What new breakthrough energy technologies are close to being realized? Energy production is one of the burning issues for humankind. There has been some talk about future energy technologies including Fusion, Anti-matter annihilation and Zero-point-energy (from most to least plausible). I'm interested in hearing what people know about developments in the field what they think will be the next real breakthrough in energy technologies.
 A: The next serious advance that is not an speculative/fringe idea is most likely to be fusion power. Harnessing the power of nuclear fusion has long been a goal for energy production since the first hydrogen bomb was created in the 1950s. Creating controlled fusion, rather than the chaotic variety has proven a rather challenging task here on Earth however. (The sun does it quite easily largely thanks to its huge gravitational field.)
Of particular note is the ITER project, currently ongoing in Southern France. It is an international collaborative project with the goal of creating a sustainable fusion reactor that produces net energy. (You can read information on the site for the specific target goals/criteria). This is essentially the last step in the 'proof of concept' stage of sustainable nuclear fusion for providing energy, and is to many looking quite promising. It is however only a scientific experiment still, and nuclear fusion power plants are a bit further off in the future, even given success of ITER.
In fact, the proposed successor to ITER is DEMO, a reactor that aims to produce over an order of magnitude more power. Once testing is complete, the idea is to turn it into the world's first fusion power plant. With any luck, this may just be the breakthrough humanity has been waiting for. It will certainly be a revolutionary event whenever it occurs, and likely render fossil fuels totally obsolete.
A: I find the idea of a breakthrough technology presented by the other answers to be extremely modest.  I'm almost positive that I'll get downvoted and flamed for my answer, but I have a strong desire to impress the potential for, and the implications of, a truly breakthrough energy technology.
I want to begin calling attention to Jevons Paradox, as well as the larger context of Jevons and other early commentators on early expanding coal power.  The fossil fuel age started with coal, and it started with steam cycles, not internal combustion engines.  Early coal engines were teeth-grindingly inefficient.  Here is an example, the Savery pump, which Jevons references.


1700s versus now, continuous flow steam cycles were a breakthrough technology
You can probably see it already, but this is a batch process.  It used coal to pump water upwards but did so with less than 1% efficiency.  It had extremely limited ability to replace labor.  When more efficient steam engines were developed, getting up to 10% efficiency, they could replace labor and do so very well.  Not only that, but those engines fueled a feedback where transportation, steel production, coal extraction itself, and a host of other economic inputs all became drastically cheaper, and led to greater demand for the fuel, coal itself.  The big picture, however, is that this lineage of efficiency improvement was a major catalyst for the entire industrial revolution.  Human population grew many-fold and this is pretty much the defining aspect of our current existence and challenges.
We have no such efficiency improvement available anymore.  We use thermal power with efficiencies of 30-60% and because of that, and the Carnot limit itself, no society-changing breakthrough will come from direct efficiency improvement.  That means that energy breakthroughs need to come from opening up new resources.
High altitude wind and solar
There are ventures currently pursuing high altitude wind with wing designs and kite designs.  There is enough resource to power the world with current 3-blade tower technology, but high altitude designs could make costs plummet and allow us to use far more of the resource.
I'll take a moment now to laugh at the idea of orbital solar power beamed back to the surface, haha.  Now, I'll mention that tethered high-altitude solar is a viable option.  Solar has several problems today, including the difficulty of pointing the PV panels or collector into the direction of the sun.  A floating platform has so such challenge, and can also rise above cloud cover which limits isolation otherwise.
Hot fusion
Tokomak is not a breakthrough technology.  Sorry.  I realize this is subjective, but if it works, then the cost of electricity from such plants will be 10-15 cents/ kWh.  Tokomak fusion will be more expensive than electricity today.  I don't know of a single person educated on this subject who argues otherwise.
Currently, national labs are studying laser inertial confinement, which could possibly be less expensive, although I doubt it would be breakthrough.  General Fusion is working on a mechanical acoustic approach that could be cheaper.  Inertial Electrostatic Confinement (IEC), using the polywell would be revolutionary if it works.  You might have noticed a trend in the examples I'm giving.  These are all becoming progressively higher payoff and higher risk, and at long last I come to focus fusion.  Focus fusion is the most risky / highest payoff energy technology I know of that has some amount of decent science and credibility behind it.
Fission power
Aim High!  LFTR reactors using Thorium is about the highest you can aim with fission power.  High risk and high payoff.  Small modular reactors, floating plant, and things like that can expand the use of our current Uranium resource which is not limited to the point of being economically prohibitive yet.
Biology advances
Craig Venter leads possibly the most technologically advanced research project in the world.  This has already led to creating "artificial life" meaning that we have cells with a genome that was printed by a computer.  He was funded to the tune of $100s of millions by Exxon.  Algae biofuels are a powerful potential technology, but the breakthrough part won't be the physical bioreactor design (although there is room to improve refining processes), it will be the algae genome design.
Sum of low probability events
If you take lots of technologies that could be breakthrough but have low probability of success, then the probability of ONE of the succeeding might be very significant.  ARPA-E was a federal program to use a competitive process to select the most likely breakthrough energy technologies.
To the question - many of these are rather close to being realized.  It differs from one to another.  Bad information, however, is quite rampant.  Everyone has their own personal opinion and the technologies listed here are presented as my own favorites unashamedly.  Things like zero-point, BlackLight Power, almost every LENR, orbital solar, and many others are hogwash.  There are too many credible potential breakthrough energy technologies to waste our times with the nonsense.  Too few people know how to disseminate between these.
An energy "breakthrough" would be a loaded development.  We already use lots of energy, and if we found it economic to use more we probably would.  The last breakthrough shift in our ability to exploit energy resources rocketed the entire planet into a new geological era, the Anthropocene.  We called this change the industrial revolution.  Some obscure project that ARPA-E funded with $500,000 could cause the next industrial revolution.  The implications of such a change would probably be beyond any of our imaginations.
A: The most scandalously neglected source of energy which is available today with no significant new discoveries required, is the energy of H-bomb explosions.
Surprisingly, H-bombs can be exploded in a cavity, and the heat confined to a fluid in the cavity, and extracted to make electricity. Such a scheme would be much cheaper than current electricity production, since the fuel costs are negligible (bombs are cheap per megaton), but such a plant would pose a high risk of proliferation and theft.
When world powers can control arms-proliferation, and can produce safeguards for peaceful thermonuclear explosions without risk of theft, one could imagine a world run by thermonuclear weapons explosion. This type of energy production is the PACER nuclear power plant, and it is explained here: How much of the energy from 1 megaton H Bomb explosion could we capture to do useful work? .
That this project, like ORION, is neglected, must be difficult for those in the nuclear weapons community, who would probably like something useful and not world-destroying horrible to have come out of their decades of research.
A: There are quite a few novel energy technologies coming through. I guess that without quantification, "breakthrough" is a subjective term. Below, I've tried to list all the energy technologies that I know of, that are not yet at commercialisation, but could be within 50 years, and that could offer at least tens of gigawatts of power. They are, in descending order of readiness:
Wave power

