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)

Hybrid Solar and Wind Energy Harvesting

This is an old project I had started about a year ago that has since been discontinued. However I am more than happy to share information to the public. The circuit schematics are all available for download from the OSHPark site

https://oshpark.com/profiles/MuonRay

Energy harvesting is a concept that essentially promises to have vast arrays of decentralized energy harvesting stations that can run everywhere and anywhere, operate independently and perhaps run devices such as Internet of things (IOT)  sensors that send data they collect to the internet in which it is in a centralized location for analysis.

The stations themselves are of largely 2 regimes - mobile and immobile. 

Mobile sensor stations would include as examples:

• Wearable sensors (i.e. clothing, watches, belts, rings, bracelets, etc) 
• Sensors for vehicles (i.e. tags or attachable modules for cars, motorcycles, bicycles, planes, drones, seafaring vehicles, etc)

Immobile sensor stations would include as examples:

• Sensors for Data Reading Stations ( i.e. outdoor weather stations, climate control for buildings, etc)
• Sensors for Appliances (washing machines, refrigerators, heating systems, etc)

In both regimes it is important to consider how to increase the autonomy of the sensor stations themselves. The field of ambient energy harvesting boasts to harvest available energy sources from the external environment and power the sensor stations themselves. In order for this to be effective, the energy harvesting must be multifaceted in order to combat the differences between different times of day and different times of year in order to work almost indefinitely. 

Multifaceted energy harvesting would work on the same basis of examining the most efficient forms of sustainable energy in the environment which make up the least amount of space and which are the least intermittent (i.e. the least amount of interruption expected in the overall energy supply).

Discussion of Solar and Wind as Energy Sources

Solar:

By far, solar and wind make up the greatest energy source in an environment and require the least space. Moreover although both can be intermittent energy sources, both systems when hybridized can allow for a reasonable amount of compensation for the relative losses experienced in sun and wind as the conditions during the day and seasons change.

Powering an IOT module from a solar charged battery source is simple in terms of installation and the main obstacle is to make sure the solar panel and energy harvesting circuitry harvest energy even in low light levels. This can be accomplished with DC-DC upconverters and/or high quality, thin film solar panels.

The latest design for my energy harvesters uses the Linear Technology's LTC3105 for energy sources such as solar energy harvesting in low light levels. A PCB board was developed using OSHPark's online ordering service and assembled in my electronics lab.

The energy harvester is tested using a thin-film, flexible silicon solar cell, made by PowerFilm Inc., held in a picture frame.

When relying on solar energy as the power source, for northern and southern latitudes the seasonal dependence on sunlight is an obvious limiting factor on energy supply. The only way to combat this is by placing the solar panels as high as possible above the ground surface to avoid obstacles blocking the light.

The added benefit of power generation by solar energy lies with efficient tracking systems along with simply examining the relative light levels in an area where a solar powered device will be planted.

Wind:

In order to harvest wind energy we require mechanical knowledge as well as electrical. There are many different wind turbine designs, however to be efficient the wind turbine must work in the direction the wind is blowing.

To do this it must either be designed to steer itself via, for example using a fin on the back of the nacelle, or be a vertical axis wind turbine (VAWT) design that can move in any direction the wind blows. The VAWT design is better for small to medium wind turbine designs for use in energy harvesting.

Here is a video showing how the VAWT was tested using a different energy harvesting circuit that boosts an input DC voltage of 0.8V-5.0V to approximately 5.0V.

By placing the VAWT inside a commercial aluminium flag pole mount, we can in principle plant the wind turbine on almost any surface and point it in the direction of the prevailing wind to harvest power.



Wind energy collection is more subtle than solar in terms of the examination of its dependencies.  The amount of wind available to harvest as energy source at a given time is itself a consequence of the solar energy available in ways that are not so direct as PV solar energy harvesting.

Wind speeds at or near the surface generally decrease after sunset because at night the surface of the Earth cools much more rapidly than does the air above the surface.

As a result of this difference in cooling ability, it doesn’t take long for the ground to become colder than the air above it.

