Solar Energy Harvesting Solution for the Wireless Sensor Platform

the UWASA Node

Thomas Höglund

, Reino Virrankoski

and Timo Mantere

Department of Electrical Engineering and Automation, University of Vaasa, FI-65101, Vaasa, Finland

Department of Computer Science, Communications and Systems Engineering Group, University of Vaasa,

P.O. Box 700, FI-65101, Vaasa, Finland

Keywords: Energy Harvesting, Energy Management, Energy Storage, Solar Power Generation, Wireless Sensor

Networks.

Abstract: This paper presents a solar energy harvester and energy management prototype developed for the UWASA

Node wireless sensor platform. The prototype was designed using a modular approach, requiring only minor

hardware modifications in order to allow harvesting from different energy sources. The primary sensor

network application for which the design was developed is wind turbine monitoring. The energy harvesting

prototype and the performance level it enables for the sensor networking are evaluated through experiments,

and methods of optimizing energy harvesting and energy management are discussed.

1 INTRODUCTION

Wireless sensor networks enable a range of

completely new kinds of monitoring and control

applications as a part of the Internet of Things

concept. Even though wireless sensor nodes have

been developed rapidly during the last decade, their

power supply still constitutes a significant

bottleneck for their applicability. Having to service a

wireless sensor node and change its battery can be

prohibitively expensive or difficult due to the

location and means of installation of the sensor

node. This greatly limits the number of feasible

applications in which wireless sensor nodes would

otherwise be perfectly suited for monitoring and

control. Different types of energy harvesting systems

have been developed to overcome this problem. A

common challenge related to them is that the energy

resources they are able to harvest usually enable a

remarkably lower sensor node performance level

compared with powering from a battery without

energy harvesting. This level might not be enough to

fill the requirements of the particular monitoring or

control application.

In this paper we present a solar energy harvesting

solution for the UWASA Node wireless sensor

platform (Yigitler et al., 2010). It was developed as a

part of our wireless automation research activities,

and it is primarily targeted for wireless sensor

network (WSN) applications for wind turbine

monitoring (Höglund, 2014a; 2014b). It would be

beneficial to collect information about different

kinds of forces and vibrations that affect the wind

turbine structures. The dimensions of the wind

turbines used for industrial-scale electricity

generation are so large that energy harvesting

capability is a necessity to make wireless sensor

nodes feasible for monitoring and control

installations. In addition to solar energy, energy

harvesting from vibrations was also considered, and

with small modifications, the developed energy

harvester could be adapted to harvest vibrational

energy.

The rest of this paper is organized as follows:

The UWASA Node wireless sensor platform is

introduced in Section 2. Methods of energy

harvesting are discussed briefly in Section 3 and

general requirements of the energy harvester in

Section 4. The developed energy harvester prototype

is described in Section 5 and the applied solar cell in

Section 6. Maximum power point tracking is

discussed in Section 7. The implemented energy

management and storage is explained in Section 8,

and the system performance is evaluated trough

experiments presented in Section 9. Finally, Section

10 concludes the paper.

Höglund, T., Virrankoski, R. and Mantere, T.

Solar Energy Harvesting Solution for the Wireless Sensor Platform the UWASA Node.

DOI: 10.5220/0005734800500057

In Proceedings of the 5th International Confererence on Sensor Networks (SENSORNETS 2016), pages 50-57

ISBN: 978-989-758-169-4

2 THE UWASA NODE

The UWASA Node, shown in Figure 1, is an open

source wireless sensor node developed by Aalto

University and the University of Vaasa (Yigitler et

al., 2010). It is a modular and stackable platform, the

software and hardware design of which allow it to be

used for different types of applications with minimal

changes to the main architecture. The possibility to

stack different slave boards onto the main board

allows the creation of custom solutions for any

application. In its simplest form, called the basic

stack, only the main module and the power module

are used. These are sufficient to comprise a wireless

sensor node that consists of processors, a wireless

communication interface, peripheral interfaces, and

power management and distribution (Çuhac, 2012;

Virrankoski, 2012).

Figure 1: The UWASA Node with power module.

2.1 The Main Module

The main module of the UWASA Node contains

two processors: one main controller and one radio

frequency controller. The radio frequency controller

can handle all computation and communication in

simple applications, and then the main controller

need not be used. For more demanding applications,

the main controller is preferable. The main

controller is an LPC2378 ARM7TDMI-S-based

high-performance 32-bit RISC microcontroller from

NXP Semiconductors.

