WHAT IS ENERGY HARVESTING?
In a time where people are consuming the Earth’s resources at the fastest rate ever, finding alternate power sources is a great way to help create sustainable energy. A sustainable energy solution such as energy harvesting provides an environmentally friendly way to harness natural energy. Energy harvesting provides several benefits and has become a proven solution for the rapidly depleting resources on our planet.
Energy harvesting is a method of generating electrical energy from normally unused energy sources available in the surrounding environment. It is also referred to as energy scavenging or power harvesting. Examples of energy sources that may be harvested and converted into electrical energy include radio waves, solar power, kinetic energy, salinity gradients, and temperature gradients. Energy harvesting techniques provide a great low power alternative replacing the use of batteries in many low power applications.
Energy harvesting can generate only very small amounts of power. This limits its use to low-energy electronics such as:
• Wearable electronics / fashion technology
• Wireless sensor networks
• Long term low power sensors
• Low power applications that generally extend beyond the capabilities of a typical battery.
Fuel sources needed to power energy harvesting devices are readily available and free to collect. For example, temperature gradients are created during the operation of the combustion engine; and, television and radio broadcasting creates electromagnetic energy. Both can be “scavenged” to create energy.
Energy Harvesting Then and Now
Humans have long searched for ways to harvest energy, and, in fact, have used wheels as a method to do so since early Roman times. Historically, water wheels have been used to harvest kinetic energy. Streams were dammed to harvest this energy to take advantage of their flowing waters. It was then converted into mechanical power that was used to run machinery and turn grinding stones in mills.
Today, heat is harvested through the use of large wheels, referred to as heat recovery wheels. They look very similar to their predecessor in both size and rotational speed.
The search for innovative energy harvesting devices continues in an effort to find a method to power mobile devices and sensor networks without the use of batteries.
Two methods are employed to harvest energy using temperature gradients – pyroelectrics and thermoelectrics (also referred to as peltiers.)The application of pyroelectrics is limited because it requires a varying temperature input. Thermoelectrics turn heat into electricity utilizing a temperature gradient. Thermoelectrics are extremely stable and can provide nonstop operation for hundreds of thousands of hours. However, thermoelectrics are very inefficient, running at about 10% of photovoltaics.
Thermoelectric Energy and Energy Harvesting
Temperature differences can be seen everywhere, in both natural and manmade environments. These differences can be used to create thermoelectric energy. It is exciting that electricity can be created by utilizing otherwise wasted heat. Thermoelectric harvesting systems could be used to convert the thermal energy found in a fluid stream into electricity. Run-off from coal and nuclear power plants would be likely sources. Waste heat from solar thermal and geothermal plants could also be harvested. Exhaust streams from common household appliances could be utilized. The possibilities are endless. According to the United States Environmental Protection Agency, “The development of robust, economically viable thermoelectric power harvesting systems will reduce the consumption of fossil fuels by increasing the overall efficiency of power producing and power consuming systems.” This in turn would provide a viable solution to the problem that most current resources used to create electrical power are unsustainable.
Thermoelectric effect (TE) is the direct transformation of temperature differences into electric voltage. Its discovery nearly 200 years ago is credited to Thomas Johann Seebeck. In a thermoelectric device, voltage is produced when differing temperatures are placed side by side. In the same way, a temperature difference occurs when voltage is applied. Because of his discovery, the ability of a material or device to generate voltage per unit of temperature is referred to as the Seebeck coefficient.
In 1834, Jean Charles Athanase Peltier found that by running an electric current through the intersection of two different conductors heating or cooling would occur. The direction of the flow determined the direction of the temperature change – up or down. The heat produced or absorbed is relative to the electrical current, and the proportionality constant is referred to as the Peltier coefficient.
Because of the discoveries of these two men, heaters, coolers, and generators (TEG’s) are created using thermoelectric materials.
Ideal thermoelectric materials will have:
• Low thermal conductivity
• High electrical conductivity
• A high Seebeck coefficient
Accumulating and Storing Harvested Energy
Current applications have limitations – they must be small and require very little power. They are also limited by the need to use battery power. To overcome these limitations, the ability to harvest, accumulate and store scavenged energy, which would allow smart sensors to operate indefinitely, becomes a necessity.
As a rule, energy is stored in batteries, capacitors or super capacitors. If an application requires huge energy spikes, a capacitor is used. If a steady flow of energy is needed, a battery is utilized.
The Future of Energy Harvesting
Energy harvesting and the opportunity it creates to use components continuously, off grid and for extended periods of time, has garnered much attention in commercial as well as military use. Future applications may include designs of high-powered output devices for use in remote locations. Additionally, the wearable electronics industry looks to find ways to create harvesting devices that can recharge or power radio communications equipment, cellphones, mobile computers and more. The ability to harvest energy from the temperature differences between night and day may one day be used to power outdoor applications. One of the challenges faced in applications such as these is creating a device robust enough to withstand long-term exposure to harsh settings. Their designs would also need to incorporate the ability to harvest energy from multiple sources of ambient energy.
Thermoelectric energy is fascinating and has great potential for future applications, both big and small. Heat sources abound and can be easily scavenged by thermoelectric generators (TEGs) for use in these applications. II-VI Marlow has lead the way for providing thermal energy harvesting products to power wireless sensors and other microdevices, thereby eliminating the need for battery-powered solutions. The EverGen series from II-VI Marlow offers a low-cost and zero-maintenance solution for wireless sensor technology.
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