Renewable Energy and OTEC in French Polynesia

Introduction
The small islands of French Polynesia in the South Pacific Ocean are threatened by climate change. As the climate warms, energy is increasing in the atmosphere, leading to storms that are more frequent and intense.[1] The increase in heat is causing melting of ice and thermal expansion of oceans, further threatening dangerous sea-level rise. The climate is warming due to increased levels of carbon dioxide in the atmosphere, which has been increasing mainly from the combustion of fossil fuels for human industry.[2] Intensifying and more frequent storms can cause expensive damage, loss of life, health risks, and a general decrease in the vitality of a country. These threats are especially apparent for small islands that do not have access to as many resources. Low-lying islands are particularly at risk for losing a significant percentage of land due to sea-level rise. As fossil fuel combustion continues the atmosphere will trap more heat and intensify these effects.[3] A cleaner renewable energy is needed to mitigate some of these effects and protect the health of small islands like those of French Polynesia.

The island of Tahiti is one candidate for application of existing and developing renewable energy technologies. Currently, Tahiti’s energy economy depends on importing oil by ship. This is expensive because of the long distances between fossil fuel sources and Tahiti, and it also increases environmental risks such as harmful oil spills. Around 66.1% of Tahiti’s electricity use comes from fossil fuels, as shown in Figure 1.


Figure 1.
Sources of power on Tahiti according to the local utility, Electricite du Tahiti (EDT). This shows that generation of hydroelectric power from dams (Hydro) makes up a significant (33.2%) portion of Tahiti’s power, while solar photovoltaic (PV) cells contribute a very minor portion and the majority comes from fossil fuels (Fuel).[4]

Of the 8,000 barrels of refined petroleum products consumed per day in Tahiti 7,190 barrels or 90% are imported.[5] Islands of French Polynesia are highly dependent on imports for their energy needs. Fuel is shipped to the islands, an example of which can be seen in Figure 2. This transportation makes energy use on islands more carbon-intensive, further contributing to climate change. It also significantly increases the cost of the energy; in Nuku Hiva, FP electricity costs around $0.40/kwh – nearly four times the cost of electricity in the US. Currently Nuku Hiva receives 70% of its energy from fossil fuels and 30 % from renewables including one hydroelectric plant and solar panels on the local elementary schools. The government of Nuku Hiva is striving to switch the island’s energy needs from fossil fuels to renewable energies by constructing a second hydroelectric plant and completing studies on wind mill use, a solar panel farm and a third hydroelectric plant.[6]


Figure 2.
A fuel truck arriving in Moorea, FP via a ferry from Pape’ete, Tahiti in March, 2014. Transporting by boat is the main way that French Polynesian islands receive fuel. Photo by S. Pollock, March 2014.

Ocean Thermal Energy Conversion
It would be beneficial for Tahiti to invest in sustainable energy that is viable for future generations and does not pose economic or environmental risks. Ocean Thermal Energy Conversion (OTEC) is one such possible energy source. OTEC creates electricity through harnessing the temperature differential created between warm surface waters (from 0 to 50 meters) and cold deep waters (depths of 1,000 meters and below).

It is estimated that 1,000 times more heat is stored in ocean waters than in the atmosphere, due to the ocean’s large volume and heat capacity.[7] Solar radiation penetrates the ocean and heats surface waters to temperatures higher than 25° C near the equator.[8] As the depth of the ocean increases, the sun’s energy is reduced and unable to penetrate deep sections of water thus temperatures decrease, creating a cold dense layer of deep water.[9] The temperature difference between cold water in the depths of the ocean and the warm surface waters provides the energy source used by OTEC.[10] Because Tahiti is situated in the tropics it receives more direct sunlight than higher latitudes. The surface ocean near Tahiti is constantly being heated, and the resulting temperature gradient between surface and deep water is between 22 and 24° C throughout the year, as seen in Figure 3.


Figure 3.
Analysis of the global average temperature gradient between surface and deep ocean water at 1000 m depth.[11] Tahiti is located in the region marked by a 22-24° C temperature gradient, sufficiently high for OTEC to utilize.

