Category Archives: Sonar-Collecttors

A 240 MW non-fossil power block

An alternative case can be made for a design which minimises array thermal losses using low temperature (200°C — 300°C) saturated steam Rankine cycle turbines. Although some effort has been made to look at low temperature trough systems using small organic rankine cycle turbines (NREL, 2002), in this temperature range, higher efficiency demands a large turbine. The array cost of the CLFR is low enough that the added cost of fossil hybridisation is relatively high. For low cost and reliability, one needs a proven system stripped of expensive fossil fuel equipment.

Such systems exist. The nuclear power industry has spent many years and huge sums developing non-fossil fuel turbines which, at about 31-33%, are more efficient than smaller organic rankine cycle plants. These turbines operate from wet steam, using steam separators to dry out the steam before entering the turbine, and they use special turbine blade design. No superheating stage is required, so the solar array needs only meet the main boiler operating temperature, which in the case of the VVET is only 250°C. If one were to design a turbine type to to suit a large solar direct steam generation array like the CLFR, it would be something close to the VVER design, although there might be a case for operating in the range 300°C — 350°C to increase thermodynamic efficiency. Operation at 250°C allows significantly lower array losses than operation at 450-500°C as proposed for advanced trough systems (NREL, 2003) and allows the use of a wider variety of air stable selective coatings on the receiver. Steam pipes are also substantially cheaper at the lower temperature range.

However, the smallest nuclear turbines one can obtain are of about 240 MWe peak capacity, which would lead to a solar plant larger than any yet built. The low temperature turbine costs used in the paper are based upon approximate estimates (VVER, 2003) supplied by JSC “Atomstroyexport” (Russia). The supply of a 240 MWe VVER steam turbine and steam separator and control equipment of about US$18
million for a single turbine, well below high temperature turbine cost. It is conservatively assumed in this paper that an additional 1/3 will be added to the turbogenerator price to cover delivery and installation. Several sites have been found in Australia with excellent solar radiation and grid access. The most attractive of these has enough spare grid capacity for a 240 MWe installation.

Optimization of cost and efficiency in concentrating. solar power technology through quality control in large. production series for solar fields

Eckhard LUpfert, German Aerospace Center (DLR), Plataforma Solar de Almeria
Klaus Pottler, German Aerospace Center (DLR), Plataforma Solar de Almeria
Wolfgang Schiel, Schlaich Bergermann und Partner (SBP), Stuttgart

Solar thermal power plants are about to continue their market introduction with large solar fields for 50 MW plants and other. The successful construction of para­bolic trough collectors in large series requires an appropriate measurement and quality control program in order to achieve the designed optical and thermal per­formance. The collector quality can be increased significantly by correct alignment of the large reflector and receiver areas. A number of tools have been developed for efficient supervision of assembly jigs, and samples during fabrication. The meas­urement techniques have been applied initially for the purpose of prototype devel­opment. Their use can be extended to quality control in series fabrication, if evalua­tion is fast enough. The available tools include digital photogrammetry, reflector and receiver flux test methods, oil flow and temperature measurements. Their appli­cation in parabolic trough collector technology results in reduced cost for quality control in series manufacturing and increased collector performance.

Overview

The optical performance of solar concentrating collectors is very sensitive to inaccuracies of components and assembly. Because of a finite sun-shape and extant imprecisions of the collector system (e. g. steel structure, tracking, receiver alignment, mirror alignment, mirror shape and mirror specularity) the interception of light at the focal receiver is af­fected. To reach maximum performance through optimal component alignment a mix of measurement techniques should be used for quality control measures. This comprises incoming inspection, mounting of the module structures, assembly of the collectors and final inspection of the collector system. High precision 3D-coordinates of important mount­ing points may be derived from close range photogrammetry, slopes by water levels or electronic inclinometers, distances by vernier callipers, gauge bars or laser range finders and surveyor’s levels. Alignment errors can be derived from the evaluation of digital pho­tos. The optical collector quality can be analysed by measuring the flux density in the vicin­ity of the receiver. Thermal performance analysis is possible for any part of the collector field with a flexible installation of temperature sensors and ultrasonic flow meters without installation needs in the heat transfer loop.

