• Energy Research

Artículos en revistas . Biomass-fueled Organic Rankine Cycle power plants in a cogeneration topping layout have been operated in Central Europe since 2000. These plants are usually integrated into a district heating system and located near to the villages whose thermal and electric energy demands are to be covered. In this paper, a technical and economic feasibility assessment of this kind of plants is presented. The energy performance has been analyzed in different scenarios. Four different typical organic fluids (two silicone oils, toluene and isopentane), subcritical and supercritical cycles and the inclusion of a recuperator have been considered. Thermal and electric energy are sold to a nearby village at competitive market prices. Spanish market prices have been used as a reference. No subsidies have been considered in the case of electricity, so that the Spanish average power pool market price has been considered. The size of the plant, the cost of biomass and the annual operation schedule have been considered for the economical analysis. According to the technical analysis, hexamethyldisiloxane (HMDSO) in recuperative cycles has turned out to be the best choice in both the subcritical and the supercritical layouts, due to its favorable global behavior (harmfulness, reliability and efficiency). The economic assessment shows a lower profitability in the case of supercritical cycles because of the fact that the increase in electric efficiency implies a decrease in the amount of produced useful heat, which is the main source of cash inflow. The size of the plant can be established according to the cost of fuel in order to achieve a similar profitability (i.e. a 1 MWe plant fueled with biomass priced at 5.5 V/MWhth has a similar internal rate of return than a 2 MWe plant fueled with biomass priced at 15.5 V/MWhth). In order to obtain a 5% internal rate of return with subcritical recuperative plants, the annual operation time must be 2750 h in the case of a 2 MWe plant fueled with biomass priced at 5.5 V/MWhth and 5500 h in the case of a 1 MWe plant fueled with biomass priced at 15.5 V/MWhth. info:eu-repo/semantics/publishedVersion

Artículos en revistas A domestic research program called TECNO-FUS was launched in Spain in 2009 to support technological developments related to a dual-coolant (He/Pb-Li) breeding blanket design concept. One of the goals of the project was the analysis of a suitable power conversion system with an enhanced coupling with the reactor heat sources. Each source has a different thermal level which generates many problems in the coupling. In previous works the authors have explored enhanced power cycles, taken from literature, which solve the differences in the thermal levels of the sources with combined or dual cycles. Although these cycles reach high efficiencies (between 45% and 47%) their layout is very complex and the use of steam is required. In this paper a new power conversion cycle is proposed. It avoids the use of complex layouts, being a variant of the supercritical CO2 Brayton cycle matched to the available thermal sources through an extra recuperator. The basic supercritical CO2 Brayton cycle has been also analyzed for comparison. The new cycle has been optimized so that efficiencies above 47% have been achieved. info:eu-repo/semantics/publishedVersion

Artículos en revistas Las centrales térmicas solares acopladas a ciclos supercríticos de CO2 parecen ser una forma de aumentar la eficiencia global de la energía solar a la eléctrica. Para ello, la tecnología solar de concentración que mejor se integra es el receptor central de sales fundidas con un almacenamiento de energía térmica asociado. Este trabajo se centra en uno de los principales desafíos de este esquema: el intercambiador de calor de la fuente que transfiere la energía térmica de la sal fundida en el campo solar al CO2 en el ciclo de potencia. Se propone un nuevo diseño, basado en la tecnología del intercambiador de calor de circuito impreso, que soporta la diferencia de presión y evita el taponamiento de la sal fundida cuando circula por los microcanales. También se calcula el modelo termomecánico de este intercambiador de calor. Este trabajo también aborda una optimización termo-económica del intercambiador de calor de circuito impreso propuesto. Para ello, se considera el rendimiento global de la planta termosolar para tres disposiciones: recompresión, interrefrigeración y ciclos de enfriamiento parcial. Esta optimización permite una gran reducción del coste de inversión de estos intercambiadores de calor de fuente, consiguiendo el menor coste en la configuración de enfriamiento parcial, seguido de la interrefrigeración y finalmente, la recompresión. Esta tendencia también se observa en el rendimiento global de la planta solar, por lo que la disposición de enfriamiento parcial es la que tiene el menor coste normalizado de electricidad; este valor es similar al de la disposición de interrefrigeración, y ambos están muy por debajo del coste en la disposición de recompresión, que resulta la configuración más cara. Solar thermal power plants coupled to supercritical CO2 cycles seems to be a way to increase the global solar-to-electric efficiency. For that, the concentrating solar technology that is best integrated is the molten salt central receiver with a thermal energy storage associated. This work is focused on one of the main challenges of this scheme: the source heat exchanger transferring the thermal energy from the molten salt in the solar field to the CO2 in the power cycle. A new design, based on the printed circuit heat exchanger technology is proposed, that withstands the pressure difference and avoids the molten salt plugging when circulating through microchannels. The thermo-mechanic model of this heat exchanger is also calculated. This work also addresses a thermo-economic optimization of the printed circuit heat exchanger proposed. For that, it is considered the global performance of the solar thermal plant for three layouts: recompression, intercooling and partial-cooling cycles. This optimization yields to a great reduction in the investment cost of these source heat exchangers, achieving the lowest cost in the partial-cooling configuration, followed by the intercooling and finally, the recompression. This trend is also observed in the global performance of the solar plant, so the partial-cooling layout is the one with the lowest levelized cost of electricity; this value is similar to that of the intercooling layout, and both are well below from the cost in the recompression layout, which results the most expensive configuration. info:eu-repo/semantics/publishedVersion

