1. Class 3: Energy Harvesting
Designing Low Power Systems using Battery and Energy Harvesting Energy Sources
Class 3: Energy Harvesting
May 6th, 2015 Warren Miller

2. Course Description
Low power systems are all around us and use is exploding- IoT, wearable devices, handheld, energy harvesting, etc Understanding how to implement low power systems, from batteries or energy harvesting, will be a key skill for engineers in these growing markets. This course will provide sufficient background to get you familiar with the key concepts, techniques and devices needed for the next generation of low power systems.

3. This Week’s Agenda
5/4/15 An Introduction to Low Power Systems 5/5/15 Battery Power for MCUs and FPGAs 5/6/15 Energy Harvesting 5/7/15 Low Power MCUs and FPGAs 5/8/15 Example Designs

4. Today’s Class Goals and Objectives Energy Harvesting- Sources
Energy Harvesting- Management
Energy Harvesting- Application fit
Energy Harvesting with FPGAs and MCUs
Design Techniques
Resources

5. Today’s Goals and Objectives
Understand the key considerations when using energy harvesting to power MCUs and FPGAs
Types of energy harvesting sources
Design considerations for MCUs and FPGAs
Typical designs for managing and controlling energy harvesting in common systems

6. Energy Harvesting Solar (Photovoltaic) Thermal (TEG)
2W peak for a 160cm2 surface area
Thermal (TEG)
Thermal to Electrical or Visa Versa
Pressure/Vibration (Piezoelectric)
5mW for .1 in displacement at 75Hz 16 grams
Air/Fluid Flow
Fans in reverse!

7. Solar Energy Solar Cell Operation (Simplified)
Photons eject electron/hole pairs
Mobile charges move to electrodes
Current flows to cancel the generated potential difference
Power Generation
Single Junction Efficiency Limit= 33.7%
Infinite Junction Limit= 86%
25% is possible (Panasonic)
"Silicon Solar cell structure and mechanism" by Cyferz at en.wikipedia
The solar cell works in several steps:
Photons in sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon.
Electrons are excited from their current molecular/atomic orbital. Once excited an electron can either dissipate the energy as heat and return to its orbital or travel through the cell until it reaches an electrode. Current flows through the material to cancel the potential and this electricity is captured.
An array of solar cells converts solar energy into a usable amount of direct current (DC) electricity.
An inverter can convert the power to alternating current (AC).
The most commonly known solar cell is configured as a large-area p-n junction made from silicon.
"Solargif1" by Freshman404
"FX-77" by Sergei Frolov

8. Solar Applications: Micro-inverter
Smaller than central inverters- individual per panel
Usually around 300W (72 cells), invert to AC
MCU control is key, many optional external components
Micro-inverters operate similarly to central inverter systems, but are installed on each individual panel and handle much less power, typically 300 W. Micro-inverters provide the benefit of scalability for those who want to start small, yet have full DC/AC conversion with MPPT, and expand later.

9. Solar Applications: Micro-converter
Single panel, convert to DC level required
MCU control is key, many optional external components
Micro-converters maximize the DC power point of a single solar panel and convert (down or up) the DC voltage to be transported downstream to a centralized AC (grid-tied) inverter. Being located on each panel, these systems are lower power (typ. 300 W) than centralized converters. These are sometimes called “optimizers” because they optimize the power of each panel, increasing the overall efficiency of the system.

10. Solar Power Management
Battery Charging via Solar Power
MCU can be connected as ‘Load’
Battery provides power when no solar energy
MCU can manage solar panel, battery, logging, etc.
MCU can manage other subsystems, communicate to host

11. Solar Application Example
Battery Charger (MCU control, Charger IC, Boost Converter)
Off grid systems
MCU control is key
Optional external components
When is energy storage needed?
Off-grid solar power systems often need to charge a battery, or array of battery cells, that provide continuous power to the load when solar energy is no longer present. Often cost sensitive, in order to optimize the size, cost and usable power of the storage elements, off-grid systems also require that the power point be maximized. However, this can be done by employing, lower power and less complex MCUs than grid-tied systems or by employing a simple fixed power point – often set at 76% of VOC. Loads such as LED lighting and motors may require additional power boosting and/or control.
MPPT (Maximum power point tracking) Kit:
Kit

