Thermochromic VO2 thin films were prepared by electron beam depositing 99.99% pure metallic vanadium onto the glass substrates followed by annealing post-treatment. The treatment was performed in oxygen pressure ranging from 1 X 10-3 to 1.5 Torr during the time from 2 to 10 h at a temperature of 723 K. Structural and optical properties of the films were measured as a function of both the annealing time and oxygen pressure. The metal-semiconductor transition of the VO2 films annealed in 0.3 Torr of gaseous oxygen, for 7 h was observed at Tc approximately equals 345 K. At the temperature below Tc these films are infrared-transparent semiconductors having monoclinic structure. At the temperature above Tc they become metallic structures exhibiting a tetragonal crystalline lattice and their optical transmittance at a wavelength of 1.2 micrometers has decreased to a value as low a 30%.
The performance of solar water heater varies depending upon many parameters, such as operating temperature, mass flow rate, solar insolation, collector orientations, solar time, wind conditions, ambient temperature, selectivity of the absorber surface, etc. A Linear Fresnel reflecting concentrator in a planar configuration using commercially available mirror strips of reflectivity approximately 0.7 in conjunction with a reverse flat plate selective and non-selective absorber surface has been designed and developed. The performance studies of the solar water heater-cum-steam cooker have been carried out. The efficiencies of the devices were found to be about 60% and 55% for selective and non-selective surfaces, respectively. This solar energy device can be made useful in the domestic sector by operating it year around in our solar insolation condition.
MnO2 thin films have been prepared by thermal evaporation technique onto glass substrate at varying deposition pressure. Electrical studies of the films deposited at about 1.5 X 10-5 Torr show a positive TCR indicating metallic characteristics, whereas a negative TCR indicating semiconducting behavior exhibit on the films deposited at a pressure of about 3.0 X 10-6 Torr. Thickness dependent electrical conductivity, activation energy, and aging effect were also carried out. Optical measurements in the wavelength range 0.3 < (lambda) < 2.5 micrometers show the films highly transparent in the visible. These studies may be of importance for the application of this material in surface coating for energy efficient devices.
A Fresnel reflecting solar concentrator fabricated from mirror strips of equal width and almost equi-spaced from one another could possibly lead to an easy design. The axes of rotation of mirror strips are parallel and lying in the same plane. The reflectivity of the mirror strips was about 0.7. Optical and time constant performance of this concentrator-receiver assembly employing mirror strips of 50 mm widths and at a 1 m focal distance in conjunction with a reverse flat plate absorber with black-board paint (non-selective coating) was studied. Employing provisions for suppressing heat loss and reducing absorber area, a time constant of 9 minutes and an overall efficiency of 46% was achieved.
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