The combination of high solar shares with high conversion efficiencies is one of the major advantages of solar gas turbine systems compared to other solar-fossil hybrid power plants. Pressurized air receivers are used in solar tower plants to heat the compressed air in the gas turbine to temperatures up to 1000°C. Therefore solar shares in the design case of 40% up to 90% can be realized and annual solar shares up to 30% can be achieved in base load. Using modern gas turbine systems in recuperation or combined cycle mode leads to conversion efficiencies of the solar heat from around 40% up to more than 50%. This is an important step towards cost reduction of solar thermal power. Together with the advantages of hybrid power plants-variable solar share, fully dispatchable power, 24 h operation without storage-solar gas turbine systems are expected to have a high potential for market introduction in the mid term view. In this paper the design and performance assessment of several prototype plants in the power levels of 1 MW, 5 MW and 15 MW are presented. Advanced software tools are used for design optimization and performance prediction of the solar tower gas turbine power plants. Detailed cost assumptions for the solarized gas turbine, the solar tower plant and further equipment as well as for operation and maintenance are presented. Intensive performance and economic analysis of the prototype plants for different locations and capacity factors are shown. The cost reduction potential through automation and remote operation is revealed.
When striving for maximum efficiencies in solar thermal central receiver systems (CRS) the use of gas turbines with bottoming cycles is inevitable. Pressurized volumetric receivers have proven their feasibility and good performance, and their integration into gas turbine cycles has been demonstrated. One disadvantage of this system is the necessity to use secondary concentrators. The sunlight has to be concentrated into the relatively small glass windows of the receiver, which leads to a limited view cone. This means that of all the possible heliostat positions around the tower, only those within the ellipse, resulting from the section boundary of the view cone with the ground plane, are usable. For small systems, for which tower costs are small, the resulting heliostat field layout is similar, with or without secondary concentrator. For large systems, which are more cost-effective, tower costs become significant, and the losses due to atmospheric attenuation and spillage dominate over the cosine losses. Thus, the purely North-oriented fields become increasingly sub-optimal. This article shall demonstrate at what power levels this problem can be alleviated by not using a single, North-oriented aperture, but up to six apertures-each of them associated with a separate heliostat field.
A completely new ray tracing software has been developed at the German Aerospace Center. The main purpose of this software is the flux density simulation of heliostat fields with a very high accuracy in a small amount of computation time. The software is primarily designed to process real sun shape distributions and real highly resolved heliostat geometry data, which means a data set of normal vectors of the entire reflecting surface of each heliostat in the field. Specific receiver and secondary concentrator models, as well as models of objects that are shadowing the heliostat field, can be implemented by the user and be linked to the simulation software subsequently. The specific architecture of the software enables the provision of other powerful simulation environments with precise flux density simulation data for the purpose of entire plant simulations. The software was validated through a severe comparison with measured flux density distributions. The simulation results show very good accordance with the measured results.
Strong efforts are being made to drive heliostat cost down. These efforts are summarised to give an update on heliostat technology comprising: determination of wind loads, heliostat dimensioning, solutions for the different sub-functions of a heliostat, a review of commercially available and prototype heliostat designs, canting, manufacturing, qualification, heliostat field layout, and mirror cleaning. There is evidence that commercial heliostat costs have dropped significantly in the past few years, with commercial suppliers of heliostat technologies now claiming heliostat field costs around 100 USD/m 2. With new approaches even target cost of 75$/m² seem to be realistic.
Solar hybrid power plants have a significant potential for cost reduction when the solar energy is introduced into a gas turbine system. The introduction into gas turbine systems could be realized with pressurized volumetric air receivers heating the compressed air of the gas turbine before it enters the combustor. A receiver module, consisting of a secondary concentrator and a volumetric receiver unit, was tested at the Plataforma Solar de Almerı´a, Spain. Air exit temperatures up to 815°C and power levels of 410 kW were achieved. Total solar test time summed up to 400 hours. Receiver efficiencies were in the range of 70%. A new secondary concentrator with improved efficiency was designed and built. Based on an inexpensive manufacturing technology, the secondary concentrator geometry was optimized to reduce the optical losses. Performance tests with this new secondary concentrator and a cold-water calorimeter proved the expected increase in efficiency of about 10%. Maximum operation power was 450 kW at the exit aperture. The dependency of performance on the incidence-angle showed good agreement with the predictions, as well as the results of a special photographic measurement campaign. Several configurations of solar-hybrid gas turbine cycles in the low to medium power range are examined for performance and costs. The results confirm the promising potential of this technology to reach competitiveness in certain power markets; a comparison between a 30 MW solar-hybrid combined cycle plant and an ISCCS power plant are presented. Future developments for system improvement and cost reduction are discussed.
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