http://hdl.handle.net/10366/138607 2026-04-21T19:08:18Z http://hdl.handle.net/10366/139972 [EN]The optimization of energy converters using different objective functions has become a relevant issue due to the energetic needs of the everyday most specialized heat devices. The stability of the operation regime of such devices is always addressed as a separate subject. Thus, the link between the robustness of the steady state and the optimization process, as well as the thermodynamic consequences of limited control on the operation regime is still an ongoing topic with phenomena yet to understand [1]. In order to provide a somewhat general view for a class of heat devices, not attached to specific heat transfer mechanism, we make use of the so-called low-dissipation refrigerator engine [2-7]. In this work we analyse random perturbations on a low-dissipation refrigerator engine’s operation regime. For the operation regime we use the Omega function, which represent a compromise between cooling power and efficiency. We found that the trajectories to the steady state follow paths in which the most relevant thermodynamic functions are improved. And under continuous perturbations the resulting stochastic trajectories follow a statistical behaviour in which the role of the thermodynamic functions play a most relevant role. The results obtained from this study are reinforced by a multi-objective optimization study of this refrigerator engine, in which it is required a simultaneous optimization of all the relevant thermodynamical quantities, that is, efficiency (coefficient of performance), cooling power, power input, entropy production and the Omega function. We also make use of the Kullback-Leibler divergence to account for statistical convergence in the obtained numerical results. REFERENCES [1] M. Bauer, K. Brandner, and U. Seifert, Optimal performance of periodically driven, stochastic heat engines under limited control, Phys. Rev. E 93, 042112 (2016). [2] M. Esposito, R. Kawai, K. Lindenberg and C. Van den Broeck, Efficiency at maximum power of low-dissipation Carnot engines. Phys. Rev. Lett. 105, 150603 (2010) [3] Calvo Hernández A, Medina A and Roco J M M 2015 Time, entropy generation, and optimization in low-dissipation heat devices New J. Phys. 17 075011 [4] Viktor Holubec and Artem Ryabov, Efficiency at and near maximum power of low-dissipation heat engines, Phys. Rev. E 92, 052125 (2015) [5] Viktor Holubec and Artem Ryabov, Maximum efficiency of low-dissipation heat engines at arbitrary power, J. Stat. Mech. , 073204 (2016). [6] J. Gonzalez-Ayala, A. Calvo Hernández, J. M. M. Roco, Irreversible and endoreversible behaviors of the LD-model for heat devices: the role of the time constraints and symmetries on the performance at maximum χ figure of merit, J. Stat. Mech. 2016, 073202 (2016). [7] J. Gonzalez-Ayala, M. Santillán, I. Reyes-Ramírez and A. Calvo Hernández, Link between optimization 2019-05-01T00:00:00Z http://hdl.handle.net/10366/139956 [EN]Concentrated solar power plants, a renewable energy technology, constitute one of the best ways of producing dispatchable and clean energy. In central receiver plants, a heliostats field concentrates the Sun energy into a receiver located in the top of a central tower. This receiver transfers the solar heat to a power cycle. The present work is focused on modeling the heliostat field of the power plant. For that, some geometric and size parameters of the receiver, the tower and the heliostats are taken into account. In the solar energy transfer process, there exist several losses factors as the solar radiation blocking between heliostats or the solar radiation attenuation due to the atmospheric particles. The heliostats field model has been implemented in Mathematica®, creating our own software. For the model validation process, Campo Code software (a standard well-accepted package) [1] has been employed. Results from our model have been compared to the ones of Campo Code getting similar values. Optical efficiency has been evaluated at the design point (see Fig. 1) and at dynamic conditions through different seasons and along a day. For the design point (21st June 2013 at 12h), this efficiency gets values around 0.674. Heliostats field efficiency maps for the different simulations show that the highest efficiency is always related to the heliostats opposite the Sun, which agrees with the results obtained by W. Stine and M. Geyer [2]. 2019-09-01T00:00:00Z http://hdl.handle.net/10366/139954 [EN]This work provides an integrated design of a small-scale hybrid solar power plant aimed at distributed generation of electrical energy. This technology may be especially interesting for remote areas with no access to electricity and advantageous solar conditions. The inherent limitations of a solar-only power plant (seasonal and meteorological sun fluctuations, nights) may be overcome with a hybrid operation mode. These systems can work uninterruptedly with an approximately constant power output, since the pressurized air of the cycle is heated from the concentrated solar irradiance and, when necessary, from the combustion of a fossil fuel. Then, the transformation of thermal energy to mechanical one is carried out by means of a Brayton thermodynamic cycle and a system of alternators. 