Solar Tower Power Plant Research Articles

Efficient optimized design of solar power tower plants based on successive response surface methodology

AbstractRecently the optimal design of the solar power tower (SPT) plants have attracted increasing attention. In this paper, an improved algorithm combing the successive response surface methodology (SRSM) and simulated annealing (SA) global algorithm was proposed to achieve the efficient optimization design of the molten salt SPT plant with 2650 heliostats in Sevilla, Spain. Based on the traditional response surface methodology (RSM) and the adaptive domain reduction method, the SRSM was established to surrogate the complex thermo-economic model of the SPT plant to the updated approximation function, which was a high-order polynomial form to define the relationship between the objective parameter of the levelized cost of energy (LCOE) and 12 design variables related to the 4 subsystems of the SPT plant. After that, the SPT plant optimization design was performed by the SA global algorithm on the basis of SRSM. According to a comparison between the results obtained by the proposed method and the actual model-based global algorithm, the high accuracy and low computation time of the proposed strategy was proved.

Modeling and simulation of a novel combined heat and power system with absorption heat pump based on solar thermal power tower plant

In this paper a novel solar combined heat and power (CHP) system incorporating absorption heat pump (AHP) driven by mid-temperature solar heat and exhaust heat is proposed to improve solar energy utilization and electricity power generation. To validate thermodynamic performance of the proposed system, a case study is carried out based on 1 MW solar tower power plant located in Beijing. Depending on the well-developed mathematical model in accordance with actual equipment, the relevant module is coded in TRNSYS application environment. Besides, control and operation strategy is deeply analyzed to efficiently utilize solar energy with principal of cascade utilization. The dynamic simulation effectively predicts thermodynamic performance under the different operation modes in a typical winter day. Results indicate that flexible heating supply of the proposed system can meet well with user-generated heating loads. The AHP system recovers the condensate heat of 914 MJ with the average COP of 1.52. The proposed system generates the additional power of 217 kWh due to the regenerative cycle. Accompanying with power generation of 6214 kWh and useable thermal energy of 14502 MJ of the proposed system, the overall utilization efficiency of solar energy is up to 15.96% with the increase of 4.55 percentages.

Starting characteristics of a novel high temperature flat heat pipe receiver in solar power tower plant based of“Flat-front”Startup model

In this study, a novel high temperature two-phase closed flat heat pipe receiver is proposed and investigated experimentally. In order to simulate the transient startup from frozen state, a transient analysis code for the novel flat heat pipe receiver has been developed. Closed-form analytical solutions for the temperature distribution along the heat pipe length are obtained and experimental tests are undertaken. These closed-form analytical solutions are in good agreement with the experimental data. The theoretical and experimental studies prove that the flat heat pipe receiver with sodium as working fluid has a well startup performance. Experiments were carried out to investigate the temperature uniformity of FHPR under the normal conditions. It is found that the FHPR has well feature of uniformity and stability through startup process experiments with constant heat flux. This research provides guidance for the research and development of flat heat pipe solar receiver, and has important significance for broadening its application in the field of heat utilization of solar energy at high temperature.

A two-parameter aiming strategy to reduce and flatten the flux map in solar power tower plants

In surrounding solar power tower plants, the collector field is designed with all the heliostats pointing to the cylindrical receiver equator to obtain maximum intercept. However, in commercial plants, the addition of thousands of energy spots at the same receiver level causes an excessively high peak heat flux of about 2 MW/m2. Therefore, this peak flux should be almost halved to avoid receiver problems due to creep and fatigue effects. The single-parameter aiming strategy (Vant-Hull, 2002) has already shown its ability to divide this high peak flux into two lower peaks; it consists in moving the hot spots up and down from the equator, in alternative heliostat rows, to create symmetric flux maps although they are clearly not very homogeneously distributed along the receiver height. In this work, a slight variation of the single-parameter aim strategy, simply proposing two aim parameters, has been successfully tested for a commercial solar power tower plant with a regular layout. The new two-parameter aiming strategy achieves not only reasonable peak flux values, but also an acceptably flattened flux profile and a slight reduction in spillage compared with the single-parameter strategy.

