Enhancing Thermal Conductivity of Phase Change Materials Using Nanomaterials and its Effectiveness in Cooling Solar Cells

Abstract

With the recent increasing reliance on renewable energy, the interest in studying energy storage technologies is increased, because renewable energy is volatile and not available all the time. The latent heat storage system is considered the most promising technology in thermal energy storage because of its many advantages. The most important feature of these systems that they have the ability to store large amounts of thermal energy without a significant change in their temperature. This technique uses phase change material (PCM) as thermal energy storage medium. However, low thermal conductivity of PCM is the major drawback of the system. Previous studies indicated that adding nanomaterial additives to PCM for enhancing the thermal conductivity is one of the most common solutions for this problem. Previous researches studied the possibility of using different types of materials as PCM, as well as the effect of different types of nano additives on their thermophysical properties. They focused on studying each nano-additive separately. Recently, limited number of studies have been published on the effect of using more than one type of nano-additives together (nanohybrid) on the thermophysical properties of PCM. Studies have shown that the addition of nanomaterials generally increases thermal conductivity of PCM. On the other hand, researches tended to study the possibility of using enhanced PCM to cool solar cells. In fact, raising the solar cell operating temperature negatively impacts its efficiency. This problem is considered one of the biggest problems of solar energy systems that researches are still working to solve it. The researches' results about the ability to use paraffin as a PCM to cool solar cells were positive. This thesis presents a study about using nano additives to improve the thermal conductivity of phase change materials and their ability to regulate the temperature of photovoltaic cell (PV) in a normal operating temperature range thus improve its efficiency. This study is a continuation of previous researches. The effect of nanomaterials' shape that was added to PCM was taken into account. Therefore, this study focused on the possibility of enhancing the thermal conductivity of paraffin by adding different shapes of silver nanomaterials to it (silver nanoparticles, silver nanowires and nanohybrid with silver nanoparticles and silver nanowires). Nanomaterials were added at volume fractions 0.5% and 1%. Then, the study examined the ability to use this improved material on regulating and reducing the temperature of solar cells. We used the "Solidworks" program to create a 3D system, and then we used the "Ansys Fluent" software to simulate five cases:  PV without PCM.  PV with PCM.  PV with PCM enhanced by nanoparticles.  PV with PCM enhanced by nanowires.  PV with PCM enhanced by nanohybrid (a mixture of nanoparticles and nanowires). The thermophysical properties of PCM (which is temperature-dependent) were defined using a user-defined function (UDF). After that, they were compiled to the Ansys fluent solver. Then, we ran the simulation under a set of hypotheses that were taken into account. The results of the five cases were analyzed and their ability to reduce PV temperature, and thus the raise in its efficiency were clarified. We found that the addition of nanowires at a volume fraction 1% led to the greatest improvement in the thermal conductivity of paraffin, but using PCM enhanced by nanowire at volume fraction 0.5% gave the best results in decreasing the temperature of PV. The average temperature of PV declined by 14.9 degrees (kelvin). In addition, the efficiency of PV rose about 7.6%. Followed by using PCM enhanced by nanoparticles at volume fraction 1%. The average temperature of PV declined by 12.9 degrees. Moreover, the efficiency of PV rose about 6.6%.