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Investigation and Selection of Materials Resistant to Temperatures and Radiation to Construct a Metallic/Ceramic Secondary Concentrator as well as Measurements at Premodels

On the one hand it containes the continuation and finally assessment of material examinations to construct a secondary concentrator (part 1 contract no. 5–370–4355), on the other hand some premodels for the possible form of concentrators and measurement results for this models are presented. Taking into account the costly production of the hard ceramic materials we tested linearized forms with and without zones with optical properties similar to ceramic.
Polished ceramic in uncoated form is partly useable, because it shows a specular reflection of about 9% with near normal incidence. But the 2π-reflection for white ceramic might be higher than 90%.
Only coatings increase the value of specular reflection clearly if the surface roughness is very small. With a coating of TiN on glass values higher than 90% are reachable, but this layer is not universal useable for the whole spectrum of sunlight. Besides this the coating has risk of oxidation. An additional gold coating might be a possible protection against it.
At the models (CPC, Trumpet) pictures of the intensity distribution in the exit aperture have been taken, partly with different angle of incidence. The intensification along some perpendicular lines and the flux in relation with and without a concentrator have been taken from this. With using of a ceramic-similar material next to the receiver area in a Trumpet model we found a reduction of the flux ratio compared with the model, only covered with mirror foil.
G. Lensch, P. Lippert, W. Rudolph

Investigations of Hard Coating and Heat Mirrors for Simultaneous Energy Conservation in a Photovoltaic/Solarthermic Hybrid System or for Use in a Secondary Reflector

The aim of this study was to develop special optical coatings with optimum spectral separation of the solar spectrum into the visible part being efficient for photovoltaic applications and in the part of thermal radiation for use in a thermal power plant. This requires high transmission in the range of lower wavelengths, and high reflectivity in the infrared (IR).
A possible application would be a power plant, consisting of an array of solar cell heliostats, which reflect the thermal radiation into a concentrator, and which make use of the photovoltaic effect directly. A secondary effect could be a lower thermal load of the solar cell, which would yield a higher efficiency as a conventional one.
We investigated two classes of materials:
hard coatings of TiN, ZrN and HfN, which exhibit near metallic characteristics, besides their wear resistance, which would be advantageous for cleaning.
layered structures of dielectric coatings with embedded thin metal films, e.g. TiO2-Au-TiO2.
We achieved hard coatings with a specular reflection of more than 90% in the IR for film thicknesses above 1µm. This could be interesting for applications in concentrators.
For optical separation purposes, three layer coatings of the type mentioned above appeared to be very promising, as very high transmission in the visible, and, simultaneously, very high reflectivity could be obtained in the IR. However, as extremely thin and homogeneous metal interlayers are needed for this, special deposition techniques are required.
R. Anton, G. Lensch, W. Rudolph, R. Ueth

Optimization of Terminal Concentrators

A terminal (secondary) concentrator provides a second optical element in the radiation concentrating system. It allows the concentration ratio to better approach the thermodynamic limit of concentrating devices. For high temperature applications high flux densities are required. In an optimized two-stage system (i) the receiver aperture is narrower than that in the optimized single-stage system, thereby reducing convection and, in particular, thermal radiation losses; (ii) the entrance aperture of the terminal concentrator is larger than the receiver aperture in the optimized single-stage system, thereby gaining flux which is lost in an optimized one-stage system. These effects have to compensate for the additional losses caused by back-reflections and non-ideal reflectivity.
OPTEC, an optimization program for terminal concentrators, was developed. It automatically optimizes thermal efficiency or flux by determining several parameters of single-and two-stage concentrating systems consisting of primary concentrator, terminal concentrator, and receiver. Various types of primaries, terminal concentrators, and receivers are selectable. A simulation input language interface has been developed for easy specification of parameters and optimization requests. Efficiencies are calculated employing full ray tracing and Monte-Carlo integration.
U. Schöffel, R. Sizmann


200 kW Stirling Engine for SSP Module Solar Stirling Receiver with Heat Storage System Analysis

