A STUDY OF ENERGY SCAVENGING STRATEGY IN A TAPPING TREE

POWER

BELAL MOHAMMED ABDULLAH ALHAMZI

A project report submitted in partial

fulfillment of the requirement for the award of the

Degree of Master of Electrical Engineering

Faculty of Electrical and Electronic Engineering

Universiti Tun Hussein Onn Malaysia

JUNE 2014

ABSTRACT

Energy harvesting has become one of the newest research attraction for both

academic and industry fields. The energy is captured from external sources (thermal,

wind, solar, vibration and hydrodynamic) and the energy source for energy harvesters is

present as ambient background and it is free. The energy can be harvested in term of

sensors or direct applying which depends on the source applied to. One of the energy

harvesting topologies is from tapping tree which is new topology that can be applied direct

to the tree by inserting electrode into the tree while the other is into the surrounding soil.

This project introduces a method for the energy to be harvested from a tree, where the

voltage obtained is conducted to three types of trees (Palm, Agrawood, and Palm Oil).

Both type of electrodes and pH value of the surrounding soil are considered in order to

get maximum voltage. Furthermore, the depth of the electrode inserted and the height from

the surrounding soil play a role for voltage variation. The maximum voltage can be

obtained is from (A1 Alloy with Al) electrodes, at neutral pH, dry mositure and in Palm

Oil Tree which is almost 0.8V - 1.2V. However, at such small energy from tree source, converter is required to increase the power. Boost converter contains of BQ25504 IC is

applied to increase the voltage from small input to 3V - 4V. As well as, buck converter with TPS6223 1 IC is introduced for the output voltage of the boost converter and can be

applied for small voltage as 1.8V. Simulation using Tina software simulate the converters

mentioned and get better results for the energy harvested with battery charging system

and energy storage. Field test is conducted with the selected converter to insure the power

observed from the plant. In addition, this work has been taken to enhance the growth of

energy harvesting from small sources and to go further for nature sources that has a high

impact for low applications in the future.

CHAPTER 1

INTRODUCTION

1.1 Project background

Over the past centuries, world economic growth is inseparable to the ever-expanding

use of hydrocarbon energy sources such as petroleum, coal and natural gas. The trend

of global economic currently hinges on increasing rates of production of these fuels.

However, petroleum oil and gas, and coal are non-renewable energy resources that will

cease in the future. The ever rising cost of fossil fuels not only causes inflation but also

hinders the economic growth as the production of goods and cost of shipment are

dependent on the cost of fuels [1]. In the view of the consequences of the normal

energy sources to the world nowadays, so both academic and industry have much

interest into the green energy by renewable energy [2,3,4].

Energy harvesting is the process by which ambient energy is captured from

external sources (thermal, wind, solar, vibration and hydrodynamic). Energy

harvesters provide a very small amount of power for low-energy electronics [5]. The

energy source for energy harvesters is present as ambient background and it is free.

The energy can be harvested in term of sensors or direct applying which depends on

the source applied to. Energy harvesting devices converting ambient energy into

electrical energy have attracted much interest in both academic and industry.

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Due to this new field of harvesting energy from such sources which generate a

green energy that replaced the normal sources, researchers are encouraged to further

deep study in this field [6]. This small source of the energy is going to be implemented

in many applications such as (fire alarms, tinny cameras, digital devices …etc). Today,

industry is advocating a suite of new and emerging technologies energy harvesting

transducers, thin film batteries, micro power integrated circuits, and Nano power

microcontrollers coupled with star network topologies, to address the pressing

challenges of energy harvesting operation.

It has been demonstrated that measureable, albeit small, electrical potentials

exist in various common plants and trees. These voltages have recently been attributed

to a pH difference between xylem tissue and soil content, types of the trees with

chemical and environmental characteristics and types of electrodes that used to

measure the potential of xylem and soil are introduced to harvest the energy from the

trees, even though, it is so small. By using the properties of living trees, a natural

source of energy is tapped to power electronic circuits, eliminating the need for

conventional batteries [7]. This energy source could foster the development of new

applications for electronics and expand the number of locations in which they operate.

Power electronics is implemented in such fields to increase the voltage harvested from

the trees which can be used for more applications. However, in this field the converter

which can be implemented should be critical and flexible for the small voltage

produced from the trees.

