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Contract No. DE-AC36-08GO28308

Design and Analysis for a 

Floating Oscillating Surge Wave 

Energy Converter 

Preprint 

Y-H. Yu, Y. Li, K. Hallett, and C. Hotimsky 

National Renewable Energy Laboratory

To be presented at ASME 2014 33rd International Conference on 

Ocean, Offshore and Arctic Engineering

San Francisco, CA

June 8‒13, 2014

Conference Paper

NREL/CP-5000-61283

March 2014
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DESIGN AND ANALYSIS FOR A FLOATING OSCILLATING SURGE WAVE ENERGY 

CONVERTER

Yi-Hsiang Yu

National Renewable Energy Laboratory

Golden, CO, USA

Email: Yi-Hsiang.Yu@nrel.gov

Kathleen Hallett

National Renewable Energy Laboratory

Golden, CO, USA

Email: Kathleen.Hallett@nrel.gov

Ye Li

National Renewable Energy Laboratory

Golden, CO, USA

Email: Ye.Li@nrel.gov

Chad Hotimsky

National Renewable Energy Laboratory

Golden, CO, USA

Email: Chad.Hotimsky@nrel.gov

ABSTRACT

This paper presents a recent study on the design and 

analysis of an oscillating surge wave energy converter

(OSWEC). A successful wave energy conversion design 

requires balance between the design performance and cost. The 

cost of energy is often used as the metric to judge the design of

the wave energy conversion (WEC) system, which is often 

determined based on the device’s power performance; the cost 

of manufacturing, deployment, operation, and maintenance; and

environmental compliance. The objective of this study is to 

demonstrate the importance of a cost-driven design strategy and 

how it can affect a WEC design. A set of three oscillating surge 

wave energy converter designs was analyzed and used as 

examples. The power generation performance of the design was 

modeled using a time-domain numerical simulation tool, and 

the mass properties of the design were determined based on a 

simple structure analysis. The results of those power 

performance simulations, the structure analysis, and a simple 

economic assessment were then used to determine the costefficiency of selected OSWEC designs. Finally, we present a 

discussion on the environmental barrier, integrated design 

strategy, and the key areas that need further investigation. 

KEYWORDS

Wave energy; oscillating surge device; cost of energy

INTRODUCTION

The development of marine and hydrokinetic (MHK)

devices that can extract energy from ocean energy resources has 

gained increasing interest in recent years. Studies have shown 

that wave energy is one of the most abundant sources for

marine hydrokinetic energy in the United States, and recent 

estimates indicated that the U.S. wave energy resource is 2,600 

TWh/year, from which 400 TWh/year of electricity could be 

generated [1]. However, one of the major challenges is that the 

cost of energy that is generated by wave energy converters 

(WECs) is high, and improving WEC designs to be more costefficient is essential for the technologies to be successful, 

particularly in the early development stages [2]. 

The U.S. Department of Energy (DOE) Water Power 

Program has, therefore, established a reference model (RM) 

project to benchmark a set of different MHK technologies, 

including wave, tidal current, river current, and ocean current 

energy. The objectives of the RM project are to: 1) provide 

designs for several MHK devices that could be used to roughly 

gauge the status of the technology and its readiness for 

commercial application; 2) identify the areas where additional 

research could be best applied to accelerate technology 

development to commercial readiness. To achieve this goal, the 

RM project team developed an integrated, cost-oriented design 

strategy to balance the performance design with economic 

assessment [3]. 

In this paper, the objective is to demonstrate the importance 

of a cost-driven design strategy and how it can affect a WEC 

design. We analyzed three oscillating surge wave energy 

converter (OSWEC) designs and used the study as the 

examples. The power generation performance of the design was 

modeled using a time-domain numerical simulation tool. A 

structural analysis was conducted to determine the mass 

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properties. We then used the results of those power 

performance simulations, structure analyses and a simple 

economic assessment to judge the design and economic 

viability for selected OSWECs. Finally, we present a discussion 

on the environmental barrier, the integrated design strategy, and 

key areas that need further investigation for evaluating the cost 

of energy of WEC designs. 

