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Review

Wave Energy Conversion Technology Based on Liquid Metal Magnetohydrodynamic Generators and Its Research Progress

by
Lingzhi Zhao
1,2,* and
Aiwu Peng
1,2,*
1
State Key Laboratory of High Density Electromagnetic Power and Systems, Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, China
2
School of Engineering Science, University of Chinese Academy of Sciences, Beijing 100190, China
*
Authors to whom correspondence should be addressed.
Energies 2025, 18(17), 4615; https://doi.org/10.3390/en18174615 (registering DOI)
Submission received: 25 June 2025 / Revised: 19 August 2025 / Accepted: 28 August 2025 / Published: 30 August 2025
(This article belongs to the Special Issue Advances in Ocean Energy Technologies and Applications)
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Abstract

Wave energy is a highly concentrated energy resource with five times higher energy density than wind and at least ten times the power density of solar energy. It is expected to make a major contribution to addressing climate change and to help end our dependency on fossil fuels. Many ingenious wave energy conversion methods have been put forward, and a large number of wave energy converters (WECs) have been developed. However, to date, wave energy conversion technology is still in the demonstration application stage. Key issues such as survivability, reliability, and efficient conversion still need to be solved. The major hurdle is the fact that ocean waves provide a slow-moving, high-magnitude force, whereas most electric generators operate at high rotary speed and low torque. Coupling the slow-moving, high-magnitude force of ocean waves normally requires conversion to a high-speed, low-magnitude force as an intermediate step before a rotary generator is applied. This, in general, tends to severely limit the overall efficiency and reliability of the converter and drives the capital cost of the converter well above an acceptable commercial target. Magnetohydrodynamic (MHD) wave energy conversion makes use of MHD generators in which a conducting fluid passes through a very strong magnetic field to produce an electric current. In contrast to alternatives, the relatively slow speed at which the fluid traverses the magnetic field makes it possible to directly couple to ocean waves with a high-magnitude, slowly moving force. The MHD generator provides an excellent match to the mechanical impedance of an ocean wave, and therefore, an MHD WEC has no rotating mechanical parts with high speeds, no complex control process, and has good response to low sea states and high efficiency under all working conditions. This review introduces the system composition, working process, and technical features of WECs based on MHD generators first. Then, the research development, key points, and issues of wave energy conversion technology based on MHD generators are presented in detail. Finally, the problems to be solved and the future research directions of wave energy conversion based on MHD generators are pointed out.

1. Introduction

With the transition of the global energy mix from highly polluting fossil fuels to low-pollution clean energy, the advantages of renewable energy are prominent, and the proportion of renewable energy generation is increasing. Solar and wind energy are the main renewable energy sources. According to the statistics of IRENA, an international renewable energy agency, solar and wind energy currently account for 41.93% and 25.47% of the total renewable energy generation, respectively [1]. Solar power generation can be divided into two technological schemes: photovoltaic (PV) and solar thermal power (STP). PV power generation uses photovoltaic conversion to convert low-grade solar radiation energy into high-grade electricity, and the main component is the solar panel. It is the fastest-growing and most widely used form of renewable energy in recent years because of its flexibility to meet a range of electricity supply needs and the low cost of USD 30.39 per MWh in 2025 [2,3]. In 2024, the global PV industry continued to grow rapidly, with 451.9 GW of new installed capacity and a year-on-year increase of 32.2%, accounting for 77.3% of the total new renewable energy for the year [1]. However, in practical applications, PV power generation systems still face a series of challenges, such as low power generation efficiency and unstable system performance [4,5,6]. STP generation uses a solar heat effect to generate electricity. It is equipped with a heat storage system and thus can realize stable, reliable, flexible, and adjustable power output. It can not only serve as a stable and peak shaving power source in the power system but also provide scarce and indispensable moment of inertia for the new power system [7,8]. However, it is still not as widely adopted as PV power generation due to relatively high construction costs and difficulties in large-scale implementation [9,10]. Wind power is also mature and rapidly developing due to a short construction period, high power generation efficiency, and applications of all sizes [1,11]. It converts wind energy into mechanical energy, usually by wind turbines, and then converts mechanical energy into electricity with generators. Now, offshore wind power construction has seen significant development due to the high density of offshore wind energy and minimal terrain restrictions for offshore wind farms [12,13]. The ocean covers about 71% of the total area of the Earth and is the largest underexploited area on the Earth. Ocean energy is a promising and important renewable energy source with abundance, comparatively strong predictability, is close to the center of loads, and has more application scenarios. Ocean power generation is of great significance to alleviating the energy crisis and environmental pollution, guaranteeing offshore energy security, and ensuring marine resource development and rights protection. Due to the harsh marine environment, compared with solar and wind power generation, the development of ocean power generation is relatively lagging behind, with tidal power generation almost in commercialization and wave power generation in the demonstration application stage [14]. Countries around the world are accelerating the development of ocean energy generation.
Ocean energy includes tidal, wave, ocean current, temperature difference energy, and so on. Wave energy, in particular, stands out for its high energy density, 24/7 availability, and greater predictability compared to wind energy. The integration of wave and offshore wind power is becoming a hot topic, with the advantages of more stable power output, as well as shared infrastructure and maintenance costs, enhancing both economic feasibility and operational efficiency [15,16,17,18,19,20]. Many ingenious methods have been explored for converting wave energy into useful electric power, and a large number of wave energy converters (WECs) have been developed, such as the point absorber [21,22,23,24,25], the oscillating water column (OWC) [26,27,28], the sharp eagle [29], the hinged raft [30], and so on. They generally can be divided into two categories: (1) traditional WECs based on rotary generators and (2) direct-drive WECs based on linear generators [31]. Ocean waves provide a slow-moving, high-magnitude force. A linear generator provides an excellent match to the mechanical impedance of ocean waves. So, direct-drive WECs do not have complex and huge intermediate systems, which can significantly improve efficiency. There are no mechanical rotating parts, and therefore, direct-drive WECs have a simple structure, high stability and reliability, and are suitable for offshore installation. Thus, direct-drive WECs are considered to be the most promising form of wave energy conversion at present [32,33,34,35,36,37]. Many internationally renowned wave energy developers, such as Ocean Power Technologies (OPT), Seabased Industry AB, and AWS Ocean Energy, are all engaged in direct-drive WECs based on linear generators. OPT has been developing a point-absorber WEC named PowerBuoy. Its PB3 PowerBuoy® has been commercialized as an Uninterruptable Power Supply (UPS) with a nominal nameplate capacity rating of up to 3 kW of peak power during recharging of the onboard batteries, and it has about 15 MWh of renewable energy production along with other generations of PowerBuoys® up to May 2024 [38]. Seabased adopts the point-absorber technology and linear generators to capture the energy passing through waves. It has built several demonstration wave power parks with a modular WEC rated at 10 kW–100 kW and is currently building a 50 MW wave power park in Martinique [39]. The Archimedes wave swing (AWS) is the first direct-drive WEC using a linear permanent generator. The fourth-generation AWS is being developed with a 1/2 prototype at the European Marine Energy Centre (EMEC) in Orkney, capturing average power over 10 kW and peaks of 80 kW during a period of moderate wave conditions in sea trials [40].
The MHD generator is another direct-drive motor for power generation. It directly converts the thermal or kinetic energy of conducting fluids into electricity based on Faraday’s electromagnetic induction law. It provides a kind of stationary power generation with no mechanical moving parts because the magnet, the generation duct, and the electrodes are all stationary. Different from traditional generators, an MHD generator uses conducting fluids instead of solid conductors. The conducting fluid traverses the magnetic field in the form of linear motion, with the flow speed easily changed by varying flow cross-sections, which makes the generator directly and easily couple to the high-magnitude and slow-moving force of ocean waves. Moreover, an MHD generator has almost constant high efficiency throughout wide working conditions with double generation ducts [41,42,43]. Thus, MHD generators provide a promising choice for direct-drive WECs. This paper concentrates on the direct-drive wave energy conversion technology based on MHD generators, which is called MHD wave energy conversion technology for convenience. It is organized as follows: The introduction of MHD WECs is illustrated in Section 2. Section 3 presents the typical research work of MHD wave energy conversion. Then, key technical issues to be solved further are discussed in Section 4. Lastly, conclusions are drawn in Section 5.