This work is going to propose some fundamental procedures to harvest weak

electric energy from plants with different conditions and tests. The difference of pH

between the xylem tissue and soil content of the tree, types of tropical trees that the

tests can be conducted to, dry and wet moisture and types of electrodes used are going

to be studied in this project to deliver a better voltage. These voltage differences are

used in attempts to monitor plant activity and hypothesized to be due to various

sources, most prominent of which appears to be the ‘‘streaming potential’’ mechanism

, which is itself related to transpiration and sap flow. The harvested energy is going to

be boosted using a boost converter to generate high output energy connected with a

buck converter for low applications. However, the output power of trees may be a

more significant physical quantity to reflect the magnitude of the electricity compared

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with the voltage or the current separately. This work is introduced to find more

comprehensive relationships between the bioelectricity in trees and their surrounding

environmental parameters.

1.2 Problem statements

In order to get an energy from the plant (tree), a few harvesting methods have been

done in the last decade. The observation from these different harvesting methods is

that, energy harvesting is a proportional to the characteristics of the tree itself and that

what at least going to increase the voltage harvested by looking into pH, Moisture,

Type of electrodes that inserted in the tree and the surrounding soil. As well as, the

type of tree with the weather play a big rule which gives the tropical trees high impact

for voltage potential difference.

While the voltage is too low, varied by time and not sufficient enough, there

should be a way to increase it and make it constant in order to get a useful and

applicable energy. Sensitivity and complication of the converter design for such small

sources are considered due to the reduction of the voltage and current by the effect of

the components and environment. However, the current is that too low as well which

makes the application of the output energy harvested from the tree not applicable in

most cases. This work is going to study and design a scavenging method that could be

more useful for some of the Nano electronics such as sensors. Such study give an

opportunity to use a natural source for small applications that can replace the limited

sources such as batteries.

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1.3 Project objectives

The major objective of this project is to scavenge energy from a tree

Its measurable objectives are as follows:

a) To explore the basic characteristics of the living tree with regards to energy

transfer.

b) To propose an energy harvesting method for a maximum voltage extraction from

selected living trees.

c) To recommend a proper converter for the harvested energy producing a useful

power.

1.4 Project scopes

This project is concerned with the scopes as following:

a) Study the characteristics of the living trees in term of pH, moisture, electrode types,

and types of trees that can conduct the test on.

b) The input voltage to the converter is to be ranged from 300mV to 1.5V due to the

small source of the plant which can produce an output of 2V to 5V from the

converter.

c) The output power obtained from this study shall be small due to low voltage and

current which reduce the possibility of the power usage in the applications.

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

Energy is harvested from a tree in different ways movement, solar or Electric Potential

Difference (EPD) between the tree and its surrounding soil. In this project, the energy

is harvested from a living tree and this chapter discusses many topics that is related to

the Tree characteristics and environment that could affect the energy harvesting. In 2.2

the study of the characteristics of the tree is described. However, 2.3 discuss the energy

transfer and the types of tropical trees, as well as, in the pH difference of the tree and

its surrounding soil is described in 2.4, types of electrodes that possible to be used in

2.5. There are different approaches for energy harvesting which mentioned and

explained in 2.6 and the various harvesting energy methods from previous work are

introduced in 2.7.

2.2 Tree architecture and characteristics

The tree has a full life cycle which depends on the environment and structure that can

affect the tree growth and life. The necessity of the tree life is summarized in six key

requirements mentioned in 2.2.1. Followed by the tree parts and Photosynthesis and

Respiration functions that can affect the growth of the tree in 2.2.2 and 2.2.3. In term

of the type of the trees, palm and non-palm trees are described in 2.2.3.

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2.2.1 The necessity of tree life

The following are the six key requirements for trees.

1. Sugars supplied by photosynthesis. Air and water are chemically recombined to

form glucose, which stores energy captured from the sun. Oxygen is a byproduct.

2. Water is required for most metabolic activities and serves as a vehicle to carry

materials through a tree. A large tree may move as much as 50-100 gallons of water

on a hot summer day.

