Hydrogen Economy Historical Background

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10 Hydrogen Economy, Geothermal and Ocean Power, and Climate Change

Hydrogen Economy

Historical Background

Henry Cavendish discovered “inflammable air” in 1766 and Antoine Lavoisier renamed it hydrogen. Hydrogen is colorless, odorless, has no taste, and burns with a pale blue flame virtually invisible in daylight. In the 1870s, Jules Verne thought that water would be the fuel of the future. In 1923, John Haldane predicted that future energy would be in the form of liquid hydrogen from rows of windmills generating electricity to produce hydrogen by electrolysis of water. Hydrogen gas would then be liquefied and stored in vacuum-jacketed underground reservoirs until needed to generate electricity when recombined with oxygen. Although his idea was ridiculed at the time, Haldane’s prediction is essentially where we are headed today.¹ Hydrogen is not as uncommon as one may think. In the nineteenth century, hydrogen was a component of coal-derived town or manufactured gas along with methane, carbon dioxide, and carbon monoxide. This mixture of gases was burned in homes and businesses decades before the discovery of natural gas fields and continued to be burned as late as the 1950s before being fully replaced by natural gas.

Fear of Hydrogen

Engine fuel on German-made Zeppelin dirigibles that carried passengers between European cities and across the Atlantic Ocean to the US varied from diesel fuel to a mixture of benzene and gasoline, augmented by excess hydrogen blow-off as a booster fuel. Hydrogen in giant bags kept zeppelins afloat in the air. Crash of Hindenburg in 1937 ended the days of dirigibles filled with hydrogen, which was replaced with helium. Hindenburg gave hydrogen a reputation for being a dangerous fuel. It was originally thought that an atmospheric electrical charge called St. Elmo’s fire, a blue glow sometimes seen around church spires, sailing masts, and airplane wings during stormy weather, ignited the hydrogen. More recent investigations into the cause of the tragedy point to other possibilities such as an electrical discharge that ignited not hydrogen, but the highly combustible coating of aluminized cellulose acetate butyrate dopant, a component of rocket fuel, which saturated the outer cotton fabric. Another possibility was leaking fuel from a propulsion engine dripping on a hot surface that started a fire within the internal structure of the dirigible. These investigations concluded that design faults and operating deficiencies made Hindenburg a bomb waiting to be detonated. Regardless of the specific cause of the fire, once hydrogen ignited, the end came quickly.

Hydrogen also has gotten a bad rap by being associated with the hydrogen bomb, which, of course, has nothing to do with combustion. On the other hand, hydrogen aficionados maintain that a tank full of hydrogen in an automobile presents no more of a hazard to a passenger than a tank full of gasoline. They argue that it may be less hazardous because a ruptured gasoline tank spills its contents on the ground, which, if ignited, will almost completely combust. If a tank filled with pressurized hydrogen were to rupture, a large portion of the fuel may escape to the atmosphere before ignition takes place. Hydrogen, the lightest of all elements, has a very high diffusion rate and disperses four times faster than natural gas and ten times faster than gasoline vapors. Moreover hydrogen radiates relatively little heat compared to burning petroleum and personal injuries are confined to those in direct contact with the flame. On the downside, hydrogen can burn when its concentration in air is between 2 and 75 percent, whereas the flammable range of gasoline vapors is between 1.4 and 7.6 percent. This means that gasoline vapors cannot ignite if their concentration is less than 1.4 percent (too little vapor to ignite) or greater than 7.6 percent (too little oxygen to support ignition). For natural gas, the flammable range is 5–15 percent, still a much more restrictive range than hydrogen. Not only does hydrogen have a wider flammable range and “ignites” easier than gasoline or natural gas, its nearly invisible flame in daylight is another element of danger. Whatever the virtues of supposedly being less hazardous than commonly used gasoline, propane, or natural gas, this is not the public’s image of hydrogen. There are real safety concerns when one is dealing with hydrogen as is the case with any flammable substance.

