17/12/2021
Parabolic-trough concentrating solar power (CSP) systems
E. Zarza Moya, in Concentrating Solar Power Technology, 2012
7.2.3 Receivers
The typical PTC receiver tube is in fact composed of two concentric pipes, an inner steel pipe containing the working fluid and an outer glass tube surrounding the steel pipe. The glass tube is made of low-iron borosilicate glass to increase its transmittance for solar radiation. The outer surface of the steel pipe has an optically selective surface with a high solar absorptance and low emittance for thermally generated infra-red radiation. The principles of such surfaces are discussed in detail in Chapter 15. The glass tube is usually provided with an anti-reflective coating to achieve a higher solar transmittance and better annual performance.
Receivers for parabolic-trough collectors can be classified as either evacuated or non-evacuated. Evacuated receivers are commonly used for temperatures above 300 °C because they have a high vacuum (i.e., 10− 5 mbar) between the steel pipe and the glass cover, thus reducing thermal losses and increasing the overall efficiency of the PTC, especially at higher operating temperatures. Figure 7.9 shows a typical evacuated receiver. The glass cover of these receivers is connected to the steel pipe by means of stainless steel expansion bellows which not only compensate for the different thermal expansion of glass and steel when the receiver tube is working at nominal temperature, but also provide a tight annular gap between both tubes to make the vacuum. One end of these expansion bellows is directly welded to the outer surface of the steel pipe, while the other end is connected to the end of the glass cover by means of a glass-to-metal welding. Shown in Fig. 7.9 are chemical ‘getters’ placed in the gap between the steel receiver pipe and the glass cover to absorb gas molecules passing from the fluid to the annulus through the steel pipe wall. Since the evacuated receivers are expensive (about 850 €/unit in 2010) due to their technical complexity, they are used only for higher temperatures, when good thermal efficiency is required and the high cost is compensated by a higher thermal output.
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Figure 7.9. A typical evacuated receiver for parabolic-trough collectors.
At the end of 2010, there were only three manufacturers of evacuated PTC receivers: Schott, Siemens and ASE. Most of the parabolic-trough solar thermal power plants implemented around the world until 2009 had receivers manufactured by either the Israeli company, Solel (purchased in 2009 by Siemens, www.energy.siemens.com), or the German company, Schott (www.schottsolar.com). In 2009, a third manufacturer, the Italian company, Archimede Solar Energy (ASE, www.archimedesolaenergy.com), announced that they were launching a new receiver tube called HEMS08, suitable for fluids up to 550 °C. The first plant using HEMS08 receivers was the Archimede Plant, located in Syracuse (Italy) and ready to operate in 2010 using molten salt (a mixture of sodium and potassium nitrate) as the receiver working fluid.
Figure 7.9 shows how these three manufacturers join the glass cover and the inner steel pipe by means of flexible bellows. The glass-to-metal welding used to connect the glass cover to the flexible bellows is a weak point in the receiver tube and has to be protected from the concentrated solar radiation to avoid high thermal and mechanical stress that could cause the welding to crack. An aluminum shield is therefore usually placed over the flexible bellows to protect the welding. Table 7.5 shows the technical parameters of the receivers manufactured by the Schott, Siemens and ASE companies.
Table 7.5. Technical parameters of the receivers commercialized by Schott, Siemens and ASE
Schott PTR-70 Siemens UVAC-2010 ASE HEMS08
Solar absorptance > 0.95 > 0.96 > 0.95
Solar transmittance > 0.96 > 0.96 n.a.
Thermal emittance < 0.1 at 400 °C < 0.09 at 400 °C < 0.1 at 400 °C < 0.14 at 580 °C
Steel pipe inner/outer diameters 70/65 mm stainless steel 70/65 mm stainless steel 70/65 mm stainless steel
Thermal losses 250 W/m at 400 °C n.a. 230 W/m at 400 °C
Glass cover Borosilicate Borosilicate Borosilicate
Active length ratio at 350 °C > 96% 96.4% n.a.
Maximum fluid temperature 400 °C 400 °C 550 °C
Non-evacuated receivers are suitable for applications with a working temperature below 300 °C, because thermal losses are not so critical at these temperatures. Although non-evacuated receivers are also composed of an inner steel pipe and a glass cover, they have neither vacuum between the steel pipe and its glass cover nor glass-to-metal welds. Selective coatings used for non-evacuated receivers are simpler than those used for evacuated receivers. Black-chrome or black nickel coatings are commonly used because they are cheap and easy to produce.
Due to manufacturing constraints, maximum receiver tube length is usually less than 5 m, so they are connected in series up to the total length of the PTC. Evacuated receivers are usually welded, while non-evacuated receivers are usually connected by special threaded joints.
