This post is the first in a series on designing energy harvesting electronics. Such devices are not very common in the maker markets. I believe this is because much less content is available on the web that discusses their design and this is generally not a focus of education systems that pump out engineers. The potential of energy harvesting is very powerful, specifically when we talk about the Internet of Things and making it the Internet of Everything as self powered devices do not require batteries, wiring, or in some cases any maintenance.
The standard approach to powering small sensor nodes is slap on a CRxxxx battery, and this works well except for when the battery dies, and when we run out of raw materials making these batteries expensive. Much like a battery a capacitor can store energy. Unlike a battery it stores energy in an electric field instead of converting chemical to electrical energy. The most important difference to consider when using a capacitor to store energy is that as the energy decreases so does the voltage available. Generally batteries maintain a minimum voltage for ~90% of their rate lifetime. Se this Panasonic CR2016 datasheet and note that the voltage is fairly constant till the end of life. The voltage available from a capacitor increases or decreases based on the integral of the current based on time. If you want to know more here Wikipedia has the details but they are not going to be covered in this discussion vs a battery.
Calculating how long a battery lasts
The calculation of the run time with any battery is fairly simple as batteries are generally measured in mAh. This means that the battery will supply x milliamps of current for some amount of time. The CR2016 battery above is rated for 90mAh. This means it will supply 90mA of current for 1 hr at 3V. If your system uses 1mA of current then it will last 90h. Generally this should be de-rated as the voltage will drop towards the last 15% of the batteries energy capacity so take a look at the data sheet. Also remember that batteries have resistance and cannot supply unlimited current. It may not be possible for the above battery to supply 360mA for 1/4 and hour depending on this value.
mAh can be converted to power by multiplying by the average voltage over the batteries lifetime. Electrical energy is voltage * current * time [V*I*t] and a batteries data sheet supplies all three of these. The power the CR2016 has is 3V * .009A * (1hr * 60min * 60sec) = 97.2J.
So how does this equate to a capacitance value?
The energy in a capacitor is defined by its capacitance, the ability to store an electric field, and the voltage on the capacitor. E=.5*CV^2. See here if you want to know why. In our system we know we need to maintain a minimum voltage so the electronics operate correctly. When the voltage is below this nothing will work. There will also be a maximum voltage based on how the energy harvesting circuitry that the capacitor can charge up to. This leaves us with a max voltage and minimum voltage. All the energy stored below the minimum voltage cannot be used so we get the following equation of usable energy in the capacitor. E[useable] = .5*C *(V[max]^2 – V[min]^2). E[useable] should match the energy in the battery and then solve for C.
Not all Capacitors are created equally.
Capacitors also enjoy a nice property known as leakage current. Different capacitors have different leakage currents, lower is better since this is wasted energy. The capacitors you use now probably wont work so well as a long term energy storage device. To further make things difficult data sheets don’t have this information available. I would not source these from ebay and would spend some time learning about the different commercially available capacitor types. Perhaps later a post can be devoted to this.
That is all for now. Next up will be a post on EH charging circuitry.