Effect of aspect ratio and dispersed PCM balls on the charging performance of a latent heat thermal storage unit for solar thermal applications

Highlights

• Charging of phase change material based packed bed storage system are studied.

• Three different aspect ratios of the storage tank were considered.

• Charging efficiency, stratification number and Richardson number are reported.

• The results are useful to design a storage system for medium temperature applications.

Abstract

A cylindrical latent heat thermal storage system having a phase change material, HS89, encased in spherical capsules and stacked in rows to form a packed bed phase change material containment was considered for the experimental investigation. The heat is stored at 89 °C which is very useful for many process industry applications using solar water heaters. Three different aspect ratios were considered for experimentation. The temperature profiles of the phase change material containment and the heat transfer fluid at various heights were measured and monitored continuously. These temperature values were used to determine the variation in charging efficiency, stratification number and Richardson number of the storage system which are very useful parameters to design a storage system. The results presented in this paper will be very useful to design a phase change material based thermal storage for solar thermal applications.

Introduction

The enormity of the importance of thermal energy storage and its usage as a backup for various energy applications, particularly in promoting the highly intermittent solar systems, has been visibly seen in the recent years for the sustainable development. To maintain continuous heat supply, thermal energy storage (TES) can be done both in sensible heat or latent heat. The latent heat thermal energy storage (LHTES) is better than sensible heat thermal energy storage (SHTES) as latent heat capacity is much denser and isothermal in behaviour when compared to sensible energy capacity [1]. There are various process industries which require heat in the temperature range of 70 °C to 90 °C for which the PCM available with transition temperature range of 80 °C to 90 °C is highly suitable. In any thermal storage system, the type of storage tank with geometrical configuration, the rate of heat transfer during charging and discharging processes and the effects of stratification are the important aspects. Various studies carried out on these aspects have been reviewed and presented in the following section.

A detailed review of 150 thermal energy storage materials used in the research performed by Zalba et al. [2] and the heat transfer is useful to understand the thermal energy storage and its applications. Sharma et al. [3] have investigated different properties of PCM types, thermal energy storage units, applications of PCM’s, different techniques for solving Stefan number problem, numerical investigation methods and the formulation of enthalpy method to the phase change problem. It is concluded that based on the different techniques and criteria, LHTES system’s design parameters for heat exchanger material should be selected carefully. On similar lines, Weiguang et al. [4] have reviewed different types of solid liquid phase change materials (organic, inorganic, and eutectics) for thermal energy storage applications. It has been reported that organic compounds have wider temperature application, where as inorganic PCMs possess higher heat storage capacities, and eutectics have an advantage in view of their melting point. Further, they found that polymerisation methods offer the best technological approach in terms of encapsulation efficiency. Regin et al. [5] reviewed the development of available LHTES technologies focussing the different aspects such as materials, the encapsulation, the heat transfer, applications and new PCM based technology innovation that has been carried out. Salunkhe et al. [6] have reviewed in detail the effect of Phase change Material (PCM) shell’s thermal conductivity. They have reported that a core to coating ratio of the shell enhances the thermal storage capacity. They found that there is a quick melting of the PCM and heat transfer enhancement in microencapsulated PCM with high thermal conductivity, a lesser shell size and high temperature of HTF. Dan et al. [7] have reviewed the existing approaches in the design, integration and applications conducted on PCM’s storage in thermal storage tank. They highlighted the existing studies on the central thermal storage, which can contribute to the design of sustainable cities. It has been reported here that there is a lack of design tool and information on cost, environmental impact, safety, and clear selection criteria for the quantity of PCM which can be resolved looking at the experimentation work done.

Haller et al. [8] have made a detailed review on the characterisation of thermal stratification in energy storage system and focused on methods that can determine the ability to store, promote and maintain stratification. Their paper analyzes different types of methods to evaluate the stratification efficiency and also the rate of entropy generation with respect to the constant high inlet temperature of HTF. The importance of the presence of stratification to increase the performance of a stratified thermal energy storage system (STESS) has been reviewed by Njoku et al. [9] along with energy, entropy and exergy analysis. The entropy generation number Ns is suggested as an effective tool for future performance studies on STESS as it gives a clear assessment of time-dependant performance of stratified thermal systems and optimizes the duration of charge, storage and discharge operations. The Entropy production for mixing and stratification are distinguished.