DEVICE DESIGN CONCEPT

Several wave energy conversion systems that utilize the 

surge motion of waves to generate electrical power have been 

proposed by manufacturers, including Oyster, EB-Frond,

WaveRoller, and Langlee. Because of the potential risk of 

permitting and regulation issues at near-shore shallow water 

regions [4] and the better wave resource at deep water sites [5], 

the study was focused on deep-water (50 m-100 m) surge 

designs, where the devices were moored to the seabed.

Figure 1. Schematic of selected OSWEC designs

Three OSWECs were selected in this study, for which the 

structural analysis and the power performance calculation for

each design were performed. The orientation for selected 

OSWEC designs is shown in Figure 1. The flap was designed to 

rotate against the supporting frame to convert wave energy into 

electrical power from the relative rotational motion, induced by

incoming waves. All three designs used a 25-m-wide flap, and 

the distance from the top of the flap to the mean water surface 

(freeboard) was equal to 1 m. The dimensions for the OSWEC 

designs are listed in Table 1, and a brief summary for each 

OSWEC design is given below: 

• Design A is a simple single-flap OSWEC system with a 

flap height of 16 m.

• Design B has two 26-m-high flaps, which sit side-by-side

(Figure 1). The purpose of the design was to increase the 

height and the total width of the device to reduce the wave 

diffraction lost under long waves so as to improve the 

power generation performance.

• Design C was inspired by the Langlee design [5], which 

used a fore flap and an aft flap to convert wave energy. In 

our study, the flap height was 16 m, and the hinge-to-hinge 

distance between the two flaps was equal to 37 m.

Table 1. List of Dimensions for the OSWEC Designs

Design A B C

Number of flaps 1 2 2

Flap width 25 m 25 m 25 m

Flap height (freeboard) 16m (1m) 26m (1m) 16m (1m)

Flap thickness 1 m 1 m 1 m

Frame and shaft diameter 2 m 2 m 2 m

Structure width 29.5 m 57 m 29.5 m

Structure length 43 m 43 m 43 m

Structure height 18 m 28 m 18 m

Note that all the flaps were connected to a 2-m-diameter 

rotational shaft, rotating against the supporting frame. The 

supporting frame was assumed to have an outer diameter of 

2 m, and the total length of the device structure for all the 

OSWEC designs was 43 m. In addition, bottom plates were

attached to the frame, and it was found to be effective, because 

it increased the frame added mass and stabilized the frame, so 

that the device power performance was improved. 

Because the power performance of the OSWEC highly 

depends on the mooring configuration, particularly for designs 

A and B, both slack mooring and taut mooring configurations 

were considered in this study. For reference purposes, the three 

OSWEC systems were also modeled by assuming the frame 

was rigidly connected to the seabed to investigate the loss of 

energy caused by the mooring connection.

POWER PERFORMANCE ANALYSIS

Modeling a WEC device involves simulating the interaction 

between waves, the WEC device motion, and the power takeoff 

(PTO) mechanism. A radiation and diffraction method is often 

used [6], particularly for predicting the device power 

performance and for optimizing the design. Typically, the 

method solves the device system dynamics in time domain, 

based on a given PTO damping representation and the 

hydrodynamics forces obtained from a frequency-domain 

solution.

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Time-Domain Numerical Method

The device system dynamic response was calculated by 

solving the Cummins equation [7], often used to represent the 

equation of motion for marine systems. The equation of motion 

for the floating-body system around the center of gravity can be 

given as

mẌ = Frx + Frad + FPTO + Fv + FB + Fm, (1)

where Ẍ is the (translational and rotational) acceleration of the 

device, m is the mass matrix, Fex is the wave excitation force, 

Frad is the force vector due to wave radiation, FPTO is the PTO 

force, Fv is the viscous damping force, FB is the net buoyancy 

restoring force, and Fm is the force due to mooring connection. 

The remainder of this section describes each of these terms in 

further detail.

Irregular waves are often characterized using a wave 

spectrum that describes the wave energy distribution over a 

range of wave frequencies. The irregular excitation force can be 

written as the real part of an integral term in frequency,

Frx(t) = Re �∫ �2S(ω)FX(ω)ei(ωt+φ)dω ∞

0 �, (2)

where Re denotes the real part of the function, S is the wave 

spectrum, and φ is a random phase. Based on the Cummins 

equation, the irregular radiation term can be calculated by 

Frad(t) = −A∞Ẍ− ∫ K(t − τ)Ẋ (τ)dτ t

0 , (3) 

where A∞ is the added mass matrix at infinite frequency, Ẋ is 

the velocity of the body, and K is the impulse response 

function. 