In order for an OTEC plant to be installed, a minimum temperature differential of 20 degrees Celsius and access to electrical power grids is necessary.[12] OTEC feasibility studies must show the presence of a constant temperature differential to drive the power plant.[13] Data on the water column temperature profile near Pape’ete, Tahiti (Figure 4) has been collected by Sea Education Association (SEA) research cruises for several years. SEA water profile data has been taken in the months of January through March using a conductivity-temperature-depth sensor (CTD; SBE19PlusV2, SeaBird Electronics, Bellevue, WA). These CTD data show the temperature differentials in the water column in Tahiti’s coastal waters. At 1000 meters temperatures are around 4° C, while surface temperatures are around 28° C. This indicates that the resource necessary for OTEC is present.


Figure 4.
SEA CTD data for temperature versus depth taken from four different cruises within 5 to 10 nautical miles of Pape’ete, Tahiti.

To ensure presence of the 20 degrees minimum temperature differential, data was collected from four different months of study on SEA research cruises and summarized in Figure 5. Temperature data were gathered and averaged for the surface and the maximum sampling depth, which is 500 meters. Figure 5 shows that the temperature gradient drops slightly below 20 degrees Celsius for the months of December and January. This is important for OTEC feasibility consideration because a change in temperature differential can have effects on OTEC gross power output[14],[15]. Further study is needed to evaluate seasonal and interannual variation at a particular site prior to plant construction.


Figure 5.
Average temperature gradients off the coast of Pape’ete, Tahiti summarized for four different months of SEA research cruises.

Additionally, bathymetry data for Tahiti shows the seafloor depth around the island. This is important for determining the appropriate location of deep water pipes. Bathymetry data were collected on the SEA S252 (2014) cruise of the seafloor beginning near Pape’ete and traveling offshore using a Knudsen CHIRP 3260 Subbottom Profiler (Knudsen Engineering Limited, Perth, Ontario, Canada). Figure 6 illustrates the depth of the water as the distance from shore increases. Figure 6 shows seafloor depths of 1000 meters or more exist within approximately 5 to 10 nautical miles from shore. This indicates that offshore OTEC plants can be located in the close vicinity of Pape’ete, making connection to electrical grids more feasible. Cold deep water is taken up from water over 1000 meters deep, and to have this water available near to shore makes the transport through a cold-water pipe less costly.


Figure 6.
Bathymetry data taken from Robert C. Seamans' CHIRP system beginning in Pape’ete, Tahiti and traveling northeast towards Rangiroa.

OTEC is still being developed, and many options for an OTEC plant exist. Some of the design considerations include plant size and location (on- or offshore). Another important consideration is whether to employ an OTEC plant that uses closed-cycle or open-cycle technology. An open-cycle OTEC plant takes up warm surface seawater, evaporates it in a low-pressure system, and uses the warm vapor to turn a turbine and generate electricity. Then water is condensed using cold deep water.[16] This desalinates the vapor and can lead to useful byproducts. The closed-cycle technology operates on the same principles, but instead evaporates and condenses a working fluid such as ammonia.[17] Closed-cycle plants are more compact and have a higher efficiency with current heat exchanger technology.[18] The efficiency of the open-cycle plant is lower than the closed-cycle plant, which means improved heat exchangers are needed for open cycle plants. There are challenges in acquiring heat exchangers with high efficiencies.[19] Since OTEC plants need 20 degrees minimum temperature difference, solar panels can be used to heat surface waters and increase the temperature gradient. Solar enhanced OTEC plants can increase efficiency up to 1.5x.[20] Hybrid OTEC designs that use a combination of the closed and open cycle systems to obtain maximum efficiency are also being studied.[21] The OTEC process can be run in reverse to refrigerate or obtain seawater air conditioning (SWAC).[22] Research and development is underway to improve the efficiencies of OTEC systems and simplify some of the complexities. Each of these options must also be investigated for their economic feasibility. While it has been shown that smaller OTEC plants currently may not be competitive with fossil fuels,[23] the markets and technology are continuing to change as OTEC is explored further. It is also important to consider various benefits of OTEC that may not have an economic value.

