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This paper aims to evaluate the potential for electricity and ethanol production in Central America using sweet sorghum, performing a techno-economic analysis. The study proposes the integration of sweet sorghum into Central American sugar mills, by using the existing machinery to process this crop during off-season. A process simulation and a cost model were developed to estimate the technical and economical feasibility of sweet sorghum integration. The data on various parameters used for techno-economic assessment were collected from an existing sugar mill and distillery in Central America. The results show that a sugar mill operating 2 months during off-season could obtain an average revenue of US$ 3 M for a crushing rate of 6500 t/d. Ethanol production costs are estimated to be 24.76 ¢US$/L. In case a new CHP plant is built, a sugar mill operating under the integrated scenario would have a payback period of 4.49 years, as compared to 7.47 years for a sugar mill using sugarcane bagasse as the only fuel. Although several studies highlight the potential of sweet sorghum for ethanol production, the results from this work prove that sweet sorghum must also be seen as a viable feedstock for electricity production. A sensitivity analysis was also performed to determine the variation of the average cost of electricity and ethanol with the variables used in the economic analysis. For all analysed scenarios the effects of installed capacity and crop yield prevailed over the increasing costs of land and transportation. [ABSTRACT FROM AUTHOR]

Abstract: One of the main problems of solar power tower plants with molten salt as heat transfer fluid is the reliability and lifetime estimation of central receivers. The receivers must withstand high working temperatures, molten salt corrosion and important solar flux transients that lead to thermal stresses and fatigue. Therefore, it is necessary optimize the design of these receivers. The aim of this work is to study the thermal behavior of a new design of central receiver. This new external receiver is composed by several vertical panels formed by bayonet tubes. Each bayonet tube presents an inner tube and an outer tube concentric to the internal one. Both are joined in one side by a bayonet end-cap. The heat transfer fluid flows through the inner tube and the annular gap between the two tubes. In addition, the outer tube will be coated in its external surface by a material of high absorptivity of the solar radiation. In absence of experimental data of the receivers both concepts of external receiver, traditional and bayonet, have been characterized using simplified numerical simulations. The results obtained have been the salt, the film and the wall temperature evolutions and the thermal efficiencies. The advantages of the bayonet receiver over the traditional tubular receiver are: soften wall and film temperatures, lower thermal stresses due to a more homogeneous wall temperature, and lower corrosion rate, that allows use more economical materials to built the receiver tubes or even reduce the receiver surface increasing the thermal efficiency. [Copyright &y& Elsevier]

Abstract: The tubes of central receiver power plants are designed to work under very demanding conditions, with heat fluxes varying between 0.2 to 1 MW/m2, depending on both the weather conditions and the operating regime of the plant. Also, the heat flux is localized along the circumferential coordinate of the tube, on the outward facing side of the tube. This results in a non-uniform temperature distribution with strong gradients both in the heat transfer fluid and the pipe walls. Predicting this distribution is important since it can induce high thermal stresses on the pipe walls and it is a key factor for the fluid decomposition. In this work we have studied different configurations to analyze the effects of tube diameter and wall thickness in the temperature distributions in the fluid and solid, for a typical concentrated solar heat flux distribution. HITEC salt and 316L stainless steel have been used as the heat transfer fluid and the material of the tubes in the receiver, respectively. A numerical method has been used to calculate the temperature distribution of the fluid and the convection coefficient along the circumference. Finally the Von-Mises thermal stresses of the tube receiver have been calculated. [Copyright &y& Elsevier]

Abstract: The bubble size, surface and volume distributions in two and three phase flows are essential to determine energy and mass transfer processes. The traditional approaches commonly use a conditional probability density function of chord-lengths to calculate the bubble size distribution, when the bubble size, shape and velocity are known. However, the approach used in this paper obtains the above distributions from statistical relations, requiring only the moments inferred from the measurements given by a sampling probe. Using image analysis of bubbles injected in a water tank, and placing an ideal probe on the image, a sample of bubble diameter, shape factor and velocity angle are obtained. The samples of the bubble chord-length are synthetically generated from these variables. Thus, we propose a semi-parametric approach based on the maximum entropy (MaxEnt) distribution estimation subjected to a number of moment constraints avoiding the use of the complex backward transformation. Therefore, the method allows us to obtain the distributions in close form. The probability density functions of the most important length scales (D, D 20, D 30, D 32), obtained applying the semi-parametric approach proposed here in the ellipsoidal bubble regime, are compared with experimental measurements. [Copyright &y& Elsevier]

