Abstract
There is a permanent interest in improving the construction and thermal properties of the planar solar collectors. Minimizing the heat losses of the collectors and improving their energy gains are some of the relevant research goals. Convective heat transfer inside the cavity of planar collectors has been determined theoretically and numerically in this conference contribution. The convective heat transfer is computed on the basis of physical relations using the correlation relations of Nusselt’s number. The Nusselt number has been expressed by means of Rayleigh’s and Prandtl’s numbers. The convective heat transfer inside the cavities of planar solar collectors represents essential part of heat losses that lower the effectiveness of these collectors. In this conference contribution the convective heat losses have been compared with the standard density of heat flow coming from the Sun and the corresponding energy ratio has been determined. It has been shown that this ratio amounts almost 10 %. A typical summer environment has been supposed with a standard sunshine reaching 1000 Wm−2. The common type of the planar solar collector with geometrical dimensions 2m x 1m x 0.05m and cavity filled by air has been investigated. The temperature of the absorber has been supposed to be 70 °C whereas the temperature of the glass plate placed on the top side of the collector has been assumed to be 35 °C.
Highlights
A solar collector is a device that collects solar radiation from the Sun and transfers it to a heat carrying medium
Assuming the normal density of energy flow from the Sun to the surface of the Earth Qsun 1000 Wm-2, we find that the convective heat loss represent almost 10 % of the Sun energy impacting on one meter squared of the Earth
In this conference contribution the air cavity of the plate solar collector has been studied from the viewpoint of thermal properties
Summary
A solar collector is a device that collects solar radiation from the Sun and transfers it to a heat carrying medium. The air inside a closed cavity is known to be a good thermal insulator [1,2]. The convective heat transfers inside the cavities of collectors represent essential heat losses that lower the effectiveness of these systems. In the present paper, only convective heat losses will be studied and their quantity compared with the energy coming from of the Sun. Section 2 of this contribution presents the basic physical relations concerning convective heat transfer occurring in the closed air cavities of collectors. The notation * implies that, if the quantity in brackets is negative, it must be set equal to zero
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