Meenakshi et al. [10] have experimented with the PCM paraffin and stearic acid individually which has been encapsulated with varying diameters and tested. The minimum diameter of 38 mm has been found to give very good heat transfer rates and better performance compared with the other higher diameter PCM capsules. Use of PCM in the application of waste heat recovery has been investigated by Pandiayaraj et al. [11] who had done experimental investigation on a shell and tube heat exchanger and a PCM based thermal storage unit and found that 10%–15% of the heat that has been wasted along the exhaust of a diesel storage unit can be successfully retrieved using this thermal storage unit. Also, the characteristics of heat lost, rate of charging, charging efficiency and the percentage of heat saved have been reported. Sharif et al. [12] have concluded that an optimization tool at the early design stage is required for creative system design such as the storage dimension, suitable PCM material with specific melting point to get a good thermal behaviour. Ling et al. [13] have concluded that the research in water tanks remains in theoretical analysis of the material itself and clear selection principles should be laid out along with forms of variety of packages for comparison with the actual engineering aspects. For increase in heat transfer, it has been summarized that a simple and effective, easy to process and low cost heat transfer is available to promote thermal energy storage. For optimization of water tanks with PCM, they presented various aspects of the thermal storage tank. It has been concluded that the optimum shape for minimum heat loss of the thermal storage tank is cylindrical, with hot water inlet and outlet to be placed at the top and that the PCM should have high thermal conductivity, so as to enable optimum stratification throughout the thermal storage tank.

Han et al. [14] have reported an idea regarding various types of thermal storage. The various influencing factors are entry and exit conditions, total thermal leakage, and static and dynamic tank operating conditions. For efficiency improvement of stratification, the impact of the numbers like stratification number, Richardson number have been analyzed and their influence on geometry has been studied. The experimental set-up and various components have been discussed in this paper for use in experimentation for thermal stratification in a (water/fuel) tank. Yang et al. [15] have studied ten different shapes and found that the thermal energy storage capacities of the sphere and barrel are the best, while the capacity of the cylinder is the least favourable. The capacities of the remaining shapes are almost equal. Thermal energy storage capacity is determined by the ratio of surface area to volume. The thermal stratification of different shapes is determined by the flow at the bottom of the water tank and the heat transfer from the fluid to the environment.

Rosen et al. [16] have shown about six different models of temperature distribution in a stratified thermal storage system. It has been concluded that improvement in stratification can increase the thermal storage efficiency and exergy storage capacity. The important aspect of thermally stratified energy storage is thermocline mixing and the thermocline thickness which Berkel [17] studied analytically and experimentally. He analyzed a short term thermally stratified energy storage system using a numerical model and reported that the fluid is withdrawn from the thermocline first by viscous drag and subsequent mixing takes place by stretching and folding of fluid particle. Gupta et al. [18] presented the computational prediction and experimental validation of the transient behaviour and performance of an active thermal storage system for domestic applications based on the use of hydrated salt PCM. They reported that heat removal performance has been identified as the limiting factor in the PCM based storage system. Hence they suggested design improvements to eliminate the various hindrances during the cooling process. Numerical investigations have been reported by Regin et al. [19] based on the solar water heating system and the fundamental equations of Schumann were used along with the enthalpy method for phase change phenomenon for PCM inside the capsules. The effects of inlet HTF temperature, mass flow rate and the phase change temperature range on the thermal performance of various radii of capsules were investigated and found that charging and discharging characteristics of PCM capsules depends on different aspects like the material used, encapsulation, heat transfer application. Kurşun et al. [20] have numerically analyzed the ratio of the tank diameter to the height (D/H) and the oblique positioning on the thermal stratification for varying the range of D/H = 0.5–1 and inclined positioning for 45° to 60°.The maximum temperature is increased by 14% due to of decreasing aspect ratio. As a result of the oblique positioning of the tank and the change of the aspect ratio, the highest loss in second law-efficiency is 4.3% and 1.56%, respectively.

