With all solar thermal collector systems there is a potential risk that the solar collector may reach an equilibrium or stagnation temperature higher than the maximum safe operating temperature. Various measures are taken for optical overheating protection. Stagnation temperatures are encountered under conditions of high radiation while no heat transfer fluid is flowing through the collector, for example during power failures, component failures, servicing, energy storage capacity limitations, or periods when little hot water is extracted from the system. More generally, stagnation conditions can be considered to be any situation under which the solar collector cannot adequately dispatch the absorbed solar heat to the heat transfer fluid. Besides any damaging effects to the system, high stagnation temperatures also place constraints on collector materials. These materials must retain their important properties during and after exposure to the high stagnation temperatures. This implies that solar collectors are generally built from high temperature resistant materials. These materials are usually expensive, heavy, and have an overall high environmental impact. Polymeric materials offer a significant cost-reduction and environmental improvement potential for solar thermal collectors and may thus benefit a broader utilization of solar energy for various heating purposes. However, the long-term service temperature of plastics is limited. Thus, for potential applications of plastics in solar absorbers an appropriate design including overheating protection is essential. Feasible ways would be a reduction in optical gain (for example, using thermotropic layers, or electrochromic devices) or an increase in system losses, by dumping of the hot water excess. In this article an alternative method to decrease the optical gain is presented. The method is based on the geometry of prisms and the phenomenon of Total Internal Reflection.
Working principle
According to Snell's law, light cannot escape from a medium when it strikes the medium boundary at an angle of incidence (θ) that is larger than the critical angle (θc), an optical phenomenon called Total Internal Reflection. The critical angle can be calculated using;
θ c = S i n − 1 ( n 1 n 2 ) , n 1 n 2 ≤ 1 {\displaystyle \theta _{c}=Sin^{-1}({\frac {n_{1}}{n_{2}}}),\;{\frac {n_{1}}{n_{2}}}\leq 1} For a polycarbonate medium, with a refraction index of n=1.59, placed in an atmosphere of air with a refraction index close to 1, Total Internal Reflection occurs when θ > θ(c,air)=39°. Consider a polycarbonate prismatic structure with an apex angle α1,2=45° placed in an atmosphere of air. A ray of light that strikes the medium boundary at normal incidence is total internal reflected, as θin=45°> θ(c,air)=39°. In presence of water, θ(c,water)=56.8° and θin=45°< θ(c,water), the incoming light is merely refracted and traverses the polycarbonate medium. As such, water acts as a switching fluid. In theory, water can be replaced by any other liquid, with an index of refraction close to that of the prismatic structure, to act as the switching fluid. The optical switch consists of a self-regulating mechanism. In its passive state the switch is filled with liquid and light is allowed to pass through the switch and heat the system behind it. As the system heats up, the switching fluid evaporates out of the optical switch and the prismatic structure starts to behave as a reflective surface. No more light passes through the switch, limiting the maximum temperature of the system to the evaporation temperature of the liquid.
Angular Dependence
Resulting from its geometry, the optical switch is sensitive to the angle of the incident beam. Depending on the shape of the prisms, the transmittance of the switch in its reflective state during a typical day shows characteristic angular dependence. This dependence can be used to find specific transmission curves for different applications, where the geometry of the prisms serves as the input variable.
… excerpt ends here. Continue reading the full article.
