Design methodology of hybrid turbine towards better extraction of wind energy

Highlights

• Systematic experimental and numerical study of Savonius and Darrieus turbines completed to identify key design parameters.

• Radius ratio of Savonius and Darrieus turbine in the hybrid configuration shown to be an important parameter.

• Based on the expression of optimal radius ratio derived here, a new design methodology for hybrid turbine proposed.

• A new index called effectiveness is proposed to evaluate the performance of hybrid turbines.

• Effectiveness of optimal radius ratio and proposed design methodology demonstrated using experimental results.

Abstract

Hybrid vertical axis turbines that combine Savonius and Darrieus turbines on a single shaft have been proposed as a way of combining the excellent starting torque of Savonius turbine with the high operational efficiency of the Darrieus turbine. Although hybrid turbines with improved starting characteristics have been demonstrated in literature, the performance of these turbines at higher tip speed ratios have been poor. In this work systematic study of stand-alone Savonius and Darrieus turbines have been carried out using experimental and numerical techniques as a precursor to studying their roles in hybrid configuration. The radius ratio of the two turbines, when combined in the form of a hybrid turbine, is identified as an important parameter that dictates the performance of hybrid turbines. An expression for an optimal radius ratio is derived and a methodology for designing hybrid turbines is proposed. The efficiency in energy conversion by hybrid turbine can be expressed in terms of a parameter called effectiveness given by the ratio of power produced by the hybrid turbine to the sum of the power produced by individual Darrieus and Savonius turbines. This idea has been verified through experiments and numerical simulations.

Introduction

Over the last decade there has been a spurt in research and development of renewable energy technologies as a result of an increased attention to climate change and a shift in the public policy towards more environment friendly power systems. This requires a significant shift from coal and other fossil fuel based power generation towards renewable energy sources such as wind energy. Wind turbines can be broadly classified into horizontal axis turbines (HAT) and vertical axis turbines (VAT). In the past, VAT has largely been overshadowed by HAT due to its superior efficiency [1]. However, the unique advantages of VAT such as simplicity of design, non-requirement of yaw mechanism, ease of manufacturing etc. have resulted in renewed attention on VAT as wind turbines for powering remote areas or for rooftop applications in urban areas. These advantages also give VAT a unique advantage for use as hydrokinetic turbines.

Darrieus and Savonius turbines are the two most common designs of VAT. Savonius turbine is a simple drag-based turbine where the pressure of the fluid stagnating within its blades causes the turbine to rotate. This turbine has high starting torque but its operational efficiency is low. Darrieus turbine is a lift-based turbine wherein the lift produced as the fluid moves past the aerofoils produces the necessary torque to rotate the turbine. Darrieus turbine can achieve high power coefficient ($C_p$). However, the aerofoils being mostly stalled at low speeds, Darrieus turbine produces very little starting torque and has difficulty in self-starting. It may be noted here that ‘starting’ has been defined by Ebert and Wood [2] as the condition when the turbine starts rotating from a stationary position and reaches a speed at which significant power can be extracted from the turbine. Although Darrieus turbine starts rotating at no-load condition even in low-velocity air streams, it is unable to accelerate to high speeds or produce any usable power due to a region of negative torque, called the ‘dead-band’, that is present at low tip speed ratio (TSR, $\lambda =R\omega /V_{\infty }$ ) [3].

The combined Darrieus-Savonius turbine or the hybrid turbine was designed by combining the two turbines with the aim of producing a turbine that has the high starting torque of Savonius turbine and the high efficiency and TSR of Darrieus turbine. A hybrid turbine can have two possible arrangements. In the first configuration, the two turbines are stacked one on top of another on a single shaft with the Savonius turbine usually at the bottom. In the second configuration, Savonius turbine is tucked away within the blades of Darrieus turbine [4].

The ERIGEN combined Darrieus-Savonius turbine was one of the first hybrid turbines to be field-tested. Ericsson Power Systems, the company that developed these turbines, installed several of them across remote locations as power source for telecommunication towers and for powering a remote light house in the Baltic Sea. The report by Åkerlund [5] on the performance of these turbines states that the turbines performed reliably and with minimal maintenance, providing nearly 80% of the power requirement of the sites. However, the efficiency of the turbines are not reported. This turbine has demonstrated the feasibility of hybrid turbines as reliable small-scale power systems especially for remote locations.

Wakui et al. [4] evaluated the two hybrid turbine configurations and observed that the configuration with Darrieus turbine stacked on top of Savonius turbine is a better choice for small scale power applications. They also noted that the hybrid turbine configuration with Savonius turbine tucked inside Darrieus turbine produces less power because of the flow interference between the two rotors. Wakui et al. [4] had also noted that the radius ratio of the two turbines is an important parameter deciding operation of hybrid turbine and that when the radius ratio is large, the power coefficient of the hybrid turbine drops sharply because Savonius turbine enters role reversal and dissipates the power being generated by the Darrieus turbine.

Kyozuka [6] fabricated a hybrid turbine with a two-bladed Darrieus turbine and a Savonius turbine with two buckets placed inside the Darrieus turbine. This hybrid turbine was tested in water and demonstrated better starting characteristics than stand-alone Darrieus turbine, but the efficiency of the combined turbine was lower than the solo-Darrieus turbine. A ratchet mechanism was introduced to separate the Savonius turbine from the Darrieus at higher speeds and prevent the role reversal of Savonius turbine from affecting the performance of hybrid turbine. However, this addition did not yield improved results. Kyozuka suggested that the reason for the lack of improvement may be due to the flow field changes made by the Savonius turbine that is affecting the Darrieus turbine.

Alam and Iqbal [7] designed and tested a hybrid turbine that used the radius ratio results from the work of Wakui et al. [4]. This hybrid turbine was tested in water and also showed good starting characteristics. But the power produced was much less than the predicted value and the turbine could not be tested at high speeds because of excessive vibration of the turbine. The authors attribute the poor performance of the turbine, in terms of power produced, to drag from the radial arms of the turbine and high bearing friction in the drivetrain.

Kou et al. [8] tested a hybrid turbine design with an overrunning clutch to disengage the Savonius turbine at high speed. This hybrid turbine has also shown improved starting characteristics than solo Darrieus turbine with the authors reporting a cut-in speed as low as 1.4 m/s for the hybrid turbine as against 4.7 m/s for the stand-alone Darrieus turbine. This hybrid turbine also matched the performance of Darrieus but the power produced was less than the value estimated theoretically for a hybrid turbine.

A survey of literature shows that hybrid turbines have successfully demonstrated better starting characteristics but have poor operating characteristics compared to solo Darrieus turbines, particularly at higher TSRs. An effective turbine design methodology that can maximise the power produced from both Savonius and Darrieus turbines is yet to be demonstrated. The objective of this work is to develop a design methodology that can effectively combine a Savonius turbine with a Darrieus turbine and thus improve the performance of hybrid turbine. Towards quantitative assessment of the performance of hybrid turbine over stand-alone Darrieus or Savonius turbines, a new performance index has been proposed. In this work, experimental and numerical analyses of Savonius and Darrieus turbines are reported. Important design constraints arising out of the study of the stand-alone turbines pave the way for an improved understanding of the design of hybrid turbines. Turbines meeting the constraint of a particular radius ratio are shown to yield a better performance index. A design methodology for hybrid turbines incorporating this radius ratio and the insights gained from study of stand-alone turbines is suggested.