technology advancements for next generation falling ... · 400 c. ho et al. / energy procedia 49 (...

Energy Procedia 49 ( 2014 ) 398 – 407

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1876-6102 © 2013 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).Selection and peer review by the scientifi c conference committee of SolarPACES 2013 under responsibility of PSE AG. Final manuscript published as received without editorial corrections. doi: 10.1016/j.egypro.2014.03.043

SolarPACES 2013

Technology advancements for next generation falling particle receivers

C. Ho,a,* J. Christian,a D. Gill,a A. Moya,a S. Jeter,b S. Abdel-Khalik,b D. Sadowski,b N. Siegel,c H. Al-Ansary,d L. Amsbeck,e B. Gobereit,e and R. Bucke

aSandia National Laboratories, P.O. 5800, Albuquerque, NM 87185-1127, USA bGeorgia Institute of Technology, 771 Ferst Drive, Atlanta, GA 30332, USA

cBucknell University, 1 Dent Drive, Lewisburg, PA 17837, USA dKing Saud University, P.O. Box 800, Riyadh 11412, Saudi Arabia

eGerman Aerospace Center (DLR), Pfaffenwaldring 38-40, Stuttgart 70569, Germany

Abstract

The falling particle receiver is a technology that can increase the operating temperature of concentrating solar power (CSP) systems, improving efficiency and lowering the costs of energy storage. Unlike conventional receivers that employ fluid flowing through tubular receivers, falling particle receivers use solid particles that are heated directly as they fall through a beam of concentrated sunlight for direct heat absorption and storage. Because the solar energy is directly absorbed by the particles, the flux limitations associated with tubular central receivers are mitigated. Once heated, the particles may be stored in an insulated tank and/or used to heat a secondary working fluid (e.g., steam, CO2, air) for the power cycle. Thermal energy storage costs can be significantly reduced by directly storing heat at higher temperatures in a relatively inexpensive, stable medium. This paper presents an overview of recent advancements being pursued in key areas of falling particle receiver technology, including (1) advances in receiver design with consideration of particle recirculation, air recirculation, and interconnected porous structures; (2) advances in particle materials to increase the solar absorptance and durability; and (3) advances in the balance of plant for falling particle receiver systems including thermal storage, heat exchange, and particle conveyance. © 2013 The Authors. Published by Elsevier Ltd. Selection and peer review by the scientific conference committee of SolarPACES 2013 under responsibility of PSE AG.

Keywords: Falling particle receiver; recirculation; air curtain; solid particles; storage; particle heat exchange; proppants; particle lift

* Corresponding author. Tel.: 1-505-844-2384; fax: 1-505-845-3366.

E-mail address: [email protected]

© 2013 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).Selection and peer review by the scientific conference committee of SolarPACES 2013 under responsibility of PSE AG. Final manuscript published as received without editorial corrections.

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C. Ho et al. / Energy Procedia 49 ( 2014 ) 398 – 407 399

1. Introduction

Conventional concentrating solar power (CSP) central receiver technologies employing molten salts have been limited to temperatures of around 600°C. At higher temperatures, nitrate salt fluids become chemically unstable [1]. In contrast, direct absorption receivers using solid particles that fall through a beam of concentrated sunlight for direct heat absorption and storage have the potential to increase the maximum temperature of the heat-transfer media to over 1,000°C [2]. Once heated, the particles may be stored in an insulated tank and/or used to heat a secondary working fluid (e.g., steam, CO2, air) for the power cycle. Thermal energy storage costs can be significantly reduced by directly storing heat at higher temperatures and with higher temperature differences for higher storage capacities in a relatively inexpensive medium (i.e., sand-like particles). Because the solar energy is directly absorbed in the sand-like working fluid, the flux limitations associated with tubular receivers are mitigated. Although a number of analytical and laboratory studies have been performed on the falling particle receiver since its inception in the 1980’s [3, 4], only one set of on-sun tests of a simple falling particle receiver has been performed [5]. Those initial tests only achieved 50% thermal efficiency, and the maximum particle temperature increase was only ~250°C.

