Thermo_Print

Human leaves an imprint on earth, in the form of energy. The project Thermo_Print aims to collect data on the energy leaving the body in motion, and propose a way to use this energy to control processes in a negative feedback loop.

Background:

Heat flux from the body in the future might be the mechanism to energize systems like watches, pulse-sensors, pacemakers etc. The motivation for this project is to control small motors, like a small fan, LED-lights amongst some, in an art installation, where the prime mover is the person in the installation. The purpose and goal is to instigate a discussion around the boarders of human and machine.

Method:

The heat flux is measured as Watts per square meter.

In order to measure the heat flux we set up a set of sensors connected to a micro-processor, myRio, this again is connected to the PC. We wrote a LabView code in order to detect the data. The code consist of a while loop that gathers data every 100 ms. In the loop the required temperature correction for the sensitivity of the sensor is embedded. There is also real-time graph window, depicting the heat flux as it is happening, and an additional graph that compiles the results when the loop is ended. There is also a slider for the user to manually feed the room temperature to enhance the accuracy of the measurements.

For the measurements we used gSKIN heat flux sensor, from greenTEG. With a precise accuracy, 3 /W, the heat flux is measured. The smallest sensors area are  4,4mm X 4,4mm and 0,5mm thick.

IN A BROADER SENSE

Considering that more than 60 % of energy produced is wasted as heat, new methods for harvesting energy is important for the future. The human body contains huge amount of energy.  Energy harvesting from human movement have drawn attention from researchers for decades. The electrical energy produced can be used to power wearable electronics, for example, a watch and a heart rate monitor. The first prototype of an electronic device powered by human movement is an electronic watch developed by SEIKO in 1986. One have also mounted energy harvesters on sneakers that generate energy from pressure on the foot sole. There are also two types energy harvesting from backpacks to generate electrical energy from backpacks. One utilizes linear vertical movement of the backpacks to generate electrical energy and the other is based on stress on the strips of the backpacks. [1]

Researchers at the University of Michigan have developed a device that harvests energy from the reverberation of heartbeats through the chest and converts it to electricity to run a pacemaker or an implanted defibrillator, hopefully obviating the need for periodic battery replacement. Research is also under way looking for ways to scavenge body heat, movement, and vibration to power other implantable devices [2].

The Sensors

A Heat Flux Sensor is a Seebeck Sensor. It is based on thermoelectric principles. The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa. A thermoelectric device creates voltage when there is a different temperature on each side. Conversely, when a voltage is applied to it, it creates a temperature difference. At the atomic scale, an applied temperature gradient causes charge carriers in the material to diffuse from the hot side to the cold side.

When diffusion is happening, we have charge separation, and whenever there is charge separation there is a potential. This potential, or to be more precise, potential energy, is what we know as voltage. Thus; when heat passes through the sensor, the sensor generates a voltage signal. This voltage signal is proportional to the heat passing through the sensor.

The type of sensors we used for our experiment come with high accuracy, and can resolve heat fluxes down to  0.01 W/m2.

The sensitivity of a Seebeck Sensor depends on the thermocouple material quality used in the sensor and the number of thermocouples used.

Most thermocouples are made of metal alloys, exploiting the voltage difference between them to detect temperature difference. This is proven to be a reliable method for measuring temperature.

In the Thermo-Print project we want to measure the heat flux from the body, and use voltage difference to drive a component. To do that we need a very sensitive measuring device; a thermocouple that consists of two separate thermopiles (n-type and p-type). The thermopiles in gSKIN® Heat Flux Sensors are based on BiTe (Bismuth Telluride). They have a high seebeck coefficient and makes them sensitive enough to measure heat flux.

The heat flux sensor are mounted to the body with an adhesive tape, and the flux detected can be reflected.

The heat flux  describes the transfer of thermal power per surface unit and is calculated using the following formula:

=U/S [W/m^2]

U is the sensors output voltage, S is the temperature-corrected sensitivity of the sensor [3].

The future:

The further development of this project would be to collect a big variety of data to determine which method can be successfully implemented in a negative feedback control loop for a control system. Our hope for the project is that it can be a part of the of hoe to harvest energy in the future

Harvesting body heat is just one example: instead of producing energy, it would simply collect energy from a passenger's seat, and redirect it to power certain aircraft functions – such as the cabin lights. 

While such concepts may sound futuristic, a proposal several decades ago for Airbus’ double-decker A380 – which has the capacity to carry more than 800 people and the efficiency of a family car – could have sounded equally fanciful [3].

References:

1.       Human movement and flow induced Vibration, Dibin Zhu, University of Southampton, Sustainable Energy Harvesting Technologies – Past, Present and Future, Edited by Yan Kheng Tan.

2.       http://eu.mouser.com/applications/energy-harvesting-new-applications/

3.       https://www.greenteg.com/

4.       http://www.airbus.com/innovation/future-by-airbus/future-energy-sources/energy-harvesting/