A new generation of absorber tubes for concentrating solar thermal (CST) systems
A. Morales, G. San Vicente, in Advances in Concentrating Solar Thermal Research and Technology, 2017
4.2.2 AR coating
The glass jackets of receiver tubes are coated by a film on both sides (inner and outer) to reduce the reflection losses in the glass, thereby increasing the optical efficiency of the receiver tube. This film, known as ARC has to satisfy two requeriments (related to coating thickness, and refractive index value) to obtain the destructive interference of light which is reflected at the glass–coating interface and at the coating–air interface. These values come from solving Fresnel's law equations for normal incidence and for a determined wavelength. The wavelength value selected is usually around 600 nm for solar applications, as this wavelength value is centered in the maximum irradiance zone of solar spectrum. In this way, the optimal ARC must have a thickness of about 150 nm and a refractive index value of 1.22.
The material most widely used as ARC on glass is silicon dioxide (SiO2), and the low refractive index value is achieved by introducing porosity in the coating. This porous nature, necessary to increase the glass transmittance, is the weak spot of this material in terms of durability. These pores easily absorb water and other volatile organic components, increasing the refractive index of the coating and lowering the transmittance. This process, mostly reversible, is known as “breathing of the coating” [26]. Another consequence of the porous structure is the weak mechanical performance coming from the weak binding force between the silica particles and substrate, as well as between the particles [19].
The technology most widely used for preparing the ARC on glass receiver tubes is sol–gel dip-coating method. It allows to coat both sides of the tubes at the same time, and it is a low-cost technology and is easy to scale up. The porous silica coatings can be produced by two routes: from colloidal solutions (obtained by basic catalysis of metal alkoxides or commercial SiO2 colloidal dispersion), which is the route used by Schott [36], or from polymeric solutions (obtained by acid catalysis of metal alkoxides) where the porous structure is achieved by adding a compound to the solution which is removed during a heat treatment, leaving pores in its place [23]. This is the route used by Archimede Solar. The first one produces coatings with excellent optical properties but with poor mechanical properties, and improvement methods have been developed to get better coating adhesion on the glass and better mechanical properties [16, 36]. The second route produces coatings with better mechanical properties, although these coatings can undergo the same process of water adsorption in the pores or quick soiling due to the porous structure.
At the end, the big challenge in the ARC is to solve the contradiction of the porous structure between low refractive index and high stability, and future trends and developments go toward this goal.
The modification of the porosity type in the silica coating can increase the durability of the ARC. For example, DSM has developed a porous silica ARC coating for photovoltaic (PV) glass covers, with close surface porosity and strong binding with the glass that increase the coating durability [7]. Other important development work is referred to develop multifunctional coatings that work not only as ARC but also as easy-to-clean or self-cleaning coating. In this way, they will increase the receiver efficiency and moreover they will minimize the costs associated to clean receivers in solar plants. The soiling of glass tube receivers in solar plants decreases significantly the optical efficiency. Moreover, the porous nature of the ARC makes it more liable to soiling than bare glass or mirrors. There are two strategies for obtaining these easy-to-clean or self-cleaning surfaces: applying a treatment or coating that makes the surface hydrophobic and applying a photocatalytic coating that produces hydrophilic surface [20]. The mechanisms that take place in both strategies are different but both give place to surface that avoid the soiling of the coatings by any cause. The key points of these multifunctional coatings are the stability of them and, overall, to producing anti-soiling behavior without decreasing the transmittance. Future researches have to focus on these restrictions.
Fundamental principles of concentrating solar power (CSP) systems
K. Lovegrove, J. Pye, in Concentrating Solar Power Technology, 2012
Limits for cylindrical and spherical receivers
Another possibility to consider is using a receiver with a circular cross-section as shown in Fig. 2.9. In this case, the diameter of the target needs to be
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2.9. Concentrating solar radiation with a perfect parabolic mirror to a circular cross-section target.
[2.29]
For a trough with cylindrical receiver, then
[2.30]
Solving for maximum geometric concentration ratio as before, the optimal trough rim angle is ϕ = 90°, and that at this angle, gives
[2.31]
Trough concentrators with cylindrical evacuated-tube receivers consequently employ rim angles approaching12 ϕ = 90°. For a dish with a spherical receiver, likewise,
[2.32]
and again the maximum geometric concentration ratio occurs at ϕ = 90°, with
[2.33]
For troughs then, the optical analysis gives a limit that is equal to the thermodynamic limit divided by π; for dishes, the result is equal to one quarter of the thermodynamic limit.
Note that this analysis is of geometric concentration ratio, and the above derivation indicates that the local contributions to focal spot size vary as a function of reflection radius x, so the geometric concentration ratio limits are less than the optical concentration ratio limits at the centre of the focal spot.
Rabl (1976) has further results giving the mirror area per aperture area for these four different collector configurations. Optimally sized troughs and dishes with flat receivers require less mirror for a given aperture than those with cylindrical/spherical receivers, since on average the glass is reflecting at closer to a normal angle.
