Solar Receiver Research Articles

Preheating of solar power tower receiver tubes for a high-temperature chloride molten salt

Enhancement of solar-to-electric efficiency in future solar power tower (SPT) plants will require an improvement of the power cycle performance. An attractive option is the use of a supercritical CO2 Brayton cycle, which needs a heat transfer fluid (HTF) temperature of around 700 °C to be competitive. However, the current HTF employed in SPT plants, i.e. nitrate salt, cannot reach this temperature target. For this reason, new HTFs with a large temperature operation range have been investigated recently, but usually their freezing temperature is comparatively high, which complicates the preheating of the tubes during the receiver start-up. In the preheating operation, the heliostat field concentrates the solar irradiation onto the receiver tubes to elevate their temperature above a minimum value, while they are still empty, to avoid the risk of freezing of the incoming flow of HTF. This is a very critical operation, as too much incident heat flux in the preheating leads to extremely high thermal-stresses, causing the creep-fatigue damage of the receiver tubes. The present work numerically characterizes the temporal evolution of the temperature and thermal-stresses during the preheating of a typical configuration of a receiver tube made of Haynes 230 alloy. The aim is to check whether it is possible to preheat the receiver tubes above the freezing temperature of a chloride salt (i.e. NaCl–KCl–MgCl2) as an alternative high-temperature HTF to the nitrate salt. The results indicate that, although the original Vant-Hull algorithm of tube preheating is suitable for the nitrate salt, it is not adequate for the chloride salt. Therefore, a series of modifications of the Vant-Hull algorithm are proposed to meet the requirements of the high-temperature HTF. In the modified Vant-Hull algorithm, the resulting peak values of the temperature gradient and the von Mises stress are reached within the first 1.2 min of preheating. Besides, in the cases analyzed, the estimated creep-fatigue damage of the tube produced with the modified algorithm is much lower than an allowable limit set to protect the structural integrity of the receiver. However, the demanding operating conditions of the chloride salt receiver lead to a lower lifespan than that of the conventional nitrate salt receiver for the same heliostat field and receiver layout. Furthermore, the modified Vant-Hull algorithm requires longer preheating times, which delays the start-up of the SPT plant and reduces its available hours of operation per day. Finally, this work also demonstrates that the modified Vant-Hull algorithm is suitable to preheat receiver tubes that operate with other high-temperature molten salts and liquid metals.

Experimental and numerical investigation on the geometric parameters of an open-to-atmosphere vortex-based solar particle receiver

We report the influence of the amount of airflow that enters and exits the receiver through the aperture, the swirl intensity, quantified with the use of the Swirl number, and flow symmetry on the measured normalised particle egress and simulated two-phase flow behaviour of an open-to-atmosphere vortex-based solar particle receiver under isothermal conditions. Here, particle egress is defined as the particle loss from the SEVR cavity to the ambient environment through the open aperture. To assess their effect, the geometric parameters of the receiver, including the aperture-to-cavity diameter ratio, the outlet-to-cavity diameter ratio, and the number of outlets, are varied. The laser Mie-scattering method was utilised to measure particle egress through the aperture, and a computational fluid dynamics model was employed and validated to understand the gas-phase flow features within the receiver. The reduction in flow ingress and egress is found to have a dominant effect on mitigating particle egress over that of the Froude number, flow symmetry and swirl intensity. More symmetric flow patterns and weaker swirl intensity are found for multiple-outlet configurations, which results in the reduction of particle egress. All these represent an important advancement toward the optimisation of the geometry of an open-to-atmosphere vortex-based solar particle receiver.

