Solar Tower Receiver Research Articles

Development of Insulation for High Flux Density Receivers

Abstract In the last years more experience has been achieved with long time operation of solar tower receivers increased in size. The insulation of these receivers shows higher complexity and importance compared to small prototypes [1,2]. Both insulation of tubular cavity receiver and inner insulation of piping are safety-relevant components as they protect e.g. support structures and pressurized tubes and connections against overheating. Besides safety, operating reliability is one of the most important requirements to meet as down times, maintenance and repairs cause high costs. This paper describes the new development of the insulation for a tubular cavity receiver with air as heat transfer medium, where costs and thermal heat loss should be reduced. The prototype has been designed, built and tested for 100 operating hours at receiver outlet temperatures of up to 800 °C at the test facility Plataforma Solar de Almeria (PSA), Spain. The results of a thermal model, of material tests and the evaluation of the thermal heat loss are presented.

Experimental system for long term aging of highly irradiated tube type receivers

Tube type receivers are being designed with codes that underestimate the thermo-mechanical fatigue and do not consider the effect of ultraviolet radiation. The effect of those phenomena in the receiver durability must be estimated from long-term aging tests, which are best carried out in a solar concentration facility. For this purpose, a solar dish, which was initially coupled to a Stirling engine, has been transformed into a test laboratory capable of reproducing the severe solar radiation concentration and temperature conditions typical of solar central tower receivers. In this facility, the temperature and the solar radiation density are controlled as independent variables. On one hand, the solar radiation density can be adapted to the test required level moving the sample along the dish optical axis. On the other hand, the temperature is controlled by an air flow, which is used as the heat transfer fluid. A test bed, including all the required elements for controlling the temperature and the solar radiation density, has been built up to satisfy certain solar irradiance and temperature ranges. So far, the solar dish has shown to be a very versatile facility for testing materials aging under high solar radiation densities.

Aging of solar absorber materials under highly concentrated solar fluxes

This article introduces an experimental approach to study the aging of absorber materials used in the receivers of CSP power plants. The term ‘aging’ is discussed in the case of solar absorber materials and a definition is suggested. A two-layer material (metal+paint coating) that is commonly used in solar tower receivers is studied. Accelerated aging tests are performed on this material with the Solar Accelerated Aging Facility (SAAF) available at PROMES laboratory (Odeillo, France). To evaluate the aging state of the samples after being solar-treated, the thermoradiative and thermophysical properties that characterize the aging of the coating are estimated with two experimental devices. The results show the influence of the mean irradiance, amplitude, period, and exposure time of the radiative cycles on the estimated properties. The absorptance appears to be the most sensitive property causing the thermal performance to decrease. Particular attention should be paid to the application of the paint coating which needs to be vitrified carefully in order to obtain optimal and stable absorption properties.

Advanced CFD&HT Numerical Modeling of Solar Tower Receivers

This paper presents an advanced methodology for the detailed modeling of the heat transfer and fluid dynamics phenomena in solar tower receivers. It has been carried out in the framework of a more ambitious enterprise which aims at modeling all the complex heat transfer and fluid dynamics phenomena present in central solar receivers. The global model is composed of 4 sub- models (heat conduction, two-phase flow, thermal radiation and natural convection) which are described. Results of the numerical model obtained so far are also presented and discussed

Accelerated Aging of a Solar Absorber Material Subjected to Highly Concentrated Solar Flux

