A particle receiver is an object placed on the top of a solar tower on which surface

solar energy Solar energy is the radiant energy from the Sun's sunlight, light and heat, which can be harnessed using a range of technologies such as solar electricity, solar thermal energy (including solar water heating) and solar architecture. It is a ...

is concentrated by means of a solar field composed of large number of mirrors, called

heliostat A heliostat () is a device that reflects sunlight toward a target, turning to compensate for the Sun's apparent motion. The reflector is usually a plane mirror. The target may be a physical object, distant from the heliostat, or a direct ...

s. The goal is to transform solar energy into

thermal energy The term "thermal energy" is often used ambiguously in physics and engineering. It can denote several different physical concepts, including: * Internal energy: The energy contained within a body of matter or radiation, excluding the potential en ...

that can be used in a heat process, thermochemical process, or in a

heat engine A heat engine is a system that transfers thermal energy to do mechanical or electrical work. While originally conceived in the context of mechanical energy, the concept of the heat engine has been applied to various other kinds of energy, pa ...

to produce electricity in a solar tower

power plant A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the electricity generation, generation of electric power. Power stations are generally connected to an electr ...

. To accomplish this, it is necessary to introduce certain material, called heat transfer medium, to the receiver that is then heated up, either directly or indirectly, by the concentrated solar energy before leaving the receiver at a higher temperature. Unlike receivers used in conventional

concentrated solar power Concentrated solar power (CSP, also known as concentrating solar power, concentrated solar thermal) systems generate solar power by using mirrors or lenses to concentrate a large area of sunlight into a receiver. Electricity is generated whe ...

(CSP), power plants which employ

molten salt Molten salt is salt which is solid at standard temperature and pressure but liquified due to elevated temperature. A salt that is liquid even at standard temperature and pressure is usually called a room-temperature ionic liquid, and molten salts ...

s as a heat transfer medium that is heated indirectly by flowing through the metal tubes that are exposed to the concentrated solar energy, particle receivers adopt solid particles which then can be heated either directly or indirectly, depending on the technology considered. One of the main advantages of adopting particles as a heat transfer medium is the possibility of direct heating, where particles are exposed directly to the incoming solar radiation, thus avoiding issues related to non-uniform heating of receiver tubes. Also, the possibility to reach temperatures above 1000 °C allows for the adoption of

Brayton cycle The Brayton cycle, also known as the Joule cycle, is a thermodynamic cycle that describes the operation of certain heat engines that have air or some other gas as their working fluid. It is characterized by isentropic process, isentropic compre ...

with supercritical CO₂ as the

working fluid For fluid power, a working fluid is a gas or liquid that primarily transfers force, motion, or mechanical energy. In hydraulics, water or hydraulic fluid transfers force between hydraulic components such as hydraulic pumps, hydraulic cylinders, a ...

which can achieve higher

thermal efficiency In thermodynamics, the thermal efficiency (\eta_) is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, steam turbine, steam engine, boiler, furnace, refrigerator, ACs etc. For ...

compared to the steam Rankine cycle which is used in the conventional CSP power plants that have a maximum temperature limit of 565 °C due to issues related with the thermal stability of the molten salts.

Directly heated particle receiver

Free falling receiver

This technology is based on a

free fall In classical mechanics, free fall is any motion of a physical object, body where gravity is the only force acting upon it. A freely falling object may not necessarily be falling down in the vertical direction. If the common definition of the word ...

ing particle curtain inside the receiver cavity that absorbs concentrated solar radiation. Idea of using falling solid particles in a concentrated solar power facility to supply high temperature heat to the power cycle or chemical process was introduced in a pioneering work carried out by Martin and Vitko during the beginning of the 1980s at

Sandia National Laboratories Sandia National Laboratories (SNL), also known as Sandia, is one of three research and development laboratories of the United States Department of Energy's National Nuclear Security Administration (NNSA). Headquartered in Kirtland Air Force B ...

