Harnessing the Sun

 

 

Unique, optically-selective surfaces that were developed at the Weizmann Institute are currently used in commercial solar collectors for heating water. Installed in Israel and in other countries, these panels are highly efficient in absorbing and converting energy and have excellent durability.

 

 

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The solar tower at the Weizmann Institute of Science is one of the world's most advanced installations for investigating and developing methods to exploit solar energy. Scientists working in the tower are developing new technologies that will make solar energy an efficient and relatively cheap alternative to the finite resources of fossil fuels which also cause environmental pollution. Research at the tower includes the following.

 

 

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Standing up to pressure

 

 

A revolutionary solar radiation receiver developed by Weizmann Institute scientists may advance the use of solar energy for electricity generation. Installed in the receiver - known as the "porcupine" - are hundreds of pins made from a ceramic material. The pins are arranged in a sophisticated geometric structure that allows maximum absorption of radiation. The structure also prevents the pins from breaking due to the expansion and contraction caused by drastic temperature variations.

 

To bring in sunlight into the porcupine, Weizmann scientists developed a special conical quartz glass window that can withstand pressure approximately five times greater than steel can endure. The way it works: Researchers pump pressurized air into the system. This flows between the porcupine's pins, heats up, and is channeled to an electricity-generating gas turbine.

 

 

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Most turbines currently used for generating electricity from solar energy are steam operated. Gas turbines are more efficient but they require heating air to much higher temperatures, i.e., above 1,000°C (1,832°F). To raise air to this temperature via solar energy, the sun's rays must be concentrated to at least 10,000 times that of sunlight reaching Earth. This is achieved using optic funnels with a unique geometric structure, developed at the Weizmann Institute.

 

An experimental system for producing electricity from solar energy is under construction at the Weizmann Institute in cooperation with the Israeli companies Rotem Industries Ltd., Ormat Industries Ltd., and the American firm Boeing, and is sponsored by the Ministry of Industry and Trade. This system will combine various technologies that originated at the Weizmann Institute and will be the first in the world of its kind. Based on this experimental system, the companies intend to develop and market commercial solar power stations.

 

 

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A novel concept developed by Weizmann Institute researchers may allow solar radiation to be converted into storable and transportable chemical energy. It is a closed-cycle, non-polluting, environmentally-friendly process based on three main stages.

 

The first is the concentration and use of solar radiation reaching the surface in desert areas to drive high-temperature thermochemical processes. The second is storage of the chemical materials created by these processes, or transportation to regions requiring the energy. In the last stage, when the energy is required, a reverse chemical process will be implemented, releasing the trapped energy as heat for industrial purposes or for turning turbines and generating electricity.

 

A complete plant such as this with a processing capacity of 500 kilowatts has already been constructed and tested at the Weizmann Institute.

 

 

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Weizmann Institute scientists are developing methods to use solar energy for converting solid organic materials, such as charcoal and wood, into gaseous and liquid fuels. Small-scale preliminary experiments have demonstrated the viability of the process.

 

In another study, Weizmann Institute scientists are developing a method for generating hydrogen, which is an efficient fuel with few polluting by-products. Using hydrogen for running vehicles and industrial machinery would contribute greatly to solving the energy crisis while protecting the environment.

 

The system is based on the thermal decomposition of water at a high temperature using concentrated solar radiation. Hydrogen is one of the components of water (each water molecule contains two hydrogen atoms and one oxygen atom). The hydrogen is separated immediately upon being produced, using a special ceramic filter.

 

 

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Another method for exploiting the sun is to concentrate its energy to achieve the high temperatures required for the reduction of metal oxides (for example, the production of zinc from zinc oxide). Zinc is used in zinc-air batteries which generate electric energy efficiently, while producing zinc oxide which is recycled to recover the zinc. Another use: When zinc reacts with water to release hydrogen, it creates a clean, efficient and environmentally-friendly fuel. The feasibility of this process, developed at the Weizmann Institute, was demonstrated in laboratory experiments.

 

 

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Sunlight should significantly improve the efficacy of a range of industrial chemical reactions. In fact, every chemical process can be assisted by light of the appropriate color.

 

To enable industry to use solar radiation of an exact color, Weizmann Institute scientists developed a system that concentrates sunlight to its practical maximum limit, which is approximately half the density of light on the sun's own surface. At this stage the researchers separate the rays into different colors (measured by their wavelength). This light is used to activate laser systems in an exact hue. The lasers may serve as an energy source for specific chemical processes or in telecommunications and remote sensors in space.

 

 

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Silicon semiconductors are sensitive to light and this characteristic is being used in a wide range of industrial applications, including the storage of optical information, microelectronics, and the production of electricity in photovoltaic cells (which produce electricity from light). The semiconductors are also used in sensors for various types of radiation, such as visible light and X-rays. Weizmann Institute scientists, in collaboration with researchers from the French National Center for Scientific Research (CNRS), have developed a novel technique to improve the efficiency of these processes.

 

The new technique is based on etching and roughening the semiconductor's surface, then illuminating it and immersing it in an electrolytic solution while passing an electric current across it. This solves two main problems which reduce the efficiency of photovoltaic cells. One is the tendency of the electrons carrying the electric current (which have a negative charge) to bind to any positive electric charges, thereby canceling out the particles' electric charge and reducing the overall current. This problem is exacerbated by pollutants on the surface of the semiconductor which disperse the electrical charges produced by the illumination, impairing their flow. The new technique enables the selective removal of these pollutants.

 

The second problem stems from the reflection of light hitting the interior of the semiconductor, which cuts the amount of electricity produced by about a third. The new method creates a minute rough inner surface (at the submicron level) that significantly diminishes light reflection. In fact, almost every photon (light particle) in the new system that hits a cell produces an electron ¾ and that means electricity.

 

 

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The Weizmann Institute's solar research complex, the Canadian Institute for the Energies and Applied Research, is one of the world's most advanced facilities for designing methods to exploit concentrated solar energy.

 

Sixty-four giant motor-driven mirrors-each measuring seven by eight meters track the sun and concentrate its energy onto the central receiving tower. The mirrors follow the sun's movements by means of a computer that calculates the sun's position relative to the earth for every second of the year.