2.2 Operating System and Software

The modularity of the UWASA Node is realized by

both the hardware and the software architectures.

The FreeRTOS (Free Real Time Operating System)

was chosen for the UWASA Node in order to enable

real-time operation and preemptive multitasking.

The UWASA Node can thus handle many

communication, measurement and control tasks

simultaneously.

Middleware has been written for the UWASA

Node to provide device drivers and hardware

abstractions that are used to establish a uniform

programming interface for both the main controller

and the radio frequency controller. The same API

functions can thus be used for programming both

controllers. Automated daemons run in the

background, taking care of tasks related to power

management, time synchronization and system

diagnostics (Çuhac, 2012).

2.3 Auxiliary Hardware

The UWASA Node can be connected to a number of

slave modules by using the hardware stack

connectors with a total of 160 pins per module.

These connectors provide all necessary inter-

modular connections for signals and power supplies.

The slave modules can be any peripherals such as

sensors, actuators and drivers.

2.4 Power Source and Energy

Management

The energy management of the UWASA Node is

handled by the power module, which is a separate

module that can be stacked onto the main module.

The power module features dynamic power path

management and is capable of choosing the most

suitable power source and charging a battery if one

is connected and sufficient power is supplied. There

is a battery monitoring chip that accurately measures

current, voltage and temperature. This can be used

for calculating the energy state of the battery and for

measuring the power consumption of different

applications (Çuhac, 2012).

The battery input of the power module is

designed for one-cell lithium ion batteries with a

nominal voltage of 3.7 V. It accepts voltages

between 1.8–4.2 V. A charger input features an

undervoltage lock-out (UVLO) that cuts the power

when the charger voltage is below 3.3 V. During

undervoltage lock-out a very small, but nontrivial

current (tens of milliamperes were measured) is

drawn from the charger input. Similarly, a very

small but nontrivial current flows into the battery

input when the battery voltage is below 1.8 V and

the charger is in short circuit mode. To eliminate this

loss, an external, very low-power UVLO circuit is

proposed for energy harvesting applications.

Solar Energy Harvesting Solution for the Wireless Sensor Platform the UWASA Node

3 METHODS OF ENERGY

HARVESTING

For outdoor WSN applications, such as wind turbine

monitoring, solar energy harvesting is the most

suitable energy harvesting method because of the

good availability of sunlight and the proven

technology of solar cells. Energy harvesters using

sunlight as their energy source can provide power on

the order of 10 mW/cm

under ideal circumstances

(Höglund, 2014a).

The most efficient method of energy harvesting

is always case-specific because of the large

differences in the availability of energy over time

from different sources and locations, and because of

the highly varying power consumption of wireless

sensor nodes (Höglund, 2014a). There are three

methods of energy harvesting that were deemed

feasible for supplying the UWASA Node with

power in wind turbine monitoring applications: solar

energy harvesting using photovoltaic (PV) cells,

vibration energy harvesting using a piezoelectric

cantilever, and wind energy harvesting using a

microscale wind turbine with an electromagnetic

generator. These three methods could also be used in

parallel in a hybrid energy harvester.

3.1 Solar Energy Harvesting

Using a PV cell as the energy source would be a safe

choice, because it is a well-established technology.

Solar cells are readily available in all sizes and in

many different configurations with conversion

efficiencies around 15% (Gilbert and Balouchi,

2008). A suitable number of photovoltaic fingers

should be connected in series in the cell to yield an

optimum nominal output voltage and more fingers

can be connected in parallel to cover the rest of the

available area. The generic 92 × 61 mm, 0.45 W

solar cell sold by SparkFun Electronics (Niwot,

Colorado) is a suitable choice, because its open

circuit voltage is approx. 5 V and its size roughly

matches that of the UWASA Node. If more energy

is required, it is possible to connect more than one

such cell in parallel to the energy harvester, while

still keeping the maximum power point (MPP)

voltage and energy harvesting circuit the same.

PV cells are made for outdoor use and are not

damaged by rain or large temperature changes.

Energy can reliably be harvested from them

whenever the ambient illuminance is sufficiently

high. A suitable harvesting schedule can be

estimated by analyzing weather data to determine

how much energy can be generated on average at a

given time of day and time of year. A large fraction

of the available energy is lost when the PV-cell is

not oriented directly against the sun, but this is

typically unavoidable. If possible, the PV-cell

should be oriented in the direction of average

maximum sunlight. The reflectiveness of the

surroundings highly influences the received energy

and a heavy cloud cover reduces the available

energy by approximately an order of magnitude

(Gilbert and Balouchi, 2008). In the worst case, the

PV-cell will experience sufficiently bright

conditions for so short a time that it cannot harvest

enough energy for the load to operate. Seasonal and

weather conditions may make it impossible to

harvest a sufficient amount of energy for a long time

and therefore it is important to store enough energy

in the sensor nodes for them to be able to operate

during such times.

