Framework for the Categorisation of Losses and Uncertainty for Wind Energy Assessments In collaboration with:
Introduction A typical wind resource and energy yield assessment derives gross generation from a site s wind speed frequency distribution and a turbine s power curve. Technical loss factors are then applied to derive expected net energy generation. Example loss factors include equipment availability, wake losses, icing losses, and electrical line losses. An uncertainty analysis is then conducted to determine the probability distribution of net energy production. Example uncertainty categories include those associated with wind speed measurement, wind shear extrapolation, modelling, and loss assumptions. However, without standard definitions for such loss factors and uncertainty categories, it is difficult to compare studies prepared by different consultants. Standardised definitions will not only facilitate direct comparison of energy estimates among different consulting studies, it will lead to more productive dialog and ultimately improved understanding of technical losses and uncertainties. It is not uncommon for more than one energy assessment to be prepared for a proposed wind power facility. Often the results of the assessments will have material differences, and these differences typically include different assumptions regarding technical losses used to derive net energy generation from gross energy generation. However, when both the definition of a loss and the value of the loss differ, direct comparisons are difficult to make. Compounding this challenge is the use of different uncertainty categories and their definitions, making it difficult to interpret and compare the results of different reports at the various probability levels (P90, P95, etc.). Developers, investors, and the consultants that support them will be able to focus on actual differences between assessments if assessments use a common set of loss and uncertainty definitions. Representatives of the organisations listed on the cover page have contributed to the process of developing the proposed loss categories and sub-categories. In addition, they have also agreed to use the uncertainty categories presented in this framework. Use of sub-categories, or addition of further sub-categories is at the users discretion. The definitions described in this document have been assembled based on consolidation of loss and uncertainty categories derived from input from the RenewableUK Wind Resource Working Group as well as industry input since circulating a final draft in October 2012. Note that every project is unique and requires specific consideration, so not every item listed in this document will apply to every project being analysed or reviewed. The definitions are not an attempt to standardise the values of losses and uncertainties used in an assessment, but instead aim to provide a framework for the definitions. Date: 5 February 2013 Page 2 of 8
Definitions Loss Categories The following seven main categories represent the first level of the proposed standard definitions of energy losses. Even if the sub-categories are not used, reporting loss factors using these seven main categories will facilitate better understanding of loss definitions and comparison between assessments. The seven main categories currently have broad adoption within the wind industry, and have been discussed previously in efforts to standardise categories among consultants. 1 1. Availability 2. Wake effects 3. Turbine performance 4. Electrical 5. Environmental 6. Curtailment 7. Other Each main category has proposed sub-categories that can be used to better detail the losses used in the assessment. Note that we assume losses are reported as a per cent of the gross energy and that gross energy includes topographic effects, air density effects, and any issues related to use of a non-standard power curve, such as one for low or high turbulence. That is, any increase or decrease in natural wind flow across a site, air density assumptions, or issues related to the selection of a power curve would already be factored into the gross energy, not included as a line item loss (or gain in some cases). For clarity, wake losses are not accounted for in reported gross energy yield. The reference power curve(s) applied to generate gross energy estimates should be reported. Net energy is the energy production as measured at the project revenue meter. The recommended sub-categories and comments on the definitions are shown in Table 1. Table 1. Recommended Loss Sub-categories, Definitions, and Comments Standard Loss Category # 1. Availability Recommended Sub-categories 1a Turbine Includes lost energy due to routine maintenance, faults, and component failures over the project lifetime. 1b Balance of plant Losses due to downtime in components between the turbine main circuit breaker to and including project substation transformer and project-specific transmission line. 1c Grid Losses due to downtime of power grid external to the wind power facility. 1 Jones, Stephen, DNV Global Energy Concepts Inc., Standard Loss Definitions for Wind Resource/Energy Assessment, American Wind Energy Association WINDPOWER Conference, Houston Texas, June 2008. Date: 5 February 2013 Page 3 of 8
