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Particle heating receivers (PHR) are a new class of receivers that use particulate materials as the heat absorption and storage medium. One variation of PHRs is the obstructed flow receiver, where particles are released from a slender slot in the form of a curtain and then forced to go through and around discrete obstructions (called chevrons) to limit their acceleration and increase their residence time, thereby increasing receiver efficiency. This design was successfully tested at the proof-of-concept scale. However, two of the potential issues with this design are dense packing of chevrons which can make the installation process cumbersome and potentially expensive, and that the velocities can be too low to the point that overheating of the chevrons can occur. This study addresses these issues by looking at the particle flow characteristics of less dense chevron configurations. For this purpose, a full-scale 2.5-m mock-up obstructed flow PHR was constructed, with a particle feeding hopper and a receiving tank. The feeding hopper is fitted with a slot at the bottom to allow a predetermined mass flow rate to go through it. Two characterization tools were used: a high-speed camera with particle image velocimetry analysis to measure the particle velocity profile, and a light source/lightmeter arrangement to measure curtain opacity. Two chevron spacing cases were considered: a spacing of 30 mm and a spacing of 60 mm. The tilt angle was also varied from 3° to 10°. Results show that a considerable tilt angle (10°) is needed to retain a majority of particles within the PHR. It was also found that the obstructed flow PHR design effectively and consistently limited particle velocity, with the highest recorded velocities being 0.7 m/s and 0.95 m/s for chevrons spacings of 30 mm and 60 mm, respectively. Furthermore, particle curtain opacity was found to be high (approximately 71%) at a chevron spacing of 60 mm, despite the low mass flow rate at which testing took place (1.53 kg/s.m). [ABSTRACT FROM AUTHOR]

Worldwide research efforts are aiming to push the operating temperature limit of CSP systems to improve efficiency and, as a result, the levelized cost of electricity. Furthermore, with higher operating temperatures, a number of new concepts become feasible, the most important of which are supercritical CO2 cycles and air Brayton cycles. Hybrid CSP-fossil fuel systems fall within the air Brayton family of concepts, where the solar field is used to preheat air, while a fossil fuel is burned to bring the air temperature to the firing temperature of a gas-fired Brayton turbine. To make this approach more attractive and environmentally friendly, it is desirable to maximize the solar "contribution" and minimize fuel assistance. This can be done by using novel receivers that increase the air temperature significantly and/or employing a recuperator, where the operating pressure and temperature are lower than conventional Brayton cycles. This paper presents information about an experimental integrated gas turbine-solar particle heating system that uses the hybrid approach. The system is located on the campus of King Saud University in Riyadh, Saudi Arabia and has a peak thermal power of 300 kW. In addition to providing details about the individual components and how they are integrated, the paper explains the start-up procedure, which consists of preheating the system with the heliostat field, followed by additional heating from the turbine itself until the nominal heat exchanger operating temperature is reached, at which point on-sun operation can commence. The study shows that solar preheating requires one to two days to complete, followed by a few hours of heating by the turbine to bring the temperature to about 560°C. Furthermore, with a temperature rise of up to 180°C in the particle heating receiver (and a drop of 10°C in the particle conveyor), the maximum particle temperature will approach 730°C, making the solar contribution significant. As larger scale systems will naturally allow for larger particle drop heights and higher temperature rises in the receiver, it is envisaged that maximum particle tempertures in those systems will approach the firing temperatures of recuperated turbines, making this solution technically, economically, and environmentally worth considering. [ABSTRACT FROM AUTHOR]