Hydrogen Today

Hydrogen has been an industrial commodity for over fifty years with about 50 million tons of hydrogen produced annually worldwide.² US production is 10–12 million tons.³ However, most of this hydrogen is a byproduct of reforming naphtha in oil refineries and is largely consumed within a refinery. Refinery hydrogen increases yield of more valuable light end products from the heavy end of a barrel and removes sulfur from petroleum products plus a relatively small portion is pipelined to nearby petrochemical plants. Only about 5 percent of hydrogen consumed by industry is merchant hydrogen, specifically produced for commercial purposes. Most merchant hydrogen is made by steam reforming of natural gas to produce a syngas of hydrogen and carbon monoxide. Only a small amount of hydrogen is made by electrolysis of water. In the future, gasification of biomass and coal may augment production. Hydrogen is transported as a compressed gas in pipelines (1,200 miles of hydrogen pipelines in the US) and in tanks carried by rail, barge, and truck. It can also be transported by rail, barge, and truck as a cryogenic liquid at a temperature of minus 253°C, only 20 degrees above absolute zero. Merchant hydrogen is used by the food industry for hydrogenation of edible organic oils and in making margarine, by the fertilizer industry for producing ammonia for nitrogen-based fertilizers, glass industry as a protective atmosphere for making flat glass sheets, and electronics industry as a flushing gas for manufacturing silicon chips.⁴ Aerospace industry relies on merchant hydrogen for fuel cells aboard manned space stations to produce electricity and potable water. The oil industry and producers and consumers of merchant hydrogen have an excellent safety record because of their understanding and appreciation of its inherent risks.

In the area of transportation, a number of experimental vehicles including submarines and torpedoes ran on hydrogen in the 1930s and 1940s. The 1973 oil crisis awakened the public to the possibility of hydrogen as a motor vehicle fuel. In 1988 the USSR and the US experimented with airplanes fueled by liquid hydrogen. The first hydrogen fuel buses were in operation in Belgium in 1994, and in 1995 Chicago tested hydrogen fuel buses. A small number of hydrogen fuel buses currently operate in several European and American cities, mainly for testing and demonstration purposes. Buses have the advantage of being able to carry large tanks of pressurized hydrogen mounted on their tops. A motor vehicle engine can be converted fairly easily from gasoline to natural gas or hydrogen. For hydrogen fuel vehicles, the problem is fuel availability, cost, and storage, not engine design. But the role of hydrogen is not limited to its potential as a motor vehicle fuel. In 1992, the first solar home that relied on hydrogen as a means to store electricity, rather than a battery, was successfully demonstrated in Germany. In 1999 Iceland announced a long-term plan to become the world’s first hydrogen economy totally eliminating fossil fuels by 2050. Icelandic motor vehicles and fishing vessels would run on hydrogen produced by electrolysis of water with electricity generated from hydro and geothermal sources. Iceland is now pursuing promotion of hydrogen rather than switching to a hydrogen-based economy through the North Atlantic Hydrogen Association (NAHA). NAHA’s members include nations with similar geographical conditions as Iceland around the North Atlantic Ocean and companies in the hydrogen business. NAHA is a mutual interest group of nations (Iceland is the only member nation) and companies interested in hydrogen applications in an energy environment of small markets in remote locations with virtually no fossil and biomass resources.⁵

