الفهرس 
Solar energy
Concentration of solar powerOnly 14 pages are availabe for public view
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Abstract
The solar energy flux reaching the earth equals about 6000 times the current global energy consumption. Although solar energy is abundant, clean and safe, its supply is intermittent and irregular. Therefore, a considerable amount of energy should be stored during the day to cover the demand during the night or from summer to winter times. Hot water storage tank is considered one of the best technologies for seasonal energy storage because of the high specific heat of water and the high capacity rate of charging and discharging. Most of these tanks are built of prestressed concrete with internal lining to prevent the water leakage. The economic studies indicated that the costs of liner are very high. Moreover, the storing temperature is low (up to 95 °C) which limits the storing capacity and therefore, the stored energy is used for heating and hot water supply only. However, to make a practical use of the stored energy such as industrial processes and steam generation, the storing temperature must be increased above 100 °C.
The main objective of this work is to study the possibility of optimizing a concrete mixture to be used in a hot water concrete tank to store solar generated energy at temperatures up to 200 ° C and pressures up to 15.5 bars. This investigation is divided into three main parts. In the first one, high density concrete mixtures have been optimized in order to prevent the leakage of water and vapour through concrete and to ensure high sustainability under hydrothermal conditions. Increasing the packing density is considered an excellent strategy to get an optimized concrete mix. In this concern, the Ideal grading curve according to Fuller has been used to achieve the maximum packing density of the solid materials and to reduce the required binder content. In addition, various cementitious materials were examined to select the most suitable mixture with high resistance to hydrothermal attack. Siliceous fillers have been added also to close the gaps between aggregate and fine materials on one hand and to enhance the stability of C-S-H phases under hydrothermal conditions on the other hand. The second part of this investigation focuses on the effect of autoclaving with 200 °C and 15.5 bars on concrete properties. Mechanical properties including compressive strength, tensile strength and rebound number have been measured at normal conditions and after autoclaving with numerous cycles. In addition, concrete porosity is measured before and after autoclaving for several cycles. Regarding concrete durability, the main three mechanisms of ingress of gases and liquids through concrete; permeability, absorption and diffusion, have been measured before and after autoclaving for several cycles. In the third part, 10 different cement pastes have been prepared and tested in order to deeply understand the effect of hydrothermal conditions on characteristics and stability of C-S-H phases. In this concern, EDX, TGA and SEM measurements have been used to study the changes in pastes morphology and properties due to autoclaving for 50 cycles. In addition, the changes in compressive strength and porosity after hydrothermal treatment have been determined.
The investigation results showed that the optimized mixes exhibited very low porosity and high mechanical properties as well as high durability at normal conditions where the grading and particle size distribution are the main factors influencing the results. However after hydrothermal exposure, mixes with low C/S ratio showed very stable performance regarding mechanical properties and durability. The addition of silica-rich materials such as fly ash, slag and quartz reduces the C/S ratio of the system and consequently strong product with low porosity and high stability is produced. On the other hand, mixes with OPC and with high C/S ratio suffered poor durability, high porosity as well as strength retrogression.