Direct Absorption Receiver Research Articles

Mitigating the losses in nanofluid-based direct solar absorption receivers

Solar thermal receivers are designed to absorb as much sunlight as possible (e.g., to increase energy inputs) while minimising heat losses and parasitic pumping energy requirements (e.g., to reduce energy losses). The field of direct absorption solar collectors has devised numerous elegant solutions for increasing energy inputs. Mitigating the energy losses, however, represents the other—generally overlooked—factor. To address this research gap, this study investigates the impact of nanofluids, superhydrophobic walls, an anti-reflective coating, and a transparent selective on the thermal efficiency of a microchannel-based solar receiver. The proposed modifications were ranked by their impact on the overall collector efficiency. It was found that compared to a pure water reference case, using a nanofluid working fluid provides the biggest reduction in optical losses (validating the literature's focus on this aspect). However, a micro-patterned surface achieved nearly the same improvement in optical efficiency. Adding a nanofluid, an anti-reflective cover, and a superhydrophobic surface (in concert) can boost the efficiency by ∼7% (compared to the reference case). Overall, this study provides a systematic approach to investigating features to mitigate energy loss mechanisms in direct absorption solar collectors via detailed component-level experimental testing and collector-level theoretical analysis.

Analysis of Nanofluid-Based Parabolic Trough Collectors for Solar Thermal Applications

Solar-to-thermal energy conversion technologies are an important and increasingly promising segment of our renewable energy technology future. Today, concentrated solar power (CSP) plants provide a method to efficiently store and distribute solar energy. Current industrial solar-to-thermal energy technologies employ selective solar absorber coatings to collect solar radiation, which suffer from low solar-to-thermal efficiencies at high temperatures due to increased thermal emission from selective absorbers. Solar absorbing nanofluids (a heat transfer fluid (HTF) seeded with nanoparticles), which can be volumetrically heated, are one method to improve solar-to-thermal energy conversion at high temperatures. To date, radiative analyses of nanofluids via the radiative transfer equation (RTE) have been conducted for low temperature applications and for flow conditions and geometries that are not representative of the technologies used in the field. In this work, we present the first comprehensive analysis of nanofluids for CSP plants in a parabolic trough configuration. This geometry was chosen because parabolic troughs are the most prevalent CSP technologies. We demonstrate that the solar-to-thermal energy conversion efficiency can be optimized by tuning the nanoparticle volume fraction, the temperature of the nanofluid, and the incident solar concentration. Moreover, we demonstrate that direct solar absorption receivers have a unique advantage over current surface-based solar coatings at large tube diameters. This is because of a nanofluid's tunability, which allows for high solar-to-thermal efficiencies across all tube diameters enabling small pressure drops to pump the HTF at large tube diameters.

Investigation of a single wall carbon nanohorn-based nanofluid in a full-scale direct absorption parabolic trough solar collector

An experimental investigation on the use of nanofluids as working fluids and direct absorbers in a full-scale concentrating collector is presented. The nanofluid consists of a suspension of single wall carbon nanohorns in distilled water with a concentration of 0.02gL−1. The thermo-physical properties are the same as those of the base fluid, but the presence of carbon nanoparticles greatly enhances the optical characteristics. A direct absorption receiver has been designed and set up to investigate the capability of the nanofluid to absorb the concentrated sunlight. The receiver exhibits a flat geometry and has been designed for installation on an asymmetric parabolic trough, where the concentrated solar flux locally reaches 100kWm−2 under clear-sky conditions. Results show that the application of a carbon nanohorn-based nanofluid in a concentrating collector displays an efficiency comparable to that obtained with a surface receiver tested in the same system. However, such performance is not maintained for a long time because of lack of stability of the absorbing fluid.

Modelling of a direct absorption solar receiver using carbon based nanofluids under concentrated solar radiation

Abstract The addition of nanoparticles in a base fluid can enhance its optical properties, in particular its absorption properties. Thus, nanofluids can be successfully used in solar collectors to absorb the solar radiation in their volume and avoid using an absorber plate. This paper investigates the application of aqueous suspensions as volumetric absorber in a concentrating direct absorption solar collector: a suspension of single wall carbon nanohorns (SWCNHs) in water is chosen as the nanofluid. A model of a solar receiver with a planar geometry to be installed in a parabolic trough concentrator is developed: the radiative transfer equation in participating medium and the energy equation are numerically solved to predict the thermal performance of the receiver. The developed model is capable to predict the temperature distribution, heat transfer rate and penetration distance of the concentrated solar radiation inside the nanofluid volume. The simulated performance of the direct absorption receiver has been compared with calculations and experimental data of two surface absorption conventional receivers under the same operating conditions.