Black and Green Hydrogen

The problem is how to produce hydrogen. Reforming it is a three-stage process that begins with the hydrocarbon (mainly natural gas, but coal and biomass can be used as well as various petroleum products) in an endothermic (heat-absorbing) reaction in presence of a catalyst to form hydrogen and carbon monoxide. Second stage is combining carbon monoxide with steam in an exothermic (heat-releasing) reaction to form additional hydrogen and carbon dioxide. Heat released by the exothermic reaction is recycled to supply a portion of the heat for the endothermic reaction. Third stage is removal of carbon dioxide and trace amounts of carbon monoxide through an adsorption process to separate hydrogen. “Black” hydrogen results if the waste product, carbon dioxide, is released to the atmosphere. An owner of a hydrogen fuel automobile who proudly announces that he or she is not polluting the environment is suffering from a case of self-delusion if the automobile is running on black hydrogen. Hydrogen from reforming is quite expensive and efforts are underway to find a different technology such as advanced ion transport membranes to reduce the cost of separating hydrogen from hydrocarbons. “Green” hydrogen results if carbon dioxide emissions from steam reformers are sequestered. Hydrogen from an integrated coal gasification combined cycle (IGCC) plant with sequestered carbon dioxide is green. Green hydrogen can be produced from ethanol, which opens up biomass as a hydrogen fuel. While this process releases carbon dioxide, growing crops to supply ethanol removes an equivalent amount of carbon dioxide from the atmosphere. This makes hydrogen from biomass essentially carbon dioxide neutral as long as energy from biomass fuels, not oil, is consumed in the growing, harvesting, and processing of biomass crops.

Another way to produce green hydrogen is electrolysis of water, in which the source of electricity is not fossil fuel (unless carbon dioxide emissions are sequestered), but nuclear, hydro, wind, solar, geothermal, tidal, or grazers (proposed floating plants on the world’s oceans generating electricity from thermal differentials). Electrolysis is flow of direct current electricity between a positive and negative electrode in pure water containing an electrolyte to enhance conductivity. Electricity splits a water molecule into its elements, oxygen, which collects at the anode, or positively charged electrode, and hydrogen, which collects at the cathode, or negatively charged electrode. Hydrogen and oxygen gases are drawn from the electrodes, dried, and stored. Hydrogen can be a fuel for specially adapted conventional motor vehicle engines or be converted to electricity in fuel cells. A fuel cell electrochemically combines hydrogen and oxygen from air to generate electricity, heat, and water with virtually no pollution emissions. A fuel cell is similar in operation to a battery with one major difference. A battery has a finite capacity to generate electricity before it has to be recharged whereas a fuel cell generates electricity as long as there is a supply of hydrogen.⁶ Oxygen from electrolysis can be pressurized in bottles and sold for various purposes or released into the atmosphere. One proposal is to install solar panels on hospital rooftops to generate electricity for the hospital and for electrolysis to produce oxygen and hydrogen. Oxygen can supply patients who need breathing assistance and hydrogen can supply fuel for ambulances and/or be sold to owners of hydrogen fuel motor vehicles. Only 4 percent of the world’s output of hydrogen is by electrolysis because of the high cost of electricity compared to steam reformers that strip hydrogen from natural gas. A cost differential of three or four times puts hydrogen by electrolysis at a severe economic disadvantage.

Promise of Hydrogen

If hydrogen is to reduce carbon dioxide emissions, electricity for electrolysis cannot come from fossil fuel plants (other than those that sequester carbon dioxide emissions). From a practical viewpoint, generation of enormous quantities of electricity necessary for the hydrogen economy would have to depend largely on nuclear power and IGCC plants augmented by hydro, wind, and solar sources. Capacity of this combination of power sources could be expanded to the point of satisfying peak electricity demand eliminating coal for conventional plants and natural gas for satisfying transient demand. As electricity demand from consumers, businesses, and industry moves off its peak, excess electricity generating capacity would be dedicated to hydrogen production. A uniform charge for electricity to consumers and to hydrogen producers would be the same low base rate, eliminating marginal electricity rate differentials for generators operating at less than base load. The prospect of nuclear power and coal for IGCC plants playing a major role in the hydrogen economy is viewed with disdain by environmentalists, but not necessarily by hydrogen enthusiasts. These two groups should not be at odds with one another because both share a mutual desire to eliminate carbon dioxide emissions.

Ocean Power

Oceans cover over 70 percent of the Earth’s surface and represent an immense reservoir of energy in the form of tides, currents, waves, and temperature differentials. Marine and hydrokinetic technologies (MHT) generate power from waves, tides, and ocean currents.

