Abdelrahman El-Leathy

High Temperature Durability of Solid Particles for Use in Particle Heating Concentrator Solar Power Systems

Using solid particulates as a heat absorption and transfer medium in solar concentrated systems is a solution for collecting and storing thermal energy. Solid particulates, such as sand, are relatively inexpensive and are much less corrosive and expensive to maintain than molten salts. Small particles may be stored easily, and can be used as a heat transfer medium for use with a suitable heat exchanger. Despite their anticipated low cost, excessive degradation of the particulates requiring replenishment or disrupting operation could impair the overall economics. Consequently, the durability of the particulates should be verified. Responding to this need, this study examines the durability of solid particulates as a heat transfer medium in a closed cycle for concentrated solar power central receiver systems. Specifically, this study analyzes the combination of attrition and sintering of sand with varying temperatures. Attrition is the reduction of a particle’s mass and sintering is a process of fusing two or more particles together to form a larger agglomerate. In a closed cycle, particularly for a concentrated solar power tower, a particle will experience typical temperatures from 600°C to 1000°C. The increase in temperature may change the physical characteristics of the particles and along with any impurities may promote lower softening point bonding. Thus, it is important to investigate particle durability at high temperatures.The experimental procedure used in this investigation involves heating and abrading particulates of a known mass and size distribution to temperatures between 600°C and 1000°C, and also at 25°C to observe attrition only. The testing is conducted using a specially designed experimental apparatus described below. The heated particulates are contained in a metal cylinder. Inside the cylinder is another cylinder made of a porous silicon carbide foam. As the temperature is held constant, the particulate sample is rotated 180 degrees around a horizontal axis every 15 seconds from a low position to a higher position so that the particulates fall and abrade against each other. This process is repeated for a known number of cycles (many thousands). Then the resulting particulate size distribution is measured to determine the amount of attrition and sintering occurred during the experiment. The particulates tested are various types of sand with varying mean diameters and composition, along with a ceramic particulate similar to hydraulic fracturing proppants. Sample composition, sample size distribution, and temperature will be used to establish parameters for rates of attrition and sintering. These rates will be used to predict the behavior of particulates in a concentrated solar power tower closed cycle.Copyright

Sun position algorithm for sun tracking applications

The efficiency of solar power systems, either concentrating or non-concentrating, can be increased by incorporating a tracking system. The tracking controller requires the position of sun in the sky which can be obtained by using a sun position algorithm that calculates sun position in a certain coordinate system. This papers presents a method to calculate the sun position which could be useful in tracking sun without sensors. The proposed algorithm is a compilation of different equations that compute solar position. The accuracy of the algorithm affects the efficiency of the tracking system but a complex high accuracy algorithm can be replaced with a simple low accuracy algorithm without affecting the accuracy to much extent as presented in this paper.

Sintering of Solid Particulates Under Elevated Temperature and Pressure in Large Storage Bins for Thermal Energy Storage

This research is a part of the DOE-funded SunShot project on “High Temperature Falling Particle Receiver.” Storing thermal energy using solid particulates is a way to mitigate the time of day dependency of concentrated solar power. Small particles may be stored easily, and can be used as a heat transfer medium to transfer heat to the power cycle working fluid through a heat exchanger. This study examines the physical characteristics of solid particulates of different materials kept inside large storage containers. Particle behavior at the expected high temperatures of the concentrated solar power cycle combined with the elevated pressure experienced within the storage container must be evaluated to assess the impact on their physical properties and ensure that the particles would not sinter thereby impacting flow through the system components particularly the receiver and heat exchanger. Sintering is a process of fusing two or more particles together to form a larger agglomerate. In the proposed concentrated solar power tower design, particles will experience temperatures from 600°C to 1000°C. The increase in temperature changes the physical characteristics of the particle, along with any impurities that could form particle to particle bonds. In addition, the hydrostatic pressure exerted on particles stored inside a storage unit increases the probability of sintering. Thus, it is important to examine the characteristics of particles under elevated temperatures and pressures.The experimental procedure involves heating particulates of a known mass and size distribution to temperatures between 600°C and 1000°C inside a crucible. As the temperature is held constant, the particulate sample is pressed upon by a piston pushing into the crucible with a known constant pressure. This process is repeated for different temperatures and pressures for varying lengths of time. The resulting particulates are cooled, and their size distribution is measured to determine the extent of sintering, if any, during the experiment. The particulates tested include various types of sand, along with alumina particles. The data from this experiment will allow designers of storage bins for the solid particulates to determine when significant sintering is expected to occur.Copyright