To support the fast growth of Concentrating Solar Power (CSP) technologies, R&D efforts need to be continued to minimize costs. Reducing manufacturing, processing and maintenance costs is key to reach a competitive low production price for electricity.From this perspective, one issue that becomes increasingly significant is the service lifetime of solar absorber materials. In solar tower receivers, absorber materials are subjected to very intense solar flux and cyclic thermal stresses. The chemical and physical aging mechanisms such as corrosion and oxidation, or the possible apparition of cracks or delamination are responsible for the change of the material's surface and bulk properties over time. This causes the thermal performance of the material to decline more or less rapidly.Taking a two-layer solar absorber material (metal + coating) that is commonly used in power tower receivers (SOLAR TWO, GEMASOLAR, SOLHYCO), we have designed several accelerated aging tests by analyzing the thermal behavior of the material in permanent and variable regimes. The experimental tests consist in subjecting the material to cyclic irradiance of variable intensity, amplitude and period to determine the fastest and most realistic tests. A Solar Accelerated Aging Facility (SAAF) was built for this purpose. It is made of a 2-meter diameter parabola concentrating the solar radiation (up to 16,000 times) onto a material sample that is cooled by air or water in direct contact with its rear face. A flux sensor coupled to a shutter allows us to apply the desired solar irradiance while an infrared pyrometer monitors surface temperature of the sample.Following different constant-irradiance solar aging treatments, the properties that are characteristic of the aging are estimated. The normal solar absorptance of each sample is evaluated by a solar optical fiber reflectometer. The thermophysical properties of the paint such as the thermal diffusivity, effusivity, conductivity and the thermal contact resistance between the paint layer and the metal substrate are estimated by inverse method using the measurements of an impulse photothermal experiment. In addition, untreated samples are characterized in order to provide rare data on the ‘not aged’ material.Careful attention should be paid to the preliminary vitrification heat treatment carried out to give the black paint its optimal absorption properties before being used. The results of the aging tests show a slight deterioration of the properties corresponding to the first aging signs of the material. The solar absorptance and the thermal effusivity of the paint coating are the most affected properties during the tests.

Simulation of Solarized Combined Cycles: Comparison Between Hybrid Gas Turbine and ISCC Plants

The present paper investigates two different solarized combined cycle layout configurations. In the first scheme, a solarized gas turbine is coupled to a solar tower. Pressurized air at the compressor exit is sent to the solar tower receiver before entering the gas turbine (GT) combustor. Here, temperature is increased up to the nominal turbine inlet value through natural gas combustion. In the second combined cycle (CC) layout, solar energy is collected by line focusing parabolic trough collectors and used to produce superheated steam in addition to the one generated in the heat recovery boiler. The goal of the paper is to compare the thermodynamic performance of these concentrating solar power (CSP) technologies when working under realistic operating conditions. Commercial software and in-house computer codes were combined together to predict CSP plant performance both on design and off-design conditions. Plant simulations have shown the beneficial effect of introducing solar energy at high temperature in the Joule–Brayton cycle and the drawback in terms of GT performance penalization due to solarization. Results of yearly simulations on a 1 h basis for the two considered plant configurations are presented and discussed. The main thermodynamic parameters such as temperatures, pressure levels, and air and steam flow rates are reported for two representative days.

Triple cycle for solar thermal power system adapted to periods with varying insolation

In the current study, a new solar-driven triple cycle is proposed to allow power generation during low insolation periods. This triple cycle integrates the solar gas-turbine top cycle, the steam Rankine cycle, and the Kalina bottom cycle. During the top cycle of the proposed system, compressed air was heated to 1000 °C or higher in the solar tower receiver. The heated compressed air was then used to drive the gas turbine to generate electricity. A Rankine cycle with a back-pressure steam turbine was utilized to recover waste heat from the gas turbine, thereby generating electricity through the steam turbine. The bottom cycle is the Kalina cycle, which comprises another back-pressure turbine and utilizes ammonia–water mixture as working fluid. After driving the steam Rankine cycle, the flue gas from the gas turbine sequentially heats the ammonia–water mixture to produce power. A new operational strategy was presented to generate electricity during low insolation period without the backup of fossil fuel. In middle insolation periods, the air is heated by the solar field and then directly drives the steam Rankine cycle, bypassing the gas turbine. In low insolation periods, the heated air directly drive the Kalina cycle, bypassing the Brayton cycle and the steam Rankine cycle. The off-design performance was investigated and the irreversibility was disclosed with the aid of the energy-utilization diagram method. Thus, the proposed system can utilize low insolation to generate electricity. This study provides a possibility to improve the solar–electric efficiency.

