Field-receiver model validation against Solar Two tests

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

Aiming strategy model based on allowable flux densities for molten salt central receivers

This study presents an aiming model to properly point heliostats at cylindrical molten salt receivers in Solar Power Tower. By means of two iterative algorithms (search and fit), the proposed strategy attempts to maximize the receiver thermal power output while preserving the receiver operational limits. Corrosion and thermal stress constraints are translated into allowable flux densities (AFD) that are handled by the model. The computer code accommodates the flux images produced by each heliostat in a field to accurately fit the AFD limit. In this paper, a Gemasolar-like field-receiver system serves to illustrate the aiming model. Compared to the equatorial aiming, receiver interception is slightly lower using the proposed strategy, but the receiver integrity is ensured; peak flux is significantly reduced up to 23%. It has been found that a favorable flux density profile generally has its peak displaced to the salt entrance at each receiver panel. Since external cylindrical receivers consist of a combination of up-flow and down-flow panels, the optimal flux profile is challenging for contiguous panels with contrary demands. In spite of that, remarkable matching is achieved by the fit algorithm. Because of its fast computation and automatic operation, the resulting tool can be applied to real-time control of existing heliostat fields and the integrated design of the coupled systems field and receiver

Feasibility study of a new concept of solar external receiver: Variable velocity receiver

The deployment of new solar power tower plants mainly depends on becoming cost-competitive with traditional forms of electricity generation. The solar field represents around 40% of the solar power tower investment cost, thus the cost reduction of such subsystems is mandatory to achieve that goal. This reduction could be done by increasing the solar flux intercepted by the receiver, which would increase the peak flux. Therefore, new concepts of solar receivers are required to accommodate such high peak flux. The proposed receiver, which withstands high peak flux, consists on a Traditional External Tubular Receiver (TETR) equipped with valves that allow the division of each panel of the receiver in two independent panels, increasing the velocity of the heat transfer fluid in specific zones of the receiver. This receiver configuration, named Variable Velocity Receiver (VVR), avoids tube overheating. Moreover, this novel receiver allows more concentrated aiming strategies, which increases the optical efficiency of the solar field and permits to reduce the number of heliostats in the field. Given a specific generation capacity, the size of the solar field required by a VVR is 12.5% smaller in comparison to a TETR. Such efficiency improvement has a negligible effect in tube mechanical stresses; even though pressure drop and parasitic consumption of the power plant increase. This new receiver configuration also gains hours of operation, even in winter: in hours with low solar irradiance all the panels can be split in two, increasing the number of passes and the velocity of the heat transfer fluid and accomplishing the transition from laminar to turbulent regime. Therefore, this receiver is able to reduce the levelized cost of energy

Aiming factor to flatten the flux distribution on cylindrical receivers

High incident flux gradients and hot spots lead to extreme thermal stresses that may damage and reduce the lifetime of central receivers. An aiming strategy based on a single parameter, k, named aiming factor, is developed to generate symmetric flux maps about the receiver equator. By means of this k factor, ranging between 3 (generally equivalent to equatorial aiming) and 0 (alternatively aiming to top and bottom borders), the solar flux incident on the receiver and the spillage losses can be controlled. For each sector in a heliostat field, the aiming factor values causing the flattest symmetric flux maps, kflat, are deterministically found with a sweep and mesh shifting procedure. Results for Dunhuang solar power tower plant show that kflat is fairly constant throughout the year, except near sunrise and sunset in east and west sectors, respectively./This work has been supported by the Spanish Ministry of Economy and Competitiveness under the project ENE2015-69486-R (MINECO/FEDER, UE)

Revised receiver efficiency of molten-salt power towers

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, [151) 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 Blot number much smaller than one for the solar receivers operation, fixed inlet and outlet salt temperature. For Solar Two reported data the hot 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

Allowable solar flux densities for molten-salt receivers: Input to the aiming strategy