3.2 A Hybrid Energy Harvester

Several different energy sources can be harvested

simultaneously by using a modular energy harvester.

Harvesting both solar and wind or vibrational energy

would reduce the downtime of the harvester and

produce power more evenly. The largest drawback

of using several sources is the increased requirement

for space. Park and Chou (2006) developed a

modular energy harvesting system called AmbiMax.

They propose to use a reservoir capacitor array, i.e. a

separate supercapacitor for each energy harvester.

These supercapacitors need to be able to reach the

same voltage in order to power the common voltage

rail. If the voltage over one of the capacitors is

higher than that of the others, only that one will

supply the voltage rail. If more than one capacitor is

used like this, diodes may be necessary to prevent

backflow from the voltage rail to the capacitors.

Diodes should be avoided when possible, because

they cause a small voltage drop and power loss. An

energy harvester that outputs less power than the

other harvesters needs to have a smaller

supercapacitor so that it can reach the target voltage

quickly enough to be efficient (reaching its

maximum power point). The voltage rail can be used

to power the wireless sensor node and/or a battery

charger.

4 ENERGY OPTIMIZATION

When using an energy harvester to power a wireless

sensor node, there are many aspects that must be

SENSORNETS 2016 - 5th International Conference on Sensor Networks

considered when seeking optimal performance to

harvest as much energy as possible and to store and

use the harvested energy as efficiently as possible. If

one part of the system is wasteful, it is not very

helpful to get another part of the system to operate

efficiently. Some of the parts of the system that must

be considered when seeking optimal overall system

performance are: harvesting location and schedule,

size of the electronics, energy conversion, voltage

conversion for storage, electrical switching, energy

storage, voltage conversion for consumption, energy

consumption of the load circuit, sensor node

program execution, wireless communication

scheme, and transmission power.

There are complex tradeoffs to be considered

when selecting components for the energy

management and storage connected to an energy

harvester. When the values of the voltage and

current of an energy source result in a maximum

power output, the circuit is said to be operating at

the maximum power point (MPP). The MPP voltage

varies with ambient conditions. This voltage may

not be optimal for the energy harvesting circuit and

voltage regulator that drain the source and supply

the voltage rail or storage device with a suitable

voltage. Thus, significant power may be lost if the

source and the harvesting circuit are not well

matched.

The energy harvesting circuit is also optimally

efficient at a certain output voltage. For example,

step-up DC/DC converters are the most efficient

when their output voltage is only slightly below the

input voltage. When such a converter is charging a

battery or a supercapacitor, its output voltage will

gradually increase as the load is charged, and the

conversion may be efficient only for a short time.

For this reason, supercapacitors are commonly used

as buffers to allow the output voltage to rapidly

climb to a suitable voltage when harvesting, and

then battery charging begins when the optimum

voltage is exceeded, thus keeping the output at this

voltage until the battery is charged to a higher

voltage, after which the efficiency again decreases

as the voltage closes in on the setpoint.

Voltage regulators supplying the sensor node

cause an additional power loss that depends on the

regulator type and its input and output voltages. It

can be very difficult to optimize the overall system

performance when so many components must be

considered. As a rule of thumb, in electronics

design, the optimum voltages of all components

should be as close to each other as possible.

A truly optimized energy harvesting system

should take into account the limitations of the

energy storage circuit and the draining schedule of

the storage over time. The wireless sensor node

needs to work intermittently and go into sleep mode

at certain times in order to conserve energy for

future measurements, data logging, and

transmissions. The schedule could also be changed

by the sensor node based on measurements of the

environment. For example, the system has to take

into account that no power is harvested from a

photovoltaic harvester at night. Schedules of

harvesting and consumption can be simulated using

computer models before they are tested in hardware

in order to make the best use of the harvested

energy.

5 THE ENERGY HARVESTING

PROTOTYPE

After measuring the typical power consumption of

the UWASA Node and investigating what forms of

energy harvesting would be suitable, the energy

harvester prototype shown in Figure 2 was designed

and built. The design was made with modularity and

expandability in mind. The harvester was designed

to work with a small solar cell, but other sources can

be added in parallel if some modifications are made

(Höglund, 2014b).