2. MHD Wave Energy Converter

Figure 1 is a typical MHD WEC, which is mainly composed of a float, a hydraulic system, a liquid metal MHD generator, a damper, a mooring system, a ballast, and so on [44,45]. It is deployed vertically, and the float rides on the surface of the sea. The float is coupled to the MHD generator by a hydraulic system. The float moves vertically with waves driving the conducting fluid to flow reciprocally in the generation duct of the MHD generator. The ballast is placed below the MHD generator to provide a downward force as the surface float rides down to the trough of the wave. The MHD generator and hydraulic system are fixed in a housing with a hydrodynamic damper to keep it relatively motionless in the water. It is the relative motion between the surface float and the housing that produces electricity, and therefore, the housing needs to remain relatively motionless.
Usually, the conducting fluid is a single-phase liquid metal with low melting points, and the conductive liquid metal MHD (LMMHD) generator with a linear generation duct is adopted. The pure liquid metal is forced to flow back and forth in the generation duct by wave forces.
In contrast to alternatives, an MHD WEC has many features and advantages.
  • Simple structure and reliable operation.
There are no high-speed mechanical moving parts nor complicated and bulky devices operating at high working pressure in an MHD WEC. The LMMHD generator has good starting performance and low working pressure, and thus, the hydraulic system runs at relatively low-pressure working conditions, and this makes the design of the power take-off (PTO) system simpler and the requirements for materials, wall thickness, and hydraulic valve control components much lower. And thus, the overall volume and mass of the MHD WEC are correspondingly reduced, the operational reliability is increased, and the maintenance is simpler.
  • High efficiency within a wide range of operating conditions.
There are no high-speed mechanical rotating parts, and no mechanical loss from the low-speed linear motion of waves to the high-speed rotary motion of generators. The LMMHD generator has high efficiency throughout a wide range of working conditions. So, an MHD WEC is efficient within a wide range of operating conditions.
  • Good response characteristics to low sea states.
There is no mechanical moving part in an MHD generator, and the whole WEC has no complicated and bulky mechanical transmission devices nor high-speed mechanical rotating parts. And thus, the inertia of an MHD WEC is very small, with a quick response to waves. It is highly compatible with all wave heights and can work with a wave height of 0.3 m.
  • Modular design and array layout.
The output of an MHD WEC is single-phase alternating current (AC) electricity with low voltage and high current because of the high conductivity of the liquid metal. Thus, usually multiple generation ducts connected in serial are adopted to obtain a usable output voltage. Moreover, an MHD WEC can have multiple modular generation units to provide different outputs according to sea conditions and users’ needs. Multiple MHD WECs can be deployed to form a large-scale wave energy conversion park.
  • Rapid and easy deployment.
An MHD generator is highly compatible with heave-based and point-absorber floating systems. And such an MHD WEC is not affected by wave directions and tide levels. It can be deployed easily with good mobility and strong storm resistance.
  • Low cost of manufacturing, operation, and maintenance.
An MHD WEC has no high-speed mechanical moving parts nor precise transmissions, such as gears, levers, turbines, drive belts, bearings, and so on. Capital needs and maintenance costs are low.
An MHD WEC is very efficient, compact, economical, and deployable. It can provide reliable deep ocean power generation systems for remote subsea wellheads. It can be applied to rapidly deployable, cost-effective power supply systems for special applications, such as unmanned underwater vehicle docking stations or temporary power for remote sites. The offshore parks of MHD WECs can power the onshore grid with megawatts of electricity.