Particle heating receivers are a promising technology that can allow operation of CSP systems at temperatures higher than what today's commercial molten salt systems can achieve, making them suitable for use in a variety of applications, including supercritical CO2 cycles, air Brayton cycles, and high-temperature process heat. One of the ways to improve cost-competitiveness of particle heating receivers is to use low-cost particulate materials, such as sand, as the working medium. Red sand is particularly attractive due to its abundance and acceptable absorptance. This paper presents the results of on-sun testing of a particle heating receiver that uses red sand as the working medium. Tests were conducted at the experimental central receiver facility at King Saud University in Riyadh, Saudi Arabia. Performance of the receiver was assessed in two ways. First, the rate of thermal energy absorption was calculated using the measured temperature rise across the receiver, particle flow rate, and red sand's specific heat. Second, receiver efficiency was calculated using the rate of thermal energy absorption and the thermal power incident on the receiver, which was estimated using a raytracing software. Results show that a temperature rise of 130°C was achieved with an incident heat flux of 230-280 kW/m². Receiver efficiency was found to range from 60% to 70%. These results are encouraging and show that red sand is a promising particulate material, especially when it is used with a proper cavity receiver design where the effect of absorptance of the particulate material becomes less significant. [ABSTRACT FROM AUTHOR]

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 measure temperature at the interfaces between all layers. Heat loss is measured across one of the layers whose thermal conductivity had already been well established using laboratory experiments. This value is used to deduce the thermal conductivity of other layers. Three interior temperature levels were considered; namely, 300℃, 500℃, and 700℃. Results show that the thermal conductivity of insulating firebrick remains low (approximately 0.22 W/m.K) at an average layer temperature as high as 640℃, but it was evident that the addition of mortar had an impact on its effective thermal conductivity. Results also show that the thermal conductivity of perlite concrete is very low, approximately 0.15 W/m.K at an average layer temperature of 360℃. This is evident by the large temperature drop that occurs across the perlite concrete layer. These results should be useful for future studies, especially those that focus on numerical modeling of TES bins. [ABSTRACT FROM AUTHOR]

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℃ 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. [ABSTRACT FROM AUTHOR]

During commissioning of the CSP test facility at King Saud University, a large temperature drop was noted when charging the thermal energy storage (TES) bin. Heat loss can be mitigated by using an airtight TES bin. In this paper, we present a small-scale TES bin, which can be isolated and sealed from its surroundings, i.e., the TES bin can be confined, isolated and sealed from its environment. There were two operational scenarios simulated, namely the start-up and steady-state scenarios. In the start-up scenario, the bin's centerline temperature was almost the same as the particle inlet temperature. Thus, the bin's wall was the sole cause of heat loss. Entrained air did not contribute to heat loss. Based on the results of the steady-state scenario, an air-tight bin can reduce the charging/discharging loss sufficiently. Additionally, no "significant" build-up of pressure was detected during bin charging. [ABSTRACT FROM AUTHOR]

The usage of solid particles as solar thermal energy absorption/storage medium requires particle heating receiver to be developed. King Saud University and Georgia Institute of Technology have been collaborating in designing and testing obstructed particle heating receiver. Size of the receiver is 1.2 m x 1.2 m; the base of it is made of duraboard on which chevron-shaped meshes, made of Inconel 601, are installed as the obstructions in a staggered arrangement. The receiver is employed in the particle-based solar power tower experimental facility at King Saud University. To evaluate performance of the receiver, on-sun tests were conducted on two types of particle material, i.e., red sand and CARBOBEAD. Results show that CARBOBEAD absorbed more solar thermal energy and achieved higher temperature rise across the receiver compared to red sand, which is, most likely, due to a higher radiation absorptivity. It was also found that red sand tended to agglomerate at the tip of the chevron mesh during the test, blocking the concentrated sunlight. Moreover, as these agglomerates crumbled, they created many smaller agglomerates that can block the filter downstream, which will, consequently, block the particle flow path. Mesh material also needs to be analyzed further to avoid particle agglomeration, because it is expected that it has a significant role in this matter. [ABSTRACT FROM AUTHOR]