Design of the Receiver System
Zhifeng Wang, in Design of Solar Thermal Power Plants, 2019
5.7.6.5 Hydrogen Permeation in the Corrugated Tube
Once the getter inside the parabolic trough receiver tube is saturated, hydrogen permeating into the receiver tube will accumulate there, and partial hydrogen escapes to the atmosphere through the corrugated tube. Because the partial pressure of hydrogen in the atmosphere is quite small, the partial pressure inside the atmosphere is nearly zero in this section. The permeation rate of hydrogen passing through the corrugated tube is:
(5.31)
in which Jann-air refers to the rate of hydrogen escaping from the corrugated tube into the atmosphere, mol/s; Abel refers to the permeation area of hydrogen passing through the corrugated tube, m2; Φbel refers to the hydrogen-permeation coefficient of the corrugated tube, mol/(m·s·MPa0.5); and lbel refers to the thickness of the corrugated tube, m.
The internal diameter of the receiver tube’s corrugated tube used in this section is about 80 mm, and the external diameter is 120 mm; each corrugated tube has five waves, and the wall thickness of the corrugated tube is 0.2 mm. Thus, total permeation area of corrugated tubes of the parabolic trough receiver tube is 0.1256 m2. In this section, the hydrogen-permeation coefficient of the corrugated tube is the same as that of pure stainless steel SS304, namely Φbel = Φs.
Once the getter inside the receiver tube is saturated, the amount of accumulated hydrogen inside the receiver tube is:
(5.32)
Introduction
Zhifeng Wang, in Design of Solar Thermal Power Plants, 2019
1.2.2.2 Parabolic Trough Solar Power Generation
Parabolic trough solar power generation (Fig. 1.9) is a technology that concentrates solar irradiation in the receiver tube mounted at the focal line of the paraboloid through linear parabolic mirrors that track the movement of the sun and thermal the heat transfer liquid for power generation. Key equipment of a parabolic trough power plant mainly includes a concentrator, a receiver tube, and thermal storage. The parabolic trough power plant is the first (1980s) thermal power generation technology to realize commercial operation, with a maximum power plant capacity of up to 80 MW while still ensuring stable operation. Certain problems with the parabolic trough power generation technology are the low concentration ratio of the paraboloid mirror (70–80), difficulty raising the working temperature of the heat transfer liquid, and restrained system efficiency.
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Figure 1.9. Parabolic trough solar power generation.
Picture provided by the Institute of Electrical Engineering, Chinese Academy of Sciences, 2017.
As shown in Fig. 1.10, a parabolic trough solar power collector consists of a parabolic trough concentrator that tracks the movement of the sun and a receiver tube mounted at the focal point of the paraboloid. The parabolic trough concentrator uses a single-axis tracking concentrator, namely a concentrator with a mirror element revolving in a one-dimensional manner by surrounding a single axis to track the movement of the sun. The surface of the parabolic mirror is the trajectory formed by a line moving along a certain parabolic curve while parallel to the fixed line. Thus with a parabolic trough concentrator that tracks the movement of the sun, DNI is constantly concentrated on the surface of the receiver tube and creates a focal line so that the heat transfer liquid inside the receiver tube can be heated. High-temperature and high-pressure steam is then generated directly or through an oil–water heat exchange system in order to participate the thermal cycle power generation system and drive the steam turbine to function and generate power or provide the requested steam for industrial processes. The heat transfer liquid of the system transfers thermal energy and is normally water/steam, synthetic oil, or molten salt.
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Figure 1.10. Structural diagram of parabolic trough solar collector.
The parabolic trough concentrator is a key component that receives and reflects solar radiation and consists of the base, bracket, mirror, power machine, transmission system, and control system. A typical parabolic trough concentrator is made up of multiple units connected in series along the axis and equipped with a power, transmission, and control system. Normally for a parabolic trough concentrator with small radiation areas, hydraulic or mechanical transmission can be applied; for one with large radiation areas, only hydraulic transmission can be applied.
A bracket is connected to the mirror through fixtures to support and ensure the stability of the parabolic mirror surface; its structure can be categorized as torque tube, torque box, and space truss types; the materials are normally metals, such as steel or aluminum products, and the processing pattern is mainly welding and punching.
Structures of the mirror can be categorized as single-layer or composite. The single-layer structure is an ultraclear glass hot-bending parabolic trough surface that is coated with silver, whereas the composite structure consists of a backboard and adhesive and reflective materials. The backboard functions to create a parabolic surface, and it can be made from steel plate, aluminum plate, float glass, and fiberglass. Reflection materials can be thin glass mirror, metal film, or firm composite materials. Adhesive materials can be PVB, neutral organic silicone, etc., in which the aluminum reflector has a high reflection rate created with an aluminum plate through the use of surface finishing and oxidation protection. A silver-coated polymer mirror is a reflection surface with a high reflection rate that is created by coating with silver on one side of the high-transmittance, strong weather-resistance polymer film; it is equipped with multiple layers of protective film that are attached to the bottom of the curved surface to create a curved mirror.