3. Nutrients. It’s not how much of a particular nutrient exists in the environment, it’s

a matter of how available the nutrient is to the tree. For example, the atmosphere is

largely composed on nitrogen, but trees can only use nitrogen in forms that have been

altered by soil bacteria and other organisms. The major chemical elements used by

plants are: carbon, hydrogen, oxygen, phosphorus, potassium, nitrogen, sulfur,

calcium, iron, and magnesium. It might be remembered by a jingle formed using the

abbreviations for these elements, Hormones and enzymes. These chemicals are critical

in the controlling the timing and activity of physiological processes. They are usually

produced in the roots or leaves. However, it is not thinkable of plants having

"hormone" deficiencies, but they are critical to the survival of any organism, including

trees [8].

4. Mycorrhizae. Pronounced "my-core-HI-zee", this a group of beneficial fungi

associated with most tree roots. It represents an ecologically symbiotic relationship

where the fungi receive food from the tree and the trees receive greatly enhanced

nutrient and water absorption. Mycorrhizae will also protect tree roots from other

invading fungi. There tends to be very specific species relationships between fungus

and tree.

5. Environmental factors. A tree needs an appropriate mix of precipitation,

temperature, sunlight, and soils in order to thrive. These factors need to occur at the

right time. Each tree species has a different set of environmental

requirements. Changing climate will lead to changing environmental factors, which

can lead to changes in forest ecosystems [8].

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2.2.2 Parts of the tree

The parts of the tree that have the high impact for the growth of the tree is explained

and listed in Table2.1.

Table 2.1: List of the Tree Parts

Leaves Broad-leaf or needles, the primary site of photosynthesis and the

production of hormones and other chemicals

Twigs &

Branches

Support structure for leaves, flowers, and fruits. Arrangement varies

from species to species by growth strategy. Can sometimes have

photosynthetic tissues. Two kinds of growth tissue, at the twig tips and

cambium under the bark.

Crown The upper region of the tree made up of leaves, twigs, branches,

flowers, and fruits. Crowns of many trees are collectively called the

"canopy".

Flowers May have both female & male parts, or only one or the other. Some

trees are either all female or all male (e.g. aspen). Flowers may have a

full complement of flower parts, or may be missing certain

elements. Conifers do not have petals and associated structures.

Fruits &

Seeds

All trees have seeds. Most trees have seeds inside fruits. Most fruits

are NOT edible, but many are, such as apples, cherries, nuts, etc.

Trunk or

Bole

Most definitions of trees include a "single bole" concept, but many of

our tree species sometimes occur with multiple stems. The main

functions of a trunk are transport and support. The trunk has growth

tissue called cambium.

Bark A highly variable tree part. The main function is to protect the sensitive

living tissues from weather and predation (by animals, insects, fungi,

etc.)

Roots Roots serve two main functions; collection of nutrients and water, and

anchoring the tree. Roots also have growth tissue, bark, and wood. Like

twigs and branches, roots have two kinds of growth tissue, at the twig

tips and cambium under the bark. Fine root hairs are where absorption

occurs

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2.2.3 Photosynthesis and respiration of a tree

All trees (most plants) both photosynthesize and respire. Photosynthesis is a process

unique to green plants and produces sugars, which are "tree food". Figure 2.1. shows

how the Photosynthesis and Respiration of a Tree can be expressed chemically.

Figure 2.1: Overview of Photosynthesis and Respiration of a Tree

Photosynthesis can be visualized in a couple ways.

Sugars produced are analogous to a "solar battery." The sugar is a chemical

way to store energy for future use (metabolism).

Trees produce their own food. We call "tree food" sugar. These sugars are

not usually of the chemical structure of refined sugar and don’t usually taste

sweet, but the basic organic components are similar.

The basic chemical formula for photosynthesis is:

Inputs: 6 carbons, 24 oxygen, 24 hydrogen

Outputs: 6 carbons, 24 oxygen, 24 hydrogen

Note: Inputs and outputs must balance in a chemical equation. In other words, what

goes in, must come out.

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Energy is stored in the bonds of sugar molecules such as "glucose" and "fructose."

Oxygen is a by-product of photosynthesis. The oxygen molecules produced by

photosynthesis are not necessarily the same oxygen molecules the plants use for

respiration [9].

