In this paper, a performance analysis model of heaving float wave energy converter/conversion (WEC) with liquid metal (LM) magnetohydrodynamics (MHD) generator (LMMHD-WEC) was developed. Performance analysis was carried out and a 5kW LMMHD- WEC prototype device with system efficiency of 27% was designed. The results reveal that working medium propery is the key factor influencing system performance. A LMMHD-WEC has different performance characteristics in different wave conditions and the best performance can be derived by adjusting loading parameters.
Ocean wave energy may become an unquestionable alternative to actual clean energy sources in the near future. The global wave energy (including that on the open ocean) is in the order of 1013W (Falnes, 2007), a quantity that is comparable with the world's present power consumption. China also has abundant wave energy resource with a theoretical value of 1.284×1011W (Wang, 2009), therefore development and utilization of wave energy converter/conversion (WEC) become the world's hotspot.
WECs usually consist of three parts, which are the wave capturing system (for the first stage conversion), the intermediate system (for the second stage conversion) and the generator (for the third stage conversion). The first stage conversion converts wave energy into the mechanical energy of wave capturing system (including oscillating water column, oscillating float, nodding duck, pendulum, raft, overtopping, et al). The second stage conversion (including hydraulic systems, turbines, gears, levers, drive belts, bearings, et al) converts the slow linear/rotation motion of wave capturing system into specific form of mechanical energy for the third stage conversion. The third stage conversion converts the former mentioned mechanical energy into electric power. The second stage conversion and the third stage conversion are collectively called power take-off mechanism (PTO).
There are mainly two problems existed in traditional WECs:
Rotating generators (high velocity, high frequency and rotating characteristics), which do not match the mechanical impedance of wave energy (low velocity, low frequency and reciprocating characteristics), are used traditionally for the third stage conversion. It is complex, bulky, low efficient and expensive as complex intermediate systems exist. What's more, linear electric generators used recently are with drawbacks of huge volume and high cost and the attractive force between stator and motor causes big problems for bearings (Polinder, 2005).
In most areas, wave energy density is so small, for example only 2.0~6.0 kW/m in the coastal areas of China, that most WECs with current technologies cannot operate or work well in such low wave conditions (Zhang, 2012).