Energy harvesting for a smart sensor with NFC capability

Autor/Author: Javier Molina
Director/Supervisor: Guillermo Robles
Master Thesis Document in pdf.

 

Abstract – Modern life’s concerns regarding unnecessary energy wasting and the unstoppable development of electrical engineering gave birth to the concept of energy harvesting. All this, along with an overwhelming number of internet connected devices, make necessary new smart devices to make easier our lives not only at home but also in industrial environments. Throughout this project, the feasibility of using a Peltier cell as a thermoelectric generator is discussed in order to scavenge energy from a heat source. This project aims at using this system in dicult access locations to create a smart sustainable system that can keep track of relevant parameters such as temperature, pressure or radiation. By implementing this self-powered system, there is no need to replace batteries when fully discharged, it is only necessary collect the data when required. In particular, this Peltier cell supplies an energy harvester module that powers a standalone microcontroller to establish a communication with a NFC module. This device embedded with a NFC tag will store the parameters measured by a sensor. This novel approach is intended to allow any NFC enabled device such as any modern smartphone to access this data to be subsequently analised and take action when needed.

 

Resumen – Las preocupaciones de hoy en da con respecto al consumo abusivo energetico sumado al gran desarollo reciente de la ingeniera electrica y electronica han dado como fruto el concepto de energy harvesting. Ademas, el mundo en el que vivimos con un mayor numero de dispositivos conectados a internet hacen necesario dispositivos inteligentes para facilitar nuestras vidas, no solo en casa, si no tambien en el entorno industrial. En este proyecto se expone la viabilidad de usar una celula Peltier que es un dispositivo termoeléctrico para proporcionar energa a partir de una fuente de calor. Este proyecto persigue usar este sistema en sitios de difcil acceso y crear un sistema sostenible que lleve a cabo un sistema de recogida de datos, como temperatura o presion. La ventaja que ofrece un sistema como este es que no es necesario cambiar la batera, puesto que el sistema se autoalimenta. Concretamente, la celula Peltier suministra energa a un modulo de almacenamiento que establece una comunicacion con un modulo NFC. Este dispositivo contiene una etiqueta NFC que almacena los datos recogidos por un sensor. Este enfoque permite a cualquier operario con un dispostivo que permita la lectura de etiquetas NFC, como por ejemplo cualquier smartphone moderno, acceder a estos datos para analizarlos y tomar decisiones si es necesario.

 

Energy harvesting and NFC tag (LTC3108 -> STM32L433 -> M24SR)

The idea behind this work was to test the capabilities of using a near-field communication (NFC) tag to store the information acquired through an analogue input of a microprocessor powered by an energy harvesting source.

The setup includes these components:

  • Peltier cell
  • Energy harvesting system
  • Microprocessor
  • Dynamic NFC/RFID tag IC
  • Temperature sensor

Energy harvesting system

The capabilities of Peltier cells to harvest energy from differences of temperature between its two sides has already been studied in other posts starting with this link, so I will not develop this part of the work here.

The energy harvesting system used in this project is now based on the outstanding Linear Technologies (now part of Analog Devices) Ultralow Voltage Step-Up Converter and Power Manager LTC3108. This device can work with four selectable output voltages: 2.35 V, 3.3 V, 4. V or 5 V to power wireless transmitters or sensors and a low dropout voltage regulator output (VLDO) to power an external microprocessor. According to its datasheet it can start harvesting energy from voltages as low as 20 mV which is precisely indicated for applications that use thermo-electric generator (TEG) such as Peltier cells. The energy is stored in a bank of supercapacitors connected to two outputs of the LTC3108. Two 1 F supercapacitors in series are connected to VOUT and charged when VAUX has reached 2.5 V. Another two 1 F supercapacitors are connected to VSTORE supporting VOUT and preventing an unexpected drop of voltage due to a high power demand by the load. A picture of the setup for this integrated circuit (IC) is:

