Transient 3D: Simulation of a Flat Plate Solar Collector in a Mild Climate Condition

Introduction

Utilization of solar energy for heating applications is one of the main scopes of renewable energies technologies (Carpaneto et al., 2015; Hussain et al., 2017; Palzer and Henning, 2014; Shepovalova, 2015; Ozoegwu et al., 2017). The solar water heating system is popular because of its simple technology and short payback time (Wang et al., 2015; Abd-ur-Rehman and Al-Sulaiman, 2016; Gautam et al., 2017; Giglio and Lamberts, 2016). Another reason for this priority is that the solar heating system needs a low temperature (Reddy, 1995; Serale et al., 2014). A significant amount of energy needed to heat drinking water and buildings (Yousefi et al., 2016). Considering the geographical conditions, this could be provided by solar energy most of the year (Lamnatou et al., 2016; Baljit et al., 2016). By employing solar collectors, even in cold regions, 40-60 percent of the annual cost of consuming energy could be reduced (Shtrakov and Batova, 2002). Simple manufacturing, the absence of movable parts and no need for maintenance resulted in the superiority of thermosiphon heaters than the other kinds of such as forced convection heater. The most important part of the solar heater is the collector. Its main role is to absorb solar radiation and transfer it to the operating fluid flowing in the pipes or canals. Flat plate collectors are usually used in solar heating systems. This collector is the simplest and the most common kind for conveying solar radiation energy to useful heat (Pandey and Chaurasiya, 2017).

Early in the morning, solar radiation starts heating the water in the collector. The heated fluid within the collector moves upward by natural convection, reaches to the container, while the cold water in the container flows from its bottom into the collector. Therefore, natural convection, where enough solar radiation exists, will be established spontaneously. In contrast, in the lack of solar radiation, since the upward body force cannot defeat the friction losses in the pipes, the circulation will stop. These systems have relatively low initial cost and because of not using many mechanical equipment, they have very high reliability (Kalogirou, 2009; Hossain et al., 2011).

Formerly, the temperature distribution on absorber of a flat plate collector, by solving one and two-dimensional conduction equations in steady state is presented (Kazeminejad, 2002). In Akhtar and Mullick research, two glass covers on the collector in the two-dimensional state are used and some analytical relations for obtaining the glass temperatures and heating loss coefficient are presented (2007). In Khoukhi et al. the efficiency of collector utilizing ordinary and low iron glass in the steady and two-dimensional state are compared (2007). Their results showed that low iron glass efficiency is higher than the ordinary glass (Kundu, 2002). It is observed that the trapeze profile is the best choice for heat transfer in the collector, although this profile is rarely seen in the designs because of its manufacturing problems (Kundu, 2002). In the most researches, collectors are geometrically simplified and modeled in the two-dimensional and steady state. Air gaps and glass layers are not included in calculations and just the effect of these parameters on absorber is considered and by applying constant flux on the absorber's plate, the temperature increase in water is established (Deng et al., 2016; Gunjo et al., 2017; Sun et al., 2016).