Nuclear fusion powers hydrogen bombs and the Sun. The technology necessary to utilize it as a power source has not yet been mastered. The reasons for trying to develop a fusion power source and the approaches being considered to achieve this end are discussed below.
Risk and relevance
Nuclear fusion, the mechanism that powers the sun, is currently used by humans only in H-bombs. Nuclear fusion typically seeks to cause heavy isotypes of hydrogen (deuterium and tritium) to merge into helium in a massive release of nuclear energy. Because water (for the deuterium) and lithium (for the tritium) are common and the energy to fuel mass ratio is so great, the fuel supply is effectively limitless. Also, unlike nuclear fission, nuclear fusion reactions do not produce large quantities of long lasting radioactive waste.
Thus, it would be far less polluting than fossil fuels or nuclear fission. In addition, large-scale accidents like Chernobyl are impossible because very little fuel is present in the reactor at one time, and the reaction itself is very fragile. If anything goes wrong, the reaction simply stops. Also, in hot fusion, the reaction is sustained only with extreme inputs of energy to run the electromagnetic field containment and/or lasers to spark the reaction. If these fail, the reaction will simply fizzle and stop due to lack of such input.
The political relevance of nuclear fusion is extreme -- if it is practical then the hydrogen economy would terminate in fusion as the primary means of humans generating energy, which would be in such abundant supply as to be free.
Ambition: free energy forever
The deuterium and tritium in the water of the world's oceans are more than sufficient to keep any conceivable human civilization operating for any time horizon that is feasible to discuss: many thousands of years, in which time, it might become practical simply to start up a "spare sun" perhaps on the other side of the Earth's orbit, reflecting its energy back towards us.
The basic technologies of hot fusion are understood in principle. The problem is that while an uncontrolled momentary nuclear fusion reaction is within our technological grasp, that controlled fusion chain reactions that take more energy to trigger than they produce have not been achieved. The so-called fusion breakeven, a sustained, controlled nuclear fusion reactor that produces net energy output which is suitable for generating electrical power has not been built because the technological challenges involved have not been overcome. Very few people in the field think that we are even close to being able to build a sustained controlled nuclear fusion reactor at any price. Multiple avenues are being explored to develop this technology, and there is no one consensus approach.
Intermittently since 1988, an international team of physicists have worked on a design for a large proof-of-concept experiment called ITER. "ITER will be the first fusion device to produce thermal energy at the level of an electricity-producing power station." One of the actions of the Republican congress that took power in 1994 was to slash DOE's research budget. US participation in ITER was one of the programs that DOE dropped.
In June of 2005 ITER finalized its decision to build the first reactor in Cadarache, France . If ITER is successful, commercial power will not be available before 2025. So no hydrogen infrastructure plans can include this on any large scale.
Also, tokamak-type reactors, such as ITER, are expensive, centralized power sources. There are good political reasons to favor simpler, more decentralized power sources such as wind and solar.
Aneuronic or "cold" fusion does not rely on the same kind of high intensity large scale fusion reaction as the Sun or a hydrogen bomb, but rather uses a magnetic, sonic, or physical/chemical technique to confine a very small number of deuterium (or tritium) atoms to an area so small they must fuse to coexist.
If a practical and safe small scale cold fusion process were invented, the energy available for heating, transport, and conversion and purification of wastes and biomass into pure water and carbohydrates, could render each individual human totally autonomous of any land or energy source other than access to seawater or some other deuterium source. A thimble of heavy water is sufficient to power a city for a year, or a particularly energy wasteful human lifestyle. There is accordingly some interest in techniques that do not require starting up a pet star or tame H-bomb, particularly in Japan, and the far smaller amounts of money necessary to do research in this area, compared to "hot fusion" have allowed research to continue despite embarassing public failures in the past.
All claims that a cold fusion process that produces net energy so far have been false alarms which have not been reproduced in laboratory experiments.
The earliest experiments focused on simply firing deuterium particles very precisely at each other to produce a controlled steady set of reactions. Some experiments in the 1980s funded to approximately a $20M level by Bob Guccione were promising, but were not continued, as the US government filed a number of actions against Guccione surrounding his historically accurate (therefore pornographic) film "Caligula". He was unable to continue funding the project and it failed.
The controversial cold fusion based on a cube made of the precious rare earth metal palladium, which was at the core of the the Pons-Fleischmann experiment which made cold fusion a household name, has been subject to extensive experiment but the source of the sporadic release of energy has never been clearly identified as fusion.
Accordingly, skepticism about this approach remains extreme and it has largely been abandoned. Speculation recently has focused on the claim that it is not the palladium, but the product of an intermediate chemical reaction within the palladium cube that appears only in certain circumstances, that causes fusion.
Approximately 20 years of research has however failed to find the source of the energy released in the Pons-Fleischmann experiment or any of the sporadic test results since that seem to verify this technique.
A phenomena known as somnoluminescence, light given off by water under certain kinds of sound wave pressure, was thought to be nuclear fusion, but the matter was not actively investigated in a controlled way until the late 1990s. The hypothesis of the approach is that soundwaves, much as magnetic waves in hot fusion, are thought to force atoms together so as to generate a fusion reaction as water bubbles collapse.
Claimed experimental successes in showing that the energy is fact coming from a fusion reaction, and can be reliably generated with inputs of sound alone have been met with skepticism from the scientific community. Mainstream scientists consider the experimental proof to date to be inconclusive, but research continues and somnoluminescence appears to be a more promising line of research than many other approaches to cold fusion.
Muon catalyzed fusion
A muon is a high energy, high mass, unstable particle in all other respects identical to an electron. It is produced in nature in the upper atmosphere when very high energy photons in cosmic rays spontaneously convert energy into matter rapidly producing a succession of decay products.
It has been known since 1957 that muons can be used to catalyze fusion reactions at low temperatures, but the energy involved in creating muons is so great that it has not been a viable process for net energy production.
This approach to cold fusion is discussed by Wikipedia.