The rmotropic materials to prevent overheating of solar collectors

Thermotropic glazings change their light transmittance behavior from transparent to light diffusing upon reaching a certain threshold temperature. This autonomous shading is induced by light scattering from domains with dimensions comparable to the wavelength of the solar spectral range which exhibit an index of refraction that is different to that of the matrix above the defined transition temperature [2,3]. Thermotropic behavior may be achieved by an alteration in the structure of the liquid crystalline phase in the case of liquid crystal systems [2]. Scattering can also be induced by the formation of local differences in the refractive index by phase separation or a change in refractive index of one or more

components [2,3]. This is the case in thermotropic hydrogels, thermotropic polymer blends and thermotropic systems with fixed domains.

For industrial applications in general thermotropic materials should fulfill a set of requirements [3,4], such as:

• high reversibility and reproducibility of the switching process

• homogeneous stability and low hysteresis

• no haze and coloring in clear state

• a uniform distribution of the turbidity in the scattering state

• a high viscosity for fluid thermotropic materials filled between glass panes

• excellent long-term stability: weatherability, UV stability, non-freezing

• that the material is safely manageable, innocuously, low or non-flammable and free of organic solvents

• availability in a large area at low costs

When thermotropic layers are applied to prevent overheating of solar thermal systems, further specifications have to be fulfilled [1,5,6]:

• a solar transmittance above 85% in clear state

• a solar transmittance below 60% in opaque state (this would allow for the application of cost-efficient plastics as absorber materials)

• switching temperatures between 55 and 60°C for thermotropic glazing

• switching temperatures between 75 and 80°C for thermotropic absorbers

• a steep and rapid switching within a small temperature range

• scattering domain sizes between 200 and 400 nm in diameter

• differences in refractive index between scattering particles and surrounding matrix above 0.03

In the following the suitability of thermotropic hydrogels, thermotropic polymer blends and thermotropic systems with fixed domains to provide adequate overheating protection of a solar collector is described.