These sugars are later broken apart and the released energy drives a variety of

metabolic actions. The process of breaking down these sugars is called "respiration" It

is the same process that animals (and people) use when they respire (not to be confused

with "breathing"). So, either the plant uses its own stored sugars, or some animal (or

decomposer) consumes the plant, and uses the stored sugars. In either case, the sugars

are valued chemicals because they contain energy, as well as important elements

(carbon, hydrogen, and oxygen) [8].

OHCOOOHC 666 2226126

Glucose + Oxygen Carbon Dioxide + Water + Energy

The tree uses its photosynthetic for (glucose and fructose) in many applications in

addition to energy storage and subsequent release.

Cell walls are made of cellulose (C6H10O5). Cellulose shows up in many plant

parts in combination with other molecular elements. It is not only vital to the tree,

but is also a very important material for people (wood, lumber, fuel, fibers,

chemical extracts, energy, etc.).

Production of carbohydrates such as sugars (C6H12O6), starches (C6H10O5),

vegetable ivory (form of hemicellulose), pectins (for jellies, jams), gums (used in

many products, including food products).

Many fats and oils are common plant products (some of which come from trees).

These are compounds of mostly carbon, hydrogen, and oxygen, but with lots

more molecules of each.

Carbon Dioxide + Water + Energy Glucose + Oxygen (2.1)

(2.2)

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Proteins are formed when the C, N, O elements are combined with nitrogen, sulfur,

and sometimes phosphorus. Certain proteins used by animals (and people) can only

be obtained by ingesting plant products.

There are numerous secretions produced by trees (and other plants) that are

important to people, such as clove oil, cedar oil, resins, pitch, gums, balsam,

camphor, natural rubber, pigments, drugs (legal and illegal), etc.

A note about energy allocation within trees. Energy is not a limitless resource for

trees. A tree will typically move energy according to these priorities. As energy in the

form of glucose becomes limited, a tree will begin to reduce resources spent beginning

with the lowest priority. As you can see, a tree with a diminishing crown will become

more vulnerable to insects and diseases rather quickly. That’s one reason why foresters

are so keen to maintain a vigorous growing environment [8, 9].

1. Maintain respiration of all parts.

2. Produce fine roots and leaves.

3. Produce flowers and seed.

4. Extend branches and roots.

5. Store energy rich chemicals.

6. Add wood to stem, roots and branches.

7. Create anti-pest chemicals for defense.

The tree zones which are roots, trunk and crown are shown in Figure 2.2 which explain

the tree growth generally.

Figure 2.2: Overview of Tree Growth Zones

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In photosynthesis, the plant uses water and nutrients from the soil, and carbon dioxide

from the air with the sun’s energy to create photosynthesis [9]. Oxygen is releases as

a byproduct as shown in Figure2.3. Table2.2 differentiate the function of

Photosynthesis and Respiration that is happening in the plant.

Figure 2.3: Necessties of Tree Growth

Table 2.2: Comparison of Photosynthesis and Respiration

Photosynthesis Respiration

Produces sugars from light

energy

Stores energy

Occurs only in cells with

chloroplasts

Releases oxygen

Uses water

Uses carbon dioxide

Requires light

Burns sugars for energy

Releases energy

Occurs in most cells

Uses oxygen

Produces water

Produces carbon dioxide

Occurs in dark and light

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2.2.4 Palm and non-palm tropical trees growth

Palms rarely get thick with age. In some palms, the very base may show a swelling

that is a result of new roots pushing out of the trunk near the ground. Broad-leaved

trees, like mango, avocado or live oak, and pine trees have thin twigs and thicker

branches and a large trunk. Old branches and trunks thicken with age. The overview

of the palm and non-palm trees are shown in Figure 2.4 which explains the function of

each part in term of nutrition and support [10].

Figure 2.4: Palm And non-Palm Trees

In broad-leaved trees the center is woody and in the very center is a pith. When

rings are present, the age of the branch or trunk can be determined by counting the

yearly growth rings. However, some tropical broad-leaved trees grow all year and

don’t have growth rings such as ficus, mango, and avocado. Figure2.5 shows the layers

of the broad-leaved trees for more understanding of the structure of these kind of trees.

