Other forms of conventional renewable energy include tidal, ocean thermal, wave, and hot fusion. Tidal energy utilizes the gravitational energy of the attraction of the Sun, Earth and Moon. Wave power converts the energy released in crashing waves, which originated in the wind, which is driven by sunlight. Ocean thermal energy exploits the greatest collector of solar energy on Earth the sea. Hot fusion is not strictly renewable since it consumes hydrogen, but hydrogen is so abundant that it can be considered limitless for human purposes. Each of these energy forms has its own advantages and disadvantages, but none of them is the answer to the looming energy crunch. We will address each of them in turn.
Tidal Energy works on the same fundamental principal as the water wheel. In the case of tidal energy, however, the difference in water elevation is caused by the difference between high and low tides. The technology involves building a dam, or barrage, across an estuary to block the incoming tide, the outgoing tide, or both. When the water level on one side of the dam is higher than the level on the other side due to a tidal change, the pressure of the higher water builds. The water is channeled through a turbine in the dam in order to get to the other side, which produces electricity by turning an electric generator.
Tidal energy is being harnessed in several countries around the world, from facilities in Russia to France with 400 kW to 240 MW capacities. Some proposed sites, however, exhibit extraordinary potential. Britain 's Severn Estuary and Canada 's Bay of Fundy have potential capacities of as much as 8,000 and 30,000 MW, respectively. The Severn Estuary averages an 8.8-meter (26-foot) tidal range and the Bay of Fundy averages a 10.8-meter (32-foot) tidal range, ideal for substantial electricity generation. But the rarity of these exceptionally high tides is the main limitation of this energy source. Considering that "a tidal range of at least 7 meters is required for economical operation and for a sufficient head of water for the turbines," few places in the world can make a facility's establishment worthwhile. Since tidal power's "estimated capacity is 50 times smaller than the world's hydroelectric power capacity," it cannot compare to other renewables.
Another constraint to the tidal system is the sheer amount of time that passes in which little electricity can be generated between the rising and falling tides. During these times, the turbines may be used to pump extra water into the basin to prepare for periods of high electricity demand, but not much else can be done in the interim to generate more electricity. By its very nature, a tidal-based energy facility can only generate a maximum of ten hours of electricity per 24-hour day. That means it cannot be expected to supply power at a steady rate or during peak times.
Although the operation and maintenance of a tidal power plant is low, the cost of the initial construction of the facility is prohibitive, so the overall cost of the electricity generated would be quite high. For example, it is estimated that the Severn tidal project with a proposed capacity of 8,640 MW will cost $1,600 per kW, or over $13.8 billion. This cost exceeds that of coal and oil facilities by a considerable amount.
In contrast to the combustion of fossil fuels , the use of tidal energy makes no contribution to global warming. But tidal energy facilities do not come without an environmental price tag. The alteration of the natural cycle of the tides may affect shoreline as well as aquatic ecosystems. Pollution that enters a river upstream from the plant may be trapped in the basin, while the natural erosion and sedimentation pattern of the estuary may be altered. Local tides could decrease by more than a foot in some areas, and the "enhanced mixing of water" could stimulate the growth of organisms, better known for their red tide effect, which paralyze shellfish. So little is known about the potential harm of a tidal energy facility that some people believe "one of the only methods of increasing our knowledge about how tidal barrages affect ecosystems may be the study of the effects after such facilities have been built." With such uncertainty, tidal power appears to be an unproven alternative energy candidate.
Assuming that the high costs and the environmental issues were circumvented, the problem of distributing the energy generated by tidal facilities would still exist. Since the collection sites are limited and fixed at unalterable locations, the power they generate must still be distributed throughout the inland areas serviced by the plant via a transmission grid system. The distribution of the energy across vast inland spaces presents formidable problems. This would make it extremely difficult to replace the existing energy infrastructure, and our entire electricity needs could never be met by tidal power alone.