. However, first step towards demonstrating the concept at the larger scale was carried out in 2009 at National Solar Thermal Test Facility in Albuquerque, NM where prototype particle receiver was placed atop the 61 meter tall solar tower with a solar field able to provide 5 MW_th. These tests resulted in receiver thermal efficiency of around 50% and maximum temperature increase of the particles of around 250 °C. More comprehensive tests were carried out in 2015 using a 1 MW_th receiver having a 1 by 1 meter aperture through which concentrated sun radiation enters the cavity. Receiver thermal efficiency ranged from 50% to 80% and temperature of the particles at the bottom of receiver reached 700 °C in some cases. Limit on the maximum temperature of molten salts used in the conventional solar tower power plants led to a workshop organized by

United States Department of Energy The United States Department of Energy (DOE) is an executive department of the U.S. federal government that oversees U.S. national energy policy and energy production, the research and development of nuclear power, the military's nuclear w ...

(DOE) in August 2016 which identified three possible pathways for the next generation CSP power plants based on the following heat transfer carriers: molten salts, solid particles, and gaseous fluids. This further led to the Generation 3 Concentrating Solar Power Systems funding program that started on May 15 of 2018 when DOE announced its intention to provide $72 million to the project where three teams are going to compete in building a system integrated with a

thermal energy storage Thermal energy storage (TES) is the storage of thermal energy for later reuse. Employing widely different technologies, it allows surplus thermal energy to be stored for hours, days, or months. Scale both of storage and use vary from small t ...

that is able to efficiently capture solar energy and provide it to the working fluid of the power cycle at temperatures above 700 °C. On March 25, 2021 DOE announced that the pathway adopting falling solid particles is the most promising one for achieving 2030 cost targets of 0.05$/kWh and awarded Sandia National Laboratories with $25 million for building, testing, and operating pilot plant adopting particle receiver at National Solar Thermal Test Facility which is expected to be completed by the end of 2024.

Obstructed flow receiver

Idea of obstructing the flow of particles while retaining the concept of direct heating is motivated by the fact that by slowing down the flow of particles, it is possible to increase thermal efficiency of the receiver by increasing the

opacity Opacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shie ...

of the particle curtain and reduce their loss through the aperture. Early tests of this concept were carried out at Sandia during the 1980s, but no analytical nor experimental studies were published until 2010s. Experiments carried out at Sandia in 2015 using chevron-shaped porous structures managed to improve heating of the particles and reduce their loss through the receiver aperture. However, there were problems related to the direct exposure of the

stainless steel Stainless steel, also known as inox, corrosion-resistant steel (CRES), or rustless steel, is an iron-based alloy that contains chromium, making it resistant to rust and corrosion. Stainless steel's resistance to corrosion comes from its chromi ...

316, used for constructing these porous structures, to concentrated solar radiation, and its wear due to particle flow over it. Another design proposes to use a spiral ramp over which particles flow due to the combined effect of the

gravitational force Newton's law of universal gravitation describes gravity as a force by stating that every particle attracts every other particle in the universe with a force that is proportional to the product of their masses and inversely proportional to the sq ...

and mechanically induced

vibration Vibration () is a mechanical phenomenon whereby oscillations occur about an equilibrium point. Vibration may be deterministic if the oscillations can be characterised precisely (e.g. the periodic motion of a pendulum), or random if the os ...

s. Tests showed that it is possible to reach particle temperature of 650 °C and thermal efficiency of around 60%. However, this design requires beam down optics that would result in additional optical losses and significant particle flow could be difficult to obtain using the proposed design.

Centrifugal receiver

Receiver installed in Solar Tower Juelich DLR

This concept is based on a rotating

cylindrical A cylinder () has traditionally been a Solid geometry, three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a Prism (geometry), prism with a circle as its base. A cylinder may ...

cavity that can be inclined with respect to the horizontal direction. Due to rotation of the cavity, particles form thin but opaque layer over the cavity inner surface. These particles are then heated by concentrated solar radiation while they descend slowly along the axial direction of the cavity due to effect of the gravitational force. Concept was proposed initially by Flamant in the late 1970s and early 1980s but there were no further developments until DLR started working on this concept in the early 2010s when they started with laboratory scale prototype testing.

Proof-of-Concept A proof of concept (POC or PoC), also known as proof of principle, is an inchoate realization of a certain idea or method in order to demonstrate its feasibility or viability. A proof of concept is usually small and may or may not be complete ...

