V.6. Marine Renewable Energy
(originally published September 4, 2007)
The final installment of Part V (renewable electricity generation) of the longer series on the Renewable Electron Economy focuses on some promising but less well-publicized types of hydropower, which can be described together as “Marine Renewable Energy”.
The world’s oceans are a great heat sink for the sun’s energy as well as a means to tap into the gravitational energy of the moon through the tides. There are three main sources of energy from the ocean that have and can in the future be harnessed to generate clean, renewable electricity: tidal power, wave energy, and thermal gradient energy (OTEC). Each of these will have a role in local and regional electricity markets as carbon emissions become priced into energy sources and these technologies mature through private and public investment. In addition, there are suggestions that the ocean’s currents and gradients in salinity can generate electricity but these remain speculative and/or with uncertain ecological effects.
Marine Power Infrastructure
Marine renewable electricity generation including offshore wind requires existing and future technologies that can handle the challenges associated with immersion in the ocean at a distance of as much as several hundred miles from land. Undersea power cables, in particular high voltage DC lines, are a mature technology. A power plant in the ocean requires individual power generators to be linked by cable to a transformer station that then sends net power plant output to the grid. Offshore wind farms in Europe link turbines along linear cables, which are then connected on one side to a transformer. Wave and thermal gradient power generators could be linked with similar infrastructure. Dealing with both the salinity and the excess kinetic energy of water during storms are common challenges for building the generating and transmission infrastructure for marine power. Durability testing of all marine power infrastructure will yield refinements and help guide selection of materials and products which can stand up to the ocean environment.
Many coastal areas with strong wind resources between 30-60 degrees of latitude in the Northern and Southern hemispheres also have substantial wave power potential. As wave energy is largely a derivative of wind energy as it pushes the surface of the ocean, offshore wind represents a much greater energy potential than wave, though wave energy is substantial. The Electric Power Research Institute estimates that the US has 2100 terawatt hours/year wave energy potential, concentrated in Alaska, Hawaii and the West Coast, estimated to be about one tenth the current US hydroelectric capacity.
There is currently a wide choice in the type of technology used to generate power from the up and down and lateral motion of waves, too many choices to review in this space. Two of the more widely discussed are the Pelamis Wave Energy Converter and the PowerBuoy. The Pelamis Wave Energy Converter is a long snake-like device that floats on top of the waves with multiple articulations that move with the motion of the waves. As the Pelamis undulates with the waves, a hydraulic fluid inside the Pelamis impels a generator to rotate and generate electricity. The Pelamis P-750, rated at 750 kW per unit, is being installed at two farms off the coast of Europe, one in Portugal and the other in Scotland. With multiple generators per installation, the output of the generators is routed to a single transformer station for the wave farm that is then connected to the grid. Another technology is the PowerBuoy that uses a piston like structure that bobs up and down with the waves to rotate a generator. PowerBuoy has been selected for installations in New Jersey, Hawaii and Spain, and estimates a capacity factor of 30-45% that compares favorably to other renewables.
Other than the challenges of durability in the ocean environment, initial cost is a challenge for wave power installations: in a regime without a carbon price, wave power installations need the support of government or utility subsidies to diminish initial capital costs. Transmission infrastructure through a marine environment is a major expense, as are the still-evolving wave energy capture devices. Eventually with economies of scale, wave power is thought to be cost-competitive with other power sources.
The tides get their energy from the gravitational pull of the moon on the ocean systems of the earth. Most tides take 12 hours to traverse a full cycle, so there are usually two high and two low tides per day. The tides are regular, highly predictable but shift in their relationship to the daily cycle of electric demand as the tidal cycle is 12 hours and 24 minutes long. The potential power of tides in any one coastal location has to do with the height difference between high (flood) and low (ebb) tides, which is shaped by local and regional coastal geography.
The tides have been used for many centuries in some tidal estuaries to drive water mills for grinding grain but contemporary use focuses on electricity generation using tidal energy. Tidal power is divided into two types, barrage tidal power and stream tidal power. Stream tidal power uses the kinetic energy of tidal streams in specific areas with a high-speed tidal current during the ebb and flood tides. Barrage tidal power traps tidal water behind an artificial barrier or barrage and then releases water to control power output.
