Parabolic trough solar collectors: A general overview of technology, industrial applications, energy market, modeling, and standards

Abstract Many innovative technologies have been developed around the world to meet its energy demands using renewable and nonrenewable resources. Solar energy is one of the most important emerging renewable energy resources in recent times. This study aims to present the state-of-the-art of parabolic trough solar collector technology with a focus on different thermal performance analysis methods and components used in the fabrication of collector together with different construction materials and their properties. Further, its industrial applications (such as heating, cooling, or concentrating photovoltaics), solar energy conversion processes, and technological advancements in these areas are discussed. Guidelines on commercial software tools used for performance analysis of parabolic trough collectors, and international standards related to performance analysis, quality of materials, and durability of parabolic trough collectors are compiled. Finally, a market overview is presented to show the importance and feasibility of this technology. We believe the compilation of reviews related to the above aspects will further provide impetus for the development of this technology in the near future.

Greek letters α sc absorptivity of the selective coating γ interception factor T Δ ab temperature difference between average absorber temperature and ambient temperature (°C) T Δ f temperature difference between average fluid temperature and ambient temperature (°C) η overall efficiency η op optical efficiency η op,0 peak optical efficiency(at normal incidence) η peak peak thermal efficiency ρ m reflectance of the mirrors τ cg transmittance of the cover glass Boundaries 1 ave fluid mean bulk 2 inner surface of pipe 3 outer surface of pipe 4 inner surface of cover glass 5 outer surface of cover glass A atmosphere C sky S outside of the brackets (extended surface)

Introduction
In the last few decades, the energy demand of industries and commercial sectors has substantially increased. Most of the energy has been generated by using fuel fossil technologies that have increased the emission of pollutants to the atmosphere, contributing to global warming and deterioration in the health of humankind. Solar energy is one of the green energy resources that can be used to reduce the consumption of fuel fossils to meet the demands of the industrial and the commercial sectors. Further, the cost of energy of solar photovoltaic (PV) and solar thermal power has tended to fall as their performance efficiency increased in late years [1][2][3]. Many technologies are used in industry to convert solar energy into thermal and electrical energies. These technologies are classified and represented in Figure 1. Solar technologies can be classified as either passive or active, depending on the type of system, with active technologies requiring external components, such as pumps or electronic control, to convert energy. The type of energy supplied to the process is the energy produced directly from the solar collection system used in the industrial process, which can be either thermal or nonthermal (e.g., PV). Solar collector type is determined based on the total radiation transmitted to the reception area compared to the radiation received in the collection area. Solar concentration is achieved by reflecting or refracting solar radiation from a large area (collection) to a smaller one (receiver) using mirrors or lenses. The concentration factor is the ratio between the collection area and the receiver area, so it is a dimensionless geometric factor having a value greater than 1 for concentrating technologies. Parabolic trough collectors (PTCs) have a common concentration ratio above 10 and lesser than 100, which is considered as "medium concentration" [4]. To realize energy conversion, concentrating technologies require external components, which are generally related to fluid transport or solar tracking, and so they are considered active. In industrial applications, solar concentration (collectors) technologies are divided into four technologies: PTCs, linear Fresnel collectors (LFCs), solar towers (STs), and parabolic dishes. Table 1 represents the principal differences between these technologies based on their functionality. The receiver can be mobile or stationary depending on the concentration process, and the shape of the focus (where the sunlight is concentrated) can be either a line or a point. Concentration factor is one of the most important parameters which differs among these technologies. This factor is usually smaller than 100 for PTCs and LFCs, whereas for the others it can be greater than 1,000. Table 2 displays the main technical and operational characteristics of these four technologies. PTC and ST have some advantages, such as maturity and typical capacity, making them the most important technologies used for solar energy conversion. Nowadays, PTC is the most deployed technology and has major share in the worldwide energy market compared to other solar concentrating technologies [1].
A PTC is basically a mirror with a cylindrical-parabolic shape and a receiver duct located along the focal line of the parabola, as represented in Figure 2. This configuration increases the energy intensity of sunlight along the receiver, where sunlight is getting converted into heat and transferred to fluid circulating through the receiver. PTCs operate at low to medium temperatures, with the working fluid reaching temperature between 50°C and 400°C [4], a temperature range where many industrial processes are carried out.
A PTC uses direct solar radiation as a heat source. As the sun's relative position changes every second, a solar tracking system is needed to improve its efficiency. Two types of solar tracking are used in PTCs namely, northsouth and east-west, as represented in Figure 3. Tracking methods are so named based on the direction of rotation of collector's aperture plane. In north-south tracking, the receiver (parallel to tracking axis) is aligned to east-west direction, so the collector's aperture plane rotates from north to south and vice versa. The opposite case is the east-west tracking type in which the receiver is aligned to north-south direction and the aperture plane rotates from east to west. The north-south tracking method has the advantage of lower tracking energy consumption, but with a higher end-effect. For east-west tracking systems, the opposite is the case (lower end effect and higher energy consumption).
These collector systems have the following advantages: -Low emissions to the environment during the system's life cycle: Right from the manufacture of components (structure, electronic components, anodized mirrors, etc.), to their use in final deposition, the environmental impact from the life cycle of these collectors is lower than that from energy technologies using fossil fuels. Burkhardt et al. [5] realized a life cycle assessment of about a 103-MW concentrating solar power (CSP) plant based on PTC when comparing some plant designs. Klein and Rubin [6] realized a similar life cycle study with a 110-MW CSP plant. Both studies reported an estimation of between 30 and 70 kg CO 2 eq/MW h for a CSP plant with storage system, which is much lower than 400 kg CO 2 eq/MW h reported for a natural gas power plant with combined cycle [6]. -Lower maintenance and operating costs: This is a major advantage of renewable energy technologies over conventional fossil fuel systems. The contribution of the fuel to the operation and maintenance (O&M) cost is important, which includes its price, availability, type, purity, and other aspects [7]. Since renewable systems do not need the kind of fossil fuels and equipment, O&M costs are much less compared to the conventional power technologies. Nevertheless, renewable energy systems require some maintenance. For example, the estimated O&M cost for solar energy generating system plants in California is USD 0.04/kW h, which is Table 2: Comparison of solar concentrating technologies [1,3] Typical capacity  considered the highest cost in terms of solar concentration with parabolic troughs [1]. -Long life span: As they operate at moderate temperatures, PTCs have longer life span.
The main disadvantages are the following: -Large area required for installation due to the diffuse nature of solar radiation (a flux of around 1 kW/m 2 on earth's surface); so large areas or land are required to collect enough heat to meet the energy load of the process. -High initial investment and medium-and long-term recovery: The total cost of manufacturing materials affects the total capital cost, recovery, and levelized cost of energy. But note that O&M costs are low as stated before.
-Intermittence of the resource: PTCs use direct solar radiation and so intermittence is a rule as energy cannot be collected at night. Further, PTCs can operate efficiently only on clear-sky days as clouds affect the energy production.
The main purpose of this review is to present the state-of-the-art of PTC technology and its role in industry. This study presents a general overview of the technology that includes both technical and economic aspects. Industrial applications are discussed in a practical approach, explaining the principal aspects and each purpose. Another objective is to compile reviews and technical and economic reports so that readers can use them as a "starting point" to focus research on a specific topic about PTC technology. Moreover, the intention of the study is to present information about modeling as a tool for designing and the status of technical standardization for practical analysis of a collector (i.e., for certification) and thermal systems.
This review consists of five sections. Section 1 presents the collector components and describes construction materials, desired properties, design data (for frameworks and receivers), and advancements in technology for each component. Section 2 describes and analyzes the status of PTC technology applications in industry, methods and processes used, and potential applications for implementation. A market overview and a brief description of the status of relevant economic parameters and future trends related to PTCs are given in Section 3. Section 4 is a summary of the mathematical methods and software tools used for performance analysis. Finally, international standards for performance analysis and materials quality are reported in Section 5. Several reviews and reports about the materials used, economic analysis, and industrial processes in solar energy conversion are discussed. To the best of our  knowledge, a review using a holistic approach considering a brief description and detailed information of components (with common properties of materials used), economics, and standards, related to PTC technology has not been reported so far.

Components of PTC
A PTC mainly consists of five major components: mirrors, a supporting structure, a receiver, working fluid, and a tracking system. Each component is designed to perform a specific purpose and is fabricated using materials based upon its functions and desired properties. These components are explained in detail as follows.

Mirrors
The principal function of mirrors is to reflect solar radiation and concentrate it onto the receiver. Mirrors are made of high-reflective material layers (aluminum or silver) with substrates and superstrates that protect the reflective layer against corrosion or abrasion so that longer life is achieved. Table 3 represents the principal types of mirrors used in applications. Silvered glass mirror, anodized sheet aluminum (sometimes coated with a polymer film), aluminized polymers, and silvered polymer films are the most commonly used materials for production. Granqvist [8] analyzed the main methods for applying reflective coatings (for mirrors) and selective coatings (SCs; for receivers), and described the general properties of some materials.
It is known that dirt, abrasion, and corrosion affect the optical performance of the mirrors, resulting in a decrease in the efficiency of thermal performance for the collector. Selection of the appropriate coating based on the desired properties of the reflective surface is a prerequisite for high thermal performance. Atkinson et al. [16] analyzed the literatures on reflective and SCs applied on concentrating solar systems as well as their main properties. Further, the deflection of the supporting structure may have a potential effect and it can decrease optical performance. Arancibia-Bulnes et al. [17] studied the main methods for evaluating the optical efficiency of solar concentrating collectors. Fernández-García et al. [18] presented the innovations made by R&D centers in recent years, under current investigation, or expectations about improvements in materials to increase optical performance of mirrors. They also considered the aspects of antisoiling coatings, high-reflective materials, high-temperature mirrors for secondary reflectors, and mirrors based on stainless steel and presented challenges, benefits, and research requirements of these concepts. SolarPACES Task III group has published a guideline about characterization of reflectance properties and methods to determine the reflectance of mirrors used in solar concentrating technologies [19]. This guideline presents a method to characterize reflectance properties of solar reflective materials with reliable results, including recommended instrumentation requirements for field measurements. Zhu et al. [20] present a method for characterization of the reflectance of mirrors in PTCs combining total specular reflectance and specularity profile of the mirror; they also presented a case of study using the proposed method. Sutter et al. [21] proposed a prototype for field measurement of reflectance based on the recommendation given in the SolarPACES Guideline. The commercial instruments used in both outdoor and indoor field measurements (with a description of prototypes under development) were analyzed by Fernández-García et al. [22].
Further, they reviewed the methods and models based on the theoretical statements or experiment parameters for estimating the reflectance of solar reflective materials.
The results of the two most used reflectometers to measure the reflectance of back-silvered glass mirrors used in CSP applications are compared by Sansom et al. [23].
Deviations or errors may occur in the collector during manufacturing and normal operation which affect the concentration and, consequently, the optical efficiency. The most important errors are shape error, slope error, receiver deviation error, specularity error, tracking deviation, and frame deformation. Shape error estimates the eccentricity of the focal line (where the receiver is) due to deviations and misalignments of the mirrors. Slope error is the deviation of the rays due to slight ripples presented in the mirror shape. The receiver is not completely aligned to the focal line, so this misalignment is measured by the receiver deviation error. Specularity error refers to the error due to the imperfect reflection of mirrors (no-ideal reflective materials). As the collector is not always perfectly pointed to the sun, tracking error occurs during operation (see Section 2.5). The other factor that affects the geometry of the collector during operation is normal loading (principally by self-weight, wind, and torsional loads), which acts on the frame of the collector and may deform it. Figure 4 represents these errors (the deviations are exaggerated to illustrate the origin of error).
Generally, manufacturing process consists of deposition of the reflective layers and protective coating over a solid (or polymer, in the case of silvered polymer reflectors) substrate. For silvered glass mirrors: the glass is heated to mold it into the parabolic shape, then tempered so it does not lose the parabolic shape, and finally coated with the reflective and protective layers in the back [24]. For aluminized reflectors, the reflective layer is deposited over a metal substrate (usually aluminum) and then coated with protective layers. Finally, the reflective silvered layer is deposited on a polymer substrate and coated in the front in silvered polymer reflectors.

