Response of a self-powered offshore floating support structure with an OWC for powering a LIDAR device

Highlights

• Self-sufficient LIDAR in SPAR buoy with an OWC device.

• 2 PTO configurations in BEM with lab validation.

• Successful application of offshore damping estimate for OWC performance.

Abstract

To give an effective response to the increasing demand to characterise wind availability and intensity for the installation of future offshore wind farms, SPAR buoy OWCs with LIDAR present a clear benefit, due to their stability, mechanical and operational simplicity and lower cost, when compared to wind measuring towers, serving as an alternative to the latter, mainly for deep water.

This paper proposes a compromise between a stable SPAR buoy for wind measurements (average speed, gusts, etc.) and a sufficiently capable energy generating OWC device for long-term self-sustainability. Key parameters such as mass distribution and geometry were accounted for, in order to achieve both outcomes.

The performance of an oceanic SPAR buoy OWC was analysed numerically and experimentally tested at 1:16 scale, under regular and irregular sea states, in order to quantify its hydrodynamic behaviour and capability to generate electrical energy, using two different PTO configurations. Depending on the sea-states, different PTO configurations result in better performance. For higher significant wave heights, the 39% porosity membrane used as a PTO performed better from a power perspective, whereas for lower significant wave height, the 70% porosity membrane presented better results. Low energy consumption wind measuring equipment may benefit from this application.

Introduction

THE main reason for the occurrence of winds is related to the irregular distribution of solar radiation. The interaction between winds and the free surface of the ocean is one of the main actors in ocean wave generation (Cruz e and Sarmento, 2004). Ocean waves are a renewable source of energy and provide a higher energy density when compared to other renewable sources (Clément et al., 2002), thus triggering a considerable amount of research throughout the years.

The offshore wind power industry is experiencing rapid growth globally and, consequently, the demand for wind resource characterisation to assess the feasibility of future wind farms, has been increasing. This characterisation can be performed with more traditional equipment to measure just the wind velocity (e.g. an anemometer), or with a more modern one where the whole wind profile is assessed (e.g. LIDAR). The anemometer is one of the most common equipment to measure the wind speed. While different types are available, anemometers only measure the wind passing at a specific height (McKeogh, 2011). More recent equipment, such as LIDAR's (Light Detection and Ranging), SODARs (Sonic Detection and Ranging) and RADAR (Radio Detection and Ranging), are able to perform remote measurements with good accuracy for different locations at the same time. These are based on the Doppler Effect, the physical phenomenon that relates the wave frequency emissions of a moving body with a stationary observer. The difference relies on the type of emitting waves (e.g., the waves used by a LIDAR are electromagnetic) (McKeogh, 2011).

Adding the necessity of a broader characterisation of the wind, to the fact that there are higher turbines (hence higher masts for the usual anemometer), LIDARs are a good alternative (Shu et al., 2016). Nonetheless, the need for LIDARs to be in a stable position, without significant vibration, poses a challenge when performing offshore wind measurements. In fact, long-term monitoring of the oceans allows for the acquisition of more reliable data regarding several parameters, such as wind average speeds and gusts. To keep costs low and to be in-line with the expected EU increase in productivity to €1 billion per year (European Economic Social Committee, 2014), non-manned, cost-effective solutions are key. This topic has driven researchers to analyse the possibility of using self-sufficient solutions to reduce costs and risks. Renewable energy harvesters, in opposition to batteries, which have to be replaced regularly, have been one of the key parameters used to address this problem yielding mixed results. Studies relating to wind and solar energy harvesters (Løken et al., 2016) have generated good results in terms of harvesting, though failing to meet the energy demand. More recent studies implemented the use of state-of-the-art triboelectric nano-generators (TENG) with improved results, though still in a very early stage of development with a low technology readiness level (TRL) (Chandrasekhar et al., 2020). Finally, wave energy converters have been successfully applied for oceanographic sensing systems in buoys, allowing for the power generation to be sufficient for the sensing system (Henriques et al., 2016).

The study presented in this paper focused on the inclusion of a LIDAR for wind characterisation on a SPAR buoy (typically known for their stable behaviour) equipped with a Wave Energy Converter (WEC) for power supply. The aim is to achieve a compromise between reduced resonance motion of the SPAR buoy and sufficient energy generation. Stability is achieved whenever low responses are obtained. Response amplitudes close or below 1 are desired for minimal movement.

