Heat/Electricity Harvesting Characteristics Of Spectral Splitting PV/T Systems Based Nanofluids

With the background of carbon neutrality in buildings,photovoltaic/photothermal systems（PV/T systems）have been widely used in buildings due to its superior performance of electricity and heat production.However,the traditional PV/T system is affected by the high thermal radiation loss and high cell temperature,and the electrical and thermal efficiencies are low.The existing technologies focus on the enhanced heat exchange technologies of PV modules to alleviate the“high temperature”effect and these technologies do not fundamentally solve this problem,resulting in the thermal/electrical efficiency not being improved.The In current study,theoretical analysis,experimental research and numerical simulation are used to investigate the spectral regulation characteristics of water-based silver nanofluids and the heat/electricity production of the spectral splitting PV/T system.Meanwhile,an PV-PV/T dual-functions switching mode based on outdoor meteorological parameters is proposed to analyze heat/electricity harvesting characteristics of this systemthe in heating season.Firstly,the optical absorption-transmission model of nanofluid is constructed based on the two-dimensional Monte Carlo method from the radiation properties of nanoparticles,considering the zero-dimensionalization characteristics of spherical particles,and the average relative error of the model is less than 3.92%compared with the experimental results.Based on the Mie theory and discrete dipole algorithm,the mechanism of multiple scattering of solar photons between nanoparticles in translucent media is clarified.Secondly,the influence of fluid parameters on the spectral absorption properties of nanoparticles is analyzed using the above optical model,and it is demonstrated that increasing the particle size and refractive index of the base fluid can cause a"red shift"of the absorption peaks of the particles.Based on these results,the regulatory mechanism of specific spectrally oriented absorption is investigated,and a nanofluid with maximum thermal/electrical energy production is proposed.Aiming at the maximum electric power output,a silver nanofluid（with a radius of 40 nm）matching the spectral response band of monocrystalline silicon cells is used as the maximum electric power-producing nanofluid,while a nanofluid with a full-spectrum broadband absorption（with a radius of 90 nm）is identified as the maximum heat-producing nanofluid to enhance the photothermal conversion of solar radiation.In addition,the heat/electricity production law of the system based on mass fraction and optical thickness regulation and the matching characteristics with the energy demand of the building are investigated.Thirdly,to further analyze the actual heat/electricity capacity characteristics of the system,an indoor experimental platform was built to discuss the mechanisms of the optical and environmental parameters on the heat/electricity capacity.The experimental results show that the volume flow rate,solar irradiation and installation inclination have small effects on the electrical efficiency,but can reduce the heat loss to improve the thermal efficiency of the system.On the contrary,since the mass fraction and the optical thickness of nanofluid have a direct effect on the spectral transmittance of the fluid,which in turn significantly affects the system electrical efficiency,both can be used as the main variables to modulate the heat/electric output ratio of the frequency-divided PV/T system.After that,comparing the energy transfer process of spectral splitting PV/T systems and traditional PV/T systems,the spectral splitting PV/T system is more advantageous in heat collection because it absorbs solar radiation using a soft solution,resulting in lower surface temperature and less heat loss.With solar radiation of1000W/m² and ambient temperature of-5℃,the total efficiency of spectral splitting PV/T systems is 13.28%higher than that of the traditional PV/T system.To improve the adaptability at low ambient temperatures,an air layer is added on the upper and an insulation layer is installed at the lower of spectral splitting PV/T systems.The results show that the thermal insulation performance of this system is optimal,when the thickness of air layer and insulation layer are 8mm and 40mm,respectively.Then,the effects of outdoor environmental factors and operating conditions on outlet temperature of nanofluid and heat/electricity output are investigated under unsteady conditions.As uncontrollable factors,high solar radiation enhances the energy harvesting by increasing the energy density,while low outdoor temperature and high wind speed reduce the yield by strengthening the heat loss.At the same time,the influence of environmental factors on the heat collection is solar radiation,air temperature and wind speed in turn.On the contrary,as an active regulation variable,when the optical thickness increases from 5mm to 40mm,the outlet temperature decreases by 8.68°C,although the thermal efficiency increased by 4.22%.While the cross-sectional flow velocity is more sensitive to outlet temperature and total efficiency.When the flow velocity increases from 4×10^-4m/s to 40×10^-4m/s,the outlet temperature and heat collection efficiency can change by 6.39℃and 15.27%.The two variables of optical thickness and cross-sectional flow rate can be synergistically regulated to meet the temperature requirements of building thermal applications and maximize capacity.Finally,oriented to buildings heat demand in winter season,the environmentally triggered PV-PT dual switching mode and maximum yield strategy are proposed to realize the full time-domain efficient energy yield.Meanwhile,the energy harvesting characteristics of this system in heating season are analyzed,using the typical meteorological year data of Zhengzhou,Beijing and Harbin.When the outlet temperature is 45℃,the calculated results show that overall efficiencies are 51.98%,57.53%and 46.67%during the heating season for three cities,and the average daily energy yield（with heat collection area of 0.61 m²）is 2.69MJ,5.01MJ and 3.12MJ,respectively.Based on the spectral regulation characteristics of nanofluids,this thesis presents a comprehensive study on the heat/electricity output characteristics of spectral splitting PV/T systems,which can provide a feasible technical solution for the operation of distributed building integrated solar systems and the application of building energy.