Introduction

Buildings account for about 40% of Europe's energy use. Smart windows that adapt to changing solar radiation conditions can reduce heating and cooling demands while improving indoor comfort. Polymer-dispersed liquid crystal (PDLC) smart windows can switch between a transparent state and a diffuse, milky state. The PDLC film consists of small liquid crystal droplets embedded in a polymer layer between two transparent, electrically conductive layers. Without an applied voltage, the droplets scatter incoming light. When a voltage is applied, the resulting electric field aligns the liquid crystals, allowing more light to pass through.

Windows strongly influence both a building's energy use and the comfort of its occupants. Large glazed areas provide daylight, but transmitted solar energy and visible light can also cause overheating and glare, respectively. Compared with traditional blinds and external screens, PDLC windows have no moving parts, are easier to clean, and give the user direct and immediate control. Research by the Norwegian University of Science and Technology (NTNU) and Stiftelsen for industriell og teknisk forskning (SINTEF) examines PDLC materials from laboratory to application scale. 

Testing PDLC films in the ZEB Test Cell 

The ZEB Test Cell on the Gløshaugen campus in Trondheim is jointly used by SINTEF and NTNU to test façade solutions and control strategies under realistic yet carefully controlled conditions. It consists of two identical test rooms, and around 500 sensors continuously log the indoor and outdoor climate. This is why it was chosen as a demonstration site, a so-called lighthouse, in the EU Horizon Europe project  Exploit4InnoMat. A PDLC film was installed on the window in the test façade to study how much sunlight and solar heat it lets in, in both of its states. Sensors measure the sunlight hitting the façade and the sunlight passing through the window, together with the indoor temperature and the light level on the desk in the right corner.

_{The left picture shows the inside of the test cell, where the PDLC film in the ZEB Test Cell is shown in its translucent state on the left, and in its transparent state on the right. The picture on the right shows the outside of the test cell, with the smart window shown in the middle. Copyright image: NTNU and SINTEF.}

From 9 July to 31 August, both states of the PDLC film let less solar radiation through than the original double-glazed window, which was measured from 15 February to 15 March. The g-value was then measured at 54.1%. The g-value tells us how much of the solar energy hitting a window ends up inside the building: both the sunlight passing straight through and the heat the glass first absorbs and afterwards releases indoors. This makes the g-value a useful way to compare how much solar heat different windows let in. 

The figure below shows the measured results. With the PDLC film in its transparent state (blue), the share of solar energy entering through the window dropped from the declared 54.1% (grey) to a measured 44.0%. In the translucent state (orange), it dropped even further, to 39.4%. However, how much the film helps depends on the window it is applied to. A g-value of 54.1% is typical for windows sold in Norway today, but windows that let in more solar heat are also available and are common in older buildings. On such windows, a PDLC film can make an even bigger difference.  
 

_{Share of solar energy entering through the window in the ZEB Test Cell: 54.1% for the window without PDLC film (grey), 44.0% with the film in its transparent state (blue), and 39.4% in its translucent state (orange). Copyright image: NTNU and SINTEF.}

Conclusions and next steps

First measurements show that a PDLC film can noticeably reduce the solar heat entering through a window, even in its transparent state. Combined with no moving parts and instant control at the push of a button, this makes PDLC a promising alternative or supplement to traditional blinds and screens. 

Next steps will examine how the film affects daylight and glare, the energy needed for heating and cooling, and how intermediate transparency states can be used. 

_{Top photo: PDLC film and conventional external screens combined in a case-study office at NTNU, Trondheim. The middle window area is in the translucent state; the left and right areas are transparent. Copyright image: NTNU and SINTEF.}