Continue reading Energy harvesting and NFC tag (LTC3108 -> STM32L433 -> M24SR)

Characterization of Peltier cells for energy harvesting applications (III)

As demonstrated in the former post, the equivalent voltage source of the cell depends on the temperature difference of the surfaces and takes a value of V_o = 0.0245 \cdot \Delta T and the internal series resistor is R_s = 2.24~\Omega. Therefore, there would be different power outputs considering the resistor load and the temperature difference. The next plot shows the delivered power to a set of loads and four temperature differences \Delta T =[5,~10,~15,~20] degree Celsius. The maximum power given by the cell is delivered to a load that equals the internal resistor, R_s=R_L and takes a value of:

P_{max}=\frac{V_o^2}{4R_L}&s=1 W

potencias.png

If a difference of temperatures of 20 ºC is achieved the voltage at the load would be 245 mV and it would draw a current of 109.4 mA, the maximum power would reach 26.8 mW when connecting a load of 2.24~\Omega. Of course, all these data are hypothetical since the assumptions are in the most optimistic side considering that R_s=R_L. Even under these circumstances a voltage booster would be needed to increase the voltage to a level according to the requirements of the MCU. For instance, the ultralow power STM32L432 ARM Cortex M4 requires at least a power supply of 1.71 V. There are two options to increase the voltage, using voltage multipliers or using DC-DC converters.

Voltage multipliers

These circuits use a combination of diodes and capacitors that allows to duplicate the voltage at the input in every stage. A common setup is the Cockcroft-Walton configuration as the shown in this paper to multiply the voltage obtained from events that create pulses that can reach peaks of 1 V or more. In the case of one or two Peltier cells connected in series, this circuit is out of the question since the Schottky diodes with the lowest forward voltage drop are close to 250 mV so they would consume the voltage provided by the cell or cells.

Cockcroft-Walton_Mod.png

Voltage boosters

Voltage boosters or DC-DC step-up converters would be the most feasible solution. The working principle is easy. The inductance L is charged closing switch S storing a magnetic field. This field will maintain the current flowing towards load R when S is opened. Since the inductance is giving energy to the load the voltage at L is effectively reversed and added to v_i(t) increasing the voltage at the output, v_o(t). The switching should be done fast to avoid a total magnetic discharge of the coil when S is open and a total depletion of capacitor C when S is closed. The diode D prevents the capacitor from discharging through S.Booster.png

This idea has been implemented in integrated circuits (IC) that scavenge small quantities of energy from the source, in our case the Pletier cell, to drive the switch and are able to increase the voltage at the output upto 3.3 V or 5 V depending on the MCU connected. Some examples of these IC and their behavior under real conditions are shown in the next post.

 

 

Characterization of Peltier cells for energy harvesting applications (II)

With the setup described in the last post, the temperature of one of the surfaces of the cells can be controlled with an electrical current, the other surface is cooled passively with a heatsink. The resistors and the two temperature sensors are connected to the same voltage source taking advantage of the wide range of voltages supported by the LM35. The outputs of the LM35 are connected directly to two multimeters to measure the differences of temperature achieved between the two surfaces. Now, the process is easy: the resistors are heated with different currents and the output voltages of the cell and the temperatures are registered. This will solve the voltage source of the equivalent electric circuit of the cell in open circuit or the Thévenin voltage. The results are represented in the next Figure showing a perfect linearity between temperature and voltage.

VthVsT.png

I_s = [0.49~0.59~0.69~0.79] A
V_s = [12.3~14.6~17.3~20] V

V_o = [62.4~140.5~252.5~414~580~775] mV

T_h = [24.8~30.2~38.4~55.0~66.5~82.7] degree Celsius
T_c = [22.5~24.5~28.0~38.0~42.9~51.0] degree Celsius

Where, I_s and V_s are the current and voltage applied by the power supply to the resistors, respectively; V_o is the output of the cell and T_h and T_c are the temperatures of the hot and cold sides of the cell, respectively. The plot shows that V_o = 0.0245 \cdot \Delta T with the slope in V / ºC.

