Energio: Zero battery IoT energy monitor

Clip-on battery-free energy monitor

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No batteries - ever.

The idea is simple. It's a shame to use batteries to monitor an energy source. Energio clips on a circuit and harvests energy to measure and communicate your energy consumption via Wi-Fi. By using Wi-Fi, the homeowner doesn't need to add a gateway to convert from another wireless protocol to IP.
The current transformer used to measure the current in an energy monitoring system simultaneously charges a supercapacitor that powers the energy monitor and radio. With only a 60 W load on the measured circuit, our goal is to report the min, max, and average energy draw once per hour. Higher draws can be reported more frequently. Lower draws, less frequently.

I have demonstrated energy harvesting and energy measurement so far, but the next step is to combine them. The ESP8266 is a fairly low power WiFi radio, but it's still pretty power greedy. I will be experimenting to determine how much capacitance I need to power it and how fast I can charge that capacitance with a low current through the primary (say 0.5A, for example). My goal is that a 60 W bulb will be able to power the radio enough to transmit monitored peak power every hour.

I test the energy harvesting with the setup shown in one of the gallery pictures. I have a 120 VAC power cord going to a switch, and the wires from the switch allow me to open the circuit to put a current meter in-line, and I can also connect a current transformer on only one wire. By doing this, I am able to plug various loads into the light socket in order to determine how much energy I can harvest.

Why Wi-Fi? It's really power inefficient relative to Bluetooth, Zigbee, and other radios, but I am staunchly anti-hub. I think it's too much to ask a user to add and provision a new hub to their network, so I want to avoid that. I have experience in low power Wi-Fi design that allowed operation of a Wi-Fi sensor node for two years from two AA batteries. I'm going to lean on that as much as I can to optimize the power.

  • Roadmap for low power ESP8266

    CaptMcAllister04/22/2016 at 15:32 2 comments

    I came across a neat how to on reducing the sleep power of the ESP8266. I haven't read it in any great detail yet, but I plan to look into it to see if there are any concepts I hadn't yet thought of:

  • Down to 34 hours charge time

    CaptMcAllister04/07/2016 at 03:26 0 comments

    I'm working on optimizing the charge time. I was able to charge the 3F supercap from 2.924 V to 3.002 V in 80 minutes with a 60 W load. By a very rough linear approximation (which I know isn't accurate, but it's easy and it's not off by that much given how early this project is in its development), this would charge from 3.0 to 5.0 V in about 34 hours. This is down from the previous estimate of 100 hours.

    There are still some surprising things going on. I still dont't have a proper rectifier running - I'm just using one diode and half wave rectifying. That's a lot of energy I'm throwing away. It's also a silicon diode instead of a Schottky. For some reason, the Schottky diode I bought is dropping the voltage to 0. I'll have to check that out. I have some really crazy TI "diodes" that only drop something like 26 mV, and I plan to try them soon. It would be great to make a bridge of these and to keep all the power I can.

  • SCT013 maximum power point

    CaptMcAllister03/31/2016 at 02:25 0 comments

    I ran a space heater through the current transformer primary today to get a strong secondary voltage, and I varied the resistance across the secondary to develop a VI curve so I could find the maximum power point. I solved for current and obtained the following graph:

    Plotted in terms of power as a percentage of the open circuit voltage, I get this graph:

    It shows me that the power harvesting hits a maximum at about 66% of the open circuit voltage. Previously, I had the harvesting circuit set to a voltage of 80% of the open circuit voltage. I should be able to obtain 0.035 W at 66% of Voc as opposed to the 0.024 W I obtained at 80% of the Voc. This should yield an efficiency improvement of about 46%. I was hoping for a doubling in one of my previous logs, but I will settle for a 46% improvement if I can actually achieve that at the output of the energy harvesting circuit.

  • Charging the supercap

    CaptMcAllister03/23/2016 at 13:15 0 comments

    I removed the built-in burden resistors from my split core current transformer (CT). This eliminates the power dissipation of that resistor and allows me to harvest as much as possible from the transformer. I was able to charge 4.5 F of supercaps from 2.9 V to 4.9 V in about one hour using a 1300 W space heater (~11 A current in the primary). This means I could charge 3 F in about 40 minutes and get about 5-6 Wi-Fi transmissions out. It looks promising.

