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====== Energy Harvesting Overview ======

By forgoing batteries, intermittent devices are required to get their energy from external energy harvesters.  Some of the most common options are listed below, as well as additional considerations that affect design decisions and performance.
===== Harvester Types =====

==== General Challenges ====

Since capacitors can hold only a fraction of the power a battery can, harvester behavior heavily dictates overall design considerations and the intended deployment can often place strict requirements on what harvesting methods are available.  However, some general challenges and considerations apply regardless of harvester type:

  * **Limited availability**: energy availability is frequently transient, and any intermittent design should be prepared to [[tutorials:checkpointing_overview|handle loss of power gracefully]].
  * **Overall efficiency**: most methods of harvesting have limits on the amount of energy they can practically harvest, even when it is otherwise plentiful.  This can be due to various factors, such as being optimized for certain frequencies or the size of the device itself limiting how large or complex a given harvester can feasibly be.  Even under optimal conditions, overall power generation can be low (down to the microwatt level for the smallest devices), heavily limiting device operation and making optimal use of what power is available critical.
  * **Event tracking vs. available energy**: a device tracking irregular, unpredictable events may not have the necessary energy to detect them when they occur.  Devices should adapt an energy harvesting method that allows them to capture as many events as possible, or failing that try and ensure that a device has sufficient reserve power to detect an event even when available ambient energy is low.
==== Photovoltaic (Solar) ====

One of the most common choices for powering intermittent devices due to their overall power output and relative ubiquity of the energy source, photovoltaic (or solar panels) harvesters convert ambient light into electricity.  Panels can be optimized for harvesting either high (sunlight) or low (indoor) lighting conditions, and should be selected based on expected deployment environment.

Reliance on light for energy has fairly obvious drawbacks: in an outdoor setting light will only be available for part of the day, even without taking weather or location into consideration.  Indoor settings can leverage artificial light without depending on the vagaries of weather, but even this has its limitations if the device is expected to be deployed in areas that are either rarely or never lit (e.g. closets, utility rooms).  Solar panels also benefit from a larger surface area, but the small size of many intermittent devices limits panel size and by extension power harvested.
==== Radio Frequency (RF) ====

Radio frequency (RF) harvesting benefits from the relative ubiquity of RF signals in many environments.  One or more antennas are connected to a matching circuit, which helps convert the voltage differential caused by a signal to power usable by the system.  Passive RFID tags operate on this principle, using the signal from a reader to activate and return information.

While able to harvest energy from across the frequency spectrum, there are notable limitations.  First, energy density is low compared to solar/photovoltaic harvesting.  Second, an antenna’s physical design will determine the optimal frequency range it can harvest, losing efficiency the further a signal falls outside of the optimal band (and while multiple antennas can overcome this issue to an extent, this incurs an increase in device size and complexity which may be impractical).  Third, while it is possible for an antenna to pull double duty as for both communications and energy harvesting, this can have negative impacts on reception.

**Examples**

  * [[https://sites.google.com/uw.edu/wisp-wiki/|WISP]]
  * [[https://web.cs.umass.edu/publication/docs/2011/UM-CS-2011-020.pdf|Moo]]
==== Power Transmission ====


A subset of photovoltaic/RF energy harvesting, a base station is used to transmit power (e.g. through targeted light/lasers at a solar panel or higher-power signal for RF devices) to batteryless devices in the vicinity: in a controlled environment this can minimize the inherent unpredictability of harvested power.  In practice, there is an upper bound to how much power can be supplied in this fashion outside of a research environment, as regulations usually limit light/laser intensity and radio transmission strength.

**Examples**

  * [[https://mobilex.cs.columbia.edu/phaser/|Phaser]]
==== Thermoelectric ====

Thermoelectric generators utilize the [[https://en.wikipedia.org/wiki/Thermoelectric_effect|Seebeck effect]]: temperature differences in a conductive material will produce a potential difference (and therefore voltage) between the hot and cold areas of the material.  Doping can be further used to enhance the effect and efficiency of the material itself, and the lack of moving parts can make for a more durable energy harvesting method than some alternatives.

Thermoelectric generators suffer from relatively low efficiency (and power generation) that is heavily environmentally dependent, as efficiency of the generator increases with the difference between the temperatures of the “hot” and “cold” areas (and vice versa).  Thermoelectrics are also dependent on some way to create or maintain a temperature differential, typically through a heat exchanger: often, the size of the heat exchanger dwarfs the thermoelectric portion itself, to the point that allowable exchanger size will dictate the thermoelectric rather than the other way around.

As a result, thermoelectric generators are most useful in situations that are both tolerant of low power and can reliably count on a temperature differential from the environment or their use case to improve efficiency (such as a wearable device powered by body heat).

