Abraham Kribus

A solar-driven combined cycle power plant

The main results of a feasibility study of a combined cycle electricity generation plant, driven by highly concentrated solar energy and high-temperature central receiver technology, are presented. New developments in solar tower optics, high-performance air receivers and solar-to-gas turbine interface, were incorporated into a new solar power plant concept. The new design features 100% solar operation at design point, and hybrid (solar and fuel) operation for maximum dispatchability. Software tools were developed to simulate the new system configuration, evaluate its performance and cost, and optimize its design. System evaluation and optimization were carried out for two power levels. The results show that the new system design has cost and performance advantages over other solar thermal concepts, and can be competitive against conventional fuel power plants in certain markets even without government subsidies.

The DIAPR: A High-Pressure, High-Temperature Solar Receiver

A solar central receiver absorbs concentrated sunlight and transfers its energy to a working medium (gas, liquid or solid particles), either in a thermal or a thermochemical process. Various attractive high-performance applications require the solar receiver to supply the working fluid at high temperature (900--1,500 C) and high pressure (10--35 bar). As the inner receiver temperature may be well over 1,000 C, sunlight concentration at its aperture must be high (4--8 MW/m{sup 2}), to minimize aperture size and reradiation losses. The Directly Irradiated Annular Pressurized Receiver (DIAPR) is a volumetric (directly irradiated), windowed cavity receiver that operates at aperture flux of up to 10 MW/m{sup 2}. It is capable of supplying hot gas at a pressure of 10--30 bar and exit temperature of up to 1,300 C. The three main innovative components of this receiver are: a Porcupine absorber, made of a high-temperature ceramic (e.g., alumina); a Frustum-Like High-Pressure (FLHIP) window, made of fused silica; a two-stage secondary concentrator followed by the KohinOr light extractor. This paper presents the design principles of the DIAPR, its structure and main components, and examples of experimental and computational results.

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:

Performance of the Directly-Irradiated Annular Pressurized Receiver (DIAPR) Operating at 20 Bar and 1,200°C

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

Inherent limitations of volumetric solar receivers

2500/kWe for solar/hybrid power plants.

A high-efficiency triple cycle for solar power generation

The Directly Irradiated Annular Pressurized Receiver (DIAPR) is a volumetric (directlyirradiated) windowed cavity receiver, designed for operation at a pressure of 10 ‐30 bar, exit gas temperature of up to 1,300°C, and aperture radiation flux of up to 10 M W/m 2 . This paper presents test results obtained under various irradiation conditions and flow rates. Inlet aperture flux was up to 5 M W/m 2 ; exit air temperatures of up to 1,200°C were obtained, while operating at pressures of 17 ‐20 bar. Estimated receiver efficiency in these tests was in the range of 0.7‐0.9. The absorber and window temperatures were 200‐400°C below the permitted maximum, indicating that higher air exit temperatures are possible. @DOI: 10.1115/1.1345844#

Analysis of Potential Conversion Efficiency of a Solar Hybrid System With High-Temperature Stage

The flow in volumetric absorbers is investigated using a simple mathematical model. It is found that there are several restrictions and failure mechanisms that are inherent to the volumetric absorber, regardless of the precise structural details, material properties, etc. The heat that the absorber can extract safely is limited by flow-related constraints. Multiple steady solutions exist for certain parameter values: a fast solution corresponding to a low exit temperature, a slow solution which is unstable, and a choked solution for which the absorber is near to stagnation temperature. The existence of multiple solutions may lead to abrupt local switching and absorber failure. For a given irradiance applied to the absorber, the existence and the character of the solutions are determined by a single dimensionless parameter, the Blow parameter B. Neglecting the variation of the hydraulic resistivity with temperature may lead to a dangerous overestimate of the receiver`s ability to sustain irradiation. For reasonable efficiencies control of mass flow or outlet temperature of the absorber, rather than pressure control, may be required.

Closed loop control of heliostats

Abstract The last three decades have witnessed a trend in solar thermal electricity generation of increasing the concentration of sunlight, the operating temperature, and subsequently the efficiency of conversion from sunlight to electricity. The current state of the art concept is a solar-driven combined cycle, with sunlight concentration ratio of a few thousands, temperatures of about 1000–1300°C, and overall annual average conversion efficiency of about 20%. A possible next step in this trend is presented: a solar triple cycle, with a high-temperature MHD generator and two additional cycles in series. This triple cycle is powered by solar heat at temperatures around 2000°C and solar concentration of about 10,000. The overall peak conversion efficiency of the solar triple cycle is shown to be significantly higher than the solar combined cycle scheme. The sensitivity of this result to several system parameters and the technological feasibility of the solar triple cycle are also discussed.

A High-Pressure Window for Volumetric Solar Receivers

The analysis is given of hybrid system of solar energy conversion having a stage operating at high temperature. The system contains a radiation concentrator, a photovoltaic solar cell, and a thermal generator, which could be thermoelectric one or a heat engine. Two options are discussed, one (a) with concentration of the whole solar radiation on the PV cell working at high temperature and coupled to the high-temperature stage, and another (b) with a special PV cell construction, which allows the use of the part of solar spectrum not absorbed in the semiconductor material of the cell (“thermal energy”) to drive the high-temperature stage while the cell is working at ambient temperature. The possibilities of using different semiconductor materials are analyzed. It is shown that the demands to the cell material are different in the two cases examined: in system (a) with high temperature of cell operation, the materials providing minimum temperature dependence of the conversion efficiency are necessary, for another system (b) the materials with the larger band gap are profitable. The efficiency of thermal generator is assumed to be proportional to that of the Carnot engine. The optical and thermal energy losses are taken into account, including the losses by convection and radiation in the high-temperature stage. The radiation losses impose restrictions upon the working temperature of the thermal generator in the system (b), thus defining the highest possible concentration ratio. The calculations made show that the hybrid system proposed could be both efficient and practical, promising the total conversion efficiency around 25–30% for system (a), and 30–40% for system (b).

Optical fibers and solar power generation

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.