Concentrated solar thermal (CST) systems are pivotal in the pursuit of renewable energy solutions to meet emissions reduction targets. They play a vital role in addressing the negative impacts of energy-intensive industrial processes, such as the high-temperature calcination step in the alumina Bayer process, requiring temperatures of approximately 1000 °C. However, achieving such high temperatures poses challenges, as radiative losses increase significantly with temperature. Current commercially available CST technologies, employing heat transfer mediums like molten salts, are constrained to temperatures below 600 °C. The emerging focus on sand-like ceramic particles, either as standalone materials or in suspension within an air stream, as heat transfer mediums signifies a promising avenue in the development of high-temperature receiver-based CST technologies. These particle-laden suspension flow receiver systems have the potential to reach operating temperatures exceeding 1000 °C.
1. Introduction
Solar energy stands out among the other alternative energy sources that have the potential to replace high-carbon-dioxide-emitting fossil fuels. This source of renewable energy is sustainable and inexhaustible, unlike finite fossil fuels. It has been predicted that the amount of solar energy falling onto the surface of the earth in just one hour could fulfil global energy needs for one year, but this requires the right technologies to effectively capture and utilize this renewable source of energy
[1]. The intermittent nature of solar resources creates a big challenge for solar energy utilization, especially for large-scale applications, which require a continuous supply of energy
[2]. In this regard, concentrated solar thermal, CST technologies are gaining importance as an alternative, clean, and renewable energy source
[3][4][5][6]. These are considered a suitable alternative to primary fossil fuels due to their potential to meet the load demands of both high- and low-temperature process heat applications
[7]. Furthermore, thermal energy storage can be integrated into a CST plant to overcome the intermittency issues related to solar energy and increase the operational reliability of the system.
Despite the potential of CST technologies as a source of heat and alternative to fossil fuels for industrial processes
[8], new CST technologies are required to achieve operating temperatures of 1000 °C or above, as this temperature is typically required for thermochemical processes such as the calcination involved in the alumina Bayer process
[9]. This is because the current commercially available CST technologies have temperature limitations in the heat transfer medium, which are molten salts. The most commonly used molten salts, such as molten nitrate solar salt (60% NaNO
3 and 40% KNO
3), have a safe working range from 220 to 565 °C, and they are more prone to corrosion
[10][11]. They freeze below 220 °C and become chemically unstable above 565 °C, requiring trace heating and limiting the operation of CST technology to below 600 °C
[12]. This eliminates the integration of CST into heavy industrial processes requiring high temperatures (>1000 °C), such as steel, aluminium, and cement manufacturing, which contribute ~15% of global CO
2 emissions
[13][14]. In Australia, the industrial processes contribute ~42% of CO
2 emissions, and ~220 PJ/year is consumed by alumina refineries alone, of which 67% is supplied using natural gas
[15][16]. Also, these processes are hard to abate as they make a huge contribution to the economy. Australia generated USD 8 billion from the alumina industry in 2018
[16]. Therefore, further technical assessments of new or existing CST technologies with the ability to achieve higher operating temperatures, as well as a better trade-off between thermal performance and operating temperature, than commercially available options are required.
To overcome the limitations of the present CST systems, particle-laden systems are being pursued for next-generation CST plants with operational temperatures >1000 °C
[10]. Recent technological innovation in the field of concentrating solar energy is opening new potential markets that can be considered in terms of their viability. In particular, high-temperature particle receivers are being developed to achieve temperatures in the order of 1000 °C for advanced power cycles and solar thermochemical processes
[17]. The use of particles as primary HTFs or in suspension within an air stream to enhance the heat transfer to the gas phase has the potential to achieve temperatures of around 1000 °C and improve the performance of the solar receiver systems, lowering the cost of energy
[18]. Also, the wide range of operating temperatures makes these receivers an enabling technology for scalability
[18][19]. There are different promising concepts of particle-laden receivers, including falling particle receivers devised by the Sandia Laboratory
[20][21][22], a centrifugal receiver devised by the German Aerospace Center (DLR)
[23][24][25][26], and tubular fluidized beds devised by the French National Centre for Scientific Research (CNRS)
[27]. The use of particles as the heat-transfer and storage media is unique
[28]. The basic difference between a high-temperature particle receiver and other commonly used central receivers is that it utilizes a directly irradiated cavity enclosure, which helps the particles, whether alone or in suspension within an air stream, to achieve a higher temperature and better conversion efficiency
[29].