Particle-based solar power tower is a promising technology that can surpass the temperature limit of today's commercial molten salt systems, which will improve the system's efficiency. King Saud University (KSU) in collaboration with Saudi Electricity Company (SEC) have built an experimental particle-based power tower system integrated with gas-microturbine at King Saud University. This test facility is, mainly, aimed to: (1) prove the concept of using solid particles as a solar thermal energy storage medium from which the gas-turbine receives its heat source; therefore, fossil fuel consumption decreases, and (2) prove the system's ability to raise the temperature of the solar-heated particles above 600 °C, surpassing the temperature limit of molten salt power tower systems, i.e., typically limited to around 600 °C. Previous on-sun tests using red sand have shown the system's ability to achieve both goals independently, in separate tests. The current paper presents further tests using red sand to reach both goals simultaneously and with better results, i.e., better solar contribution and higher temperature of solar-heated particles. Results show that the system is able to raise the particles temperature to a maximum temperature of 630 °C and to increase the compressed air temperature by around 17 °C before entering the combustor by utilizing the heat from solar-heated particles. It was also found that one of the issues that can decrease or prevent the solar contribution is the temperature drop on the hot particles as they flow from the particle heating receiver (PHR) outlet to the inlet of particle-to-working-fluid-heat exchanger (PWFHX). This can happen if the thermal energy storage (TES) bin, located upstream of the PWFHX, is nearly empty/partially filled, allowing outside air to be entrained with the particle flow into the bin; on the other hand, fully filled TES bin blocks the air to enter it, which will reduce the temperature drop significantly. [ABSTRACT FROM AUTHOR]

This paper explores one important issue related to the operation of particle-based concentrating solar power (PBCSP) systems which is the heat that might be lost through the main components comprising such systems. Tests conducted at the PBCSP test facility at King Saud University (KSU) in Riyadh, Saudi Arabia reveal significant heat loss in several components, including the thermal energy storage (TES) bin, the particle-to-working-fluid heat exchanger (PWFHX), the particle lift system, and the air piping. On-sun tests results show that only a small portion (16%) of the net thermal energy captured in the particle heating receiver (PHR) was transferred to the turbine's air, which was then converted into electrical power. The remaining amount of energy was either leaked or consumed to heat up the thermal mass of facility's components. The air entrained with particles flowing to the TES bin resulted in a heat loss across the bin amounted to 24% of the collected energy. The large thermal mass of the PWFHX consumed more than half of the collected energy to be heated up, although some heat might had leaked to the concrete platform on which the PWFHX is placed. The reasons for this enormous loss are explained, and recommendations for ensuring efficient construction and operation of such systems are suggested. [ABSTRACT FROM AUTHOR]

This paper presents a thermal performance evaluation of a pilot 50 kWth moving packed-bed particle-to-air heat exchanger. The exchanger has been integrated into the particle-based concentrating solar power (CSP) test facility located at the campus of King Saud University (KSU) in Riyadh, Saudi Arabia. The configuration is a shell-and-tube design in which solid particles move downward by gravity as a dense packed-bed inside the vertical tubes whereas air flows on the shell-side. The detailed description of the heat exchanger and integration process is presented. Tests were conducted to investigate the effect of particle flow rate on the heat exchanger performance. Although the heat exchanger was tested at off-design conditions using a recuperated air of a gas turbine as a heat source, the results show high values of the overall heat transfer coefficient (up to ∼120 W/m2-°C) in accordance with shell-and-tube indirect contact heat exchangers. The heat exchanger will be tested at the design conditions in the near future when the weather conditions in Riyadh city improve and the direct normal irradiance (DNI) increases. The particle heating receiver (PHR) will be used as a heat source in the upcoming test campaign. [ABSTRACT FROM AUTHOR]