Central receiver endurance in solar power tower plants is a critical aspect to ensure their viability. In this work, two receiver configurations, with and without flow path crossover, are compared in thermal and mechanical performance. Considering their thermomechanical limits during operation, results show that panels in the crossover configuration would endure from 1.22 to 2 times that in the no-crossover, just marginally penalising its thermal efficiency. Additionally, two plant operation strategies are compared: continuous operation and nighttime strategy. The former results in 1.89-times more damage but roughly a 1.9-times increase in energy output, obtaining a practically constant trade-off between damage and energy generation for these two strategies. Lastly, the effect of considering either a linear or bilinear (interaction point set at 0.05,0.05) creep/fatigue damage interaction is explored. While producing more conservative results, linear interaction error is below 3 % in the most critical receiver regions. Such a low discrepancy is caused by the creep-damage dominance over fatigue in the most critical regions. Other areas show heavier creep-fatigue interaction, resulting in larger error, but these areas obtain times to failure much longer than expected receiver lifespan. [ABSTRACT FROM AUTHOR]

Validation is a crucial aspect in the reliability assessment of models. Validation accuracy is measured with respect to experimental data. In this regard, there is not any experimental technique with sufficient spatial resolution capable of measuring the incident solar flux onto the receiver of solar power tower plants to model validation. Therefore, the individual optical efficiency of the field and the thermal performance of the receiver cannot be accurately obtained experimentally. To calculate these efficiencies, the development of numerical models is mandatory. Although, numerous receiver models can be found in the literature, the accuracy of most of them is not checked because of the scarcity of experimental data to compare with. In this study, the simulations of a model that includes the heliostat field and an external tubular receiver, taking into account all the receiver tubes, have been compared with available experimental data from Solar Two plant, in order to check its accuracy. It was obtained that the model overestimates 1.42% the total mass flow rate and 0.73% the global efficiency of Solar Two, which is almost negligible. Besides, the field-receiver efficiency can be calculated with confidence using Solar Two experimental data, and then it can be taken into account in the validation of the model. The model error, with respect to the experimental data are of 1.1% at full load and 1.4% at 50% partial load. • A field-receiver model (FluxSPT-FGM) has been validated with Solar Two tests. • The average error in the global SPT efficiency is 0.73% points. • The average error in the mass flow rate is 1.76%. • The field efficiency using FluxSPT is 9 and 3% points lower than with RCELL and POM, respectively. • The receiver efficiency using FGM differs 0.42 and 5.18% points at full and 50% partial load from POM. [ABSTRACT FROM AUTHOR]

Solar radiation numerical models need the implementation of an accurate method for determining cast shadows on the terrain or on solar collectors. The aim of this work is the development of a new methodology to detect the shadows on a particular terrain. The paper addresses the detection of self and cast shadows produced by the orography as well as those caused by clouds. The paper presents important enhancements on the methodology proposed by the authors in previous works, to detect the shadows caused by the orography. The domain is the terrain surface discretised using an adaptive mesh of triangles. A triangle of terrain will be under cast shadows when, looking at the mesh from the Sun, you can find another triangle that covers all or partially the first one. For each time step, all the triangles should be checked to see if there are cast or self shadows on it. The computational cost of this procedure eventually resulted unaffordable when dealing with complex topography such as that in Canary Islands thus, a new methodology was developed. This one includes a filtering system to identify which triangles are those likely to be shadowed. If there are no self shadowed triangles, the entire mesh will be illuminated and there will not be any shadows. Only triangles that have their backs towards the Sun will be able to cast shadows on other triangles. Detection of shadows generated by clouds is achieved by a shadow algorithm using satellite images. In this paper, Landsat 8 images have been used. The code was done in python programming language. Finally, the outputs of both approaches, shadows generated by the topography and generated by clouds, can be combined in one map. The whole problem has been tested in Gran Canaria and Tenerife Island (Canary Islands – Spain), and in the Tatra Mountains (Poland and Slovakia). [ABSTRACT FROM AUTHOR]

