Solar Thermal Utilization Research Articles

Design and preparation of salt hydrate/ graphene oxide@SiO2/ SiC composites for efficient solar thermal utilization

Efficient solar thermal utilization has attracted extensive attention because it effectively alleviates carbon dioxide emissions and environmental pollution. Phase-change material (PCM) energy storage technology has been regarded as the most promising in improving its efficiency. Toward higher energy utilization efficiency, how to realize conversion and storage of solar energy into PCM is becoming progressively important. Herein, we developed low-cost and stable sodium acetate trihydrate (SAT, salt hydrate PCM) with ultra-fast solar energy charging efficiency by integrating photothermal and thermal conductivity materials. Specifically, a graphene oxide@SiO 2 (GS) nanocomposite fabricated by a novel modified Stober method was employed as the nucleating agent and photothermal conversion enhancers for SAT. The supercooling degree of SAT can be reduced to 0.5 °C from 32.0 °C with the addition of 1.0 wt% GS, while the solar energy charging efficiency of SAT was increased by 92.5% when it is synergistically augmented by integrating GS and silicon carbide (SiC) foam. This study results indicate that the prepared SAT-GS-SiC composite with an enthalpy of 193.2 J/g can be potentially used to efficiently utilize solar energy. A novel PCM composite with low supercooling, high thermal conductivity, and photothermal conversion efficiency was facilely synthesized. • Graphene oxide and SiO 2 (GS) composite with excellent photothermal conversion efficiency was synthesized. • Supercooling degree of sodium acetate trihydrate (SAT) could be reduced to 0.5 °C with the addition of 1.0 wt% GS. • Thermal conductivity of SAT improved by 184.5% after integrating GS and silicon carbide (SiC) foam. • Photothermal conversion efficiency of SAT-GS-SiC composites could reach to 92.5%.

Three-dimensional directional cellulose-based carbon aerogels composite phase change materials with enhanced broadband absorption for light-thermal-electric conversion

Energy shortage, environmental and ecological issues have prompted a focus on effective solar energy utilization. Using composite phase change materials (PCMs) as an energy storage medium is an important method to achieve efficient solar energy conversion. Biomass materials are highly desirable for solar applications due to their wide range of sources, low cost, environment-friendly, and natural porous structure. In this study, cellulose is selected as a carbon source precursor. Three-dimensional (3D) directional cellulose-based carbon aerogels (CBCA) are constructed through an immersion expansion, orientation, freeze-drying, and carbonization process. Taking tert-butanol/ deionized water as co-solvent, the aerogel shows high specific surface and productivity as well as low structural shrinkage. 3D directional graphitized porous network and graphene guarantee excellent broadband absorption, efficient heat transfer pathways, and effective thermal storage ability. Stearic acid (SA) and graphene are melt blending, followed by a vacuum-impregnating process for the formation of 3D composite PCMs. The thermal storage capability of PCMs is greater than 96%, accompanied by the highest thermal conductivity of 1.17 W/(m·K) with 0.5 wt% graphene. Moreover, the highest light-thermal conversion efficiency can reach 90.3%. It also has a maximum light-thermal-electric energy conversion output power of 1.80 mW. This work provides a feasible, economical strategy for highly efficient solar energy storage and thermal energy utilization.

Absorption properties of a multilayer composite nanoparticle for solar thermal utilization

As the primary way of solar thermal utilization, direct absorption solar collector (DASC) is attracting widespread attention. Nanofluids have good development prospects in DASC due to their excellent absorption and heat transfer properties. In this study, a three-layer cylindrical Ag-SiO2-Ag composite nanoparticle is proposed. The advantages of Ag-SiO2-Ag nanoparticle are proved by comparing the absorption performance with other materials and the same structure of sterling silver. The maximum absorption peak of the nanoparticle is 2.28 times that of the silver nanoparticle. At the same time, the influence of the size parameters of composite nanoparticle on the absorption performance is also discussed, which has an important reference value for the sensitive tuning of composite nanoparticle in different scenarios. Finally, the reasons for the enhanced absorption performance of the composite nanoparticle are revealed by analyzing the electromagnetic field distribution around the nanoparticle. The composite nanoparticle makes full use of various electrical and magnetic resonance modes to excite the three absorption peaks in 390 nm, 470 nm and 780 nm, thus improving the absorption performance of the nanoparticle. The composite nanoparticle proposed has positive significance for promoting the development of high-performance DASC.

