Energy Harvesting Estimates

TEG produces an electric power from heat flows across a temperature gradient. As the heat flows from hot to clod side free charge carriers in the material are also driven to the cold end. The resulting voltage is proportional to the temperature difference (ΔT) via the Seebeck co-efficient ‘α’ (V = α ΔT). A good thermoelectric material has a Seebeck co-efficient between 100µV/K to 300µV/K. Thus in order to achieve a few volts at the load many thermo electric couples need to be connected in series to make the thermo electric device. Series connection of TEG’s increases the output resistance of the generator.

Photovoltaic cells, TEGs (thermoelectric generators) and fuel cells are high impedance alternative power sources.

Output resistance of the TEG (Thermo Electric Generator) is very high (more than 10KΩ).

The key feature that you need to look for in the power converter that you will be using the charge the super cap is soft start as this will manage the inrush current.

Calculating the watt hour of a capacitor

To begin, what is measured in Farads is capacitance C. What is measured in Ah is the charge that can be stored in a battery or a capacitor.

From the definition of capacitance, the charge on the walls of a capacitor with capacitance C and potential difference V is

Q = CV

So you obtain the value you're interested in. However, if C is in Farad and V is in volts, q will be measured in Coulombs.

Ampere = Coulomb / Second

So

1 Ampere hour = 1X (Coulomb/Second) X hour = 3600 Coulombs

Ampere hour to watt hour conversion

1 watt hour = 1 Ampere hour X Voltage

Requirements

1) What should be the output voltage of the harvester?

18V to 3.3V

2) What is the peak current requirement? (Under Pulsed Load)

50mA

3) What is the average current requirement?

1uA

4) Where the module will be used? (Available energy sources)

Automobiles, Human wearables.

5) Optimal size.

6.25 Cm2

6) Is there any Weight Constraint?

7) What is the cost constraint?

Duty Cycle of operation is given as 2%.

Consider period as one day. Then the system is in active mode for 28.8 minutes, in sleep mode for 1411.2 minutes.

System operates in 5 operating modes.

1. Deep sleep mode (5uA)2. Hibernate mode (2uA)3. Dormant mode (900nA)4. Idle mode (1mA)5. Active mode (16mA)

It is given that while sending the data module consumes a current of 50mA and in idle mode consumes 1mA @3.3V.

Super Capacitor Selection.

Current = 50mA

Voltage = 3.3V

Energy Stored in a capacitor = 12CV 2

---- Joules or Watt Seconds

(Ampere x hour) x Voltage = (Watt x hour) = Joules X 60

= 12

(CV 2 ) x60

(50mA x 28.8/ 60) x 3.3V = 12C x 3.32 x 60

C = 0.8727 Farads.

Alternatively

An approximate time can be calculated from the following expression:

T=[C x (V o−V min−V Drop ) ]

I Seconds

C : Double layer capacitor capacitance (Farad)

Vo : Voltage charged in double layer capacitor (VDC)

VDrop : Voltage drop by DC resistance with double layer capacitor (VDC)

Vmin : Minimum required voltage for backup circuit (VDC)

I : Backup current (Amp)

The voltage drop is determined by the DC resistance of the capacitor and backup current VDrop

(VDC) = DCR * I … DCR = Ω & I = A

Example #1: 1.0F 3.3VDC, DCR = 40mΩ, DMF4B5R5G105M3DTA0

Circuit Back-up Current; I = 50mA

VDrop = DCR * I = 40m * 50mA = 0.002

Vo= 3.3V

Vmin = 1.8V

Back-up Time (T) = 30 seconds (0.5 minutes)

In our case:

Back up time required (T) is 28.8 seconds.

DCR0.04 1 3.3 1.5 0.01 0.0004 179.960.05 2 3.3 2 0.02 0.001 129.9

0 #DIV/0!0 #DIV/0!0 #DIV/0!0 #DIV/0!

Capacity in Farads

Charged Voltage

Minimum Discharged Voltage

Discharged Current

Voltage drop in DCR

Discharge Time in Seconds

Surveyed super capacitor

http://www.mouser.com/ds/2/315/ABC0000CE8-462878.pdf

EECHW0D226 22F/2.3V

EECHW0D306 30F/2.3V

EECHW0D506 50F/2.3V

EECHW0D706 70F/2.3V

http://www.mouser.com/catalog/specsheets/Maxwell_Technologies_k2_2_85v_ds_3000619en_1.pdf

http://www.yuden.co.jp/productdata/catalog/en/capacitor02_e.pdf

Surveyed regulators/Harvester

TPS61291 DRVT/TPS DRVR (2.25 $ USD)

LTC 3330/LTC3331 (5.78 $ USD)

LTC3107 (6.28 to 7.41 $ USD)

LTC3105 (5.78 $ USD)

LTC3107

Block Diagram

LTC3330

Features:Dual Input, Single Output DC/DCs with Input PrioritizerEnergy Harvesting Input: 3.0V to 19V Buck DC/DCPrimary Cell Input: 1.8V to 5.5V Buck-Boost DC/DC

Zero Battery IQ When Energy Harvesting Source is AvailableUltralow Quiescent Current: 750nA at No-LoadLow Noise LDO Post RegulatorIntegrated Super capacitor BalancerUp to 50mA of Output CurrentProgrammable DC/DC and LDO Output Voltages,Buck UVLO, and Buck-Boost Peak Input CurrentIntegrated Low Loss Full-Wave Bridge Rectifier

Typical Application

Block Diagram

LTC 3105

Features:

High efficiency step-up DC/DC converterLow Start-Up Voltage: 250mV Maximum Power Point ControlWide VIN Range: 225mV to 5VAuxiliary 6mA LDO RegulatorBurst Mode Operation: I= 24µA

Typical Application Diagram.

Block Diagram

LTC3108

Operates from Inputs of 20mVSelectable output voltage of 2.35V, 3.3V, 4.1V or 5V LDO: 2.2V at 3mA

SPV1050 Transformer less thermoelectric generators and PV modules energy harvester High efficiency for any harvesting source Up to 70 mA maximum battery charging current Fully integrated buck-boost DC-DC converter Programmable MPPT by external resistors 2.6 V to 5.3 V trimmable battery charge voltage level (± 1% accuracy) 2.2 V to 3.6 V trimmable battery.

BQ25504

Ultra Low-Power With High-Efficiency DC-DC Boost Converter/Charger

Continuous Energy Harvesting From Low-Input Sources: VIN ≥ 80 mV (Typical)

Ultra-Low Quiescent Current: I Q < 330 nA (Typical)

Cold-Start Voltage: VIN ≥ 330 mV (Typical)

Programmable Dynamic Maximum Power Point Tracking (MPPT)

Integrated Dynamic Maximum Power Point Tracking for Optimal Energy Extraction from Variety of Energy Generation Sources

Input Voltage Regulation Prevents Collapsing input source.

Functional Block Diagram.

Typical application.

Coil craft : LPR6235-752SML (1:100 Ratio) LPR6235-253PML (1:20 Ratio) LPR6235-123QML (1:50 Ratio)

Würth www.we-online 74488540070 (1:100 Ratio) 74488540120 (1:50 Ratio) 74488540250 (1:20 Ratio)