Modeling of Flow and Heat Transfer in a Molten Glass Mini-Film for High Temperature Heat Collection in a Falling-Film Solar Central Receiver

ABSTRACTIn concentrating solar power plants, there is a strong incentive to increase the collection temperature and the overall exergy efficiency of the system. Some molten glass mixtures are attractive working fluids for high temperature solar thermal heat collection because optimized glass mixtures can be more stable, less-toxic, and less-corrosive than, for example, molten salts at high temperatures (≥1000°C). A specific phosphorous pentoxide glass mixture is considered in this study to explore its performance in a molten glass falling film central receiver design for collection of heat at conditions resulting in a mini-film with a thickness less than 3mm. In our falling molten glass thin film, the phosphate glass flow is treated as a laminar, Newtonian and gravity-driven flow over a slightly inclined flat plate using an explicit finite difference scheme to evaluate its heat transfer performance for a direct absorption receiver concept. One of the main challenges of modeling transport in the molten glass is the strong dependence of its viscosity on temperature. To incorporate this effect in our numerical analysis, a temperature-dependent viscosity model is used in the momentum equation to model the fluid behavior as it flows down the surface and is progressively heated. An exponential function is used to model the viscosity as it changes with temperature to properly fit the measured the viscosity data provided by Halotechnics. Also, a variable film thickness model analysis is implemented to treat the variation in film thickness that results from the viscosity variation with temperature. In order to avoid stability issues, the finite difference scheme is organized in terms of nondimensional parameters that include all important properties that govern the system. The results of our model indicate that thinning of the film as it flows over the heated surface enhances the heat transfer performance on the lower portion of the receiver system. The heat transfer coefficient increases instead of remaining constant (as normally expected for fully developed laminar flows) on the lower portion of the heated surface. The results further indicate that using a thin mini-film of molten glass for solar thermal heat collection provides high heat transfer performance and enhances the exergy collection.

Assessment of Improved Molten Salt Solar Tower Plants

The temperature level increase of molten salt solar tower plants is one important task on the development schedule to increase the system's overall efficiency. In the conventional power plant technology, modern supercritical steam power plants work with life steam temperatures near 620°C. To apply these modern power blocks in molten salt solar tower plants, salt temperatures near 650°C are required. Today's molten salt tower plants reach salt temperatures of 565°C. To follow the positive development tendencies of fossil power plants, in the present work the combination of supercritical steam power plants with solar towers is analyzed. As stated in recent literature (Kolb, Kelly, Singer), the state of the art tubular central receiver concept coupled with increased steam parameters of the power block (300bar/600°C/610°C) shows a marginal potential to decrease LEC. The identification of future concept innovations with significant cost reduction potential in the field of molten salt tower plants, the enhancement of the receiver efficiency as well as the assessment of critical aspects related to their feasibility is the task of this paper.Therefore this work first focuses on the state of the art molten salt tower technology and related improved concepts with molten salt. The selected improvements follow on the one hand the aim to increase the receiver's maximum temperature level to approximately 650°C to be able to feed modern supercritical power blocks, on the other hand the aim to require a low development effort. After the reference concept is characterized, the objective is the systematic comparison of receiver technologies, which show a potential of improved thermal efficiencies.The assessed receiver candidates are tubular and direct absorption receivers, either external or located in a cavity. The analyzed power level is 125 MWel, while the steam process parameters are varied from low temperature level (550°C) and subcritical steam to high temperature level (620°C) and supercritical steam. With this, selected molten salt power plants were specified and then modeled with different sizes of solar fields and different storage capacities and analyzed on an annual basis. The results show, that the assessed concept options close to the state of the art require drastically decreased investment costs to end up with significant LEC reduction and cost-competitiveness. The increase in overall efficiency of these improved concepts is compensated by their higher financial effort. Results of further concept assessments as well as the sensitivity analysis of parameters and costs are described.

