Solar transparent insulation materials: a review

Abstract

This paper presents a status report on solar transparent insulation materials (TIM). It covers a survey of the literature, various physical and other properties of TIM devices, their classifications, applications, fabrication procedures, availability and cost trends. The global resurgence of research is clarified. Subsequently, the development of TIM cover systems (often referred to as advanced glazing) from such products as polymer sheets, capillaries and cellular profiles, is discussed. Their design and performance characteristics are investigated; results corresponding to experimental measurements, as well as computational models, are presented. An explicit comparative study of absorber parallel and absorber perpendicular configurations of TIM cover systems is presented. The TIM covers with black end cover plates, and cellular walls of high emissivity, as well as those with selective cover plates and cellular walls fully transparent to IR radiations, have relatively lower heat loss coefficients.

Introduction

Thermal insulation is the simplest means of preventing heat losses and achieving economy in energy usage. In industry, thermal insulation serves several important functions such as preventing heat leakage, saving energy, control of temperature and thermal energy storage. Conventional insulation materials are often opaque and porous, and can be classified into fibrous, cellular, granular and reflecting types [1]. The thermal characteristic of these insulation materials is specified in terms of thermal conductivity. Stagnant air is a good insulating material; it has a thermal conductivity of 0.026 W/m·K. Primitive people used the underlying principle of air insulation by lining their garments with fur to protect them from severe winter weather. Some commonly used thermal insulation materials, such as glass fibre (thermal conductivity: 0.0325), alumina silicate (0.035), mineral wool (0.0407) and calcium silicate (0.057) have low thermal conductivity, which depends on the number of air cells packed at the cores of solid media. The diameter of air cells is about 0.09 μm, which is smaller than the mean free path of air. Heat is transferred through the insulation by conduction in solid media, convection as well as radiation across air cells. There are always some losses from thermal energy systems insulated with opaque materials.

Transparent insulation materials (TIM) represent a new class of thermal insulation wherein air gaps and evacuated spaces are used to reduce the unwanted heat losses. It consists of a transparent cellular (honeycomb) array immersed in an air layer. The air layers are similar to conventional insulation materials with regard to the placement of air gaps in the transparent solid media. TIM are solar transparent, yet they provide good thermal insulation. They hold great promise for application in increasing the solar gain of outdoor thermal energy systems. Solar transmittance and heat loss coefficient are the two parameters used for their characterization. The fundamental physical principle used in TIM is the wavelength difference between solar radiation which is received by the absorber and IR radiation which is emitted by the absorber. This review presents a status report on TIM technology and applications.