3. Research Development of MHD Wave Energy Conversion

An MHD generator was first used for energy conversion of the kinetic energy of seawater driven by wave-powered air compressors and/or water pumps in 1979 [46]. An MHD WEC with a point absorber for generating electricity from the heave motion of ocean waves was proposed in 1992 [47]. And a combined power generation system was put forward to recycle the remaining potential energy of seawater flowing out of a hydroelectric generator by an MHD generator to increase the power generation capacity and improve the efficiency of hydropower generation [48]. In all of these applications, seawater MHD generators were used. However, seawater as the MHD interaction fluid is not nearly conductive enough to generate any reasonably attainable electromagnetic force density, and inefficiency becomes a major problem of such MHD WECs.
In 1998 or so, an LMMHD generator was proposed to be used in wave energy conversion for higher efficiency and larger power density by Scientific Applications & Research Associates, Inc. (SARA) [49]. From then on, more attention from the USA, China, Japan, Mexico, Malaysia, and other countries has been paid to MHD wave energy conversion, and a lot of research work has been carried out focusing on performance analysis, operation control, prototype development, and experiments/tests. And quite a few new MHD wave energy conversion methods have been proposed.

3.1. Performance Analysis

An MHD WEC is a wave-mechanism-fluid-electricity coupling system, and how to make wave energy drive the liquid metal flowing in the generation duct is very important. Usually, a point absorber floating on the water is adopted to utilize the heave force of waves [44,45,49,50,51]. And a hydraulic system in Figure 2a or a mechanical system, such as the return spring of Figure 2b, is used to couple to the LMMHD generator. The mathematical model of an MHD WEC system includes the wave model, the float’s motion equation, the hydraulic system model or spring model, and the LMMHD generator model [44,52,53]. The coupling relationship is shown in Figure 3. The velocity of the liquid metal in the generation duct is obtained from the motion equation of the float, and then the output power of the system can be calculated with the LMMHD generator model based on Kirchhoff’s laws. When there is an array of floats, the interaction among floats is considered by the sum of the diffraction wave force acting on float n [54].
As to an MHD WEC of Figure 2a, a conductive LMMHD generator with a linear generation duct is used, and the generation duct has a rectangular cross-section. The wave energy captured by the float is as follows:
P w = p w D f = 981 H 2 T D f ,
pw = 981H2T is the wave energy power density, H is the wave height, T is the period, and Df is the diameter of the float.
The output power Pe can be expressed as follows:
P e = U e I e = σ L M B 0 2 u 2 k ( 1 − k ) a b L M H D ,
Ue is the load voltage, Ie is the load current, σLM is the conductivity of the liquid metal, B0 is the applied magnetic flux density, k is the load factor, a is the distance of electrodes, b is the width of electrodes, LMHD is the length of the electrode, and u is the average velocity of the liquid metal in the generation duct.
The system efficiency of wave energy to electricity, η:
η = P e P w = = σ L M B 0 2 u 2 k ( 1 − k ) a b L M H D 981 H 2 T D f
So, for an MHD WEC, the important structural parameters include the diameter of the float Df, the length LMHD, the width b, and the distance a of electrodes. The applied magnetic flux density B0, the load factor k, the wave condition H and T, and the conductivity of the liquid metal σLM are the main operating parameters. The density and viscosity of the liquid metal and the diameter of the master cylinder Ds are also important parameters that affect the flow loss and velocity of the liquid metal u. In order to investigate the relationship between η, Pe, and the main structural parameter, dimensionless parameters are defined as follows [55]:
α = S p S M H D = u u s ,
β = a L M H D ,
γ = b L M H D ,
χ = a b ,
where Sp = 0.25π(Ds)2, SMHD = ab, and us is the velocity of oil in the master cylinder.
With only a single parameter changed and ideal conditions, the system efficiency η and output power Pe increase with α, β, γ, k, and B0 first and then decrease, and there is an optimal value maximizing η and Pe. η and Pe almost do not vary with χ [55]. And when NaK 78 is used and H = 1.3 m, T = 5.2 s, α = 22, k = 0.9, β = 0.0149, γ = 0.41, and χ = 0.036, Pe and η obtain the maximum value when B0 is from 0.6 to 1.5 T, corresponding to Df changing between 2 m and 7 m. The liquid metal is involved in both the power generation and flow process in an LMMHD generator. And so liquid metals with high electric conductivity, low density, and viscosity are required to ensure high power density and low electromagnetic damping. And NaK alloy is the ideal working medium for an MHD WEC in terms of solving safety issues [55,56]. An MHD WEC has different performance characteristics in different wave conditions, and the maximum output power can be obtained by adjusting load parameters [55].
Equations (2) and (3) show the output power and system efficiency in an ideal condition. In fact, there are end effects, induced magnetic fields, contact resistance between the moving liquid metal and stationary solid electrodes, and so on, within an LMMHD generator. The short-circuit current in the inlet and outlet zones of the generation duct and the eddy current caused by the spatial gradient of the applied magnetic field along the flow direction cause joule heating loss and reduce efficiency [57,58]. The induced magnetic field generated by the current in the generation duct, that is, the armature current, distorts the applied magnetic field of the air gap B0 [41,42]. When the permanent magnet is used, it also has a serious demagnetization effect on the permanent magnet, which affects the operational stability of the generator [59]. The internal resistance of the LMMHD generator is very low because of the high conductivity of the liquid metal. And the resistance of electrodes, electrode leads, and the contact resistance between the moving liquid metal and the stationary electrodes are comparable to or even greater than the internal resistance, which greatly increases joule heating power and is considered to be one of the reasons for the large error between the experimental efficiency and the calculated one [60,61,62].

