Solar Central Receiver Systems Research Articles

Comparative Analysis of Rankine Cycle Linear Fresnel Reflector and Solar Tower Plant Technologies: Techno-Economic Analysis for Ethiopia

The need to meet the world’s growing demand for energy in an environmentally sustainable manner has led to the exploration of various renewable energy (RE) resources for power generation. The objective of this study is to examine the techno-economic potential of concentrated solar power plants (i.e., linear Fresnel reflector (LFR) and central receiver system (CRS) for electricity generation in Eastern African countries with a case study on Ethiopia. The study was conducted using the System Advisor Model (SAM). In order to estimate the economics of the two power plants, the Levelized cost of energy (LCOE) and the net present value (NPV) metrics were used. According to results obtained from the simulations, the LFR produced annual energy of 528 TWh at a capacity factor (CF) of 60.3%. The CRS also produced a total of 540 TWh at a CF of 61.9%. The LCOE (real) for the CRS is found to be 9.44 cent/kWh against 10.35 cent/kWh for the LFR. The NPV for both technologies is found to be positive for inflation rates of 2% and below. An inflation rate above 2% renders the two power plants financially impracticable. A real discount rate above 9% also renders both projects economically unviable. Based on the obtained results, the CRS system is identified as the best technology for electricity generation under the Jijiga climatic condition in Ethiopia.

Numerical investigation to assess the techno-economic feasibility of solar central receiver system for off-grid power in Saint Martin's Island, Bangladesh

Bangladesh is a small country, located in south-east Asia having a fast-progressing economy and exemplary development in industry, architecture and engineering sectors. With such developments, demand of power is also on the rise exponentially. Although the generation capacity of the country has seen a major boost, lack of proper distribution, negligence towards renewable energy resources and dependance on perishable fossil fuels (natural gas, coal, mineral oil etc.) pose a major risk of a significant power crisis in future. The only tourist favorite island of the country, known as Saint Martin's Island lacks national grid connection and stable source of power to mitigate the inhabitants' electricity need. This study intends to analyze the technical and economic feasibility of solar powered central receiver system to provide steady and cheap means of power generation at the island in an environment friendly manner. A detailed and optimized model has been designed for simulating a 1 MW solar powered central receiver plant using SolarPILOT and System Advisor Model (SAM) software. Modelled power plant can generate 3304 MWh electricity yearly with a gross efficiency and capacity factor 12.29% and 41.9% respectively at 38.61 Cents/kWh levelized cost of electricity (LCOE) which is much cheaper than the current rate at Saint Martin's Island. The promising performance of the powerplant designed in this study would encourage future research and innovations in case of off-grid electricity generation employing solar thermal energy in Bangladesh to ensure sustainable development.

Development of A New Methodology for Evaluation of Beam Accuracy and Optical Quality of Solar Central Receiver Systems

In central tower concentrated solar power plants, the concentrated flux distribution at the receiver is controlled by the aiming strategy of thousands of heliostats in the field to accomplish the specific requirements of solar-thermal conversion processes. The main goal of this work is to evaluate the reflected solar flux from a heliostat or from a group of heliostats on a calibration target. The solar flux map at the target receiver is constructed using the information embedded in a captured high dynamic range image of the evaluated scene. In the present article, a methodology has been proposed to evaluate the optical performance of solar central receiver systems in terms of beam accuracy and optical quality of reflected light on calibration target. The developed system is a simple, automatic and effective tool used to accurately analyse the reflected solar flux without limitations of previous methods, such as using specific type of calibration target. The capability of the present methodology for accurate analysis is demonstrated through experimental indoor and outdoor tests.

Accurate interpolation methods for the annual simulation of solar central receiver systems using celestial coordinate system

Heliostat field layout optimization bases on simulations of the annual energy production. To reduce the computation time of the optimization process, one can try to reduce the number of simulation points of the annual domain, while keeping similar accuracy. For the temporal domain, there exist already different approaches as aggregation of days. To further reduce the number of needed simulation points, in this paper we decouple the power computation from the irradiation, such that we just regard the computation of the power plant efficiency. This time-dependent parameter is transformed into a celestial coordinate system where the solar angle-dependent efficiency will be approximated using suitable multi-dimensional interpolation methods.We distinguish between an accurate approximation of the received annual optical energy and the electrical energy of each moment of a year. These methods are demonstrated for the existing heliostat field layouts PS10 and Gemasolar in Seville, while using realistic weather conditions. With this new approach just around 40 simulation points suffice to reach an accuracy of 99.9% for the received power for smaller power plants as PS10, and for larger plants as Gemasolar. Compared to a state-of-the-art method, this investigation helps to accelerate the simulation by factor three.