A low temperature low cost storage system

The proposed plant uses the concept of Underground Thermal Energy Storage (UTES), which we will refer to in this paper as ‘cavern storage’. Pressurised water cavern storage appears to have been first proposed by R&D Associates in 1977, but the original reference is no longer available. The oldest extant major analysis is a 1983 report (Copeland and Ullman, 1983; Dubberly et al, 1983) from the Solar Energy Research Institute SERI (which later became NREL). The SERI report was a study of different storage options prepared for the U. S. Department of Energy (DOE) in the early 1980’s. Cavern storage involves storage of water under pressure in deep metal lined caverns where the pressure is contained by the rock and the overburden weight. There are no heat exchangers, and a low cost makeup water tank is provided on the surface. The array supplies steam to the cavern water, and steam is flashed directly from the cavern into the turbine, in a very similar manner as steam is evaporated from a nuclear boiler vessel into a nuclear turbine. Fourteen organizations were involved in deriving the comparative rankings, which indicated quite definitively that UTES for a large system was the cheapest storage method.

Because costs have changed greatly in some areas, Tanner (2003) has produced, at the suggestion of one of the authors, an engineering thesis report on cavern storage applied to the case of the CLFR. This study investigates, using estimates supplied by experienced engineering and excavation companies, the current costs of a steel lined caverns at depths of 200m and 400m using modern excavation techniques. This report indicates that cavern storage is now much cheaper than other currently proposed storage methods at installed costs under US$3 per kWht. This report is being rewritten for publication. With low cost storage, there is a tendency for total system delivered electricity costs to be reduced as the capacity factor increases.

Digital Close Range Photogrammetry on Jigs and Collectors

High precision photogrammetry is an appropriate tool to measure coordinates of concen­trator support points and mirror surfaces, especially for the analysis of large concentrators. The photogrammetric method directly delivers coordinates of selected test points and thus allows geometrical assessments of the concentrator. Previous work has described the ap­plication of photogrammetry to the characterization of solar collectors [1]. Close-range photogrammetry involves the use of a network of multiple photographs of a targeted object taken from a range of viewing positions, to obtain high-accuracy, 3-dimensional coordinate data for the object being measured. A significant advantage of photogrammetry is that it is a rapid non-contact technique that can readily be applied to many kinds of measuring ob­jects. With appropriate retro-reflective targets and flashlight it can be performed during the
day, even under bright sun. After application of the targets, actual measurement time for a set of 10 to 30 photos is short. Until now the techniques have been applied on R&D level to measure the collector assembly jig and EuroTrough collector modules.

Figure 1: Left: Photo acquisition with tripod. Right: Resulting camera positions (cones) and measured target locations (dots) of the EuroTrough assembly jig

The collector assembly jig is an essential part for the precise assembly of EuroTrough space frames. Therefore it needs to be rigid and all support points well adjusted. Photo — grammetry has been used to test the jig setup. Figure 1 demonstrates the photographing with a 5m tripod and the resulting camera positions.

The results have been compared to results obtained from conventional measurement and levelling techniques and have demonstrated the advantage of photogrammetry in 3­dimensional measurements. For manufacturing quality control photogrammetry will always have to be combined with the other techniques.

Figure 2: Space frame of a EuroTrough module with measurement targets on the mirror support points (left), and measured deviations of test points from design heights in mm (right)

The assembly of the collector modules is done on these kinds of jigs. Checking of the as­sembled steel structures can be performed very well with photogrammetry. Figure 2 shows the space frame of a 12 m long collector module prepared for the check. Retro-reflective targets are placed on all 112 of the mirror support points. The colour graph on the right side of Figure 2 represents a measurement result.

