3.2. Parabolic Dish Reflectors
The main components of a PDR system (shown in
Figure 3b) include a parabolic dish concentrator mirror and a thermal receiver mounted at the focus point of the parabolic dish
[57][63]. Similar to PTR technology, PDR also requires the incident direction of the parallel sunlight to be perpendicular to the aperture of the parabolic dish, thus enabling effective solar energy harvesting. Therefore, a two-axis sun tracking system is also indispensable for PDR. Generally, PDR systems are characterized by a high efficiency, autonomous operation and excellent modularity
[57][63]. Many PDR-based power plants have been manufactured worldwide, which exhibit high solar-to-electric conversion efficiency and have the potential to become one of the most effective renewable energy utilization approaches
[44][49][57][44,49,63].
3.3. Solar Power Towers
In recent decades, as a representative of high-temperature concentration engineering, SPT has been a popular topic in CSP engineering applications
[58][68]. As shown in
Figure 3c, in the SPT system, a large number of heliostats are arranged in an oval shape around the heat collecting tower. The heliostat could automatically track the movement of the sun and focus sunlight to the receiver located above the tower, thereby converting light energy into heat flux. The solar concentration ratio of a SPT can reach the equivalent of hundreds to thousands of suns, thereby achieving ultra-high operating temperatures. In recent years, to achieve the goal of saving energy and reducing emissions, SPT-based photo-thermal power plants employing the Rankine or Brayton cycle have been intensively studied and deployed
[59][69].
3.4. Linear Fresnel Reflectors
In the past few decades, LFRs have also been studied and developed, and they have been widely used in photo-thermal power generation projects
[60][61][70,71]. As shown in
Figure 3d, the main structure of an LFR includes segmented flat mirrors which are mounted in parallel (as a primary mirror) and a secondary mirror located above the primary mirror array. The light receiver is mounted at the focus position of the secondary mirror. Each row of primary mirrors has a rotation axis, and all the primary mirrors track the position of the sun in the sky under the control of the driving mechanism, focusing the sunlight on the secondary mirror
[7]. The shape of the secondary mirror is a compound parabolic collector (CPC) that can capture and reflect incoming sunlight in all directions to its focal point
[62][72]. CPC, as a supporting CSP technology for LFR, plays a crucial role in improving the efficiency and heat flow uniformity of LFR systems. CPC is usually formed by a variety of shape combinations (such as involute, parabola, etc.), thus endowing it with the unique capability to collect beam radiation and diffuse radiation within a specific acceptable half-angle and without the utilization of a complex solar tracking system
[63][73].
4. CSP Technologies Applied in Photocatalysis
4.1. Solar Concentrated Type
4.1.1. Parabolic Trough Reflector-Based Photoreactors
At present, solar-concentrated types of photoreactors mainly apply three technologies, including PTR, PDR and LFR. As a compact and efficient configuration for harvesting solar energy, it is not surprising that PTRs have been widely adopted as the basis for the construction of photocatalytic reactors
[64][65][66][67][68][69][70][71][72][74,75,76,77,78,79,80,81,101]. As early as 1991, Anderson et al. reported a PTR-based photocatalytic reactor for groundwater remediation, which was widely considered to be the first on-site application project
[72][101]. This project was built at the Lawrence Livermore Laboratory in the United States. The system structure consisted of rows of parabola troughs arranged in a field to form a trough array, which could reflect solar rays onto the reaction tube filled with photocatalyst particles. Subsequently, from 1993, a series of PTR-based photoreactors employing TiO
2 as a photocatalyst for the degradation of various organic pollutants was tested and evaluated at the Plataforma Solar de Almeria (PSA), Spain. For example, Minero et al. established a large solar plant for the photocatalytic degradation of pentachlorophenol, which achieved a water treatment capacity of cubic meters per hour
[68][78]. The system consisted of a total of twelve heliostat modules that could track solar motion, and every heliostat module was equipped with four PTRs. The nominal aperture area of each heliostat module was 32 m
2, thus providing sufficient light energy for the reaction tube. They also carried out the photocatalytic degradation of atrazine by employing one of the heliostat modules in the above solar plant, which exhibited an atrazine removal ratio of 98% in 2 h. Other photocatalytic experiments using PTR-based photoreactors include the inactivation of
Escherichia coli [65][75], the elimination of methylene blue and rhodamine b
[70][80] and the removal of oxalic acid
[71][81].
