Wind power is one of the most promising green energy sources, especially when produced in offshore power
plants. Corrective operations in wind turbines cause a considerable part of the maintenance costs of such plants.
One preventive action for reducing such operations is the periodic off-line control of oil samples from the wind
turbines. The time delay between sampling and availability of the results is a major disadvantage of this kind
of controlling. In-situ condition monitoring is a solution to this problem. In-situ monitoring allows real time
detection of random, time discrete events, thus enabling a better scheduling of preventive actions and reducing
costs and downtime.
Fluorescence spectroscopy is a complementary technique to absorption spectroscopy. Due to absorption of
UV or visible light, the electrons of specific molecules are excited from a ground electronic state to a vibrational
state of higher energy. By collision with other molecules, the excited electron looses a part of the acquired
energy and relaxes to a lower vibrational state. The remaining acquired energy is emitted during the electron's
transition to the ground state. The resulting frequency shift between excitation and emission energy, known as
Stokes shift, is unique and characteristic for each active molecule.
In this paper gear-oil condition monitoring based on fluorescence spectroscopy is proposed. Three typical
commercial gear-oils for wind turbines were studied. The spectra gained by UV excitation of the samples were
analyzed by means of partial least square (PLS) regression. Good prediction results were obtained for the total
acid number (TAN). The latter is a measure for the oil acidity and is considered to be a proxy variable for oil age.
Other parameters delivering information about gear-oil additive depletion and the related oil aging condition,
like phosphor, sulfur and molybdenum concentration, were also analyzed.