Figure 2: Developed energy harvester prototype.

The chosen implementation is based on the

AmbiMax system described by Park and Chou

(2006). It is an entirely analog energy harvesting

system that was relatively efficient when it was

made in 2006, but the power consumption of

common, low-power digital controllers has since

dropped significantly, making them a viable

alternative. The maximum power point tracking was

not implemented in the same way in this project as

in the AmbiMax. The LTC3105 energy harvesting

IC was chosen to perform the harvesting. Since 2013

when this choice was made, some even more

Solar Energy Harvesting Solution for the Wireless Sensor Platform the UWASA Node

efficient energy harvesting ICs have become

available (Höglund, 2014b). The energy

management was designed with components similar

to those of the AmbiMax, but slightly more efficient

(Höglund, 2014b).

The architecture of the AmbiMax platform is

shown in Figure 3, reproduced from Park and Chou

(2006). It consists of a comparator with hysteresis

that performs MPPT with the aid of a sensor and

controls a boost regulator. The regulator charges a

supercapacitor that is connected to the voltage rail.

All of these components can be grouped as a

subsystem and used in parallel if more than one

energy source is used. The supercapacitors are

connected to the voltage rail via optional protection

circuitry, and the voltage rail powers the sensor

node. If the voltage of the voltage rail increases

above a certain threshold, the battery is charged

from the voltage rail via a current limiter.

Conversely, if the voltage of the voltage rail drops to

below a certain threshold, the battery feeds the

voltage rail as long as its voltage is above another

fixed threshold. This, in short, is how the AmbiMax

and the developed energy harvester work.

Additionally, a low-power undervoltage lock-out

circuit and a real-time clock-controlled latch switch

were designed to cut off the voltage rail from the

sensor node when it drops below a threshold or

when the node signals it to shut down for a length of

time; these were not part of the AmbiMax.

Figure 3: The architecture of the AmbiMax Platform (Park

and Chou, 2006).

The LTC3105 energy harvesting IC by Linear

Technology (Milpitas, California) was chosen from

many alternatives to be the energy harvester used in

the prototype of this work. It is listed as a 400 mA

step-up DC/DC converter with MPP control and 250

mV start-up voltage. It is capable of supplying up to

5.25 V. The prototype is designed to supply 4.2 V,

which is the maximum voltage of a one-cell lithium-

ion battery. The very low start-up voltage of the

LTC3105 allows it to harvest from a photovoltaic

cell that outputs a low voltage due to low ambient

illuminance. The low input voltage compatibility can

also be useful for other types of energy harvesting

sources such as thermoelectric, electromagnetic, or

magnetostrictive sources, which output a low

voltage.

6 THE SOLAR CELL

A 92 mm × 61 mm solar cell, with a nominal power

of 0.45 W, was chosen for the energy harvester

prototype, because its open circuit voltage is

approximately 5 V, which is suitable for the

LTC3105 and the battery, and its size is

approximately that of the UWASA Node’s. If more

energy is needed, it is possible to connect more than

one such solar cell in parallel with the other cells to

the energy harvester, while still keeping the MPP

voltage and energy harvesting circuit the same.

Protection diodes could be used to allow operation

with solar cells of higher voltages, but the LTC3105

operates most efficiently at input voltages slightly

lower than its output voltage, and therefore the MPP

voltage of the solar cell should be lower than the

desired output voltage.

In order to measure the MPP of the solar cell, it

was connected to a potentiometer used as a variable

load. It was then placed under a constant illuminance

of 2.8 klx and its output current and voltage were

measured while varying the load. The output power

was calculated, and the result is plotted in Figure 4.

The MPP occurs at approximately 3.6 V and 6.3

mW. The MPP varies slightly with the illuminance,

but after a few attempts at maximum power point

tracking, it was decided that a fixed MPP voltage is

sufficient for this application.

Figure 4: Power vs. voltage for the 0.45 W solar cell at 2.8

klx.

SENSORNETS 2016 - 5th International Conference on Sensor Networks

7 MAXIMUM POWER POINT

TRACKING

Maximum power point tracking (MPPT) aims to

adapt the energy harvesting load to the ambient

conditions so that the input voltage of the energy

harvester is always equal to the MPP voltage as it

varies, in effect performing impedance matching.

