| Among renewable energy resources, solar energy is by far the largest exploitable resource. It can be used free of charge, and does not need to be transported. The applications of solar energy include solar thermal utilization, photovoltaic power generation and photochemical conversion. Solar thermal utilization is the most popular application among them.The critical problem for solar thermal utilization is how to improve the ef?ciency of the solar collector. It can be performed with optimizing the structure of collector or developing a new type of working medium. Currently, solar thermal collectors capture light energy on an absorbing surface, which must then transfer the energy via convection to a circulating fluid. This solar thermal collecting mode will lead to the lower ef?ciency of solar collectors.In order to minimize the heat loss, the direct absorption collector was proposed in the 1970 s. The nanofluids absorb the solar thermal energy directly, which will reduce the heat loss and enhance the thermal efficiency. When the nanoparticle is with magnetic iron, cobalt, nickel or its oxide magnetic particles, etc. The nanofluids is become magnetic nanofluid. Magnetic nanofluid is a special kind of nanofluids, it has magnetic and fluidity characteristic. When the magnetic nanofluid is under a magnetic field, the distribution of the nanoparticle structure will change. It makes the magnetic nanofluid has many special optical properties.Applied magnetic field is expect to strengthen the photothermal properties of nanofluids. The main purpose of this study is to discuss the effect of the applied magnetic field on the photothermal properties of magnetic nano?uids, and investigate the efficiency of a direct absorption solar collector with magnetic nano?uids as its working fluid. The following research works were carried out.1. Preparation of Co-H2 O nanofluids.The effect of addition of dispersant and p H value on the stability of the nanofluids dispersion were performed. The Zeta potential and absorbance were used to representate the stability of nanofluids. The results show that the best amount of SDBS is 0.03wt% for Co-H2 O nanofluids(0.04wt%). When the p H value is 8.5, the absorbance and absolute value of Zeta potential are the highest, the corresponding dispersion stability of Co-H2 O nanofluids is good.2. Thermal physical properties of Co-H2 O nanofluids.The effect of mass fraction, nanoparticle size, p H value, temperature on the thermal conductivity, specific heat and viscosity were investigated. The data indicates that the thermal conductivity of nano?uid increases nonlinearly with mass fraction of the nanoparticles. When the mass fraction is 0.01wt% and 0.1wt%, thermal conductivity is increased up to 6.23% and 18.12% at 30℃, respectively. The thermal conductivity of 30 nm Co-H2 O nanofluids is larger than that of 50 nm Co-H2 O nanofluids. The Co nanoparticle added in water will reduce the specific heat of water. The specific heat of nanofluids decreases with the increase of particle mass fraction and size. The principle model of tradition mixture can not used to calculate the specific heat of nanofluids.The viscosity of Co-H2 O nanofluids was tested by vertebral plate viscometer. The results showed that the mass fraction and temperature have a major influence on the viscosity.3. Optical property of Co-H2 O nanofluids.Based on the analysis of the theory of the optical properties, the transmittance of nanofluids over solar spectrum(250 to 2500nm) was measured by the UV-Vis-NIR spectrophotometer based on integrating sphere principle. The factors influencing transmittance of nanofluids, such as particle size, mass fraction and optical path were investigated. Adding nanoparticle to pure water can change the solar energy spectrum absorption characteristics of water. Nanoparticles with excellent optical absorption make the transmittance of nanofluid in 250~1370nm wavelength range lower than that of pure water. Transmittance decreases along with particle size increase, and decreases with mass fraction increase. The transmittance of Co-H2 O nanofluids(0.1wt%) is closes to zero, which Indicates that the mass fraction to nanofluids as heating material performance is the best.4. Photothermal properties of Co-H2 O nanofluids.Nanoparticles excellent light absorption performance makes the photothermal properties of nanofluids strengthen. The particle mass fraction had a greater influence on the thermal characteristics of nanofluids. The temperature rise rate increased with the increase of particle mass fraction. But particle content also shoulds not be too much. In this experiment the optimum mass fraction is 0.1wt%. The highest temperature of it is increased up to 30.3% than that of water. In addition, the particle size will also affect the photothermal properties of nanofluid, the more small particle size, the stronger the light absorption ability.Apply magnetic field to magnetic nanofluids can change the distribution of nanoparticle and enhance the the solar energy spectrum absorption characteristics of nanofluids. There is a best magnetic field intensity values. In this experiment, the optimum magnetic field intensity is 30 Gs. The highest temperature of Co-H2 O nanofluid(0.04wt%, 30 Gs, 50nm) is increased up to 39.5% compared with deionized water.5. Solar-thermal applications of Co-H2 O nanofluidsTwo small size direct absorption solar collectors were made. The effect of Co-H2 O nano?uids on the ef?ciency of a direct absorption solar collector was investigated experimentally. The influencing factors such as mass fraction, nanoparticle size, mass flow rate, p H value and applied magnetic field were analyzed. The effect of Co-H2 O nano?uids on the ef?ciency of a ?at-plate solar collector was also investigated experimentally.The maximum efficiency of direct absorption solar collector with Co-H2 O nanofluid under 30 Gs magnetic field(0.1wt%, 30nm) is increased up to 51.70% and 13.24% compared with water and Co-H2 O nanofluid without magnetic field, respectively. The smaller size of nanoparticle has a more effective to strengthen the collection efficiency. The maximum instantaneous efficiency for 30 nm Co-H2 O nanofluid is increased up to 12.29% compared with 50 nm Co-H2 O nanofluid.The thermal performance of the traditional flat collector indicates that the efficiency of Co-H2 O nanofluid(0.1wt%) is increased up to 22.31% compared with water. The highest temperature and highest heat gain of water in the nano?uid(30nm,0.1wt%) tank can be increased up to 11.18% and 22.35% compared with water tank, respectively. |
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