12. Thermal Energy Source Thermal Electric Generation (TEG)
Thermal to Electrical (Seebeck effect)
Pairs of N and P doped pellets (Bi2Te3)
Output: 10mV/K to 50mV/K
Source Resistance: 0.5 to 5 Ohm
Load matching, max(Vmax x Imax)
15 to 30uW/K-cm2
Thermal Considerations
TEG BASICS
Thermoelectric generators (TEGs) are simply thermoelectric modules that convert a temperature differential across the device, and resulting heat flow through it, into a voltage via the Seebeck effect. The reverse of this phenomenon, known as the Peltier effect, produces a temperature differential by applying a voltage and is familiarly used in thermoelectric coolers (TECs).The polarity of the output voltage is dependent on the polarity of the temperature differential across the TEG. Reverse the hot and cold sides of the TEG and the output voltage changes polarity.
TEGs are made up of pairs or couples of N-doped and P-doped semiconductor pellets connected electrically in series and sandwiched between two thermally conductive ceramic plates. The most commonly used semiconductor material is bismuth-tellurium (Bi2Te3). Figure 4 illustrates the mechanical construction of a TEG.
Some manufacturers differentiate between a TEG and a TEC. When sold as a TEG, it generally means that the solder used to assemble the couples within the module has a higher melting point, allowing operation at higher temperatures and temperature differentials, and therefore higher output power than a standard TEC (which is usually limited to a maximum of
125°C). Most low power harvesting applications do not see high temperatures or high temperature differentials.
TEGs come in a wide variety of sizes and electrical specifications. The most common modules are square, ranging in size from about 10mm to 50mm per side. They are usually 2mm–5mm thick.
A number of variables control how much voltage a TEG will produce for a given ∆T (proportional to the Seebeck coefficient). Their output voltage is in the range of 1 0 mV/K to 50mV/K of differential temperature (depending on the number of couples), with a source resistance in the range of 0.5Ω to 5Ω. In general, the more couples a TEG has in series, the higher its output voltage is for a given ∆T. However, increasing the number of couples also increases the series resistance of the TEG, resulting in a larger voltage drop when loaded. Manufacturers can compensate for this by adjusting the size and design of the individual pellets to preserve a low resistance while still providing a higher output voltage.

13. Thermal Energy Harvesting
LTC3108 Power Converter/Manager
Step-up Transformer plus Internal MOS FET (Resonant Oscillator)
20mv start-up with 1:100 transformer ratio
VAUX via
Charge pump and Rectifier
2.2V LDO for MCU
Stable first
VOUT on COUT
Programmable level
Sensors, analog, RF, supercap or battery
Current reservoir for energy bursts
VOUT2: Controlled Output
Power Good

14. Thermal Application Example
Wireless Sensor Application (MCU, Sensors, RF Link)
LTC3108 Power Converter/Manager
Harvest and convert
Power storage
Power to MCU
Fast
Power to sensors
Controlled
Power to RF link
Power bursts needed
MCU
Sensors, Communication, Logging, Encryption, Compression, etc.

15. Piezoelectric Effect Material deformation results in a voltage
Works in reverse too
Electric Dipole Moment Change
Examples
Guitar pick-up, drum pads
RPG fuse
Engine control, machine sensors
Ultrasonic generator
Motors (Micromo, etc)
Lead Titanate

16. Vibration Vibration (Piezoelectric) Source vibration characteristics
5mW at 75MHz
.1 in displacement with 15.6 grams
Source vibration characteristics
Known?
Measure and FFT
Tuning mass
Ring out
Power level

17. Piezoelectric Management
LTC Energy Harvesting Supply
High Output Impedance energy sources
Piezoelectric, Solar, Magnetic
Sub 1uA Quiescent current
Up to 100mA output
Selectable voltages- 1.8/2.5/3.3/3.6V

18. Resources

19. Additional Resources
Microchip XLP Technology ST Micro Ultra Low Power MCUs Silicon Labs Battery Operated MCUs TI Ultra Low Power MCUs RL78 MCUs Renesas RL78 Low Power Evaluation Kit Lattice iCE40 Ultra FPGAs Microsemi IGLOO2 FPGAs WE Energy Harvesting Brochure WE Reference Solution Kits CEC Course- Low Power MCUs

20. This Week’s Agenda
5/4/15 An Introduction to Low Power Systems 5/5/15 Battery Power for MCUs and FPGAs 5/6/15 Energy Harvesting 5/7/15 Low Power MCUs and FPGAs 5/8/15 Example Designs