SolSwitch roof experiment and Weather station site

Link to results of the survey: "Beeldvorming van duurzame energie in nederland" (Only available in Dutch).

SolSwitch; an efficient optical switch


Research at the condensed matter physics group of VU University Amsterdam on superconducting materials has lead to the discovery of "hydrogen switchable mirrors" in 1996. These are thin film mirrors which optically change when some hydrogen gas is applied near the mirror. This optical change can either be to a transparent state or to a light absorbing "black" state.

A double glazing unit of which one of the glasses is coated with a thin film mirror, based on a Magnesium-Gadolinium alloy. The coating is inside the double glazing unit. The same Magnesium-Gadolinium mirror but now some hydrogen gas is applied inside the double glazing unit. The windmill behind the mirror becomes visible.

Since the discovery of the switchable mirror effect there is an increasing interest in the fundamental understanding of this switching effect and a search for applications which make use of it. Potential applications are: hydrogen detectors, privacy windows and optical switches for preventing overheating in solar applications. The last named application inspired prof. Griessen to search for a different method of creating an optical switch; hydrogen and heat are usually not good companions and in solar applications the costs matter.

After a small "Friday afternoon" discussion and a few table-top experiments we found that it is possible to make an optical switch just with a well designed window surface and plain water as a switching medium.

VU University developed a low-cost optical switch. The switch consists of a panel which is transparent when filled with water and light reflecting when empty.
Movie of the "Optische schakelaar" (optical switch) on
Critical angle and total internal reflection:

The optical switching effect is created by a physical effect called "total internal reflection":

Light which travels through a medium such as polycarbonate (PC) at incidence angles larger than the so-called “critical angle (θc)” cannot escape from the medium. This effect is called “Total internal reflection”. The critical angle can be derived from the ratio of the index of refraction of the medium (n1) and the surrounding (n2) by Snells equation.

θc = arc sin (n2 / n1)         [n2 / n1 < 1]

Polycarbonate has an index of refraction (n) of 1.59. When it is placed in an environment of air (n=1) the critical angle θc = 39°.
When air is replaced by water (n=1.33) the critical angle rises to θc = 57°.

For angles θ1 smaller than the critical angle light can escape from the medium (red line). For angles larger than θc light can not escape, "total internal reflection" occurs. Light falls on an PC-air interface at 45° (A), which is larger than θc and will be reflected (twice). When air is replaced by water (B) the incidence angle is smaller than θc and light passes through.
The optical switch consists of a double glazed unit of which one plate contains prismatic structures with angles of 90°. Light which falls orthogonal on the surface (normal incidence) enters the prisms and is "total reflected" at the prism-to-air interface since the incidence angle θ (45°) is larger then the critical angle θc (39°). A second prism interface re-directs the light back, out of the structure. The structure is now in the reflecting state. When air is replaced by water the critical angle changes to 57°. This means that light can now pass the prism-to-water interface. The optical switch is now in the transmitting state.

Solar applications: Diurnal and seasonal changes

The light reflecting state of the optical switch depends on the incidence angle of light. Since the prism-to-air interface is 45° tilted with respect to the surface and θc = 39° there is a band of + 6° and -6° around normal incidence where total internal reflection occurs and almost all light will return. For larger or smaller angles some light will pass the prisms, an increasing angle means more transmission of light, although light which escapes from a prism might still be reflected back by a neighbouring prism. Light wich escapes the prisms is slightly changed in direction. Therefore a second layer of prisms can be used to enhance the light reflection over a larger amount of angles.
When the optical switch is used in solar applications it is needed to know that the incidence angle of sun light is not only changing during the day but that there is also an incidence angle shift depending on the seasons. Depending on the application it is possible to find an optimal shape of the prismatic structure and a proper defined orientation of multiple prismatic structured layers. In some applications it is needed to block sunlight efficiently around noon, when the sun is most powerful, while other applications ask for a good  light reduction which is constant during seasonal changes.

The effect of light reflection is optimal for light rays at or near normal incidence (a, b). However, in a double prismatic layer light is efficiently reflected for a wide interval of incidence angles. Optimal configurations can easily be designed for various applications. Some prism configurations are efficient light blockers around noon, all year long. Other prism configurations are better in blocking light during a larger part of the day.


An obvious application of an optical switch would be in  windows for houses and office buildings. The amount of sunlight entering the building can be selected by filling the double glazed unit with water. Reducing the amount of sunlight will reduce the temperature inside the building on sunny days. For these kind of applications it is important to know that light slightly changes its direction while passing the unit in the transmission state. Therefore the image will be distorted when you look trough the window. This makes the window interesting to use as privacy window. In reflection mode it  avoids light to enter the building while in transmission mode light can enter the building, but without a clear view.

The SolSwitch can be used as a privacy window which can be switched to a light blocking state.

An other application of the SolSwitch is overheating protection. The optical switch can be integrated in thermal solar collectors to prevent them from overheating when no thermal energy is drained from the collector on sunny days. Currently solar collectors are made from expensive materials like aluminium and copper because these materials can withstand the high stagnation temperatures (above 200 °C !!) inside the collector. The cost of a thermal solar collector could drop with a factor 10 when the collector is based on a full plastic design. In order to allow a plastic design it is needed to limit the temperature of the collector below the degradation temperature of the used polymer (60 °C - 120 °C).

The temperature inside the collector is limited according the "fail save" principle when the optical switch is integrated in the top plate of the collector. When water is evaporated or drained from the collector the optical switch is filled with air, which brings the switch in the reflecting state. With a proper alignment towards the sun the collector will reflect all light (when needed) around noon, when the sun is most powerful. The maximum temperature inside the collector is the evaporation temperature of the fluid inside the collector.  In case of water the temperature is limited to 100 °C, but when an alcohol based fluid is used the temperature is even limited at a lower value. Alcohol based fluids also prevent the solar collector from freezing in winter. The collected solar power is extract from the switching fluid and fluid channels inside the (plastic) solar absorber plate via a heat exchanger and stored in an insulated tank until needed. 

SolSwitch can be used in low-cost all-polymer solar collectors to prevent stagnation due to overheating. a) Thermally isolated prismatic structure. b) Switching fluid reservoir. c) Plastic solar absorber with (optional) thermal fluid channels.

The SolSwitch can also be used in roofs of greenhouses. There are many advantages of switching the greenhouse roof from transparent to reflective:

 The roof of the greenhouse is transparent when the optical switch is filled with water  Sunlight is reflected on the greenhouse roof when water is drained from the optical switch


[1]. M. Slaman, R. Griessen, Solar collector overheating protection, Solar Energy 83 (2009), 982-987

[2]. Patent: Solar collector overheat protection, filed in February 2008, Inventors: R. Griessen and M. Slaman. (owned by VU). Filed as Optical switch for greenhouse applications in august 2008, OCN-2001271 (6017675NL).

Download A4 poster (.pdf) (675 KB)

Download opportunity sheet solar collectors (.pdf) (225 KB)

Download opportunity sheet greenhouses (.pdf) (200 KB)