PowerWindow: A transparent and colourless energy converting window

Somewhere in a small sized country a small city houses a large tech incubator with many promising innovative start-ups. The country is the Netherlands, the city is Delft, the incubator is Yes!Delft and one of those promising and highly technological development driven start-ups is; PowerWindow.

PowerWindow operates by the vision of ‘sustainable innovation without compromise’. By this vision the founders, Ferdinand Grapperhaus en Willem Kesteloo, are developing clean electricity generating windows. Friday the 18th they will be celebrating their first anniversary and TTalents went out to explore what they have achieved in their first year of existence.


A PowerWindow consists of three main components; first, a luminescent coating absorbs part of the (sun)light and re-emits this light at a different wavelength, second, glass transports this re-emitted and invisible light towards the edges of the window and finally, customized PV-strips convert the redirected light into usable electricity.

This concept is referred to as a LSC (Luminescent Solar Concentrator) and the very first mention of it dates back to the 70s. By that time it wasn’t possible to find materials that didn’t have a bright colour emission or could achieve sufficient efficiency. During MSc graduation projects at Delft University of Technology, Willem, Ferdinand and their professor Erik van der Kolk discovered new material properties, which enable scaling of an efficient colourless and transparent electricity-generating window – a PowerWindow.

Intellectual Property

In the first months after graduating and founding PowerWindow BV, negotiations and strategy determination resulted in an ‘exclusive license agreement’ of the filed patent that protects the technical know-how of PowerWindow. With the TU Delft as a minority shareholder, the young and promising company strengthened themselves with a strong partner.

PowerWindow: A transparent and colourless energy converting window

Business Development

With a promising technology, a patent and a partner in the pocket, one would think the road is paved. However it is only where the adventure started. The PowerWindowTeam made it their mission to ‘go outside the building’ as much as possible. They have spoken with large construction companies, architects, real estate developers, financers, possible customers, electricity suppliers and so on. This resulted in a strong business model, which helped the team win the second price of the New Venture 2015 competition.

Pitching and Prices

During their first year WnF – how they refer to themselves – have joined (and won) several competitions. Some examples are All Energy Day ’14, Climate KIC stage 1, Entrepreneur Forum ’15, Hello Tomorrow ’15, Living Daylights Innovation ’15, Shell Rising Star ’15, etc. The amount of these start-up competitions may seem in abundance, however they all contribute to the possibility of financially bootstrapping your business in an early stage. Making the Netherlands a well-fit location for the first year of existence.

PowerWindow: A transparent and colourless energy converting window


Besides going out of the building, WnF stayed inside the lab to work together with their former thesis supervisor Erik van der Kolk and several Applied Physics students. Their focus was not only to improve and analyse the existing luminescent coating but probably even more demanding was to construct a working Proof-of-Concept. Together with Fraunhofer ISE (customized solar cell) and Gregor van Egdom (frame work and assembly) they constructed the world’s first working 5x5cm transparent and colourless energy converting window. The POC demonstrated architects and real estate developers the appearance of the coating and on the other side made it possible to measure future efficiencies. These measurements predict an efficiency of 15-25 W/m2 resulting in the urge to scale up to a real-size prototype.

Scaling up

Scaling up us next, and the name itself correctly entails it is not just a linear step in technological development. The PowerWindowTeam has access to a magnetron-sputtering machine with which no larger than 5x5cm glass panes can be coated. Therefore PowerWindow is currently in the search for the right partner to co-produce a real size (1x1m) prototype. PowerWindow looks for a partner with the operational capability of sputtering coatings on glass, but even more important, the future partner should have a pragmatic and entrepreneurial state of mind. When one wants to make such disruptive technology successful it is essential to accept challenges with great motivation.

PowerWindow A Window of Opportunities


Both Willem and Ferdinand envision producing a real size prototype on short term. With such prototype in hands they dare to dream of the company’s future. They project realisation of the first commercial buildings with PowerWindows, making them energy neutral, in the Netherlands. Not only because it is home, but moreover because of it’s demanding market with accompanying possibilities. Quickly they will proceed to enter new markets, in Europe, the states and most certainly the mid-East. Of course they will need investments by then, but first they focus on realisation of their prototype.

For now we advice them to focus on celebrating their first birthday. Because in the hectic world of starting your own (hardware tech) company, it might be a lesson for life to ‘celebrate your successes’.

For more information check their website! PowerWindow: A transparent and colourless energy converting window!


Categories: Solar,Startups