This paper aims to quantify the benefits of switching from a system dependent on traditional biomass to systems running on more efficient fuels and technologies. It is estimated that even when open fires burning fuelwood are replaced by improved cooking stoves (ICSs) and liquefied petroleum gas (LPG) stoves, and biomass is processed in dedicated biomass power plants, a net reduction in CO 2 emissions is still obtained. The ICS/LPG stove/biomass combustion power plant configuration could provide an average net reduction of 84 kg-C e /tDM. Meanwhile, a net reduction of 105 kg-C e /tDM could be obtained when implementing a ICS/LPG stove/biomass gasification power plant scheme. Main factors influencing the net reduction of CO 2 emissions are technology efficiency and the fraction of non-renewable fuelwood use. The switch from traditional biomass to modern biomass in traditional biomass intensive countries must not only be done to reduce CO 2 emissions but also to avoid indoor pollution and energy poverty. Health improvements should be more important than energy savings. Results also indicate that the use of modern biomass systems not only could provide a reduction of local environmental pollution, but also could boost the local economy by the creation of biomass infrastructures. [ABSTRACT FROM AUTHOR]

This paper reviews and assesses conditions for increased and efficient use of biomass in Central America (CA), providing an overview of conditions for biomass supply in each country. Then, a Fuzzy Multi-Actor Multi-Criteria Decision-Making (MCDM) method is applied to identify a portfolio of biomass conversion technologies appropriate for CA, considering technical, economic, environmental and socio-political aspects. The work is motivated by the relatively large availability of biomass in CA at the same time as current conversion of biomass is carried out in inefficient processes. The assessment of technologies includes thermochemical processes (pyrolysis, combustion and gasification) for production of different energy carriers, including improved cooking stoves (ICSs). The most promising biomass feedstocks in the region are residue based; animal (manure), forest and agricultural origin. We show that around 250 PJ/year could be available for the energy sector, which is equivalent to 34% of primary energy supply for CA. It is concluded that in the short term promoting and implementing ICSs will give the largest improvement in the efficiency of biomass use, whereas on the long term small combustion plants seem to be the best choice for transforming CA׳s biomass into a clean and sustainable energy carriers, boosting economy and industrial development. Results show that the introduction of ICSs will result in an annual saving in the range of 4–8 Mt of fuelwood (59–113 PJ). Moreover, even when the investment cost of the cooking stoves is considered, ICSs yield economic savings to fuelwood consumers compared to traditional stoves. The total savings during the first year of implementation would be in the range of 19–152 US$/stove. [ABSTRACT FROM AUTHOR]

This paper presents a thermal analysis of thin-wall pipes under non-uniform heat flux in the circumferential direction, with a turbulent flow in statistically stationary state inside. The temperature distribution in the solid and in the fluid is obtained using an spectral method that solves the conjugate heat transfer problem. Special attention is paid to the inner wall fluid temperatures and the thermal stresses on the solid, that are compared to predictions based on 1D models in which the circumferential heat transfer is neglected. The comparison shows that, even if at sufficiently large Biot numbers ( Bi ≳ 0.3 ) the 1D model gives a reasonable prediction of the inner wall fluid temperatures (less than 5% of error), the 1D model for the thermal stresses is only appropriate for very large Biot numbers ( Bi ≳ 10 ), giving qualitatively wrong results for Biot numbers below 0.3. [ABSTRACT FROM AUTHOR]