The air in close contact with the ground — say in the lowest 300 feet of the atmosphere — then becomes colder than the air above it.

This circumstance leads to the development of what is known as a temperature inversion. Inversions dramatically reduce the amount of mixing that occurs between different vertical layers of the atmosphere. As a consequence, once the inversion sets up (after sunset), it is much harder for fast-moving air above the ground to mix down to the surface, where it could appear as a gust of wind.

This is why fogs appear typically after sunset or before sunrise. The inversion prevents mixing that would disrupt the fog.


On a clear day when the sun is shining and is heating up the ground much more quickly than the air above it then air near the ground is heated more than the air above say 300 feet of atmosphere and it then becomes very easy for currents to form, as hot air rises and pushed cold air down, which in effect causes the air to mix and can form a cycle of surface gusts. This can be then harnessed by a surface windmill.

However, at sunset the ground will begin to cool rapidly and a temperature inversion will occur as the air at the surface cools faster than the more insulated air in the upper atmosphere. The winds will then be low or non-existent at night after a clear sunny day, in summer for example.

The ground always cools faster than the air but if the temperature inversion is negated somehow the winds will blow day or night.

In some cases cloud and temperature structures exist, in storms for example, that can often overrule the tendency for inversions to set up at night.

Low pressure systems can negate temperature inversions to set up at night by hot,moisture laden air rising from the surface. This frequently happens around large bodies of water. Water can retain the heat from the sun for a long time after sunset and can thus halt temperature inversion at night.

Late autumn and winter can also bring cold clouds and air in the upper atmosphere that counter the temperature inversion at night and create a relative inversion where the air in the upper atmosphere is even colder than the air near the surface even during low levels of lower atmospheric heating where the sun is not as intense in these colder seasons. Hence windy conditions are more common in autumn and winter.

Along coastlines air near the surface of the water can remain warmer for longer near the surface than would happen inland. Hence air will move from the relatively warmer coastline towards inland. This becomes particularly apparent in autumn and winter.

So, in general, if we can expect high levels of clear sunshine then we can expect low levels of wind, and if we can expect high levels of wind then we can say that sunlight will most likely be very dissipated, along with a lot of moving cloud cover.

Hence having a hybrid solar and wind system can cover a great deal of weather conditions for powering an IOT module as continuously as possible.

Hybrid Solar and Wind Energy Harvesting Station Prototype

The hybrid solar-wind powered unit showcased here uses an integrated solar panel, vertical axis wind turbine (VAWT), energy harvesting cricuitry and a 3.7V Lithium-Ion polymer battery incased within a hollow but strong and durable clear perspex tube.

The tube is clear for solar energy to be gathered inside the tube during the day and to function as a "light pipe" of sorts for an energy efficient LED for illumination purposes as a demonstration.

The system is designed to be integrated together by using a small thin film silicon solar panel that has been folded inside the cylindrical perspex tube so that it can gather light from any possible angle without the need for tracking.

Although this technique would be inefficient for conventional solar panels, the thin-film flexible solar panel can generate voltage from diffuse light hitting it from any direction, so that it can generate power from dawn to dusk by solar energy while the VAWT generator can provide a power source whenever the wind is blowing.

Altogether, this design saves on space for installation for a energy harvesting power source for use in lighting, USB device charging, small IOT sensor stations, signal boosters/repeaters and so forth.

We can even place the pipe in conventional fittings, such as commercially available aluminium flagpole mounts. that allow us to move the direction of the pipe to work at whatever angle we figure is to be the best for energy gathering.

The union between energy harvesting and IOT devices has several hurdles to jump across. Hopefully by examining and incorporating more ways to harvest energy from the surrounding environment these hurdles can be worked on by engineers who want to make, in essence, networks of self-powering, efficient and highly versatile electronics.

Who Truly Benefits from Science?

Science is often portrayed in media as a self-evidently benevolent enterprise. Moreover, it is constantly portrayed in mainstream media, both in news and popular documentaries, as a continuously and eternally progressing enterprise where each new development somehow brings us to a world of wonder and whimsy with external consequences which can be either ignored or simply adapted to.

