In this paper, the general design philosophy for a large pure solar storage plant is discussed. The proposed stand alone plant design will use the same low cost Compact Linear Fresnel Reflector (CLFR) array system previously reported (Mills et al, 2003; Hu et al, 2003) as is being constructed for a coal fired plant preheating project of 35 MWe integrated with a coal-fired plant. This current coal saver project has been now been re­estimated to be 40 MWe. The project, being built for Macquarie Generation, is composed of three stages; a proving array of 1100 m2, an intermediate array of 20236 m2, and a final array of 134909 m2. After stage 3 is built, it will be the largest solar electricity plant built since the last LS3 parabolic trough field built in California in 1990, and will provide a solar electricity capacity about 3 times the current PV capacity of Australia. The kWh cost of the first plant is expected to be similar to, or below, current

wind technology in Australia.

The array system is linear like a parabolic trough collector, but it has many advantages over troughs which allow significant cost reductions, such as a long focal length with allows elastically bent flat standard glass reflector to be used.

Fig. 1. The Stage 1 array and tower line produced by SHP at the Liddell power plant site.

The array technology used in this project is of the Linear Fresnel type and was originally developed at the University of Sydney (Mills and Morrison,1999). It is called the Compact Linear Fresnel Reflector (CLFR) technology. In this approach, ground level reflector rows aim solar beam radiation at a downward facing receiver mounted on multiple elevated parallel tower lines. The technology is innovative in that it allows reflectors to have choice of two receivers so that a configuration can be chosen which offers minimal mutual blocking of adjacent reflectors and minimum ground usage. However, there are also many
supporting engineering innovations in the commercial product, including highly rigid space frame mirror supports with 360° roatation capability, long horizontal direct steam generation cavity receivers, and array fine tracking control electronics. The design of the CLFR array design incorporates high volume production elements to reduce engineering cost.

The authors have previously described some of the cost advantages of the CLFR array system (Mills et al, 2003) of the current trough technology, but have not discussed the general issue of overall stand-alone solar plant design. The traditional approach to the design of a line focus solar plant is to use a parabolic trough system to the supply of heat at between 320°C and 400°C to the main boiler and superheater of a conventional turbogenerator (NREL, 2003). Some higher cost trough designs utilise fossil fuel in off — solar hours, not only to increase the plant capacity factor, but to lower the overall cost of delivered energy. The present CLFR design can also be straightforwardly adapted in this direction. However, in trough and CLFR systems, thermal losses can rise rapidly with array operating temperature, partially cancelling out improvements in thermal conversion efficiency. In addition, the traditional path of using a superheated turbine requires more highly efficient and durable selective coatings, thicker-walled tubing for steam pressure containment, and the use of oil instead of water as a heat transfer fluid if operating above the water triple point.

The use of Fresnel-Collectors in power plant configurations with low or zero CO2-emission has been analysed in this paper. Both, the solar-biomass hybrid plant and the solar-gas hybrid plant are very promising concepts with respect to technical, economical and ecological aspects. The hybrid operation would be very useful to handle the fluctuating solar resource and facilitate operation.

Depending on feed in tariffs the hybridisation of a solar thermal power plant with biomass or with small shares of natural gas can be economically very interesting. The ecological advantage of a solar-biomass power plant is evident, since it would be a zero CO2- emission plant. Beyond that biomass is a limited source, especially in regions with high solar irradiance.

More difficult to evaluate is the fossil hybridisation since it could be argued, that it would be better to fire the gas in a highly efficient combined cycle power plant instead of a low efficient steam cycle. It has been shown that it is favourable to build a hybrid solar plant, as from one kWh fuel over 200% electricity can be taken out in the configuration with 15% fossil share. In comparison a high-efficiency combined cycle will reach in maximum 60%. Of course in case it is technically and economically feasible to run a Fresnel plant without gas co-firing, from an ecological point of view this option should be realized, since a combined cycle can transform gas more efficiently.

Therefore the long term strategy should be working with very low gas shares or coupling solar fields to other steam plants than gas like coal or as described above to a biomass vessel. Another long-term option is thermal storage which is not yet feasible for direct steam generating systems.

The herein examined gas hybrid variants are by far more favourable than Integrated Solar Combined Cycle Systems (ISCCS), which have been previously examined. Since much higher solar shares can be reached this is a forward-looking technology. Furthermore the solar field does not act as a disturbing factor as opposed to the ISCCS concept, where the efficiency of the sophisticated CC system is reduced due to suboptimal dimensioning of components. In other words: It is better to build the suggested hybrid plant with low gas share and CC plants instead of ISCCS plants. The resulting solar levelised electricity costs of both options are approximately the same.

The calculated levelised electricity costs for the examined configurations of between 11 and 13 ct/kWh are very promising results. The calculations were made based on cost assumptions for the collector of 130 €/m2 which seems realistic not for the first project but for the »third« plant. As a next step in the commercialisation of the Fresnel-technology demonstration and test collectors must be built, such as by the Australian company Solar Heat and Power, to validate the technical and economic assumptions.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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].

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

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.