# Modeling and Simulation of a Wave Energy Converter _Nwave_
## 1 Introduction  
Since industrialization, the use of fossil fuel energy has increased exponentially. Consequently, the awareness and concern for the entirety life on Earth is growing due to side effects, such as global warming. As a result, the importance of renewable energy is on the rise, and energy sources that use wind, sun and geothermal heat are at the commercialization stage [1]. Among the sources for renewable energy that await commercialization in the near future is wave energy [2]. Although the idea of wave energy has existed for a long time, the commercialization has been slower than other sources of renewable energy. Unlike sunlight or wind that have relatively uniform energy flow, wave energy has a reciprocating motion, high energy density with slow speed and seasonal variations in heights and periods [3]. As a result, wave energy converters (WECs) must have a complicated mechanism, solidity and flexibility corresponding to each attribute. In addition, since waves generally have higher energy density at a deep water depth and the density becomes lesser as they get closer to the shore, it is appropriate to install the WECs in the distant sea, which has a deep water depth [4]. Hence, the accessibility is poor, and a long sub-sea transmissionline is needed. As the accessibility is low, WECs should be more robust so that less maintenance is required. To reduce the proportion of the cable installation cost in the capital expenditure of the WEC, a large-capacity wave energy farm should be built. Consequently, it requires more complete initial verification than other renewable energy sources for commercialization [5]. One of the aforementioned disadvantages of wave energy is that the area of high wave energy density is far away from the on-grid. However, depending on the location, it can actually work as an advantage. One of these locations would be remote islands that are off-grid and therefore do not receive electricity from the mainland. Diesel generators are used for sourcing electricity in these islands. Since the power production unit price of a diesel generator is higher than that of the mainland, renewable energy sources in the islands are cost competitive. This has been represented in the example of Samse Island in Denmark [6]. Wave energy can be seen as the most suitable option for creating an energy self-sufficient island. In consideration of such a situation, the wave power company INGINE has created a wave power generator called _INWAVE_. Currently, there are three full-scale prototype devices installed in Jeju Island as a test run [7]. The company seeks to start commercial operations in the latter half of 2016. This device, a tight moored moving body-type WEC, shares similar concepts with [PERSON]'s Lifesaver WEC, in that the buoy has six degrees of freedom for movement and is connected to the sea-floor via stiff mooring ropes. While the Lifesaver [8] is installed at a medium water depth of about 50 m, _INWAVE_ is installed along the coast. The _INWAVE_ device can follow the theoretical analysis of the Lifesaver since the basic concept is similar to this device. In the papers dealing with the Lifesaver, however, there is no modeling description for the multi-mooring and multi-degree motion of the WEC, but a point absorber modeling with heave motion is only described. In [9], meanwhile, a multi-point tight moored body-type WEC is presented. However, only the frequency domain analysis is applied to the optimal control theory. Hence, there are limits to using this modeling approach directly. In order to realize the modeling of the _INWAVE_ device, it is necessary to derive the kinematic relations of mooring ropes between the buoy and PTO (power take off) module and to build up the ratchet gear modeling in the PTO module. The ratchet gear is a mechanical element that converts the reciprocating motion of the mooring rope into a uni-directional rotation to the generator. This process is determined by the relationship between the angular velocities of the rope drum and the generator.