Experimentation using PCM in thermal storage for a water tank has been done by Mawire et al. [21]. They have experimentally investigated three varieties of cases for simultaneous charging and discharging with originally unstratified storage tank, stratified storage tank, initially unstratified at the top and stratified at the bottom of the PCM containment. It has been found that water could be boiled at the end of 2 hours and adequate amount of energy could be stored in the containment. The experimental analysis using the phase change materials by Fazilati et al. [22] has shown a maximum of 39% increase in the heat storage density in a solar water heater and also compared the result with various flow rates and various temperatures. Nallusamy et al. [23] have constructed, designed a thermal storage unit and integrated with constant temperature bath as a constant source/varying source solar collector to study the performance of the latent and sensible thermal storage units using paraffin as the PCM. Charging with various temperature inlets and continuous and batch-wise discharging characteristics have also been reported. Mazman et al. [24] have experimentally investigated thermally stratified solar hot water tanks and their short-term application of storing hot water. Three PCM mixtures (PP, PS, SM) were tested in cooling and reheating experiments. The average drop in temperature was maximum for PS and minimum for SM mixture. During reheating, 3 kg of PCM could increase the temperature of 14 L–36 L of water by 3 °C - 4 °C and it has been concluded that the best thermal performance enhanced was by using PS by 74%.

Huang et al. [25] thoroughly investigated the effects of sodium acetate trihydrate as PCM in solar water tank with HTF inlet at the bottom. The thermal stratification characteristics at various flow rates (0.06, 0.18, 0.3, 0.42 & 0.54 m³/ hr) with increase in dimensionless time were compared. The fill efficiency was analyzed with exergy efficiency, mix number, and Richardson number. It was concluded that the thermal stratification of ordinary tank was superior to that of a PCM tank as the inlet of the hot water was placed at the bottom of the tank which is not usual for most of the experiments and it was found that stratification was eventually high when the PCM was placed close to the inlet towards the bottom of the HWT. Also, the tank energy was higher when the inlet temperature was raised from 278.15 K to 353.13 K. Sharp et al. [26] have shown that 5–15% of increase in improvement in the system performance can be obtained if stratification is maintained in the storage tank till the end. An experimental set-up by Zhao et al. [27] with the PCM embedded in open cell metal foams and expanded graphite was constructed. They reported that there is a clear indication of the doubling of the heat transfer using metal foam both during melting and solidification for paraffins and tripling calcium chloride hexahydrate. The metal foams can also improve the operational performance of PCM by effectively reducing the supercooling of hydrated salts. Kumar et al. [28] conducted experimental investigation by placing phase change material on the top of the thermal storage tank which enhanced the stratification and the charging efficiency. By maintaining various HTF inlet temperatures and varied flow rates, the stratification has been analyzed in the PCM containment by using non-dimensional numbers like Stratification number, Richardson number, charging efficiency and cumulative charge fraction.

Kaygusuz [29] conducted an experimental and theoretical investigation to determine the performance of PCM based solar water heating systems. The variation of outlet fluid temperature with different values of NTU and the variation of the stored energy with time for the PCM CaCl₂–6H₂O are investigated. Based on the simulation studies, it was reported that there is a marginal gain in solar fraction achieved with PCM based storage system for higher storage capacities per unit collector area of more than 20 kg/m². However the solar gain drops when the storage capacity per unit collector area is lesser than 5 kg/m². Mehling et al. [30] reported that adding a PCM module at the top of the water tank would give a system with higher storage density and compensate heat loss in the top layer. In order to assess their statements they performed numerical and experimental investigation using different cylindrical PCM modules in the storage tank. They concluded that the energy density could be improved by 20–45% and the measured time gains to delay heat loss was in the order of 50–200%. Oro et al. [31] conducted an experimental investigation on a cylindrical storage tank with a capacity of 3.73 L filled with spherically encapsulated PCM. The various parameters such as degree of stratification first law efficiencies, second law efficiencies, Mix number, stratification number, and Richardson number were used to characterize the storage system and they reported that energy and exergy efficiencies have no relationship with thermal stratification. Similar concept was also investigated by Cabeza et al. [32] through the experimental solar pilot plant to test the PCM behaviour in real conditions. Granular PCM graphite was chosen as the PCM for the experiments. In contrast to Mehling et al., [2003] and Cabeza et al., [2006] Padovan et al. [33] reported that solar domestic water heater with paraffin based PCM do not find improvements when compared to hydrated salts for enhanced heat storage through their experiments conducted with a number of long cylindrical PCM modules placed at the top of a hot water tank along the height of the tank. It was observed from the literature survey that some positive and negative aspects were reported while including the PCM on the top of the storage tank. This could be due to various geometries used and quantity of PCM used in the various investigations. Hence the major objectives of the present work was formulated to construct a packed bed thermal storage tank using HS89 encapsulated balls in a PCM containment and to investigate the charging characteristics and the stratification behaviour under 3 different aspect ratios (ratio of height to diameter) in the thermal storage tank.

