Entries Tagged ‘Energy Harvesting’

Low Power Design: Energy Harvesting

Friday, March 25th, 2011 by Robert Cravotta

In an online course about the Fundamentals of Low Power Design I proposed a spectrum of six categories of applications that identify the different design considerations for low power design for embedded developers. The spectrum of low power applications I propose are:

1) Energy harvesting

2) Disposable or limited use

3) Replaceable energy storage

4) Mobile

5) Tethered with passive cooling

6) Tethered with active cooling

This article focuses on the characteristics that affect energy harvesting applications. I will publish future articles that will focus on the characteristics of the other categories.

Energy harvesting designs represent the extreme low end of low power design spectrum. In an earlier article I identified some common forms of energy harvesting that are publicly available and the magnitude (typically in the μW to mW range) of the energy that are typically available for harvesting.

Energy harvesting designs are ideal for tasks that take place in locations that are difficult to deliver power. Examples include remote sensors, such as might reside in a manufacturing building where the quantity of devices might make performing regular battery replacements infeasible. Also, many of the sensors may be in locations that are difficult or dangerous for an operator to reach. For this reason, energy harvesting systems usually run autonomously, and they spend the majority of their time in a sleep state. Energy harvesting designs often trade-off computation capabilities to fit within a small energy budget because the source of energy is intermittent and/or not guaranteed on a demand basis.

Energy harvesting systems consist of a number of subsystems that work together to provide energy to the electronics of the system. The energy harvester is the subsystem that interfaces with the energy source and converts it into usable and storable electricity. Common types of energy harvesters are able to extract energy from ambient light, vibration, thermal differentials, as well as ambient RF energy.

The rate of energy captured from the environment by the energy harvester may not be sufficient to allow the system to operate; rather, the output of the energy harvester feeds into an energy storage and power management controller that conditions and stores the captured energy in an energy bladder, buffer, capacitor, or battery. Then, when the system is in an awake state, it is drawing energy from the storage module.

The asymmetry between the rate of collecting energy and consuming energy necessitates that the functions the system needs to perform are only executed on a periodic basis that allows enough new energy to be captured and stored between operating cycles. Microcontrollers that support low operating or active power consumption, as well as the capability to quickly switch between the on and off state are key considerations for energy harvesting applications.

A consideration that makes energy harvesting designs different from the other categories in the low power spectrum is that the harvested energy must undergo a transformation to be usable by the electronics. This is in contrast to systems that can recharge their energy storage – these systems receive electricity directly in quantities that support operating the system and recharging the energy storage module.

If the available electricity ever becomes insufficient to operate the energy harvesting module, the module may not be able to capture and transform ambient energy even when there is enough energy in the environment. This key condition for operating means the decision for when and how the system will turn on and off must take extra precautions to avoid drawing too much energy during operation or it will risk starving the system into an unrecoverable condition.

Energy harvesting applications are still an emerging application space. As the cost continues to decrease and the efficiency of the harvesting modules continues to improve, more applications will make sense to pursue in an analogous fashion that microcontrollers have been replacing mechanical controls within systems for the past few decades.

Storing Harvested Energy

Friday, June 25th, 2010 by Robert Cravotta

Systems that harvest ambient energy on an anticipated basis do not always have a 1-to-1 correlation between when they are active and operating and when there is enough ambient energy to harvest. These systems must include mechanisms to not only harvest and convert the ambient energy, but they must also be able to store and manage their energy store. Energy storage is essential to allow systems to continue to operate during periods of insufficient ambient energy. Energy storage devices can also enable a system to support instant-on capabilities because the system does not have to be able to harvest enough energy from the environment to start operation.

Like many emerging technologies, including touch screens, fully integrated modules may integrate component parts, such as the harvesting transducers and storage technologies, from different companies within the same module. As the energy harvesting device market matures, designers will have access to more options that are fully integrated systems. For now, many of the fully integrated options available to designers include components from multiple companies.

The different types of storage technologies appropriate for energy harvesting applications include thin film micro-energy storage devices, supercapacitors, lithium-ion or lithium polymer batteries, high capacity batteries, and traditional capacitors. Capacitors are able to support applications that need energy spikes. Batteries leak less energy than capacitors, and they are more appropriate for applications that need a steady supply of energy. Thin-film energy storage cells support high numbers of charge/discharge cycles.


(Caption: Rechargeable and nonrechargeable storage technologies. Cymbet, Infinite Power Systems (IPS), Cap-XX, Saft, and Tadiran are listed as some of the companies providing different storage technologies (source: Adaptive Energy).)

