Wind Farm Repowering: Industry and researchers come together

06 Dec 2018

When the blades stop turning

What are we to do as onshore wind farms reach the end of their consented life? In our latest research we seek to understand how to support repowering of onshore wind farms sustainably.

Onshore wind, as the biggest source of renewable electricity in the UK, is still considered a crucial part of the UK renewable energy portfolio, with a keenness to grow this industry. Just last week international stakeholders gathered for the Renewables UK Onshore 2018 conference “Growing Industry, Powering the Future” 

The options

How to grow this industry as sustainably as possible, with minimal cost increase to the customer, and without government subsidy?

One way is to maximise energy generation from existing infrastructure at minimal cost, a timely consideration as older onshore wind farms approach the end of their consented life. Currently there are three scenarios for these sites:

  • decommission the wind farm,
  • implement an end-of-life extension, or
  • repower the existing wind farm.

Repowering increases electricity generation per unit area and wind speed capacity, by the installation of a larger or more efficient wind turbine. So, end-of-life extension and repowering can help meet the demand for growth of onshore wind renewable provision. But how to address the issue of sustainability?

Building on firm ground

In collaboration with the renewable industry, engineers, regulators, and others with interest in the end-of-life planning, we tackled how to repower sustainably through focusing on turbine foundation construction.

This was our focus as foundations require considerable resources, and current designs will not be fit-for-purpose for larger turbine generation capacity and size. As such this research was funded by from the Construction Scotland Innovation Centre.

Our first order question was whether to reengineer existing foundations, or build new foundations? There is no blueprint to answer this question – repowering of the scale needed and in relevant locations has not taken place.

Minimal soil disturbance is important, particularly in Scotland, which hosts most UK onshore wind renewable capacity, with approximately 50% of wind farms are on carbon-rich peatland soils. Disturbance can reduce capacity of these soils to sequester carbon and affect payback time (the number of years of renewable energy generation to offset fossil-fuel emissions from construction activities).

Growing pains

The report uses, as a framework, foundation design, but covers so much more.

Current foundations cannot be reused for larger turbines, and likely would need reengineered just to be replaced with the same size turbines, as beyond their design life. Reengineering existing foundations is possible, and the technical drawings (from Arup) for these reengineering foundations are in the report.

However, on almost everything, from the size of excavation to the quantity of materials used to build a foundation for a larger turbine, the environmental footprint of reengineering a foundation is mostly larger than a new foundation (detailed in the report).

Industrial collaborators considered reengineering the foundation would be 2- 3 times more expensive than a new, which translates into a multi-million pound increase in the cost of a typical >50MW wind farm. So, reengineering a foundation challenges cost control.

Carbon

The report outlined the environmental impacts that must be considered, but with no repowered sites this was based on transferable understanding. We still do not have a concerted body of knowledge that clearly identifies how quickly, or otherwise, carbon-rich soils hosting wind farms return to sequestering carbon after disturbance.

This is even more important with repowering if foundations are ‘abandoned’. Constructing a new foundation does cause more surface disturbance (to the vegetated zone that fixes carbon) than reengineering the existing foundation.

We identified how the ‘carbon-payback calculator’ could be easily adapted for wind farm repowering, but we did not quantify differences in payback time. Understanding than projecting an environmental response is still needed.

Conclusion: a promising opportunity

Onshore windfarm repowering will proceed, informed by research like this. And if industry and environmental regulators want, there is now an opportunity to use the early repowered sites as natural laboratories to assess both effective restoration and C-payback time.

Access the report in full here.