Direct Solar Steam Research Articles

Numerical analysis of the heat transfer performance of the absorber tube of a parabolic trough solar collector using the swirling flow technique

In the present work, the absorber tube of the DISS (Direct solar steam) solar plant in Plataforma Solar de Almeria (Spain) is powered by fluid flows in a swirl motion. Numerical simulations are used to predict the performance of a parabolic trough solar collector with inlets that allow the working fluid to flow in a swirl motion inside the absorber tube. There are numerous sites for tangential inlets distributed along the absorber tube. There is one inlet placed up, one located down, two located up, and two located down. High Reynolds numbers between 2.2 × 105 and 6.2 × 105 are used for the simulation. When the collector with the tangential inlet is compared to a conventional collector, the results reveal that using one inlet down causes a 9.1K decrease in the average temperature of the lower surface of the absorber tube. Compared to axial flow, one inlet down increases useful heat and heat transfer by 12.7 % and 41.7 %, respectively. The average friction factor of axial flow is 16.4 % higher than that of one inlet down. The collector with a tangential inlet reduces surface area by 10.9 % over Reynolds ranges when compared to an axial flow collector. Therefore, the thermal strain on the absorber tube and collector surface area is reduced by the swirling flow in the parabolic trough solar collector. As a helpful design tool, thermal performance correlations of absorber tubes with tangential inlets are constructed.

Alternative Approach for Thermo-Hydraulic Modeling of Direct Steam Generation in Parabolic Trough Solar Collectors

Abstract The implementation of direct steam generation in linear concentrators is limited mainly by the complexity and the high demand for computational resources of the models developed to predict the installation behavior. With this in mind, we introduce an innovative methodology to characterize the thermo-hydraulic behavior of direct steam generation in parabolic trough solar collectors, with a strong focus on two-phase flow phenomena. Our proposed approach has resulted in a generalized function that eliminates the need for the convective coefficient h and enables accurate prediction of the flow pattern within the receiver. By comparing our model with experimental data from the literature, we achieved relative squared errors (RSEs) values of less than 3% for temperature and pressure calculations, thus validating the robustness of our methodology. The Taitel and Dukler diagram confirms an appropriate flow pattern, while intermittent flow is observed initially during boiling; the pressure drop, although slightly elevated compared to direct solar steam (DISS) loop results, remains within acceptable limits; and the model demonstrates suitability for assessing liquid water, phase change, and superheated steam temperature evolution along the loop. Moreover, we further showcased the practical application of our developed model by applying it to a specific case study conducted in Agua Prieta, Sonora (Northwest México). The validated model exhibits versatility and is applicable to various cases, including both concentrating systems for electricity production and solar heat for industrial processes.

Cost-effective seawater desalination by photothermal membranes composed of biopolymers-doped graphene oxide using direct solar steam generation strategy

The current study detailed the impact of the qualitative and quantitative addition of biopolymers, besides the influence of the drying method on the efficacy of the prepared photothermal membranes in seawater desalination using the direct solar steam generation (DSSG) strategy. Three natural polymers (NP) (i.e., cellulose acetate (CA), gum Arabic (GA), and sodium alginate (SA)), one synthetic polymer (i.e., polyvinyl alcohol; PVA) were used in membranes' preparation at different (NP/PVA) weight ratios, while graphene oxide (GO) and silica aerogel (SG) were added as a photothermal absorber and to enhance membrane floatability, respectively. The solar-driven evaporation experiments revealed that the evaporation rates (ER) were enhanced by increasing the NP/PVA ratio. Freeze-dried photothermal membranes demonstrated higher performances (ER = 1.59–3.62 kg/m2/h) compared with the freezing-thawed membranes (ER = 1.36–2.95 kg/m2/h) under 1-sun illumination when the NP/PVA ratio increased from 1:1 to 2:1. Interestingly, among all freeze-dried membranes, SA-based membranes at (SA/PVA = 3:1) ratio reflected the highest performance (ER = 4.33 kg/m2/h) at 1-sun illumination. The recorded high ER can be interpreted by several features: increasing the hydrophilic character of the composite membranes at high NP/PVA ratios, the high microporous structure of the internal channels when applying the freeze-drying method; the high GO content which effectively converts light to thermal energy; adequate membrane thickness (∼0.5 mm); besides the presence of SG to maintain the membrane floatability inducing heat localization. Thus, the study provided low-cost freshwater production at affordable rates using biopolymers-doped graphene oxide for desalination using DSSG systems.