Generally, the effect of viscosity on the system dynamics 

for a WEC device needs to be considered, and neglecting the 

effect may lead to overestimating the power generation 

performance of the system, particularly when a linear model is 

applied. A common way of modeling the viscous damping 

effect is to add a (Morison-equation-type) quadratic damping 

term to the equation of motion,

FV = 0.5CdρAD�Ẋ − V0��Ẋ − V0�, (4) 

where Cd is the viscous drag coefficient, AD is the 

characteristic area, and V0 is the undisturbed flow velocity. 

However, the viscous damping coefficient for the device needs 

to be carefully selected [4,5]. In this study, a drag coefficient of 

8 was selected for the flap [8] and a drag coefficient of 4 was 

used for the heave motion of the plate [9], which was attached 

to the frame. 

The mooring load was represented using a linear quasistatic mooring stiffness, which can be calculated by

Fm = −KmX, (5)

where Km is the stiffness matrix for the mooring system.

The PTO mechanism was represented as a linear springdamper system, where the generated power was proportional to 

the relative motion and velocity,

FPTO = −KPTOXrrr − CPTOẊ

rrr, (6) 

where KPTO is the stiffness of the system, CPTO is the damping 

coefficient, and Xrel and Ẋ

rrr are the relative motion and 

velocity between the two bodies. Note that we did not optimize 

KPTO and assumed KPTO is equal to 0 in the study.

WEC-Sim Model

WEC-Sim is an open-source, computer-aided engineering

tool developed for the analysis and optimization of WECs. The 

tool is currently being developed by the National Renewable 

Energy Laboratory and Sandia National Laboratories [10,11]. 

In the WEC-Sim model, a frequency-domain boundary 

element method (BEM), WAMIT [12], was applied to model 

the hydrodynamics of the system by solving the velocity 

potential. The corresponding hydrodynamic coefficients, 

obtained from the BEM simulation, along with viscous 

damping, mooring, and the PTO mechanism were converted 

into time-varying forces in the pre-processing module. The 

time-varying six degrees-of-freedom hydrodynamic responses 

and the corresponding power generation of the system were 

then obtained by solving Eq. (1) using the MATLAB 

Simulink/SimMechanics Toolbox with a 4th-order Runge-Kutta 

algorithm for time marching.

Figure 2. An example of a WEC-Sim model for simulating a 

floating WEC device and the top level of hydrodynamics 

modules

Figure 2 shows an example of using WEC-Sim to simulate a 

floating WEC system and the top level of hydrodynamics 

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modules for the WEC-Sim model. Each separate module on the 

right side of the figure corresponds to the hydrodynamics 

calculation of wave radiation, excitation, net buoyancy 

restoring, viscous damping, and mooring forces.

WEC-Sim Model Validation

To validate the numerical model, WEC-Sim was applied to 

simulate a bottom-hinged OSWEC design (Figure 3). All the 

mass properties and dimensions (Table 2) were given based on 

the experimental study of Hoff [13]. The WEC-Sim simulation 

was performed at three different peak periods (Tp) and four 

different significant wave heights (Hs). For each case, the 

simulation duration was 125 Tp long with a ramp time of 25 Tp

and a time step size of 0.01 Tp. 

Figure 3. Schematic of a bottom-hinged OSWEC and mesh 

(half of the flap) used for the hydrodynamics calculations

Table 2. Mass Properties and Dimensions

Parameters Values (Unit)

Dimensions (width x thickness) 18 m x 1.8 m

Hinge to water surface 8.9 m

Moment of Inertia (at Hinge) 1.85 x106 kgm2

Restoring Stiffness (at Hinge) 6.4 MNm/rad

Figure 4. Mechanical power performance from WEC-Sim 

and experimental data (in full scale)

To compare WEC-Sim simulation results with those 

obtained from existing experimental measurement data [13], 

the averaged mechanical power was calculated. The values are 

plotted against the energy period Te in Figure 4, where Te is 

approximately 1.16 times of Tp [14]. Overall, the numerical 

simulations results agreed well with those measurements from 

the wave tank test. 

