purchase and maintenance costs of the required radiometers [2]. On the other hand, solar radiation measurements on inclined surfaces are indeed rare [3] and, if they exist, they are performed for a limited time period. Therefore, solar radiation models for this purpose have been developed since the 1950s and are now numerous in the international literature. A good survey of most existing solar radiation models is given in [4]. Internationally, there have been many studies carried out in order to assess the best performance of either stationary (fixed tilt angle and constant orientation) or dynamic (solar tracking) systems. Efforts from researchers to give answers about the most suitable tilt angle or angles and southerly orientations throughout the year at various locations in the northern hemisphere for the best performance of solar energy flat-plate convertors are numerous. Most of them deal with the estimation of the most appropriate tilt and azimuth angles for static flat-plate convertors to receive the maximum solar energy in a year [1; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27], within seasons [28], or for static photovoltaic (PV) flat-plate collectors in a year [29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44]. Such studies mostly used solar radiation models or statistical methods [16; 17; 27] to derive the optimum tilt angle and orientation for the maximum solar energy received on a solar flat-plate convector. Others used a combination of ground-based solar data and modeling [1], the retrieval of solar data from international data bases [45], or even satellite-derived solar data [46] with the purpose of deriving the maximum solar energy. Fewer studies, however, used solar radiation data from ground stations, because such stations are sparse. The present study aims to investigate the optimal tilt angle with a southern orientation over Greece, but it confronts this matter in a different and mostly innovative way. It focuses on stationary solar flat-plate convertors (as Part I of the study), while two future studies intend to investigate the energy gain from dynamic solar flat-plate convectors (single-axis systems in Part II and dual-axis systems in Part III). As far as Greece is concerned, several works have appeared in the literature in the context of the aim of the present study. [PERSON] and [PERSON] [43] found that for south-facing PV arrays, the optimum tilt angles for azimuths of \(\pm 30^{\circ}\) (\(0^{\circ}\) due south) receive solar power about 30% greater than those at tilt angles equal to the latitude of the site. On the contrary, [PERSON] [47] estimated the optimum tilt angles with southern orientations for Athens to be \(25^{\circ}\) S in the case of solar flat-plate panels and \(30^{\circ}\) S for concentrated solar cells (CPC) all year round. [PERSON] and [PERSON] [48] repeated their calculations for Athens and found that a tilt angle equal to the latitude of the site collected the maximum energy all year round. [PERSON] et al.