Predictive machine learning models for optimization of direct solar steam generation

Direct solar steam generation (DSSG) has gained significant consideration in the recent decade because of its ability to generate freshwater, relying on renewable solar energy. Despite experimental data abundance, it is still difficult to optimize DSSG under certain conditions regarding fluid surface temperature changes (Ttop) and evaporation efficiency (η). This study investigates six predictive machine learning models, including multilayer perceptron (MLP), support vector regression (SVR), decision tree (DT), random forest (RF), adaptive boosting ensemble (ADA-BE), and combinations of them, to model Ttop and η in interfacial and volumetric DSSG systems. The models are trained on experimental data, and their performance is evaluated using various metrics. Based on the findings of the study, the DT (total R2 = 0. 9900) and DT-SVR combo (total R2 = 0.9829) are the best models to predict η in interfacial and volumetric systems, respectively. Results show that interfacial DT-MLP combo (total R2 = 0.9964) and volumetric DT-ADA-BE (total R2 = 0.9870) models predict Ttop more accurately. The study predicts that the ηmax of 85 ± 5 % and 90.91 ± 5 % will be obtained under one sun (1 kW/m2) using GNP-MWCNT with 0.015 weight percentage in volumetric and using Au-HT-wood with a thickness of 14.78 mm in interfacial approaches, respectively.

Preparation of Carbon/PVA/Chitosan Aerogel and Its Application to Direct Solar Steam Generator

Direct Solar Steam Generator（DSSG）is a technology that is currently attracting attention due to its advantages over conventional evaporative desalination technologies, such as the fact that it does not use fossil energy and has a photothermal conversion efficiency of almost 100%. The DSSG system is composed of steam generating membrane including photothermal and water transportation materials. Thus, the performance of DSSG system depends on the photothermal and water transportation materials, and heat management of system. Recently, carbon materials are considered as good photothermal materials under one sun irradiation（1 kW/m2）. Carbons synthesized by the solution plasma（SP）method show good feature such as porous structures, large surface area, and fast transportation of water. In this study, we synthesized photothermal carbon materials by SP method and prepared films composed of poly（vinyl alcohol）（PVA）and chitosan aerogel（CSA）with high affinity to water by freeze-drying, and composited carbon and PVA/CSA film（Cx/PVA/CSA: x＝0.1，0.5，1.0）．The morphologies and evaporation performance of the Cx/PVA/CSA were investigated. The evaporation rate and evaporation efficiency of C0.5/PVA/CSA were estimated to be 1.29 kg/m2･hand 86.1%, respectively. These values showed excellent evaporation performance within the range of C additions investigated. Finally, the desalination test of DSSG system using the C0.5/PVA/CSA was performed and the DSSG system showed desalination performance of more than 99.3% for Na＋, Ca2＋ and Mg2＋ ion concentrations in sea water obtained from Toyosu canal.

Cavity-shaped direct solar steam generator employing conical helical tube for high-temperature application: Model development, experimental testing and numerical analysis

Concentrating solar thermal technologies are drawing more attention since it can substantially contribute to a carbon-neutral society. The solar receivers are essential in this technology to convert solar energy efficiently into thermal energy. Moreover, high-temperature steam generation is the promising application for concentrated solar power plants or industirial processes. Therefore, the direct solar steam generator has been gaining more attention due to its advantages of low operation and maintenance costs. Most solar steam generator designs consist of a tube with helical configuration because of its high heat exchange performance and compactness. However, only few studies address solar steam generators with conical helical tubes. Thus, both experimental and simulation data of this design are scarce. This paper shows the successful development and experimental testing of a solar steam generator with a conical helical tube. The experimental results proved that the developed solar steam generator can produce high-temperature steam of 600 °C at an inlet pressure and mass flow rate of 150–200 kPa and 2.5 kg/h, respectively. The overall calculated energy efficiency (thermal and optical efficiency) was 60–62%. In addition, a coupled 1D-3D numerical model was implemented to analyze the solar steam generator’s performance. The model consists of a 3D cavity heat transfer model and a 1D two-phase fluid flow model. The numerical analysis demonstrated the ideal generator’s performance (energy efficiency of 68–69%) and the great impact of convection in the heat losses (50% of the total energy losses). Although more research of the convection is required, the presented results provide a basis for designing further, upscaled solar steam generators employing conical helical tubes.