Supporting structure
The main purpose of the supporting structure is to fix the components of the collector, which provides rigidity and stability to the whole system. The structure is generally made of structural materials such as steel or aluminum. PTC supporting structure consists of three parts from a structural perspective: the main support (columns, piles, and box), the frame, and the receiver brackets ( Figure 5).
-The main support serves as the structural anchor of a collector and holds it. Structurally, the main support will have the capacity to withstand wind loads as the aperture of the collector is exposed to the wind [25]. -The frame gives rigidity to the mirrors, allowing them to retain its parabolic shape at all times. It also transmits the torque of the tracking system. -The brackets are attached to the support of the mirrors and the receiver is fixed at the focal line of the parabola. An insulating material is usually placed between both components to reduce heat losses from the receiver to the bracket.
The parabolic shape of the mirrors and the location of the receiver at the focal line of the parabola should be maintained always to prevent misalignments, which will result in performance losses. So a proper design of the structural framework is very important. The most important mechanical properties that are considered during manufacturing are bending and torsion of the framework, which are principally produced by self-weight and wind forces. These two parameters will affect the performance if the frame is not properly designed. Giannuzzi et al. [25] proposed structural design criteria for parabolic trough solar collectors and presented a methodology to calculate loads for structural designs based on European codes. Other studies on wind loads are based on experiments and computational fluid dynamics (CFD). Hosoya et al. [26] estimated experimentally wind loads of a 7.9-m section, 5 m aperture scaled PTC facing the wind as isolated single module and array configuration in a wind tunnel with a turntable platform (to determine wind loads with different wind directions). Like Hosoya et al., Randall et al. [27] conducted a similar experimental study, but the collector was tested under different geometric parameters. The effect of wind loads using windbreaks in PTCs was estimated according to Torres-García et al. [28]. Mier-Torrecilla et al. [29] conducted a CFD analysis to obtain the wind loads in a single model PTC under different yaw and pitch angles, and the results were compared with wind-tunnel experiments. Like Mier-Torrecilla, Paetzold et al. [30] made a similar CFD analysis but used three different focal length of the parabola. Naeeni and Yaghoubi [31] performed a two-dimensional CFD analysis of a PTC under different wind velocities and pitch angles to observe the flow behavior around the collector. Naeeni and Yaghoubi [31], similar two-dimensional CFD analysis was conducted by Zemler et al. [32] and Hachicha et al. [33] but focused on wind loads and both aerodynamics and heat transfer, respectively. Many patents of frameworks are found in the literature but the most important are those used in CSP plants around the world. The principal characteristics of the most commonly used parabolic trough frameworks are presented in Table 4.

Receiver
The principal function of the receiver is to absorb as much radiation as possible and to transfer this energy to the heat transfer fluid (HTF) efficiently. The receiver can be fabricated from metal (heating) or glass (solar disinfection) depending on its application. For heating applications, the receiver has three principal components: the pipe receiver, the cover glass, and the SC. The cover glass minimizes the heat losses on the pipe and protects it from degradation. The SC is applied to the external surface of the pipe to increase the solar heat flux absorption. A vacuum is applied in the annular region between the glass and the tube to minimize the heat losses and the receiver is sealed to prevent vacuum losses. These kinds of receivers are called evacuated receivers. The bellows are the airtight part of the receiver, and they also allow thermal expansion of the pipe and   glass cover. It is a known fact that the pipe and the glass cover do not expand at the same rate in the entire operation. Bellows allow thermal expansion without losing vacuum in the annulus because of high temperature difference with the atmosphere. The principal function of the hydrogen getters is to adsorb residual hydrogen that can be formed due to thermal degradation of the HTF. The glass-to-metal seal is the joint between pipe and glass cover, and its main function is to reduce mechanical stresses that occur due to differences in thermal expansion. Figure 6 represents the components of an evacuated receiver. Some prefabricated receivers and their characteristics are shown in Table 5. Small-aperture collectors and prototypes usually have nonevacuated receivers. Table 6 displays the characteristics of the most representative small-aperture collectors available in market.
The ideal material selected for a pipe receiver should have high resistance to corrosion, low thermal expansion, and high thermal conductivity, and stainless steel is the most commonly used material. Stainless steels have low thermal conductivity and a high resistance to corrosion, and they are malleable (so fabrication of tubes is easy).
The cover glass is made of a material with high transmittance, low reflectance, and low refractive index. It is very important that it should transmit the highest possible amount of the incident radiation reflected from the mirrors. Deubener et al. [62] analyzed the most important properties of glasses used in solar thermal, solar PV, and solar chemical applications and reported techniques to improve the optical performance of those materials. Antireflective coatings (ARCs) are  applied to the external surface of the cover glass for enhancing its transmittance. Further, they create a transition on the refractive index from air to the cover [16]. Silica and low-iron glasses are the most common types of glasses used in solar applications (for example, borosilicate is extensively employed in solar applications) [62]. The measurement of transmittance is performed by standard spectrophotometry at near-normal incidence (e.g., ASTM-G173-03 direct) [24]. The SCs should absorb as much solar radiation as possible and transmit it to the pipe receiver. They are applied to the external surface of the pipe to increase heat flux absorption. The SC should have the properties such as high short-wave absorptance, low long-wave emittance, good surface adhesion, and chemical stability in the working temperature range of collectors. Atkinson et al. [16] and Wijewardane and Goswami [63] reported state-of-the-art reviews of SC and focused on the capabilities, designs, and fabrication methods of coatings used for solar applications. The measurement of reflectance of the SC is performed using standard spectrophotometry at near-normal incidence (e.g., ASTM-G173-03 direct), and absorptance is obtained assuming opaque conditions ( = − ) α ρ 1 [24]. Receivers used are entirely different in solar disinfection. Here, heat losses are not important, but the low transmission of UV radiation is important. In this case, the receiver is a tube made of a chemically inert material with a high-UV-radiation (short-wave) transmittance and low reflectance, like the cover glasses mentioned earlier.
The manufacturing process consists of coating the pipe (by methods shown in Ref. [8]) with an SC and the glass cover with an ARC. Prior to coating the glass, it is welded to the seals and treated in the surface for stress releasing and with other surface treatments. The pipe usually is treated on the external surface for a better application of the SC. Pipe, glass, getters, and vacuum indicators are then assembled in one piece. Finally, the annulus is evacuated. Figure 7 represents a basic diagram about the manufacturing process. Nonevacuated receivers are easier for assembly. The cover glass, the pipe, and glass-to-metal support are assembled or connected once the pipe and the glass were coated.