The experimental set-up as illustrated in Fig. 1a consists of a TES tank, a circulating hot water bath, a heater to simulate the actual solar heat, an HTF loop and the data acquisition system. Three cylindrical storage tank were fabricated with different aspect ratios (height to diameter) 320 mm: 330 mm for 1:1, 560 mm: 250 mm for 2:1 and 720 mm: 240 mm for 3:1 as shown in Fig. 1a, Fig. 1b, Fig. 1cc. In the storage tank, the top 50 mm for header and the bottom 25 mm space is kept idle and the PCM balls are placed in between these two regions. The storage tank was divided into several layers and each layer was separated using steel mesh. In the case of the aspect ratio 1:1, 11 balls were used in each layer and the tank was divided into 4 layers, whereas the tanks were divided into 7 layers and 9 layers in the case of the aspect ratio 2:1 and 3:1 respectively. In all the above said three cases, a total of 45 PCM balls were distributed uniformly within the layers. The PCM HS89 was procured from Pluss advanced technologies Pvt. Ltd., New Delhi and was used as the storage medium. The thermo-physical properties of the PCM are presented in Table 1. The phase transition temperature, specific heats, liquid and solid densities and the latent heat were taken from the technical data sheets. The spherical encapsulations were made of stainless steel with an average outer diameter of 80 mm and a thickness of 2 mm. The storage tank was insulated with 40 mm thick Polyurethane foam to minimize the heat loss.

The hot water bath was maintained with 30 L HTF. The bath was fitted with two immersion heaters of 1.5 kW to simulate the solar heat input to the storage tank. The temperature of the incoming water was raised continuously till it reached around 95 °C and it was maintained uniformly with the use of a thermostat control. Make up water was added whenever water level in the bath went down. In the HTF Loop, a 0.25 kW centrifugal pump was used to circulate the water from the storage tank to the hot water bath and if the water levels in the hot water bath increased beyond the maximum limit, it was returned to the storage tank naturally through gravity. In the thermal storage tank, separate inlet and outlet provisions were made to circulate the water for discharging applications.

The water level was measured by a glass tube attached from the bottom of the water heater as shown in Fig. 1a. A rotameter which is having a measurement range of 0–2 L/min and with an accuracy of 5% was placed in the HTF loop to measure the mass flow rate of the fluid. J type thermocouples were used to measure the temperature of the PCM capsule and HTF. The accuracy of the thermocouples used are 0.4%. All the thermocouples were connected to the Data Acquisition system [Make: NI 9213] through which the temperature measurements were continuously monitored. In each layer, one thermocouple was located inside the PCM ball and there were 4 thermocouples equally spaced in the HTF region in the tank with 1:1 aspect ratio and 5 thermocouples spaced equally in the remaining two tanks with the aspect ratio of 2:1 and 3:1.

In the present work, two sets of experiments were conducted. In the first set of experiments, the storage tank was filled only with water and in the second set of experiments, the tank was filled with water and PCM capsules. In both these sets of experiments, 3 different experiments were conducted for different aspect ratios (1:1, 2:1, and 3:1). In all these experiments, initially the water in the storage tank was maintained approximately at 30 °C. The heater in the hot water bath was switched on and the water was allowed to circulate through the storage tank with a flow rate of 1 L/minute. The temperature of water and PCM measured at different positions as shown in Fig. 1a, Fig. 1b, Fig. 1c in the storage tank were increased slowly with respect to time and these increase in temperatures with respect to time was monitored continuously using the Data Acquisition system. When the temperature increased beyond 90 °C, the PCM kept in the spherical capsules melted and during this melting period it was ensured that the temperature of water in the hot water bath did not go beyond 95 °C. This experiment was continued till complete phase change was ensured. Similar experiments were repeated for all the tanks with different aspect ratios.

The transient temperature variation obtained in the PCM and the HTF were used to analyze the instantaneous and cumulative heat transfer, charging and discharging efficiencies and also used to analyze the level of stratification in the storage tank. Further experiments were also conducted in all the 3 tanks to determine the overall heat loss coefficient for which the water in the storage tank without PCM was allowed to drop from 95 °C to 45 °C by keeping the tank under idle conditions. The temperature in the storage tank was monitored continuously during the experiment. The overall heat loss coefficient was determined using these temperatures, ambient temperature and the time taken for the temperature drop of the heat transfer fluid in the storage tank.