4.1.2. Parabolic Dish Reflector-Based Photoreactors
Due to the high efficiency and compactness of PDR technology in concentrating solar energy, unique PDR-based photoreactors have also been explored. Oyama et al. presented a batch-mode photoreactor system aiming to achieve the photocatalytic degradation of commercial detergent under real sunlight
[73][82]. The system consisted of a round concave mirror (parabolic dish reflector) with an aperture diameter of 1.0 m and a flask-type reaction vessel. The parabolic dish reflector employed could achieve a geometric concentration ratio of 70 suns. The reaction vessel was mounted at the focal point of the parabolic dish for adsorbing the incident solar energy. TiO
2, as the photocatalyst, was used in suspension and flowed inside the system circulation loop. The photocatalytic degradation of the refractory detergent driven by concentrated sunlight exhibited a much higher treatment efficiency than conventional biodegradation.
4.1.3. Fresnel Condenser-Based Photoreactors
Linear Fresnel reflectors, due to their excellent solar energy harvesting capabilities and widely acknowledged low costs, have also shown strengths in the structural exploration of photocatalytic systems. Zhang et al. developed an LFR-based photoreactor for the photocatalytic treatment of various organic contaminants including organic dye, antibiotics and pathogenic microorganisms
[7]. In this system, six flat primary mirrors were mounted on the pedestal of the reactor and driven by stepper motors. The system was located in the north–south direction, with the primary continuously tracking and reflecting solar rays to the reaction tubes. The quartz glass reaction tubes were located above the primary mirror array, which could be moved in a north–south direction to accommodate the solar altitude angle in different seasons. Different from previous studies employing TiO
2 particles, this LFR-based photoreactor utilized TiO
2-coated silicone beads as the photocatalyst, which filled the inside of the reaction tubes. Rhodamine b, amoxicillin and
Escherichia coli were successfully eliminated under actual weather conditions including a sunny day, cloudy day and rainy day, which demonstrated the feasibility of the LFR configuration for providing solar energy to photocatalysis.
4.1.4. Problems in Solar-Concentrated Photoreactors
Although solar energy-concentrated photocatalytic reactors have demonstrated the capability of improving photocatalytic efficiency, their inherent disadvantages appeared with further research. First, solar-concentrated photoreactors may provide excessively high light irradiance and reaction temperatures for photocatalytic reactions
[17][74][17,102].
Figure 47a shows the ability of some typical PTRs
[45][75][76][77][78][45,62,103,104,105], PDRs
[49][79][80][81][49,106,107,108], SPTs
[59][69] and LFRs
[60][62][82][70,72,109] to concentrate solar energy. Usually, these solar concentrators are used to concentrate solar energy efficiently to produce high temperatures which can be employed in thermal engineering processes such as in photo-thermal power plants. A solar concentration ratio equivalent to hundreds or thousands of suns was achieved, resulting in ultra-high light irradiance (
Figure 47a).
Figure 47. Solar thermal energy provided by typical CSP technologies. (
a) Concentrated light irradiance
[45][49][59][60][62][75][76][77][78][79][80][81][82][45,49,62,69,70,72,103,104,105,106,107,108,109]; (
b) temperature
[18][19][44][49][57][58][59][61][62][75][77][79][81][83][84][85][86][87][88][89][90][91][92][18,19,44,49,62,63,68,69,71,72,104,106,108,110,111,112,113,114,115,116,117,118,119].
Figure 47b illustrates the temperature in some typical CSP projects, including PTR
[18][19][62][75][77][83][84][85][86][87][18,19,62,72,104,110,111,112,113,114], PDR
[44][49][57][79][81][44,49,63,106,108], SPT
[58][59][88][89][68,69,115,116] and LFR
[61][84][90][91][92][71,111,117,118,119]. For example, de Risi et al. optimized a PTR-based solar collector, which achieved a nanofluid outlet temperature of 650 °C
[87][114]. Khan et al. introduced a next-generation SPT system with a temperature of more than 1000 °C
[58][68]. Additionally, a dish Stirling system was developed by Abbas et al., who demonstrated its ability to achieve a temperature in excess of 1500 °C
[44].