One class of suspension flow solar particle receiver technology that has received significant attention employs direct irradiation to heat a vortex of air and particles in a cylindrical cavity, known as the solar expanding-vortex receiver (SEVR). This configuration of the receiver has the potential to act as an air heater, with the option of using suspended particles to reheat returned air from the integrated storage, which is already at an elevated temperature (>300 °C). Although high-temperature particle-laden receiver technology has the potential to achieve operating temperatures of over 1000 °C, the performance of these directly irradiated receivers needs to be assessed based on their transient operation using real-time solar irradiance data. Also, the operating temperature above the melting temperature of mild steel requires the use of refractory linings, similar to non-solar rotary kilns, or expensive high-temperature metals. The use of refractories allows the operating temperature to exceed 1000 °C, but these are brittle and have high thermal mass. This makes the transient response of a refractory-lined directly irradiated receiver more challenging than those of tubular receivers. This highlights the need for heat and mass transfer models to understand the influence of long-term solar transients on the thermal performance of these particle-laden receivers.
The practical implementation of refractory-lined solar receivers requires greater insight into the potential approaches required to manage the transient heat inputs. This is because the high thermal inertia of the refractory lining can potentially result in a significant fraction of the solar resource being needed to heat the cavity to the required operating temperature. While refractory-lined solar receivers are expected to have longer start-up time, little attention has been paid to the means through which this time might be reduced when considering long-term solar resource variability
[30]. Hence, there is a need to better understand the start-up behavior of these receivers and the potential options through which the start-up time might be reduced, considering the influence of transients in the incoming concentrated solar radiations
[31][32][33]. Insufficient information being available to guide the selection of the refractory type, thickness, or management strategy in response to these challenges provides further motivation for the present research project.
The integration of high-temperature solar central receiver systems has been reported in the literature, albeit only for the generation of electricity using steam and gas turbine cycles
[34][35][36][37][38][39][40]. There are limited studies of the use of integrated systems to produce high-temperature heat, i.e., above 1000 °C, for heavy industrial applications such as the production of alumina. Also, the integration of new technologies and concepts, such as refractory-lined suspension flow systems, is more challenging to customize and requires physical models of the sub-systems, which are able to predict the transient system behavior, to change the design and operating parameters. This highlights the need to perform a system analysis by integrating the physical sub-models into heat and mass transfer equations for each component to understand their combined performance, considering site-specific solar variability. Further, the combined effects of key design parameters on the thermo-economic performance these high-temperature CST systems, considering transient losses through a year of operation at a potential plant site, are still unclear. Hence, there is a need for an assessment that provides a new understanding of the system-level performance of these CST systems, along with a preliminary assessment of the levelized cost of the solar component of the energy.
2. CST to Power Industrial Decarbonization
Carbon dioxide (CO
2) emissions are increasing at a rapid rate due to rising energy consumption being required to fulfill different commercial and residential needs. These emissions are a major contributor to global warming and climatic changes. Global CO
2 emissions related to the energy sector increased to the highest recorded value of 36.3 Gt in 2021
[41][42]. About 90% of total CO
2 emissions occur because of the burning of fossil fuels for power generation and use in the transport sector
[41][43]. Consequently, it has been projected that CO
2 emissions from energy-related processes will double by 2050
[43]. The progressive increase in these harmful emissions is causing environmental imbalances, such as extreme weather conditions, around the world. Meanwhile, the prices of diminishing fossil fuels are increasing rapidly and projected to further rise in the next 20 years. Furthermore, the gap between energy supply and demand is expected to widen due to this rapid increase in global energy demand
[41][43][44]. Many developing countries around the world are already facing a deficiency of power because of this increasing demand and supply gap. The issues associated with the excessive use of fossil fuels for different commercial and industrial applications require the development and implementation of new technologies that are both environmentally friendly and cost-effective. In this regard, concentrated solar thermal (CST) technologies are gaining importance as alternative, clean and renewable energy sources, particularly in regions with higher solar irradiance
[3][4][5]. The growth in solar thermal installed capacity has shown an encouraging trend over the last few years
[45][46], with greatest share of growth gained by Spain, the US, China, South Africa, and Morocco
[47]. Despite some of the positive indicators noted, more work is required to increase the share of this vast renewable energy technology, especially for high-temperature heat applications.
