Solar Selective Absorber Design

There are several ways of designing the solar-selective absorbing surfaces on substrate. The different designs result in the different optical absorption mechanisms such as optical trapping absorbers, metal-dielectric multilayer absorber, absorber-reflector tandem, quantum size effects, etc. The descriptions about various selective absorbers synthesised by different methods can be found elsewhere [52-58]. Generally, the dark mirror absorber- reflector tandem is the most common commercially available selective absorber design [58- 61].

An absorber-reflector tandem configuration is obtained by superimposing one or more layers on the top of substrate in which the layer and the surface have different optical properties. If the layers are highly-absorbing in the solar region and the substrate is non- selective highly-reflecting material then this configuration is known as a dark mirror. Figure 2.3 describes the dark mirror absorber-reflector tandem design. If the layer is solar transparent material and simultaneously also infrared reflector material whereas the surface is highly solar absorber, then it is known as a heat mirror. In the dark mirror coatings, the

13

absorber layer on the top of substrate can be a semiconductor, absorber particles embedded in dielectric matrix or selective paint. Several designs of materials for the construction of the dark mirror absorber-reflector tandem are given below.

Figure 2.3. Dark mirror absorber-reflector tandem design

Semiconductor thin film coatings

A low band gap semiconductor coating has the characteristic of absorbing the solar radiation and transmitting the mid-far infrared. It is because the semiconductor absorbs photons having energies greater than the band gap and it will raise the material’s valence electrons into the conduction band. Photons with energies less than the band gap energy are transmitted through the coating [62]. If a semiconductor is deposited on the top of highly infrared reflecting metal substrate, then a spectrally selective semiconductor coating will be obtained.

According to the equation (2.14), to absorb all solar radiation below wavelengths of λ=2.5 µm, the semiconductor should have a band gap (Eg) of 0.5 eV. However, the main

14

obstacle is the difficulty finding a suitable semiconductor. Most semiconductors have too large band gaps, corresponding to too short wavelengths. Lead sulphide (PbS) is a suitable semiconductor with a band gap of 0.4 eV [52]. Unfortunately, this material is very poisonous for humans and the environment and is not commercially feasible.

hc E_g (2.14)

where h is Planck’s constant and c is light speed in vacuum.

Another problem with semiconductors is that they have a high refracting index which results in low absorptance in the air-coating surface interface. To obtain high solar absorptance, the refractive index of the semiconductor should be as low as possible. The absorptance can be increased by controlling the thickness of the coating to reduce the interference effect or by applying an antireflection layer. An example of a semiconductor metal tandem selective absorber is the chemical vapour deposition of silicon in a stack of SiO2/Si33N4/Si/Cr2O3/Ag/Cr2O3 on stainless steel with an antireflection coating on top of the

multilayer stack [62, 63].

Composite thin film coatings

Certain metallic clusters embedded in a ceramic/dielectric matrix (cermet) composite coating such as Cr-Cr2O3, Ni-Al2O3, Mo-Al2O3, or Ni-NiOx exhibit good solar spectral

selective absorption. The coatings strongly absorb solar radiation and are almost transparent in the infrared region. The spectral selectivity of a cermet coating is enhanced by using a highly infrared reflecting (poor thermal emitter) metal substrate [42, 64]. The concept of using a cermet material to form a tandem structure with a poor thermal emitter metal substrate has been investigated both theoretically and practically [18, 65].

Cermet selective absorbers usually consist of nanometer-sized metal particles (1–20 nm) [32] and the effective medium theories can be used to model the optical properties of the

15

film [66, 67]. Simulations have proved that a ceramic-metal solar absorber with an AR layer could achieve absorptance values of 0.91–0.97 and emittance values of 0.02–0.07 [68]. The metallic particles are usually transition metals which are uniformly distributed in the matrix or gradient index with gradually increasing particles content from the upper-limit of the matrix towards the substrate surface. Figure 2.4 shows the microstructures of two different examples of composite coating solar selective absorbers. In the nickel pigmented anodic aluminium oxide (Ni-Al2O3) microstructure, the particles are uniformly distributed in the

matrix, while in the sputtered nickel/nickel oxide (Ni-NiOx) microstructure the particles are

arrayed with graded index composition [18, 69, 70]. The metal particles in the cermet act as a modifier for the optical response of the ceramic phase [71, 72]. The absorption in a cermet coating is a result of light scattering by the boundaries between the metallic phase and the oxide (dielectric) phase [73, 74].

The cermet system offers a high degree of flexibility with optical parameters which can be tuned by controlling the metal content, the shape, orientation and size of the small metallic clusters as well as the optical constants of the constituents. The thickness and chemical nature of the dielectric phase can be adjusted to obtain the desired spectral selectivity. The type of matrix also influences the quality of the film. In this regard, a porous matrix is the optimum host for metal particle inclusion [2, 22, 42, 64, 75]. The surface morphology of the cermet also plays a significant role in determining the surface absorptance and can favour multiple reflections in the surface, thus enhancing the solar radiation absorption [8]. By varying many of the parameters listed above, countless combinations can be created, thus the required spectral selectivity can be easily achieved [22].

16

Figure 2.4. Microstructure pictures of two examples of solar absorber composite coatings (adapted from [62] and [76]).

Selective paint, spinels and metal oxide absorber coatings

The selective paint absorber is a simple and less expensive selective absorber because it can be produced by using the low-cost sol-gel process. This type of absorber is usually used for a façade coating with a certain purpose. Factors determining the optical performance of selective paint type absorber include intrinsic optical constants, particle size-dependent scattering and paint binder [29, 62]. Generally, the selective paint absorber has a low selectivity due to high thermal emittance.

The selective paint coating is composed of pigment absorber particles dispersed in a resin/binder agent where they uniformly form the coating matrix. Some pigments, mostly from transition-metal oxides, have high solar absoption which is due to the existence of numerous spin-allowed electron transitions between partially filled d-orbital [29]. Polymer

binder, such as silicone, siloxane resin or phenoxy resin, is usually used in the selective paint coating. Unfortunately, the binder agent absorbs strongly in the thermal IR range increasing

17

the thermal emittance significantly. Another disadvantage is that it is impossible to make paint coatings thinner than 1–2 µm because the thickness of the paint layer is limited by the size of the ground pigment particles [29]. Usually the pigment particles will agglomerate and their size will be comparable to or larger than the incident wavelength of light, reducing the paint performance. An example of commercial selective solar-absorbing paints is the Solarect-ZTM which is synthesised using siloxane resin and an inorganic pigment of

FeMnCuOx with a pigment volume fraction of about 0.2 [77].

Efforts to decrease the emittance value of paint coating have been done by other researchers [20, 29]. They prepared the CuCoMnOx pigment coating without binder via the

sol-gel method, forming a spinel-type absorber. Other researchers prepared solar absorber which consisted of less than three components of transition metal forming a spinels or metal oxide absorber coating, and even the CuMn spinel oxide coating has reached a promising performance for use on an industrial scale [24, 38, 78]. The review of metal oxides, spinels and composite selective absorber coatings synthesised using the sol-gel method can be found in Chapter 3.