2019-10-01T00:00:00Z http://hdl.handle.net/10366/139953 [EN]The present work performs a techno-economic analysis of an innovative solar-hybrid combined cycle composed of a topping gas turbine coupled to a bottoming packed bed thermal energy storage at the gas turbine exhaust, which runs in parallel to a bottoming steam cycle. Plant performances have been evaluated in terms of the capacity factor, the specific CO2 emissions, the capital expenditure, and the Levelised Cost of Electricity. The influence of the combustion chamber outlet temperature, solar multiple and energy storage capacity has been assessed by means of a sensitivity analysis. The present study also compares the previously listed performance against that of conventional molten salt tower Concentrating Solar Power plants and traditional combined cycle gas turbine power plants with equivalent installed capacities and load factors. The results show that it is worth hybridizing the system, particularly at high combustion chamber outlet temperature, large storage size and solar multiple. Furthermore, plant configurations leading to a Levelised Cost of Electricity lower than 110 $/MWh can be achieved for a capacity factor of about 60%. Under these working conditions, the proposed configuration would be only 1.66 times more costly than an equivalent size CCGT. At the same time, it would yield less than half of the emissions of the latter. Simultaneously, the proposed layout is considerably cheaper than an equivalent molten salt Concentrating Solar Power plant. 2019-01-01T00:00:00Z http://hdl.handle.net/10366/139620 [EN]Small-scale hybrid parabolic dish Concentrated Solar Power (CSP) systems coupled to a micro-gas turbine are a promising option to obtain electrical energy in a distributed manner. During the day, solar energy is used to produce electricity and the absence of sunlight can be overcome with the combustion of a fossil or renewable fuel. This study presents the technical feasibility and thermo-economic model of a hybridized power plant in different regions of Spain, considering the local climatic conditions. The implemented model aims to provide a realistic view of the behaviour of the system, using a reduced number of selected parameters with a clear physical meaning. The irreversibilities taking place in all subsystems (solar part, combustion chamber, micro-gas turbine, and the corresponding heat exchangers) have been considered in the model, developed in Mathematica® language. The model considers the instant solar irradiance and ambient temperature dynamically, providing an estimation of the power output, the associated fuel consumption, and the most relevant pollutant emissions (CO2, CH4 and NO2) linked to combustion, for hybrid and combustion only operating modes at selected geographical locations in Spain. The considered power output ranges between 7 to 30 kWe which is achieved by varying the design specifications. The levelized cost of electricity (LCoE) indicator is estimated as a function of investment, interest rate, maintenance and fuel consumption actual costs in Spain. The electricity costs from hybrid parabolic dish are between 22% and 27% lower compared to pure combustion power plant, while specific fuel consumption and therefore CO2 emissions can be reduced up to 33%. This model shows the potential of hybrid solar dishes to become cost-competitive against non-renewable ones from the point of view of electricity costs and significant reduction in gas emission levels in regions with high solar radiation and low water resources. 2019-06-01T00:00:00Z http://hdl.handle.net/10366/139619 [ENConcentrated solar power (CSP) is one challenging renewable technology for the future production of electricity. Within this concept central receiver solar plants combined with gas turbines are being investigated because of their promising efficiencies and reduced water consumption. Hybrid plants incorporate a combustion chamber in such a way that in periods of low solar irradiance power output can be kept approximately constant and so, electricity production is predictable. An integrated, non-complex solar thermodynamic model of a hybrid multi-stage gas turbine solar plant is developed employing a reduced number of parameters with a clear physical meaning. The solar subsystem is modelled in detail, taking into account the main heliostats field losses factors as cosine effect, blocking, or attenuation. The model is implemented in our own software, developed in Mathematica® language, considering as reference Gemasolar solar field (Seville, Spain). First, an on-design analysis is performed for four different working fluids (dry air, nitrogen, carbon dioxide, and helium), for different number of expansion and compression stages, and for recuperative and non-recuperative modes. Moreover, heliostats field configuration is determined for the design point and its associated efficiency is computed. A pre-optimization process is carried out regarding the pressure ratio of the gas turbine for different configurations. Some significant efficiency and power rises can be obtained when pressure ratio is adapted for