Modeling of Transient Energy Loss From a Cylindrical-Shaped Solid Particle Thermal Energy Storage Tank for Central Receiver Applications

The use of solid particles as a heat transfer and thermal energy storage (TES) medium in central receiver systems has received renewed attention in recent years due to the ability of achieving high temperatures and the potential reduction in receiver and TES costs. Performance of TES systems is primarily characterized by the percentage of heat loss they allow over a prescribed period of time. Accurate estimation of this parameter requires special attention to the transient nature of the process of charging the TES bin during solar field operation and discharging during nighttime or at periods where solar field operation is interrupted. In this study, a numerical model is built to simulate the charge-discharge cycle of a small cylindrical-shaped TES bin that is currently under construction. This bin is integrated into the tower of an experimental 300-kW (thermal) central receiver field being built in Riyadh, Saudi Arabia, for solid particle receiver research, most notably on-sun testing of the falling particle receiver concept within the context of a SunShot project. The model utilizes a type of wall construction that had been previously identified as showing favorable structural characteristics and being able to withstand high temperatures. The model takes into account the anticipated charge-discharge particle flow rates, and includes an insulating layer at the ceiling of the bin to minimize heat loss by convection and radiation to the receiver cavity located immediately over the TES bin. Results show that energy loss during the full charge-discharge cycle is 4.9% and 5.9% for a 5-hour and 17-hour discharge period, respectively. While large, these energy loss values are primarily due to the high surface-to-volume ratio of the small TES bin being investigated. Preliminary analysis shows that a utility-scale TES bin using the same concept will have an energy loss of less than 1%.Copyright

Development and Design Prototype 300 kW-Thermal High-Temperature Particle Heating Concentrator Solar Power System Utilizing Thermal Energy Storage

The advantages of high temperature central receiver particle heating solar heat supply systems in concentrator solar power (CSP) have been recognized in recent years. The use of particulate as the collection medium provides two critical advantages: (1) Ordinary particulate minerals and products will allow higher collection temperatures approaching 1000°C compared with conventional molten salts, which are limited to around 650°C, and (2) the low cost high temperature particulate material can also be used as the storage medium in a highly cost effective thermal energy storage (TES) system. The high operating temperature allows use of high efficiency power conversion systems such as supercritical steam in a vapor power cycle or supercritical carbon dioxide in a Brayton cycle. Alternatively, a lower cost gas turbine can be used for the power conversion system. High conversion efficiency combined with inexpensive TES will yield a highly cost effective CSP system. The 300 kW-th prototype is being constructed as a solar heat supply system only, deferring the power conversion system for later demonstration in a larger integrated CSP system.This paper describes the general design and development efforts leading to construction of the 300 kW prototype system located in the Riyadh Techno Valley development near King Saud University in Riyadh, Saudi Arabia, which is the first sizeable solar heat supply system purposely designed, and constructed as a particle heating system.An important component in a particle heating system is the particle heating receiver (PHR), which should be durable and efficient while remaining cost-effective. A critical enabling technology of the PHR being implemented for this project was invented by researchers on our team. In our version of the PHR, the particulate flows downwards through a porous or mesh structure where the concentrated solar energy is absorbed. The porous structure will reduce the speed of the falling particulate material allowing a large temperature rise on a single pass. The new design will also increase the absorption of solar energy and mitigate convective heat loss and particle loss. Other innovative aspects of this design include low cost thermal energy storage bins and a cost effective particle to working fluid heat exchanger. Certain features of these design elements are subjects of ongoing patent applications. Nevertheless, the overall design and the development process of the prototype system is presented in this paper.Copyright

Simulation of sun tracking system for point focus fresnel collector

This work presents simulation of a sun tracking system that was developed for a Point Focus Fresnel Collector (PFFC) installed at King Saud University. The tracker is dual axis with a concentration ratio of 160 and a small acceptance angle of 2 degrees, hence a precise tracking control is required. Two control schemes were developed and tested, the open loop and closed loop. A sun position algorithm was used for open loop control whereas a sun sensor was used for closed loop control. The control system were simulated using Matlab/Simulink. It was found that open loop tracking is simpler to implement and consumes less power but is also less accurate and tracking error accumulates over time. On the other hand, closed loop control is more complicated to implement, consumes more power is much more accurate than open loop control. So, it is recommended to use the closed loop controller for PFFC system. The individual components of the control system were modelled using generic Simulink models. The complete model was operated in both open loop and closed loop configurations. The results of the models were analysed to estimate the operating limits of the tracker.

