Interaction between solar light/radiation and matter is studied and applied at least since the Classic Antiquity. During the siege of Syracuse (circa 213–212 BC), Archimedes may have used mirrors acting collectively as a parabolic reflector to concentrate the sun’s rays in order to burn the Roman ships attacking the city. Nowadays, a single Fresnel lens, available on the market, for example, with dimensions 1400 × 1050 mm, fabricated from acrylic glass (PMMA polymer) and mounted in a structure equipped with an automatic sun-tracking system, is capable of concentrating the radiation on a small focal spot of approximately 8 to 10 mm diameter, and then temperatures of circa 1500 °C, or even higher, are easily attainable depending on the characteristics of the material that is being irradiated.
Either for residential, commercial, or industrial applications, solar heating and cooling technologies are available to provide hot water, space heating and cooling, pool heating, etc., but, on the contrary, the so-called high-concentration or high-flux solar applications, which pertain to the attainment of higher temperatures, are much less used. In fact, the current state-of-the-art on the use of concentrated solar energy applied to materials science and metallurgy is far from being so widely known. The main reason for it being less widespread is because high-concentration solar furnaces or other high-flux devices are relatively more difficult to build in order to operate under steady conditions. Although available in several labs distributed throughout the world, until now all high-flux solar furnaces have been designed and constructed individually, i.e., on a one-by-one basis because there are several possible optical configurations [1
] which must take into account the geographical location [2
] and the maximum power to be achieved.
Until now, the so-called “solar furnaces” are mainly point-focusing solar concentration facilities located throughout the world at universities, research institutes, and companies, operating at concentrations relevant to solar thermochemistry, materials processing and thermal treatments. The design of solar furnaces is mainly based on the concepts of geometrical optics, or ray optics, that describe light propagation uniquely in terms of rays [1
The origin of current solar furnaces dates back to the beginning of the 20th century, when the Portuguese Catholic priest Manuel António Gomes created and patented “an apparatus for making industrial use of the heat of the sun and obtaining high temperatures such as are required in metallurgical and chemical researches” [3
]. He was a very tall man, and for this reason was nicknamed ‘‘Father Himalaya”. In both his British [3
] and United States [4
] patents, Manuel “Himalaya” described his invention as comprising a parabolic reflecting-surface arranged to cause solar rays to converge upon a confined focus placed in the centre of a furnace, crucible, or other receiver. Two of the figures included in the Manuel Himalaya’s British patent application are depicted in Figure 1
. For the 1904 World’s Fair of St. Louis (Missouri, MO, USA), he built a huge device he called the Pyreliophorus, which was apparently similar to a large convex lens that can concentrate the sun’s rays onto a small area, but in which thousands of mirrors over a surface of 80 m2
concentrated solar energy up to a temperature of around 3500 °C, enough to melt many different types of materials, including metals and rocks. The Pyreliophorus installation was one of the main attractions at the Saint Louis world’s fair, where it was awarded with two gold medals and one silver medal [5
Manuel Himalaya’s British [3
] and United States [4
] accepted patents (“for making industrial use of solar heat, particularly in the metallurgical and chemical arts which necessitate the use of temperatures higher than those of ordinary furnaces, including the electrical furnace”) date back to 1901 and 1905, respectively, and during those years he resided in France. More than 40 years later, and by an initiative of the French chemist Félix Trombe and his team, a so-called solar furnace was built (circa 1950) at the town of Mont-Louis, a commune in the French Pyrénées-Orientales. Then, based on the 50-kW prototype built in Mont-Louis and using the results obtained there, a much more powerful solar furnace of 1000 kW was designed to be placed (not very far from Mont-Louis) at Odeillo (Pyrénées-Orientales and Cerdagne near the Spanish border in the south of France). Work on the construction of the Odeillo solar furnace lasted from 1962 to 1968 and was commissioned in 1970 [7
]. Nowadays, this 1 MW solar furnace continues serving as an installation for studying materials at very high temperatures.
Instead of providing a state-of-the-art review of the development of Concentrated Solar Power (CSP) technologies over the last decade [8
] or analysing the so-called “solar thermochemical processes” [9
], this review intends to highlight the most recent achievements, but also shortcomings of the existing solar furnaces to be used for materials processing. Based on the author’s own experience, some guidelines, proposed solutions, and prospects for future research work will be also presented.
