I had the chance to analyze a solid-state gas engine a few months ago, based on this technology: http://web.mit.edu/newsoffice/2011/sun-free-photovoltaics-0728

I had the chance to analyze a solid-state gas engine a few months ago, based on this technology: http://web.mit.edu/newsoffice/2011/sun-free-photovoltaics-0728.html http://web.mit.edu/newsoffice/2011/sun-free-photovoltaics-07... The solid-state engine has much lower efficiency than a regular power engine would (a hypothetical 12%/realized 5% vs. a realized efficiency of about 30%), but it can be made much smaller - and even at a 5% conversion rate, the energy density just blows everything else out of the water. Aside from being energy dense, it's also much cheaper than high-end lithium-ion batteries; and unlike regular fuel-cells, the technology is just based on regular photovoltaics, and is relatively easy to manufacture.