Secondly, although PTR, PDR, SPT and LFR can achieve effective solar energy convergence under sunny conditions with sufficient direct light, their photothermal harvesting capability will plummet on cloudy days, thus greatly limiting the efficiency of the photocatalytic reaction
[1][7][1,7]. Thirdly, for these solar concentrator configurations, complex mechanisms for driving these structures are unavoidable due to the need to actively track the incidence angle of the solar rays
[17]. This would increase the overall design difficulty, construction investment, operation and maintenance costs of photocatalytic reactors. Due to the above issues, traditional CSP-based photoreactors, mainly PTRs, are widely considered to be an outdated technology. In order to find photoreactors that can achieve appropriate enhancement of natural solar energy for achieving promoted photocatalytic efficiency, non/low-concentrated photoreactors began to be developed.
4.2. Non/Low-Concentrated Photoreactors
4.2.1. V-Groove-Based Photoreactors
The V-groove, as a variant of the PTR, simplifies the profile of the parabola into two flat mirrors, thereby dramatically reducing the solar concentrating ratio. V-groove-based photoreactors were mainly studied in 2004 by McLoughlin et al. and Bandala et al. to explore the advantages and disadvantages of PTR, V-groove and CPC technologies in photocatalytic applications
[65][71][75,81]. TiO
2 was adopted as the photocatalyst for the degradation of various organic targets including oxalic acid and
Escherichia coli under real sunlight. Due to the low concentration of sunlight, the reaction temperature of the V-groove-based photoreactor never exceeded 38 °C. In comparison experiments, the CPC-based photoreactor clearly demonstrated a higher photocatalytic efficiency compared to the V-groove-based photoreactor. This may be due to the fact that the V-groove has no optical focus; therefore, the flat mirror cannot effectively provide the reflected light to the reaction tube, thereby resulting in its low efficiency. As a transitional photocatalytic reactor technology, the V-groove-based photoreactor was quickly eliminated by CPC-based photoreactors.
4.2.2. Compound Parabolic Collector-Based Photoreactors
As mentioned above, in traditional applications, CPC was used as a secondary mirror for LFR, which has the unique advantage of being able to focus the light incoming from any direction to its focus point
[17]. Applying a similar principle, under sunny conditions, a fixed CPC could focus direct solar light with different incidence angles at its focal point or reflect scattered light at its focal point on cloudy days. Due to the excellent adaptability of CPC to actual weather conditions, currently, CPC-based photoreactors have gradually become the mainstream configuration of photocatalytic reactors, and a large number of CPC photoreactor experiments have been conducted
[38][46][48][63][65][69][71][93][94][95][96][97][98][99][100][101][102][103][104][105][106][107][108][109][38,46,48,73,75,79,81,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100].
Afterwards, another CPC-based treatment plant was established by Vidal et al. for detoxification and disinfection of contaminated water
[93][84]. Various typical organic pollutants such as pesticides (EPTC, butiphos and γ-lindane) and microorganisms (
Escherichia coli,
Enterococcus faecalis) were selected as the treatment targets. All the organic contaminants were nearly 100% removed after 30 min of sunlight exposure, further demonstrating the applicability of CPC photoreactors to different treatment targets. Furthermore, a variant of CPC photoreactors has also been explored to promote its photocatalytic reaction performance.
5. Relevant Problems in CSP-Based Photoreactors
5.1. Instability of Real Weather
Although solar energy is an ideal energy source that is widely distributed and could be utilized at a low cost, the actual weather inevitably causes instability in the natural solar energy reaching the ground
[7]. For the photocatalytic process, appropriate light irradiance is required to maximize the excitation of the photocatalyst and favorable heat energy is required to promote the reaction rate
[7]. Therefore, natural variations in solar energy could inevitably affect the photocatalytic reaction rate. It also needs to be pointed out that modern industrial processes pursue the high robustness of system operation, and uncontrolled decline in the production rate is usually unacceptable. Therefore, if solar-energy-driven photocatalysis technology is to be truly extended to large-scale industrial processes (such as wastewater treatment, hydrogen production, biomass conversion, etc.), searching for solutions that could mitigate or even avoid the effects of natural solar variability on photocatalytic rates needs to be prioritized.