DEVICE STRUCTURE DESIGN

This section presents a preliminary structural analysis to 

determine the mass properties of the OSWEC designs. Each 

design contained one or two flaps and a supporting frame that 

was made of steel tubes. Each flap was assumed made of 

fiberglass tubes and was connected to a steel rotational shaft. 

When a WEC is deployed under operation, all structural 

components endure not only the physical stresses imparted by 

the hydrodynamic pressure that is a product of water depth, but 

also the stresses from the force of waves as they pass over the 

device and drive the PTO system. The structural calculations 

were first performed using the pressure distribution data from 

WAMIT, where the force and moment distributions over each 

of the WEC fiberglass and steel tubes were evaluated. For 

simplicity, the tube was divided into multiple "bins" and simple

beam theory was applied to calculate the moment distribution 

over the tube. Each cylinder tube was designed to withstand its 

maximum internal moment under an extreme wave amplitude 

of A = 10 m. Figure 5 shows an example of the pressure 

distribution and the corresponding moment distribution on one 

of the flaps for design B.

Figure 5. Hydrodynamic pressure distribution on a flap 

tube (left); corresponding moment distributions (right) 

Several other checks were also required to ensure that the 

tubes will hold up to adverse stress conditions apart from 

transverse bending caused by hydrodynamic pressures

(buckling). In addition, the thickness of the tube was also 

constrained by its displacement, which was limited to no more 

than 0.5 m under the given moment distribution. Linear 

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displacement was assumed with thickness, so direct 

proportionality was used to find the final thickness necessary 

for the design. Note that a dynamic factor of safety was applied 

to all calculations, which was set at a conservative value of 2.0.

More details on the structural design methodology were 

described in [15]. 

Based on the structural analysis, we determined the 

thickness of the fiberglass and steel tubes. Additionally, the 

bottom plate thickness was determined by following the plate 

design for a two-body floating point absorber [3]. The mass 

properties of the selected OSWEC designs are listed in Table 3, 

which were obtained from SolidWorks mass properties 

calculations. Note that the listed values for the flap and shaft 

only accounts for the mass properties for a single flap.

Table 3. List of Design Mass Properties

Design A B C

Flap & 

Shaft

Fiberglass Mass (Ton) 90 112 90

Steel Mass (Ton) 64 64 64

CG (m) (0,0,-10) (0,0,-15) (±18.5,0,-10)

MOI (106kgm2) 14.11 17.30 14.11

Supporting 

Frame

Steel Mass (Ton) 322 870 351

CG (m) (0,0,-15) (0,0,-23) (0,0,-14)

MOI (106kgm2) 67.66 188.86 61.30

CG: Center of gravity

MOI: Moment of inertia at the CG in pitch

MODELING OF A FLOATING OSWEC

Numerical Model Setup

WEC-Sim was used to model the hydrodynamic response of 

the selected OSWEC designs and calculate the corresponding 

power performance. The model setup in WEC-Sim for different 

designs is shown in Figure 6. 

Figure 6. WEC-Sim model setup for designs A & B (left) 

and design C (right)

For each design, the simulation was performed for a range 

of irregular sea states, which were characterized by Hs and Tp, 

and were represented using a Brechtschneider spectrum. The 

hydrodynamic response of the device was simulated in the time

domain for a duration of 125 Tp with a ramp time of 25 Tp and 

a maximum time step size of 0.01 Tp. 

In the simulations, the supporting frame was ballasted so 

that the device was located at its equilibrium position without 

any pre-tension when the device was connected to slack 

mooring lines. On the other hand, the frame was ballasted to the 

weight of the frame when the device was connected to taut 

moorings, where the pre-tension was equal to the total net 

buoyancy force from the flap and shaft. Accordingly, the 

moment of inertia for the slack moored and taut moored cases

was adjusted. For designs with a slack mooring configuration, a 

mooring stiffness of 40 kN/m was specified for the surge 

direction to keep the device in position. For designs with a taut 

mooring connection, an optimal mooring stiffness was selected 

based on device power performance for a typical sea state

(Te=8.7 sec; Hs=1.75 m) from the reference site. 