The HTF
The HTF, also known as working fluid, is a substance that can absorb heat coming from the receiver and uses it as process energy. This fluid should have high thermal capacity and thermal conductivity, low thermal expansion, low viscosity, minimal corrosive activity, low toxicity, and thermal and chemical stability throughout its entire operating temperature range. Liquids are extensively used as HTFs. Table 7 shows the most common HTFs used in industry and their properties, typical operating temperature range, and advantages and disadvantages in operation.
Water is mostly used in low enthalpy and direct steam generation (SG) applications, while thermal oils (such as Syltherm or Therminol) are used in solar power generation along with a heat exchanger to generate steam for use in a Rankine cycle. Both water and thermal oils are the most used HTFs in industrial applications.
Pressurized air is commonly used for drying and heating in buildings using flat-plate collectors, but it has been studied only as an option for using in PTCs [58]. In fact, some studies suggested that pressurized air has a good performance in power plants with solarassisted gas turbines. Amelio et al. [78] constructed a simulation of a combined cycle in which the PTC solar field preheats the air before entering the combustion chamber, resulting in a yearly global efficiency increment of 15.5% under certain operational conditions when compared with a reference plant without the solar field. Bellos et al. [79] obtained the optimization of a solarassisted gas turbine using parametric analysis. In addition, they found that the solar field lead to a fuel savings up to 64%, with a small reduction in power due to pressure losses. Ferraro et al. [80] proposed a thermal model to analyze the thermal behavior of a solar-assisted gas turbine with two modes of operation: with constant inlet temperature/variable air flow rate and constant air flow rate/variable inlet temperature, all of them with and without reheating achieved an annual average efficiency of 17.8%. Cipollone et al. [81] analyzed the use of CO 2 and air as HTF for CSP based on PTC with a solar-assisted gas turbine. The energetic and exergetic performances of six different gases as HTF in PTCs were studied and compared by Bellos et al. [82]. They concluded that helium has the higher exergetic performance, and carbon dioxide is the most appropriate gaseous HTF at higher temperatures.
Recent investigation has increased attention in using supercritical carbon dioxide (s-CO 2 ) as a potential HTF in PTC applications [77]. Its main advantage is thermal enhancement when compared with subcritical CO 2 as it     behaves as a hybrid liquid-gas fluid. Unfortunately, there is a lack of experimental research about s-CO 2 . But there are many numerical studies about its performance in the literature. A feasibility study about the usage of CO 2 at high temperature and pressure as a potential HTF in PTCs with experimental test was presented by Muñoz-Anton et al. [83]. Aguilar et al. [77] proposed a one-dimensional numerical model for comparing thermal performance of a PTC with thermal oil (Syltherm 800), subcritical CO 2 , and s-CO 2 and compared the model and experimental results. Both results are more or less similar and in numerical data results, there was a 2.9% deviation in temperature prediction (for thermal oil and subcritical CO 2 ), and the prediction of thermal behavior was within 2.7% when compared with [82]. Biencinto et al. [84] proposed a preliminary concept of power plant with s-CO 2 as a working fluid and molten salts as a secondary HTF. The results show that it is possible to reach up to 12.87% annual net efficiency of the plant and up to 6% of saving in cost of energy when compared to conventional thermal oil-based power plants.
Recently, molten salts and ionic liquids with good heat transfer capabilities have been mentioned in the literature. However, these HTFs should overcome some practical challenges. The main challenge for ionic liquids is their cost compared to the cost of conventional thermal oils used [68] and the operational aspects (such as freeze protection, impedance heating, and materials considerations) for molten salts [85].
Nanofluids have been developed for solar energy applications in recent times. A nanofluid is a fluid containing suspended solid nanometer-sized particles called nanoparticles, which enhance the heat capacity and thermal conductivity of the mixture. These particles are commonly metals (in natural form or oxides) and two types of nanofluids are used: (a) Mono-nanofluids: a base fluid is doped with a single material as nanoparticles (b) Hybrid nanofluids: a base fluid is doped with two or more materials as nanoparticles.
Thermal enhancement (improvement in properties such as thermal conductivity, heat transfer coefficient, and heat capacity) is the main advantage of nanofluids. The other important characteristic of nanofluids is its stability. The proper mixture of base fluid and nanoparticle will be effectively used in applications with PTCs. Krishna et al. [86] made an analysis about HTFs used in PTC technology. They reported that an enhancement of up to 27.9% in thermal efficiency and up to 234% on convective heat transfer coefficient is achieved with liquid-based nanofluids. They concluded that the addition of particles such as CuO, Ag, SiO 2 , carbon nanotubes, or hybrid mixtures will improve thermal conductivity of the based fluid for the applications using PTCs. Ahmadi et al. [87] have given a report about the use of hybrid nanofluids in solar applications, and they concluded that thermal enhancement up to 60% can be reached using hybrid nanofluids. According to Shah and Ali [88], stability studies of hybrid nanofluids in a practical approach are not available, which necessitate further research on the performance of these HTFs for solar energy applications.
Some studies related to its applications in concentrated solar technology are available in the literature on nanofluids. Singh et al. [89] reported an analysis about the use of some common metallic nanoparticles added to organic thermal fluids. Mahian et al. [90], Kasaeian et al. [91], and Verma and Tiwari [92] presented an analysis of the applications of nanofluids in solar energy systems.

Solar tracking system
Solar tracking systems are needed because PTCs work with direct beam radiation. A solar tracking system aligns the collector with the sun to maximize collector performance in its one-axis rotation, as described before (see Figure 3). Solar trackers can be classified as either passive or active. The thermosiphon effect is used by passive trackers to align the collector, whereas active trackers use electronic signal conversion. But passive trackers are not commonly used in PTCs because misalignment occurs by the forces of wind during operation.
Active trackers are mostly used in concentrating solar systems and are divided into closed-loop and open-loop trackers. The principal difference between these types of active trackers is the use of signal conversion method. Closed-loop trackers use electronic signal conversion with feedback control. Open-loop trackers use algorithms and prerecoded data with a computer and timing controls to track the sun. Figure 8 shows the operation of each type of active solar tracking.
Active closed-loop trackers consist of three parts: a light sensor, a control system, and a drive system ( Figure  8a). The sensor is active when the collector is misaligned with the sun (it measures a high light intensity when the collector is aligned), the misalignment triggers it to send a signal to the control system. The control system converts this signal so that movement of the driver occurs for transmitting torque to the structure until the collector is correctly aligned. A pair of light receiving diodes or PV cells is the most used sensors for closed-loop tracking. The main advantage of this tracking method is its high tracking accuracy but can be affected by shadowing. But the main disadvantage is the inability of the control system to recover or find the direction of the sun under long cloudy periods.
Active open-loop trackers consist of a controller and a drive ( Figure 8b). During operation, the algorithm programmed on the controller signals the drive to move the collectors. Open-loop trackers can be classified into timed trackers and altitude/azimuth trackers. Timed trackers use algorithms based on time, so that the collector is controlled by incremental movements. Altitude/azimuth trackers use algorithms based on astronomical data depending on location and time. The disadvantage of these methods is the accuracy of the equations used in the algorithms, which can result in high misalignments.
Hybrid-loop tracking has been developed to overcome the disadvantages of both closed-loop and openloop trackings. The fundamental strategy of hybrid-loop tracking is the use of algorithm (open loop) to track the sun and to reduce the misalignments by sensors (closed loop). Typical hybrid-loop tracking would be to calculate the position of the sun by the algorithm and activate the drive and later to correct misalignments by the sensor in the case that it is sensed.
Lee et al. [93] and Sumathi et al. [94] presented the report of active closed-loop and open-loop trackers, analyzing some designs published in the literature and showing the general description of the electronic control methodology used in each of them. Further, the strategies employed for light sensing for solar trackers were compiled by Moga et al. [95]. The information on solar tracking algorithms, computer code, and software available on the internet for use in solar trackers was studied and presented by Prinsloo and Dobson [96].

Industrial applications
PTCs are used in a variety of industrial applications and it is divided into three main groups, as shown in Figure 9. The heat collected is used directly for a given process in thermal applications. The solar energy is mainly used in heating and cooling applications in the industrial and commercial sectors, but concentrating PVs are currently under development. It is very important to note that there are small-scale installations and enterprises around the world producing PTC for PV applications. Each of these application groups is described in the following sections.

Heating
The application of solar energy in the heating application of industries is the extensively studied literature and PTCs are used in various types of industries. The collector transfers heat to the HTF, which is used as a source of energy for a given process (heating a fluid as main purpose of the PTC system). Heating applications can be divided into two groups based upon the temperature achieved by the HTF. Low-temperature applications are for a maximum temperature of 100°C and medium-temperature applications usually reach temperatures up to 400°C. Low-temperature PTC systems are commonly used in preheating and drying processes in commercial, residential, and industrial sectors. SG and CSP are the principal applications for medium-temperature PTC systems. Kalogirou [97] presented a list of potential industrial processes for implementing PTC systems, and Table 8 shows some of the processes. The temperature ranges of the processes are in the operational range of temperature of the PTC systems. The International Energy Agency (IEA), in collaboration with the AEE-Institute for Sustainable Technologies (AE-INTEC), has created a database of solar heating industrial processes (SHIPs) [98] and a guide to integrating solar-assisted systems with industrial heating processes [99]. Figure 10 shows the location of general-purpose nonpower-generation heating plants using PTCs around the world, according to the IEA SHIP database. In addition, Hisan Farjana et al. [100] reviewed the state-of-the-art of SHIPs' focus on identifying the potential of different solar thermal technologies in various industrial processes.
CSP is the most popular PTC application for electricity generation. CSP plants generate electricity using solar-assisted turbines or integrated systems with combined cycles, usually with a thermal storage block. Both direct and indirect SGs are the most commonly used methods in the world for electricity generation with PTCs. These systems generate steam to run a turbine, by direct heating (water as HTF of the PTC system) or by indirect heating (i.e., oil as HTF and heat is transferred to water through a heat exchanger). Figure 11 presents a diagram of a CSP plant with direct SG (a) and indirect SG integrated to a combined cycle (b). The thermal storage block increases the efficiency and the capacity factor, making it possible to generate electricity even when there is no solar resource (nights or cloudy days). Tian and Zhao [101] and Alva et al. [102] presented the analysis of materials used in thermal storage systems for solar    plant was presented by Manzolini et al. [106]. They observed and found that the energy costs and CO 2 emissions are lower than conventional cogeneration plants.

