Special Issue: Solar Energy

At a symposium entitled "From Basic Research to Industry" held at the Weizmann Institute on April 15, 1996, scientists, industrialists and government officials heard reports from four teams working at Weizmann's solar research facilities. Highlights of the symposium are presented below.

Patents on the technologies described have been registered by Yeda Research and Development Co. Ltd., which deals with the commercialization of Weizmann Institute research.

The process of harnessing concentrated solar energy begins with a receiver - a device that collects concentrated sunlight in order to heat a gas which can then drive a turbine to generate electricity or be converted into energy-rich chemical fuel.

However, the efficiency of systems based on standard solar receivers has been low because these receivers are unable to operate at the high temperature and pressure required by modern power generation equipment.

A dramatic increase in efficiency has now been attained with a new receiver developed by Weizmann Institute's Dr. Jacob Karni, Dr. Abraham Kribus and Rahamim Rubin, in cooperation with a team headed by Dan Sagie of Rotem Industries Ltd.

Sunlight enters the device -- referred to as the directly-irradiated annular pressurized receiver (DIAPR) -- through a special cone-shaped window made of quartz that can withstand five times more pressure than steel. The rays are absorbed by hundreds of ceramic pins which line the receiver's inner walls, pointing towards the incoming light.

This light-absorbing matrix -- nicknamed "kippod," the Hebrew word for porcupine -- is designed in a sophisticated manner to absorb maximum sunlight while preventing cracks resulting from expansion and contraction caused by extreme changes in temperature. Gas, which fills the entire device, flows across the pins and removes the heat at around ten times the rate achieved in existing receivers.

The unique elements of the new receiver's design, which now make it possible to reach extremely high temperature and pressure, open the way to major industrial applications.

In photochemical reactions, solar energy is converted directly into chemical energy without intermediate conversion to heat - much like in photosynthesis, the process underlying plant life. However, each such photochemical reaction uses a different fraction of the solar spectrum with a precisely defined wavelength, or color.

To adapt the photochemical methods for efficient industrial applications, Weizmann Institute Prof. Amnon Yogev and his team have developed a technology that converts sunlight into laser light, which can then be selectively tuned to various colors.

Their system now achieves the maximum feasible sunlight concentration -- about half the density of light on the surface of thesun itself. A portion of this concentrated beam is then transformed into laser light.
 

Such solar-powered lasers may serve as a source of energy for various chemical processes. They may also be used by sensory and communications devices in outer space. For example, small satellites in a polar orbit may use the Weizmann method for converting sunlight to laser light that can be transmitted to the atmosphere as communications signals.

Prof. Yogev holds the Stephen and Mary Meadow Chair of Laser Photochemistry.

A major obstacle to the exploitation of solar energy on the industrial scale is the need to store and transport it over long distances. These goals may be best accomplished by converting the sun's radiation into energy-rich chemicals in a closed-loop, environmentally-friendly system developed at the Weizmann Institute.

Known as the chemical heat pipe, the system has three stages: (1) sunlight collected in desert areas is concentrated and used to drive chemical processes that run only at high temperatures; (2) the gases formed during this processing are cooled and then stored or transported to areas where the energy is needed; (3) once these gases reach their destination, the chemical processes are reversed, releasing heat that can be used to produce steam for industry or to power electricity-generating turbines.

This approach, pioneered by Weizmann Institute Profs. Israel Dostrovsky and Moshe Levy, is currently being implemented by a team headed by Engineer Michael Epstein. Recently, the concept was advanced with the construction of a chemical heat pipe that has the ability to absorb about 500 kilowatts of power. The first stage of the system, known as reforming technology, is currently being considered for scaling up to suit various industrial applications.

Epstein's team is also working on solar-powered conversion of solid organic materials such as coal and wood into liquid or gas fuels. Lab experiments have demonstrated the feasibility of this process, and an expanded conversion facility is now being designed.

Another application of solar energy explored by Prof. Yogev Epstein, and colleagues is based on using heat from the sun to extract metals from oxides, such as zinc from zinc oxide. Zinc can be used in batteries that store electrical energy while generating zinc oxide, which can again be processed by the sun's heat.

This process has been proven successful in preliminary lab experiments, and a joint research project by the Weizmann Institute and Israel's Ministry of Energy is to be launched in the coming year.
 

Most turbines used today to produce electricity run on steam. Seeking to improve efficiency, electrical companies have recently started replacing steam with gas. However, state-of-the-art gas turbines require high-pressure air at temperatures exceeding 1,000° C.

 

Heating air to such temperatures with solar energy -- rather than by burning fossil fuels -- demands a ten-thousand-fold increase in the concentration of sunlight reaching the earth.

To achieve such concentrations, Weizmann Institute researchers have developed optical "funnels" with a unique geometrical structure. These funnels collect the sunlight and concentrate it to levels approaching the feasible maximum.

With the transition to industrial development, Dr. Abraham Kribus and colleagues have also begun to test the optical devices involved at all stages of solar collection and concentration systems with the goal of increasing the overall efficiency of such systems.

Work on optimization of optical and thermal systems is currently funded by Israel's Ministries of Energy and of Science and the Arts.

The Weizmann Institute of Science is a major center of scientific research and graduate study located in Rehovot, Israel.