Experimental and techno-economic analysis of two innovative solar thermal receiver designs for a point focus solar Fresnel collector

In this paper, techno-economic analysis and the experimental campaign for a new solar concentrating system are presented. Two novel receivers (Jet impingement receiver with mesh structure & helical channel receiver) are tested on a novel collector technology. The collector technology integrates the two established models of solar concentrating technologies, which are linear Fresnel reflector technology and central receiver technology, into the new concept called the Point Focus Fresnel Collector (PFFC). It is found that the PFFC system with jet impingement receiver and mesh structure provides higher thermal efficiency and manufacturing ease compared to parabolic dish systems. Maximum thermal efficiency of jet impingement receiver is highest at 87% whereas that for helical channel receiver is 83%. Average thermal efficiency of jet impingement receiver is 61% whereas that for helical channel receiver is 58%. Economic analysis of the system reveals that the discounted payback time for the PFFC system is only 5.5 years and less than one year compared to diesel and electricity powered steam generators respectively at an inflation rate of 2.5%. Substantial saving is predicted with PFFC system compared to diesel and electricity powered steam generator. For all discount rates in the range of 2.5–25%, the levelized cost of energy (LCOE) for PFFC system is much lower than that of LCOE in Saudi Arabia for conventional system. Since the LCOE in most of the Middle East and North Africa (MENA) countries is higher than the LCOE in Saudi Arabia, therefore, the PFFC system is also suitable for other MENA countries where average direct normal irradiation values are comparable to Saudi Arabia.

Numerical study on influence of wind on thermal performance of a solid particle solar receiver

The effects of wind speed and wind attacking angle on the thermal performance of a solid particle solar receiver (SPSR) are studied by numerical simulation. In addition, the effect of aerowindow on the efficiency of the solar receiver is also explored. The results show that under different wind speeds and attack angles, wind can differently prevent hot air from flowing out and carry cold air into the solar receiver. These two effects compete with each other, resulting in the complex influence law of wind on receiver efficiency. Generally, the increasing wind speed will eventually lead to a negative effect on the thermal efficiency of the receiver. At the incident radiation aperture, the aerowindow formed by the air nozzle can effectively reduce convection heat loss and significantly improve the receiver efficiency. Under the simulation conditions of the present study, there is an optimal air jet velocity to maximize the protection effect of the aerowindow. The heat receiver efficiency increases from 41.7 to 58.2% when the wind is 2.5 m/s and the wind attacking angle is 135°. In the analysis of the energy balance inside the solar receiver, the proportion of radiation heat loss is about one-third, and it is not affected by the aerowindow. Thus, reducing radiation heat loss is very important for further improving the receiver’s efficiency of SPSR.

Modelled annual thermal performance of a 50MWth refractory-lined particle-laden solar receiver operating above 1000°C

The paper reports on the thermal performance of the sub-system for a solar thermal particle technology used to generate high temperature air, including refractory-lined particle-laden receiver, particle separator, particle storage and particle feeder. These assessments are made with a transient mathematical model developed to calculate the heat and mass transfer within the cavity of the receiver together with the thermal losses to the surroundings, incorporating the influence of solar transients during start-up, turndown or shutdown periods. New insights are provided of the influences of the variables of refractory configuration and of the potential operating controller parameters to manage the influence of solar variability on the annual thermal performance of the system, considering the useful thermal gain of hot air. The model is further used to advance the understanding of the sensitivity of the thermal performance to the mass flow rate of inlet air and mass loading of particles in the receiver on the sensible energy harnessed. The influence of the returned air temperature on the receiver thermal performance is also assessed, to provide insights on the suitability of the present configuration to re-heat already hot air in a CST system. Further to this, the thermal outputs are compared with available CFD data for this configuration, and with that reported for a cavity reactor, to provide information on the model validation.

Thermal response of multilayered refractory-lined solar receivers to transient operation

The thermal response of a multilayered refractory-lined particle-laden solar receiver during start-up, turn down, and shut down has been assessed with a transient model and long time-series of variable solar resource data. The influence of operating conditions and geometric parameters on the temperature of the particles and cavity wall, together with thermal stress distribution, are reported for cold and hot starts from 27 to 800 °C. Both the influence of covering the aperture during shutdown periods of sufficiently low solar resource, and geometric scale on thermal performance are also reported. The results provide insights into optimisation of the refractory-lining and insulation thickness of industrial-scale particle-laden receiver with regard to balancing the trade-off between thermal stability, temperature drop overnight, warm-up time, useful daily solar hours and allowable thermal stresses.