each specific configuration and working fluid. Three particular plant configurations are chosen for the off-design analysis due to their interesting behaviours. For these configurations, a dynamic study is performed for four representative of each season. Then, efficiencies and solar share are plotted against time. In addition, fuel consumption and greenhouse emissions are computed for all seasons. Heliostats efficiency varying with the season and the solar time is also forecasted. Keywords: Dynamic analysis, On-design pre-optimization, Multi-stage gas 2019-06-01T00:00:00Z http://hdl.handle.net/10366/138855 [EN]A thermodynamic model for a hybrid thermo-solar externally fired gas turbine power plant fueled with biomass is presented. This technology represents a fully renewable way of obtaining electric power relying mainly in biomass with an extra advantage of decreasing its consumption at good radiative conditions. A concentrated solar power plant is implemented considering the Solugas project situated in Abengoa’s Solúcar Platform near Seville. To predict its behavior two models were implemented and compared with experimental data. Typical daily evolution of the output power, biomass mass flow, overall efficiency and biomass conversion efficiency for a typical Uruguayan year is presented. In addition, global results are presented leading to a 1.5% biomass saving increasing the economic efficiency a 0.34% what denotes the solar field and thermal power of receiver seem to be undersized. 2018-11-01T00:00:00Z http://hdl.handle.net/10366/138639 [EN]The necessity to diversify the energy sources in power generation and to look for renewable ones is undoubted. Thermosolar power plants, which constitute one of the main ways of solar energy exploitation, are competing with other renewable energy sources for generating clean electrical energy, reducing fuel consumption. Hybrid thermosolar plants combine two great advantages on electricity generation: the emissions reduction of thermosolar energy, as well as the stable supply of power output to the grid of conventional power plants, avoiding the use of storage systems. For those reasons in the last years a big effort has been done in the development of prototypes and experimental plants in order to investigate the viability of thermosolar hybrid Brayton cycle plants. A working fluid, usually air, is preheated by concentration solar energy, before entering a combustion chamber. Then, the fluid performs a thermodynamic cycle (in this case, a Brayton cycle), generating electrical energy indirectly. In this way fossil fuel and the associated emissions are reduced. It is important to note that apart from being easily scalable, gas-turbines can be combined with other cycles like bottoming Rankine. Also they do not require too much water for operation, which makes them suitable for electrical generation in arid regions, and are extremely versatile [1]. Experimental projects and prototypes developed up to date show that this technology is viable, but they also reveal that it is necessary to improve their efficiency, in order to generate electricity at competitive prices. Apart from R+D projects, prototypes, and experimental installations, several research works have been published in the last times. Some of them make use of commercial simulation environments, which allow a detailed description of all plant components and specific calculations on the solar subsystem. However, it is not easy to extract direct physical information about the main losses sources in the plant and to perform a global optimization of the plant design. Because of this reason, in this paper the next modus operandi is followed instead of this one. A second type of strategy is to build a theoretical model of the plant, in terms of a reduced number of parameters, allowing a simple but realistic picture of plant operation and to estimate its performance records. Thermodynamic analyses can provide an integrated point of view of all subsystems and their importance in the overall efficiency. Moreover, they help to predesign future generations of plants based in this concept because of their flexibility to survey the adequate intervals of key parameters for optimal plant operation. There are several theoretical works that start from the ideal Brayton cycle and thereafter refinements are included in the analysis of the thermodynamics of the cycle in order to recover realistic output records. Usually, in these works, the model for the concentrated solar subsystem, although including the main heat transfer losses, is simple. This allows to obtain closed analytical expressions for thermal efficiencies and power output, and then check the model predictions for particular design point conditions, with fixed values of direct solar irradiance and ambient temperature. But also by means of this thermodynamic model, a dynamic analysis that varies solar irradiance and external temperature conditions with time can be carried out. And in a possible step forward to suggest and guide optimization strategies. 