Theoretical and Experimental Investigations on Passive Thermal Management of Solar Photovoltaic Systems using PCM Metallic Cellular Absorbers
Environmental conditions such as irradiance, temperature, humidity and dust accumulation have impact on PV performance and reliability. In this paper, we will focus on the effect of temperature and how to mitigate this effect by using passive cooling approach. Qatar is rich with solar irradiance favored for photovoltaic; however, the high temperature (a module temperature of 50degC was measured during summer months) has a negative impact on the power output of a PV module. Operation of solar PV systems under extremely high temperatures and high humidity in hot climates represents one of the major challenges to guarantee higher system's reliability. Therefore, thermal management in hot climates is crucial for reliable application of PV systems, as it has a potential to increase the efficiency and life expectancy and to stabilize the output power characteristics. Apart from that, dust accumulation on PV module is known to be also as one of the challenges that affect the PV power output. Due to the difference in ambient temperature between day and night, water condensation on PV modules was observed. Dust accumulated on a PV module together with water condensation may cause a thick layer of mud that is difficult to be removed. In this paper, we will show that, condensation of water on the cell surface during night can be prevented by maintaining the temperature of solar PV panels above the dew point during night. Application of Phase Change Materials (PCM) for passive or combined active passive cooling systems offers various options for adequate thermal management solutions. The present research focuses on utilization of PCM for passive thermal management of solar systems. Passive cooling can be realized by integration of PCM layers with the back side of PV panels. Passive cooling use the high temperature differences between day and night in arid desert regions, due to sky radiation in the night. The high thermal capacity of PCM accumulates coolness during night to keep the PV cells at a moderate temperature during the day. This also can help maintaining the PV panel temperature well above the dew point to prevent condensation during day and night, thereby avoiding mud formation on the panel surface. In some instances, active cooling may still be needed during peak solar radiation hours around noon time in summer, however integration of PCM can also reduce significantly the pumping power required to circulate the cooling medium as well as the external thermal/cool storage size and cost. Although PCM provides high energy storage density and nearly isothermal behavior around the phase change temperature, they suffer from low thermal conductivity, which limits the power density during charging and discharging. The low thermal conductivity of a PCM can be increased by combining them with highly conductive heat transfer structures. One advanced option is the application of cellular materials like metal fibers, which allow a significant enhancement of the PCM absorber thermal conductivity by more than 100 times. Hence it is proposed to hybridize PCM with cellular metallic matrices to enhance the thermal conductivity and provide a practical solution for easy encapsulation and integration with the PV panels. The main focus of this study is therefore to explore the effect of utilization of PCM based cooling elements incorporating cellular metallic heat conducting structures on the thermal behavior of solar PV panels. Preliminary laboratory experimental investigations have been carried out to characterize the thermal resistances between the PV panel and the PCM matrix absorber using different coupling mechanisms attached to the backside of PV panels. The coupling mechanisms include mechanical clamping, adhesive bonding, and double side thin and thick adhesive tapes. Based on the measured data, design recommendations for the desired performance will be discussed. The outcomes of the laboratory experimental investigations provide important input parameters that are needed in numerical analysis and design optimization of such systems under weather conditions in Qatar and elsewhere. Beside the laboratory experimental work, theoretical analysis to optimize the properties of the PCM matrix absorber for application of solar PV systems in Qatar has been carried out. The simulation model has been developed using homogenization based on volume averaging techniques and interpenetration continua approach. Due to complexity of the underlying transport phenomena, solution of highly nonlinear coupled system of equations with moving boundaries is required as a function of space and time. Hence, numerical modeling and optimization of large scale PCM storage is both challenging and computationally expensive. However, in engineering systems microscopic details are neither easy to be captured nor needed, instead, the macroscopic aspects are much more interesting. Therefore, dealing with a large scale PCM storage, a fundamental question arises as how to bridge the computational scale and reduce the problem to a simple one. A simplified modeling approach and numerical procedures shall be proposed to determine the macroscopic transport and time history of the PCM temperature field in a PCM thermal storage. The model is fairly general to be applied as a design and optimization tool for thermal energy storage and thermal management systems. Preliminary results of the numerical simulation shall be presented and discussed. Due to similarity of climatic conditions in the GCC, the solution can be easily adapted to suit other countries in the Gulf. The fibrous porous structure can be manufactured using wastes of metals processing such as in manufacturing aluminum profiles. PCM candidates with low temperature and desired thermo physical properties, such as paraffin waxes, are abundantly available with reasonable cost. Further cost reductions for manufacturing of PCM matrix absorbers can be achieved by integration with the PV support structure. This an important part of the ongoing research in collaboration with local industry partners in Qatar in order to produce these systems locally on a commercial scale effectively with lower costs. Preliminary analysis shows that the passive thermal management can increase both instantaneous conversion efficiency by 3 5% while it can considerably increase the life span of PV modules and reduces maintenance and cleaning costs. These factors hold a great promise for supporting the economic viability of passive thermal management using PCM matrix absorbers. However, detailed technoeconomic analysis will be elaborated within the framework of this project and will be published later.