One of the greatest intrinsic values of OTEC is that it can provide renewable, clean energy to Tahiti, allowing energy independence and pride in the fact that Tahiti’s energy source is significantly safer than fossil fuels for the health of the environment. Desalinated water, a byproduct of the OTEC process, can be used for clean drinking water, agriculture, and mariculture. The cold water brought up from the deep sea can also be used to efficiently and renewably provide air conditioning for buildings.[24] Each of these has the potential to improve the quality of life and increase energy production on Tahiti.

There are also many challenges currently facing the implementation of OTEC. Several technical, engineering problems still face the technology. Keeping offshore, floating plants moored to the seafloor is one such challenge.[25] Transporting the amounts of water necessary for an onshore OTEC plant is another difficulty. The long, cold-water pipe must be very large and reach deep enough waters for a thermal gradient to be achieved. According to one study, approximately 4 m3/s of warm seawater and 2 m3/s of cold seawater (ratio of 2:1) are required as inputs per MW of exportable or net electricity.[26] The designs of these water pipes are complex engineering challenges.[27]

Through OTEC, discharge plumes are created from different water masses released from open-cycle OTEC plants. Understanding the oceanographic characteristics of the surrounding environment is critical background information for modelers tasked with determining the behavior of an OTEC discharge plume.[28] Since drawn from a depth of 1000m, the discharge water will be cooler than the surrounding water, and the chemical characteristics of the plume will be significantly different.[29] The change in chemical compositions can alter the surrounding ecosystem by fueling phytoplankton blooms, therefore increasing primary production and influencing food web dynamics.[30] Because of the influence on the marine ecosystem, OTEC plants should not be placed near fishing grounds or reef habitats that could be affected by this change in water composition.[31] Placement of OTEC plants must also take into account the possibility of the water intake pipes accumulating and trapping marine organisms if a plant is placed in a region of high biological activity.  To prevent impingement, of marine organisms pipes are being designed to have the seawater entering the pipe at low velocities.[32]

Regardless of location, water intake pipes will be in contact with seawater which contains marine organisms. An effect called fouling presents another environmental challenge. Fouling occurs when the pipes, which transport the water to the heat exchangers and condensers, develop a film of microbial organisms, which decrease water flow and thus disrupt efficiency.[33] Pipes are either manually cleaned of the films or a coating, called a biocide, is applied to minimize fouling.[34] In U.S. waters, the amount of biocides needed to maintain efficiency must be less than the maximum discharge amount permitted under the Clean Water Act.[35]

Desalinated water produced through the open-cycle process could be used by the surrounding community for municipal or agricultural use. Mariculture, used to grow algae, fish and shellfish, and seawater air-conditioning can use the cold-water released from the system.[36]

A power transmission cable must connect to shore, thus close proximity to an electrical grid is necessary to efficiently transport electricity produced from the plant. The distance to cold, deep seawater must also be minimized to decrease the length and cost of the cold-water pipe.[37] The cold-water pipe makes up most of the cost of an OTEC plant and is one of the largest challenges of the plant.

The cost of electricity from OTEC decreases with increasing plant size,[38] and it is estimated that a 50 MW or greater plant could produce electricity at a rate competitive with the price of electricity produced with fossil fuels.[39] Plants of this size have not been developed or tested, but as the market for this technology grows OTEC will continue to become more competitive. There are many logistical issues to be worked out that prevent OTEC from being a more widespread energy technology. It has a limited scope of where it can be used, but in the correct location OTEC can be a source of clean and sustainable energy.

Ongoing Renewable Energy Projects
Various efforts to implement renewable energy technologies are occurring across French Polynesia. Projects such as Tonne Equivalent Petrole Valorisation des Energies Renouvelables et Transfort d'Experience et de Savoir faire (TEP VERTES) have been funded by the European Union to increase access to renewable energy, as well as improve lifestyle and create jobs, in French Polynesia and other overseas countries and territories (OCT’s).[40] French Polynesia’s tropical location is well suited to obtaining consistent and direct energy from the sun, year-round. Solar power and OTEC are two emerging technologies that take advantage of this source of energy.[41] The government of Tahiti has shown interest in OTEC from an OTEC feasibility study completed through DCNS.[42] Already 33% of Tahiti’s energy is supplied by renewable hydropower according to Tahiti’s electrical supplier.[43] Wind and solar energy are also in development for French Polynesia. Solar electricity is economically competitive with fuel, and it can be seen across French Polynesia for residential, business, and governmental energy use, as shown in Figure 7.


