Collector Efficiency Curves considering an Adapted Calculation Approach for Convective Heat Transfer in Insulating Gas Layers of Solar Collectors
Abstract
Recent investigations of the heat transfer in a gas layer between parallel plates heated from below indicate that there is a convective flow also below the critical Rayleigh number of 1708 in the so-called conductive flow regime. Based on experimental investigations, a correction term is proposed in this study that better characterizes the heat transfer below the critical Rayleigh number. The aspect ratio of the insulating gas layer is identified as the dependent variable of the correction function, which has a significant influence on the flow regimes and thus the heat transport phenomena that occur. This paper presents collector efficiency curves of a glass collector. Taking into account the convective flow at the critical Rayleigh number, the Nusselt number is increased by about 23 % for an aspect ratio of 48. This leads to an increase of the collector efficiency by nearly 3 % for a temperature difference of 40 K between the collector fluid and ambient temperature, and more than 6 % for a temperature difference of 60 K.