Wind data management
DETERMINING THE physical resource potential of a site is the single most important variable in determining the viability of a major renewable energy investment. The availability of authenticated resource potential for a multitude of sustainable energy technologies is pivotal for the growth of this nascent industry. Pakistans sustainable energy sector is trying hard to consolidate its presence in the countries energy mix, and the basis of this consolidation is highly sensitive on the accuracy of our resource potential measurements. However, it has been highly unfortunate that we as a nation havent carried out a detailed real time resource assessment analysis of our country. All we significantly depend on are the regional NREL maps of wind and solar data, based on advanced numerical simulation and skewed data from the existing metrological stations in Pakistan. People working in the renewable energy industry as “off-grid system integrators” of wind and solar energy continue to proudly using these regional NREL maps as reliable and accurate means of resource data. Engineers, technicians and analysts should understand the difference between the data obtained from numerically simulated procedures and that from the actual real time measurements before they are used, as there can be substantial differences in the performance of renewable energy systems, this could end up in debilitating consumer trust. However, the increasing numbers of small private renewable energy organizations working in different regions of Pakistan are not exactly the ones to blame, as carrying out a real time detailed resource assessment is the duty of the government (AEDB).
To achieve the highest quality and authenticity of the wind data collected it is essential that a site visit is carried out to understand the surrounding terrain. The wind data has to be collected for at least 12 months without gaps and must be checked for plausibility. While some project developers may insist on a larger sample size, 12 months is the least that should be measured. The international body which is responsible for accrediting institutions for carrying out “harmonized and recognized measurements in wind energy” is named as MEASNET. It should, however, be noted that selecting the right sensors is of prime importance to achieve an accurate assessment. Anemometers complying with IEC 61400 should be used after they are calibrated by an accrediting body (member of MEASNET); calibration should be done based on ISO-3966 1977 standards. In terms of importance the primary measurements that have to be made is of wind speed and wind direction and the secondary variables to be determined are of atmospheric pressure, temperature and relative humidity. It is best to measure wind speeds at hub heights of the wind turbine. If this isnt possible then the effects of wind shear must be incorporated in the power law equation to determine the wind speeds at different heights. Commercial anemometers which are used for weather forecast have a tolerance of ±0.3m/s to 0.5 m/s; these may be reasonably accurate anemometers for weather forecast but such tolerance is not acceptable for wind turbine installation. In a really well designed measuring project, the anemometers are also calibrated for a second time after their use. This makes it sure there have been no changes while measuring.
Wind direction can be determined by “wind vanes” at any height unlike wind speed and its measurement plays a vital role in wind farm layouts. While the measurement masts are installed on site it should be made sure that wind vanes are not installed at the same height of anemometers as this might disturb the stream of air available to the anemometers. All wind sensors should be absolutely vertical and even slight deviation can lead to skewed wind to strike the anemometer.
The atmospheric pressure can be measured at a convenient height and should be conveniently placed in a shelter box used for the data logger. Temperature sensors are protected from solar radiation and should be at least 10 meters from the ground to avoid the effects of heat radiation.
Once the data is collected in a data logger installed at site or transferred to another storage medium via SCADA (Supervisory Controls and Data Acquisition) installations, data is checked for linearity. Measured wind data is also correlated with existing databases like MERRA (Modern Era Retrospective analysis For Research and Applications).
Standard IEC 61400-12 titled as “Power performance measurements of electricity producing wind turbines “gives comprehensive details on how to collect, reject and correct wind data. Once the database is formed, the data is normalized. The database normalization is the process of organizing the fields and tables of a relational database to minimize redundancy and dependency. The measurement of temperature and atmospheric pressure helps to determine the density of the winds. The power curve is then formed after which the power coefficient is calculated depending on the type of manufacturer. WAsP is a commonly used programme, (developed by RISOE DTU) which is primarily used for wind data processing.
At the end of the day, no matter how accurate we are with our measurements, there is always an element of uncertainty in the annual energy production (AEP). “Exceedance Probabilities” like (P50, P75. P90 etc) are used to evaluate the revised AEPs. AEP levels with increasing Exceedance probabilities decrease as it is more difficult to be certain of a higher electric power production levels. Determining highly probable AEP levels is crucial for predicting the profitability of the project.
Thus we see how wind data is managed from being measured accurately from a mast to being processed for crucial and conceivable results, and more importantly depicting the viability of wind power projects.