Economic Chances and Technical Risks of the Internal Direct Absorption Receiver

Increased receiver temperatures of solar tower power plants are proposed to decrease the plants levelized electricity costs (LEC) due to the utilization of supercritical steam power plants and thus higher overall plant efficiency. Related to elevated receiver temperatures preliminary concept studies show a distinct LEC reduction potential of the internal direct absorption receiver (IDAR), if it is compared to liquid in tube (LIT) or beam-down (BD) receiver types. The IDAR is characterized by a downward oriented aperture of a cylindrical cavity, whose internal lateral area is illuminated from the concentrator field and cooled by a liquid molten salt film. The objective is the further efficiency enhancement, as well as the identification and assessment of the technical critical aspects. For this a detailed fluid mechanic and thermodynamic receiver model of the novel receiver concept is developed to be able to analyze the IDAR's operating performance at full size receiver geometries. The model is used to analyze the open parameters concerning the feasibility, functionality and performance of the concept. Hence, different system management strategies are examined and assessed, which lead to the proposal of a cost optimized lead-concept. This concept involves a rotating receiver system with inclined absorber walls. The spatial arrangements of the absorber walls minimize thermal losses of the receiver and enhance film stability. The centrifugal forces acting on the liquid salt film are essential to realize the required system criteria, which are related to the maximal molten salt temperature, film stability and droplet ejection. Compared to the state of the art at a 200 MWel power level the IDAR concept can lead to a LEC reduction of up to 8%. The cost assumptions made for the assessment are quantified with sensitivity analysis.

Economic Potential of Innovative Receiver Concepts With Different Solar Field Configurations for Supercritical Steam Cycles

The cost reduction potential of solar power towers (SPT) is an important issue concerning its market introduction. Raising the steam process temperature and pressure can lead to a cost reduction due to increased overall plant efficiency. Thus, for new receiver configurations, a supercritical steam cycle operated at 300 bar/600 °C/610 °C live steam conditions was assumed. The considered systems include innovative direct absorption receivers, either with conventional or beam down heliostat field layouts. For the beam down option, the receiver is assumed to be a cylindrical vessel with a flow-through porous absorber structure at the internal lateral area of the cylinder. The direct absorption receiver option consists of a cylindrical barrel with downwards oriented aperture, whose absorber structure at the internal lateral area is cooled by a molten salt film. For the assessment, CFD based methods are developed and able to examine the receiver efficiency characteristics. Based on the receiver thermal efficiency characteristics and the solar field characteristics, the annual performance is evaluated using hourly time series. The assessment methodology is based on the European Concentrated Solar Thermal Roadmap (ECOSTAR) study and enables the prediction of the annual performance and the levelized cost of electricity (LCOE). Applying appropriate cost assumptions from literature, the LCOE are estimated for each considered SPT concept and compared to tubular receiver concepts with molten salt and liquid metal cooling. The power level of the compared concepts and the reference case is 200 MWel. The sensitivity of the specific cost assumptions is analyzed. No detailed evaluation is done for the thermal storage, but comparable storage utilization and costs are assumed for all cases. At optimized plant parameters, the results indicate a LCOE reduction potential of up to 0.5% for beam down and of up to 7.2% for the direct absorption receiver compared to today's state of the art molten salt solar tower technology.

A study of solid particle flow characterization in solar particle receiver

The solid particle receiver (SPR) is a direct absorption receiver in which solar energy heats a curtain of falling ceramic particle to a temperature in excess of 1000 °C. A small scale test platform was built to investigate particle flow properties. The curtain was comprised of approximately 697 μm ceramic particles that were dropped within the receiver cavity of the test platform. Tests were conducted to experimentally determine the distribution of particles velocity, curtain thickness, and curtain opacity along a drop length of approximately 3 m. Velocity data were measured using a high speed digital camera to obtain images of the particle flow at 1000 frames per second with an exposure time of 100 μs. Five mass flow rates ranging from 1 kg/s-m to 22 kg/s-m were examined, and it was found that all flows approached a terminal velocity of about 6–7 m/s in a vertical drop distance of 3 m. The experimental results were validated with computational results and were found in excellent agreement with the simulation results. In addition, a similar study was performed with various sizes of the particles to better understand how the particle flow characteristics were affected by the size of the particles.

