Jacob Karni

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.

High-temperature solar thermochemistry: Production of iron and synthesis gas by Fe3O4-reduction with methane

Criteria for selecting thermochemical processes that use concentrated solar radiation as the energy source of high-temperature process heat are reviewed. We have thermodynamically examined the system Fe3O4 + 4CH4. At 1 atm and temperatures above 1300 K, the chemical equilibrium components consist of metallic iron in the solid phase and a mixture of 66.7% H2 and 33.3% CO in the gaseous phase. The total energy required to effect this highly endothermic transformation is about 1000 kJ/per mole of Fe3O4 reduced. We conducted exploratory experimental studies in a solar furnace using a solar receiver (with internal infrared mirrors) containing a fluidized bed reactor. Directly irradiated iron oxide particles, fluidized in methane, acted simultaneously as radiant absorbers and chemical reactants, while freshly produced iron particles acted as reaction catalysts. The proposed process offers simultaneous production of iron from its ores and of syngas from natural gas, without discharging CO2 and other pollutants to the environment.

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.

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

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#

An astigmatic corrected target-aligned heliostat for high concentration

Conventional heliostats suffer from astigmatism for non-normal incidence. For tangential rays the focal length is shortened while for sagittal rays it is longer than the nominal focal length. Due to this astigmatism it is impossible to produce a sharp image of the sun, and the rays will be spread over a larger area. In order to correct this the heliostat should have different curvature radii along the sagittal and tangential direction in the heliostat plane just like a non axial part of a paraboloid. In conventional heliostats, where the first axis, fixed with respect to the ground, is vertical while the second, fixed with respect to the reflector surface, is horizontal such an astigmatism correction is not practical because the sagittal and tangential directions rotate with respect to the reflector. We suggest an alternative mount where the first axis is oriented towards the target. The second axis, perpendicular to the first and tangent to the reflector, coincides with the tangential direction. With this mounting sagittal and tangential direction are fixed with respect to the reflector during operation. Therefore a partial astigmatism compensation is possible. We calculate the optimum correction and show the performance of the heliostat. We also show predicted yearly average concentrations.

Development Steps for Parabolic Trough Solar Power Technologies With Maximum Impact on Cost Reduction

Besides continuous implementation of concentrating solar power plants (CSP) in Europe, which stipulate cost reduction by mass production effects, further R&D activities are necessary to achieve the cost competitiveness to fossil power generation. The European Concentrated Solar Thermal Roadmap (ECOSTAR) study that was conducted by European research institutes in the field of CSP intends to stipulate the direction for R&D activities in the context of cost reduction. This paper gives an overview about the methodology and the results for one of the seven different CSP system concepts that are currently under promotion worldwide and considered within ECOSTAR. The technology presented here is the parabolic trough with direct steam generation (DSG), which may be considered as an evolution of the existing parabolic systems with thermal oil as heat transfer fluid. The methodology is explained using this exemplary system, and the technical improvements are evaluated according to their cost-reduction potential using a common approach, based on an annual performance model. Research priorities are given based on the results. The simultaneous implementation of three measures is required in order to achieve the cost-reduction target: Technical improvement by R&D, upscaling of the unit size, and mass production of the equipment.

Solar Gasification of Biomass: A Molten Salt Pyrolysis Study

A novel solar process and reactor for thermochemical conversion of biomass to synthesisgas is described. The concept is based on dispersion of biomass particles in a molteninorganic salt medium and, simultaneously, absorbing, storing and transferring solarenergy needed to perform pyrolysis reactions in the high-temperature liquid phase. Alab-scale reactor filled with carbonates of potassium and sodium was set up to study thekinetics of fast pyrolysis and the characteristics of transient heat transfer for celluloseparticles (few millimeters size) introduced into the molten salt medium. The operatingconditions were reaction temperatures of 1073–1188 K and a particle peak-heating rateof 100 K/sec. The assessments performed for a commercial-scale solar reactor demon-strate that pyrolysis of biomass particles dispersed in a molten salt phase could be afeasible option for the continuous, round-the-clock production of syngas, using solarenergy only. @DOI: 10.1115/1.1753577#Keywords: Biomass, Pyrolysis, Molten salt, Kinetics, Solar reactor, Thermal storage

A High-Pressure Window for Volumetric Solar Receivers

The absorbing matrix of a volumetric (directly irradiated) solar receiver must be exposed to the concentrated incoming sunlight. Most applications require that the receiver operates at an elevated pressure and in many cases the working fluid is not air. These requirements can be met only if the receiver is equipped with a transparent window. A novel frustum-like high-pressure (FLHiP) window, made of fused silica, is presented. Optical, mechanical, and thermal analyses, over 1,000 hours of accelerated life-time tests and several hundred hours of tests in a solar receiver, show that this window satisfies the required criteria for operation in a volumetric solar receiver whose operating pressure and peak absorber temperature reach 30 bar and 1,700 C, respectively.

Optical fibers and solar power generation

Abstract A study of the potential use of optical fibers for solar thermal power generation is presented. The main performance characteristics (numerical aperture and attenuation) and typical costs of currently available fibers are discussed. Several approaches to the application of fibers are presented, for centralized (tower, central receiver) and distributed (dish–engine) systems. The overall system design-point efficiency and overall system cost are estimated. A scaling relation between system size and the cost of the fiber component is identified, which severely limits the applicability of fibers to small systems only. The overall system cost for centralized systems is found to be higher than the currently competitive range, even under optimistic assumptions of mass production of major components. A significant reduction in fiber cost is required before the use of fibers for centralized solar power generation can become competitive. In distributed generation using dish/engine systems, however, the use of fibers does achieve competitive performance and costs, comparable to the costs for conventional dish systems.

Solar energy: The thermoelectric alternative

Results show that achievable improvements may make solar thermoelectric generators competitive with other solar power conversion methods.