Remote sensing best practice

Remote sensing technologies such as SoDAR and LiDAR have proved to be cost-effective methods of acquiring critical project datasets from locations that present access challenges. As a result they are increasingly being adopted in the offshore environment where their compact, portable and robust characteristics can be maximally leveraged to acquire data that previously would have represented greater cost or would not have been available at all. The codification of best practice in relation to remote sensing has made significant progress and currently is the focus of efforts made by, for example, the IEC, the IEA and MeasNet, among others. This process is providing both informative recommended guidelines and a regulatory framework of normative procedures which remote sensing practitioners will need to comply with to ensure, for example, the robustness of calculations of project uncertainty, representing in a complete and unbiassed manner all sources of project uncertainty. This paper outlines the status of the regulatory environment regarding the use of remote sensing as well as discusses the benefits that accrue through compliance. In addition key sources of uncertainty are discussed and best practice described in the light of the developing normative framework. Figure 1: various models of SoDAR EWEA Offshore 2011 PO.0322 Introduction The constraints that apply to this conference paper in terms of length mean that it cannot serve as a single comprehensive stand-alone guide to best practice when operating remote sensing devices to acquire wind data for wind power applications. However, there is a rich and diverse literature already where sufficient guidance can be found. This paper should be viewed as a high level review, a sort of road-map through the existing literature detailing guidelines, recommendations, and normative standards relating to the use of remote sensing in the wind power industry. The author refers the reader to [2, 4, 6, 7, 9 and 11] for detailed discussions of best practice. In addition, this paper will outline some general considerations which have an important bearing on the development of the regulatory environment governing the use of remote sensing in the wind power industry. Figure 2: various models of LiDAR What is remote sensing? Remote sensing typically refers to the use of SoDAR and LiDAR. Some examples of these are shown in Figure 1 – 3. These use the interaction of laser or acoustic pulses with the atmosphere to acquire measurements. References [1 – 11] and the works cited therein provide a detailed description of their operating principles. These measurement principles differ from those represented by conventional mast mounted cup anemometry. The measurements themselves may therefore differ under certain circumstances. The standards and guidelines governing measurements in the wind power industry are written in terms of measurements made using IEC compliant cup anemometers whose performance is understood in terms of observations made at wind tunnel facilities that have been inspected and found to comply with national standards. Confidence in remote sensing devices requires that their performance can be understood in terms of the measurements that would have been made by these cup anemometers had they been deployed instead of the remote sensing device. However wind EWEA Offshore 2011 PO.0322 tunnel facilities that are suitable for observing the performance of both cup anemometers and remote wind sensing devices are not available and so comparisons between remote sensing devices and reference instruments comprising mast mounted anemometers configured and calibrated in an IEC compliant manner are necessary. Figure 3: Galion LiDAR Remote sensing complements mast measurements, rather than replaces them. There are technical reasons for this arising from, for example, the complementarity of the different measurement principles and the differing degrees of autonomy and deploy-ability under the circumstances prevailing on site. However the most compelling argument for complementarity is economic. The objective of many measurement campaigns is to achieve the greatest reduction in project uncertainty for a given investment in data acquisition, and the way the costs of the different measurement techniques are structured leads to them being used in different roles. For example, the cost of a mast may be heavily weighted towards installation, in which case it makes financial sense to leave the mast in place to acquire one or two years of wind data for use in, for example, MCP studies. Remote sensing often represents a cumulative cost associated with equipment hire, resulting in the most advantageous cost-benefit accruing from shorter term deployments across multiple locations, exploiting the compact and portable nature of the devices to achieve objectives of the measurement campaign that may be more difficult or costly with a mast. In general remote sensing needs to be used to acquire measurements that would be difficult, impossible, or even inconceivable with masts, in order to unlock the value of remote sensing. In many instances, if mast-like measurements are required, the most cost-effective way of acquiring them, from the point of view of the cost per percentage point reduction in project uncertainty, is to install a mast. Remote sensing is effectively used to undertake assessments that are beyond the limitations of masts, such as wind shear extrapolation across the entire rotor