Field of the Invention The present invention relates to storage of thermal energy in heat exchangers. More particularly, the present invention relates to storage of thermal energy in phase change material of a horizontal heat exchanger.

Background of the Invention

Efforts for rational and effective management, as well as environmental considerations increase the interest in utilizing renewable energy sources, especially solar energy. Because of discrepancy between the energy supply and demand in solar heating applications, a thermal energy storage device has to be used. Solar thermal for domestic hot water heating is one of the most cost effective and efficient areas of renewable energy exploitation. A solar domestic heat water (SDHW) system collects energy when solar radiation is available and exchanges this energy to preheat domestic water for use in a building when there is a demand for hot water.

Thermal energy storage plays an important role in the rationale use of energy as it allows the decoupling between production and demand of energy. In applications with intermittent energy generation, such as solar thermal systems or waste heat recovery, an appropriate thermal storage system is essential. Energy storage is the storing of some form of energy that can be drawn upon later to perform some useful operation. Thermal energy storage can refer to a number of technologies that store energy in a thermal reservoir for later reuse. The thermal reservoir may be maintained at a temperature above (hotter) or below (colder) than that of the ambient environment.

There are three main methods of thermal energy storing, namely sensible, latent and thermochemical heat storage. Thermal energy storage (TES) is commonly used to bridge this gap between energy availability and demand. Using phase change materials (PCMs) for TES can solve this problem by reducing the weight and space required for energy storage. PCMs are used as energy storage mediums: energy is stored during melting and released during solidification . The thermal storage technology based on the use of phase change materials (PCMs) has recently raised an important practical interest. This is mainly due to the high- energy storage density during phase change within a very narrow temperature range. The solidification or melting periods of a particular PCM must be known in order to design a latent heat storage unit. The operating circumstances and the storage configurations also must be known in order to forecast the heat transfer coefficients during the process of phase change. The most common configuration is shell and tube heat exchanger.

Because of the change in volume of phase change materials (PCM) in solidification and melting, researchers have two options either using vertical type or using horizontal type with 70% of shell volume (because more PCM than this value will be break the shell).

Hence, it is advantageous to provide a horizontal type heat exchanger of an energy storage device that is capable of using 100% shell volume. By utilizing 100% shell volume, the phase change material (PCM) of the energy storage device can cover whole of finned tube with symmetric position for both processes, i.e. melting (charging) and solidification (discharging).

Summary of the Invention

It is an object of the present invention to provide an energy storage device that is capable of enhancing energy saving between 15% and 30%.

It is another object of the present invention to provide an energy storage device that is capable of extending solar thermal energy saving time.

It is also another object of the present invention to provide an energy storage device consisting of a horizontal heat exchanger.

Accordingly, the present invention provides an energy storage device for the introduction of thermal energy into a phase change material and removal of thermal energy from the phase change material, in which the phase change material is maintained in a stationery container and surrounds a horizontal heat exchanger. The horizontal heat exchanger is preferably a horizontal finned-tube. It is advantageous to provide a phase change material that fully surrounds or covers whole of the finned-tube with symmetric position during both steps of melting (charging) and solidification (discharging) of the phase change material, which advantageously enhances saving energy of the energy storage device between 15% and 30%.

The 15% to 30% energy saving of the energy storage device depends on the type of phase change material. The phase change material includes paraffin, palmitic acid, salt hydrate and myristic acid. Preferably, the phase change material is paraffin.

Brief Description of the Drawings

Fig. 1 a is a cross-section view of an energy storage device during solidification process (discharging), according to an embodiment of the present invention ;

Fig. 1 b is a cross-section view of an energy storage device during melting process (charging), according to an embodiment of the present invention ; Fig. 2 is a perspective diagram of an energy storage device, according to an embodiment of the present invention;

Fig. 3 is a diagram of a control system implementing an energy storage device according to an embodiment of the present invention;

Figs. 4a-4b are graphs showing temperature measurement records in the melting process (charging) for Re=1000 (Figure 4a), Re=1500 (Figure 4b), Re=2000 (Figure 4c); Figs. 5a-5d are diagrams showing the melting process (charging) of the phase change material (PCM) for Re=1000 at 1000s (Fig. 5a), 4000s (Fig. 5b), 5000s (Fig. 5c) and 6000s (Fig. 5d);

Figs. 6a-6c are graphs showing temperature measurement records in the solidification process (discharging) at Re=1000 (Fig. 6a), Re=1500 (Fig. 6b) and Re=2000 (Fig. 6c); and

Figs. 7a-7d are diagrams showing solidification process of the phase change material (PCM) for Re=1000 at 200s (Fig. 7a), 600s (Fig. 7b), 1200s (Fig. 7c) and 1800s (Fig. 7d).

Detailed Description of the Invention

An embodiment of a heat storage device 100 according to the present invention will now be described in detail with reference to Figs. 1 a-1 b and Fig. 2.

