KM3NeT 2.0: A carbon neutral research facility13/02/17
Maarten De Jong of Nikhef profiles the next-generation KM3NeT research infrastructure
Unlike traditional telescopes, KM3NeT will detect neutrinos instead of light. Neutrinos are subatomic particles that interact only very weakly with matter; in everyday life, we are unable to detect their presence, nor do we realise the impact of this interaction. We are very much aware, though, that the atoms in our bodies would fall apart without the electro-magnetic interaction. We also know that the nuclei inside these atoms are held together by a different, much stronger force. Since Newton’s famous brainwave in 16871 we understand that the same gravitational force which keeps our feet on the ground makes the Earth steadily orbit around the Sun. One may argue that this is all, and one does not need another force – but because of this other force, the Sun keeps burning. Without it, we would be sitting in the cold and, due to it, exactly the right mixture of atomic nuclei was formed in the early Universe to trigger the formation of stars and, eventually, Life on Earth.
Without this other interaction, however weak, we simply would not exist. Its fundamental role makes us want to observe neutrinos and learn more about the way the Universe was shaped.
A different look at the Universe
For such a different look at the Universe, the weak interaction is both a blessing and a curse. In principle, neutrinos are ideal messengers from the cosmos. Because they interact only weakly with matter, neutrinos can travel through the Universe without being cloaked by stars or even entire galaxies. Because they carry no electric charge, neutrinos are not deflected by galactic or intergalactic magnetic fields, but instead travel in a straight line from their source to the Earth, thereby producing a sharp image of the sky. In practice, however, the weakness of their interaction with matter makes it very hard to detect them. To overcome the weakness of this interaction with matter, we need a new generation of giant telescopes.
KM3NeT is a research infrastructure comprising a network of neutrino telescopes located on the bottom of the Mediterranean Sea. Several cubic kilometres of deep seawater will be instrumented by a three-dimensional array comprising several thousands of omnidirectional ‘eyes’. Note that one can look for neutrinos coming from above as well from below, as the Earth is transparent to them. Even with such a huge detector the feebleness of their interaction with matter makes it necessary to collect data for many years in order to see a significant signal. The deep sea telescopes are controlled from a shore station that is equipped to deliver power, to house the computing hardware for the real-time processing of the data and to provide a fast internet connection to the outside world. For the interested reader, an introduction to the relatively new field of neutrino astronomy has been presented in Pan European Networks: Science & Technology, issue 21 (2016), pages 28-31.
The eyes of the telescope have been designed to fit the envisaged lifetime of ten years (or more), thereby avoiding costly and technically challenging repairs. The eyes consist of a glass sphere with a diameter of 42cm, housing 31 photo-sensors and associated readout electronics. The glass sphere can withstand the pressure of the water and is transparent for the faint light that must be detected to see the traces of a rare neutrino interaction in the seawater. To maximise the reliability and longevity, one needs to keep the hardware inside these glass spheres cool. Contrary to what you may think, cooling in the deep sea is not straightforward. The few materials that are resistant against corrosion, such as glass, titanium and polyethylene, are poor heat conductors. So, to keep the temperature inside the glass sphere low, the power consumption is kept to the minimum (about six watts per eye). This low power consumption duly matches with our ambition to make the operation of the KM3NeT research infrastructure carbon neutral.
Three suitable deep sea sites have been selected, namely offshore of Toulon, France, of Capo Passero, Sicily, and of Pylos, Greece. These sites were selected for the optical properties of the deep seawater, the short distance to shore, and the availability of local infrastructure. For each site, the power consumption of the neutrino telescope and the shore station is less than 150kW. For 24h/day and 7d/week operation, this translates to about 1.3GWh per year, or the equivalent of around 300 households. This amount of energy can easily be supplied by wind turbines or solar panels, meaning that the operation of the KM3NeT research infrastructure can have a zero-carbon footprint for its entire life.
The average wind profile at the three sites is quite favourable for electrical power generation. The observed wind speeds at the selected sites vary throughout the year with an average speed of about 7-10m/s. At such wind speeds, standard wind turbines are capable of delivering 600kW each. Solar panels could be an alternative solution as the selected sites also benefit from significant sunshine throughout the year. Typically, the sunshine ranges from four hours in the winter to 12 hours in the summer. Solar panels rated at 150W/m2 peak are quite common and would provide 1.2kWh per day averaged throughout the year. This would conservatively translate to around 4,500m2 of solar panels per site, about the size of a football pitch, to provide the required power.
Recently, a feasibility study for the envisaged zero-carbon footprint of the KM3NeT research infrastructure has been funded by the EU in the framework of the Horizon 2020 programme. Furthermore, the KM3NeT research infrastructure will be operated remotely, which will minimise power consumption in other areas such as travelling. The power consumption of the computing hardware for the analyses of the data can be reduced by developing faster algorithms (the faster the algorithm, the less computing power, the less CO2). So, stay tuned for a completely carbon neutral discovery of a neutrino source in the sky.
1 The story goes that when newton saw an apple falling from a tree, he realised that the well-known gravitational force and the mysterious long-range force between celestial objects are the same (Philosophiæ Naturalis Principia Mathematica)