The demonstration power plant Solar Two was the pioneer design of a molten-salt power tower. In the report “Final Test and Evaluation Results from the Solar Two Project” (Pacheco, 2002, [15] ) the efficiencies of the three main subsystems: heliostats, receiver and power block were measured or estimated. The efficiency of the global plant and the power block could be obtained with confidence, whereas the efficiencies of the heliostat field and the receiver could only be estimated because the solar flux reflected by the heliostats and intercepted by the receiver cannot be measured. The receiver efficiency was estimated using the Power-On Method. The authors themselves highlighted that this method contain an important assumption: the temperature distribution on the receiver surface is independent of the incident power level. This assumption is equivalent to have a Biot number much smaller than one for the solar receivers operation, fixed inlet and outlet salt temperature. For Solar Two reported data the Biot number is of order unity, and then the external tube temperature depends on the receiver load; and the thermal losses vary linearly with the incident solar flux rather than constant. Besides, our results show that receiver efficiency is around 76% for full load and 69% for half load instead of 87% and 80% reported when external tube temperature was assumed to be independent on the incident power. [ABSTRACT FROM AUTHOR]

The aim of this work is to explore the capabilities, from heat transfer and structural point of view, of a novel header and coil steam generator for a 100 MWe solar tower plant using molten-salt as heat transfer fluid. A methodology for the design and economic optimization of the header and coil steam generator is presented, paying special attention to the structural assessment which considers complex phenomena such creep-fatigue and stress relaxation due to the high working temperatures of solar tower plants. The results showed that header and coil steam generators provide economically effective overall heat transfer coefficients with lower pressure drops on the shell side compared to conventional shell-and-tube steam generators, leading to a reduction in the annual pumping costs of around 3.6 times. The structural assessment reveals that the critical points are located in the headers of superheater, reheater and evaporator. Different redesign actions have been performed to increase the lifetime in the critical points without affecting to the optimum thermo-economic solutions. Finally, the results showed that the header and coil steam generator is able to operate with fast daily startups at 6.1 K/min, a ramp-up 2.4 times higher than conventional shell-and-tube steam generators. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the effect of a novel rotating distributor for fluidized beds on the bubble size is studied. The distributor is a perforated plate that rotates around the vertical axis of the column. The formation of the bubbles on the rotating distributor is theoretically analyzed. The pierced length of the bubbles ascending in the bed were measured using optical probes. The probability distribution of bubble diameter was inferred from these experimental measurements using the maximum entropy method. The radial profile of the bubble diameter is presented for the static and rotating configurations at different gas velocities. The frequency of bubble passage and the distribution of bubbles in the cross section of the bed are also reported. Results were finally shown for different heights above the distributor. A radial decrease in the bubble size when the distributor rotates is found. The bubble growth with the bed height is also lower in the rotating case. [Copyright &y& Elsevier]

Abstract: The problem of non-isothermal absorption of vapour into freely expanding liquid sheets is addressed in this study. This is done in the context of four models that characterise the coupled heat and mass transfer in the liquid phase: a nonlinear model retaining the effect of sheet growth, an approximate model for slowly increasing mass flow rate in the sheet, a large Lewis number model and finally, a boundary layer model. These models have been numerically or analytically solved and applied to the comparative analysis of two different working pairs, LiBr–H2O and LiNO3–NH3, under conditions representative of adiabatic absorption in refrigeration systems. The limits of applicability of each model have been assessed and the sensitivity of the results to the sheet aperture angle, heat of absorption and initial subcooling has also been tested. For equal initial mass fraction and subcooling, the models indicate that Sherwood number and the rate of absorption in laminar expanding sheets for the LiNO3–NH3 solution are always superior to those for the LiBr–H2O solution. [Copyright &y& Elsevier]

Abstract: This work presents a new methodology, based on the maximum entropy method, to obtain bubble characteristics in fluidized beds. The probability distributions (PDF) of bubble pierced length and velocity are obtained applying the maximum entropy principle to experimental measurements. In addition, the bubble diameter distribution has been inferred from experimental pierced length measurements. This method is applied to characterize bubbles in fluidized beds for the first time and the most general bubble geometry, a truncated spheroid, is considered. The distance between probes, , which is the minimum pierced length that is possible to measure accurately using intrusive probes, has been introduced as a constraint in the derivation of the size distribution equation. The maximum entropy method is applied to experimental measurements of bubble characteristics carried out using optical and pressure probes in a three-dimensional fluidized bed of Geldart B particles. Results on bubble size obtained from pressure and optical probes are very similar, although optical probes provide more local information and can be used at any position in the bed. The maximum entropy principle has been found to be a simple method that offers many advantages over other methods applied before for size distribution modeling in fluidized beds. [Copyright &y& Elsevier]

