Energy Harvesting is a technology for collecting (i.e.: harvesting) small amounts of energy from various, unconventional sources such as light, heat, vibrations, and radio waves occurring in the immediate surroundings of the device. It is also sometimes called environmental power generation.

Unlike large-scale solar and wind installations that generate large amounts of energy, energy harvesters collect small amounts of energy from their immediate surroundings. When incorporated into mobile handsets or other small devices, they allow these devices to generate their own power supply.

Thermal energy harvesting (Also known as energy scavenging or ambient power) refers to the process of capturing and converting energy from the surroundings to electricity. The energy can either be used immediately or be stored for future use. Thermal Energy harvesting works by harnessing small amounts of ambient energy, which is otherwise dissipated or wasted in the form of heat, vibration, light, etc.

Thermal energy harvesting is the process of capturing heat which is either freely available in the environment or which is waste energy given off by engines, machines and other sources and putting it to use. Thermal energy which is harvested may be used as heat to pre-heat water for domestic use or industrial processes. Alternatively, it can be converted into mechanical or electrical energy.

Types Of Thermal Energy Harvesting

Piezoelectric Harvesting

In this harvesting method, sources are those materials that receive pressure from such things as human motion, low-frequency vibrations or acoustic noise. By using piezoelectric harvesting we can make such things as:

• Batteryless remote controls

This device uses the human force used to press down its buttons to generate enough electric charge to power itself. Because it is using the energy emitted through its functionality, it is completely self-sustaining. A great example of this is the infrared batteryless remote developed by Averni in France. This model features a large central button that once pressed down creates the power to send up to 9 signals to your television.

• Piezoelectric floor tiles

When pressure, in this case, footsteps, is applied to some materials, the atoms that make up their structure are disturbed and a small electrical voltage is created. Through piezoelectric harvesting, it is now possible to collect this electrical potential and put it to good use. This kind of kinetic floor tile was used in the Paris Marathon in 2013; their usage resulted in the generation of 4.7 kWh of electricity, enough to power a laptop computer for 48 hours. This was generated solely from the runners’ footsteps.

Thermoelectric Harvesting

Between two selected dissimilar objects, an electric voltage can be created through temperature opposition, which is harvested through this method. The two materials must remain at a relatively constant temperature, as the interceptor relies on this to receive a steady voltage. By using this method we can develop technologies such as:

• Temperature-powered phone charger

Developed by Epiphany Labs, the ‘onE Puck’ thermoelectric charger has taken full advantage of thermoelectric harvesting to create a self-sustaining system using everyday items as its source. This device works by placing a hot or cold item, preferably a drink, on the coaster-style product. This initiates the electrical voltage and as such allows you to charge your phone.

• Thermoelectric generator for cars and lorries

Cars and lorries create and thus lose a large amount of heat in their general operation, which creates an opportunity for thermoelectric harvesting. These generators can be installed into cars and lorries to reuse some of the heat given off by their vehicle. This puts less stress on the engine and thereby increases efficiency levels. Studies have also shown that cars and lorries installed with this type of generator reduced their fuel consumption by around 5%.

Pyroelectric Harvesting

Meaning: Harvesting electricity from materials that gain current from temperature change over time.

This method is currently pretty restricted in its usage. Its voltage source comes from temperature change which at the moment is primarily only applied to sensors. It should be noted that this method is not quite ready for commercial systems.

• Pyroelectric sensor

Pyroelectric harvesting can be used in passive infrared (PIR) sensors which pick up heat signals from approaching motion, such as in outdoor lighting. The pyroelectric element inside the sensor creates a small voltage from the approaching heat signal emitted from the approaching person, which is sufficient enough to power the light.

The energy harvesting techniques listed above are still being used at a very low level, so what about the huge amounts of energy that are lost every day from large-scale energy creation and output? Well, although predictably we lose the majority of our wastage to the environment, energy harvesting and other renewable schemes aim to reduce this amount.

Two of the main ways in which this is being carried out on a large scale are solar and wind energy generation. This might not typically fit into the energy harvesting category, but it works on the same principle: using energy that would otherwise go to waste to fuel our society.

How does thermal energy harvesting work?

A TE energy-harvesting system takes advantage of any temperature difference between its two surfaces. Temperature gradients are everywhere. We encounter a wide range of equipment operating at temperatures much higher than the ambient environment. Our own bodies are relatively warm considering core body temperature is 37°C. Skin temperatures are typically in the range of 32°C.