At present, some efforts have been put into solving this problem by researchers. In the development of a photocatalytic reactor based on a linear Fresnel concentrator constructed by Zhang et al., a novel solar energy regulation strategy was employed
[7]. Traditional linear Fresnel concentrators utilize multiple primary mirrors to track the sun’s movement and reflect the sunlight to the focus to obtain ultra-high energy. Meanwhile, in their reactors, the innovation is that each primary mirror was controlled to rotate through its own independent control system. Under different weather conditions and at different times during the day, there were different numbers of primary mirrors used to focus sunlight. On sunny days when natural sunlight is strong, only two or three primary mirrors focus sunlight on the photocatalytic reaction tube, thus providing superior light irradiance and reaction temperatures for maximizing photocatalytic efficiency.
5.2. Nighttime Operation
Photocatalytic reactions require continuous input of light and heat energy to proceed smoothly. Considering the requirement of high robustness for actual industrial process, it is also a challenge to maintain efficient photocatalytic processes at night when there is no natural sunlight. The adoption of artificial light sources for nighttime photocatalytic lighting is straightforward in theory, but in the actual engineering project design, it is necessary to integrate the natural sunlight collector with the artificial light source.
5.3. Configuration of Photocatalyst Immobilization Substrate
Most CSP-based photoreactors utilize suspended photocatalysts, which may lead to complex post-separation and increased operating costs. In order to facilitate the large-scale application of photocatalytic technology, the immobilized photocatalyst needs to be equipped in the photoreactors.
In the previous studies, Villén et al. tested the efficiency of two CPC prototypes with different substrate configurations of photocatalysts, namely coaxial- and a fin-type photocatalysts
[97][88]. The results demonstrated that the fin-type photochemical reactor always showed higher inactivation of waterborne bacteria than the coaxial-type reactor. This phenomenon might be due to the lower moving rate of water flowing through the fin-type system, which may lead to a higher number of bacteria attached to the immobilized photocatalyst.
It is worth noting that the above-mentioned development and optimization of the supporting configuration of the immobilized photocatalysts are all oriented towards specific application scenarios (e.g., sterilization and microalgae processing). The conclusions obtained may not be applicable to a wide range of application scenarios. For other application scenarios (such as gas-phase treatment, water treatment, water splitting, etc.) and different working conditions (such as reactant concentrations, flow rates, etc.), appropriate catalyst substrate configurations are needed. Generally, in a CSP-based photoreactor, the immobilization of photocatalyst may need to meet the following requirements: (1) achieve effective incident light energy utilization; (2) avoid obstructing the flow of the reactant and (3) load a large amount of photocatalyst. Based on the above requirements, an immobilization substrate and configuration that are suitable for CSP-based photocatalytic reactors need to be designed correspondingly.
5.4. Economic Analysis
An economic evaluation is one of the criteria used to judge whether a new technology can be applied to practical applications. Due to the absence of practical application examples of large-scale photocatalytic facilities, an economic analysis of photocatalytic reactors is required. In a study by Vidal et al., a large-scale CPC photocatalytic wastewater treatment plant with an occupied area of 500 m
2 was assumed
[93][84]. The operation cost of this plant was estimated to be 0.7 USD for 1 m
3 water treatment (based on 1997 construction cost indices), which exhibited excellent cost-effectiveness compared with conventional technologies (around 1.0 dollars/m
3). Then, Vela et al. carried out an economic comparison of CPC photoreactors using different photocatalysts (ZnO and TiO
2)
[107][98]. Assuming 365 sunny days in a year and 8 h of system operation per day, the cost per cubic meter of water treated by the TiO
2-adopted CPC photoreactor was 1.45-fold higher than that of the ZnO system. The difference in process cost caused by the different catalysts was mainly due to the different times the catalysts required for pollutants degradation.
6. Conclusions
Efficient photocatalytic reactions require appropriate light irradiance and temperatures, and the low energy density of natural solar energy highlights the necessity of combining CSP technology with photocatalysis. Although existing CSP-based photoreactor technology has significantly improved the practical availability of photocatalysis, the challenges in unstable real weather, nighttime operation, post-separation and economic concern remain to be solved. Based on the latest reports, the adoption of a solar energy control strategy, employment of UV-visible responsive photocatalyst and immobilization substrate, innovations in reactor structure, economic evaluation of systems and establishment of reactor industry standards may be favorable for technological breakthroughs and engineering applications of future CSP-based photoreactors.