Figure 1. Conceptual illustration of a concentrating solar power plant employing a falling particle receiver system.

The objective of this work is to make advancements in key areas of falling particle receiver technology, including (1) advances in receiver design with consideration of particle recirculation, air recirculation, and interconnected porous structures; (2) advances in particle materials to increase the solar absorptance and durability; and (3) advances in the balance of plant for falling particle receiver systems including thermal storage, heat exchange, and particle conveyance. Technical innovations are being developed in these areas to meet the technical targets set forth by the United States Department of Energy for development of advanced receivers: (1) temperature of HTF exiting receiver ≥ 650°C, (2) annual average receiver thermal efficiency ≥ 90%, (3) number of thermal cycles without failure ≥ 10,000, and (4) cost of receiver subsystem ≤ $150/kWth.

2. Receiver designs

Designs for a high-temperature falling particle receiver must consider a number of factors that impact the irradiance and heating of the particles, including residence time within the concentrated beam of sunlight, convective and radiative heat losses, and stability of the particle flow. The following sections describe several advancements that address these factors to increase particle temperatures and improve the thermal efficiency of the receiver.

400 C. Ho et al. / Energy Procedia 49 ( 2014 ) 398 – 407

2.1. Particle recirculation

One option being pursued to increase the residence time of particles in the concentrated sunlight is the development of a system that recirculates particles back to the top of the receiver for multiple passes through the concentrated beam. This concept was first introduced by Sandia in 2006 [6], and analytical studies since the introduction of this concept have shown that recirculation can be effective at increasing the particle temperatures to >800°C. “Smart” recirculation patterns can be developed to take advantage of the spatially varying irradiance distribution at different times of the day [7, 8]. The general idea is to increase the thermal efficiency by initially releasing particles in locations within the cavity where the irradiance is lower to preheat the particles before being dropped through the high-flux regions. Figure 2 shows an example of a recirculation pattern that proved to yield the optimal efficiency for both north-facing (north heliostat field) and face-down (surround heliostat field) at solar noon. Figure 2 also shows the simulated particle tracks colored by temperature using computational fluid dynamics with consideration of radiation, convection, and discrete-phase particle participation with turbulent flow and entrainment. Simulated particles are exceeding 800°C [9], and face-down designs have resulted in thermal receiver efficiencies above 90% [10]. Additional improvements are being sought, and features that are being evaluated and optimized include aperture size, nod angle, alternative geometries, particle flow rate per unit length (opacity), particle size, release location, and inclusion of an air curtain ([11, 12]).

Figure 2. Left: Particle recirculation pattern identified for north-facing and face-down receivers at solar noon [7, 8]. Right: Simulated particle

flow lines colored by particle temperature (K).

2.2. Air recirculation

External winds can enter the cavity receiver and destabilize the falling particles, ejecting the particles from the receiver [13]. In addition, as heated air within the cavity is exchanged with cooler ambient air, significant convective heat losses can occur. Convective losses from a north-facing falling-particle receiver have been simulated to account for ~20% of the incident power on the receiver [7]. In order to reduce the destabilization of particles and convective heat loss by external winds, air curtains (or aerowindows) near the aperture of the receiver have been proposed [14, 15]. A high-temperature blower is used to inject and recirculate a stream of air flow across the aperture. Tan et al. [16] performed computational simulations that showed the thermal efficiency could be increased by nearly 10% with the presence of an air curtain, depending on the wind velocity; however, no tests were performed.

In this work, a prototype system has been developed to generate a recirculating air curtain across the receiver aperture (Figure 3). A particle release bin holds the particles above the receiver and releases them through a hydraulically actuated slot in the bottom. Another bin is located beneath the receiver to collect the particles and transfer them to the base of a bucket elevator, which lifts the particles back to the top of the particle release bin. The as-built receiver dimensions are 1.3 m x 1.3 m x 1.3 m. Figure 4 (left) shows the measured air velocity distribution across the aperture at the highest valve setting. Results from preliminary testing (and computational fluid dynamic simulations [17]) showed that when 0.7 mm CARBO HSP particles were dropped near the aperture at a high blower


flow rate, the particle flow was stable, although there was some spreading of the particle curtain caused by the air flow from the blower (Figure 4, right image)

Figure 3. Prototype solid particle receiver including top and bottom particle bins, air recirculation system, and particle lift elevator (behind).

