# Optical overheating protection

With all solar thermal collectors 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 of the temperature of the collector, for example during failures, component failures, servicing, energy storage capacity limitations, or periods. [1] More generally, stagnation can be considered to be a situation in which the solar collector can not adequately absorb the heat transfer.

In addition to damaging effects to the system, high stagnation also places constraints on collector materials. These materials must retain their important properties during and after exposure to high stagnation temperatures. This implies that solar collectors are built from high temperature resistant materials. These materials are usually expensive, heavy, and have an overall high environmental impact. [2]

Polymeric materials offers a significant cost-reduction and environmental improvement for solar thermal collectors. 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. [3] 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 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

Working Principle of the Prismatic Optical Switch. When liquid is present in the switch, the switch behaves as if transparent. When there is no liquid present, the switch becomes reflective

According to Snell’s law , the angle of incidence (θ) is greater than the critical angle (θ c ), an optical phenomenon called Total Internal Reflection . The critical angle can be calculated using;

{\ displaystyle \ theta _ {c} = Sin ^ {- 1} ({\ frac {n_ {1}} {n_ {2}}}), \; {\ frac {n_ {1}} {n_ {2 }}} \ leq 1}

For a polycarbonate medium, with a refractive index of n = 1.59, in a 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 α 1,2 = 45 ° in an air atmosphere. A ray of light that strikes the medium boundary at normal incidence is total internal reflected, as θ in = 45 °> θ (c, air) = 39 °. In presence of water, θ (c, water) = 56.8 ° and θ in = 45 ° <θ (c, water) , the incoming light is merely refracted and traverses the polycarbonate medium. As such, water acts as a fluid switching. In theory, with an index of refraction to the prismatic structure, to act as the fluid switching.

The optical switch consists of a self-regulating mechanism. In its passive state the switch is filled with liquid and is allowed to pass through the switch and heat the system behind it. As the system heats up, the moving 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 temperature of the system to the evaporation temperature of the liquid. [4]

## Angular Dependence

Transmittance of a prismatic array against day rotation (δ) (δ = 0 stands for 12.00h, 15 ° equals 1 hour). The prisms are aligned such that the angle of incidence (θ) in mid-summer at noon. (AT); Vertical alignment of the prismatic array for a mid-summer day (a) for a mid-autumn & mid-spring day (b), and for a mid-winter day (c). B; Horizontal alignment of the prismatic array for a mid-summer day (d), a mid-autumn & mid-spring day (e), and for a mid-winter day (f).

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 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.

## Applications

The main application for which the optical switch has been developed is overheating protection for solar thermal collectors. [4] The prismatic geometry can be integrated into the collection of the collectors to prevent them from overheating, or by self-regulation through evaporation, or by draining the water out of the switch at a specified maximum temperature. Temperature limitation would allow for the use of polymeric materials within solar collectors, dramatically reducing cost-price and increasing market penetration.

Another application of the switch is for windows for both housing and office buildings. The amount of sunlight entering the building can be controlled by the liquid switching. Preventing the amount of sunlight entering a building will reduce the temperature inside the building on sunny days.

Finally, the switch can be used within roofs of greenhouses . The plants in the greenhouse can be protected in the reflective state. Currently greenhouses are covered with a chalk layer to protect the plants during summer from excessive sunlight. Applying the chalk layer is time consuming and bad for the environment. Once the chalk is applied, it also blocks sunlight during less sunny days. The optical switch could be used to resolve this issue.

The temperature inside the greenhouse can be regulated by a certain amount of roofing in the reflective state. Also the fluid flow inside the roof can be circulated to extract heat from the greenhouse. These coefficients remain that the climate ( relative humidity and elevated CO2 levels) remain optimal and constant.

The Switching Fluid in the Greenhouse can be used as a filter for a part of the solar spectrum . Water allows so-called “PAR” light ( Photosynthetically active radiation , the light that plants use to grow) to pass, while “NIR” (Near Infra Red ) light is absorbed. The amount of NIR light to be absorbed by solving micro-particles of copper sulphate or clay in the fluid switching. In that way optimum growth conditions can be selected.

Some greenhouse products, like flowers, are grown by using artificial light during the night. This artificial light causes so-called light pollution in the environment of the greenhouse. When a greenhouse is made of the light of day, which keeps the artificial light inside the greenhouse. As a side effect there is an effective mirror.

## References

1. Jump up^ SJ Harrison Q. Lin and LCS Mesquita. Integral Stagnation Temperature Control for Solar Collectors, SESCI 2004 University of Waterloo Conference Waterloo, Ontario, Canada August 21st-25th, 2004
2. Jump up^ M. Köhl et al. Durability of Polymeric Glazing Materials for Solar Applications, Solar Energy 79 (2005) 618-623.
3. Jump up^ GM Wallner, K. Resch and R. Hausner. Solar Energy Materials & Solar Cells 92 (2008) 614-620 Property and Performance Requirements for Thermotropic Polymer Flat-plate Collectors
4. ^ Jump up to:b M. Slaman, R. Griessen. Solar Collector Overheating Protection, Solar Energy 83 (2009) 982-987