Cooling
Solar cooling refers to the use of solar radiation as a thermal energy source to cool a fluid or a space. Absorption and adsorption are two main solar cooling methods and both the method will replace the mechanical compressor in a conventional cooling process with a thermal compressor. The absorption process is the most commonly used method for solar cooling with PTCs. Absorption uses a fluid-fluid mixture (also called working pair) as the refrigerant. The fluids of the working pair make a strong solution when mixed at low temperatures and can be separated when the mixture is heated. The solute is converted into a gas and the solvent remains in liquid state when the mixture is heated. The absorption cycle takes place as follows: 1. The mixture is separated in the generator by heating. 2. The solute (gas) is condensed, and it rejects heat to the ambient space. The solute is then expanded and later evaporated by heat collected from the refrigerated space (as in traditional cooling systems). 3. The heated solute is then mixed with the solvent in the absorber. 4. The mixture is pumped to the generator and preheated by a heat exchanger with pure solvent coming from the generator. 5. The mixture is heated again in the generator, closing the cycle.
Single-effect and double-effect cycles are the most common processes in absorption cooling. Figure 12 shows a schematic diagram of single and double-effect absorption cycles. The most commonly used working mixture in solar absorption systems are lithium bromide and water ammonia. Marcriss et al. [107] also reported on other combinations of working mixtures.
Solar adsorption cooling processes are entirely different from the absorption processes. The physical principle behind adsorption cooling is a surface-based phenomenon in which a porous material (adsorbent) captures vapor from a fluid (refrigerant) and the adsorbent is regenerated by heating. Adsorption processes differ from absorption because the working mixture consists of a solid-fluid combination and moreover heating is intermittent, not a continuous one (the adsorbent is heated whenever it is saturated) [108]. Adsorption cycles require very low or no mechanical or electrical input but need thermal input (e.g., from the collector field) which is very important, and it works intermittently with the solar resource. A simple adsorption system cycle diagram is shown by Figure 13 and the cycle of working is as follows: 1. The refrigerant is evaporated by heat from the refrigerated space. 2. The vaporized refrigerant is adsorbed in the adsorption chamber. 3. When the adsorption chamber gets heated, the vapor is released and then condensed, resulting in the rejection of heat to the ambient space. 4. The condensates are stored in a tank. 5. Finally, the condensates are evaporated again, closing the cycle.
In solar adsorption cooling, nonconcentrating technologies (such as flat plates or evacuated tubes) are most frequently used. Nevertheless, some literatures suggest good performance for an adsorption cooling pilot plant with PTC technology compared to nonconcentrating technologies. Li et al. [109] conducted an experimental study about an ice maker using a 20-m 2 PTC and 30 kg of CaCl 2 + activated carbon, obtaining a coefficient of performance (COP) up to 0.15 and a production of 50 kg under typical summer day in Shanghai. Further, a thermal model of a cooling system using PTCs as generator and activated carbon methanol as working mixture was proposed by Tashtoush et al. [110]. Fadar et al. [111,112] conducted a theoretical and optimization study about an adsorption cooling system using PTCs and activated carbon ammonia, realizing an optimum operational COP of up to 0.18. In addition, a prototype of solar cooling system driven by PTCs for use in remote areas was designed successfully by Abu-Hamdeh et al. [113]. They realized a statistical optimization of the system obtaining a 3.5-to 5-m 2 PTC and an average COP of 0.18-0.2. The most commonly used adsorbent materials are zeolite, activated carbon, and silica gel, and the most commonly used refrigerants are ammonia, methanol, and water. Sumathy et al. [114], Masesh [115], and Fernandes et al. [116] analyzed and reviewed solar adsorption processes as well as the material characteristics of adsorption working mixtures.
Chemical adsorption is another method for cooling. It is based on a reaction between the adsorbent and refrigerant that form a strong chemical bond, increasing heat transfer. The main disadvantage of chemical adsorption is that it cannot be easily reversed as higher amount of energy is needed to break the chemical bond, and the chemical reaction alters the state of both adsorbent and refrigerant in a continuous operation. The most used adsorbent for chemical adsorption is calcium chloride, which can make a working pair with either water or ammonia.
The general advantage of solar cooling technologies is lower energy consumption compare to the conventional vapor-compressor systems. The working pair (in a liquid phase) is pumped in solar absorption cooling rather than using a compressor as in conventional cooling processes, and there is almost no mechanical input in adsorption cooling, as described earlier. So the energy consumption is drastically reduced here. Other advantages are low noise and low vibration because of fewer moving parts. The principal disadvantage is their low COP, with a COP of around 0.7 for single-effect absorption, 1.2 for double-effect absorption, and 0.1-0.2 for adsorption [117], compared with a COP of 3-4 for conventional vapor-compression systems.
Cabrera et al. [118] analyzed the use of solar cooling technologies with PTC systems. They also have given a list of PTC-based solar cooling facilities installed around the world, where most of them used small PTCs. The authors state that the advantages of PTCs over conventional collectors used in solar cooling are their lower thermal losses, higher working temperature, smaller collector area, and no risk of stagnation. The principal disadvantages are the increase in capital cost (due to tracking and other components), geographical limitations, and interruption during windy conditions. Further, they also concluded that the cost of energy of cooling systems using PTCs are the same or lower compared with conventional nonconcentrating technologies used in solar cooling.

Seawater desalination
Seawater desalination is a process in which minerals are separated from seawater to produce fresh water. Many methods are used for separating salts from seawater in industry, and they are classified into four groups as represented in Figure 14. Thermal processes (phase change) uses thermal energy to separate the brine and the most commonly used methods are multistage flash and multieffect distillation (MED). Single-phase processes use mechanical separation by passing seawater through filter membranes where salts are trapped, and reverse osmosis (RO) and forward osmosis are the most frequently used methods. Electric processes are based on cationic and anionic ion-exchange effects. In this electrolysis process, cathode and anode membranes are arranged alternately and exposed to an electric field, so that salt particles are trapped during the process and separated from seawater. The principal methods of electric processes are electro-dialysis, ion exchange, and capacitive deionization. Hybrid processes usually mix phase change with single-phase processes, such as membrane distillation method.
Solar seawater desalination with PTC technology participates directly only in phase-change processes as PTCs provide thermal energy. However, there are some reports on single-phase processes powered by thermal cycles based on PTCs. Ortega-Delgado et al. [119] performed computational simulations for thermoeconomic comparison analysis of an MED and RO systems coupled to a Direct Steam Generation-CSP plant driven by PTCs that produced 5 MWe.
Further, a preliminary analysis of an RO plant driven by a solar-heated Rankine power system was performed by García-Rodríguez and Delgado-Torres [120]. They concluded that these types of systems show lower specific solar energy consumption when compared to solar distillation and PV-driven RO systems, resulting in an interesting costeffective development. Delgado-Torres et al. [121] made a preliminary design of an RO plant which was coupled to a solar thermal-powered Rankine cycle and compared the use of two PTC models (LS-3 and IND300) in the solar system. Nafey and Sharaf [122] simulated a solar-assisted organic Rankine cycle that powers an RO system and then analyzed and compared the parameters using flat-plate, compound parabolic, and parabolic trough solar collectors and different working fluids. They concluded that "according to the current techno-economic framework, the PTC system is the best option." Buenaventura and García-Rodríguez [123] conducted a study about market potential of thermal-powered desalination technology, concluding that a stand-alone RO desalination plant driven by one-axis concentrating collectors (PTCs and LFCs) coupled to a ORC cycle has very good marketing opportunities. Iaquaniello et al. [124] presented a model that the potential of integrating CSP technology into a combined fossil-fueled/solar plant to power an MED/RO desalination plant. They concluded that desalination processes powered by solar (especially CSP) technology have become very attractive in the Middle East and North African countries. According to Ref. [116], with the proposed configuration plant, it is possible to produce fresh water with costs under USD 1.2/m 3 . It is important to note the impact of PTC technology on CSP plants and others solar concentrating systems (see Section 3.1).
Brine and fresh water are separated by heating seawater in thermal processes and the process is represented in Figure 15a. Seawater is heated so that water is vaporized, which is physically separated from salt because of its lower boiling point. The water vapor then condenses, leaving fresh water. A Rankine cycle provides the necessary electrical energy to pressurize the seawater and pump it through the membranes in single-phase processes, as shown in Figure 15b. The Rankine cycle operates as a CSP plant (see Section 3.1).
Both thermal processes and membrane processes have advantages and disadvantages over each other. The main ones are described as follows [125]: -Thermal systems' advantages over membrane systems: proven and established technology, higher quality product, less rigid monitoring required than for membrane processes, less impacted by quality changes in feed water, and no membrane replacement costs.

Phase Change
Single Phase Electrical Hydrib processes Figure 14: Classification of seawater desalination processes.
-Membrane systems' advantages over thermal systems: lower capital cost and energy requirements, smaller footprint and higher space/production ratio, higher recovery ratios, modularity allows for upgrades or downgrades and minimal interruption to operation when maintenance or a membrane replacement is required, less vulnerable to corrosion and scaling due to ambient temperature operation, and membranes reject microbial contamination. RO is the mostly used desalination method worldwide, because of its low energy consumption compared with the phase-change processes. This technology has a market share of around 59% of worldwide desalination capacity [126]. Thermal processes are suitable for mediumand small-scale plants. Sharon and Reddy [127] made a review of solar desalination and they reported information by comparing the performance of desalination methods with factors such as capacity, energy consumption, and efficiency. The data from Sharon and Reddy on the three methods mentioned in this work are exhibited in Table 9. Li et al. [128] reviewed the recent investigation of solarassisted desalination, and they also discussed modeled and experimental pilot plant installations of desalination using PTCs and other solar technologies. Further Reif and Alhalabi [129] analyzed and presented a review of potential benefits and challenges in cost-effectiveness, energy efficiency, and requirements and other parameters of solar-driven desalination. According to Aboelmaaref et al. [130], hybrid MED/RO desalination systems using PTC technology is a potential option for desalination since it has higher reliability, higher efficiency, and lower cost.

Water decontamination
Decontamination is the process of removing hazardous compounds such as heavy metals, organic compounds, and chemical substances from water. Advanced oxidation processes (AOPs) offer a feasible and sustainable alternative for disinfection of water. General applications of AOPs include the degradation of resistant material prior to a biological treatment and treatment of refractory organic compounds [131]. The methods for treatment of hazardous organic and inorganic compounds are reviewed by Kabra et al. [132]. They presented a list of previous studies on the removal of heavy metals using photocatalysis process. AOPs generate a high concentration of oxidants (usually hydroxyl radical, OH − ) to oxidize polluting matter that would not be easy to separate by biological degradation. There are some AOPs that use UV radiation as an energy source to produce the oxidants. But heterogeneous photocatalysis (HPC) with TiO 2 and photo-Fenton process (PFP) is the most commonly used  methods in solar applications. These processes generate hydroxyl radicals (*OH) when UV radiation activates the catalyst in an atmosphere with oxygen. A review on decontamination by photocatalysis and comparison of HPC and PFP reactor design requirements were presented by Malato et al. [133]. Solar decontamination is usually realized in batch mode [134,135], as shown in Figure 16. General purposes of water decontamination are performed for drinking water or agricultural applications [136]. Initially the catalyst is mixed in water to suspend particles in the fluid and then the mixture is pumped to the solar field to undergo the chemical processes before returning to the mixer. This process repeats continuously until the pollutants are degraded. The collector has the same characteristics as explained before, but the receiver is replaced with a glass pipe that is transparent to solar UV radiation. Ironically, the first photoreactors developed were based on PTC technology only [133], but nonconcentrating collector (NCC) and compound parabolic collector technologies have improved a lot in the few decades. Table 10 represents the comparison of these technologies. It is not very difficult to compare solar decontamination systems against conventional systems, which is an obstacle to industrial application. But solar photocatalysis is a promising technology when compared to others because of its low impact, according to Malato et al. [133] analysis.