Figure 4. Left: measured air velocity distribution in the air stream across the receiver aperture with blower on high setting. Air flow is from bottom to top. Right: side view of particles falling through the receiver (aperture is to the right) with blower on high setting. The particle flow is

highlighted with brown arrows, and the blower airflow is highlighted with a blue arrow.

2.3. Porous structures

Another concept to increase the residence time of the particles in the concentrated beam of the receiver is the use of highly porous interconnected or discrete structures. The particles fall through or over the structures, which will increase the residence time of the falling particles, thereby increasing the amount of absorbed concentrated solar radiation while providing additional containment that minimizes the amount of particle attrition due to wind and dispersion.

Aperture Width (m)

Ape

rture

Hei

ght (

m)

0.05 0.25 0.50 0.74 0.940.05

0.25

0.50

0.74

0.99

2

4

6

8

10

12

14

Particle release bin

Particle collection bin Blower nozzles

Suction plenum

Blower

Cavity

Air

velo

city

(m

/s)


Figure 5. Use of interconnected porous panels in the falling particle receiver to contain and increase the residence time of falling particles (patent pending).

Lab-scale evaluation of particle flow through ceramic porous structures showed that this concept is feasible without clogging. Tests were performed to evaluate the accumulation of particles in stacked silicon-carbide ceramic porous material (10 pores-per-inch). Stacks of one, two, four, six and eight square-faced samples were tested by pouring sand on top of the stack, allowing it to pass through, and then weighing the stack after each set of 10 pours. The fraction of void volume occupied by the sand was determined by the mass of the sand retained in the stack, the particle density, and the void volume of the porous blocks, which was measured using a water displacement method. It was observed that the sand accumulated only on very wide ligaments and horizontal cell walls. It did not appear to get trapped within pores. Furthermore, there was no systematic difference between the number of stacked blocks and fraction of void volume occupied. An apparatus has been built to test particle flow through the porous structures at elevated temperature (up to 1000°C) for thousands of cycles.

3. Particles

3.1. Radiative properties

In a falling particle receiver, the particles serve as both a heat transfer fluid (solar energy absorber) and as a thermal storage medium. The radiative properties of the particles have a strong influence on the magnitude of the thermal losses from the receiver and thus on the efficiency of the receiver itself; increasing the solar weighted absorptance of the particles, while (to a much lesser degree) reducing thermal emittance, directly increases receiver and system level performance. In addition, some of the compounds used to produce solid particle media can be thermally reduced to regenerate the solar absorptance after a period of time.

The solar absorptance and thermal emittance of several candidate particles, including sintered bauxite proppants and sand, were measured (as-received) using a Surface Optics Corporation 410 Solar reflectometer Table 1 [18]. The radiative properties of high-temperature paint (Pyromark 2500) used on central receivers is also shown in the table for reference. The absorptance of some of the as-received sintered bauxite proppants was quite high (>0.9). Heating the particles was shown to reduce the solar weighted absorptance from as high as 93% to 84% after 192 hours at 1000 °C. Particle stability was better at 700 °C, and the solar absorptance remained above 92% after 192 hours of exposure. Analysis using x-ray diffraction (XRD) showed evidence of multiple chemical transformations in the sintered bauxite particle materials, which contain oxides of aluminum, silicon, titanium, and iron, following heating in air. However, the XRD spectra show only small differences between as-received and heat treated particles leaving open the possibility that the observed change in radiative properties results from a change in oxidation state without a concomitant phase change. Regardless of the specific degradation mechanism, the solar weighted absorptance of the particles can be increased beyond the as-received condition by chemically reducing the

Particle curtain

Aperture

Particle curtain

Aperture

Falling Particles

Porous Ceramic Foam

Insulation


particles in forming gas (5%H2 in N2 or Ar) above 700 °C, providing a possible means of periodically rejuvenating degraded particles in situ.