3.2. Control of MHD WECs

The single-phase AC output of MHD WECs with low frequency (10−1 Hz), low voltage (10−1–100 V), and high current (103–105 A) is significantly different from that of rotary or linear generators. At such high currents of thousands of amps, the output voltage of the order of 10−1–100 V can hardly overcome the on-voltage drop of existing power electronic devices. For example, the current rating of Mitsubishi Gate Turn-off Thyristors FG6000AU-120D is 6000 A, and the peak on-state voltage drop is 6 V. The high output current of the LMMHD generator will result in large heat loss of the entire power conversion system. The fluctuation of waves makes the output electricity rise or decrease sharply, which in turn generates a large impulse voltage or current, affecting the reliability and safe operation of the power conversion system. Furthermore, in order to make full use of wave energy, it is necessary to ensure WECs always operate at the maximum power point with the required output power. So, high-current and low-voltage power conversion and maximum power output tracking/optimization of MHD WECs are mainly investigated.
Research on high-current and low-voltage power conversion of MHD WECs focuses on the general scheme and the overall circuit topology with low on-state loss and large voltage transmission ratio based on the existing power electronics [63,64,65,66]. Generally, a series of electric arrays of multiple MHD generation units is adopted to increase the input voltage, and multiple parallel modules are used in the power conversion overall circuit to decrease the input current–voltage ratio and on-state loss. Figure 4 shows an MHD marine energy conversion system (MECS) interconnected with distribution electric networks [66]. It consists of an MHD generator array and AC/DC, DC/DC, and DC/AC power electronics converters, including a robust control to transfer the produced power into an AC infinite bus without voltage variations. The simulation results show a robust and effective methodology generating a total output power of 290 kW from the MHD MECS into distribution electric networks after a short period, with barely 0.4 s of instability.
As with a heaving float WEC, it is common to adjust the damping of the PTO system in real time to make the float and the wave close to or reach a resonant state. It is generally necessary to change the electromagnetic force of the LMMHD generator by controlling the applied load impedance to obtain the maximum power output of a designed and manufactured MHD WEC. A theoretical approach to optimize the power output of an MHD WEC, shown in Figure 2b, was demonstrated by using Pontryagin’s maximum principle [44]. A control strategy based on an improved hill-climbing method was proposed to obtain the maximum power of the MHD WEC shown in Figure 2a [67]. As shown in Figure 5, the current of the boost circuit is chosen as the perturbation term dIL, the optimal reference current is determined by the hill-climbing method, and the MOSFET is controlled by the model prediction of the MHD WEC to trace the optimal reference current. When the wave height decreases, the judgment of the average power reduction threshold is added in case the direction of disturbance may be judged wrong by the hill-climbing method. The simulation results show that the improved hill-climbing method can correctly determine the disturbance direction and can adaptively adjust the equivalent load of the boost circuit under any wave condition, and the tracking time is more than 10 s.
In order to further improve the response speed and accuracy of the output power control system of an MHD WEC, it was proposed that the equivalent load was controlled by real-time adjustment of the output current with the output voltage as a reference signal [63]. As shown in Figure 6, the wave condition is judged based on the absolute value of the measured output voltage Uao of the LMMHD generator first. Then, look up the table calculated in advance to obtain the corresponding current Iref related to the output power of the judged wave condition, and Iref is used as the reference value for the closed-loop control. Next, compare the measured output current, Iao and Iref, to obtain the current error signal, which is sent to the proportional integration controller for error compensation, and the control signal of pulse width modulation (PWM) is obtained after limiting the compensated signal. The simulation results show that the control system can control the output current to track the change of the output voltage in real time with the same phase, both at the beginning and the moment when wave conditions change. And the tracing time is less than 5 s.
As for WECs with a hydraulic PTO system, the hydraulic energy storage method is dominant to realize the absorption, storage, and centralized release of unstable wave energy to achieve stable electricity output [68,69,70]. A 50 kW array-type MHD WEC with a multi-source hydraulic energy storage system was proposed, and the flow pulsation was reduced to 1/7 of that without accumulators [54].

3.3. Prototype Development and Experiments/Tests

In the last few decades, quite a few MHD WEC prototypes at different scales have been set up. Although most of them were proof-of-principle experimental devices with simulated wave forces and even NaCl solutions as the conducting fluid, the first 100 kW MHD WEC lab prototype was manufactured and tested by SARA, and the first sea trial of a 10 kW MHD WEC prototype was successfully carried out by the Institute of Electrical Engineering, Chinese Academy of Sciences (IEECAS).