Tidal Power

Tides result from the gravitational interaction of Earth and moon with about two high and two low tides each day. Time between high tides is 12 hours and 25 minutes. The shift in maximum tidal power output on a daily basis, while predictable, may not correspond to timing of peak demand for electricity. Tides are also affected by relative placement of sun and moon with respect to the Earth, which causes spring (maximum) and neap (minimum) tides. The elliptical path Earth traces around the sun, plus weather and other influences, affect tides, as does topography of the shoreline. Unfortunately, coastal estuaries that create tides of up to 50 feet are located at high latitudes, far from population centers. Tidal power output must be viewed as a supplemental power source available only about eight to ten hours a day, although some tidal dams can catch both outbound and inbound tidal flows extending electricity generation up to 14 hours per day.

The concept of tidal power is fairly old; waterwheels powered by tidal currents ground grain in eleventh-century England. Modern day tidal turbines are quite similar in principle to wind turbines, but are more substantially built with a smaller blade diameter as water is 784 times denser than air.⁷ While more expensive to build, tidal turbines have a much higher power output. Tidal turbines are incorporated in different types of barrages (tidal dams).⁸ For one type, sluice gates open to allow incoming tide to fill a body of water behind the barrage. When the tide turns, sluice gates are shut entrapping water followed by a 1–2 hour hiatus until there is sufficient depth differential to direct water through electricity generating turbines. Generation stops when the tide turns and sluice gates are once again opened to allow water to be trapped behind the barrage. Another type of barrage has no sluice gates. During flood tide, water passes through turbines installed within a barrage generating electricity in addition to raising the water level behind the barrage. When the tide turns, generation of electricity stops until a differential in depth between each side of the barrage comes into being before water is passed through the turbines in reverse direction again generating electricity. In this case, turbines are generating electricity during both flood and ebb tides. The lagoon type barrage does not depend on a water depth differential. Water simply flows through the barrage during flood and ebb tides generating electricity without creating a head of water (difference in depth) on either side of the dam.⁹

A tidal dam must be located where there is a marked difference between high and low tides. One favored area proposed for building a tidal dam is Bay of Fundy in eastern Canada where the difference in water level between tides is over 50 feet, the highest in the world. Other areas with pronounced tides in the northern hemisphere are Cook Inlet in Alaska, White Sea in Russia, and coastline along eastern Russia, northern China, and Korea. In the southern hemisphere, potential sites are in Argentina, Chile, and western Australia. With electricity generation limited to between eight and fourteen hours a day, and with maximum tidal flow restricted to just a few hours, a tidal barrage has an effective output of only 35 percent or so of rated capacity. Moreover, maximum output may not coincide with peak electricity demand and a substantial investment may be necessary to transmit electricity from remote sites conducive to tidal dams to population centers.

The largest tidal barrage is in South Korea where an existing seawall between Sihung City and Daeboo Island created saltwater Lake Sihwa. Without an outlet, Lake Sihwa became polluted. Constructing an outlet through the sea wall to flush the lake during tides also allowed for the installation of turbines to take advantage of tidal flows. Electricity is not generated when there is a sufficient head of water on one side of the dam; rather tidal flow through turbines generates electricity. Completed in 2011 with an output of 254 mW, Lake Sihwa is the largest tidal power project in the world.¹⁰ South Korea has also embarked on another tidal power project at Incheon, near Seoul. Incheon Bay Tidal Power Plant will consist of 44 water turbines of 30 mW (40,000 horsepower) and a 4.8 mile barrage connecting four islands. It will have a maximum output of 1,320 mW and will be the largest tidal source of power when completed in 2017.¹¹

The first major and now the second largest tidal dam in operation is at La Rance estuary in France, built in 1960s, capable of producing 240 mW of electricity. It has a maximum tidal range of 26 feet, operates at 26 percent of rated capacity on average (540 gWh), requires low maintenance, and is in service 97 percent of the time. The dam has 24 bulb turbines each rated at 10 mW and weighing 470 tons with 5.4 m diameter blades. Bulb turbines imbedded in the dam allows generation to take place both on ebb and flood tide; that is, when its lagoon is filling or emptying. Three other tidal dams are far smaller; an 18 mW tidal dam at Annapolis Royal, Canada (Bay of Fundy), which serves the local area, a 3.2 mW dam in eastern China at Jiangxia, and a 0.4 mW dam in the White Sea at Kislaya, Russia.