Concentrating PVs
Concentrating PV (CPV) generation is the direct conversion of solar energy into electrical energy using semiconductor materials. Photoelectric effect is the basic principle behind the operation of the solar cell and that effect generates a potential difference within a semiconductor when it is exposed to sunlight. The information about PV cells appeared in the late nineteenth century but the first performance test records of PV cells under concentrated light occurred in the early 1960s [137]. Research on PV concentrator (PVC) technology has increased since then, and now PTC technology can actively participate in this application due to its advantages and maturity. PVC systems with PTCs have the same equipment as flat-plate PV collectors such as converters, batteries, and voltage regulators because both collectors provide direct current.
PVC operates in the same way as thermal concentrator but works with a modified receiver with PV cells on its surface. Concentrated sunlight strikes the PV cells, and PV cells convert solar radiation into electricity. The cells generate more energy under concentrated sunlight due to the incident energy density, and it is shown in Figure 17a. It is a fact that PV cells do not convert all the incident energy into electricity and most of this rejected energy is converted into heat, which lead to temperature of the cell to increase, affecting its efficiency (Figure 17b). The fluid used in the cells will be flowing inside the receiver. These collectors are known as thermal-PVCs (T-PVCs). PV cells used in concentrating PVs are designed to resist high incident radiation, so the use of Si-based cells fully depends on the concentration factor of the PVCs, as represented in Table 11.
Principal advantages of PVCs over nonconcentrating PV systems is that they have a higher efficiency and require fewer PV cells. Today, a number of PV cells operate under high concentrated sunlight with higher efficiencies compared to the common Si-based cells, as shown in Figure 18. Green et al. [139] presented a list of efficiencies of PV cells and modules for PV technologies. CPVs is still under development and it is not yet a commercially proven technology. But it has a good future potential. Karathanassis et al. [140] designed a parabolic trough of 2 m 2 aperture area T-PVC prototype evaluated the thermal and optical performance. They also described the design procedure and technical challenges of the technology. They obtained an overall efficiency of 50% (thermal efficiency of 44% and electrical efficiency of 6%), a temperature rise of 3.5-4 K, and an energy cost of around €1.75/W. The optical performance of a twostage T-PVC with spectral beam splitting filter using a ray-tracing technique was evaluated by Jiang et al. [141]. They found a reduction in heat loads pf up to 20.7% in the cells. The performance of a prototype using four different solar cells in the array (mono-crystalline Si, poly-crystalline Si, Super cell, and GaAs) was evaluated by Li et al. [142,143]. Further, a technical-economic analysis of combined heat and power solar generators was performed by Quaia et al. [144], concluding that the combined efficiency of this technology is remarkable but not worthy when evaluated separately in terms of the cost of energy. Yazdanifard et al. [145] simulated a parabolic trough T-PVC model focusing on energy and exergy performance with laminar and turbulent flow regimes using nanofluids. It is found that nanofluids can increase the performance (both energetic and exergetic) in T-PVC when compared with NCCs. Mendelsohn et al. [146] made a review of the development of the PV technology in the United States (concentrating and nonconcentrating) and reported a total gain of 471 MW under contract projects.

Market overview and economics
This section presents a general status of PTC technology in energy market and economics. There is also a brief description of economic concepts such as cost of energy, estimated prices of components, investment, and O&M of energy systems using PTC. In addition, the future trends of the technology from the economical point of view are also described.

General status in power-generation market
The utilization of renewable technologies in the international power-generation market has increased steeply in recent years all around the world. The principal reason is the growing interest of governments in implementing renewable energy generation systems to mitigate greenhouse gas (GHG) emissions and provide better energy security. Around 150 countries implemented energy improvement policies, and approximately 128 countries established energy efficiency strategies and targets at the end of 2015 [147]. Figure 19 represents the evolution of countries with renewable energy targets around the world. One of the most important applications for PTC technology is generation of electricity (CSP plants). The installed capacity of CSP plants has increased in the last decade (as shown in Figure 20), with the United States and Spain being the major contributors in solar thermal power generation. PTC-based solar thermal systems are mostly used in electricity generation systems, accounting for approximately 85% of total current installed capacity worldwide [1]. The US national renewable energy laboratory (NREL), in collaboration with the international program SolarPACES, has compiled data on existing and projected power plants that use solar concentration. A list of those plants is available on the internet and can be classified by technology, country, or status [148]. Figure 21 shows the statuses of installed, under construction, and projected CSP plants.

General status in SHIP market
The demand of thermal energy in industry was around 85.3EJ in 2014, equivalent to 74% of industrial energy needs [149]. Nearly 52% of this demand is involved in low-to medium-temperature heating applications, and this shows that there is a high potential of SHIPs for industrial thermal energy supply. As shown in Ref. [100], there are many industrial processes where PTC technology can be applied.
PTCs are one of the technologies used in SHIPs that have recently been developed and implemented in small-to medium-scale plants around the world. Small-aperture collectors are the mostly used PTCs in these applications, which can reach temperatures up to 250°C [150]. Nowadays, the technology applied to SHIPs is still under development. But a number of installations and collectors with substantial technical improvements have been reported all around the world (see Section 3.1 and Ref. [98]), with good performance and economics during the last years [151]. Figure 22 displays the evolution of every-year installed thermal power, collector area, and number of projects of PTC systems for SHIPs around the world, based on Ref. [90]. But a falling trend occurred between 2008 and 2011, and this trend could be caused by worldwide crisis known as "the great recession" that started on 2008 in the United States and also affected some European states in 2010s. Spain for example (one of the major contributors of developing solar technology) was affected due to this crisis until 2014. In addition, it has been reported that 18 companies produce PTCs, which represent approximately the 25.4% of enterprises that provide solar collectors for SHIPs [140]. IEA SHC Task 49 has made an effort in research to integrate solar heating technology and industrial heat supply systems efficiently.
The principal advantages of this technology are its reduced risk (compared with volatility of fossil fuel prices), zero fuel cost, localized production, and low GHG emission. Nonetheless, it still needs to overcome the barriers of the investment cost and complexity of the systems to have a good penetration into industry. Lack of transfer of technical information, suitable design guidelines, and analysis tools are other barriers [152].

General status in cooling and desalination applications
Nowadays, the market of cooling systems is generally dominated by nonconcentrating technologies. Up to 3% of the installed capacity is driven by concentrated technologies [153]. Nonetheless, concentrating technologies are suitable to be connected to solar-assisted double-effect absorption systems in locations where there is high solar radiation (e.g., Southern Europe or North America) [154]. IEA SHC Task 53 is making effort to assist in the sustainability of solar-driven cooling systems and heating in buildings [155].
In desalination applications, it is noted that PTC technology is still under development for proven installations. The potential of solar energy application in desalination applications was also reviewed by Buenaventura and García-Rodríguez [156]. They also compared the different methods and technologies used. They concluded that RO/PTC system-driven organic Rankine cycle has great potential in the market.

Economics
Four key economic factors are used in market analysis. The first (and most important) factor is the levelized cost of energy (LCOE). This cost indicates the equivalent payback for energy generation during the lifetime of the project. LCOE consists of the investment expenditures, O&M costs, fuel costs, the discount rate, and total energy generated. Figure 23 displays a comparison of the LCOE of different renewable technologies and their behaviors over the past few years. It is found that (thermal and PV) the trend is downward for solar energy. Despite this tendency, solar thermal power generation with PTCs is still not competitive to conventional fossil fuel technologies. But PTC technology has a good potential for development and implementation in power generation due to its advantages over other CSP technologies. The second factor is the capital cost (also called capital expenditure or CAPEX), which represents around 84% of LCOE [158]. The CAPEX represents the direct costs (materials and equipment), indirect costs (i.e., contingencies), and the owner's costs (land preparation, project development, and others) of the system. Table 12 gives an idea about the estimated breakdown of the capital costs for a 50-MW/7.5 h TES PTC-based CSP plant [2]. The major cost is contributed by the cost of the PTC system (38.5%), where the steel construction, receivers, mirrors, and HTF system represent the other important components of capital cost (29.6% approximately). The investment cost of equipment for PTCs is related to CAPEX. This cost is usually represented as cost of collectors per unit of aperture area. Table 13 gives the breakdown of the average cost of collectors based on two designs with costs between    [159]. The framework, receivers, and mirrors represent almost 70% of the cost of the collector, according to this information. The third factor is the O&M cost and it is around 11% of the LCOE [158]. O&M costs are estimated to be between USD $0.02 and USD $0.04/kW h, as shown in Figure 24. But O&M costs depend on the capacity of the plant and the TES system. Fixed costs depend upon the capacity of the plant, and variable costs are related to energy generation costs. Replacement of receivers (e.g., because of a glass breakage) and replacement and cleaning of mirrors are the most significant components of the O&M costs. Finally, the installed cost is another important factor in economics, which is the CAPEX cost per unit power. Table 14 shows a description of these costs. Table 15 shows some estimated prices of PTCs used in SHIPs in the case of small-aperture PTCs. Most of the PTC systems used in these applications can produce up to 2 MW. Figure 25 shows the evolution of every-year total investment and average costs of PTC systems for SHIPs around the world based on [98]. It shows an increment in investment for installing new systems while the cost tends to slow down.

Future trends
Installed costs and LCOE are predicted to fall around 30% over the next decade. The total reductions and shares predicted for the total installed cost, LCOE, indirect and owner's costs and investment based on a 160-MW/7.5 h TES CSP plant with PTCs is shown in Table 16 [158]. Technological improvements and installation and operational experiences from long-scale plants are the contributing factors for cost reduction. As an example, the transition of thermal oil to molten salt as HTF could lead to a more efficient operation. Heat exchangers for thermal oil and molten salts are no longer needed under this condition (but require an antifreeze system), and it will also permit operation of the TES block at the same time with fewer requirements [158]. Recent advances in glass materials have reduced the rate of breakage, which further decreased the O&M costs, but advances in mirrors must also be made. Improvements in automatization can also be a significant reducer of O&M costs.
The LCOE of CSP plants is expected to be competitive with other solar technologies in the near future. CPV technology has a good potential for implementation, even with its current uncertainty in market and technology   development. The LCOE of concentrating technologies is predicted to be competitive when compared with conventional solar PV electricity generation, as shown in Figure  26. The principal reason of cost reduction is the requirement of lower amount of material for conversion and the decrease in cost of PV cell in recent years. For small-aperture PTCs, the technology still needs to be upgraded for effective utilization in industry, taking their advantages and using experiences in SHIPs and other small-to medium-scale applications. However, the actual tendency shows an overall increment in interest, investment, and development of this technology [152], principally for industrial thermal energy supply. IEA considers that there is potential market and technological development as there is around 28% of the overall demand for thermal energy in European Union countries in the heat processes with temperatures below 250°C [162]. For efficient use of solar-assisted cooling, the major efforts lie on the integrability of the solar collector system to thermal-driven cooling systems and economic competitiveness when compared with the conventional cooling systems.