Figure 7.
Solar photovoltaic (PV) panels in use on the local police station in Rangiroa (left), for a residential home in Nuku Hiva (center), and to power street lights outside a solar panel distributor in Pape’ete, Tahiti (right). Solar panels commonly supply energy to a variety of establishments in French Polynesia. Photos by S. Pollock, March-April 2014.

Owner of Pacific Promotion Tahiti S.A., Teva Sylvain, was interviewed about his renewable energy supply company’s use of solar panels. Sylvain was concerned with the economic welfare of his company because of the high price of electricity. To aid in this problem Sylvain began to provide solar power for residents in Tahiti because he believes solar panels have the potential to supply all the energy needed for a typical home in Pape’ete. The company’s building uses a combined power supply including solar panels. After the installation of this system, Sylvain was producing more electricity than was used by the building, and they sold electricity back to the grid. Currently the company provides and maintains solar panels for 110 different installation sites on Tahiti with a combined power of 2.4 megawatts. When asked about SWAC and OTEC, Sylvain stated that he has knowledge of SWAC and thinks it is necessary and possible for Tahiti, though not currently economically feasible.[44]

A downfall of solar and wind energy is that they often do not provide a reliable source of energy due to the small, non-interconnected nature of the electrical grids in use on these islands.[45] An alternative way that many French Polynesians harness the sun’s abundant energy is solar water-heaters, as can be seen in Figure 8.


Figure 8.
Solar water heaters on the roof of the Mai Tai Resort on Rangiroa.  These heaters are popular because they harness energy from the sun without needing connection to an electrical grid – which may not always be available in French Polynesia. Photo by S. Pollock, March 2014.

French Polynesian residents also use solar and wind power on smaller scales to power their boats.[46] This energy does not need to be connected to a grid; the electricity simply goes directly towards the energy needs of their boat. This has much potential to decrease costs and fuel emissions in the fishing industry of French Polynesia. An example of a renewably powered boat can be seen in Figure 9.


Figure 9.
Local boat in Nuku Hiva with wind turbine and solar panel directly below on the stern. These forms of renewable energy are popular for use on boats because the electricity can directly supply a boat’s modest energy needs. Photo by S. Pollock, April 2014.

The sea is one of the most consistent and reliable resources for renewable energy in French Polynesian islands, if technologies can be developed to harness it. OTEC has the ability to provide electricity from the ocean temperature gradient, without release of damaging greenhouse gases. OTEC has much potential to provide energy for an island such as Tahiti. Oceans near Tahiti have a large temperature gradient, appropriate seafloor depth and proximity to electrical grids needed to connect to the OTEC plant. With extensive study of these requirements, Tahiti could benefit from a renewable energy source such as OTEC. Marine renewable energy sources such as OTEC, and tidal or current (hydrokinetic) energies are being developed, but are not yet available for the large-scale application needed in French Polynesia.[47] One successful renewable energy project has been the air conditioning of the InterContinental Bora Bora Resort by cool seawater using SWAC.[48] Solar power, wind, hydroelectric, OTEC, and power from currents are all being researched and developed as potential future energy sources. Each will continue to improve and will aid in French Polynesia’s transition to sustainable energy sources.

Sonia Pollock, Macalester College
Catherine Puleo, Miami University
2014

References

[1]  IPCC, 2013: Summary for Policymakers. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

[2]  IPCC, 2013.

[3]  IPCC, 2013

[4]  Foo, M. C. 2012. From Fuel Based Electricity to Renewable Energy: The difficult path of a power generation operator on small isolated islands. PECC Seminar: Oceans as a Source of Renewable Energy.

[5]  The World Factbook 2013-14. Washington, DC: Central Intelligence Agency, 2013.

