When working with energy harvesting, the most important step is the power budget analysis.
Indeed, to build a viable system, enought power must be harvested to supply the target application.
Here below, we propose to study the power budget for choicing the adequate elements.
This study is composed of steps to define the parameters useful in the storage and source choice.
Those steps have no real order; and different ways for using them exist.
- On ways is starting from the load consumption to define the source and the storage element size.
- Another way is starting from the source to estimate the available left power for the load and the storage element size.
Please note that below, TS represents the time with available power at the source; and TNS represents the time without power available at the source.
If starting from load consumption :
- STEP EC : Evaluate the energy consumed EC by the load during a repeatable period (a day, a week, a month, …). This period must be chosen to include all TS and TNS time. For example, one day is a typical period for outdoor solar application since light is coming back every day. A week could be adapted for indoor application without light during the week-end.
The formula below includes 2 different active current peak [A1 and A2] and passive current [P]. Units below are A for current, V for voltage and s for the time.
EC = (IA1 x VA1 x TA1 + IA2 x VA2 x TA2 ) + ( IP x VP x TP ) [ J ]
Active consumption is mostly due to radio communication or processing. Passive consumption is the sleep power or any constant consumption required by the load.
- STEP E BOOST : Calculate the energy to be harvested EBOOST in order to supply the load.
The formula below includes converter used between the storage and the load. If using internal LDO converters, their efficiency is provided in the AEM datasheet. For external converter, the efficiency should be given in its datasheet.
Please note that the internal leakage of the storage should be taken into account.
E BOOST = EC / n LDO (or n DCDC) + E leak [ J ]
- STEP E STORE : Calculate the required energy E STORE to be stored. Based on the autonomy and the load consumption, size of the storage must be define.
The formula below implies an autonomy egals to TNS – time with no power available at the source.
E STORE = ( TNS / TNS + TS ) x E BOOST [ J ]
For longer/smaller autonomy, the ratio R must be adapted in order to store the energy consumed during the autonomy defined.
E STORE = R x E BOOST [ J ]
If the autonomy required is 2 days, the ratio R is defined as 2*864000 / (TNS+TS) = 2 days / 1 day = 2
-> See FAQ about storage size : How to estimate the storage element size ?
- STEP P SRC : estimate the P SRC source power required to supply the load. Based on the time with available power at the source, the efficiency on the boost; the required power from the source is calculated.
Please note that, if longer autonomy is required, the additional energy to be stored must be harvested first.
Here below, we will define E BOOST* as the energy to boost for the autonomy required which is egal to E BOOST + E BOOST additional.
This E BOOST additional is defined as E BOOST * Z with Z = add time / (TNS+TS).
If the additional time is one day, E BOOST* = 2 x E BOOST.
P SRC = E BOOST / (TS x n Boost ) [ W ]
For RF system, the efficiency over the RF path must be include in that step to estimate the power to be received.
P RECEIVED = E BOOST / (TS x n RF global)
This n RF global efficiency is provided in the AEM30940/AEM40940 datasheet for the e-peas solution. For custom design (external rectifier with associated matching network), this efficiency must be characterized.
If starting from source power :
In that case, the first is to estimate the available power at the source.
This information is given in the harvester’s datasheet for solar, thermal or vibration harvester.
For RF source, the losses in the air must be evaluated to estimate the received power P RECEIVED based on the emitted power.
P RECEIVED = P EMITTED – Losses air
- STEP E BOOST : Calculate the energy harvested EBOOST from the source.
E BOOST = P RECEIVED x TS x n RF global [ J ]
E BOOST = P SRC x TS x n Boost [ J ]
- STEP EC : Evaluate the available energy for the load based on the energy harvested.
EC = E BOOST x nLDO – Eleak [ J ]
The size of the battery is estimate based on E STORE = E BOOST.