| Solar energy has great potential and vast application prospects because of its non-polluting nature,abundance and inexhaustibility.Photovoltaic(PV)technology and solar thermal utilization are two kinds of common way to exploit solar energy.A kind of hybrid photovoltaic-thermal system,which combines photovoltaic technology and solar thermal utilization together,has been proposed.Building further on this foundation,a new PV/T accumulator is developed by integrating phase change material into a PV/T collector.The PV/T accumulator converts sunlight directly into electricity and heat.Thermal energy is stored by PCM layer and can be extracted when needed.In this paper,Chapter One briefly discusses the present situation and future trend of energy and introduces the development of the hybrid PV/T systems.Chapter Two performs comparative analysis on the dynamic performance of PV/T solar collector with dual channel for different fluids.Chapter Three focuses on preparing a composite PCM(CPCM)for work conditions of PV/T collector and analyzes the physical and chemical properties of CPCM.Chapter Four proposes to integrate PCM layer into PV/T collector,and studies the effect of the variation in the relative position and the thickness of PCM layer on the dynamic performance of the PV/T accumulator.By changing some design parameters of PCM layer,Chapter Five compares and analyzes the numerical simulation results of PV/T accumulator and presents some optimal methods to maximize the energy output.Finally,some suggestions have been made for further work.A proposed numerical model have been developed using one dimensional energy balance method.Basic performance parameters like solar cell temperature,photovoltaic efficiency,outlet temperature,electrical efficiency and thermal efficiency are calculated under given climate conditions.This method is compared commonly used way for the analysis of dynamic performance of the PV/T systems.The main conclusions of each section are as follows.1.Dynamic performances of the PV/T collector with dual channels for different fluidsFor the PV/T collector with dual channels,the solar cell temperature,electrical power,thermal power and thermal efficiency increase with the increase in intensity of solar radiation while the photovoltaic efficiency and electrical efficiency decrease with the increase of in intensity of solar radiation.Besides,the outlet temperature of the fluid follows the variation of environment temperature while the overall efficiency remains relatively stable value about 80-83%.Water-water cooled PV/T collector is most excellent in both electrical and thermal performance.Water-water cooled PV/T collector can provide the most amount hot water(39.4℃,0.1 kg/s).The temperature of water in air-water case is the highest(40.9℃,0.05 kg/s).Air-air cooled PV/T collector may provide the most amount hot air which its temperature is the highest(45.6 ℃,0.1 kg/s).The increase in mass flow rate of water and the height ratio between the upper pipe and the lower pipe lead to a rise in electrical efficiency and overall efficiency.The optimized value of the mass rate and the height ratio is 0.15kg/s and 3:1,respectively.2.Preparation and analysis of composite phase change materialIn the composites,the n-octadecane and stearic acid(SA-ODE)eutectic mixtures act as phase change materials(PCMs)for thermal energy storage,the HBN is introduced to improve the thermal conductivity.The chemical bonding,crystalloid phase and morphology of the CPCM are tested,and the thermal properties,thermal stability and thermal conductivity are also measured.The results demonstrate that the SA-ODE eutectics are uniformly absorbed on the HBNs with no chemical interaction.The prepared CPCMs have relatively high latent heat,which is reduced lineally with the increase of the HBN content.According to TGA analysis,the weight losses of the CPCMs are less than 0.1%at its work temperature(under 70℃).It means that the CPCMs have good thermal stability.3.Dynamic performances of the PV/T/PCM accumulatorWith the increase of solar radiation intensity,the solar cell temperature increases to 76.7℃ and the electrical power increases to 106.1 W,while the photovoltaic efficiency decreases to 9.2%and electrical efficiency decreases to 6.7%,respectively.The rate of useful thermal energy follows the variation of solar radiation intensity and reaches a peak value of 435 W at 12:00 in upper PCM mode.Besides,the outlet temperature follows the variation of ambient temperature and reaches maximum 42.4℃ at 12:00 in no PCM mode.It is demonstrated that the comprehensive performance of the PV/T collector in upper PCM mode is optimal among three structures,and overall efficiency of the PV/T collector in upper PCM mode is 10.7%higher than that in no PCM mode.For a PV/T collector with upper PCM,it is verified that 3 cm-thick PCM layer is excellent both in electrical and thermal performance.4.Maximizing the energy output of the PV/T/PCM accumulatorThe PV/T collector with PCM of 30℃ melting point has the best electrical performance.The 60℃ melting point case can provide the warmest water for a while,and the outlet temperature and thermal power reach peaks of 30.24℃ and 1096 W,respectively,at 17:30.The 40℃ melting point case can provide maximum up time with warm water from 17:00 to 22:00.To maximize the thermal energy outputs,the PCM layer with 40℃ melting point is optimal for the PV/T collector in this work.The PV/T collector gets a maximum of overall energy output by incorporating 3.4cm-thick phase change material layer with 40℃ melting point. |
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