| Solar water heater collector is the unique renewable energy utilization facility, which can compete with conventional fossil fuel. The high performance collector is very important for solar energy heat utilization systems. The performance of solar collector mainly weighed by collector efficiency, in order to improve the collector efficiency, the best principle is to keep solar radiation absorption more and keep heat loss less.In this paper, a new concept of direct absorption type black nanofluid solar collector (BNSC) is proposed. Black nanofluid refers to a new kind of recycle fluid for solar collectors, which collect and transport solar energy by suspending black nano-particles in base fluid. Black nanofluid is used to absorb the solar radiation directly in BNSC, so the highest temperature of BNSC is exist in black nanofluid, and reduce the solar collector thermal loss effectively.In order to obtain adaptive black nanofluid used for BNSC, this study presents a preparation method of black nanofluid by directly mixing nano-particles and Ethylene Glycol aqueous solutions, some auxiliary dispersants and disperse means are necessary to obtain the even distributed and stabilized black nanofluid. With this method, several black nanofluids (include nanoscaled black carbon, copper powder and carbon-coated copper powder) with different content have been prepared. The results show that some factors affect the stability and evenness of black nanofluid, such as the property of nano-particles, the property and content of dispersants, disperse means and disperse time. The prepared black nanofluid suspensions have a good photo-thermal stability, and have no corrosive for some common materials of solar energy water heater, such as silicon rubber and stainless steel. The corrosion of brass in black nanofluid is only surface dezincification.By means of thermophysical property (heat conductivity, viscosity, specific heat capacity) measurement experiments, combined with the analysis based on particular physicochemical behaviors of nano-particles, the thermophysical performances of black nanofluid, a novel class of potential solar collector recycle fluid, and their relations to the species and content of nano-particles, the species and content of dispersant, temperature and suspension stability have been deeply investigated. The results show that the heat conductivity and viscosity increase almost linearly with an increase in the content of nano-particles, but the ratio of heat conductivity and viscosity increase is different for different nano-particles and dispersants. In addition, temperature is an important influencing factor for the viscosity of black nanofluid. Since Brownian motion and quantum effect of the nano-particles in black nanofluid, the conventional mathematic model, which used for calculate heat conductivity and viscosity of solid-liquid mixture can not reflect the true energy transfer process and rheologic behaviour of black nanofluid. Consider the size effect of nano-particles and the salvation and electroviscosity effect of black nanofluids, a new viscosity modelobtained:μeff=μf{1+2.5(1+△r/r)3[1+1/Lμfr2(εζ/2π)2]φ}(1+350φ/r).The specific heatcapacity decrease with an increase in the content of nano-particles and disprsants, but the decrease ratio of specific heat capacity is different for different nano-particles and dispersants. The influence of temperature to specific heat capacity is very slightly. Since microeffect of black nanofluid, the specific heat capacity of black naonfluid is not satisfy additivity. The surface vibration enhanced of nano-particles and modification of solid-liquid interface free energy state in black nanofluid is the main reason why the specific heat capacity of black naonfluid different from convention solid-liquid mixture.Because of combined action of absorbing and scattering from nano-particles, the black nanofluid has strong absorption for solar radiation in full-wavelength. The insolation experiment results show that the photo-thermal transition property of black nanofluid is better than Ethylene Glycol aqueous solutions without nano-particles and conventional black absorbing coating. Since high heat conductivity and low specific heat capacity, carbon-coated copper black nanofluid is considered the best recycle fluid for BNSC.According to "GB/T4271-2000: Testing Method for Thermal Property of Flat-Plate Solar Collector", collector efficiency of BNSC is measured. The results show that BNSC improve the collector efficiency obviously, the highest momentary efficiency accessible in testing is 74.688%, which is 10% more than conventional flat-plate type solar collector.Because of the excellent photo-thermal properties and flow performances, black nanofluid is used as recycle fluid for direct absorption type BNSC may improve the thermal efficiency greatly. Black nanofluid provides a new idea for the development of solar water heater collectors. The research results indicate that the black nanofluid has extensive application prospect and potential economic value.
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