As one focuses on evaluating accurate value of the expected net energy yield, it should be noted that accuracy in this case is a function of uncertainty, says Andy Oldroyd, technical director at Oldbaum Services (pictured)

By Ritesh Gupta

The forecast of probable energy yield is a key factor in ascertaining the performance of any project. Accordingly the offshore wind industry has been finding ways to improve its reliability on energy yield analysis for overall project optimisation.

As further accuracy and enhancements are achieved in wind calculation and measurement, the focus now turns from the top line to the bottom line as project stakeholders seek to hike yield and reduce cost. As highlighted by Wind Energy Update’s recently released white paper, Big Data – how applied analytics can enhance your O&M programmes, while these two variables would seem to be in conflict, a diligently planned O&M programme can achieve precisely that by proactive measures that will increase yield through lower downtime, gained at a probably lower cost.

Also, quantified analysis of asset reliability results in more precise estimate of earning possibility through life, facilitating better grasp of the affect of asset underperformance on financial performance. These outputs in turn feed financial modelling, asset valuations, energy yield predictions and cost of energy calculations.

Accuracy

As one focuses on evaluating accurate value of the expected net energy yield, it should be noted that accuracy in this case is a function of uncertainty, says Andy Oldroyd, technical director at Oldbaum Services.

“Once an array has been chosen and a yield value obtained, losses and uncertainties need to be applied to calculate the yield and the risk associated with that yield. This is sometimes called the Net Yield P50 and standard uncertainty,” says Oldroyd.

He says in some cases losses and uncertainties have standard values used throughout the wind industry - 2% uncertainty on cup anemometry being an example, and there are some initiatives to standardise terms within the wind industry.

In practical terms, losses and uncertainties have to be determined where possible based on the project variables and route to obtaining the base data for analysis.

Oldroyd adds that whereas some of these terms are commercial in nature, such as turbine availability etc, issues such as uncertainty in wake estimation is a much more complex and uncertain area. For clarity, wakes in this case are defined as the loss of momentum through a wind farm development due to energy extraction of the preceding turbine, and its affect on the incident wind of each subsequent turbine. He adds that for wakes the issue is two-fold.

The first being ability of the software to accurately describe the affect of wakes in a large array and the susceptibility to yield estimation change of a wind farm due to initial data assumptions (wind distribution, direction distribution and candidate turbines being some of the influencing parameters).

To address these issues the wind industry has a number of initiatives, such as The Carbon Trust Offshore Wind Accelerator campaign, where Oldbaum is instrumenting and acquiring wind speed data offshore using Leosphere and Avent scanning and Nacelle mounted LiDAR systems.

“This helps to address base model issues (or point 1 above), but not the latter. For point 2, a systematic multi variant analysis has to be performed to look at yield variation,” says Oldroyd.

Overcoming shortcomings

Indeed the industry has been pursuing ways to address the scarcity of data that has inhibited the development of wake effects models for offshore wind.

For its part, The Carbon Trust earlier this year came up with a research project focused on curtailing the cost of offshore wind power by stepping up the energy yields from Round 3 wind farms that is to be located up to 290km from the UK coast. This £2m wake effects measurement project is going to offer comprehensive measurement data for one to assess how the wind behaves in complex situations offshore.

As per the information available, specialist wakes monitoring equipment has been installed on the Rødsand 2 windfarm in Denmark and will measure wakes passing through the wind farm for at least six months.

Collaboration

The Carbon Trust has highlighted that industry collaboration on key innovation projects that offer scope for dramatic cost reduction is the best way to go in this arena.

Several companies and university spin-offs are working on improved systems for wind forecasts, says Stephan Schnitger, head of innovation and optimisation, ENERGIEKONTOR.

Future

“There is still uncertainty to forecast accuracy,” says Ralf Neulinger, Director Operations Renewable Energies, EnBW. He says “because contrary to onshore where - depending on locations – we have a high density of met masts and also wind turbines, offshore a sufficient data basis is missing,” he says.

The future system should provide a world-wide accepted base for yield forecasts to support the international structures of the power grid as well as financing.

Specific areas like wakes effects measurement should be considered in order to augment energy yields from offshore wind farms. This way the industry can also look at offering financiers greater certainty on the economics of the projects. Also, it should be highlighted that data garnered will have a striking impact on the future of offshore wind energy.