Direct particulate heating receivers (DPHR) are the most important part of the particle-based concentering solar power (PBCSP) systems in which the particles are exposed to direct high-concentrated sunlight. One variation of DPHRs is the obstructed flow receiver, which King Saud University (KSU) and the Georgia Institute of Technology have been developing for years. In this type of PHR, the particles fall freely in the form of a curtain through some obstruction to decelerate flow and allow the falling particles to absorb a considerable amount of thermal energy. The discrete structure presented in this design uses chevron-shaped mesh made of metallic alloy (Inconel 601) that was fixed on a Duraboard HD (an alumina-silica ceramic fiberboard). The design has been successfully tested using red sand and CARBOBEAD as heat transfer media. However, the interaction between the particulate and chevron mesh screen needs to be considered carefully at high operating temperatures. Careful consideration is the softening of the tip of the metallic chevron facing the sun. The tip of the mesh becomes soft at high temperatures and deforms which causes the particles to agglomerate on the chevrons, clog the chevron opening, and prevent particles from moving down to the end. Also, the particles get exposed to high flux for a long time which leads to particle fusing. Finally, a high degradation in the thermal efficiency of the system occurs as a result of this agglomeration and stops the whole operation of the plant. So, it is very important to study the interaction between the particulates and obstacles at high temperatures before installing it in the PHR cavity. This paper addresses these issues by presenting a study of the interaction between several chevron candidate materials and high-temperature candidate particles. The test was conducted in two stages: On-sun test and the lab test. Lab test is done by immersing the mesh screens with particulate materials and then heating at a specific temperature for several hours. For the chevron material, three candidates were considered: (a) stainless steel 316, (b) Inconel 601, and (c) ceramic mesh (WHIPOX). Three candidate particulate materials were considered: (a) Riyadh red sand, (b) Riyadh white sand, and (c) CARBOBEAD CP. Three temperature levels were considered in the lab test: (a) 800℃, (b) 1000℃, and (c) 1200℃. [ABSTRACT FROM AUTHOR]

Utilizing the solid particles as a heat transfer media in concentrated solar power systems offers many advantages over other current commercial heat transfer media like steam, oil, air, and molten salt. These advantages are: high operating temperature (>1000℃), abundance, low cost, high thermal energy storage capacity, no freezing limit, and its ability to be irradiated directly. The solid particles do not need to be pumped to the system facilities. Friction properties play the main role in the transfer of particles from one place to another. However, it is important to study the flow kinematics in which the particle flow behavior is affected by friction or interparticle properties such as the angle of rest and the internal angle of friction. Knowing such angles is important to design components like chutes, hoppers, particle-to-fluid heat exchanger, etc., and understanding the characteristics of particle flow. In this study, the verification of friction properties for different particles was studied based on its dynamic resting angle, internal angle of friction, and static resting angle. The revolving cylinder method was used to measure the dynamic angle of repose while a transparent glass cylinder with a small hole in the center of the bottom surface was used to measure the internal angle of friction. The static angle of repose was verified using two methods: The hollow cylinder method and the fixed funnel method. Three natural particulate materials with different particle sizes and sphericity were used namely: Riyadh white sand, Riyadh red sand, US olivine sand (Green Diamond) and one engineered particulate material called Carbobead. [ABSTRACT FROM AUTHOR]

Utilizing an efficient and reliable thermal energy storage (TES) bin in a concentrated solar power (CSP) systems can help to overcome the variability in the thermal energy supply due to the bad weather condition or during the off-sun period. At King Saud University (KSU), Saudi Arabia, a multilayered-wall TES bin has been developed to be used in a solid particle central receiver system. In this system, the multilayered-wall TES bin can reach a very high temperature and experience a thermal cycle. Therefore, the thermal expansion and compression will be significant which can cause cracks on the walls of the TES. One of the important layers used to avoid such a failure is the expansion joint layer. This research work attempts to further investigate various expansion joint materials for the TES bin, additional to the one which has been used in the previous design. Compression tests were conducted to find the suitable expansion joint materials. There were six (6) materials tested, i.e., asphalt, fibre, ceramar, sponge rubber, cork, and wooden fiber. Initially, room temperature compression tests were done. The room temperature tests were done in two stages, i.e., the preliminary stage and the advanced stage. The preliminary stage was a short test without cycling to quickly screen the prospective materials. The advanced stage was done to further test the best materials concluded from the first stage test. This test was done with longer compression holding-time and with cycling. The most promising materials from the room temperature tests were, then, tested at a higher temperature to simulate the real condition under which they will be utilized. The go/no-go criteria were how far the material recovers to its original thickness (referred to as the recovery value) and how fast after the compression it recovers (referred to as the recovery speed). The results showed that, at room temperature, sponge rubber has the highest recovery value and speed. However, at a higher temperature, all the materials have relatively low recovery values with the wooden fiber outperforms the others. [ABSTRACT FROM AUTHOR]