"Worldwide, approximately 3000 gigawatts (1 gigawatt = 1 GW = 1 billion watts) of energy is continuously available from the action of tides. Due to the constraints outlined above, it has been estimated that only 2% or 60 GW can potentially be recovered for electricity generation." Despite tidal power's inability to replace conventional energy sources, it will not be dismissed in the near future. Britain , India , and North Korea have planned to supplement their grid with this renewable energy source. Meanwhile, "a university study in January  said New Zealand could become the first country in the world to run solely on fossil fuel-free power if it exploited the tides on its long coastlines as well as its plentiful wind and sunshine. But while the wind may not constantly blow and the sun may not shine 24 hours a day, the advantage of the tides is that they never cease."
Wave Energy, like tidal power, will always be available, but there are current constraints that limit its contribution to the electrical grid. Areas with the strongest winds will produce the highest concentrations of wave power a low-frequency energy that can be converted to a 60-Hertz frequency. The best areas are on the eastern sides of the oceans (western side of the continents) between the 40 and 60 latitudes in both the northern and southern hemispheres. The waters off California and the UK are regarded as the best potential sites. " California 's coastal waters are sufficient to produce between seven and 17 MW per mile of coastline."
There are several drawbacks of wave energy . While the "wave power at deep ocean sites is three to eight times the wave power at adjacent coastal sites," constructing and mooring the site and transmitting the electricity to shore would be prohibitively costly. Especially considering that "a wave power unit will probably not have much more than three times the output of a single wind turbine." Once in place, the device could be a dangerous obstacle to navigational craft that cannot see or detect it on radar, while fishermen may have trouble with the underwater mooring lines. Conversely, an onshore wave energy system or offshore platform would have a significant visual impact. Scenic views would be replaced by industrial activity.
Wave energy has received little attention in comparison to other renewable sources of energy. Though 12 broad types of wave energy systems have been developed combinations of fixed or moveable, floating or submerged, onshore or offshore s cientists have not fully investigated this technology. "Many research and development goals remain to be accomplished, including cost reduction, efficiency and reliability improvements, identification of suitable sites in California, interconnection with the utility grid, better understanding of the impacts of the technology on marine life and the shoreline. Also essential is a demonstration of the ability of the equipment to survive the salinity and pressure environments of the ocean as well as weather effects over the life of the facility." Even a successfully built and operated wave power facility could not provide extra power for peak demand, nor would it be a reliable source of energy.
There is a handful of wave energy demonstration plants operating worldwide, but none produces a significant amount of electricity. Projects have been discussed for various sites in California San Francisco, Half Moon Bay , Fort Bragg , and Avila Beach but no firm plans have been made. While government agencies in Europe and Scandinavia are sponsoring research and development, "wave energy conversion is not commercially available in the United States . The technology is in the early stages of development and is not expected to be available within the near future due to limited research and lack of federal funding."
Ocean Thermal Energy Conversion (OTEC) seems to be a promising source of renewable, non-polluting energy for the future. The oceans comprise over two-thirds of the earth's surface, meaning they collect and store an enormous amount of solar energy. The raw numbers show that if even 0.1% of this stored energy could be tapped, the output would be 20 times the current daily energy demands of the United States .
Ocean thermal energy conversion exploits the temperature gradient between the varying depths of the ocean, requiring at least a 36F difference from top to bottom, as is found in tropical regions. This difference in temperature is the "heat engine" for a thermodynamic cycle. There are three types of OTEC designs: open cycle, closed cycle, and hybrid cycle. In an open cycle, seawater is the working fluid. Warm seawater is evaporated in a partial vacuum, expanding through a turbine connected to an electrical generator. The steam then passes through a condenser that uses cold seawater from the depths of the ocean, and the result is desalinated water that can be used for other purposes. New seawater is used in the next cycle. In a closed cycle, a low boiling point liquid such as ammonia or refrigerant is used as the working fluid, vaporized by warm seawater. After expanding through a turbine connected to an electrical generator, cold seawater is used to condense the vapor back into a liquid to start the process again. A hybrid cycle combines the two processes, in which flash-evaporated seawater creates steam, which in turn vaporizes a working fluid in a closed cycle. The vapor from the working fluid powers the turbine while the steam is condensed for desalinated water, as in an open system. The hybrid system continues to process seawater and produce electricity.