Heat transfer enhancement of latent heat thermal energy storage in solar heating system: A state-of-the-art review

Latent heat thermal energy storage (LHETS) has been widely used in solar thermal utilization and waste heat recovery on account of advantages of high-energy storage density and stable temperature as heat charging and discharging. Medium and low temperature phase change materials (PCMs), which always with their low thermal conductivity, are used in solar thermal utilization system, resulting in unsatisfactory heat storage performance. This paper summarizes the PCMs used in solar thermal utilization and their thermal physical parameters in different operating temperature ranges (low, medium, and high). Considering the specific cold and heat sources of the solar thermal system, the thermal performance evaluation index of the LHTES is put forward. Also several methods concerning to LHTES performance improvement are reviewed, which involve three aspects: material optimization, structure optimization and system optimization. Finally, through further discussion, some deeper problems about these optimizations are proposed and some recommendations for future research were put forward.

Solar‐Initiated Frontal Polymerization of Photothermic Hydrogels with High Swelling Properties for Efficient Water Evaporation

Hydrogel‐based photothermal materials for enhanced solar water evaporation have drawn increasing attention due to their unique hierarchical nanostructure, reduced latent heat, and high evaporation rate. However, the laborious preparation process and high energy consumption restrict the on‐site applicability and immediacy, seriously obstructing practical application of hydrogel evaporators. Herein, a low‐cost hydrogel evaporator ($1.85 m−2) with high swelling ratio (≈2445%) prepared via solar‐initiated frontal polymerization (FP) is demonstrated. Solar‐initiated FP based on solar thermal technology not only realizes the field rapid hydrogel polymerization without extra energy input and complicated equipment, but also achieves the homogeneous interconnected macroporous structure to enhance water uptake (≈70 times evaporator weight) and light absorption (≈1% transmittance and ≈2% reflectance). Due to the lower vaporization enthalpy, the evaporator presented an evaporation rate of 2.42 kg m−2 h−1 with a light‐to‐vapor efficiency of ≈92.8% under 1 sun. More importantly, the high swelling ratio imparts the hydrogel evaporator with 2700% and 900% expansion in volume and surface area, resulting in a dramatically promoted solar evaporation rate compared with non‐swelling photothermic materials. This feasible approach realizes the all‐process solar thermal utilization, from material synthesis to final application, which would be significant for sustainable freshwater production in remote/off‐grid areas.

Phase change material heat storage performance in the solar thermal storage structure employing experimental evaluation

One of the most investigated and broadly used mediums in the solar thermal storage systems is using phase change materials. In this research, a comprehensive performance test bench for solar thermal utilization system using a controllable heater to substitute different levels of solar input was established. The test bench is not limited by the weather and equipped with alternative heat storage tanks for different PCMs. The heat storage structure and the performance of paraffin in low temperature system was examined using numerical simulation method. The results showed that the heating power received by PCM was stable at 6–8 kW under the heating condition of 85 °C. At the stage of incompletely melting, the temperature difference between the inside and outside was as high as 31.6, which can reduce the loss of heat to a great extent.

Numerical analysis and multi-objective optimization design of parabolic trough receiver with ribbed absorber tube

Parabolic trough collector (PTC) is the most cost-effective and mature concentrated solar power (CSP) technology for solar thermal utilization. However, the highly concentrated solar irradiation on the bottom of the absorber tube causes high local temperature, thermal stress and deformation, further leading to damage and performance degradation of PTC. In order to alleviate the above problem, a novel parabolic trough receiver (PTR) with ribbed absorber tube is proposed to improve the thermo-mechanical performance of PTC Moreover, the multi-objective optimization is employed to determine the optimal parameter combinations (slant angle (β), size (e), pitch (p), and number of the ribs (N)) of ribbed absorber tube coupled with 3D numerical simulation and the genetic algorithm (GA). Based on the TOPSIS method and performance evaluation criteria (PEC) value, the ribbed absorber tube with the optimal geometric parameters of β=15°, N=6, e=4.5 mm and p=20 mm is selected in the Pareto Front as the suggested optimal design. Furthermore, the performance of the PTRs at different inlet temperature of the fluid (Tin=400–600K), direct normal irradiance (DNI=300–1000 W/m2) and mass flow rate (M=0.62–3.72kg/s) are evaluated. It is found that the ribbed absorber tube can effectively disturb the fluid near the boundary and induce longitudinal vortexes flow, which results in significant improvement in heat transfer between fluid and absorber tube, thereby enhancing the performance of the PTC. Compared to the plain tube, the heat transfer and flow resistance of the optimal ribbed absorber are enhanced by 57%–225% and 222%–630%, and the heat loss is lessened by up to 79.3%, resulting in 2.3% and 2.2% improvement in the heat collecting efficiency and overall efficiency. The findings in this paper may provide practical guidelines for developing efficient PTC.

