Ocean Thermal Energy Conversion (OTEC)

• Week 8 discusses Ocean Thermal Energy

Principles of OTEC

• Ocean Thermal Energy Conversion (OTEC) utilizes temperature differences between warm surface ocean water and cold deep ocean water for electricity generation
• To produce net output energy, the temperature difference between surface water and water at around 1000m depth needs to be about 20°C
• OTEC functions as a heat engine operating at the available modest temperature differential
• Ammonia is a suitable working fluid due to its vaporization and condensation properties at ocean temperatures

OTEC System

• Has cold temperature (Tc) and hot temperature (Th = Tc + ΔT)
• The working fluid circulates in a closed cycle and takes up heat from the warm water using a heat exchanger
• Fluid expansion drives a turbine, which in turn drives a generator
• The working fluid is then cooled by the cool water to continue the cycle

OTEC Types

Closed Cycle

• Employs a low boiling point working fluid such as ammonia
• Composed of an evaporator, turbine, condenser, and pump
• Involves warm surface seawater evaporating the working fluid in the evaporator
• Afterwards, vaporized working fluid drives a turbine that connects to a generator
• Cold deep seawater then condenses the vapor back to the liquid phase
• Then the pump recirculates the liquid working fluid back to the evaporator

Open Cycle

• Warm, surface seawater is placed in a low-pressure container to cause boiling, which turns a turbine as the steam expands
• Subsequently it is exposed to cold, deep seawater that condenses to make freshwater

Hybrid Cycle

• Contains elements of both closed and open cycles
• Generates electricity through a closed cycle
• Uses discharge water from the closed cycle to produce the fresh water, which is done via open cycle flash evaporation and condensation

OTEC Advantages

• Electricity is produced without fossil fuels, resulting in no carbon emissions or other harmful byproducts and utilizes vast thermal energy that is replenished by solar heating
• Unlike intermittent resources such as solar or wind, it can continuously provide power, making it appropriate for baseload networks
• Valuable byproducts include desalinated water (open-cycle systems), cooling for air conditioning/refrigeration, and aquaculture via nutrient-rich seawater
• Enormous amount of thermal energy due to oceans covering 70% of the earth with estimates suggesting OTEC could potentially generate 3-5 terawatts of baseload power

OTEC Disadvantages

• Construction and deployment require significant upfront investment which is more challenging for developing nations
• Only viable in regions with at least a 20°C temperature difference between surface/deep waters
• It is also a complex and specialized system requiring specialized engineering
• May have some impacts on marine ecosystems
• Features low thermal efficiency and can pose power transmission challenges

OTEC Development

• The world's largest operational OTEC power plant is located at the Natural Energy Laboratory of Hawaii Authority (NELHA) in Kailua-Kona, Hawaii
• NELHA has a 100kW annual power generation capacity, sufficient to power approximately 120 homes
• NELHA was connected to the US grid in August 2015 and provides baseload power
• An OTEC project in São Tomé and Príncipe anticipates completion in 2025, with a planned capacity of 1.5 MW
• The São Tomé and Príncipe project will be the first commercial-scale OTEC platform and designed to provide 17% of the nation's energy consumption

Tidal Energy

• A renewable source harnessing the natural rise and fall of ocean tides to generate electricity
• There are three systems commonly used
• Tidal Barrages use dam-like structures across estuaries
• Tidal Stream Generators use underwater turbines
• Dynamic Tidal Power involves theoretical dams extending from the coast into the ocean

Tidal Projects

• Sihwa Lake Tidal Power Station in South Korea has a 254 MW capacity
• The Inner Sound of the Pentland Firth in Scotland has a planned tidal energy project that may generate 398MW

Wave Energy

• Is a renewable source with kinetic energy to generate electricity
• Wave Energy Converters (WECs) are used to capture wave motion
• There are designs for platforms, columns, absorbers, and overtopping devices

Wave Projects

• Mutriku Wave Power Plant is located in Spain and has 300kW capacity
• Uses oscillating water column technology
• Perth Wave Energy Project is located off the coast of Garden Island with the aim to integrate wave energy into Australia's grid
• Okinawa Wave Energy Plant is designed to withstand the powers of mother nature and demonstrates Japans commitment to renewable energy