Solar reforming of methane in a direct absorption catalytic reactor on a parabolic dish: II—Modeling and analysis

The CAtalytically Enhanced Solar Absorption Receiver (CAESAR) test was conducted to determine the thermal, chemical, and mechanical performance of a commercial-scale, dish-mounted, direct catalytic absorption receiver (DCAR) reactor over a range of steady state and transient (cloud) operating conditions. The focus of the test was to demonstrate “proof-of-concept” and determine global performance such as reactor efficiencies and overall methane conversion. A numerical model was previously developed to provide guidance in the design of the absorber. The one-dimensional, planar, and steady-state model incorporates the following energy transfer mechanisms: solar and infrared radiation, heterogeneous chemical reaction, conduction in the solid phase, and convection between the fluid and solid phases. Improvements to the model and improved property values are presented here. In particular, the solar radiative transfer model is improved by using a three-flux technique to more accurately represent the typically conical incident flux. A spatially varying catalyst loading is incorporated, convective and radiative properties for each layer in the multilayer absorber are determined, and more realistic boundary conditions are applied. Considering that this test was not intended to provide data for code validation, model predictions are shown to generally bound the test axial thermocouple data when test uncertainties are included. Global predictions are made using a technique in which the incident solar flux distribution is subdivided into flux contour bands. Reactor predictions for anticipated operating conditions suggest that a further decrease in optical density (i.e., extinction coefficient) at the front of the absorber inner disk may improve absorber conditions. Code-validation experiments are needed to improve the confidence in the simulation of large-scale reactor operation.

High-temperature solar energy utilization after 15 years R&D: kick-off for the third generation of technologies

Today we look back on 15 years of international research and development of solar tower, parabolic trough and parabolic dish technologies. Many countries have contributed to this effort. The market is growing. Some 300 MW e from solar thermal parabolic trough power plants are being fed into electric grids and doing peaking/intermediate load duty under commercial conditions. It is reported that several hundred additional megawatts are under contract or being planned. Approxomately 3–5 billion DM have been spent worldwide on R&D. Only the parabolic trouh power plants have managed to get beyond the R&D stage. Here the accumulated experience is about 1000MW e ×a (1989) power plant operating years. Solar tower and parabolic dish technologies still have to demonstrate their market readiness or are not yet at the pilot project stage. A consortium of European and USA companies is preparing an air-colled 30 MW e solar tower power plant, PHOEBUS, for Jordan. The foundations are being laid for high-temperature solar chemistry. Fifteen years after the initiation of research activities, the international community of scientists and engineers at work on solar thermal energy conversion needs a nwe perspective for the next 15 years in order to avoid technological gaps. There are even more incentives for this work than before: ♭ Over five billion people live on earth; six billion are likely in 2000. The population doubles in only 30 years now! All need to be fed and clothed, all of them need a roof over their head. Since by far the majority lives in or near the highly insolated parts of the world, solar energy utilization is a natural choice. ♭ Investments for energy supply are a significant economic factor. In 1986 they amounted to 320 billion US$ (not including countries with centrally planned economies), which is equivalent to 11% of the world's capital investments. In the future, the greatest demand for energy supply projects will be in the developing countries, most of which are located in the earth's sun belt. For these countries the World Bank estimates a need for annual power plant investments of over 100 billion US$ for the next 20 years. At present about 20 billion US$/a is being invested by the energy utilities, by development banks, and as a result of bilateral agreements. This leaves a balance of over 50 billion US$ for which new financing has to be found. ♭ The use of solar energy contributes only very marginaly, if at all, to environmental damage. Solar energy does not require energy raw materials and as a consequence does not prodcue any associated pollutants. Those aspects of solar energy technologies having effects on teh environment, such as land area, material and eneryg intensities, are becoming less and less relevant because of progress in R&D. ♭ By contrast, the finite nature of the fossil energy raw materials and their irreversible for quasi-irreversible damage to the environment are becoming ever more obvious. The atmosphere is burdened with 22.5 × 10 9 tons of anthropogenic CO 2 annually (198). There is no containment technology yet nor is an economically applicable one likely. Research and development for the third generation of solar thermal high-temperature technologies has to be started and carried out with perseverance, in international cooperation. It is hoped that the International Energy Agency (IEA) can once again provide the necessary protective climate. Examples for future projects are: ♭ lightweigth membrane heliostats, troughs and dishes; ♭ industrialized solarized prime movers in the several hundred kilowatt to a few hundred megawatt range; ♭ receiver-reactors for dishes; ♭ thermal/latent heat storage for intermediate-load duty solar thermal plants; ♭ advanced air-cooled or direct particle absorption receivers; ♭ direct solar water vaporization receivers/loops. This paper views the state-of-the-art of high-temperature solar thermal energy conversion, raises questions about the relevant future technologies, outlines appropriate organizational and management structures, and estimates the financial requirements and the time scale.

