Sensible and Latent Storage Heat

Copyright © 2011 Praise Worthy Prize S.r.l. - All rights reserved International Review of Mechanical Engineering, Vol. 5, N. 2 Special Issue on Heat Transfer 280 heaters with paraffin as thermal energy storage material. The first system had tank in tank type storage and the second had integrated type of storage using a reflector. Both systems were able to deliver hot water during the night and in morning on a 24 h cycle basis the two systems were found to be 45 and 60 efficient respectively. Canbazoglou et al [12] presented some results of investigations on solar energy storage performance using sodium thiosulfate pentahydrate in a conventional solar water heating system. In this study, authors used an experimental, open-loop conventional passive solar energy system with a natural circulation to provide domestic hot water. The system consisted of solar collectors, hot and cold water tanks and was equipped with sensors to take measurements. Al-Hinti et al [6] made an experimental study of the performance of water-phase change material PCM storage for use with conventional solar water heating systems. Paraffin wax contained in small cylindrical aluminum containers was used as the PCM. The containers were packed in a commercially available, cylindrical hot water storage tank on two levels. The PCM storage advantage is demonstrated with controlled energy input experiments with the aid of an electrical heater on an isolated storage tank, with and without the PCM containers. It was found that the use of the proposed configuration can provide a 13–14 o C advantage in the stored hot water temperature for extended periods of time. In this work, typical results from a solar installation developed at Technological Educational Institution of Halkida to test PCM effect on solar energy storage under Greek climate conditions are presented and discussed. Results show clearly enhancement of solar thermal energy storage performance compared to a conventional system. The suggested storage configuration is simple and can be easily used with existing conventional systems without major or expensive modifications.

II. Sensible and Latent Storage Heat

he basic methods for storing thermal energy are sensible heat storage and latent heat storage. In sensible heat storage heating a liquid or a solid takes place without changing phase. Sensible heat storage systems utilize the heat capacity and the change in temperature of the material during the process of charging and discharging. In sensible heat storage, the temperature of the medium changes during charging or discharging of the system. The amount of energy stored depends on the temperature change of the material and can be expressed by: 2 1 T p T Q m c dT = ∫ 1 where m is the mass and c p the specific heat at constant pressure. T 1 and T 2 represent the lower and upper temperature levels between the storage operation. In latent heat storage during heating, the medium temperature remains more or less constant since it undergoes a phase transformation. Latent heat storage systems offer high storage capacity as compared to sensible heat storage and also involve low heat losses. The amount of energy stored Q in this case depends upon the mass m and latent heat of fusion of the material. The storage operates isothermally at the melting point of the material. If isothermal operation at the phase change temperature is difficult, the system operates over a range of temperatures T i to T f that includes the melting point. The storage capacity of a latent heat storage system with a PCM medium is given by: f m m L,PCM p p i m m L,PCM sp m i lp f m Q m Q mC dT mC dT m Q C T T C T T α α = + + = ⎡ ⎤ = + − + − ⎣ ⎦ ∫ ∫ 2 where, α m is the PCM fraction melted, Q L,PCM is heat of fusion Jkg, m, is mass of heat storage medium kg, T f is final temperature °C, T i is initial temperature °C, T m is melting temperature °C, C p is specific heat Jkg K, C lp average specific heat between T m and T f Jkg K, C sp is average specific heat between T i and T m Jkg K. The selection of the storage method should consider among others the temperature range, over which the storage has to operate; the rate of charging and discharging; the effect of storage capacity on the operation of the rest of the system; a smaller storage unit operates at a higher mean temperature; cost of the storage unit.

III. Experimental Facility