Numerical and One-Dimensional Studies of Supersonic Ejectors for Refrigeration Application: The Significance of Wall Pressure Variation in the Converging Mixing Section

This paper studies the pressure variation that exists on the converging mixing section wall of a supersonic ejector for refrigeration application. The objective is to show that the ejector one-dimensional model can be improved by considering this wall’s pressure variation which is typically assumed constant. Computational Fluid Dynamics (CFD) simulations were used to obtain the pressure variation on the aforementioned wall. Four different ejectors were simulated. An ejector was obtained from a published experimental work and used to validate the CFD simulations. The other three ejectors were a modification of the first ejector and used for the parametric study. The secondary mass flow rate, m˙s, was the main parameter to compare. The CFD validation results indicate that the transition SST turbulence model is better than the k-omega SST model in predicting the m˙s. The results of the ejector one-dimensional model were compared before and after incorporating the wall pressure variation. The comparison shows that the effect of the pressure variation is significant at certain operating conditions. Even around 2% change in the average pressure can give around 32% difference in the prediction of m˙s. For the least sensitive case, around 2% change in the average pressure can give around 7% difference in the prediction

Turbulent Convective Heat Transfer and Pressure Drop of Dilute CuO (Copper Oxide)- Water Nanofluid Inside a Circular Tube

Abstract Turbulent forced convective heat transfer and pressure drop of 0.01 vol. % CuO-water nanofluid was assessed experimentally. The nanofluids were made flow into a heated horizontal tube under uniform constant heat flux within Reynolds number range of 11,500 to 32,000. The first objective is to know how close traditional correlation/formula for, both, heat transfer and pressure drop can predict nanofluid’s heat transfer and pressure drop. The second is to know how nanofluid’s convective heat transfer and pressure drop are compared to those of its base fluid; in this case water. The results showed that the abovementioned characteristics of the nanofluid can be predicted by the traditional correlation available. It is also found that the nanofluid’s Nusselt number and friction factor, which represent the heat transfer rate and pressure drop, respectively, are close to those of water. Hence, there is no anomaly due to the dispersed nanoparticles within the water

Thermal Performance Evaluation of Two Thermal Energy Storage Tank Design Concepts for Use with a Solid Particle Receiver-Based Solar Power Tower

This paper presents the results of an extensive study of two thermal energy storage (TES) systems. The goal of the research is to make solar energy cost-competitive with other forms of electricity. A small-scale TES system was first built. The inner to outer layers were made of firebrick (FB), autoclaved aerated concrete (AAC) and reinforced concrete brick (CB). The experiments were conducted at temperatures of up to 1000 °C for sustained periods of time. AAC was found to be prone to cracking at temperatures exceeding 900 °C; as a result, AAC was eliminated from the second TES system. The second, larger-scale TES system was subsequently built of multiple layers of readily available materials, namely, insulating firebrick (IFB), perlite concrete (PC), expansion joint (EJ), and CB. All of the surfaces were instrumented with thermocouples to estimate the heat loss from the system. The temperature was maintained at approximately 800 °C to approximate steady state conditions closely. The steady state heat loss was determined to be approximately 4.4% for a day. The results indicate that high-temperature TES systems can be constructed of readily available materials while meeting the heat loss requirements for a falling particle receiver system, thereby contributing to reducing the overall cost of concentrating solar power systems

Characterization of Low-Cost Particulates Used as Energy Storage and Heat-Transfer Medium in Concentrated Solar Power Systems

Utilizing solid particles as a heat-transfer medium in concentrated solar power applications has gained growing attention lately. Unlike molten salts, solid particles offer many benefits, which include: high operating temperatures (greater than 1000 °C), a lack of freezing issues and corrosivity, abundant availability, high thermal energy storage capacity, a low cost, and applicability in direct irradiation. Comprehensive knowledge of thermophysical and optical properties of solid particles is essential to ensure an effective harnessing of solar energy. The most important considerations when selecting solid particles include: thermophysical and optical properties, thermal resistance, crack resistance, satisfactory health and safety risks, availability, and low cost. It is also imperative to consider optical and thermophysical characteristics that might change from what they were “as received” after cyclic heating for a long period. Therefore, the knowledge of thermal performance of particulate materials becomes significant before using them as a heat-transfer medium. In this study, some particulate materials were chosen to study their feasibilities as heat-transfer and storage media for a particle-based central receiver tower system. These particulate materials included white sand, red sand, ilmenite, and Carbobead CP. The candidate particulate materials were heated at high temperatures for 6 h and then cooled to room temperature. After that, cyclic heating was performed on the particulate materials for 500 h at 1200 °C. The optical properties were represented by weighted solar absorptance, and the thermophysical properties of the particulates were measured “as received” and after cyclic heating (aging). EDX and XRD were conducted to quantify the chemical composition and interpret the changes in appearance associated with the particulate materials after cyclic heating. The results showed a considerable agglomeration in all particulates except for white sand in the 6 h heating test, and high agglomeration in the ilmenite. A slight decrease in the optical properties in the white sand and Carbobead CP was found after the aging test. The specific heat was decreased for red and white sand. The EDX and XRD results for white sand and Carbobead CP showed chemical stability, indicating high durability and reliability

