For his graduation project, TU Delft student of Sustainable Energy Technology Stefan Roest developed a new type of hybrid solar collector with a higher efficiency and a longer lifespan than the current hybrid systems. Hybrid solar collectors combine photovoltaic solar cells that convert sunlight into electricity with a solar heater that provides warm water.

Roest built a prototype and also built an actual solar simulator that he used to test the efficiency of his prototype. There turned out to be considerable commercial interest in this solar simulator. This motivated Roest and a partner to start the TU Delft spin-off company Eternal Sun, so they could put the solar simulator on the market. Eternal Sun recently came out on top at the European finals of the BE.Project, a competition for student-entrepreneurs.

Solar collector

A hybrid solar collector is a combination of a photovoltaic solar panel and a thermal solar collector. The residual heat from the PV solar panel is used to heat water. The water flows through a system of pipes on a copper sheet. A great deal of heat is needed to heat the water in the pipes. That is why the solar collector has been fitted with a transparent cover that helps to retain the heat. Unfortunately, the material used in the PV solar cell degrades quickly under temperatures of around 120 degrees. As a result, its efficiency is reduced by around 20 per cent and it has a lifespan of between five and ten years.

For his graduation research as part of a Master's degree in Sustainable Energy Technology, Stefan Roest developed a new type of hybrid solar collector with increased electrical efficiency and a longer lifespan. For a start, Roest's solar collector does not require a transparent cover. The water flows through a large number of small aluminium channels directly under the solar panel instead of through copper tubing and a copper sheet. Consequently, less heat is required to heat the water sufficiently for household use. Roest also chose not to use a crystalline silicon PV solar panel, opting for a thin film solar panel instead. It is easier to draw heat from this type of solar cell. Getting rid of the cover meant that the heat of the solar panel could be limited to around 80 degrees.

An additional benefit of thin film solar panels is that these perform relatively well at high temperatures. At a temperature of 80 degrees, an efficiency loss of around 10 per cent occurs, instead of the 20 per cent in the case of crystalline silicon solar panels. Roest's hybrid solar collector has an estimated lifespan of 15 to 20 years.

Roest developed the new solar collector under the supervision of the professor of Photovoltaic Materials and Devices, Miro Zeman, who comments: "This innovative design could play an important role in the development of affordable and efficient hybrid systems for household use."

Solar simulator

Roest developed a special solar simulator to measure the efficiency of his prototype. Almost immediately, there was commercial interest in this simulator and the relevant technology was quickly patented by TU Delft. Roest and his partner Chokri Mousaoui have since introduced the simulator onto the market via their TU Delft spin-off company Eternal Sun. Eternal Sun recently came out on top in the European finals of the BE.Project competition for students from top universities with an innovative business case, which was organised by the technology consulting company BearingPoint. The Eternal Sun team has now grown to include six students and recent graduates, and five solar simulators have already been sold since January.

Roest's affinity with solar energy goes back quite a while. In 2007, he was the team leader of the Nuon Solar Team that won the World Solar Challenge in Australia with the solar car Nuna4.