Table 1. Measured solar absorptance and thermal emittance of candidate particles and Pyromark 2500 paint (for reference).

Material Name Type Solar weighted absorptance

Thermal emittance*

Selective Absorber Efficiency**

Carbo HSP Sintered Bauxite 0.934 0.843 0.864

CarboProp 40/70 Sintered Bauxite 0.929 0.803 0.862

CarboProp 30/60 Sintered Bauxite 0.894 0.752 0.831

Accucast ID50K Sintered Bauxite 0.906 0.754 0.843

Accucast ID70K Sintered Bauxite 0.909 0.789 0.843

Fracking Sand Silica 0.55 0.715 0.490

Pyromark 2500 High-Temperature Paint 0.97 0.88 0.897

*Spectral directional reflectivity values were measured at room temperature. The total hemispherical emittance was calculated assuming a surface temperature of 700 °C.

**Selective absorber efficiency, , is defined as = ( Q - T4)/Q, where is the solar absorptance, Q is the irradiance on the receiver (W/m2), is the thermal emittance, is the Stefan-Boltzmann constant (5.67x10-8 W/m2/K4), and T is the surface temperature (K). Q is assumed to be

6x105 W/m2 and T is assumed to be 700 °C (973 K).

3.2. Particle durability

In a falling particle receiver, the impact of the particles with the collection hopper, structures, or other particles may cause abrasion and attrition of the particles. To measure the attrition at different particle velocities, devices for three different drop heights were developed. Plexiglas pipes with 0.5 m, 1.72 m, and 10 m length were used. The 0.5 m and 1.72 m pipes were filled with a certain amount of particles and closed with steel plates. Manual half turning of the pipes resulted in the drop of the particles. The process was repeated until a measurable amount of attrition occurred. For the 10 m drop height a stationary pipe was used and particles were transported to the top externally for each cycle. Particles were sieved with an automatic sieve machine and measured with an analysis scale (accuracy 0.1 mg) before the initial filling and after the particle drops.

Particles were dropped 50 – 100 times until a measurable mass loss occurred. Repeated experiments showed a standard deviation of 10% of the average measurement. The particle attrition per drop (measured mass loss / total mass released during all the drops) is shown in Figure 6 as a function of particle speed as measured by particle velocimetry. The data indicate that a mass loss of <0.004% was achieved for particle velocities lower than ~7 m/s. It is difficult to extrapolate the limited data to attrition rates at higher particle speeds, but assuming an exponential increase in attrition with particle velocity indicates that as particle speeds approach ~10 m/s or greater, the attrition rate may exceed ~0.01% of the mass flow. There is a possibility that attrition is only high for the first cycles as the outer layer of the particles is perhaps not very well sintered. Then attrition would be reduced after the outer layer was removed. A possible mitigation could be a device or method to buffer the particle impact, such as the porous structures being investigated. A curved structure made of resistant ceramics that would slowly deflect and decelerate the particles in the collection bin could also be used.


Figure 6. Measured particle attrition per drop (measured mass loss / total mass released during all the drops) for two different sintered bauxite particle sizes.

4. Balance of plant

4.1. Thermal storage

Solid particles have three distinct advantages as storage media relative to more conventional materials such as molten salts: (1) they are chemically inert and stable beyond 1100oC, (2) they are capable of storing energy over a greater temperature span (effectively increasing storage density in a sensible energy-based system), and (3) they are relatively low cost. Assuming energy storage between 400 °C – 1000 °C, the current cost for candidate particles is estimated to be ~$3-$7/kWhth, depending on materials (e.g., silica, alumina oxide, SiC), delivery costs, and inflation [19]. This is considerably lower than the current cost of refined nitrate salts that store energy between 280 °C – 565 °C at a media cost of nearly $12/kWhth [20]. Although it is difficult to predict the cost of a complete particle storage system, the fact that the media costs are low compared to current competing technology, combined with the chemically benign nature of the materials, makes it likely that the cost of energy storage can be significantly reduced relative to current CSP systems.