Converting kinetic energy from (near-)surface waves to electricity. First grid-connected prototypes went live in 2008, off the coast of Portugal. Theoretical UK resource (at 100% efficiency) is thought to be 40GW (at 50% efficiency), but this is subject to revision as better data is collected. 
Tidal stream

Converting kinetic energy from daily / twice-daily tidal movements, with underwater turbines. First grid-connected prototypes went live in 2008. UK resource estimated in the range 4GW-400GW at expected efficiencies.
Superconductors
Not a generation technology in itself, of course, but a technology offering potential breakthroughs within the energy industry.  Superconductors are being trialled as fault-current limiters; may have a role to play in HVDC circuit-breakers; and offer lots of potential in all sorts of turbines: trials are being done now with superconductors in run-of-river hydro turbines; and in wind turbines.
Deep (100m-700m depth) offshore wind
Offshore wind turbines, floating, moored to the seabed at depths of 100-700m. The first operational full-size prototype, HyWind, was deployed in 2009 in deep water off the coast of Norway. Theoretical UK resource at expected efficiency rates is over 1000GW. Global resource is one or two orders of magnitude higher.
Osmotic power
Generating electricity from salinity gradients. First prototype went in the water in Norway, 2009. Expected global resource is about 200GW.
Novel photovoltaics (PV)
Huge amounts of work going on in labs around the world on enhancing photovoltaics:


*

*Photonics & plasmonics looks to harness quantum effects to increase
efficiency by scattering the light with metallic nanoparticles, or by
using quantum wells / quantum dots;

*BIPV integrates PV into building materials;

*inkjet-printing and solar dyes look to give high-speed, low-cost production.