Control of the flux distribution on a solar tower receiver using an optimized aiming point strategy: Application to THEMIS solar tower

Life time of components is one of the technological bottle-necks in the development of solar tower power plant technology. The receiver, which is subjected to high and variable concentrated solar flux density is particularly affected: High, variable and non-homogeneous solar flux on the solar receiver walls results in strong stresses because of high temperatures, thermal shocks and temperature gradient that contribute to the reduction of the life time of this key component. This work aims to present an open loop approach to control the flux density distribution delivered on a flat plate receiver for a solar power tower. Various distributions of aiming points on the aperture of the receiver are considered. The flux density distribution on the aperture is simulated by a computer code. A specific neighborhood is defined for the TABU optimization meta-heuristic according to the size of the image of each individual heliostat. This modified algorithm is implemented to select the best aiming point for each heliostat. This approach has been validated using the example of THEMIS solar power tower in Targasonne, France.

A numerical thermal approach to study the accelerated aging of a solar absorber material

Solar power tower receivers are exposed to highly-concentrated solar flux. The strong flux variations that they are exposed to during their service life enhance the physical–chemical aging mechanisms and cause the decrease of the material’s thermal performance. A material that is commonly used for these applications was selected for our study. It is used for the absorber tubes of a solar power tower receiver. This material is made of an alloy (Inconel 625) that is coated with a silicone-based paint coating (Pyromark 2500 black) to increase the solar radiation absorption capacity.With the aim of determining the optimal conditions to accelerate the aging of this two-layer material (metal+paint coating), an axisymmetric 2-D model reproducing its thermal behavior was developed. Several thermal indicators (temperature, thermal gradients, temporal gradient), which are representative of the thermal aging factors and the material’s thermal performance, are analyzed in different configurations of boundary conditions. One configuration was defined according to the normal working conditions of a particular solar power plant application; the others are associated with various boundary conditions with the potential to increase or modify the thermal stress factors to accelerate the aging mechanisms. Other aging factors such as humidity, pollutants, and dust are not investigated in this paper. Several simulations were run in permanent and variable regimes. The most influencing boundary conditions and material properties were highlighted by a sensitivity study. To design relevant aging tests, particular attention should be paid to the incident solar power and the cooling characteristics of the material. The surface total absorptance, the thermal conductivities and the thermal contact resistance between the paint and the metal layers are the parameters that most affect the material’s thermal behavior. The evolution of those material properties characterizes the aging and should be monitored. Irradiance cycles were simulated for their potential to increase the thermal fatigue. Depending on the average, the amplitude and the period of the cycle, the evolution of the thermal indicators were analyzed. They were compared to select the most appropriate aging tests that are to be performed. From there, two aging strategies were determined. To put them in practice, an experimental Solar Accelerated Aging Facility (SAAF) was built and is described in this paper. It enables to perform accelerated aging experiments with a 2-m-diameter parabola concentrating the sun radiation up to 16,000 times. A preliminary experiment to validate the model confirmed that the simulation results are in good agreement with the experimental.

An integrated solar thermal power system using intercooled gas turbine and Kalina cycle

A new solar tower thermal power system integrating the intercooled gas turbine top cycle and the Kalina bottoming cycle is proposed in the present paper. The thermodynamic performance of the proposed system is investigated, and the irreversibility of energy conversion is disclosed using the energy–utilization diagram method. On the top cycle of the proposed system, the compressed air after being intercooled is heated at 1000 °C or higher at the solar tower receiver and is used to drive the gas turbine to generate power. The ammonia–water mixture as the working substance of the bottom cycle recovers the waste heat from the gas turbine to generate power. A concise analytical formula of solar-to-electric efficiency of the proposed system is developed. As a result, the peak solar-to-electric efficiency of the proposed system is 27.5% at a gas turbine inlet temperature of 1000 °C under the designed solar direct normal irradiance of 800 W/m2. Compared with a conventional solar power tower plant, the proposed integrated system conserves approximately 69% of consumed water. The results obtained in the current study provide an approach to improve solar-to-electric efficiency and offer a potential to conserve water for solar thermal power plants in arid area.

