And now...liquid ammonia???

Researchers at Texas Tech University believe they have found a new solution to storing surplus energy from renewable sources - liquid ammonia!

It is claimed that by using a new kind of transformer the researchers expect to produce hydrogen at almost half the cost of traditional electrolysis methods. The hydrogen is then pumped into a compression chamber causing it to heat up to around 400°C. It is then vented into a second chamber compartment where an iron oxide catalyst starts the conversion to ammonia, using nitrogen from the air. The ammonia and leftover hydrogen is then allowed to cool down and decompress in a third chamber. Some of the decompression energy drives a piston that helps in the compression part, reducing the overall electric consumption. The ammonia must then be cooled to -75°C and pumped into a tank for use.

It is claimed that many modern cars can use ammonia as a petrol additive without modification (up to 10%) and flex-fuel cars could be modified to use ammonia in conjunction with ethanol, allowing for a mixture of 85% ammonia. The TTU team are variously reported as saying the fuel could end up costing as little as between 75c and US$1 a (US) gallon as the raw materials are essentially free. It's not clear if this includes any amortisation of the plant needed to make and store the liquid ammonia.

The overall process is offered as a way of creating a liquid energy store from intermittent renewable energy sources, such as wind or PV.

I'm dubious about the TTU process. My knowledge of chemistry is a little ancient, but I never recall ammonia being touted as a possible fuel, especially if it has to be cooled to -75°C to liquefy: it does have a reasonable energy density (about half that of petrol) but is a nasty, caustic and poisonous substance. (Not that different from some other fuels, I guess, and easier to handle than liquid hydrogen!) And although there are many ways of creating a hydrocarbon (or similar) from air and water (usually using CO2 rather than N2 as the air component) but they almost all fall down on the overall system efficient. Sure, in the context of a way of storing surplus renewable energy from wind or solar to drive the electrolysis and compression/cooling cycles it doesn't have to be quite as efficient as if it were part of a fossil fuel storage cycle, but I'd still be surprised if the system efficiency came anywhere near some of the other storage mechanisms. On the plus side, water and air are pretty widespread raw materials, and liquid fuels for cars may be easier to market than electric vehicles. The idea of using bioethanol as the base in a flex fuel vehicle is also intriguing, and if it reduced the land take for biofuels by five-sixths, would surely be welcome.