The tubes are bonded to the absorbing plate in various designs [30; 31]. The FPSCs can be classified into four sub-categories (Figure 1); however, the serpentine design is the most suitable type [32]. A serpentine tube includes a one-way flow channel that provides higher fluid temperature differences through the system because the water stays more in the collector given the greater tube length, which can be implemented. The serpentine designs benefit the lower manufacturing costs and stay efficient even at low flow rates, improving thermal stratification in the water tanks [30; 33]. Figure 2 indicates the FPSC with serpentine tubes [30]. Water is regarded as the most typical working fluid used through solar thermal systems that benefit from its cooling characteristics, abundance, economic viability [34]. Due to the higher thermal capacity of water, the water-based systems have higher efficiency than the air-based systems [35; 36]. The solar thermal collectors can be integrated with PV panels to form a hybrid photovoltaic thermal (PV/T) system that produces both heat and electricity. Several studies have proved that due to heat dissipation from PV panels, a PV/T collector performance as a single combined system is superior to a photovoltaic system and a thermal collector being employed separately [17; 18; 37; 38; 39; 40]. The heat transfer rate (the useful energy gain) is the major contributing factor to the design of any solar thermal system and is mainly affected by tube configuration and thermal properties of the working fluid. Increasing the contact surface between the absorber plate and working fluid improves the heat transfer rate, generally [41]. [PERSON] et al. [42] numerically and experimentally studied the cross-section of the different thermal collector designs. They found that square cross-sectional tubes have a better heat transfer rate than round tube collectors due to the greater contact area, which suffers from a smaller contact surface in their design. [PERSON] et al. [43] investigated the heat transfer characteristics of the circular and flattened tubes in an experimental study. They found that the heat transfer coefficient was enhanced notably for flattened tube collectors at the same flow conditions compared to circular tubes. [PERSON] et al. [23] theoretically investigated the round,  
Figure 1: The major types of FPSCs. Figure 2: The main components of an FPSC [30]. square, and rectangular tubes. They argued that rectangular cross-sections had superior heat transfer performance to round and square counterparts. Literature review revealed that only a few studies focused on developing serpentine collector designs with different and effective cross-sections (Table 1), while for typical cooling applications such as solar thermal collectors, heat transfer enhancement greatly depends on the tube cross-section. Analytical studies regarding appropriate tube configurations before the manufacturing stage will lead to designing the foremost and most effective solar thermal systems. This research aims to theoretically analyze the heat transfer characteristics of serpentine solar thermal collectors with elliptical and circular cross-sections. The solar thermal collector performance will be evaluated in terms of the mass flow rate, Reynolds number, Nusselt number, heat transfer coefficient, heat removal factor, useful energy gain, and thermal efficiency. These heat transfer controlling parameters must be calculated and determined before any manufacturing process to achieve a highly efficient and cost-effective design. This process paves the way for future research as well as encourages public investment in solar thermal collectors' technologies.