3.3.1. SARA

The concept of LMMHD WEC for cost-effective and rapidly deployable WECs used in unmanned underwater vehicle docking stations or temporary power for remote sites was first put forward by SARA.
Figure 7 shows SARA’s main research work on MHD WECs.
In 2001, a proof-of-principle demonstration device succeeded in lighting light bulbs.
And then a 100 kW LMMHD WEC was designed and patented in 2005 [71]. It comprises an array of 16 LMMHD generation cells for converting mechanical power into electric power. The upper/lower bellow reservoirs of the generation cells are enclosed in an upper/lower chamber connected to an upper/lower piston. The chambers are filled with hydraulic oil. In operation, the pistons are forced in and out; that is, the upper/lower piston goes in as the lower/upper one goes out. The in-and-out movement pressurizes the hydraulic oil, which then compresses the bellow reservoirs and forces NaK 78 to flow through the cell. The electric power is thus generated at the output terminal of the generator. The overall scale size is roughly 4 m in diameter and 12 m in height. The designed output power is 125 kW with a mechanical–electric efficiency of 55%, 10.2 V output voltage, and 12.2 kA output current. Given that a heave-based WEC device can be as much as 50% efficient in wave capture, the system efficiency is 27.5%. One generation cell is an LMMHD generator with a linear generation duct, a pair of bellow reservoirs, and a dipole permanent magnet. The LMMHD generator itself has very few moving parts and would be built to have a 20+ year life span. This 100 kW LMMHD WEC can be deployed using nearly any ship of opportunity with a sufficient capacity overboarding crane. When it is moored in 200 m or less water depth, the ballast requirements can be added after deployment, or ballast material can be dredged from the bottom. It is expected that several of these LMMHD WECs can be deployed in a “farm” arrangement, so that they can be interconnected, and the electric loads on each device can be impedance-matched to an overall electric load.
In 2008, the 100 kW LMMHD WEC lab prototype was constructed and tested, demonstrating ~50% efficiency in converting mechanical power into electricity across almost the entire range tested. It is very regrettable that there has been no public coverage of SARA’s work since then. However, SARA’s work is very meaningful and referenced whenever MHD wave energy conversion technology is mentioned.

3.3.2. IEECAS

Some LMMHD WEC prototypes have been set up and tested in IEECAS since 2005, as shown in Figure 8.
In 2008, a proof-of-principle demonstration device with a set of hydraulic systems to simulate the wave motion was developed. About 160 W output power was obtained with mercury as the working fluid, a 0.5 T magnet, and a pure resistive load when the simulated wave was 0.35 m in height and 3.2 s in period [72].
In 2011, a 1 kW lab prototype with the same hydraulic system was developed and tested. The simulated wave was 0.3 m in height and 2 s in period. When the liquid metal was U alloy 47 with a 45 μΩ resistive load, the measured output power was 1.069 kW with the output current of 3250 A and output voltage of 0.33 V [73]. And when the working fluid was Ga alloy, the measured output power was 2.1 kW.
In 2015, the first 10 kW MHD WEC sea-trial prototype was constructed and tested for 2 months near Wanshan Island, Zhuhai, Guangdong, China [74,75]. As shown in Figure 9, it is a point-absorber WEC with a dual-module LMMHD generator set. It mainly comprises an annular float, a PTO system, a spar, trusses, a damper, an anchor system, and a control and load system. It is 15 m in total length and 19 tons in total weight in air. The designed output power is 10 kW@2.5 m (wave height) and 5.2 s (wave period). The PTO system includes a hydraulic system and a dual-module LMMHD generator set, fixed inside the spar and connected to the float via trusses. Two modules of the LMMHD generator set are connected in parallel for the liquid circuit and in series for the electric one. The damper is at the bottom to guarantee the relative movement between the float and the spar. The working liquid metal is a kind of Ga alloy that changes from liquid to solid at 8.0 °C and from solid to liquid at 3.9 °C. The designed working pressure of the LMMHD generator is 5 MPa, which is about 1/2 that of a 10 kW floating array buoy WEC based on rotory generators [68]. It was deployed vertically in the sea with a depth of 24 m and an offshore distance of ~200 m. The sea trial test was from February to March 2015. The output power of 2.3 kW with the generator efficiency of 57% under the wave condition of 0.54 m in height and 4.8 s in period was obtained, and the system efficiency was 17.8%. It also demonstrated that it generated about 100 W of power output with a wave height of 0.3 m. The work was regarded as “New Progress of Ocean Energy Conversion Technology of China in 2016” by Ocean Energy Systems.
In 2022, an MHD wave energy conversion platform was constructed, and this work had not been published before. It is an energy supply system integrating MHD wave energy conversion, energy storage, a microgrid, and intelligent control. The platform is 32 m in length, 23 m in width, and 5 m in height with a total weight of 100 tons in air. Ten floats are symmetrically arranged along the two long sides of the platform, and every float is connected to a separate hydraulic cylinder, as shown in Figure 10. Ten hydraulic cylinders are linked to a manifold and then an accumulator set. A 50 kW LMMHD generator with double generation modules is adopted, and the pressurized hydraulic oil from the accumulator set drives Ga alloy flowing in the generation duct. The accumulator set, the microgrid system, and the generator are installed in the equipment cabin in the middle of the upper deck of the platform. It is planned to be deployed near Wanshan Islands, Zhuhai, Guangdong, China, and about 40,000 kWh will be generated per year with a wave height of 1.8 m and a period of 6 s. However, for some reason, it has not been deployed to the planned site for sea trials so far. And Figure 11 shows that the platform has been in the river for more than three years.

3.3.3. Others

A small-scale LMMHD WEC device, as shown in Figure 12, was developed by the National Autonomous University of Mexico [76]. It adopted a reciprocating system including an electric motor and a connecting rod–crank sliding mechanism to emulate the oscillatory motion of marine waves transferred to the liquid metal and a precise measurement system based on an ultrasonic Doppler velocimetry (UDV) system, a Hall effect sensor, and a four-wire technique to characterize the flow pattern and the electrical performance of the device. In the experiment, the maximum RMS value around 5 mW and the isotropic efficiency of about 20% were reached with gallium–indium–tin (Galinstan) as the working fluid, a 0.18 T magnetic field, a 0.88 load factor, and an oscillation Reynolds number of 273. Currently, a new experimental prototype able to operate in more realistic conditions is under construction with the objective of testing an LMMHD generator in a wave channel.
A 2W proof-of-principle demonstration device for deep-sea sensor power supplies with an LMMHD generator was manufactured by Beijing University of Technology [64,77]. A low-voltage, high-transformation-ratio power conversion system was developed and tested, realizing power conversion from 0.4 V and 0.16 Hz single-phase AC to DC 12V with a conversion efficiency of about 50%.
Some proof-of-principle experiments for MHD marine energy conversion with seawater as the working fluid and superconducting magnets to supply a strong magnetic field were also carried out [78,79].

