Looking through this paper I feel like someone is doing a lot of handwaving. Maybe I am just too rooted in my fundamentals, but I can't figure out how you harvest a temperature differential without putting something between the two end points, which immediately forces me into a sphere encompassing the planet. This takes me to the more mundane question of "Why am I trying to harvest the little side ?" (Solar influx is how many orders of magnitude bigger than IR out?)
It's like living next to niagara falls and coming up with ways to "get enough water from the air" and building massive condensers to do it and then purposely avoiding the moisture rich air at the base of the falls...
If I had to guess what is going on with this EEH (emissive energy harvester) idea, I would firstly note that it looks like a potentially very good idea for some small-scale niche application. I can imagine somebody wanting a form of renewable energy for some low power requirement which is more or less continuously available, even at night (bearing in mind that in some parts of the world night can last as long as several months).
However it might be difficult to acquire funding from the various government agencies that fund renewable energy R & D if it was being presented as for some small-scale niche application (other than a military niche application). On the other hand, if you presented your proposed development work as being an attempt to tap into some previously unrecognised large global source of renewable energy that will greatly benefit mankind, you might be more likely to get the funding.