How do CSP work?

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1. Solar thermal system

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Developed in the early 1980s, solar thermal electricity has seen a large expansion and successful operations. Many Solar Concentrating Power Plants have been installed all over the world to provide electricity. Concentrated Solar Power (CSP) technology captures and concentrates the solar radiation to provide the heat required to generate electricity.

The CSP system consists to use mirrors to focus sunlight upon a receiver. The receiver has a critical function in CSP; it fixes the efficiency of the system. The receiver needs to absorb a maximum of the solar radiation and converts it to mechanical power and then into electricity. Many Solar Concentrating Power Plants were installed all over the world to provide electricity.

Different systems with different efficiency were developed. Four mains systems commercialized:

 A- Parabolic through

B- Dish sterling

C- Linear Fresnel

D- Tower receiver.

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1.1 Solar receiver

The solar receiver is in general a metallic or glass substrate covered/coated by a black coating absorber. Large varieties of coatings  were developed and have been commercialized. These coatings are mainly deposited from wet deposition, electroplating, and physical vapor deposition .

Selectivity of certain coatings prepared by PVD. Electroplating coating is very good and present thermal stability at a certain temperature.

Depending on the system used (dish/stirling, power tower or trough-electric) temperature of use varies between 200 to 500°C. Next generation of CPS aims to increase the operating temperature to increase its efficiency and reduce its cost. Novel generation of CSP needs receiver to operate at a temperature higher than 600°C.

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                   Picture 1: CSP Tower                                    Picture 2: Dish Sterling

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1.2 Requirements of the solar receiver

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At high temperature, the receiver has to present an Absorptance above 0.95 and needs to be stable to the extreme conditions such as high irradiation, high temperature and strong thermal shocks. These factors accelerate ageing mechanisms that are responsible for the degradation of the material's thermal performance and affect the lifetime of the solar absorber.

Most of the materials in use today present drawbacks, coatings developed by PVD methods are generally not stable at a temperature higher than 600°C or required to work under vacuum. Same for electroplating coatings those are limited at lower temperature. Best results regarding heat resistance, durability and weathering stability have been reported with paint based on silicone resins. Paint is a cheap alternative to sophisticated surface treatment (electrochemical, chemical/physical vapor deposition) and very easy to apply.

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2. Solar Selective Absorbing Coating

Black paints Black paints as black solar absorber coatings have been reported at the beginning of 1980. The solar coatings are made of a mixture of a black inorganic pigment, and metal oxides, and a transparent matrix, usually an oxide, which serves as a binder.

Black absorber paints have been categorized as non-selective (NS), Thickness Insensitive Spectrally Selective (TISS)(14-16) or Thickness Sensitive Selective (TSSS).

It is possible to obtain a high Absorptance of about 0.95 for all the three types of paint but with significant difference in thermal emittance. A number of factors affecting the selective absorptivity of the film have been determined (substrate metal emittance, particles size, dispersion paint, surfactant,refractive index , addition of aluminum flakes .

Commercial selective black paints are nevertheless limited a by low a relatively low Absorptance 0.95 and be stable at high temperature. Various ceramic/pre-ceramic binders have been suggested for high temperature applications because of their high melting point and chemical durability. These preceramic precursors contain: silicate resins (such as siloxanes, poly-borosiloxanes, polysilazanes , polycarbosilanes), titanium, zirconium, or hafnium metal boride, carbide, oxide, nitride, and silicide. These materials have some of the highest melting points with high hardness, improved wear corrosion, and oxidation resistance.

Disadvantages of the black paint is when the temperature exceeds 800°C; the spinel began to decompose, forming various oxides of copper, iron and manganese. When the sample was cooled, the spinel did not reform, indicating that heating above 860°C would irreversibly destroy the pigment. We also notice diffusion of the metal through the coating at working temperature above 800°C. 

Protection from the drastic environmental conditions, High percent of humidity and salt accompanied with high temperature causes damage to the receiver. The formulation of the black paint will depends from the needs of the coating: - Protective coating from corrosion - Coating with optical properties.

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3. Characterization of the coating

Standards test or coating evaluation and characterization

Adhesion: ASTM 3359 Standard Test Methods for Rating Adhesion by Tape Test

Scratch resistance: ASTM D7027-20 Standard Test Method for Evaluation of Scratch Resistance of Polymeric Coatings and Plastics Using an Instrumented Scratch Machine

Thermal cycling: D6944-3 Standard Test Method for Resistance of Cured Coatings to Thermal Cycling D3363 Standard Test Method for Film Hardness by Pencil Test1

Salt fog test: the salt spray test and its use in ranking stainless steel D2485-91

Standard Test Methods for Evaluating Coatings for High Temperature Service Absorptance: E903 -96 Standard Test Method for Solar Absorptance, Reflectance, and Transmittance of Materials Using Integrating Spheres Abrasion: ASTM 4060 Abrasion resistance of organic coatings by the Taber Abraser

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4. Conclusions

You wish more information; please contact CZ Green Technologies Consulting

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