The heat storage device 100 is comprised of a finned-tube 102 that allows flow of water from an inlet point 104, which travels across the tube 102 until it exits the tube 102 through an outlet point 106. The finned-tube 102 is enclosed by an elongated polymeric tube 108, which is spaced apart from the outer wall of the finned-tube 102 to create a space or gap 110 to accommodate a phase change material.

The phase change material includes paraffin, palmitic acid, salt hydrate and myristic acid. In a preferred embodiment, the phase change material is paraffin.

In another preferred embodiment, the finned-tube 102 is made of a material having high conductivity, for example copper. The elongated polymeric tube 108 is configured to surround the finned-tube 102 in a way that it is joined by a pair of parallel rectangular plates 112 that connects the tube 108 to the top portion 114 of the device 100. In this manner, the space 110 to accommodate the phase change material is extended between the pair of parallel rectangular plates 112. The upward extension of the space 110 allows movement of the phase change towards the top portion 114 of the device 100 during melting process (charging) of the phase change material.

The heat storage device 100 is preferably enclosed by a rectangular casing 116 having insulation properties. The casing 116 is disposed with an inlet valve 118 to allow water to flow into the device 100 and an outlet valve 120 to allow water to flow out of the device 100.

The present invention will now be described in further detail by way of a non-limiting example.

EXAMPLE

The use of the heat storage device 100 can be implemented and regulated in an exemplary system, as shown in Figure 3. Referring to Figure 3, there are three (3) loops running in the system, namely a main loop 122, primary loop 124 and secondary loop 126.

The main loop 122 comprises a data acquisition unit, which consists of a computer 128 and a data logger 130, a plurality of connecting pipes 132, the heat storage device 100, a flow-adjustment valve 134, an inlet valve 135, an outlet valve 136 and a constant temperature bath 138. The constant temperature bath 138, which controls the heat transfer fluid's (HTF's) inlet temperature delivers as much as 15L/min of water, has an accuracy of ±0.1 ^S C, and its temperature range is -20 to +100 ^S C.

The primary loop 124 comprises an inlet valve 118 of the heat storage device 100, an outlet valve 120 of the heat storage device 100 and a constant temperature bath 140.

Pre-heating process was performed by circulating warm water in the primary loop 124. By applying this process, the paraffin temperature increased more and approached the phase change value for paraffin (53-57 ^s C). This temperature range holds the paraffin in the mushy zone and to make a more homogeneous temperature distribution in (around) the paraffin. At the start of the water circulation inside the heat storage device 100, the outlet valve 120 was closed and warm water at 55 ^S C was allowed to flow through the inlet valve 118 to the rectangular casing box 116 of the device 100. Upon water reaching 300 mm height in the box 116, the outlet valve 120 was opened and the warm water was allowed to start circulation until the temperature of the storage unit 100 reaches around 53 ^S C. Following this process, the inlet valve 118 is closed and the outlet valve 120 remains open until the water reached a height of 100 mm in the storage unit in near-vacuum conditions. The heat transfer fluid (HTF) is preferably distilled water.

The heat transfer fluid (HTF) that was derived from a constant-temperature bath 138 first circulated and flowed through the secondary loop 126 so as to attain the desired inlet temperature prior to entering the heat storage device 100. This thermal bath 138 serves as a heat source replacing solar heat in real application. Next, the HTF circulates in the main loop 122 and the process of charging within the heat storage device 100 begins. This process is deemed to have finished once all temperature recordings of the heat storage device 100 showed higher values than PCM's melting temperature and the entire paraffin melted. This cycle is repeated subsequent to the charging process, and the energy stored is extracted through the discharging process (the circulation of cold water). The solidification process stopped when the whole amount of PCM was solidified and the test module temperature was less than PCM's solidification temperature.

Figures 4a, 4b and 4c show the temperature gradient of the phase change material (PCM) at different times and Reynolds numbers, Re=1000 (Figure 4a), 1500 (Figure 4b) and 2000 (Figure 4c). It can be seen that the phase change material (PCM) takes a short time to reach the melting temperature with a higher Reynolds number. According to results, the gradient temperature fluctuations began from 470s, 276s and 270s for Re=1000, Re=1500 and Re=2000, respectively. It is clear that the fluctuation of gradient temperature decreased and became smooth with increasing Reynolds number. In conclusion, by increasing the Reynolds number in the charging process, and the theoretical efficiency rises too. As theoretical efficiency approaches unity, by definition, (for example in the melting process) the energy required to melt the whole PCM is provided. For a clearer explanation, Figures 5a-5d depict the results of Re=1000 as an example of ordinary and normal behavior of a transition. As witnessed, melting begins peripherally, near the HTF tube's wall and it continues to spread radially outwards. When the charging process is initiated, the PCMs close to the HTF tube's wall surface reach the melt, which is due to the conduction of the finned tube. The rapid melting of the PCMs near the walls is mostly because of greater temperature differences of PCM in the surrounding areas of the tube wall. Nevertheless, it is worth noting that the melting behavior of PCMs in the upper areas differs from those in lower regions. The molten PCM moves up towards the storage container's top areas due to the natural convection current. The melting region extends radially upward. As a result, the areas in the upper sections attain melting temperature more rapidly than the lower parts.

