A review of phase change material based thermal energy accumulators in small-scale solar thermal dryers

Abstract

Solar thermal energy is usually of intermittent and dynamic character and the possibility to use it during non-sunshine periods is one of the current interest of researchers. Phase change materials as thermal energy accumulators are attractive because of their high storage density and ability to release thermal energy at a constant temperature corresponding to the phase transition temperature. This paper reviews the recent state-of-the-art small-scale solar thermal dryers integrated with phase change material as energy accumulators. This is an intensive field of investigation for more than 30 years with importance for the agriculture and the food industry, especially in hot climate. A variety of commercial small-scale solar dryers are offered as a low-cost, zero-energy solution for small farmers. And yet, there are no commercial systems using latent thermal storage because at the present level of development this unit will increase unacceptably the price of the system. The solution needs a very simple design, accessible materials and optimal conditions for operation. The aim of the present work is on the basis of the recent developments to identify the requirements for this new solution. Among the great number of designs, devices and materials, it compares the most cost and energy effective solar dryer systems with thermal storage. At the same time it makes an overview of the methods for theoretical evaluation and prediction, which are used to design and assess them. The resulting conclusions from the collected and compared information will serve as prerequisites for a novel solution of a cost-effective thermal energy storage for a small-scale solar dryer, which will lead to improved efficiency of the drying process due to controlled temperature and longer operation time. This information might serve also in the development of the wider field of thermal energy storage, which is an important part of the technologies of renewable and waste energy conversion.