A review of high-temperature particle receivers for concentrating solar power
Introduction
Higher efficiency power cycles are being pursued to reduce the levelized cost of energy from concentrating solar power tower technologies [1]. These cycles, which include combined air-Brayton, supercritical-CO2 (sCO2) Brayton, and ultra-supercritical steam cycles, require higher temperatures than those previously achieved using central receivers. Current central receiver technologies employ either water/steam or molten nitrate salt as the heat-transfer fluid in subcritical Rankine power cycles. The gross thermal-to-electric efficiency of these cycles in currently operating power-tower plants is typically between 30% and 40% at turbine inlet temperatures < 600 °C. At higher input temperatures, the thermal-to-electric efficiency of the power cycles increases. However, at temperatures greater than 600 °C, molten nitrate salt becomes chemically unstable, producing oxide ions that are highly corrosive [2], which results in significant mass loss [3].
Technical challenges and requirements associated with high-temperature receivers include the development and use of geometric shapes (e.g., dimensions, configurations), materials, heat-transfer fluids, and processes that maximize solar irradiance and absorptance, minimize heat loss, and have high reliability at high temperatures over thousands of thermal cycles [4]. Advantages of direct heating of the working fluid include reduced exergetic losses through intermediate heat exchange, while advantages of indirect heating include the ability to store the heat-transfer media (e.g., molten salt, solid particles) for energy production during non-solar hours.
Ho and Iverson [4] showed that a high solar concentration ratio on the receiver and reduced radiation losses are critical to maintain high thermal efficiencies at temperatures above 650 °C. Reducing the convective heat loss is less significant, although it can yield a several percentage point increase in thermal efficiency at high temperatures (note that the convective heat loss in cavity receivers can be a factor of two or more greater than that in external receivers because of the larger absorber area [5]). Increasing the solar absorptance, α, and/or decreasing the thermal emittance, ɛ, can also increase the thermal efficiency.
Particle receivers are currently being designed and tested as a means to achieve higher operating temperatures (>700 °C), inexpensive direct storage, and higher receiver efficiencies for concentrating solar power technologies, thermochemical reactions, and process heat [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. Unlike conventional receivers that employ fluid flowing through tubular receivers, particle receivers use solid particles that are heated—either directly or indirectly—as they fall through a beam of concentrated sunlight. Once heated, the particles may be stored in an insulated tank and used to heat a secondary working fluid (e.g., steam, CO2, air) for the power cycle (Fig. 1). Particle receivers have the potential to increase the maximum temperature of the heat-transfer media to over 1000 °C. Thermal energy storage costs can be significantly reduced by directly storing heat at higher temperatures in a relatively inexpensive medium (i.e., sand-like particles). Because the solar energy is directly absorbed in the particles, the flux limitations associated with tubular central receivers (high stresses resulting from the containment of high temperature, high pressure fluids) are significantly relaxed. The falling particle receiver appears well-suited for scalability ranging from 10 to 100 MWe power-tower systems.
Previous studies have considered alternative particle receiver designs including free-falling [18], obstructed flow [24], [25], centrifugal [20], [21], [26], [27], flow in tubes with or without fluidization [15], [22], [23], [28], [29], [30], [31], multi-pass recirculation [9], [17] north- or south-facing [6], [11], and face-down configurations [32]. In general, these particle receivers can be categorized as either direct or indirect particle heating receivers. Direct particle heating receivers irradiate the particles directly as they fall through a receiver, while indirect particle heating receivers utilize tubes or other enclosures to convey and heat the particles. The following section summarizes both direct and indirect particle heating receivers and presents advantages and challenges associated with each.
Section snippets
Free-falling particle receivers
The most basic form of a direct particle heating receiver consists of particles falling through a cavity receiver, where the particles are irradiated directly by concentrated sunlight. The particles are released through a slot at the base of a hopper above the receiver, producing a thin sheet (or curtain) of particles falling through the receiver (Fig. 2).
A number of assessments and studies have been performed on direct free-falling particle receivers since its inception in the 1980s [6], [7],
Particles
A variety of ceramic and silica-based particles have been investigated for high-temperature falling particle receivers. Commercially available ceramic particles that are used for hydraulic fracturing are well-suited for falling particle receivers because of their durability, high solar absorptance, and low cost. Ceramic particles appear best suited for direct-heating particle receivers to absorb as much concentrated sunlight as possible. Table 3 shows the optical properties of several different
Cost estimate
The costs associated with the receiver component of a 100 MWe high-temperature falling-particle receiver system with 9 h of thermal storage are estimated in this section. For a 100 MWe plant, the parameter values in Table 4 are used to calculate costs associated with components specific to the falling particle cavity receiver. The System Advisor Model [83] was used to determine some parameters such as tower height, aperture size, and required thermal input power, which impact costs.
Costs for
Summary
Particle receivers enable temperatures significantly higher than conventional receivers employing molten nitrate salts, which are limited to less than ∼600 °C. Direct particle receivers include free-falling, obstructed-flow, centrifugal, and fluidized designs that irradiate the particles directly. Advantages include the potential for high efficiencies due to direct irradiance of the heat transfer media. Challenges include maintaining and controlling sufficient mass flow and reducing particle
Acknowledgements
This paper is based upon work supported in part by the DOE SunShot Program (Award DE-EE0000595-1558) and the US-India Partnership to Advance Clean Energy-Research (PACE-R) for the Solar Energy Research Institute for India and the United States (SERIIUS), funded jointly by the U.S. Department of Energy (Office of Science, Office of Basic Energy Sciences, and Energy Efficiency and Renewable Energy, Solar Energy Technology Program, under Subcontract DE-AC36-08GO28308 to the National Renewable
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