Numerical investigation on porous media heat transfer in a solar tower receiver

In order to investigate the steady heat transfer characteristics of a porous media solar tower receiver developed in China, this paper applies the steady heat and mass transfer models of the porous media to solar receivers, chooses the preferable volume convection heat transfer coefficient model, solves these equations by using the numerical method, and analyzes the typical influences of the porosity, average particle diameter, air inlet velocity, and thickness on the temperature distribution. The following conclusions have been drawn: in the same position or relative position along the downstream, the bigger the average particle diameter is, the higher the solid matrix dimensionless temperature is, the higher the air dimensionless temperature is. The bigger the porosity is, the lower the solid matrix dimensionless temperature is, the bigger the porosity is, the higher the air dimensionless temperature is. The bigger the thickness is, the lower the solid matrix dimensionless temperature is, the higher the air dimensionless temperature is. In a certain depth, the bigger the air inlet velocity is, the higher the solid matrix dimensionless temperature is. After a certain depth, the bigger the air inlet velocity is, the lower the solid matrix dimensionless temperature is, and the bigger the air inlet velocity is, the higher the air dimensionless temperature is. The paper can provide a reference for this type of receiver design and reconstruction.

Thermal resistance of nanoporous antireflective coatings on silica glass for solar tower receivers

Quarter-wave anti reflectance (AR) coatings were applied on silica glass by a sol–gel dip coating process to minimize reflective losses of the receiver cover materials for solar tower power plants. To investigate the thermal resistance of the coatings prepared from silica sols of 20 nm, mean particle sizes, solar transmittance, thickness, and density were determined as a function of isothermal heat treatments using UV–vis–NIR spectroscopy, profilometry, and X-ray reflectometry, respectively. The initial solar transmittance τ of the AR coated silica glass, cured at 773 K, was 0.973 for a film thickness d=111 nm. Heat treatments in the range from 1273 to 1423 K for 15–240 min reduce both film thickness and solar transmittance of the crystal-free films. The isothermal shrinkage rate of the film thickness was described for a relative density >0.634 using Mackenzie–Shuttleworth (MS) model. Viscosity data calculated from the sintering rate constant of the MS model were compared with literature data of silica glasses of known water content prepared by sintering and fusion. This analysis indicates that the sintering kinetics of the nanoporous film were controlled by viscous flow of amorphous silica particles of a water content of ≈1100 ppmw associated with an activation energy of 321 kJ mol −1.

Calibration corrections of solar tower flux density measurements

The PSA flux density measuring system PROHERMES measures the concentrated solar radiation in the entrance aperture of solar tower receivers with a white rotating bar as target and a CCD-camera taking images. The calibration is done with commercial flux gauges placed in the measurement plane. To im prove the calibration of the system and to reveal systematic errors, measurements are performed with two different types of commercial flux gauges (Thermogage sensors with and without quartz window) and a large custom-made calorimeter used as reference. The comparison shows that the sensors without quartz window measure about 5–8% higher and the sensors with quartz window about 100% higher. This error is explained with the differences in the spectral composition of the radiation and different angles of incidence between the manufacturer calibration and the solar measurements and corrections are proposed. Spectral changes of the sunlight during the day and year can affect the measurements by more than 10%. By selecting a correction filter adapted to the camera sensitivity, this influence can be reduced to less than 2.5%. Due to the reflective properties of the target coating, changes in angle of incidence can affect the measurements. In standard solar field conditions, this error is less than 0.5%, but for special conditions a correction of the systematic error of up to 8% is proposed.

COMPARATIVE PERFORMANCES OF `TOWER-TOP' AND `TOWER-REFLECTOR' CENTRAL SOLAR RECEIVERS

The optics of various types of solar tower configurations and receivers are analyzed. The power values that are finally available for the process, for a given field and different optical arrangements at specified operating temperatures (above 1100 K), are compared. At these temperatures the use of secondary concentrators placed in front of the receivers is peremptorily justified from optical and energetical points of view. A model for calculating the efficiencies of central solar receivers with secondary concentrators is presented. This model permits the optimization of these secondary concentrators taking into account the different losses of real devices. The effect of heliostat field density and the performance of `tower-top' and `tower-reflector' solar plants are discussed and compared.