why ammonia is not a competitive energy carrier? reading this article about Germany planning to store renewable energy as hydrogen gas, i'm left wondering why they deem so great this plan, compared with other alternatives like ammonia  as a energy carrier for storage? 
Is there a comparison chart between the different processes to store electric energy in chemical format?
 A: (I tried to post this response with chart and links, but I'm new here, so the system won't let me include images or more than two links. Please cut and paste the other links to view them in your browser.)
This diagram from the NH3 Fuel Association (taken from www.nh3fuelassociation.org/about-us--why-nh3) (scroll down the page) may answer part of your question: it compares ammonia with hydrogen and various other fuels. It shows that ammonia contains twice as much hydrogen energy as pure hydrogen (see "H2 Density" column), so, assuming efficient conversion and cracking processes, it can certainly make more sense to store hydrogen as ammonia.
MattZ ... the website you link to used to be the official site for the NH3 Fuel Association, but they changed their name and the url you cite has since been taken over by spiderbots. Please visit http://www.nh3fuelassociation.org for links to the various studies you mention. Please also note that an updated version of their website will be relaunched in a few weeks.
As to why Germany plans to store hydrogen ... ammonia's toxicity may make it politically complicated for governments to promote, and more needs to be done to establish safe handling standards and address confidence issues for ammonia. The argument for hydrogen in Germany may also be a local one: if there is no need to transport the energy carrier long distances, then hydrogen may do fine. As I understand it, in Germany, they're planning on keeping and using it more or less in place. This wouldn't be the case if transporting large distances: the benefits of the existing ammonia infrastructure - pipelines, storage, safety, training - may outweigh the cost of building a new hydrogen infrastructure from scratch.
Addressing Martin Beckett's points, re: toxicity, ease of handling, and emissions ...
Toxicity: certainly, yes it is toxic, but rather than dismiss ammonia for this fact alone, it is worth expanding the view of our risk analysis. For instance, it's easy to argue that the death rate from our current fuel use poses far greater and more unacceptable toxicity (tailpipe emissions cause approximately 21,000 early deaths and 3,000 lung cancer deaths every year in the US alone, according to the Clean Air Task Force, from 2005 (www.catf.us/resources/publications/view/83). Exploring alternatives with an open mind is worthwhile. See Quest Analytics' Risk Analysis (PDF) from 2009 (www.energy.iastate.edu/Renewable/ammonia/downloads/NH3_RiskAnalysis_final.pdf), which compares the risks of NH3, LNG and gasoline in automobiles, and concludes that "the hazards and risks associated with the truck transport, storage, and dispensing of refrigerated anhydrous ammonia are similar to those of gasoline and LPG." Don't take my word for it - read the Risk Analysis.
Handling: ammonia is easier to handle than hydrogen, because it doesn't require high pressure (saving significant money and reducing risk of explosion) and also because of the hydrogen embrittlement problem: hydrogen is such a small atom it leaches into steel and causes it to fracture, requiring a more expensive storage / pipeline (en.wikipedia.org/wiki/Hydrogen_embrittlement). Farming and refrigeration industries have been handling ammonia safely for decades. Perhaps we need to define "safely." I don't have access to FACT's full database, but they record global industrial accidents. Two quick searches showed that in the last four years there has been one fatal accident involving ammonia, but three fatal accidents involving hydrogen. Search the FACT database here: www.factsonline.nl/browse-chemical-accidents-in-database. If anyone has access to more precise data, I'd be very grateful to learn more. Comparing ammonia to any other energy carrier should be done rationally, without a "scare factor". Another advantage to ammonia: it stinks. This makes it unfavorable at a simple level but actually offers a crucial benefit: the early warning signal. If you have a hydrogen leak, you won't know until it's too late (and you can't add a smell to the hydrogen, or it won't function in fuel cells). If you have an ammonia leak, you'll know.
NOx emissions: true if you're using it carelessly and inefficiently, but not true if you have a balanced input (correct proportions of oxygen / ammonia to produce a clean, stochiometric reaction), and catalytic converter to remove emmissions. See the Hydrogen Engine Center (www.hydrogenenginecenter.com/aspx/news/news.aspx) for detailed examples of successfully using ammonia for internal combustion engine and genset power. Ammonia is also an important ingredient in many systems that clean tailpipe emissions - for Diesel Exhaust Fluid, see BlueSky (www.blueskydefna.com) - so any proper ammonia fuel system is designed to clean emissions using the onboard fuel.
John Rennie: there are green ammonia projects out there, well beyond the labs, using renewable electrochemical ammonia synthesis, not just Haber-Bosch, in much the same way as the German project is producing hydrogen. This eliminates the natural gas feedstock and can reduce electricity consumption (because it mitigates the high temperatures and pressures necessary for the H-B process) despite relying upon electrolysis to provide the hydrogen - efficiency advances are being made all the time. For a few up-and-running examples, see:
Freedom Fertilizer:
freedomfertilizer.com/Nitrefinery.html
GreenNH3:
www.greennh3.com/
Wind To Green:
www.conocophillips.com/EN/tech/energyprize/Pages/IngramSwearengen.aspx
University of Minnesota (they built its ammonia plant, but I can't find a more up-to-date link than this one:
www.license.umn.edu/Products/Ammonia-Production-without-Molecular-Hydrogen-for-use-in-Fertilizer-Production__Z07182.aspx
Let me know if I can provide more details. Best,
  TB.
A: Making ammonia is an absolute bugger because nitrogen is so unreactive. The normal route is the Haber process, http://en.wikipedia.org/wiki/Haber-Bosch, but this requires high pressures and temperatures.
Hydrogen is easy to produce from electricity, and I guess that's why it's the first choice for storage of energy generated by unreliable sources like wind or solar. If there were an industrial scale process for using electricity to make something more easily stored, like methane, I guess that would be a lot easier to store. However I don't know of any such process that works outside the lab.
A: Ammonia is also toxic, tricky to handle and burning it is going to generate a whole bunch of the same nitrous-oxides you get from fossil fuels.
A: Actually, Ammonia IS a better energy storage option than hydrogen. there are a ton of studies and a few demonstration projects that have already shown this - www.nh3fuelassociation.org
A: There are a number of other proposed fluid (mostly liquid) fuels that could be used as energy stores. Ammonia is one, but people also talk about methanol, and even methane. There are known techniques for making any of these using spare electricity (and some common input such as air or water).
Hydrogen is different - it's more awkward to handle than all of the above, but its big advantage is that the electrical conversion is two-way. You can make all the alternatives using electricity, but you have to burn them to get the energy back. Hydrogen can be used in fuel cells, and can thus be used as a compact electricity source. In the example of powering a car, if you use hydrogen you can use a fuel cell and then an electric drivetrain, which the associated advantages (efficiency, simplicity). If you use a different fluid then you're back to having an internal combustion engine with lots of moving parts.