Thermal performance and modeling
Thermal performance analysis provides the quantity of energy that a collector system can supply to a load, which is important in the design of systems used in industries.  There are many studies on thermal analysis and computational tools for measuring PTC system performance used in industry. Thermal performances are evaluated by either mathematical models or experiments. Mathematical models use thermodynamics formulas to compute thermal performance. Experiments are based on field measurements taken to obtain a more realistic performance. The principal advantage of experiments is that they consider complex phenomena (implicit in the measurements) that may be difficult to incorporate into models. The advantages of mathematical models are their simplicity and low cost when compared to experiments. These two models or groups are classified as shown in Figure 27.
Experimental methods may be based on indoor or outdoor analysis, as shown in Figure 27. Indoor analysis is used to testing of receivers for heat losses, whereas outdoor testing is for thermal performance of collectors. Atmospheric factors, such as wind or ambient temperature, are either totally or partially controlled in indoor testing. However, these factors are not at all controlled for outdoor experiments. Many studies in the literature described experiments using indoor and outdoor testing related to PTC technology. Table 17 shows some of these studies found in the literature. Standards are recommended to provide guidelines about instrument quality and how to proceed with the measurements to obtain accurate results. ASHRAE 93, ISO 9806, and SRCC 600 are the most commonly used standards for thermal performance analysis with PTCs. These and other standards are described in the next section. Several studies about the methodologies of mathematical modeling of thermal performanceare available in the literature; some of them include validation with experiments. Table 18 shows a few of these studies with a brief description of the approaches used for thermal modeling of PTC technology. Yilmaz and Mwesigye [205] performed a review about thermal and optical performance studies for thermohydraulic analysis with PTC, which included inserts, nanofluids, and SG. The literatures about thermal performance of PTC system adapted to desalination processes are reported by Aboelmaaref et al. [130]. A summary about thermal and optical modeling implemented in simulation tools of direct SG systems using PTC technology was provided by Sandá et al. [206]. Daneshazarian et al. [207] have given a report about thermal and electrical performance assessments for T-PVCs by comparing some types of collectors. It is found that CPV systems integrated to a PTC can reach up to 70 and 25% as the overall thermal and electrical efficiencies, respectively.
Thermal performance analysis with thermal resistance modeling and thermal parameter characterization (performance curve) are the most frequently used methods in energy balance for a macroscopic approach. But CFD modeling is the most common methodology used for a microscopic approach. One-dimensional and steady-state heat transfer is the most common assumption made throughout the literature in modeling of the thermal behavior of PTCs. Unfortunately, there are no design criteria in the literature for thermal performance evaluation. However, the methods discussed here will be useful for performance analysis of new design or models of collectors.
Thermal resistance modeling consists of simulating the heat transfer phenomena (convection, conduction, and radiation) using thermal circuits, which are analogous to electric circuits. In this model, surfaces are represented by  Pempeintner et al.                2012 [195] x x x √ CFD simulation of a receiver used in direct SG plant. Superheated steam is the HTF. Surface temperature measurements are used as experimental validation Zaversky et al.
2012 [196] Probabilistic thermal model to obtain useful energy of a PTC system using Latin hypercube method Lobón et al.
2013 [197] x x x √ Thermal analysis of a PTC system for direct SG. Different conditions of pressure, temperature, incident radiation, and mass flow are analyzed. The method used is CFD simulation using k-ε turbulence model Silva et al.
2013 [198] x 2014 [175] x Experimental methodology to obtain the thermooptical performance of a PTC with outdoor tests Xu et al.
2014 [199] x Comparison of three experimental methods (ASHRAE 93, EN 12975-2 and a dynamic method presented by the authors) to determine thermal performance Biencinto et al.
2015 [200] x x x x Thermal performance analysis of a PTC system for direct SG. A quasi-dynamic method is used. It is based on finite-difference method in temporal thermal nodes, temperature differences correspond to voltage potentials, and heat transfer rates correspond to the currents. Figure 28 displays the thermal model used by Forristal to analyze a glass-covered receiver [184]; in which an energy balance is made at the outer and inner surfaces of the glass and the pipe of the receiver, which is represented by Eq. 1-4. This model used experimental correlations for convection heat transfer and theoretical equations for conduction and radiation. Salgado et al. [208] analyzed thermal performance of PTCs with a description of the mathematical models used in the literature.
Thermal parameter characterization uses mathematical models of thermal performance as expressed by standards, i.e., Eq. 5, which is the quasi-dynamic thermal model of standard ISO 9806-2017 for concentrating collectors with a glass-covered receiver and a concentrating factor (C r ) higher than 20 (usual for PTCs). The constants η peak , a 1 , a 3 , and a , 5 and the function ( ) K θ of the model are obtained by a regression analysis of experimental data. Each collector model has its own constants, so using this method it is not possible to design a system with a noncharacterized PTC. Characterized collector and receiver models (e.g., LS-2 or EuroTrough) are discussed in the literature, which make it possible to obtain the thermal performance constants. Table 17 gives specific thermal performance equations for a variety of PTC models and receivers available in the market. It is noticed that there are some mathematical models developed for thermal performance of PTCs, i.e., URSSA Trough uses a 4th-power polynomial steady-state model for thermal efficiency [175], SkyTrough uses a temperature-based cubic model in which the effect of the wind on thermal losses is included [177], and other collectors (LS-2, IST, CAPSOL, and EuroTrough) use a quadratic steady-state model. Figure 29 shows thermal efficiency curves of some large-sized PTCs based on experimental data reported in references presented in Table 17 (URSSA trough is shown using [175] under normal incidence and I b = 1,000 W/m 2 ). Differences in the thermal behavior of these PTCs are noticed.
Discretization, which consists of approximating partial differential equations using a system of algebraic equations, is the important aspect for microscopic analysis. The control volume of analysis (continuous domain) is divided into smaller volumes (discretized domain) and then the differential equations are solved for each small volume. Finite-difference analysis (FDA), finite volume analysis (FVA), and finite-element analysis (FEA) are the common methods used in microscopic analysis of thermal performance. FDA uses truncated series expansions for partial derivatives (usually a Taylor series) and a regular discretization of the domain. This method is very simple and easy to apply in simple geometric shapes. But it is not suitable for cases when many elements are required or a higher order in an expansion series is needed to increase accuracy. FVA and FEA are useful for irregular shapes of control volume domain or when elements of different sizes or shapes (mesh) need to be used.
As discussed in Section 2.4, nanofluids enhance thermal performance of collectors. Nevertheless, other methods increase heat transfer coefficient, and here the heat transfer coefficient is increased by using turbulators. A turbulator is an element used to create turbulence in the flow inside the receiver. In the literature, many studies are available about thermal enhancement using turbulators, and many of them are reported in Ref. [209][210][211][212].
The most common turbulators are listed as follows: Inserts are generally elements fixed to a smooth pipe. The objective of these elements is to enhance thermal performance by swirling the flow of the HTF. Generally twisted tapes are used as the most common inserts. Vortex generators are protrusions or dimples from the internal surface of the receiver. The flow adopts the shape of the protrusion, so turbulence is generated, enhancing thermal performance.
Baffles are a type of turbulator that deviate the flow of the HTF. The enhancement is made by interrupting both thermal and hydraulic boundary layers and forcing the flowing fluids to hit the internal surface of the pipe. Baffles can also be hollowed to reduce a high drop in pressure in the flow.
Fins are extended surfaces in the internal surface of the receiver. Fins enhance thermal performance by increasing the heat transfer surface area. They can also be twisted to generate a double effect on the swirled flow (i.e., wire coils).
The other important parameter affecting the thermal performance of PTCs is optical efficiency, which measures the quantity of energy in the receiver compared to the energy flux in the collection area. The optical efficiency of a collector (η op ) depends on the transmittance of the cover glass (τ cg ), the absorptivity of the SC (α sc ), the reflectance of the mirrors (ρ m ), and the incident angle modifier ( ( ) K θ ), as expressed in Eq. 6. It is known that the shape of mirrors and receiver alignments are not perfect in practical applications, so it also involves a random error. This error is measured by the interception factor (γ), which combines the effects of misalignments and slope random error in mirrors.
Ray-tracing techniques are employed to obtain an estimation of interception factor of a PTC. These methods simulate the propagation of light through media and surfaces with specific optical properties such as reflection, refraction, diffraction, and scattering. The most commonly used method is Monte Carlo ray tracing, which is based on probability distribution functions for predicting light ray paths. Photogrammetry is widely used for experimental measurement of interception factor. This method and other experimental methods were described by Arancibia-Bulnes et al. [17]. Osório et al. [213] compared different ray-tracing software available for linear focusing collectors, where the comparison of simulations was conducted for LFCs and PTCs. Section 2.1 addressed the guidelines and gave some hints about reflectance measurement. The measurement of absorptance and transmittance was addressed in Section 2.3. All the optical properties including the interception factor and the normal-incidence optical efficiency (when the incidence angle is 0°) are estimated.
Parabolic through solar collectors: A general overview of technology  631 The incidence angle modifier ( ) K θ takes into account the optical losses due to no-normal incidence during operation (such as end-effect losses). This factor is usually measured by experiments. The other important factor is the soiling factor, F c , which represents the ratio between the real-operation and nominal-clear mirror reflectance. During normal operation, cleaning the mirrors (washing) is a requirement for good performance, and this affects reflectance. For PTCs, the common values of soiling factor are between 0.95 and 1 [58].
Commercial software tools are available in the market to realize thermal, thermoeconomic, or optical analysis in PTCs. Software with PTC systems usually uses the thermal parameters of collectors for thermal analysis. Table 19 exhibits these tools and their characteristics and capabilities. Many software tools are available for CFD analysis (such as STAR-CD, Autodesk CFD, SimFlow, etc.), but Fluent and Comsol are the most popular CFD software tools for FVA and FEA, respectively.
It is important to estimate the losses when focusing on optimizing a specific parameter or strategy in maintenance, which can lead to a better performance of the PTC system. Forristal [184] reported about contribution of thermal and optical losses in a PTC using the mathematical model developed by him. He proved that the heat losses by conduction through the brackets can go up to 3% of the overall heat losses. Convective losses in the receiver by wind can go up to 12 and 105% in conditions of loss vacuum and broken glass, respectively. Incidence angle and reflectivity do have a large impact on performance. The report indicated that thermal performance can reduce around 15% at an incidence angle of 30°a nd 65% at an incidence angle of 60°. It was also found that a drop of 0.15 in reflectivity can lead to a decrement of 24.5% thermal performance, and an error of 5% in estimating mirror reflectivity is similar to a change of 7% in thermal performance.
Apart from the thermal and optical performance, it is important to analyze the structural performance. The design of supporting structure impacts not only performance but also the costs (see Section 4.2). Structural analysis is important to design a rigid structure, so that it can maintain a parabolic shape of the receivers and ensure that they are located in the focal line of the cylindrical parabola, as fabricated with the EuroTrough PTC [36]. Design criteria for structural analysis of PTCs were also proposed by Giannuzzi et al. [25]. They presented a methodology for estimating structural loads due to wind, snow, or temperature based on European standards for structural analysis.