Tidal Lagoon Power is to begin construction of a tidal flow barrage in 2015 for completion in 2018 at Swansea Bay in Wales. Water will flow through the barrage during incoming and outgoing tides and will power turbines built within the barrage measuring 6 m in height and 18 m in length. They will be capable of generating 320 mW of electricity at peak tidal flow for 14 hours of electricity generation with a cost of $248 per megawatt-hour.¹² The company is also proposing another tidal flow plant at Cardiff, about 35 miles away from Swansea Bay, which will have an output of 2.8 gW with a capital cost of £92 ($144) per megawatt-hour, the same price quoted for building a nuclear plant.¹³

One proposal under sporadic consideration since the 1980s is to build a 16 km (10 mile) barrage across the Severn estuary in the UK (Severn Barrage Tidal Power Project). It would have a maximum output of 8.6 gW, employing 214 electricity generating turbines of 40 mW each, and would be capable of supplying about 5 percent of UK electricity demand.¹⁴ The project once again is in limbo as government subsidies to support its $40 billion cost were rejected by the UK government in 2013.¹⁵ Another proposal made in 1990 for a 48 mW tidal dam near Derby in northwestern Australia has made little progress to date.¹⁶

Obviously an effective and less costly way to harness tidal power is channeling tidal flow through a restricted waterway so that tidal flow, not difference in heights of water on either side of a tidal barrage, powers turbines during incoming and outgoing tides. This “double flow” system provides electricity generation whenever tides are running, but not during a change in tides. Moreover peak power output does not last very long before power drops during waxing and waning of tidal flows. Though the double flow system has a higher effective output than a tidal barrage where electricity is only generated when trapped water from high tide is flowing to a lower level, electricity generation is still not continuous and may not be timed to accommodate demand. One proposal, now defunct, was to build a tidal fence two and a half miles long (4 km) across the San Bernardino Strait in Philippines between the islands of Samar and Dalupiri. A tidal fence would contain 274 turbines capable of generating 2,200 mW (2.2 gW) at peak tidal flow.¹⁷ A unique tidal stream power installation has been built in Strangford Narrows in Northern Ireland by Marine Current Turbines. No barrage is built, turbines are anchored to the sea bottom to remain stationary and are powered by tidal flows through the turbines generators connected to shore by an underwater cable. When the tide is running, two tidal stream or flow turbines provide 1.2 mW sufficient to power 1,000 homes.¹⁸

“Double basin” tidal power provides a continuous supply of electricity because water flows continually from a higher basin to a lower basin. Water in the upper basin is replenished during high tide and water accumulating in the lower basin is drained during low tide. Continuous power is also possible by installing turbines in rivers as proposed for Mississippi and Niagara Rivers where power generation would not be interrupted as with tidal flows. Tidal or river currents with an optimal speed is 4.5–6.7 miles per hour (2–3 m per second) turn a propeller that drives a generator whose output is transmitted to shore via an underwater cable. In principle, the propeller and generator are similar to a wind turbine, but with a smaller propeller and a lower rate of rotation to take advantage of higher density of water. Unlike birds, fish learn to live with rotating propellers whose speed is slow enough for them to escape contact. In 2003 a turbine built by the Norwegian company Hammerfest Strom became the first to send free-flow hydropower to a grid. Anchored in deep water off Norway’s coast where currents are strong and constant, Hammerfest turbine is massive with three 30’ foot blades capable of generating 300 kW of electricity to supply a local community of 35 homes. The UK has similar plans as Norway to take advantage of its abundant ocean currents. Marine Current Turbine installed a double propeller turbine measuring 45 feet across off the Devon coast.¹⁹ Verdant Power installed six tidal turbines in New York City’s East River to send free-flow hydropower to the grid on Roosevelt Island and in 2012 received permission to install thirty.²⁰