Optical analysis of a multi-aperture solar central receiver system for high-temperature concentrating solar applications.

A multi-aperture solar central receiver system is optically analyzed for increasing the net power to the receiver in a wide temperature range of 600-1800 K. A model system comprises a tower, a multi-aperture receiver with compound parabolic concentrators, and heliostat sub-fields. Optical modeling is performed using in-house developed Monte-Carlo ray-tracing programs. The heliostat sub-field geometrical configuration, the number of receiver apertures and optical properties of reflective surfaces are varied in the parametric study. Increasing the number of apertures from one to four increases the maximum net receiver power from 116 MW to 332 MW. The use of more than four apertures results in only limited further gain of the net receiver power but significantly decreases the overall optical efficiency and the solar-to-thermal efficiency. The optimal temperature for the maximized annual solar-to-exergy efficiency is found in the range of 1100-1200 K. This optimal temperature decreases slightly with an increasing number of apertures.

Temperature-based optical design, optimization and economics of solar polar-field central receiver systems with an optional compound parabolic concentrator

Energetic and economic characteristics are studied for solar central receiver systems consisting of a polar heliostat field, a tower, a single-aperture cavity receiver, and an optional compound parabolic concentrator (CPC). System characterization and optimization are performed with a numerical model combining an in-house developed Monte-Carlo ray-tracing optical model, a simplified receiver heat transfer model, and a cost model based on the System Advisor Model (SAM). Based on the model, the effects of receiver temperature on the optical configuration of cost-optimal systems are elucidated, along with the benefits of using a CPC for improved energetic and economic performance. Under the assumptions made in this study, it is found that the overall minimum levelized cost of exergy is obtained by a non-CPC system with a receiver operated at approximately 900 K. A CPC benefits both the energetic and economic performance of systems only at elevated temperatures. The working temperature thresholds at which the energetic and economic performance benefit from the addition of a CPC are identified as 900 K and 1200 K, respectively. The general formulation of the model and broad range of values of the investigated parameters provide a universal predictive capability for studying techno-economic performance of concentrating solar thermal systems.

Design of thermal power plant modernization and rehabilitation model for the new market demands and challenges

The article presents the model for the rehabilitation of existing conventional thermal power plants in order to lower the consumption of fossil fuels. Instead of them, the model uses an alternative energy source ? Sun irradiation. The proposed rehabilitation model is theoretically calculated and designed. The model in the software environment MATLAB SIMULINK was developed, based on previous calculations and determined parameters. In this article, the combination of Clausius-Rankine process and solar central receiver system is presented. The model enables simultaneous calculations of exit model parameters for the complete model, based on predetermined entering parameters of the model.

Novel hybridization of solar central receiver system with combined cycle power plant

This paper presents a novel hybridization of a solar central receiver system via an open volumetric air receiver with a combined cycle. The exhaust gases from a gas turbine before entering a heat recovery steam generator are heated up by the solar heat in a volumetric receiver working as a supplementary firing, in contrast to the solarized gas turbine where the solar heat is introduced into an open or closed Brayton cycle. This hybridization does not require significant modifications in the gas power block. In this study the thermodynamic and economic issues are investigated and complemented with an optimization to demonstrate its technical feasibility. The obtained results reveal noticeable enhancements in solar energy conversion and power plant performance, where the net solar energy ratio may reach up to 23.5% and the overall efficiency about 64.4%. Assessment of the levelized cost of energy led to an estimated value of 0.0335 $/kWh which could be further reduced through an optimization which when the environmental issue is considered becomes more competitive. Furthermore, there is an important fuel saving of 7.07 Million $ and less pollutant emission about 0.33 Million ton over 30 years of operating service.