The natural convection has a greater effect in the upper regions of the annular storage container. Aydin et al. offered a good, comprehensive description of the physical attributes displayed by PCM in the charging process whereby two areas coexist. These two areas are the solid phase non-melted PCM and liquid phase melted PCM. Due to buoyancy forces produced by the density gradients that resulted from differences in temperature, the melted PCM recirculation is spurred by the heat transfer through convection when the PCM's solid matrix melts. The heating and mixing of the molten PCM is elevated by recirculation inside the test section and the fact that it takes less time for the areas closer to the upper regions to reach melting temperature than lower regions. However, it is worth noting that PCM has a lower density in the molten phase compared to the solid phase. In cases with larger operation, the PCM molten area spreads to cover larger areas of the PCM container. In line with the assumptions, the melting within will increase with rising Reynolds number.

Determination of Heat Transfer to the Phase Change Material (PCM)

As shown in Figure 3, the temperature measurement system consists of thermocouples 139 and data logger 130. It is essential to measure the temperatures of the finned tube wall 102, the fin tip and base tube to accurately determine the heat transfer to the phase-change material. In this study, K-type thermocouples were used, whose locations are depicted in Figures 7a-7d. During this experiment, the temperatures at twenty different points in the finned tube 102 were measured and recorded. To enhance experiment accuracy and collect additional temperature data, many more thermocouples 139 were applied among the fins. The thermocouples 139 were located at three parts of the device 100. Sixteen thermocouples were used on the finned tube 102, two thermocouples were installed to measure the HTF temperatures at the device's inlet 104 and outlet 106 and 2 more thermocouples measured the ambient temperatures of the energy storage unit 100 and PCM Container 112. The output data was recorded by the help of a HP type 34970A data logger with ±1 °C accuracy. This data logger measured the thermocouples' millvolt outputs. The signal output from the data logger was transferred and documented in a personal computer.

For this process, HP-Benchlink software was used. The temperature values were scanned by the data logger every 10 seconds during both melting and solidification. All thermocouples were installed on the finned-tube 102 by brazing. The HTF's volumetric flow rate was controlled by controlling the adjustment valve gate that was situated at the inlet of the main loop. All experiments were conducted in a conditional room with ambient temperature of 30 ^S C. The experiments were carried at no less than three times, and three different Reynolds number values of heat transfer fluid with similar outcomes were considered, namely 1000, 1500, and 2000. The entire recorded transient temperatures were repeated in a range of ±3%. In an attempt to put a stop to loss into the vicinity, the system was covered with fiberglass insulation that was 37 mm thick and had an aluminized outer surface cladding. The average thermal conductivity of this cover layer was 0.038W/m K. It is important to remember that studies on charging and discharging were both conducted at a constant inlet temperature. The discharging studies were carried out after the charging studies were conducted and were performed at Ti = 63 and 47 ^S C. Based on the studies on charging and discharging outcomes, eight thermocouples were justified in graphs owing to the large number of measured temperatures.

Discharging Process

Experimentations on the discharging process were carried out by reversing the heat transfer direction between HTF and PCM. Hereby, the cold water was circulated through the system. The temperature measurement records for different thermocouples at varying times are presented in Figs. 6a-6c. The initial temperature of PCM, when it is liquid, is 63 ^S C. It is then introduced to a sudden temperature change in the circulating system during the discharging process as a result of the high temperature differences between HTF and PCM. Due to the sensible type of heat transference taking place within the solid PCM, volume increases until complete solidification occurs at solidification temperature. In accordance with the expectations, the points closer to the inlet part and HTF wall solidified faster than the other areas. In an attempt to provide a more comprehensive perception of the physics of the process, Figs 6a-6c present the temperature distribution's temporal variation at the lengthy points that are similarly distanced from the tube wall of HTF.

The PCM temperature at particular positions reduces slowly with time, as demonstrated before; temporal variation becomes almost uniform after some time due to the initial impact and effectiveness of natural convection on heat transfer. After that, conduction becomes the only dominating mechanism in heat transfer, with lower convection impacts in contrast to the melting or charging instances. This happens due to the melted PCM's circulation quantity that is influenced by the decreasing natural convection with the increase in time as a result of the outward HTF tube's solidification. In the same vein, the process of solidification will speed up with enhancing Reynolds number in the discharging process. For a clearer explanation, Figs. 7a-7d depict the results for Re=1000 where the ordinary and normal behavior of a transiention. It is clear that solidification occurred homogeneously with increasing time. INDUSTRIAL APPLICATION

This energy storage device of the present invention can be used in solar domestic heat water (SDHW) and solar devices to decrease energy wasting and increase time of energy saving.