Standards for performance evaluation of PTCs
Experiments are required to validate data from numerical models and provide information on the performance of PTCs under field operation conditions and for ensuring their reliability. Standards prescribe the requirements for good and reliable operation and also the performance measurement data for this method of testing. A number of standards are related to PTC technology, but a brief explanation of the important ones is presented in this study. Standards can be divided into three groups: standards for collectors (as one element), standards for systems (principally related with CSP generation), and standards for components and materials.

Standards for reliability and performance of collectors
These groups of standards provide guidelines to evaluate the reliability and/or performance by considering the collector as a separated element (unit), and they do not consider the subsystems or other components related to the final application of the collectors. Reliability tests simulate critical conditions that can severely damage a collector under normal operation. The requirements and conditions of the tests are stated in each standard. Performance tests are related specifically with thermal performance (and electrical performance, in the case of T-PVCs). Each standard specifies the procedures of the tests and instruments to be used (including their recommended quality). The principal difference among all the standards is the required tests and conditions used for testing. Fernández-García et al. [221] presented the development of a test loop for thermal performance evaluation of a small-sized PTC model and a comparison under different testing conditions (with experimental results) of standards applicable for thermal performance of PTCs. They report some experimental conditions (i.e., time sampling, or uncertainties of instrumentation) which are recommended for optimal thermal performance testing. Table 20 gives an idea about the recommended tests among the standards related to thermal performance of PTC. Each standard is explained in the following sections.

ASTM E-905
This standard describes a methodology for determining the thermal performance of collectors with tracking systems and concentrating factors greater than or equal to seven, so that  the effects of diffuse irradiance become insignificant. It can be applied to collectors with one-axis or full-tracking, singleinlet/single-outlet of single-phase fluid and in steady-state conditions. The standard does not include safety or durability. The latest update was made in 2013. This standard provides four tests for one-axis tracking concentrators: response time (to determine the time constant), incidence angle modifier, near-normal incidence angular range for determining heat gain, and heat gain near-normal incidence [222]. Two methods to determine the response time are by abruptly shading the collector and abruptly irradiating the collector. Procedures similar to ASHRAE 93 are presented for heat gain and ( ) K θ . The standard divides the tests for each type of collector evaluated (single axis or full tracking).

FSEC 102
The Florida solar energy center (FSEC) presented a standard for the evaluation of reliability and thermal performance of solar collectors. It also provides methodologies for reliability tests and thermal performance. It can be applied for both liquid and air heating collectors and also for concentrating and NCCs. Three types of water heating collectors are municipal water (called "street pressure solar collectors"), lowpressure service hot water/swimming pool, and hybrid/alternative fluid solar collectors. The pressure and connection to service water differ in all three types of liquid heating collectors.
This standard presents the minimum requirements for certification of collectors regarding its reliability. It is similar to SRCC standard 600 and also has the same minimum tests as SRCC 600, except for the impact and protection system test. The standard divides the analysis for thermal performance according to the fluid used. For all parameters (time constant, thermal efficiency, and ( ) K θ ), the procedures established in ASHRAE 93 are used in air heating collectors, and in liquid heating collectors the procedures are used according to ISO 9806 [223]. The last update of the standard was made in 2010.

ISO 9806
ISO 9806 is another important evaluation standard. This standard also provides a methodology for evaluating the reliability and thermal performance of the collector. It is applicable for nonconcentrating and concentrating solar collectors with a single-phase fluid (liquid or air) or hybrid-type (thermal-PV), single-inlet/single-outlet of single-phase HTF, and without storage as part of the collector [224]. This standard replaced EN 12975 to unify methods for evaluating thermal and durability in a single methodology [225]. This standard presents various methods for evaluating thermal performance, but concentrating collectors are evaluated using a quasi-dynamic method. The mathematical model is similar to the model used in EN 12975. In 2017, the last update was made to the standard.
This standard divides the test into two parts, namely, reliability tests and thermal performance tests. The reliability tests evaluate the durability and resistance of each part of the collector. Thirteen tests evaluate the potential failure modes of collectors, such as cracking, corrosion, buckling, and rain penetration. The key difference with other standards is the number of tests and the conditions stated for conducting tests, as shown in Ref. [226].
Further, this standard evaluates the same characteristics for thermal evaluation as ASHRAE 93 (which is withdrawn) but under different conditions and the time constant is optional. The procedures are similar to the standards mentioned earlier; the difference is the state of the conditions and measurements done during the test, as shown in Table 20 and Ref. [226]. This standard also presents a method for estimating the pressure drop across the collector. It also provides a data sheet for recording the test results.

SRCC standard 600
The SRCC standard 600 provides minimum requirements to certify a collector as safe, reliable, and efficient. It is applicable for concentrating collectors with active or passive controls and without thermal storage as part of the collector. Even flat-plate collectors with lenses or reflective panels are considered concentrating collector by this standard. It is also applicable for collectors with hybrid thermal-PV collectors. The last update of the standard was made in 2014.
SRCC standard 600 mainly references to ISO 9806 and EN 12975. It establishes that both durability and thermal performance tests should be performed as described in ISO 9806, but it only presents seven durability tests as the minimum criteria (plus protection system test) [227]. The thermal procedures required by the standards are based on the quasi-dynamic model, but the main differences are the wind speed and type of testing done after the exposure test. For T-PVCs, the performance is divided into electrical and thermal performance. Electrical performance test is based on International Electrotechnical Commission (IEC) 61925 with open circuit.

IEC 62108
This standard establishes the minimum requirements for the qualification of a concentrating PV module with crystalline silicon solar cells for terrestrial use. This standard evaluates the thermal, mechanical, and electrical reliability of a concentrator in long-exposure climates. The methodology of analysis involves the testing sequence for characterization and after that the collector is tested under aging conditions. Finally, it is again characterized by the testing sequence to determine degradation. The sequence of the tests is similar to the procedures described in IEC 61215 (standard for flat-plate PV collectors) but features of CPV receivers such as tracking alignment, high current density, and rapid temperature change are also considered. For example, the thermal cycling test consists of analyzing change in temperature gradient of the receivers and verifying whether it can withstand large temperature gradients due to variable incident radiation flux that occurs during normal operation (cloudy days) [228]. It is not exclusive for PTC but may be generally used for any type of concentrator with a PV or thermal-PV output [229]. The latest update was made in 2007, but now a revised version of this standard is available [230].

Standards for solar thermal systems with PTCs
Contrary to the above standards, this standard provides guidelines to evaluate the performance of a solar thermal system driven by PTCs as a unit. They are applicable in power generation, so the tests consider not only the solar field but also the TES system or thermal-to-electrical conversion systems. A brief explanation of the principal standards related to the CSP plant performance are presented in the following sections.  [232]. It also gives general loading/unloading procedures for characterization of TES. Furthermore, the standard provides quality requirements for the instruments used for measuring and data processing methods. -UNE 206014: This standard gives procedures to test the solar system of a PTC-based CSP generation system under quasi-stationary conditions [233]. Information about instrumentation and measurement techniques of available solar irradiant energy and solar field net thermal energy are given in the standard. This standard is applicable for CSP plants with solar fields where the solar energy is its principal energy source. Other equipment of the power plant is not at all considered by this standard.

ASME PTC 52
The American Society of Mechanical Engineers (ASME) is developing a Performance Test Code standard 52 (PTC 52) to evaluate the performance of CSP plants. The standard will provide procedures and methods to evaluate only thermal performance (solar-to-thermal conversion) of the solar field, considering only concentrating technology (it is not exclusive for PTCs, like UNE standard) [24]. The first version of this standard will be available soon, and it is expected to replace NREL Guidelines for evaluating the CSP plants.
The main driver of this standard is the expected improvements in baseline in evaluating and comparing different technologies and projects. "The code should simplify the tendering process for CSP projects by standardizing testing methodologies and acceptance requirements among power companies, financial institutions, developers, and contractors" [234]. The key benefits of this standard to CSP technology will be the improved bankability, expected wider participation, and longer life cycle.

ASME PTC 46
This standard provides test methods and procedures to determine thermal performance and net electrical output of the overall power plant. It is applicable to any power cycle that uses heat as the energy resource and so it can also be applied to CSP plants. In fact, it is used to test the conventional fossil-fueled power plants, including cogeneration systems. The standard does not consider components separately for the analysis, but considers the whole power plant as one element. However, thermoeconomic analysis or comparing with other technologies is not included in this standard. The power plant can be of any size including the parameters, using a specific configuration, operating disposition, fixed power level, and at reference conditions. The procedures are employed to determine corrected net power, corrected heat rate (efficiency), and corrected heat input [235]. Performance (thermal and electrical) evaluations are based on steady-state models, and tests are conducted for a short-term duration (at least 24 h) due to the thermal stability requirements of TES systems [24]. Performance-related issue tests, such as emission, operational demonstration, and reliability, are out of the scope of this standard.

IEC TC 117
The committee TC 117 from IEC is working to develop standards for solar electric plants driven by solar concentrating technologies [231,236]. These standards will provide specifications for the test collectors to be used in CSP plants. The developing standards related to PTC technology are listed as follows.
-IEC 62862-3-1 provides general requirements for the design of parabolic trough solar thermal electric plants. -IEC 62862-3-2 provides general requirements and test methods for large-size PTC. This standard is pending for publication [237]. This standard will provide guidelines for evaluating optical performance, thermal performance, and tracking accuracy of large-size PTCs under outdoor conditions with single-phase HTFs. -IEC TS 62862-3-3 provides general requirements for systems and components and general requirements and test methods for solar receivers.
Every solar concentrating technology is considered in this standard, including tests for TES systems. The standard defines design, safety, power quality issues, and installation requirements and performance test methods [236][237][238]. It analyzes not only the collector field but also the subsystems and components of a CSP plant.

