We present the results of our investigation on cylindrical honeycomb solar collector. The honeycomb has been fabricated with transparent cellulose triacetate polymer sheets. Insulation characteristics of the honeycomb were studied by varying the separation between the honeycomb and the absorber plate. The optimal value of the separation was found to be 3.3？mm for which the heat transfer coefficient is 3.06？W？m？2？K？1. This supports result of previous similar experiments. Further we test the honeycomb through a field experiment conducted in Delhi (28.6°N, 77°E) and found that when the incident angle of the solar radiation is within 20° then the performance of the system with the honeycomb is better than the one without the honeycomb. 1. Introduction A solar collector converts the solar energy into useful heat in the most efficient way. The solar collector absorbs the whole of the spectrum, unlike the photovoltaic cell which converts a part of the solar spectrum. However the efficiency of solar collectors is low as a large fraction of heat is dissipated [1]. The most significant part of heat loss from a solar thermal device is due to convection [2]. The most important challenge connected with solar collector is to reduce this loss of heat from the collector surface which may be as high as 40% [3] and therefore it has attracted the attention of scientists working in this area. In order to reduce this heat loss, generally one or more transparent plates are used to cover the collector surface. In more recent devices the space between the collector surface and the cover plate is evacuated. This makes the manufacture of solar devices expensive and energy intensive. Furthermore, the maintenance of vacuum poses additional problem for large collector surfaces that are required for building or for industrial applications. The other alternative which has been explored since the 1970s is to use a honeycomb layer between the cover plate and the collector plate [4, 5]. The honeycomb suppresses the convection and also reduces the loss of energy due to radiation while it allows the solar energy to go through. Many variations of honeycomb have been attempted. These are classified as transparent insulation materials (TIM). A good review of such materials can be found in the paper by Wong et al. [6]. The honeycombs have been applied for water heating, air heating, and building application like windows [7–10]. In Israel, panels fitted with honeycombs are available commercially. In countries like India, the full potential of honeycombs has not been fully utilized. Honeycombs
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