Concentrating solar power technologies. The DESERTEC megaproject

The European strategies on energy have been searching for years to reduce the dependency of Europe from fossil fuels. Underlying this effort, there exist geopolitical, economic, environmental reasons and the reality that oil reservoirs will dry out some day. Renewable energies have become a milestone of this strategy because their huge potential has emerged after years of uncertainty. One of the better developed renewable sources, which is nearer to commercial maturity is solar-thermal energy. In this paper, the current state of this technology will be described as well as the developments that may be expected in the short and mid terms, including the thermoelectric solar megaproject DESERTEC, a German proposal to ensure energy resources to the mayor areas of the EU-MENA countries. The reader will acquire a picture of the current state of the market, of the technical challenges already achieved and of the remaining ones

Comparison between s-CO2 and other supercritical working Fluids (s-Ethane, s-SF6, s-Xe, s-CH4, s-N2) in Line-Focusing Solar Power Plants with supercritical Brayton power cycles

Thermosolar power plants with linear solar collectors and Rankine or Brayton power cycles are maturing as a competitive solution for reducing CO2 emissions in power plants as an alternative to traditional fossil and nuclear fuels. In this context, nowadays a great effort is being invested in supercritical Carbon Dioxide Brayton (s-CO2) power cycles for optimizing the line-focusing solar plants performance and reducing the cost of renewable energy. However, there are other working fluids with similar properties as s-CO2 near critical point. This researching study was focused on assessing the solar plants performance with alternative supercritical working fluids in the Balance Of Plant (BOP): Ethane, Sulfur Hexafluoride, Xenon, Methane and Nitrogen, see [1, 2, 3]. The integration between linear solar collectors (Parabolic or Fresnel), Direct Moten Salt (MS) as Heat Transfer Fluids (HTF) and a Simple Brayton cycle with Recuperation and ReHeating were studied in this paper. Main innovation in this researching study is the Brayton power cycle parameters optimization at Design-Point via the Subplex algorithm as proposed in John Dyreby Thesis [4]. After obtaining the optimum reheating pressure, compressor inlet pressure, recompression fraction, and other optimized variables, the solar power plants performances were simulated and detail designed with Thermoflow software [5], providing a first approach about the Solar Fields (SF) effective areas and investment costs. As main conclusion, we deducted the importance of heat exchangers conductance (UA) for increasing the Brayton power plants efficiency and reducing the SF effective area and investment cost. The pinch point at recuperators exit is the main constrain for increasing the UA in s-CO2 cycles. This limitation is overcome with the other working fluids proposed in this study providing higher plant efficiency but requiring higher UA in the recuperators. In future studies the heat exchangers detailed design constitute a great challenge for increasing the UA and optimizing these equipments cost. The material corrosion and equipments dimensions and cost is another key issue discussed for selecting the optimum energy transfer fluid in Brayton power cycles

Thermodynamic optimisation of supercritical CO2 Brayton power cycles coupled to Direct Steam Generation Line-Focusing solar fields

In this paper a new generation line-focusing solar plants coupled to a s-CO2 Brayton power cycles are studied. These innovative CSP will increase the plant energy efficiency, and subsequently optimizing the SF effective aperture area and SF investment cost for a fixed power output. Two SF configurations were assessed: the Configuration 1 with a condenser between the SF and the Balance Of Plant (BOP), for Turbine Inlet Temperatures (TIT) up to 400oC, and the Configuration 2, for higher TIT up to 550oC, with steam compressors in SF for pressure drop compensation. Both alternatives are interchangeable in the same CSP, and boosting with a backing boiler to warranty the plant performance. In relation to the BOP three configurations were studied the Recompression cycle (RC), the Partial Cooling with Recompression cycle (PCRC), and the Recompression with Main Compression Intercooling cycle (RCMCI), all these solutions without ReHeating. The methodology considered the thesis developed by Dyreby [1] as starting point, fixing the Brayton cycles recuperator conductance (UA), and optimizing the power cycles performance by means of the SUBPLEX [2] algorithm. The cycles optimal operating parameters were calculated with a “Windows” desktop application, called Supercritical_CSP (SCSP), calling the supercritical fluids properties database REFPROP, developed in C#, calling Fortran compiled dynamic linked libraries. The results obtained from the Brayton cycles optimizations were exported to Thermoflow [3] for SF simulation and design. The mathematical algorithms UOBYQA [4] and NEWOUA [5] were also integrated in the SCSP tool, for validating the SUBPLEX results. The HTF studied was Direct Steam Generation (DSG) in the SF, and the solar collectors simulated were PTC and LF. The plant net power output, the net efficiency, the SF effective aperture, were computed at DesignPoint. As main conclusion obtained it is confirmed minimum Pinch Point in heat exchangers is the main constrain, reaching a threshold in the net plant efficiency, when increasing the Low Temperatura Recuperator (LTR) and High Temperature Recuperator (HT) conductances UA. The shell-tubes heat exchanger types are the most suitable solution to couple the Balance Of Plant (BOP) and the SF. The target of future works will be aligned with the analysis of innovative linear solar collectors, as the Norwich Technologies company solution, for getting higher TIT as provided by Central Tower CSP. The s-CO2 BOP equipments detail design and detailed cost estimation are pending items under industrial development. Finally, the annual plant performance calculation, considering the variable ambient temperature and Direct Normal Irradiance (DNI), and the TES integration, are future researching works for calculating the Levelized Cost Of Energy (LCOE) in this new generation line-focusing solar power plants

