A major part of future renewable energy will be generated in offshore wind farms. The used turbines of the 5 MW class and beyond, often feature a planetary gear with 1000 liters lubricating oil or even more. Monitoring the oil aging process provides early indication of necessary maintenance and oil change. Thus maintenance is no longer time-scheduled but becomes wear dependent providing ecological and economical benefits. This paper describes two approaches based on a linear variable filter (LVF) as dispersive element in a setup of a cost effective infrared miniature spectrometer for oil condition monitoring purposes. Spectra and design criteria of a static multi-element detector and a scanning single element detector system are compared and rated. Both LVF miniature spectrometers are appropriately designed for the suggested measurements but have certain restrictions. LVF multi-channel sensors combined with sophisticated multivariate data processing offer the possibility to use the sensor for a broad range of lubricants just by a software update of the calibration set. An all-purpose oil sensor may be obtained.

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.