Investigation of an infrared sea surface temperature measurement as part of a system for determining offshore atmospheric stability
Richard Fruehmann, Richard Foreman, Thomas Neumann
UL International GmbH DEWI, Wilhelmshaven, Lower Saxony, Germany
With the rapid development of large scale offshore wind farms, stability in the offshore atmospheric boundary layer has found increasing interest due to its importance with regard to the length of wind turbine wakes. A significant challenge has hitherto been the accurate in situ measurement of temperature profiles in the lowest few hundred meters of the boundary layer due to the small temperature differences involved, the remote location and harsh environment. Since the summer of 2015, new air temperature measurements with significantly improved precision have been installed on the FINO 1 research platform in the North Sea [Frühmann et al., DEWI magazin Vol 48, 02/2016]. Since May 2016 this has been augmented by an infrared based method for obtaining the sea surface temperature (SST) with the same temporal resolution of 1 Hz as the atmospheric measurements.
The infrared based measurement offers easy installation and low maintenance costs on existing offshore structures in comparison with measurement buoys. In combination with the existing set of atmospheric measurements at FINO 1, several methods of assessing atmospheric stability can be applied in a comparative study. The aim of this work is to present the capabilities of the infrared SST measurements and to demonstrate the potential of a simple temperature parameter obtained through the new improved measurements. Thus atmospheric stability in offshore locations is investigated with a view to developing a simple measurement setup that could be used in future to estimate wind turbine wake effects offshore.
The infrared SST measurement is based on two temperature calibrated infrared sensors, one pointed towards the sea, the second pointed towards the sky to enable compensation for background radiation. The air temperature measurements utilise standard PT100 sensors with a relative calibration of sensors and measurement modules.
The infrared SST measurements are compared with measurements from a Waverider buoy that represents the current standard for SST measurement to assess various influencing factors (including background radiation, sea state, atmospheric conditions). Subsequently, the SST and air temperature measurements are used to estimate atmospheric stability and the results are compared with measured and modelled profiles and fluxes of temperature and humidity, e.g. using a Richardson number approach.
A comparison with a nearby Waverider buoy in the first six months of deployment has demonstrated the infrared method to be robust and largely independent of atmospheric conditions and sea state. The largest factor influencing the difference between the two systems has been shown to be the background radiation emanating from the sky. Applying the correction, the two systems agree to within 0.16 °C with a standard deviation of 0.25 °C. Comparisons with the bulk model are still in progress at the time of writing.
A setup and methodology for accurately determining SST via non-contacting infrared sensing is presented. The robustness of the method is shown by demonstrating the measurements to be independent of sea state and atmospheric conditions. Potential sources of error and the corresponding correction methods are described. The suitability of the measurements for assessing atmospheric stability offshore is verified by applying the measurements to two methods of estimating atmospheric stability.
The paper addresses some of the challenges in obtaining reliable estimates of atmospheric stability offshore. An approach to accurately determine SST via a non-contacting method is demonstrated, thereby providing a cost effective means of obtaining this critical mesaurement. The utility of the measurement for estimating atmospheric stability is assessed in light of existing models, such as the bulk Richardson number, and in situ measurements of wind profiles under a range of conditions and wind directions including wake and non-wake influenced wind directions. In light of the increasing density of offshore wind farms, such a system could contribute to improved energy yield predictions by providing a parameter on which to base wake length estimates.