Proposal and performance analysis of solar cogeneration system coupled with absorption heat pump

In this paper, a new kind of solar cogeneration system coupled with absorption heat pump (AHP) to recover condensate waste heat from exhaust of steam turbine in solar thermal power (STP) tower plant is proposed. Depending on mathematical models of the proposed system, several design parameters including direct normal irradiance (DNI), heat recovery ratio and improved power cycle are tested to evaluate their effects on the thermodynamic performance. Thermoeconomic investigations are also studied based on the existing AHP system installed in the pilot STP tower plant in Beijing. The results show that thermodynamic performance of the proposed system is improved due to energy and exergy contributions from steam storage tank (SST) and AHP. With the increase of DNI, the energy efficiency decreases accompanying with the increasing of exergy efficiency. As the condensate waste heat is completely recovered by AHP, the energy efficiency can reach up to 80.50%. Integrating with the improved regenerative cycle, the energy and exergy efficiencies can be increased from 16.90% to 17.17% and from 15.76% to 16.05%, respectively. Under the present base case, the unit heating cost decreases from 8.1 ¢/kWh to 2.8 ¢/kWh with the increase of operation time from 228 h to 2880 h.

Inverse heat problem of determining unknown surface heat flux in a molten salt loop

Inverse heat transfer problems typically rely on temperature measurements for estimating unknown boundary heat flux, such as that in the water tubes of steam boilers or central receivers in solar tower power plants. In this work, an experimental facility consisting of a molten salt loop that simulates a tube of a solar tower receiver is presented to obtain the outer tube surface temperatures under solar tower power plant operating conditions. The external surface f the pipe in the test section is heated in a controlled manner with an induction heater, which provides a very high nonuniform heat flux. An inverse thermal method has been applied to obtain the incident heat flux onto the receiver tube from the outer surface temperature measurements. To solve the inverse problem, a transient two-dimensional numerical model of a circular pipe flowing molten nitrate salt and subjected to a nonhomogeneous circumferential heat flux has been developed. The heat flux calculation with the inverse method is in accordance with the heat flux estimation based on the calibration of the induction heater. A good agreement between the experimental and calculated temperatures is observed. Furthermore, the deflection of the tube caused by the nonhomogeneous heat flux is measured and is compared to the deflection calculated from the radial temperature profile from the inverse problem solution, and a good agreement between both results is observed.

Atmospheric Transmittance Model Validation for CSP Tower Plants

In yield analysis and plant design of concentrated solar power (CSP) tower plants, increased uncertainties are caused by the mostly unknown solar attenuation between the concentrating heliostat field and the receiver on top of the tower. This attenuation is caused mainly by aerosol particles and water vapor. Various on-site measurement methods of atmospheric extinction in solar tower plants have been developed during recent years, but during resource assessment for distinct tower plant projects in-situ measurement data sets are typically not available. To overcome this lack of information, a transmittance model (TM) has been previously developed and enhanced by the authors to derive the atmospheric transmittance between a heliostat and receiver on the basis of common direct normal irradiance (DNI), temperature, relative humidity and barometric pressure measurements. Previously the model was only tested at one site. In this manuscript, the enhanced TM is validated for three sites (CIEMAT’s Plataforma Solar de Almería (PSA), Spain, Missour, Morocco (MIS) and Zagora, Morocco (ZAG)). As the strongest assumption in the TM is the vertical aerosol particle profile, three different approaches to describe the vertical profile are tested in the TM. One approach assumes a homogeneous aerosol profile up to 1 kilometer above ground, the second approach is based on LIVAS profiles obtained from Lidar measurements and the third approach uses boundary layer height (BLH) data of the European Centre for Medium-Range Weather Forecasts (ECMWF). The derived broadband transmittance for a slant range of 1 km ( T 1 k m ) time series is compared with a reference data set of on-site absorption- and broadband corrected T 1 k m derived from meteorological optical range (MOR) measurements for the temporal period between January 2015 and November 2017. The absolute mean bias error (MBE) for the TM’s T 1 k m using the three different aerosol profiles lies below 5% except for ZAG and one profile assumption. The MBE is close to 0 for PSA and MIS assuming a homogeneous extinction coefficient up to 1 km above ground. The root mean square error (RMSE) is around 5–6% for PSA and ZAG and around 7–8% for MIS. The TM performs better during summer months, during which more data points have been evaluated. This validation proves the applicability of the transmittance model for resource assessment at various sites. It enables the identification of a clear site with high T 1 k m with a high accuracy and provides an estimation of the T 1 k m for hazy sites. Thus it facilitates the decision if on-site extinction measurements are necessary. The model can be used to improve the accuracy of yield analysis of tower plants and allows the site adapted design.