Evaporator fabricated with accessible photothermal material derived from waste fallen leaves for highly efficient desalination

Direct solar steam generation (DSSG) has been considered to be one of the most promising technologies to alleviate the significant threat of freshwater scarcity worldwide. Among these, the solar-thermal evaporator based on carbonized natural materials is a very promising and environmentally friendly alternative. However, due to the high energy consumption and expensive equipment requirements in the preparation of conventional carbon-based materials, developing simple and low-cost material preparation methods without additional energy input is still challenging. Fallen leave is a widely distributed and renewable waste biomass that is easily accessible, which can be made into evaporators for desalination. Herein, the biomass waste fallen leaves were selected as raw material to synthesize carbonized fallen leaves (CFL) by simple sulfuric acid carbonization without energy input. Then the CFL were sprayed onto artificial deerskin towels (ADT), and finally, the portable CFL@ADT evaporators were constructed. Our CFL@ADT evaporator with 1 wing shows a solar absorptance of 97.4 % and an evaporation rate of 1.50 kg m−2h−1 in water with an evaporation efficiency of up to 95 %. The CFL@ADT evaporators provide an attractive alternative to conducting biomass waste resource utilization as well as achieving efficient and economical desalination during the day and self-cleaning at night.

Co‐Hydrothermal Carbonization of Cotton Stalks and MnO2 for Direct Solar Steam Generation with High Efficiency

Biomass‐based photothermal materials for solar steam power generation have been a recent research hotspot. However, despite the many research advances that have been made, many challenges remain. For example, high temperature carbonization leads to the loss of hydrophilicity, the surface coating is easy to peel off, hydrothermal carbonization (HTC) does not guarantee the photothermal performance, and hydrothermal carbonization fluid (HTC‐F) is difficult to handle. Herein, a new material (H‐CS/MnO2) is prepared by co‐hydrothermal carbonization of cotton stalks (CS) and MnO2, and a stable and effective evaporation system is designed. While both ensure super hydrophilicity, the stable loading of MnO2 leads to an excellent photothermal performance. Experimental results show that H‐CS/MnO2 reaches 93.46% evaporation efficiency and the evaporation rate can reach 2.43 kg m−2 h−1 under 1 sun. Moreover, by means of recycling, not only the treatment of HTC solution is solved, but also the carbon dots are continuously enriched and exhibit certain photothermal effects during the recycling process. In addition, H‐CS/MnO2 exhibited excellent salt resistance and corrosion resistance. The obtained condensate heavy ion concentration removal rate reached more than 99.9%. Therefore, the H‐CS/MnO2 fabricated herein has the possibility of practical application in the future.

High-rate long-lasting solar desalination towards hypersaline brine enabled by introducing a siphon-drop mode

Direct solar steam generation (DSSG) is well-proven to be a sustainable and effective technique to provide high-quality freshwater supply. However, it customarily suffers from severe salt accumulation, especially for treating strong brine, becoming a fatal bottleneck for approaching its commercial applications. Herein, we developed a DSSG with potent siphon-drop mode to achieve powerful self-absorbing and self-desalting, in which the 3D cone-shaped self-supporting polydopamine and polyethyleneimine co-coated poly(acrylonitrile-styrene-acrylate) (PDA-PEI@ASA) used as photothermal material (PTM). Source water can be spontaneously and continuously pumped to the top of water-evaporating surface of PTM via strong siphon effect, and the top-down active water diffusion on evaporating surface can effectively take the salts away from the system. Such above features give the DSSG real-time antifouling nature and hence address the fatal issue of salt accumulation. Accordingly, when continuously treating a near-saturated brine (20% NaCl) for at least 168 h, no obvious salts deposition can be found with a steady water evaporation rate of ∼2.550 kg m–2 h−1. This work could help shed novel light on the design of practical DSSG systems toward efficient lasting freshwater extraction, holding great promise for global water scarcity.