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Figure 2.5: Layers of Broad- Leaved Trees

Trees grow wider by producing new wood. The new wood grows from a thin

soft layer of dividing cells, called the cambium (X), that covers the outside of the wood

and is protected by the bark to the outside of the trunk. The wood (xylem) gives the

tree strength and transports water from roots to leaves. The inner bark (phloem)

transport sugars and amino acids produced by the leaves down to the roots. The

cambium produces new inner bark to the outside. When a tree is cut, the wood (xylem)

and the bark (phloem and cork) are recognized as in Figure 2.6.

Figure 2.6: Cross Section of Xylem and Phleom

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Pencil points to the cambium which is split in this dried log. Dark bark is to

outside and wood (both sapwood and heartwood) is to the inside. The inner bark is

protected by the outer bark and cork. Both inner and outer bark can be peeled from the

trunk because the cambium is a soft and delicate layer that can be easily split [11].

Birch bark for Indian canoes and cork for wine bottles can be removed because

of the fragile cambium layer. A few bottle palms and even the royal palm have swollen

regions of the trunk due to localized cell enlargement but not due to new vascular

tissues (xylem and phloem). These barks of Oak and Bursera are shown in Figure 2.7.

2.3 Plant energy transport

Plants are classified as autotrophs because they manufacture their needed nutrients

by photosynthesis, converting carbon dioxide and water to sugar fuels with the

addition of energy from the Sun. In times of rapid photosynthesis, the main product

is glucose, but it is usually converted to the larger sugar sucrose. These sugars that are

synthesized in the leaves must be transported to other parts of the plant. Other

structures in the plants such as roots and flowers require the energy but cannot

manufacture it. Also, sugars may be stored in the roots and stem [12].

Bark of gumbo limbo (Bursera). Bark of live oak

The bark is smooth, reddish in color and the thin The thick bark is deeply fissured and makes a

layers of cork peel off in thin sheets. good surface for the resurrection fern to grow

on.

Figure 2.7: Overview Bursera and Oak Plants

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The sugar and other organic molecules are transported through the plant by means

of a special layer of tissue called phloem. Phloem is composed of living cells that

transport a water solution of sugars that we commonly call sap. This movement is

modeled by the pressure-flow theory, a part of which is that the sugar-containing fluid

is moved through sieve tubes by fluid pressure. By this means, nutrients can be moved

from the photosynthetic site (the source) to the place where the sugar is being used

(the sink) whether it is up or down the stem of the plant.

2.3.1 Energy flow in tropical trees

The energy flow in a tropical rainforest with sunlight and water is very different from

any other biome. Because the tropical rainforest receives so much sunlight and rainfall,

plants grow to be bigger, taller, and stronger than plants in any other biome. Plants

becoming very large prevent most sunlight from reaching the bottom layer of the

rainforest. As shown in the diagram below, 100% of the rainfall reaches the rainforest.

50% of that rainfall is consumed, 25% of it evaporates, and the other 25% of it is

surface runoff. This rainfall cycle is shown in Figure2.8 and known as the water cycle

continues every day and produced rainfall and water to the plants and animals of the

tropical rainforest biome [13].

Figure 2.8: Energy Flow in the Rainfull Trees

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2.3.2 Electrical activity of excitable membranes

Nerve cells in animals and phloem cells in plants share one fundamental property.

They possess excitable membranes through which electrical excitations, in the form of

action potentials, can propagate. These propagating excitations are modeled

theoretically as traveling wave solutions of certain parameter dependent nonlinear

reaction-diffusion equations coupled with some nonlinear ordinary differential

equations. These traveling wave solutions can be classified as single loop pulse,

multiple loop pulses, fronts and backs, or periodic waves of different wave speed. This

classification is matched by the classification of the electrochemical responses

observed in plants. The experimental observations also show that under influence of

various pathogens, the shapes and wave speeds of the electrochemical responses

undergo changes. From the theoretical perspective, the changes in the shapes and wave

speeds of the traveling waves can be accounted for by appropriate changes in

parameters in the corresponding nonlinear differential equations [14].