As shown in Fig. 1.11, the receiver tube of the parabolic trough solar collector is a core component of the parabolic trough collector and is typically about 4 m long. The interior tube is a commercial-type metal receiver tube with an external diameter of 70 mm, whereas the exterior tube is a glazed shield tube with an external diameter that falls in the range of 115–125 mm. Due to the metal receiver tube and glazed shield tube having different coefficients of expansion and thermal intensities during operation, high-temperature-resistant glass and metal sealing pieces are required as transition pieces to ensure an airtight connection. In addition, a metal corrugated pipe is used as the thermal stress buffer section to relieve the longitudinal thermal expansion difference between the metal receiver tube and the glazed shield tube. To ensure degree of vacuum degree in the vacuum interlayers of the receiver tube, a getter must be mounted between the metal receiver tube and the glazed shield tube. Furthermore, with any focusing solar irradiation, the seal undertakes great thermal stress that may easily invalidate the sealing of glass and metal. Therefore, thin-walled materials with good reflection performance are required as a solar shade to block radiation while reflecting it to the metal receiver tube.
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Figure 1.11. Structural diagram of receiver tube of parabolic trough solar collector.
Both the thermal properties and life of the parabolic trough receiver tube are determined by the vacuum degree of the vacuum interlayer. If the vacuum environment is damaged, not only will the respective heat losses rapidly increase, but also the selective receiver film of the metal receiver tube surface will deteriorate due to oxidation, which may result in severe reduction of the receiver tube's optical efficiency. Under the special working conditions of high temperature and strong radiation, CSP performance and vacuum life can only be ensured when the materials and properties of these components satisfy certain requirements:
1.
Glazed shield tube. Due to day–night alternation and temporary cloud occlusions, alternating stress may be generated at the seal, which thus requires high hardness and thermal stability as well as corrosion resistance. Materials that are widely applied at the present include borosilicate glasses such as Pyrex glass, the expansion coefficient of which is 3.3 × 10−6/K while featuring high hardness, good optical properties, and acid and alkali corrosion resistance. It also has the disadvantages of having no corresponding sealing metal, and its softening temperature approximates 820°C, and thus the temperature is extremely high during sealing operations.
2.
Metal receiver tube. The temperature of the metal receiver tube under concentration effect will be much higher than 400°C. Thus it is necessary that it is equipped with high-temperature and corrosion resistance. To eliminate the influences of axial expansion on the collector bracket, the expansion coefficient shall be as small as possible. Due to thermal and gravitational influences, downward deflection may occur, so there must be a sufficient distance between the exterior wall and the interior wall of the glass tube. Currently, high-temperature-resistant 316L stainless steel is normally used with an external diameter of 70 mm, a wall thickness of 3–5.5 mm, a standard length of 4060 mm, and a mean roughness of less than 0.5 μm.
3.
Glass-metal sealing transition piece. A certain sealing alloy is applied to solve the inconsistency of the expansion coefficients of the interior metal tube and exterior glass tube. Therefore, both expansion coefficients shall be as close to each other in value as possible in order to satisfy matched sealing and easier welding to the corrugated pipe.
4.
Thermal stress buffer section. This buffer is required in order to compensate the expansions of the metal receiver tube and the glazed shield tube. Thus it is necessary that is has good flexibility, excellent tension fatigue strength and life, high-temperature resistance, and acid and alkali corrosion resistance. The respective length shall be as short as possible to increase the effective concentration length of the receiver tube.
5.
Getter. A getter is used to absorb the residual gases in the vacuum interlayer after sealing and the released gases of components under high-temperature working status to ensure a satisfactory vacuum state. A getter accomplishes the target of absorbing residual gases by mainly by relying on physical and chemical absorption.
6.
Selective absorption film. According to its working mechanism, it can be categorized as optical interference, intrinsic absorption, metal ceramic, or multilayered gradient film. As a general requirement, for temperatures below 400°C, its absorptivity shall be not less than 95%, and its reflectivity shall be less than 14%. The most widely applied selective absorption film is composite material absorption film, including multilayered gradient metal ceramic film and double-layered absorption film. The multilayered gradient metal ceramic film has a metal substrate, and the absorption layer is made of metal and dielectric gradient film, whereas double-layered absorption film creates two absorption layers and one or two dielectric antireflection layer(s) on the high-reflectance metal substrate to achieve low reflectance without reducing the absorption rate.
As shown in Fig. 1.12, in 2017, a 9000-m2 parabolic trough solar collector was completed at the Beijing Badaling CSP experimental base. The collector was arranged horizontally along the 3000-m2 north–south axis and 6000-m2 west–east axis with a tracking length of 300 m. The bracket was mounted by applying a torque tube structure and selecting an independently developed sandwich-structure glass mirror, the technical parameters of which are shown in Table 1.3.
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Figure 1.12. Beijing parabolic trough solar power collector.
Provided by the Institute of Electrical Engineering, Chinese Academy of Sciences, 2017.