This paper presents results of an exploratory study aimed to investigate heat loss during charging the thermal energy storage (TES) bin with hot solid particles. Tests were performed at the pilot particle-based power tower plant in King Saud University in Riyadh, Saudi Arabia. In this plant, a small TES bin is installed between the particle heating receiver and the particle-to-working-fluid heat exchanger. Two types of experiments, the on-sun test and the steady-state test, were conducted; in each experiment, two cases were considered, namely the empty bin and full bin. Results show that in the case of empty TES bin, the air, entrained as a result of particles falling, can cause a significant temperature drop in the particle side. Moreover, the use of several particle-feeding lines can promote the so-called chimney effect. In the case of full bin, the amount of entrained air was reduced significantly, thus the temperature drop in the particle side was shrunk greatly. However, results also show that the air occupying the empty region (due to the particle's angle of repose) inside the full bin recorded lower temperature than that of particles which implies that the chimney effect is still present. [ABSTRACT FROM AUTHOR]

The current paper presents experimental observations on the particle-based integrated gas turbine-solar heating system located on the campus of King Saud University in Riyadh, Saudi Arabia. The system was built to achieve three primary outcomes: (1) use the concentrated solar energy to raise the particle temperature to more than 600°C. (2) achieve a significant solar contribution, which can help in limiting fossil fuel consumption. (3) demonstrate the ability to store the thermal energy in the solid particles for later use. However, only two objectives have been fulfilled; due to some deficiencies and limitations in the current system, the first objective has been successfully fulfilled while the second is partially fulfilled. The paper outlines the system deficiencies and the imperative recommendations towards efficient execution of the system. The system deficiencies mentioned above include the heat loss on PHR due to exposure to the wind, particle agglomeration tendency on some regions of the PHR, heat loss while charging the thermal energy storage, and heat loss during particle circulation. However, despite all deficiencies above, the system was able to bring the particle temperature (red sand particles) to more than 600°C. Furthermore, the results show that harnessing solar energy is possible if such deficiencies were overcome. In general, the results are encouraging, and additional precautions are currently added to the system to push the particle temperature even higher and achieve higher solar contribution. [ABSTRACT FROM AUTHOR]

Using solid particles as a heat transfer media (HTM) in concentrated solar power applications gained growing attention lately. Unlike molten salt, solid particles offer high operating temperatures (greater than 1000°C), no freezing issues, abundantly availabile, high thermal energy storage capacity, low-cost, non-corrosive, and applicability of 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 (1) thermophysical and optical properties, (2) thermal resistance, (3) crack resistance (4) satisfactory health and safety risks (5) availablity, 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 time. Therefore, the knowledge of the thermal performance of particulate materials becomes significant before using them as HTM. In this study, some particulate materials have been chosen to study their feasibility to be used as heat transfer and storage media for the particle-based central receiver tower system. These particulate materials include red sand, white sand, ilmenite and carbobead CP. The cyclic heating was performed on the particulate materials for 500 hours at 1200°C. Weighted solar absorbtance and specific heat have been measured for the candidate particulates as received and after cyclic heating (aging) for a specified duration of time. [ABSTRACT FROM AUTHOR]

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. Riyadh is a region that is well known for high dust-loading, especially during the summer. Two days of different atmospheric conditions were selected in September for comparative analysis. Results show that this method produces repeatable results, and that the CSR can increase significantly due to high dust loading and passing clouds. The CSR is found to be a strong function of DNI, ranging from about 4.5% at DNI values above 800 W/m2 and increasing to as much as 8.5% when DNI drops to about 400 W/m2, due to passing clouds. Furthermore, the results show that circumsolar ratio tends to be high in the early morning and late afternoon due to the high air mass, while its values tend to be lowest around solar noon when the air mass is lowest. [ABSTRACT FROM AUTHOR]