There’s a lot of discussion these days on IoT applications, architectures, communication, security and more, all very good stuff, but little debate on how these devices will be powered. If you can plug them in, this maybe isn’t an issue (though we may need to think about increased demand on our overstrained power generation infrastructure). However, for mobile and remote applications, the question is commonly dismissed as something that can be resolved through energy-harvesting, without detailed investigation of how practical that option really is, at least from what I have read.
I too thought that energy harvesting was a really exciting direction, but was frustrated by many popular articles that, while starting from real research/applications, seem to point without support to wildly-extrapolated implications. So I did my own literature survey, with a bias to journal articles rather than popular articles wherever possible and what I found wasn’t quite so promising.
A quick summary:
· Inductive charging and wireless-beamed power are very practical but limited by heat generation and safety and of course by proximity to a wall-plug powered source
· Piezo- and thermo-electric charging are limited to order of uA/cm[SUP]2[/SUP]
· Biochemical charging can get to mA/ cm[SUP]2[/SUP] but the enzymes required for this method have to be replaced every couple of years
· Ambient wireless charging value is negligible unless very close to an antenna (if we could get this to a realistic charging level, I would worry about being cooked while I charge)
· Nuclear, surprisingly, is order of uA/cm[SUP]2[/SUP] or less apart from radio-thermal which is not considered safe for public use
· Biomechanical varies widely depending on the method used but suffers both from being a very intermittent source and potentially being tiring to use (you are the real power source)
· There are some scaled-up sources, such as piezo harvesting traffic pressure in roadways, building vibrations and vibration in railway tracks that can generate meaningful power (~100W for regenerative shock absorbers in cars and on tracks to ~10kW from skyscraper building dampers).
· One bright spot is photovoltaic (solar). Power density is still at the uA/cm[SUP]2[/SUP] level but we know how to scale up solar panels. A 10cm[SUP]2[/SUP] panel (a large watch face) could in principle power an LTE radio with a 1% duty-cycle. But of course solar is limited to outdoor applications.
None of these numbers is very exciting. Apart from solar, the problem seems to be that tapping new sources starts with a relatively small amount of practically and theoretically accessible energy which is then substantially reduced by unavoidable limits and inefficiencies in conversion. One way to offset this problem is through massive scale-up, but then you are limited to platforms like buildings and bridges. And even in those cases, the amount of energy that can be generated is dwarfed by the daily needs of the structure (a skyscraper for instance). I might wish the reality were different, but I suspect outside of the commercial successes we already know, this aspect of the green movement will have a rather short life. All of which may mean we’ll be stuck with the grid, batteries and proximity charging (inductive and beamed) for the foreseeable future.
I published my findings in a couple of LinkedIn blogs, the first on limitations to powering wearables and the second on limitations in locally-generated power (power generated close to the consumer of power, without connection to the grid). What I hope is different between my research and what appears in popular articles is that I tried to survey widely and I documented all my sources, so you can check what I found. I would be very happy to see supported counter-examples, because it really would be fantastic to find that there are practical local-scale harvesting technologies.
The link on limits to wearable power is HERE (the blog starts with power consumption in wearables; the generation part is in the second half of the blog). The link on limits to locally-generated power is HERE.