Thermal Energy Harvesting Projects

Introduction to Energy Harvesting

Energy harvesting is the process for collecting energy from the surrounding environment and converting it to electricity, and is gaining interest as a future next-generation energy source. Conventionally, electricity is supplied by a centralized power source, which is either a power plant or a reserve power source, such as a battery, which has the advantage of having a small amount of electrical wiring, which loses electricity through electrical resistance, and allows components to be easily replaceable and to be optimized in the circuit board for space, power, utility, etc.

In recent years, the idea of using ambient energy in the forms of light, motion, vibration, heat, radio waves, etc. has become increasingly attractive, and a number of methods to produce electricity in a decentralized system from these different kinds of energy sources have been developed. Energy harvesting technology could eliminate the need for replacing batteries and power cords for some, but by no means all, devices.

Background

It is first important to clarify that electrical power that can be generated by energy harvesting from surrounding light, vibration, heat, radio waves, etc is minute compared to what is available from power plants or even small scale generators. However, there are power sources, such as ambient light, heat and vibration that surround us are not negligible sources and with recent developments in low power electronics these power sources may to be exploited to do useful work in certain devices.

Nevertheless, as these developments take time in order for us to operate current electrical equipment by energy harvesting, we need to develop devices that can both generate more power at low voltage levels and design systems that can store more and more small and intermittent periods of power generation. Hence power generation systems, as far as energy harvesting is concerned, need to be designed to be smart at managing power.

Therefore we need to look at what are the main sources of energy in our surrounding environment, the most efficient technology we have to harvest this energy into usable electrical power and how each source relates and differs from the rest.

The main sources of ambient energy, which we can generate usable work from are from light, which is harvested with solar panels, temperature gradients, which are harvested with peltier elements, vibrations, which are harvested with piezoelectric elements, and action-reaction forces which can be harvested with inertial electrical induction motors:

Another source of power, which is ambient, are the radio waves which we use to transmit signals. These too have a non-trivial source of power, however the source of the energy is not as such harvested as the radio transmission itself is generated from a conventional power source, which is usually centralized, and is not harvested from the ambient environment. Nevertheless this does relate to wireless energy transmission, which is covered in another article.

For example, the main energy harvesting sources such as temperature variations, external inertial forces, light intensity, motion and vibrations are not always available in the ambient environment. Therefore, there is a growing demand for devices that can efficiently derive energy from the surrounding environment at any time, thereby enabling the devices to be used at all times, which would require a multifaceted energy harvesting approach.

Some energy sources which can be exploited from everyday life we be explored here and technological challenges to combine some of these systems will be mentioned.

Wearable Body Heat Thermoelectric Energy Harvester - Waste Heat to Electricity 

Thermoelectricity produced from bismuth-selenide peltier elements by the Seebeck effect can produce a low voltage and a more or less stable current when a constant temperature differential is created across the element. This can be accomplished by having a constant heat source and a constant heat sink, i.e. a cooling apparatus. The low voltage can be boosted using a standard low voltage boosting circuit which boosts the voltage from millivolts to around 5V-9V. 

Such circuits have become more and more widespread as the field of energy harvesting grows, with thermoelectric flashlights such as the one demonstrated here being developed for mass production for sale to the public in the near future as people become more aware of renewable energy and as the renewable energy economy grows which will only increase the demand for such devices in the electronics industry over the next decade.

Harvested power is stored temporarily in a supercapacitor bank, which allows for fast energy harvesting in a low volumetric power density, which was then transferred to a lithium ion battery which allows for slower energy transfer in a larger volumetric power density. This means that the although the supercapacitor is able to harvest the energy quickly, it also dissipates quickly and has less volume to store large amounts of power for a longer period of time. Lithium ion battery however may take longer to charge but they retain power much longer and have a much larger power density. 

Making thermoelectric energy harvesting devices that charge devices with a more intensive energy demand than LEDs is a bigger challenge and requires the device to have more elements and a faster yet more continuous method of harvesting the energy as the temperature differential is diminished as is the case for devices which are cooled by the air as the cooler side eventually heats up itself.