A typical quality control plan will include such measurements with a decreasing sample testing rate over the duration of the construction phase. Typically statistical information is
gathered from the measurements such as drifts and outliers in order to produce constant assembly quality.

Figure 3: EuroTrough module with reflecting targets on the mirrors, configuration (left) and measured deviations from the design heights in expanded scale (right)

The final result of the assembly process after absorber mounting might be affected by the intermediate assembly steps. This is why a final inspection of the collector geometry should be introduced in certain intervals. For final checking targets are attached onto the mirror surface for the photogrammetry and can be removed easily after the measurement (Figure 3, Figure 4). The results of such measurements on EuroTrough collectors proved the excellent quality of the assembly procedures in terms of geometric accuracy.

Figure 4: EuroTrough collector prepared for final close-range photogrammetric geometry analysis

A typical configuration for a photogrammetry system for quality control consists of a fix frame of several digital cameras and flashes, which transmit the captured images directly to the evaluation computer system. The fix set-up in a workshop reduces the effort for the photogrammetry evaluation, so that the results can be obtained from automated software in very short response time and with constant accuracy.

Flux Distribution Analysis

Main and principal criterion for a trough collector is its ability to concentrate sunrays effi­ciently and economically on the absorber tube in order to heat the thermal fluid. It is obvi­ous that the knowledge of the flux distribution on the absorber tube is very useful to assess and improve the collector. The objective is to analyze the influence of different parameters of the collector geometry on the collector output. One approach has been made with the PARASCAN (PARAbolic through flux SCANner) system [3, 4], which measures the solar flux distribution at a distance of about 10 cm around the absorber tube along its longitudi­nal axis, resulting in 2-dimensional flux distribution maps. It has a high spatial resolution and provides the result for a 3.5 m long focal area between two absorber tube supports. Figure 5 shows a plot of a PARASCAN measurement.

4000 3500 3000 2500 2000 1500 1000 500 0

tube length [mm]

Figure 5: PARASCAN-Fluxmap result at a distance of 10 cm around the 70 mm diameter absorber tube on the EuroTrough collector

For fast and easy flux distribution analysis the Camera-Target-Method shown in Figure 6 can be used. With this method the flux distribution on a plane perpendicular to the receiver axis can be visualized and sunrays, which pass the absorber tube, can be detected. The digital pictures taken of the diffuse reflecting target are evaluated basing on long-time ex­perience with indirect flux density measurements. Quantitative results of intercept values over the length of large collector areas are possible.

Both methods allow for checking and documentation of proper trough collector alignment and intercept factor impact on receiver performance data. The results of this technique help to quantify the effects of tracking accuracy on the collector performance [4].

Clamp-on thermal efficiency measurements

Figure 7: Ultrasonic flow meter probe, mounted to the pipe (Flexim)

Figure 6: Camera-Target-Method method: diffuse reflecting target perpendicular in the fo­cal line of the linear concentrator (left) and flux density map after image rectification and intensity corrections (right)

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After correct assembly of the collectors and positive testing of the flux distributions in the focal line, thermal tests should complete the acceptance tests. In order to reduce cost for sensor mounting and reduce the risk of leak­age of heat transfer fluid, clamp-on sensors might be preferred for this testing, in spite of the lower precision. For temperature meas­urements, thermally well-insulated and cali­brated surface resistance thermo probes (PT100) in four-wire technique, designed for temperatures up to 400 °C are used. An ultra­sonic flow meter can be used for working ranges up to 300°C on a wide range of pipe diameters. One ultrasonic sensor attached to a pipe is shown in Figure 7. The instrument also determines the wall thickness of the pipe in use, which is necessary because this value can vary significantly. Knowing the heat capacity, the specific density, the sound velocity and the viscosity of the fluid for the measured temperature range, mass flow rate can be measured with an accuracy of 1 to 3 percent.