Hodgkin and Huxley formulated a membrane model that accounts for

potassium, sodium, and ion leakage channels in squid giant axons (Figure 2.9 a). The

membrane resting for each ion species is treated like a battery. A

variable resistor models the degree to which the channel is open. Fromm and Spans

wick found that electric stimulation of the plant is followed by ion shifts that are most

striking in the phloem cells. While the content of potassium and chloride was

diminished after stimulation, the amount of cytoplasmic calcium increased slightly.

These displacements lead to the conclusion that calcium ion influxes, as well as

potassium and chloride ion effluxes are involved in the propagation of action

potentials.

The main difference between the propagation of action potentials in animals

and plants is that in an axon there is the potassium/sodium ion trans membrane

transport, but in phloem cells the potassium/calcium ion channels are involved in this

process (Figure 2.9 b).

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Figure 2.9: The Hodgkin-Huxley (HH) equivalent circuit for an axon (a) and the

modified HH circuit for sieve tubes in phloem (b).

2.4 The plant pH influences

pH is simply a measure of how acid or alkaline a substance is, and soil acidity or

alkalinity (soil pH) is important because it influences how easily plants can take up

nutrients from the soil. Many gardening books and catalogs list the preferred pH for

specific plants. The good news is that, with a few exceptions, most plants will tolerate

a fairly wide range of soil pH [14].

2.4.1 Effect of pH of the soil to the plant

There are some parameters of the soil pH that can effects the plant such as Nutrient

uptake and pH when the plant roots absorb mineral nutrients such as nitrogen and iron

when they are dissolved in water. If the soil solution (the mixture of water and nutrients

in the soil) is too acid or alkaline, some nutrients won’t dissolve easily, so they won’t

be available for uptake by roots. Most nutrients that plants need can dissolve easily

when the pH of the soil solution ranges from 6.0 to 7.5. Below pH 6.0, some nutrients,

(a) (b)

http://www.organicgardening.com/learn-and-grow/soil-nitrogen-content

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such as nitrogen, phosphorus, and potassium, are less available. When pH exceeds 7.5,

iron, manganese, and phosphorus are less available.

2.4.2 Measurement of soil pH

The term pH stands for the potential (p) of the hydrogen ion (H+) in water. It is actually

a way of reporting the concentration of H+ in solution using an electrical "potential"

to measure H+. The pH of any solution is one of the easiest laboratory measurements

to make using a pH meter and an electrode specifically designed to measure hydrogen

(pH electrode). Color indicators and litmus paper are a quick alternative for less precise

measurements. By mixing a quantity of soil with demineralized or distilled water

(usually a 1:1 mixture), we can measure the pH of the water solution in equilibrium

with the soil [15].

The pH measurement is based on a scale from 1 to 14 (pH is reported as

the negative logarithm of the hydrogen ion activity). At a pH of 7.0, there is an equal

balance of hydrogen (H+) ions and hydroxyl (OH-) ions, and the soil (actually the soil-

water suspension) is said to be neutral. Because the pH measurement is logarithmic,

each unit change in pH represents a ten-fold increase in the amount of acidity or

basicity. That is, a soil solution with a pH of 6.0 has 10 times as much active H+ as one

with a pH of 7.0.

Table 2.3: pH of Solution and Hydrogen Ion Activity Performance

pH of solution Hydrogen ion activity (g/liter)

9.0 (very alkaline) 10-9 (0.000000001)

8.0 (alkaline or basic) 10-8 (0.00000001)

7.0 (neutral, pure water) 10-7 (0.0000001)

6.0 (slightly acid) 10-6 (0.000001)

5.0 (very acid) 10-5 (0.00001)

4.0 (extremely acid) 10-4 (0.0001)

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2.4.3 Acid soil infertility

When the pH falls below 6.0, the availability of nutrients such as phosphorus,

potassium, calcium, and magnesium decreases. The availability of the metallic

micronutrients, however, like zinc, manganese, copper, and iron increases as the pH

decreases. Plants don't need aluminum to grow. It's not an essential plant nutrient.

Aluminum, however, is one of the prominent mineral components of silt and clay.

Therefore, the earth's crust is naturally high in aluminum. Like zinc, manganese,

copper and iron, the more acid the soil, the more aluminum will be dissolved into the

soil solution. If the pH is allowed to drop much below 5.5, the availability of

manganese and aluminum is increased to the point that they could become toxic to

plants. Aluminum toxicity to plants is the main concern we have with acid soils in our

region. The problems in very acid and alkaline soils which can affect the plant are

discussed in Table 2.4 [16].