An optimal velocity-dependent PTO damping force for each 

sea state was determined by tuning the device performance with 

different PTO damping coefficients. Note that we assumed the 

same PTO damping value for the fore flap and the aft flap when 

modeling design C. 

Power Matrix and Estimated AEP Calculation

The annual energy production for a WEC can be determined 

based on the wave statistics at a reference site, the device 

performance power matrix, and the loss between absorbed 

mechanical power and electrical power output.

The wave statistic at the reference site is often represented 

by the joint probability distribution (JPD) of the waves, 

characterized by Hs and Te (or Tp). The JPD at Humboldt Bay, 

CA (Table 4) was used in the study, and it was obtained based 

on the measurements from a nearby National Data Buoy Center

buoy (#46212).

Table 4. JPD for the Wave Resource at Humboldt Bay

The power generation performance of WECs in irregular 

seas is often represented using a power matrix. The mechanical 

power matrix was obtained by modeling the OSWEC design at 

each given sea state and by calculating the averaged power 

performance. To estimate the electrical power matrix, we 

multiplied the mechanical power matrix with a PTO conversion 

efficiency and limited the maximum electrical power output at 

the rated power (also referred to as capacity) to reduce the size 

of generator and cost. The electrical power that can be 

generated by the OSWEC design under each given sea state can 

be obtained from

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Pe = Pm × η1, (7) 

where Pm and Pe are the estimate mechanical and electrical 

power that can be generated under each given sea state, and η1

is the PTO efficiency that accounts for the losses between 

absorbed mechanical power and electrical power output.

We assumed a hydraulic PTO system was used for the 

OSWEC designs and the conversion efficiency was 80% [16]. 

The rated power was estimated based on a capacity factor of 

30%, and details of the electrical power matrix calculation were 

described in [3]. An example of the resulting electrical power 

matrix for design A with a taut mooring connection is shown in 

Table 5. The annual averaged electrical power Pae was then 

obtained by summing the product of the electrical power matrix

and the JPD for the reference site.

Table 5. Electrical Power Matrix for Design A (taut) 

Parametric Studies

The power generation performance for the selected OSWEC 

designs was calculated based on the methodology described in 

the previous section. To compare the power generation 

performance for the OSWEC designs with different mooring 

settings, the resulting annual averaged electrical power output 

for different scenarios is plotted in Figure 7. Note that, for 

design C, we did not consider a taut-moored system in the 

study because the slack-mooring configuration worked well. In 

addition, the fixed frame results are also included.

Figure 7. Annual averaged electrical power output for 

different OSWEC designs

As expected, the power generation increased as the number 

of flaps was increased (designs B and C) for the same mooring 

configuration. Design B had better performance when the 

supporting frame was fixed. For each design, the fixed-frame 

system provided the best power generation performance. For 

designs A and B, the taut mooring connection helped reduce the 

frame rotation. Therefore, the device with a taut-mooring 

connection performed better than the one with a slacked 

mooring system. 

To further investigate the influence of the frame stability on 

the design power generation performance, we compared the Pe

on a single flap for the selected floating design to the fixed 

frame simulation results. The values are plotted against 

different Te in Figure 8. In addition, the corresponding ratios 

between the optimal CPTO values for the fixed frame cases and 

those used for the floating design cases were calculated. The 

ratios are listed in Table 6. 

Figure 8. Electrical power output for selected floating and 

fixed frame OSWEC designs (Hs= 2.75 m) 

Table 6. Fixed Frame to Floating Designs 

Optimal Cpto Ratio

Designs CPTO (Fixed) / 

CPTO(Floating)

Designs A (Fixed/Taut) 4.2-12.5

Designs B (Fixed/Taut) 8.3-15

Designs C (Fixed/Slack) 1-1.5

When the flap rotated against a fixed frame, a larger optimal 

PTO damping value was applied. When the supporting frame 

was connected to mooring lines, the frame would rotate with 

the flap, and the optimal PTO damping value would be smaller, 

which reduced the device power generation performance, 

particularly for designs A and B. Because the fixed frame 

OSWEC operates as a terminator, a wider flap (e.g., design B) 

typically reduces the diffraction loss and performs better under 

a large wave period [17]. However, the power performance for 

design B decreased significantly when the frame was connected 

to moorings, particularly for large-wave period scenarios, 

where design B was less effective than design A, as shown in 

Figure 8. The results also implied that the taut mooring 

connection was not very effective for a wider OSWEC design.