2017-01-01T00:00:00Z http://hdl.handle.net/10366/138638 [EN]During the last years a significant R&D&i effort to develop energy production technologies capable to provide clean and efficient generation on one side, and reliable, non-intermittent, and predictable on the other is being done. In thermosolar power plants with a central tower receiver a working fluid runs a thermodynamic heat engine as a Rankine or Brayton one (or a combination of both) [1]. In these plants it is feasible to include a combustion chamber in series with the solar receiver in such a way that during low irradiance periods due to meteorological conditions or during the nights, the combustion chamber can provide the heat input required to keep approximately constant the turbine inlet temperature. And so, the plant power output. These plants are not completely carbon free, because of the combustion of a fossil fuel (usually natural gas) but produce 2018-09-01T00:00:00Z http://hdl.handle.net/10366/138636 [EN]This work provides an integrated design of a small-scale hybrid solar power plant aimed at distributed generation of electrical energy. This technology may be especially interesting for remote areas with no access to electricity and advantageous solar conditions. The inherent limitations of a solar-only power plant (seasonal and meteorological sun fluctuations, nights) may be overcome with a hybrid operation mode. These systems can work uninterruptedly with an approximately constant power output, since the pressurized air of the cycle is heated from the concentrated solar irradiance and, when necessary, from the combustion of a fossil fuel. Then, the transformation of thermal energy to mechanical one is carried out by means of a Brayton thermodynamic cycle. The main purpose in this work is to analyse the performance of the system for any real environmental and geographical conditions, through a thermodynamic model based on a reduced number of parameters, each one of clear physical significance. This is complemented with a thermoeconomic analysis, allowing 2018-09-01T00:00:00Z http://hdl.handle.net/10366/138634 [EN]En este trabajo se realiza una simulación válida para plantas termosolares híbridas, de ciclo Brayton y de torre de concentración, desde un punto de vista termodinámico. Estas plantas híbridas emplean dos fuentes principales de energía para su funcionamiento: la energía termosolar y la energía proveniente de una cámara de combustión. Por lo que con este tipo de plantas de generación de energía eléctrica se pretende reducir el consumo de combustibles y la emisión de contaminantes, así como conseguir una potencia neta de salida estable. Se presenta un modelo termodinámico para una planta multietapa, con la idea de valorar la eficiencia y generación de energía en diferentes configuraciones de la planta y para diversos fluidos de trabajo, proponiendo mejoras respecto a una planta termosolar híbrida tipo Brayton monoetapa. 2018-09-01T00:00:00Z http://hdl.handle.net/10366/138633 [EN]Este trabajo presenta un modelo termodinámico que reproduce el funcionamiento de una planta termosolar de disco parabólico híbrido tipo Brayton que incluye recuperación. Este sistema puede operar ininterrumpidamente, ya que la energía necesaria para su funcionamiento proviene de dos fuentes diferentes: una renovable e inagotable, la energía solar, y otra convencional, que es la combustión de un combustible. La gran ventaja de este tipo de planta es que puede trabajar de manera autónoma y descentralizada, permitiendo su instalación en cualquier parte del mundo con alta irradiación solar, dando energía accesible a toda la población. El modelo analiza el rendimiento de cada subsistema y el global de la planta termosolar de manera dinámica en cualquier condición ambiental y ubicación geográfica, pudiendo así evaluar y optimizar cada componente.; [EN]In this work we present a thermodynamic model that describes the performance of a power plant based on a hybrid recuperative Brayton-type parabolic dish. One of the characteristics of parabolic dishes operating in hybrid mode is that they can work uninterruptedly, since the energy necessary for their operation comes from two different sources: a renewable one, the solar system, and a conventional one, the combustion of a fuel. The idea that motivates the development and optimization of these systems is the distributed generation of electrical energy, that is, the installation of electricity producing sources near where it is going to be used. The model analyses the efficiency of each subsystem and the overall thermosolar plant performance in a dynamic way , in any geographical location and environmental condition, allowing the evaluation and optimization of the different components of the plant. 