1. Field of the Invention
The present invention relates to a solar heat collecting apparatus for collecting solar heat, and in particular to a solar heat collecting apparatus in which solar heat is collected by a flat plate-shaped heat collecting panel installed in a vacuum space.
2. Description of the Related Art
Conventionally, various types of solar heat collecting apparatus have been suggested. One known solar heat collecting apparatus is a solar heat collector which collects solar heat using a heat-collecting panel in the form of a flat plate installed within a vacuum space. Such a solar heat collecting apparatus of vacuum and flat plate type is disclosed in, for example, U.S. Pat. No. 6,119,683 given to the same applicant of this invention and Japanese Patent Laid-Open Publication No. Hei 11-14162. In such a solar heat collecting apparatus of vacuum and flat plate type, the vacuum space is formed by a casing body and a window glass serving as a lid of the casing body at high vacuum. Inside the vacuum space, a heat collecting panel is placed at a position almost parallel to the window glass. Further, a plurality of pillars are disposed in the vacuum space to support the window glass receiving atmospheric pressure. A plurality of the pillars pass through the heat collecting panel.
Here, as the heat collecting panel absorbs solar heat, the heat collecting panel swells due to thermal expansion, which may cause the heat collecting panel to make contact with the pillars and a side wall of the casing body. When the heat collecting panel touches the pillars and the side wall of the casing body, collected heat will be released to the outside via the casing body and the pillars due to conductive heat transfer properties of the casing body and the pillars, which results in a decrease in efficiency of the heat collecting apparatus. Further, in some cases, the pillars, the casing body, and the heat collecting panel might be damaged. Accordingly, clearance gaps of a sufficient size are provided between the side wall of the casing body and the heat collecting panel, and between the pillars and the heat collecting panel, so as to prevent physical contact with the heat collecting panel from occurring due to thermal expansion of the heat collecting panel.
However, provision of such clearance gaps allows infrared rays of secondary radiation emerging from the undersurface of the heat collecting panel to escape through the clearance gaps to the outside, which causes loss due to radiant heat transfer, that is, so-called radiative leakage. This radiative leakage brings about a state similar to so-called cavity radiation because an area of the clearance gap is far less than that of the undersurface of the heat collecting panel. In cavity radiation, a large amount of radiative leakage is developed from opening of a small area, as described on Pages 385-386 of “Dennetsu Gairon (An Introduction of Heat Transfer)”, 29th edition, Jul. 20, 1990, written by Yoshiro Kattou, and published by yokendo Co., Ltd. Because a state similar to cavity radiation arises in the solar heat collecting apparatus, opening of a small area, in other words, the clearance gaps from the heat collecting panel to the side wall of the casing body and to the pillars can cause a large amount of radiative leakage. Thus, by creating the clearance gaps from the heat collecting panel to the side wall of the casing body and to the pillars, efficiency of the solar heat collecting apparatus is reduced.
In conventional apparatus, therefore, it has been desired to improve the efficiency by suppressing external radiation of infrared rays.