The table documents some of the companies providing storage devices as well as the voltage and maximum current levels that these different technologies support. Energy density is the amount of energy stored per unit mass. The higher a device’s energy density, the larger the amount of energy it can store in its mass. Power density is the maximum amount of power that the device can supply per unit mass. The higher a device’s power density, the larger the amount of power it can supply relative to its mass.

In addition to the energy and power density for each type of storage technology, the temperature ranges that your application will operate in will affect the appropriateness of one approach versus another. For example, in high temperature environments, a lithium polymer battery is generally not a good choice, while for low temperature environments, thin film batteries exhibit lower maximum current ratings. Another consideration as to which storage technology to use relates to the anticipated number of charge/discharge cycles you will subject the device to. For example, a designer using a rechargeable storage approach, such as a battery or capacitor, may run into trouble if the storage mechanism is unable to maintain sufficient performance characteristics while undergoing high numbers of charge/discharge cycles.

The purpose of the energy storage component in an integrated energy harvesting module is to accumulate and preserve the energy captured by the harvester and conversion electronics. In order to deliver maximum storage efficiency, designers should couple the storage technology with the conversion electronics to maximize the effectiveness of storing the energy charge coming from the harvester component. Lastly, for many applications, the storage component should exhibit a slow leakage characteristic so that it can store energy for long periods to accommodate the periods of energy starvation that the system may experience.

If you would like to be an information source for this series or provide a guest post, please contact me at Embedded Insights.

[Editor's Note: This was originally posted at the Embedded Master]

Energy Harvesting Sources

Friday, June 18th, 2010 by Robert Cravotta

In my previous post about RF energy harvesting, I focused on a model for intentionally broadcasting RF energy to ensure the ambient energy in the environment was sufficient and consistent enough to power devices on demand that were located in difficult, unsafe, or expensive to reach locations. This approach is the basis for many RFID solutions. Using an intentional model of delivering energy by broadcasting can also simplify the energy harvesting system when the system only needs to operate in the presence of sufficient energy because the device may not need to implement a method of storing and managing the energy during periods of insufficient energy to harvest.

In addition to harvesting RF energy, designers have several options, such as thermal differentials, vibrations, and solar energy for extracting useful amounts of ambient energy. Which type(s) of energy a designer will choose to harness depends significantly on the specific location of the end device within the environment. The table identifies the magnitude of energy that a properly equipped device might expect to extract if placed in the appropriate location. The table also identifies the opportunities of extracting energy from a user by a wearable device. The amount of energy available from a human user is typically two to three orders of magnitude lower than that available in ideal industrial conditions.

Characteristics of ambient and harvested power energy sources (source: imec)

The Micropower Energy Harvesting paper by R.J.M. Vullers, et al., provides a fair amount of detailed information about each type of energy harvesting approach that I summarize here. Solar or photovoltaic harvesters can collect energy from both outdoor and indoor light sources. Harvesting outdoor light offers the highest energy density when the device is being used in direct sun;however, harvesting indoor light can perform comparably with the other forms of energy harvesting listed in the table. Using photovoltaic harvesting indoors requires the use of fine-tuned cellsthat accommodate the different spectral composition of the light and the lower level of illumination than compared to outdoor lighting.

Harvesting energy from motion and vibration may use electrostatic, piezoelectric, or electromagnetic transducers. All vibration-harvesting systems rely on mechanical components that vibrate with a natural frequency close to that of the vibration source, such as a compressor, motor, pump, blowers, or even fans and ducts, to maximize the coupling between the vibration source and the harvesting system. The amount of energy that is extractable from vibrations usually scales with the cube of the vibration frequency and the square of the vibration amplitude.

Harvesting energy with electrostatic transducers relies on a voltage change across a polarized capacitor due to the movement of one moveable electrode. Harvesting energy with piezoelectric transducers relies on motion in the system causing the piezoelectric capacitor to deform which generates a voltage. Harvesting energy with electromagnetic transducers relies on a change in magnetic flux due to the relative motion of a magnetic mass with respect to a coil that generates an AC voltage acrossthe coil.

Harvesting energy from thermal gradients relies on the Seebeck effect where the junction made from two dissimilar conductors causescurrent to flow across the junction when the conductors are different temperatures. A thermopile, a device formed by a large number of thermocouples placed between a hot and cold plate, and which are connected thermally in parallel and electrically in series, is the core element of a thermal energy harvester. The power density of this energy harvesting technique increases as the temperature difference increases.

The majority of these harvesting systems has a relatively large size and is fabricated by standard or fine machining. The advances in research, development, and commercialization of MEMS promise to decrease the cost and increase the energy collection efficiency of energy harvesting devices.

If you would like to be an information source for this series or provide a guest post, please contact me at Embedded Insights.

[Editor's Note: This was originally posted on the Embedded Master]