Performance Impact of Geometric Precision

The optical performance of a parabolic trough collector is determined by the optical proper­ties of its key components, the mirrors and the receiver tubes. But of course their proper­ties have to match, and the concentrating collector as a whole has to be manufactured on the appropriate precision level to reach the design performance. The methods for geomet­
ric evaluation of concentrating collectors presented in the previous sections provide infor­mation about the actual geometry of the product. However the classification in pass/fail categories has been very difficult at some stages. Apart from the common criteria to fit components together, there is a need for appropriate criteria and tolerances that have to be fulfilled in order to reach the design energetic performance of the final product.

Ray tracing has being used to model the capture fraction of the reflected sunrays on the absorber tube. A detailed approach uses finite mirror facet elements and Monte-Carlo methods with millions of rays to find out the intercept factor of the solar radiation. If well modelled it reveals the optical efficiency and also the flux distribution of a part of a large collector under certain geometric conditions. This method is not practical for the analysis of large collector fields over longer time periods (e. g. one year).

So different ray-tracing techniques, as proposed by Rabl [6], have been used for the more extended annual analysis of solar collector fields. Certain simplifications reduce drastically the computational effort required. As usual for studies with a large number of independent, stochastically varying inputs the individual input will be replaced by the statistical model of a Gaussian distribution characterized by the standard deviation. So the beam spread oc­curring to the sunrays when interacting with the imperfect concentrator is represented by its standard deviation. The same can be applied, within a certain range of validity, for the sunbeam spread due to the size of the solar disc. Basing on this model the effect of irregu­larities can be respected in dependence of their frequency distribution. The individual ef­fects sum up with their weighted squares:

2 2 ^total = ai Wi

ct total in mrad

Figure 8: Intercept factor dependence of the total beam spread for a parabolic trough col­lector geometry acceptance function (EuroTrough-geometry, 70 mm absorber tube)

This equation also suggests that the standard deviation for each component (e. g. struc­ture, mirror) is the quality measure, which can be assessed easily from large quantities of measurement results. As given by this theory, the intercept factors for line focusing collec­tors have a dependence of the total beam spreads. The result for the EuroTrough geome­try (and because of identical concentrator and receiver geometry also for the LS 3- collector) is shown in Figure 8.

Conclusions

The systematic analysis and specific measurement systems used until now in solar ther­mal concentrating technology used to serve for the evaluation and qualification of proto­types in test or demonstration installations. Numerous techniques have been developed and used for measuring and optimizing the performance of prototypes. At the moment of the continuous transition from research and development work to market introduction in large series fabrication, the role of measurement techniques change. Their former applica­tion experience however is the basis for its further deployment in concepts for the quality control in large-scale projects.

The experience from tedious manual work in geometric measurements, leveling, photo — grammetry and flux density measurements has contributed to the collection of very de­tailed knowledge about the EuroTrough collector. The fastest and most reliable techniques from R&D experience are now transferred to quality control tools in order to assist the manufacturing and assembly of thousands of trough collector modules for the large solar power plant projects in Spain. Close-range photogrammetry is among the favorites. The contact-less measurement with digital camera equipment has been identified to fulfill the precision requirements of trough collector structure assembly. Further effort is underway with the objective to automate the caption and evaluation processes. The work on flux measurement and intercept factor analysis has identified the significant potential of im­provement in collector quality, which can be exploited basing on the detailed knowledge gained of the complexity of a concentrating solar collector.

The application of the proposed quality control concepts will reduce the effort on meas­urements and reworking. But even more: The potential in solar field performance gain amounts to several percent, and savings are reflected in cost reduction for less solar field area needed. In addition the knowledge that has been gathered on how to check and ver­ify in efficient manner the good performance of large parabolic trough collector fields will help to reduce the risk for construction companies and thus cut down solar field cost sig­nificantly.

The authors gratefully acknowledge financial support by the German Federal Ministry for the Environment (BMU) within the scope of “PARASOL/OPAL", the contributions by G. Johnston, S. Ulmer, and K.-J. Riffelmann, and the collaboration with the SKAL-ET project partners.