NREL guidelines
In 2011, NREL published performance acceptance test guidelines as a standard to evaluate the performance of solar plants due to a lack of standards at that time. These guidelines initially focused on PTC technology. In 2013, similar guidelines were presented for ST power plants. The guidelines only evaluate the solar-to-thermal conversion system (solar field and HTF subsystems) excluding TES system, heat exchangers, and the power block. The guideline recommends ASME standards for other subsystems and elements. The main objectives of the test are determining solar thermal power output and thermal efficiency, comparing the measured parameters with projected parameters, monitoring inlet and outlet temperature of the HTF, and measuring parasitic power consumption [239].
Two types of tests mentioned in this standard are the following: (a) Short-time steady-state test is conducted under clear sky conditions during a short time when the thermal steady-state conditions exist. (b) Multiday continuous energy test is conducted for a long-term period to gather thermal energy output during continuous operation. Clear sky and partly cloudy days are considered for this test. The functional performance of the solar field should be considered, from the start-up, to normal operation and till shutdown. The minimum recommended period is 10 days in a continuous operation.

Quality standards for materials and components
To fabricate a collector with high thermal and mechanical quality, qualified materials should be selected by a standard based on their function. The main structural steel grades for mechanical parts of the structural support are ASTM A36 and ASTM A588 for steel beams and plates and ASTM A53 for structural pipe. The most commonly used materials for the receiver are stainless steel (AISI 316L and AISI 304L) and copper (ASTM B42) pipes (used in some prototypes [240]). All these standards provide required properties of materials for their effective function under normal operation. There is a standard for evaluating the quality of optical properties of materials (e.g., mirrors). The ASTM E-903 standard describes the methodology for determining and measuring the optical properties (transmittance, reflectance, and absorptance) of the materials used, and it is applicable for materials with diffuse and specular optical properties. The main standards applied to concentrating solar technology are the SolarPACES guidelines [19] for reflectance measurement (mirrors) and ASTM standard G-173-03 Section 7.2 for solar-spectralweighted transmittance and absorptance measurement (cover glass and SC). IEC TS 62727-5 is used for evaluating tracking systems. Important features of these standards are discussed subsequently. Sallaberry et al. [241] made a compilation of other quality standards related to solar concentrating collectors, including standards for qualifying thermal performance and durability and standards related to the quality of HTFs.

AENOR UNE standards for quality of HTFs and reflectors
Two UNE standards characterize the properties of materials of PTC technology used in CSP plants [242]. UNE 206015 determines the physicochemical properties and measurement techniques for characterization of HTFs (mostly organic synthetic) used in in-service plants. Further UNE 206016 determines test methods and minimum requirements for performance of reflectors used in concentrating technologies, and it is applicable for any kind of mirror used in the CSP technology.

ASTM G-173-03
This standard presents a compilation of terrestrial solar spectral irradiance distribution data. It is basically a reference of both direct and diffuse hemispherical solar spectral irradiance in the wavelength range of 280-4,000 nm. It is applied at a reference angle of 37°tilted surface, which is applicable for most part of the United States [243]. The data represent clear sky conditions and applied for determining relative optical performance of materials used in solar energy conversion. Section 7.2 of this standard illustrates how to obtain solar spectral weighted averages of wavelengthdependent properties (such as reflectance, absorptance, or transmittance) based on solar spectral irradiance given in the tables as described previously.

ISO 9845-1
Standard ISO 9845-1 provides modeled direct-normal and hemispherical solar spectral irradiance distribution to determine solar-weighted properties. The data can be applied for 5.8°field-of-view angle (for direct irradiance) at a 37°tilted surface, air mass of 1.5, an albedo of 0.2, and clear sky conditions [244]. The irradiance data are given in a wavelength range between 305 and 4045 nm. Similar to ASTM G173, Section 3 of standard ISO 9845-1 provides methods to evaluate solar-weighted properties for absorptance, reflectance, and transmittance.

ISO 9050
This standard provides guidelines and a methodology for determining light and energy transmittance of glasses used in building and solar applications. It is applicable for nearnormal incidence (irradiated beam should not exceed 10°f rom the normal direction of the surface) [245]. Section 3.5 describes the formulation for solar transmittance, reflectance, and absorptance of glasses. Standard ISO 9050 reference standard ISO 9845-1 for solar spectral irradiance, where spectral data with wavelength between 300 and 2,500 nm are used for obtaining solar properties.

IEC TS 62727-5
This IEC standard provides guidelines and specifications for evaluating a solar tracking system (single tracking or full tracking) and recommendations for measurement techniques to be used in measuring critical tracking-related parameters. It is applicable to PV systems but may be used for other solar applications. It evaluates tracking system's accuracy, functionality, and mechanical performance. Some tests include a pointing error test, data analysis (collection, filtering, and calculation), corrosion test, and durability test [246]. In 2012, the latest update was made.

SolarPACES Guidelines for reflectance measurement
These guidelines provide a methodology for characterizing reflective materials used in solar concentrating technologies [18]. It is also applicable for comparing reflectance characterization of different materials but not used for simulation like ray tracing. The main measurements that are considered in the guidelines are hemispherical reflectance, specular reflectance, solar-weighted reflectance, UV-weighted reflectance, and soiling. The guidelines specify the evaluation of the reflectance of the material in the wavelength range of 280-2,500 nm for solar-weighted reflectance. Solar-weighted reflectance is obtained in accordance with ASTM G173-03 Section 7.2. These guidelines also give recommendations regarding the measuring process to be used for reflectance and the requirements for instrumentation. A draft methodology for measuring reflectance losses of aged/soiled mirrors is included in the latest version [18].

Standards for terminology of solar technology
Standards for terminology define the terms and concepts used in solar energy standards. They may include terms for both thermal and PV applications. The usual concepts included in these standards are related to solar geometry, components of solar conversion system, instrumentation used to measure solar radiation, solar properties of materials, and definition of different types of solar collectors. The most used standards for solar terminology are listed below.
-ASTM E772 (standard terminology of solar energy conversion) and ISO 9488 (solar energy: vocabulary): These standards provide the definitions of terms used in general solar thermal/PV applications. These standards are referenced in thermal performance testing (ASTM E905 and ISO 9806 respectively) as "vocabulary." -IEC 62862-1-1 (solar thermal electric plants -Part 1: terminology) and UNE 206009 (solar thermal electric plants. Terminology): The main objective of these standards is defining all terms related to solar thermal electric plants, which means that it is mostly applicable to solar concentrating technology.

Discussion
Current status of PTC technology is presented in this review. The materials used, physical properties, relevant information about the reliability of each component, and the design aspects of collectors are discussed in detail. Industrial applications, common processes and methods used in energy conversion, and the details of the application of the collectors are also presented. A general economic and market overviews were described, emphasizing the impact of this technology on the energy market and its economic status and trends. An exposition of methods to evaluate the performance of PTC systems, describing mathematical and experimental methods for testing thermal and optical performance are also presented and analyzed. And finally, the principal standards used to test the performance of collector, with a brief explanation of the approaches and methods used in each standard are described.
Four main ideas stand out in our present analysis. First, it is important to mention the relevance and potential of this technology in the energy market and in industry. The principal highlights are the impact that the PTC systems have in electricity generation using solar resources, their applicability in general-purpose thermal applications, and the potential use in nonthermal industry applications. It is also important to mention the integration of solar systems into conventional power and thermal systems to mitigate GHG emissions. Second, it is important to mention about the influence of materials on cost analysis. Further research should be done to develop cost-effective materials to reduce manufacturing and installation costs, so that this technology is cost competitive with conventional fossil fuel technologies. Third, the impact of PTC system design and evaluation is important. The chronological list of studies presented in this review shows that the implementation and integration of mathematical modeling, experimental validation, and computational tools improve the performance analysis of energy systems. The implementation of software tools benefits the design process, making it possible to quickly and more precisely evaluate a collector than with experiments. Finally, it is important to establish measurement and performance evaluation processes for PTC systems, so that it is possible to compare different collector or system designs and to standardize the processes for energy conversion.

Conclusions
From the analysis presented on this review of the PTC, the following conclusions can be drawn: 1. A general overview of the technology was presented.
The different aspects of functionality, applicability, and development of PTC technology were discussed from an industrial approach. 2. Even though this technology is mature, still research has to be done to improve its components. Major research efforts must be focused on mirrors and thermal enhancement (with nanofluids or inserts). Other improvements may be changing the design of the receiver, like a rotating receiver proposed by Norouzi et al. [247], or a V-shape receiver proposed by Rafiei et al. [248]. 3. The most developed applications of PTC technology are CSP plants (power generation) and SHIPs. Further, the promising areas of applicability of this technology are desalination or concentrating PVs. 4. In recent years, this technology has improved its cost of energy. The general trend of LCOE for a CSP plant is downward, making it more competitive as an alternative to provide thermal energy for power generation instead of using fossil fuels in the following years. 5. The development on modeling and testing has increased in the last decade. Designing tools and standards applicable to industrial systems has played an important role in improving the quality and applicability of this technology in industry.

Future work
The authors believe it is relevant to mention three main ideas for future work considering our analysis of PTC technology. The first is a proposal to realize design criteria for PTC systems due to a lack of information about normalized processes in designing and evaluating the performance of a system. It was shown in this review that standardization of thermal performance for collectors and CSP plants is currently established or will be available soon. But performance and reliability tests for complex systems or solar collector fields integrated with another industrial application is minimum or not existing (e.g., desalination or photocatalytic decontamination systems using PTCs). The second idea is that it is also important to report the impact, benefits, and drawbacks of PTC technology in applications where this technology can be implemented but has a low penetration in market. Some potential applications are described, but most of the reports mentioned are from laboratory-scale or low-scale systems. The theoretical analysis reports mentioned can be a premise to be implemented for testing systems under real conditions. As mentioned in Section 4.2, SHIPs using PTC technology have increased in recent years due to the improvements in technology and gained experience. It may be possible to follow the same way for other industrial applications as shown in this review. The third idea is to develop computational software tools for performance analysis, especially for designing new collectors or systems as most of the software tools available realize performance analysis based on tested large-aperture PTC designs. The implementation of these kinds of computational tools can help to decrease time and resources needed to design collectors, which can lead to a reduction in manufacturing costs. For example, a computational tool that can simulate a solar field using nontested small-aperture PTCs under transient conditions can be applied for estimation of thermal net output or thermoeconomic analysis in SHIPs applications or mediumscale DSG plant. R&D centers, like NREL, are working on this simulation tools, but they are focused on heating applications with large-aperture PTC and mostly for CSP plants. IEA SHC Task 49 Subtask B reported about the integrability of solar energy systems to industrial heating processes using software tools, concluding that the introduction of solar systems simulation interfaces into process tools can enhance the integrability of both solar energy and heating processes [249].