A detailed analysis of the heat transfers and fluid flows within a direct thermosyphonic solar-energy water-heater has been undertaken. The collector energy equations when cast in a two-dimensional form enabled heat transfer and thermal capacitance effects to be simulated accurately at the small flow rates encountered commonly in such systems. An investigation of thermocline relaxation processes within the store indicated negligible mixing at the store inlet over a wide range of Richardsons numbers (43,608 < Ri < 729,016). Thermal relaxation under conditions of no flow was shown to be due predominantly to axial conduction along the store wall. The use of an appropriate non-isothermal friction factor correlation when calculating frictional losses in the collector's riser pipes, produced predicted steady-state flow rates which were corroborated experimentally to within 2%. An indoor test facility, monitored and controlled by a microcomputer, enabled 'real' operating conditions to be simulated. The predicted responses of the system to identical conditions showed good agreement with the corresponding experimental observations, the predicted heat delivery being within 2.8% of the measured value. A technique for correlating the daily performances of thermosyphon solar-energy water-heaters has been developed. The five dimensionless groups which form the basis of the correlations and the functional relationships between these groups were derived from an analytical solution of a linear first-order differential transient heat balance carried out on a generic system. Thermal performance data used in the correlations was generated by the numerical simulation using representative U. K. hourly weather data and operating conditions. The minimum amount of data required to establish a characteristic curve for an individual system was found to be thirty days. Using such a curve, the total annual solar fraction agreed with that predicted by the high level model to within 3%. Two universal curves were determined in which the gradients of characteristic curves were correlated against the derived dimensionless groups. The accuracy of the resulting two-stage algorithm in determining annual solar fractions was established as ranging from 5.5% for predominantly multiple-pass systems to a mean of 10.5% for single-pass systems.

The development of products which enable passive solar-energy air-heating to be integrated into the heating strategies of public, commercial and industrial buildings is described. These buildings are, in general, only occupied significantly during the day; consequently the bulk of heating demand coincides with the period of solar gain. In these circumstances collected solar heat should be delivered with the minimum of delay. The design and operation of units which are capable of supplying solar heated air in this manner is outlined. These are passive, naturalcirculation air-heating collectors, also known as natural-convection air-heaters, or thermosyphoning air panels. Four methods of retrofitting such solar collectors to non-domestic buildings have been identified, one of which, the overcladding collector, has not been proposed previously. Problems associated with the successful installation and operation of these units have also been considered. The relative merits of a number of methods of testing passive solarenergy air-heating collectors have been investigated. A method of determining instantaneous collector efficiency based on the measurement of glazing temperature, inlet and outlet air temperature, ambient temperature and insolation has been developed. Three novel design proposals have been presented: i) a collector constructed with the insulation fitted outside, rather than inside, so that the metal body of the collector may provide more symmetrical heating of the air flow than the conventional arrangement, ii) an absorber which consisted of parallel ducts to increase the rate of heat transfer to the air, heating it symmetrically, (iii) a hinged air-deflector for conversion from the heating to the ventilation mode.

Two types of solar cell AlGaAs-GaAs structures which are heteroface and triple heterojunction are investigated in this study. A complete theoretical study including optimisation for the optical properties ( transmission and reflection) of the heteroface Alo.sGao.2As- GaAs space solar cell is presented. The grid shadow and window layer effects, angle of incidence and the effects of the layer design parameters for AR-coating and window layer on the optical properties are considered in the calculations. A new structure for space solar cell which consists of double heterojunction AlGaAs­GaAs structure with GaAs/AlGaAs heterojunction back surface field (triple heterojunction(TIIJ))-to enhance the performance of the existed double heterojunction solar cell- is proposed. The analytical model for this TIU cell is presented as a function of all the cell's design parameters ( such as _layers doping, thicknesses, etc). The calculated results for this structure is compared with the experimental results for the previous double heterojunction structure. The effects of the design parameters of all layers including the AR-coating on the cell's output performance and the optimisation conditions are studied as well. The techniques of the light trapping and the photon recycling( which are gocxl for space solar cells) are applied for the THJ thin film AlGaAs-GaAs structure to improve further the efficiency . The change of the optimisation conditions due to the usage of these two techniques is also discussed.

No description available.

No description available.