Table 2.4: Problems in Acid and Alkaline Soils

Problems in very acid soils Problems in alkaline soils

*Aluminum toxicity to plant roots *Iron deficiency

*Manganese toxicity to plants *Manganese deficiency

*Calcium & magnesium deficiency *Zinc deficiencies

*Molybdenum deficiency in legumes *excess salts (in some soils)

*P tied up by Fe and Al *P tied up by Ca and Mg

*poor bacterial growth *bacterial diseases in potatoes

*reduced nitrogen transformations

Some plants such as alfalfa, spinach, and lettuce require high levels of calcium

and potassium and can tolerate high salt levels that may occur in near neutral to

alkaline soils. White potatoes will do well in near neutral to slightly acid soils but are

usually grown in more acid soils (pH less than 6.0) because of "scab", a bacterial

disease. As already noted, bacteria don't thrive in very acid soils.

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2.4.4 Factors Affecting Soil pH

Soils are not homogenous and the pH can vary considerably from one spot in the field

to another. It also varies with depth. Soils in different geographic regions, as already

mentioned, may have different pH's because of the five soil forming factors: (1) parent

material, (2) climate, (3) living organisms, (4) topography, and (5) time.

Parent material soils of the Piedmont and Sandstone Plateau regions of

Alabama are very acid because of the acid nature of the rocks (granites and sandstones,

respectively) which formed these soils. Limestone valley soils were formed from basic

rocks (limestones) but may be acid on the surface because of time and weathering.

Some Black Belt Prairie soils may be alkaline because the Selma chalk (soft limestone)

which formed the soils is alkaline [17]. Rainfall/leaching also affects soil pH. Water

passing through the soil leaches basic cations such as calcium (Ca2+), magnesium

(Mg2+), and potassium (K+) into drainage water. These basic cations are replaced by

acidic cations such as aluminum (Al3+) and hydrogen (H+). For this reason, soils

formed under high rainfall conditions are more acid than those formed under arid

conditions.

Fertilizers both chemical and organic fertilizers may eventually make the soil

more acid. Hydrogen is added in the form of ammonia-based fertilizers (NH4+), urea-

based fertilizers [CO(NH2)2], and as proteins (amino acids) in organic fertilizers.

Transformations of these sources of N into nitrate (NO3-) releases H+ to create soil

acidity. Therefore, fertilization with fertilizers containing ammonium or even adding

large quantities of organic matter to a soil will ultimately increase the soil acidity and

lower the pH.

Plant uptake. Plants take up basic cations such as K+, Ca++, and Mg++. When these are

removed from the soil, they are replaced with H+ in order to maintain electrical

neutrality

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Raising Soil pH (Liming Acid Soils) Soils are limed to reduce the harmful

effects of low pH (aluminum or manganese toxicity) and to add calcium and

magnesium to the soil. The amount of lime needed to achieve a certain pH depends on

the pH of the soil and the buffering capacity of the soil. The buffering capacity is

related to the cation exchange capacity (CEC). The higher the CEC, the more

exchangeable acidity (hydrogen and aluminum) is held by the soil colloids. As with

CEC, buffering capacity increases with the amounts of clay and organic matter in the

soil. Soils with a high buffering capacity require larger amounts of lime to increase the

pH than soils with a lower buffering capacity. Most soil testing laboratories use a

special buffered solution to measure the exchangeable acidity. This is the form of soil

acidity that must be neutralized for a change in soil pH. By calibrating pH changes in

the buffered solution with known amounts of acid, the amount of lime required to bring

the soil to a particular pH can be determined [18].

Lime reduces soil acidity (increases pH) by changing some of the hydrogen

ions into water and carbon dioxide (CO2). A Ca++ ion from the lime replaces two

H+ ions on the cation exchange complex. The carbonate (CO3-) reacts with water to

form bicarbonate (HCO3-). These react with H+ to form H2O and CO2. The pH

increases because the H+ concentration has been reduced.