As the user moves around, the change in the external environment will affect the amount of power produced. For example, walking or running outside with the device exposed will not only allow more heat to be harvested as the body expels waste heat but the movement will move air across the coolers which will sustain the temperature differential. In cold weather and environments the device will also work very efficiently. In warm weather or if the user is indoors and not moving the amount of power produced will be reduced. Hence the power harvested is intermittent.

Ultra-capacitors/Super-capacitors are therefore useful for storing the intermittent periods of a strong temperature differential forming across the elements as a voltage which is contained in the capacitor bank as long as the circuit is not switched. When power is required, the circuit is switched on and the capacitors are drained. The circuit is then switched off to let the charge rebuild again. 

The voltage from the supercapacitor must be regulated properly before charging the lithium-ion battery. In this early prototype, an energy inefficient 7805 5 Volt regulator IC is used. 

In later versions, this is replaced with a lm2596 switcher which uses less power to regulate the voltage.

The more advanced thermal energy harvester I developed used the LTC3108 for millivolt sources such as thermal energy generators under a fairly low temperature differential across the Thermoelectric Generator (TEG) element.

The PCB Board Above is available for download from the OSH Park Online PCB ordering service:

https://oshpark.com/profiles/MuonRay


The full circuit schematic and components are listed at the bottom of the page.



In the following demonstration the TEG can be heated on one side with a low to medium heat source and cooled on the other side using an aluminium heat sink. 

Using magnets on the side of the heat sink we can hold the TEG module and circuit onto a metal appliance for energy harvesting, such as a steel central heating boiler, radiator or piping.

In this demonstration, the energy harvester circuit is used to power a colour cycling RGB LED. 

It is hoped that small energy harvesters will soon be universally incorporated within microelectronics such as IOT devices, sensor stations, WiFi routers, drones and wearable electronics. 

Incorporating energy harvesting helps devices become more autonomous as by including the ability to charge and power themselves using the environment and waste energy sources such as waste heat produced by machines.

This design is optimal for use in arduino-type devices and their derivatives. Low power consuming Atmel chips could be run purely from harvested energy contained in a supercapacitor and small Li-Po cell battery, which makes them a very attractive option for small devices. 


In the initial demonstrations of the thermoelectric energy harvesting devices, we used the commercially available pelteir elements to show the proof of principle. The power is transferred periodically into a commercially available lithium-ion power watch which stores the electricity for further use in the range of portable rechargeable devices that people have grown accustomed to in the modern age. 

As progress continues, devices like this will be improved upon more and more until they are eventually commercially available and affordable for people to incorporate such technology into wearable devices and clothing to have more sustainable sources of energy to power potable technologies on the go without having to depend on power outlets. 

Personal Note: 

Much of these projects were started by myself in 2013 and as of the current date - November 2017 there has been much progress in independent circuit designs that have created a growing availability in energy harvesting circuitry that is affordable and can be bought online and integrated into low power consuming devices.


I have since discontinued working in this area of engineering and have specialized in something else but I still have a few project applications of energy harvesting that I am interested to share online regarding the designs and applications I have found to be somewhat unique and interesting. 

LTC 3108 Energy Harvester Schematic 



The Following Circuit Schematic Corresponds to the PCB provided on the OSH Park Link.

Components List:

• LTC3108 Energy harvesting IC from Linear electronics in the SSOP 16 Lead package
• Some headers 2.54mm spacing 
• A supercapacitor (e.g. 1F, 0.33F) (for charging) 
• A PCB prototype board 
• Some wire for connections 
• A Thermoelectric Generator cell

·        Coilcraft 1:100 transformer

·        x2 100 Ohm SMD resistors (R1,R2)

·        330pF capacitor (SMD) (C3)

·        1nF capacitor (SMD) (C2)

·        2.2uF capacitor (SMD) (C6,C7)

·        220uF Capacitor (SMD) (C1,C5,C4,C8)

·        470uF capacitor (SMD) (C1)