A comparative study of the effect of phase change material (paraffin wax) on volumetric and surface direct solar steam generation

BackgroundThe rate of evaporation in direct solar steam generation can be increased by employing several techniques. One of the passive techniques that can be used for the improvement of system performance is the use of phase change materials (PCMs). MethodsAn experimental study was carried out to explore the role of PCMs in energy storage and continuous evaporation in a direct solar steam generation (SSG) system. For this purpose, volumetric and surface methods have been employed and compared in the absence and presence of PCMs. A nanofluid composed of MWCNT@GNP (a nanocomposite of graphene and multiwall carbon nanotubes) and water was used as a photoabsorber in the volumetric method, whereas poplar wood coated with MWCNT@GNP was chosen for the surface method. Significant resultsThe results indicated that paraffin wax PCMs around the bulk fluid with benefiting from excellent capability of energy storage gave a superb performance for continuance of vapor production in both methods. The PCMs have a significant impact on maintaining the evaporation rate at night. However, in the presence of PCMs, the photoabsorber in the volumetric method represented much better performance compared to the surface method.

Three-dimensional thermal modelling and heat transfer analysis in the heat collector element of parabolic-trough solar collectors

The prediction of thermal distributions around the heat collector element (HCE) is a key issue for the safety and efficiency in parabolic-trough solar collectors. Obtaining 3D temperature fields involving the non-uniform heat flux distribution (NUHFD) around the receiver becomes an essential matter for modelling and simulation tools. Several 3D numerical studies have been implemented using computational fluid dynamics (CFD) commercial software, but with high computational effort. As an alternative, 3D HCE models coupled to 1D heat transfer fluid (HTF) problem results in a much lower computational cost and accuracy enough. In the present work, a realistic 3D HCE − 1D HTF model under an unsteady formulation of the partial differential equations is implemented to properly calculate the receiver thermal distribution. The model is solved using the finite volume method, involving the NUFHD through a Monte Carlo ray-tracing method implemented in SolTrace. Its main novelty is to involve a correction factor (CF) in the standard heat transfer coefficient (HTC) correlations for uniform boundary conditions (BC), due to their inability to correctly predict the absorber thermal profiles. The suggested CF is based on the azimuthal local Nusselt reported in past studies for circumferentially-varying BCs, and on the absorber experimental data from the Direct Solar Steam (DISS) test facility. The model is validated in the AZTRAK platform and the superheated steam region of the DISS facility under steady-state conditions. The heat transfer variables mean deviations are lower than 2.4% and 7.0% for AZTRAK and DISS facilities, respectively. The involvement of a CF in the DISS facility improves the accuracy of absorber cross-section thermal gradients predictions, reducing the mean deviations from 22.2% (without considers it) to 6.9%. Otherwise, the verifications against previously models in AZTRAK platform certify the necessity to correct the standard HTC, but the absence of absorber thermal profiles experimental data inhibits its validation.

Energy Matching for Boosting Water Evaporation in Direct Solar Steam Generation

Energy matching strategy is verified as an effective method for significant improvement of evaporation rate in direct solar steam generation (DSSG). By adjusting the solid–liquid interface through bilayer structure, the water transport speed from reservoir to evaporation surface is controlled to reduce the required energy (RE) for closely matching with that of input energy (IE). The highest evaporation rate can reach up to 2.22 kg m−2 h−1 under one‐sun illumination, which is improved by 40% through energy matching. A solar‐to‐vapor energy conversion efficiency of 93.2% is obtained. It is also highly effective in practical applications including desalination, sewage treatment, oil–water separation, and heavy metal ions treatment. This concept displays a balance between the energy needed for water waiting to be evaporated and the IE. It can inspire more ideas for researchers to accelerate the development of DSSG.