However, the reverse of the above process can also occur. An acid soil can become

more acid as basic cations such as Ca2+, Mg2+, and K+ are removed, usually by crop

uptake or leaching, and replaced by H+.

22

2.4 The effect of the electrode potential

When talking about the electrodes potential effect we ask why do different metals

develop different voltages across their electrode surfaces, in contact with the same

electrolyte? What other factors besides electrode material affect electrode

potentials? The surface of a metal electrode in contact with an electrolyte is shown in

Figure 2.10.

Figure 2.10: The Surface of a Metal Electrode in Contact with an Electrolyte

A voltage develops across the metal/solution interface because charges are

distributed differently on either side of the interface. Suppose some of the metal cations

move from the metal side to the solution side. If the metal is zinc, we can write an

equation to describe this process [19].

Zn = Zn2+ + 2e‾ (2.5)

Two electrons are left behind in the metal; the charge on the metal strip becomes more

negative. The solution becomes more positive. The voltage between the solution and

metal will increase because the difference in charge will increase. If the electrons are

removed somehow, the process can continue, and the zinc is seen to be stripped

gradually dissolve. The reverse process also occurs and the metal cations is moved

from the solution to the metal. This process could be described as

Zn2+ + 2e- = Zn (2.6)

If more electrons are supplied, this process can continue, and most of the zinc

ions in solution will end up deposited on the surface of the zinc strip. Capture of metal

cations by the metal will occur at the same time other metal cations are leaving the

23

voltage across the metal/solution boundary will depend on which of these two

processes dominates.

The first process (conversion of metal atoms into metal cations in solution)

often dominates for metals that give up electrons easily. Lots of things affect how

easily a metal will give up its electrons. For example, metal atoms at the bottom of a

column on the periodic table tend to give up electrons more easily than atoms at the

top of the column. The outer electrons are farther from the nucleus. The inner electrons

tend to wrap around the nucleus and hide its full positive charge from the outer

electrons, too. Because the attraction between the nucleus and the outer electrons is

weakened, the electrons are more easily lost. That's why potassium is more reactive

than sodium [19].

To predict the standard electrode voltages, you have to consider more than just

energetics (how much energy does it take to pry an electron out of the metal? How

much energy is released when an extra electron is added to the metal?) Entropy effects

play an important role as well- but that's another question.

Many other factors affect electrode potentials is when the concentration of

electrolytes in the solution, or the concentration of metal cations in the solution are

changed, the charge difference across the boundary is changed. Ion concentrations in

the solution have a strong effect on the voltages across the metal/solution boundary.

24

2.6 Energy harvesting approaches

Since the energy harvesting from small sources has become wide in academic research,

there are different approaches to harvest the energy from various sources such as

motion and vibration, temperature difference, photolytic, and RF.

2.6.1 Harvesting energy from motion and vibration

For converting motion or vibration, the established transduction mechanisms are

electrostatic, piezoelectric or electromagnetic. In electrostatic transducers, the distance

or overlap of two electrodes of a polarized capacitor changes due to the movement or

to the vibration of one movable electrode. This motion causes a voltage change across

the capacitor and results in a current flow in an external circuit. In piezoelectric

transducers, vibrations or movement cause the deformation of a piezoelectric capacitor

thereby generating a voltage. In electromagnetic transducers, the relative motion of a

magnetic mass with respect to a coil causes a change in the magnetic flux. This

generates an AC voltage across the coil.

If the energy source is a slow, long-stroke movement, it may be possible to

anchor one of the two parts of the transducer to a fixed reference and the other to the

source of movement [20]. In most cases however, this is not possible and the principle

of inertia has to be used. The transducer is inserted in a frame, one part of it is fixed to

the frame itself, and the other can move. The frame is attached to the moving or

vibrating object and relative motion of the parts of the transducer is controlled by the

law of inertia. This approach is the most widely used for harvesting energy from

vibrations [21], in most cases the system is made resonant by means of suspending the

moveable part to a spring. It can also be used for motion energy harvesters [22, 23] in

which case no spring is used and a non-resonant system is the result.

Resonant vibration harvesters are by far the most widely investigated in the

literature. Fine-machined versions are the earliest emerging commercial devices while

micro machined versions on the other hand are far less mature. Their power levels

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