CST technologies are considered a suitable alternative to primary fossil fuels due to their potential to meet the load demands of both high- and low-temperature applications. The CST can be integrated into a high-thermal-energy storage system, which can be used during the night or on cloudy days, to supply the required input energy and overcome solar intermittency issues. Presently, CST technologies are commercially available for power production in the lower temperature range, i.e., less than 600 °C
[48]. There is a push to develop solar thermal technologies for use in higher temperatures than currently commercially available to integrate them into processes requiring higher temperatures, such as the alumina Bayer process, which requires an operating temperature in the order of 1000 °C
[9]. In this regard, solar particle-laden (suspension flow) receivers are a promising class of technology for operating at high temperatures, i.e., above 1000 °C
[23][24][49][50]. This receiver technology employs sand-like ceramic particles as heat transfer fluids, as standalone elements, or in suspension within an air stream, which are stable at such high temperatures and cost-effective
[23][24][49][51]. These receivers can help to overcome the issue of temperature limitation associated with other commonly used heat transfer fluids, such as molten salts and oils
[27].
Recently, Rafique et al.
[2][16][33][48] made a noteworthy contribution to the assessment of refractory-lined solar receivers for high-temperature industrial processes. Their study introduced a comprehensive approach to analysing and optimizing the thermal performance of a refractory-lined particle receiver in response to solar resource variability. The study extended its impact by conducting a detailed examination of time-dependent temperature fields within the receiver cavity. Furthermore, the thermal response of a multilayered refractory-lined solar receiver was investigated across critical operational phases, encompassing start-up, turn-down, and shut-down scenarios for both cold and hot starts. The study went further by evaluating the annual thermal performance of a cutting-edge CST technology featuring the solar expanding-vortex receiver. Noteworthy insights emerged, shedding light on the influence of refractory materials in particle-laden receivers, the efficacy of potential operating controllers in terms of mitigating solar variability effects, and the impacts of particle loadings and heat transfer medium temperatures on the system’s overall thermal performance throughout the year. The research culminated in a demonstrated approach for analysing and optimizing a high-temperature CST plant by utilizing a particle-laden receiver and sensible thermal storage.
Despite the above strides made in advancing the understanding of refractory-lined solar receivers, it is essential to stress that this field still presents avenues for further exploration and research. The complexity of solar receiver systems, particularly in the context of particle-laden receivers and high-temperature CST plants, calls for continued investigations. Future research endeavors could delve into refining mathematical models, considering additional parameters, and expanding their scope. Moreover, there remains a need for comprehensive studies that integrate advanced control strategies to enhance the adaptability of refractory-lined receivers to dynamic solar conditions. The development of novel materials with improved thermal characteristics, coupled with a deeper exploration of their behavior under varying operational conditions, is also an area that requires further investigations. As the field of solar energy continues to evolve, ongoing research efforts will be crucial in terms of advancing the efficiency, reliability, and overall performance of refractory-lined solar receivers, ultimately contributing to the broader goal of achieving sustainable and efficient solar power utilization.