Table 1.3. Parameters for Parabolic Trough Solar Power Collector
Item Parameters Item Parameters
Total area/m2 9000 Glazed shield tube wall thickness/mm 3
Aperture area/m2 9000 Single-piece receiver tube length/mm 4060
Aperture width/m 5.76 Total length of receiver tube/mm 97,440
Focal length/mm 1.71 Total length of collector/m 1500
Glass thickness of mirror/mm 4 Tracking precision/(°) ±0.1
Thickness of glass mirror/mm 3.2 Maximum operating temperature/°C 400
Exterior diameter of metal receiver tube/mm 70 Maximum operating pressure/MPa 1.6
Exterior diameter of glazed shield tube/mm 120 Tracking axis direction 3000 m2 north–south
6000 m2 west–east
Provided by the Institute of Electrical Engineering, Chinese Academy of Sciences, 2017.
As shown in Fig. 1.13, the collector is of torque tube-type, the support arm is made from rectangular steel pipe by welding, and the parabolic mirror is made by gluing together the hot-bending glass paraboloid and the ultrathin glass mirror. A parabolic trough solar power collector contains 24 pieces of vacuum receiver tubes; the metal absorber pipe inside the cover glass tube is made from 316L stainless steel with a high-temperature-resistant metal ceramic selective absorption coating on the exterior surface. The thermal stress buffer section is made from stainless steel corrugated pipes, the displacement of which is calculated based on the thermal expansion differences generated by the metal receiver tube at 450°C and the glazed shield tube at 0°C when the length is 4 m. Heat transfer oil is used as the heat transfer fluid inside the receiver tube, the type of which is selected according to minimum ambient air temperatures in different seasons: Dowtherm A by Dow Chemical is used, the main ingredients of which are diphenyl and diphenyl ether.
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Figure 1.13. Structural diagram of parabolic trough collector.
Provided by the Institute of Electrical Engineering, Chinese Academy of Sciences, 2010.
Introduction to concentrating solar power technology
Keith Lovegrove, Wes Stein, in Concentrating Solar Power Technology (Second Edition), 2021
1.2.1 Parabolic trough
Parabolic trough-shaped mirrors produce a linear focus on a receiver tube along the parabola’s focal line as illustrated in Fig. 1.1. The complete assembly of mirrors plus the receiver is mounted on a frame that tracks the daily movement of the sun on one axis. Relative seasonal movements of the sun in the other axis result in lateral movements of the line focus, which remains on the receiver but can have some spill at the row ends. It also results in a slightly longer incident path that creates a slightly defocused image.
Trough systems using thermal energy collection via evacuated tube receivers are currently the most widely deployed CSP technology. In this configuration, an oil heat transfer fluid is usually used to collect the heat from the receiver tubes and transport it to a central power block. Chapter 7 examines trough systems in detail.
Absorber materials for solar thermal receivers in concentrating solar power systems
Werner J. Platzer, Christina Hildebrandt, in Concentrating Solar Power Technology (Second Edition), 2021
14.1.5 Optical and thermal operating requirements
For linearly concentrating systems, the operating requirements on the tube receivers are dependent on the chosen HTF of the solar field which determines the maximum temperature. For the three currently-used types of HTFs, typical different maximum fluid temperatures and fluid pressures are listed in Table 14.1.
Table 14.1. Maximum fluid temperatures and related pressures for general categories of heat transfer fluids.
Fluid Max fluid temp. (°C) Typical fluid pressure (bar)
Synthetic thermo-oil 393 20–50
Molten salts 550 20–50
Steam 480 120
The operating pressure in the solar field for thermo-oil and molten salt is due to the required pumping power to drive the fluid through the field and is therefore dependent on the solar field design and hydraulics. Steam is used as the HTF where it is generally intended to be the working fluid in a power cycle, thus high pressures are dictated by the requirements of the turbine chosen. If steam is intended for process heat applications, elevated pressures are likely to be chosen to improve heat transfer, reduce pressure drop, and increase the boiling temperature. Due to the higher pressure of steam, the wall thickness of the absorber tubes has to be increased compared to the other fluids. In addition, the heat transfer from the inner tube surface to the fluid is relatively low for dry steam. As a consequence, the temperature differential is larger between the absorber coating and the fluid itself, depending on the momentary heat flux. For the absorber system, therefore, the thermal stresses can be larger for a direct steam generation even when the operating temperature of the circulating fluid is the same as for thermo-oil.