2018-01-01T00:00:00Z http://hdl.handle.net/10366/138632 [EN]In this work a thermodynamic model that describes the performance of a power plant based on a hybrid recuperative Brayton-type parabolic dish is presented. The model is capable to analyse the performance of such plants at off-design conditions. One of the characteristics of parabolic dishes operating in hybrid mode is that they can work uninterruptedly, since the energy necessary for their operation comes from two different sources: a renewable one, the solar system, and a conventional one, the combustion of a fossil fuel. The use of a renewable energy source allows for the reduction of the fuel consumption and consequently of the emissions of pollutant gases. The distributed generation of electrical energy (that is, the installation of electricity production sources near where it is going to be used) motivates the development and optimization of these systems. The transformation of thermal energy to mechanical one is carried out by means of a Brayton thermodynamic cycle. The irreversibilities taking place in all subsystems (solar part, combustion chamber, micro-gas turbine, and the corresponding heat exchangers) have been considered in the model with home-software elaborated using Mathematica. The model is validated by comparing with several results from the literature. Subsequently, an analysis is made for two operating conditions: with and without solar contributions. Four days are analysed, each of them corresponding to a season and for four different micro-turbine power outlets (30, 23, 15 and 7 kWe). In addition, an off-design study of the behaviour of the system is made for a representative day. An estimation of the greenhouse effect emissions is made, comparing the operation with and without solar power input. 2018-01-01T00:00:00Z http://hdl.handle.net/10366/138626 [EN]A thermodynamic model for hybrid Brayton thermosolar plants is proposed with the aim to analyze possible configurations with improved performance. In these plants an array of mirrors with a two-axis tracking system gathers solar power and redirects it to a central receiver. In turn the receiver acts as a heat exchanger that heats up a gaseous working fluid that runs a Brayton-like cycle. These plants also include a combustion chamber that ensures an approximately constant power output even during night or in periods with poor solar irradiance. Throughout the last years it has been demonstrated by means of experimental projects and prototypes that this concept is technically feasible but still R+D+i efforts are required in order to reach commercial feasibility. From the thermodynamic viewpoint it is necessary to increase overall plant efficiency. The model proposed in this paper is an extension of previous studies from our group that takes into consideration multi-stage configurations with an arbitrary number of compression steps with intercooling and expansion stages with reheating between turbines. The model is comprehensive and includes the main sources of losses in real plants: pressure decays in heat absorption and release, losses in compressors, turbines and heat exchangers, non-ideal recuperators and, of course, losses in the solar subsystem and combustion chamber. A numerical application is done taking as reference the data from the project Solugas, developed by the Abengoa Solar at the south of Spain. Several plant configurations are analyzed and also different working fluids checked, including air, nitrogen, carbon dioxide, and helium at subcritical conditions. It is concluded that for air, nitrogen and carbon dioxide, plant configurations with 2-3 compression/expansion steps are capable of achieving improved overall plant thermal efficiency (about 25% above single step plants) and also fuel conversion efficiency, i.e., lead to a considerable increase in power output without an appreciable increase in fuel consumption. 2018-01-01T00:00:00Z http://hdl.handle.net/10366/138624 [EN]A thermodynamic model for a Brayton-like microturbine in combination with a solar parabolic dish is analyzed in order to evaluate its efficiency under any ambient condition. The thermodynamic cycle is a recuperative Brayton cycle with internal irreversibilities in the recuperator, compressor and turbine and external losses associated to the heat transfers in the solar receiver, the combustion chamber, and the environment. All the irreversibilities have been taken into account in the model with home-software elaborated using Mathematicaâ. The model validation is done by comparison with results provided by Semprini et al. [1]. An analysis of hybrid and sunless performance is carried out for four different microturbine power outlets (30, 23, 15 and 7 kWe) and for four days of the year (corresponding to each season). The greenhouse emissions are also calculated for both off-design performance and for the four power output levels. 2018-03-19T00:00:00Z http://hdl.handle.net/10366/138623 [EN]In this communication a general thermodynamic model for hybrid Brayton thermosolar plants is proposed. The model is flexible and capable to include multistage configurations and recuperation. During the last years was proved that this kind of plants is technically feasible but R+D efforts need to be done in order to improve its commercial feasibility. From the thermodynamic viewpoint it is necessary to increase its overall efficiency. A general model that allows to simulate recuperative or non-recuperative plants, with an arbitrary number of stages and working with different subcritical fluids is presented. Numerical results for multi-step configurations are compared with those for a reference experimental plant, developed during the last years near Seville. Different working fluids and several plant layouts are analyzed. 2018-03-19T00:00:00Z