3. High-Temperature Particle Receiver Designs
Different types of particle-laden receiver designs have been investigated for achieving better performance by reducing thermal losses and achieving higher operating temperatures
[25][52][53][54]. The most widely investigated designs include free-falling
[55][56][57], obstructed flow
[58], and centrifugal
[23][24][25][59] and tubular receivers (fluidized)
[60][61][62]. The basic design of particle-laden receiver technology is the falling particle receiver, which has been studied both analytically and experimentally
[56][57]. Each design of these high-temperature receivers is investigated by implementing various geometric configurations to improve the achievable temperature and thermal efficiency
[63]. Siegel et al.
[20] achieved a thermal efficiency of 50% by performing one of the first on-sun tests of a simple free-falling particle receiver. Ho et al.
[55] concluded that the thermal efficiency and achievable output temperature are strongly dependent on the mass flow rate through a receiver and solar irradiance. Higher thermal efficiency is achieved through an increase in the particle mass flow rate, but the outlet temperature decreases
[55]. The obstructed or recirculating design can achieve higher outlet temperatures due to the increased residence time
[64]. Ho et al.
[55] concluded that the continuous circulation of particles will increase the particle outlet temperature to over 700 °C, with a thermal efficiency of 50–80%.
Using the primary concept of these designs, researchers have considered different configurations to be used for the particles in central receiver technologies. These secondary designs include placing a V-shaped mesh
[65], a porous structure
[66], an inclined plate
[67], and spiral tubes
[68] inside a receiver cavity to increase the residence time of particles to ensure higher absorption efficiency.
3.1. Cylindrical Cavity Receivers
A directly irradiated particle-laden receiver utilizes a cavity enclosure to contain concentrated solar radiations and reduce the thermal losses
[6]. The purpose of the cavity receiver is to direct the reflected solar radiations from the heliostat field to the heat transfer fluid contained in a box or cylindrical shape through an aperture. One type of cavity receiver that has gained particular attention in recent years forms a vortex of particles inside a cylindrical cavity using a carrier gas. Concentrated solar radiations through the aperture directly heat a vortex of particles. This type of cavity receiver is commonly known as a solar vortex receiver (SVR)
[69]. SVR comprises an aperture covered by a quartz window placed normally in relation to the cavity axis, through which concentrated solar radiations fall onto the particles, being injected tangentially along with a gas to form a vortex. The heated particles and gas exit from the rear along the axis of the cavity
[70][71]. Due to the formation of a vortex, the heat transfer medium absorbs more solar radiation falling through the aperture and helps to achieve better mixing inside the receiver cavity.
Theoretical and experimental studies have been carried out for cylindrical cavity receivers, and most of them have shown the advantage of achieving better heat transfer to the particle-laden flow
[9][72]. The output temperature in the order of 1000 °C can be achieved using these receivers, and they have the flexibility to be configured in different ways
[9]. Z’Graggen et al.
[70][73][74] reported a carbon conversion efficiency of 87% from the laboratory scale testing of a 5 kW solar receiver for hydrogen production via steam gasification. The solar cavity was directly injected with petcoke water slurry to form a vortex inside the cavity, which was directly exposed to the solar radiation falling through the aperture covered by a glass window.
Numerous studies of high-temperature cavity receivers have used a rather complex computational fluid dynamics (CFD) method to study the effects of different geometric parameters and operating conditions on their performance. Xiao et al.
[75] numerically and experimentally analyzed a cylindrical cavity receiver using a two-stage solar dish reflector via which the sun rays were reflected twice before reaching the aperture. The particles inside the receiver moved into a spiral shape and were heated through the solar irradiance directly falling onto them. The spiral movement of the particles increased their residence time to achieve higher thermal efficiency. The results of the study revealed that with an average solar flux of 150 kW/m
2, a particle temperature of up to 1100 °C can be achieved. Meier
[56] employed a CFD computational tool to analyze a 1.5 MW cavity receiver for limestone decomposition. The study revealed that an output temperature of >900 °C can be achieved with a solar flux of less than 1000 kW/m
2. Ozalp et al.
[76][77][78] analyzed the flow features and temperature distributions in a solar cavity receiver–reactor. Similarly, Dai et al.