Development of a New High Dynamic Range Technique for Solar Flux Analysis Using Double-CCD Optical Camera

In solar central receiver (SCR) systems, camera-based tracking and control strategies have been widely developed and tested. The camera, usually with a single CCD, captures images of the sun reflected by thousands of heliostats on the central receiver. The captured images are then analysed to allow the evaluation of solar flux. For accurate analysis process, the camera must have a high dynamic range (HDR) to give real and accurate information about the captured scene. In the present article, a new methodology has been developed using a double-CCD optical camera for improvement of dynamic range of the captured solar flux image. An experimental calibration set-up is adopted for accurate estimation of the camera response function. The double-CCD camera is used for acquiring two images at the same instant with two different levels of exposure. The two captured images are then merged into a single HDR image using a new proposed weighting function. The enhanced optical performance of the proposed weighting function has been demonstrated by comparison with the previous HDR image generation weighting functions at different levels of illumination. Investigation of the capability of the proposed methodology for the analysis of beam accuracy and optical quality of SCR systems show that the present proposed method leads to more accurate solar flux analysis because of the improvement in the resolution of details of the final merged image.

Wind tunnel measurements of forced convective heat loss from multi-megawatt cavities of solar central receiver systems

We investigated the forced convective heat loss from a model of a multi-megawatt cavity receiver of a concentrated solar power (CSP) tower system in a high-pressure wind tunnel. Measurements of 5 geometrical configurations of this model as well as measurement uncertainties are reported in this contribution. The experiment covered a Reynolds number range of between 2·106 and 8·106, based on the external dimensions and flow field. In general, the measured values are highly sensitive to the geometrical configuration, the wind velocity, and the wind direction. The results show that the maximum forced convective heat loss for all configurations occurs when the wind blows from frontal directions of between 60° and 80° relative to the tower symmetry plane. We found that the peak location does not vary for different inclinations, but does vary for different aperture openings. Also, the results show that the direction of the wind causes the forced convective heat loss to vary with a factor of up to 6.1, but at least with a factor of 2.6. Last but not least, our power-law correlation of the dependency of the forced convective heat loss on the Reynolds number matches literature values.

A two-layered solution for automatic heliostat aiming

The efficiency and safety of a solar central receiver system depend on the flux distribution reflected by the heliostat field on its receiver. Thus, the field must be carefully controlled to avoid dangerous radiation peaks and temperature gradients while also maximizing the efficiency of the system. Control tasks include deciding which heliostats to activate and where to aim them. The field is usually under direct human supervision, which is a potential limitation, and automatic aiming procedures are of great interest. This work proposes a general aiming methodology for flat-plate receivers. It intends to cover heliostat selection and aim point assignation to replicate any given reference flux distribution on the receiver. The methodology, which addresses this situation as a large-scale optimization problem, defines two consecutive stages. The first one handles heliostat selection by applying a specific genetic algorithm. The second one, based on a local gradient descent, assigns a final aim point to every active heliostat. The proposed methodology, in contrast to other existing methods in the literature, is not limited to achieve any specific target distribution. It exploits the analytical characterization of the considered field to minimize the accumulated squared error between any reference flux distribution and the achieved one. The results show very good replication quality and, considering its execution time, this method is suitable for preliminary and high-resolution field configuration.

Hybrid solar flameless combustion system: Modeling and thermodynamic analysis

In this paper, the idea of using flameless combustion in a hybrid solar combustion system is investigated by modeling and thermodynamically analyzing a gas turbine system. In this regard, a gas turbine system coupling with hybrid solar flameless combustion including heliostat solar field, central receiver, flameless combustor and power generation system are modeled. In conditions that sun power is not adequate to heat up the combustion air over auto-ignition temperature of the fuel, air is passed through the first stage combustor. To provide a basic cycle for comparison, a common gas turbine with preheater is modeled as well. Energy and exergitic-based analyses of various systems are evaluated and environmental footprints reduction of proposed optimum cycle is assessed and compared to the basic case. The results illustrate that Nitrogen Oxides formation in hybrid solar flameless combustion is significantly lower than common gas turbine system. While the gas turbine with preheater generates 67 μg Nitrogen Oxides per kWh, hybrid solar combustion system produces less than 7 μg Nitrogen Oxides per kWh. In comparison to gas turbine system, fuel consumption decreases in hybrid solar flameless combustion system from 0.1875 kg/s to 0.16 kg/s (about 14.7%) when solar share is considered just 40%. Since the inlet air of flameless combustor is charged from solar heater outlet, increasing air temperature enhances the share of solar energy in the system which results in overall exergy reduction in the system. The proposed system shows significant environmental benefits and based on the available technologies, it is suitable for high temperature solar towers.