Curiously, it is also assumed that the rate of progress in science is completely linear and that in the next 100 years we are told we shall see rates of progress greater than or equal to the progress seen last century. So it is said in virtually all media, both in fiction and fact contexts.

It is strange therefore to compare this idea to the cycles of growth, maturity, decay and decline seen in the record of history itself where we see complex, albeit not scientific, societies emerge, grow and decay as a matter of record. All complex societies as they growing inevitably require more resources, more specification of the roles of participants leading to less freedom and ultimately more methods of taxation to continue the standard the civilization reaches at optimum once resources have been exhausted beyond a certain critical point.

The scientific method is the basic definition of what science is as a function of acquired knowledge. However, as scientific culture inevitably grew more complex, so too did specialization emerge and in the vernacular use of the term science really means many different things to different people be it right or wrong. At the very least, to some science is experimentation, to others theory. Moreover in ordinary comprehension science can be presented as popular science or often as simply technical wizardry. Computer science for example is in reality engineering but to very many people computer technology is a science as an example, the same can be said of social science.

More troubling is that in this age the products of science, namely technical gadgets, are often so much lauded and are in so much abundance that they begin to eclipse the methods of science that produced them. Often technology produced by industry is used as a kind of logo equated with the "benevolent" visions of science. To the appearance of many therefore science and industry have merely become one and the same. Therefore, we might ask ourselves, who benefits from what is seen in the modern sense as "Science".

In much of my own experience over the last few years working in research and industry, science and physics in particular, appears to be mainly functioning as at best as a service for industry. Very little science is done that do not have direct applications to industry and the marketplace. In effect science has all but become a research and development wing for corporations, with very few exceptions.

The corporations involved with making profit from science have it very much their own way, with very little risk, which is one of the main advantages of corporate structure to begin with. As mentioned before, the banner of "science" being self-evidently benign is a strong dogma in the minds of the public and politicians with very few exceptions. Therefore many corporations can conduct their "scientific" R&D operations under a kind of saintly halo unless costly and time-consuming investigations are launched, which are rare.

Moreover, the image of science being a benign enterprise leads governments to directly fund scientific research out of taxpayer funds. Hence corporations can easily use taxpayer funding as a continuous resource to garner future profits for themselves based on new discoveries and new techniques painstakingly generated in the lab often by highly intelligent, but often institutionalized, hardworking scientists.

Furthermore, the apparently self-evidently benign image and prestige of scientists shown in the media also leads to a demand on universities to educate more in the STEM (science, technology, engineering, maths) without questioning why it is better to educate a young person to spend their time studying one subject over another without invoking the condition that "Subject A is more useful than subject B" -one question being "more useful to whom and to what?" - It is of obvious benefit to high tech multi-national corporations; The STEM educational system trains and educates their future workers and ensures future profit keeps flowing.

Opportunity of employment and empowerment with STEM, undoubtedly true, is often cited as the main reason why STEM is so popular a topic to discuss in the context of education. Interestingly though there is always an increasing demand for STEM graduates and a bemoaning of large dropout rates, particularly among engineering majors. If there is such a demand for STEM educated workers from the side of corporations above what students want to study and are capable of studying in college then it raises suspicions as to which party the educators are working in the best interests of.

Much of the modern scientific enterprise therefore has a troubling amount of cyclical reasoning behind it. Science is funded by working taxpayers, those workers are encouraged to study STEM by taxpayer funded education, the workers operate in STEM fields earn a salary which is then garnished to provide funding for future workers and developments. The future developments are real and do benefit people, but it can be a selective few. After all, it is still a market system that modern science and modern technology exists in and profits they make are increasingly in the hands of the many over the few. There are many in the over-exploited regions of the world today that not only do not benefit largely from much of the scientific progress but are burdened disproportionately by the consequences of pollution and the weapons systems science has created in a not-so-neutral fashion.

By and large we might live more prosperously than our ancestors have thanks to modern scientific developments but we have carried with that prosperity an enormous burden and put a tremendous strain on the limited resources of both fragile humans and the Earth to accomplish this. We might all have to ask ourselves, is science really beneficial to everyone equally?