TFE4188 - Lecture X

Energy Sources

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Goal

Why do we need energy sources?

Introduction to Energy Harvesting

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Why

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Lithium Battery

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Energy density \(\Rightarrow [250,693]\text{ }mWh/cm^3\)

1 year \(\Rightarrow\) \([29,80]\text{ }\mu\)W/cm\(^3\)

10 year \(\Rightarrow\) \([2.9,8]\text{ }\mu\)W/cm\(^3\)

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Thermoelectric

Photovoltaic

Piezoelectric

Electromagnetic

Triboelectric

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Thermoelectric

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Seebeck coefficient

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12

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Radioisotope Thermoelectric generator

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Thermoelectric generators

In A 3.5-mV Input Single-Inductor Self-Starting Boost Converter With Loss-Aware MPPT for Efficient Autonomous Body-Heat Energy Harvesting [@bose21] they use a combination of both switched capacitor and switched inductor boost.

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Photovoltaic

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I_D = I_S\left(e^\frac{V_D}{V_T} - 1\right)

I_D = I_{Photo} - I_{Load}

V_D = V_T ln{\left(\frac{I_{Photo} - I_{Load}}{I_S} + 1 \right)}

P_{Load} = V_D I_{Load}

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#!/usr/bin/env python3
import numpy as np
import matplotlib.pyplot as plt

m = 1e-3  
i_load = np.linspace(1e-5,1e-3,200)

i_s = 1e-12  # saturation current 
i_ph = 1e-3  # Photocurrent

V_T = 1.38e-23*300/1.6e-19  #Thermal voltage

V_D = V_T*np.log((i_ph - i_load)/(i_s) + 1)

P_load = V_D*i_load

plt.subplot(2,1,1)
plt.plot(i_load/m,V_D)
plt.ylabel("Diode voltage [mA]")
plt.grid()
plt.subplot(2,1,2)
plt.plot(i_load/m,P_load/m)
plt.xlabel("Current load [mA]")
plt.ylabel("Power Load [mW]")
plt.grid()
plt.savefig("pv.pdf")
plt.show()

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ANYSOLAR

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In A Reconfigurable Capacitive Power Converter With Capacitance Redistribution for Indoor Light-Powered Batteryless Internet-of-Things Devices [@cheng21] they include a maximum power point tracker and a reconfigurable charge pump to optimize efficiency.

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Piezoelectric

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An example of piezoelectric energy harvester can be found in A Fully Integrated Split-Electrode SSHC Rectifier for Piezoelectric Energy Harvesting [@du19]

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Electromagnetic

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"Near field" harvesting

Near Field Communication (NFC) operates at close physical distances

Reactive near field or inductive near field

\text{Inductive} < \frac{\lambda}{2 \pi}

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Standard	Frequency [MHz]	Inductive [m]
AirFuel Resonant	6.78	7.03
NFC	13.56	3.52
Qi	0.205	232
Bluetooth	2400	0.02

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Ambient RF Harvesting

Extremely inefficient idea, but may find special use-cases at short-distance.

Will get better with beam-forming and directive antennas

AirFuel RF

dBm	W
30	1
0	1 m
-30	1 u
-60	1 n
-90	1 p

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Assume \(P_{TX}\) = 1 W (30 dBm) and \(P_{RX}\) = 10 uW (-20 dBm)

D = 10^\frac{P_{TX} - P_{RX} + 20 log_{10}\left(\frac{c}{4 \pi f}\right)}{20}

Freq	\(20 log_{10}\left(c/4 \pi f\right)\) [dB]	D [m]
915M	-31.7	8.2
2.45G	-40.2	3.1
5.80G	-47.7	1.3

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Triboelectric generator

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Take a look in A Fully Energy-Autonomous Temperature-to-Time Converter Powered by a Triboelectric Energy Harvester for Biomedical Applications [@tan21] for more details.

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Comparison

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Energy source	Power density	Frequency	Characteristics
Solar / PV	10 uW/cm\(^2\) indoor, 15 mW/cm\(^2\) outdoor	DC	Requires exposure to light
RF	0.1 uW/cm\(^2\) GSM, 0.01 uW/cm\(^2\) WiFi	380 MHz--5 GHz	Poor indoors and out of line of sight
Thermal, body heat	40 uW/cm\(^2\)	DC	Requires a high temperature difference
Piezoelectric	4 uW/cm\(^2\)	> 30 Hz	Not limited to indoors or outdoors
Triboelectric (TENG)	1 uW/cm\(^2\)	1 Hz	Not limited to indoors or outdoors

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Summary

Harvesters deliver power, batteries deliver energy: a harvester design starts from the average load current, not the peak
Thermoelectric generators give millivolts per kelvin of gradient - real designs start below 100 mV and need a boost converter that can cold-start from that little
A photovoltaic cell is the photodiode from this chapter run in the fourth quadrant: half a volt a cell, current proportional to light
Piezo and electromagnetic harvesters turn vibration into AC that must be rectified; triboelectric is the same story with contact charging
Ambient RF sounds free until Friis has spoken: microwatts at best, and only near the transmitter
Every source needs power management sized to its impedance - maximum power transfer is the whole game

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Would you like to know more?

[1] Towards a Green and Self-Powered Internet of Things Using Piezoelectric Energy Harvesting [@shirvanimoghaddam19]

A 3.5-mV Input Single-Inductor Self-Starting Boost Converter With Loss-Aware MPPT for Efficient Autonomous Body-Heat Energy Harvesting [@bose21]

A Reconfigurable Capacitive Power Converter With Capacitance Redistribution for Indoor Light-Powered Batteryless Internet- of-Things Devices [@cheng21]

A Fully Integrated Split-Electrode SSHC Rectifier for Piezoelectric Energy Harvesting [@du19]

Current progress on power management systems for triboelectric nanogenerators [@hu22]

A Fully Energy-Autonomous Temperature-to-Time Converter Powered by a Triboelectric Energy Harvester for Biomedical Applications [@tan21]

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Thanks!

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