The UK PV resource estimated at over 400GW, using 5% of land at 40% efficiency. Globally, the resource is orders of magnitude higher than global power demand.
Artificial photosynthesis
Building novel synthetic living photosynthesisers from basic genetic building blocks; or novel catalysts to harness sunlight to release hydrogen from water; various labs working on different routes. Potential resource: greater than current global power demand.
Nuclear fusion
For work in progress, see JET, ITER and CCFE. And our local giant fusion reactor, the sun, can provide us with absolutely vast amounts of energy for a very long time to come. 
Tidal benthic friction
The hypothesis is that tidal water movements are mostly dissipated as friction against the bottom of the sea, and that by placing energy harvesters on the seabed, the kinetic energy can be converted to electricity. Specific harvesting technologies, scale of the resource, and the impact on benthic ecology, are currently unknown.
A: I will add to the list of promising energy technologies for 21th century cold fusion. Short answer on question what is it would be it's a room-temperature fusion of atomic nucleus. But the real picture is bit more complicated because the so called cold fusion can comprise whole set of fissions in host metal. In no way it can be interchanged for a more popular hot fusion because the conditions it requires to happen are considerably different. Some people prefer alternative names chemically assisted nuclear reactions (in short CANR) or low energy nuclear reactions (in short LENR). We can say that this area of research is in its early stage. Even when we have a number of experimental results confirming its reality (we can count them to several thousands) there are many unexplained aspects of this group of phenomenons. We have more competing theories trying to explain cold fusion but none is to this day accepted from mainstream physics community.
Now some words from history of cold fusion. Cold fusion invention is traditionally attributed to group of electro-chemists from University of Utah, Salt Lake City, USA. These were Stanley Pons and Martin Fleischmann. They reported on March 23, 1989 on press conference that they found a new way for fusion at room temperature. Their paper was published on April 10, 1989 in J. Electroanalytical Chemistry and Interfacial Electrochemistry. Their experiment was from conceptual point of view very simple. It was essentially a heavy water electrolysis cell with platinum as anode and palladium as cathode. This doesn't mean it is amateurish. Successful setup of experiment requires considerable practical experiences in many at first sight unconnected areas like physics of subatomic particles and electro-chemistry. Researcher must intimately know various types of calorimeters and theirs pro-and-con (not common thing for most of atomic scientists). Japanese associate professor of nuclear engineering Tadahiko Mizuno needed 8 months (page 59, second paragraph) to prepare his electrochemical cell! Main observed things in Fleischmann-Pons type of experiment were:


*

*anomalous heat in volume that couldn't be attributed to any know chemical reaction

*production of tritium

*production of neutrons three time above that of cosmic ray background


Even when original experiment has some inaccuracies in neutron measurements pointed by MIT Plasma Fusion Center director Ronald Parker, experiments all around world showed that this small amount of neutrons can indeed be produced. From all of them I will mention work of Tadahiko Mizuno:
Mizuno, T., T. Akimoto, and N. Sato: "Neutron Evolution from Annealed Palladium Cathode in LiOD-D2O Solution", Denki kagaku, 57, No. 7, (1989) 742-743
Very precious kind of work in measuring excess heat was carried out by John Appleby and colleagues from Texas A&M University using world class micro-calorimeter. This device is so accurate that it can detect heat in sub-microwatt range and measure even extremely small heat resulting from steel oxidation!
Main problem in the first years of cold fusion was the reproducibility. In fact, back in 1989, estimates for the initial reproducibility of the Pons/Fleischmann effect ranged between 5-10%. This means that about 90-95% of the scientists who attempted to re-create the Pons/Fleischmann experiment did not succeed. Nothing happened. No heat. No particles.  Nothing. This is now after more than 20 years of research much better (> 70 % but it depends on experimental setup and materials involved). Long-time researcher in this field Michael McKubre from SRI International in his lecture asserts that he personally witnessed excess heat in hundreds of experiments (he deals mainly with original experiment with palladium).
Practical feasibility of this phenomenons is another issue. Even though heat is released in copious amounts in relation to weight of samples, this is a matter of weeks to months. Net power output is in most cases a few watts. But there are documented results like that of professor Piantelli from University of Sienna that this could be increased at least one order higher.
Excellent website with hundreds of scientific papers about cold fusion ready for download is lenr-canr.org. An accessible manual for beginners about cold fusion was written by retired Los Alamos National Laboratories researcher Edmund Storm. Recently also NASA confirmed that it believes cold fusion research can lead to practical applications. For NASA it is especially interesting as long-term power source for space probes. Currently they are using plutonium-238 heat source but this is problematic not only because of high prize but quickly reducing supplies. Jim Adams, deputy director of planetary science at NASA, says that there is enough of the fuel for NASA missions to around 2022. He says if NASA does not get more after that, "then we won't go beyond Mars anymore. We won't be exploring the solar system beyond Mars and the asteroid belt.
