Exergy analysis of particle dispersed latent heat thermal storage system for solar water heaters

Abstract

Phase change material (PCM) based latent heat thermal storage (LHTS) systems offer a challenging option to be employed as an effective energy storage and retrieval device if the energy source is intermittent and time dependent. However, the performance of LHTS systems is limited by the poor thermal conductivity of PCMs employed. The addition of high conductivity particles is proposed as one of the promising performance enhancement techniques for LHTS. In our study, an attempt is made to investigate the performance enhancement of a shell and tube storage unit applicable to solar water heaters due to the addition of high conductivity particles (copper) through exergy analysis. The enthalpy based two dimensional-transient equations are solved numerically for discharging process using FLUENT—a commercial computational fluid dynamics code. The numerical results show good agreement with experimental results available in the literature. The thermal behavior and performance of particle dispersed PCM unit in terms of exergy efficiency and total exergy recovered are compared with that of pure PCM unit. It is observed that the discharging time is reduced by around 28% by adding particles of volume fraction 0.1 and by around 85% with 0.6 volume fraction. However, the decrease in discharging time is not much appreciable beyond the particle volume fraction of 0.4. Similarly, the exergy efficiency and total exergy recovered are increased by 12% and 15%, respectively, with 0.1 volume fraction and by 50% and 60%, respectively, with 0.4 volume fraction. Different Reynolds numbers (i.e., Re=200–10 000) are considered in the numerical trials to investigate the influence of mass flow rate of heat transfer fluid (HTF) on the performance of the system. The results reveal that the role of HTF mass flow rate in increasing the overall thermal performance of LHTS unit is insignificant.

Energy and exergy performance assessments for latent heat thermal energy storage systems

“…Using two PCMs can make an increase in the overall exergetic efficiency up to 19.0-53.8% compared with a single PCM [37] An appreciable enhancement in the exergy performance in the latent TES unit containing dispersed high conductivity particles in the PCM [27] The particles addition enhances the quality of recovered energy despite reducing the storage capacity of the unit. The higher heat transfer rates can minimize the exergy destruction.…”

“…[42] Energy charge/discharge rate for parallel flow is 5% more than counter-current flow mode [43] PCM melting temperature, number and additives Temperature distribution during the freezing process has been investigated in detail [18] Both the efficiency and the percentage of stored latent energy increase by decreasing the PCM melting temperature. The energy efficiency of the system is strongly affected by the melting temperature [22] The PCM with lower melting temperature (paraffin) has the higher stored energy as compared with the PCM with higher melting temperature (P-wax) [23] From an energetic point of view, the transition temperature of the PCM should be as low as possible [24] A low transition temperature leads to an increased efficiency of the solar heat collection system [25] Muitiple-PCMs with reasonable arragements can achieve better charge-discharge performance [26] An appreciable enhancement can be achieved in the energy storage containing dispersed high conductivity particles in the PCMs [27] Effective thermal conductivity calculated by employing paraffin with lessing rings can be ten times greater than the thermal conductivity of paraffin) [44] Heat brush/PCM composite can be inferior to the fiber-chip/PCM composite in terms of heat transfer rate of the bulk but the former can be superior to the latter in terms of overall heat transfer rate [45] Flat plate PCM encapsulate can decrease the fan power consumption by 50%,response time much lower 50% [46] Inserting Carbon fiber brushes to PCMs can contribute to saving space and reducing the cost of the tanks [47] Inserting Carbon fiber clothes to the PCM can make a reduction in the thermal contact resistance and improve the heat exchange rate of the tank [48] Melting temperature difference between the multiple PCMs plays an important roles in storage performance [49] Energy charged-discharged in a cycle is 34.7% more when using five PCMs [50] Three PCMs can improve the receiver performance, reduce the fluid temperature fluctuation, and reduce the heat receiver weight [51] Three PCMs can have a higher phase change fraction and storage/discharge capacity [52] Storage unit dimension 2D directional measurements along the radial and angular directions are investigated to reveal the temperature variation for the TES performance [18] The total melting time of the PCM corresponding to cylindrical shell containers are considerably less than those of the rectangular containers having equivalent volume and surface area of heat transfer [29] Increasing shell diameter from 114 mm to 190 mm can cause an increase in the total stored energy ...…”

“…One latent TES system consists of a bank of tubes arranged in a particular pattern inside an insulated shell [27]. The shell side is loaded with either pure PCM or PCM dispersed with high conductivity particles and the tube side carries the HTF that flows at a constant hot inlet temperature to initiate the melting process.…”

“…A numerical study is conducted to investigate the performance enhancement of a latent TES unit of shell and tube configuration due to the dispersion of high conductivity particles in the PCM during the melting process [27]. The effect of particles on the cumulative energy stored during the melting process is shown in Fig.…”

Performance of Loaded Thermal Storage Unit with a Commercial Phase Change Materials based on Energy and Exergy Analysis

“…Thus, exergy analysis depends on second law has been adopted by many researches (Erek and Dincer , 2009 ;Aghbalou and Badia, 2006) to overcome the inadequacy of energy analysis. Jegadheeswarm and Pohekar (2010) numerically analyzed the energy and exergy of a particle dispersed latent ESS. They found that the exergy analysis based on the performance evaluation of latent heat was more perspective measures than that of energy analysis and reflected the true potential of the system under different input parameters.…”