**Examples**

  * [[https://dl.acm.org/doi/10.1145/3384419.3430722|REPUBLIC]]
==== Kinetic ====

A broad category of harvesters that use various means to capture movement or vibrations as energy.  Some of the more notable types:
=== Electromagnetic ===

Electromagnetic energy harvesters utilize the fact that changes in a magnetic field acting on a coil will produce current, and by extension power.  The exact design varies, but a common implementation is a free-moving magnet in a tube surrounded (or adjacent) to one or more sets of coils: movements or vibrations cause the magnet to move, which in turn produces changes in the magnetic fields on the coils that can be harvested for power.  Pendulum generators operate on similar principles, but instead produce magnetic fluctuations using magnets attached to a pendulum instead.

=== Piezoelectric ===

Piezoelectric generators generate power from the voltage differential caused by the physical deformation or stress (such as vibration) on certain materials.  A common example (that readers may have unknowingly interacted with) are the push-button igniters on some barbeque grills: force is applied to a piezoelectric to generate a spark, which ignites the fuel source.

Similar to RF harvesters, piezoelectric generators have an optimal frequency at which they most optimally convert input force to energy, which should be taken into account when implementing a design.

**Examples**

  * [[https://dl.acm.org/doi/10.1145/3384419.3430722|REPUBLIC]]
==== Microbial Fuel Cells ====

Microbial fuel cells are designed around the behaviors of [[https://en.wikipedia.org/wiki/Exoelectrogen|exoelectrogenic bacteria]], which release electrons as part of their own biological processes.  MFCs are designed to harvest the electrons released by these bacteria, converting it to power in the process.  The exact medium can vary depending on deployment, but soil-based batteries are the most “common” type of design.

While promising as a renewable, environmentally-friendly power source, there are several issues that have stymied deployment outside of a research environment.  Maintaining sufficient microbial activity is a major issue: without some way to replenish nutrients or bacteria, an MFC ultimately acts more as a battery than as an energy harvester.  Power output is also low (usually in the microwatts range) relative to harvester size, and soil MFCs are heavily affected by soil moisture, with power output dropping when soil is dry.

**Examples**

  * [[https://dl.acm.org/doi/10.1145/3631410|Soil-Powered Computing]]

===== Making Harvested Energy Useful =====

Harvested energy can rarely be taken directly from the harvester to the load: beyond the natural variations of harvested energy (usually requiring at least a capacitor to provide sufficient buffer if nothing else), the energy itself may require further handling before it can be safely utilized by a device.
==== Voltage and Current Management ====

Voltage requirements for individual components can vary wildly: the operating range of the MSP430 series is generally in the 1.8-3.6V range, whereas energy harvesters can often produce peaks of voltage significantly higher than even the max safe operating range.  Smaller capacitors may also be damaged by sufficiently high voltages, and peripherals with their own safe operating voltage ranges can complicate the picture even further.  Converters can be used to step the voltage up (or down) to the appropriate level, but add additional weight and complexity to the circuit in addition to losing some energy in the conversion due to non-ideal circuit behavior.
==== DC/AC Harvesting and Rectifiers ====

Most intermittent devices run on DC power, but that may not be what the harvester provides.  The type of power being provided should be checked for each individual energy harvester, but as a general rule photovoltaic and thermoelectric harvesters tend to supply DC power, whereas the other harvester types tend to supply AC.  If a harvester supplies AC power, a rectifier circuit will be required to convert the power to DC.
==== Capacitor Charge Affects Energy Harvesting ====

Further complicating matters is that performance of an energy harvester depends not only on the current energy being harvested, but the current charge of the capacitor.  While true in all devices using harvested power to an extent (as a battery or capacitor will resist changes in voltage, which will cause harvester power to change accordingly), using capacitors instead of batteries makes the behavior more noticeable because capacitor voltage changes more frequently compared to a battery.  Because of this, intermittent device behavior has a much greater impact on harvester performance than an equivalent battery-powered device, as the more frequent voltage fluctuations can lead to more unpredictable harvested energy if not properly accounted for.

===== References and Further Reading =====


  * [[https://www.sciencedirect.com/science/article/pii/S1364032115012629|Energy harvesting in wireless sensor networks: A comprehensive review]]
  * [[https://www.sciencedirect.com/science/article/pii/S0360544219302993|Efficiency in RF energy harvesting systems: A comprehensive review]]
  * [[https://thermoelectrics.matsci.northwestern.edu/publications/pdf/TEEnergyHarvestingBookChapter.pdf|Thermoelectric Energy Harvesting]]
  * [[https://www.sciencedirect.com/science/article/pii/S2211285524009479|Piezoelectric energy harvesting and ultra-low-power management circuits for medical devices]]
  * [[https://www.sciencedirect.com/science/article/pii/S2352431617300482|On the efficiency of piezoelectric energy harvesters]]
  * [[https://dl.acm.org/doi/pdf/10.1145/3631410|Soil-Powered Computing: The Engineer's Guide to Practical Soil Microbial Fuel Cell Design]]
  * [[https://patpannuto.com/pubs/marcano2021einkbiobattery.pdf|Soil Power? Can Microbial Fuel Cells Power Non-Trivial Sensors?]]