The Effect of In Situ Synthesis of MgO Nanoparticles on the Thermal Properties of Ternary Nitrate.

The multiple eutectic nitrates with a low melting point are widely used in the field of solar thermal utilization due to their good thermophysical properties. The addition of nanoparticles can improve the heat transfer and heat storage performance of nitrate. This article explored the effect of MgO nanoparticles on the thermal properties of ternary eutectic nitrates. As a result of the decomposition reaction of the Mg(OH)2 precursor at high temperature, MgO nanoparticles were synthesized in situ in the LiNO3–NaNO3–KNO3 ternary eutectic nitrate system. XRD and Raman results showed that MgO nanoparticles were successfully synthesized in situ in the ternary nitrate system. SEM and EDS results showed no obvious agglomeration. The specific heat capacity of the modified salt is significantly increased. When the content of MgO nanoparticles is 2 wt %, the specific heat of the modified salt in the solid phase and the specific heat in the liquid phase increased by 51.54% and 44.50%, respectively. The heat transfer performance of the modified salt is also significantly improved. When the content of MgO nanoparticles is 5 wt %, the thermal diffusion coefficient of the modified salt is increased by 39.3%. This study also discussed the enhancement mechanism of the specific heat capacity of the molten salt by the nanoparticles mainly due to the higher specific surface energy of MgO and the semi-solid layer that formed between the MgO nanoparticles and the molten salt.

Heat collection performance analysis of corrugated flat plate collector: An experimental study

Heat collection through flat plate collector is a common technology in solar thermal utilization. In order to improve the heat collection efficiency of flat plate collector and determine the best inclination angle and glass plate thickness, this paper establishes a collector indoor experimental system, and completes the research of solar radiation intensity, collector inclination angle, mass flow rate and glass thickness on the heat collection performance of corrugated flat plate collector. The result shows that when radiation intensity is 510W/m2, the temperature difference between the inlet and outlet in the radiation decreasing stage is 0.25 °C higher than the radiation increasing stage, and the heat collection in the radiation decreasing stage is 157.5W higher than the radiation increasing stage. The heat collection efficiency under different inclination angles decreases with the increase of radiation intensity, the best inclination angle of the collector is 45° and the maximum heat collection efficiency is 0.89. When the radiation intensity is more than 1000W/m2, heat collection of flat plate collector with glass thickness of 8 mm is 6% higher than that with the thickness of 6 mm, insulating effect of the glass is more obvious. The optimum thickness of the glass plate is 4 mm, the maximum temperature difference is 1.5 °C and the maximum heat collection efficiency is 0.89. The heat collection efficiency at different mass flow rates increases first and then decreases with the increase of radiation intensity. The maximum value of the heat collection efficiency at low mass flow rate is delayed. When the mass flow rate is 0.15 kg/s, 0.13 kg/s and 0.1 kg/s, the maximum heat collection efficiency is 0.89, 0.86 and 0.73, respectively; and the corresponding radiation intensity are 670W/m2, 850W/m2 and 1000W/m2, respectively.

