A single transparent glass cover of dimensions 4 mm thick, transmits the heat flux to the absorber plate, ensures the reduction of thermal losses due to the heat exchange by convection and radiation and protects the absorber from the outside environment.

A thin copper absorber plate with a black chrome finish on it, which was 1.5 mm thick.

A polystyrene sheet with a thickness of 40 mm, placed alongside a wooden case to minimize the heat loss from the back and sides of the collector.

The solar air collector was mounted on the south. The incident radiation is partially reflected and absorbed by the transparent glass cover of the solar collector,

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Figure 6 provides a detailed overview of heat exchanges in the solar air collector. However, the equivalent circuit diagram is given by figure 7; this implicit scheme is very useful to the determination of heat losses at different levels. The transposition of the thermal problem into an electrical problem is called thermo-electrical analogy. Working this analogy, the nodal method leads to the setting up of an electrical network. The nodes, which in an electrical meaning symbolize equipotentials, correspond to isotherm lines. On the outside of the transparent glass cover, the heat-balance equation is written as follow:

Figure 7: here QSun_G: The heat ﬂux emitted by the sun incident on the transparent glass cover (W), it is given by the following equation: QSky_G: The heat ﬂux emitted by the sky on the transparent glass cover (W), it is given by the following equation: The radiation transfer heat coefficient between the transparent glass cover and the sky is calculated by the following equation [40]:

The sky temperature could be determined by Swinbank equation [41]: QGround_G: The radiation heat transfer emitted by the ground on the transparent glass cover (W), is given by the

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Different numerical methods are used to integrate the differential governing equations. The choice of the numerical method which will be joined with the influence of calculations steps in terms of accuracy are discussed in results section.

However, an equivalent circuit diagram, shown in figure 7, was established regarding the energy balance analysis on solar collectors to investigate variations of various parameters affecting their performance.

According to ASHRAE standard [44], the useful thermal power Qu is defined as the difference between the collected heat ﬂux and the losses, can be evaluated using equation 43. It may be also expressed as function of the gained and lost energy of the absorber as given by equation