An experimental study of solar receiver under cloudy weather conditions

The performance of the central solar receiver should be properly examined for an effective design of a solar power or solar heating system. The performance of solar receiver is affected by three parameters. These are the amount of solar radiations, the state of the heating medium and the temperature of the surrounding air. Also, while evaluating the performance of a solar receiver, the ambient weather conditions should be taken into account. As a result, when a large amount of direct light strikes a solar receiver, the power generated increases as well. In contrast, as the amount of direct light falling on the solar receiver diminishes, so does the amount of electricity generated. As a result, it is generally understood that on sunny days, the performance of the central solar receiver will be higher, while on overcast days, the performance will be reduced. The solar receiver collects heat from diffused or reflecting light (when the sun is obscured by clouds) and absorbs reflective light from glossy or light-colored surfaces. The experimental results reveal that solar radiation collection performance is dependent on climatic circumstances; thus, when installing a solar receiver, it should be suitable for the specific region depending on weather conditions.

Energy performance of a solar cavity receiver with neural output flow regulation under controlled conditions

This article presents the theoretical and experimental performance of a Solar Cavity Receiver (SCR) which operates under controlled conditions of radiation and flow regulation using an Artificial Neural Network (ANN); the objective of this application of neural networks is to obtain the maximum temperature of a fluid in an open hydraulic circuit that presents variations in pressure and temperature of the inlet fluid. The SCR is evaluated for two cases: In the first case, inlet fluid presents a constant temperature and pressure variations of 62–96 kPa, and in the second case, the system presents variations in pressure 55–89 kPa and temperature 29 °C to 31 °C; it was found that an ANN presents a response with a varied range of 2–14%, with average temperature variations lower than 6% respect to the set-point, with the advantage of being self-regulated through ANN training.

Negatively Biased Solar Cell Optical Receiver for Underwater Wireless Optical Communication System With Low Peak Average Power Ratio

Charging batteries in underwater scenarios is generally very expensive and impractical, and solar cell (SC)-based underwater simultaneous lightwave information and power transfer (SLIPT) systems are a powerful solution. However, silicon SC receiver devices have limited bandwidth and are prone to deep signal-to-noise (SNR) degradation during underwater light fading effects. For these problems, this manuscript proposes a negative-biased SC optical receiver scheme to increase the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-3$</tex-math></inline-formula> dB bandwidth of silicon SC from 440 kHz to 780 kHz. For the deep fading of SNR caused by various degradation effects in the water environment, a low peak average power ratio (PAPR) discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-S-OFDM) modulation scheme is employed to counteract the deep fading phenomenon in the system. Achieved a communication rate of 15.2 Mbps in a 60 cm underwater environment with a fading factor of 0.403 and a bit error rate (BER) of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$1.59\times 10^{-3}$</tex-math></inline-formula> under perturbation. Also, the performance of DFT-S-OFDM and orthogonal frequency division multiplexing (OFDM) modulation systems in water environments with different turbidity (absorption characteristics and scattering) and presence of disturbances are compared separately, and the DFT-S-OFDM system is more robust. Finally, we complete the energy harvesting during the communication process, and the experiments show that the total battery power efficiency of the energy harvesting system can be increased by 1.87 times under the white light-emitting diode (LED) continuous irradiation for three hours.

Improving overall heat transfer performance of parabolic trough solar receiver by helically convex absorber tube

The parabolic trough collector (PTC) system has been widely applied in concentrating solar power (CSP) technology. The absorber tube of PTC has a significant impact on the solar energy conversion efficiency. In order to improve the overall heat transfer performance of PTC system, the idea of using helically convex tube as a metal absorber tube of the parabolic trough receiver (PTR) is proposed. Combined with Monte Carlo ray tracing method, the Finite Volume Method is used to establish the numerical analysis model. The effects of structural parameters (p, h, and r) of helically convex absorber tube on the heat transfer and flow performance are studied. The results show that by designing the helically convex structure of tube, the heat transfer and flow properties of parabolic trough receiver can be effectively optimized. Compared with the traditional PTR, the overall heat transfer performance is improved by up to 34%.