3.4. New MHD WEC Concepts

Besides a point absorber floating on the water utilizing the heave force/motion of waves via a hydraulic or mechanical system to drive the liquid metal through a magnetic field, as shown in Figure 2, there are also some new MHD WEC concepts with other wave capturers and PTO systems, as shown in Table 1.
Floating MHD WECs semi-submerged on the surface of the sea utilize the wave pitching moment to cause a relative motion between the conducting fluid and the magnet. There are two ways. One is that the conducting fluid is coupled to the wave absorber via a hydraulic or a mechanical system; the wave absorber may be a duck pendulum [80] or there may be multiple tube sections linked with joints [81]. Another is by a floating flexible shell filled with the liquid metal [82]. The flexible shell is deformed by waves, directly forcing inner conducting fluid to flow in the magnetic field, as shown in Figure 13b; there is no other mechanical or hydraulic system realizing a direct drive of the conducting fluid.
A submerged MHD WEC is deployed vertically under the surface of the sea and adopts a submerged float to capture the heave motion of waves. The float is coupled to the MHD generator via a mechanical or hydraulic system, forcing the conducting fluid to flow through the magnetic field. Usually, the MHD generator is fixed to the seabed or is almost relatively motionless with an anchor or a damper. In Figure 14, the float is a cylindrical air-filled chamber, and the bellow reservoir is connected to it by a shaft [83]. The bellow reservoir can be replaced by a magnetic coupling piston with the advantage of no leakage caused by mechanical fatigue of the bellow reservoir [84].
In order to make full use of wave energy, multiple wave-absorbing methods are used together in one WEC, which can capture the heave and pitching motions of waves [85,86]. Figure 15 shows a combined MHD WEC that is a vertically semi-submerged device and is anchored to the seabed [85]. It is mainly composed of a floating and converging wave capturer, a suspended LMMHD generator, a reciprocating compression system, and a mooring system. The reciprocating compression system is above the floating and converging wave capturer and is coupled with the LMMHD generator and the capturer. The semi-submerged floating and converging wave capturer combines the advantages of a float, a contouring raft, and a tapered channel, which can absorb the vertical and horizontal energy of waves, improving the absorption efficiency. The LMMHD generator is vertically suspended above the sea and in the middle of the reciprocating compression system. Under the action of the return spring, the floating and converging wave capturer moves up and down with waves and then pushes the reciprocating compression system to move vertically, driving the liquid metal to flow back and forth in the generation duct.
In view of problems such as complex structure, difficult maintenance, and the high cost caused by check valves of conventional OWC WECs, an OWC MHD WEC was proposed [87]. It is a semi-submerged offshore device and vertically deployed. As shown in Figure 16, the buoy includes a sealed end and an open end; there is a nozzle on the sealed end, and the open end vertically extends into the water body, forming an air chamber between the water surface and the sealed end. The bellow reservoirs and an LMMHD generator are located vertically in the upper part of the air chamber. The bellow reservoirs are connected to the generation duct and filled with the liquid metal. As the water surface fluctuates, an upward or downward airflow is formed in the air chamber. The bellow reservoirs undergo elastic deformation, extruding the liquid metal to flow in the magnetic field.
The above MHD WECs all adopt MHD generators with linear generation ducts. An LMMHD generator with a disk generation duct was proposed to convert wave energy into electricity [50]. A float riding on the sea surface moves up and down, making the liquid metal to flow circumferentially in the annular generation duct by a propeller coupled to the heaving motion of waves via a mechanical system.
Besides wave energy, MHD generators are also used in tidal energy conversion. A turbine [88] or an oscillating foil [89,90] is adopted to absorb tidal energy.

4. Some Technical Challenges

MHD wave energy conversion adopts LMMHD generators to have a simple structure, good response characteristics to low sea states, high efficiency under almost all working conditions, stable and reliable operation with hydraulic storage systems, good applicability to sea states, and power rates based on the modular design and array layout. It provides a new and promising way to efficient and reliable wave energy conversion with wide working conditions. However, MHD wave energy conversion is still in the lab and field prototype development stage. Currently, types of wave energy conversion technology are relatively scattered, and they are generally in the demonstration application stage. Furthermore, MHD wave energy conversion technology started late relative to other technologies. In addition to common problems such as survival and economy, there are special key technical challenges that MHD WECs need to address for demonstration applications.

