Steam Based Charging-Discharging of a PCM Heat Storage
Abstract
Latent heat storage and efficient heat transport technology helps to utilize the intermittent solar energy for continuous and near isothermal applications. Nonetheless, latent heat storages face challenges of storage charging, heat retaining, and discharging the stored heat. The focus point of solar concentrators is the ideal position to collect maximum thermal power; however, this place is not convenient. Therefore, it is important to transport the concentrated heat from the focus to some place efficiently in order to utilize it safely. This paper addresses the challenges of heat transportation and storage charging-discharging issues. The heat transportation from the receiver over some distance is carried out with a pipe lines and water as heat transfer fluids. However, the charging-discharging process was carried by pipe less method with the help of fins. In addition, the stored heat was retained for a couple of days by using appropriate insulation material. The latent heat is stored in a phase change material (PCM) of nitrate salt (mixture of 60%NaNO3 and 40%KNO3), which melts at 222oC and has 109 J/g specific heat of fusion. The storage was designed to supply nearly isothermal heat during the liquid-solid phase transition and the sensible heat stored in the solid and liquid form is used to perform additional applications that do not require uniform heat. The low thermal conductivity of the PCM is improved by using extended fins, which enhance the thermal conductivity within the storage. In this article, two-phase loop thermosyphon of steam was used to manage long distance heat transportation between the receiver and the storage. The steam in the thermosyphon flow was restricted to a maximum working temperature of 250oC. Steam is selected for its highest heat capacity, easily availability and stable nature. The steam carries the heat from the collector focus point and condenses in a coiled pipe imbedded in aluminum plate placed on top of the storage. Many fins have solidly attached to this plate to conduct this heat down to the PCM inside the storage during charging. This design configuration avoids pressure development inside the PCM storage. The charging-discharging temperature of the storage is recorded in three zones (top, middle and bottom) of the storage. The experimental and numerical results show the heat transportation, retention and charging-discharging methods are effective.
Keywords
Solar energy, Pcm storage, Latent heat storage, Two-phase thermosyphon