Experimental performance of gas-solid countercurrent fluidized bed particle solar receiver with high-density suspension

• A particle solar receiver based on high-density countercurrent fluidized bed proposed. • The lab-scale single tube prototype for high-temperature testing established. • Particles downward stably under the fluidized state of countercurrent flow with bubbling. • Stable operation, high solids holdup and particle temperature increase achieved simultaneously. The particle-based concentrated solar power plant has attracted more attention since its higher working temperature. A particle solar receiver based on the gas-solid countercurrent fluidized bed was proposed to realize the high-density suspension and high outlet temperature of particles. To demonstrate the feasibility of this concept, a lab-scale single tube prototype was established and an electromagnetic heating furnace with an electric power of 10–40 kW el was selected to provide heating power. Based on a series of experimental testing, the results revealed that the countercurrent fluidized bed receiver could operate stably in the fluidized state as gas-solid countercurrent flow with bubbling and the average solids holdup of ∼40% was achieved, resulting in a particle temperature increase ranging between 101 and 312 °C. The global wall-to-bed heat transfer coefficient with a range of 311 to 1481 W/(m 2 ·K) was obtained by the experimental data. The effects of the input heat flux fluctuation on the countercurrent fluidized bed particle solar receiver were studied to reveal its operating characteristics. These results demonstrated that the receiver may combine the merits of the up bubbling fluidized bed and the Downer, which are long residence time, high solids holdup, lower solids back mixing, avoid slugs and excellent heat transfer. The suspension operating in the fluidized state of countercurrent flow with bubbling has the potential to open a new domain of gas-solids two-phase flow. The novel concept appears to provide a potential technique to applicate in concentrated solar power generation.

Reticulated porous lanthanum strontium manganite structures for solar thermochemical hydrogen production

Porous ceramic foams are utilized in various fields and applications due to their advantageous properties such as high porosity, high specific surface area, and controlled permeability. Non-stoichiometric oxide foams are of particular interest for the solar thermochemical production of hydrogen in directly irradiated solar receivers due to their enhanced heat transfer capabilities and structural integrity. We report the fabrication of lanthanum strontium manganite replica foams and their solar thermochemical water splitting performance. Highly porous (>80%) foams were produced with controlled structure and tested in a high temperature water splitting furnace to demonstrate stability and overall performance. The temperature profile of the burnout and sintering of the foam structure were altered based on simultaneous differential scanning calorimetry and thermal gravimetry analysis. Two-step sintering was used both to retain micron-sized pores (D50 = 0.4 μm) on the foam's struts and to limit grain growth to a grain size of ∼1 μm. The structural characteristics were confirmed by porosimetry and scanning electron microscopy. The foams were repeatedly cycled, producing approximately 200 μmol/g of H2 per cycle. Water splitting performance was tested for 50 cycles and approximately 35 h at 1400 °C. No statistical difference in the effective hydrogen production per cycle was observed, which is promising for long-term redox cycle stability.

Performance improvement methods for quartz tube solid particle fluidized bed solar receiver

The conditions to improve performance of quartz tube silicon carbide (SiC) solid particle fluidized bed solar receiver was investigated with computational fluid dynamics (CFD) simulations. The difficulty of experimenting all possible operating conditions was overcome by preparing CFD base input with appropriate models and parameters. The amount of SiC in the bed, the size of particles, and the air inlet velocity were considered as variables. After model verification, in order to evaluate the effect of particle addition, bed without solid particles were simulated first. Outlet temperature of single-phase receiver was calculated as 421 K. Outlet temperatures of 913 K, 895 K, and 881 K were obtained for 400 μm diameter particles in 0.3 m bed height for air inlet velocities of 0.25, 0.3, and 0.35 m/s. Air outlet temperature decreases as air inlet velocity increases. On the other hand, too much reduction at inlet velocity retards the system performance since it affects fluidization. For 400 μm particle diameter and bed height of 0.2 m, outlet temperatures of 994 K, 974 K, and 955 K were found for the same air inlet velocities above. As bed height decreases, air outlet temperature increases. For particle diameters of 300 and 500 μm for bed height of 0.3 m, outlet temperatures of 980 K and 878 K were calculated for appropriate minimum fluidization velocities. Outlet temperature increased with decreasing particle size.