In the case of the LTC3105, the MPPT is

integrated on the chip and there is a pin named

MPPC. The LTC3105 keeps the source voltage the

same as the voltage on the MPPC pin, which

constantly outputs 10 µA. If MPPT is not necessary,

this pin can be connected via a fixed resistor R

MPPC

to ground in order to set the MPP to a fixed voltage

MPPT

) according to (1). In the prototype, a 360 kΩ

resistor was used to achieve a U

MPPT

of 3.6 V.

MPPT

=10 µA* R

MPPC

(1)

The datasheet of LTC3105 proposes to use a diode

thermally coupled to the solar cell for MPPT, but

this is unlikely to work well over the large

temperature range of this application; it would also

be difficult to achieve thermal coupling.

MPPT could also be performed digitally by a

low-power microcontroller, digital signal processor,

or field-programmable gate array. It is easier to

calculate the MPP digitally and take several factors

into account, such as illuminance and temperature,

but unless such a digital control system is carefully

designed, not much power can be saved.

8 ENERGY MANAGEMENT AND

STORAGE

The energy management part of the circuit takes

care of routing the power in an optimal way between

the energy harvester and sensor node components

for maximum performance and optimal schedule of

operation. The energy management of the energy

harvester prototype consists of supercapacitors, a Li-

ion polymer battery, nanopower voltage

comparators, a logical AND gate, two MOSFETs

and a few current limiters.

The LiPo battery is charged by two

supercapacitors connected in series when the

supercapacitors reach a voltage threshold. The

charge current is limited by a current limiter that

also works as a switch. Charge current flows

intermittently due to a configured hysteresis, until

4.2 V is reached. The voltage thresholds at which

power is transferred in the prototype between the

supercapacitors, the battery, and the UWASA Node

are governed by LTC1540 nanopower voltage

comparators by Linear Technology (Milpitas,

California). These comparators feature an ultralow

quiescent current of nominally 0.3 µA, a voltage

reference, and a hysteresis, both adjustable by

resistor voltage dividers.

One comparator is used for activating the current

flow from the voltage rail (supercapacitor) to the

battery when the rail voltage is more than 3.7 V.

Another comparator is used in the undervoltage

lock-out (described in Section 8.3), and a pair of

comparators with an AND gate is used for activating

the current flow from the battery to the voltage rail.

All comparators were configured for a hysteresis of

approximately 100 mV.

The comparator that activates battery charging

and the AND gate that activates battery draining are

connected to the enable pins of two separate current

limiters that, when enabled, permit a limited current

flow through them in one direction. These current

limiters were implemented using TPS2030D power

distribution switches by Texas Instruments (Dallas,

Texas). They allow 300 mA to pass through them

when activated.

8.1 Supercapacitors

Supercapacitors can act as a buffer and be used to

store the first energy delivered by the energy

harvester until there is enough energy to begin

charging the battery or supplying the sensor node.

The voltage of the supercapacitor can rise quickly to

a voltage where the step-up (or step-down) converter

operates the most efficiently because of its much

lower capacity compared with a battery. Connecting

the harvester directly to the battery would cause its

voltage to rise very slowly and energy would be

harvested less efficiently because of the step-up

inefficiency at lower voltages. Supercapacitors can

also smooth out the wide dynamic range of energy

harvesters and the node load, especially if more than

one harvester subsystem is connected in parallel.

Another advantage of using supercapacitors is that

they can be used to preferentially supply the sensor

node before the battery is needed. This keeps the

battery voltage more even, which slows down

battery aging.

According to Mars (2009), (2) gives an

approximation for the necessary capacitance C of the

supercapacitor assuming there is a constant load

current I

and that the supercapacitor needs to be

able to supply I

for time t. When current is drawn

from a supercapacitor, there is an instantaneous

Solar Energy Harvesting Solution for the Wireless Sensor Platform the UWASA Node

voltage drop due to its equivalent series resistance

ESR

. The load voltage is allowed to decrease from

max

to U

min

. Equation (2) shows that an

approximately 12 F supercapacitor is necessary to

supply 250 mA for 60 seconds with the voltage

limits of the developed prototype. In the actual case,

the current would vary significantly over time, but

this equation provides a useful indication of how

large a capacitor is required.

max

-U

min

-I

ESR

250*10

-3

A*60 s

4.2 V-2.9 V-250*10

-3

A*200

-3

Ω

=12 F

(2)

8.2 Switch Controlled by Real-Time

Clock

A real-time clock (RTC) was added to the prototype

so that the sensor node can cut off its own power

supply in order to avoid consuming any energy on

the node side while it is in sleep mode. The RTC has

an alarm output that can be set to trigger at the point

in time when the node should be powered on. The

RTC consumes only a few microamperes of current.