Slag as an Inventory Material for Heat Storage in a Concentrated Solar Tower Power Plant: Design Studies and Systematic Comparative Assessment

By using metallurgical slag from an electric arc furnace that is otherwise not recycled but deposited as an inventory material in thermal energy storage for concentrated solar power plants, it is possible to make a significant step forward in two transformation processes: energy and raw materials. As this type of slag has not been considered as an inventory material for this purpose, it is important to clarify fundamental questions about this low-cost material and its storage design. In this paper, design studies of slag-based thermal energy storage are carried out. Different slag-specific design concepts are developed, calculated and evaluated by a method based on established management tools. Finally, concepts for further investigations are defined. The highest aptitude value and the lowest risk value are achieved by the vertical storage design with axial flow direction. Therefore, it is taken as the lead concept and will be considered in complete detail in further research. Also, a closer look, but not as detailed as the lead concept, is taken at the horizontal storage with axial flow and the vertical storage with radial flow direction.

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.

Quick design of regular heliostat fields for commercial solar tower power plants

The design of the collector field for commercial solar power tower plants, including tower height, receiver dimensions and the layout of thousands of heliostats, keeps on being a complex and time consuming task. The details of the full layout optimization for a big surrounding field plant, as Noor III (150 MWe, 7400 heliostats), performed with Campo code, are presented here by the first time in order to contribute suggestions that simplify the optimization without losing accuracy. After the validation of Campo, it is shown that it is able to optimize the whole radial staggered layout of thousands of heliostats with only two parameters. Running more than one hundred and fifty designs of the collector field, a ‘regular’ layout has been found. For big plants, this ‘regular’ layout may be used as a general one for any combination of tower height and receiver dimension tested thus simplifying greatly the process. The differences between the field efficiency for the ‘regular’ layout and that of the actual optimum layout are well less than 1%. Finally, it is found that, for big plants and tall towers, dense layouts work better than expanded layouts, in spite of that blockings are better for the latter.

Three-dimensional numerical study of direct steam generation in vertical tubes receiving concentrated solar radiation

Concentrated solar thermal energy is a good alternative to mitigate the environmental impacts generated by the growing use of energy. There are several commercial solar power tower plants in the world operating with direct steam generation. These plants operate with cavity-type and external tubular receivers. The conventional tubular receivers consist of a set of vertical circular tubes through which the working fluid circulates. This study is focused to analyze in detail the direct steam generation in a vertical tube receiving concentrated solar radiation. The mathematical model is based on the equations of continuity, momentum and energy for each phase. The modified model of Rensselaer Polytechnic Institute was used for the conditions of critical heat flux coupled to a Eulerian two fluid model. In the modified model of Rensselaer Polytechnic Institute, the total heat flux is divided into four components from the wall to the liquid: (a) liquid-phase convective heat flux, (b) heat transfer due to quenching, (c) heat flux due to evaporation and (d) convective heat flux of the vapor phase. The mathematical model was solved with a computational fluid dynamics software. The results were validated with experimental data reported in the literature and a parametric study was carried out to determinate the effect of the mass flux and heat flux incident on the wall, on: the steam quality, the volumetric fraction, the enthalpies and temperatures of liquid and steam. The reduction of mass flux by 44% increased the output mass flow of steam 4.28 times, whereas the increase of concentrated solar flux by 40% increased the output mass flow of steam 4 times.

Preliminary comparison of two potential test methods for molten salt receiver under uniformly and Gaussian-shape distributed incident power

Molten salt receiver is a critical equipment in solar power tower plant. It is essential to establish a test method to evaluate receiver performance. This paper applies the standard test method of solar collector to molten salt receiver. Inspired by the collector test method, a new test method is proposed based on the operating characteristics of solar power tower plant. Multiple simulations with different receivers and test conditions are run to compare the performance of the two methods. The comparisons show that both methods should are feasible with acceptable relative errors. However, the relative error of single test can be significant. It is suggested that both methods should be used with causion if test number is not sufficient.

Cogeneration using multi-effect distillation and a solar-powered supercritical carbon dioxide Brayton cycle

Supercritical carbon dioxide (sCO2) Brayton cycles are considered to be a potentially viable option for reducing the cost of electricity generation from solar power tower plants, owing to theoretically high efficiency, low compressor work, and compact turbomachinery. These cycles have relatively high heat rejection temperatures (>70 °C), which are ideal for integration with multi-effect distillation (MED) for distillate generation from seawater, without being a parasitic load to the power plant as compared to cogeneration with steam Rankine cycle. The intermittency of solar radiation reduces the annual capacity factor of concentrating solar power (CSP) plants to about 50–60% even with a molten salt storage tank of 10-h. This reduces the annual distillate generation and increases the cost of the distillate. A novel concept to reduce distillate cost is introduced, whereby the waste heat from the sCO2 Brayton cycle is stored. A two-tank demineralized water storage tank is selected and is shown to allow for the integration of a smaller MED system operating at an increased capacity factor. The optimal storage tank design reduces the distillate cost by 19% and increase the MED capacity factor from 46.4% to 75%. Different coastal locations with good solar resource are studied for cogeneration with the CSP-sCO2 Brayton cycle. Techno-economic analysis shows the cost of distillate produced by MED is 16% cheaper than the distillate produced from reverse osmosis system for Yanbu, Saudi Arabia.