    There is one big caveat so far. I want to get one report an hour with 0.5 A in the primary (equivalent to one 60 W bulb on the circuit). I did try to charge 3 F with 0.5 A in the primary and it looked like it was going to take about 100 hours. That's not a huge surprise. The energy harvesting circuit is pretty inefficient down at the low secondary current levels. Plus, I had nothing optimized. Finally, the current transformer is not really intended for harvesting, so the turns ratio is a bit off. I do have plans to try to fix these though. A factor of 100 looks bad, but in my experience it isn't necessarily that bad.

    First, I'm going to characterize the current transformer I do have. The BQ25504 has a maximum power point (MPP) setting where you set the operating voltage to be a fraction of the open circuit voltage. This is essential to get solar cells operating at their MPP. I will also use it for the current transformer. It is probably operating too close to the short circuit current, and I need to find the MPP. I will generate the IV curve by varying the burden resistance on the CT secondary to find this point. Then I will tune the resistor network on the BQ25504 to hit exactly that maximum power point. I might be able to get a factor of 2 out of this tuning, if I'm lucky. At some point, I will almost certainly get a different and more efficient current transformer that is more optimized to the task at hand.

    Another thing I need to do is look at my rectification. Right now, I'm just running the CT secondary through a single diode because the diodes I had on hand dropped the voltage too much when I tried to rectify the CT secondary with a full bridge. I didn't even have a smoothing capacitor There was almost nothing left at the output of the bridge for the charging circuit. If I can get the CT output voltage a bit higher by operating closer to the MPP of the CT, I am hopeful a bridge will work, and that will probably get me a factor of 2. I'll also use Schottky diodes for their lower voltage drop.

    Finally, I may have to drop the BQ25504 and look at the Linear Technologies line of harvesters. They are specified to harvest all the way down to 20 mV, whereas TI needs 330 mV to cold start the harvesting. It could just be a difference in how these are specified, but it's something I have to look into. I'm not sure what kind of efficiency improvement I can get out of this. That's the point of the testing.

    The other thing I'll need to get clever about is optimizing the Wi-Fi power consumption. I have done no work at all there right now. I'm just running a webserver that autorefreshes periodically. I might be able to get a factor of 10 improvement out of the Wi-Fi by optimizing power use and possibly by selecting a different radio. The ESP8266 is just a good easy one to start with.

  • Running ESP8266 from 4.5F capacitance

    CaptMcAllister03/19/2016 at 18:24 0 comments

    I managed to get the ESP8266 to run a webserver for 90 seconds off 4.5F of supercaps. It's kind of fun to charge them, because I"m using my BooSTick. Now that I have a bunch of them around, I find myself using them all the time. You can check that project out by clicking here.

    So now the question is, how fast can I charge several farads of capacitance off a current transformer? That's the next step.

  • Harvesting energy

    CaptMcAllister03/18/2016 at 16:23 0 comments

    I bought a couple current transformers and TI's BQ25504 energy harvesting kit. I was able to charge a 1.5F supercap with a space heater. It took quite a while (tens of minutes) because the current transformers were actually rated for 20 kilohertz, and didn't seem very efficient at 60 Hz. I plan to remove the ballast resistor from a split core transformer like the one pictured and I will try to harvest the higher voltage available from that circuit.

View all 6 project logs

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HclX wrote 03/25/2016 at 21:40 point

ESP8266 requires a LOT current to transmit a thing, I'd suggest you try some light weight RF thing, such as Nordic's NRF24L01. Of course, you then need a "gateway" to bridge it to the internet, but that should greatly reduce your power requirement...

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CaptMcAllister wrote 03/25/2016 at 21:46 point

I completely agree.  ESP8266 is a stretch goal, and I'll scale back to something lighter weight if I can't harvest enough energy.  Right now I'm trying to go big or go home :)

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electrobob wrote 04/27/2016 at 15:57 point

yeah, the Radiohead library that can be used with a lot of low power modules has been ported to the ESP, so you can use the ESP as gateway. 

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CaptMcAllister wrote 04/27/2016 at 18:49 point

Cool - thanks for the tip!  I'm still trying to optimize harvesting, so I haven't gotten too far into the Wi-Fi.  Your note is perfectly timed!

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K.C. Lee wrote 03/23/2016 at 12:14 point

It works better if you have only the Live wire going through the current transformer instead of both Live and Neutral.  Something about the wires have opposite current directions and their magnetic fields cancels out.

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CaptMcAllister wrote 03/23/2016 at 12:37 point

Yeah, stock photos are a funny thing.  I noticed it when I pulled it off the internet to set up the skeleton project.  I'm surprised you're the first one to comment.

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