15 kW_th centrifugal receiver was designed, built, and tested at the DLR facility in

Cologne Cologne ( ; ; ) is the largest city of the States of Germany, German state of North Rhine-Westphalia and the List of cities in Germany by population, fourth-most populous city of Germany with nearly 1.1 million inhabitants in the city pr ...

and the results showed that it is possible to reach outlet particle temperatures of over 900 °C. However, due to problems in measuring particle mass flow rate it was not possible to determine receiver thermal efficiency. Further experimental campaigns obtained thermal efficiency of 75% for particle outlet temperature equal to 900 °C with an incident heat flux of 670 kW/m². As of 2018 this receiver concept is installed at Jülich Solar Tower test facility.

Fluidized receiver

This receiver can operate on two possible working principles. First one was proposed by Flamant at the same time when he proposed the centrifugal receiver, and it is based on a transparent silica wall through which concentrated solar energy passes and heats solid particles that are suspended by means of

compressed air Compressed air is air kept under a pressure that is greater than atmospheric pressure. Compressed air in vehicle tires and shock absorbers are commonly used for improved traction and reduced vibration. Compressed air is an important medium for t ...

. Goal was to heat the suspended particles in order to perform decarbonation of CaCO₃ an therefore the conversion of solar energy to the thermochemical one. Tests carried out showed that it is possible to reach particle temperatures above 1200 °C. Second working principle was proposed by Hunt during late 1970s and it is based on the injection of very small particles inside the stream of compressed air that absorb concentrated solar energy, and due to their large surface area they immediately transfer that heat to the surrounding air. Heating of the mixture is performed until the particles vaporize and then the air is sent to the Brayton cycle to produce electric energy.

Indirectly heated particle receiver

Gravity-driven particle flow through enclosures

This receiver concept is composed of an enclosure with horizontal tubes with their external side inside of the enclosure and having their internal side irradiated with concentrated solar energy. The idea is that particles are going to flow down inside the enclosure due to the gravitational force and around the staggered array of tubes while being heated. Tests showed that

heat transfer Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...

between particles and tubes was reduced in the areas where particles lost contact with the tubes, however no data regarding the temperatures and thermal efficiencies were reported. Advantages of this concept include absence of particle loss due to the presence of the enclosure, however issues related to the thermal stresses on the enclosure may arise due to indirect heating of the particles.

Fluidized particle flow through tubes

This concept is similar to that of the directly heated fluidized receiver, only difference is that now tubes are not transparent and particles are heated indirectly by the metal tubes. Flamant proposed and demonstrated this concept and obtained suspension temperatures up to 750 °C, however he did not report thermal efficiencies. Possible problems related to this idea include electric energy consumption for the fluidization of particles in the receiver and possible hotspots and high surface temperatures that can increase radiation losses to the environment.

Particle selection

Important factor in evaluating thermal performance of the particle receiver and economic viability of the whole plant is the type of particles used. Desired properties include low cost, high thermal stability, and in case of directly heated receivers,

optical properties The optical properties of a material define how it interacts with light. The optical properties of matter are studied in optical physics (a subfield of optics) and applied in materials science. The optical properties of matter include: *Refractiv ...

. Natural materials are considered due to their low price, however attention should also be paid to the composite materials that can be synthesized to enhance desired properties even though they have a higher price. As in directly heated receivers solid particles serve as the solar

absorber In Particle physics, high energy physics experiments, an absorber is a block of material used to Absorption (electromagnetic radiation), absorb some of the energy of an incident Subatomic particle, particle in an experiment. Absorbers can be made ...

s, their optical properties become crucial in evaluating receiver performances. It has been shown that increasing solar absorptivity is more important than reducing thermal

emissivity The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Thermal radiation is electromagnetic radiation that most commonly includes both visible radiation (light) and infrared radiation, which is n ...

, meaning that particles with high emissivity can still be considered as a good candidate if they have high absorptivity, but particles having low absorptivity cannot be considered as a good candidate even if they have low emissivity. Sintered bauxite particles have high solar absorptivity, and it has been shown that despite certain degradation due to prolonged heating, they are able to keep it above 90%. It has also been shown that they are the most durable ones among the other candidate particles. Therefore it has been concluded that they are the best candidate for the use in directly heated particle receivers, and in particular intermediate-density casting media "Accucast" has been deployed at Sandia's National Solar Thermal Test Facility.

References

External links

Falling particle receiver pilot plant

DLR particle receivers

SolarPACES on particle receiver
Solar thermal energy Solar energy