Flame-retardant and form-stable phase change composites based on MXene with high thermostability and thermal conductivity for thermal energy storage

Phase change materials (PCMs) are promising candidates for enhancing the efficiency of solar thermal energy utilization owing to their excellent capacity of storing thermal energy. However, the issues of leakage, poor thermal conductivity, and high flammability have hindered their application. Here, we have successfully designed and prepared flame-retardant PCMs through chemical modification of stearyl alcohol (SAL) with a phosphorus-containing molecule. Form-stable phase change composites were then fabricated through a vacuum impregnation method, where an MXene with a porous architecture serves as the supporting skeleton for PCMs. As expected, benefiting from the high aspect ratio and strong capillary force of the MXene aerogel as well as the interfacial interaction between the PCM molecule and MXene, the resulting MXene-based PCMs (PSM-4) exhibit a large thermal conductivity (0.486 W m−1 K−1) and are form-stable upon heating up to 90°C. Additionally, the combination of phosphorus and MXene further strengthens the flame retardancy of PCMs, e.g., the peak heat release rate and total heat release are reduced by 42.8% and 32.1%, respectively. The improvement of flame retardancy can be assigned to the catalytic charring and barrier effect in the condensed phase as well as to the effect of free radical quenching in the gas phase. Hence, the obtained MXene-based flame-retardant PCMs can be potentially utilized for safe and efficient applications of solar energy storage.

An alternative electron-donor and highly thermo-assisted strategy for solar-driven water splitting redox chemistry towards efficient hydrogen production plus effective wastewater treatment

An alternative technology has been sought for highly efficient and sustainable hydrogen production and wastewater treatment worldwide. This paper is focused on the intersection of solar energy, wastewater treatment and hydrogen production. The solar thermo-assisted and electron-donor alternative strategy was conducted for water-splitting redox chemistry to target highly efficient hydrogen production and effective wastewater treatment. The thermodynamic calculation displays that the solar thermal application and shuttle sulfide electron donor alternative are favorable for the hydrogen production and sulfide-to-sulfate oxidation by the falling trend in the potential (from 1.5V to 0.6V). The curves revealed that high current peaks in the low potential range were dominated by the thermal-assisted process and the alternative of the oxidation half-reaction. The experiments illustrated that solar thermal utilization and sulfide donors were dominant factors for the high efficiencies. Under the outdoor sunlight, the high hydrogen production rate and sulfide conversion efficiency were experimentally achieved at rates of 0.25 mL/h H2 and 93% sulfide-to-sulfate oxidation, reasonably more than that of the direct electrochemical water splitting to hydrogen (nearly no evolution) under the same applied bias (＜1.2V). The system realized that one sustainable technology conducts simultaneously both highly efficient hydrogen production and highly effective water treatment.

A Review on Solar Thermal Utilization for Industrial Heating and Cooling Processes: Global and Ethiopian Perspective

A substantial share of the total energy in various countries is consumed by industries and manufacturing sectors. Most of the energy is used for low and medium temperature process heating (up to 3000C) as well as low and medium cooling capacity (up to 350kW). To meet the demand, the industrial sector consumes most of its energy in either thermal (heat) or electrical energy forms. The use of fossil fuels accounts for about half of the overall share. This resulted in a necessity to commercialize local and clean renewable energy sources efficiently considering the reduction of economic dependence on fossil fuels and greenhouse gases emission. As such, solar energy has proven potential and resulted in considerable development and deployment of solar heating industrial processes (SHIP) and solar cooling systems in recent times. Thus, an attempt to present a review of the available literature on overall energy intensiveness, process temperature levels, solar technology match, and solar thermal system performance and cost have been made in this paper. The review also includes identifying the potential and relevance of involving solar thermal for industrial heating and cooling demand. As a result, at least 624 SHIP including promising large-scale plants and 1350 solar cooling systems most of them in small and medium capacities in operation are identified. Though limited data is available for solar cooling potential and installation, investigations projected the global SHIP potential to 5.6 EJ for 2050. Consequently, given the presence of many low and medium temperature heating processes and cooling capacities in industries with immense solar energy potential, developing counties such as Ethiopia can take experience and pay attention to the development of sustainable industrial systems.