This report describes aspects of Solar Stirling engineering. It is principally concerned with tentative methods of computation, and designs used to further develop, for use in solar farms, the 6-cylinder Stirling engine conceived by MAN Technologie.
In this connection ways have already been marked out, inter alia [1], which promise successful conversion for solar applications, and the first receiver concepts have been worked out. A brief summary of the results achieved so far is given here, and the technical characteristics of the main components used in the system described.
Process control is illustrated by means of a flow diagram. Then Stirling solar farm modules for electrical outputs of 200 kW and 400 kW respectively, are introduced. In both cases the at present customary heliostats, arranged around a solar tower in which the Stirling energy conversion system is installed, are used.
Power requirements, efficiency and performance under part-load conditions are arrived at analytically. Two examples illustrate the relationships.
An analytic analysis of the system shows the importance of cavity temperature and aperture diameter for receiver performance. On the other hand the various thermo-optical characteristics (ε,ρ) of the cavity receivers used here are of littleinfluence.
Furthermore, receiver configuration, tower height and the heliostat field area required are estimated by elementary geometrical optics methods. The derived analytic relationships offer the possibility of obtaining an adequately exact record of the system’s main parameters. Moreover, this also makes it easier to understand the results of sophisticated computer-aided calculations and to apply them in the right directions.
On this basis the incident flux density for a cylindrical cavity receiver is calculated in the aperture plane and on the absorber surface, and the time curves for the energy radiated into the aperture and the field efficiency are plotted. These computations were carried out with the aid of the HEFLD programme developed by MAN Technologie for the GAST project.
To design the thermal energy storage (TES) a method is adapted by means of which the temperature hysteresis, and thus the operating behaviour of a latent heat storage system, can be defined. To give a general survey, data has been compiled on suitable storage materials. In addition the main design data for lithium fluoride storage systems with a thermal capacity of 730 MJ have been computed for various capsule diameters. An example shows the time curve for temperature hysteresis. This shows that the storage can be fully charged in about 2.3 hours if the temperature of the boiling liquid metal is only 2 K above the lithium fluoride’s melting point. This data is supplemented by two draft designs for storage capsules, a report on lab tasks and an overview of storage configurations. The documentation is comprehensive so as to allow the design engineer to design the storage system quickly.
The further investigation of thermohydraulic problems to be solved in connection with the storage system requires even more specific computations of natural circulation. Not until then should it be decided whether the thermically induced flow will be adequate or whether it will have to be boosted by induction pumps [2]. At present the aim is to find pump-free solutions for heat pipe receivers. But this does not exclude the possibility of pumps being used at temperatures below operating temperature. Induction pumps should only be used if the hydrodynamic friction in the heat storage proves to be disturbing at operating temperature, or if the problems regarding incipient boiling superheat can be solved more simply than by injecting inert gas. Work is still being carried out on these problems.
The provisional design data and estimated costs have been brought up to date. It is shown that the solar Stirling system with a liquid metal temperature of about 850°C has under nominal conditions a system efficiency of 26.7 %. In the case of series manufacture, specific installation costs of about 9,500 DM/kWe are to be expected.
The final section of the paper touches on alternative concepts with induction pumps. It appears likely that the internally irradiated spiral-tube cylinder receiver can seriously compete with the heat pipe receiver, provided that the induction pump operates reliably in the temperature range around 850°C. This is being investigated at present. Of particular note are the receiver’s considerably simpler construction and the flexible absorber area. Thus a further promising receiver concept for the solar Stirling system is available, and this too is being exhaustively investigated in the course of the current development project.
The paper concludes with an explanation of a Stirling steam process which increases overall efficiency and utilizes the hot air flow of a volumetric high-temperature receiver. To help the process and improve efficiency, supplementary heating by gas is of advantage.
H. Michel

The Construction of a Volumetric Receiver with a Staggered Structure

A new volumetric receiver concept is presented, consisting of a ceramic foil receiver which is covered by a matrix structure of quartz glass. The complete receiver is cooled by ambient air in an open cycle.
An analysis of the steady state energy balance of a simplified one-channel receiver model shows an increase of more than 6% in temperature and COP in comparison with the single ceramic receiver and a possible air outlet temperature of 1000°C
A sensitivity analysis indicates that a big uncertainty in the optical and thermal properties in the calculation only has influence on the absolute value of the predicted receiver efficiency, but is nearly independent of the efficiency improvement by the quartz glass structure.
A variation of geometrical parameters of the quartz glass structure was performed in order to find a compromise between efficiency improvement and prevention of overheating of the quartz glass.
The evaluated parameters were translated into a receiver construction adapted to the test set up in the solar power plant in Almeria (Spain).
R. Pitz-Paal, J. Morhenne, M. Fiebig

Analysis of Convective Heat Transfer in Volumetric Receivers Built of Porous Media