The intensity distribution on the absorber surface also has an impact on the stresses on the absorber system. Due to the concentrator system usually, one side of the absorber tube receives an overwhelming fraction of the total irradiation on the tube. For PTC, the outside part of the pipe receives just unconcentrated radiation (say 900 W/m2) whereas the side facing the reflector in the average receives around 35–40 kW/m2. For solar tower receivers, this effect is even larger as the solar concentration is even higher and the backside of the receiver tubes do not receive any solar radiation. For linear Fresnel receiver designs with a secondary concentrator, the difference between downward-facing side and the upper side of the tubes might not be so extreme, however, the thermal stresses associated with the local surface temperatures being above fluid temperatures are still appreciable, especially in the steam case where heat transfer coefficients may be very different depending on the position in the tube. The surface temperatures depend on the thermal conductivity and thickness of the steel tube wall as well as the thermal resistance of the fluid boundary layer inside the tube. Local temperature differences approaching 70–80 K compared to the steam temperature have been calculated for a stainless steel absorber for a LFC, with about 30 K of the temperature difference occurring over the wall thickness (Hildebrandt, 2009). On the upper opposite side of the tube for the same case, only a few degrees Kelvin excess temperature are predicted. This means that the temperature distribution around the absorber tube has an azimuthal variation of 60–70 K. A coating has to be able to cope with the mechanical stresses associated with that in addition to the effects of the average high receiver temperature. Furthermore, the coating also has to withstand mechanical stress imposed due to the temperature cycling during night and day.
Sustainable Water & Energy Systems
Y. Dai, J. Chen, in Encyclopedia of Sustainable Technologies, 2017
Linear concentrator plants
Linear concentrating solar power collectors reflect and focus the sunlight onto a linear receiver tube with mirrors. The fluid inside the receiver is then heated by the focused sunlight and turns into superheated steam that drives a turbine and the connected generator to produce electricity. Alternatively, the steam can also be generated by using a heat exchanger to collect the thermal energy in the solar field. Linear concentrating collector fields are typically aligned in a north–south orientation to maximize annual solar energy collection, consisting of a large number of collectors connected in parallel. The tracking system for linear concentrating collector is usually a single-axis sun-tracking system, which turns the mirrors from east to west to track the sun. In this way, the sun reflects on the receiver all the while.
Parabolic trough power plants (Fig.6) focus the sun’s rays on tubular receivers with the aid of parabolic mirrors that track the sun in a single axis. The heat generated is removed, for example, using circulating thermal oil, and used to produce steam. The receiver tube is positioned along the focal line of each parabolic-shaped reflector.
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Fig. 6. Parabolic trough power plants configuration.
Reprinted from Guney, M. S. (2016). Solar power and application methods. Renewable and Sustainable Energy Reviews 57, 776–785.
Like parabolic trough power plants, Fresnel power plants (Fig.7) are based around a trough structure which focuses the sun’s rays on a line. Unlike the parabolic trough, in which the receiver tube is situated in the focus of the parabolic-shaped mirror and tracks the sun with the mirror, in Fresnel power plants the receiver is in a fixed position above the solar array so that only the mirrors track the movement of the sun. Furthermore, this system allows the use of flat mirrors, which are considerably cheaper. The heat carrier used is water, allowing the progressive direct evaporation technique to be used. The steam is used directly to drive a turbine in a standard Rankine cycle to produce electricity, avoiding the need for a heat exchanger to produce steam from some other high-temperature fluids.
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Fig. 7. Fresnel power plants configuration.
Reprinted from Guney, M. S. (2016). Solar power and application methods. Renewable and Sustainable Energy Reviews 57, 776–785.
Solar energy
Nikolay Belyakov, in Sustainable Power Generation, 2019
17.2.2.1 Parabolic trough
Parabolic trough is a set of concave mirrors that concentrate solar rays on the receiver tube that is located in the focus. These troughs can track the Sun around one axis, typically oriented north–south to ensure the highest possible efficiency. The fluid flows through this tube and absorbs heat from the concentrated solar energy.
Similar to a parabolic trough is a linear Fresnel system. These collectors resemble parabolic troughs but use long flat Fresnel mirrors. This technology is much cheaper to install but has lower efficiency. Examples are limited to a few plants.
Synthetic oil or molten salt mixture is usually used to circulate through the heat exchanger tubes and absorb heat concentrated by the mirrors. This fluid usually gets heated up to 400–. The tubes then deliver this oil to the process.
Design of the Concentration System
Zhifeng Wang, in Design of Solar Thermal Power Plants, 2019
4.10.2.6 Wind-Induced Vibration of Evacuated Receiver Tube
1.
Definition of formula. The peak wind-induced vibration of the evacuated receiver tube of the concentrator tested and analyzed in this section can be used to describe the receiver’s structural reliability under wind environment effects, which can be expressed as follows:
in which A refers to the peak receiver tube vibration; and respectively refer to the maximum values of the accelerated vibration of receiver tube on Fx and Fz within the unit time interval (which is selected to be 3 s in this section); and and respectively refer to standard deviations of the accelerated vibration of receiver on Fx and Fz within the unit time interval (which is selected to be 3 s in this section). The accelerated vibration of the receiver tube and the respective vibration damage are mainly caused by gust. Therefore, a gust duration of 3 s is selected in this section as the time interval of vibration analysis for the vacuum heat-absorbing tube.
2.
