I will use this to plug my favorite books on all things which involve energy "Sustainable Energy – without the hot air" by David MacKay. In one of the appendices he works out that the energy efficiency of airship transport is on the order of 10 times more efficient (wrt energy per weight and distance transported) than conventional airplane. He also points out why there are theoretical limits to how efficient a plane can ever be.

The efficiency of the airship turns out to be on the same order as train transport, really quite remarkable!

http://withouthotair.com/cC/page_280.shtml