Thermoeconomic analysis of multi-stage recuperative Brayton cycles: Part II – Waste energy recovery using CO2 and organic Rankine power cycles

Multi-stage gas turbine cycle configurations can greatly contribute to improve the economic and environmental aspects of gas turbine power plants. In the first part of this work, the hybridization of multi-stage recuperative Brayton power cycles with a solar power tower system that used a pressurized air receiver was evaluated. In this work, a detailed thermodynamic and economic analysis was conducted to evaluate the waste energy recovery from a non-solar multi-stage recuperative Brayton cycle configuration that showed the lowest levelized cost of electricity (LCOE) in the first part. A comparative analysis between different organic Rankine cycles (ORCs) and supercritical CO2 power cycles was conducted for waste energy recovery from both intercooling stages and the exhaust gas. After designing the systems, a transient analysis was conducted to consider the effect of off-design conditions on the cycles thermo-economic performance. The obtained results showed that waste energy recovery using an ORC is the most economical option. By recovering the available heat in intercooling and exhaust gas stages by ORCs that use Isobutane and Toluene as working fluids, respectively, an additional power of 3508 kW was generated. Also, thermal efficiency was increased from 53.6% to 59.9% and the LCOE was reduced from $57.80/MWh to $54.77/MWh. Overall, the results demonstrated that waste energy recovery from intercooling stages and exhaust gas of a multi-stage gas turbine power plant is an attractive option for improving its thermal and economic performance. Also, considering the high price of a solar power tower plant and the technological pressure and temperature limitation of a pressurized air receiver, waste energy recovery is a more practical and beneficial option at current time than hybridization with a solar power tower plant.

Dinitrogen tetroxide and carbon dioxide mixtures as working fluids in solar tower plants

The competitiveness of concentrated solar power technology in the near-future electricity generation scenario, requires a substantial reduction of the Levelized Cost of Energy which can be achieved with an increase of the energy conversion efficiencies while maintaining or reducing the investment costs. This paper discusses the use of pure Dinitrogen tetroxide N2O4, and N2O4/CO2 mixture, as working fluids in supercritical Brayton cycles applied to solar tower power plants. When N2O4 is combined with CO2, the resulting mixture has a comparatively higher critical temperature than pure CO2, allowing a condensing cycle even at the fairly high ambient temperatures of desert areas, where solar power plants are typically installed. This allows the adoption of simpler cycle configurations than the one used in sCO2 cycles (cost reduction) while achieving very high thermodynamic efficiency (47% at 700 °C). The N2O4/CO2 mixture with optimized composition, integrated in a solar tower unit, increases the solar-electric efficiency by 1% with respect to commercial plants based on steam cycle with 550 °C maximum temperature (22.3% vs. 21.3%). At 700 °C, the overall solar-electric efficiency can reach 24.5% which is slightly higher than supercritical CO2 cycles, yet with a foreseeable reduction of the investment costs as consequence of the simpler plant lay-out.

Analysis and optimization the size of heliostat field and thermal energy storage for solar tower power plants

Abstract The optimal sizing of solar tower power (STP) plants with thermal energy storage (TES) is critical for increasing the system reliability and reducing the investment cost. However, it is still unclear to determine the key design parameters for sizing the STP system, including design direct normal irradiance (DNI), solar multiple (SM) and TES hours. In this study, a thermo-economic analysis of the effects of these parameters on the capacity factor and levelized cost of energy (LCOE) of a 50 MW STP plant is presented using System Advisor Model. Optimal combinations of these parameters are obtained with respect to the minimal LCOE at two locations in China. The results show that the optimal design DNI is dependent on both the annual radiation level and the distribution of solar irradiance, which differs from the recommended values obtained from traditional methods. It is found that the irradiation received by heliostats at the optimal design condition accounts for a specific percentage of annual irradiation. The percentage is close to 73% for the cases in this study, and the corresponding optimal SM and TES hours are 2.7-2.8 and 15-16 hours respectively. The results can provide a theoretical reference for the optimal sizing of STP with TES systems under different solar resources.

