Experimental Investigation of a Novel Direct Mechanical Drive Wave Energy Converter Vishnu Vijayasankar1, Gundi Amarnath2, S. A. Sannasiraj3, Abdus Samad4 Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai 600036, India [email protected] [email protected] [email protected] [email protected] Abstract— In the present work, a unique design using a direct mechanical drive based power takeoff system is Indirect Drive proposed, designed, fabricated and tested for a point absorber. The power take-off mechanism in the design Rotary Air Chamber/ Air Turbine/ Generator consists of rack and pinion arrangement, which converts Accumulator Hydro Turbine the bidirectional reciprocating motion to unidirectional rotation. This mechanism is fixed on top of an oscillating G Direct Mechanical buoy. The model is tested in a 4m wave flume to measure Drive various parameters like Response Amplitude Operator of the buoy, capture width, electric power output and Direct Drive Linear Generator absorption efficiency for different wave height and Direct Electrical Drive periods. The model is also tested for friction outside the To Grid flume. The experiments revealed some promising results Fig. 1 Flow paths for different PTO mechanisms that can be taken forward for further investigations. Out of all PTO mechanisms, the most widely used one is the air turbine coupled to an oscillating water column, which Keywords— wave energy, point absorber, rack and pinion, is an indirect drive mechanism. Even though it has high power take-off mechanism. reliability due to simplicity in working, the efficiency is very low [2][6]. Pelamis [7] uses another indirect drive method I. INTRODUCTION using accumulators and rotary generators. However, compared The types of wave energy converters (WECs) are to OWCs, this system lacks reliability and is prone to leakage categorized based on their principle of operation, shape or the problems. location. Articles like Cruz [1], Falcao [2] and Clement et al. Since, in direct drive systems, only up to three energy [3] will provide an insight into various existing energy conversions are required, these systems have higher efficiency. converters. Point absorbers are wave energy converters with A direct electrical drive system usually consists of a linear its horizontal length much smaller than the wavelength and generator that directly converts the reciprocating motion into convert the heave motion of the buoy imparted from the electricity. This PTO is successfully used in different projects waves into linear or rotational motion, which in turn is used to like Lysekil Project [8]. However, in the case of direct run an electric generator with the help of a power takeoff electrical system, the highly precise design is required since mechanism (PTO). These type of WECs are advantageous fine air gaps are to be maintained between the stator and since they can extract energy from a wavefront of length much translator. Thus, direct mechanical system gets an edge over more than their geometric width [4]. other forms of PTO in terms of efficiency and simplicity. For PTO mechanism, which is the most fundamental the same reason, direct mechanical drive System is chosen to component of a WEC, converts the absorbed wave energy into be the PTO mechanism for the present work. electricity. Wave energy is absorbed by using systems like Some notable work is been done in the field of direct Oscillating Water Columns (OWC) or buoys. The PTO mechanical drive PTO mechanisms. In 2011, H. Karayaka et system not only determines the efficient conversion of al [9] proposed a system using components of an IC engine to absorbed power but also affects the mass, the size and the drive a rotary generator. But, the system was very complex structural dynamics of the whole system [5]. and it was idle in the occasion where the wave breaks before The flow paths of different PTOs are shown in Fig. 1. reaching the system. In 2013, a project named Lifesaver was proposed by J. Sjolte et al [10] that uses a winch and rope system coupled to a rotary generator as the PTO system. The main disadvantage of this system was that the generator can only produce power during upwards motion. In 2015, J. Ai et A. Equation of Motion al [11] proposed a system that uses a mechanical motion Let us consider a system having a coefficient of damping rectifier mechanism as the PTO. Compared to other direct 푏푑 and an external harmonic force with an amplitude 퐹퐴 and mechanical drive systems, this system proved to be simpler angular frequency 휔 is applied on it. Then, the equation of and efficient. But, here the weight of the PTO and generator is motion for the system will take the form: supported by buoy, reducing system