Dual Loop Line-Focusing Solar Power Plants with Supercritical Brayton Power cycles

Most of the deployed commercial line-focusing solar power plants with Parabolic Troughs (PTC) or Linear Fresnel (LF) solar collectors and Rankine power cycles use a Single Loop Solar Field (SF), Configuration 1 illustrated in Fig. 2, with synthetic oil as Heat Transfer Fluid (HTF) [1, 2]. However, thermal oils maximum operating temperature should be below ~400ºC for assuring no oil degradation, hence limiting the power cycle gross efficiency up to ~38%. For overcoming this limitation Molten Salts (MS) as HTF in linear solar collectors (PTC and LF) were recently experimented in pilot facilities [3, 4]. Direct MS main drawbacks are the equipments and components material corrosion and the salts freezing temperature, requiring heat tracing to avoid any sald solidification, hence increasing the Solar Field (SF) capital investment cost and parasitic energy looses. Concentrated Solar Power plants (CSP) with Dual Loop SF are being studied since 2012 [5] for gaining the synergies between thermal oils and MS properties. In the Dual Loop SF the HTF in the primary loop is thermal oil (Dowtherm A) [6] for heating the Balance Of Plant (BOP) working fluid from ~300ºC up to ~400ºC, and a secondary loop with Solar Salt (60% NaNO3, 40% KNO3) as HTF, for boosting the working fluid temperature from ~400ºC up to 550ºC [7, 8, 9]. The CSP Dual Loop state of the art technology includes Rankine power cycles, the main innovation of this paper is the integration between Dual Loop SF and the supercritical Carbon Dioxide (s-CO2) Brayton power cycles [10], see Configurations 2 and 3 illustrated in Fig. 3a, Fig 3b. A secondary innovation studied in this paper is the integration between thermal oil HTF (Dowtherm A) in linear solar collectors, a widely validated and mature technology, with the s-CO2 Brayton power cycles. This technical solution is very cost competitive with carbon steel receiver pipes, low SF operating pressure, and no requiring any heat tracing. Two main conclusions are deducted from this researching study. Firstly we demonstrated the higher gross plant efficiency ~44.4%, with 550ºC Turbine Inlet Temperature (TIT), provided by the Dual Loop with the Simple recuperated s-CO2 Brayton cycle with reheating, in comparison with 41.8% obtained from the Dual Loop SF and subcritical water Rankine power cycle. And finally the second conclusion obtained is the selection of the most cost competitive plant configuration with a Single loop SF with Dowtherma A and a s-CO2 Brayton power cycle due to the receiver material low cost and no heat tracing for the thermal oil

New generation Line-Focusing Solar Power Plants with Molten Salts and Supercritical Carbon Dioxide Joule-Brayton Cycles

Nowadays there is no dominant technology for the concentrated solar power plants that means there is still a way to go. Within this context, new concepts for solar fields and power cycles are being studied. One of them is the proposed on this paper: the integration of line-focusing solar field, with parabolic trough or linear Fresnel solar collectors, with molten salts as heat transfer fluid and supercritical carbon dioxide Joule-Brayton power cycles. This concept works as a feasible design solution to increase efficiency and reduce final energy cost in solar electricity production. In this work, four Joule-Brayton cycles configurations were assessed and compared with the considered reference, a concentrated solar power plant with direct steam generation in the solar field and a Rankine power cycle. The studied Joule-Brayton cycles are: simple cycle, recompression cycle, partial cooling with recompression cycle and recompression with main compression intercooling cycle. The common operation conditions for all the configurations are that at design-point the high pressure turbine inlet temperature value is 550ºC, this limit was established considering maximum temperature allowed by selective coating material in linear receivers. Also is analyzed the hypothetical scenario of increasing the turbine inlet temperature to 650ºC, extrapolating the receivers heat losses regressions. The innovative configurations of solar field and supercritical carbon dioxide power cycles increase plant efficiency, for recompression cycle configuration, up to 46.84% (550ºC turbine inlet) and 50.85% (650ºC turbine inlet), and reduces required solar field effective aperture area and land area for a fixed plant power output. Proposed configurations, parabolic trough collector and linear Fresnel coupled with a Joule-Brayton cycle decreases the solar field required for the same net power. Relating to power block, the supercritical carbon dioxide higher density in comparison with water steam, reduces turbines and compressors dimensions, footprint and final cost, but is a technology nowadays under industrial development and final turbo machines cost could not be assessed in this study. Another important keystone in JouleBrayton cycle costs are the heavy duty heat exchangers required. Printed circuit heat exchangers are the most advisable solution proposed for supercritical carbon dioxide recuperators, mainly due to higher compactness and better heat transfer coefficient inside channels. However, in this paper it is demonstrated how common shell & tube heat exchangers, with AISI 347 (austenitic) stainless steels, are competitive and feasible solutions for the primary and reheating molten salts – carbon dioxide heat exchangers