Hydrothermal carbonization process can transform wet bio-wastes into value-added products. This work aims to hybridize a concentrating solar technology and a biomass reactor for the continuous and sustainable valorization of biomass. The novel technology proposed integrates a linear beam-down solar field with a twin-screw reactor for continuous HTC process. The solar field consists of two reflections that concentrate linearly the sun energy on the ground, where the twin-screw reactor is placed. A mathematical model is proposed to solve both the heat transfer and HTC kinetics for a co-rotating twin-screw reactor. The incoming heat flux from the solar field (8–20 kW/m2), the reactor length (L/D = 30–60 where D is the diameter) and the rotating velocity of the screw (25–100 rpm) are the main variables used to process the biomass up to the desired severity factor. The simulation results of different lignocellulosic biomasses (loblolly pine, sugarcane bagasse, corn stover and rice husk) are validated against literature data. The developed model shows good agreement with experimental results shown in the literature. The proposed technology foresees hydrochar yields of 64–78% for severity factors of 4.2 and 5.3, respectively, in agreement to the experimental results of 63–70% shown in literature. [Display omitted] [ABSTRACT FROM AUTHOR]

Supercritical CO 2 central-receivers must withstand high temperatures and pressures combined with cyclic operation, which makes the solar receiver susceptible to creep-fatigue failure. In this work, a creep-fatigue analysis of a sCO 2 Inconel 740H tubular receiver of a 2 MW e solar tower plant has been accomplished to study the influence of the tube size on the receiver and solar field design. A 2D numerical model of the tubular receiver that accounts for the thermal conduction in both radial and circumferential directions was developed to determine the sCO 2 and wall temperature profile, which is crucial for the creep-fatigue calculations. The receiver flux distribution, which is an input to the model, was obtained with SolarPILOT, while a conventional recompression model was used to calculate the cycle efficiency and inlet temperature to the receiver. Comparison of the results of the 2D model with those of a 1D model showed that the 1D model overestimates the creep fatigue rupture time by two orders of magnitude. Furthermore, the efficiency and costs of the heliostat field and receiver were calculated for different receiver tube sizes. Smaller tubes allowed a higher maximum heat flux leading to smaller receiver and heliostat field designs, which resulted in higher overall efficiency of the power plant and lower material costs. For a design ensuring 25 year receiver lifetime the minimum sCO 2 solar receiver cost, 345 €/kW th , was obtained for the smallest pipe diameter. [Display omitted] • A creep-fatigue analysis of a sCO 2 central-receiver is presented. • Circumferential thermal conduction is not negligible due to the thick receiver tubes. • The cost and thermo-mechanical behavior of the power plant for different receiver tubes size are studied. • Smaller tubes result in better performance and lower costs of sCO 2 tower power plants. [ABSTRACT FROM AUTHOR]

• A low-computation cost procedure is proposed to assess the solar receiver lifetime. • The lifetime of a solar receiver similar to Gemasolar power plant is investigated. • The stress relaxation effect has a strong impact on the receiver lifetime. • The receiver operation based on UTS/3 stress limit leads to a huge economic penalty. One of the major uncertainties in the design of molten-salt solar receivers is the estimation of the lifetime because solar receivers have to withstand high temperature, thermal stress and corrosive-media in addition to daily star-up and shutdown. The first step to estimate the receiver damage and therefore, to monitor and manage the receiver life cycle, is to obtain properly the tube stresses under daily operating conditions. Whit this aim, we propose a straightforward low-computational cost procedure to determine the stresses under elastic-plastic regime. In addition, the stress relaxation due to visco-elastic behavior of the material, is also calculated. The analytical thermo-elastic, elastic-plastic and relaxation stress models for the high-nickel alloy Haynes 230 were verified against finite element simulations showing a good accuracy with a low-computational cost. For exemplification purposes, a reference operation day of a molten-salt solar receiver like Gemasolar is investigated. A receiver creep-fatigue damage assessment was developed for a flat aiming strategy. The results showed that the fatigue damage is 0.03% of the total damage. The average-field-receiver efficiency was 42.18% with a predicted lifetime higher than the expected. On the contrary, the receiver lifetime increases unnecessarily and the field-receiver efficiency drops to 26.25% when the receiver is operated limiting the stress as one third of the ultimate tensile strength (UTS/3). This limit results in a loss of revenues of around 250,000€ per month. Hence, the excessive level of conservatism produced by stress limit methods are not suitable for solar receivers due to the resulting economic penalties. [ABSTRACT FROM AUTHOR]