4.1. High-Performance LMMHD Generator

The LMMHD generator is a key component of MHD WECs. Today, the efficiency of LMMHD generator prototypes adopted in MHD WECs is about 50%, which is much lower than the theoretical one, and the output power is ten to one hundred kilowatts, which is far from meeting the demands of large-scale applications. Moreover, the output voltage is too low and the output current is too high to obtain efficient power conversion and transmission. So, improving the generator efficiency, output power, and practicability of the output electricity of LMMHD generators is a key issue that needs to be addressed urgently.
In the actual operation of LMMHD generators, there are field–circuit coupling, unsteady reciprocating flow under the action of randomly varying wave forces, leakage and induced current in end zones of the generation duct, induced magnetic field and armature reaction, electrodes’ resistance, and contact resistance that cannot be ignored. All of these actual phenomena and factors will cause losses and degrade the performance of generators and should be considered in the performance analysis and design of LMMHD generators. Some approaches have been proposed to mitigate the end effects and induced magnetic field [41,42,57,58,91]. However, there is still much to be done for the unsteady flow in the generation duct, dynamic conductive characteristics of the liquid–solid interface between the moving liquid metal and electrodes, the precise generator’s equivalent circuit model, and the multiphysics structure-coupling design approach to generation ducts.
The output voltage of LMMHD generators is Ue = kU0, and U0 is the induced electromotive force (EMF). To increase Ue, U0 must be increased if k is constant. For a linear generation duct with a rectangular cross-section, U0 = B0va. Presently, the main methods to increase Ue are to increase the electrodes’ distance a, connecting multiple generation ducts or multiple segmented electrodes in series, which essentially increases the characteristic distance of the applied magnetic field in the direction of the induced current. However, increasing the electrodes’ distance a with constant β, γ, and χ defined in Equations (5)–(7) will increase the size and mass of the magnet greatly. Therefore, only increasing the characteristic distance of the magnetic field in the direction of the induced current in a limited space has practical value and significance, and new topologies of LMMHD generators should be developed to fundamentally increase Ue/U0.

4.2. Efficient Low-Voltage and High-Current Power Conversion

The output of MHD WECs is single-phase AC with low voltage, low frequency, and high current, and is not practical for usage in electric devices. Therefore, it is necessary to convert it into the power required by the user. The simulation results show that high conversion efficiency can be obtained when the input voltage is 100–101 V [63,65,66,92]. Experimental results show that the conversion efficiency is about 50% when converting 0.4 V and 0.16 Hz single-phase AC into DC 12V [77]. So far, there is nearly no low-voltage and high-current power conversion system tested along with an MHD WEC prototype. So, efficient low-voltage, low-frequency, and high-current power conversion is also a challenge to overcome.
Presently, it is generally converted from the low-frequency, low-voltage, high-current and unstable single-phase AC of MHD WECs to stable high-voltage DC power first, and then DC-AC conversion, or directly to power the DC load, is supplied according to user needs, and energy storage links will be added as needed. For low-voltage and high-current power conversion, power electronic devices with low threshold voltages, low on-resistance, and large current ratings should be adopted. However, the commercial power electronic devices with current ratings of 103 A and more also have a high on-state voltage drop of 100 V and are not suitable for the power conversion of MHD WECs. SiC- and GaN-based power electronic devices have low switching losses and low on-state resistance and are gaining significant attention. However, the current rating of commercial 650–1700 V SiC power MOSFETs is 5–600 A [93,94,95]. GaN-based power electronic devices are more suitable for high-frequency applications, and the current rating is 101 A on the market [95].
Fortunately, multilevel topologies and series-connected configurations of low-voltage semiconductor devices, such as the modular multilevel converter (MMC), have been proposed and put into practical use for various medium to high voltage applications [96,97,98], which enlightens the low-voltage and high-current power conversion of MHD WECs. It is believed that the multiple parallel topologies, combined with SiC-/GaN-based power electronic devices, can be a suitable solution to efficient low-voltage and high-current power conversion of MHD WECs. Furthermore, the current rating of SiC-/GaN-based power electronic devices will become higher and higher with the development of related technologies.

4.3. Issues with Materials

In addition to the requirements of the marine environment for materials, a special material issue of MHD WECs is the liquid metal and its material compatibility. At present, typical liquid metals used in LMMHD generator prototypes are mercury [72], U-47 [73,99], NaK alloys [71], gallium alloys [61,74,75], liquid sodium [100], and liquid gallium [101]. From the perspective of the output performance, NaK alloy is the best choice because of its very low density, melting point, and viscosity. Although its chemical properties are extremely lively, NaK alloy was used in SARA’s 100 kW LMMHD WEC lab prototype and experiments and has been applied as a coolant for nuclear reactors. Gallium alloys are the most common working liquid in LMMHD generator prototypes due to their good operability, low melting point, and comparatively low density and viscosity.
Contact between solid and liquid metals leads to corrosion, embrittlement, and fluid cavitation damage [102,103,104,105]. As for MHD WECs, the liquid metal mainly flows in the generation duct. There are electrodes and electrically insulated walls in the generation duct. Electrodes are usually highly conductive solid metal materials. The electrically insulated walls also need to withstand pressures of MPa. In addition, electrodes, electrically insulated walls, and liquid metal should be non-magnetically conductive. Furthermore, the liquid metal could also come into contact with sealing materials and hydraulic fluid. And thus, the corrosion resistance of these materials toward liquid metals should be considered for the service performances and lifetime of the MHD WECs. Some work on gallium alloy and its compatibility has been carried out [55]. There is still much work, especially on NaK alloy and its compatibility, to be carried out.
Seawater temperature and marine environmental protection requirements are also important for the liquid metal selection of MHD WECs. The melting point should be lower than the seawater temperature in the sea where MHD WECs are deployed, and the liquid metal should be non-toxic to avoid environmental pollution.