For design C, the aft flap converted less energy than the fore 

flap due to wave diffraction from the fore flap when the frame 

was rigidly connected to the seabed. When design C was 

connected to slack moorings, the fore flap and the aft flap 

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extract about the same amount of energy from waves. We 

believed that the distance between the fore and aft flaps 

resulted in a phase shift on the wave excitation force and 

increased the relative motion between the aft flap and the 

frame, which improved the power performance of the system. 

As a result, the power extraction loss due to the connection of 

moorings for design C with the slack moorings was minimized. 

Note that the complex nonlinear wave and flaps interaction, 

particularly for the aft flap, was not fully captured in the WECSim model. Neglecting those effects may lead to overestimating 

the device power output. Although design C’s power generation 

performance predicted from WEC-Sim is promising, further 

analysis, such as experimental wave tank tests or computational 

fluid dynamics simulations, are needed to validate the results.

DISCUSSION

From a hydrodynamics point of view, the power generation 

performance of the OSWEC systems can be improved by 

increasing the flap height and width and by stabilizing the 

frame using a taut mooring connection or a fixed supporting 

frame structure. Design improvements that increase power 

generation performance often incur costs—increased capital 

costs and/or operation costs. The levelized cost of energy 

(LCOE) metric reflects the necessary tradeoffs between 

minimizing costs and maximizing energy production. In 

practice, LCOE is often used as the metric to judge the design 

and economic viability of an energy conversion system.

Understanding LCOE and how to reduce it is essential to make 

renewable energy designs that are successful and competitive 

with other electricity resources. As a result, the cost 

implications of the design need to be addressed during the 

design process.

To assess the tradeoffs between improving power generation 

performance and lowering costs of the selected OSWEC 

designs, we evaluated the device annual energy production 

(AEP) to characteristic mass ratio. This ratio serves as a proxy 

for LCOE in the absence of detailed cost data. Because the 

study was focused on the floating OSWEC design, the fixed 

frame designs were excluded from the comparison. The 

estimated AEP [in megawatt-hours (MWh)] was calculated by

AEP = Pae × 8766 × β, (8) 

where β=η2xη3 is the parameter, accounting for the losses 

caused by transmission efficiency η3=0.98 (98%) and device 

availability η2. A device availability value of 95% was used for 

designs A and B [3]. Because design C contains a fore flap and

an aft flap, we assumed the design will most likely require two 

sets of PTO systems, which may increase the downtime due to 

maintenance. A device availability value of 90% was therefore 

used for design C.

The characteristic mass (mch) of the device was determined 

from

mch = (ms + mf) × βm, (9) 

where ms is the mass of the steel for the device, mf is an 

equivalent steel mass value for the fiberglass part, which was 

determined based on the cost difference between the fiberglass 

and the steel. We assumed mf was equal to 2.3 times the mass 

of the fiberglass used in the design. The value was selected 

based on the experience from the wind energy blade design. 

The variable βm was used to account for the mooring cost. We 

assumed βm was equal to 1.2 for a typical slack-moored 

floating WEC device. The value was determined from the 

Carbon Trust [18], which estimated the percent contributions to 

the cost of energy for early commercial WEC designs. For a 

taut-moored WEC device, we assumed the total cost of mooring 

and anchor was doubled [19], and βm was therefore equal to 

1.4.

The AEP to characteristic mass ratio for different floating 

OSWEC designs are plotted in Figure 9. For designs A and B, 

the taut-moored system was more cost-efficient than the slackmoored system. Overall, design C had the best performance in 

terms of AEP per characteristic mass among all the floating 

OSWEC designs. However, the manufacturing and mooring 

cost (which are represented by the AEP to characteristic mass 

ratio) only represent about 30% of the WEC design cost [18]. 

