• Energy Research

In solar thermal power plants, at least the solar part undergoes a daily start-up process. Since duration and energy consumption of the start-up depend on irradiance and temperature boundary conditions, differences occur between the individual days and especially between the seasons. For a good representation in annual electricity yield calculations, the start-up process should be modeled on a daily basis. This paper presents a closed approach for the calculation of start-up and cool-down in solar thermal power plants especially designed for annual calculations. It is demonstrated by one example how the required parameters can be obtained and how the methodology is applied. A sensitivity analysis reveals the large impact of start-up consumption on the annual yield.

AbstractOne option to improve the cost-effectiveness and environmental friendliness of parabolic trough power plants is the utilization of water/steam in the solar field, the so-called direct steam generation (DSG). First commercial stand-alone plants using DSG are now in operation in Thailand (parabolic trough with superheating by Solarlite) and in Spain (linear Fresnel for saturated steam by Novatec Solar). To further bring down the costs of a DSG solar field, the research project DUKE aims at the development and demonstration of a commercially applicable once-through mode design.The demonstration will be done at the DISS test facility at the Plataforma Solar de Almería (PSA), Spain, by the DLR-Institute of Solar Research in close collaboration with the Spanish CIEMAT. For this purpose, the DISS test facility was upgraded to a length of 1000 m by three new collectors. It now has commercial-scale size and, in addition, is able to stand 500°C and 110bar at the outlet [1].The construction and commissioning of the new plant has been completed (see Fig. 1) and the first test period has started in May 2013. The paper examines two aspects: illustrate the design changes of the solar field and show the performance of the two new main components: the DSG receivers as well as the new collectors.

Abstract Direct steam generation (DSG) is one alternative to the current oil-based parabolic trough solar thermal power plants. Within the German research project ITES, the dynamic behavior of a DSG collector field and the interactions with the conventional power block are assessed in detail. A transient solar field model developed by DLR is used to simulate the steam temperature behavior. Artificial irradiance disturbances as well as real irradiance data are used as input to the system. The resulting main steam temperature gradients are then analyzed by Siemens considering the standards for steam turbines. This paper presents the transient simulation results of the steam temperature as well as the corresponding results of the steam turbine analysis. It is found that the occurring temperature gradients are challenging for a safe turbine operation, if a conservative control system is used. Therefore, the use of an additional thermal inertia to stabilize the steam temperature is suggested. Its impact is also analyzed and discussed in this paper.

AbstractYield analysis is a crucial task during project deployment of solar thermal power plants. Currently, many different modeling approaches and computer tools for yield analysis are used. Within the SolarPACES project guiSmo, aiming at the development of guidelines for the annual yield prediction of solar thermal power plants, relevant effects for the steady-state modeling of plant sub-systems have been identified [1].The target of the ongoing project phase is the development of general definitions for all relevant effects. Furthermore, suitable modeling approaches have to be identified and described unambiguously. For every effect, the impact on the predicted electricity yield is estimated in order to assess the significance of the considered effect. Since the most significant effects have to be considered in a subsequent uncertainty analysis, reasonable model and parameter uncertainties have to be defined, too. Finally, default model parameter for state-of-the-art components or subsystems will be edited.Beside the present status of the project, this paper presents investigations on the effects of the optical losses of the sub-system collector field on the annual yield. These effects are namely the peak optical efficiency, incidence angle modifier (IAM), shading and end losses. For each effect, a precise definition is presented and relevant modeling approaches are identified and their pros and cons are discussed. To investigate the influence of these effects, a reference solar thermal power plant with parabolic troughs is defined. The annual yield is simulated for this reference system investigating the identified modeling approaches and assessing their significance.

In addition to the commercial parabolic trough power plants using synthetic oil, the direct steam generation (DSG) is one option for future trough plants. Besides its higher efficiency and lower environmental impact, the overall levelized electricity costs (LEC) will be decisive for the future application of DSG. This paper focuses on the thermodynamic and economic comparison of synthetic oil and DSG plants including the aspect of thermal energy storage – which gains more and more in importance by industry and investors.

Technical and economical evaluation of solar thermal power plants constantly gains more importance for industry and research. The reliability of the results highly depends on the assumptions made for the applied parameters. Reducing a power plant system to one single, deterministic number for evaluation, like the levelized cost of electricity (LCOE), might end in misleading results. Probabilistic approaches can help to better evaluate systems [1] and scenarios [2]. While industry looks for safety in investment and profitability, research is predominantly interested in the evaluation of concepts and the identification of promising new approaches. Especially for research, dealing with higher and hardly quantifiable uncertainties, it is desirable to get a detailed view of the system and its main influences. However, to get there, also a good knowledge on the stochastic interrelations and its interpretation is required. Therefore, this paper mainly assesses the influences of basic stochastic assumptions and suggests a methodology to consider suitable stochastic input, especially for parameters of systems still under research. As examples, the comparison between a parabolic trough plant with synthetic oil and direct steam generation is used.

The direct steam generation (DSG) in parabolic trough collectors is a promising option to improve the mature parabolic trough solar thermal power plant technology of the solar energy generating systems (SEGS) in California. According to previous studies [Langenkamp, 1998, “Revised LEC Projections and Discussion of Different DSG Benefits,” Technical Report No. DISS-SC-QA-02, Almeria, Spain; Price, et al., 2002, “Advances in Parabolic Trough Solar Power Technology,” ASME J. Sol. Energy Eng., 124(2), pp. 109–125; Zarza, E., 2002, “DISS Phase II Final Report,” Technical Report EU Contract No. JOR3-CT98-0277, Almeria, Spain], the cost reduction in the DSG process compared with the SEGS technology is expected to be 8–25%. All these studies were more or less preliminary since they lacked detailed information on the design of collector fields, absorber tubes required for steam temperatures higher than 400°C, and power blocks adapted to the specific needs of the direct steam generation. Power blocks and collector fields were designed for four different capacities (5 MWel, 10 MWel, 50 MWel, and 100 MWel) and different live steam parameters. The live steam temperature was varied between saturation temperature and 500°C and live steam pressures of 40 bars, 64 bars, and 100 bars were investigated. To assess the different cases, detailed yield analyses of the overall system were performed using hourly data for the direct normal irradiation and the ambient temperature for typical years. Based on these results, the levelized costs of electricity were determined for all cases and compared with a reference system using synthetic oil as heat transfer fluid. This paper focuses on two main project findings. First, the 50 MWel DSG system parameter comparisons are presented. Second, the detailed comparison between a DSG and a SEGS-like 100 MWel system is given. The main result of the investigation is that the benefit of the DSG process depends on the project site and can reach an 11% reduction in the levelized electricity cost.