Solar-assisted biomass chemical looping gasification in an indirect coupling: Principle and application

A new configuration of solar-assisted biomass chemical looping gasification (CLG), in which solar heat is indirectly absorbed and stored in the form of sensible heat in solid particles (oxygen carrier and support material), was developed (i.e., indirect coupling). A steam Rankine cycle was integrated to further recuperate the thermal energy of the high-temperature solid particles at the outlet of the steam reactor. Compared with the direct solar energy utilization approach, the proposed configuration can circumvent the fluctuating working conditions of the CLG and downstream processes. Additionally, since the solar receiver and reactor are uncoupled, efficient thermal integration of the developed system is relatively easy to achieve. Thermodynamic analysis of the proposed configuration was conducted, and the indirect coupling of solar energy with CLG was investigated. A case study of methanol synthesis was conducted to compare the annual output of the proposed configuration with that of an auto-thermal CLG. Results indicate that the energy and exergy efficiencies of the present configuration were 66.16% and 77.68%, respectively. The optimal solar multiple was 2.8, which corresponds to the thermal energy storage and designed heliostat size of 10 h and 110.74 × 103 m2, respectively. The case study shows that the present configuration-based methanol synthesis route produced more methanol and electricity but emitted less carbon than the auto-thermal CLG-based method.

Numerical study of convective heat transfer to supercritical CO2 in vertical heated tubes

Supercritical heat transfer is important to design and operate advanced power generation system. Non-uniform heating is frequently encountered such as for sCO2 boiler and solar receiver applications. Here, 3D numerical simulations are performed in turbulent regime, coupling solid wall and sCO2 in tubes. Non-uniform heating is simulated by applying heat via a thin metal layer at the outer tube wall, covering half circumference of tube wall. Calculation results matched experimental data well. Heat transfer is compared between uniform heating and non-uniform heating. Numerical results are re-processed by the pseudo-phase transition theory, including a liquid-like region in tube core and a gas-like layer near the tube wall. The two layers of structure are interfaced by a terminating temperature T+ which is determined by thermodynamics. Hence, supercritical heat transfer is evaluated based on the relative importance of thermal resistances induced by the liquid-like region and the gas-like film region. It is found that wall temperatures for uniform heating are higher than those for non-uniform heating, which is explained by larger thermal resistance of the gas-like film for uniform heating than that for non-uniform heating. This difference ensures better heat transfer for non-uniform heating than uniform heating. We conclude that in turbulent regime, the gas-like film dominates the magnitudes of wall temperature and the occurrence of heat transfer deterioration for both uniform and non-uniform heating. Our work is important for heat transfer facilities in supercritical domain, especially under non-uniform heating condition.

Thermodynamic analysis of CO2–SF6 mixture working fluid supercritical Brayton cycle used for solar power plants

To overcome the poor cycle thermal efficiency of supercritical CO2 Brayton cycle caused by the high rejection temperature used for solar power plants, mixture of CO2–SF6 as working fluid for the Brayton cycle system is proposed and evaluated. Detailed thermodynamic analysis is conducted to the CO2–SF6 mixture Brayton cycle compared with the referenced s-CO2 cycle. Influence of MC inlet temperature and mole fraction of SF6 on the thermodynamic performances is discussed. Moreover, exergy efficiencies and exergy loss distribution of the components in the CO2–SF6 mixture Brayton cycle are analyzed. The results suggest that the system cycle thermal efficiencies of CO2–SF6 mixture cycle and s–CO2 cycle both decrease with the increase of the MC inlet temperature, however, the thermal efficiency of CO2–SF6 mixture cycle is significantly higher than that of s-CO2 cycle. An optimal mole fraction of SF6 exists in the CO2–SF6 mixture for the CO2–SF6 mixture cycle. The optimal value is 0.356 in this context. The heliostat field and solar receiver has the lowest exergy efficiency among all the components of the CO2–SF6 mixture cycle, which is as low as 31.59%.