Vibration analysis. Under strong wind conditions, the receiver may result in wind-induced vibration. Such long-term vibration may reduce receiver service life and solar concentration precision. When the vibration amplitude is too large, it may result in collision of the receiver tube’s glass exterior and metal interior, further breaking the glass exterior. Based on this, vibration of the glass exterior under strong wind conditions is tested and analyzed in this section, and conclusions are drawn for reference in design and improvement.
Fig. 4.52 shows the time history curve of accelerated receiver vibration and the relationship between wind speed and wind direction within 30 min under strong wind conditions in the concentrator field. Fig. 4.53 shows the variation relationship curve of peak receiver vibration along with gust wind speed variation. Through the analysis, four conclusions can be drawn: (1) Gust wind speed increases from 4 to 13 m/s and the peak vibration basically stays unchanged, with no tendency of variation accordingly. In this section, the reason is inferred to be that along with the increase in gust wind speed, peak vibration may surge; as 13 m/s is slower than the boundary wind speed for the surging of peak vibration, no obvious variation rule between peak vibration and gust wind speed has been obtained in this test. (2) Within the section of 4–13 m/s for gust wind speed, peak vibration values corresponding to each pitch angle at various measuring points are basically stable. For example, for a pitch angle of 180°, the variation values at measuring point 1 concentrate within 0.4–0.6, while values at measuring points 2, 3, and 4 concentrate within 0.2–0.4. Based on this, peak vibration values of various measuring points under different pitch angles can be obtained through statistical analysis, and the respective rules can be summarized. (3) Under the condition that wind speed is fixed, different pitch angles may severely influence the peak receiver vibration amplitude. For example, when the pitch angle is 25°, peak vibration at measuring point 3 exceeds that in 180° by 45%. (4) By focusing on different pitch angles, position points of the maximum peak vibration amplitudes of receiver tube are different from each other. For example, when the pitch angle is small, the position point of the maximum amplitude is at measuring point 3; when the pitch angle is large, it is at measuring point 1.
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Figure 4.52. Relationship between the time history of accelerated vibration of receiver and the corresponding wind speed and wind direction.
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Figure 4.53. Relationship between peak vibration of receiver and wind speed.
Fig. 4.54 shows the relationship curve of peak vibration and pitch angle corresponding to different measuring points under the condition that the gust wind speed is 8 m/s. When the pitch angle falls within the range of 45°–120°, the respective peak vibration is comparatively small, and the peak acceleration value falls within a range of 0.2–0.35; when the pitch angle is between 150° and 180°, the respective peak vibration is comparatively large, and the peak acceleration value is between 0.3 and 0.5; when the pitch angle is between 0° and 25°, the peak vibration reaches the maximum value, and the peak acceleration value is between 0.35 and 0.6. This is attributable to the direct impact of the incoming wind on the receiver; in addition, the mirror is located at the leeward direction of receiver, which may further lead to increased wind speed on the receiver tube.
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Figure 4.54. Relationship between peak vibration and pitch angle.
HCE-7 and HCE-8 refer to the serial numbers of receiver tubes in a parabolic trough collector.
3.
Natural vibration frequency. Modal parameter identification research means to identify the inherent modal parameters of the structure based on experimental vibration test results, including natural vibration frequency, vibration mode, and damping ratio, which aims at solving relevant issues in terms of structural dynamics by using identification results, such as vibration control, dynamic response analysis, and fault diagnosis. The conventional modal identification methods have already been widely applied in aviation, aerospace, automotive, and many other fields. These methods require the utilization of excitation and response signals simultaneously in order to achieve frequency response function or pulse response function and infer the system frequency, vibration mode, and damping ratio based on these functions. However, in terms of a large-scale civil structure, it is very difficult to perform excitation. Thus, modal parameter identification techniques on the basis of environmental excitation (namely wind load excitation) have been enjoying significant development. The power spectrum peak value method is applied in this section to analyze vibration test data under wind load excitation and identify the receiver’s modal parameters.
Power spectrum peak value method is a kind of frequency domain analyzing method based on structural modal parameters of environmental excitation rapid identification, the fundamental principle of which is to obtain natural vibration frequency of the structure through power spectrum peak values of a random response. For a structure with low damping and discrete natural vibration frequency, the natural vibration frequency can be easily identified. As this method is easy to operate and convenient to use (only needing to take advantage of the Fourier transformation and transform time history data into a power spectrum), it has been widely applied in civil engineering.
Fig. 4.55 offers acceleration power spectrum densities in both Fx and Fz directions and separately compares and analyzes the working conditions corresponding to pitch angles of 5° and 165°, which can be used to research the influencing effects of different pitch angles on the receiver’s modal parameters. By applying the power spectrum peak value method and focusing on the power spectrum peak value indicated in Fig. 4.55, the respective frequency values are extracted as the natural vibration frequencies of the structure. Table 4.8 has listed the first five order values for the receiver tube’s natural vibration frequency.
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Figure 4.55. Acceleration power spectrum density. PSD, power spectrum density.
