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[en] Else of solar energy as substitute for conventional fuels at a competitive cost requires efficient conversion from solar radiation to usable forms of energy. In solar thermal or thermochemical applications, high efficiency usually re- quires high temperature and high concentration of incoming radiation. The main form of energy loss from high temperature solar central receivers is thermal emission ('re radiation'), at an effective temperature close to the maximum receiver temperature. This loss is reduced if the aperture is divided into segments, most of which are maintained at lower temperatures. A two-stage partitioned receiver demonstrating this concept is under construction at the Weizman Solar Tower. The high-temperature stage is the DIAPR (Directly Irradiated Annular Pressurized Receiver). The low-temperature stage is made of tubular cavity receivers of simpler design. Preliminary optical and thermal design of the partitioned receiver is presented. For the design exit temperature of 1500 K, the aperture size of the partitioned receiver is about 60% of the equivalent single-stage receiver, indicating a significant increase of conversion efficiency. The exit temperature of the low-temperature stage is around 1100 K, allowing simpler design and inexpensive construction. (authors)
[en] Initial tests of the 50 kw DIAPR solar receiver have been conducted in the WIS Solar Furnace, at a power level of 11 kw. These tests show that the receiver components, and especially the novel window, are capable of stable operation at extreme conditions. A total of about 50 hours under solar load, including about 20 on-off cycles, were completed. all experiments were at high pressure, in the range of 15-25 bars. Asymmetric temperature distribution and accidental contamination of the window during the first run created thermal stresses on the window which exceeded those expected in normal operation at full load, but neither failure nor deterioration of the window were detected. Internal modifications of the receiver are planned that will reduce the temperature gradients and allow more efficient operation. Work is now in progress to modify and install the receiver in the WIS solar tower and test it at full 50 kw load in the summer of 93. The two-dimensional numerical simulation code is nearly completed, and produces energy balance and temperature range consistent with the experimental results. When extended to three dimensions it will serve to analyze the experimental results, to perform parametric studies and to assist in up scaling design. (authors). 2 refs., 7 figs., 1 tab
[en] Recently, Chen and his team were active in the theoretical and practical study of a new heliostat for the use of solar energy. This work represents the first innovation in the area of heliostats after many years of little progress. The mathematical development of the tracking and concentration optics principles, and the practical implementation and demonstration of the technology, are both very interesting advances in this field. Many applications are possible for this technology such as generation of solar electricity and solar industrial process heat.
[en] Many cases of radiation transport in nature and technology can be described as a continuum transport process through an interacting Participating Medium, governed by the Radiative Transfer Equation. Current numerical methods for solving the RTE, in particular Discrete-Ordinates based methods, are in a process of rapid development during recent years. However, they are still lacking in generality and sometimes suffer from performance limitations in three-dimensional problems involving strong coupling between ordinate directions. We present a new numerical solution procedure for the Discrete Ordinates approximation of the RTE, including treatment of anisotropic optical properties and generalized boundary conditions. The numerical schemes used here are well established in CFD, but have not been applied previously to the solution of the RTE. The new scheme is guaranteed to converge, subject to a numerical stability condition. We demonstrate the validity of the developed code on a series of verification cases
[en] In thermionic energy converters, electrons in the gap between electrodes form a negative space charge and inhibit the emission of additional electrons, causing a significant reduction in conversion efficiency. However, in Photon Enhanced Thermionic Emission (PETE) solar energy converters, electrons that are reflected by the electric field in the gap return to the cathode with energy above the conduction band minimum. These electrons first occupy the conduction band from which they can be reemitted. This form of electron recycling makes PETE converters less susceptible to negative space charge loss. While the negative space charge effect was studied extensively in thermionic converters, modeling its effect in PETE converters does not account for important issues such as this form of electron recycling, nor the cathode thermal energy balance. Here, we investigate the space charge effect in PETE solar converters accounting for electron recycling, with full coupling of the cathode and gap models, and addressing conservation of both electric and thermal energy. The analysis shows that the negative space charge loss is lower than previously reported, allowing somewhat larger gaps compared to previous predictions. For a converter with a specific gap, there is an optimal solar flux concentration. The optimal solar flux concentration, the cathode temperature, and the efficiency all increase with smaller gaps. For example, for a gap of 3 μm the maximum efficiency is 38% and the optimal flux concentration is 628, while for a gap of 5 μm the maximum efficiency is 31% and optimal flux concentration is 163