Korrelationen zur Bestimmung von Wärmeübergangskoeffizienten und Druckverlust in volumetrischen Receivern, aufgebaut aus porösen, geordneten und ungeordneten Festbettstrukturen, wurden untersucht. In Abhängigkeit der Reynoldszahl sind unterschiedliche Korrelationsgleichungen notwendig, um eine gute Übereinstimmung mit veröffentlichten Meßdaten zu erhalten. Für den Bereich kleiner Reynoldszahlen (Re < 500) wird die Korrelation von Martin empfohlen, die auf dem sogenannten ‘Channel-Model’ basiert. Wärmeübergangskoeffizienten, bestimmt aus der Analogie von Wärme- und Impulsübertragung, sind für Receiver nicht verwendbar. Für eine Bestimmung der charakteristischen, hydrodynamischen Längen von Festtbettstrukturen, die wegen der unregelmäßigen Struktur nur sehr schwer zu bestimmen sind, kann jedoch der gemessene Druckverlust herangezogen werden. Bei der Berechnung des Druckverlustes von Matrixstrukturen liefert die von Kast im VDI-Wärmeatlas veröffentlichte Gleichung die geringsten Abweichungen zu publizierten Meßdaten und kann sowohl für Kugelschüttungen als auch für Schüttungen von ungleichmäßigen, scharfkantigen Körpern verwendet werden. Die Abweichungen von Meßwerten sind kleiner als 50 % und werden zu niedrig berechnet.
J. Morhenne, R. Pitz-Paal

Solar Chemistry

Thermochemical H 2- Production with Sulfur-Iodine Process and Solar Energy Adaptation

A thermochemical cycle for hydrogen production is a process in which water is used as a feedstock along with a non-fossil high temperature heat source to produce H2 and O2 as product gases. The water splitting process is accomplished through a closed loop sequence of chemical reaction steps in which the chemical reagents are continuously recycled and reused in the process with essentially no loss of material. Practical thermochemical cycles, as currently envisioned, require input temperatures of 1200K for the highest temperature chemical step, and operate at a thermal efficiency of about 50%. Here, the thermal efficiency is defined as the higher heating value of the H2 produced divided by thermal heat per mole of H2 delivered by the high temperature heat source. High temperature gas-cooled reactors have been considered as heat source for these cycles. Electrical energy for process equipment is required in addition to high temperature heat for operation of thermochemical hydrogen plants.
K. F. Knoche

The Use of Thermal Solar Energy to Treat Waste Materials

The processes employed in the various production sectors of trade and industry give rise to waste materials containing substances that can harm the environment to a greater or lesser extent. The volume of such waste has been reduced in recent years by modifying these processes, choosing different raw materials and feeding waste products back into the production. However even pollution control facilities such as central wastewater treatment plants, improved gas cleaning systems and secondary raw material treatment plants in turn produce waste materials that require disposal.
H. Effelsberg, B. Barbknecht

Solar Thermal Energy Cycle Based on Sulfur and Sulfide Oxidizing Bacteria

New solar energy technologies for the production of biomass for the purpose of synthesizing methane, hydrogen and proteins could be based on the solar generation of the inorganic energy source (Fe2+, metal sulfides) of chemoautotrophic bacteria (Thiobacilli). The resulting energy cycle is identical with the life sustaining processes coupled to black smokers in the deep sea with the difference that solar energy replaces geothermal energy. Progress and difficulties are discussed for energy and fuel generation on the basis of a solar driven sulfur cycle involving sulfate reduction using solar energy and sulfide oxidation by Thiobacillus ferrooxidans. The rate determining steps for catalysis of sulfate reduction and interfacial oxidation of sulfides are discussed. The potential advantages of this energy cycle for space exploration, terrestrial carbon dioxide fixation and technologies for energy and material production are outlined.
C. C. Bärtels, H. Tributsch

Experiments on the Decomposition of Toxic and Hazardous Chemicals under High Pressure and High Temperature Light Flux Conditions

A high temperature, high pressure cell coupled to a HPLC system had originally been considered for experiments in which a laser was supposed to provide the radiation energy needed for the decomposition of chemicals. This set up has been abandoned in favour of a reaction cell directly integrated into a mirror oven having two 400W halogen lamps in its focuses. In all cases, decomposition of the toxic chloro-organic compounds to graphit like materials plus gaseous HCl has been observed. No other volatile products were detected using capillary GC with FID and ECD detection.
H. J. Möckel, W. Knauf, S. Fiechter, H. Tributsch


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