5. Conclusions

Direct-drive wave energy conversion technology based on LMMHD generators provides a new and promising way to efficient and reliable wave energy conversion with wide working conditions. After more than three decades of development, the 100 kW MHD WEC lab prototype was manufactured and tested, a sea trial of a 10 kW MHD WEC prototype was successfully carried out, and a 50 kW floating-array buoy MHD wave energy conversion platform was developed. Furthermore, many new MHD WEC concepts have been proposed.
MHD wave energy conversion is still in the lab and field prototype development stage. In addition to common problems such as survival and economy, special key technical challenges, such as a high-performance LMMHD generator, efficient low-voltage and high-current power conversion, and material compatibility of liquid metals, need to be addressed for its demonstration applications.
In order to reduce comprehensive costs and utilize the sea space intensively and economically, MHD WECs can be integrated with offshore wind energy, mariculture, seawater desalination, marine observation, and so on.
MHD wave energy conversion involves multiple disciplines. Its development will greatly promote the related disciplines and technologies, such as the environmentally friendly liquid metal with a low melting point, density, and viscosity; insulation materials with high strength and good material compatibility; power electronic devices with current ratings of 103 A and low on-state resistance; efficient low-voltage and high-current power conversion technology; high-performance LMMHD generators; and so on.

Author Contributions

Writing—original draft preparation, L.Z.; writing—review and editing, A.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

No new data were created or analyzed in this study.

Acknowledgments

The authors want to express their gratitude to Weinan Liu and Jian Li.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Diagram of an MHD wave energy converter.
Figure 1. Diagram of an MHD wave energy converter.
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Figure 2. Schematic of MHD WECs with LMMHD generators. (a) With a hydraulic system [52,53]. (b) With a return spring [44].
Figure 2. Schematic of MHD WECs with LMMHD generators. (a) With a hydraulic system [52,53]. (b) With a return spring [44].
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Figure 3. Coupling among the mathematical models of an MHD WEC.
Figure 3. Coupling among the mathematical models of an MHD WEC.
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Figure 4. General scheme of an MHD MECS interconnected into the distribution electric networks.
Figure 4. General scheme of an MHD MECS interconnected into the distribution electric networks.
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Figure 5. Schematic diagram of maximum power tracking based on an improved hill-climbing method.
Figure 5. Schematic diagram of maximum power tracking based on an improved hill-climbing method.
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Figure 6. Block diagram of the control system of an MHD WEC.
Figure 6. Block diagram of the control system of an MHD WEC.
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Figure 7. Research progress on MHD WECs of SARA.
Figure 7. Research progress on MHD WECs of SARA.
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Figure 8. Research progress on MHD WECs of IEECAS.
Figure 8. Research progress on MHD WECs of IEECAS.
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Figure 9. A 10 kW MHD WEC prototype and sea trial. (a) Sea trial; (b) 10 kW MHD WEC prototype; (c) PTO system; (d) LMMHD generator module.
Figure 9. A 10 kW MHD WEC prototype and sea trial. (a) Sea trial; (b) 10 kW MHD WEC prototype; (c) PTO system; (d) LMMHD generator module.
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Figure 10. Top view of the MHD wave energy conversion platform.
Figure 10. Top view of the MHD wave energy conversion platform.
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Figure 11. MHD wave energy conversion platform in the river.
Figure 11. MHD wave energy conversion platform in the river.
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Figure 12. Experimental device of the National Autonomous University of Mexico [76].
Figure 12. Experimental device of the National Autonomous University of Mexico [76].
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Figure 13. Floating MHD WEC. (a) Duck pendulum [80]; (b) flexible shell [82].
Figure 13. Floating MHD WEC. (a) Duck pendulum [80]; (b) flexible shell [82].
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Figure 14. Submerged MHD WEC [83].
Figure 14. Submerged MHD WEC [83].
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Figure 15. Combined MHD WEC [85].
Figure 15. Combined MHD WEC [85].
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Figure 16. OWC MHD WEC [87].
Figure 16. OWC MHD WEC [87].
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Table 1. New MHD WEC concepts.
Table 1. New MHD WEC concepts.
ConceptWave AbsorberOcean Energy UtilizedDeployment LocationPTO System
Floating MHD WECDuck pendulum [80]Wave pitching momentSemi-submerged on the surface of the waterHydraulic or mechanical system
Multiple tube sections linked with joints [81]
Flexible shell filled with the liquid metal inside [82]No mechanical or hydraulic system
Submerged MHD WEC [83,84]Submerged floatHeave force of wavesSubmerged verticallyMechanical or hydraulic system
Combined MHD WECFloat, contouring raft, and tapered channel [85]
Float and buoyancy pendulum [86]		Heave and surge forcesSemi-submergedMechanical system
OWC MHD WEC [87]OWCHeave force of wavesSemi-submerged verticallyAir, hydraulic system
Point-absorber WEC with disk LMMHD generator [50]Floating point absorberHeave forceSemi-submerged on the surface of the waterMechanical system
Vertical-axis LMMHD ocean current power generation device [88]TurbineKinetic energy of ocean currentVertical in the waterMechanical system
Oscillating hydrofoil tidal energy generator [89,90]Oscillating foilKinetic energy of tidal energyVertical in the waterHydraulic system
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Zhao, L.; Peng, A. Wave Energy Conversion Technology Based on Liquid Metal Magnetohydrodynamic Generators and Its Research Progress. Energies 2025, 18, 4615. https://doi.org/10.3390/en18174615

AMA Style

Zhao L, Peng A. Wave Energy Conversion Technology Based on Liquid Metal Magnetohydrodynamic Generators and Its Research Progress. Energies. 2025; 18(17):4615. https://doi.org/10.3390/en18174615

Chicago/Turabian Style

Zhao, Lingzhi, and Aiwu Peng. 2025. "Wave Energy Conversion Technology Based on Liquid Metal Magnetohydrodynamic Generators and Its Research Progress" Energies 18, no. 17: 4615. https://doi.org/10.3390/en18174615

APA Style

Zhao, L., & Peng, A. (2025). Wave Energy Conversion Technology Based on Liquid Metal Magnetohydrodynamic Generators and Its Research Progress. Energies, 18(17), 4615. https://doi.org/10.3390/en18174615

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