2. Other Recent Overviews
A review conducted by Bushra and Hartmann [10
] has shown that most articles published on reflective two-stage solar concentrators deal with applications for power generation using solar cells, and thermo-electric generators. In a work published in 2017, Levêque et al. [11
] reviewed the designs and characteristics of high-flux optical systems (HFOSs), as well as challenges and opportunities in the area of HFOSs for solar thermochemical applications. This review [11
] provides an exhaustive list of point-focusing on-sun HFOSs, showing the characteristics and peak fluxes of existing solar furnaces worldwide (at universities, research institutes, and companies) which use real sunlight, but also of indoor solar simulators. The discrepancy between the spectral power distributions of the solar simulators compared to real solar radiation is normally considered a reasonable compromise, in order to allow for easily controlled and repeatable experiments. When the radiation is provided by xenon arc bulbs, some significant divergence occurs in the infra-red region due to the Xe emission lines, as discussed by Petrasch et al. [12
] (2007) and Alxneit and Schmit [13
More recently (2018), the state-of-the-art technology on the use of concentrated solar energy applied to materials science and metallurgy has been comprehensively reviewed by Fernández-González et al. [14
], and it was concluded that solar energy offers a great potential in applications of high temperature. As opposed to conventional processes used in metallic and non-metallic materials processing (in which the higher the temperature, the higher the energy consumption), concentrated solar energy has costs that are not dependent on the temperature, and in this way, could be competitive with other high-energy technologies (laser, plasma, etc.). The most known drawback of solar energy is the impossibility of working 24 h per day because of the sun availability, but the work of Fernández-González et al. [14
] has pointed out that concentrated solar energy could find application in the recovery of wastes, as well as in the processing of a short series of products (as for instance in obtaining of hard refractory ceramics), high purity materials (as for instance the production of lime for the chemical and pharmaceutical industries) or in materials recently discovered (as for instance fullerenes and carbon nanotubes). However, it is worthwhile to note remarks made by the authors [14
]: “nowadays, the interest of solar energy is mainly focused on the field of energy, both thermal and electric, except for several research projects where the possible applications of solar energy in materials science are explored. The problem observed in most of these researches connected with materials science is the lack of continuity (not in all of them but is the general keynote), meaning that a certain field is explored and then abandoned, without an attempt of scaling up to pilot plant or industrial scale (searching for a commercial application). The reason for abandoning the topic is perhaps the lack of interest by the industrial companies (generally solar processes are competitive in quality, time and temperatures with traditional methods in a laboratory scale but they are not industrially proved, and apart from that, these traditional methods are well known and currently installed in competitive plants) or the lack of promising results (but this question was not observed, in general). In fact, few projects were scaled from laboratory to pilot scale, and none of them are commercially used on an industrial scale”.
Two other review works, by Alonso et al. [15
] and by Ho [16
], published in 2017 and 2016, respectively, are also worth being mentioned. Alonso et al. [15
] reviewed the rotary kiln technology and focused on the employment of these devices for thermal and thermochemical processes conducted by concentrating solar energy. Among the solar devices, a novel rotary kiln prototype for thermochemical processes was presented and compared with a static solar reactor. In fact, rotary movement favours the radiation heat transfer; then, some practical conclusions on the design and operation of solar rotary kilns are remarked upon, and an analysis of their current limitations is presented.
The review work authored by Ho [16
] provides an analysis of high-temperature particle receivers for concentrating solar power. It reveals the concern of the technical-scientific community to find out more efficient ways to generate high temperatures and to control them adequately. Although most studies are focused in the production of electricity by concentrating solar power in a receiver located at the top of a central tower (i.e., concentrating solar power tower technologies), the use of advanced receivers, of fluidized bed type or of the so-called “particle receivers”, may contribute to the evolution of the technologies for other important industrial sectors that need the attainment of high temperature. The falling particle receivers use solid particles that are heated directly as they fall through a beam of concentrated sunlight, with particle temperatures capable of reaching 1000 °C and higher. There are several alternative designs for the particle receivers, which include: free-falling, obstructed flow, centrifugal, flow in tubes with or without fluidization, multi-pass recirculation, north- or south-facing, and face-down configurations [16
Furthermore, in 2018, a bibliographic analysis was carried out by Rosa & Rosa [17
] on the database named Web of Science™ Core Collection, and the searches were done year by year, starting in 2008, i.e., with PY = Year Published = 2008, and ending with PY = 2017. This bibliographic analysis [17
] considering the period from January 2008 till December 2017, aimed to provide a vision of recent applications of the so-called solar furnaces, reactors or process chambers and corresponding accessories, used for physical, chemical or metallurgical processes requiring high (>400 °C) and very high (>1500 °C) temperatures. Research works/publications dedicated to the production of electricity by solar-thermal or photovoltaic technologies, as well as other research topics like those dealing with desalinization technologies, and works aiming for the development of systems for thermal storage, namely those that make use of molten salts or phase change materials (PCMs), were not included in the metrics of the bibliographic analysis [17
]. The results of the analysis have indicated the main topics under investigation, the institutions and countries involved, as well as the scientific journals where the works have been published. It could be also concluded that the usage of concentrated solar radiation to obtain high (>400 °C) and very high (>1500 °C) temperatures, needed for physical, chemical or metallurgical processes, is presently characterized by a low percentage of publications in comparison to the other research topics in which solar energy is also used.
5. On-Going Activities and Prospects on Future Trends for Solar Processing of Materials
Two examples of research activities carried out at the University of Lisbon are outlined: (1) development of indirect heating techniques to improve the temperature homogeneity conditions inside reaction chambers for materials processing in solar furnaces; and (2) design of new modular systems, practical and flexible, for capture, concentration, control and conduction of solar radiation.