Small-scale thermal energy storage (TES) designs and experiments for measuring heat loss were developed as part of this work [21]. Early designs investigated a layered bin consisting of regular firebrick (FB), autoclaved aerated concrete (AAC), and reinforced concrete (RC) on the outside, and an LPG burner was used to simulate the presence of hot solid particles inside the TES bin. Despite the durability and low cost of firebrick, the thermal performance was not suitable due to its high thermal conductivity. In addition, the AAC was found to be unsuitable due to cracking at high temperatures. In a new design, FB and AAC were replaced by insulating firebrick (IFB) and perlite concrete (PC), respectively. RC remained as the external layer. Furthermore, an expansion joint (EJ) was installed between PC and RC to relieve stress. The entire bin was ~ 3 m x 3 m x 3 m.

The experiment was continuously run for nearly 45 hours, during which the temperature was maintained at about 800 °C, and steady state conditions were closely approximated. Simulation and experimental results of the temporal and spatial temperature distributions were in close agreement. Results indicate that the transient response time of the system was of the order of 2 days, and the simulation gave a reasonably close agreement of the temperature distribution during that transient startup. The steady-state heat loss was determined to be approximately 4.4% over a 24 hour period. Furthermore, inspection of the materials used to construct the TES system showed that they remained intact and did not show signs of cracking or wearing. These results show that high-temperature TES

0.0E+00

5.0E-06

1.0E-05

1.5E-05

2.0E-05

2.5E-05

3.0E-05

3.5E-05

4.0E-05

0 2 4 6 8

Attr

ition

per

cyc

le/d

rop

[-]

Particle speed [m/s]

Carbo HSP 12/18 particles,1.291mm average diameter

Saint Gobain Norpro SinteredBauxite 30/50 particles,0.458mm average diameter


systems can be constructed of readily available materials and yet meet the heat-loss requirements for a falling particle receiver system, thereby contributing to reducing the overall cost of concentrating solar power systems.

(a) (b)

Figure 7. Experimental and simulation results of TES bin: (a) temperature vs. time, and (b) steady-state temperature distribution across layers (black lines represent experimental data and red lines represent simulation results) [21].

4.2. Particle heat exchange

Heat exchangers suitable for coupling particulate media to process fluids such as pressurized air and steam have been developed in the past. However, many of these systems have not been evaluated and optimized for the temperatures, particle materials, and flow rates being considered for high-temperature falling particle receivers. Initial bench-scale tests were performed on particle-to-fluid heat exchangers employing shell-and-tube designs, where the particles reside on the shell side and slowly flow across heated tubes [22]. The effect of input power levels and sand velocities were studied for silica and olivine sand, and the heat transfer coefficient values ranged from 120 to 180 W/m2-K. [23]. Additional tests are being performed to investigate different particle sizes and types (sand and proppants). In addition, a larger (~1 m3) shell-and-tube system has been designed with finned serpentine tubes containing either heated air or water. Tests are being conducted to evaluate the impact of different particle sizes, flow rate, and heat input on the overall heat transfer coefficient.

4.3. Particle conveyance

Particle recirculation may be required with a free-falling particle receiver and may even be desirable in a porous-matrix receiver to provide additional heating to the particles. Available technologies that could be used for this relatively demanding and high temperature operation include the OLDS elevator and the conventional bucket lift. The OLDS elevator employs a circular casing rotating around a stationary screw or helix. A small-scale OLDS elevator was found to have an efficiency of around 4.5%. OLDS engineers also scaled up their calculations to an 8.23 m (27 ft) elevator yielding a projected efficiency of around 5.5%. However, because the recirculation lift height is relatively small, the parasitic power consumed is also small.