Supercritical Steam power cycle for Line-Focus Solar Power Plants

The supercritical Rankine power cycle offers a net improvement in plant efficiency compared with a subcritical Rankine cycle. For fossil power plants the minimum supercritical steam turbine size is about 450MW. A recent study between Sandia National Laboratories and Siemens Energy, Inc., published on March 2013, confirmed the feasibility of adapting the Siemens turbine SST-900 for supercritical steam in concentrated solar power plants, with a live steam conditions 230-260 bar and output range between 140-200 MWe. In this context, this analysis is focused on integrating a line-focus solar field with a supercritical Rankine power cycle. For this purpose two heat transfer fluids were assessed: direct steam generation and molten salt Hitec XL. To isolate solar field from high pressure supercritical water power cycle, an intermediate heat exchanger was installed between linear solar collectors and balance of plant. Due to receiver selective coating temperature limitations, turbine inlet temperature was fixed 550ºC. The design-point conditions were 550ºC and 260 bar at turbine inlet, and 165 MWe Gross power output. Plant performance was assessed at design-point in the supercritical power plant (between 43-45% net plant efficiency depending on balance of plantconfiguration), and in the subcritical plant configuration (~40% net plant efficiency). Regarding the balance of plant configuration, direct reheating was adopted as the optimum solution to avoid any intermediate heat exchanger. One direct reheating stage between high pressure turbine and intermediate pressure turbine is the common practice; however, General Electric ultrasupercritical(350 bar) fossil power plants also considered doubled-reheat applications. In this study were analyzed heat balances with single-reheat, double-reheat and even three reheating stages. In all cases were adopted the proper reheating solar field configurations to limit solar collectors pressure drops. As main conclusion, it was confirmed net plant efficiency improvements in supercritical Rankine line-focus (parabolic or linear Fresnel) solar plant configurations are mainly due to the following two reasons: higher number of feed-water preheaters (up to seven)delivering hotter water at solar field inlet, and two or even three direct reheating stages (550ºC reheating temperature) in high or intermediate pressure turbines. However, the turbine manufacturer should confirm the equipment constrains regarding reheating stages and number of steam extractions to feed-water heaters

Does Two-Class Training Extract Real Features? A COVID-19 Case Study

Diagnosis aid systems that use image analysis are currently very useful due to the large workload of health professionals involved in making diagnoses. In recent years, Convolutional Neural Networks (CNNs) have been used to help in these tasks. For this reason, multiple studies that analyze the detection precision for several diseases have been developed. However, many of these works distinguish between only two classes: healthy and with a specific disease. Based on this premise, in this work, we try to answer the questions: When training an image classification system with only two classes (healthy and sick), does this system extract the specific features of this disease, or does it only obtain the features that differentiate it from a healthy patient? Trying to answer these questions, we analyze the particular case of COVID-19 detection. Many works that classify this disease using X-ray images have been published; some of them use two classes (with and without COVID-19), while others include more classes (pneumonia, SARS, influenza, etc.). In this work, we carry out several classification studies with two classes, using test images that do not belong to those classes, in order to try to answer the previous questions. The first studies indicate problems in these two-class systems when using a third class as a test, being classified inconsistently. Deeper studies show that deep learning systems trained with two classes do not correctly extract the characteristics of pathologies, but rather differentiate the classes based on the physical characteristics of the images. After the discussion, we conclude that these two-class trained deep learning systems are not valid if there are other diseases that cause similar symptoms.Junta de Andalucía and FEDER research project MSF-PHIA (US-1263715

Verificación experimental de las correlaciones de transferencia de calor por ebullición en película, en piscina, en torno a esferas