Table 4.8. Natural Vibration Frequency of Receiver
Working Condition (Pitch Angle) First-Order Modal/Hz Second-Order Modal/Hz Third-Order Modal/Hz Fourth-Order Modal/Hz Fifth-Order Modal/Hz
Fz 5° 4.7 12.9 19.7 29.1 32.4
165° 4.7 14.9 20.3 28.9 33.5
Fx 5° 4.8 9.1 20.2 29.6 33.6
165° 4.6 8.8 20.2 29.5 32.8
4.
Vibration modes of collector receiver. For framed structure equipment supported on both ends, wind-induced vibrations can be divided into three categories: flutter, buffeting, and vortex-excited vibration. Flutter is a kind of aerodynamic instability occurring under certain wind speeds in which aerodynamics and the vibration structure jointly create a dynamic system with an interaction feedback mechanism. In this case, aerodynamics mainly represent a self-excited force. Under continuous interactive feedback effects, in the case of the positive damping value of the vibration system created by the combined actions of the structure and circumfluent airflow that generates self-excited aerodynamic force approaching a negative value, energy absorbed by the vibration system will exceed its own energy consumption capability and result in vibration system and motion divergence that further leads to the damage of the framed structure. Buffeting refers to the stochastic forced vibration of the structure under effects of natural wind fluctuation components. It is subject to an amplitude-limited vibration; unlike the divergence nature of flutter, it normally does not result in any catastrophic instability damage to the structure. The current framed structure buffeting analysis is mainly focused on the structural buffeting caused by characteristic turbulence in the atmospheric boundary layer.
Frame structure vortex-excited vibration is subject to an important aeroelasticity phenomenon that may easily occur under low wind speeds. When a blunt-body structure is influenced by airflow, a vortex will be generated at the back end. In the case of periodic shedding of vortex from both sides of the structure, a Karman vortex is generated. In this case, the periodically shedding vortex will generate an alternative and periodic excited-vibration force (a vortex-excited force) to the structure, which leads to the periodic vibration of the structure. Such a vibration is referred to as a vortex-excited vibration. When the vortex shedding frequency approaches the natural vibration frequency, the structure generates a significant amplitude vibration; in addition, vortex-excited resonance often occurs on bridge components, such as stayed-cable. Vortex-excited vibration mainly has five features: (1) a limited amplitude vibration occurring under low wind speeds; (2) occurring only within a certain wind speed section; (3) significant dependence of the maximum amplitude on damping; (4) vortex-excited response being very sensitive to the subtle changes of section configuration; and (5) vortex-excited vibration being able to excite flexural vibration and torsional vibration.
By analyzing wind vibration damage and section configuration and the receiver’s physical properties, buffeting and vortex-excited vibration are two major sources of wind-induced receiver vibration; vortex-excited vibration easily occurs under low wind speeds, which influences intensity and fatigue of the structure. Receiver tube vortex-excited vibration is discussed to a certain extent in this section in order to lay a solid foundation for subsequent works.
During the flowing process, inertial and viscous forces play leading roles on fluid particles. The ratio of inertial force to viscous force is referred to as the Reynolds number, the expression of which against air is as follows:
in which ρ refers to the air density; v refers to the wind speed; l refers to the characteristic dimension of the structure; and μ refers to the dynamic viscosity.
Based on the value of the Reynolds number, three critical ranges have been divided, including a subcritical range, which normally takes the values of 3 × 102 < Re < 3 × 105; a supercritical range, which normally takes the values of 3 × 105 ≤ Re < 3 × 106; and a transcritical range, which normally takes the values of Re ≥ 106.
Within subcritical and transcritical ranges, vortex shedding follows a quite definite frequency on a periodic basis; whereas within the supercritical range, vortex shedding is disordered and ruleless. Therefore, only subcritical and transcritical ranges are checked for vortex-excited resonance. Vortex shedding frequency refers to the quantity of vortices shedding from the flow per second. When vortex shedding frequency approaches the natural vibration frequency of the structure, the structure generates large amplitude vibrations, which can be expressed as follows:
in which v refers to the wind speed; l refers to the characteristic dimension of the structure, namely receiver diameter, which represents the maximum scale of the object section in the direction perpendicular to the flow; Sr refers to the Strouhal number, which is a function of the geometrical shape of the object and the Reynolds number; the Strouhal number for a cylindrical structure like the receiver tube is selected to be 0.2. Table 4.9 shows vortex shedding frequencies of the vacuum tube corresponding to different wind speed conditions. The first-order frequency of the vacuum tube approximates 5 Hz, whereas the second-order frequency is 10 Hz; both remain in the subcritical range. It is inferred that vortex-excited resonance conditions exist under low wind speeds.
Table 4.9. Vortex Shedding Frequencies Corresponding to Receiver Tube
Wind Speed (m/s) l = 0.12 m
Re ns/Hz
1 8.28 × 103 1.67
5 4.14 × 104 8.33
10 8.28 × 104 16.7
15 1.242 × 105 25.0
20 1.656 × 105 33.4
