To set up an energy harvesting system, you need a harvester, a storage element, and an AEM. The AEM is the central component of the system. It extracts the maximum power provided by the harvester and stores it in the storage element, so that this energy can be used later — and in some cases, it can even be used to directly power the application.
Q&A
This page tries to answer any questions you may have. If you’re still in doubt, please feel free to send us a message!
WHAT ARE THE DIFFERENT ELEMENTS REQUIRED TO BUILD AN ENERGY HARVESTING SYSTEM?
WHAT ARE THE POWER RANGES IN AN INDOOR AND OUTDOOR ENVIRONMENT?
The power generated depends on the type of harvester, its size, and the environment.
For example, in the case of Photovoltaic energy harvesting, the output power will depend on the size of the photovoltaic cell, the light intensity, and the PV cell technology used.
- Indoor Photovoltaic energy harvesting generates power in the range of hundreds of microwatts. (These values may vary depending on the size of the cell.)
- Outdoor Photovoltaic energy harvesting generates power that can reach tens of milliwatts. (These values may vary depending on the size of the cell.)
Other types of harvesters include thermal, RF, and vibration harvesters.
- Thermal energy harvesting (TEG) relies on temperature differences and typically generate power in the range of a few microwatts to several milliwatts, depending on the thermal gradient and the efficiency of the thermoelectric materials.
- RF (Radio Frequency) harvesting capture energy from ambient radio waves, such as those emitted by Wi-Fi, GSM, or TV signals. The power levels are usually very low, often in the microwatt range, and highly dependent on the distance from the emitter and the frequency band.
- Vibration or piezoelectric harvesters convert mechanical vibrations into electricity. The harvested power can vary from a few microwatts up to several milliwatts, depending on the frequency and amplitude of the vibrations as well as the mechanical structure of the harvester.
Each harvester type has its strengths and limitations, and the choice depends on the available energy in the environment and the application’s power requirements.
CAN I HARVEST ENERGY FROM MY WIFI?
Wi-Fi transmissions are limited by regulations to a maximum of +20 dBm at the transmitter. In most cases, this power level is insufficient for effective energy harvesting.
Significant losses occur as RF energy propagates through the air and along the RF path on the PCB. These losses are frequency-dependent and tend to be lower at lower frequencies.
To have full control over the emitted power, antenna characteristics, and transmission duty cycle, we recommend using a dedicated RF transmitter.
CAN THE APPLICATION BE POWERED DIRECTLY BY AN AEM?
Some AEMs have a regulated output to power an application circuit. Depending on the AEMs, different converter architectures are available. Some are designed with an LDO, with a BUCK, or with a BUCK_BOOST architecture.
Sometimes, the application circuit can also be supplied directly from the storage element or thanks to an external DCDC connected on the storage element and driven by the status of the AEM. In this case, a regulated output is not mandatory.
HOW CAN WE CONFIGURE AN AEM?
AEMs can be configured using the GPIO pins by connecting them to a high state or to GND. However, some AEMs include I²C communication for the configuration and monitoring. This allows overriding the configurations set by the configuration pins and allows accessing to all the AEMs configurations, enabling wider range of settings and allowing live system monitoring
WHAT IS THE MAXIMUM POWER POINT VOLTAGE OF A HARVESTER?
The Maximum Power Point voltage is the voltage at which a power source (like a Photovoltaic cell or TEG) delivers its maximum power output. To optimise the extraction of the energy from the harvester, the AEM Source will regulate at this voltage.
HOW CAN WE FIND THE RIGHT SOURCE CONFIGURATION BASED ON THE SELECTED HARVESTER?
To find the right configuration for the source, it is crucial to understand the voltage/power behaviour of the harvester under various environmental conditions. The most straightforward approach is to perform a voltage sweep across the harvester terminals in a controlled environment and measure the current provided by the harvester at different voltages. By multiplying the voltage by the current, the power generated at different source voltages can be calculated. This process should be repeated for different environmental conditions.
Once the harvester is characterized under various conditions, it’s possible to determine whether constant voltage regulation or regulation based on the open-circuit ratio is required.
HOW CAN WE MEASURE THE EFFICIENCY OF THE AEM?
To measure the efficiency of the AEM simply connect an SMU to the input of the converter and another to the output. By dividing the output power by the input power, the efficiency of the converter can be determined.
For example, for measuring the system efficiency between STO and LOAD of the AEM10920, the input of the converter will be the STO pin, and the output will be the LOAD pin.
This efficiency measurement will include the quiescent current of the AEM and the power converter efficiency.
HOW CAN WE KNOW IF THE AEM IS AWAKE?
To determine if the AEM is awake, the AEM internal supply voltage can be measured of the AEM (Vbuck for the AEMx094x, VINT for the others). If the voltage is around 2.2V, the AEM is awake.
WHAT IS THE DIFFERENCE BETWEEN A STORAGE CHARGER ONLY AEM AND ANOTHER AEM?
Storage chargers only AEMs do not have a regulated output voltage to power directly the application circuit. The primary objective of a storage charger only AEM is to efficiently charge the storage element.
WHICH AEM (PMIC) – POWER MANAGEMENT INTEGRATED CIRCUIT – DO I NEED FOR MY APPLICATION?
All our AEMs (PMICs) share the same purpose: manage the power from an harvester to a storage and/or a load.
However, their coldstart, MPPT ratio and timing might be different to be adapted to the source they should harvest from.
For more details, please see “What is the MPPT ratio?” question.
To choose wisely your PMIC, please keep in mind it must be adapted to your harvester and its behavior.
HOW TO ANALYZE THE POWER BUDGET OF AN ENERGY HARVESTING SYSTEM?
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 - 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 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.
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