For typical indoor air temperatures, a harvester attached to a person’s skin offers ∆T(difference between temperatures) up to 10°C. Waste heat from a human body is insignificant compared to most of the machines we use every day. Anything warm to the touch, such as your laptop, will provide more harvesting potential than the 10° or so available from human skin at standard indoor air temperature. At the extreme end of readily available waste heat is internal combustion engine exhaust.

The exhaust gases themselves run up to several hundred degrees, which is well above the melting temperature of the solder used to construct the thermoelectric modules, but this example illustrates how easily accessible very high temperature gradients are. It is also not too difficult to imagine a harvester with insulation designed to protect the TE module.

Whether a thermoelectric generator (TEG) is intended to harvest a temperature gradient of a few degrees or a hundred, the same set of design principles apply. Chief among these is the idea that the temperature gradient needs to be maintained. The charge is moved in a TE device by the flow of heat. Without a temperature difference, there is no flow of heat and no electrical output from the generator.

Consider a TEG mounted on the surface of equipment with a surface temperature on the housing of 50°C. Initial output would be 5 V for a device with specifications similar to the example module mentioned earlier. However, the temperature of the entire TEG would continue to rise until it reached equilibrium with the equipment housing. Although a small gradient might continue, it is unlikely to be sufficient to produce a useful output power. A good heat sink design is critical to optimize the effectiveness of the TEG modules.

How efficient is thermoelectric generator?

The theoretical maximum efficiency of a thermoelectric generator is the Carnot efficiency. But no thermoelectric generator is NEARLY this efficient. Most are maybe 2–5% efficient, depending on the material and temperature difference between the hot and cold sides. 

One major issue with thermoelectric devices is that, to generate sufficient power, the cold side must be kept very cool. This is a problem since, due to low efficiency, thermoelectric devices must reject so much heat compared to their power output. Active cooling with fans and a heatsink is pretty much required. And the fans usually consume a huge fraction of the already meagre power output. At that point, the big question is whether the whole thing is truly worthwhile.

A good example here is the Biolite Stove.

This device uses the heat of a wood fire used for cooking to charge USB devices such as Smartphones. The charging current tops out at around half an amp at 5V, for about 2.5W. The fan probably uses about this much power to keep the cold side cool enough – half the total output. Imagine an automotive engine that uses half of its total output to drive the radiator fan! On this device, this is not too much of a problem because the fan is also needed to keep the fire going. But on other applications where the fan is simply a parasitic loss, it would be a deal killer.

Energy harvesting materials 

Energy harvesting is emerging as a viable method for electronic devices to pull ambient energy from their surrounding environment (e.g., solar power, thermal energy, wind energy, salinity gradients, and kinetic energy, also known as ambient energy) and convert it into electrical energy for stored power. This coveted technology has the potential to serve as an alternative power supply for batteries that are ubiquitous in small, mobile, and autonomous wireless electronic devices, like those used in wearable electronics and wireless sensor networks.

The discipline of energy harvesting is a broad topic that includes established methods and materials such as photovoltaics, and thermoelectrics, as well as emerging technologies that convert mechanical energy, magnetic energy, and waste heat to electricity.

Innovative materials are vital to the development of all these energy-harvesting technologies. There are several promising micro- and nano-scale energy-harvesting materials (including ceramics, single crystals, polymers, and composites) and technologies currently being developed, such as thermoelectric materials, piezoelectric materials, pyroelectric materials, and magnetic materials.

Conclusion

As energy harvesting gains in popularity, the challenges they must overcome are also increasing.

The power from energy harvesting is often weak and unstable. Only devices that operate on very little power can be coupled with energy harvesting. It is possible that the power to the device stops completely, so measures against sudden shutdowns must be considered. In addition, if the devices being used are costly to manufacture and install, the energy harvesting capacity must be quite large to match the cost. For widespread application of energy harvesting, high efficiency, high output, and low-cost harvesting equipment will be necessary.

Reduction of power consumption of the devices is also necessary.  Energy harvesting is an essential technology for building networks of countless IoT and M2M devices. To help realize the smart cities, smart homes, and smart factories of the future, which will require many kinds of data from many kinds of interconnected sensors and devices, development of more advanced energy harvesting technologies will continue.

Energy harvesting is not going to save the world or end the energy crisis, but it can make a valuable contribution toward it. The ability of this technology to tap energy sources that would otherwise go to waste can help reduce demand on other energy generation methods, in particular fossil fuel or nuclear power consumption.

Development is also underway on thermal energy storage systems. This technology could be combined with energy harvesting systems to capture heat when it is available to then release it and use it for heating or convert it to another form of energy on demand.