It is generally accepted nowadays that a significant cost reduction in terrestrial solar cell application could be brought about by investigating alternative fabrication techniques for solar cells. It is believed that screen printing (or the so called thick film technique) is one such technique which promises a potentially low cost method for fabricating flexible, large area solar energy conversion cells. The active research on this technique started in 1976 in Japan. However, it was not until 1983, that wide interest developed when the Matsushita group in Japan reported an efficiency of 12.8% for their entirely screen printed CdS/CdTe solar cells. This was the highest reported efficiency for any thin film solar cell. However, the details of the fabrication processes of these cells were not reported and several scientific groups in the world started to explore this technique. The first published report was in 1985. In the last few years these groups have reported results on various aspects of this technique. Nevertheless there are still major parameters to be investigated. This thesis represents a concise reference for the application of the screen printing technique to solar cells. In the course of this study many new investigations have been made which supplement the previous work by other groups. Starting with a pure CdS powder with suitable grain size and distribution is a prerequisite for achieving the best morphological and electrical behaviour of screen printed layers of CdS. Careful paste mixing is of uppermost importance which can override any other parameters involved in the fabrication processes. It is essential to impose restricted sintering conditions for adequate utilization of the doping and fluxing function of the CdCl(_2) material. Standardization of the printing, preparation and sintering conditions involved in the fabrication processes were necessary to ensure reproducible CdS layers. Good quality screen printed layers were fabricated on soda lime substrates. The significance of other substrate materials for CdS preparation was also investigated and optimum substrate choice is suggested. The properties of the CdS screen printed layers were investigated by forming simple Schottky devices and more complicated heterojunction solar cells. Good rectification behaviour of the Schottky diodes was achieved. The CdS/CdTe solar cells revealed a wide spectral response. However, the photovoltaic behaviour was relatively poor largely due to the high resistivity of the CdTe part of the cell structure.

Thin film solar cells with a band gap graded in the thickness direction were prepared by vacuum evaporation of various alloys of Cadmium Sulphide (CdS) and Cadmium Telluride (CdTe) onto glass substrates. Theoretical computer analyses were carried out and show that a solar cell with a graded band gap surface layer ought to be a more efficient photovoltaic converter than a conventional homojunction device. A vacuum evaporator capable of multiple simultaneous co-evaporations was built and used initially to deposit only the CdS and CdTe films and subsequently mixed and graded films of Cadmium Sulphide Telluride (CdS x Te 1-x). The composition of the mixed and graded films was controlled by a set of shutters built above the sources, rather than by the control of the temperature of the sources. The electronic properties of the films were adjusted by co-evaporation of dopant materials such as Cadmium (Cd), Indium (In) and Copper (Cu), and measured by Hall Effect measurements. The physical properties of the films such as the band gap, crystal phase and grain size were investigated by optical transmission measurements, X-ray analysis and scanning electron microscopy respectively. Finally, p-n junctions with n-type graded band gap surface layers on top of either pure p-type CdTe films or mixed p-type CdS0.5Te0.5 films were prepared. However, the photoresponse of these structures was low, probably due to very short minority carrier life-times, lack of low resistance contacts and cross diffusion of dopant materials. Nevertheless, the characteristics of individual materials and the spectral response of the devices indicated that if these problems could be solved, a successful solar cell could be made.

This thesis is a comparative study of nickel oxide based thin films for use as a counter electrode in a variable transmittance electrochromic device. Coatings have been prepared using anodic electrodeposition, colloidal precipitation and radio frequency (r.f.) sputtering. Systematic studies of the effect of deposition process parameters on optical and electrochromic properties of such films have been undertaken. Optimum conditions for the deposition of coatings deposited by colloidal and anodic deposition have been determined. A novel process for the colloidal deposition of electrochromic a-Ni(OH)2 coatings using a simple one dip process is reported. Also the electrochromic properties of coatings anodically deposited from aqueous solutions containing NiS04 and NH40H were improved by the addition of the non-ionic surfactant polyoxyethylene sorbitan monolaurate. Spectroscopic and electrochemical analytical techniques were used to identify the chemical composition of the coloured and bleached states. It was found using Fourier transform infra-red spectrophotometry (FTIR) that coatings deposited by anodic and colloidal deposition contained f3-Ni(OH)2 and a-Ni(OH)2 respectively in the as-deposited and transparent states. For coatings deposited by both techniques 13 or y-NiOOH was detected in the coloured state using FTIR. Using Raman spectroscopy, y-NiOOH was detected in the coloured state for coatings deposited by anodic deposition from solutions containing the additive polyoxyethylene sorbitan monolaurate. I3-Ni(OH)2 was also detected in the transparent state of r.f. sputtered coatings that were electrochemically cycled in 1M KOH(aq). Using cyclic voltammetry the oxidation of nickel hydroxide to the oxyhydroxide was detected during colouration for coatings produced using anodic electrodeposition, colloidal precipitation and r.f. sputtering (after cycling sputtered films for 1 hour in 1M KOH(aq)). This information has been compared for films prepared using the different deposition techniques to enable the respective colouration mechanisms to be elucidated. Prototype electrochromic devices have been constructed and their performances assessed. It can be concluded that nickel oxide based coatings can be used as suitable counter electrodes for hydrated electrochromic devices.

No description available.

No description available.