Barrage tidal power
In specific coastal and estuarine locations, the shape of the coast and the strength of the tides offers the opportunity for building one or more barriers (barrages) that capture the tidal in and out-flows and then allow water from the resulting in and outflow to turn turbines in a controlled manner, thereby generating electricity. The largest barrage tidal power plant is 40 years old, built in 1967 at La Rance, France has a peak capacity of 240MW and generates 600 GWh of electricity a year, yielding a capacity factor of 28%. There are coastal locations in Great Britain, Northern Europe, Russia, East Asia, and North America which could, if a large enough barrage were built, create tidal power plants with peak capacities close to 90 GW in specific locations around the world. Barrage tidal power plants can be constructed that produce power 24 hours a day with multiple lagoons.
Similar to very large hydropower plants and dam projects, a barrage tidal plant is a very expensive undertaking that in addition disrupts a large coastal or estuarine area. In Great Britain, a very large barrage project at the Severn estuary on the border of Wales and England has been under discussion, on and off for over a century. There have been numerous proposals that range in power output from 15 GW to less than one GW depending upon the area of this large tidal inlet that would be encompassed by the new barrier. The most developed proposal at 8.6GW would provide 6% of Great Britain’s power, largely carbon-free upon operation and potentially available for over a century. A more recent proposal by a Welsh businessman, Gareth Woodham, has proposed a 15 GW plant and the creation of a Severn Lake with marinas, a roadway, and various amenities. Cost for the project is estimated at around $28 billion. In addition the barrier could help insulate the area from rising sea levels, storms. What speaks against this type of development is the transformation of the coastal and estuarine ecosystems that is inevitable with changing the natural flow of water and sediment in this area. For instance, the Royal Society for the Protection of Birds has opposed a number of these proposals, as the impact on shorebirds would be, at least in the beginning, very negative. Counterproposals from some environmental groups suggest that power can be generated from the tides at Severn with less environmental impact. Disagreements between environmental groups in favor of preservation of this marine environment, and in the last 10 years, those wishing for development and/or concerned about global warming have helped to keep plans for Severn in limbo.
Stream Tidal Power
Much less controversial though less productive per unit width of the installation are stream tidal power installations that use submerged rotors in areas with high-speed tidal streams or in deep rivers to generate power. Similar to underwater wind turbines, these generators are moored to the bottom of the stream and connected to the shore via a transmission cable. Putting a shroud around the turbine can increase output though confines the turbine to generating power only in one direction. The turbines in tidal and river streams move relatively slowly, thereby presenting a lesser danger to fish than a faster moving device might. Nevertheless the relative density of water gives each turbine more power per unit area and speed.
Notable stream tidal power projects in the US are in New York City’s East River and a proposed project at the Golden Gate off San Francisco. In the East River, a tidal river between Long Island Sound, the Hudson River, and New York Harbor, Verdant Power is constructing a tidal power farm that will generate power in both directions of the tidal stream. Using 36kW unshrouded turbines, the full project is to generate 10 MW at peak capacity. Currently Verdant and Con Edison, the New York City electric utility, are now testing six of these 3-blade axial turbines before full scale operation can begin in coming years. The City of San Francisco and Pacific Gas and Electric (PG&E) are exploring the possibility of building a 400 MW tidal power plant in the waters underneath the Golden Gate, the strait linking the Pacific Ocean, the San Francisco Bay and the outflow of the Sacramento and San Joaquin river systems. While only in investigative stages, the full plant could produce power for 45000 homes.
Thermal Gradient Power
An experimental, somewhat controversial source of power is ocean Thermal Gradient Power or Ocean Thermal Energy Conversion (OTEC). A function of the great difference in the temperature of tropical or sub-tropical surface ocean water, heated by the sun, and the cold of deeper water layers, thermal gradient power has a very large potential for power generation in areas where there is a difference of 20 degrees Celsius (36 degrees Fahrenheit) at around 1000m depth. A number of OTEC technologies use this difference in water temperatures to generate electricity as well as create a host of other services including cooling, water desalination, and mineral content of deep ocean water.