Performance comparison of SPT systems integrated with various supercritical CO2-based mixture Brayton cycles based on multi-objective optimization

The adoption of other gas with supercritical carbon dioxide (S-CO2) cycle is an effective way to improve the performance of the solar power tower plant (SPT). Among various supercritical CO2-based mixture cycles, it is important to select a suitable layout for the SPT system. In this paper, the ability to generate work and compatibility between the thermal storage system and power cycle are investigated simultaneously. First, the effects of key parameters on the system overall exergy efficiency and specific work are discussed separately. Second, the specific work and temperature difference of the thermal storage system are considered simultaneously with the efficiency based on the multi-objective optimization. Finally, the optimal performance of various cycles are compared. The results show that the supercritical CO2-Xe cycles yield excellent performance and present obvious advantages in generating more specific work and good compatibility that compares to the S-CO2 cycles. While adding butane has reverse effects. The inter-cooling and recompression cycles are advantage in exergy efficiency with maximum 34.1% and 33.9%, respectively. The performance comparison of different cycles based on the multi-objective optimization can be helpful in selecting the suitable cycle layout.

Optical efficiency improvement of solar power tower by employing and optimizing novel fin-like receivers

The optical efficiency of the solar receiver is important to the global efficiency of the solar power tower plant. However, the present commercial cylindrical receiver causes a considerable optical loss because most of the sunrays reflected by the receiver surface are lost to the environment. To improve optical efficiency, three novel receivers with vertical finned structures and two novel receivers with horizontal finned structures are proposed in this paper. The optimal configurations of these receivers are obtained after parametric optimization. The optical efficiency and solar fluxes distributions of the optimal novel receivers are compared with those of the traditional cylindrical receiver at the noon of spring equinox. The results show that there is an optimal fin number of 6 for all the receivers with vertical finned structures. The receiver with vertical finned structures and the flat bottom shows the maximal optical efficiency of 63.9%. It indicates an improvement of 3.2 percentage points compared with that of the traditional cylindrical receiver. The receiver with horizontal fins with flat bottom shows the minimal optical efficiency of 60.3% at the noon of the spring equinox. It indicates a decrease of 0.4 percentage points compared with that of the traditional cylindrical receiver. Additionally, the peak solar fluxes on the novel receivers are quite close to each other and are nearly double that of the traditional cylindrical receiver. In the end, a multi-point aiming strategy is applied to the optimal novel receivers, and the results show that the peak fluxes can be obviously reduced. It is concluded that optical efficiency can be effectively improved by using the receivers with vertical finned structures.

Structural reliability analysis of a heliostat under wind load for concentrating solar power

A heliostat is a structure whose function is to reflect sunlight to a target receiver. It represents an important element in solar tower power plant (STPP). Heliostat deformations, due to stochastic wind loads and geometrical properties, can degrade optical pointing accuracy and fatigue the structural components. This paper reports on an analytical and numerical program with a goal to improve understanding of the response to wind loading on heliostats reliability.To this end, the main heliostat components, such as pedestal, torque tube and frame, have been modeled for different azimuth and elevation angles. The obtained mathematical models have been used as performance functions in order to evaluate the structural reliability of heliostat. Subset simulation method has been considered to assess this reliability. The uncertainties in wind speed, heliostat dimensions (diameter, thickness, section…) and heliostat physical properties (yield stress and Young’s modulus), have also been taken into account in this study. The results show that the external loads (wind speed) and geometrical properties, such as pedestal and torque tube diameters and truss member sections, have represented an important effect on the heliostat reliability. Therefore, for small heliostats sited in higher wind speed, it is more reliable to use thin torque tube with large diameters. However, a thick torque tube with small diameters are suitable for heliostat placed in lower wind speed.

Heliostat Cluster Control for the Solar Tower Power Plant Based on Leader-Follower Strategy

As one of the most promising power generation technologies using renewable energy sources, and the solar thermal electric power generation may be developed to alleviate the energy crisis. But it still faces the obstacles of excessive cost and low energy efficiency nowadays. To this end, a method of using heliostats cluster control is proposed to solve the main contradiction in solar tower power plants. In this approach, the entire heliostats field will be divided into a few groups, and a leader heliostat with high-precision closed-loop control is arranged in each group while the remaining heliostats as follower heliostats. On this basis, a leader heliostat tracking model is established and a leader-to-follower heliostat conversion mode is designed. Meanwhile, the posture information of adjacent leader heliostats is communicated to guarantee the tracking reliability. According to the simulation results of this paper, this method not only improves the automation level of solar tower power plants at this stage, but also reduces the computing complexity of the heliostats field control algorithm. Moreover, the proposed method improves the precision of the control, and also reduces the control cost, making the closed-loop control of the heliostats field possible.