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Energy Harvesting: Alternative Power Sources

Alternative energy sources such as solar energy currently power larger power using systems and products such as construction signs. Portable electronics have changed our lives in many ways. Mobile phones, laptops, MP3 players, headphones, tablet PC’s, e-books, games and much more continue to advance at an astonishing rate, freeing us from a tethered lifestyle or working environment. The only downside is the reliance on the availability of an energy source for recharging. Green, sustainable energy sources of electric power are available all around these portable electronics and we are only just beginning to develop systems to run on this power.

Alternative and Sustainable Wireless Power:

At the moment alternative and sustainable power is not available in most mobile devices. In every airport around the world you’ll see people searching the walls for a free plug to charge their devices. So what is the solution? The answer may sound a little Sci-Fi, but alternative power sources are starting to emerge that will ultimately allow devices to be self powered through innovations in Energy Harvesting.

Energy Harvesting is the ability use alternative energy sources to gather energy from a device’s environment in order to completely power or partially power the device. This is not entirely new! Wrist watches have been able to do this for years, using the kinetic energy of arm movement to run the watch and we’ve had solar powered calculators since 1978. The barrier to transitioning this alternative energy technology into mainstream equipment is directly associated to the higher power required to operate such devices.

Case Studies

Over the last 2-3 years we have seen power usage drop significantly for certain applications, to the point where they can now be powered by types of energy surrounding the device.

Building an Energy Harvesting solution requires five 5 elements: an energy harvester, DC-DC circuit to charge a storage device, DC-DC for the application and the application itself. For example, a solar panel, the DC circuit to boost or decrease voltage to charge a super capacitor, the DC-DC to take the voltage from the super capacitor to supply a sensor and radio with the right voltage to operate.

Types of Power Sources:

Depending on your application, you can turn to RF, Solar, Kinetic, Thermoelectric and Piezoelectric (to name a few) for your power sources.

RF Energy Harvesting uses the energy from radio signals to generate electricity. This RF energy can come in two forms: intentional where an RF signal is transmitted to power the device, or unintentional where RF signals in a particular location are strong enough to power the device.

Solar Photovoltaic Energy Harvesting started in calculators, which utilize amorphous photovoltaic solar cells that were designed for indoor low light environments in conjunction with low power ASICs (Application Specific Integrated Circuits). Solar cells technology has advanced significantly since the days of the first calculator and just as importantly, pricing has fallen to the point where solar power generation has become affordable. Coupled with the use of lower power chips, the advantages of photovoltaic energy are now starting to be used in a variety of solar powered devices entering the market. Advantages of solar energy include it’s abundance, low cost and ability to integrate photovoltaic panels into systems. As a renewable energy source, the sun is the cheapest green energy source available. Solar Energy cons, however are the fact that at night no solar power is available to harvest. Indoor solar energy is still possible to use, however the amount of power is low. Solar energy storage from daytime sun is hopefully enough to run the system overnight.

Kinetic Energy is energy derived from movement and can be a very viable energy source. The Sports and Medical industries have taken the lead in utilizing motion to generate sufficient power to drive instruments and associated devices which not only liberates the user from a power perspective but perhaps more importantly, it eliminates the need for larger and heavier batteries.

Thermoelectric (Thermal energy) is another potential energy source, using the difference between two temperatures to create electricity. This is called the Seebeck Effect, which uses the reactions of different metals to generate electricity. Unfortunately the amount of power generated by thermal energy techniques is quite low, typically in the microvolts range and conversion efficiency is also low, at 5% with a temperature difference of 100 Kelvin and 20% efficiency for 800 Kelvin. Depending on your application, thermal energy may be a sufficient energy source for various systems using the latest semiconductor technology.

The Piezoelectric Effect produces electricity via mechanical strain on a substance. This strain can come from several sources such as vibration or more traditionally from pressure. Again, the piezoelectric energy generated is small but it is sufficient to perform actions such as activating sensors or sending a command signal (TV remote, for example). A recent example is a T-shirt that contains a large piece of piezoelectric material that was integrated into the shirt. The T-Shirt contains the electronics necessary to charge a mobile phone while at a concert courtesy of the sound vibrations resonating from the speakers. Piezo electric energy harvesting is also used in low power systems by harvesting the energy from pushing a button or turning a key, for example. Enough energy is produced to power a wireless signal to open a door or turn off a light.

Sustainable Energy Design:

The first step in the design cycle leading to the use of energy harvesting as an energy source is obvious; you must ensure the solution is as energy efficient as possible, as any unnecessary drain on power will severely impact the potential for sustainable power.

Next you must consider the environment in which your device will operate. Multiple external factors will have dramatic impact on the quality or power you can sustain, such as temperature, the availability of sunlight or the consistent source of motion.

Once these factors are determined, you need to consider the type of energy storage your application will require. Do you only need short bursts of energy or will the system be in constant operation?

All of these questions are very general, but are a good start to building energy harvesting into different systems.

Frequently Asked Questions

Questions

There are typically 4 main sections in a power chain of an Energy Harvesting system, excluding the application circuit.

1. Source section such as solar panel, vibration, RF, therma,l etc.
2. Energy extraction circuit and storage charge management
3. Storage section such as battery or super capacitor
4. Energy extraction circuit from storage to application circuit

Each section must be carefully designed/selected in order to make sure that enough energy gets stored to accommodate proper operation of the application circuit.

Energy from ambient light, whether it is the sun or indoor light

Energy from vibrations, typically using a piezoelectric device

Energy from RF signals, which consists of taking the energy from electro-magnetic wave radiations (Wifi, AM/FM radio, cell phones, radio frequencies, etc.)

Energy from thermal gradient, typically using a Seebeck Effect device

Energy from wind

There is virtually no limitation on how much energy can be harvested and/or used.  Energy harvesting systems can tackle a whole range of applications, very demanding or not, such as solar power plants (MW), hybrid electric vehicles (kW), stand alone lighting systems (W) or electronic calculators (mW).  Each application will bring its own set of requirements for each block in the power chain and each block must be scaled appropriately to fulfill those requirements.

First, sufficient energy is achieved if the desired operation autonomy of the application circuit can be achieved.  The time it will take to harvest sufficient energy will vary with respect to the source strength, storage capacity and application circuit power consumption vs. autonomy required.  If a strong source is used but the application circuit is very low power, then sufficient energy can be achieved possibly in matter of seconds if the storage element is small, possibly minutes if the storage element is bigger.  Another extreme would be to have a weak source powering a demanding circuit, where the source is going to take much longer time, such as hours, to charge the storage element enough to get minimal autonomy of operation on the application circuit.

There are many advantages to use such a system, but the overall benefits will depend on the application.  In HEV applications, the advantages are a reduced carbon footprint, fuel/money savings and perhaps extra horse-power when needed, in some cases.  In a context of remote sensor applications, the advantage could be the costs savings tied to the reduction in maintenance for battery replacement or even the savings tied to the use of free renewable energy as opposed to more costly traditional forms of energy sources.

Remote circuits/systems

Inaccessible areas that would/could utilize sensors, such as a rotary sensor in a motor shaft which requires to be entirely wireless

Ever powered devices, such as scientific calculators, computer keyboards, wrist watches, Blinking LEDs in sport shoes and other similar devices

Roadside signs and emergency blinkers

Many more…

Virtually any storage devices can be used, but not all storage elements are good for all applications or scenarios of use.  There are lots of variable/parameters to take into account when deciding which storage will be the best fit, such as: operation voltage and current ranges, storage capacity, physical composition (Lead or Cadmium are not always acceptable), self discharge characteristics and many more. The most common storage elements are batteries and super capacitors.