Jan Fabian Feldhoff

Advancements in the Field of Direct Steam Generation in Linear Solar Concentrators—A Review

Direct steam generation in parabolic trough or linear Fresnel collectors represents one interesting technological option for concentrating solar electricity production. Todays state of the art characterized by the first commercial plants in operation is a result of more than 20 years of intensive research on this topic. This article provides a review on the key results from research that includes physical effects like heat transfer and pressure drop in horizontal boiler tubes, plant layout considerations, and thermal storage options. An overview on test and demonstration facilities as well as on commercial plants is given, leading to an outlook on the next generation of direct steam generation systems.

THERMAL MODELLING AND SIMULATION OF PARABOLIC TROUGH RECEIVER TUBES

Receiver tubes (or heat collecting elements — HCE) are a key component of parabolic trough solar thermal power plants. They are mounted in the focal line of the collectors, absorb the concentrated solar irradiance and transfer the absorbed energy to the heat transfer fluid flowing through them. During the design phase of the receiver tubes and for the performance prediction of solar thermal power plants it is helpful to derive their technical properties, like the thermal losses or the temperature field in the receiver tubes, from their physical and geometrical properties. For this purpose, several models have been developed in the past [1–3]. In this paper, the different existing models are presented, compared and assessed. It is found that a simple analytical model is a helpful tool for the fast prediction of the temperature distribution in the receiver tube. Furthermore, a 2-dimensional and a 3-dimensioanl model are compared regarding the heat losses of a HCE at different operation conditions. Both tools show a good agreement with available measurements. Finally with these tools the efficiency factor F′ is calculated that considers the heat losses of an irradiated receiver compared to that of an un-irradiated receiver. According to the performed calculations, the efficiency factor of parabolic trough receivers is higher than expected.Copyright

Start-Up Modeling for Annual CSP Yield Calculations

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.

Energetic Comparison of Linear Fresnel and Parabolic Trough Collector Systems

Within the last years, Linear Fresnel (LF) collector systems have been developed as a technical alternative to parabolic trough collector (PT) systems. In the past, LF systems focused on low- and medium temperature applications. Nowadays, LF systems equipped with vacuum receivers can be operated at the same temperatures as PT systems. Papers about the technical and economical comparison of specific PT and LF systems have already been published, [1–3]. However, the present paper focuses on the systematic differences in optical and thermodynamic performance and the impact on the economic figures.In a first step the optical performance of typical PT and LF solar fields has been examined, showing the differences during the course of the day and annually. Furthermore, the thermodynamic performance, depending on the operating temperature, has been compared.In a second step, the annual electricity yield of typical PT and LF plants are examined. Solar Salt has been chosen as heat transfer fluid. Both systems utilize the same power block and storage type. Solar field size, storage capacity, and power block electrical power are variable, while all examined configurations achieve the same annual electricity yield. As expected for molten salt systems, both systems are the most cost-effective with large storage capacities. The lower thermodynamic performance of the LF system requires a larger solar field and lower specific costs in order to be competitive. Assuming specific PT field costs of 300 €/m2 aperture, the break-even costs of the LF system with Solar Salt range between 202 and 235 €/m2, depending on the site and storage capacity.Copyright

Analysis and potential of once-through steam generators in line focus systems – Final results of the DUKE project

The direct steam generation in line focus systems such as parabolic troughs and linear Fresnel collectors is one option for providing ‘solar steam’ or heat. Commercial power plants use the recirculation concept, in which the steam generation is separated from the superheating by a steam drum. This paper analyzes the once-through mode as an advanced solar field concept. It summarizes the results of the DUKE project on loop design, a new temperature control strategy, thermo-mechanical stress analysis, and an overall cost analysis. Experimental results of the temperature control concept at the DISS test facility at Plataforma Solar de Almeria are presented.