efficiency. In the present work, the same idea is conceived for PTO, but the design is 푑2푧 푑푧 altered so that weight of generator is supported separately by 푚 + 푏 + 푘푧 = 퐹 sin 휔푡 (1) 푑푡2 푑 푑푡 퐴 the spar. Thus the buoy will experience lesser damping and enhanced heave response. II. NOMENCLATURE Abbreviations WEC Wave Energy Converter k b PTO Power Take-off d OWC Oscillating Water Columns RAO Response Amplitude Operator DAQ Data Acquisition System MMR Mechanical Motion Rectifier m z SS Stainless Steel F Symbols 푏 damping coefficient [kg rad/s] Fig. 2 A mass-spring-damper system 푑 퐹퐴 amplitude of harmonic force [m] On solving, we get the critical damping coefficient, 휔 angular frequency [rad/s] 푘 stiffness [N/m] 푏 = 2 푘푚 = 2푚휔 (2) 푚 mass [kg] 푐 푛 휔 natural frequency [rad/s] 푛 Where, the natural frequency of the system is given by: 푏 critical damping coefficient [kg rad/s] 푐 damping ratio 휁푑 퐹 excitation force [N] 푘 푒푥 (3) 휔푛 = 퐹푟푎푑 radiation force [N] 푚 퐹 hydrodynamic damping force [N] 푕푦푑푑푎푚푝 퐹푒푥푑푎푚푝 external damping force [N] The ratio of the damping coefficient to the critical damping 퐹푟푒푠 restoring force [N] coefficient is called the damping ratio, 휁 3 푑 휌 density of water [kg/m ] ζ heave amplitude [m] 푏푑 η wave amplitude [m] 휁푑 = (4) 푏푐 휆 capture width [m] 푃푎푏푠 absorbed power [W] The motion of the concerned point absorber is assumed to 푃푤푎푣푒 wave power [W] be heaving mode only. If the buoy makes a vertical 퐹푓푟푖푐 frictional force [N] displacement of z from mean position, the equation of motion of point absorber is: 푚푡 accelerated mass [kg] 푚푠 standard mass [kg] 2 푧 vertical displacement [m] 푑 푧 푚 = 퐹 + 퐹 + 퐹 + 퐹 + 퐹 (5) 푧 vertical acceleration [m/s2] 푑푡2 푒푥 푟푎푑 푟푒푠 푕푦푑푑푎푚푝 푒푥푑푎푚푝 푡 time [s] Where, m is the combined mass of the buoy and the load 푑2푧 above it and is its acceleration in heave direction. 퐹푒푥 is III. HYDRODYNAMICS OF POINT ABSORBERS 푑푡 2 the excitation force , 퐹푟푎푑 is the radiation force, 퐹푟푒푠 is the The behavior of a point absorber can be studied similar that restoring force given by the difference between buoyancy of a mechanical mass spring damper system. The system is force and gravity force, 퐹 is the hydrodynamic considered to have one degree of freedom, i.e heave and to be 푕푦푑푑푎푚푝 acted upon by an external excitation force, i.e wave force. damping and 퐹푒푥푑푎푚푝 is the external damping force imparted These concepts are extensively discussed in [12] and [13]. by the PTO mechanism. For smaller diameter buoys, radiation effects are negligible uses a much more efficient rotary generator. Also, when therefore radiation force, 퐹푟푎푑 is overlooked in the present compared to other indirect drive mechanisms, the proposed work. However, during scaling up, one may have to consider system proved to be more efficient. the effects of radiation as well. Stainless steel SAE 316 was chosen as the fabricating material owing to its corrosion resistance, weldability, 퐹푟푒푠 = 휌푉 푡 − 푚푔 (6) strength, workability, and availability. A scaling factor of 1:6.6 is chosen to fabricate a prototype for the proposed Where 휌 is the density of water and 푉 푡 is instantaneous system. All the experiments and simulations are done for the draft volume. scaled model. A CAD design of the prototype used for testing Assuming damping forces to be linear, is shown in the left side of Fig. 3 and the PTO mechanism used is shown on the right side of Fig. 3. 푑푧 퐹 = 푏 (7) A. Buoy 푕푦푑푑푎푚푝 푕푦푑 푑푡 The buoy is manufactured in the shape of a cylinder with a 푑푧 diameter of 600mm, a height of 400mm and a thickness of 퐹 = 푏 (8) 푒푥푑푎푚푝 푒푥 푑푡 2mm. The buoy weighs around 30kg and can provide buoyancy more than 50kg. A linear bearing is fitted inside the Where, 푏푕푦푑 and 푏푒푥 are called damping coefficients. buoy so that it can reciprocate on the spar with minimal friction. Two clamps are fitted on top of the buoy in order to B. Response Amplitude Operator fix the PTO mechanism. The stability test was often The Response Amplitude Operator is the ratio of the heave performed on the buoy during fabrication. amplitude of the buoy to the wave amplitude. RAO establishes B. Spar a relationship between the input and output characteristics of a point absorber. RAOs play an important role in designing a Spar is a cylindrical component passing through the buoy WEC with maximum power absorption. For a point absorber, about which the buoy reciprocates. It supports the weight of since heave motion is responsible for power generation, RAO generator and gears. In the full-scale design, the spar is to be can be calculated as: made separately buoyant from the buoy. However, in the ζ prototype, the spar is made from a Stainless Steel hollow tube 푅퐴푂 = (9) η with 48mm outer diameter, 42mm inner diameter, and 2m in length.