The next test will determine the equivalent series resistance, R_s, of the cell loading it with a known resistor, R_L = 13.5~\Omega. In this case, the current given by the cell provokes a voltage drop in the internal resistor R_s so the voltage applied to R_L, V_L, is smaller than the open circuit voltage, V_o > V_L.  A new set of measurements is conducted injecting the same current to the heating resistors to determine this voltage and, then, the internal resistance of the cell:

V_L = [51.9~121.1~ 211.7~ 344.1~ 486~ 639] mV

T_h = [27.6~ 33.2~ 40.7~ 53.0~ 65.0~ 79.3] degree Celsius
T_c = [25.5~ 27.7~ 30.7~ 36.7~ 41.9~ 48.6] degree Celsius

VLVsT

The blue plot represents the voltage in open circuit, V_o, and the red plot the voltage at the load, V_L. Dividing this voltage by the load resistor yields the current given by the cell, I_L. Therefore, the internal resistor is calculated applying Ohm’s Law knowing that the voltage drop is V_o - V_L and the current is I_L. This is done for several points along the experimental results in the plot giving a constant value for the resistor, R_s = 2.24~\Omega. The same process is repeated for another cell of the same type and the result for R_s differ from the first cell giving R_s = 4.75~\Omega. Even when the former internal resistor is double the latter one, the results have been re-checked and confirmed and are in agreement to other results found in the literature.

Once the cell has been characterized, it is possible to determine the current that it will give to a known load. The next step is to devise a method to store the energy delivered by the cell or to explore the possibility of boosting the voltage to drive an MCU (microcontroller unit) directly. This will be explained in the next post.

Characterization of Peltier cells for energy harvesting applications (I)

Módulo Peltier, 32.8W, 6A, 8.8VPeltier cells are usually applied to cool surfaces when connected to an electric power supply but they can also convert differences of temperature between their sides into a voltage, known as the Seebeck effect. Therefore, it is possible to have a voltage at the ends of the wires of a Peltier cell by applying heat to one of the sides and attaching a heatsink to the other side. In terms of energy harvesting, the heat should come from a residual source such as an electrical or mechanical machine or, simply, the sun using Fresnel lenses. The cooling of the other side should be passive to minimize the energy consumption, hence the use of a heatsink. It is important to know how much energy can be obtained with a single cell as a function of the difference of temperatures, for this reason, the characterization of the cell must be the first step in the design of applications scavenging energy.

20170324_162157.jpgThe mounting scheme is shown in the side Figure where the heatsink is clearly seen on top of the cell. This is hidden by some pieces of thermal insulating foam but the two wires are visible. Finally, an aluminium plate has been adhered to the cell with a thermal conductive bonding paste.

The aim of this work is to obtain the Thévenin equivalent of an Adaptive ETH-071-14-15 Peltier cell but the process is valid for any other type of cell. The datasheet details the working curves when the cell is used as load but nothing is said about its characteristics when used as a source. The characterization requires the application of a known difference of temperatures, ΔT, and this is achieved injecting current to a pair of ceramic resistors  of 47 Ω in parallel attached to the aluminium plate with silicone.

20170324_162012

20170324_161932_001Two LM35 temperature sensors read the temperature of the aluminium plate and the heatsink. Notice that, once the plate and the heatsink are attached to the cell, its sides are no longer reachable so the sensors have to be connected to the closest surfaces to the cell. There will be an uncertainty in the measurements but we can asume that it is negligible or, at least, that it affects the two sensors in the same manner so the difference of temperatures is the same as in the surfaces of the cell. The mounted cell is enclosed in methacrylate box coated with thermal insulating panels ensuring that, once a constant difference of temperatures, ΔT, is achieved, it is maintained along the experiment so the output voltage can be directly related to the selected ΔT.

Next: Measurements.