OTEC taps energy in a consistent fashion, producing what "is probably the most environmentally friendly energy available on the planet today." Unfortunately, the realization of this promising potential is largely experimental in nature for the time being. In fact, the only ocean thermal energy conversion plant in the U.S. was an experimental facility the Natural Energy Laboratory of Hawaii (NELHA), which was closed at the end of a successful test in 1998.
The technology is still far from being developed to an extent to make this type of innovation viable as a widespread alternative energy source. The facility in Hawaii , for instance, produced the highest amount of electricity to date with a 210 kW open-cycle OTEC experimental facility that operated from 1992 to 1998. When considering the capacity of conventional combustion turbines, ranging from a typical output of 25 MW to a maximum 220 MW, this technology is not even in the running. It is most applicable on small islands that depend on imported fuels. This system would render an island more self-sufficient while improving the sanitation and nutrition standards, with an abundance of desalinated water that could be used to grow aquaculture products.
It will be some time before OTEC technology is in a position to partially phase out the use of fossil fuels. The location limitations stall any worldwide progress, and the ability of the technology to produce the quantity of energy needed to supply the world energy demands is still largely theoretical.
Nuclear fusion has been called "the Holy Grail of the energy field." It is the diametrically opposite process of nuclear fission, in which an atom of the heavy isotope Uranium-238 is split in a collision with an accelerated neutron, releasing some of the energy from inside the atom. Fusion involves combining light atoms, which releases an enormous amount of energy. The waste product of this reaction is helium and it is precisely this process which fires most stars, in particular our sun. "Fusion is attractive as an energy source because of the virtually inexhaustible supply of fuel, the promise of minimal adverse environmental impact, and its inherent safety."
The atoms fused together in a reaction are not ordinary hydrogen atoms that contain only one proton in the nucleus. They are the heavy isotopes of deuterium or tritium that contain one or two neutrons along with the protons in their nucleus. These isotopes are somewhat rare in nature "about one part [deuterium] in 6000 is found in ordinary water" but the technology exists to isolate them in great abundance.
The fundamental problem with traditional nuclear fusion is that the fuel, the heavy hydrogen, must be raised to over one hundred million degrees. At such a tremendous temperature, the electrons are stripped away from the heavy hydrogen atoms leaving a fully ionized state called "plasma." This plasma must then be held together in order to produce useful amounts of electricity. There are no known construction materials that can withstand such temperatures, so the plasma must be contained by magnetic or inertial confinement. "Magnetic confinement utilizes strong magnetic fields, typically 100,000 times the earth's magnetic field, arranged in a configuration to prevent the charged particles from leaking out (essentially a 'magnetic bottle'). Inertial confinement uses powerful lasers or high energy particle beams to compress the fusion fuel."
Another fundamental problem with hot fusion revolves around "whether a fusion system producing sufficient net energy gain to be attractive as a commercial power source can be sustained and controlled." While fusion power production has increased from less than one watt to over 10 million watts over the years, we still have yet to witness a net energy gain. Even if this were to be achieved in the near future, the metallurgical requirements that must be met by the surrounding structural materials are extremely demanding and cost prohibitive. Accomplishing a net energy gain in hot fusion will involve the construction of a $1 billion device for experimenting with burning plasma. Add to this the estimate of $300 million per year that the fusion community in the US will require for "significant enhancements of the program" up from the current $230 million. The US is not alone in its fusion expenditure. Concerned about reliance on imported energy, Japan and Europe, respectively, have allotted 1.5 and 3 times the budget that the US currently spends for hot fusion.
The incredible complexity and cost of this process is the precise reason why the announcement of a "cold fusion breakthrough" at the University of Utah a few years ago met with such enthusiasm. If the process could be brought about at room temperatures, the complexity that now prevents the generation of power based on nuclear fusion would disappear.
While billions of dollars and decades of research have been devoted to hot fusion, we are far from mastering this type of energy generation. " Optimistic projections do not suggest that fusion energy will contribute significantly to energy supply until well into the next century." Nevertheless, the US Department of Energy's August 1999 Final Report of the Task Force on Fusion Energy concluded "that we should pursue fusion energy aggressively." .