[79] also employed the CFD simulation tool to analyze the effects of different operating and geometric parameters on the performance of a 10 kW SVR for CO
2 coal gasification. These CFD studies have significantly contributed to investigating the flow features inside a cavity receiver, but the influence of solar transients on the long-term thermal performances of these receivers, considering start-up, turn down, and shutdown periods, is still unknown.
Some analytical models have also been developed for high-temperature particle-laden receivers
[80][81][82], but these have not considered the transient effect on the annual thermal performance. A recent study by Davis et al.
[80] employed a steady-state analytical model developed for a cylindrical cavity high-temperature particle receiver to observe its thermal performance under varying operating conditions. The authors assumed a constant solar flux of 2000 kW/m
2. It was revealed that the vortex receiver has the potential to achieve a thermal efficiency of up to 88%. Although these vortex-based receivers have high conversion efficiency, they have two major drawbacks, which are as follows
[76]:
3.2. Solar Expanding-Vortex Receiver
To overcome the drawbacks associated with the SVR, Chinnici et al.
[29][84][85] presented a new configuration of a cylindrical cavity receiver known as the solar expanding-vortex receiver (SEVR). This novel configuration has a modified geometry, with the introduction of a conical section. The particles and air exit in the direction that is radial to the axis of the receiver cavity. The residence time is dependent on the particle size in this new configuration, which helps to achieve a uniform outlet temperature. This configuration is also helpful for avoiding the deposition of particles onto the aperture window. The authors evaluated the effect of receiver geometry on the particle trajectories and residence times using a CFD model
[29]. Furthermore, a particle’s deposition onto the aperture window was also assessed both experimentally and numerically
[84]. It was found that for a fixed aperture size, an increase in the cone angle is favorable as it generates a larger vortex at the aperture plane. This is why a larger cone angle is favorable for smaller size particles, as it will lower the particle flowing toward the aperture. The optimum value of the cone angle was predicted to be 40° for the studied conditions. Chinnici et al.
[84][85] further confirmed that a well-established vortex flow inside the cavity is generated via the SEVR. It was also found that a reversed flow formed in the vortex core region, whereas at the inlet and outlet regions of the cavity, a processing vortex core (PVC) structure was observed
[85].
While the above-mentioned studies have analyzed the thermal performances of high-temperature particle-laden receivers and indicated their potential advantages, they have not considered the transient effect on the receiver thermal performance. The solar flux changes significantly from one location to another, which means that the actual performance of the receiver can only be assessed based on the real-time climatic conditions of each location, considering transient losses. The distribution of minutely, hourly, daily, and monthly DNI for a specific location significantly varies. This is why hourly or minutely performance evaluation is recommended to ensure a reasonably good estimation of system performance. Therefore, the transient performances of the high-temperature particle-laden receivers need to be assessed based on the actual solar DNI data for a specific site. This requires the development of analytical transient models to be solved for hourly or minutely solar irradiation data over a longer period.
The reliance on steady-state assumptions for predicting the performances of high-temperature particle receivers introduces significant uncertainties, rendering them unsuitable for use in accurate yearly performance assessments. This underscores the imperative to analyze and design these receivers based on transient operation, incorporating real-time solar resource data specific to a site and time of year. Implementing a constant value to represent the overall yearly performance, accounting for solar resource variability, risks generating inaccurate predictions and design specifications.
The recent advances in CST technology, notably the advancements in high-temperature particle-laden receivers achieving temperatures of approximately 1000 °C, offer a promising avenue for exploring new markets and applications. In particular, high-temperature particle-laden receivers are being developed to achieve temperatures in the order of 1000 °C for advanced power cycles and solar thermochemical processes
[17]. The integration of these solar central receiver systems has been widely reported in the literature, albeit only for the generation of electricity using steam and gas turbine cycles. However, there are limited studies of the integrated systems used to produce high-temperature heat, i.e., above 1000 °C, for heavy industrial applications, such as the production of alumina and hydrogen.