Atlantis Resources has developed two commercial sized 1 mW and 1.5 mW marine tidal turbines secured by a gravity based foundation. A test program with a 1.5 mW turbine has begun in Pentland Firth in the far north of Scotland. Of the estimated 29 tWh of available tidal energy in the UK, 11 tWh are at Pentland Firth whose currents are double the speed of other locations. If this test program is successful, more 1.5 mW turbines will be installed to bring maximum power output up to 400 mW. The company is pursuing similar projects at another site in the UK, Nova Scotia in Canada, China, and India.²¹ A much larger tidal energy project is envisioned by ARCADIS, a global engineering consulting firm, to build dynamic tidal power system in China to take advantage of strong tidal flows that run parallel to the shore. A tidal barrage would run perpendicular from the shore to an offshore island and would be fitted with 4,000 turbines to generate 15 gW when the tidal current is in full flow.²² The Gulf Stream, some 50 miles off the east coast of the US, would seem to be a logical place for tidal turbines. One problem is that the Gulf Stream changes its track. However, there might be suitable sites for installing tidal turbines such as where the Gulf Stream is confined by Florida and Bahamas Islands.

European Marine Energy Centre is a testing and certification organization for marine power developers. Tidal devices being built or proposed to capture tidal energy are horizontal and vertical axis turbines, oscillating hydrofoil, enclosed tips, Archimedes screw, and tidal kite plus various means to connect tidal devices to the sea bottom.²³ Minesto Deep Green technology is effectively an underwater kite with a low velocity turbine attached to it. It is tethered to the bottom and “flies” in an underwater current generating electricity that is fed by underwater cable to land. A working model has already been successfully tested. Next step is to install the first commercial scale 0.5 mW power plant off coast of Wales in 2017. If successful, this will be expanded to a 10 mW array to deliver power to over eight thousand Welsh households at a cost lower than fossil fuel plants.²⁴ The largest water current electricity generating scheme ever contemplated is to dam the Mediterranean at Gibraltar where the waterway is about 10 miles wide. There are two currents at Gibraltar; a surface current of outgoing water and a deeper current of incoming water (submarines during the Second World War would “sneak” into the Mediterranean by drifting in the deeper current).²⁵ Net flow of water is incoming as more water evaporates from Mediterranean and Black Seas than is entering via Nile, Danube, Dnieper, and other rivers. Obviously gates would be necessary for the passage of ships. Enormous turbines would be installed in the dam powered by the incoming water current. Electricity generating potential is nearly without limit, but cost and obvious concerns over its environmental impact have stymied the project.

Wave Power

Waves are caused by wind and their enormous power potential can be tapped by using hydraulic or mechanical means to translate up-and-down motion to rotate a generator. Calm weather and severe storms affect operation of these devices, but when in operation, electricity can be delivered to shore via underwater cables. While one may feel that this energy source is futuristic, tens of thousands of navigational buoys have long relied on wave motion to power their lights and sound their horns. Height of a column of water in a cylinder within the buoy changes with up and down motion of the buoy, creating an air pressure differential that drives a piston powering a generator to supply power for lights, sound devices, and other navigational aids of the buoy. A battery is kept charged by wave motion in case of calm weather. One wave power system has been in operation since 1989, producing 75 kW for a remote community at Islay in Scotland.

Pelamis Wave Power built sausage looking semisubmerged, articulated cylindrically shaped wave generators where internal hydraulic rams, driven by wave motion, pumped high pressure oil through hydraulic motors to drive generators. Electricity was collected in an underwater cable for transmission to shore. Load control maximized output in quiet seas and limited output in severe weather. Precautions had to be taken to keep ships away from wave generators. The company had three 750 kW units installed offshore Portugal in 2008, but became inoperative a few months later when the primary investor became insolvent. Although Pelamis Wave Power won several awards for its environmental achievements and engineering ingenuity with several projects in the planning stage, the company went out of business in 2014.