Measurement of circumsolar ratio in high dust loading regions using a photographic method

Performance of concentrating solar power (CSP) plants is highly affected by direct normal irradiance (DNI). However, it is also important to consider circumsolar radiation in any simulation of a CSP plant, especially in desert regions where dust loading in the atmosphere is expected. There are a number of methods to measure circumsolar radiation. However, most of them require expensive instrumentation. This work introduces a simple method to estimate circumsolar radiation. It involves taking high-resolution photographs of the sun and processing them using a computer code that identifies the sun’s disk. The code then uses pixel intensities to obtain the solar intensity distribution across the sun’s disk and in the aureole region. The solar intensity distribution is then used to obtain the circumsolar ratio (CSR) which represents the shape of the sun. To test this method, numerous photos of the sun were taken during the month of April and September 2016 at King Saud University in Riyadh, Saudi Arabia. Riyad...

Experimental measurements of thermal properties of high-temperature refractory materials used for thermal energy storage

This paper builds on studies conducted on thermal energy storage (TES) systems that were built as a part of the work performed for a DOE-funded SunShot project titled “High Temperature Falling Particle Receiver”. In previous studies, two small-scale TES systems were constructed for measuring heat loss at high temperatures that are compatible with the falling particle receiver concept, both of which had shown very limited heat loss. Through the course of those studies, it became evident that there was a lack of information about the thermal performance of some of the insulating refractory materials used in the experiments at high temperatures, especially insulating firebrick and perlite concrete. This work focuses on determining the thermal conductivities of those materials at high temperatures. The apparatus consists of a prototype cylindrical TES bin built with the same wall construction used in previous studies. An electric heater is placed along the centerline of the bin, and thermocouples are used to ...

Experimental study of compatibility of reduced metal oxides with thermal energy storage lining materials

Solid particles have been shown to be able to operate at temperatures higher than 1000 °C in concentrated solar power (CSP) systems with thermal energy storage (TES). Thermochemical energy storage (TCES) using metal oxides have also found to be advantageous over sensible and latent heat storage concepts. This paper investigates the compatibility of the inner lining material of a TES tank with the reduced metal oxide. Two candidate metal oxides are investigated against six candidate lining materials. XRD results for both the materials are investigated and compared before and after the reduction of metal oxide at 1000°C in the presence of lining material. It is found that the lining material rich in zirconia is suitable for such application. Silicon Carbide is also found non-reacting with one of the metal oxides so it needs to be further investigated with other candidate metal oxides.

Computational Analysis of Particulate Storage Bin for High Temperature Thermal Energy Storage

The Riyadh Techno Valley Solar Tower, an innovative type of concentrator solar power plant, is being developed by King Saud University (KSU) and Georgia Tech (GT). The facility is being constructed at the Riyadh Techno Valley development near the KSU campus and will store thermal energy collected from the sun in solid particles, which can be heated to higher temperatures than is currently possible using molten salts. The particles must be well insulated to stop energy loss to the environment. Hence, GT and KSU have incorporated an insulated storage bin into the plant design.The bin will be constructed in several layers: an inner layer of firebrick, which can endure direct exposure to the heated particles; a specially prepared refractory insulating concrete, which maintains good insulating value at high temperatures; and a conventional structural concrete shell surrounding the entire bin. This paper presents a thermal analysis of this storage device and discusses structural analyses. Simplified analytical solutions are compared with the finite element results from a 3D ANSYS model of the entire bin. A temperature distribution is obtained, and heat loss through the bin is also evaluated.Modeling of rebar and concrete cracking are described, and methods of reducing stress on the outer concrete shell are considered. Structural support for an access tunnel into the bin is also explored. The current tunnel design involves a material with a relatively high thermal conductivity, necessitating modifications to the bin. Finally, material selection is considered, particularly with regard to the insulating concrete layer. Limitations on the use of Portland cement based insulating concretes are discussed, and alternative base materials are evaluated.Copyright