Standard Loss Category # 2. Wake effects 3. Turbine performance 4. Electrical 5. Environmental Recommended Sub-categories 1d Other Other availability losses not accounted for above or in other categories below (e.g. availability correlation with high wind events). 2a 2b Internal wake effects External wake effects Losses within the turbine array that is the subject of the energy assessment. Losses on the turbines that are the subject of the energy assessment, from identified turbines that are not the subject of the energy assessment, which either already operate or which are expected to operate at commissioning of the facility being studied. 2c Future wake effects Losses due to additional development in the vicinity of the turbines being studied, but which would occur after commissioning of the turbines being studied. 3a Power curve Losses due to the turbine not producing to its reference power curve within test specifications (for which the turbines typically perform most favourably). 3b Wind flow Losses due to turbulence, off-yaw axis winds, inclined flow, high shear, etc. These represent losses due to differences between turbine power curve test conditions and actual conditions at the site. 3c High wind hysteresis Losses due to shutdown between high-wind cut-out and subsequent cut-back-in. 3d Other Other turbine performance losses not accounted for above. 4a Electrical losses Losses to the point of revenue metering, including, as applicable, transformers, collection wiring, substation, transmission. 4b 5a 5b 5c 5d Facility parasitic consumption Performance degradation not due to icing Performance degradation due to icing Shutdown due to icing, lightning, hail, etc. High and low temperature Losses due to parasitic consumption (heaters, transformer no-load losses, etc.) within the facility. This factor is not intended to cover facility power purchase costs, but does include the reduction of sold energy due to consumption behind the meter. Losses due to blade degradation over time (which typically gets worse over time, but may be repaired periodically), and blade soiling (which may be mitigated from time to time with precipitation or blade cleaning). Losses due to temporary ice accumulation on blades, reducing their aerodynamic performance. Losses due to turbine shutdowns (whether by the local turbine controller, project-wide control system, or by an operator) due to ice accumulation on blades, lightning, hail, and other similar events. Losses due to ambient temperatures outside the turbine s operating range. (Faults due to overheating of components that occur when ambient conditions are within the turbine design envelope would be covered under turbine availability category above.) Date: 5 February 2013 Page 4 of 8
Standard Loss Category # 6. Curtailment 7. Other 5e 5f 6a Recommended Sub-categories Site access and other force majeure events Tree growth or felling Wind sector management Losses due to difficult site access (for example: snow, ice, or remote project location). Note that this environmental loss and some other environmental losses may be covered under the availability definition, above. However, these environmental losses are intended to cover factors outside the control of turbine manufacturers. Losses due to growth of trees in the facility vicinity. This loss may be a gain in certain cases where trees are expected to be felled. Losses due to commanded shutdown of closely spaced turbines to reduce physical loads on the turbines. 6b Grid and ramp-rate Losses due to limitations on the grid external to the wind power facility, both due to limitations on the amount of power delivered at a given time, as well as limitations on the rate of change of power deliveries. This can be ongoing control of the wind farm output over the project lifetime or temporary curtailment until grid reinforcements are carried out early in the project. 6c Offtaker curtailment Losses due to the power purchaser electing to not take power generated by the facility. 6d Environmental (noise, visual, bird/bat) 7a Wind speed energy relationship Losses due to shutdowns or altered operations to reduce noise and shadow impacts, and for bird or bat mitigation. This would include use of a low-noise power curve versus a standard one from time to time. Any loss due to the non-linear relationship of wind speed to energy. 7b Air density Air density correction if treated downstream of gross energy calculation. 7c Other This would cover anything that does not fit into the above six loss categories. Definitions Uncertainty Categories The following seven main categories represent the first level of the proposed framework uncertainty definitions. Even if the sub-categories are not used, reporting uncertainties using these seven main categories will facilitate better understanding of uncertainty definitions and comparison among assessments. The aim is to allow comparisons between assessments prepared by different parties. While all categories ultimately reflect uncertainty in the final P50 energy estimate, these may be presented as uncertainties on wind speed and/or uncertainties on energy production for ease of reporting and increased transparency. 1. Site measurement 2. Historic climate 3. Vertical extrapolation 4. Future variability 5. Spatial variation Date: 5 February 2013 Page 5 of 8
