Design of a Multi-Source Offshore Renewable Energy Platform
Authors: Engelmann Georg , Robinson Roy 
Published: April 2019

The paper will present the design of a floating platform incorporating the following systems:

Conventional Wind Turbine

Long and Short Period Wave Energy Capture

Ocean Thermal Energy Conversion (OTEC)

Open Flow Current Turbines

Energy Storage

The focus will be integration of the systems from a structural standpoint; effects on the cost of each system and the resulting LCOE and overnight cost; and the nameplate and peak power for given conditions.

Energy mechanisms in the marine environment are the wind, waves, water currents, and seawater temperature differences. An assessment and rating of the energy resource potential of a given development site is used to inform the renewable energy technology system selection process. Offshore Renewable Energy (ORE) technologies can be summarized into the following groups:

Offshore Wind Turbines are the prevalent ORE technology exploiting the present market, similar to onshore wind turbines, but mounted upon a fixed or floating offshore platform.

Ocean Thermal Energy Conversion (OTEC) uses the temperature differential between surface water and seabed water to drive heat engines.

Marine Hydro-Kinetic (MHK) devices convert energy from waves or fluid flow.

Wave Energy Converters (WEC) are oscillating/reciprocal/pressure driven systems operating at or near the ocean surface or bottom mounted in shallow waters.

Flow Energy Converters (FEC) are used in areas where velocity and direction of water flow is relatively constant or highly predictable if intermittent (tidal).

Unlike an onshore wind energy site, offshore wind energy systems (especially floating ones) are surrounded by these other energy sources; the integrated renewable energy facility design process addresses selecting systems that will complement each other while capturing the energy resident in the operating environment, as well as leveraging the wind turbine supporting structure and infrastructure to reduce the costs of the WEC, FEC and OTEC systems.

The amount of CAPEX spent on non-power generating equipment can be optimized by leveraging the floating system structure cost to host various ORE technologies.

Between 50% and 70% of the overnight cost of a typical MHK or OTEC facility will consist of equipment and activities that do not generate power. This is one of the key differences with offshore wind which has an overnight capital cost overhead of roughly 30%. By combining multiple technologies into a single platform, it is possible to reduce the MHK overhead costs to 18 to 36%, with little or no effect on the offshore wind overhead costs.

The resulting design is novel in configuration which takes the form of a Multi-source Articulated Spar Leg (MASL) platform and can reduce the Levelized Cost of Energy (LCOE – the economic measure used to compare energy systems) by at least 25%; can be fabricated and pre-commissioned in port; is fully configurable to the local conditions; is more stable than the current floating wind designs in use; and can be scaled up to carry any sized wind turbine.

Both cost savings and an increase in revenue can be realized using integrated ORE facilities given the higher average availability factor offered by blended ORE systems and reduction of individual system OPEX relative to stand-alone ORE systems, and example of which is shown in Illustration of Results

A single MASL platform prototype is expected to produce power as cost effectively as the only commercial floating wind farm consisting of 5 spar-type platforms that comprise the Hywind Project. Using published information, the internal rate of return (IRR) of Hywind is between 8% and 10%. The estimated return for the MASL prototype is 8.7%. Both based on a realized electricity price of $0.25/kWh and design life of 25 years.