Ocean Power Technologies produces a conventional looking buoy capable of converting wave energy to a mechanical stroking action that drives an electricity generator for servicing a shore community. Small generating buoys are deployed off the coast of New Jersey and Spain with an effective power output of 9–14 kW. An order of magnitude improvement has been achieved by APB350 with sufficient storage capacity to provide a steady 350 kW of output.²⁶ Other proposed types of wave generators are entirely underwater to avoid being damaged by waves.²⁷ One example of this is entirely submerged floating buoys, 11 m in diameter, manufactured by Carnegie Wave Energy. CETO-5 buoys, of which three have been installed, are capable of generating 240 kW each, located 3–6 feet under the surface to protect them from pounding waves. In an unusual arrangement, buoys are physically connected by a rod to an underwater pump on the ocean bottom. A hydraulic pump, powered by the up and down motion of the buoy, pressurizes water that is piped ashore for generating electricity. Pressurized water can also be a salt water supply to a desalinization plant where it is forced through membranes to become fresh water. After generating electricity, low pressure water is pumped back to the buoy for repressurization. The latest model CETO-6 has an electricity generating capacity of 1 mW with an underwater buoy double the diameter of CETO-5. The up and down motion of CETO-6 will power an electricity generator with connecting underwater cables to shore in order for the buoy to be located in deeper waters. Three units are expected to be built in 2016.²⁸

One other method of extracting energy from the ocean is to take advantage of temperature differentials. Warm temperature ocean surface water can be used to vaporize a working fluid, such as ammonia, which boils at a low temperature, to drive a turbine to generate electricity. The working fluid is cooled and condensed for recycling by deep cold water whose temperature differential is of the order of 80°F. Warmed cold water must be pumped back into the ocean’s depths to prevent cooling the surface. Ocean thermal energy conversion (OTEC) systems are located in the tropics, where warm surface waters lie over deep cold waters. This provides the greatest temperature differential for operating a turbine; even so, the efficiency of heat transfer at these relatively small temperature differentials is only 5 percent, a technical challenge that requires building and operating a heat exchanger large enough to produce a significant amount of electricity. Demonstration plants have been built, including one in Hawaii that produced up to 250 kW of electricity for a number of years. However technical problems associated with ocean thermal energy still pose a significant barrier to developing this source of energy on a commercial scale. But there is incipient interest in “grazing plants,” which are located far from shore where temperature differentials are the greatest. Since an underwater cable cannot connect the grazing plant to shore, generated electricity would produce hydrogen by electrolyzing water. So-called energy islands combine an OTEC with solar and wind installations to produce even larger quantities of hydrogen as stored electricity.²⁹ Hydrogen is shipped from grazing plants in specially designed liquefied gas vessels to shore-based terminals for further distribution as a power source for fuel cells in vehicles and homes.³⁰

1 Jeremy Rifkin, The Hydrogen Economy: The Creation of the Worldwide Energy Web and the Redistribution of Power on Earth (New York, NY: Jeremy P. Tarcher/Penguin Group, 2002).

2 “Report of the Hydrogen Expert Panel: A Subcommittee of the Hydrogen & Fuel Cell Technical Advisory Committee” (May 2013), Web site www.hydrogen.energy.gov/pdfs/hpep_report_2013.pdf.

3 “An Overview of Hydrogen Production and Storage Systems with Renewable Hydrogen Case Studies,” Clean States Energy Alliance (May 2011), Web site www.cesa.org/assets/2011-Files/Hydrogen-and-Fuel-Cells/CESA-Lipman-H2-prod-storage-050311.pdf.

4 “Periodic Table: Hydrogen,” RSC Publishing, Web site www.rsc.org/periodic-table/element/1/hydrogen.

5 Icelandic New Energy Web site www.newenergy.is/en/aboutine.

6 “How Do Fuel Cells Work,” FuelCell Energy, Web site www.fuelcellenergy.com/why-fuelcell-energy/how-do-fuel-cells-work.