A Comprehensive Physical Profile for Aqueous Dispersions of Carbon Derivatives as Solar Working Fluids

The application of nanofluids in direct solar absorption, heat transfer or direct solar steam generation entails carrying out a comprehensive study taking into account several physical quantities. Long-term stability, rheological, thermophysical and optical properties of dispersions must be known to assess their potential for envisaged applications. Two low-concentration nanofluids, 0.005 and 0.05 wt%, of sulfonic acid-functionalized and polycarboxylate chemically modified graphene nanoplatelets in water were considered in this work. Elemental analyses of the nanopowders and pH evaluations of the colloids were carried out. The rheological behaviour of dispersions at different temperatures was studied by rotational rheometry. Thermal conductivities were measured by the transient hot wire method and densities by the oscillating U-tube technique. Additionally, a brief report of the optical properties was included to provide a comprehensive physical analysis.

Thermal and fluid dynamic analysis of Direct Steam Generation Parabolic Trough Collectors

Direct Steam Generation (DSG) has attracted considerable attention in comparison with conventional technologies based on Heat Transfer Fluids (HTF) for electricity generation due to its higher temperatures. In this context, a detailed numerical model has been developed, not only for comparison and assessment purposes but also for optimization analysis, in order to obtain an accurate comprehensive analysis of this type of systems.This paper explains in detail a numerical model developed to accurately predict the phase change and the heat transfer in the DSG Parabolic Trough Collector (PTC) system. The whole numerical model has been validated using experimental data of Direct Solar Steam (DISS) test facility from scientific literature. The model results have been compared also to the measurements of an experimental DSG PTC setup installed at Ecole Nationale d’Ingénieurs de Tunis (ENIT) running under different working and climate conditions. In all cases, both numerical results and experimental data present a reasonable agreement. The numerical code is used to perform a parametric study pointing out the influence of inlet conditions on the performance of DSG PTCs. Emphasis is placed on the determination of the optimal mass flow rate range leading to a maximum thermal efficiency of the solar collectors. Annual thermal performance of the solar collectors is featured including useful energy and thermal efficiency evaluated under Tunisian climatic conditions.

Comparative study of different functionalized graphene-nanoplatelet aqueous nanofluids for solar energy applications

The optical properties of nanofluids are peculiar and interesting for a variety of applications. Among them, the high light extinction coefficient of nanofluids can be useful in linear parabolic concentrating solar systems, while their properties under high light irradiation intensities can be exploited for direct solar steam generation. The optical characterization of colloids, including the study of non-linear optical properties, is thus a needed step to design the use of such novel materials for solar energy exploitation. In this work, we analysed two different types of nanofluids, consisting of polycarboxylate chemically modified graphene nanoplatelets (P-GnP) and sulfonic acid-functionalized graphene nanoplatelets (S-GnP) dispersed in water, at three concentrations from 0.005 wt% to 0.05 wt%. Moderately stable nanofluids were achieved with favourable light extinction properties, as well as a non-linear optical behaviour under high input solar intensities.

Flame-treated and fast-assembled foam system for direct solar steam generation and non-plugging high salinity desalination with self-cleaning effect

The utilization of solar energy has been expanded in various fields, such as photoelectricity, photochemical reaction and photo-thermal conversion. As a kind of fast-developing solar energy photo-thermal applications, direct solar steam generation (DSSG) have been considered as one of the promising and environmentally friendly technologies for desalination to deal with global water scarcity. Recent years, the photo-thermal material and structure of the DSSG system have been researched to accomplish satisfactory DSSG efficiency (more than 80% under one sun). However, the drawbacks of the material of the DSSG system, such as high cost, complex preparation, impede the practical implication of the DSSG technology. Here, we fast prepare the flame-treated melamine foam (F-MF) by a simple method and reassemble an F-MF system for DSSG. The efficiency of the F-MF setup can reach as much as 87.9% under 2.0-sun. For desalination with high concentration seawater (3.5–15 wt%), the evaporation rate of the system is more than 2.39 kg m−2 h−1 under 2.0-sun without channel blocking. This study presents a low-cost and high-efficient F-MF system which can contribute to the solar-driven desalination application.