Multivariable Closed Control Loop Methodology for Heliostat Aiming Manipulation in Solar Central Receiver Systems

Maintaining receiver’s thermal stresses and corrosion below the material limits are issues that need careful attention in solar thermal towers. Both depend on heliostats’ aiming points over the central receiver and available direct solar radiation at any instant. Since this technology relies on an unavoidable time-changing resource, aiming points need to be properly manipulated to avoid excessive hot spots. This paper proposes a new aiming point strategy based on a multivariable model predictive control (MPC) approach. It shows an alternative approach by introducing an agent-based group behavior over heliostats’ subsets, which makes possible either concentrating or dispersing solar radiation as required by the MPC algorithm. Simulated results indicate that it is feasible to develop a closed-loop control procedure that distributes solar irradiance over the central receiver according to the predefined heat flux limits. The performance of the proposed approach is also compared with the results found in the available literature that uses a different methodology.

A novel sun-tracking and target-aiming method to improve the concentration efficiency of solar central receiver systems

Abstract The solar central receiver (SCR) system is an important candidate for solar thermal utilization. The cosine loss and astigmatism of heliostats are two major factors which lead to the low concentration efficiency of SCR systems. Most public discussions of concentration efficiency improvement of SCR systems focused on astigmatism correction; however, not so many studies were given to reducing cosine loss of heliostats. In this paper, we proposed a new sun-tracking and target-aiming method for SCR systems in order to eliminate the cosine loss and enhance the concentration efficiency. We calculated the tracking formulae of the sun-tracking strategy and target-aiming strategy by using coordinate rotation transformation method, designed an aspheric lens in target-aiming device by using non-imaging optics principles, and finally constructed the optical model of the proposed system in TracePro software to simulate its concentration performance. The computational results demonstrated the kinematic feasibility of the sun-tracking strategy and target-aiming strategy. The simulation results showed that the proposed method can improve the concentration efficiency of SCR systems obviously in some cases. This novel sun-tracking and target-aiming method sheds light on the field efficiency improvement of SCR systems.

Experimental Method to Find the Flux and Temperature Distribution on the Flat Receiver of Small Central Receiver System

The central receiver concept for solar energy concentration and collection is based on a field of sun-tracking mirrors (heliostats) that reflect the incident sunshine to a receiver (boiler) at the top of a centrally located tower. A solar central receiver system is used for medium temperature process heating applications or in solar thermal power plants. A flat receiver plate was experimentally investigated with a small scale solar central receiver system in order to study the effects of receiver temperature and flux distribution on the receiver plate. Flux density distribution at the focal region of the concentrator can be estimated, if the temperature on the focal region is measured. In the present work, a thermocouple method is used to estimate the flux density distribution on the flat receiver plate. The variation of temperature on the receiver plate is experimentally estimated by varying the number of heliostats that are focused on the receiver plate. It is seen that for all cases of heliostat numbers, the radiation reflected from the heliostat field on the receiver surface generates a variation of flux density distribution. The peak flux is obtained at a single point on the receiver surface. For the small central receiver system of 7 heliostats, the peak flux value is found around 4110 W/m2 (± 2%) and the average value of flux of the receiver plate is 1984 W/m2(± 2%).