• High-resolution calculation of SPT central receivers exergy efficiency. • Close examination to the various exergy terms of the balance. • The heliostat field is responsible for the greatest exergy loss, around 40%. • Every 5% degradation of the coating absorptivity lowers a 1% the exergy efficiency. • The ambient temperature regard can aid the site selection, preferring a low one. A high-resolution method to analyse the exergy of the SPT external tubular receivers is presented, examining the different heat transfer process involved individually. This sheds light on the role that each irreversibility source plays in the outcome, aiding in the receiver design and the facility location selection. The exergy efficiency is around 32% in the base configuration. Besides the exergy loss in the heliostat field, over 40%, it is found that the biggest exergy destruction cause are the radiation emissions and absorptions in the tube outer surface, around 17%. From the remaining ones, the greatest are the exergy destructed in the HTF and the one escaping to the ambient (over 4% each). Then, the exergy balance for a variety of strategies and ambient conditions is performed: optical properties of the tubes coating, peak and flat aiming strategies, DNI and ambient temperature. The heliostat field exergy loss rate only varies when changing the aiming. However, the emission and absorption losses and the ones in the HTF suffer the greater modifications with all the parameters studied. The impact of the optical properties degradation, 1% descent in the efficiency per 5% degradation, would advise repainting works in order to avoid greater exergy destruction. The surroundings temperature modification impacts considerably the exergy efficiency, showing the suitability of locations with low ambient temperature and a moderate DNI: descends of over 0.35% occur every 5 °C increase of the temperature for a fixed DNI. [ABSTRACT FROM AUTHOR]

• Fast, accurate and reliable estimation of thermal stresses in solar receivers. • Tube deflection can be obtained using the beam theory. • Temperature-dependent properties must be used for a reliable stress estimation. • The axial stress in receiver tubes is the most critical stress. The aim of the design of central solar receivers is to withstand the high non-uniform solar-heat-flux and temperature during the solar-power-plant lifetime. This high non-uniform tube temperature causes high thermal stress, producing creep and fatigue damage. Therefore, is necessary to obtain an accurate estimation of the tube stresses during the receiver operation. In the same way, to ensure the panel integrity, the frontal and lateral tube deflections must be obtained to avoid excessive panel bowing and warpage, respectively. The huge number of simulations needed to perform the creep-fatigue analysis precludes the use of high time-consuming CFD-FEM simulations. To resolve this drawback, a reliable, accurate and fast procedure to obtain the tube stresses, using analytical stress estimation, is proposed. The procedure considers the temperature dependence of the thermo-mechanical properties. The temperature-dependent hoop stress is estimated using the solution for constant mechanical properties whereas the radial stress is estimated taking constant the Young modulus only. The temperature-dependent axial-bending stress is obtained using the non-homogeneous beam equation subjected to the movement restriction produced by tube clips. When the tube displacement is restricted by tube clips, the equivalent stress difference is less than 2% taking temperature-dependent properties and slightly higher than 10% for constant properties. The proposed stress estimation is enough accurate to perform a reliable fatigue-creep analysis and two order of magnitude faster than the CFD-FEM simulations. Finally, the tube deflection and displacement, restricted by tube clips, are derived straightforward using the temperature-dependent tube curvature and the beam theory. [ABSTRACT FROM AUTHOR]