Fresnel-Collectors in hybrid Solar Thermal Power Plants with high Solar Shares

Fig. 15 Constant pressure concept; external depressurization,

Saturated Steam

Fig. 16 Steam accumulator with integrated latent heat storage material.

Although steam accumulators exhibit only a small storage capacity, the availability of these buffer storage systems can contribute to reduce the investment costs for storage capacity if they are combined with storage systems intended for longer periods of discharge. By reducing the requirements regarding response time and discharge rate the specific costs for storage systems with several hours of heat capacity can be reduced.

Acknowledgement

Part of the work presented in this paper has been funded by the German Federal

Environment Ministry under the contract code PARASOL/WESPE and part by the European

Commission within the 5th Framework Programme on Research, Technological

Development and Demonstration under contract no. ENK5-CT-2001-00540.

The authors are responsible for the content of this publication.

References

[1] Tamme, R., Laing, D., Steinmann, W. D., Zunft, S., 2002, "Innovative Thermal Energy Storage Technology for Parabolic Trough Concentrating Solar Power Plants”, Proceedings EuroSun 2002, The 4th ISES Europe Solar Congress, Bologna, Italy

[2] Tamme, R., Steinmann, W. D., Laing, D., 2003, „High Temperature Thermal Energy Storage Technologies for Power Generation and Industrial Process Heat", Proceedings FUTURESTOCK 2003, 9th International Conference on Thermal Energy Storage, 1.-4. Sept. 2003, Warsaw, Poland.

[3] Tamme, R., Laing, D., Steinmann, W. D., 2004, „Advanced Thermal Energy Storage Technology for Parabolic Trough", ASME-J. of Solar Energy Engineering, Vol. 126, May 2004.

[4] Eck M., Zarza E., Eickhoff M., Rheinlander J., Valenzuela L.: Applied Research concerning the Direct Steam Generation in Parabolic Troughs, Solar Energy, Vol.

74 (2003) pp. 341-351

[5] Beckmann, G., Gilli, P. V. (1984): "Thermal Energy Storage", Springer Verlag

Hansjorg Lerchenmuller, Max Mertins, Gabriel Morin

Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, 79110 Freiburg e-mail: hansjoerg. lerchenmueller@ise. fraunhofer. de

Dr. Andreas Haberle

PSE GmbH, Solar Info Center, 79072 Freiburg, Germany e-mail:ah@pse. de

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Dr. Stefan Bockamp, Dr. Markus Ewert, Matthias Fruth, Thomas Griestop E. ON Energie AG, Brienner Str. 40, 80333 Munich, Germany e-mail: markus. ewert@eon-energie. com

Dr. Jurgen Dersch

German Aerospace Centre (DLR), 51147 Cologne, Germany e-mail: juergen. dersch@dlr. de

Over the last few years Fresnel-Collectors have attracted a lot of attention within the solar thermal power sector. The main reason is comparatively low investment costs through simple components. The Fraunhofer Institute for Solar Energy Systems,

E. ON Energie AG and German Aerospace Centre (DLR) have carried out a feasibility study in order to assess the technology with respect to technical, economical and ecological aspects.

The mid to long term strategy of solar thermal electricity generation must aim at technical solutions with high solar shares. Thermal storage is not yet technically proven for direct steam generating systems. Therefore special configurations of hybrid operation are an interesting option from a technical and economical point of view. Full load hours of the power plant increase and allow for more stable plant operation. Based on Fresnel-Collectors, two different types of power plant configurations with low or zero CO2-emission are analysed in this paper:

• Hybrid operation of a solar field and a biomass vessel

• From the starting point of a Solar Only power plant, natural gas hybrid operation will be considered and the trade off between high solar share and low cost electricity production will be analysed in detail.

Calculations for this study were carried out in three steps:

• Thermodynamic calculations of the water/steam cycle were done with the commercial process simulation tool Ebsilon [1].