[6]  Schmith, T. and C. Utia. 10 April 2014. Taiohae Government. Taiohae, Nuku Hiva. Personal Interview.

[7]  Rahmstorf, S. 2006. Thermohaline Ocean Circulation. Encyclopedia of Quaternary Sciences. Edited by S.A. Elias. Elsevier, Amsterdam 2006. 10 p.

[8]  Etemadi, A., A. Emdadi, O. AsefAfshar and Y. Emami. 2011. Electricity Generation by the Ocean Thermal Energy. Urmia University of Technology, Urmia, Iran. Energy Procedia Journal. 12: 936-943.

[9]  Etemadi et. al., 2011

[10]  Etemadi et. al., 2011

[11]  Etemadi et. al., 2011

[12]  Boye, H. 2012. An Overall Picture of the Marine Renewable Energy and Innovation Policies. PECC International Project. Hawaii, USA. 6 p.

[13]  Foo, M. C., 2012

[14]  Etemadi et. al., 2011

[15]  Nihous, G. 2012. Ocean Thermal Energy Conversion (OTEC): Challenges and opportunities. Department of Ocean and Resources Engineering: University of Hawaii, Honolulu, Hawaii, USA. 6 p.

[16]  Comfort, M. and L. Vega. 2011. Environmental Assessment of Ocean Thermal Energy Conversion in Hawaii. Hawaii National Marine Renewable Energy Center. p 1-8.

[17]  Vega, L. 2003. Ocean Thermal Energy Conversion Primer. Marine Technology Society Journal. 6(4): 25-35.

[18]  Etemadi et. al., 2011

[19]  Nihous, 2012

[20]  Yamadaa, N., A. Hoshi and Y. Ikegami. 2009. Performance simulation of solar-boosted ocean thermal energy conversion plant. Renewable Energy. 34(7):1752–1758.

[21]  Etemadi et. al., 2011

[22]  Nihous, 2012

[23]  Vega, L. 2010. Economics of Ocean Thermal Energy Conversion (OTEC): An Update. Offshore Technology Conference 21016. 18 p.

[24]  Makai Ocean Engineering. 2010. Cold Seawater Air Conditioning. <http://www.makai.com/pipelines/ac-pipelines/>

[25]  Mario, R. 2001. Ocean thermal energy conversion and the Pacific islands. South Pacific Applied Geoscience Commission (SOPAC). Miscellaneous Report 417. 16 p.

[26]  Vega, 2010.

[27]  Nihous, 2012

[28]  Comfort and Vega, 2011

[29]  Comfort and Vega, 2011

[30]  Etemadi et al., 2011

[31]  Etemadi et al., 2011

[32]  Comfort and Vega, 2011

[33]  Nihous, 2012

[34]  National Ocean and Atmospheric Administration (NOAA). 1981. Office of Ocean & Coastal Resource Management. OTEC Final Environmental Impact Statement.

[35]  NOAA, 1981

[36]  Etemadi et. al., 2011

[37]  Vega, 2003

[38]  Mario, 2001

[39]  Vega, 2010

[40]  European Union - OCT (Overseas Country and Territories). 2009. Design and Evaluation of the Technical assistance on the TEP VERTES program design.

[41]  Foo, 2012.

[42]  DCNS. 2012. Ocean Thermal Energy Conversion. <http://en.dcnsgroup.com/energy/marine-renewable-energy/ocean-thermal-energy/>

[43]  Foo, 2012.

[44]  Sylvain, T. 24 March, 2014. Pacific Promotion Tahiti S.A.. Pape’ete Tahiti. Personal Interview.

[45]  Foo, 2012.

[46]  Ludovic, C. 24 March, 2014. Solar Edwards. Pape’ete Tahiti. Personal Interview.

[47]  Foo, 2012.

[48]  Makai Ocean Engineering. 2010.

How to cite this entry:
Sonia Pollock and Catherine Puleo. 2014. "Renewable Energy and Ocean Thermal Energy Conversion in French Polynesia." Atlas on Sustainability of Polynesian Island Cultures and Ecosystems. Sea Education Association, Woods Hole, MA. Web. [Date Accessed] <html>