The cost of the PTO system, deployment (installation), grid 

connection, and operation and maintenance (O&M) also 

depend on the WEC device designs, which were not considered 

in the study. In addition, the design strategy may be altered 

because of environmental permitting and regulation.

Figure 9. AEP to characteristic mass ratio for selected 

OSWEC designs

In fact, the original OSWEC design proposed in the RM 

project was a bottom-fixed system, in which the flap rotated 

with respect to a frame rigidly connected to the seabed. From 

the power generation performance point of view, the bottomfixed OSWEC designs also have greater power generation 

performance, as shown in the results presented in the previous 

section. However, the design was changed to a floating system 

before significant engineering or cost analysis was conducted, 

based on concerns raised by the environmental assessment [4]. 

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The potential risk to the marine environment from the devices 

could have a significant effect on near-shore marine animals, 

habitats, and ecosystem processes, particularly for a 

commercial-scale deployment in shallow water (10- to 20-m 

water depth) with sand bottom, where shoreline sediment drift 

and shore-form creation also depends on the incoming waves. 

One of the RM project objectives is to investigate the 

commercial application of the design. Therefore, the OSWEC 

design was changed to a floating system.

Because LCOE is often used to judge the design and 

economic feasibility of an energy conversion system, we 

suggested a more sophisticated iterative design strategy, 

proposed in the RM project [3], as shown in Figure 10. This 

approach involves four modules that can be used to determine 

the device design, considerations for manufacturing and 

deployment (M&D), strategies for O&M, and environmental 

constraints. These four modules can be used to estimate the 

parameters needed to calculate LCOE: AEP, capital 

expenditures (CapEx), and operational expenditures (OpEx).

Incorporating this multifaceted design strategy into the 

development process at an early stage, rather than exclusively 

focusing on the hydrodynamic performance, facilitates a more 

efficient path towards designing for an optimized LCOE. As 

shown in the study, changing the OSWEC design for different 

sizes will affect the structural design as well as the 

hydrodynamic power performance, which provides a different 

cost of energy for each design. Developing an optimal costefficient design and a pathway to reduce LCOE in the future are 

important for the success of wave energy technology.

Figure 10. An iterative design strategy for WEC designs

CONCLUSIONS AND FUTURE WORK

The cost-efficiency for three OSWEC designs were 

evaluated in this paper by calculating the power generation 

performance of the designs using a time-domain numerical 

simulation tool and by evaluating the characteristic mass of the 

designs. The study also showed that it is essential to consider a 

cost-oriented design strategy to balance the performance design 

with economic assessment, as well as with the environmental 

issues. Incorporating this design strategy into the development 

process at an early stage can facilitate a more efficient path 

towards designing for an optimized LCOE. We used OSWEC 

as the example in this paper, and this multifaceted design 

strategy can be used by other ocean energy systems. 

To fully understand the potential LCOE for WEC designs, 

further analyses are still needed, particularly for the extreme 

load prediction and the cost estimate for deployment and O&M. 

Specifically: 

• The WEC devices are designed to survive under extreme 

storm waves. The nonlinear extreme wave load is essential 

for the structural design. However, nonlinear wave and 

body interaction increase the complexity for estimating the 

extreme wave load on the device and on mooring lines. 

8

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.
Further investigations on the extreme load measurement in 

a wave tank test and prediction in numerical simulations 

are needed.

• The evaluation of the cost and the strategy for deployment 

and O&M is very difficult at this nascent stage in the wave 

energy industry. Most of the cost estimation methods and 

strategies for WEC deployment and O&M come from 

experience with offshore oil and gas platforms and offshore 

wind energy experience. To date, no commercial wave 

energy farms have been deployed; representative data in 

these areas are not yet available and need to be investigated 

further.

ACKNOWLEDGMENT

This work was supported by the U.S. Department of 

Energy under Contract No. DE-AC36-08GO28308 with the 

National Renewable Energy Laboratory. Funding for the work 

was provided by the DOE Office of Energy Efficiency and 

Renewable Energy, Wind and Water Power Technologies. We 

would like to thank Vincent Neary from Sandia National 

Laboratories for his efforts and Andrea E. Copping from Pacific 

Northwest National Laboratory for her efforts on the RM

project.

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9

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.