OLDS is at present designing and building a small capacity 10.7 m (35 ft) elevator for King Saud University (KSU) to be part of the Particle Heating Receiver system near the KSU campus in Riyadh, Saudi Arabia. This system is being designed for 815 °C (1500 °F) operation; the vendor is confident that such a unit of comparable lift can operate at the desired operating temperature.

Sandia has a conventional bucket lift that was used in the first large-scale on-sun test of a falling particle receiver [5]. This bucket lift has been modified to operate in a scale model test of the air recirculation system for this project. The efficiency was recently measured by recording the time required to fill a 19 liter (5 gallon) bucket to an approximate level and then weighing the bucket. The test was performed 10 times, and the modified bucket lift, with a total lift height of 6 m, had a measured flow rate of 1.35 kg/s with a standard deviation of 0.008, showing good agreement among the 10 tests. The voltage, current, and power factor were measured and showed the lift to require 1.02 kW during lift operation. This yielded a bucket lift efficiency of 7.8%. This efficiency is higher than Olds type elevators, but is expected to decrease for a high temperature bucket lift due to the use of chains instead of a belt and the need to insulate the buckets and lift more bucket weight without increasing the flow rate. Manufacturers of bucket lifts have, however, expressed confidence in building a 500 °C bucket lift, so this technology is still a candidate for particle recirculation. A commercial design for a bucket lift with 120 m transport height at ~2.4x105 kg/hour (260 tons/hour) and 200°C with a 171 kWe motor results in a 50% efficiency.

Available technologies to lift particles from the base of the tower to the top includes: mine hoist, bucket elevator, pocket elevator, screw conveyor, OLDS elevator, pneumatic conveyors, conveyor belts, cleated conveyor belts, metallic belted conveyors, en masses elevators, bucket wheels, linear induction motor powered elevators, and electromagnetic field conveyors. The current most likely candidate would be the use of a mine hoist with an insulated container (also called a “skip”). The most basic mine hoist consists of a skip at the end of a wire rope which is wrapped around a large motor powered drum. Material is loaded into the skip at the bottom of a shaft, the skip is raised, and the material is discharged at the top. Mine hoist efficiency is largely determined by the weight of material hoisted, and the speed at which it is hoisted. Several examples are given by de la Vergne [24], which shows an overall lift efficiency of 0.748. These calculations are for two skips hoisting in balance on a single drum. With reasonable insulation (0.3 m of high temperature mineral wool), it is expected that the thermal loss from the skip can be less than 0.1% of the heat collection rate. A further advantage of the insulated mine hoist system is the ability to recover potential energy of the hot particles on the way down from the receiver, further reducing the power consumption of the lift system.

The second most promising concept appears to be a so-called pocket elevator. These appear to be similar to conventional bucket elevators, in that a series of specially shaped containers (pockets) travels along a loop of steel cable between two pulleys. Material falls into the pockets at the bottom pulley and is discharged at the top pulley when the pocket is inverted. Pocket elevators are mentioned to be able to handle materials up to 500 °C and 930 °C by Ilchmann-Fordertechnik [25] and Falcone [19], respectively.

5. Summary

Falling particle receivers employ solid particles that are directly irradiated by concentrated sunlight from a field of heliostats. These systems have the potential to increase temperatures beyond 600 °C, which is the current limit for conventional molten-salt central receiver systems. In addition, thermal energy storage costs can be reduced by directly storing heat at higher temperatures in a relatively inexpensive, stable medium (e.g., sand or proppants). Important issues include increasing the heating of the particles in the receiver while mitigating heat losses. In addition, the design of balance-of-plant components specific to particle heating, exchange, and storage is necessary to enable a working particle receiver system. In this paper, an overview of advancements for next-generation falling particle receivers has been presented. These include (1) advances in receiver design with consideration of particle recirculation, air recirculation, and interconnected porous structures; (2) advances in particle materials to increase the solar absorptance and durability; and (3) advances in the balance of plant for falling particle receiver systems including thermal storage, heat exchange, and particle conveyance.


Acknowledgements

This work was funded by the U.S. Department of Energy, SunShot Initiative, Project #DE-EE0000595-1558. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

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