Modelling, Simulation and Assessment of Solar Thermal Power Plants: A First Step Towards Definition of Best Practice Approaches

With 620 MWel in operation [1] and more than 2.000 MWel under construction, concentrated solar power (CSP) experiences a renaissance mainly in Spain and the USA, but also in many other countries in the earth’s sunbelt. Due to their large capacity (50 MWel and more) and thus large investment, CSP projects are characterised by an extensive project development process. In several stages of this process, mathematical models of the system predicting its energy yield are required, among others to: • assess single CSP projects (e.g., feasibility or due diligence studies), • compare different CSP concepts (e.g., technology, site), • optimise a project (e.g., solar field size, storage capacity), • investigate the influence of component characteristics (e.g., receiver characteristics), • optimise the operation strategy (e.g., on-line surveillance) or to • assess system performance during commissioning. The models used for these different tasks differ in complexity and accuracy, e.g. the accuracy of a model used for project assessment during commissioning has to be higher than a model used for a pre-feasibility study. At the moment, numerous modelling approaches exist and every project developer uses his own system model and assessment methodology. This confusing situation hinders the acceptance of CSP technology by potential investors. This paper presents a methodology for structuring systems into sub-systems. This is the first step towards a standardized modelling approach for CSP systems. It is not the intention of the authors to present a final model and assessment methodology but to start a broader discussion on this important topic. In fact, it aims at initiating an international working group, devoted to the definition of guidelines for modelling, simulation and assessment of CSP systems, covering all CSP technologies such as solar towers, parabolic troughs, linear Fresnel collectors and solar dishes.Copyright

Standardization of CSP performance model projection - latest results from the guiSmo project

Electricity yield projection is one of the essential tasks related to concentrating solar thermal power plants. Currently, the project engineers and other stakeholders cannot refer to standardized methods for calculation of the annual electricity outcome of such plants. In autumn 2010, the first steps were taken to initiate an international project within SolarPACES Task I that targets the development of reliable methodologies for yield projection [1, 2]. More than 80 international participants from academia, industry, and finance agreed to work together within the guiSmo project (Gui delines for CS P performance mo delling). Intended as an open project with publicly available results, CSP stakeholders are invited to join the team. Since initiation of the project, a coordination team and supervisory board have been founded. The challenging task is subdivided into ten “work packages” that cover all aspects of CSP project analysis — from elementary plant models to transient effects and financial evaluation approaches. This paper provides an overview of recent achievements and upcoming activities in the guiSmo project. The focus is on the definition of three quality levels for yield projection as they will be required during the various project development phases.Copyright

Economic Potential of Solar Thermal Power Plants With Direct Steam Generation Compared to HTF Plants

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 [1-3], the cost reduction of the DSG process compared to the SEGS technology is expected to be 8 to 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. To bridge this gap, a detailed system analysis was performed within the German R&D project DIVA. Power blocks and collector fields were designed for four different capacities (5, 10, 50 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, 64 and 100 bar 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 to a reference system using synthetic oil as heat transfer fluid (HTF). 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 SEGSlike 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 of the levelized electricity cost (LEC).

Guidelines for CSP yield analysis-Definition of elementary terms

With the success of CSP technology in the last years more players are active in the market, inducing the need for harmonization of technical terms and methodologies. The mission of the SolarPACES “guiSmo” project which was started in 2010 is to develop a guideline for CSP yield analysis [1]. Activities carried out so far have shown that people have different understandings of many terms used in daily CSP practice. Especially for the development of guidelines, the essential terms need to be clearly defined in order to avoid inconsistencies within the same project. A first version of a nomenclature has been compiled by the “guiSmo” team and will undergo final discussion. The aim is to come to a harmonized version by Summer 2013 which will then be presented at the ASME Energy Sustainability conference. The compilation so far includes essential definitions of terms like direct normal irradiance, incident angles, heat flows, and efficiencies on a system level. The definitions presented will be discussed together with existing standards like the ISO 80000 (physical quantities and units of measurement), the ISO 9488 (Solar energy-vocabulary) and other relevant sources. Although the list of terms is primarily put together for the work in the “guiSmo” project, it might serve as a basis for standardization in the official councils. An international group of solar experts is involved in the preparation of the document in order to ensure high quality and international support for the results.

Considering Uncertainties in Research by Probabilistic Modeling

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.