Amnon Yogev

Solar “tower reflector” systems: A new approach for high-temperature solar plants

Abstract During the last few years, considerable research efforts have been directed at the Weizmann Institute towards development of high-concentration, high-temperature solar energy systems. This included optical methods and devices, thermal receivers for solar thermal electricity generation, and thermo-chemical processes for solar energy storage and solar fuel production. Some of these efforts are now mature enough for transfer to industry, and programs are starting to affect the transfer and upscale the new technologies to commercial levels. Feasibility studies carried out during 1995 in cooperation with industry have shown the advantage of the new high-concentration system approach. The costs of high-quality solar energy are attractive, even before application of government subsidies:

Solar-pumped Er,Tm,Ho:YAG laser.

800/kWth for high-temperature process heat applications, and

Closed loop control of heliostats

2500/kWe for solar/hybrid power plants.

Excitation‐emission study of continuum Raman and the photofragmentation mapping of IBr

Direct solar illumination was used to pump a 5-mm-diameter 62-mm exposed-length rod of Er,Tm,Ho:YAG to achieve a quasi-cw lasing of the Ho ion at an average power of 12 W. The solar radiation was chopped at a 20% duty cycle to avoid overloading of the cooling system. The peak power output was more than 65 W during the choppers open times. The slope efficiency is 3.8%, and the threshold input energy is approximately 100 W. The laser was operated for long times (up to hours) while maintaining its performance. This is, to our knowledge, the first directly solar-pumped laser operating at 77 K.

Photochemistry in the electronic ground state. Quantitative electrocyclic isomerization induced by multiphoton absorption of infrared laser radiation

Tracking control in current heliostats is performed with an open loop, without any verification that the radiation is actually arriving at the desired target. Errors due to open-loop tracking control are often around 1–2 mrad and can accumulate during operation. A significant reduction of tracking error by closing the control loop is presented. The method includes a dynamic measurement of the actual radiation incident around the receiver’s aperture (spillage), detection of aiming errors, and feedback of a correction signal to the tracking algorithm. The measurement does not interfere with the receiver operation. The detection method can distinguish among different heliostats in the field, producing individual corrections to each heliostat. The closed loop control system was developed and successfully operated at the Weizmann Institute heliostat field. Both large errors and gradual drift errors were detected and corrected automatically. Resolution of the closed loop detection algorithm can reach 0.1 mrad, which is insignificant in the overall heliostat beam quality.

Theory and operation of the IBr-photodissociation Br laser

Experiments and theory of the excitation‐frequency dependence of the continuum‐Raman signal of IBr are described. The theoretical computations, which do not follow the Kramas–Heisenberg expression, are extremely successful in predicting the wavelength dependence of each Stokes line and the ratio between the different overtones. The ‘‘reflection principle’’ or ‘‘photofragmentation mapping’’ of absorption spectroscopy, which postulates the equivalence between the topology of an initial vibrational wave function and the frequency dependence of the cross section, is shown to hold also for continuum‐Raman spectroscopy. Our experimental excitation–emission spectra for the low‐lying overtones of IBr are a clear reflection of the final nuclear density. In contrast, we show that the excitation‐frequency dependence of the high overtones is mainly sensitive to the shape of the intermediate excited electronic states.

Determination of the molecular distribution in anisotropic media by polarized absorption and emission spectroscopy

Abstract The electrocyclic isomerization of hexafluorocyclobutene (HFCB) induced by transversely excited infrared laser radiation was studied. A quantitative conversion of HFCB to the less thermodynamically stable hexafluorobutadien (HFB) was achieved in the presence of excess helium gas. The wavelength and light intensity dependence were studied.

Nonimaging optics and solar laser pumping at the Weizmann Institute

IBr as a potential candidate for solar-powered lasers is discussed. Theory and operation of a Br-atom laser obtained by photodissociation of IBr are described. Computed Br* : Br ratios are predicted to increase with pump-photon energy — in good agreement with experiment. Lasing peak power of up to 50 kW at an efficiency of 0.03% with input electrical energy as low as 90 J was demonstrated, and effects of pressure and input energy, on the laser pulse and onset were investigated.

Photochemistry of Oriented Molecule: The Photodimerization of Tetraphenylbutatriene Incorporated in Polyethylene and Nematic Matrices

Abstract A theory for the interpretation of polarized absorption and emission spectra of uniaxially oriented samples, including overlap of different polarized transitions in both processes, is described. Using this method for the interpretation of experimental data, the distribution of molecules possessing different geometrical anisotropy, incorporated in stretched polyethylene films, and the partial contribution of different polarized transitions to the total emission intensity can be estimated.

Site-selective liquid-prase vibrational overtone photochemistry of hydroxyhexadiene

The applicability of non imaging concentrators for solar thermal devices and laser pumping in a central receiver facility is discussed. A kilowatt scale solar pumped laser is described.