A New Methodology for Building-Up a Robust Model for Heliostat Field Flux Characterization

The heliostat field of solar central receiver systems (SCRS) is formed by hundreds, even thousands, of working heliostats. Their adequate configuration and control define a currently active research line. For instance, automatic aiming methodologies of existing heliostat fields are being widely studied. In general, control techniques require a model of the system to be controlled in order to obtain an estimation of its states. However, this kind of information may not be available or may be hard to obtain for every plant to be studied. In this work, an innovative methodology for data-based analytical heliostat field characterization is proposed and described. It formalizes the way in which the behavior of a whole field can be derived from the study of its more descriptive parts. By successfully applying this procedure, the instantaneous behavior of a field could be expressed by a reduced set of expressions that can be seen as a field descriptor. It is not intended to replace real experimentation but to enhance researchers’ autonomy to build their own reliable and portable synthetic datasets at preliminary stages of their work. The methodology proposed in this paper is successfully applied to a virtual field. Only 30 heliostats out of 541 were studied to characterize the whole field. For the validation set, the average difference in power between the flux maps directly fitted from the measured information and the estimated ones is only of 0.67% (just 0.10946 kW/m2 of root-mean-square error, on average, between them). According to these results, a consistent field descriptor can be built by applying the proposed methodology, which is hence ready for use.

ビームダウン式太陽集光装置のための機械撹拌式顕熱蓄熱装置の開発

One of the merits of a solar thermal utility is that the cost of heat storage is lower than that of an electrical battery. Furthermore, a power generation system using solar thermal energy with heat storage equipment is capable of stabilizing its output. For a parabolic trough solar concentrator, molten salt is adopted as the heat storage material using latent heat. However, molten salt is applicable only up to 600 °C; this temperature is not sufficiently high for adaptation to a solar central receiver system. Therefore, we designed and produced sensible heat storage equipment for a beam down solar concentrator (a central receiver system) to offer higher temperature storage. Heat storage materials in this device are mechanically mixed, and receive the concentrated sunlight on the focal plane of the solar concentrator. First, the radiation flux distribution under CPC (Compound Parabolic Concentrator) was measured using thin film heat flux sensors to obtain the input energy for the storage equipment. Next, an experiment was carried out using the storage equipment and the beam down solar concentrator. The maximum temperature was found to reach 1070 °C at the center of the heat storage tank in this experimental conditions, and the stored energy was approximately 14 % of the incident energy.

A parallel Teaching–Learning-Based Optimization procedure for automatic heliostat aiming

The flux distribution generated by the heliostat field of solar central receiver system (SCRS) over the receiver needs to be carefully controlled. It is necessary to avoid dangerous radiation peaks and temperature distributions to maximize the efficiency and keep the system in a safe state. These tasks imply both selecting the subset of heliostats to be activated and assigning each one to a certain aiming point at the receiver. The heliostat field is usually under human control and supervision, what is a potential limiting factor. Thus, there is an active research line to define automatic aiming procedures. In fact, a general and autonomous methodology is being developed by the authors of this work. However, the mathematical modeling leads to face a complex large-scale optimization problem. In this work, applying Teaching---Learning-Based Optimization (TLBO), a population-based large-scale optimizer, is considered. It is intended to serve to perform large explorations of the search-space to finally deploy further local optimizers over the most promising results. Considering the computational cost of the objective function, a parallel version of TLBO has been developed. It significantly accelerates the procedure, and the possibility of being included in a more complex process remains viable. Additionally, the parallel version of TLBO is also linked as a generic open-source library.

Optics of solar central receiver systems: a review.

This article reviews the state of the art in optical design, modeling and characterization of solar central receiver systems.

High performance computing for the heliostat field layout evaluation

In Solar Central Receiver Systems (SCRS), the heliostat field is generally the most important subsystem in terms of initial investment and energy losses. Therefore, heliostat field layout needs to be carefully designed and optimized when deploying this kind of power facilities. This optimization procedure can be focused on multiple and heterogeneous criteria depending on particular factors that lead to define different optimization problems based on specific objective functions. However, objective functions defined for this problem are, in general terms, computationally very expensive. This fact may make an exhaustive optimization process infeasible, specially depending on the available resources, and forces particular simplifications at some steps of the process. Fortunately, some of the objective functions defined can benefit from parallelization, even though this idea is not usually pointed out or discussed, and then, become affordable in better conditions. In this paper, the heliostat field optical efficiency, which is a common objective function in this area, is analyzed to be parallelized by three different approaches.