(Digital Presentation) Photo-Thermo-Electrochemical Cells for on-Demand Solar Power and Hydrogen Generation

Converting solar energy into power and hydrogen provides a promising pathway to fulfilling instantaneous electricity demand (power generation) as well as continuous demand via storing energy in chemical bonds (hydrogen generation). Co-generation of power and hydrogen is of great interest due to its potential to overcome expensive electricity storage in conventional PV plus battery systems. Both solar thermochemistry processes and photo-electrochemical cells (PECs) are extensively explored technologies to produce solar hydrogen. The key challenges for solar thermochemistry processes are extremely high operating temperature (~ 1500 oC) and low demonstrated efficiency (< 1% for hydrogen generation). For PECs, the limited solar absorption together with sluggish electrochemical reactions, especially for OER, leads to limited theoretical solar fuel generation. Operating PECs at high temperature will lead to decreased photovoltage and interface stability. Inspired by the thermally regenerative batteries, we propose a photo-thermo-electrochemical (PTEC) device that uses the solid oxide-based moderate high temperature cell (~1000 ℃) as the photo-absorber for simultaneously converting concentrated solar radiation into heat and generating fuel or power electrochemically driven by the discharging power from the low temperature cell (~700 ℃). PTEC device enables full solar spectrum utilization, highly favorable thermodynamics and kinetics, and cost-effectiveness. A continuous PTEC device has two working modes, which are voltage differential (VD) mode and current differential (CD) mode. The current-voltage characteristics of a PTEC device are shown in Figure 1. It mainly consists of five parts. A high temperature cell (HTC) serves as a solar absorber and a low temperature cell (LTC) serves as heat recovery. Besides, the opposite electrochemical reactions take place in two cells meaning that HTC and LTC can also function as a hydrogen production as well as an electricity generator component, respectively. Heat exchanger(s) is placed between the HTC and LTC and hot fluids pass through a heat exchanger before entering LTC to reduce heat losses to environment as well as reducing input solar energy. The VD mode and CD mode can be realized in PTECs via controlling of DC-DC converter. In order to identify the main parameters, we develop a multi-physics model based on finite element method, including mass, heat and charge transfer, and electrochemical reactions. In addition, heat exchange is modeled by solving energy balance equation, DC-DC convertor is assumed by constant efficiency, and a lumped parameter model is used to describe solar receiver including energy losses of conduction and reradiation. This framework also allows us to provide design guidelines for PTEC devices with high solar-to-electricity (STE) efficiency and solar-to-hydrogen (STH) efficiency. The maximum STE and STH efficiency under reference conditions of PTEC device was found to be 4 % and 2 %. A further improved performance in terms of STE and STH efficiency are about 19 % and 16 %, respectively, via optimizing temperature configuration between HTC and LTC and material properties. It is also interesting to note that STH can reach higher than 80 % of STE at a large temperature difference, which shows a promising energy storage device by storing excessive electrical power in form of hydrogen. The main results show that the temperature of HTC and efficiency of heat exchange are key parameters to optimize PTEC efficiency. The performance of DC-DC convertor dominates STH efficiency. Besides, ionic conductivity of electrolyte can contribute to significantly expanding the operating current density range. The PTEC is a promising technology for solar energy conversion and storage as it is able to produce electricity and hydrogen in a single device. The solar conversion efficiency predicted with our numerical model supports that by optimizing the design and operational conditions, this technology can compete with existing solar fuel pathways. Figure 1

Heat transfer enhancement of supercritical CO2 in solar tower receiver by the field synergy principle

