Offshore Wind Farm Optimisation

Abstract

Wind farm layouts in industry show a range of patterns with an overall trend from regular to irregular patterns over time. Wind farm layout optimisation studies in literature generally result in irregular wind turbine patterns. Review of existing literature shows that the performance of irregular and regular wind farm layouts has not been compared in a consistent way. Although there are indications that irregular layouts outperform regular layouts, it is yet to be determined if this is the case for the overall performance and if so, to what degree. In this research the effect of regular and irregular wind farm layouts on selected performance indicators is quantified. This quantification is performed through means of a comparative case study.
The performance of both regular and irregular wind farm layouts is assessed on the basis of three performance indicator groups: (1) power performance; (2) wake-induced tower fatigue; and (3) inter-array cabling system. The performance indicators in these groups are affected by a change in wind farm layout, feasible, site independent, and technical as concluded from a multi-criteria decision analysis.
The irregular wind farm layout has a higher annual energy production and a higher persistence to wind direction. The net present value of this increase in cash flow over the lifetime of the wind farm is estimated at 10 million Euros. A higher persistence to wind direction means that the power output is less sensitive to fluctuations in wind direction. This characteristic increases the predictability of the wind farm power, which can indirectly lead to a decrease in imbalance cost on the electricity market.
The wake induced tower fatigue is found to be negatively impacted by an irregular wind farm layout. Implementation of the Frandsen model shows that the maximum effective turbulence of the irregular wind farm is 23.8% higher than that of the regular wind farm layout. For fatigue-driven tower design, this leads to an increase in tower wall thickness, which in turn results in an increase in tower material consumption. The increase in tower cost in the wind farm is estimated at 4 million Euros. Application of a minimum inter-turbine spacing to ameliorate the negative effect on effective turbulence is can lead to a decrease of 20 % (as compared to the case study).
The inter-array cable design results show a marginal increase in cable cost of 1.15 % for the irregular wind farm. The analysis reveals that this performance indicator is strongly dependent on site-specific input data. Due to this marginal change and dependency on site specific input data, this performance indicator is omitted from further conclusions.
Comparing the negative effect of the tower cost and the increase in revenue due to the higher AEP, the net present value is computed. With a discount rate of 5 %, the net present value of the AEP reduced by the increase in tower cost results in an increase of 6 million Euros.
For improved performance in future wind farm layouts, the implementation of irregular wind turbine patterns is advisable. That is, with the proviso that the minimum inter-turbine spacing is taken into consideration with respect to wake-added turbulence levels.