The narrative is enriched by several case studies illustrating the potential of integrated high-temperature receiver systems for use in electricity generation. Noteworthy examples include the analysis by Rovense et al.
[34] of a 150 MW power plant with a multi-tower upward fluidized bed solar receiver, the study of a 20 MW power plant with a fluidized particle-in-tube receiver by Behar et al.
[35], and the investigation of a dense particle suspension flow solar receiver by Reyes-Belmonte et al.
[36]. These studies showcase diverse technological approaches, each with its unique thermal efficiencies, overall plant efficiencies, and solar-to-electric efficiency metrics. González-Portillo et al.
[37] delve into the techno-economics of a CSP system incorporating a free-falling particle receiver, ground-based thermal storage bins, and a supercritical CO
2 cycle, demonstrating a potential reduction in the levelized cost of electricity (LCOE). Similarly, Albrecht et al.’s
[38] simulation of the annual energy production and LCOE estimation for commercial-scale CSP systems featuring a falling particle receiver, moving packed-bed heat exchanger, storage bin, particle lift, and recompression supercritical CO
2 cycle underscores the economic viability of particle-based CSP technology. Buck et al.
[39] reported that the multi-tower CSP system with a centrifugal particle receiver operating at 700–1000 °C adds further diversity to the technological landscape. Despite these advancements, a critical gap in knowledge exists regarding the industrial-level performance of these solar thermal systems. Specifically, there is a lack of comprehensive data addressing the reheating of returned warm air from a thermal storage system, accounting for transient losses over an entire year of operation at a potential plant site.
Although these solar thermal systems with central receiver technology have been proven to be suitable for driving high-temperature power cycles, there is a lack of available data regarding their industrial-level system performance in terms of their suitability to re-heat returned warm air from a thermal storage system, considering transient losses through a year of operation at a potential plant site.
4. Current review on Recent Advancements in High-Temperature Solar Particle Receivers
In addition to the discussed advancements, it is crucial to consider persistent challenges in the design and operation of high-temperature particle-laden receivers. Given their design temperature surpasses the melting point of mild steel, necessitating a refractory lining with inherent brittleness and high thermal inertia, a fundamental departure from the transient response observed in tubular receivers is evident
[86]. Although challenges during transient operation have been recognized in various contexts, comprehensive assessments, especially for refractory-lined high-temperature particle-laden receivers, are notably absent, emphasizing a critical research gap
[87][88][89][90][91][92]. The higher thermal inertia of refractory linings compared to metal tubes further complicates the understanding of their behavior under unsteady conditions. Addressing this gap is imperative for identifying how these receivers respond to fluctuations in solar resource availability during transient operation.
Moreover, there is a noticeable gap regarding investigations into overnight heat loss for refractory-lined cavity receivers across diverse geometric parameters and operating conditions. The intricacies of receiver shutdown strategies and the optimal combination of insulation thickness and inner refractory thickness for enhanced thermal gains remain inadequately explored. This highlights the pressing need for further investigations into the thermal behavior of refractory-lined receivers. A nuanced understanding of the trade-offs between operating temperature, overnight temperature drop, and start-up time, as informed by extended time-series solar input data, is essential for advancing the efficiency and practical applicability of these receivers in the context of industrial decarbonization. Bridging these knowledge gaps will not only enrich the comprehension of high-temperature solar technologies but also contribute to the holistic integration of these systems into sustainable industrial practices.
As researchers traverse the landscape of renewable energy solutions, these advancements not only exemplify the transformative potential of high-temperature solar technologies in decarbonizing industries but also accentuate the necessity of sustained research and innovation in this dynamic field. The integration of high-temperature solar particle receivers into industrial processes holds the promise of not only reducing carbon emissions but also fundamentally reshaping production methods towards cleaner, more sustainable paradigms. This underscores a critical juncture in the collective pursuit of a low-carbon future, where the deployment of high-temperature solar particle receivers provides a beacon of hope, embodying the tangible contributions that science and technology can make toward a global transition to a more sustainable energy-based future.