Integrated CSP-PV hybrid solar power plant for two cities in Saudi Arabia

Solar energy has the potential to provide most of the electricity needed by mankind sustainably into the indefinite future. Concentrated Solar Power (CSP) has conventionally been considered more applicable than photovoltaic (PV) for baseload power since thermal storage is far cheaper than battery storage. However, the solar fields for CSP are relatively expensive. On the other hand, PV plants without storage deliver electric power at a much lower cost than CSP plants of comparable capacity without storage. Integrating both technologies is an attractive approach towards solar baseload power with affordable levelized cost of energy (LCOE). This study, which investigates the two cities of Saudi Arabia, consists of simulation and optimization in three main parts: The first part is a simulation of the CSP parabolic trough (CSP-PT) standalone plant and integrating the output parameters with an economic model to calculate the LCOE. The second part is the simulation of combined CSP-PT with PV, with a design strategy of utilizing all PV power for daytime use, and supplementing the PV output with thermal power as needed to maintain baseload operation in the daytime. The third part is the simulation of combined CSP-PT with a design strategy of providing all or nearly all daytime power with PV, with the utilization of excess energy from PV to supply heat to the thermal storage system. The results show that for a target capacity factor of 79%, the CSP plant alone requires a solar multiple of 6 in Riyadh and 3.5 in Tabuk. For both locations, the introduction of the hybrid concept substantially reduced the solar multiple. In Riyadh, the solar multiple ranged from 2.9 to 3 with the PV portion of the plant having a nameplate capacity equal to that of the CSP portion and 1.95 for a case with the PV nameplate capacity 60% greater than the CSP portion. For these same cases in Tabuk, the solar multiples were 1.78–1.85 and 1.6 simultaneously. Generally, hybridization is of greater benefit in Riyadh than in Tabuk owing to the greater fraction of solar resource in the form of diffuse sunlight which can be collected by the PV plant but not by the CSP plant. The LCOE was reduced by hybridization in both locations, but again the benefit was greater for Riyadh. Clearly, from a technological perspective, it is important to study the hybridization for an individual city based on its weather data as the incorporation of PV into CSP plant designs should be considered for all locations in order to reduce the cost to provide baseload power from solar energy, and the application of the hybridization concept can extend the applicability of CSP technology to regions with less direct sunlight than would be economically feasible with CSP alone

An experimental investigation of chevron-shaped discrete structure configuration on the particle flow behavior of particle heating receivers

One of the main components of particle-based power tower (PBPT) systems is the particle heating receiver (PHR), through which solid particles are heated by concentrated sunlight. An obstructed-flow PHR (OF-PHR) developed by King Saud University is a type of PHR that has inverted V-shaped obstructions made of Inconel meshes, called chevrons, allowing for a longer time of sunlight exposure on the particles; hence, overcoming one of the limitations of free-fall PHRs. Two problems have been observed with this OF-PHR: (1) a considerable amount of the falling particles bounces forward and leaves the chevrons region; (2) it has a high packing density of chevrons that results in a very low falling particle velocity, which can lead to chevrons overheating. The present research attempts to resolve these issues and improve the PHR performance by understanding deeply the falling-particles behavior of OF-PHRs. The effects of (i) vertical spacing of chevrons and (ii) porosity of chevron meshes, (iii) PHR tilt angle, (iv) particle mass flow rate, and (v) type of particulate material were studied, at room temperature, concerning the following metrics: (1) particle velocity, (2) particle retention, and (3) particle curtain opacity. It was found that the particle velocity profile was nearly identical throughout the OF-PHR, which means that the obstructions successfully limited the particle falling speed. As for the PHR tilt angles, values below 10° were not sufficient to effectively retain the particles flowing through the chevron meshes since many particles bounced and left the PHR (low particle retention). Chevron's vertical spacing showed the most significant effect on particle opacity, with 30 mm spacing resulting in opacity values greater than 94 %. Two types of particulate material were tested, olivine sand and CARBOBEAD; the particle curtain opacity values using these materials were found to be comparable. By having high particle retention, sufficiently low particle falling speed, and high particle curtain opacity, the number of falling particles “controlled” by the obstructions (chevrons) is high; hence, a high number of particles has increased residence time, which will lead to higher PHR outlet temperatures. Moreover, high particle opacity ensures that most of the sunlight strikes the particles instead of the back side of the PHR and the obstructions (chevrons), ensuring optimal absorption of the solar energy and protecting the PHR construction from overheating/damage

An Experimental Demonstration of the Effective Application of Thermal Energy Storage in a Particle-Based CSP System

Tests were performed at the particle-based CSP test facility at King Saud University to demonstrate a viable solution to overcome the limitations of using molten salt as a working medium in power plants. The KSU facility is composed of a heliostat field, particle heating receiver (PHR) at the top of a tower, thermal energy storage (TES) bin, a particle-to-working fluid heat exchanger (PWFHX), power cycle (microturbine), and a particle lift. During pre-commissioning, a substantial portion of the collected solar energy was lost during particle flow through the TES bin. The entrained air is shown to be the primary cause of such heat loss. The results show that the particle temperature at the PHR outlet can reach 720 °C after mitigating the entrained air issue. Additionally, during on-sun testing, a higher temperature of the air exiting the PWFHX than that of the air entering is observed, which indicates the effective solar contribution. Half-hour plant operation through stored energy was demonstrated after heliostat defocusing. Lastly, a sealable TES bin configuration for 1.3 MWe pre-commercial demonstration unit to be built in Saudi Arabia by Saudi Electric Company (SEC) is presented. This design modification has addressed the heat loss, pressure build-up, and contamination issues during TES charging