• Thermal and electrical yields were calculated with ColSim [2] for different solar field sizes and different options of hybridization. The simulations are based on the efficiencies of the power cycles — depending on ambient temperature and load — and hourly meteorological data for a site with a DNI of 2’247 kWh/(m2a) [3].

• Based on economic assumptions and on the results of the previous steps, calculations of levelised electricity costs (LEC) and profitability were carried out.

Geometry Optimization of Fresnel-Collectors with economic assessment

Dipl.-Ing. Max Mertins, University of Karlsruhe, Englerstr. 7, 76128 Karlsruhe Dipl.-Phys. Hansjorg Lerchenmuller, Fraunhofer ISE, Heidenhofstr. 2, 79110 Freiburg Dr. Andreas Haberle, PSE GmbH, Solar Info Center, 79072 Freiburg Dr. Ing. habil. Volker Heinzel, University of Karlsruhe, FZK, 76131 Karlsruhe

Abstract

The Fresnel solar collector is a promising concept to reduce the electricity cost price in solar thermal power plants. The optical performance of a Fresnel collec­tor depends on material properties, on its geometric layout and on the level of op­tical accuracy that can be obtained. A variety of geometric parameters, e. g. the height of the absorber, the number, size and distance of primary mirrors influence the shading and blocking of rays and the amount of rays missing the absorber. To evaluate the influence of the parameter variation regarding the electricity cost price and to yield an optimization, the optical performance is assessed with an annual simulation based on hourly weather-data. To permit a consideration of changes in collector cost according to different geometric layouts, cost factors where allocated to geometric parameters. The paper presents the method and the simulation re­sults of the optimization under different boundary conditions and shows how the developed simulation tool can lead to an optimum collector design with respect to cost price of electricity. The sensitivity of the results will be discussed.

Introduction

Similar to the parabolic trough system the linear Fresnel-collector, which is a piecewise approximation to the parabola, is suitable to produce steam for use in solar thermal power plants. The collector comprises of slightly elastic curved mirror-stripes, which reflect the sunlight to a fully stationary receiver (see figure 1).

secondary concentrator primary mirrors

insulation glass plane

-W

The receiver consists of a secondary CPC — type concentrator and a selective coated tubular absorber with no need of a vacuum insulation. Principally the collector is not lim­ited in aperture width[6], therefore a wide range primary mirrors glass plane —

of free geometry parameters is possible. The width of the primary mirrors B has to be

TOC o "1-5" h z coordinated with their gaps D, their number :

N and, the hight H of the receiver. Several studies ([1], [2]) have certified promising cost

perspectives of the linear Fresnel-concept ——- —

but at less specific energy yield. Hence an W ■

optimization of the geometry and the field Figure 1: Principle °f the linear Fresnel size is only meaningful with an economic collector

assessment.

The collector considered in this paper is intended to produce superheated steam at 440 °C

and 50 bar, therefore the collector-field is divided into sections for preheating, evaporating and superheating.

(a) cosine-losses (b) shading (c) blocking

Figure 2: Geometric losses of Fresnel type collectors

The main geometric losses of the Fresnel-concept are shown in figure 2. These effects can be decreased by heightening the absorber and by widening the gap between primary mirrors. On the other hand geometric changes cause losses due to inaccuracy of assembly and tracking of the primary mirrors.

Approach

For analyzing and optimizing the geometry, the optical efficiency is not of primary interest. The optimization for maximum performance of the collector at noon would differ from the one for lowest LEC[7]. The difference would be up to 10% in the LEC between the different optimization targets. Therefore an integral view on the whole system is essential. A link between the optical, thermodynamic and cost models is necessary to take the main influences into account. The electricity yield of the power-block is calculated via annual simulation based on hourly weather-data of a certain site under specific boundary conditions. After consideration of the specific cost the LEC is evaluated and chosen to assess the configuration. Hence arbitrary geometries and receiver-concepts can be investigated.