• The secondary flow generated in the horizontal tube can enhance the heat transfer. • The field synergy principle is used to determine the location of the enhancement structures. • The non-uniform enhanced heat transfer structures are proposed. • The triangular ribs tube is the best heat transfer enhancement structure. The supercritical CO 2 used as heat transfer fluid (HTF) in solar tower cavity receiver has higher pressure and operating temperature, and lower investment than traditional heat transfer fluid, such as water steam, molten salt, etc. Many researchers have analyzed the effects of mass flow rate, heat flux, tube diameter, operating pressure and inlet temperature near the critical point on the heat transfer performance of supercritical CO 2 . However, there are few studies on the convective heat transfer characteristics of supercritical CO 2 in tower solar receivers under the non-uniform heat flux boundary. In this contribution, in order to improve the convective heat transfer performance of supercritical CO 2 under non-uniform heat flux, we firstly analyze the effect of the arrangement of the receiver tube on the flow and heat transfer performance of supercritical CO 2 . Then, we apply the field synergy principle to guide the flow field enhancement, and design the non-uniform structure at the position with larger field synergy angle. Finally, we further analyze the effect of the non-uniform structure on the heat transfer performance of supercritical CO 2 . The results show that the triangular ribs tube is the best heat transfer enhancement structure, e.g., when the mass flow rate is 0.1 kg·s −1 , the Nusselt number is 956.11, which is 163% higher than that of the smooth tube. This contribution can provide a guidance for the structural enhancement of a practical supercritical CO 2 solar receiver to improve the heat transfer performance.

A novel approach for volumetric solar receiver performance assessments

A novel design and an efficient operation of solar volumetric receivers remain critical for successfully integrating such receivers into solar energy harvesting devices. The performance of the solar volumetric receiver is highly dependent on the geometric connectivity of the actual pore-sites in the SiC open-cell foams. Although the performance of SiC foam receivers has been investigated previously considering the homogeneous foam structure, further studies are needed to explore the influence of the actual path of the pores and their connectivity on the flow and thermal performance of the receiver. Hence, the present study considers the novel design of a solar volumetric absorber incorporating the actual SiC foam receiver geometric features. Accordingly, the thermal and flow behavior of the working fluid within the receiver is examined considering the 3D connectivity of the pore sites. Moreover, the performance of the solar volumetric receiver is evaluated for different Reynolds numbers, solar concentrations, and foam porosities. A 3D-dimensional numerical investigation is carried out incorporating the interactions between convection, conduction, and radiation heat transfer for which the discrete ordinate radiation model is used. Porous SiC foam is placed in a channel having an optically transparent wall at the top and air is used as the working fluid extracting heat from foam. The overall heat transfer coefficient, normalized Nusselt number, and thermal effectiveness are evaluated for various operating conditions. Findings revealed that the receiver thermal performance improves at high Reynolds numbers. The convection heat transfer is found to be the apparent heat transfer mode within the porous SiC foam. In addition, the pumping power to overcome the flow resistance in the channel is considerably less than that of the rate of heat transfer gain.

Preliminary investigation on the adoption of CO2-SO2 working mixtures in a transcritical Recompression cycle

This paper investigates the interest and potential of using working fluids based on Carbon and Sulphur Dioxide mixtures (CO 2 -SO 2 ) in a transcritical Recompression cycle. In order to assess the actual thermodynamic potential of the concept proposed, the influence of dopant (SO 2 ) content is assessed for two different turbine inlet temperatures (550 °C and 700 °C). The results obtained are compared with other CO 2 mixtures already proposed in literature (CO 2 -C 6 F 6 and CO 2 -TiCl 4 )) and for two alternative cycle layouts ( Recuperated Rankine and Precompression ). The results pf the analysis reveal that, at high ambient temperature, the Recompression cycle operating on CO 2 -SO 2 , with Sulphur Dioxide content between 20% and 30%(v), is a very interesting option for Concentrated Solar Power plants, able to achieve thermal efficiencies ≈ 45% and ¿51% at 550 °C and 700 °C respectively. At a minimum cycle temperature of 50 °C, the proposed configuration leads to thermal efficiency gains of 6% and 2% with respect to the Brayton and Recompression cycles working on pure CO 2 . This performance enhancement of the Recompression cycle with CO 2 -SO 2 is comparable to or higher than that enabled by other CO 2 mixtures proposed in literature, but with significantly higher specific work (smaller footprint) and temperature rise across the solar receiver (lower installation costs). • CO 2 mixtures enable thermal efficiencies higher than 50% at high ambient temperatures. • Recompression is the most interesting layout to exploit CO 2 -SO 2 mixtures. • CO 2 -SO 2 mixtures enable a good compromise between thermal efficiency and specific work.