The Oscillating Water Column (OWC) emerges as one of the most studied operational principles for wave energy harvesting. The first device based on the OWC principle was initially named Masuda device, honouring its inventor Yoshio Masuda, who started exploring the concept of a navigational buoy powered by wave energy in the late 1940s (Heath, 1959). In general, an OWC device consists of a partly submerged structure that encloses a volume of air between the free surface and its top (Fig. 1), as well as an opening to the ocean (Heath, 1959; Falcão, 2010). This particularity allows the free surface inside the structure to vary its position due to the action of waves, thus causing the variation of pressure and volume inside the pneumatic chamber (Falcão e and Henriques, 2016). The effects of compression and decompression inside the chamber forces the air to flow in or out through a turbine, coupled to an electrical generator, installed at the top of the device (Falcão, 2010; Falcão e and Henriques, 2016).

To accommodate the oscillatory characteristics of the airflow, turbines usually require rectifying valves for air guidance, in order to maintain the turbine rotating in the same direction independently of the flow orientation (Falcão e and Henriques, 2016). In the mid-1970s, Alan Arthur Wells invented a self-rectifying turbine, also called the axial-flow Wells turbine, with a design that eliminated the need for rectifying valves, thus providing a more cost-effective and robust solution (Falcão, 2010; Takao e and Setoguchi, 2012). Recent developments were made in radial-flow self-rectifying impulse turbines, more specifically in bi-radial turbines. These have shown good efficiency over a wide range of flow rates, due to the aerodynamic performance. However, these turbines add considerable mechanical complexity (Falcão, 2010; Falcão e and Henriques, 2016; Falcão and Nunes, 2012). OWC devices are more advantageous when compared to other Wave Energy Converters (WECs), due to their mechanical simplicity and because all the sensible parts of the power take off system are not in direct contact with the seawater.

Single Point Anchor Reservoirs (SPARs) are a widely studied type of floating structure due to their high stability. Geometrically, these structures are typically considered slender bodies owing to their reduced sectional area at the waterline and significant draught. Commonly the body is axisymmetric, allowing for an equal performance independent of wave direction, with its mass mainly concentrated on the bottom. This set of characteristics provide SPARs with high stability and low response amplitude to the excitation of the waves.

The use of floating platforms in place of research towers, to monitor the wind resource, has been attracting the scientific community. The simplicity and stability of SPAR structures make them strong candidates to compete directly with measuring towers. In addition, their long-term deployment capability in deep waters, as well as easy transport and low maintenance costs, make SPARs an appealing solution.

When considering either fixed or floating structures, a clear challenge is how to power the monitoring equipment. In line with this, an OWC SPAR buoy is presented in this study as an alternative to address the need for a self-sustained wind-resource monitoring solution. An axisymmetric SPAR buoy with reduced diameter was studied, aiming to decrease the amplitude of its response to incident waves and thus minimize the influence of its movements in the quality of the measurements. Inside the chamber region of the SPAR buoy, high response amplitudes, mainly in the heave motion, were desired, such that energy to supply the monitoring equipment on board could be harnessed through the OWC principle.

Due to its simplicity, the OWC principle has been widely studied (Sheng et al., 2014a) where, besides the technical advances in experimental testing, numerical methods were employed for better physical representation. Among these, a prevalent method is the Boundary Element Method (BEM) with the use of ANSYS AQWA (Gaebele et al., 2020; Gao e and Yu, 2018; Cruz e and Sarmento, 2004; Clément et al., 2002), due to the good compromise between accuracy and computational cost. This method represents the water portion as a fixed body and has been providing reasonable results. As such, this method can be applied to different WEC devices, such as point absorbers (Gao e and Yu, 2018) and floating wind turbines (Chen et al., 2019). OWC SPAR buoy dimensioning and testing has been scarce. Numerical dynamic and parametric optimization of an OWC SPAR buoy geometry has been studied, as well as the corresponding optimized air turbine (Falcão et al., 2012; Falcão et al., 2014). Nonetheless, the long-term deployment of a SPAR buoy for wind measuring purposes (requiring low response amplitude operators (RAO) for stability of the measurements) with a self-sustainable energy system (requiring high response amplitude operators (RAO) for energy generation) has not been conducted yet.

In this paper, the numerical study of the OWC SPAR buoy is carried out using a BEM approach and validated with experimental results from physical model tests. The scaled model of the SPAR buoy was tested for selected regular and irregular sea states, defined based on typical values offshore the Portuguese West coast. The data collected allowed for the quantification of the pressure variation inside the pneumatic chamber, as well as for the measurements of airflow rate and the internal water displacement. These variables were used to estimate the power generation capacity. The tests allowed for the characterisation of the hydrodynamic behaviour of the body. To simulate the Power Take-Off (PTO), porous membranes were used to replicate the behaviour of the air-turbine. The air-turbine has linear characteristics, since the generated power is linear proportional on the airflow and averaged pressure, similar to a Wells Turbine.