The alarm output is connected to a latch IC that

turns on or off the current flow through two

MOSFETs that supply the node with power. The

sensor node can request the RTC to activate the latch

at a specific time in the future, turning the power

supply on at that time, and then use the reset line of

the latch to shut itself down. An SHT11 temperature

and humidity sensor was also included on the PCB

on the same I

C bus as the RTC because temperature

and humidity measurements are needed in wind

turbine monitoring.

8.3 UnderVoltage Lock-Out Circuit

An undervoltage lock-out (UVLO) circuit was

designed to cut off the power from the sensor node

when the voltage rail is below 2.9 V. There is a 20

MΩ feedback resistor that creates an extra high

hysteresis of 350 mV to allow enough energy for the

sensor node to wake up and measure the voltage

without allowing the turn-on current surge and any

startup tasks to drain the voltage rail below the

UVLO threshold again. The switching is done using

a 2N7002 small signal N-channel MOSFET and an

IRLML6401 P-channel power MOSFET.

9 THE PERFORMANCE OF THE

PROTOTYPE

The lowest level of illuminance at which the

LTC3105 was able to harvest was a few hundred

lux, depending on the voltage of the supercapacitor.

The energy harvester prototype was tested in a long-

term test that lasted six continuous days. The solar

cell was located on a roof where it was not

shadowed by any object at any time of the day.

There was no load connected to the prototype. A

data logger was connected and used for measuring

time, illuminance, and the voltages of the

supercapacitor, battery, solar cell, and MPPC pin of

the LTC3105.

Figure 5: Energy harvester performance over six days.

SENSORNETS 2016 - 5th International Conference on Sensor Networks

During the test, the temperature was a few degrees C

below the freezing point. Figure 5 shows how the

prototype performed. On average, the energy

harvester was active for 9.0 hours per day (the sunny

hours) and harvested at 35.6 mW. On average, 1.16

kJ was harvested per day, or 2.14 J per minute

active. In 6 days, the total energy harvested was 6.9

kJ, which corresponds to 51% of the capacity of the

1,000 mAh, 3.7 V LiPo battery. Once the battery

was fully charged, the voltage rail reached the set

point of the energy harvesting circuit and the solar

cell was automatically disconnected, causing a

voltage of more than 5 V over the solar cell.

For most applications, one solar cell of the type

tested should be sufficient and the 1,000 mAh

battery capacity is useful to have to ensure the

sensor node can operate during days of low

illuminance. The solar cell, battery and energy

harvester of the prototype were well-dimensioned.

Regarding the energy consumption of the

UWASA Node, experiments showed that the startup

and initialization of wireless communication and a

few sensors consumes between 0.7 and 1.5 J.

Measuring three voltages 10,000 times using the

internal ADC consumes approx. 400 mJ (no

peripherals turned off). Transmitting 100 bytes of

data consumes ~850 mJ. Measuring 3-axis, 10-bit

acceleration at a sample rate of 500 Hz for 2 s

consumes 1.82 J. A typical program reading several

sensors at a high rate will consume approx. 3-10 J

for measurements and 50-100 J for transmission of

thousands of bytes. If few bytes are transmitted, the

node will consume less than 5 J and can thus operate

intermittently at an interval of 3-4 minutes on

harvested power.

10 CONCLUSIONS

The goal of this work was to build and test a small

energy harvester and power management prototype

optimized for the UWASA Node for outdoor use in

cold weather, primarily for wind turbine monitoring

applications. The developed energy harvester was

tested using only a solar cell, but the prototype was

designed so that more energy harvesting sources can

easily be added. Every part of the energy harvester

and power management was chosen to operate at

voltages optimal for the UWASA Node with power

module. The energy measurements presented in

Section 9 can be useful for energy harvester

developers. The presented prototype is an

improvement on the AmbiMax system described by

Park and Chou (2006). By integrating the RTC

switch on the energy harvesting PCB, the power

consumption of any connected sensor node can be

eliminated when inactive.

Powering the UWASA Node by energy

harvesting is a useful idea, as it makes the node self-

sufficient and allows it to operate in places where

servicing would be prohibitively expensive or

impossible. By using energy harvesters, wireless

sensor nodes can potentially operate independently

for several years, if the rest of the software and

hardware platform is sufficiently robust.

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<http://web.eecs.umich.edu/~prabal/teaching/eecs598-

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