6. Plant performance and losses 7. Other Each main category has example sub-categories that can be used to better detail the uncertainty used in the assessment. The example sub-categories and comments on the definitions are shown in Table 2. Table 2. Recommended Uncertainty Sub-categories, Definitions, and Comments Uncertainty Category Description # 1. Site measurement 2. Historic wind resource Includes sensor uncertainty and quality of project data Includes MCP uncertainty of local and reference data, historic variability and reliability of long term reference 1a 1b 1c 2a Example Subcategories Instrument accuracy Measurement interference Data quality and metadata Period of data representative (of long term) Sub-category Variability in measurement of wind speed by individual instruments. Includes calibration uncertainty. Uncertainty due to effects of masts on measurements or other types of interference. For masts, includes tilted anemometers, separation concerns or any other mounting issues. For remote sensing, includes physical or atmospheric interference on the measurement. Uncertainty regarding possible bias due to any removed or missing data or lack of quality metadata. Includes uncertainty due to non-encrypted data or non-traceable data sources, or for inconsistencies/ contradictory metadata. Representativeness of period of data relative to project period of interest. 2b Reference site Correlation uncertainty and uncertainty associated with consistency and quality of long-term references, measured or modelled. 2c 2d Wind speed frequency distribution past On-site data synthesis For a given mean wind speed, the distribution can have variability. Uncertainty regarding measured distribution relative to true long-term distribution. Uncertainty associated with filling in data. Date: 5 February 2013 Page 6 of 8
Uncertainty Category Description # 3. Vertical extrapolation 4. Future wind variability 5. Spatial variation 6. Plant performance and losses Includes uncertainty related to the estimate of wind speeds at elevations above ground level for which there are no direct measurements. This includes uncertainty associated with estimating the true vertical profile of horizontal wind speed across the height range of interest. Includes future variability of resource. Considers both the variability of the wind climate. Includes the uncertainty of extrapolating from met tower locations to individual turbine locations. Includes the uncertainty of estimating turbine production, efficiencies with respect to atmospheric conditions, and all plant losses including wake losses. 3a 4a Example Subcategories Extrapolation to hub height Interannual variability Sub-category Uncertainty of representativeness of shear or other extrapolation model up to hub height. Also could include the extent to which the hub-height wind speed is not representative of the rotor plane average wind speed. Uncertainty regarding whether the true long-term mean wind speed will occur over the project life. 4b Climate change Local and global climate patterns (long- and mediumterm phenomena). 4c Wind frequency distribution future Uncertainty/variability in year-to-year distribution in future. 5a Model inputs Quality of terrain data, roughness maps. Includes degree of site uniformity with respect to terrain and vegetation. 5b Horizontal extrapolation Representativeness of masts relative to turbine locations (exposure, terrain, vegetation, elevation, etc.). 5c Other uncertainty Includes model bias, model quality and error, complexity of terrain, quality of Weibull fit used in model. 6a Availability Turbine and balance of plant over project lifetime. 6b Wake effects Includes uncertainty in the model inputs (including wind direction), and that associated with model performance and appropriateness for site. Also includes uncertainty related to any proposed neighbouring sites (construction timing, layout, turbine type). Date: 5 February 2013 Page 7 of 8
Uncertainty Category Description # 7. Other 6c Example Subcategories Turbine performance Sub-category Includes power curve uncertainty, uncertainty on performance under site conditions for which power curve is not valid. Includes impact of atmospheric stability. 6d Electrical losses Line loss, metering. 6e Environmental Blade soiling, blade degradation, weather effects. 6f Curtailment Includes uncertainty with respect to wind sector management, grid and ramprate, offtaker, and/or environmental curtailment. 6g Other losses Includes uncertainties associated with other project specific losses. 7 Includes other uncertainties that do not fit into the above six uncertainty categories. Conclusions Various entities have come together to propose that the wind industry adopt a common framework for discussing both energy losses and uncertainties around energy assessments. The goal for this framework is to lead to more transparency and ultimately advancements in the quantification of project losses and uncertainties. Date: 5 February 2013 Page 8 of 8