Listening to MIT’s Sanjay Sarma present a few weeks ago led me to think a bit about how we’re going to power all these connected devices in the Internet of Things. Mr. Sarma’s primary point is that RFID didn’t become ubiquitous until the lifetime cost of tags was reduced to pennies. Had the industry stuck with the conventional thinking around battery-powered devices with more powerful processors, it is unlikely RFID would have become as far-reaching.
The cost of batteries is dropping, and the lifespan we’re able to stretch out of them is increasing, but I don’t foresee lifetime costs (which combines the cost of the battery and the cost of replacing it when it fails) dropping to the under 10 cents range that Mr. Sarma was targeting. Even the best lithium battery technology can only be reliably stretched to a few years under constant load (even under low power) – and these batteries are still relatively expensive.
Some of the newer wide band communication protocols like 6LoWPAN and mesh networking like that proposed by Iotera can certainly help by reducing the energy requirements needed for long range communication to well below what is required by things like WIFI, Bluetooth, or Zigbee. But the reduced power needs don’t change the equation enough to meet Mr. Sarma’s goal for lifetime costs.
Energy Harvesting offers promise as a battery booster. Ambient energy can be harvested from heat, light, motion, pressure, chemical reaction, or other sources. These military boots that harvest energy from marching soldiers are a good example, but this technology is in its infancy and still relatively expensive, well outside of Mr. Sarma’s range.
But what Mr. Sarma proposed as a solution is actually quite interesting. He went back to his roots, and asked us to consider passive tags. Passive tags have no inherent power source. They are powered by a signal coming from a nearby “reader”. When the reader passes by, the tag is powered up and sends a response to the reader. The data transmitted from the tag can be anything it is able to sense.
At first blush, you may think (as I did), “didn’t we already discount Near Field Communication in IoT?”
Well on second thought, perhaps we really didn’t, and perhaps it does deserve some more consideration. If we are to get to 50 billion connected things in the next 5 years, we’ll need to expand beyond the consumer market. Businesses and governments will need to deploy lots of stuff, in all likelihood mostly sensors, and they’ll need to do it fast.
Some quick math, if you believe the 50 billion number, and then you assume 25% of those will be consumer devices (almost two for every human on earth), that leaves 38 billion for businesses. If you assume that 80% of those will be deployed by the Global 3000, that would mean each of these companies would be on average deploying almost 10 million connected things. At $10 each in five year cost, that would be $100 million. That seems to me to be on the high side (of course my assumptions are based on little science, but you get the point).
Passive tag technology reduces this number to $1,000,000 (assuming 10 cents per tag). And now it seems quite reasonable all of a sudden… in fact 50 billion devices now seems kind of low.
So could passive tags be the answer? Mr. Sarma raised the example of passive tags being used to detect termites, using one regular antenna on the tag and one antenna fashioned from wood. When only a single signal is detected from the sensor, you can deduce that the wooden antenna has been eaten and therefore you have a termite. Since the tags are so cheap, you can afford to put them everywhere, which enables you to average out the anomalies, and in the case of termites, pinpoint exactly where they are likely hiding out. Passive tag sensors now exist for everything from heat sensors, to chemical sensors, to moisture sensors.
I do believe that Mr. Sarma is on to something – at least for a specific class of sensor. It makes sense to me that if there is a cheaper sensor, that is what businesses will use. That said, there are many IoT scenarios that need much more than what passive sensors can provide, so there will likely be a mix of passive tags and more advanced applications with MCUs and batteries (and likely as much harvesting as is economically sensible).
So I take away two things from this: 1) Don’t discount the power of the passive tag. I believe it will play a role in IoT. 2) If passive tags do begin to dominate the sensor space, 50 billion is likely way too low of an estimate for the number of connected things.