5.1. Novel Approach Based on Heating by Indirect Irradiation
For the majority of operations for materials processing at the industrial scale (e.g., firing of ceramics, heat treatment of metallic parts, furnaces for glass melting or for glass fritting, calcination furnaces, etc.) it is indispensable that temperature control and temperature homogeneity conditions are guaranteed. Pursuing these objectives, innovative works by Li et al. [69
] and Oliveira et al. [70
] have provided some guidelines for improving the design of new indirect heating setups. Figure 7
depicts a setup used for indirect heating. Different setup configurations were investigated for improving temperature uniformity and high-temperature experiments using graphite receiver discs were carried out up to 1400 °C. Graphite discs were used in both vacuum and flowing argon atmospheres [70
]. Other materials were also used as receivers, namely AISI 310 stainless steel, and molybdenum disilicide (MoSi2
): an intermetallic compound with very attractive properties for use as a high-temperature heating element. MoSi2
is very refractory with a melting point of 2030 °C, has excellent resistance to oxidation and a moderate density (6.24 g/cm3
Recent work by Pereira et al. [1
] has shown that theoretically there exists the possibility to attain homogenous distribution of concentrated solar flux by means of double reflexion using two paraboloid surfaces. As schematically presented in Figure 8
, the parallel rays of solar light are reflected by the “concentrator” (concave paraboloid); then, before being concentrated to the focal point F, if these rays are reflected by a smaller paraboloid (with the same focal point of the “concentrator”)—which can be convex such as paraboloid reflector A or concave such as paraboloid reflector B—our calculations show that the reflected rays are simultaneously highly concentrated and equally distributed over the illuminated region (with a very small hole in the middle due to the shadow produced by the second paraboloid, probably too small to be detectable). Consequently, new devices, e.g., [71
], can be designed to homogenise the flux of radiation near the sample (in order to equilibrate the temperatures).
5.2. Modular Systems for Capture, Concentration, Control and Conduction of Solar Radiation
depicts a computer-aided view of a modular-type system that was conceived to capture, concentrate, control and conduct solar radiation. The presented system is composed of two Fresnel lenses which, side by side, are mounted on a structure. Each Fresnel lens has, on its focus, a receiver device so that the concentrated solar radiation can be captured and then conducted by an optical waveguide made of low-loss optical fibers. Since in this case there are two Fresnel lenses, there will be two fiber-optical cables which conduct the concentrated solar radiation till the place or places where the radiation is needed.
These systems can be autonomous, stationary or mobile, and easily transported and deployed on the site where they can collect the direct solar radiation. They can be applied to a variety of processes. The main advantages of such types of systems derived from the fact that fiber optical cables are flexible and, if adequately manufactured, can provide highly concentrated solar radiation to places far from the site where the solar light was collected. Additionally, as the fiber optical cables are flexible, they can be used to illuminate the target not only from one single direction, but by using several cables it is possible to illuminate the target with radiation that is coming from different directions, thus circumventing one of the major problems with traditional high-flux concentrators: the unidirectionality of the radiation pattern.
The structure of the prototype depicted in Figure 9
is designed to follow the Sun during the day and to keep the receiver device as the focus of the Fresnel lens. For control/screening of the radiation that will be transmitted by each of the fiber optical cables, on the top of each Fresnel lens there is a “shutter” or “attenuator” (similar to a venetian blind) made of very light and stiff slats. The slats are rotated by an electrical motor that can be powered by a photovoltaic source. The control of the radiation is made by the opening or closing movement of the slats. Techniques of online/robotic adaptive control are being used [72
] thus allowing dealing with the presence of fast perturbations on sunlight induced by clouds. The ongoing activities are also grounded on the work done by other researchers to demonstrate both capabilities and benefits of the usage of advanced optical fibers [75
], which allow for the transferring and directing of the concentrated solar radiation to the place of solar energy utilization.
Receiver devices to be inserted at the entrance/inlet of the fiber optical cable are now being developed to satisfy the acceptance angle of the optical cables (see Figure 10
). The current research results are very promising demonstrating that fiber optical cables (bundles) can be specially designed to transmit concentrated sunlight.
This review was intentionally focussed on solar-driven high-temperature technologies for (solid) materials processing. Solar thermal technologies for the production of electricity, as well as many so-called “solar thermochemical processes” for production of gases or liquids are outside the scope of this review. Nevertheless, despite some current shortcomings for attainment of homogeneous distribution of temperature in the processed materials (due mainly to the unidirectionality of the radiation pattern and the non-homogeneous distribution of the highly concentrated radiant flux), the examples analysed in this work reveal the tremendous potentialities of the usage of solar heat for materials processing.
There is a need for innovative systems for capture, concentration, control and conduction of concentrated solar radiation. Using optical waveguide transmission lines made of low loss optical fibers, it is possible to direct the concentrated solar radiation to the place of utilization of the high-flux solar energy. Therefore, it is foreseen that most of the new systems will take advantage of the capabilities and benefits of the usage of (advanced) optical fibers, which allow for the transferring of solar radiation to locations difficult to get to.