The concept of OTEC has been experimented with and discussed for over a century though the cost of constructing an OTEC system has been prohibitive and only recently have resources been directed towards developing these systems. The US renewable energy laboratory, NREL, estimates that OTEC will develop first around islands in the Pacific where power is expensive and where freshwater is in short supply.
There are a number of variations of OTEC depending on where the actual power plant is located and the technologies used to do the work desired by the plant designers. OTEC plants can be based on land, on the continental or island shelf or floating in deep water. There are also three different technologies that have been developed depending upon the end products desired from the OTEC process: closed cycle, open cycle, and hybrid closed/open cycle.
Closed Cycle OTEC
Able to be sited onshore or offshore, a closed cycle OTEC plant is designed to provide electric power alone. The closed-cycle refers to the use of a closed loop of ammonia, used for its low boiling point that is alternately heated and cooled by ocean water. Warm ocean water from the surface is passed through a heat exchanger to evaporate ammonia which then drives a turbine and electric generator. The ammonia is then cooled and re-condensed by passage through another heat exchanger cooled by deep ocean water. The National Energy Laboratory of Hawaii has built a number of prototypes for closed cycle OTEC plants but none have gone into production due to lack of funding.
Open Cycle OTEC
Open Cycle OTEC utilizes the tendency of water to evaporate under low pressure and produces both fresh water and power. Warm water from the surface of the ocean is pumped into a vacuum chamber, evaporates and drives a turbine, producing power. In a heat exchanger, cold deep-ocean water cools the steam which is then a source of fresh water. Thus in Open Cycle OTEC both power and desalination are achieved within the same process. Open cycle OTEC can be sited either onshore or offshore.
In Hybrid OTEC the strengths of Open and Closed Cycle OTEC are combined. As with open-cycle OTEC, warm ocean water is pumped into a vacuum chamber that produces steam. Rather than drive a turbine directly as in open cycle, the steam passes through a heat exchanger which boils ammonia that in turn drives an electric turbine. Cold ocean water then is used in heat exchangers to condense both the ammonia and the now fresh water. Hybrid OTEC produces both fresh water and power and utilizes the higher vapor pressure of ammonia to generate more power than steam alone.
Controversies and challenges around OTEC
OTEC shows promise as a source of both carbon-free power and fresh water. There are questions though whether extracting cold water from the deep ocean layers is the wisest strategy in a warming planet where ocean currents and temperatures are key to the climate system. On the other hand, the net exchange of heat with the ocean as a whole in some OTEC schemes is approximately zero, as heat is extracted from the upper layers of water before some is rejected into the cooling water from the depths that is then released into the upper layers of the sea. There is therefore some climate risk associated with intervention in the layering of temperatures in the ocean though advocates of OTEC counsel spacing of OTEC installations to avoid a concentrated effect of siphoning cold deep water in any one area.
More immediate challenges for OTEC are the high expense and technical challenges associated with the extraction of water from the depths. OTEC demonstration plants have so far been unable to be commercialized due to the high expenses associated with the new technology. With fuel prices relatively high, OTEC may receive needed funding and private investment.
If OTEC can be commercialized and any climate risk associated with it can be assessed or quantified, balancing the diminishment of climate risk from substituting OTEC for oil, nuclear or fossil generation vs. the unknowns of using cold water from the deep ocean is a matter for a future assessment. On the surface, it appears as though the risk may be well worth taking as known climate altering agents can be displaced by OTEC. A full assessment would require monitoring of changes in currents and temperatures around a large OTEC installation as well as a fuller understanding in climate science upon the role of ocean layers in the climate system.
Summary of Marine Renewables
Marine renewable energy is an inevitable part of the larger portfolio of renewable energy alternatives to fossil fuels. Which marine renewable technologies in addition to offshore wind develop depends in part on political processes within governments and the environmental movement, government R&D funding, the prices of fossil fuels, and the development of transmission infrastructure that could receive the output of marine renewable energy projects. Also important, in the case of tidal barrages and OTEC, are scientific studies of the environmental impact and impact mitigation alternatives for these two more invasive interventions in natural processes. OTEC, in particular, holds out the possibility for an alternative development path for the global South that can depend upon renewable electricity rather than oil or coal.