Akiba Segal

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 optics of the solar tower reflector

Abstract The concept of the reflective solar tower is based on inverting the path of the solar rays originating from a heliostat field to a solar receiver that can be placed on the ground. This system is based on the property of a reflective quadric surface to reflect each ray oriented to one of its foci to its second focus. Two shapes of surfaces can be used: one is concave — the ellipsoid, and the other is convex — the hyperboloid (with two sheets). This study analyzes the achievable optical performances of these types of reflectors and proves that the hyperboloidal surface is a superior reflector. The optics of the hyperboloid are analyzed in detail.

A two junction, four terminal photovoltaic device for enhanced light to electric power conversion using a low-cost dichroic mirror

A low-cost dichroic mirror can be used successfully for solar spectrum splitting to enhance solar to electrical energy conversion. The mirror is optimized for use with a polycrystalline silicon photovoltaic cell pc-Si. With the dichroic mirror simultaneous excitation of a medium-efficient 11.1% commercial pc-Si and a custommade high band gap GaInP cell 12.3%, yields 16.8% efficiency, with both cells operating at maximum power. Our results clearly show that what is missing for this simple low-cost enhancement of Si solar cell efficiency are low-cost high band gap cells.

Chemical reactions in a solar furnace 2: Direct heating of a vertical reactor in an insulated receiver. Experiments and computer simulations

The performance of a solar chemical heat pipe was studied using CO2 reforming of methane as the endothermic reaction. A directly heated vertical reactor, packed with a rhodium catalyst was used. The solar tests were carried out in the Schaeffer solar furnace of the Weizmann Institute of Science. The power absorbed was up to 6.3 KW, the maximal flow rates of the gases reached 11,000 1/h, and the methane conversions reached 85%. A computer model was developed to simulate the process. Agreement of the calculations with the experimental results was quite satisfactory.

Theoretical modeling of a directly heated solar-driven chemical reactor

Abstract A theoretical formulation for calculating the performances of a solar-driven catalytic chemical reactor was developed. It accounts for the spatial distribution of the deposition of primary energy within the receiver, the heat transfer into the catalytic bed and the thermochemical endothermic reaction, chemical composition and flow distribution within the reactor. The theory set forth was applied to analyze results obtained in a solar furnace with a directly heated U-shaped tubular reactor, wherein catalytic carbon dioxide reforming of methane occurred. We find that the receiver/reactor assembly acts as a self-regulating system. Beyond a fractional catalytic bed length of 0.14, solar energy can be converted primarily into chemical enthalpy. The fluid temperature gradient monitors the heat balance by adjusting the overall rate of conversion to the rate at which energy is being transferred through the reactor walls. Under certain circumstances, the process may be heat-transfer limited or controlled by chemical thermodynamics. A good fit between theory and experiment and accountability of all the intricate details in the various calculated performances of the receiver/reactor system support the theoretical model set forth in this study. We offer it as a tool for simulating future experimental results and for designing solar-driven reactors.

EXTENSION OF THE HERMITE EXPANSION METHOD FOR CASSEGRAINIAN SOLAR CENTRAL RECEIVER SYSTEMS

Abstract An extension of the Hermite Expansion Method for performance simulation of central receiver plants is presented. This extension allows simulation and parametric study for a solar central receiver plant based on Solar Concentration Off-Tower (SCOT, or Reflective Tower) design. The extension includes mapping the physical receiver aperture into a Virtual Receiver located near the fields aim point, and performing the Hermite Expansion calculation on the Virtual Receiver. The calculation of aperture intercept/spillage and additional losses due to the Tower Reflectors finite size are discussed. Validation of the extension by comparison to ray-tracing simulation is presented for single heliostats, a group of heliostats and a complete surround field. The results match closely, showing the validity of the method and of its implementation.

Truncation of the Secondary Concentrator (CPC) as Means to Cost Effective Beam-Down System

A central solar plant based on beam-down optics is composed of a field of heliostats, a tower reflector (hyperboloid mirror), and a ground receiver interfaced at its aperture with one or a cluster of secondary concentrators (compound parabolic concentrator). In previous publications, a method was presented, illustrating the correlation between the tower reflector position and its size on one hand and the geometry, dimensions, and reflective area of the secondary concentrator on the other hand, both related to the heliostat field reflective area. Obviously when one wishes to reduce the size of a tower reflector by locating it closer to the upper focal point, the image created at the lower focus will be broader, resulting in a larger secondary ground concentrator. The present paper describes a method for substantial decrease in the dimensions of the ground secondary concentrator cluster (and, implicitly, the concentrators area) via truncation and some geometrical corrections without significant sacrifice of the optical performance. This offers a method for cost effective design of future central solar plants, utilizing the beam-down optics.

Solar chemical heat pipe in closed loop operation: mathematical model and experiments

Abstract A computer model was developed for closed loop operation of a solar chemical heat pipe, to include both the endothermic methane reforming and the exothermic methanation of synthesis gas. The modeling results showed reasonably good agreement with experiments. The model can be used for design of larger systems as well as other chemical heat pipes.

Approximated secondary CPC, built of planar facets, adjustable for two solar central receivers

Decreasing the aperture of a solar central receiver operating at high temperatures contributes significantly to the increase of the efficiency of energy absorption. However, decreasing the aperture also decreases the collection efficiency. A simple solution is using a 3D- CPC as secondary element for augmentating the energy collection, while the aperture can remain relative small. Nevertheless, the receiver aperture as well as the secondary concentrator are usually rather large. For practical considerations an approximate solution may be chosen at times, designing the concentrator by a series of truncated cones[1}. However, in particular cases, the solution of truncated cones remains expensive and unpractical and therefore we designed a CPC approximated by a series of trapezoidal planar facets. Under technical restrictions, there is an optimum for choosing the partition of the purecPc in truncated pyramids and the paper presents the method for a good solution. The design of the concentrator is such that it can accomodate two different receiver apertures (60 cm and 47 cm respectively). The final design of the concentrator provides an entrance diameter of 121 cm and consists of four truncated pyramids, each composed of 12 trapezoidal facets, when connected to the 60 cm diameter aperture of the receiver. A fifth ring of facets is added when the concentrator is used in conjunction with the second receiver. . Some considerations about the materials used in construction are presented in the final section of the paper.

Modular solar chemical heat pipe for a parabolic dish: conceptual design and model calculations

Abstract A conceptual design for a 100 kW methane reforming, solar chemical heat pipe, to be mounted on a large parabolic dish is described. Detailed computer calculations, based on previous experimental work, show that such a design can yield over 80% chemical conversions and efficient energy absorption into the reacting medium. A unit like that has the advantage of being modular, so that a number of such units can be installed in a distributed parabolic dish central plant, to provide high temperature heat for an industrial site or a small community.