La ebullición en película es el mecanismo de transferencia de calor básico que acopla térmicamente un líquido y una superficie caliente cuando existe una gran diferencia de temperatura entre ambos. El conocimiento preciso del comportamiento de este mecanismo térmico en torno a esferas es necesario para el análisis de seguridad de escenarios industriales en los que exista contacto entre un líquido y un material fundido fragmentado, generalmente en piezas esferoidales, y resulta esencial para garantizar la seguridad de los reactores nucleares ante escenarios accidentales de muy baja probabilidad, pero de gran severidad, en los que se postule la rotura en guillotina del circuito de refrigeración del reactor simultánea al fallo total activo del sistema de refrigeración de emergencia del núcleo. En tal hipotético escenario, se produciría la fusión del combustible en el plazo de algunas horas, con dispersión y relocalización de fragmentos sólidos esferoidales a muy alta temperatura. Para hacer frente a este escenario se precisa desarrollar procedimientos automáticos y manuales de operación de emergencia, resultando imprescindible disponer de modelos térmicos confiables, con un calificado soporte experimental, que permitan analizar de forma realista la refrigeración por ebullición en película, en modo ebullición en piscina, de las geometrías esferoidales sólidas resultantes. En el presente trabajo UNET-UPM abordan la verificación de las correlaciones más conocidas para ebullición en película en piscina en torno a esferas, mediante la comparación de las mismas con los resultados experimentales obtenidos por Liu-Theofanous. Algunos de los aspectos resaltantes son la limitada aplicabilidad de la correlación empírica de Frederking-Clark, el buen ajuste que muestran las correlaciones de Tou- Tso y de Grigoriew frente a los datos experimentales usados, y las deficiencias que muestran las correlaciones que toman en cuenta el subenfriamiento del líquido, sobre todo a altos niveles de subenfriamiento. En una futura segunda fase de esta investigación, de tipo analítica-numérica-experimental, se abordará en el desarrollo de nuevas correlaciones semi-empíricas, de mejor ajuste, que permitan una mejor capacidad predictiva en los modelos

Comparison of Different Technologies for Integrated Solar Combined Cycles: Analysis of Concentrating Technology and Solar Integration

This paper compares the annual performance of Integrated Solar Combined Cycles (ISCCs) using different solar concentration technologies: parabolic trough collectors (PTC), linear Fresnel reflectors (LFR) and central tower receiver (CT). Each solar technology (i.e. PTC, LFR and CT) is proposed to integrate solar energy into the combined cycle in two different ways. The first one is based on the use of solar energy to evaporate water of the steam cycle by means of direct steam generation (DSG), increasing the steam production of the high pressure level of the steam generator. The other one is based on the use of solar energy to preheat the pressurized air at the exit of the gas turbine compressor before it is introduced in the combustion chamber, reducing the fuel consumption. Results show that ISCC with DSG increases the yearly production while solar air heating reduces it due to the incremental pressure drop. However, air heating allows significantly higher solar-to-electricity efficiencies and lower heat rates. Regarding the solar technologies, PTC provides the best thermal results.Authors acknowledge the financial support of the Spanish Ministry of Economy and Competitiveness to ENE2015-70515-C2-1-R and ENE2015-70515-C2-2-R projects

Comparativa de distintos métodos para el cálculo de la incidencia de potencia en campos solares de torre central

El objetivo fundamental del trabajo que se presenta es la propuesta de herramientas i formáticas de simulación y post-proceso de plantas termosolares de torre que, derivando de las técnicas ray-tracing aporten, sin merma apreciable de la precisión, una reducción de los tiempos de ejecución, pues se trata de uno de los principales inconvenientes del método. En primer lugar se exponen las hipótesis de partida de los códigos desarrollados. Se continuará realizando un acercamiento a los errores ópticos involucrados en los problemas de simulación. Posteriormente se exponen los esquemas de operación de los códigos presentados. Finalmente se llevará a cabo un estudio de los tiempos de ejecución y de precisión. En el presente documento se presentarán dos códigos de simulación de plantas termosolares de torre con receptor de cavidad. Ambos se han desarrollado en lenguaje de programación propio de MATLAB. El primero de los cuales se considera un código ray-tracing convencional (código RT), optimizado para cálculo paralelo y basado en la proyección de un solo rayo incidente encada punto de cálculo elegido sobre la superficie reflectante del heliostato. Sus prestaciones y resultados se emplearán como modelo de referencia. El segundo código presentado (código RT-HR) considera que en un mismo punto de la superficie reflectante del heliostato se aplican más de un rayo incidente.Los autores desean agradecer al Ministerio de Economía y Competitividad la financiación proporcionada al trabajo, a través de los proyectos de Plan nacional de I+D+i ENE2012-37950-C02- 01 y ENE2012-37950-C02-02