Solar Power Tower technology requires accurate models and tools to assist in design and operation stages. The heliostat field aiming strategy seeks the maximization of the thermal output from the receiver, while preventing its permanent damage because of thermal stress and corrosion in molten salt receivers. These two limitations are translated into Allowable Flux Densities (AFD), which can be handled by the aiming strategy. This paper explains the methodology to determine AFDs, and analyzes the influence of tube geometry and material. AFD by corrosion is slightly lower in Haynes 230 than Inconel 625 and austenitic alloys. On the contrary, HA230 has better performance than In625 under thermal stress. Increment of tube wall thickness diminishes the AFD: slightly by corrosion, but significantly by thermal stress. The generated AFD databases feed the aiming model, herein applied to Gemasolar case study. In the cylindrical receiver, first northern panels are limited by thermal stress, while the last ones by corrosion. Under optimized aiming, HA230 receiver tubes produce equivalent thermal output than In625.This study has been supported and financed by Iberdrola España Foundation under the program Ayudas a la investigación en Energía y Medio Ambiente 2018, project: “Diseño y evaluación de un Nuevo receptor solar exterior de tubos ovalados”. The authors acknowledge to 11CNIT (XI National and II International Engineering Thermodynamics Congress) committee for selecting the previous conference contribution for further dissemination through this special issue

Saving assessment using the PERS in solar power towers

The improvement of the solar power tower using solar salt is one of the main goals of researchers. Any method or invention to improve the efficiency of this technology contributes to promote the renewable energies. The use of a Potential Energy Recovery System (PERS) in two different solar power tower plants of 20 and 100 MW has been analysed. The PERS is formed, at least, by one turbine, located at the hot salt pipe coming from the receiver. The turbine is engaged to the shaft of the feed pump, which raises the heat transfer fluid from the cold tank to the receiver. It reduces the parasitic power consumption of the plant, and increases its global efficiency. Different PERS configurations have been modelled. Based on an energetic and economic analysis, the optimal configuration is a geometrical similar turbine of three times the volume flow rate of one feed pump. The PERS has been proven to be a cost reductive and clean tool. For a 100 MW power plant of 30-year lifetime the investment cost is 1.26 M$and the annual cash flow is 0.89 M$, while for a plant of 20 MW these values are 0.26 M$and 0.19 M$, respectively.The financial support from CDTI and S2m Solutions for the MOSARELA Project (Molten salt receiver lab), whose reference is IDI-20120128. Finally, the financial support of the ENE2012-34255 Project is also acknowledged

Frequency estimation in DSOGI cells by means of the teager energy operator

Second Order Generalized Integrator (SOGI) cells are used for notch filtering due to their simplicity and their harmonic rejection capability. SOGI and Dual SOGI (DSOGI) filter cells, combined with Frequency Locked Loops (FLL) to adjust the notch frequency, are commonly used in both 1f and 3f grid following (GFL) power converters for synchronization, i.e. SOGI-FLL and DSOGI-FLL, respectively. The FLL relies on a gradient descent method to minimize a cost function built up around one inner SOGI cell variable, e.g., the in-quadrature voltage estimation, and one outer variable, i.e. the error signal due to the SOGI filter cell. As a result, the FLL manages relatively large DC offsets and harmonic distortion passing through the outer SOGI cell variable, which deteriorates the frequency estimation and then, the SOGI-FLL performance. To attenuate such issues, the method proposed in this digest only uses inner SOGI cell variables. It minimizes the deviation between the estimated grid frequency and the frequency of the signal across the SOGI cell, which is detected through the Teager Energy Operator (TEO). The proposal is validated in simulation and experimentally.This work has been supported by the Ministry of Science and Innovation through the project RTI2018-095138-B-C31:"Electrónica de potencia aplicada a la red eléctrica y a procesos industriales": PEGIA

Envelope-based modeling for single-phase grid-following and forming converters

The study of the interaction with the grid, including synchronization, controller design and stability assessment for 1o grid-following (GFL) and grid-forming (GFM) power converters requires an efficient modeling tool to design universal grid-connected converters considering the different grid scenarios. From the initial time-periodic system, approximated linear time-invariant (LTI) models are obtained through dynamic phasors, linearization of variables represented in a virtual synchronous rotating reference frame (RRF) or linearization in the frequency domain, i.e. harmonic linearization. The accuracy and complexity of the obtained model depend on the method used. This work proposes to use the well-known envelope modeling approach used for resonant converters but requiring the time periodic input to generate its related phase synchronization for the model. The result is a simple and accurate LTI model of 1º GFL/GFM power converter for such stability studies. The proposed 1o modeling approach is valid for any application with phase locked loop (PLL) synchronization. Simulation results validating de proposal are provided.This work has been supported by the Ministry of Science and Innovation through the project RTI2018-095138-B-C31:"Electrónica de potencia aplicada a la red eléctrica y a procesos industriales": PEGI