Research progress on novel solar steam generation system based on black nanomaterials

AbstractAmong all the renewable energy sources, solar irradiation has the greatest potential to meet the world's future energy demands. The solar‐driven generation of water steam has emerged as a promising route for solar energy utilization in the fields of global water cycle, seawater desalination, high‐temperature sterilization, and wastewater treatment. A typical solar thermal energy converting process involves heat harvesting by solar thermal absorber and heat stimulating the evaporation of water via sensible heat and latent heat. In this review, the novel direct solar steam generation systems based on black nanomaterials are presented. These direct solar steam production systems can be categorized into the following two types: nanofluid‐based evaporation systems and suspended interface evaporation systems. We describe the physical model and experimental results of these systems, as well as the determination of optimal evaporation system design according to excellent evaporation performance. Finally, we elucidate the dominant factors influencing the evaporation efficiency and propose improved approaches to further increase solar steam production efficiency for the discovery of practical applications.

A Novel Ink‐Stained Paper for Solar Heavy Metal Treatment and Desalination

High‐performance, flexible and non‐toxic ink‐stained paper is successfully prepared by a two‐step ultrasonic process. Owing to the superior features of light absorption, wettability and heat management via our homemade mimetic tree system, the prepared ink‐stained paper‐black with an average pore size of 2 µm (ISP2‐Black) shows outstanding steam generation performance (85.8% for 1.25 kg m−2 h−1 and 85.5% for 3.70 kg m−2 h−1 at 1 and 3 suns, respectively), and good durability. Furthermore, an outdoor prototype system is realized by fresh water generation up to 1.24 kg m−2 d−1 for direct solar steam generation. Moreover, the ISP2‐Black also exhibits good performance in solar heavy metal treatment and desalination. This work provides a facile method to fabricate ink‐stained paper and a practical route for more cost‐efficient fresh water production and wastewater/polluted water treatment.

A mimetic transpiration system for record high conversion efficiency in solar steam generator under one-sun

Currently, direct solar steam generator based on suspending plasmonic solar absorber and floating porous photothermal convertors still suffered from high optical concentrations, poor productivity rates, high expenses and complicated fabrication processes. Here we designed a mimetic transpiration system imitating the water transport and transpiration of tree, the hydrophilic airlaid paper acting as the root of tree, the commercial expanded polyethylene foam as the soil for reducing the parasitic heat loss efficiently, and black carbonized wood with tremendous vertical channels as the photothermal convertor. A proof-of-concept mimetic transpiration system generator with carbonized wood achieved a record high conversion efficiency of 91.3% under one-sun, originating from the excellent thermal management of system and >97% carbonized wood solar absorptance and green house effect of vapor. Mimetic transpiration system can be fabricated by easy and scalable processes, providing a high-efficiency solar thermal conversion system for desalination, promising their industrial applications.

Comparison of Two-Phase Flow Correlations for Thermo-Hydraulic Modeling of Direct Steam Generation in a Solar Parabolic Trough Collector System

Direct steam generation (DSG) in parabolic trough collector (PTC) is an efficient and feasible option for solar thermal power generation as well as for industrial process heat supply. The two-phase flow inside the absorber tube complicates the thermo-hydraulic modeling of the DSG process. In the present work, a thermo-hydraulic model is developed for the DSG process in the receiver of a solar PTC. The two-phase flow in the evaporating section is analyzed using two empirical correlations of heat transfer and pressure drop, and a flow map integrated heat transfer and pressure drop model. The results of the thermo-hydraulic simulation using different two-phase heat transfer and pressure drop correlations were compared with experimental data from the direct solar steam (DISS) test facility at Plataforma Solar de Almeria (PSA), Spain. The test facility has collectors with aperture width of 5.76 m, focal length of 1.71 m, and absorber tube with inner and outer diameters of 50 mm and 70 mm, respectively. The simulation results using the aforementioned two-phase models were found to be satisfactory and consistent within the experimental uncertainty. The flow map based heat transfer model predicted the mean fluid temperature with root-mean-square error (RMSE) of 0.45% and 1.40%, for the cases considered in the present study. Whereas the flow pattern map based pressure drop model predicts the variation of pressure along the length of the collector with RMSE of 0.5% and 0.14%. Moreover, the flow pattern map based model predicts the different flow regimes paving a better understanding of the two-phase flow and helps in identifying the critical sections along the collector length.