Heat transfer in molten salt direct absorption receivers

Efficient absorption of concentrated solar energy in a falling film of molten salt is critical to the success of the direct absorption receiver (DAR) concept. This paper presents experimental data on the heat transfer between the molten salt film and the surface which supports the film. The results demonstrate that operation of the film in the turbulent regime (Reynolds number greater than approximately 3000) will give heat transfer coefficients high enough to ensure that the support surface temperature will not be excessive. In particular, we conclude that for a commercial receiver delivering 570°C nitrate salt with peak flux of 1380 kW/m 2, the support surface will not exceed 585°C. These results indicate that the addition of a dopant to the salt to enhance volumetric absorption of the solar flux may not be necessary. Adding 0.25 wt% cobalt oxide particles of nominal 40 μ size, increased the apparent heat transfer coefficient by about 140%.

Experiments and Analysis on the Molten Salt Direct Absorption Receiver Concept

This paper presents results of experiments on the Direct Absorption Receiver (DAR) concept using molten salt as the working fluid. The DAR concept may result in a solar central receiver that costs 50 percent less than the current tube receiver and has significantly lower operational and maintenance costs. These experiments were aimed at determining whether the DAR concept is technically feasible and were carried out at the Advanced Components Test Facility, Atlanta, GA. Results are based on several days of operating with solar flux ranging up to 50 W/cm2 and also on a numerical model that is capable of predicting the thermal performance of the DAR salt film. Issues relating to thermal efficiency, absorber-to-salt heat transfer, and salt film stability are addressed.

Experimental investigation of the direct absorption receiver concept

This paper describes the results of an experimental analysis of a flowing-film liquid direct absorption receiver (DAR) concept. Results of a numerical model of the thermal performance are also included. Experiments were conducted with concentrated solar flux at the Advanced Components Test Facility (ACTF) in Atlanta, Ga. Data are presented that support analysis of the film stability at high and low flow rates, thermal efficiency, and heat transfer (between the salt and the absorber plate). Comparisons of the data with analytical predictions of thermal efficiency and heat transfer were favorable and suggest a thermal efficiency of 80–90%, depending on operating temperature and flux. Heat transfer coefficients (for the undoped salt) of about 3000 W/m 2 °C were observed. No flow instabilities at high flow rates (Reynolds number up to 1200) were observed; however, a dry-out problem for flow rates below that was observed, which may be caused by experimental artifacts.

Optical properties of high-temperature materials for direct absorption receivers

Recent solar central receiver designs emphasize direct absorption receiver (DAR) concepts primarily because of their ability to absorb high flux densities. An attractive design utilizes molten salt as the transport/storage fluid, which is pumped to the receiver and allowed to flow over a high-temperature absorber surface. As the salt runs down the absorber surface in a thin film, concentrated solar flux heats the salt to 900°C, either directly (blackened fluid) or by convective heat transfer with the irradiated absorber (clear fluid). The feasibility of such a design depends on the optical efficiency of the absorber/fluid combination. The optical properties of candidate absorber materials and transport fluids are therefore required at appropriate elevated temperatures. Because such salts can be extremely reactive at high temperatures, it is important to measure optical properties as a function of exposure history of the salt/absorber. Optical characterization of a clear molten carbonate salt (Li 2CO 3Na 2CO 3K 2CO 3 ternary eutectic) and a high-temperature metal alloy (Inconel 600) has been carried out at elevated temperatures using a recently developed integrating sphere solar spectrometer. Reflectance measurements of oxidized Inconel 600 alone and covered with several thicknesses of molten salt have been made. Measurements were also made of the reflectance of Inconel 600 samples that had been exposed to molten salt under high cyclic temperatures.