Thermodynamic performance analyses for CCHP system coupled with organic Rankine cycle and solar thermal utilization under a novel operation strategy

The combination of an organic Rankine cycle (ORC) and solar collector (ST) with combined cooling heating and power (CCHP) system has the potential to improve its thermodynamic performances. Therefore, in this paper, fully considering thermal cascade utilization, a novel CCHP system combining ORC and ST is proposed, which uses solar energy and exhaust heat as ORC driving heat sources. Compared with traditional CCHP and CCHP-ST systems, the advantages of the CCHP-ORC-ST system are demonstrated. A collaborative optimization method considering both system configuration optimization and operation strategy is proposed, which realizes the improvement of system energy efficiency and economy. Based on the collaborative optimization method, the CCHP system with different integrated configurations is applied to commercial and office buildings. Through parameter analysis, the effects of parameters (such as the rated capacity of internal combustion engine and ORC evaporation temperature, etc.) are discussed. The findings portrays that the cost per unit supply area of the CCHP-ORC-ST system in typical years of office and commercial buildings is about 19.8 $/m2 and 25.9 $/m2. Compared with the CCHP system, the cost-saving rates of the CCHP-ORC-ST system in commercial and office buildings increase by 15.0% and 27.0%, respectively. The CCHP-ORC-ST system can flexibly utilize ORC to generate more electricity, improving energy efficiency and reducing wasted heat. Consequently, the proposed CCHP-ORC-ST system based on the collaborative optimization method can determine the optimal system configuration and operation design for energy savings and consumption reduction.

Unsaturated polyester resin supported form-stable phase change materials with enhanced thermal conductivity for solar energy storage and conversion

Solar energy absorption, conversion, transportation and storage are crucial for high-efficiency solar thermal utilization. It is positive and promising to develop novel phase change materials (PCMs) with good shape stability, excellent photo-absorption and thermal-physical properties in the practical solar thermal application. Polymer supported PCMs with good heat transfer performance show an extensive application prospect. This research introduces a straightforward and practical strategy to prepare flexible poly (ethylene glycol) (PEG) form-stable phase change materials (FSPCMs) by taking cross-linked unsaturated polyester resin as skeleton and expanded graphite (EG) as thermal conductivity additive. The results show the FSCMs have excellent stability, thermal reliability and can be kept from leaking at 80 °C. The efficiency of photo-thermal conversion of FSPCMs doped with 7 wt % EG is up to 93.3%. The thermal conductivity is 1150% higher than PEG. Comparing to PEG, FSPCMs doped with 7 wt % EG can save time of 67.1% and 51.4% during the heating and cooling processes, respectively. The unsaturated polyester resin as common industry materials will provide more possibility for FSPCMs with enhanced thermal conductivity in practical solar thermal application.

Machine Learning-Based Stealing Attack of the Temperature Monitoring System for the Energy Internet of Things

With the development of the Energy Internet of Things (EIoT), it is of great practical significance to study the security strategy and intelligent control system for solar thermal utilization system to optimize the operation efficiency and carry out intelligent dynamic adjustment. For buildings integrated with solar water heating systems, computational fluid dynamics simulation was used in analyzing the process of solar energy output. A method based on machine learning is proposed to predict energy conversion. Besides, the simulation and analysis are carried out in combination with the possible safety problems such as the vibration of the control system. This paper proposed a novel platform of EIoT for machine learning-based cybersecurity study and implemented the platform for the temperature monitoring system. After the evaluation of the machine learning-based cybersecurity study, the EIoT system demonstrated a high performance with the Extreme Gradient Boosting (XGBoost) training algorithm.

Ultra-broadband selective absorber for near-perfect harvesting of solar energy

Efficient absorption of solar spectral radiation is a key requirement in solar heat utilization. In an effort to achieve this goal, this study investigated slow light effect and magnetic polaritons, and analyzed other physical model coupling mechanisms in photonic crystals. To construct a class of two-dimensional layers of Ni-Al2O3 as a pyramid array solar energy absorber, the high melting points of Ni and Al2O3 were utilized to create an arrangement of absorbers with good thermal stability, low sensitivity to angle of incidence, and polarization independence. Based on AM1.5 solar spectral data, a simulation study of the photothermal absorption characteristics of this type of absorber in the 0.3–2.5 µm band was conducted. The results showed that the photothermal conversion efficiency is as high as 96.45%. Compared with the traditional single physical effect design, the conversion efficiency of this photonic crystal structure is increased by ~11%. Furthermore, increasing the concentration factor allows the absorber to maintain high efficiency even at high temperatures, which provides theoretical evidence for the efficient use of solar radiation. This work will be beneficial in several areas, including solar thermal utilization, thermal photovoltaics, absorber design, and radiator design in radiant refrigeration systems.

