Abstract
Tidal energy is a predictable and reliable renewable energy resource with significant potential to contribute to a diverse and sustainable electricity mix. As integral components of tidal energy converters, tidal turbine blades operate in challenging conditions, where constant submersion in seawater and high, cyclic loads necessitate thick wall sections (10–100 mm), particularly at the root of the blade.
Tidal turbine blades are typically manufactured from fibre-reinforced thermoset-matrix composites using vacuum infusion processing. Although thermoset matrices have traditionally been preferred for composite applications, the cross-links that provide their desirable mechanical properties also prevent melting and reshaping, limiting recyclability compared to thermoplastics. Increasing interest in sustainable alternatives has driven the development of recyclable, room-temperature processable liquid thermoplastic resins (LTPRs) to replace thermosets in vacuum infusion, offering the potential for more sustainable material solutions.
To validate their suitability for tidal energy applications, it is essential to understand how these LTPRs perform during the manufacture of thick-section composites. Challenges typically associated with composite laminate manufacture are exacerbated in the production of thick-section laminates, including managing the thermal effects of polymerisation to avoid boiling and defects. It is therefore critical to understand the thermal profiles experienced during processing and the resulting laminate quality. Additionally, given the harsh marine operating environment, the gravimetric and microstructure effects of long-term submersion must be evaluated.
This thesis investigates the suitability of LTPRs for the production of thick-section composites used in tidal turbine blades. Current blade manufacturing practices are reviewed, and alternative techniques explored to advance production methods. Two low-exotherm, acrylic-based LTPRs, Elium® 188 XO (E188 XO) and Elium® 191 XO/SA (E191 XO/SA), designed specifically for vacuum infusion, are considered. Thermal analysis studies using embedded thermocouples demonstrated that both LTPRs remained below their boiling points during the manufacture of 10-ply glass fibre-reinforced laminates. E188 XO exhibited a higher peak temperature and interlaminar thermal gradient but reached this peak more quickly, implying a higher manufacturing rate. Further studies with 20- and 30-ply laminates, produced using E188 XO, indicated that interlaminar temperature increases with laminate thickness. Nonetheless, peak temperatures did not exceed the resin’s boiling point, suggesting a minimised risk of exotherm-induced defects. These findings provide critical insights into polymerisation behaviour and support process optimisation.
Variations of vacuum infusion processing were used to fabricate four E188 XO 52-ply laminates, with the baseline setup producing a laminate with a polymerised thickness of 42.5 mm. Surface temperatures varied between configurations, but remained below the boiling point of the resin. Notwithstanding, higher internal temperatures may have been reached, but external instrumentation enabled enhanced vacuum integrity during processing. Thickness variation was observed in all polymerised laminates. Fibre volume fraction and void content varied along the length and through the thickness of each laminate but were found to be relatively uniform across the width in most laminates. Of the four configurations assessed, notably lower void contents were observed in laminates produced with a semi-permeable membrane and the baseline setup. Quantitative results suggest a variation in laminate quality along the length as well as through the thickness of thick-section laminates.
The long-term water uptake behaviour of the two LTPRs was assessed through an experimentally simulated 20-year service period. All 10-ply glass fibre-reinforced laminate samples reached saturation within this time. E191 XO/SA exhibited substantially faster water uptake, with considerable deviation in behaviour between samples. Samples extracted closer to the resin inlet end of the laminate exhibited rapid increases in mass that continued over time, while those from nearer the vacuum outlet showed a more gradual mass increase, followed by a steady decrease. Sample position from the laminate appeared to have a stronger influence on water uptake than delamination caused by cutting of the samples from the laminate. The variation in behaviour between samples from the same laminate is attributed to excessive voids towards the inlet end. Samples from E188 XO 20- and 30-ply laminates did not reach saturation, though inlet-end samples again showed higher water uptake and void content in the 30-ply laminate.
These results highlight the challenges in thick-section manufacture and the complex relationships between materials, manufacturing, microstructure, and long-term behaviour in a marine environment. While variations in quality through the thickness are apparent, this work highlights that variations across laminates, including both length and thickness, should be carefully considered and managed during production. Overall, this thesis provides novel insights into the polymerisation behaviour, laminate quality, and water uptake performance of LTPRs in thick-section composites. These findings offer value to academics and industrial practitioners to support informed and strategic choices to improve and advance tidal turbine blade production.