7 Jeff Haby, “Comparing the Density of Air to Water,” Web site www.theweatherprediction.com/habyhints/216.

8 “Tidal Energy,” National Geographic Education, Web site http://education.nationalgeographic.com/encyclopedia/tidal-energy.

9 David Appleyard, “Turning the Tide on Barrage Technology,” Renewable Energy World (April 15, 2014) . See also “How a Barrage Works,” Wyre Tidal Energy, Web site http://www.wyretidalenergy.com/tidal-barrage/how-a-barrage-works.

10 Planetorium Science Center Web site www.bibalex.org/psc/en/home/sciplanetdetails.aspx?id=76 for videos on Sihwa Lake Tidal Power Plant.

11 Malte Kollenberg, “World’s Largest Tidal Power Plant Threatens the Old Man and the Sea,” Yonhap News (August 27, 2012), Web site http://english.yonhapnews.co.kr/n_feature/2012/08/23/3/4901000000AEN20120823003100315F.HTML.

12 Swansea Bay Tidal Lagoon Web site www.tidallagoonswanseabay.com.

13 Louise Downing, “Tidal Lagoon’s Next Plant May Produce Power on Par with Nuclear,” Bloomberg (March 20, 2015), as reported on Web site www.renewableenergyworld.com/rea/news/article/2015/03/tidal-lagoons-next-plant-may-produce-power-on-par-with-nuclear.

14 Web site www.reuk.co.uk/Severn-Barrage-Tidal-Power.htm for current status of this project.

15 Louise Downing, “U.K. Snubs $40 Billion Severn Barrage Tidal-Power Project,” Bloomberg (September 18, 2013), Web site www.bloomberg.com/news/articles/2013-09-18/u-k-rejects-40-billion-severn-barrage-tidal-power-development.

16 EPA Web site http://epa.wa.gov.au/EPADocLib/904_B942.pdf for 1999 Derby Tidal Power Project report.

17 Nonilo Pena and Albert Marino, “Marine Current Energy Initiatives in the Philippines,” East Asian Seas Conference (2009), Web site http://pemsea.org/eascongress/international-conference/presentation_t4-1_pena.pdf describes several straits in Philippines considered for tidal fence power projects.

18 Case Study Strangford Lough Website http://geography.exeter.ac.uk/beyond_nimbyism/deliverables/reports_StrangfordLough_Final.pdf describes Strangford Narrows tidal stream power plant.

19 Lucid Energy Web site www.lucidenergy.com/news/alexanders-marvelous-machine.

20 Verdant Power Web site www.verdantpower.com/rite-project.html.

21 Atlantis Resources Web site http://atlantisresourcesltd.com.

22 “Large-scale generation of tidal energy in China edges closer with ARCADIS help,” ARCADIS (June 10, 2014), Web site www.arcadis-us.com/press/Large_scale_generation_of_tidal_energy_in_China_edges_closer_with_ARCADIS_help.aspx.

23 European Marine Energy Centre (EMEC) Web site www.emec.org.uk/marine-energy/tidal-devices.

24 Minesto Web site http://minesto.com/deep-green describes Deep Green technology.

25 The 1981 movie Das Boot shows a submarine taking advantage of this current.

26 Ocean Power Technologies Web site www.oceanpowertechnologies.com.

27 European Marine Energy Centre Web site www.emec.org.uk/marine-energy/wave-devices for description of wave generating devices.

28 Carnegie Wave Energy Web site www.carnegiewave.com/ceto-technology/ceto-overview.html.See also Amy Tee, “From Ocean Waves, Power and Potable Water,” The New York Times (April 23, 2015).

29 Michael Schirber, “How Floating ‘Energy Islands’ Could Power the Future,” lifescience (November 12, 2008), Web site www.livescience.com/3063-floating-energy-islands-power-future.html.

30 Chris Holt, “Energy from the Oceans,” Web site www.iop.org/activity/groups/subject/env/prize/file_40766.pdf.

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