# Experimental Validation of a Gyroscope Wave Energy Converter for Autonomous Underwater Vehicles
## 1 Introduction  
Autonomous underwater vehicles (AUV) are a type of unmanned underwater vehicle (UUV) which are irreplaceable in a variety of deep-sea resource surveys. With the rapid advancement of technology, AUV has been vigorously developed and is increasingly being valued by military marine technology departments in developed countries. However, some technologies still need to be improved, such as motion control, navigation, and power supply, etc., where power technology plays a crucial role in the completion of AUV tasks [1]. However, AUVs cannot perform long-term underwater missions because of their limited energy storage [2]. In recent years, the development of power systems and new energy sources has made considerable strides thanks to the efforts of various institutions and scientists. Currently, the solutions for power sources fall into three main categories, including batteries, docking stations and environmental energy [3]. Among them, the battery power supply includes primary batteries [4], secondary batteries [5], fuel cells [6], semi-fuel cells [7] and a hybrid power system of batteries [8], which have the advantages of a small size, high energy density, and direct power supply. However, its energy is limited, battery leakage can cause pollution to the marine environment, and it takes too long to replace the battery. Docking stations can extend the service time of the AUVs, thus ensuring mission accomplishment. However, AUVs need an additional navigation system to reach the designated site, which increases commercial costs. Environmental energy mainly includes solar energy [9], thermal energy [10] and wave energy [11], which has huge energy storage and is widely regarded as the main source of energy for AUVs in the future [12]. Among them, solar energy can directly convert energy into electricity, but its conversion efficiencyis only 10-12%, which restricts the development of charging systems [13]. Thermal energy comes from solar energy, which is less affected by environmental changes, but is limited by geographic location [14]. As for wave energy, it is only related to wave height and period, so its application is more extensive [15]. Wave energy has attracted widespread attention from the scientific community and the energy industry because it is easy to predict and available. Wave energy resources and potential have been evaluated in many recent studies, such as coastal sea areas [16], sea areas with different energy flow densities [17] and lakes [18]. The collection of wave energy depends on the development of wave energy converter (WEC) technology. The first device to convert wave energy into electrical energy was created in France in 1799 [19]. In recent years, breakthroughs in wave energy technology have accelerated the development of the industry, and maritime trials have also achieved promising results [20]. The general wave energy converters are point absorbers [21; 22], oscillating water columns (OWC) [23; 24] and flaps [25]. The annual energy varies from 6.90 to 15.64 GWh for Wave Dragon WEC and from 0.45 to 0.97 GWh for Pelamis WEC in the sea area with an energy flow density of 7.83 to 29.94 kW/m [26]. The feasibility study of energy harvesting in the Italian seas shows that large WECs are idle for most of the year, but smaller WECs have significantly higher capacity factors [27], especially suitable for energy in the lower waters [28]. With the widespread application of gyroscopes on ships for stabilization [29; 30], a gyroscope wave energy converter (GWEC) device has become a research hotspot. At present, Japan [31; 32; 33], Italy [34; 35] and Spain [36] are relatively mature in this technology area. The application of small GWEC to AUVs has become a future development trend due to it being sensitive to external stimuli. In this paper, a novel wave energy conversion system for AUV is described and mathematically modeled. The hydrodynamic response of the hull to various cycles of waves is obtained using ANSYS/AQWA software.