A critical review on thermophysical and electrochemical properties of Ionanofluids (nanoparticles dispersed in ionic liquids) and their applications

The problems associated with the ever-increasing energy consumption have urged researchers to develop systems that contain renewable, sustainable, and green synthesis operations. Nanotechnology as an applied field has evolved in numerous areas with multiple directions during recent decades. Nanotechnology utility in ionic liquids has helped introduce a new class of liquids formally known as Ionanofluids (INFs). INFs belong to an indigenous class of nanofluids possessing extraordinary enhancement in thermophysical and electrochemical properties. This study summarizes the synthesis of INFs, influence of temperature and nanoparticles concentration of INFs in terms of thermophysical and electrochemical properties, before enlisting wide applications related to these properties such as solar thermal utilization, electrolytes, lubrication, catalysis, and separation processes. The key focus of this article is to address parameters such as thermal and electrical conductivity, (specific) heat capacity, viscosity, and density as a result of the temperature variation, different mass loadings of nanoparticles and the ionic liquid nature. A comparison is drawn between different INFs on the basis of their combination and composition. Moreover, this study also enlists the reasons behind their proposed applications. Future challenges and hurdles associated with INFs, especially stability, cost, and mass production issue of INFs are also addressed.

Polyoxyethylene based phase change materials with enhanced mechanical property, thermal conductivity and photo-thermal energy charging capacity

Phase change materials are widely used as the thermal capacitors in solar thermal systems. For solar thermal utilization, the phase change materials should have large enthalpy, thermal conductivity, good mechanical property and photo-thermal performance. Polymer based phase change materials usually have good mechanical property. In this paper, we first using polyolefin elastomer to prepare a phase change material satisfied above properties. In addition, the mechanism of photo-thermal conversion of phase change materials is rarely reported. Here, the photo-thermal energy charging rate and photo-thermal efficiency of phase change materials are investigated with a combined numerical and experimental method. The effect of mass fraction, solar concentration and phase change materials height is systematically evaluated. The phase change composite has melting enthalpy of 126.7J⋅g−1, thermal conductivity of 1.413 W⋅m−1⋅ K−1, and photo-thermal efficiency of 58.2%. This work not only provide a method to prepare a phase change material with great photo-thermal performance, but also provide a sight to design the phase change materials for solar thermal utilizations via numerical results.

Performance analysis of an improved 30 MW parabolic trough solar thermal power plant

As a promising application of solar energy, parabolic trough solar thermal power generation technology is one of the most important methods of solar thermal utilization. This paper takes the SEGS VI parabolic trough plant as the research object and proposes an improved 30 MW parabolic trough solar thermal power plant. An optimization model is established for the overall plant efficiency, and the design of the solar field of the improved plant is presented with an explanation in this paper. This paper uses the commercial software Ebsilon to establish the simulation models of the SEGS VI Plant and the improved plant and conducts a performance analysis of the two plants under design and operating conditions. The results show that the improved plant based on sectional heating can reduce the average working temperature of the thermal oil. The solar field efficiency increases by 0.52% and the overall plant efficiency increases by 0.24% under the design conditions. The daily mean solar field efficiency data increase by 0.53%, and the overall plant efficiency increases by 0.22% under operating conditions. Simultaneously, the number of collectors in the solar field also decreases, which has good application prospects.

Light sources selection for solar simulators: A review

Solar energy has certain limitations such as seasonal variations, cloudy weather etc. Because of these limitations, it is very difficult to perform the experiments in Rainy and winters seasons. To tackle this problem, we have to use solar simulators. It is source of artificial lights, very much similar to concentrated sunlight. It can be analyzed that increased demand in manufacturing and development of solar simulators for testing and simulation of solar photovoltaic and solar thermal energy utilization. This paper reviews the solar simulator light sources for testing photovoltaic panels as well as for thermal applications. Light intensity, cost, durability and stability were included as a criterion for comparing solar spectrum with lamp wavelength spectrum. The classification of solar simulator depend on the light source application and their technological innovations were considered according to the literature. Also, carbon arc lamp, argon lamp, high pressure sodium lamps, quartz tungsten halogen lamp, mercury xenon lamps, xenon arc lamps, metal halide lamps, LEDs and super continuum laser are discussed in details. Also, multi light source solar simulator discussed as a separate topic.