Multi-rotor wind turbines: A review of modern research efforts and challenges

The wind turbine industry faces mounting challenges in material usage and disposal, as well as in meeting the growing demands of the renewable energy market. Scaling up single-rotor wind turbines presents significant challenges, including the manufacturing, testing, transportation, operation, and maintenance of blades exceeding 100 meters in length. As these challenges grow, rising capital and operational costs (CapEx, OpEx) may offset the energy gains of larger rotors, making their impact on levelised cost of energy (LCoE) uncertain. Multi-rotor wind turbines (MRWTs) show great promise in reducing LCoE through their advantageous scaling laws. Their smaller individual rotors enable the use of more sustainable materials and streamlined operations and maintenance (O&M), addressing both economic and environmental shortcomings of single-rotor designs. Despite the potential for MRWTs to address these issues, the concept is still in the early stages of validation, with little empirical data supporting their reported advantages. Published MRWT research over the past decade spans numerous disciplines, including aerodynamics, materials, structures, design, control, sustainability, reliability, and maintenance, amongst others. This review aims to provide a broad yet concise overview of the field, catering to both newcomers and experts alike. It seeks to raise awareness of MRWTs, highlight their advantages and ongoing challenges, as well as identify under-explored areas for future investigation. Key recommendations include quantification of the environmental impact benefits of MRWTs, improved characterisation of rotor–rotor interaction effects, exploration of novel control strategies, the investigation of aerodynamic noise, comprehensive aero-servo-elastic design studies and many more. • Comprehensive aero-servo-elastic design studies on multi-rotors are scarce. • Experiments on the effects of rotor interactions are rare and limited in relevance. • Reported multi-rotor LCoE savings are inconsistent and lack authoritative evidence.