Experimental investigation on the energy storage/discharge performance of xylitol in a compact spiral coil heat exchanger

Highlights

• Experimented charging and discharging of xylitol in a vertical double spiral coil unit.

• Investigated temperature variation, liquid fraction and rate of energy storage/discharge.

• Charging and discharging were dominated by natural convection and conduction, respectively.

• Provide understandings and direction for designing double spiral coil heat exchangers.

Abstract

The main challenge ahead of widespread application of renewable energy sources is their availability intermittence preventing continuous power supply. In order to circumvent the temporal mismatch between their supply and demand, thermal energy storage systems can be utilized. In this study, a compact spiral coil thermal storage unit was investigated to analyze the impact of operating parameters such as inlet temperature and flow rate of the heat transfer fluid and explain the physics of heat transfer during the phase change process of xylitol. A concentric double spiral coil was inserted into a storage unit to ensure an improved heat transfer performance. Using experimental data, average temperature variation, heat storage/discharge rate and liquid fraction of xylitol in the storage unit were calculated. It was found that when the PCM melted, the lower density liquid PCM created buoyancy forces resulting in natural convection. On the other hand, discharging process was mainly governed by conduction. In the present storage unit, xylitol stored 450 kJ of heat in 35 min for an inlet temperature and flow rate of 130 °C and 2.5 lpm during charging and discharged 345 kJ in 50 min at an inlet temperature and flow rate of 45 °C and 2.5 lpm. The outcomes of this analysis are expected to be greatly applicable for the design of phase change material based spiral coil units to be used for instance in solar heating systems.

Introduction

Extensive use of fossil fuels has resulted in a steady increase in the average atmospheric temperature bringing the phenomenon of global warming to an alarming extent. Recently, renewable energy sources and waste heat recovery received growing attention mainly because of the growing demand for energy. However, the main problem is that continuous power supply cannot be offered due to the intermittent availability of such sources. To solve this issue, energy storage can be used to circumvent the temporal mismatch, ensuring an uninterrupted power supply [1]. Phase change materials (PCMs) are suggested as potential candidates in developing efficient heat storage systems for the operation of various thermal systems [[2], [3], [4]]. Industrial heat is wasted in the range of 100–200 °C [5]. Similar temperature ranges are encountered for instance in solar energy applications [6,7]. By the proper installation of energy storage systems, the energy can be effectively utilized, considerably decreasing the energy demand [8].

Sugar alcohols are considered a promising class of PCMs because of their excellent thermal properties [[9], [10], [11], [12], [13], [14]]. Wide temperature range and high energy storage density are the crucial characteristics of sugar alcohols. These materials can effectively store energy during its availability, and this stored energy can be reused later for different applications. They also possess thermal cycling stability and have been investigated for medium temperature applications [10,[15], [16], [17]]. For instance, Kumaresan et al. [7,18] developed a PCM based cooking unit for solar cooking applications. In order to replace conventional LPG, performance of the cooking unit was investigated using D–mannitol (a sugar alcohol), resulting in 41% efficiency and 100 W/m²K of the average heat transfer coefficient. Sharma et al. [6] experimentally investigated the performance of erythritol (a sugar alcohol) based solar cooker. Performance of the solar cooker under different load and climatic conditions was analyzed and reported.

The major disadvantages associated with PCMs are incongruent melting, thermal cycling as well as chemical stability problems, corrosion possibility, high cost and volume change during phase change. Regarding sugar alcohols, low thermal conductivity and possible subcooling tendency are the main drawbacks. Some options have been proposed/investigated to resolve this issue which can be categorized as [19]: introduction of additives/metal matrices possessing high thermal conductivity, PCM microencapsulation, using multiple PCMs, increasing heat transfer surface area by utilization of extended surfaces, etc. Among them, the surface extension has received greater attention primarily due to the manufacturing simplicity together with low cost [20].

The geometrical configuration of the storage is another major factor which greatly influences its performance. Different configurations such as tube-in-tube [21], shell and tube [[22], [23], [24]], finned tube [[25], [26], [27], [28]], triplex tube [[29], [30], [31]] and spiral coil [17,32] have been tested and many reports summarize the effects of geometrical parameters on the melting and solidification process. Among them, spiral coil configuration has received greater attention in recent studies [17,[33], [34], [35], [36], [37], [38]]. Basically, spiral coil heat exchangers are compact systems similar to shell-and-tube configuration where the tube is helically twisted with a certain diameter to increase the heat transfer surface area. In such configuration, the heat transfer surface area can be further extended using a double spiral coil (compared to conventional single spiral coil) [17,39].

Table 1 shows a summary of the studies on the spiral coil heat exchangers. According to the table, thermal performance of paraffin wax in spiral coil heat exchangers (including the effects of operating parameters on the phase change performance) has been extensively analyzed [38,[40], [41], [42], [43]]. Moreover, numerical investigation of paraffin and paraffin based composite PCMs (paraffin and expanded graphite as well as nano-enhanced paraffin) using spiral coil heat exchangers have been conducted [[34], [35], [36], [37],44]. Yamac and Koca [45] numerically investigated melting and solidification characteristics of three different types of PCMs (i.e. hydrated salt S21, paraffin RT21 and a eutectic mixture of capric acid, myristic acid, expanded perlite and expanded graphite) using a spiral coil storage unit. The results indicated that the hydrated salt (i.e. S21) outperformed other PCMs for long term applications. The effect of spiral coil diameter on the phase change process of paraffin (RT35) in a storage unit was studied by Rahimi et al. [46]. They observed reduction in melting time by increasing the helical diameter of the HTF tube. Hu et al. [39] experimentally and numerically analyzed the phase change characteristics of calcium chloride in a double spiral coil thermal energy storage system for heat pump applications. According to the table, sugar alcohols have been rarely investigated in the literature. For instance, mannitol and erythritol were investigated in single and double spiral coil storage units by Ling et al. [47] and Anish et al. [17], respectively. The results showed that the PCMs could store a large amount of thermal energy and that the performance was influenced by the inlet temperature and flow rate of the heat transfer fluid (HTF). It has been reported that 100 L of water could be heated from 30 to 50 °C in 6 h using the heat stored in 14 kg of mannitol [47]. However, there are other potential sugar alcohols (such as xylitol) which have never been investigated for thermal storage in a spiral coil configuration. Xylitol has promising characteristics such as high energy storage capacity in the temperature range of 70–100 °C [[9], [10], [11], [12],48]. Therefore, it is suitable for industrial waste heat recovery applications, solar cookers, water heating and space heating applications, etc. [6,49,50].

As reported in the literature, using spiral coil heat exchangers is beneficial from the viewpoint of heat transfer performance; however, according to Table 1 (and to the best of the authors’ knowledge), there is a gap in the literature in terms of experimental investigation of double spiral coil heat exchangers using medium temperature PCMs. Overall, the existing limitations of the literature can be summarized as: (1) Few experimental studies are available using double spiral coil storage units. (2) Most of the earlier studies focused on PCMs with the phase change temperature range of 30–70 °C and a few experimental studies are available for the temperature range of 70–100 °C (see Table 1). (3) Experimental study of energy storage/discharge performance of xylitol as PCM is missing from the literature. Therefore, to address these shortcomings, in this study, the phase change behavior and effects of natural convection during charging and discharging of xylitol were experimentally investigated in a vertical double spiral coil heat exchanger. The outcomes of this analysis are expected to explain the physics of heat transfer during the phase change process of xylitol. It can also provide understandings and direction for designing double spiral coil heat exchangers.