disc of a large wind turbine, or the horizontal extrapolation of wind resource across multiple proposed wind turbine locations. Remote sensing devices are amenable to onshore and offshore use. There is a massive diversity of tools and techniques associated with remote wind speed sensing, including but not limited to  “mast replacement” techniques such as Velocity Azimuth Display (VAD) and Doppler Beam Swinging (DBS),  techniques that acquire data from regions not immediately above the device, such as arc/sector scans, position plane indicator (PPI) and range height indicator (RHI) scans, and  multiple aperture, convergent beam, dual Doppler, bistatic techniques that are more suitable for complex terrain, such as crossed RHI (XRHI) and co-planar RHI configurations. EWEA Offshore 2011 PO.0322 These techniques offer a huge scope for reducing project uncertainty and risk, both by directly addressing the contributions to project uncertainty that arise through the incomplete understanding of the variation in wind conditions across the site, and by the acquisition of critical site suitability information that may have otherwise remained undetected during the development phase, such as, for example, the incidence of wind shear anomalies like low level jets. However performance must be related to existing tools and techniques, such as cup anemometry. Nevertheless, best practice must take into account the versatility and flexibility of the available options. A much greater degree of variety and diversity exists within the array of remote sensing tools and techniques currently available than is evident within cup anemometry. The standards and guidelines that govern the use of cup anemometry can afford to be more prescriptive about the devices to be used as a result of this relatively lesser degree of diversity. When developing guidelines and standards for remote sensing devices it is important to avoid imposing a similar level of prescriptiveness as this will result in the exclusion of valid solutions from the scope of the guidance. Figure 4: Boulder Protocol hierarchy of guidance and recommendations In addition, the most cost-effective use of remote sensing may not be just as a simple surrogate for conventional methods based on cup anemometry. There are occasions where simple analogies between the methods cannot be drawn, such as, for example, when considering flow visualisation methods such as PPI and RHI scans. This should be recognised when developing recommendations and guidelines for remote sensing best practice. In general, the much more diverse range of remote sensing solutions than are available with regards to conventional mast mounted instruments means best practice must therefore  focus on results, data, and outcomes, rather than details of methodology, and  not stifle innovation and flexibility with excessive prescriptiveness based on the limited experience of the authors who may not have been exposed to the full range and capabilities of the available remote sensing solutions. Traceability of the accuracy of remote sensing devices requires comparison with conventional instruments. With the above considerations in mind, these comparisons must be performed on the basis of the measurements of key wind flow parameters by the remote sensing and EWEA Offshore 2011 PO.0322 reference instruments, while allowing the method employed by the remote sensing device to acquire these measurements to remain a “black box”. Remote sensing in the wind industry A key signpost in terms of guidance in the use of remote sensing to acquire measurement of key wind flow parameters of relevance to the wind power industry is the Boulder Protocol [1–5]. This was proposed at the 59 th IEA Topical Expert Meeting in Boulder, Colorado in October 2009. This is a framework within which to understand and develop best practice: a set of generic objectives whose achievement the guidelines for the technology used (SoDAR/LiDAR) and instrument specific (user manual) guidelines and recommendations should enable. This framework is illustrated in Figure 4. This framework is set within a hierarchy of inter-related guidance. The framework describing a general approach to conducting the comparison of remote sensing devices with reference instrumentation and their subsequent field deployment is augmented by technology specific guidelines which deal with particular considerations that arise in the context of the category of remote sensing device used, such as SoDAR or LiDAR. These guidelines are ultimately augmented with model specific guidelines, or user manuals. The framework of the Boulder Protocol requires that  instrument performance is verified by comparison with a suitable reference,  the instrument is subsequently operated in the mode in which it has been verified,  the circumstances in which it is operated are equivalent to those in which verification has been achieved,  bankability is ensured by deriving annual energy production (AEP) percentiles from the resulting data with reference to a complete, robust and unbiased uncertainty analysis that incorporates all sources of uncertainty, and  other relevant guidance is followed, in particular the technology and model specific recommendations. The development of technology specific guidelines commenced in 2007 at 51 st IEA Topical Expert Meeting, and continued at the 59 th in 2009. The SoDAR guidelines [6] are now in their 5 th Edition (July 2011). These SoDAR guidelines discuss  siting (fixed echoes, complex terrain),