The article suffers a bit from the fog that covers most popular science articles but I think I get the picture. As others have said this particular technology is based on photovoltaics, in particular narrow-bandgap photovoltaics typically referred to as Thermophotovoltaics [1][2].

Bad news first, unfortunately the Carnot limit to efficiency still applies [3] and as an aside the limit for simple, single-junction photovoltaics under one sun illumination is about 31%. Since we're not dealing with either the sun nor necessarily single junction TPVs there's more to say on the topic.

The scheme proposed here seems to be very similar to a design proposed by one of the awards of the DARPA RIMS [4] (later MIPS [5]) program. Specifically they're proposing a combustion cell for the butane, a tungsten photonic crystal [6] emitter to reduce long wavelength "waste" emissions and THEN a TPV. A TPV consists of one or more pn junctions consisting of narrow bandgap semiconductors, typically horrendously complicated structures of ternary or quaternary materials like InGaAsP. [8] Please note that they don't even try to address the structure of the TPV in the article.

Some practical engineering difficulties include the fact that the nanosctructured tungsten emitter is quite reactive in oxygen at elevated temperatures (probably still true for their proposed structure). I should know, I reduced an incredibly expensive photonic crystal emitter to a very pretty layer of rust while trying to measure its emission spectrum while it was under argon. It turns out that it's incredibly difficult to purge oxygen from this structure after it's been exposed even ONCE. If you're using this as a battery replacement for soldiers you have to ensure that it can really take a beating and since the TPV is quite brittle (yep, killed a few of those too) you have to have an enclosing structure that is quite rigid, adding to your weight. The combustion cell is also going to be an engineering challenge since you have to maintain pretty good temperature control to keep your conversion efficiency up. How do you control your fuel-air mixture? What happens at high altitudes? How are you minimizing parasitic heat loss?

The reason I think a lot of people are enamored with this fairly niche piece of engineering is due to an erroneous Sandia Labs announcement that they had excess emission in a particularly favorable wavelength region. [7] That and the fact that DARPA is throwing an increasing quantity of money at a problem space with relatively few solutions.

Despite all the problems I listed this sort of TPV structure is still a very attractive option in at least one market namely powering deep space missions like JUNO or New Horizons [9]. Currently these are powered by something called an RTG [10] using thermoelectrics, but more on thermoelectrics another day. Below I have linked to a very good article on the subject of TPVs for space written by some of my former co-workers. [11]

Sorry for the lengthy reply I got excited that I have something to contribute to one of these discussions!

[1] http://en.wikipedia.org/wiki/Thermophotovoltaic
[2] http://gcep.stanford.edu/research/factsheets/ultrahigh_therm...
[3] eprints.soton.ac.uk/69617/1/chapter_2_with_header.pdf
[4] https://www.fbo.gov/index?id=c57c017b04923ce367344c9fdd369ee...
[5] http://www.darpa.mil/Our_Work/MTO/Programs/Micro_Isotope_Pow...
[6] http://lib.semi.ac.cn:8080/tsh/dzzy/wsqk/Nature/nature417-05...
[7]http://www.sandia.gov/news-center/news-releases/2003/other/p...
[8] http://iopscience.iop.org/0268-1242/18/5/309
[9] http://pluto.jhuapl.edu/
[10] http://en.wikipedia.org/wiki/Radioisotope_thermoelectric_gen...
[11] http://lib.semi.ac.cn:8080/tsh/dzzy/wsqk/selected%20papers/j...