Artículos en revistas . This century power engineering is facing up to one of the greatest challenges ever posed to humankind: the achievement of a sustainable energy system. In order to respond to this challenge, nuclear technology is designing a new generation of power plants termed Generation IV, among them high temperature gascooled reactors stand out for their potential capability to achieve an excellent thermal performance. This paper investigates the thermal and economic performance of several direct Brayton cycle configurations that could be used in future HTGRs, with special attention to the effects of inter-cooling and reheating. Among the hypotheses and assumptions taken, the adoption of the PBMR reactor parameters and settings as a reference is particularly important. All inter-cooled layouts have shown thermal efficiencies near or even higher than 50%, which means a substantial improvement with respect to non-intercooled baselines with no economic penalties. Reheating has been shown not to affect remarkably the thermal or economic plant performance under base-load operation, but it provides the plant with such a flexibility that allows its operation under the load-follow regime without heavily taxing the thermal or economic performance. Anyway, use of a multiple axes configuration instead of a single one seems to worsen plant economics and not to entail any thermal benefit. info:eu-repo/semantics/publishedVersion

Artículos en revistas . This paper investigates Brayton power cycles for fusion reactors. Two working fluids have been explored: helium in classical configurations and CO2 in recompression layouts (Feher cycle). Typical recuperator arrangements in both cycles have been strongly constrained by low temperature of some of the energy thermal sources from the reactor. This limitation has been overcome in two ways: with a combined architecture and with dual cycles. Combined architecture couples the Brayton cycle with a Rankine one capable of taking advantage of the thermal energy content of the working fluid after exiting the turbine stage (iso-butane and steam fitted best the conditions of the He and CO2 cycles, respectively). Dual cycles set a specific Rankine cycle to exploit the lowest quality thermal energy source, allowing usual recuperator arrangements in the Brayton cycle. The results of the analyses indicate that dual cycles could reach thermal efficiencies around 42.8% when using helium, whereas thermal performance might be even better (46.7%), if a combined CO2 H2O cycle was set. info:eu-repo/semantics/publishedVersion

This work presents a novel design of microchannel central receiver for pressurised gases and supercritical fluids in solar tower plants. It consists of a radial arrangement of vertical absorber panels that converge on the central axis of the tower. The absorber panels comprise compact structures, whose compactness is increased in one flow pass compared to the previous one, as the fluid is heated. This concept reduces radiation heat losses due to its light-trapping geometry and increases heat transfer to the thermal fluid without over penalising its pressure drop. For the receiver assessment, it has been developed a thermal resistance model characterising the fluid heating along the panel height and the temperature gradient between parallel channel rows of the compact structure across the panel thickness. Once the thermal and optical boundary conditions are defined, an optimisation analysis of the main geometrical parameters of the receiver has been accomplished. The receiver performance is evaluated by means of a global exergy efficiency referred to the solar subsystem, which computes the receiver heat losses, the fluid pressure drop and the optical efficiency of the heliostat field in which the receiver is integrated. For each parametric optimisation, the configuration that maximises this efficiency is identified.

Solar thermal power plants are an alternative for the future energy context, allowing for a progressive decarbonisation of electricity production. One way to improve the performance of such plants is the use of supercritical CO2 power cycles. This article focuses on a solar thermal plant with a central solar receiver coupled to a partial cooling cycle, and it conducts a comparative study from both a thermal and economic perspective with the aim of optimising the configuration of the receiver. The design of the solar receiver is based on a radial configuration, with absorber panels converging on the tower axis; the absorber panels are compact structures through which a pressurised gas circulates. The different configurations analysed keep a constant thermal power provided by the receiver while varying the number of panels and their dimensions. The results demonstrate the existence of an optimal configuration that maximises the exergy efficiency of the solar subsystem, taking into account both the receiver exergy efficiency and the heliostat field optical efficiency. The evolution of electricity generation cost follows a similar trend to that of the exergy efficiency, exhibiting minimum values when this efficiency is at its maximum.